Nuclear thermal rockets
We're building nuclear spaceships again – for real this time.
The military and NASA appear to be serious about building demonstration equipment.
07/22/2024, Jacek Krivko, arstechnica.com
Phoebus 2A, the most powerful space nuclear reactor ever built, was fired up at the Nevada Test Site on June 26, 1968. The test lasted 750 seconds and confirmed that it could carry the first humans to Mars. But Phoebus 2A never got anyone to Mars. It was too big, too expensive, and didn’t fit Nixon’s vision that there was nowhere to go beyond low Earth orbit.
But it wasn't NASA that first proposed nuclear-powered rockets. It was the military, which wanted to use them for intercontinental ballistic missiles. And now the military wants them again.
Nuclear powered ICBM
Work on nuclear thermal rockets (NTRs) began with the Rover program, initiated by the US Air Force in the mid-1950s. The concept was simple on paper. Take tanks of liquid hydrogen and use turbopumps to pump that hydrogen through a nuclear reactor core to heat it to very high temperatures and force it out a nozzle to create thrust. Instead of causing the gas to heat and expand by burning it in a combustion chamber, the gas was heated by coming into contact with a nuclear reactor.
The key advantage was fuel efficiency. “Specific impulse,” a parameter that is a bit like a rocket’s fuel consumption, could be calculated from the square root of the exhaust temperature divided by the molecular weight of the fuel. This meant that the most efficient fuel for rockets was hydrogen, since it had the lowest molecular weight.
Chemical rockets had to mix hydrogen with an oxidizer, which increased the overall molecular weight of the fuel, but it was necessary for combustion. Nuclear rockets do not require combustion and can run on pure hydrogen, making them at least twice as efficient. The Air Force wanted to efficiently deliver nuclear warheads to targets around the world.
The problem was that getting stationary reactors running on Earth was one thing, but getting them to fly was quite another.
Problems of the space reactor
Fuel rods made of uranium-235 oxide dispersed in a metal or ceramic matrix form the core of a standard fission reactor. Fission occurs when a slow-moving neutron is absorbed by a uranium-235 nucleus and splits it into two lighter nuclei, releasing enormous amounts of energy and excess, very fast neutrons. These excess neutrons usually do not cause further fission because they are moving too fast to be absorbed by other uranium nuclei.
The chain reaction that keeps a reactor running depends on them being slowed down by a moderator, such as water, to reduce their speed. This reaction is kept at a moderate level by control rods made of neutron-absorbing materials, usually boron or cadmium, which limit the number of neutrons that can cause fission. Reactors increase or decrease power by moving control rods in and out of the core.
Translating all this into a flying reactor is a complex task. The first problem is fuel. The hotter you make the exhaust gas, the more the specific impulse increases, so NTR required the core to operate at temperatures reaching 3,000 K – nearly 1,800 K hotter than terrestrial reactors. Making fuel rods that could withstand such temperatures proved extremely difficult.
The next problem was hydrogen itself, which is extremely corrosive at such temperatures, especially when interacting with the few materials that are stable at 3000 K. Finally, the standard control rods had to be abandoned, since on Earth they would be dropped into the core by gravity, and this would not work in flight.
The Los Alamos Scientific Laboratory proposed several promising NTR designs that addressed these issues in 1955 and 1956, but the program really gained momentum after it was handed over to NASA and the Atomic Energy Commission (AEC) in 1958. There, the idea was renamed NERVA, for Nuclear Engine for Rocket Vehicle Applications. NASA and the AEC, with nearly unlimited budgets, set about building space reactors—and lots of them.
Kiwi tries to fly
The first of these reactors was called Kiwi-A. A test on July 1, 1959, proved the concept worked, but the devil was in the details. Vibrations caused by the hydrogen flow damaged the reactor after just five minutes of operation at a relatively modest 70 megawatts of power. Temperatures reached 2,683 K, causing hydrogen corrosion of the rods and the ejection of parts of the core through the nozzle, a problem known as “sloughing.”
On the other hand, the rotating drums placed around the core that replaced the standard control rods worked well. They were long tubes made of a neutron-absorbing material, with one side coated with a material that reflected neutrons back into the core.
The reactor power was increased by turning the drums so that their reflecting side faced the core, and decreased by turning the neutron-absorbing side toward the core.
Over 18 years, NASA, AEC, and industrial contractors like Aerojet Corporation built and tested a total of 23 reactors. “The last engine in the Rover/NERVA program was the XE Prime. They tested it in a vacuum environment and got it to TRL 6,” said Dr. Tabitha Dodson, a program manager in DARPA’s Tactical Technology Office. TRL 6 stands for “Technology Readiness Level 6” — reaching 7 would mean flying a demonstration engine in space.
