Aluminum slurry as a shuttle-booster fuel.

Aluminum slurry as a shuttle-booster fueld

Once the solid propellant used in the shuttle's "strap-on" booster rockets has been ignited, it burns nonstop until gone. Liquid-propellant rockets are more complex, requiring pumps and other systems, but NASA has been studying their use for the shuttle boosters in part because they would allow the motors to be throttled down or even stopped and restarted if necessary. In addition, however, researchers are investigating a propellant that is not exactly liquid or solid.

Instead, it is a "slurry," such as mud, consisting of an insoluble solid in a liquid. Stephen R. Turns, at NASA's Center for Space Propulsion Engineering at Pennsylvania State University in University Park, is studying a combination of powdered aluminum in standard kerosene rocket fuel, or RP-1. "With other factors equal," he says, "the heavier the fuel per unit volume, the greater the payload capacity." Furthermore, he says, changing to slurries "would allow the shuttle's basic configuration to remain unchanged." He is now studying the numerous "microexplosions" that occur before the aluminum ignites, potentially shortening the slurry's burning time.