Coilgun

 From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Coilgun

Simplified diagram of a multistage coilgun with three coils, a barrel, and a ferromagnetic projectile

A coilgun, also known as a Gauss rifle, is a type of mass driver consisting of one or more coils used as electromagnets in the configuration of a linear motor that accelerate a ferromagnetic or conducting projectile to high velocity. In almost all coilgun configurations, the coils and the gun barrel are arranged on a common axis. A coilgun is not a rifle as the barrel is smoothbore (not rifled). The name "Gauss" is in reference to Carl Friedrich Gauss, who formulated mathematical descriptions of the magnetic effect used by magnetic accelerator cannons.

Coilguns generally consist of one or more coils arranged along a barrel, so the path of the accelerating projectile lies along the central axis of the coils. The coils are switched on and off in a precisely timed sequence, causing the projectile to be accelerated quickly along the barrel via magnetic forces. Coilguns are distinct from railguns, as the direction of acceleration in a railgun is at right angles to the central axis of the current loop formed by the conducting rails. In addition, railguns usually require the use of sliding contacts to pass a large current through the projectile or sabot, but coilguns do not necessarily require sliding contacts. While some simple coilgun concepts can use ferromagnetic projectiles or even permanent magnet projectiles, most designs for high velocities actually incorporate a coupled coil as part of the projectile.

History

The oldest electromagnetic gun came in the form of the coilgun, the first of which was invented by Norwegian scientist Kristian Birkeland at the University of Kristiania (today Oslo). The invention was officially patented in 1904, although its development reportedly started as early as 1845. According to his accounts, Birkeland accelerated a 500-gram projectile to approximately 50 m/s (110 mph; 180 km/h; 160 ft/s).

In 1933, Texan inventor Virgil Rigsby developed a stationary coilgun that was designed to be used similarly to a machine gun. It was powered by a large electrical motor and generator. It appeared in many contemporary science publications, but never piqued the interest of any armed forces.

Construction

There are two main types or setups of a coilgun: single-stage and multistage. A single-stage coilgun uses one electromagnetic coil to propel a projectile. A multistage coilgun uses several electromagnetic coils in succession to progressively increase the speed of the projectile.

Ferromagnetic projectiles

A single stage coilgun

For ferromagnetic projectiles, a single-stage coilgun can be formed by a coil of wire, an electromagnet, with a ferromagnetic projectile placed at one of its ends. This type of coilgun is formed like the solenoid used in an electromechanical relay, i.e. a current-carrying coil which will draw a ferromagnetic object through its center. A large current is pulsed through the coil of wire and a strong magnetic field forms, pulling the projectile to the center of the coil. When the projectile nears this point the electromagnet must be switched off, to prevent the projectile from becoming arrested at the center of the electromagnet.

In a multistage design, further electromagnets are then used to repeat this process, progressively accelerating the projectile. In common coilgun designs, the "barrel" of the gun is made up of a track that the projectile rides on, with the driver into the magnetic coils around the track. Power is supplied to the electromagnet from some sort of fast discharge storage device, typically a battery, or capacitors (one per electromagnet), designed for fast energy discharge. A diode is used to protect polarity sensitive components (such as semiconductors or electrolytic capacitors) from damage due to inverse polarity of the voltage after turning off the coil.

Many hobbyists use low-cost rudimentary designs to experiment with coilguns, for example using photoflash capacitors from a disposable camera, or a capacitor from a standard cathode-ray tube television as the energy source, and a low inductance coil to propel the projectile forward.

Non-ferromagnetic projectiles

Some designs have non-ferromagnetic projectiles, of materials such as aluminium or copper, with the armature of the projectile acting as an electromagnet with internal current induced by pulses of the acceleration coils. A superconducting coilgun called a quench gun could be created by successively quenching a line of adjacent coaxial superconducting coils forming a gun barrel, generating a wave of magnetic field gradient traveling at any desired speed. A traveling superconducting coil might be made to ride this wave like a surfboard. The device would be a mass driver or linear synchronous motor with the propulsion energy stored directly in the drive coils. Another method would have non-superconducting acceleration coils and propulsion energy stored outside them but a projectile with superconducting magnets.

Though the cost of power switching and other factors can limit projectile energy, a notable benefit of some coilgun designs over simpler railguns is avoiding an intrinsic velocity limit from hypervelocity physical contact and erosion. By having the projectile pulled towards or levitated within the center of the coils as it is accelerated, no physical friction with the walls of the bore occurs. If the bore is a total vacuum (such as a tube with a plasma window), there is no friction at all, which helps prolong the period of reusability.

Switching

A multistage coilgun

One main obstacle in coilgun design is switching the power through the coils. There are several common solutions—the simplest (and probably least effective) is the spark gap, which releases the stored energy through the coil when the voltage reaches a certain threshold. A better option is to use solid-state switches; these include IGBTs or power MOSFETs (which can be switched off mid-pulse) and SCRs (which release all stored energy before turning off).

A quick-and-dirty method for switching, especially for those using a flash camera for the main components, is to use the flash tube itself as a switch. By wiring it in series with the coil, it can silently and non-destructively (assuming that the energy in the capacitor is kept below the tube's safe operating limits) allow a large amount of current to pass through to the coil. Like any flash tube, ionizing the gas in the tube with a high voltage triggers it. However, a large amount of the energy will be dissipated as heat and light, and, because of the tube being a spark gap, the tube will stop conducting once the voltage across it drops sufficiently, leaving some charge remaining on the capacitor.

Resistance

The electrical resistance of the coils and the equivalent series resistance (ESR) of the current source dissipate considerable power.

At low speeds the heating of the coils dominates the efficiency of the coilgun, giving exceptionally low efficiency. However, as speeds climb, mechanical power grows proportional to the square of the speed, but, correctly switched, the resistive losses are largely unaffected, and thus these resistive losses become much smaller in percentage terms.

Magnetic circuit

Ideally, 100% of the magnetic flux generated by the coil would be delivered to and act on the projectile; in reality this is impossible due to energy losses always present in a real system, which cannot be eliminated.

With a simple air-cored solenoid, the majority of the magnetic flux is not coupled into the projectile because of the magnetic circuit's high reluctance. The uncoupled flux generates a magnetic field that stores energy in the surrounding air. The energy that is stored in this field does not simply disappear from the magnetic circuit once the capacitor finishes discharging, instead returning to the coilgun's electric circuit. Because the coilgun's electric circuit is inherently analogous to an LC oscillator, the unused energy returns in the reverse direction ('ringing'), which can seriously damage polarized capacitors such as electrolytic capacitors.

Reverse charging can be prevented by a diode connected in reverse-parallel across the capacitor terminals; as a result, the current keeps flowing until the diode and the coil's resistance dissipate the field energy as heat. While this is a simple and frequently utilized solution, it requires an additional expensive high-power diode and a well-designed coil with enough thermal mass and heat dissipation capability in order to prevent component failure.

