Refueling Rockets

On the campus of NASA’s Ames Research Center in Mountain View, Calif., you periodically hear–and feel–a thunderous roar as engineers ignite experimental rocket engines that are chock full of the same paraffin wax that illuminates candlelight dinners. The idea of using wax as rocket fuel isn’t new. People tried it years ago but couldn’t get the wax to work well enough to launch a heavy rocket into space. The engineers now bracing against the roar of their wax-filled engines suspect, however, that their predecessors were indeed onto something. If that’s true, paraffin wax could become the world’s cheapest, safest, most environmentally friendly rocket fuel.

Fuel for thought

It all started with a trip to San Diego in 1995. Arif Karabeyoglu, then a graduate student at Stanford University, and David Altman, a longtime rocket scientist affiliated with the university, were checking out research presentations at a conference on rocket propulsion. A talk on an Air Force rocket-fuel project set their own research trajectories in a new direction.

Unlike the standard solid or liquid fuels used today, the Air Force material was part solid, part liquid. The solid portion was frozen pentane, a hydrocarbon, and the liquid was pure oxygen. Just as a fire needs oxygen-bearing air to burn, all rocket fuels require an oxidizer for combustion. In this case, the oxidizer was the oxygen. Inside an engine, as the liquid oxygen became gaseous and blew across the fuel’s solid component, the pentane burned.

So-called hybrid-rocket fuels have been around for half a century, but they haven’t taken off, so to speak. Compared with more widely used fuels, they don’t burn quickly enough or provide enough thrust to launch heavy loads. Moreover, the solid component of a hybrid needs to be molded into complicated and often fragile shapes to provide a great enough surface area for burning.

NASA and other rocketeers usually choose liquid or solid fuels. Liquid fuels include hydrogen and kerosene. Here, the oxidizer is either oxygen or another compound that accepts electrons readily enough and fast enough to drive the heat-releasing chemical reactions that underlie burning. Liquid fuels take up a lot of precious space on a rocket, as the large external tank on the space shuttles vividly shows. Some liquid fuels also require refrigeration. Nonetheless, they have an important benefit: Their combustion can be easily switched on and off to provide reliable and safe control.

The two booster rockets flanking the space shuttle’s liquid-fuel engine run on solid propellant. These materials are typically made of an aluminum fuel with an ammonium perchlorate oxidizer built right into them, so they’re very compact.

Unfortunately, this setup also makes them dangerous. Like July 4th sparklers, once a solid rocket is ignited, it’s hard or impossible to stop. Safety concerns make production, storage, and transport of solid fuels enormously expensive, says Brian J. Cantwell, an aeronautical engineer at Stanford.

Moreover, solid fuels are made of toxic materials. Perchlorates, which have been linked to thyroid problems, may end up in ground water during fuel production or after a launch (SN: 07/29/00, p. 77: Living routes to toxic routs). As they burn, solid fuels may also produce hazardous emissions, such as hydrogen chloride, which forms hydrochloric acid when it encounters water vapor, and aluminum oxide, an abrasive white powder.

So, researchers continue to study hybrid-rocket fuels. Some produce mostly carbon dioxide and water when they burn. Moreover, these fuels are more compact than liquids, and compared with solid fuels, hybrid-rocket fuels are safer and their thrust can be regulated.

A particularly attractive feature is that hybrid-rocket-fuel combustion can be completely shut off and restarted. When the first puff of smoke appeared from a solid rocket booster on Challenger’s doomed takeoff in 1986, the spacecraft was still sitting on the launch pad. Even if mission controllers had suspected a problem at that moment, they couldn’t have prevented the shuttle from lifting off, says Cantwell. Nothing could stop the solid fuel that was already burning.

A hybrid rocket could have been shut off with the flip of a switch.

What has kept hybrid-rocket fuels out of shuttle designs, as well as out of any existing plans for rockets that carry heavy loads, is the fuels’ slow burning rate. And that’s what sparked Karabeyoglu’s and Altman’s interest in the Air Force fuel. For some reason, it burned three times faster than other hybrid fuels.

Better burning

After learning about the Air Force fuel at the propulsion conference, Karabeyoglu and Altman began searching for an explanation for the fuel’s accelerated rate of combustion.

As it turns out, the answer lay in the library. After extensive searches of the chemistry and physics literature, Karabeyoglu finally traced the reason for the quick burning rate to a property of some thin liquid films that was first reported in 1966. From what he could gather, Karabeyoglu surmised that a 100-micrometer-thick layer of liquid pentane had formed on the surface of the Air Force fuel’s solid pentane while the fuel was burning. Meanwhile, he theorizes, oxygen vapor blowing over this thin layer created a spray of small pentane droplets. The large collective surface area of these droplets caused the fuel to burn especially quickly.