This didn't mean there wouldn't be problems, however. Fuel shedding and cracking problems persisted in all NERVA engines to varying degrees. But what ultimately killed NERVA in 1973 was NASA's shift in goals from deep space to low Earth orbit. And NERVA wasn't needed for that.
Nuclear Mars Express
More than 40 years passed before NASA again considered nuclear propulsion, first in the short-lived Jupiter Icy Moon Orbiter project and then in reference architecture for human exploration of MarsPowering Mars missions with a compact reactor could cut the time to reach Mars by more than half, to three to four months, compared to the six to nine months projected for chemical rocket engines. Less time in space means less exposure to radiation for astronauts and fewer supplies for the trip.
So in 2017, NASA started a small NTR research program. The budget was only a little over $18 million, but it was something. Two years later, Congress passed an appropriations bill that provided $125 million to develop NTR. Things were moving forward, but it was mostly paperwork, followed by more paperwork, followed by more paperwork.
And then on June 17, 2020, DARPA “came into the chat room” and said, “We want a nuclear missile.” Not just another paper study, but a demonstrator.
In pursuit of Sputnik 2.0
DARPA's website says the agency has always had a single mission: investing in breakthrough technologies for national security. What does a nuclear-powered spacecraft have to do with national security? The military's view was hinted at by General James Dickinson, an officer at U.S. Space Command, in his testimony to Congress in April 2021.
He said that “Beijing is pursuing space superiority through space attack systems,” and cited intelligence gathered about Shijian-17, a Chinese satellite equipped with a robotic arm that could be used to “capture other satellites.” That may seem like a ridiculous exaggeration, but it was enough to get the go-ahead for a nuclear-powered spacecraft.
And the obvious concern about hypothetical threats continues. The goal of the Demonstration Rocket for Agile Cislunar Operations (DRACO) project, stated in its environmental assessmentwas “providing space assets to deter strategic attacks by adversaries.” Dickinson's concerns about China were also cited there.
“Let's say you have a mission where you need to get from point A to point B quickly in cislunar space, or you need to monitor another country that's doing something near or around the moon, and you need to act very quickly. With a platform like DRACO, you can do that,” DARPA's Dodson said.
Two years after DARPA’s intervention, the preliminary design phase was complete, and Lockheed won a half-billion-dollar contract to build DRACO. But DARPA wasn’t the only one paying. NASA chipped in, too. The two agencies made DRACO a joint project, splitting the costs 50-50.
Next Generation NERVA
But building DRACO will also present another challenge: operating it. “There are a number of regulatory and technical challenges,” said Kirk Shireman, the Lockheed Martin Space vice president who oversees the DRACO project. For starters, firing nuclear engines in the open air somewhere in the Nevada desert would be out of the question. Building facilities that meet all the rules would take years.
Then there was the fuel. NERVA reactors ran on highly enriched uranium, the same stuff used to make nuclear weapons. If something went wrong during launch, about 700 kilograms of weapons-grade uranium would suddenly fall from the sky. And to make a bomb, you only need about 25 kilograms.
That's why DRACO will use a new fuel called high-assay-low-enriched uranium (HALEU) — a fissile material made by mixing highly enriched uranium with low-enriched uranium to below 20 percent enrichment. “You can relax some of the safety requirements by going to HALEU,” said Joe Miller, vice president of BWXT Technologies, a company that specializes in naval reactors and was chosen by Lockheed Martin to build the reactor for DRACO. And while building a bomb with HALEU is still a possible under certain circumstancesthis is much more difficult than with highly enriched uranium, which was mandatory in all NERVA reactors.
With the fuel issue solved, BWXT began designing the reactor itself. “Using HALEU controls the internal geometry of the reactor,” Miller says. To avoid reinventing the wheel, Miller’s team began by reviewing reams of reports from the NERVA program. But compared to NERVA’s designs, his team used different channels to route hydrogen through the reactor core and the thermal control systems that transfer heat to the hydrogen.
“Our chief engineer was a bit of a historian and librarian, so he would dig up all these reports, scan them, and integrate them into our design reviews. Lots of black-and-white photographs. Lots of old charts from testing. We learned from that. It was extremely relevant,” Miller said.
One of the key things BWXT found in the NERVA reports was evidence of reactor fuel cracking when exposed to hydrogen. “We passed [отчеты] “We gave it to our young materials scientists and they were able to use it as a springboard for early design decisions that they were making,” Miller said. The result, he said, was a coating that could withstand reactor temperatures without cracking. “We created our own internal nuclear fuel formula, which I can't talk about publicly,” he said.
