jacksaff t1_j2c1ufb wrote
Reply to comment by boytoy421 in ELI5 why would we not be able to venture into space using rockets If Earth was 50% larger in diameter? by ShoulderHuge420
The problem with getting to orbit is that you need very high velocity to stay there. Altitude and atmosphere resistance are much less relevant than speed.
People often get the idea that if you launch from 15000m up and flying at several hundred km/h, you are a long way towards space and above a lot of the atmosphere. Unfortunately, overcoming altitude and the atmosphere are fairly small fractions of what you need to stay in orbit. You need to accelerate to around 27000 km/h to achieve orbit, so the benefits of a high launch are not as great as you would think. You are generally better off with a bigger rocket launched from the ground.
boytoy421 t1_j2c2ccl wrote
Well yeah but we're assuming increased gravity so the weight of the fuel makes that prohibitive. I'm saying if you had that restriction I think you could get around it (inefficiently) by using a separate ground based 1st stage to get closer to that 27000 kmh without having to carry the fuel on the "1st stage"
jacksaff t1_j2c4bti wrote
It's not the weight of the fuel fighting against gravity that is the problem. It is the inertia of the fuel preventing you from accelerating your rocket up to orbital speed. The main effect of more gravity is to increase the required orbital velocity. You need to go even faster, requiring more fuel, requiring even more fuel to accelerate the fuel and so on.
There will be more loss fighting gravity if the earth were bigger, but it is the increase in required final velocity that makes it impossible to achieve orbit with chemical rockets in this case.
Accelerating stuff at ground level definitely helps - see Spin Launch. Unfortunately, Earth having an atmosphere places a big limit on how fast you can get until you are above most of the air. You could spin launch to orbit (with small rocket corrections) on a huge planet, as long as there was no atmosphere. You could even launch people if you had a long linear accelerator rather than a spinning one.
boytoy421 t1_j2c8wpl wrote
And I suppose heat shielding still adds to the weight and there's no way to ditch the extra shielding before you're air/spaceborne
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