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Monday, April 6, 2015

Interstellar travel


From Wikipedia, the free encyclopedia


A Bussard Ramjet, one of many possible methods that could serve as propulsion for a starship.

Interstellar space travel is manned or unmanned travel between stars. Interstellar travel is much more difficult than interplanetary travel: the distances between the planets in the Solar System are typically measured in standard astronomical units (AU)—whereas the distances between stars are typically hundreds of thousands of AU, and usually expressed in light-years. Because of the vastness of those distances, interstellar travel would require either great speed (some percentage of the speed of light) or huge travel time (lasting from decades to millennia).

The required speeds for interstellar travel in a human lifespan are far beyond what current methods of spacecraft propulsion can provide. The energy required to propel a spacecraft to these speeds, regardless of the propulsion system used, is enormous by today's standards of energy production. At these speeds, collisions by the spacecraft with interstellar dust and gas can produce very dangerous effects both to any passengers and the spacecraft itself.

A number of widely differing strategies have been proposed to deal with these problems, ranging from giant arks that would carry entire societies and ecosystems very slowly, to microscopic space probes. Many different propulsion systems have been proposed to give spacecraft the required speeds: these range from different forms of nuclear propulsion, to beamed energy methods that would require megascale engineering projects, to methods based on speculative physics.

For both unmanned and manned interstellar travel, considerable technological and economic challenges would need to be met. Even the most optimistic views about interstellar travel are that it might happen decades in the future due to the exponential advances in technology; the more common view is that it is a century or more away.

Challenges

Interstellar distances

The basic challenge facing interstellar travel is the immense distances between the stars.

Astronomical distances are measured using different units of length, depending on the scale of the distances involved. Between the planets in the Solar System they are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some 150 million kilometers (93 million miles). Venus, the closest other planet to Earth is (at closest approach) 0.28 AU away. Neptune, the farthest planet from the Sun, is 29.8 AU away. Voyager 1, the farthest man-made object from Earth, is 130.83 AU away.

The closest known star Proxima Centauri, however, is some 268,332 AU away, or 9000 times farther away than even the farthest planet in the Solar System.

Object A.U. light time
The Moon 0.0026 1.3 seconds
Venus (nearest planet) 0.28 2.41 minutes
Neptune (farthest planet) 29.8 4.1 hours
Voyager 1 130.83 18.1 hours
Proxima Centauri (nearest star)    268,332    4.24 years

Because of this, distances between stars are usually expressed in light-years, defined as the distance that a ray of light travels in a year. Light in a vacuum travels around 300,000 kilometers (186,000 miles) per second, so this is some 9.46 trillion kilometers (5.87 trillion miles) or 63,241 AU. Proxima Centauri is 4.243 light-years away.
Another way of understanding the vastness of interstellar distances is by scaling: one of the closest stars to the Sun, Alpha Centauri A (a Sun-like star), can be pictured by scaling down the Earth–Sun distance to one meter (~3.3 ft). On this scale, the distance to Alpha Centauri A would be 271 kilometers (169 miles).

The fastest outward-bound spacecraft yet sent, Voyager 1, has covered 1/600th of a light-year in 30 years and is currently moving at 1/18,000th the speed of light. At this rate, a journey to Proxima Centauri would take 80,000 years.[1]

Some combination of great speed and long travel time are required. The time required by propulsion methods based on currently known physical principles would require years to millennia.

Required energy

A significant factor contributing to the difficulty is the energy that must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the kinetic energy K =  1⁄2 mv2 where m is the final mass. If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the lower bound for the required energy is doubled to mv2.

The velocity for a manned round trip of a few decades to even the nearest star is several thousand times greater than those of present space vehicles. This means that due to the v2 term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least 450 PJ or 4.5 ×1017 J or 125 terawatt-hours (world energy consumption 2008 was 143,851 terawatt-hours), without factoring in efficiency of the propulsion mechanism. This energy has to be generated on-board from stored fuel, harvested from the interstellar medium, or projected over immense distances.

Manned missions

The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned interstellar probe. For instance, the first space probe, Sputnik 1, had a payload of 83.6 kg, whereas the first spacecraft carrying a living passenger (the dog Laika), Sputnik 2, had a payload six times that at 508.3 kg. This underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a closed-cycle life support system. As technology continues to advance, combined with the aggregate risks and support requirements of manned interstellar travel, the first interstellar missions are unlikely to carry life forms.

A manned craft will require more time to reach its top speed as humans have limited tolerance to acceleration.

Interstellar medium

A major issue with traveling at extremely high speeds is that interstellar dust and gas may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Various shielding methods to mitigate this problem have been proposed.[2] Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects, and methods of mitigating these risks, have been discussed in the literature, but many unknowns remain.[3]

Travel time

An interstellar ship would face manifold hazards found in interplanetary travel, including vacuum, radiation, weightlessness, and micrometeoroids. Even the minimum multi-year travel times to the nearest stars are beyond current manned space mission design experience.

The habitual illumination energy requirement for each person is estimated to be 12 kilowatts.[4][5] Other long-term energy requirements are still being investigated.[6]

More speculative approaches to interstellar travel offer the possibility of circumventing these difficulties. Special relativity offers the possibility of shortening the travel time through relativistic time dilation: if a starship could reach velocities approaching the speed of light, the journey time as experienced by the traveler would be greatly reduced (see time dilation section). General relativity offers the theoretical possibility that faster-than-light travel could greatly shorten travel times, both for the traveler and those on Earth (see Faster-than-light travel section).

Wait calculation

It has been argued that an interstellar mission that cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity, not yet having reached the limit, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more-advanced propulsion (the incessant obsolescence postulate).[7] On the other hand, Andrew Kennedy has shown that if one calculates the journey time to a given destination as the rate of travel speed derived from growth (even exponential growth) increases, there is a clear minimum in the total time to that destination from now (see wait calculation).[8] Voyages undertaken before the minimum will be overtaken by those who leave at the minimum, whereas those who leave after the minimum will never overtake those who left at the minimum.

One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.

Communications

The round-trip delay time is the minimum time between an observation by the probe and the moment the probe can receive instructions from Earth reacting to the observation. Given that information can travel no faster than the speed of light, this is for the Voyager 1 about 36 hours, and near Proxima Centauri it would be 8 years. Faster reaction would have to be programmed to be carried out automatically. Of course, in the case of a manned flight the crew can respond immediately to their observations. However, the round-trip delay time makes them not only extremely distant from, but, in terms of communication, also extremely isolated from Earth (analogous to how past long distance explorers were similarly isolated before the invention of the electrical telegraph).

Interstellar communication is still problematic – even if a probe could reach the nearest star, its ability to communicate back to Earth would be difficult given the extreme distance. See Interstellar communication.

Prime targets for interstellar travel

There are 59 known stellar systems within 20 light years of the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:[9]

Stellar system Distance (ly) Remarks
Alpha Centauri 4.3 Closest system. Three stars (G2, K1, M5). Component A is similar to the Sun (a G2 star). Alpha Centauri B has one confirmed planet.[10]
Barnard's Star 6 Small, low-luminosity M5 red dwarf. Second closest to Solar System.
Sirius 8.7 Large, very bright A1 star with a white dwarf companion.
Epsilon Eridani 10.8 Single K2 star slightly smaller and colder than the Sun. Has two asteroid belts, might have a giant and one much smaller planet,[11] and may possess a Solar-System-type planetary system.
Tau Ceti 11.8 Single G8 star similar to the Sun. High probability of possessing a Solar-System-type planetary system: current evidence shows 5 planets with potentially two in the habitable zone.
Gliese 581 20.3 Multiple planet system. The unconfirmed exoplanet Gliese 581 g and the confirmed exoplanet Gliese 581 d are in the star's habitable zone.
Gliese 667C 22 A system with at least six planets. A record-breaking three of these planets are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life.[12]
Vega 25 At least one planet, and of a suitable age to have evolved primitive life [13]

Existing and near-term astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.

