Government, Industry Explore Nuclear, Solar Space Engines

SPACE

DARPA concept

COLORADO SPRINGS, Colorado — More commercial and military activity is taking place in space, and the Defense Department and industry are investing in emerging propulsion technologies to move systems in orbit faster, farther and more efficiently.

Traditionally, spacecraft have used chemical reactions to release energy and generate thrust. However, this method is far less efficient than using nuclear thermal propulsion, said Lockheed Martin NextGen Strategy and Business Development Senior Manager Lisa May.

While nuclear thermal propulsion has the same thrust as chemical, “it is two to four times more efficient,” May told reporters recently. Using specific impulse, or ISP — the measure of efficiency for a propulsion system — chemical has around 400 seconds of ISP, whereas nuclear has “beyond 700, up to 900” seconds, “which is what NASA has been talking about for getting humans to Mars,” she said.

In 2021, the Defense Advanced Research Projects Agency selected Lockheed Martin as one of three prime contractors — along with General Atomics and Blue Origin — for Phase 1 of its Demonstration Rocket for Agile Cislunar Operations, or DRACO, program to showcase the potential of a nuclear thermal propulsion system in space, a DARPA release said.

This January, NASA announced it had partnered with DARPA on the DRACO program, describing a nuclear thermal rocket engine as “an enabling capability for NASA crewed missions to Mars.” The goal is to demonstrate the system in orbit in fiscal year 2027, with the Space Force providing the launch vehicle for the DRACO mission, a DARPA statement said.

The program is about to enter Phase 2, which “will primarily involve building and testing on the non-nuclear components of the engine” such as valves, pumps, the nozzle and “a representative core without the nuclear materials in it,” DARPA’s program manager for DRACO Tabitha Dodson said during a panel discussion at the Space Foundation’s Space Symposium in April. Dodson said then a Phase 2 decision is “quite close.” However, at press time in mid-July, no contracts have been awarded.

Phase 3 of DRACO “will involve assembly of the fueled [nuclear thermal rocket] with the stage, environmental testing and launch into space to conduct experiments on the [nuclear thermal rocket] and its reactor,” DARPA’s website stated.

“There are no facilities on Earth that we could use for our DRACO reactor’s power test … so we’ve always baselined doing our power test for the reactor in space,” Dodson said. Once in space, DARPA will “very gradually” ramp up the system to “full power thrust,” she said.

As far as Defense Department applications for the system, the nuclear thermal engine “will perform missions typically reserved for upper-stage rockets … but it does those missions better,” carrying heavy payloads “faster [and] farther without needing a super heavy-lift first stage” rocket booster, she said.

“Rapid maneuver is a core tenet of modern Department of Defense operations on land, at sea, and in the air,” DARPA said. “However, rapid maneuver in the space domain has traditionally been challenging because current electric and chemical space propulsion systems have drawbacks in thrust-to-weight and propellent efficiency, respectively.”

DRACO’s nuclear thermal propulsion system “has the potential to achieve high thrust-to-weight ratios similar to in-space chemical propulsion and approach the high propellent efficiency of electric systems,” the release said. “This combination would give a DRACO spacecraft greater agility to implement DoD’s core tenet of rapid maneuver in cislunar space.”

However, like most nuclear technology, space propulsion systems could run into regulatory red tape, said Kirk Shireman, vice president of the Lockheed Martin Lunar Exploration Campaign.

“There is the political [perception] and the regulations in general and there’s public perception about nuclear reactors, and launching them off the planet seems to scare people,” Shireman said during the panel. “I do believe we can do that safely. We’ve been flying [radioisotope thermoelectric generators] in space, launching them, and I think we can launch reactors.”

Doing demonstrations like DRACO will help educate operators not only on the technical challenges of using nuclear propulsion in space, but also help them “understand all the issues that you have to work through [in] the regulatory environment,” he said. “So, I’m worried about it, but I know we can do it.”

Dodson said DARPA is “very comfortable and confident working within” existing regulatory and safety frameworks for DRACO, and noted the program’s “safety approach is grounded in four general design criteria” — never turning the reactor on until it gets into space; doing everything possible to prevent the reactor from turning itself on accidentally; preventing radiological risks to the public in accordance with National Security Presidential Memorandum-20; and disposing of the reactor while in an orbit in space, in line with the U.S. government’s Orbital Debris Mitigation Standard Practices.

