8 ms·

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

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?

loceng 12y ago
Good question. Hope someone cues in to answer.
Yizahi 12y ago
I would guess that higher speeds would require exponentially more energy and bigger accelerator.
iblaine 12y ago
Best answer I could fine. The short answer is no. http://www.quora.com/Is-it-possible-to-use-a-big-gun-railgun-to-shoot-satellites-or-spacecraft-into-orbit-around-Earth-or-to-escape-velocity http://www.quora.com/Is-it-possible-to-use-a-big-gun-railgun...
jessriedel 12y ago
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 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.
rm_-rf_slash 12y ago
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.
- mikeash 12y ago
  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."
empy 12y ago
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 https://twitter.com/elonmusk/status/559629011983147008
- reitzensteinm 12y ago
  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.
  - stolio 12y ago
    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.
  - tjradcliffe 12y ago
    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...)
    - 12y ago
sparkman55 12y ago
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-possibility-of-a-railgun-assisted-orbital-launch http://physics.stackexchange.com/questions/35139/what-is-the...
- mikeash 12y ago
  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.
Dylan16807 12y ago
The problem remains that getting to space is only a small fraction of the energy it takes to get to orbit.
- Symmetry 12y ago
  Thankfully using "escape velocity" covers all that energy cost.
  - Dylan16807 12y ago
    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.
Symmetry 12y ago
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.

loceng 12y ago

Good question. Hope someone cues in to answer.

Yizahi 12y ago

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

iblaine 12y ago

Best answer I could fine. The short answer is no. http://www.quora.com/Is-it-possible-to-use-a-big-gun-railgun-to-shoot-satellites-or-spacecraft-into-orbit-around-Earth-or-to-escape-velocity http://www.quora.com/Is-it-possible-to-use-a-big-gun-railgun...

jessriedel 12y ago

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 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.

rm_-rf_slash 12y ago

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.

mikeash 12y ago
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."

empy 12y ago

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 https://twitter.com/elonmusk/status/559629011983147008

reitzensteinm 12y ago
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.
- stolio 12y ago
  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.
- tjradcliffe 12y ago
  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...)
  - 12y ago

sparkman55 12y ago

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-possibility-of-a-railgun-assisted-orbital-launch http://physics.stackexchange.com/questions/35139/what-is-the...

mikeash 12y ago
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.

Dylan16807 12y ago

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

Symmetry 12y ago
Thankfully using "escape velocity" covers all that energy cost.
- Dylan16807 12y ago
  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.

Symmetry 12y ago

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.