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Specifically: not a "space gun" for launching payloads to space. This subject has been discussed a lot, and I know of the slew of problems and their partial solutions enough.

I mean a device mounted on a craft, using electricity for launching macroscopic pieces of metallic reaction mass at hypervelocity, in the opposite direction the craft is meant to move.

Railguns operating in atmosphere have limited applications and their own slew of problems. Some of them should vanish in void, while others might remain or get exceberated. Nevertheless, if EDO-1 ever reached the planned 7000m/s exit velocity, it would constitute an engine of over 700s of ISp, and that's surely not the end of it - if we don't strive for heavy projectiles of aerodynamic shape and armor piercing properties, just focus on getting whatever shape ejected as fast as possible, this could see even further improvements.

Of course there would be problems. Pulsed propulsion stress, energy supply, heat dissipation and so on. But you have to admit 700s of ISp sounds very tempting.

And of course I'd be very surprised if I was the first to think of it. So - has this been attempted, developed, researched? Some unforseen problems that make it totally impossible or otherwise a misguided endeavor?

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    $\begingroup$ Hm, I wonder if we could pilot an asteroid like this by launching its own mass away... $\endgroup$ – called2voyage Jun 7 '16 at 19:38
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    $\begingroup$ There was a scheme with mass drivers using asteroidal material as reaction mass. Google MADMEN asteroids. $\endgroup$ – HopDavid Jun 8 '16 at 3:53
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    $\begingroup$ I wonder how long it would be before spitting out a gigantic quantity of small fast-moving objects near Earth or installations in space would be a problem... $\endgroup$ – kim holder Jun 8 '16 at 15:00
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    $\begingroup$ @kimholder: I imagined it would be, so the craft would likely be sending projectiles into the atmosphere to burn up. Or at least into escape trajectory. Besides, I imagine with the energies involved, the projectiles might be leaving the engine as vapor, or at least a very fine liquid mist. $\endgroup$ – SF. Jun 8 '16 at 16:55
  • $\begingroup$ Mass-throwers are worth evaluating for specific applications. ISP and thrust/weight are the key performance factors when launching from a gravity well and carrying all of your propellant. ISP is a function of exhaust velocity and, for a rocket, the molecular weight of your propellant. So you are giving up ISP by throwing rocks or metallic slugs, not hydrogen. However if you are moving an asteroid then your system is not mass-limited, it's energy-limited and launch-system-mass limited. The Wikipedia article on ISP links to another on energy efficiency, worth checking out. $\endgroup$ – Kengineer Jun 9 '16 at 19:29
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Yes, Made on Space is researching a similar idea. Plan to Turn Asteroids Into Spaceships

the propulsion system might be some sort of catapult that launches boulders or other material off the asteroid in a controlled way, thereby pushing the space rock in the opposite direction (as described by Newton's Third Law of Motion)

While the Made in Space plan involves much simpler forms of object expulsion then a rail gun to create propulsion, the basic principles still apply.

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I suspect that if you have the electric power available to fire macroscopic projectiles from a railgun, you're better off accelerating Xenon gas with it. Current ion thrusters yield exhaust velocities above 30,000m/s. https://en.wikipedia.org/wiki/Ion_thruster

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  • $\begingroup$ I was wondering if this might be the case. Xenon is expensive, though. $\endgroup$ – called2voyage Jun 7 '16 at 20:22
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    $\begingroup$ At $850/kg it's not going to break the bank for small probes, and MPDTs can get similarly efficient performance on hydrogen. $\endgroup$ – Russell Borogove Jun 7 '16 at 20:36
  • $\begingroup$ You might want to link to another resource like this then, because your Wiki article claims that Xenon is in short supply. $\endgroup$ – called2voyage Jun 7 '16 at 20:40
  • $\begingroup$ 30,000m/s at what thrust? Also, iron asteroids are a dime a dozen, while finding xenon in space is going to be challenging... $\endgroup$ – SF. Jun 7 '16 at 21:38
  • $\begingroup$ @called2voyage There's plenty of xenon in the atmosphere. It is global production that limits supply. Xenon is often discarded in the process of liquefying oxygen and nitrogen from our atmosphere. If there were a large demande for it, it wouldn't be hard to ramp up production. $\endgroup$ – HopDavid Jun 8 '16 at 3:41
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From a science fiction perspective, there is a book Heart of the Comet by Gregory Benford and David Brin about an attempt to corral Halleys comet into Earth orbit. At apogee, they use mass-slingers, basically railguns firing mined mass from the comet to transfer momentum and 'fly the comet'.

Great story regardless, but Brin and Benford are actual scientists, who often pay at least superficial care to the science in the story.

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  • $\begingroup$ 'Heart Of The Comet' is a good read. I recommend it. $\endgroup$ – Howard Miller Jun 10 '16 at 23:19

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