Building a space reactor isn't easy, but at least it's been done before. What hasn't been done yet is building a spacecraft based on it.
The first nuclear spacecraft
DRACO will be a medium-sized spacecraft, less than 15 meters long and less than 5.4 meters in diameter — dimensions dictated by the size of the standard payload fairing of the Vulcan Centaur rocket it will likely launch on. “We’re familiar with liquid hydrogen, spacecraft systems design and integration. We have the skills and the people to build this thing,” Shireman said.
DRACO will operate like a NERVA-type rocket, with hydrogen tanks located at the front of the engine bay, and turbomachines feeding that hydrogen through the core (mounted just behind them), but separated from the core by a radiation shield. The HALEU reactor will be surrounded by control drums and positioned ahead of the exhaust nozzle. According to DARPA requirements, DRACO will have at least 700 seconds of specific impulse, which is 300 seconds better than the RL-10, the most efficient chemical space engine we have.
“The main technical challenge here is working with liquid hydrogen stored at 20 K — very, very cold, really slippery molecules that like to slide out of wherever you put them,” Shireman said. For DRACO, Lockheed opted for passive hydrogen cooling. The tanks will be thermally insulated to keep the sun from heating them up. That way, the hydrogen will have to stay at 20 K long enough to complete all the tests. For longer missions, nuclear spacecraft will have to rely on active cooling.
Test drive DRACO
Because there is a nuclear reactor on board, Lockheed and BWXT ensured that the risks of any potential catastrophic accident were kept to an absolute minimum, with contingency plans developed for every scenario.
What if the launch platform fails and DRACO crashes somewhere near the Florida launch site? This would be no more of a problem than a conventional engine failure, since the reactor would only be activated by its control drums once it had reached a safe orbit at least 700 kilometers from Earth.
Falling into the ocean? That's a little trickier, because water is a moderator and would start a fission chain reaction, essentially turning on the reactor no matter what the control drums do. But DRACO is designed to prevent that, too. In that case, a neutron poison, a material that absorbs neutrons and immediately stops the reaction, would be deployed right into the core.
The actual test drive will begin when DRACO reaches its target orbit. “We’ll first run a series of checks, making sure all the sensors and actuators are working. Then, slowly, we’ll start turning on the reactor,” Dodson said. This will be the moment of truth for DRACO, as the program does not include any ground tests with the reactor running.
“Because DRACO uses lower enriched uranium than NERVA, we need to use more moderator. We also expect a phenomenon we call negative temperature feedback, where the reactor reduces power as it heats up. That's one of the interesting unknowns in this project, and we hope to gather more data on how that works,” Dodson says.
“It’s like a new high-performance car. You don’t go full throttle right from the start. We’re going to gradually increase the performance, and eventually, if we can show something significant, maybe we’ll go full throttle,” said Dr. Anthony Calomino, NASA’s space nuclear technology portfolio manager. That “something significant” is a specific impulse high enough to get humans to Mars. But that’s not all.
Lazy rivers
The problem with getting to destinations like the moon or Mars is that we can’t get there in a straight line. You can’t just point your regular rocket at the moon and fire away like Jules Verne and expect it to get there. “These rockets can’t move entirely on their own. They use complex fractal orbits that hug Lagrange points — gravitational eddies in lunar space — ‘lazy rivers,’ as I like to call them,” Dodson said.
Imagine boarding a tiny boat in Liverpool with just enough fuel to reach the nearest ocean current, because you figured that current would eventually take you to New York. That's how we travel through space today. DRACO is intended to be the first step toward nuclear space cruisers.
“There are also civilian applications,” Calomino said. “You're talking about delivering payloads from Earth to low orbits, where a space tug can pick them up and ferry them to the moon, and back.” Such nuclear space tugs, he suggested, would be the basis for a new lunar transportation system.
And perhaps the best thing about these space tugs is that the reactors can run for years. “We know there’s water on the surface of the moon. You can recycle that water to make hydrogen and use it to fuel your ship, just like you fuel a car. The reactor itself will last a very long time,” Calomino said.
Aside from refueling, cars and nuclear spaceships have one other thing in common: we can charge them.
Nuclear spacecraft with supercharged engine
“My background is in hypersonic fluid dynamics, mostly reentry vehicles. I attended lectures at NASA about problems with going to Mars that even NTRs couldn’t solve,” said Ryan Goss, a professor of the practice in the University of Florida’s Herbert Wertheim College of Engineering. Goss and his team figured they could solve some of those problems by outfitting NTRs with superchargers.