Proposed methods

Slow, uncrewed probes

Slow interstellar missions based on current and near-future propulsion technologies are associated with trip times starting from about one hundred years to thousands of years. These missions consist of sending a robotic probe to a nearby star for exploration, similar to interplanetary probes such as used in the Voyager program. By taking along no crew, the cost and complexity of the mission is significantly reduced although technology lifetime is still a significant issue next to obtaining a reasonable speed of travel. Proposed concepts include Project Daedalus, Project Icarus and Project Longshot.

Fast, uncrewed probes

Nanoprobes

Near-lightspeed nanospacecraft might be possible within the near future built on existing microchip technology with a newly developed nanoscale thruster. Researchers at the University of Michigan are developing thrusters that use nanoparticles as propellant. Their technology is called “nanoparticle field extraction thruster”, or nanoFET. These devices act like small particle accelerators shooting conductive nanoparticles out into space.[14]

Michio Kaku, a theoretical physicist, has suggested that clouds of "smart dust" be sent to the stars, which may become possible with advances in nanotechnology. Kaku also notes that a large amount of nanoprobes would need to be sent due to the vulnerability of very small probes to be easily deflected by magnetic fields, micrometeorites and other dangers to ensure the chances that at least one nanoprobe will survive the journey and reach the destination.[15]

Given the light weight of these probes, it would take much less energy to accelerate them. With on board solar cells they could continually accelerate using solar power. One can envision a day when a fleet of millions or even billions of these particles swarm to distant stars at nearly the speed of light and relay signals back to Earth through a vast interstellar communication network.

Slow, manned missions

In crewed missions, the duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways.[16] They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.

Generation ships

A generation ship (or world ship) is a type of interstellar ark in which the crew that arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.[17][18][19][20]

Suspended animation

Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. Although neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long duration of the voyage.[21]

Extended human lifespan

A variant on this possibility is based on the development of substantial human life extension, such as the "Strategies for Engineered Negligible Senescence" proposed by Dr. Aubrey de Grey. If a ship crew had lifespans of some thousands of years, or had artificial bodies, they could traverse interstellar distances without the need to replace the crew in generations. The psychological effects of such an extended period of travel would potentially still pose a problem.

Frozen embryos

A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of an artificial uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots that would replace human parents.[22]

Mind uploading

A more speculative method of transporting humans to the stars is by using mind uploading or also called brain emulation.[23][24] Frank J. Tipler speculates about the colonization of the universe by starships transporting uploaded astronauts.[25] Hein presents a range of concepts how such missions could be conducted, using more or less speculative technologies, for example self-replicating machines, wormholes, and teleportation.[23][26] One of the major challenges besides mind uploading itself are the means for downloading the uploads into physical entities, which can be biological or artificial or both.

Island hopping through interstellar space

Interstellar space is not completely empty; it contains trillions of icy bodies ranging from small asteroids (Oort cloud) to possible rogue planets. There may be ways to take advantage of these resources for a good part of an interstellar trip, slowly hopping from body to body or setting up waystations along the way.[27]

Fast missions

If a spaceship could average 10 percent of light speed (and decelerate at the destination, for manned missions), this would be enough to reach Proxima Centauri in forty years. Several propulsion concepts are proposed that might be eventually developed to accomplish this (see section below on propulsion methods), but none of them are ready for near-term (few decades) development at acceptable cost.[citation needed]

Time dilation

Assuming one cannot travel faster than light one might conclude that a human can never make a round-trip farther from Earth than 20 light years if the traveler is active between the ages of 20 and 60. A traveler would never be able to reach more than the very few star systems that exist within the limit of 20 light years from Earth. This, however, fails to take into account time dilation. Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship's engines were powerful enough the ship could reach mostly anywhere in the galaxy and return to Earth within 40 years ship-time. Upon return, there would be a difference between the time elapsed on the astronaut's ship and the time elapsed on Earth. A spaceship could travel to a star 32 light-years away, initially accelerating at a constant 1.03g (i.e. 10.1 m/s2) for 1.32 years (ship time), then stopping its engines and coasting for the next 17.3 years (ship time) at a constant speed, then decelerating again for 1.32 ship-years, and coming to a stop at the destination. After a short visit the astronaut could return to Earth the same way.
After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to those on Earth, the ship comes back 76 years after launch.

From the viewpoint of the astronaut, on-board clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship-year. The universe would appear contracted along the direction of travel to half the size it had when the ship was at rest; the distance between that star and the Sun would seem to be 16 light years as measured by the astronaut.

At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of the Milky Way (30 kly from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 lightyear per Earth year, so, when back home, the astronaut will find that 60 thousand years will have passed on Earth.[citation needed]

Constant acceleration

This plot shows a ship capable of 1-gee (10 m/s2 or about 1.0 ly/y2) "felt" or proper-acceleration[28] can go far, except for the problem of accelerating on-board propellant.

Regardless of how it is achieved, if a propulsion system can produce acceleration continuously from departure to destination, then this will be the fastest method of travel. If the propulsion system drives the ship faster and faster for the first half of the journey, then turns around and brakes the craft so that it arrives at the destination at a standstill, this is a constant acceleration journey. If this were performed at nearly 1g, this would have the added advantage of producing artificial "gravity". This is, however, prohibitively expensive with current technology.[29]

From the planetary observer perspective the ship will appear to steadily accelerate but more slowly as it approaches the speed of light. The ship will be close to the speed of light after about a year of accelerating and remain at that speed until it brakes for the end of the journey.

From the ship perspective there will be no top limit on speed – the ship keeps going faster and faster the whole first half. This happens because the ship's time sense slows down – relative to the planetary observer – the more it approaches the speed of light.

The result is an impressively fast journey if you are in the ship.

By transmission

If physical entities could be transmitted as information and reconstructed at a destination, travel at nearly the speed of light would be possible, which for the "travelers" would be instantaneous. However, sending an atom-by-atom description of (say) a human body would be a daunting task. Extracting and sending only a computer brain simulation is a significant part of that problem. "Journey" time would be the light-travel time plus the time needed to encode, send and reconstruct the whole transmission.[30]

Propulsion

Rocket concepts

All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, the ratio of initial (M0, including fuel) to final (M1, fuel depleted) mass.

Very high specific power, the ratio of thrust to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.[31] Some heat transfer is inevitable and a tremendous heating load must be adequately handled.

