A rocket engine is simple in concept. Take your fuel and your oxidizer, bring the two together, toss in a spark, channel the resulting combustion through an exhaust nozzle, and Voilą! You’ve got lift-off.

The oxidizer reacts with the fuel, destabilizes it. The spark sets the whole thing in motion. The heat generated by the products of combustion is converted to kinetic energy by the nozzle. Pressurized gas squeezing through the nozzle pushes the rocket ship into the sky.

Because liquid fuel (typically hydrogen) is volatile to begin with, a liquid engine requires two compartments for holding the components of combustion apart until the big moment. Open a valve during countdown and the two are combined.

A solid rocket engine works differently. Solid fuel is inert. "Fuel and oxidizer particles are mixed, like sugar and salt," says Marty Chiaverini, a graduate student in mechanical engineering. The two are held together by a binding agent in a solid block, awaiting ignition.

Each design has its advantages. "Traditionally," Chiaverini says, "the military has liked solid propellants, because they’re easier to store. NASA likes liquids, because of their higher efficiency." But safety concerns and the development of new fuels have led engineers toward a new, hybrid design, one that combines the best of liquid and solid.

In a hybrid engine, a liquid oxidizer, typically liquid or gaseous oxygen, is injected through a hollow in the center of a solid cylinder of fuel. That fuel can be almost anything, so long as it’s inert and contains a lot of energy. Experimenters have tested coal, wood, wax, and yes, even Italian salami. These days they mostly use something similar to tire rubber.

In performance, hybrid engines prove comparable to liquid and solid counterparts. And, Chiaverini says, the hybrid design is less complicated, cheaper, cleaner, and more reliable. But the major advantage is in safety -- during manufacture, storage, transport, and operation of the rocket engine. "Because the fuel is inert, there is almost no possiblity for explosion or catastrophe." And because fuel and oxidizer are kept separate, the combustion reaction can be controlled.

"Once you turn on a solid motor, you have no control over it." Chiaverini explains. "You can’t stop it or change the thrust. With a hybrid, you can control it by throttling the oxygen valve. The fuel by itself won’t burn." With a hybrid engine, he says, "the Challenger disaster presumably could have been avoided. The motor could have been shut down."

Engineers are currently seeking a better understanding of the fundamental physical processes involved in hybrid combustion. One key is something called regression rate.

Regression rate, Chiaverini explains, is the velocity at which solid fuel burns away from the central hollow where the oxidizer is injected. "Regression rate determines thrust, overall motor performance, and fuel utilization," Chiaverini says. An inaccurate calculation could, for example, leave extra fuel at the end of the mission -- an added payload, and expense.

While regression rate has been frequently modeled, data to check those models has been lacking. Developing and testing an experimental hybrid engine has been a long-term focus for mechanical engineering professor Kenneth Kuo and his students at the Penn State high-pressure combustion laboratory.

"Generally," says Chiaverini, "people measure regression rate by weighing fuel before and after a test. This only yields an average figure."

The engine that Kuo and his students have designed, by contrast, offers a cutaway view of its combustion chamber, which is visible by x-ray through a graphite window. Fuel slabs are bolted into place and -- after engineers scurry from the well-bunkered test cell -- oxidizer is injected into the chamber and ignited. A sophisticated video set-up records the fuel burning in real-time. Computerized image processing yields 400 to 600 localized regression rates. And, says Chiaverini, "our lab motor is in the realistic operating range for a real rocket, which is unusual for a lab."

From recent tests, Chiaverini reports, "we’ve determined that the classical theory works well within certain ranges. But we’ve also seen that regression rate varies substantially over time and across the length of the fuel slab, to a greater degree than has been accounted for."

In addition, they have used the system to investigate fuel additives for enhancing burning rates. One of these additives, an ultra-fine aluminum powder, increased burning rate by up to 70 percent in initial tests. "We want to look at this powder further, find the mechanism for this increase," Chiaverini says. Also, he hopes to develop better regression-rate figures for a variety of hybrid fuels.

"Eventually, we’d like to develop the necessary correlations and come up with a better formula for predicting."

Martin J. Chiaverini is a doctoral student in mechanical engineering, 233 Reseach Building East, University Park PA, 16802; 814-863-2264. He presented some of the work described above at the Fourth International Symposium on Special Topics in Chemical Propulsion in Stockholm, Sweden in May 1996. Chiaverini's adviser, Kenneth K. Kuo, Ph.D., is distinguished professor of mechanical engineering and director of the Penn State High Pressure Combustion Laboratory. Funding for the hybrid rocket motor project is from the National Aeronautic and Space Administration’s Marshall Space Flight Center.