High Quality Content by WIKIPEDIA articles! 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.

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