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.

Saturday, April 4, 2015

Rockets V: Things that aren't actually rockets

See also parts IIIIIIIV, and VI.

We've covered a bunch of ways of moving ships around in space by shoving stuff out their backs.  But there are some ways of moving around in outer space that actually don't involve the rocket equation at all.  When you fly in a plane on Earth you can push around all that nice air that surrounds you in your environment in order to fly.  Well, you can if you have a plane.  It's very convenient in terms of not having to carry around huge amounts of fuel.  There isn't any air in space but that doesn't mean that space is entirely featureless either.  There are basically three things I know of that you can push off against in order to go places in space: the light of the sun, the solar wind, and planetary magnetic fields.

The principle behind solar sails is pretty simple.  You still have sunlight in space and it's very bright too, at least within Earth's orbit.  By Einstein's good old e=mc2 we know that since light has energy it has to have mass as well and thus momentum to impart when it's been deflected.  Even non-solar sail spacecraft have to take into account the pressure of sunlight if they're going to reach their destinations.  The effect is small since most spacecraft don't have large cross sections in comparison to their masses but if you made your spacecraft very thin you could reasonably use this as your main method of travel.

You might think that solar sails would only be useful in moving away from the Sun, since that's the direction the light is going.  Thankfully orbital dynamics comes to the rescue.  In order for a satellite around Earth to stay in orbit and not fall back down it has to be traveling around 7,800 m/s.  By the same principle the Earth is only able to avoid falling into the sun because it's traveling at 30,000 m/s around the sun.  By deflecting light in the same direction it's traveling a solar sail can slow down in its orbit around the sun and fall into a lower orbit.

And just like ion drives have already been used by Dawn a solar sail has already been used by a Japanese space probe, IKAROS, sent sunwards to go take a look at Venus and to test out solar sails.  Look here for a picture of the probe with its sail deployed.  What's really nifty is that it's got solar panels and LCDs build right into the sail.  The cells are for power and the LCDs enable IKAROS to control its orientation.  The sunlight will exert more force on reflective surfaces than non-reflective ones and by changing the LCDs from white to black the probe can control the forces on different parts of itself.  The entire probe is spinning slowly so that the centrifugal force keeps the sail deployed.

How fast does it go?  Well according to Wikipedia the radiation pressure from Sunlight around Earth is 9.8 µN/m2.  That's for absorption so double it to 19.6 µN/m2 for a perfect reflector.  The sail IKAROS has weighed 10 g/m2 so a square of that material would accelerate at about .002m/s2 if it's out in space by itself.  That's more than the solar ion drive spaceship we looked at but less than the nuclear ion drive one.  Of course there's still the mass of the everything else.  IKAROS weighed 315 kg all told and had 200 m2 of sail.  That gives .000012 m/s2 of acceleration which is pretty tiny but then again it uses literally zero fuel.  Also these guys get better the closer you get to the sun.  Around the orbit of Mercury they accelerate 6 times faster than out here around Earth.  So if you want to take a solar sail to the outer planets it makes sense to drop in near the sun, pick up speed there, and then coast to your destination where you'll need to find some other form of propulsion for stopping.

On to other things.  Besides light the sun spits out a stream of charged particles, the solar wind.  Unlike with sunlight we don't tend to notice because the Earth has a gigantic magnetic field that intercepts these particles and traps them in the Van Allen belts.  That's a good thing because these particles are a form of ionizing radiation and you'd accumulate an unhealthy dose after being exposed to it for a couple of years.  The astronauts at the ISS are safely inside the Earth's magnetic field but the Apollo astronauts were exposed to it for a week and it'll be a big concern for any astronauts going to Mars.

But wait, we just said that are all these fast moving particles that are being stopped by Earth's magnetic field.  Doesn't that mean that they're giving their momentum to Earth?  Yes it does.  The idea behind a magnetic sail is that you use a huge magnetic field to deflect these particles and use the momentum to propel your ship.  The solar wind has much less momentum to give than sunlight but since you just need to intercept them with a magnetic field rather than a mirror the idea is that you can make your sail much bigger for the same mass and so end up accelerating faster.  Another idea is to use electric fields to repel the solar wind rather than magnetic fields and the electric sail is something being worked on now by the European Space Agency.  It's easier to make but harder to control the direction of thrust.

Finally we have the direct use of a planet's magnetic field.  If you run a current through a wire that is in a magnetic field you get a force.  If you're got a nice electrodynamic tether in orbit you can either trade the kinetic energy of your spacecraft for electrical energy or use electrical energy to increase the kinetic energy of your spacecraft.  When you're boosting the trade offs are essentially identical to those of other sorts of electrical drives except you have to do it in a magnetic field and you don't have to use propellant.  When you're braking, on the other hand, you get paid to slow down and that's really nice.  You do need a handy source of electrons to grab to run through the wire but thankfully they don't call it the ionosphere for nothing.  There's at least one company seriously working on these.

So there you have it, three sorts of useful space non-rockets.  They're not as far along as electric thrusters but they're certainly possible.

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