Pretty straight forward question really. What are rail guns, how do they work and how likely is it that they will replace stage one rockets in the future? Why have they not done so already? Is it feasible? Or would the astronauts just get pasted due to the extreme acceleration?
In: Technology
Rail guns are a type of electric gun design. By applying a voltage between two rails and short circuiting them with a projectile while continuously supplying them with massive currents will create a force that pushes the projectile forward. The problems we have is related to supplying it with enough current without melting and damaging components. This does include the projectile which have a tendency to melt, boil and then turn into a ball of plasma before it can leave the rails. But even if you can solve these issues in the gun and were able to shoot a projecile at orbital speeds it would compresse the air in front of it heating it up so it would melt, boil and then turn into a ball of plasma. And satellites do tend to work slightly better if you manage to keep them from turning into plasma. Similarly astronauts do tend to feel better about the launch knowing that their bodies will be able to retain the same state of matter after the launch as before.
A rail gun is basically a gun that uses electromagnets to accelerate the payload rather than rockets or an explosion. They can theoretically be used to accelerate things to far greater speed than conventional guns, but that comes with some caveats:
a) With the level of technology we have right now, the wear on the rails inside the gun is horrific–they need to be replaced after every couple of shots.
b) They can’t really be used for anything very large yet either.
As for using them to launch stuff into space? The problem you have there is the atmosphere. Orbital velocity is around 17,000mph, so to fire an object into space with any type of gun, it has to be going quite a bit faster than that in order to compensate for gravity and air resistance. Anything hitting the atmosphere at those sort of speeds will rapidly heat up–you can see this when spacecraft re-enter the atmosphere from orbit, and bear in mind that those are hitting the really thin atmosphere high up, not the thick stuff lower down. Chances are trying to fire something at 17,000mph anywhere near sea level would immediately destroy it. And then, even if you could somehow get around all that, it wouldn’t remove the need for whatever you’re launching to have its own propulsion system, because once it gets out into space it needs some way to circularise the orbit–otherwise, it’ll just come straight back down.
Others have already talked about the technical challanges, but yes, the extreme acceleration is a prohibitive problem for most tasks:
In order to reach space, and then stay in space, you need a speed of 8km/s, or 5 miles per second. Say we build a really long railgun, lets say 2000 meters long. In order to go from zero to 8km/s within 2000 meters, you need an acceleration of 1600g. Not only is that enough to kill any astronaut a hundred times over, its too much for pretty much anything except raw materials. You certainly cannot use it for something like a satellite or a space station module.
So, even if we solved the other problems like air friction, we would either have to build them hundreds or thousands of kilometers long, or would be restricted to use them for raw materialy only.
The most fundamental issue is that either the acceleration would be high enough to turn astronauts into a paste, or you’d have to make the thing miles long. Neither option is really any better than current rocket tech.
Rail guns work on the principle of electromagnetism. Basically anything with magnetic properties can be launched via railgun. Voltage and Amperage applied to the rails (solenoids, but that’s another conversation) will determine the strength of the field that propels the magnetic material.
So why dont we use it for space launches?
Well as soon as you leave the rails the acceleration effects stop. It would be like trying to drive to the store but only hitting the gas on your driveway and coasting the rest of the way.
To accelerate fast enough and would apply a considerable amount of gforces that not only would humans not survive, but quite probably the craft as well.
The other big reason is that its not exactly energy efficient. Rail guns are actually very inefficient when it comes to converting electrical energy to mechanical energy. Using rocket engines is way more efficient.
I suppose you could have rails erected all the way out of the atmosphere, but at that point, why launch anything at all?
Rail guns us a law of electricity to shoot.
But to go fast you need a lot of power, and just like your PC gets hot when using a lot of power so does the thing you’re shooting. This is a big limitation.
You need to go really fast to go into space, like really really fast. Wind resistance slows things down just like it slows down a baseball or a bullet.
Rockets overcome this by constantly pushing. Just like driving a car. You need to have the throttle down or you just keep slowing down.
Rail guns use electromagnets to accelerate things to high speeds.
An electromagnet is just lots of loops of wire, often around an iron core. When you pump electricity through it, it becomes magnetic. When you turn the electricity off, it stops being magnetic.
