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Sirhc978 t1_j296qrr wrote
They basically mean, we can't do it with our current rocket technology. Our rockets do the bare minimum to get into low earth orbit as it is. They are also horribly inefficient. Most rockets are something like 95% fuel, and 5% 'rocket'. If earth was 50% larger, we would need some alternative to chemical rockets. A lot of people suggest nuclear powered ones, which are possible, but have never really been tested.
In the end, rockets would not be impossible, they would just need to be different from the technology we currently use.
dimonium_anonimo t1_j297p18 wrote
Also, I think there's some treaty that says we can't. I mean, I'm just going off of what my high school physics teacher said, but I remember him telling us about a ship that rides the wave of nuclear fission, but it never got to fly because we can't detonate nukes in space or something.
Sirhc978 t1_j2984zp wrote
Yeah it is called The Outer Space Treaty. However there is something called a nuclear salt-water rocket which doesn't use explosions.
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
mmmmmmBacon12345 t1_j29d9xj wrote
The rocket equation is a butt and as mass/gravity scale up you start needing a rocket the size of the Saturn V just to put Sputnik into orbit
Earth having a 50% greater diameter means it has 3.375x the mass because mass scales with volume and that results in a surface gravity about 50% higher (~14.7 m/s^2 )
That higher gravity means that you need stronger(heavier) rocket engines since a rocket needs a Thrust to Weight ratio of at least 1 to leave Earth and if we still with Earth weights it'd need a TWR of 1.5 to leave the bigger Earth. The Saturn V rocket had a TWR of 1.02 at launch so it'd never make it, and the Falcon 9 is pretty zippy off the pad today but it only has a TWR of 1.4 so it would also never make it.
This then pairs with the tyranny of the rocket equation where the higher gravity means we need to go faster which means we need more fuel which needs to be lifted with more fuel and you need to lift that fuel with more fuel. Today the ISS orbits at a speed of 7.66 km/s, but around our new Earth even at the same 400 km up it'd need to be going 11.6 km/s and it'd take a bit more to get to orbit
Today it takes about 10 km/s of DeltaV(how much a rocket can change its speed, basically range) to get from the surface to the ISS, that means lifting 1 ton requires 29 tons of fuel so the rocket is 96.66% fuel. If you need to get up to 14 km/s then the rocket needs to have 115 tons of fuel(99.1% of overall mass)
So we start needing more fuel just to get to orbit and stronger rocket engines which require yet more fuel which means stronger heavier engines and more fuel and more fuel for the fuel....
TheCheshireCody t1_j2a199z wrote
More mass means higher gravity. Higher gravity requires more speed required to escape the atmosphere. More speed requires more fuel, or more fuel efficiency, or a lighter rocket. We can't reduce the weight of the rocket itself too much because of the materials we're working with. We can't make the fuels we're using much more efficient because that's just the way chemical combustion is. So the only way to get more power is more fuel, which means more weight.
We can currently juuuuust balance the amount of fuel (and how much force that fuel can generate) against the weight required to lift that fuel and the rocket and a payload. The Saturn V was the biggest rocket we could make at the time of the Apollo launches, and we needed every bit of it to get to the Moon. In sixty years we've not been able to make massive advances in rocket tech, so the SLS is very close to the Saturn V in overall specs, relatively speaking. If we're just ahead of breaking even on that now, you can see how an increase in the requirements would put us behind the curve.
That doesn't mean a 50% larger Earth would result in us never leaving it, though. What it would mean is we'd need to come up with better solutions to one of the above variables - better materials that weigh less, more-efficient fuels, or a better design than igniting a ton of fuel and pointing the exhaust at the ground.
[deleted] t1_j295yxj wrote
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eloel- t1_j295vgy wrote
If earth had more gravity, we'd need more force to get us out. I don't know if the 50% figure is a tight limit, but at SOME limit, the amount of fuel you need to escape gravity is more than the amount you can carry, so you stay grounded. That's of course a current-tech thing, future tech might fix it.
A larger earth means a larger gravity, hence the connection