Satellite manufacturers may soon have a second source of green propellant for their spacecraft thrusters, if space testing of a U.S.-developed alternative to highly toxic hydrazine monopropellant goes well. Ball Aerospace and Aerojet Rocketdyne say their system, which uses a hydroxyl ammonium nitrate (HAN) mixture designated AF-M315E, with a special catalyst, has greater density than hydrazine for better storage efficiency, and produces better performance than both hydrazine and a different “green” spacecraft propellant already tested by Sweden. It has passed ground testing, paving the way for a satellite flight test as early as 2015.

“When we look at this compared to a hydrazine monopropellant type of system, where we have a single fluid driving the system on the spacecraft, we have a 50% increase in performance over the standard hydrazine,” says Christopher McLean of Ball, principal investigator on NASA's upcoming Green Propellant Infusion Mission (GPIM).

GPIM is designed to fly as a secondary payload on a SpaceX Falcon 9 Heavy, using a Ball Configurable Platform (BCP) 100 spacecraft bus and an Aerojet Rocketdyne thruster system that combines a 22N (5-lb.) thruster with four 1N units, all burning the green fuel to put the satellite testbed through the maneuvers an operational small satellite would see.

“These were selected because they have the largest market share, [so] we are developing the technologies that really meet the needs of the marketplace for this type of attitude control on a spacecraft,” McLean says.

Ecological Advanced Propulsion Systems (Ecaps), a unit of the Swedish Space Corp., has tested a different green propellant—based on ammonium dinitramide—in space. The fuel was used in tandem with a hydrazine system on the Prisma mission's Mango satellite to maneuver in formation with a smaller spacecraft.

Despite the completion of space qualification, sales of the Ecaps system have been slow to take off. Roger M. Myers of Aerojet Rocketdyne's Redmond, Wash., facility says the performance of the U.S. system in the ground test is better than the Swedish approach, and suggests there may be safety issues with the by-products of its evaporation.

Ultimately, Aerojet Rocketdyne hopes to “infuse” its new green technology into applications other than small satellites, including tactical missiles and large geostationary satellites, Myers says. As this photo of an Aerojet Rocketdyne technician with a beaker of AF-M315E shows, the green propellant requires none of the special handling or equipment mandatory for hydrazine.

“We can move, we think, to a shirtsleeve environment with this new fuel,” says Michael Gazarik, associate administrator for NASA's Space Technology Mission Directorate, which is funding the GPIM mission. “That means less ground-processing time [and] less ground-processing cost in order to load the spacecraft with the fuel.”

Swedish space officials say they were able to fuel the green system on Prisma in one day, meeting Russian safety requirements for their launch on a Dnepr-1 rocket without wearing hazmat suits. During the formation-flying operations, the Mango satellite fired thrusters using hydrazine and the LMP-103S green propellant interchangeably (AW&ST Nov. 1, 2010, p. 69).

Ball's GPIM spacecraft will use all five Aerojet Rocketdyne thrusters simultaneously to demonstrate attitude control, spacecraft point and hold, orbit lowering and inclination change. It will also characterize the thruster performance in space for future improvements, which could include shipping fueled spacecraft instead of fueling them at the launch site.

The U.S. Air Force Research Laboratory developed the HAN fuel/oxidizer blend, but it wasn't until Aerojet Rocketdyne worked with AFRL to perfect the catalyst necessary for it to fire that a spacecraft propulsion system became practical.

“These are high-temperature propellants; they burn at a higher temperature than hydrazine,” says Myers. “So the catalyst lifetime is limited by the high-temperature operations, so we developed a new catalyst. That was the key breakthrough, a new catalyst material, a new catalyst processing capability to enable the catalyst to survive long life.”

For now, hydrazine remains qualified for longer service life than the U.S. green propellant. Myers says the ground test ran the thruster for 11 hr. continuously, which should qualify it for many small NASA, military and commercial missions. Basically, McLean says, the technology was designed for ESPA-class rideshare spacecraft, although it can be scaled to larger spacecraft and other applications.

“We're looking to demonstrate about 7 kg (15 lb.) of fuel out of the entire propulsion system. That really is just for a very small spacecraft. However, as part of the overall program that we're doing, we're not just testing the engines and the flight engines during the overall qualification program to those levels of fuel. We're actually testing them to 30 kg of throughput, especially on the 22N, so that we will demonstrate its compatibility with longer-life missions.”