Thus, for interstellar rocket concepts of all technologies, a key engineering problem (seldom explicitly discussed) is limiting the heat transfer from the exhaust stream back into the vehicle.[32]

Nuclear fission powered

Ion engine

The NASA spacecraft Dawn was the first to use an ion engine. In an Ion engine, electric power is used to create charged particles of the fuel, usually the gas xenon, and accelerate them to extremely high velocities. The exhaust velocity of conventional rockets is limited by the chemical energy stored in the fuel’s molecular bonds, which limits the thrust to about 5 km/s. Ion engines are in principle limited only by the electrical power available on the spacecraft, but typically the exhaust speed of the charged particles range from 15 km/s to 35 km/s.[33]
Fission-electric
Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, have the potential to reach speeds much greater than chemically powered vehicles or nuclear-thermal rockets. Such vehicles probably have the potential to power Solar System exploration with reasonable trip times within the current century. Because of their low-thrust propulsion, they would be limited to off-planet, deep-space operation.
Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only small accelerations, would take centuries to reach for example 15% of the velocity of light, thus unsuitable for interstellar flight during a single human lifetime.[34]
Fission-fragment
Fission-fragment rockets use nuclear fission to create high-speed jets of fission fragments, which are ejected at speeds of up to 12,000 km/s. With fission, the energy output is approximately 0.1% of the total mass-energy of the reactor fuel and limits the effective exhaust velocity to about 5% of the velocity of light. For maximum velocity, the reaction mass should optimally consist of fission products, the "ash" of the primary energy source, in order that no extra reaction mass need be book-kept in the mass ratio. This is known as a fission-fragment rocket. thermal-propulsion engines such as NERVA produce sufficient thrust, but can only achieve relatively low-velocity exhaust jets, so to accelerate to the desired speed would require an enormous amount of fuel.
Nuclear pulse
Modern Pulsed Fission Propulsion Concept.

Based on work in the late 1950s to the early 1960s, it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse (space travel's equivalent of fuel economy) and high specific power.[35]

Project Orion team member, Freeman Dyson, proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion that used pure deuterium fusion detonations with a very high fuel-burnup fraction. He computed an exhaust velocity of 15,000 km/s and a 100,000-tonne space vehicle able to achieve a 20,000 km/s delta-v allowing a flight-time to Alpha Centauri of 130 years.[36] Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).[37] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the maximum speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[38] Alternative designs utilizing similar principles include Project Longshot, Project Daedalus, and Mini-Mag Orion. The principle of external nuclear pulse propulsion to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming and for very high-performance interplanetary flight.

In the 1970s the Nuclear Pulse Propulsion concept further was refined by Project Daedalus by use of externally triggered inertial confinement fusion, in this case producing fusion explosions via compressing fusion fuel pellets with high-powered electron beams. Since then, lasers, ion beams, neutral particle beams and hyper-kinetic projectiles have been suggested to produce nuclear pulses for propulsion purposes.[39]

A current impediment to the development of any nuclear-explosion-powered spacecraft is the 1963 Partial Test Ban Treaty, which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space. This treaty would therefore need to be renegotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require international cooperation on at least the scale of the International Space Station.

Nuclear fusion rockets

Fusion rocket starships, powered by nuclear fusion reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light.[40] These would "burn" such light element fuels as deuterium, tritium, 3He, 11B, and 7Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0 .1="" 4="" are="" available="" c.="" correspondingly="" energetically="" exhaust="" fission="" for="" fuel="" higher="" mass-energy.="" maximum="" nbsp="" of="" p="" potentially="" s="" than="" the="" typically="" velocities="">However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, although these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.

Daedalus Interstellar Engine.

Early studies include Project Daedalus, performed by the British Interplanetary Society in 1973–1978, and Project Longshot, a student project sponsored by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II",[41] designed and optimized for crewed Solar System exploration, based on the D3He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10−3 g, with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. Although these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% burnup fraction it could achieve a pure fusion product exhaust velocity of ~3,000 km/s.

Antimatter rockets


File:Beamed-core starship design concept.

An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket. If energy resources and efficient production methods are found to make antimatter in the quantities required and store it safely, it would be theoretically possible to reach speeds approaching that of light. Then relativistic time dilation would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, reducing the trip time experienced by human travelers.

Supposing the production and storage of antimatter should become practical, two further problems would present and need to be solved. First, in the annihilation of antimatter, much of the energy is lost in very penetrating high-energy gamma radiation, and especially also in neutrinos, so that substantially less than mc2 would actually be available if the antimatter were simply allowed to annihilate into radiations thermally. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of mc2 yield of nuclear fusion, the next-best rival candidate.

Second, once again heat transfer from exhaust to vehicle seems likely to deposit enormous wasted energy into the ship, considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming biological shielding were provided to protect the passengers, some of the energy would inevitably heat the vehicle, and may thereby prove limiting. This requires consideration for serious proposals if useful accelerations are to be achieved, because the energies involved (e.g. for 0.1g ship acceleration, approaching 0.3 trillion watts per ton of ship mass) are very large.

Rockets with an external energy source

Rockets deriving their power from external sources, such as a laser, could replace their internal energy source with an energy collector, potentially reducing the mass of the ship greatly and allowing much higher travel speeds. Geoffrey A. Landis has proposed for an interstellar probe, with energy supplied by an external laser from a base station powering an Ion thruster.[42]

Non-rocket concepts

A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it very massive, in accordance with the rocket equation. Some concepts attempt to escape from this problem ([43]):

Interstellar ramjets

In 1960, Robert W. Bussard proposed the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton–proton fusion reaction, and expel it out of the back. Later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design. Yet the idea is attractive because the fuel would be collected en route (commensurate with the concept of energy harvesting), so the craft could theoretically accelerate to near the speed of light.

Beamed propulsion


This diagram illustrates Robert L. Forward's scheme for slowing down an interstellar light-sail at the destination [44] system.

A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft's payload. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, whereas the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.[45]

A magnetic sail could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium.[46][47]

The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:[48]

Mission Laser Power Vehicle Mass Acceleration Sail Diameter Maximum Velocity (% of the speed of light)
1. Flyby - Alpha Centauri, 40 years
outbound stage 65 GW 1 t 0.036 g 3.6 km 11% @ 0.17 ly
2. Rendezvous - Alpha Centauri, 41 years
outbound stage 7,200 GW 785 t 0.005 g 100 km 21% @ 4.29 ly
deceleration stage 26,000 GW 71 t 0.2 g 30 km 21% @ 4.29 ly
3. Manned - Epsilon Eridani, 51 years (including 5 years exploring star system)
outbound stage 75,000,000 GW 78,500 t 0.3 g 1000 km 50% @ 0.4 ly
deceleration stage 21,500,000 GW 7,850 t 0.3 g 320 km 50% @ 10.4 ly
return stage 710,000 GW 785 t 0.3 g 100 km 50% @ 10.4 ly
deceleration stage 60,000 GW 785 t 0.3 g 100 km 50% @ 0.4 ly

Pre-accelerated fuel

Achieving start-stop interstellar trip times of less than a human lifetime require mass-ratios of between 1,000 and 1,000,000, even for the nearer stars. This could be achieved by multi-staged vehicles on a vast scale.[40]
Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.[49]

Speculative methods

Quark matter

Scientist T. Marshall Eubanks thinks that nuggets of condensed quark matter may exist at the centers of some asteroids, created during the Big Bang and each nugget with a mass of 1010 to 1011 kg.[50] If so these could be an enormous source of energy, as the nuggets could be used to generate huge quantities of antimatter—about a million tonnes of antimatter per nugget. This would be enough to propel a spacecraft close to the speed of light.[51]

Hawking radiation rockets

In a black hole starship, a parabolic reflector would reflect Hawking radiation from an artificial black hole. In 2009, Louis Crane and Shawn Westmoreland of Kansas State University published a paper investigating the feasibility of this idea. Their conclusion was that it was on the edge of possibility, but that quantum gravity effects that are presently unknown may make it easier or make it impossible.[52][53]

Magnetic monopole rockets

If some of the Grand unification models are correct, e.g. 't Hooft–Polyakov, it would be possible to construct a photonic engine that uses no antimatter thanks to the magnetic monopole that hypothetically can catalyze the decay of a proton to a positron and π0-meson:[54][55]
p \rarr e^{+} + \pi^0
π0 decays rapidly to two photons, and the positron annihilates with an electron to give two more photons. As a result, a hydrogen atom turns into four photons and only the problem of a mirror remains unresolved.

