If we assume we are in a perfect vacuum in space, where there’s no friction or anything to slow us down, at what point would bursts of fuel from thrusters stop increasing our speed? And would they even?
Now, I understand that by einstein, we need infinite energy to accelerate to speed of light, but i cant intuitively see how a rocket wouldnt constantly speed up in a vacuum when given constant bursts of rocket thrust. Im sure im missing something pretty simple here, but nevertheless.
In: 4
Kind of. The fuel’s *specific impulse* is a measure of how quickly the fuel could make its own weight go. You might think you can’t make it go faster than that.
However, by adding more stages, the rocket can go faster. If your specific impulse is 1000m/s then 1000 tonnes of fuel can make 1000 tonnes of stuff go at 1000m/s. But that 1000 tonnes of stuff could be another rocket with 500 tonnes of fuel moving 500 tonnes of stuff, at 1000m/s faster than how fast the first rocket made it go. And so on.
But then again, it’s exponential. If you *double* the weight of the rocket it only goes 1000m/s faster again, and that’s the theoretical limit. So if you want it to go 100,000m/s you’re out of luck. You can’t make a rocket that big.
It also explains why they want to make satellites and space probes as lightweight as possible, because at the bottom of this stack of rockets – and the deciding factor for the overall weight of the stack – is the thing you’re actually trying to get into space. If you can halve the weight of that you can use that extra weight to make a small rocket and then you can delete the biggest rocket from the stack – halving its weight
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