InertialLepton t1_j29c552 wrote
This idea comes from the rocket equation.
The basic idea of which is this: you want to get some amount of mass into space to do this you need to be going fast enough to get into orbit and to do this you need to burn fuel. Simple enough. If you know how much mass you need to launch you can work out how much fuel you need.
The problem is you have to take that fuel with you in the rocket. That fuel also has mass.
So now you need even more fuel to get that mass of the fuel you need into space as well. And you need even more fuel to account for the mass of that fuel.
Here's the wikipedia article for the rocket equation. Give it a read if you want to see the actual equation and the derivations - it's surprisingly readable.
In any case we can derive an equation for how much fuel we need for any rocket.
What we can also do is look at the equation the other way round. I think I found the article that originated the 50% idea and they explain their reasoning pretty well:
>Let us assume that building a rocket at 96% propellant (4% rocket)... is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter [while maintaining the same density], we would not be able to venture into space, at least using rockets for transport.
boytoy421 t1_j2a4zzo wrote
Couldn't you get around that by doing a ground-based acceleration stage? I'm thinking basically a big-ass track on the ground that you could use to accelerate the second stage (using idk magnets, or even just like a detachable rocket sled) So that by the time you ignite the 2nd stage you're already going X speed without the rocket using any fuel? I mean you could also presumably use a big-ass cannon but the acceleration would turn the astronauts into people salsa presumably
InertialLepton t1_j2bahgw wrote
Sure!
A ground based acceleration on it's own isn't really an option - the speed needed for escape velocity is well into hypersonic speeds so you'd burn up, but launching into a low earth orbit with only small rockets to provide corrections is physically possible. As you mention though, that wouldn't work for astronauts, just cargo.
But a part approach with a ground launch then rockets is absolutely possible to claw back some of the limits from the rocket equation.
I believe there's a company called Spinlaunch who are developing a system like this (not for humans sadly) where the rocket is accelerated by a spinning arm in a vacuum sealed chamber before being launched.
In general, getting objects going fast enough without an enormous launch tube is a bit of an engineering challenge. Also, I guess given we are not in the 50% larger scenario people are happy with rockets.
boytoy421 t1_j2bb6a9 wrote
I mean didn't the amerikabomber do a horizontal first stage and then use aerodynamics to gain altitude without fuel usage and then skip across the atmosphere?
My thinking is that you swap out the bomb bay for a 2nd stage rocket and fire that at the apogee to get into orbit.
jacksaff t1_j2c1ufb wrote
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
Viewing a single comment thread. View all comments