Some designs attempt to recover the energy stored in the magnetic field by using a pair of diodes. These diodes, instead of being forced to dissipate the remaining energy, recharge the capacitors with the right polarity for the next discharge cycle. This will also avoid the need to fully recharge the capacitors, thus significantly reducing charge times. However, the practicality of this solution is limited by the resulting high recharge current through the equivalent series resistance (ESR) of the capacitors; the ESR will dissipate some of the recharge current, generating heat within the capacitors and potentially shortening their lifetime.

To reduce component size, weight, durability requirements, and most importantly, cost, the magnetic circuit must be optimized to deliver more energy to the projectile for a given energy input. This has been addressed to some extent by the use of back iron and end iron, which are pieces of magnetic material that enclose the coil and create paths of lower reluctance in order to improve the amount of magnetic flux coupled into the projectile. Results can vary widely, depending on the materials used; hobbyist designs may use, for example, materials ranging anywhere from magnetic steel (more effective, lower reluctance) to video tape (little improvement in reluctance). Moreover, the additional pieces of magnetic material in the magnetic circuit can potentially exacerbate the possibility of flux saturation and other magnetic losses.

Ferromagnetic projectile saturation

Another significant limitation of the coilgun is the occurrence of magnetic saturation in the ferromagnetic projectile. When the flux in the projectile lies in the linear portion of its material's B(H) curve, the force applied to the core is proportional to the square of coil current (I)—the field (H) is linearly dependent on I, B is linearly dependent on H and force is linearly dependent on the product BI. This relationship continues until the core is saturated; once this happens B will only increase marginally with H (and thus with I), so force gain is linear. Since losses are proportional to I², increasing current beyond this point eventually decreases efficiency although it may increase the force. This puts an absolute limit on how much a given projectile can be accelerated with a single stage at acceptable efficiency.

Projectile magnetization and reaction time

Apart from saturation, the B(H) dependency often contains a hysteresis loop and the reaction time of the projectile material may be significant. The hysteresis means that the projectile becomes permanently magnetized and some energy will be lost as a permanent magnetic field of the projectile. The projectile reaction time, on the other hand, makes the projectile reluctant to respond to abrupt B changes; the flux will not rise as fast as desired while current is applied and a B tail will occur after the coil field has disappeared. This delay decreases the force, which would be maximized if the H and B were in phase.

Induction coilguns

Most of the work to develop coilguns as hyper-velocity launchers has used "air-cored" systems to get around the limitations associated with ferromagnetic projectiles. In these systems, the projectile is accelerated by a moving coil "armature". If the armature is configured as one or more "shorted turns" then induced currents will result as a consequence of the time variation of the current in the static launcher coil (or coils).

In principle, coilguns can also be constructed in which the moving coils are fed with current via sliding contacts. However, the practical construction of such arrangements requires the provision of reliable high speed sliding contacts. Although feeding current to a multi-turn coil armature might not require currents as large as those required in a railgun, the elimination of the need for high speed sliding contacts is an obvious potential advantage of the induction coilgun relative to the railgun.

Air cored systems also introduce the penalty that much higher currents may be needed than in an "iron cored" system. Ultimately though, subject to the provision of appropriately rated power supplies, air cored systems can operate with much greater magnetic field strengths than "iron cored" systems, so that, ultimately, much higher accelerations and forces should be possible.

Formula for exit velocity of coilgun projectile

An approximate for the exit velocity of a projectile having been accelerated by a single-stage coilgun can be obtained by the equation

${\displaystyle v_{exit}=\sqrt{\frac{2}{m}V{\mu }_0{\chi }_m{n}^2{I}^2}}$

m being the mass of the projectile, defined as kg

V being the volume of the projectile, defined as m³

μ₀ being the vacuum permeability, defined in SI units as 4π × 10⁻⁷ V·s/(A·m)

χ_m being the magnetic susceptibility of the projectile, a dimensionless proportionality constant indicating the degree of magnetization in a material in response to applied magnetic fields. This often must be determined experimentally, and tables containing susceptibility values for certain materials may be found in the CRC Handbook of Chemistry and Physics as well as the Wikipedia article for magnetic susceptibility.

n being the number of coil turns per unit length of the coil, which can be found by dividing the total turns of the coil by the total length of the coil in meters.

and I being the current passing through the coil in amperes.

While this approximation is useful for quickly defining the upper limit of velocity in a coilgun system, more accurate and non-linear second order differential equations do exist. The issues with this formula being that it assumes the projectile lies completely within a uniform magnetic field, that the current dies out instantly once the projectile reaches the center of the coil (eliminating the possibility of coil suckback), that all potential energy is transferred into kinetic energy (whereas most would go into frictional forces), and that the wires of the coil are infinitely thin and do not stack on one another, all cumulatively increasing the expected exit velocity.

Uses

A M934 mortar round is adapted for experimental coilgun launch with a conformal armature tail kit, to be fired through a barrel composed of short solenoidal electromagnets stacked end to end

Small coilguns are recreationally made by hobbyists, typically up to several joules to tens of joules projectile energy (the latter comparable to a typical air gun and an order of magnitude less than a firearm) while ranging from under one percent to several percent efficiency.

In 2018, a Los Angeles-based company Arcflash Labs offered the first coilgun for sale to the general public, the EMG-01A. It fired 6-gram steel slugs at 45 m/s with a muzzle energy of approximately 5 joules. In 2021, they developed a larger model, the GR-1 Gauss rifle which fired 30-gram steel slugs at up to 75 m/s with a muzzle energy of approximately 85 joules, comparable to a PCP air rifle.

In 2022 Northshore Sports Club in Lake Forest, Illinois brought to market a compact, magazine fed coil gun, with the approximate maximum muzzle energy of 15 joules. Marketed originally as the E-Shotgun. The initial launch of this product followed its debut on the hit YouTube channel Demolition Ranch. Full-scale production is expected to reach 5000 units per year.

Much higher efficiency and energy can be obtained with designs of greater expense and sophistication. In 1978, Bondaletov in the USSR achieved record acceleration with a single stage by sending a 2-gram ring to 5000 m/s in 1 cm of length, but the most efficient modern designs tend to involve many stages. Above 90% efficiency is estimated for some vastly larger superconducting concepts for space launch. An experimental 45-stage, 2.1 m long DARPA coilgun mortar design is 22% efficient, with 1.6 megajoules KE delivered to a round.

 

A large coilgun concept, a coaxial electromagnetic launcher firing projectiles to orbit

Though facing the challenge of competitiveness versus conventional guns (and sometimes railgun alternatives), coilguns are being researched for weaponry.

