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Question about nuclear pulse propulsion
August 14, 2012 7:17 AM   Subscribe

As I understand it, the idea behind "nuclear pulse propulsion" is that you have a spaceship where you explode nukes behind the ship, and then the ship sort-of "rides the shockwave", the nuclear explosions propelling the ship forward. But how can that possibly work in the vacuum of space? If there's no medium, like air, there's no shockwave, right? So what, exactly, would be propelling the ship forward?
posted by gkhan to Science & Nature (16 answers total) 5 users marked this as a favorite
 
Some of the pusher plate ablates under the radiation blast, giving a high velocity rocket effect. Also you can pump inert reactant onto the plate to increase this effect. Presumably the expanding gas shell from the nuke contributes something as well.
posted by ROU_Xenophobe at 7:22 AM on August 14, 2012


There is a shockwave. The matter making up the bomb is vaporized and expands in all directions. The total mass involved is not very great but the velocity is extremely high, so the impulse would still be significant.
posted by Chocolate Pickle at 7:22 AM on August 14, 2012 [4 favorites]


The nuclear bomb is made of matter. When the bomb goes off, particles move in all directions with high kinetic energy. A small fraction of those particles hit the deflection plate on the rear of the vehicle, pushing against it just like a chemical rocket.
posted by b1tr0t at 7:23 AM on August 14, 2012 [4 favorites]


Here are some details on the conceptual Orion spaceship, which would use nuclear pulse propulsion.
posted by audi alteram partem at 7:25 AM on August 14, 2012


Each bomb would be a shaped charge, launching its matter into the ship's pusher plate. Essentially, you're pushing the ship with plasma:

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

A bomb with a cylinder of reaction mass expands into a flat, disk-shaped wave of plasma when it explodes. A bomb with a disk-shaped reaction mass expands into a far more efficient cigar-shaped wave of plasma debris. The cigar shape focuses much of the plasma to impinge onto the pusher-plate.
posted by damn dirty ape at 7:25 AM on August 14, 2012 [1 favorite]


Conservation of momentum.

From the Wikipedia article on nuclear pulse propulsion:

"The idea of Orion was to react small directional nuclear explosives against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to specific impulses in the range of 6,000 seconds, or about twelve times that of the Space Shuttle Main Engine."
posted by vacapinta at 7:25 AM on August 14, 2012


See also, the awesome John McPhee book, The Curve of Binding Energy.
posted by shothotbot at 7:26 AM on August 14, 2012 [2 favorites]


Assuming FTL travel to be impossible, which seems likely, what other options for (more or less practical) manned interstellar travel exist besides this and perhaps laser cannons?
posted by Guy_Inamonkeysuit at 7:29 AM on August 14, 2012


>Assuming FTL travel to be impossible, which seems likely, what other options for (more or less practical) manned interstellar travel exist besides this and perhaps laser cannons?

Project Prometheus studied the ability to put a proper nuclear reactor in space which would give something like 10x to 20x the power of an RTG. More than likely you'd use all the power to run an ion thruster, like the one on Deep Space 1.

Project Prometheus was cancelled in 2003 before they could build any sort of working space-hardened prototype. Oh well, maybe the next race of intelligent mammals or lizards or whatever will figure this stuff well after we got extinct.
posted by damn dirty ape at 7:34 AM on August 14, 2012


Not only is the nuclear blast hitting the pusher-plate with its matter, but the radiation-- energy particles can also impart momentum. Think of your basic "solar sail" design in which radiation can be used to push a suitable "sail" with the pressure of light itself. The Gamma and X-Rays produced by your basic nuke would hit the Orion's pusher-plate with a massive amount of physical force.

Hopefully if we ever sent one of these into space, we'd have much better nuclear-explosive/propulsive technology so that each bomb-blast wouldn't be a blast of broad spectrum radiation that explodes inefficiently and dirtily, but rather an efficient, well-tuned blast that has the desired directionality, the efficient fuel consumption (recall that your basic nuke's requires a highly compressed metal, but it blows itself apart, eliminating the compression that the triggering blast worked so hard to achieve) and safe, stable construction, so that we can keep rack upon rack of these bombs sitting in a cargo hold in space (and the radiation hazards that exist in space) without threatening the crew, ship, and mission.
posted by Sunburnt at 7:51 AM on August 14, 2012


Did you, perchance, read the XKCD "What if" this today?
posted by AwkwardPause at 8:43 AM on August 14, 2012


If there's no medium, like air, there's no shockwave, right?

Just to be clear: this is actually correct, since a detonation shock wave is usually thought of as a wave in a medium (air, water, etc.) that travels at a supersonic velocity. ("Supersonic" here, of course, means "faster than the speed of sound in that medium".) However, as noted above, the mass & radiation ejected during the explosion can still impart momentum to the pusher plate in the vacuum.
posted by Johnny Assay at 8:47 AM on August 14, 2012 [1 favorite]


Did you, perchance, read the XKCD "What if" this today?

Yes, but I've been wondering this for years, the post only reminded me :)
posted by gkhan at 10:01 AM on August 14, 2012


Despite conflicting answers, Johnny Assay is absolutely correct, there is no shock. A shock is not just material moving fast, it is very specifically material moving faster than the ambient sound speed.

There are shocks in space, but they usually occur at the interface between something like fast moving supernova ejecta and the interstellar medium. But the densities involved are much different than in what we think of as a shock on Earth, so they're not very comparable.
posted by kiltedtaco at 3:50 PM on August 14, 2012


IIRC, the Orion drive was supposed to be used in 2001: A Space Odyssey, but it was killed as oddly inappropriate to a film that contained the subtext that nukes where bad.
posted by SPrintF at 7:03 PM on August 14, 2012


ROU_Xenophobe: Some of the pusher plate ablates under the radiation blast, giving a high velocity rocket effect. Also you can pump inert reactant onto the plate to increase this effect. Presumably the expanding gas shell from the nuke contributes something as well.
All of this would merely decrease the inertia imparted by the bomb, making it a less efficient rocket.

Inertia transferral from the bomb to the spacecraft is the entire mechanism. The ablation plate sacrificially protects the spacecraft from damage.
kiltedtaco: Despite conflicting answers, Johnny Assay is absolutely correct, there is no shock. A shock is not just material moving fast, it is very specifically material moving faster than the ambient sound speed.
The material of the bomb leaving the site of the atomic blast is indeed moving faster than the ambient sound speed in the material of the bomb itself, and of the rocket.

So, yes, there is a shock wave; it travels (for picoseconds) through the bomb shell and then through the rocket itself. There is effectively no shock wave outside of the rocket - all the expelled matter is travelling in roughly the same direction and speed. Because there is no acceleration of the dispersed bomb materials after they exit the rocket, these particles are nearly still with respect to one another, but slightly expanding away from their center (as the blast cone widens).
posted by IAmBroom at 8:12 AM on August 16, 2012


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