The Quest for CATS

It's a funny thing about rockets in science fiction: They all stay in one piece. Well, they might get blown up by missiles, or chopped up by death rays, in those whooshing swooping space battles. But in normal operation, they don't come apart. They just leap off the pad, cruise around in space, then come back and land.

Aerospace companies have long sought to build a spaceship like that — one that could reach orbit and return intact. By that, I mean that the vehicle that lands is the same one that lifted off. So far, the immutable laws of chemistry and physics have prevented it. The reason lies in one of those "Cold Equations" that Tom Godwin 1 wrote about. This one is known as the Rocket Equation.

To put it in English, the rocket equation says that given the strength of Earth's gravity, the energy available from the best chemical fuels, and the weight of those propellants plus the best structural materials, it's not feasible to boost a single vehicle up to orbital velocity. It's physically possible, but said vehicle can't carry enough payload to make it worthwhile.

If we lived on a smaller planet, other things being equal, we could build that long-sought spaceship. (But of course other things would not be equal.) And we cannot lower Earth's gravity, or screen it out. Until the laws of physics are amended, antigravity will remain a fantasy — despite the stream of "scientific" papers 2 claiming a "breakthrough". Such amendment is unlikely. A good overview of these claims was at the Quantum Cavorite Web site (http://www.inetarena.com/~noetic/pls/gravity.html). Unfortunately, that whole domain has disappeared.3 A cursory discussion of Podkletnov's alleged antigravity effect is found in the course of Physicist Milton Rothman's article Pseudoscience on the Internet (December 1996.)

More powerful fuels would do the trick. Alas, we are already using the most powerful fuels it's practical to use. More powerful combinations exist, but they tend to be nasty to handle. Either they blow up if you look at them cross-eyed, or they eat through any tanks you try to store them in, or they burn so hot that they melt the engines they're used in. As if that weren't enough, most of them are toxic, or have toxic exhaust products. There's still a lot of fuel research going on, but the problem will not be easy to lick. 4

Better engines would help. By fine-turning the designs, a few percentage points of improvement can be gained. Push the combustion-chamber pressure higher. Shave some weight from the nozzle. Burn hotter. All these changes cut your safety margins, and reduce reliability. As with the fuel combinations, if improvements were easy, we'd have them already.

That leaves materials, and again there is some hope. Replacing metal fuel tanks with ones made of composite materials brings a substantial weight reduction. The same is true for many structural components. Some types of composites can even resist high temperatures, offering hope that they could be used within the engines. Questions about manufacture, cost, and reliability exist, but experience should teach us how to make effective use of composites and lower the dry weight of space vehicles enough to beat the rocket equation.

For the present, though, that equation makes us resort to a costly trick to get into orbit. That trick is the staged vehicle. Staging gets around the rocket equation by splitting the vehicle into two or more pieces. The first and largest piece carries most of the fuel, along with a smaller and lighter upper stage. The first stage boosts the whole assemblage partway to orbit, then drops away — leaving the upper stage a short sprint to the finish line.

We've been putting satellites into orbit that way for a long time. People, too. It works. However, when you talk about routine access to space, its shortcomings become evident. They're best explained by analogy with an airline. Could Amalgamated Airlines afford to throw a 747 away on each transatlantic flight, with only a Piper Cub carrying four passengers making landfall in Europe?

No, the truth is the present stable of expendable launch vehicles is never going to permit routine access to space. The only thing that will is the type of spaceship that so far exists only in science fiction. But now, with new, lighter composite structural materials, more heat-resistant alloys, compact solid-state avionics, and improved engines, we are on the verge of being able to build a ship that can achieve that dream of routine, affordable space flight. The concept is called SSTO: Single Stage To Orbit. Here's how it might look when (not if) it finally comes to pass.

The next page briefly reviews the history of launch vehicles. The one beyond that describes some of the recent contenders for the SSTO prize.

1 Tom Godwin's prize-winning story is called The Cold Equations. In it, a young girl stows away on a spaceship carrying vital medicines to a plague-ridden planet. She only wants to see her brother on that world. The pilot has to explain to her that her presence dooms his mission, for the ship has insufficient fuel to complete its trip with her aboard. After tearful protests, she does the honorable thing.

2 It used to be eccentric rotation that was going to give us the true "space drive". The Dean Drive was the last gasp of that particular technobabble. Now it's high-temperature superconductors, reputed to generate "gravity pulses" or "gravitomagnetic fields" when subjected to "nonequilibrium conditions". Search for papers by Podkletnov and Modanese for examples. I'd love to say they're onto something. But they're not.

3 But you can find it at the Wayback Machine. Just enter the URL and check the years 2001-2003.

4 For a vivid explanation of why this is so, look up IGNITION! An Informal History of Liquid-Propellant Research by John Clark. It's available on-line.

5 If so, they will only be used well away from Earth. Nuclear energy is still a political hot potato; a NERVA engine would never be allowed to launch anything from the surface.

To Open The Sky

THE QUEST FOR CATS