Practical Starships

You don't often see practical spacecraft capable of manned flight between stars. And you won't see it here either, because the ship shown above is not a practical starship. But it is a lot more practical than pretty much anything else I've seen in science fiction since the days of 2001: A Space Odyssey. This is, of course, the ISV VentureStar from the movie Avatar. What makes it more practical than other things in sci-fi-dom?

For one thing, it isn't superluminal. It doesn't go faster than the speed of light, so it doesn't have to contend with things like warp drive, hyperdrive, hyperspace, wormholes, or any of that other hoohah. It gets there the hard way, by covering every damn kilometer between here and Pandora over a one-way flight of five-plus years.

Here's the basic mission profile. When it leaves Earth, it is propelled by enormously powerful lasers shining onto an equally enormous solar sail. When it gets to the halfway point, it furls or jettisons the solar said and decelerates using its own matter-antimatter engines. No warp drive here, folks, the matter-antimatter engines are just reaction motors of extremely high specific impulse. Once its affairs at Pandora are ended, it accelerates toward Earth using its aforementioned matter-antimatter engines, and then at the halfway point on the way back it deploys an enormous solar sail (or redeploys the old one) and is decelerated by the same lasers that drove it toward Pandora.

The proposal contains several interesting technical features. One is that the ship is a tension structure - the engines and the solar sail attach point are ahead of the rest of the ship, meaning that the thrust of the engines or sail pull the ship rather than push it. It's easier to make a tension structure light than it is to make a compression structure light.

Another feature is the whopping size of the radiators. Really advanced spacecraft engines have a problem in that they aren't able to eliminate enough heat in their exhausts to keep them cool. Chemical rockets can, and up to a point nuclear-thermal rockets can, but engines of this sort tend to produce way more heat than they can dump through the exhausts. So advanced spacecraft propulsion is often more a matter of heat-sinking and radiator design than anything else. (Incidentally, they knew this when they made 2001 and Discovery was intended to have radiators of similar size, but they eliminated them for the sake of visual cleanliness.) In this artist's conception, the radiators are still glowing red-hot as they dump the heat from several years of engine operation.

Another interesting idea is the use of what is called "r-squared" shielding instead of a "shadow shield". Matter-antimatter engines will produce a lot of pretty harmful radiation, probably lots of high-energy particles and even more hard x-rays. One method of shielding the crew from this nastiness is to put a hockey-puck-shaped shield between the engine and the crew compartment - a "shadow shield", so-called because it makes a "radiation shadow". But shields are heavy, and lugging a sixty-ton lead shield to Pandora and back isn't efficient. So the ship uses "r-squared" shielding, which means that you simply put the people as far away from the engine as you can, because the radiation drops at the square of the distance between the engine and the crew. (Again, 2001 had this right; the design of the Discovery is just right to exploit r-squared shielding.)

The ship also employs a recognizable variant of Whipple shields. An unavoidable fact of life is that colliding with dust motes and even hydrogen atoms at a high percentage of the speed of light is a bad idea. In Star Trek this problem is dealt with by the navigational deflector, which moves such gleefus aside so it doesn't hit the ship. In Star Wars, this problem is apparently not dealt with at all. The VentureStar uses Whipple shields, which amount to stacked layers of aluminum foil. The dust mote hits the foil and blows the hell out of it, but doesn't get through to riddle the ship (it's almost like spaced armor or ERA on tanks).

But it still isn't practical. The design contains at least four industrial-strength hand-waves.

The first is that achieving the accelerations required for relatively brief interstellar flight (say, seven years) with a solar sail is hard. The propulsion lasers would have to be both numerous and incredibly powerful, to say nothing of the "pointing problem", keeping all those gigawatt-class lasers pointed at a solar sail that might be only on the order of ten miles in diameter at distances of two or three light years. It isn't impossible, but it isn't something we can do right now, and probably won't be able to do until the advent of cheap and reliable fusion reactors and probably several hitherto unknown breakthroughs in free-electron lasers.

The second is that the hundred or so passengers remain in suspended animation throughout most of the flight - not for their convenience, but so that the ship doesn't have to carry food, water, and oxygen for them. Only four people remain awake during the voyage. Is that sort of suspended animation possible? I'm no biologist, but my sense is that it isn't impossible in principle, but the details are liable to be a bitch.

The third is that the ship uses matter-antimatter engines, largely as a means of getting around the depressing reality that lower-energy engines either don't generate enough thrust to achieve a reasonable flight time, or consume so much fuel or energy that the ship can't actually carry anything but fuel. Matter-antimatter engines are not impossible. I myself have indulged in the subtle joys of matter-antimatter reactions; every time I get a PET scan to monitor my cancer, chemicals in my body are undergoing beta decay and producing positrons, which are antimatter. They collide with electrons, they annihilate, and 511 KeV X-rays go shooting off through my tissues. So a matter-antimatter engine isn't impossible by any means. The chief problem is collecting enough antimatter to fuel a starship, and containing it years without significant decay.

This leads to the fourth and final hand-wave, which is that the ship is said to employ the "unobtainium" mined on Pandora to contain the requisite amounts of antimatter for the requisite time. Unobtainium is an excellent plot device, but its physical properties on the face of it appear to violate the laws of physics. But don't take my word for that. I have a problem understanding the energy dynamics of magnets, which to my mind also appear to violate the laws of physics. You hold a magnet over a nail and suddenly the nail flies up to the magnet, against the pull of gravity. Okay, now where the hell did that energy come from?? The nail gains both potential and kinetic energy, and I can't for the life of me figure out where it came from. So given this critical failure in my understanding of ordinary physics, I may not be the person best qualified to say whether levitating unobtainium is bullshit or not. But I think it is.

Still, though, the VentureStar is an acutely interesting design and good food for thought, if nothing else. And I rather like the Valkyrie shuttles too, because they manage to get around all that hokey VTVL nonsense by employing dual-cycle engines capable of breathing air. But that's a whole different rant, innit?