The 1986 Challenger disaster highlighted for all the world just how risky it is to launch a rocket. All seven people aboard the shuttle died when unseasonably cold conditions caused one of its solid rocket motors to fail. NASA made many changes in response to the catastrophe, yet the solid motor that powered the shuttle is still used to launch rockets today.
Is there a safer alternative? Since 1997, Stanford Professor Brian Cantwell has been working on one. Currently, solid propellants combine their fuel and oxidizer into a single material that, once ignited, must burn continuously in a controlled fashion or the rocket may explode. Cantwell’s approach is to develop a hybrid propulsion system that stores the fuel as a solid and the oxidizer as a liquid so the rocket motor can be throttled back to avoid disaster should a launch problem occur. “Our fuel is based on paraffin wax, one of the most ancient fuels known to mankind,” says Cantwell.
The hybrid rocket concept is not new. In the first Soviet rocket launch on August 17, 1933, a group of scientists tested a small hybrid launcher based on liquid oxygen and a gelled form of gasoline. The rocket reached 400 meters but it could not be scaled to larger sizes and the group moved on to develop the large liquid rockets that were eventually used to launch Sputnik.
Twenty years ago, Arif Karabeyoglu, who was then one of Cantwell’s PhD students, made an advance when he discovered that paraffin burned quickly enough to release the enormous thrust required to launch a large rocket. Ever since, Cantwell’s lab and others around the world have been developing paraffin-based hybrid rocket motors.
Conceptually, Cantwell’s hybrid motor might be likened to a large candle with a hollow core where the wick would usually be. Once an oxidizing agent is sprayed into the core and ignited, a thin layer of paraffin melts all along the inner surface, releasing waves of fast-burning liquid droplets. Turning off the oxidizer flow snuffs out the candle.
The latest and largest prototype paraffin hybrid is the Peregrine rocket being developed and tested at NASA Ames by Greg Zilliac with help from several Stanford students. The rocket stands 35 feet tall, measures about 2 feet in diameter and produces about 15,000 pounds of thrust. The first launch to 100 kilometers is planned for later this year and if successful would set the stage for a wide range of applications.
Now NASA is considering the technology as part of its next flagship mission to search for evidence of life on Mars. The space agency is looking at Cantwell’s hybrid to provide the propulsion necessary to take soil samples gathered on the Martian surface and lift these into orbit with a powerful single-stage rocket. The stakes are high: No propulsion system has launched to orbit from a planetary body other than Earth since December 14, 1972, when the Lunar Ascent Engine lifted the Apollo 17 astronauts off the moon.
Cantwell said NASA is considering a hybrid rocket for the liftoff from Mars in part because paraffin can withstand temperature swings more readily than a solid fuel. As he explained, temperatures on Mars can dip as low as -100 degrees Fahrenheit at night then soar to 20 F during the day. In such extremes a solid propellant would tend to develop cracks, which could cause the motor to burn unevenly and explode. Designers could insulate a solid rocket against these swings by using a thermal blanket, but that would add weight that would be better used to do more science. Cantwell said paraffin is less vulnerable to temperature swings, making the hybrid system lighter and safer. Soon NASA will have to decide whether to equip the forthcoming Mars Ascent Vehicle (MAV) with the new paraffin-fueled hybrid or a heavier two-stage solid rocket with a known track record.
For Cantwell, the NASA decision brings his career full circle. Four years before the Apollo 17 mission, as a young graduate propulsion engineer, Cantwell spent a summer internship in Houston at what was then called the Manned Spacecraft Center. There, he helped test the liquid fueled engine that lifted the astronauts off the moon. “Little has really changed in basic rocket design since I worked at NASA in 1968,” Cantwell said. Now, decades later, he hopes to convince the space agency to consider a new launch technology. “I think fast-burning hybrids could be the safe, low-cost choice of the future for a wide variety of uses in space exploration beyond the Mars Ascent Vehicle.”