Saturday, April 11, 2015

Rockets VI: Very nuclear rockets

See also parts IIIIIIIV, and V.

Chemical rockets would be really nice if they were just a bit more energetic.  The energy that goes into pushing the propellant out comes from the propellant itself, so there's stuff you can do to minimize the transmission of heat from the propellant to the rest of your engine.  That helps you get around some of the problems of rockets where you heat up the propellant from outside.  And the fact that you're still basically using heat means you don't have to suffer the efficiency losses that happen turning heat to electricity when you use an electric rocket.  But what sort of reactions are there that we might cause in our propellant that are higher energy than chemical reactions?  I think you've all seen the post title and know that I'm about to say "nuclear."

Now I should make sure to say that unlike the other categories I've mentioned nobody is actively working on any of these.  In the case of some versions that's because there are... problems.  In the case of others it's because we have no idea how to actually build them.  Fusion rockets fall into that later category.  We can do fusion with current technology but the only way we have to get more energy out than we put in is by igniting it with an atomic bomb.

But lets say we solve this problem with cleverness involving lasers or magnetic fields.  We could just inject a little bit of deuterium and tritium into our reaction chamber, ignite it, and then use the resulting plasma to produce thrust.  According to Wikipedia a D-T fusion reaction will produce a helium atom flying out at 13,000,000 m/s and a neutron screaming out at 52,000,000 m/s.  Since the helium has more mass it contributes more to the average velocity, which is 20,000,000 m/s.  Of course that assumes you can cause all of the hydrogen reacts and that the neutron goes in the direction you want it to go.  I have no idea how close an actual rocket could get to this but there it is.

Unfortunately the neutron produced is a bit of a problem with using this reaction for propulsion.  Neutrons tend to transmute other materials which also tends damage whatever the engine is made out of.  Depending on the material it might become radioactive as well or if the rocket is taking off from a planet it might make the launchpad radioactive.  Now there are other sorts of fusion reactions such as between deuterium and helium-3 that don't produce any neutrons at all.  That sort of reaction is harder to create but we don't really know how to do any sort of fusion in a controlled fashion.

What sort of drive in this category do we know how to create?  Well, back in the heady days of the 1950s some people theorized that you could use atomic explosions to send a ship into space.  There was actually a nuclear test in '57 that launched 2,000 pound metal lid into the air at escape velocity.  That wouldn't be an ideal method of getting to space, though, since any crew would be pulped by the acceleration.   The idea that Stanislaw Ulam came up with was to have a large metal pusher plate separated from the main spacecraft by a large shock absorber.  The name of the design was Project Orion.

Now there are a number of disadvantages to this scheme.  The pusher plate and shock absorber are going to be very heavy.  You have all the dangers of neutron activation and protecting the crew from radiation you get from fusion.  Oh, and you've got atomic bombs exploding outside your spaceship.

That's mostly a problem if you're using this thing to take off from Earth.  In space there aren't many things things to damage and space is pretty radioactive anyways outside planetary magnetic fields so nobody is going to worry about the fallout.  On Earth the fallout is much more a cause for concern.  We exploded a lot of big big atomic bombs in the atmosphere when nukes were being developed before the Test Ban Treaty and they increased the typical person's radiation exposure by about .11 Millisieverts per year.  Now, you typically get a couple of mS or radiation a year or maybe 6 if you live somewhere high in elevation like Denver.  But that extra bit could still mean the difference between cancer and no cancer.  The .11mS figure was from around 200 megatons of nuclear fission.  The bombs Orion would use are only 3 kilotons but you'd need a couple of hundred of them to reach orbit so that comes to 600 kilotons of nuclear explosion.  So figure a global radiation dose of .00033 mS per person.  Assume a linear no threshold dose model and a Sievert giving you a 5.5% chance of getting cancer and multiply by the global population and you'd expect 127 cases of cancer per launch.  So not an option unless we need to prevent a giant asteroid from hitting earth or something.  Also the Test Ban Treaty I mentioned prohibits setting off bombs in the atmosphere, I trust you can see why.

This is all something of a shame because the Orion would be a really good rocket.  You'd get an effective ve of over 20,000 m/s combined with really high thrust. Maybe you could assemble one in orbit and use it to take people to Saturn or Mercury or something but as I mentioned the pusher plate system is really heavy and it's hard to get it up to orbit if it's not pushing itself.

So I don't see Orion being a viable option any time soon and fusion isn't something we know how to do.  Maybe someday but not here and now.

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