The DARPA Electromagnetic Mortar program is an example of potential benefits, if practical challenges like sufficiently low weight can be managed. The coilgun would be relatively silent with no smoke giving away its position, though a coilgun projectile would still create a sonic boom if supersonic. Adjustable yet smooth acceleration of the projectile throughout the barrel can allow somewhat higher velocity, with a predicted range increase of 30% for a 120mm EM mortar over the conventional version of similar length. With no separate propellant charges to load, the researchers envision the firing rate to approximately double.

In 2006, a 120mm prototype was under construction for evaluation, though time before reaching field deployment, if such occurs, was estimated then as 5 to 10+ years by Sandia National Laboratories. In 2011, development was proposed of an 81mm coilgun mortar to operate with a hybrid-electric version of the future Joint Light Tactical Vehicle.

Electromagnetic aircraft catapults are planned, including on board future U.S. Gerald R. Ford class aircraft carriers. An experimental induction coilgun version of an Electromagnetic Missile Launcher (EMML) has been tested for launching Tomahawk missiles. A coilgun-based active defense system for tanks is under development at HIT in China.

Coilgun potential has been perceived as extending beyond military applications. Challenging and corresponding to a magnitude of capital investment that few entities could readily fund, gigantic coilguns with projectile mass and velocity on the scale of gigajoules of kinetic energy (as opposed to megajoules or less) have not been developed so far, but such have been proposed as launchers from the Moon or from Earth:

• An ambitious lunar-base proposal considered within a 1975 NASA study would have involved a 4000-ton coilgun sending 10 million tons of lunar material to L5 in support of massive space colonization (cumulatively over years, utilizing a large 9900-ton power plant).
• A 1992 NASA study calculated that a 330-ton lunar superconducting quenchgun could launch 4400 projectiles annually, each 1.5 tons and mostly liquid oxygen payload, using a relatively small amount of power, 350 kW average.
• After NASA Ames estimated how to meet aerothermal requirements for heat shields with terrestrial surface launch, Sandia National Laboratories investigated electromagnetic launchers to orbit, in addition to researching other EML applications, both railguns and coilguns. In 1990, a kilometer-long coilgun was proposed for launch of small satellites.
• Later investigations at Sandia included a 2005 study of the StarTram concept for an extremely long coilgun, one version conceived as launching passengers to orbit with survivable acceleration.
• A mass driver is essentially a coilgun that magnetically accelerates a package consisting of a magnetizable holder containing a payload. Once the payload has been accelerated, the two separate, and the holder is slowed and recycled for another payload.

Space gun

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Space_gun 

 

The Quicklauncher spacegun

A space gun, sometimes called a Verne gun because of its appearance in From the Earth to the Moon by Jules Verne, is a method of launching an object into space using a large gun- or cannon-like structure. Space guns could thus potentially provide a method of non-rocket spacelaunch. It has been conjectured that space guns could place satellites into Earth's orbit (although after-launch propulsion of the satellite would be necessary to achieve a stable orbit), and could also launch spacecraft beyond Earth's gravitational pull and into other parts of the Solar System by exceeding Earth's escape velocity of about 11.20 km/s (40,320 km/h; 25,050 mph). However, these speeds are too far into the hypersonic range for most practical propulsion systems and also would cause most objects to burn up due to aerodynamic heating or be torn apart by aerodynamic drag. Therefore, a more likely future use of space guns would be to launch objects into Low Earth orbit, at which point attached rockets could be fired or the objects could be "collected" by maneuverable orbiting satellites.

In Project HARP, a 1960s joint United States and Canada defence project, a U.S. Navy 410 mm (16 in) 100 caliber gun was used to fire a 180 kg (400 lb) projectile at 3,600 m/s (12,960 km/h; 8,050 mph), reaching an apogee of 180 km (110 mi), hence performing a suborbital spaceflight. However, a space gun has never been successfully used to launch an object into orbit or out of Earth's gravitational pull.

Technical issues

The large g-force likely to be experienced by a ballistic projectile launched in this manner would mean that a space gun would be incapable of safely launching humans or delicate instruments, rather being restricted to freight, fuel or ruggedized satellites.

Getting to orbit

A space gun by itself is not capable of placing objects into a stable orbit around the object (planet or otherwise) they are launched from. The orbit is a parabolic orbit, a hyperbolic orbit, or part of an elliptic orbit which ends at the planet's surface at the point of launch or another point. This means that an uncorrected ballistic payload will always strike the planet within its first orbit unless the velocity was so high as to reach or exceed escape velocity. As a result, all payloads intended to reach a closed orbit need at least to perform some sort of course correction to create another orbit that does not intersect the planet's surface.

A rocket can be used for additional boost, as planned in both Project HARP and the Quicklaunch project. The magnitude of such correction may be small; for instance, the StarTram Generation 1 reference design involves a total of 0.6 km/s (1,300 mph) of rocket burn to raise perigee well above the atmosphere when entering an 8 km/s (18,000 mph) low Earth orbit.

In a three-body or larger system, a gravity assist trajectory might be available such that a carefully aimed escape velocity projectile would have its trajectory modified by the gravitational fields of other bodies in the system such that the projectile would eventually return to orbit the initial planet using only the launch delta-v.

Isaac Newton avoided this objection in his thought experiment by placing his notional cannon atop a tall mountain and positing negligible air resistance. If in a stable orbit, the projectile would circle the planet and return to the altitude of launch after one orbit (see Newton's cannonball).

Acceleration

For a space gun with a gun barrel of length (${\displaystyle l}$), and the needed velocity (${\displaystyle v_e}$), the acceleration (${\displaystyle a}$) is provided by the following formula:

${\displaystyle a=\frac{v_e^2}{2l}}$

For instance, with a space gun with a vertical "gun barrel" through both the Earth's crust and the troposphere, totalling ~60 km (37 miles) of length (${\displaystyle l}$), and a velocity (${\displaystyle v_e}$) enough to escape the Earth's gravity (escape velocity, which is 11.2 km/s or 25,000 mph on Earth), the acceleration (${\displaystyle a}$) would theoretically be more than 1,000 m/s² (3,300 ft/s²), which is more than 100 g-forces, which is about 3 times the human tolerance to g-forces of maximum 20 to 35 g during the ~10 seconds such a firing would take.This calculation does not take into account the decreasing escape velocity at higher altitudes.

Practical attempts

Two sections of the Project Babylon gun

 

Project HARP, a prototype of a space gun.

V3 Cannon (1944-45)

The German V-3 cannon program, during World War II was an attempt to build something approaching a space gun. Based in the Pas-de-Calais area of France it was planned to be more devastating than the other Nazi 'Vengeance weapons'. The cannon was capable of launching 140 kg (310 lb), 15 cm (5.9 in) diameter shells over a distance of 88 km (55 mi). It was destroyed by RAF bombing using Tallboy blockbuster bombs in July 1944.

Super High Altitude Research Project (1985-95)

The US Ballistic Missile Defense program sponsored the Super High Altitude Research Project (SHARP) in the 1980s. Developed at Lawrence Livermore Laboratory, it is a light-gas gun and has been used to test fire objects at Mach 9.