A magnetic monopole engine could also work on a once-through scheme such as the Bussard ramjet (see below).

At the same time, most of the modern Grand unification theories such as M-theory predict no magnetic monopoles, which casts doubt on this attractive idea.

Faster-than-light travel


Artist's depiction of a hypothetical Wormhole Induction Propelled Spacecraft, based loosely on the 1994 "warp drive" paper of Miguel Alcubierre. Credit: NASA CD-98-76634 by Les Bossinas.

Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light.
Even the most serious-minded of these are speculative.

It is also debated whether this is possible, in part, because of causality concerns, because in essence travel faster than light is equivalent to going back in time. Proposed mechanisms for faster-than-light travel within the theory of general relativity require the existence of exotic matter.
Alcubierre drive
According to Einstein's equation of general relativity, spacetime is curved:
G_{\mu\nu}=8\pi\,GT_{\mu\nu} \,
General relativity may permit the travel of an object faster than light in curved spacetime.[56] One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the warp drive concept.
In physics, the Alcubierre drive is based on an argument that the curvature could take the form of a wave in which a spaceship might be carried in a "bubble". Space would be collapsing at one end of the bubble and expanding at the other end. The motion of the wave would carry a spaceship from one space point to another in less time than light would take through unwarped space. Nevertheless, the spaceship would not be moving faster than light within the bubble. This concept would require the spaceship to incorporate a region of exotic matter, or "negative mass".
Artificial gravity control
Scientist Lance Williams thinks that gravity can be controlled artificially through electromagnetic control.[57]
Wormholes
Wormholes are conjectural distortions in spacetime that theorists postulate could connect two arbitrary points in the universe, across an Einstein–Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity that allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical.[58]
However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.[59] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.[60]

Designs and studies

Enzmann starship

The Enzmann starship, as detailed by G. Harry Stine in the October 1973 issue of Analog, was a design for a future starship, based on the ideas of Dr. Robert Duncan-Enzmann.[61] The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium to power 12–24 thermonuclear pulse propulsion units.[61] Twice as long as the Empire State Building and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probes and telescopic observation of target star systems.[61][62]

Project Hyperion

Project Hyperion, one of the projects of Icarus Interstellar.[63]

NASA research

NASA has been researching interstellar travel since its formation, translating important foreign language papers and conducting early studies on applying fusion propulsion, in the 1960s, and laser propulsion, in the 1970s, to interstellar travel.

The NASA Breakthrough Propulsion Physics Program (terminated in FY 2003 after a 6-year, $1.2-million study, because "No breakthroughs appear imminent.")[64] identified some breakthroughs that are needed for interstellar travel to be possible.[65]

Geoffrey A. Landis of NASA's Glenn Research Center states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with extensive sails, propelled by laser light to about one-tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years,[66] whereas a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.

100 Year Starship study

The 100 Year Starship (100YSS) is the name of the overall effort that will, over the next century, work toward achieving interstellar travel. The effort will also go by the moniker 100YSS. The 100 Year Starship study is the name of a one-year project to assess the attributes of and lay the groundwork for an organization that can carry forward the 100 Year Starship vision.

Dr. Harold ("Sonny") White[67] from NASA's Johnson Space Center is a member of Icarus Interstellar,[68] the nonprofit foundation whose mission is to realize interstellar flight before the year 2100. At the 2012 meeting of 100YSS, he reported using a laser to try to warp spacetime by 1 part in 10 million with the aim of helping to make interstellar travel possible.[69]

Other designs

Non-profit organizations

A few organisations dedicated to interstellar propulsion research and advocacy for the case exist worldwide. These are still in their infancy, but are already backed up by a membership of a wide variety of scientists, students and professionals.

Skepticism

The energy requirements make interstellar travel very difficult. It has been reported that at the 2008 Joint Propulsion Conference, multiple experts opined that it was improbable that humans would ever explore beyond the Solar System.[76] Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated at least the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.[76]
at

Spacecraft propulsion


From Wikipedia, the free encyclopedia


A remote camera captures a close-up view of a Space Shuttle Main Engine during a test firing at the John C. Stennis Space Center in Hancock County, Mississippi.

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. This sort of engine is called a rocket engine.

All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some (such as the Pegasus rocket and SpaceShipOne) have used air-breathing engines on their first stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south stationkeeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall effect thrusters (two different types of electric propulsion) to great success.

Requirements

Artificial satellites must be launched into orbit and once there they must be placed in their nominal orbit. Once in the desired orbit, they often need some form of attitude control so that they are correctly pointed with respect to Earth, the Sun, and possibly some astronomical object of interest.[1] They are also subject to drag from the thin atmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital stationkeeping).[2] Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.[3] A satellite's useful life is over once it has exhausted its ability to adjust its orbit.

Spacecraft designed to travel further also need propulsion methods. They need to be launched out of the Earth's atmosphere just as satellites do. Once there, they need to leave orbit and move around.

For interplanetary travel, a spacecraft must use its engines to leave Earth orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.[4] In between these adjustments, the spacecraft simply falls freely along its trajectory. The most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit: the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.[5] Special methods such as aerobraking or aerocapture are sometimes used for this final orbital adjustment.[6]

Artist's concept of a solar sail

Some spacecraft propulsion methods such as solar sails provide very low but inexhaustible thrust;[7] an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun or constantly thrusting along its direction of motion to increase its distance from the Sun. The concept has been successfully tested by the Japanese IKAROS solar sail spacecraft.

Spacecraft for interstellar travel also need propulsion methods. No such spacecraft has yet been built, but many designs have been discussed. Because interstellar distances are very great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival will be a formidable challenge for spacecraft designers.[8]

Effectiveness

When in space, the purpose of a propulsion system is to change the velocity, or v, of a spacecraft. Because this is more difficult for more massive spacecraft, designers generally discuss momentum, mv. The amount of change in momentum is called impulse.[9] So the goal of a propulsion method in space is to create an impulse.

When launching a spacecraft from Earth, a propulsion method must overcome a higher gravitational pull to provide a positive net acceleration.[10] In orbit, any additional impulse, even very tiny, will result in a change in the orbit path.

The rate of change of velocity is called acceleration, and the rate of change of momentum is called force. To reach a given velocity, one can apply a small acceleration over a long period of time, or one can apply a large acceleration over a short time. Similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations but runs for a long time can produce the same impulse as a propulsion method that produces large accelerations for a short time. When launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.

Earth's surface is situated fairly deep in a gravity well. The escape velocity required to get out of it is 11.2 kilometers/second. As human beings evolved in a gravitational field of 1g (9.8 m/s²), an ideal propulsion system would be one that provides a continuous acceleration of 1g (though human bodies can tolerate much larger accelerations over short periods). The occupants of a rocket or spaceship having such a propulsion system would be free from all the ill effects of free fall, such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones.

