Space launch is the earliest part of a flight that reaches space. Space launch involves liftoff, when a rocket or other space launch vehicle leaves the ground, floating ship or midair aircraft at the start of a flight. Liftoff is of two main types: rocket launch (the current conventional method), and non-rocket spacelaunch (where other forms of propulsion are employed, including airbreathing jet engines or other kinds).
Issues with reaching space
Definition of space
Space
has no physical edge to it as the atmospheric pressure gradually
reduces with altitude; instead, the edge of space is defined by
convention, often the Kármán line of 100 km. Other definitions have been created as well. In the US for example space has been defined as 50 miles.
Energy
Therefore, by definition for spaceflight to occur, sufficient altitude is necessary. This implies a minimum gravitational potential energy needs to be overcome: for the Kármán line this is approximately 1 MJ/kg.
W=mgh, m=1 kg, g=9.82 m/s2, h=105m.
W=1*9.82*105≈106J/kg=1MJ/kg.
In practice, a higher energy than this is needed to be expended
due to losses such as airdrag, propulsive efficiency, cycle efficiency
of engines that are employed and gravity drag.
In the past fifty years spaceflight has usually meant remaining
in space for a period of time, rather than going up and immediately
falling back to earth. This entails orbit, which is mostly a matter of
velocity, not altitude, although that does not mean air friction and
relevant altitudes in relation to that and orbit don't have to be taken
into account. At much, much higher altitudes than many orbital ones
maintained by satellites, altitude begins to become a larger and larger
factor and speed a lesser one. At lower altitudes, due to the high speed
required to remain in orbit, air friction is a very important
consideration affecting satellites, much more than in the popular image
of space. At even lower altitudes, balloons, with no forward velocity,
can serve many of the roles satellites play.
G-forces
Many cargoes, particularly humans have a limiting g-force
that they can survive. For humans this is about 3-6 g. Some launchers
such as gun launchers would give accelerations in the hundred or
thousands of g and thus are completely unsuitable.
Reliability
Launchers vary with respect to their reliability for achieving the mission.
Safety
Safety is
the probability of causing injury or loss of life. Unreliable launchers
are not necessarily unsafe, whereas reliable launchers are usually, but
not invariably safe.
Apart from catastrophic failure of the launch vehicle itself other safety hazards include depressurisation, and the Van Allen radiation belts which preclude orbits which spend long periods within them.
Trajectory optimisation
Trajectory optimization is the process of designing a trajectory that minimizes or maximizes some measure of performance
within prescribed constraint boundaries. While not exactly the same,
the goal of solving a trajectory optimization problem is essentially the
same as solving an optimal control problem. This problem was first studied by Robert H. Goddard and is also known as the Goddard problem.
The selection of flight profiles that yield the greatest
performance plays a substantial role in the preliminary design of flight
vehicles, since the use of ad-hoc profile or control policies to
evaluate competing configurations may inappropriately penalize the
performance of one configuration over another. Thus, to guarantee the
selection of the best vehicle design, it is important to optimize the
profile and control policy for each configuration early in the design
process.
For example, for tactical missiles, the flight profiles are determined by the thrust and load factor (lift) histories. These histories can be controlled by a number of means including such techniques as using an angle of attack
command history or an altitude/downrange schedule that the missile must
follow. Each combination of missile design factors, desired missile
performance, and system constraints results in a new set of optimal
control parameters.
Sustained spaceflight
Suborbital launch
Sub-orbital space flight is any space launch that reaches space
without doing a full orbit around the planet, and requires a maximum
speed of around 1 km/s just to reach space, and up to 7 km/s for longer
distance such as an intercontinental space flight. An example of a
sub-orbital flight would be a ballistic missile, or future tourist
flight such as Virgin Galactic, or an intercontinental transport flight like SpaceLiner.
Any space launch without an orbit-optimization correction to achieve a
stable orbit will result in a suborbital space flight, unless there is
sufficient thrust to leave orbit completely.
Orbital launch
In addition, if orbit is required, then a much greater amount of
energy must be generated in order to give the craft some sideways speed.
The speed that must be achieved depends on the altitude of the orbit –
less speed is needed at high altitude. However, after allowing for the
extra potential energy of being at higher altitudes, overall more energy
is used reaching higher orbits than lower ones.
The speed needed to maintain an orbit near the Earth's surface
corresponds to a sideways speed of about 7.8 km/s (17,400 mph), an
energy of about 30MJ/kg. This is several times the energy per kg of
practical rocket propellant mixes.
Gaining the kinetic energy is awkward as the airdrag tends to
slow the spacecraft, so rocket-powered spacecraft generally fly a
compromise trajectory that leaves the thickest part of the atmosphere
very early on, and then fly on for example, a Hohmann transfer orbit
to reach the particular orbit that is required. This minimises the
airdrag as well as minimising the time that the vehicle spends holding
itself up. Airdrag is a significant issue with essentially all proposed
and current launch systems, although usually less so than the difficulty
of obtaining enough kinetic energy to simply reach orbit at all.
Escape velocity
If the Earth's gravity is to be overcome entirely then sufficient
energy must be obtained by a spacecraft to exceed the depth of the
gravity potential energy well. Once this has occurred, provided the
energy is not lost in any non-conservative way, then the vehicle will
leave the influence of the Earth. The depth of the potential well
depends on the vehicle's position, and the energy depends on the
vehicle's speed. If the kinetic energy exceeds the potential energy then
escape occurs. At the Earth's surface this occurs at a speed of
11.2 km/s (25,000 mph), but in practice a much higher speed is needed
due to airdrag.
Types of space launch
Rocket launch
Rocket launch is the only current way to reach space. In some cases
an airbreathing (jet engine) first stage has been used as well.
Non-rocket launch
Non-rocket space launch is a launch into space where some or all of the needed speed and altitude are provided by something other than expendable rockets. A number of alternatives to expendable rockets have been proposed. In some systems such as Skyhooks, rocket sled launch, and air launch, a rocket is used to reach orbit, but it is only part of the system.
