Rolls-Royce Holdings is getting into the spaceflight industry. The British aerospace engineering company says it's developing a micro-nuclear reactor that the company hopes could be a source of fuel for long trips to the Moon and Mars.
As humanity begins to venture back into space, with crewed missions scheduled to visit the Moon and Mars within the next two decades, the technology that moves us throughout the solar system will be a pivotal part of that journey. Last week, Rolls-Royce teased the design of its Rolls-Royce micro-reactor for spaceflight with a digital mockup posted to Twitter last week.
[...] Rolls-Royce Holdings announced in 2021 its intent to develop nuclear reactor technology, having obtained $600 million in public and private funding to develop its business. Since the nuclear reactor won't have to carry as much fuel as a chemical propulsion rocket, the entire system will be lighter allowing for faster travel or increased payloads. The company says that the reactor could serve as both a new form of propulsion and a power source for bases on the Moon or Mars, and Rolls-Royce claims that they will have a nuclear reactor ready to send to the Moon by 2029.
Rolls-Royce is not the only party working on rocket propulsion outside of traditional chemical fuel. NASA and the Defense Advanced Research Projects Agency announced a collaboration to develop a thermal rocket engine that could improve the time it takes to get to deep space. Likewise, NASA had a successful test of a rotating detonation rocket engine, which uses less fuel and provides more thrust than current propulsion systems.
[...] A previous version of this article stated that Rolls-Royce is entering the nuclear reactor business. While this is the company's first public effort at space-based nuclear reactors, it has been supplying submarines with small reactors since the 1960s.
(Score: 2) by Fluffeh on Friday February 10, @01:37AM (12 children)
So, clearly I've missed something obvious, but my understanding was that chemical rockets used the spent fuel as the propulsion mass driver. Engine burns fuel, points burning fuel out the back at high speed, making thrust, pushing rocket forward.
With this, there's a source of good power, but what is the mass that is expelled out the back end of the rocket to create the thrust itself, or is this article more along the lines of saying "We've got a new power source for throwing any old shit out the back for thrust!" ?
(Score: 3, Informative) by Anonymous Coward on Friday February 10, @01:46AM (1 child)
You still need a fuel. In nuclear rockets you're using the heat to heat a gas to high temperatures before it get expelled. Typical designs use liquid hydrogen. The nuclear core gets really really hot and you pass the liquid through that.
(Score: 3, Interesting) by Fluffeh on Friday February 10, @02:24AM
Awesome, that makes more sense, and I was having one of those "What the heck am I missing" moments. Good chance I wasn't the only one after reading the summary and article, so thanks!
(Score: 3, Informative) by khallow on Friday February 10, @02:37AM (9 children)
Your last part is correct. This is a power source for high power propulsion like electric propulsion. I see no indication that they intend to heat up a propellant directly with the waste heat from the reactor. Typical propellants for electric propulsion include mercury vapor, xenon, and hydrogen - anything that is easy to ionize by stripping off one or more electrons. What makes it better than chemical propulsion is that the exhaust velocity is much higher and hence, for a given delta-v, one needs a much smaller portion of the overall vehicle's mass devoted to propellant.
A catch here is that these would be used in space, not in getting from a planetary (or lunar) surface to space. You still need a way to get into orbit.
(Score: 4, Interesting) by ElizabethGreene on Friday February 10, @02:32PM
That's exactly what they're doing in the Nuclear Thermal project. They replace the combustion chamber of a chemical rocket with a critical fission reactor core. The reaction mass flows into the reactor, gets obscenely hot, expands in a De Laval nozzle, and yields an engine with a specific impulse about triple that of a chemical rocket. The downside is the engine, nozzle, and exhaust are all radioactive.
(Score: 5, Informative) by Immerman on Friday February 10, @05:59PM (7 children)
Apparently Rolls-Royce are working on *both* nuclear-thermal rocket engines *and* space-worthy nuclear power reactors, which no doubt leads to a lot of not-entirely-clear articles about them.
They both have their place.
High-power nuclear-electric propulsion has a serious heat problem: thermodynamics laws mean that even in the best case scenario, for every one MW of electricity you generate, you need to shed around two MW of waste heat at the lowest temperature possible (the higher the temperature, the less electricity you get per unit of waste heat). And since you're in space, the only way to shed heat is via infrared radiators. Massive radiators for that kind of power - I believe much larger than those on the ISS.