The law of conservation of momentum means that in order for a propulsion method to change the momentum of a space craft it must change the momentum of something else as well. A few designs take advantage of things like magnetic fields or light pressure in order to change the spacecraft's momentum, but in free space the rocket must bring along some mass to accelerate away in order to push itself forward. Such mass is called reaction mass.

In order for a rocket to work, it needs two things: reaction mass and energy. The impulse provided by launching a particle of reaction mass having mass m at velocity v is mv. But this particle has kinetic energy mv²/2, which must come from somewhere. In a conventional solid, liquid, or hybrid rocket, the fuel is burned, providing the energy, and the reaction products are allowed to flow out the back, providing the reaction mass. In an ion thruster, electricity is used to accelerate ions out the back. Here some other source must provide the electrical energy (perhaps a solar panel or a nuclear reactor), whereas the ions provide the reaction mass.[10]

When discussing the efficiency of a propulsion system, designers often focus on effectively using the reaction mass. Reaction mass must be carried along with the rocket and is irretrievably consumed when used. One way of measuring the amount of impulse that can be obtained from a fixed amount of reaction mass is the specific impulse, the impulse per unit weight-on-Earth (typically designated by I_{sp}). The unit for this value is seconds. Because the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass. This alternate form of specific impulse uses the same units as velocity (e.g. m/s), and in fact it is equal to the effective exhaust velocity of the engine (typically designated v_{e}).
Confusingly, both values are sometimes called specific impulse. The two values differ by a factor of gn, the standard acceleration due to gravity 9.80665 m/s² (I_{sp} g_\mathrm{n} = v_{e}).

A rocket with a high exhaust velocity can achieve the same impulse with less reaction mass. However, the energy required for that impulse is proportional to the exhaust velocity, so that more mass-efficient engines require much more energy, and are typically less energy efficient. This is a problem if the engine is to provide a large amount of thrust. To generate a large amount of impulse per second, it must use a large amount of energy per second. So high-mass-efficient engines require enormous amounts of energy per second to produce high thrusts. As a result, most high-mass-efficient engine designs also provide lower thrust due to the unavailability of high amounts of energy.

Methods

Propulsion methods can be classified based on their means of accelerating the reaction mass. There are also some special methods for launches, planetary arrivals, and landings.

Reaction engines

A reaction engine is an engine which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force".
Examples include both duct engines and rocket engines, and more uncommon variations such as Hall effect thrusters, ion drives and mass drivers. Duct engines are obviously not used for space propulsion due to the lack of air; however some proposed spacecraft have these kinds of engines to assist takeoff and landing.

Delta-v and propellant


Rocket mass ratios versus final velocity, as calculated from the rocket equation

Exhausting the entire usable propellant of a spacecraft through the engines in a straight line in free space would produce a net velocity change to the vehicle; this number is termed 'delta-v' (\Delta v).

If the exhaust velocity is constant then the total \Delta v of a vehicle can be calculated using the rocket equation, where M is the mass of propellant, P is the mass of the payload (including the rocket structure), and v_e is the velocity of the rocket exhaust. This is known as the Tsiolkovsky rocket equation:
\Delta v = v_e \ln \left(\frac{M+P}{P}\right).
For historical reasons, as discussed above, v_e is sometimes written as
v_e = I_{sp} g_{o}
where I_{sp} is the specific impulse of the rocket, measured in seconds, and g_{o} is the gravitational acceleration at sea level.

For a high delta-v mission, the majority of the spacecraft's mass needs to be reaction mass. Because a rocket must carry all of its reaction mass, most of the initially-expended reaction mass goes towards accelerating reaction mass rather than payload. If the rocket has a payload of mass P, the spacecraft needs to change its velocity by \Delta v, and the rocket engine has exhaust velocity ve, then the mass M of reaction mass which is needed can be calculated using the rocket equation and the formula for I_{sp}:
M = P \left(e^{\Delta v/v_e}-1\right).
For \Delta v much smaller than ve, this equation is roughly linear, and little reaction mass is needed. If \Delta v is comparable to ve, then there needs to be about twice as much fuel as combined payload and structure (which includes engines, fuel tanks, and so on). Beyond this, the growth is exponential; speeds much higher than the exhaust velocity require very high ratios of fuel mass to payload and structural mass.

For a mission, for example, when launching from or landing on a planet, the effects of gravitational attraction and any atmospheric drag must be overcome by using fuel. It is typical to combine the effects of these and other effects into an effective mission delta-v. For example a launch mission to low Earth orbit requires about 9.3–10 km/s delta-v. These mission delta-vs are typically numerically integrated on a computer.

Some effects such as Oberth effect can only be significantly utilised by high thrust engines such as rockets, i.e. engines that can produce a high g-force (thrust per unit mass, equal to delta-v per unit time).

Power use and propulsive efficiency

For all reaction engines (such as rockets and ion drives) some energy must go into accelerating the reaction mass. Every engine will waste some energy, but even assuming 100% efficiency, to accelerate an exhaust the engine will need energy amounting to
\frac {1} {2} \dot m v_e^2[11]
This energy is not necessarily lost- some of it usually ends up as kinetic energy of the vehicle, and the rest is wasted in residual motion of the exhaust.

Due to energy carried away in the exhaust the energy efficiency of a reaction engine varies with the speed of the exhaust relative to the speed of the vehicle, this is called propulsive efficiency

Comparing the rocket equation (which shows how much energy ends up in the final vehicle) and the above equation (which shows the total energy required) shows that even with 100% engine efficiency, certainly not all energy supplied ends up in the vehicle - some of it, indeed usually most of it, ends up as kinetic energy of the exhaust.

The exact amount depends on the design of the vehicle, and the mission. However there are some useful fixed points:
  • if the I_{sp} is fixed, for a mission delta-v, there is a particular I_{sp} that minimises the overall energy used by the rocket. This comes to an exhaust velocity of about ⅔ of the mission delta-v (see the energy computed from the rocket equation). Drives with a specific impulse that is both high and fixed such as Ion thrusters have exhaust velocities that can be enormously higher than this ideal for many missions.
  • if the exhaust velocity can be made to vary so that at each instant it is equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space [1] and has no kinetic energy; and the propulsive efficiency is 100%- all the energy ends up in the vehicle (in principle such a drive would be 100% efficient, in practice there would be thermal losses from within the drive system and residual heat in the exhaust). However in most cases this uses an impractical quantity of propellant, but is a useful theoretical consideration. Anyway the vehicle has to move before the method can be applied.
Some drives (such as VASIMR or Electrodeless plasma thruster) actually can significantly vary their exhaust velocity. This can help reduce propellant usage or improve acceleration at different stages of the flight. However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed.
Proposed ion and plasma drives usually have exhaust velocities enormously higher than that ideal (in the case of VASIMR the lowest quoted speed is around 15000 m/s compared to a mission delta-v from high Earth orbit to Mars of about 4000m/s).

It might be thought that adding power generation capacity is helpful, and although initially this can improve performance, this inevitably increases the weight of the power source, and eventually the mass of the power source and the associated engines and propellant dominates the weight of the vehicle, and then adding more power gives no significant improvement.

For, although solar power and nuclear power are virtually unlimited sources of energy, the maximum power they can supply is substantially proportional to the mass of the powerplant (i.e. specific power takes a largely constant value which is dependent on the particular powerplant technology). For any given specific power, with a large v_{e} which is desirable to save propellant mass, it turns out that the maximum acceleration is inversely proportional to v_{e}. Hence the time to reach a required delta-v is proportional to v_{e}. Thus the latter should not be too large.