The availability of a fast-burning hybrid fuel was great news, but the fuel still had a big drawback. To stay frozen, the pentane would require a refrigeration system that could maintain the pentane at nearly 200C below the freezing point of water.

“You don’t want to have to take your rocket and put it in a deep freeze before launch,” says Cantwell. “It’s not practical.”

Now that Karabeyoglu had figured out why the pentane in the Air Force fuel burned so quickly, he and his colleagues wondered whether they could find a related material that burns as fast but is solid at room temperature.

The new fuel would also need to have the appropriate viscosity and surface tension to create an unstable, thin film of liquid on its surface that would rustle up a spray of fuel droplets when oxygen or some other oxidizer blew over it. The material that could meet all of these criteria turned out to be a mechanically strong paraffin, or wax. Like pentane, paraffins are alkanes–hydrocarbon molecules that have as many hydrogen atoms as the molecule’s carbon backbone can accommodate. To chemists, they’re known as fully saturated hydrocarbons. Pentane has 5 carbon atoms and 12 hydrogens. Soft paraffins with Vaseline-like consistency have about 15 to 20 carbon atoms, and stronger paraffins have about 25 to 30 carbons.

The wax that the Stanford scientists chose isn’t exactly dinner-table candle wax, but it’s pretty close. It includes a small amount of carbon black, a fine soot, to block radiation from heating and softening the inside of the material. It also contains other additives, which the scientists won’t reveal, to improve the material’s structural properties. A person can hold the wax, carve it, and melt it, and–unlike liquid or solid rocket fuel–there’s no risk of explosion or fire. In fact, the researchers have shipped the material by FedEx.

Cantwell notes that rocket science has considered wax before. The California Rocket Society launched a small rocket with a synthetic wax fuel in 1938. There aren’t any accounts of what happened, but it “probably didn’t work out,” he says.

Cantwell suspects the wax used then would have softened and not burned well.

Sixty-five years later, there may be a more promising ending to the wax fuel story. With a small motor on a laboratory bench top in a basement at Stanford, Karabeyoglu, Cantwell, and Altman have performed more than 200 tests on their wax fuel with gaseous oxygen as their oxidizer. In doing so, they’ve optimized the combustion of their room-temperature wax so that it now burns as fast as the frozen pentane hybrid did.

Bigger and better

The big question is whether this laboratory promise can translate into launch pad reliability. A basement at Stanford is not the place for taking the next steps in that direction.

So in 2000, the Stanford team convinced engineer Greg Zilliac of NASA’s Ames Research Center in nearby Mountain View to buy in on the project. “It looked like an exciting new avenue of research for our [Ames] group,” says Zilliac.

At Ames, Zilliac had a large motor constructed, and the researchers have now completed 41 test burns. This January at an aerospace meeting in Reno, Nev., the researchers reported that the fuel burned at the same brisk rate at Ames as it had in the smaller Stanford trials–three to four times that of conventional hybrid fuels. Tests using nitrous oxide instead of oxygen as an oxidizer yielded similar results.

“Stanford has brought the [paraffin] technology a long way,” says Martin Chiaverini of Orbitec, a Madison, Wis., company that does aerospace research and development.

Meanwhile, other researchers continue to work on conventional hybrid-rocket technologies, he notes. For example, at the Pennsylvania State University in State College, where Chiaverini did his graduate studies, engineers are adding nanoscale aluminum particles to a conventional hybrid rocket fuel–hydroxyl-terminated polybutadiene–in order to improve the fuel’s performance. And at Orbitec, scientists have found a way to create a double vortex–like a tornado inside a tornado–in a hybrid-rocket engine, which increases the fuel’s burning rate.

Combining varied hybrid-rocket research may yield even better burn rates, says Chiaverini, who hopes to test Orbitec’s double-vortex system on the Stanford paraffin.

These are signs that wax fuel could soon become suitable for full-scale use in a suborbital research rocket, where the material’s compactness may reduce drag, says Zilliac. Later, wax could potentially fuel larger rockets. And someday, Cantwell says, it’s conceivable that safe wax fuel may even replace the solid fuel in space shuttle booster rockets.

To Zilliac, the optimal use for the wax fuel would be in robotic crafts on multiyear missions to explore the solar system. Wax offers some great benefits, he says. It’s compact and doesn’t need refrigeration. It’s also easily turned off and restarted, even after 2 years or so.

Before the wax-fuel saga takes off in those directions, more tests will have to be done at Ames and other NASA centers. A new facility at Ames has just been completed for studying the physics of how the fuel melts and burns, and the first tests are now under way. The new testing complex has sapphire windows for researchers to set up detailed optical studies of the burning process. With that additional knowledge, the researchers expect to fine-tune these fuels. If they succeed, candlepower could become part and parcel of leading-edge rocket science.

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