Several companies are working on advanced electric propulsion technologies that could reduce the cost of sending cargo to Mars or even provide shorter travel times for crewed missions. (credit: MSNW LLC)
It’s a technology looking for a new mission.
The technology is solar electric propulsion (SEP), which NASA has identified in recent years as a key enabler for eventual human missions to Mars. SEP, the agency argued, could be used to propel cargo missions to Mars in advance of crewed missions much more efficiently than conventional chemical propulsion systems.
High-power SEP was to be tested in interplanetary space on the Asteroid Redirect Mission (ARM), powering the robotic spacecraft that would travel to a near Earth asteroid, grab a boulder off its surface, and fly back to cislunar space. However, NASA announced earlier this year it planned to cancel ARM, and Congress, never much of a fan of the mission, has shown no signs of opposing it.
When releasing its budget request in May, NASA’s acting administrator, Robert Lightfoot, emphasizes that aspects of ARM like SEP would continue in some form. “While we ended the Asteroid Redirect Mission going forward, we will take those technologies, with solar electric propulsion as probably the best example, and move forward with them and morph those into a different mission,” he said in a “state of the agency” speech times to the rollout of the budget request.
So how will NASA test ARM now? One possibility is using it on its proposed Deep Space Gateway, a cislunar habitat that NASA is discussing assembling in cislunar space using the excess cargo capacity on Space Launch System missions starting with Exploration Mission 2 in the early 2020s (see “A gateway to Mars, or the Moon?”, The Space Review, March 27, 2017).
Lightfoot said a hearing on the NASA budget proposal by the House Appropriations Committee June 8 that the first element of the Deep Space Gateway would be a power and propulsion module that incorporates SEP. “It would build right off of the bus that we had for the Asteroid Redirect Mission,” he said, although perhaps somewhat smaller than what was envisioned for ARM.
The Deep Space Gateway concept, though, remains just a concept: it was not included in the agency’s 2018 budget request as a standalone program, only as one possibility it is pursuing for missions in the 2020s that might make use of SEP.
Solar-electric propulsion has also been proposed for use on future robotic Mars missions, including an orbiter proposed for launch in 2022. Concepts discussed last year included options that used an “exploration” version of SEP systems that would allow the Mars orbiter to change orbits around the planet, rendezvous and collect a sample placed in orbit by a Mars sample return mission, and bring that sample back to Earth.
However, there’s no money for a 2022 Mars orbiter of any kind, let alone one with advanced SEP, in NASA’s proposed budget (see “The future (or lack thereof) of NASA’s Mars Exploration Program”, The Space Review, this issue.) One ray of hope, though, is a decision by House appropriators last week to provide $62 million for future Mars missions—NASA requested less than $3 million—that can keep alive a 2022 orbiter mission of some kind, although whether or not it uses an advanced SEP system remains to be seen.
Despite this uncertainty about how advanced SEP systems will be used, there’s still remarkable optimism about the future of that and other in-space propulsion technologies. “We are on a cusp of a giant leap in space transportation technology,” said Rep. Brian Babin (R-TX), chairman of the space subcommittee of the House Science Committee, at a June 28 hearing on in-space propulsion. “Advances in in-space propulsion system hold the promise of radically altering space exploration.”
At that hearing, Bill Gerstenmaier, NASA associate administrator for human exploration and operations, reiterated the importance of advanced SEP. “As we prepare for missions in the vicinity of the Moon, and ultimately Mars, electric propulsion will be a key enabling technology,” he said, citing the role of SEP in supporting a cislunar habitat.
He also looked ahead to Mars. “We believe that using electric propulsion to pre-position key large elements will be necessary for human Mars-class missions,” he said.
Joe Cassady, executive director for space in Aerojet Rocketdyne’s Washington office, agreed at the hearing, calling SEP a critical element of future sustainable Mars mission architectures “by enabling efficient transfer of cargo, habitats, and payloads to deep space destinations in advance of astronaut arrival.”