Energy


Plot of instantaneous propulsive efficiency (blue) and overall efficiency for a vehicle accelerating from rest (red) as percentages of the engine efficiency

In the ideal case m_1 is useful payload and m_0-m_1 is reaction mass (this corresponds to empty tanks having no mass, etc.). The energy required can simply be computed as
\frac{1}{2}(m_0-m_1)v_\text{e}^2
This corresponds to the kinetic energy the expelled reaction mass would have at a speed equal to the exhaust speed.
If the reaction mass had to be accelerated from zero speed to the exhaust speed, all energy produced would go into the reaction mass and nothing would be left for kinetic energy gain by the rocket and payload. However, if the rocket already moves and accelerates (the reaction mass is expelled in the direction opposite to the direction in which the rocket moves) less kinetic energy is added to the reaction mass. To see this, if, for example, v_e=10 km/s and the speed of the rocket is 3 km/s, then the speed of a small amount of expended reaction mass changes from 3 km/s forwards to 7 km/s rearwards. Thus, although the energy required is 50 MJ per kg reaction mass, only 20 MJ is used for the increase in speed of the reaction mass. The remaining 30 MJ is the increase of the kinetic energy of the rocket and payload.

In general:
d\left(\frac{1}{2}v^2\right)=vdv=vv_\text{e}dm/m=\frac{1}{2}\left(v_\text{e}^2-(v-v_\text{e})^2+v^2\right)dm/m
Thus the specific energy gain of the rocket in any small time interval is the energy gain of the rocket including the remaining fuel, divided by its mass, where the energy gain is equal to the energy produced by the fuel minus the energy gain of the reaction mass. The larger the speed of the rocket, the smaller the energy gain of the reaction mass; if the rocket speed is more than half of the exhaust speed the reaction mass even loses energy on being expelled, to the benefit of the energy gain of the rocket; the larger the speed of the rocket, the larger the energy loss of the reaction mass.

We have
\Delta \epsilon = \int v\, d (\Delta v)
where \epsilon is the specific energy of the rocket (potential plus kinetic energy) and \Delta v is a separate variable, not just the change in v. In the case of using the rocket for deceleration, i.e. expelling reaction mass in the direction of the velocity, v should be taken negative.

The formula is for the ideal case again, with no energy lost on heat, etc. The latter causes a reduction of thrust, so it is a disadvantage even when the objective is to lose energy (deceleration).

If the energy is produced by the mass itself, as in a chemical rocket, the fuel value has to be \scriptstyle{v_\text{e}^2/2}, where for the fuel value also the mass of the oxidizer has to be taken into account. A typical value is v_\text{e} = 4.5 km/s, corresponding to a fuel value of 10.1 MJ/kg. The actual fuel value is higher, but much of the energy is lost as waste heat in the exhaust that the nozzle was unable to extract.

The required energy E is
E = \frac{1}{2}m_1\left(e^{\Delta v\ / v_\text{e}}-1\right)v_\text{e}^2
Conclusions:
  • for \Delta v \ll v_e we have E\approx \frac{1}{2}m_1 v_\text{e} \Delta v
  • for a given \Delta v, the minimum energy is needed if v_\text{e}=0.6275 \Delta v, requiring an energy of
E = 0.772 m_1(\Delta v)^2.
In the case of acceleration in a fixed direction, and starting from zero speed, and in the absence of other forces, this is 54.4% more than just the final kinetic energy of the payload. In this optimal case the initial mass is 4.92 times the final mass.
These results apply for a fixed exhaust speed.

Due to the Oberth effect and starting from a nonzero speed, the required potential energy needed from the propellant may be less than the increase in energy in the vehicle and payload. This can be the case when the reaction mass has a lower speed after being expelled than before – rockets are able to liberate some or all of the initial kinetic energy of the propellant.

Also, for a given objective such as moving from one orbit to another, the required \Delta v may depend greatly on the rate at which the engine can produce \Delta v and maneuvers may even be impossible if that rate is too low. For example, a launch to LEO normally requires a \Delta v of ca. 9.5 km/s (mostly for the speed to be acquired), but if the engine could produce \Delta v at a rate of only slightly more than g, it would be a slow launch requiring altogether a very large \Delta v (think of hovering without making any progress in speed or altitude, it would cost a \Delta v of 9.8 m/s each second). If the possible rate is only g or less, the maneuver can not be carried out at all with this engine.

The power is given by
P= \frac{1}{2} m a v_\text{e} = \frac{1}{2}F v_\text{e}
where F is the thrust and a the acceleration due to it. Thus the theoretically possible thrust per unit power is 2 divided by the specific impulse in m/s. The thrust efficiency is the actual thrust as percentage of this.

If e.g. solar power is used this restricts a; in the case of a large v_\text{e} the possible acceleration is inversely proportional to it, hence the time to reach a required delta-v is proportional to v_\text{e}; with 100% efficiency:
  • for \Delta v \ll v_\text{e} we have t\approx \frac{m v_\text{e} \Delta v}{2P}
Examples:
  • power 1000 W, mass 100 kg, \Delta v= 5 km/s, v_\text{e}= 16 km/s, takes 1.5 months.
  • power 1000 W, mass 100 kg, \Delta v= 5 km/s, v_\text{e}= 50 km/s, takes 5 months.
Thus v_\text{e} should not be too large.

Power to thrust ratio

The power to thrust ratio is simply:[11]
\frac {P} {F} = \frac { \frac {1} {2} {\dot m v^2}} {\dot m v} = \frac {1} {2} v
Thus for any vehicle power P, the thrust that may be provided is:
F = \frac {P} {\frac {1} {2} v} = \frac {2 P} v

Example

Suppose we want to send a 10,000 kg space probe to Mars. The required \Delta v from LEO is approximately 3000 m/s, using a Hohmann transfer orbit. For the sake of argument, let us say that the following thrusters may be used:
Engine Effective Exhaust Velocity
(km/s)		 Specific impulse
(s)		 Fuel mass
(kg)		 Energy required
(GJ)		 Energy per kg
of propellant		 minimum power/thrust Power generator mass/thrust*
Solid rocket 1 100 190,000 95 500 kJ 0.5 kW/N N/A
Bipropellant rocket 5 500 8,200 103 12.6 MJ 2.5 kW/N N/A
Ion thruster 50 5,000 620 775 1.25 GJ 25 kW/N 25 kg/N
* - assumes a specific power of 1kW/kg

Observe that the more fuel-efficient engines can use far less fuel; its mass is almost negligible (relative to the mass of the payload and the engine itself) for some of the engines. However, note also that these require a large total amount of energy. For Earth launch, engines require a thrust to weight ratio of more than one. To do this with the ion or more theoretical electrical drives, the engine would have to be supplied with one to several gigawatts of power — equivalent to a major metropolitan generating station. From the table it can be seen that this is clearly impractical with current power sources.

Alternative approaches include some forms of laser propulsion, where the reaction mass does not provide the energy required to accelerate it, with the energy instead being provided from an external laser or other Beam-powered propulsion system. Small models of some of these concepts have flown, although the engineering problems are complex and the ground based power systems are not a solved problem.

