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Spacecraft launch energy
Intermediate
Energy needed to reach orbit vs escape.
To reach low orbit one needs about 32 MJ/kg of kinetic energy plus 9 MJ/kg potential, but rocket inefficiencies and gravity losses make the needed Δv about 9.4 km/s.
Escape requires 62.6 MJ/kg.
The rocket equation makes small increases expensive.
In more detail
Launching a spacecraft requires energy to lift it out of Earth's gravitational well and to reach the required orbital speed. Most of the energy goes into reaching orbital speed, not just altitude.
Real-world uses
- Launch vehicles are sized for the energy needed to reach orbit.
- Mission planners optimise launch energy for interplanetary trajectories.
- Launch engineers compare the energy needed to reach low orbit with the energy needed to escape the planet entirely.
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Frequently asked questions
How much energy is needed to reach orbit?
Most of the energy goes into speed, since orbital speed is about 7.8 km/s.
Why not launch straight up?
Because a rocket must be going sideways at high speed to stay in orbit.
Is reaching orbit the same as escaping?
No. An orbit needs less energy than escape, so most launches target orbit and only deep-space missions need escape speed.
Related topics in Gravitation & Orbits
Galaxy and cosmic structure · Space elevators and tethers · Gravity on other planets