“I want to try and envision a future where people become just as used to seeing, and as comfortable with, the concept of nuclear reactors in space, as we are to seeing the Navy employ them in ships and subs,” she said. “The Navy has a perfect safety record with reactors in the sea … they set a great example from going from production to manufacturing to installment, experiment and deployment. So, I think we could and should endeavor to just follow their example” when demonstrating nuclear capabilities in space.

Despite DARPA’s commitment to safety, nuclear propulsion systems face an uphill battle getting deployed on spacecraft at scale, said Joel Sercel, founder and CEO of TransAstra, a space technology company.

“Nuclear reactors are the most efficient power source that we have, and the engineer in me loves them,” Sercel said in an interview. “Problem is, society has pretty much said we’re not going to do that. And when the engineering enthusiasts … get very excited about this, they can convince certain levels of decision-makers that this is the right thing to do. But as soon as it starts to break through the billion-dollar funding level, the adults in the room say, ‘And how are you going to get this in space? And how are you going to test this?’”

Overcoming the physical barriers — the needed facilities and testing to prove the technology is safe — and the psychological barriers hasn’t been tenable, he said.

In May, the Space Force awarded TransAstra a Phase One Small Business Innovation Research contract to explore new applications for the company’s propellant-agnostic Omnivore thruster.

The Omnivore thruster uses solar reflectors to focus sunlight onto a solar absorber, which then superheats the system’s propellant to generate thrust “typically six times faster and eight times cheaper than electric systems,” a company release said.

Additionally, TransAstra calculated an Omnivore thruster “using liquid hydrogen propellant … will perform similarly to nuclear rockets, but without nuclear materials, costs or risk.”

Sercel said Omnivore has “80 percent of the performance of nuclear at 1 percent of the cost.” The system is essentially nuclear powered, “but the nuclear reactor in question is the fusion reactor at the center of the solar system called the Sun,” he added.

“The nice thing about nuclear reactors is that you have a small, compact reactor versus large deployable solar reflectors, but the basic performance of solar thermal rockets and nuclear rockets is about the same,” he said. And with Omnivore “you don’t have all these safety concerns and radioactive material and reactor control issues and so on. So, we think it’s a much more practical approach.”

Omnivore could have multiple mission applications for the Defense Department, Sercel said. Using liquid hydrogen propellant, the thruster “can deliver hundreds of kilograms” of spacecraft to geosynchronous orbit “on small launch vehicles, and the Space Force seems to be very excited about this,” he said. The system could also deliver spacecraft weighing more than 100 kilograms to cislunar space, he said.

Additionally, TransAstra has an Omnivore variant that uses water as the propellant, the solar absorber superheating the water vapor and releasing the gas through a nozzle to generate thrust.

The water-based variant can be placed on the company’s Worker Bee small orbital transfer vehicles, about 25 of which can fit on a single Falcon 9 rocket, Sercel said.

“Each [Worker Bee] could deliver up to six small [satellites] to their orbital destinations. So, we can deliver a full constellation of 100 small or micro [satellites] to all different inclinations, and you would get global coverage in one launch.”

TransAstra has conducted research and testing on Omnivore using a variety of propellants at the company’s Los Angeles laboratory, and the goal is to demonstrate the thruster in orbit in two years, “as soon as we secure the last increment of funding to make that happen,” Sercel said. The company has been encouraged by its relationship with the Space Force so far, he added.

“As a private company, sometimes it’s a little hard to understand exactly what the Space Force is looking for,” he said. As part of the SBIR award, the service provided TransAstra “a contract representative who’s an engineer at the Air Force Research Lab to work with us to help us find the users within the Space Force, and we think that will help make these connections to the market.”

The Space Force “has been brilliant and forward-looking [in] how they’re working differently with non-traditional suppliers to use commercial-like contracts,” he continued. “Instead of the way traditional government acquisition — which hasn’t been very efficient — has worked, [which] is the government will study a problem, come up with a solution and then tell the contractors what the solution is and ask them how much it costs.

“Instead, the Space Force is saying what the problem is, and letting private sector innovators come up with solutions,” he said. “And we think that’s going to pay huge dividends for the taxpayer.” ND

Topics: Emerging Technologies, Defense Department

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Of course the biggest advantage of nuclear is the fact you don't have to haul a massive load of Water or Oxygen to orbit for fuel. A Nuclear Core can provide high thrust for a very very long time at a fraction of the fuel mass. In space, worrying about radioactivity is pointless, as Space is already loaded with radioactivity and any radioactive thrust output by a NERVA engine is statistically meaningless.