Instead, a much smaller, less powerful generator may be included which will take much longer to generate the total energy needed. This lower power is only sufficient to accelerate a tiny amount of fuel per second, and would be insufficient for launching from Earth. However, over long periods in orbit where there is no friction, the velocity will be finally achieved. For example, it took the SMART-1 more than a year to reach the Moon, whereas with a chemical rocket it takes a few days. Because the ion drive needs much less fuel, the total launched mass is usually lower, which typically results in a lower overall cost, but the journey takes longer.

Mission planning therefore frequently involves adjusting and choosing the propulsion system so as to minimise the total cost of the project, and can involve trading off launch costs and mission duration against payload fraction.

Rocket engines

SpaceX's Kestrel engine is tested

Most rocket engines are internal combustion heat engines (although non combusting forms exist). Rocket engines generally produce a high temperature reaction mass, as a hot gas. This is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber. The extremely hot gas is then allowed to escape through a high-expansion ratio nozzle. This bell-shaped nozzle is what gives a rocket engine its characteristic shape. The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy. Exhaust speed reaching as high as 10 times the speed of sound at sea level are common.

Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.

Ion propulsion rockets can heat a plasma or charged gas inside a magnetic bottle and release it via a magnetic nozzle, so that no solid matter need come in contact with the plasma. Of course, the machinery to do this is complex, but research into nuclear fusion has developed methods, some of which have been proposed to be used in propulsion systems, and some have been tested in a lab.

See rocket engine for a listing of various kinds of rocket engines using different heating methods, including chemical, electrical, solar, and nuclear.

Electromagnetic propulsion


This test engine accelerates ions using electrostatic forces

Rather than relying on high temperature and fluid dynamics to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or electromagnetic forces to accelerate the reaction mass directly. Usually the reaction mass is a stream of ions. Such an engine typically uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities.

The idea of electric propulsion dates back to 1906, when Robert Goddard considered the possibility in his personal notebook.[12] Konstantin Tsiolkovsky published the idea in 1911.

For these drives, at the highest exhaust speeds, energetic efficiency and thrust are all inversely proportional to exhaust velocity. Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel.

For some missions, particularly reasonably close to the Sun, solar energy may be sufficient, and has very often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are called nuclear electric rockets.

With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits the amount of thrust that can be produced to a small value. Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle.

Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun. Chemical power generators are not used due to the far lower total available energy. Beamed power to the spacecraft shows some potential.

6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory.

Some electromagnetic methods:
In electrothermal and electromagnetic thrusters, both ions and electrons are accelerated simultaneously, no neutralizer is required.

Without internal reaction mass


NASA study of a solar sail. The sail would be half a kilometer wide.

The law of conservation of momentum is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible). But space is not empty, especially space inside the Solar System; there are gravitation fields, magnetic fields, electromagnetic waves, solar wind and solar radiation. Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux density P of an EM wave is quantitatively 1/c times the Poynting vector S, i.e. P = S/c, where c is the velocity of light. Field propulsion methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft. However, because many of these phenomena are diffuse in nature, corresponding propulsion structures need to be proportionately large.

There are several different space drives that need little or no reaction mass to function. A tether propulsion system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object.[13] Solar sails rely on radiation pressure from electromagnetic energy, but they require a large collection surface to function effectively. The magnetic sail deflects charged particles from the solar wind with a magnetic field, thereby imparting momentum to the spacecraft. A variant is the mini-magnetospheric plasma propulsion system, which uses a small cloud of plasma held in a magnetic field to deflect the Sun's charged particles. An E-sail would use very thin and lightweight wires holding an electric charge to deflect these particles, and may have more controllable directionality.

As a proof of concept, NanoSail-D became the first nanosatellite to orbit Earth.[14][full citation needed] There are plans to add them[clarification needed] to future Earth orbit satellites, enabling them to de-orbit and burn up once they are no longer needed. Cube sail aims to tackle space junk.[15][full citation needed]

Japan also launched its own solar sail powered spacecraft IKAROS in May 2010. IKAROS successfully demonstrated propulsion and guidance and is still flying today.

A satellite or other space vehicle is subject to the law of conservation of angular momentum, which constrains a body from a net change in angular velocity. Thus, for a vehicle to change its relative orientation without expending reaction mass, another part of the vehicle may rotate in the opposite direction. Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum,[16] so secondary systems are designed to "bleed off" undesired rotational energies built up over time. Accordingly, many spacecraft utilize reaction wheels or control moment gyroscopes to control orientation in space.[17]

A gravitational slingshot can carry a space probe onward to other destinations without the expense of reaction mass. By harnessing the gravitational energy of other celestial objects, the spacecraft can pick up kinetic energy.[18]
However, even more energy can be obtained from the gravity assist if rockets are used.

Planetary and atmospheric propulsion


A successful proof of concept Lightcraft test, a subset of beam-powered propulsion.

Launch-assist mechanisms

The conceptual ocean-located Quicklauncher, a light-gas gun–based space gun

There have been many ideas proposed for launch-assist mechanisms that have the potential of drastically reducing the cost of getting into orbit. Proposed non-rocket spacelaunch launch-assist mechanisms include:

Airbreathing engines

Studies generally show that conventional air-breathing engines, such as ramjets or turbojets are basically too heavy (have too low a thrust/weight ratio) to give any significant performance improvement when installed on a launch vehicle itself. However, launch vehicles can be air launched from separate lift vehicles (e.g. B-29, Pegasus Rocket and White Knight) which do use such propulsion systems. Jet engines mounted on a launch rail could also be so used.
On the other hand, very lightweight or very high speed engines have been proposed that take advantage of the air during ascent:
  • SABRE - a lightweight hydrogen fuelled turbojet with precooler[19]
  • ATREX - a lightweight hydrogen fuelled turbojet with precooler[20]
  • Liquid air cycle engine - a hydrogen fuelled jet engine that liquifies the air before burning it in a rocket engine
  • Scramjet - jet engines that use supersonic combustion
Normal rocket launch vehicles fly almost vertically before rolling over at an altitude of some tens of kilometers before burning sideways for orbit; this initial vertical climb wastes propellant but is optimal as it greatly reduces airdrag. Airbreathing engines burn propellant much more efficiently and this would permit a far flatter launch trajectory, the vehicles would typically fly approximately tangentially to Earth's surface until leaving the atmosphere then perform a rocket burn to bridge the final delta-v to orbital velocity.

Planetary arrival and landing


A test version of the MARS Pathfinder airbag system

When a vehicle is to enter orbit around its destination planet, or when it is to land, it must adjust its velocity. This can be done using all the methods listed above (provided they can generate a high enough thrust), but there are a few methods that can take advantage of planetary atmospheres and/or surfaces.
  • Aerobraking allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere at the low point of the orbit. This can save a considerable amount of fuel because it takes much less delta-V to enter an elliptical orbit compared to a low circular orbit. Because the braking is done over the course of many orbits, heating is comparatively minor, and a heat shield is not required. This has been done on several Mars missions such as Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter, and at least one Venus mission, Magellan.
  • Aerocapture is a much more aggressive manoeuver, converting an incoming hyperbolic orbit to an elliptical orbit in one pass. This requires a heat shield and much trickier navigation, because it must be completed in one pass through the atmosphere, and unlike aerobraking no preview of the atmosphere is possible. If the intent is to remain in orbit, then at least one more propulsive maneuver is required after aerocapture—otherwise the low point of the resulting orbit will remain in the atmosphere, resulting in eventual re-entry. Aerocapture has not yet been tried on a planetary mission, but the re-entry skip by Zond 6 and Zond 7 upon lunar return were aerocapture maneuvers, because they turned a hyperbolic orbit into an elliptical orbit. On these missions, because there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.
  • A ballute is an inflatable drag device.
  • Parachutes can land a probe on a planet or moon with an atmosphere, usually after the atmosphere has scrubbed off most of the velocity, using a heat shield.
  • Airbags can soften the final landing.
  • Lithobraking, or stopping by impacting the surface, is usually done by accident. However, it may be done deliberately with the probe expected to survive (see, for example, Deep Impact (spacecraft)), in which case very sturdy probes are required.