Meanwhile, using our most powerful ion thrusters, 1 MW of power can only deliver about 5N (1.1lbf) of thrust. It can keep it up for a lot longer with a given mass of propellant, eventually reaching much higher speeds, but you've also added all the fixed mass of electrical generators and radiators that reduce your mass budget for other things, like propellant.
For comparison, a single Falcon 9 Merlin engine produces about 1.4 GW of power and 900kN (200,000lbf) of thrust, and has no serious cooling issues because most of the heat is carried away by the propellant, and the remainder can be easily and efficiently radiated away at temperatures high enough to make the rocket bell glow white-hot. (radiative power increases with the 4th power of absolute temperature - so even just twice the temperature translates only 1/16th the required surface area to shed the same amount of heat)
A nuclear thermal rocket has all the cooling advantages of a chemical rocket, comparable thrust, and considerably higher specific impulse to drastically reduce the propellant needed for a given flight profile. Nowhere close to nuclear-electric thrusters, but it offers MUCH higher thrust with a MUCH lower up-front mass and price penalty, since you're eliminating all the heavy, complicated radiators, electric generators, and electric thrusters with exotic propellants.
And of course, if you're concerned with making the trip as fast as possible because you're carrying passengers or other goods that are being damaged by cosmic radiation for the duration of the trip, then you want as much thrust as possible delivered as quickly as possible.
There's still a tipping point where nuclear-electric's long duration low thrust will win the race - but it's probably somewhere out past the asteroid belt. At least until we develop better nuclear reactors that don't convert the energy to heat as an extremely inefficient intermediate step. (e.g. one of the big draws of proton-boron fusion is that it should be relatively easy to convert most of the reaction energy directly to electricity)
(Score: 2, Interesting) by khallow on Saturday February 11, @02:13AM (4 children)
There are other electric propulsion thrusters that deliver better thrust, such as hall effect thrusters and VASIMR (two historical examples with working prototypes).
It would still need shielding mass. I know about these advantages of nuclear thermal propulsion, but I also know a bunch of disadvantages:
(Score: 3, Informative) by Immerman on Saturday February 11, @03:30PM (3 children)
No, thermal radiators can't double as reactor shielding - you need vast surface area for radiators, football fields worth for any serious power. As thin as possible to minimize the mass penalty, and positioned so that they can't shine on each other. While for reactor shielding you want a small, dense concentration of hydrogen-rich mass directly in front of the reactor, with a footprint just large enough to keep the passenger portion of the ship in its neutron shadow. Something which can be made easier with a scaffolding separating the engine and passenger portions of the ship - with empty space both diffusing the radiation hazard (inverse square law), which reduces the thickness of shielding needed, as well as shrinking the diameter of the shielding disc needed to keep the passenger area in its shadow. (the highly radioactive reactor chamber itself is usually quite small) an
Both types of nuclear ships use reactors - the question is only whether the heat from the reactor is used to super-heat propellant, or heat a working fluid to drive a heat engine to generate electricity to power electric thrusters. I'm sure there are some always-on nuclear thermal engine designs, but most are much more controllable - they have to be or they'd be useless for anything but high-power single-use launches.
And both types can be designed for operation as close or far from the material limits as desired. Operating nuclear thermal rockets near the material limits will give the best specific impulse, but just like with chemical rockets the associated risk of engine damage is not necessarily worth it, especially in a highly reusable scenario - which is where almost everything looks to be heading (finally!).
Generator-reactors may be multi-purpose in theory - but in practice they're really only good for powering the ship they're mounted on. A ship that won't have the power to land or take off even if you were willing to risk an atmospheric reactor breach. They're going to be too radioactive to want to try to remove from the ship (especially since you don't want to waste ship mass shielding the rear or sides), meaning that repurposing them is going to mean retiring the ship. In almost all cases you'd be better off just bringing a second, fully fueled and minimally shielded reactor to install on-site, and then use local materials for shielding before you start it up the first time and it becomes significantly radioactive.
