Just What’s in that Gas Tank?

Whether it’s liquid hydrogen or Mr. Fusion, our various spacemen of SF are always playing around with some exotic energy sources, leaving our old petroleum ways to the dinosaurs. But just what energies are they using?

Pedestrian but Practical

Let’s start with some pretty tame ones, namely those already in use. Our own rockets use all manner of fuels, ranging from liquid hydrogen and oxygen to exotic hypergolic fuels such as hydrazine plus dinitrogen tetroxide. These have been used since the fifties in a variety of chemically driven spacecraft.

The only nuclear power currently used in space is via radioactive decay in what they call a radioisotope thermoelectric generator. These turn radioactive decay into heat and then heat into electricity. Most use Plutonium-238 which emits alpha particles at a half-life rate of 87 years, giving your spacecraft reasonably steady power for decades. It’s great for all those interplanetary probes we’ve sent out over the years, but it’s unfortunately quite toxic – handle with care.

We currently use solar power in a couple of different ways. Notably, we use it for pure electricity via photovoltaic cells. A number of satellites and inner-system probes have used it. Another use of solar power is to use the solar wind for thrust via a lightweight solar sail. This has long been hypothesized and was recently tested on two different flights, one a probe to Venus and the other a testbed for Earth-orbit maneuvers.

New Nuclear

Various ideas for nuclear propulsion have been around since the late 1940’s. My reactions to them have ranged from “are you kidding me?” to “but of course”. I think the area where I have the biggest problem is in those designs that talk about setting off small nuclear explosions behind the ship. I’ve read enough to know that these were serious and reasonable designs, but I confess they fill my head with visions of Wile E. Coyote at the helm of an ACME Atomics rocket sled.

The most notable of these was the old Project Orion which intended to set off small fission explosions against a large steel plate mounted on big shock absorbers. As crazy as that sounds to me, it is still considered a reasonable design, capable of reaching Mars in a month. However, various complications in the design and the, ahem, fallout, mean we’ll likely never see this one. A slightly less crazy version involved throwing the bombs forward towards something like a solar sail which would catch more of the explosions energy than a steel plate, and the cables holding the sail would act as more effective shock absorbers than anything they had before.

A later proposal, Project Daedalus, suggested using nuclear fusion to provide thrust. Instead of bombs, tiny fusion-fuel pellets would be passed into the target zone of a laser or electron-beam array, compressing them to the point of fusion. The resulting explosion of fusion-driven plasma would be controlled by magnetic field and mostly shoved out the back of the engine, thus providing thrust. This seems to be a much more controlled ride than dropping bombs out the back door, but in truth, its main advantage is the additional energy from fusion.

Both of these engine types were much more efficient than current chemical rockets. Specifically, they were ten to two hundred times as efficient as the main engines on the shuttle. (Geekery: engine efficiency is given in “specific impulse” which is the number of seconds you can get a certain thrust out of a certain mass of fuel. This is critical in engines where you have to carry your fuel with you.)

A somewhat less exotic form of nuclear propulsion is a nuclear thermal rocket. You put a conventional fission reactor on your ship, and you use its heat to heat some liquid propellant into a hot gas, likely hydrogen, and shoot the hot gas out the back of your engine. The design of the nuclear reactor varies, and the efficiencies range from twice to ten times as efficient as the current chemical rockets. However, the weight of the reactor makes this not terribly good for initial lift-off. It’s more useful out in space where you’re more concerned about efficiency over prolonged thrust than you are with the high thrust-to-weight ratios necessary for lift-off.

And then going back to the other end of the exotic is the Bussard Ramjet. Here, you’re actually less worried about the efficiency of the engine because you don’t have to carry your fuel with you. Instead, you scoop it up out of the interstellar medium, that not-quite-vacuum of interstellar space. Typically, you would use a large electromagnetic field to direct the gas into your vessel, and then you would use fusion to heat it further and shoot it out the back. (Or back out the front once you’re slowing down.) There are a number of technical difficulties here, notably with the make-up and density of interstellar gas, but Larry Niven made this a staple of SF in his Known Space stories. Even if it turns out to be as impractical as warp drive, it will forever work in fiction.