Project Babylon (1988-90)

The most prominent recent attempt to make a space gun was artillery engineer Gerald Bull's Project Babylon, which was also known as the 'Iraqi supergun' by the media. During Project Babylon, Bull used his experience from Project HARP to build a massive cannon for Saddam Hussein, leader of Ba'athist Iraq. Bull was assassinated before the project was completed.

Quicklaunch (1996-2016)

After cancellation of SHARP, lead developer John Hunter founded the Jules Verne Launcher Company in 1996 and the Quicklaunch company. As of September 2012, Quicklaunch was seeking to raise $500 million to build a gun that could refuel a propellant depot or send bulk materials into space.

Ram accelerators have also been proposed as an alternative to light-gas guns. Other proposals use electromagnetic techniques for accelerating the payload, such as coilguns and railguns.

In fiction

The firing of a space gun in Jules Verne's From the Earth to the Moon

The first publication of the concept may be Newton's cannonball in his 1728 book A Treatise of the System of the World, although it was primarily used as a thought experiment regarding gravity.

Perhaps the most famous representations of a space gun appear in Jules Verne's 1865 novel From the Earth to the Moon and his 1869 novel Around the Moon (loosely interpreted into the 1902 film Le Voyage dans la Lune), in which astronauts fly to the Moon aboard a ship launched from a cannon. Another famous example is used by the Martians to launch their invasion in H. G. Wells' 1897 book The War of the Worlds. Wells also used the concept in the climax of the 1936 film Things to Come. The device was featured in films as late as 1967, such as Jules Verne's Rocket to the Moon.

In the 1991 video game Ultima: Worlds of Adventure 2: Martian Dreams, Percival Lowell builds a space gun to send a spacecraft to Mars.

The 1992 video game Steel Empire, a shoot 'em up with steampunk aesthetics, features a space gun in its seventh level that is used by the main villain General Styron to launch himself to the Moon.

In Hannu Rajaniemi's 2012 novel The Fractal Prince, a space gun at the "Jannah-of-the-cannon", powered by a 150-kiloton nuclear bomb, is used to launch a spaceship from Earth.

The 2015 video game SOMA features a space gun used to launch satellites.

Gerald Bull's assassination and the Project Babylon gun were also the starting point for Frederick Forsyth's 1994 novel The Fist of God. In Larry Bond's 2001 novella and 2015 novel Lash-Up, China uses a space gun to destroy American GPS satellites.

In the 2004 role-playing game Paper Mario: The Thousand-Year Door, a village of Bob-ombs operates a space gun to send Paper Mario and company to the X-Naut's base on the Moon.

Gerald Bull and Project Babylon are integral to the plot of Louise Penny's 2015 novel The Nature of the Beast.

Mass driver

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Mass_driver 

 

Artist's conception of a mass driver on the Moon

A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. Existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets, though a rotary mass driver has also been proposed. Sequential firing of a row of electromagnets accelerates the payload along a path. After leaving the path, the payload continues to move due to momentum.

Although any device used to propel a ballistic payload is technically a mass driver, in this context a mass driver is essentially a coilgun that magnetically accelerates a package consisting of a magnetizable holder containing a payload. Once the payload has been accelerated, the two separate, and the holder is slowed and recycled for another payload.

Mass drivers can be used to propel spacecraft in three different ways: A large, ground-based mass driver could be used to launch spacecraft away from Earth, the Moon, or another body. A small mass driver could be on board a spacecraft, flinging pieces of material into space to propel itself. Another variation would have a massive facility on a moon or asteroid send projectiles to assist a distant craft.

Miniaturized mass drivers can also be used as weapons in a similar manner as classic firearms or cannon using chemical combustion. Hybrids between coilguns and railguns such as helical railguns are also possible.

Fixed mass drivers

See also: Space gun

Mass drivers need no physical contact between moving parts because they guide their projectiles by dynamic magnetic levitation, allowing extreme reusability in the case of solid-state power switching, and a functional life of – theoretically – up to millions of launches. While marginal costs tend to be accordingly low, initial development and construction costs are highly dependent on performance, especially the intended mass, acceleration, and velocity of projectiles. For instance, while Gerard O'Neill built his first mass driver in 1976–1977 with a $2000 budget, a short test model firing a projectile at 40 m/s and 33 g, his next model had an order-of-magnitude greater acceleration after a comparable increase in funding, and, a few years later, researchers at the University of Texas estimated that a mass driver firing a 10 kilogram projectile at 6000 m/s would cost $47 million.

For a given amount of energy involved, heavier objects go proportionally slower. Light objects may be projected at 20 km/s or more. The limits are generally the expense of energy storage able to be discharged quickly enough and the cost of power switching, which may be by semiconductors or by gas-phase switches (which still often have a niche in extreme pulse power applications). However, energy can be stored inductively in superconducting coils. A 1 km long mass driver made of superconducting coils can accelerate a 20 kg vehicle to 10.5 km/s at a conversion efficiency of 80%, and average acceleration of 5,600 g.

Earth-based mass drivers for propelling vehicles to orbit, such as the StarTram concept, would require considerable capital investment. The Earth's relatively strong gravity and relatively thick atmosphere make the implementation of a practical solution difficult. Also, most if not all plausible launch sites would propel spacecraft through heavily-traversed air routes. Due to the massive turbulence such launches would cause, significant air traffic control measures would be needed to ensure the safety of other aircraft operating in the area.

With the proliferation of reusable rockets to launch from Earth (especially first stages) whatever potential might have once existed for any economic advantage in using mass drivers as an alternative to chemical rockets to launch from Earth is becoming increasingly doubtful. For these reasons many proposals feature installing mass drivers on the Moon where the lower gravity and lack of atmosphere greatly reduce the required velocity to reach lunar orbit, also, lunar launches from a fixed position are much less likely to generate issues with respect to matters such as traffic control.

Most serious mass-driver designs use superconducting coils to achieve reasonable energetic efficiency (often 50% to 90+%, depending on design). Equipment may include a superconducting bucket or aluminum coil as the payload. The coils of a mass driver can induce eddy currents in a payload's aluminum coil, and then act on the resulting magnetic field. There are two sections of a mass driver. The maximum acceleration part spaces the coils at constant distances, and synchronizes the coil currents to the bucket. In this section, the acceleration increases as the velocity increases, up to the maximum that the bucket can take. After that, the constant acceleration region begins. This region spaces the coils at increasing distances to give a fixed amount of velocity increase per unit of time.

Based on this mode, a major proposal for the use of mass drivers involved transporting lunar-surface material to space habitats for processing using solar energy. The Space Studies Institute showed that this application was reasonably practical.

