To sum up, I understand fully that fuel is an issue, I’m talking theoretical.
In space there is little to no resistance because of how few particles floating about. But if you were always running and engine, pushing matter out behind you, youd have a constant acceleration. So it seems like we should be able to travel near speed of light (combining this constant acceleration with abusing gravity for slingshot maneuvers). Yet people always say nothing can travel the speed of light.
In: Physics
There are 2 problems at work here. One is based on thr rockets themselves, and one on the fundamentals of physics.
Chemical rockets can only ever go so fast. This limit is the speed of the exhaust gasses going the other way. This is one of the reasons you might want a hydrogen based rocket, since it is very light and thus its exhaust can reach a very high speed.
The second problem is somrthing called the gamma factor. As we go faster, the energy we need to accelerate increases by an amount equal to the gamma factor. However, as we get closer to the speed of light thr gamma factor increases exponentially, and even becomes infinite at the speed of light. And there isn’t infinite energy in the universe, so we won’t be able to get anything to move that fast.
(This is also where we get the term relativistic mass from. You multiply mass with the gamma factor (it only becomes relevant at high relativistic speeds) and hey presto, it is relativistic mass. Keep in mind though that the mass doesn’t actually change. It is shorthand to make it easier on students.)
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