Now imagine having a row of these, with a ball bearing sitting at one end. You turn on the first 3 magnets, say, and the ball begins to roll. When the ball reaches the first magnet, you turn it off and turn on mag number 4. The ball accelerates more. It reaches 2, which you turn off and turn on 5. Rinse and repeat. This makes the ball accelerate a lot, until it shoots past then end of the row of (now all off) electromagnets. This is essentially how a rail gun works. It speeds things up by using electromagnets. (This is also how the Large Hadron Collider works, but in a loop.)
Now to rockets. Rockets work by constantly fighting against the force of gravity. They throw a load of expanding gas out the back, and this has pushes them up the way. (Contrary to common belief, they aren’t pushing on the ground at all. It just happens to be in the way.)
The term “launch” is probably a bit misleading, because it gets used to describe releasing rockets *and* projectiles, like our rail gun friend. A projectile is something that gets launched, but doesn’t then have any propulsion. It’s essentially falling. You *could* launch something in to space by throwing it really, *really*, hard. But the speed you need is called the escape velocity. On Earth, the escape velocity is over 11km/s. That is, to launch something to space by throwing it, you would need to make it go over 11km in a second. And that’s forgetting about drag.
So, the reason we don’t use rail guns is because you’d need the make the rocket go stupidly quick through the atmosphere. Either that or build a rail that goes all the way in to space. On the other hand, using rockets to constantly fight against gravity, spaceships can accelerate at a much gentler pace (relatively speaking) the whole way up.
This isn’t really an ELI5-answerable question, but I’ll try.
> What are rail guns, how do they work
A rail gun is a gun that uses electricity to shoot something instead of a chemical like gunpowder. This is similar to a gauss gun, but a gauss gun uses the electricity to create magnets that shoot the something instead of electricity directly.
If you’re not actually 5, see https://en.wikipedia.org/wiki/Lorentz_force for more details.
> how likely is it that they will replace stage one rockets in the future? Why have they not done so already? Is it feasible? Or would the astronauts just get pasted due to the extreme acceleration?
Almost zero chance. A “gun” of any sort is pretty bad at putting things into orbit. First, things shot by guns start out really fast then slow down due to friction. This is opposite the best way to get into space because it gets easier to go fast the higher you go, so a gun wastes a lot of power going fast when it’s hard and slows down when it gets easy.
Second, starting fast only gets you *into* space. To stay there for any length of time you have to go fast again. So to use a gun you would need to shoot a rocket so the rocket could do the second go-fast part. This is more complicated and less efficient than just using rockets the whole way.
Finally, because guns get slower they have to go really, really fast at first and that’s hard to live through. Especially for humans, but also for delicate electronics and other things you might find on a space ship.
There is a really good reason most space ships use rockets. To move around while in space you need one anyway, and making a rocket take care of the launch as well is relatively simple.
ELI15:
A gun system has zero acceleration ability after the projectile is fired, that is, the projectile is in *free-fall* after being shot. This means your speed is highest immediately after firing, which is when the atmosphere is thickest and losses due to friction are the worst. As the atmosphere thins out and friction decreases, the projectile has already slowed down and has no ability to take advangate of it.
To get into a stable orbit *around* the earth instead of just acting like a really-fast baseball and crashing back to earth you need a second acceleration phase–usually called the “circularization burn” near your peak altitude. A projectile itself cannot provide this, so you would have to shoot something like a rocket anyway. While you’re building a rocket, it’s relatively simple to build a second one for the launch instead of a completely different launch system. Also, as we’ve seen recently, the first stage is the easiest to recovery and reduce costs on, so the “waste” of a stage can be pretty low.
Finally, the acceleration profile of a gun-launch would be massive. Earth’s escape velocity is about 11 km/s. Ignoring atmospheric friction, that’s the speed your projectile would need to be moving when it left the gun. If we assume a 5 km-long gun, that requires an acceleration of over 1000 Gs (12 km/s^2). There aren’t many machines, and no humans, that can survive that. This part is a bit hand-wavy as escape velocity is for *leaving* Earth, and most space flight is concerned with *orbiting* Earth, but the fact that we’re at least two orders of magnitude away from “safe” even without having to deal with friction should be pretty telling.
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