In some designs, the payload would be held in a bucket and then released, so that the bucket can be decelerated and reused. A disposable bucket, on the other hand, would avail acceleration along the whole track. Alternatively, if a track were constructed along the entire circumference of the Moon (or any other celestial body without a significant atmosphere) then a reusable bucket's acceleration would not be limited by the length of the track – however, such a system would need to be engineered to withstand substantial centrifugal forces if it were intended to accelerate passengers and/or cargo to very high velocities.

On Earth

In contrast to cargo-only chemical space-gun concepts, a mass driver could be any length, affordable, and with relatively smooth acceleration throughout, optionally even lengthy enough to reach target velocity without excessive g forces for passengers. It can be constructed as a very long and mainly horizontally aligned launch track for spacelaunch, targeted upwards at the end, partly by bending of the track upwards and partly by Earth's curvature in the other direction.

Natural elevations, such as mountains, may facilitate the construction of the distant, upwardly targeted part. The higher up the track terminates, the less resistance from the atmosphere the launched object will encounter.

The 40 megajoules per kilogram or less kinetic energy of projectiles launched at up to 9000 m/s velocity (if including extra for drag losses) towards low Earth orbit is a few kilowatt-hours per kilogram if efficiencies are relatively high, which accordingly has been hypothesized to be under $1 of electrical energy cost per kilogram shipped to LEO, though total costs would be far more than electricity alone. By being mainly located slightly above, on or beneath the ground, a mass driver may be easier to maintain compared with many other structures of non-rocket spacelaunch. Whether or not underground, it needs to be housed in a pipe that is vacuum pumped in order to prevent internal air drag, such as with a mechanical shutter kept closed most of the time but a plasma window used during the moments of firing to prevent loss of vacuum.

A mass driver on Earth would usually be a compromise system. A mass driver would accelerate a payload up to some high speed which would not be enough for orbit. It would then release the payload, which would complete the launch with rockets. This would drastically reduce the amount of velocity needed to be provided by rockets to reach orbit. Well under a tenth of orbital velocity from a small rocket thruster is enough to raise perigee if a design prioritizes minimizing such, but hybrid proposals optionally reduce requirements for the mass driver itself by having a greater portion of delta-v by a rocket burn (or orbital momentum exchange tether). On Earth, a mass-driver design could possibly use well-tested maglev components.

To launch a space vehicle with humans on board, a mass driver's track would need to be almost 1000 kilometres long if providing almost all the velocity to Low Earth Orbit, though a lesser length could provide major launch assist. Required length, if accelerating mainly at near a constant maximum acceptable g-force for passengers, is proportional to velocity squared. For instance, half of the velocity goal could correspond to a tunnel a quarter as long needing to be constructed, for the same acceleration. For rugged objects, much higher accelerations may suffice, allowing a far shorter track, potentially circular or helical (spiral). Another concept involves a large ring design whereby a space vehicle would circle the ring numerous times, gradually gaining speed, before being released into a launch corridor leading skyward.

Mass drivers have been proposed for the disposal of nuclear waste in space: a projectile launched at much above Earth's escape velocity would escape the Solar System, with atmospheric passage at such speed calculated as survivable through an elongated projectile and a very substantial heatshield.

Spacecraft-based mass drivers

A spacecraft could carry a mass driver as its primary engine. With a suitable source of electrical power (probably a nuclear reactor) the spaceship could then use the mass driver to accelerate pieces of matter of almost any sort, boosting itself in the opposite direction. At the smallest scale of reaction mass, this type of drive is called an ion drive.

No absolute theoretical limit is known for the size, acceleration or muzzle energy of linear motors. However, practical engineering constraints apply for such as the power-to-mass ratio, waste heat dissipation, and the energy intake able to be supplied and handled. Exhaust velocity is best neither too low nor too high.

There is a mission-dependent limited optimal exhaust velocity and specific impulse for any thruster constrained by a limited amount of onboard spacecraft power. Thrust and momentum from exhaust, per unit mass expelled, scales up linearly with its velocity (momentum = mv), yet kinetic energy and energy input requirements scale up faster with velocity squared (kinetic energy = +1⁄2 mv²). Too low an exhaust velocity would excessively increase propellant mass needed under the rocket equation, with too high a fraction of energy going into accelerating propellant not used yet. Higher exhaust velocity has both benefit and tradeoff, increasing propellant usage efficiency (more momentum per unit mass of propellant expelled) but decreasing thrust and the current rate of spacecraft acceleration if available input power is constant (less momentum per unit of energy given to propellant).

Electric propulsion methods like mass drivers are systems where energy does not come from the propellant itself. (Such contrasts to chemical rockets where propulsive efficiency varies with the ratio of exhaust velocity to vehicle velocity at the time, but near maximum obtainable specific impulse tends to be a design goal when corresponding to the most energy released from reacting propellants). Although the specific impulse of an electric thruster itself optionally could range up to where mass drivers merge into particle accelerators with fractional-lightspeed exhaust velocity for tiny particles, trying to use extreme exhaust velocity to accelerate a far slower spacecraft could be suboptimally low thrust when the energy available from a spacecraft's reactor or power source is limited (a lesser analogue of feeding onboard power to a row of spotlights, photons being an example of an extremely low momentum to energy ratio).

For instance, if limited onboard power fed to its engine was the dominant limitation on how much payload a hypothetical spacecraft could shuttle (such as if intrinsic propellant economic cost was minor from usage of extraterrestrial soil or ice), ideal exhaust velocity would rather be around 62.75% of total mission delta v if operating at constant specific impulse, except greater optimization could come from varying exhaust velocity during the mission profile (as possible with some thruster types, including mass drivers and variable specific impulse magnetoplasma rockets).

Since a mass driver could use any type of mass for reaction mass to move the spacecraft, a mass driver or some variation seems ideal for deep-space vehicles that scavenge reaction mass from found resources.

One possible drawback of the mass driver is that it has the potential to send solid reaction mass travelling at dangerously high relative speeds into useful orbits and traffic lanes. To overcome this problem, most schemes plan to throw finely-divided dust. Alternatively, liquid oxygen could be used as reaction mass, which upon release would boil down to its molecular state. Propelling the reaction mass to solar escape velocity is another way to ensure that it will not remain a hazard.

Hybrid mass drivers

A mass driver on a spacecraft could be used to "reflect" masses from a stationary mass driver. Each deceleration and acceleration of the mass contributes to the momentum of the spacecraft. The lightweight, fast spacecraft need not carry reaction mass, and does not need much electricity beyond the amount needed to replace losses in the electronics, while the immobile support facility can run off power plants able to be much larger than the spacecraft if needed. This could be considered a form of beam-powered propulsion (a macroscopic-scale analogue of a particle beam propelled magsail). A similar system could also deliver pellets of fuel to a spacecraft to power another propulsion system.

Another theoretical use for this concept of propulsion can be found in space fountains, a system in which a continuous stream of pellets in a circular track holds up a tall structure.

