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Old 23-02-2018, 09:30 AM   #10
st jimmy
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Quote:
Originally Posted by skulb View Post
Well, in vacuum they use electrical engines rather than the chemical ones they use for liftoff on earth. https://en.wikipedia.org/wiki/Electr...aft_propulsion
As far as I know these engines are just powered by a battery inside the craft and expel matter to propel the craft forward. Or to reverse thrust when you want to stop. Not much thrust but that obviously works in vacuum and uses almost no energy.
(...)

But how do you stop? The only way I know of is a retrograde burn to slow you down enough to enter an orbit, or using the atmosphere itself to gradually slow down over the course of many orbits. But you still have to slow down a lot with burns before this even becomes possible. Without it you either crash into something eventually or skip off the atmosphere of the planet you're trying to get to and disappear in space.
I have the impression that we agree that it would be impossible to descend to the moon, because it’s impossible to slow down enough for a soft landing.

I’ve been thinking about the theoretical possibility of an engine that ignites in vacuum. The first problem I think of is lack of oxygen.
Even if it would be possible to build it in a way to supply the oxygen, I doubt if this could ever do anything besides make the temperature very hot.
The engine would have to be in front of the direction of the rocket (instead of behind it, to make slowing down possible). The rocket would become very, very hot as a result…

The Lunar Module would need huge tanks just to carry the oxygen alone which would increase the mass of the module with additional problems as a result.
I don’t know how much oxygen they would need, but this could very well be the size of the Empire State Building. Do you see the huge tanks on the Lunar Module?
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