Question

what are the gain speed

Answer 1

Answer:

By "burning fuel" you probably mean usage of the law of conservation of momentum. So you should just throw away something - not necessary gases of burning fuel, but probably radiation, or accelerate gas by electro magnetic field like in Ion thruster or just explode atomic bombs like in Nuclear pulse propulsion

Another option is Solar Sail - by using Sun radiation pressure

You could go around planets in order to gain in speed. This is called gravitational assist or the slingshot effect, where the orbital speed of the moving planet boosts up your final velocity and modifies your direction.

hope this answer is helpful thank you

Answer 2

Fosill fule

please Flow Me

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