Mass drivers as weapons

Small to moderate size high-acceleration electromagnetic projectile launchers are currently undergoing active research by the US Navy for use as ground-based or ship-based weapons (most often railguns but coilguns in some cases). On larger scale than weapons currently near deployment but sometimes suggested in long-range future projections, a sufficiently high velocity linear motor, a mass driver, could in theory be used as intercontinental artillery (or, if built on the Moon or in orbit, used to attack a location on Earth's surface). As the mass driver would be located further up the gravity well than the theoretical targets, it would enjoy a significant energy imbalance in terms of counter-attack.

Practical attempts

One of the first engineering descriptions of an "Electric Gun" appears in the technical supplement of the 1937 science fiction novel "Zero to Eighty" by "Akkad Pseudoman", a pen name for the Princeton physicist and electrical entrepreneur Edwin Fitch Northrup. Dr. Northrup built prototype coil guns powered by kHz-frequency three-phase electrical generators, and the book contains photographs of some of these prototypes. The book describes a fictional circumnavigation of the moon by a two-person vehicle launched by a Northrup electric gun.

Later prototype mass drivers have been built since 1976 (Mass Driver 1), some constructed by the U.S. Space Studies Institute in order to prove their properties and practicality. Military R&D on coilguns is related, as are maglev trains.

SpinLaunch, a company founded in 2014, conducted the initial test of their test accelerator in October 2021.

United States gravity control propulsion research

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American interest in "gravity control propulsion research" intensified during the early 1950s. Literature from that period used the terms anti-gravity, anti-gravitation, baricentric, counterbary, electrogravitics (eGrav), G-projects, gravitics, gravity control, and gravity propulsion. Their publicized goals were to discover and develop technologies and theories for the manipulation of gravity or gravity-like fields for propulsion. Although general relativity theory appeared to prohibit anti-gravity propulsion, several programs were funded to develop it through gravitation research from 1955 to 1974. The names of many contributors to general relativity and those of the golden age of general relativity have appeared among documents about the institutions that had served as the theoretical research components of those programs. Since its emergence in the 1950s, the existence of the related gravity control propulsion research has not been a subject of controversy for aerospace writers, critics, and conspiracy theory advocates alike, but their rationale, effectiveness, and longevity have been the objects of contested views.

Evidence of existence

Mainstream newspapers, popular magazines, technical journals, and declassified papers reported the existence of the gravity control propulsion research. For example, the title of the March 1956 Aero Digest article about the intensified interest was "Anti-gravity Booming." A. V. Cleaver made the following statement about the programs in his article:

What are the facts, insofar as they are publicly known, or (as at this date) knowable? Well, they seem to amount to this: The Americans have decided to look into the old science-fictional dream of gravity control, or "anti-gravity," to investigate, both theoretically and (if possible) practically the fundamental nature of gravitational fields and their relationship to electromagnetic and other phenomena – and someone (unknown to the present writer) has apparently decided to call all this study by the high-sounding name of "electro-gravitics." Unknown, too – at least unannounced – is the name of agency or individual who decided to encourage, stimulate, or sponsor this effort, also in just what way it is being done. However, that the effort is in progress there can be little doubt, and, of course, it is entirely to be welcomed.

The gravitics programs had not been evinced by any technological artifacts, such as the Project Pluto Tory IIA, the world's first nuclear ramjet. Commemorative monuments by the Gravity Research Foundation have been the artifacts attesting to the early commitments to finding materials and methods to manipulate gravity. The endeavor had the resources and publicity of an initiative, but writers from that period did not describe them with that term. Gladych stated:

At least 14 United States universities and other research centers are hard at work cracking the gravity barrier. And backing the basic research with multi-million dollar secret projects is our aircraft industry.

The writings about the gravity control propulsion research effort had disclosed the "players" and resources while prudently withholding both the specific features of the research and the identity of its coordinating body. Publicized and telecasted conspiracy theory anecdotes have suggested much higher levels of success to the G-projects than mainstream science.

Histories

Recent historical analysis and reports have attracted attention to the agencies and firms that had participated in the gravity control propulsion research. James E. Allen, BAE Systems consultant and engineering professor at Kingston University, referred to those programs in his history of novel propulsion systems for the journal Progress in Aerospace Sciences. Research by Dr. David Kaiser, Associate Professor of the History of Science, Massachusetts Institute of Technology, manifested the contributions made by the Gravity Research Foundation to the pedagogical aspects of the golden age of general relativity. Dr. Joshua Goldberg, Syracuse University, described the Air Force's support of relativity research during that period. Progress reports and anecdotes and Internet resumes of former visiting and staff scientists have been the sources of the history of the Research Institute for Advanced Study (RIAS). Former aviation editor of Jane's Defence Weekly, Nick Cook, drew attention to the antigravity programs through worldwide publications of his book, The Hunt for Zero Point, and subsequent televised documentaries. Mainstream historical accounts of the G-projects have been supplemented with conspiracy theory anecdotes.

Coetaneous literature

Lists of the research institutes, industrial sites, and policy makers along with statements from prominent physicists were provided in five comprehensive works that had been published during the early years of the gravity control propulsion research. Aviation Studies (International) Limited, London, published a detailed report about those activities by the Gravity Research Group that was later declassified. The Journal of the British Interplanetary Society and The Aeroplane published the propulsion survey and critical assessment of the American gravitics research by the internationally recognized astronautics historian A. V. Cleaver. The New York Herald Tribune and Miami Herald published a series of three articles by one of the world's greatest aviation journalists of the twentieth century, Ansel Talbert. Talbert's two series of newspaper articles took place in the midst of the policy-by-press-release era. Neither his, nor the writings that followed the five prominent works from that period, yielded denials and/or retractions.

UFO and conspiracy theory literature

Gravity control propulsion research had been the subject of widely published UFO literature. The documented testimonies of whistleblowers edited by Dr. Steven Greer, Director of the Disclosure Project; anecdotes and schematics by Mark McCandlish and Milton William Cooper; and the reports by Philip J. Corso, David Darlington, and Donald Keyhoe, famous UFO researcher, have suggested incorporation of reverse engineering of recovered extraterrestrial vehicles with the anti-gravity propulsion projects had enabled them to continue beyond 1973 to successfully manufacture antigravity vehicles. Branches of the military and defense agencies have denied and refuted such claims.

Theoretical research agencies

Talbert indicated the rationale for the intensified interest in gravity control propulsion research had stemmed from the works of three physicists. They were Bryce DeWitt's prize-winning Gravity Research Foundation essay; the book Gravity and the Universe by Pascual Jordan; and presentations to the International Astronautical Federation by Dr. Burkhard Heim. DeWitt's essay discouraged the pursuit of materials that shield, reflect, and/or insulate gravity and emphasized the need to encourage young physicists to pursue gravitational research. He opened his essay with the following paragraph:

Before anyone can have the audacity to formulate even the most rudimentary plan of attack on the problem of harnessing the force of gravitation, he must understand the nature of his adversary. I take it as most axiomatic that the phenomenon of gravitation is poorly understood even by the best of minds, and the last word on it is very far indeed from having been spoken.

