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After partnering up with the Defense Advanced Research Projects Agency (DARPA) earlier this year to develop a nuclear rocket engine, officials from the two agencies, lead contractor Lockheed Martin and BWXT Technologies shared important details about the nuclear reactor that will fly on top of a rocket to space. This reactor is part of the two agencies' DRACO program, which will test a nuclear reactor based rocket in space. Under DRACO, NASA is responsible for developing the nuclear engine, while DARPA will be responsible for other aspects of the mission.

NASA & DARPA Assure That Nuclear Rocket Engine Has Small Probability of Failure That Adheres To Existing Regulations

NASA and DARPA had teamed up in January as part of an announcement that planned a test flight for a nuclear rocket in 2027. This rocket will use a small reactor and hydrogen to heat the latter and generate thrust. The agreement, called Demonstration Rocket for Agile Cislunar Operations (DRACO), will see NASA lead the engine and rocket development while DARPA will be responsible for an experimental nuclear thermal rocket vehicle (X-NTRV).

The pair chose Lockheed Martin as the primary contractor for the vehicle and its engine, which will use uranium to demonstrate propulsion. NASA's portfolio manager for nuclear technologies, Dr. Steve Calomino, explained that key aspects of the DRACO mission will be to test the complex turbomachinery equipment on the engine, understand the reactor's performance, manipulate the engine's performance to power it up, restart it and throttle it. The test will aim to gather data to verify NASA's models on the ground. These models will provide the space agency with the "engineering foundation" to understand what role nuclear propulsion and rockets can play in a journey to Mars.

A key limitation of the DRACO test is the choice of hydrogen as its fuel. While traditional rocket engines use liquid Oxygen as an oxidizer to burn a fuel such as kerosene, the nuclear engine will use a reactor's heat to heat cryogenic hydrogen to high temperatures. Due to this, hydrogen is the limiting factor for the test, as the DRACO rocket's performance will be limited by the time that DARPA and NASA can keep the hydrogen cool while in orbit. However, hydrogen's mass provides significant advantages as well since it is less dense than conventional rocket fuels. For the DARPA rocket, the test will include two thousand kilograms of hydrogen and one hundred kilograms of uranium.

NASA' render of the Mars Transportation Habitat (MTH). The MTH is a separate project and is unrelated to the DRACO engine. Image: NASA

Data is an important resource for NASA since it will allow the agency and the Defense Department to justify an investment to get the reactor hot enough to gather reliable data. DARPA's DRACO program manager Dr. Tabitha Dodson shared that NASA's NERVA nucelar rocket program in the late 1900s was quite advanced and was manufacturing three reactors per year after it had already tested the engine on the ground.

DARPA is quite confident in the nuclear engine's capability to remain safe while it is on the ground before launch. This is mirrored by NASA's sentiments, with Dr. Calomino explaining in quite a detail the risks involved with the flight and the choice of fuel:

We've mentioned radioisotope systems. I want to make it clear that fission systems are not radioisotope systems. They're very very much different. Radioisotope systems are radioactive from the time that they are prepared to go into an electric generator all the way up to when they are incorporated into the payload and sitting on the launch pad. You know, plutonium it's just basically a radioactive material. Uranium 235 is, without it having been fissioned and actually being surrounded by radioactive products is basically a metal. It is safe to work around, it is safe to be around, it doesn't need the protection measures that need to be in place for plutonium. if you have a mishap during launch or on the launch pad itself, there is the debris that would potentially be generated by that isn't any worse than the debris that would be produced by the turbomachinery that might also be dispersed in such an accident or placed anywhere else. It's just, it is not a radioactive material at that point in time.

I think we did talk about one of the major risks, or another major risk, one of the key risks that we're looking at and we're looking at carefully is the reactor actually going to a water immersion. We call it a water immersion event where you can actually increase the criticality on the reactor, and it could, if it not prevented from doing so, it could go, active. But I think Joe talked to those in a pretty detailed way and we have ways of putting poison wires and poison systems in place, to make certain that event, if it happened, would still be able to be under the threshold of having that reactor be a hazard to the biosphere or to some other Earth sort of exposure event.

Dr. Dodson talked about the probability of such an event and commented:

I wanted to add another thing here. So Anthony mentioned an accident scenario which we consider to be almost impossible. The possibility is so tiny. We have to consider every range of possibilities when it comes to accidents of course. So the idea that the reactor could turn itself on, for instance, if it fell in the water, in a lot of instances is contrived.

. . . The probabilities are very low, and even in the event that there were an accident, the radiological release to the public would be enormously tiny. And this is in compliance with NSPM-20 which was mentioned earlier in the phone call. So there's a table that says as long as you are almost impossible with respect to the probability of an accident or extremely teeny tiny release to the public, so you're basically at the background radiation, then you're good to go to launch. So DRACO has already done all of our preliminary analysis across the entire spectrum of possibilities for accidents and found that we are all the way down in the low probability and all the way down in the teeny tiny amount of release.

A Rolls-Royce design concept of a nuclear reactor intended to be used on the Moon.

As for the engine itself, it uses a standard expander cycle which makes it likely that the hydrogen will first be heated and then used to power the turbomachinery. NASA is aiming for 800 to 900 second specific efficiency (isp) and Dr. Calomino explained that producing thrust will involve heating hydrogen at 20 degrees Kelvin to 2,700 degrees in a second and then expanding the heated hydrogen out of the engine nozzle. NASA has looked at the costs of testing on the ground and these are higher than the costs of conducting the test in space. Primary cost drivers are capturing the engine's effluent and a need to ensure that fission products are not released into the atmosphere.

While it is possible to achieve this on the ground, it costs too much money and time; however, according to the NASA portfolio manager, ground testing will be necessary if the agency wants to transport cargo and humans to Mars with a nuclear engine.

The orbit for the test can range between 700 kilometers and 2,000 kilometers and the reactor's manufacturer BWXT explained that the company has demonstrated the ability to manufacture the materials and components for the reactor.