The National Aeronautics and Space Administration (NASA) and the Defence Advanced Research Projects Agency (DARPA) have announced a new partnership to develop advanced rocket technologies that use nuclear power for propulsion. Despite significant advances in aerospace engineering over the decades, the amount of thrust a rocket can generate is still limited by conventional fuels such as kerosene and hydrogen. This constrains the speed that a vehicle can reach, making longer distance missions challenging and strenuous, particularly for the crew involved.
NASA Will Be Responsible For Developing Nuclear Rocket Engine With DARPA Focusing On Vehicle Operations
NASA announced the engine development as part of the American Institute of Aeronautics and Astronautics (AIAA) SciTech Forum in Maryland today. At a fireside chat at the event, DARPA's director Ms. Stefanie Tompkins explained that recent advances in nuclear technology had enabled her agency to take more 'risks' with it. She outlined that the shift to High-Assay Low-Enriched Uranium (HALEU) has a higher proportion of enriched Uranium in the fuel mixture when compared to the fuel that is currently used in light-water nuclear reactors. This allows it to generate more power; however, currently, the concentration is still lower than what is required for nuclear submarines, aircraft carriers and weapons.
NASA has signed an Interagency Agreement (IAA) with DARPA, which delegates responsibility for demonstrating nuclear propulsion in space to both parties. As part of the agreement, NASA will be responsible for designing what it dubs a nuclear thermal rocket (NTR) technology and the NRT engine. This includes building and developing the nuclear reactor, all aspects of the engine, testing the engine on the ground, helping DARPA procure HALEU, and vehicle integration.
The engine developed by NASA will have to be integrated into a vehicle, which is where DARPA comes in. This vehicle is called an experimental NTR vehicle (X-NTRV), and DARPA will integrate the launch vehicle into the X-NTRV (implying that a traditional rocket will launch the NTR-equipped vehicle), operating and disposing the X-NTRV and perform all associated activities. Additionally, all systems developed under NASA's part of the agreement will be unclassified.
A central issue for nuclear propulsion is safety, which also creates regulatory hurdles for the technology. At this front, NASA's deputy administration Pam Melroy explained that
I think that probably the biggest hurdle for regulatory is actually been with commercial and HALEU is absolutely going to help that. SPD-6, the Space Policy Directive from the White House provided a lot of clarity in this area. I think the government has always been capable of doing what it wanted to do, if you know, have to find the authorities to do it. But I think the clarity of the agreement between and DARPA and DOE, where DARPA has the oversight authority, is absolutely going to move this faster. So I think there's a lot of different pieces that came together in this policy environment, but for me the really big outcome is using HALEU is going to streamline a lot of that because it's not considered weapons-grade material and so that means also the potential for commercial spin-off is also there.
Ms. Tompkins added that when it comes to safety, the system will be designed in a manner that the engine will not be operating until it reaches space and that it will use an orbit that will not 'degrade' until the engine itself is safe to enter Earth again. The engine itself will not emit any radioactive exhaust, with only gaseous hydrogen coming out of a potential nozzle. A couple of minutes later, Ms. Melroy also shared more details about the engine, explaining that:
There's a couple of key things. For nuclear thermal, you do have a hydrogen tank. For, if you had a traditional rocket, you would have to have two tanks. You'd have to have a fuel and an oxidizer. So in this case, the hydrogen is actually pumped into the reactor with the turbopump that looks actually like a traditional rocket pump. And then it's heated up and it's ejected out of the nozzle. But the fact that you're not carrying two, you know both the fuel and the oxidizer, um, certainly provides um some, some efficiencies, you talked about ISP. Some of the things that make it more efficient. So there's potential for mass savings in the end. So it's um, it's just, you know like as you pointed out very very high ISP.
Currently, the NASA-DARPA agreement calls for a launch readiness review, one of the final reviews before launch in the fiscal year 2027 (roughly four years from now). The X-NTRV will be flown at a high orbit, and according to the NASA official:
It's critically important for us to get a high enough altitude that the material will no longer be radioactive by the time it reenters right. So that's critical for us. So that's sort of minimum at the 700-kilometer threshold, and maybe up to as high as 2,000 kilometers - both of which are well above the International Space Station. So 300 years+, for reentry.