Nuclear space gets hot
by Jeff Foust
|“The benefits of a nuclear thermal propulsion reactor, such as the one that DRACO is developing, in the space domain are understood and accepted by the power and propulsion communities as a step change in current capability,” said Dodson.
There had been only modest progress in nuclear power and propulsion systems, though, a combination of cost, need, and concerns about safety. However, recent developments show that there is a rising tide for space nuclear systems, although it is not necessarily lifting all boats evenly.
The biggest development for nuclear systems in space came last Wednesday, when NASA and DARPA announced they had selected Lockheed Martin to develop a spacecraft to demonstrate NTP technologies. NASA and DARPA are evenly splitting the $499 million cost of the agreement for the Demonstration Rocket for Agile Cislunar Operations (DRACO) program, which the two agencies announced plans to cooperate on in January.
“The benefits of a nuclear thermal propulsion reactor, such as the one that DRACO is developing, in the space domain are understood and accepted by the power and propulsion communities as a step change in current capability,” Tabitha Dodson, DRACO program manager at DARPA, said during a briefing about the selection of Lockheed to develop the spacecraft. DRACO, she said, will “help to take a variety of programmatic and technical questions off the table.”
DRACO draws up visions of a spacecraft zipping through the solar system, or at least in cislunar space, thanks to its high-efficiency, high-thrust propulsion system. (DARPA started DRACO because of a Defense Department interest in greater mobility in cislunar space; NASA joined because it sees NTP as a key technology for human Mars missions.) But DRACO will instead be a relatively modest test of the technology that not only will not leave Earth orbit, but will not maneuver much there.
“It's a flying test stand, essentially,” she said. After launched into an operational orbit, likely between 700 and 2,000 kilometers high, the spacecraft will not make any major maneuvers like raising its orbit.
Instead, NASA and DARPA plan to test the reactor, including its use of high-assay low-enriched uranium (HALEU), a fuel less potent than the highly-enriched uranium previously used for space nuclear systems but without the nuclear weapons proliferation issues associated with it. “This will be the primary focus of the DRACO demo, and the act of collecting data on the HALEU reactor will define mission success,” Dodson said.
NTP systems have been tested in the past on the ground, such as NASA’s NERVA program a half-century ago at the Nevada Test Site. Anthony Calomino, space nuclear technologies portfolio manager at NASA, said the agency looked into testing the system on the ground, either by reactivating those old test stands or building new ones. But the requirements to prevent venting of potentially radioactive exhaust into the atmosphere meant that “the costs of that are actually higher than what we’re estimating is going to be to conduct this test in space.”
Neither government nor industry officials at the briefing offered many specifics about the DRACO spacecraft, which Lockheed is building in cooperation with nuclear power company BWXT. Dodson described the spacecraft as similar in size to a launch vehicle upper stage, which can be accommodated inside the payload fairing of a Falcon 9 or Vulcan Centaur (the Space Force is providing the launch as its contribution to the program.)
They did not disclose the thrust that the vehicle will produce, although Calomino said it will have a specific impulse of about 700 seconds, better than chemical engines but less than other NTP designs intended to reach up to 900 seconds. “For the DRACO mission, we’re right at the level where we can get that engineering relevance that we need for a better understanding for higher-thrust engines,” he said.
|“If you have a mishap during launch or on the launch pad itself, the debris that would potentially be generated by that isn’t any worse than the debris that would be produced by the turbomachinery,” said Calomino.
DRACO will use liquid hydrogen as its propellant, heated up by the reactor and expelled through a nozzle to create thrust. That will keep the demonstration limited to a couple months because of the challenges of limiting boiloff of liquid hydrogen. “Keeping the hydrogen around is a big challenge, so we will want to expedite the checkout of the spacecraft and of the nuclear reactor,” said Kirk Shireman, vice president of lunar exploration campaigns at Lockheed Martin.
However, he and government officials did not rule out refueling DRACO to extend its mission; Lockheed is developing a “cislunar transporter” that will refuel Blue Origin’s Blue Moon lunar lander with liquid hydrogen and liquid oxygen (see “A lunar lander makeover”, The Space Review, May 22, 2023). Refueling likely won’t be needed to meet DRACO’s test requirements, Shireman said, “but I’d love to refuel it and keep it around and use it for years to come.”
The DRACO agreement is done as a fixed-price, milestone-based agreement. Both Shireman and Joe Miller, president of BWXT Advanced Technologies, said on the call their companies were making significant investments into the program but did not disclose specific figures.
Much of the discussion on the call focused on safety. The reactor will be launched “cold” and will not be started until after it is in orbit. “It is safe to work around, it is safe to be around,” Calomino said. “If you have a mishap during launch or on the launch pad itself, the debris that would potentially be generated by that isn’t any worse than the debris that would be produced by the turbomachinery.”
