Hacker News Reader

Comment by abstractbill | Hacker News Reader

This can make a projectile move at mach 7, which is a little more than 20% of the Earth's escape velocity.

Are there physical limits that would prevent us from building a bigger one and using it to launch small things into space? Or "just" engineering problems?

View on HN

A space railgun is theoretically possible, but you'll still need a rocket to circularize the orbit.

In practice, a rocket goes up (out of most of the atmosphere) before going fast. A projectile fired out of a railgun would be going at its fastest while still in the atmosphere, and would thus be very hot / inefficient at the gun's "muzzle"

Heinlein's "The Moon is a Harsh Mistress" discusses such a solution for sending mined material back from the moon, which seems like a more feasible application.

The whole system is indeed possible, though. See this discussion:

http://physics.stackexchange.com/questions/35139/what-is-the...

If you wanted to get really clever, you could reach at least some orbits without a rocket (other than small corrections) by taking advantage of three-body interactions with the Moon or Sun. This would require even more velocity than just getting into low-Earth orbit, I imagine.

This video is a bit goofy, but has a good introduction to a lot of the relevant factors.

https://www.youtube.com/watch?v=Moo5nuLWtHs

The short answer is that it is a plausible mechanism for g-hardened payloads, but probably not for soft payloads (like human beings). The problem is that accelerating up to orbital velocity at the maximum survivable continuous rate (~4 or 5 g) still takes hundreds of miles, which is probably an infeasible length for a barrel. Additionally, if it's not somehow supported so that it rises above the atmosphere, the de-acceleration from drag at the exist of the barrel is also too extreme.

Elon Musk @elonmusk · Jan 26 Final one: anything launched by a railgun (if you could ever reach ~ Mach 27) would explode upon exiting the barrel in our dense atmosphere https://twitter.com/elonmusk/status/559629011983147008

No doubt that's true in the individual case, but surely you'd attempt to run a "train" into the atmosphere with this kind of thing? Meaning that each projectile is slipstreaming behind the one in front.

It still might not be possible, but it seems like friction would be much less of a problem. Although starting up would be a significant issue with many thousands of projectiles hitting earth at high velocity - you'd want to run it 24x7 indefinitely.

That doesn't sound like a low cost project, though; certainly well beyond our current capabilities. But you might achieve very nice energy efficiency with it.

This could maybe be an xkcd "what if" problem....I suspect a train of mach 27 projectiles would have some interesting effects on our atmosphere.

This is my imagination vs your imagination, so neither of us really has anything resembling an argument, but I'm wondering if in such a scenario the following projectiles might not find themselves running into the plasma wake left by the preceding projectiles. As such, your scenario might actually be worse than firing things one at a time.

The plasma wake will be less dense than the atmosphere displaced by the leading projectile, but it'll be much hotter and comprised of [#] much heavier atoms.

[#] "comprised of" is a correct usage, for anyone sucked in by the poor guy obsessed with editing it out of Wikipedia (many words have opposing meanings and some great English poetry depends on this; the term causes no confusion amongst native speakers; etymology is irrelevant to modern usage...)

In my mind, I'm picturing this being used to shoot pretty much blocks of solid materials, not anything delicate, so the only issue is how much they are slowed down; by the time we could build something like this, fabricating in space is not going to be an issue.

Of course, by the time we could build this (assuming its even possible), mining asteroids could already have surpassed it.

>[#] "comprised of" is a correct usage, for anyone sucked in by the poor guy obsessed with editing it out of Wikipedia (many words have opposing meanings and some great English poetry depends on this; the term causes no confusion amongst native speakers; etymology is irrelevant to modern usage...)

The tyranny of the minority, in this case, is comprised of a single man, whose idiosyncratic reign of grammatical terror is now reaching out from Wikipedialand into the Greater Internet.

I find this all very amusing.

But why can't we use the railgun to replace just the first stage of a rocket? Due to the tyranny of the rocket equation that would mean outsized savings in fuel and payload.

It would be exiting the barrel at far less than mach 27 so this wouldn't be an issue.

Even without a first stage, wouldn't that mean that the railgun would need to account for dealing the energy needed to accelerate a couple hundred thousand tons of rocket fuel to any reasonable speed? As per Newtons 3rd law?

This is correct. And the power needed for larger rockets is well beyond what we currently generate with traditional power plants on earth.

For instance, the space shuttle with SRBs and SSME's firing would require more than five Hoover Dams to match (11.7 gigawatts, vs 2.074 gigawatts). The Three Gorges Dam puts out 18.3 gigawatts, but the Saturn V first stage did somewhere around 190 gigawatts.

You could get away with less of course, since with rockets the first stage must lift the immense mass of itself, but with a railgun you will have significant efficiency losses with all of the energy conversions involved. I would expect a heavy-lift railgun to consume at least a significant percentage of a large nation's entire power supply. So we'd be looking at ludicrously sized capacitor banks, or some very large power plants that sit idle without anything to do while we aren't launching rockets.

What if you have an enclosed track in a partial vacuum that runs up along a mountain and exits where the air is thinner? Everest is 8.8km tall, and air at that height has 1/3 the pressure as at sea level [1]. If you can build a 1km tower on top to extend the track, you can get down to 1/4 the pressure.

[1]: http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...

Sounds like a modified hyperloop

Best answer I could fine. The short answer is no.

http://www.quora.com/Is-it-possible-to-use-a-big-gun-railgun...

Other people have mentioned why you wouldn't want to use a railgun to get a payload into orbit in one go. However, the exponential nature of the rocket equation means that every km/s you can get to start with has a large effect on what fraction of your rocket has to be fuel.

The more likely limitation is the human body. We can only take so much acceleration until we lose consciousness or worse. This would necessitate a much longer "runway" for people to use safely. No comment on unmanned use.

And note that "lose consciousness" is around 10g (depending on orientation, health, and many other factors) and "die" is something like 20-40g for more than a small period of time. A gun like this generates thousands or tens of thousands of g, which is more along the lines of "turned into a fine paste smeared on the back wall of the capsule."

I would guess that higher speeds would require exponentially more energy and bigger accelerator.

The problem remains that getting to space is only a small fraction of the energy it takes to get to orbit.

Thankfully using "escape velocity" covers all that energy cost.

It covers the speed you need, but not the velocity. For most tasks, launching directly away from Earth with minimal post-launch fuel capacity is not very helpful.

Good question. Hope someone cues in to answer.

Reply on news.ycombinator.com