Elon Musk @elonmusk ยท 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
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.
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 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...)
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?
infogulch|11 years ago
[1]: http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...
Phlarp|11 years ago
unknown|11 years ago
[deleted]
reitzensteinm|11 years ago
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.
tjradcliffe|11 years ago
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...)
stolio|11 years ago
mrfusion|11 years ago
It would be exiting the barrel at far less than mach 27 so this wouldn't be an issue.
benihana|11 years ago