Space launch

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 at the start of a flight. Liftoff is of two main types: rocket launch, the current conventional method, 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 100km. Other definitions have also been made, 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 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.

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.

afety

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.

ustained spaceflight

uborbital launch

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. 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 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.

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