Hypothetical methods


Artist's conception of a warp drive design

A variety of hypothetical propulsion techniques have been considered that would require entirely new principles of physics to be realized or that may not exist. To date, such methods are highly speculative and include:[citation needed]
A NASA assessment is found at Marc G Millis Assessing potential propulsion breakthroughs (2005) and an overview of NASA research in this area is at Breakthrough Propulsion Physics.

Table of methods

Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods.

Four numbers are shown. The first is the effective exhaust velocity: the equivalent speed that the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However:
  • if the delta-v is much more than the exhaust velocity, then exorbitant amounts of fuel are necessary (see the section on calculations, above)
  • if it is much more than the delta-v, then, proportionally more energy is needed; if the power is limited, as with solar energy, this means that the journey takes a proportionally longer time
The second and third are the typical amounts of thrust and the typical burn times of the method. Outside a gravitational potential small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period. (This result does not apply when the object is significantly influenced by gravity.)

The fourth is the maximum delta-v this technique can give (without staging). For rocket-like propulsion systems this is a function of mass fraction and exhaust velocity. Mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight. For a system to achieve this limit, typically the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower.

Propulsion methods
Method Effective
Exhaust
Velocity
(km/s)		 Thrust
(N)		 Firing
Duration		 Maximum
Delta-v
(km/s)		 Technology
readiness
level
Solid-fuel rocket <~ 2.5 <~ 107 minutes ~ 7 9:Flight proven
Hybrid rocket minutes > 3 9:Flight proven
Monopropellant rocket 1 – 3[citation needed] 0.1 – 100[citation needed] milliseconds–minutes ~ 3 9:Flight proven
Liquid-fuel rocket <~ 4.4 <~ 107 minutes ~ 9 9:Flight proven
Electrostatic ion thruster 15 – 210[22][full citation needed] months/years > 100 9:Flight proven
Hall effect thruster (HET) 8–50[citation needed] months/years > 100 9:Flight proven[23]
Resistojet rocket 2–6 10−2–10 minutes ? 8:Flight qualified[24]
Arcjet rocket 4–16 10−2–10 minutes ? 8:Flight qualified[citation needed]
Field Emission Electric Propulsion (FEEP) 100[25]–130 10−6[25]–10−3[25] months/years ? 8:Flight qualified[25]
Pulsed plasma thruster (PPT) ~ 20 ~ 0.1 ~2,000–10,000 hours ? 7:Prototype demoed in space
Dual mode propulsion rocket 1 – 4.7 0.1 – 107 milliseconds–minutes ~ 3 – 9 7:Prototype demoed in space
Solar sails 299792:Light
145–750:Wind		 9/km2 at 1 AU
230/km2 at 0.2AU
10−10/km2 at 4 ly		 indefinite > 40 9:Light pressure attitude-control flight proven
6:Deploy-only demoed in space
5:Light-sail validated in lit vacuum
Tripropellant rocket 2.5–5.3[citation needed] 0.1–107[citation needed] minutes ~ 9 6:Prototype demoed on ground[26]
Magnetoplasmadynamic thruster (MPD) 20–100 100 weeks ? 6:Model—1 kW demoed in space[27]
Nuclear thermal rocket 9[28] 107[28] minutes[28] > ~ 20 6:Prototype demoed on ground
Mass drivers (for propulsion) 0–~30 104–108 months ? 6:Model-32MJ demoed on ground
Tether propulsion N/A 1–1012 minutes ~ 7 6:Model—31.7 km demoed in space[29]
Air-augmented rocket 5–6 0.1–107 seconds–minutes > 7? 6:Prototype demoed on ground[30][31]
Liquid air cycle engine 4.5 103–107 seconds–minutes ? 6:Prototype demoed on ground
Pulsed inductive thruster (PIT) 10[32]–80[32] 20 months ? 5:Component validated in vacuum[32]
Variable Specific Impulse Magnetoplasma Rocket (VASIMR) 10–300[citation needed] 40–1,200[citation needed] days–months > 100 5:Component—200 kW validated in vacuum
Magnetic field oscillating amplified thruster 10–130 0.1–1 days–months > 100 5:Component validated in vacuum
Solar thermal rocket 7–12 1–100 weeks > ~ 20 4:Component validated in lab[33]
Radioisotope rocket 7–8[citation needed] 1.3–1.5 months ? 4:Component validated in lab
Nuclear electric rocket(As electric prop. method used) Variable Variable Variable ? 4:Component—400kW validated in lab
Orion Project (Near term nuclear pulse propulsion) 20–100 109–1012 several days ~30–60 3:Validated—900 kg proof-of-concept[34][35]
Space elevator N/A N/A indefinite > 12 3:Validated proof-of-concept
Reaction Engines SABRE[19] 30/4.5 0.1–107 minutes 9.4 3:Validated proof-of-concept
Magnetic sails 145–750:Wind 70/40Mg[36] indefinite ? 3:Validated proof-of-concept
Magnetic sail#Mini-magnetospheric plasma propulsion 200 ~1 N/kW months ? 3:Validated proof-of-concept[37]
Beam-powered/Laser(As prop. method powered by beam) Variable Variable Variable ? 3:Validated—71m proof-of-concept
Launch loop/Orbital ring N/A ~104 minutes >>11–30 2:Technology concept formulated
Nuclear pulse propulsion (Project Daedalus' drive) 20–1,000 109–1012 years ~15,000 2:Technology concept formulated
Gas core reactor rocket 10–20 103–106 ? ? 2:Technology concept formulated
Nuclear salt-water rocket 100 103–107 half hour ? 2:Technology concept formulated
Fission sail ? ? ? ? 2:Technology concept formulated
Fission-fragment rocket 15,000 ? ? ? 2:Technology concept formulated
Nuclear photonic rocket 299,792 10−5–1 years–decades ? 2:Technology concept formulated
Fusion rocket 100–1,000[citation needed] ? ? ? 2:Technology concept formulated
Antimatter catalyzed nuclear pulse propulsion 200–4,000 ? days–weeks ? 2:Technology concept formulated
Antimatter rocket 10,000–100,000[citation needed] ? ? ? 2:Technology concept formulated
Bussard ramjet 2.2–20,000 ? indefinite ~30,000 2:Technology concept formulated
Method Effective
Exhaust
Velocity
(km/s)		 Thrust
(N)		 Firing
Duration		 Maximum
Delta-v
(km/s)		 Technology
readiness
level

Testing

Spacecraft propulsion systems are often first statically tested on Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully. Rockets are usually tested at a rocket engine test facility well away from habitation and other buildings for safety reasons. Ion drives are far less dangerous and require much less stringent safety, usually only a large-ish vacuum chamber is needed.

Famous static test locations can be found at Rocket Ground Test Facilities

Some systems cannot be adequately tested on the ground and test launches may be employed at a Rocket Launch Site.
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