Insert Tech Here

Moving past fission and fusion, we start getting into some real hand-waving fuel technologies. Let’s start with anti-matter. I’ll grant you that anti-matter is quite real and is a terrific energy source, but most anti-matter stories ignore the sticky question of where it comes from. I know of one author (Poul Anderson) who did at least talk about the laborious processing of collecting anti-matter from solar-wind impacts, but for most folks it’s simply there. They do frequently talk about the dangers of storing it (a.k.a. “we’ve got an anti-matter containment breach!”), but for all I know they order their anti-matter through Amazon.

Related to anti-matter power is quite simply “matter conversion”. While you get anti-matter power by annihilating matter and anti-matter, matter conversion skips a step and simply converts matter directly into energy. As magic as this sounds, there has actually been a fair amount of research into this occurring naturally in a process called proton decay. So far, however, proton decay has never actually been observed. (Though I once saw a nifty Law & Order episode where competing proton decay theories formed the motive for murder. Major geek-out!)

Not quite as potent as matter conversion is fission via the manipulation of the weak force. The idea is to cause many (or all) of the neutrons in the atomic nucleus to undergo beta decay, emitting an electron and thus becoming protons. A heavy element (say lead on up) suddenly converted to an all-proton nucleus would fly apart almost instantly. This gives you more energy than typical uranium fission, possibly even more than deuterium-tritium fusion. (I think the best yield comes from when the resulting particles end up in the iron to nickel range as they have the lowest nuclear energy states.) The only time I’ve ever seen this in fiction was in a mild reference in Niven’s old Protector novel, and frankly I’m surprised I haven’t seen it more often. Then again, it may be too much of an uncomfortable marriage between the hard science of nuclear physics with the hand-waving of unspecified force manipulation.

Then there’s zero-point energy. The basics of zero-point energy are weird but fairly well understood by physicists in the appropriate fields, and the basics are that there is a certain energy level in any given system that you cannot drop below. That is, no matter how low energy a system gets, it is required to still have some in place. Of course, there’s nothing that says you can get that energy out. In fact, you kind of can’t – that’s the very point of zero-point energy. The energy limbo bar doesn’t go any lower. But then there are a few ways of looking at it that suggest the energy available there is actually infinite. The physics of that are beyond me at the moment, but with SF hand-waving glee, I can inverse polarity on the ZPE matrix and bind it to an altered ground state, thus making that energy easily accessible in my hand-held infinite-energy ZPE battery. The best attempt I’ve seen at the technical appreciation of this was in Arthur C. Clarke’s Songs of Distant Earth, in which a bored physicist on a dying Earth reviewed the millennia old conclusion that zero-point energy was inaccessible and discovered a simple math error so that the various terms did not, in fact, zero out. “Mankind was handed the keys to the universe – and barely a century in which to use them.”

An odd variation on ZPE that I ran into (though I’ve forgotten where) was that while the amount of energy you could extract from empty space was negligible, once you got up to higher and higher speeds and passed through more and more empty space, that negligible amount starts to pile up. I have to give it points for style, but somehow this seems to violate the spirit of special relativity (i.e. ain’t no such thing as a true frame of reference”) in so many ways that it stops being funny.

But as long as we’re waving our hands, one idea that I’m toying with is tachyon energy. In my various SF stories, I like to use the idea of tachyon sails for FTL propulsion. (Don’t worry, I’ll say more about that in a future survey on different types of FTL.) The idea being that the universe is awash in tachyons (FTL particles) left over from the inflation stage just after the big bang. As long as I’m using them for riding between the stars, I may as well extract some energy from them, a bit like having a windmill atop the masts of your sailing ships.

While there are certainly advantages to sticking with the standard tropes of SF, I imagine there are hundreds more alternative energy sources tucked into Spaceman Spiff’s rocket pack. What have you guys run into that I haven’t?