He likened it to military deployments, where large items are pre-positioned by cargo ships or other transports, with the troops arriving later by air. “SEP systems are the equivalent to the cargo ship for deep space missions,” he said.
Aerojet Rocketdyne is heavily involved in SEP today, working on systems of varying power for use on NASA missions, including one that had been planned for ARM. It is one of three companies that have Next Space Technologies for Exploration Partnerships (NextSTEP) awards from NASA to work on in-space propulsion technologies, awarded in 2015.
The other two companies with NextSTEP propulsion awards were also at the hearing. However, they have ambitions beyond providing the propulsion for cargo ships, instead developing technologies that they believe can shorten the travel time for human missions as well.
“High-power electric propulsion is a key technology for humanity’s sustained presence in deep space,” said Anthony Pancotti, director of propulsion research at MSNW LLC. “We need to break today’s ‘impulse-and-coast’ approach and advance to continuous, direct burns to destinations in our solar system.”
The company, based near Seattle, is working on one such technology, called the Field Reverse Configuration (FRC) thruster. Based on technology originally developed as part of fusion power research, FRC holds the promise, according to MSNW, of providing high-power propulsion as high specific impulses, and also able to use a wide variety of propellants, including those obtained from in-situ resources on Mars or other solar system bodies.
The third company with a NextSTEP propulsion award is Ad Astra Rocket Company, the Houston firm founded by former astronaut Franklin Chang Diaz that has been working for years on a concept called Variable Specific Impulse Magnetoplasma Rocket, or VASIMR (see “VASIMR: hope or hype for Mars exploration?”, The Space Review, September 7, 2010). That system, like MSNW’s technology, could conceivable enable continuous-thrust missions with short travel times to Mars.
“It is an electric rocket that fits squarely in the high-power niche,” Chang Diaz said at the hearing. The company has tested a 200-kilowatt engine, the VX-200, in its lab, which he said is at a technology readiness level of four or five. “The lion’s share of this development has been achieved” by the company, he said, relying primarily on private funding.
The three companies are scheduled to complete their NextSTEP awards next year, after which NASA will decide how to use those technologies for its future cislunar and Martian exploration plans.
Future uses of those technologies will face technical and other issues, though, that go beyond the propulsion systems themselves. One is that, as power levels increase from the hundreds of kilowatts into the megawatts, SEP will have to become NEP—nuclear electric propulsion. That will be especially true for missions that venture farther into the outer solar system.
“These rockets will first be solar electric, and later, as we move outwards from the Sun, they must transition to nuclear electric power,” Chang Diaz said.
Nuclear power, though, carries with it its own technical and policy complications. NASA is doing a limited amount of work on nuclear power in space, primarily in the form of a small nuclear reactor concept intended to support surface operations. But, for the near term, SEP may be sufficient.
“We don’t need to go to a megawatt to be ready to go to Mars. We can do it with 100 to 200 kilowatts,” Cassady said, which can better fit into what he foresees as long-term constrained budgets. “What is the minimum that we need to ensure we can do this mission, and make this mission close? For the cargo part of the mission, we can live with about 200 kilowatts.”
Members of the space subcommittee seemed fascinated by the technology, even if they were not immediately volunteering to seek additional funding for it. “I feel like a student in office hours with professors,” said Rep. Ami Bera (D-CA), ranking member of the subcommittee, after chairman Babin offered members a rare second round of questions at the hearing.
“This has been a very fascinating hearing, one of the best ones that I believe I’ve had since I’ve been in Congress,” Babin, elected in 2014, said as the hearing ended.
The message the witnesses tried to leave with the subcommittee was the utility of advanced electric propulsion, no matter what the specific approach, for future human missions beyond Earth orbit, even as the specific architectures remain unclear. “No matter what we do in deep space,” Pancotti said, “we are going to need advanced propulsion.”