for 30 years. In this design, Myers says, a “very fancy hair dryer”—actually a tungsten-rhenium heat exchanger—is attached to the exhaust of a standard chemical rocket, heating it further.
To explain how this actually saves fuel, Myers is happy to give an impromptu course on basic rocket science, which he’s able to ramp up or down, depending on his audience. The important point here is that a chemical rocket “is limited in its efficiency by the chemical bond energy in the chemical in the propellant, and the mass of the propellant,” he says.
By introducing electric energy—gathered from a spacecraft’s solar panels—that limit is removed without adding the extra weight of additional propellant.
It can save weight in other ways, too. Aerojet Rocketdyne makes monopropellant chemical rockets that use hydrazine, and more powerful bipropellant rockets in which two chemicals—mono-methyl hydrazine and nitrogen tetraoxide—are combined, combusting on contact. A resistojet can give a monopropellant rocket the same efficiency as a similarly sized bipropellant rocket, without the additional weight of a second set of tanks, pipes, and valves to carry the second propellant.
A newer generation of electric rockets replaces the “hair dryer” with an arc of electric current—like in an arc welder—coursing through the exhaust gas. This allows the gas to be heated even more, because the melting temperature of the material is no longer a constraint. An arcjet, as it’s called, can deliver twice the fuel efficiency of a similarly sized bipropellant or resistojet rocket, Myers says.
Art Veyna, program manager for resistojet programs, overhears Myers explaining the products and pulls us into his office. After checking my credentials—jokingly, I think, though Aerojet Rocketdyne does provide propulsion systems for satellites used in defense and intelligence—Veyna unrolls a poster-size diagram of a resistojet. He points out the fine components, many of which took years of development to perfect, manufactured to exacting tolerances.
“This is one of the most complex engines that we’ve built,” Veyna says, clearly proud of the work. “There isn’t anything in this engine that is simple or easy.”
Aerojet Rocketdyne builds other electric propulsion systems that use electrical energy only, rather than using electricity to augment chemical energy. These have the potential to triple and quadruple efficiency again, though these gains come at the expense of lower thrust levels, Myers says.
Gridded ion thrusters, for example, strip an electron off of an atom of xenon gas, resulting in xenon ions that drift toward a pair of gridded plates, set 30 thousandths of an inch apart with a 2,000-volt charge between them. The xenon ions are attracted by the gridded plates, creating the exhaust that accelerates the rocket.
NASA recently completed a record 48,000-hour (five-and-a-half-year) life test of a gridded ion thruster built by Aerojet Rocketdyne and NASA’s Glenn Research Center in Cleveland, where Myers previously led electric propulsion research. It would take more than 10,000 kilograms of conventional rocket fuel to produce the same impulse the thruster in the test achieved using only 870 kilograms of xenon.
This is all possible thanks to increasing solar panel efficiency, and the shrinking size and power requirements of electronics, making more energy available for electric propulsion.
Greener Pastures and Fuel
In addition to electric energy sources, Aerojet Rocketdyne is working on a project to use less-toxic chemical propellants.
For decades, hydrazine has been a rocket fuel of choice for the monopropellant engines Aerojet Rocketdyne makes. “We’ve got 40 years of experience with it, and it has many very appealing characteristics,” Myers says. “On the other hand, it’s toxic so you have to handle it very carefully.”
That means added costs for the additional training and safety measures required to work with hydrazine.
The NASA Green Propellant Infusion Mission, led by Boulder, CO-based Ball Aerospace with Aerojet Rocketdyne and researchers from the U.S. Air Force and NASA as co-investigators, would use a hydroxyl ammonium nitrate blend to replace hydrazine. The replacement promises a lighter environmental footprint, improved fuel efficiency, and reduced safety hazards, among other advantages, according to NASA. The space agency aims to perform orbital maneuvers