Finally, a quick search says we don't actually have any experience with fission reactors in space, only with RTGs, which are a completely different beast. It sounds like NASA's KiloPower reactor may be the first operational prototype we've created, and it doesn't sound like its actually powered a mission yet.
(Score: 1) by khallow on Sunday February 12, @01:36AM (2 children)
That's my point. You can do better than that because you don't need to minimize the mass penalty to that degree. The shielding and at least part of the radiator occupy the same space. Might as well dual use where they overlap.
Any thermal engine design is an always-on when you're in the right failure mode, say if a micrometeorite takes out your ability to control it. This has long been a big concern with any vehicle which is propelled directly by a fission reactor - what happens when you can't turn it off? Space is relatively safe for this sort of problem since you can point such a vehicle in a direction away from the inner Solar System and just let it go. There have been some scary Earth-side designs (nuclear powered cruise missiles and airplanes, for example) where a key problem was what happens if the vehicle can't be stopped, ends up flying in the air for weeks, and crashing in some random part of the globe.
Here's one example [wikipedia.org]. Sounds like it's one of a series of 33 nuclear powered satellites (31 using the BES-5 reactor [wikipedia.org] and 2 the TOPAZ reactor [wikipedia.org]), not all which made it to orbit (2 launch failures, according to Wikipedia).
(Score: 2) by Immerman on Sunday February 12, @04:04PM (1 child)
Every gram of shielding or radiator translates directly to lower acceleration for the ship, so you still want to minimize it. Your design goal is to reach a particular acceleration for your payload, additional mass means you need a more powerful engine and reactor, which requires more shielding and radiators... The rocket equation doesn't just include the propellant, it also includes the mass of the engine, reactor, etc. Which for an electric power generating facility can add up fast.
Except they don't.
The shielding is likely to need to be several feet thick, and ideally covers only a few square feet - maybe a few square yards, directly between the reactor and the passenger segment of the ship. Shuttles need to just not get anywhere near the glowing end - easy enough when the complexities of orbital mechanics mean you have to micro-manage your approach anyway.
The radiators meanwhile must cover football fields worth of flat-or-convex area with nothing in front of them. Geometry dictates that only a fraction of a percent can be anywhere near the reactor, and they'd be useless between the reactor and passenger module (where the shielding is) since the heat they're trying to radiate to space would instead be absorbed by the passenger module and radiated right back to them.
If you want to service the reactor it's likely much more mass effective to have a heavily shielded servicing suit or vehicle rather than wrapping the whole reactor in radiation shielding. Especially since you'll want that anyway to be able to service the reactor internals if something goes seriously wrong.
That's just as true for a reactor based failure. In which case you have to leave the engines on as well to consume the power and avoid a meltdown.
A severe reactor failure on a nuclear powered spaceship almost certainly means all hands lost simply because a rescue mission to intercept and return them isn't feasible. The details beyond that really don't matter much.
Awesome, thanks! I stand corrected.
(Score: 1) by khallow on Sunday February 12, @07:29PM
So what? You have to have those grams for a viable mission. My point remains that you can dual purpose a lot of grams this way.
(Score: 1) by khallow on Saturday February 11, @03:01AM (1 child)
There's a lot more exotic propellants out there in the human-accessible world than there is hydrogen. You have to go past the Asteroid Belt before water ice (the most common ore of hydrogen there is) is common on the surface of small asteroids. Oxygen OTOH is quite common. It will work in a ion drive, for example, despite heavy corrosion of exposed metal parts. But running oxygen as propellant through a nuclear rocket would be extremely dangerous.
(Score: 2) by Immerman on Saturday February 11, @02:48PM
Oxygen's ubiquity would indeed make it an excellent propellant if we could keep it from eating the engines - and not just the metal parts, just about *everything* corrodes rapidly in the face of hot oxygen. Even if an ion thruster doesn't offer the same risk of catastrophic failure I suspect it'd be hard to keep the thing running for more than a month or two - barely enough to start most missions where ion drives would be useful.
Huh... the frost line really is beyond the asteroid belt (arguably at least, sounds like it's still under debate). I figured obviously not, because Ceres, but it sounds like ice becomes more stable on larger objects.
It sounds like the most common C-type asteroids are still expected to be rich in hydrated minerals, but that's likely a lot more difficult to mine than free ice.