Space launch
From Wikipedia, the free encyclopedia
Space launch is the earliest part of a flight that reaches space and may commonly refer to
- Liftoff, when a rocket or other space launch vehicle leaves the ground at the start of a flight. It has two main types:
- Rocket launch, the current conventional method
- Non-rocket spacelaunch
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[edit] Issues with reaching space
[edit] 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 100km. Other definitions have also been made, in the US for example space has been defined as 50 miles.
[edit] Energy
Therefore, by definition for spaceflight to occur, sufficient altitude is necessary. This implies a minimum specific gravitational potential energy needs to be overcome: for the Kármán line this is approximately 1 MJ/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.
[edit] G-forces
Many cargoes, particularly humans have a limiting g-force that they can survive. For humans this is about 3-6 g. Launchers such as gun launchers are unsuitable for such cargo.
[edit] Reliability
Launchers vary with respect to their reliability for achieving the mission.
[edit] 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.
[edit] Sustained spaceflight
[edit] Suborbital launch
[edit] Orbital launch
In addition, if orbit is required, then much higher energy is needed as some sideways speed is needed. The speed needed depends on the altitude, less speed is needed at high altitude; however allowing for the extra potential energy due to altitude, overall, far more energy is needed to orbit at high altitude than lower.
The speed needed to maintain an orbit, near to the Earth's surface corresponds to a sideways speed of about 7.8 km/s, an energy of about 60MJ/kg.
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.
[edit] 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 vehicles speed. The kinetic energy exceeds the potential energy then escape occurs. At the Earths surface this occurs at a speed of 11.2 km/s, but in practice a much higher speed would be needed due to airdrag.
