A handful of private firms submitted testimony to Congress touting the potential uses of solar electric propulsion at a hearing on in-space propulsion held last week by the U.S. House of Representatives’ subcommittee on space. SEP systems could replace chemical propulsion for use in deep space exploration, but experts say the technology needs more improvement to become a viable propulsion option.
“We are on the cusp of a giant leap in space transportation technology,” said subcommittee chairman and Texas Representative Brian Babin, at the hearing. The development of SEP technology will enable “an across-the-board change in technology on par with the jump from sailing vessels and steam-powered ships,” Babin said. “That long-term vision is still quite a ways off and will require further work, but the promise is exciting.”
Commercial participants at the hearing included Franklin Chang-Diaz, CEO and founder of Ad Astra Rocket Company; Anthony Pancotti, director of propulsion research at MSNW; and Joe Cassady, executive director for Space Programs at Aerojet Rocketdyne. All participants are part of the NASA’s Next Space Technologies for Exploration Partnerships (NextSTEP) program, and are working on SEP systems.
Solar electric propulsion systems combine solar energy technology with ion propulsion. SEP uses electricity generated from solar panels to ionize atoms of xenon propellant. The system uses a strong electric field to repel the ions out of the back of the spacecraft, which can produce enough thrust to propel a spacecraft forward in space. The benefit of SEP is that it radically reduces the amount of fuel that a spacecraft needs to carry to complete its mission, while delivering higher efficiency specific impulse — the average exit velocity of the gas coming out of the spacecraft — than chemical propulsion systems like those used in launch vehicles.
“Our transportation workhorse, the chemical rocket, has reached an exquisite level of refinement — but it has also reached its performance limit,” said Ad Astra Rocket Company’s Chang-Diaz, speaking at the hearing. “That technology will not provide us with a sustainable path to deep space.”
Ion thrusters are used today in geosynchronous satellites for things like stationkeeping and orbit raising, but SEP systems haven’t been widely adopted. That’s because SEP systems have only been able to deliver up to 5 kW of power, which doesn’t translate into much thrust. Solar arrays would need to generate upwards of hundreds of kilowatts of power to generate enough thrust to move heavier spacecraft through space.
“For this approach to be effective, high specific impulse systems are needed,” said MSNW’s Pancotti. “This metric ensures that a large fraction of the expensive mass we launch into orbit are payload, and not just more propellant to get the job done.” Pancotti said a trip from Earth to Mars, using today’s electric propulsion technology and “the world’s largest solar array” would take over 10 years.
NASA has been working on SEP systems since the 1960s. The technology is being used in the Dawn mission around Ceres. NASA’s Glenn Research Center in Cleveland, Ohio, leads the SEP project for the agency and is preparing a system-level flight demonstration to launch later this decade. Glenn Research Center has been developing a 50 kW system that was slated to be part of the Asteroid Redirect Mission — a science mission to relocate an asteroid near Earth for further study. That mission was recently scrapped as part of President Trump’s 2018 budget.
Cassady told members of Congress that SEP systems will become vital logistics components for future missions to Mars. He said SEP-powered craft will be able to act as cargo ships, sending as much as 75% of supplies needed for a Mars mission ahead of manned spacecraft. “SEP systems are equivalent to cargo ships for deep space missions,” Cassady said.
NASA says SEP systems could reduce the amount of propellant needed for deep space missions beyond the Moon by a factor of 10.
Last year, NASA awarded $65 million to Aerojet Rocketdyne to develop advanced SEP systems for deep space exploration. Aerojet Rocketdyne is now working on three high-power electric propulsion systems for NASA: NEXT-C xenon ion engine for planetary missions; Advanced Electric Propulsion System (AEPS) for deep space cargo missions; and NASA’s NextSTEP 100kW Nested Hall Thruster for future technology insertion.
Cassady stressed that development of SEP technology should continue to be a funding priority for NASA if it wants to keep its Mars mission promise. “As NASA looks to expand human presence in the solar system, starting with missions to lunar orbit and onto Mars, development of efficient in-space transportation systems is critical,” Cassady said. “We are well on our way to having efficient in-space transportation with SEP. We must continue to adequately fund these development efforts to ensure that we will have the first human footprints on Mars in the 2030s.”
The participants at the hearing hoped to sway Congress in making more money available to research and development of SEP systems, as NASA tries to figure out how to salvage as much technology from the scrapped ARM program as possible.
“Smart investments, focused exploration goals, and constancy of purpose will maintain U.S. leadership in not only in-space propulsion, but also space exploration more broadly,” Babin said.
UPDATE: 11 July 2017 Aerojet Rocketdyne conducted a series of successful hot-firing tests to its power processing unit for its AEPS on July 6. The tests were held in conjunction with NASA’s Technology Development Unit thruster and a Xenon Flow Control Unit. Aerojet Rocketdyne claims its AEPS can deliver twice the thrust of its XR-5 Hall thrusters, which are used on commercial and government satellites to reach geosynchronous orbit.