“Part of the design is ensuring safety during launch,” Miller said. That includes a scenario where the reactor goes into the ocean, where water could trigger criticality. The reactor includes design features to prevent that, he said, which will be reviewed extensively as part of the government’s launch approval process for nuclear systems.
Zeno Power won a NASA Tipping Point award to develop a new type of radioisotope power system that could power future lunar missions. (credit: Zeno Power)
The DRACO announcement came a day after NASA’s space technology directorate made $150 million in awards through its Tipping Point program to 11 companies. The awards are designed to advance technologies, of interest to both NASA and other customers, to the point of flight demonstrations.
Among the winners was Zeno Power, a startup that received $15 million to work on a radioisotope power system that could be used on the Moon. The Harmonia project is led by the startup with a team that includes Blue Origin, lunar lander developer Intuitive Machines, and NASA’s Glenn Research Center and Marshall Space Flight Center, among others.
The technology is closer to the radioisotope thermoelectric generators (RTGs) that NASA has flown for decades than a nuclear reactor, but with some key differences. Unlike a conventional RTG, which uses thermocouples to convert the heat from decaying radioactive materials into electricity, that heat instead drives a Stirling engine to generate power, with three to four times higher efficiency. NASA has studied radioisotope Stirling generators for years but never flown one, and in recent years halted funding for one such project because of budget constraints.
“We will be building upon a lot of this work that NASA Glenn and commercial entities have done and, for the first time, actually match the Stirling engine with a radioisotope source for use in space,” said Tyler Bernstein, co-founder and CEO of Zeno Power.
|“How we can bring different entities together and have sustainability with systems that operate for years instead of days?” asked Bernstein.
The other major difference is the radioisotope. Rather than plutonium-238, Zeno Power plans to use americium-241. That isotope, not yet flown in space, doesn’t generate as much power per gram as plutonium-238 but is readily available. ESA, which lacks access to plutonium-238, has previously looked at using americium-241 for future radioisotope power systems.
The company sees the technology as key to enabling long-term lunar operations, particularly through the two-week lunar night. “There are a number of missions that can use this technology to enable them to operate and survive through the lunar night, and operate in permanently shadowed regions,” Bernstein said.
The goal of the project is to get the power system ready for a 2027 lander mission by Intuitive Machines with support from Blue Origin. “We’re looking at harmony on the lunar surface,” he said of the project’s name, Harmonia. “How we can bring different entities together and have sustainability with systems that operate for years instead of days?”
NASA has been supporting other nuclear power efforts, including development of fission nuclear reactors to provide power on the lunar surface. The agency awarded $5 million study contacts in June 2022 to teams led by Lockheed Martin, Westinghouse and IX, a joint venture of Intuitive Machines and X-Energy, to work on initial designs for those systems.
The goal of the Fission Surface Power (FSP) program is to develop a 40-kilowatt reactor that can operate for 10 years. Like DRACO, the reactor would use HALEU fuel. However, both NASA and its partner, the Department of Energy, included few other design requirements for the systems.
“We are able to get really innovative ideas from the three partnerships,” said Lindsay Kaldon, FSP program manager at NASA Glenn, during a panel session earlier in the month at the American Astronautical Society’s Glenn Memorial Symposium. “We’re able to see some out-of-the-box thinking.”
Those studies were intended to be phase 1 of the program, but it’s uncertain when NASA will be able to proceed with phase 2, to turn one of the designs into flight-qualified hardware. The agency hasn't stated when it plans to issue a request for proposals for that second phase.
|“We’ve got a lot of momentum right now in nuclear space,” Bilardo said, supporting Artemis. “We want to go to stay this time.”
Industry officials are worried there could be a gap between the end of phase 1 and beginning of phase 2. Such a gap “causes us on the industry side to have to reassign our teams,” said Vince Bilardo, an industry consultant supporting the IX team.
With the focus on programs like DRACO, and limited funding, he was skeptical that NASA would be able to obtain the funding to proceed with FSP in the next few years. That’s particularly the case where NASA is expecting flat budgets, at best, for fiscal years 2024 and 2025 (see “A chaotic trajectory for NASA’s budget”, The Space Review, June 19, 2023.) “In that current environment, my take on that is it’s going to be a challenge for NASA to secure formal new start project authorizations for these different Artemis elements,” he said.
He suggested NASA find ways to extend the phase 1 studies to keep the industry teams active, while also working in parallel on key enabling technologies for fission power systems: “We know what these systems need to look like, and we know what the list of enabling technologies is.” There may also be ways to leverage technologies being developed for nuclear propulsion systems for power systems on the lunar surface.
Despite the near-term issues facing FSP, Bilardo was optimistic about the status of space nuclear development in general. “We’ve got a lot of momentum right now in nuclear space,” he said. It was, he argued, an essential capability for Artemis to enable long-term stays on the Moon. “We want to go to stay this time.”
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