Several articles cited his essay during and after the gravity control propulsion research period. Within a few years facilities emerged embodying the theme of DeWitt's call for increased stimuli for research.

Physical principle surveys by Cleaver and Weyl stated the antigravity research was not based on any recognized theoretical breakthroughs. Cleaver's skepticism suggested an alternative rationale for establishing that research was based on a science fiction novel. Weyl charged publishers with poor journalism; attacked their terminology; and gave the highest rating for prospective physical principles for gravity control propulsion to Burkhard Heim's works. Stambler leveled harsh criticisms against Gluraheff's gravitation hypothesis. Talbert and other authors listed the following three agencies as the principal facilities that had conducted the theoretical research:

Gravity Research Foundation

Several articles contained expressions of gratitude for the support to the gravity control propulsion endeavor by the Gravity Research Foundation. Even though the Foundation was a humble, non-profit organization, its creator, Roger Babson, used his wealth and influence to mobilize industries; raise private and government funding; and motivate engineers and physicists to conduct research in gravity shielding and control. According to his autobiography: "The purpose of the Foundation is to encourage others to work on gravity problems and aid others in obtaining rewards for their efforts."

During Babson's lifetime, the Foundation conducted Gravity Day Conferences each summer; established a library on gravity; solicited essays that addressed (1.) various prospects for shielding gravity, (2.) the development and/or discovery of materials that could convert gravitational force into heat, or (3.) methods of manipulating gravity; and installed monuments at various universities that cited its antigravity focus.

Aerospace Research Laboratories

In September, 1956, the General Physics Laboratory of the Aeronautical Research Laboratories (ARL) at Wright-Patterson Air Force Base, Dayton, Ohio, commenced an intense program to coordinate research into gravitational and unified field theories with the hiring of Joshua N. Goldberg. Creation by ARL of Goldberg's program may have been coincidental to Talbert's disclosures of commitments to gravity control propulsion research. The precise rationale for creating the program and justifying its budgets and personnel may never be determined. Neither Goldberg nor the Air Force's Deputy for Scientific and Technical Information, Walter Blados, were able to locate the founding documents. Roy Kerr, a former ARL scientist, stated the antigravity propulsion purpose of ARL was "rubbish" and that "The only real use that the USAF made of us was when some crackpot sent them a proposal for antigravity or for converting rotary motion inside a spaceship to a translational driving system." The December 30, 1957 issue of Product Engineering closed its report with the following statement:

Nevertheless, the Air Force is encouraging research in electrogravitics, and many companies and individuals are working on the problem. It could be that one of them will confound the experts.

During the following sixteen years, its name was changed to the Aerospace Research Laboratories. The ARL scientists produced nineteen technical reports and over seventy peer-reviewed journal articles. The Air Force's Foreign Technical Division, and other agencies, investigated stories about Soviet attempts to understand gravity. Such actions were consistent with the paranoia of the Cold War.

The funding for the military components of the gravity control propulsion research had been terminated by the Mansfield Amendment of 1973. Black project experts, conspiracy theorists, and whistleblowers had suggested the gravity control propulsion efforts had achieved their goals and had been continued decades beyond 1973.

Research Institute for Advanced Study (RIAS)

Further information: Research Institute for Advanced Studies

The Research Institute for Advanced Study (RIAS) was conceived by George S. Trimble, the vice president for aviation and advanced propulsion systems, Glenn L. Martin Company, and was placed under the direct supervision of Welcome Bender. The first person Bender hired was Louis Witten, an authority on gravitation physics. Talbert's article had announced Trimble's completion of contractual agreements with Pascual Jordan and Burkhard Heim for RIAS. Subsequent hires yielded a half dozen gravity researchers known as the field theory group. Arthur C. Clarke and others stated that RIAS' assembly of talent was qualified for the task of discovering new principles that could be used to develop gravity control propulsion systems.

The quest for propulsion through gravity control was vaguely implied in various publications. Works by Cook and Cleaver summarized statements in the RIAS brochures. Cook had equated the broad range of RIAS's mission statements with those of Skunk Works. In 1958, Mallan reported "the control of the force of gravity itself for propulsion" was one of the unorthodox goals initiated by Trimble for RIAS.

RIAS was renamed the Research Institute for Advanced Studies during the sixties when the American-Marietta Company merged with Martin to become the Martin Marietta Company. The 1995 merger that yielded the Lockheed Martin Company modified its goals, but not its name.

Aerospace firms

Talbert's newspaper series and subsequent articles in technical magazines and journals listed the names of aerospace firms conducting gravity control propulsion research.

The Gravity Research Group indicated those companies had constructed "rigs" to improve the performance of Thomas Townsend Brown's gravitators through attempts to develop materials with high dielectric constants (k). Gravity Rand Limited provided a set of guidelines to help management conduct research and nurture creativity. Articles about the gravity propulsion research by the aerospace firms ceased after 1974. None of the companies featured in those publications had filed retractions. The following aerospace firms have been cited in the works published from 1955 through 1974:

• Bell Aircraft, Buffalo, New York.
• Boeing Aircraft.
• Clarke Electronics, Palm Springs, California.
• Convair, San Diego.
• Douglas Aircraft.
• Electronics Division, Ryan Aeronautical Company, San Diego.
• General Electric.
• Glenn L. Martin Company, Baltimore, Maryland.
• Gluhareff Helicopter & Airplane Corporation, Manhattan Beach, California.
• Grumman Aircraft.
• Hiller Aircraft.
• Hughes Aircraft.
• Lear Incorporated, Santa Monica, California.
• Lockheed Aircraft Corporation.
• Radio Corporation.
• Sikorsky Division of United Aircraft.
• Sperry Gyroscope Division of Sperry Rand Corporation, Great Neck, Long Island.

Reported breakthroughs

None of the reported experimental breakthroughs published during the 1950s and 1960s have been recognized by the aerospace community.

Experimental

Brown's gravitator

Various reports indicated Brown's gravitators were the main experimental focus of the gravity control propulsion research. According to G. Harry Stine and Intel, research on Brown's gravitators became classified immediately after demonstrations of 30% weight reductions. Thomas Townsend Brown had obtained a British patent for high voltage, symmetric, parallel plate capacitors, that he called gravitators, in 1928. Brown claimed they would produce a net thrust in the direction of the anode of the capacitor that varied slightly with the positions of the Moon. The scientific community rejected such claims as products of pseudoscience and/or misinterpretations of ion wind effects.

Independent research found small amounts of lift from Brown's gravitator based on an inefficient use of ionic propulsion. The devices were named Ion Lifters or Ionocraft and were reported to be able to lift the empty shell of a vehicle under ideal conditions, but not the additional machinery required to generate the electric field. Gravity effects were not found in the independent research.

