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this post was submitted on 15 Mar 2024
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Uhhh
Can someone tell me what velocity you need to launch an object at in order to reach orbit? And what accelerative forces you need in order to achieve that?
How big is this railgun? How are they handling heat from atmospheric friction? You have the same problem going up too quickly as you get coming down too quickly.
It's probably a high speed launch assist, not the entire source of escape velocity.
That sounds so funny though.
How would you even do that? Build something like a particle accelerator except it's a launching railgun the size of China?
I mean yeah why not if it means you can partially reach escape velocity with like solar powered electricity vs going the entire way 100% on burning rocket fuel
NASAs version of this is called StarTram, it's feasible but there are some major technical hurdles.
In the prototype stage, the launch velocity at the end of the gun would not be anywhere near the 10 km/s you would need to achieve orbit. But you could reach a sizable fraction, say 3 km/s. Becuase the velocity gained during rocket flight is not linear with mass, having that 3 km/s inital boost would reduce your overall rocket mass by like 80%. The rocket would clear the atmosphere and then fire its motor to gain a stable orbit.
We can achieve 3 km/s with current technology, it would require like a 100 km long maglev in a very low pressure tube, likely built on the side of a mountain. It could probably reduce space flight costs by a factor of 10 if completed.
Building the proposed goal of a human rated version capable of reaching near 10 km/s at the end point would be much more difficult. The tube would have to span over 1,000 km and I don't believe we have the technology yet to maintain the vacuum or supply enough power quickly enough to power it.
China is likely proposing to build a testbed system to start researching the concept. Probably on a larger scale than NASA has but nowhere near a complete system.
You'd still need engines and fuel onboard, or you couldn't circularize the orbit. The idea is more just get something out of the atmosphere with something that doesn't itself have to be dragged along for the ride, then do the rest of it once you don't have atmospheric drag and you aren't fighting directly against gravity anymore.
It's kind of the same as the idea of "what if the first stage was a big air-breathing plane that just, like, flew really high and really fast?" that keeps cropping up, just finding a way to make the first part of the process less absurdly expensive.
But no matter what you can't put something into orbit with a single input of velocity unless that was enough to remove it from the Earth's sphere of influence entirely, because you can't make the lowest point in an orbit higher than the point you're currently at.
Absolutely impossible. The problem isn't the angle, the problem is that orbits are circular. The "orbit" would have to go around in circular motion until until it reaches the point where it exited the railgun (after accounting for the rotation of the earth and all the other nonsense you need to account for).
What you said, but the orbits are elliptical. The periapsis (lowest orbital altitude) would be the point where the craft exits the rail launch - located within earth's atmosphere.
I think the next evolution after the raillaunch would be a hybrid engine that breathes air and functions in a vacuum so we could have small SSTO craft.
What if the interior of the railgun accelerator was all a vacuum and the exit point was really really really really tall? So that impact with atmosphere at the exit of the gun was lower.
This is what StarTram is, the launch system maintains a vacuum. The system won't work without that feature.
The problem is that you still need an additional acceleration in order to get into an real orbit. In any theoretical orbit (without an acceleration) you'll always go around in a circular motion until you reach your starting point, if your starting point is in the atmosphere then you'll go around your orbit until you hit your starting point in the atmosphere again. This will obviously cause your orbit to decay. There are a few solutions.
I love the idea of combining the rail launch with a skyhook. Skyhooks always sounded like they would fail due to atmospheric concerns which this would alleviate.
The Delta-V (change in velocity) to reach the ISS (400km) in Low Earth Orbit (LEO) is 9.4km/s. This is the absolute minimum so it doesn't include things like drag or other inefficiencies. Getting to space takes only about 1.4km/s, just about all the rest of that delta-v is used to accelerate the spaceship to 7.6km/s. This is about the minimum speed, altitude, and delta-v to get into orbit and stay there. Going much lower than the ISS will have your orbit decay too much to stay in orbit for more than weeks or months. I think it would take about 8000m/s to orbit right at 100km but your orbit would decay really quickly at that altitude, you would only get a few hours of orbiting.
I think I answered you below about why you can't to orbit using only a railgun.
Edit: Going from earth orbit to other places doesn't take as much delta-v as you might think. Going from earth orbit to lunar orbit only takes about 3km/s and going from earth orbit to mars orbit takes 5.7km/s.