Goss's idea was based on the use of a wave rotor. “In cars, this is called a compressor or supercharger,” Goss explained. In his NTR concept, the wave rotor is installed between the reactor core outlet and the exhaust nozzle to further increase the exhaust gas temperature.
“The limiting factor for NTR is the reactor core temperature. Today, it’s about 3,000 K, which gives you about 900 seconds of specific impulse,” Goss said. The wave rotor, he calculates, should increase that time to 1,400 seconds — twice as long as DRACO. Goss and his team proposed the concept to NIAC, a NASA program that funds early-stage innovative ideas, and were awarded funding in 2023 to conduct a detailed feasibility study.
But the wave rotor isn’t the only unique thing about Goss’s spacecraft. The real magic happens when the NTR engine finishes firing. The ship will turn around, flying nozzle first. It will then switch the reactor into power plant mode, redirecting its heated hydrogen from the nozzle into a closed loop of turbines that generate electricity, and use the electricity to power a special form of ion engine that’s attached to the opposite end of the spacecraft. They’ll increase the specific impulse from 1,400 seconds to more than 10,000 seconds.
Getting big and keeping cool
The first mention of a bimodal propulsion system like this was towards the end of the NERVA program. There were two problems, however.
For one thing, electric motors have always been used to power small, unmanned spacecraft. Scaling them up to accommodate the thousands of megawatts generated by nuclear reactors would require huge spacecraft. “Modern electric motors can only reach about 100 kilowatts. If you tried to use them in our spacecraft, you’d need so many of them that it would be impractical. It’s not a trivial problem, like, ‘Well, just get a thousand 100-kilowatt motors and that’s it,’” Goss said. “So we’re looking at magnetoplasmadynamic (MPD) thrusterswhich have much higher energy density and have been demonstrated to operate at megawatt levels of power.”
The second problem is cooling. NTR has no problem with waste heat because the hydrogen acts as a coolant for the reactor and is then expelled from the ship. In nuclear electric propulsion (NEP) mode, the coolant flows in a closed loop, meaning the heat builds up in the spacecraft. That's why all NEP designs have huge radiators. In NASA's NEP reference chemical architecture, a single radiator would have to be over 2,000 square meters. Goss's bimodal wave rotor ship would need a radiator five times larger.
But it would be very fast. “The reference NTP spacecraft would need to get to Mars in 297 days and weigh more than 600 tons. The chemical/NEP design would need 382 days and weigh 418 tons,” Goss said. His bimodal wave rotor concept is fast enough to launch when Mars and Earth are closest together, reaching Mars in just 45 days with a mass of 530 tons.
“By flying a little slower, doing a 65-day trip, we can get down to 273 tons,” Goss said.
Baby steps
But that idea won't be tested at DRACO. “The crawl-walk-run strategy is what we really want to do here,” Calomino said. “The main thing is to get the NTP engine running, get confidence, understand the reactor, get the reactor stable, so let's focus on that. Let's do that.”
Once we know it works, it will be time to evaluate whether adding complexity to MPD engines makes sense. When you do both electric and thermal nuclear propulsion, you have two systems with different requirements, even if they are powered by the same reactor. Then you have to add up the mass of both and compare that to using only one system and feeding it more fuel. Adding complexity also increases risk.
Some Defense Department officials believe much depends on successfully demonstrating a simple system. “Think about the Navy. The best way to move heavy payloads across the oceans is with huge battleships with big engines. Nuclear propulsion is the best option. Same thing with space. The Defense Department doesn’t have that capability right now,” Dodson said. “But once we have it, our ships will be able to move through space the same way they move through the oceans.”
Putting aside the problems with that statement (the Navy has never had nuclear battleships, and space travel is not at all like ocean travel), the question is whether we need nuclear space battleships at all.
The main reason we don’t launch NTRs today is that they’ve never been a technology that enables anything we’ve tried to do. Every time their proponents said something couldn’t be done without nuclear missiles, they were proven wrong. Nuclear warheads? With chemical missiles. Landing on the moon? With chemical missiles. Hunting Chinese interceptor satellites? In 2021, Russia destroyed the satellite using a chemical-powered rocket launched from Earth.
Giant space tugs shuttling between Earth, the moon, and Mars? Our need for them remains an open question. Whether we’ll ever need nuclear space battleships to keep them safe is even more remote. But some of the people involved are clearly thinking long-term.
“DRACO has really great potential for the future, for the world. It could really open up something. It's the beginning of a journey that maybe your grandchildren will complete. We hope to make history,” Shireman said.
Translation: Alexander Tarlakovsky (blog tay-ceti)
Original: We're building nuclear spaceships again – this time for real