Kaplan's gravity-like impulses

In July 1960, Missiles and Rockets reported Martin N. Kaplan, Senior Research Engineer, Electronics Division, Ryan Aeronautical Company, San Diego, had conducted anti-gravitational experiments yielding the promise of impulses, accelerations, and decelerations one hundred times the pull of gravity. Neither comments nor criticism of the report appeared in subsequent articles during the period of intensified gravity control propulsion research (see Section 1 of tractor beam for similar reports).

Theoretical

Forward's rotational field

Robert L. Forward, Hughes Research Laboratories, Malibu, described the theoretical generation of dipole gravitational fields by accelerating a super-dense fluid through pipes wound around a torus. The proposed mechanism relies on the use of a superconducting fluid such as supercooled mercury, being quickly rotated within a circular tube whilst under a high electrical current. It is believed that this creates a powerful electromagnetic force as a torus field to envelop a craft thus effectively reducing its mass and G-forces to near zero allowing almost instantaneous acceleration and deceleration under propulsion.

Legacies

Many of the contributors to general relativity have been supported by and/or associated with the ARL, RIAS, and/or the Gravity Research Foundation. The decades preceding the 1955 revelation of the gravity control propulsion research were a low water mark for general relativity. The following summarizes how the components of that research had stimulated the resurgence of general relativity:

Gravity Research Foundation

Even though some of the physicists who attended the Gravity Day Conferences quietly mocked the anti-gravity mission of the Foundation, it provided significant contributions to mainstream physics. The International Journal of Modern Physics D has featured selected papers from the Gravity Research Foundation essay competition. Many have been incorporated with the collections of the Niels Bohr Library. A few of the Foundation essay contest winners became Nobel laureates (e.g., Ilya Prigogine, Maurice Allais, George F. Smoot). Foundation essays have been among the resources graduate students check for new ideas. Kaiser summarized the Foundation's influence in the following manner:

Despite the vast conceptual gulf separating Babson from the new generation of relativists, we are left with intriguing, and perhaps ironic associations: by organizing conferences, sponsoring the annual essay contests, and making money and enthusiasm widely available for people interested in gravity, the eccentric Gravity Research Foundation may claim at least some small amount of the credit for helping to stimulate the postwar resurgence of interest in gravitation and general relativity.

Foundation trustee, Agnew Bahnson, contacted Dr. Bryce DeWitt with a proposal to fund the creation of a gravity research institute. DeWitt had won the first prize for the 1953 essay contest. The proposed name was changed to the Institute for Field Physics and it was established in 1956 at the University of North Carolina at Chapel Hill under the direction of Bryce and his wife, Cécile DeWitt-Morette.

The peer reviewed physics journal, Physica C, published a report by Eugene Podkletnov and Nieminen about gravity-like shielding. Although their work had gained international attention, researchers were not able to replicate Podkletnov's initial conditions. But, analyses by Giovanni Modanese and Ning Wu indicated various applications of quantum gravity theory could allow gravitational shielding phenomena. Those achievements have not been pursued by the scientific community.

Aerospace Research Laboratories (ARL)

The list of prominent contributors to the golden age of general relativity, contains the names of several scientists who had authored the nineteen ARL Technical Reports and/or seventy papers. The ARL sponsored papers were published in the Proceedings of the Royal Society of London, Physical Review, Journal of Mathematical Physics, Physical Review Letters, Physical Review D, Review of Modern Physics, General Relativity and Gravitation, International Journal of Theoretical Physics, and Nuovo Cimento B. Some of the ARL papers were written in collaboration with RIAS, the U.S. Army Signal Research and Development Laboratory at Fort Monmouth, New Jersey, and the Office of Naval Research. The ARL had provided significant enhancements to general relativity theory. For example, Roy Kerr's description of the behavior of space-time in the vicinity of a rotating mass was among those works. Goldberg concluded: "However, it should be recognized that, in the United States, the Department of Defense played an essential role in building a strong scientific community without widespread encroachment on academic values."

Research Institute for Advanced Studies (RIAS)

The growth of nonlinear differential equations during the fifties was stimulated by RIAS. One of the leading groups in dynamical systems and control theory, the Lefschetz Center for Dynamical Systems, was a spinoff from RIAS. After the launch of Sputnik, world-class mathematician Solomon Lefschetz came out of retirement to join RIAS in 1958 and formed the world's largest group of mathematicians devoted to research in nonlinear differential equations. The RIAS mathematics group stimulated the growth of nonlinear differential equations through conferences and publications. It left RIAS in 1964 to form the Lefschetz Center for Dynamical Systems at Brown University, Providence, Rhode Island.

UFO and conspiracy theories

On May 9, 2001, Mark McCandlish testified on the televised news conference held by the Disclosure Project, at the National Press Club, Washington, D.C. He stated gravity control propulsion research had started in the 1950s and had successfully reverse engineered the vehicle retrieved from the Roswell crash site to build three Alien Reproduction Vehicles (ARVs) by 1981. McCandlish described their propulsion systems in terms of Thomas Townsend Brown's gravitators and provided a line drawing of its interior. The diagram closely resembled the drawing provided earlier in Milton William Cooper's book. Another Disclosure Project whistleblower, Philip J. Corso, stated in his book the craft retrieved from the second crash site at Roswell, New Mexico, had a propulsion system resembling Thomas Townsend Brown's gravitators. And, Corso's book featured several gravity control propulsion statements made by Hermann Oberth.

Soon after the end of the Cold War, a small group of scientists and engineers openly expressed their desire to use technologies developed by black projects for civil applications. Steven Greer formed the Disclosure Project in 1995 to help those and other research whistleblowers share their information with and to petition Congress. By 2001, it had provided reports to two Congressional hearings and had acquired over 400 members from branches of the military and aerospace industry.

During the early 1960s, Keyhoe published excerpts from a letter by Hermann Oberth that presented explanations for the flight characteristics of UFOs in terms of gravity control propulsion. Prior to Oberth's letter, Keyhoe had supported arguments for magnetic forces as the source of propulsion for UFOs. The letter caused him to search for the existence of gravity control propulsion research programs. The following is a segment of his findings he had released in his 1966 and 1974 publications:

When AF [air force] researchers fully realized the astounding possibilities, headquarters persuaded scientists, aerospace companies and technical laboratories to set up anti-gravity projects, many of them under secret contracts. Every year, the number of projects increased. In 1965, forty-six unclassified G-projects were confirmed to me by the Scientific Information Exchange of the Smithsonian Institution. Of the forty-six, thirty-three were AF-controlled.

During his press conferences on February 2, 1955, in Bogotá and February 10, 1955, in Grand Rapids, Michigan, aviation pioneer William Lear stated one of his reasons for believing in flying saucers was the existence of American research efforts into antigravity. Talbert's series of newspaper articles about the intensified interest in gravity control propulsion research were published during the Thanksgiving week of that year.