Nuclear Reactors in space

[Branjoman]

Hello, I was thinking about all these controversies related to things like VASIMR's or PPT's and their quite ludicrous power usages. Would having a small nuclear reactor onboard provide the power necessary? How much I know that all the radiation shielding on a manned craft, combined with the low thrust on these kinds of engines render them impractical, but surely on an unmanned craft, none of that is strictly necessary - a small shielding would be necessary to protect rom micrometeorites, but that's it, right? Do safety issues or guidelines prohibit an unshielded nuclear reactor from being in Earth orbit, from fear of it deorbiting and spreading radiation?

[Bill Phil]

It'd probably be better to just use NTRs.

[RainDreamer]

I don't think we have safety guidelines or such relating to that. There are laws preventing WMD in space, but not using nuclear power for other purposes. They did that sometimes in the past too:

https://en.wikipedia.org/wiki/Kosmos_954

That resulted in huge clean up operation, and the USSR had to paid C$3 billion for Canada because of that.

Later designs include a core ejection mechanism to shoot the radioactive core to a safety disposal orbit in case of failure. Still, I don't think people have been using nuclear power again for fear of being hit massive lawsuits.

[peadar1987]

Unmanned probes are generally in no particular hurry, so don't need large amounts of thrust. They can get on just fine with miniscule thrust provided by a standard ion engine powered by solar panels, or use energy-efficient, but slow, gravitational slingshots. An unshielded nuclear reactor would probably be more trouble than it was worth for things like this, unless you were doing something crazy, like a Sedna orbiter, which would need lots of thrust, and delta-V, all being called upon in the dark outer expanses of the solar system where solar panels are all but useless.

[Streetwind]

Hello, I was thinking about all these controversies related to things like VASIMR's or PPT's and their quite ludicrous power usages. Would having a small nuclear reactor onboard provide the power necessary? How much I know that all the radiation shielding on a manned craft, combined with the low thrust on these kinds of engines render them impractical, but surely on an unmanned craft, none of that is strictly necessary - a small shielding would be necessary to protect rom micrometeorites, but that's it, right? Do safety issues or guidelines prohibit an unshielded nuclear reactor from being in Earth orbit, from fear of it deorbiting and spreading radiation?

Your questions are difficult to answer because nuclear technology is politically "unpopular" and thus doesn't have any funding or manpower worth mentioning assigned to development. There are/were some smaller programs like NASA's Safe Affordable Fission Engine program, running on discretionary leftovers of the lab's funding, but as far as I know nobody has made a real effort to research and launch these since the... uuhh... 70's? I'm not exactly sure when the Russians decided to stop.

[Kryten]

Nobody's had a mission that's needed a nuclear reactor for decades, though. The Russians only used them because of the huge power requirements of first-gen radar satellites, nowadays you can fit the same capability in a solar-powered bird of a couple hundred kg.

[Streetwind]

That doesn't invalidate my point though, does it?

[Kryten]

That doesn't invalidate my point though, does it?

What, you want another reason why people wouldn't bother to develop something they don't need in the first place? By the way, your initial point is incorrect; NASA had an active effort to launch a nuclear reactor as part of the JIMO mission in the mid-2000s. They only stopped it because somebody pointed out expecting $20 billion for a planetary mission was insane, not because of the scary nuklears.

[cantab]

There's no technological reason a perfectly well shielded nuclear reactor couldn't be flown in space. Powering some sort of "ion drive" it would be more efficient and probably safer than a nuclear thermal rocket, and better for an interplanetary trip too because ion drive propellant doesn't boil off like liquid hydrogen. In my view it's probably the way to get people to Mars and back.

The barriers are political, which means I'm not allowed to discuss them on Squad's forums.

[peadar1987]

There's no technological reason a perfectly well shielded nuclear reactor couldn't be flown in space. Powering some sort of "ion drive" it would be more efficient and probably safer than a nuclear thermal rocket, and better for an interplanetary trip too because ion drive propellant doesn't boil off like liquid hydrogen. In my view it's probably the way to get people to Mars and back.

The barriers are political, which means I'm not allowed to discuss them on Squad's forums.

There are also technical problems with dumping large amounts of waste heat into space, making large space-based nuclear reactors very heavy, and diminishing the TWR.

[Kryten]

The barriers are political, which means I'm not allowed to discuss them on Squad's forums.

I'm sorry, but if you want to blame the lack of a mars mission/a moonbase/insert other sf fantasy here exclusive on anti-nuke sentiment, then you're at best completely unaware of the costs involved involved and how budgetary decisions get made in the space program, and at worst a conspiracy theorist. It's the exact same impulse that leads to somebody to blame the 'ignorant anti-nuclear public' for everything as for somebody to blame the reptilians for all the world's ill; it makes a complicated situation into a simple one, and makes you into somebody's who Seen The Truth and is above the Sheeple. It's ....ing tiring.

[cantab]

the costs involved involved and how budgetary decisions get made in the space program

The budget of government space agencies is a political topic itself, therefore according to Squad not a subject for discussion here. No conspiracy theories needed.

[Laie]

Back to topic:

RTGs are simple and reliable (look mommy, no moving parts) but inefficient.

The next step up would be some sort of engine that uses the heat to expand gas and drive a proper generator (probably something closed-cycle like a stirling engine). More weight, more possible failure modes, but also a lot more efficient. If you need enough power, the efficiency gains will offset the weight for the engine. I guess this might be in the area of a few kW. Still, I wouldn't expect these to keep working for decades like RTGs do.

A proper reactor is another huge step up in terms of extra mass and complications. I don't think it's a question of "can it be done", but more a question of "what for?"

[shynung]

A proper reactor is another huge step up in terms of extra mass and complications. I don't think it's a question of "can it be done", but more a question of "what for?"

^This. If it's to power something like VASIMR, or activities that require a lot of energy (say, ISRU refinery), then it's probably worth sending a reactor along. Though, if it's just for deltaV, a nuclear thermal rocket can do the job just as well.

[cantab]

There are also technical problems with dumping large amounts of waste heat into space, making large space-based nuclear reactors very heavy, and diminishing the TWR.

All valid concerns but not insurmountable ones. Full-blown nuclear reactors have already flown and operated in space, though I believe most used solid-state methods to turn heat into electricity a la RTGs, rather than turbines or other moving parts. The Russian Topaz reactor massed 320 kilograms and produced 10 kW of power. I don't know what their shielding was like, but considering radiation is an issue for electronics and they needed to be fitted onto a rocket on the ground I'd be surprised if they were totally unshielded. Scaling up may be a challenge and mass will probably be the major limit on available reactor power, but ultimately we're talking about proven technology here.

As for TWR, well ion drives already have low TWR. Interplanetary journeys are long anyway giving the drive enough time to do its work. NASA's Dawn probe was capable of around 10^-5g acceleration and it spent around half its travels coasting, though it wasn't in a hurry, but a sensible duration Earth-Mars transfer still only needs a TWR around that magnitude. It's a bigger issue for Moon trips or if you want to make an "express" transfer to another planet.

[Streetwind]

RTGs are simple and reliable (look mommy, no moving parts) but inefficient.

The next step up would be some sort of engine that uses the heat to expand gas and drive a proper generator (probably something closed-cycle like a stirling engine). More weight, more possible failure modes, but also a lot more efficient. If you need enough power, the efficiency gains will offset the weight for the engine. I guess this might be in the area of a few kW. Still, I wouldn't expect these to keep working for decades like RTGs do.

Such a thing exists: https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope_generator

...or rather, existed. It was cancelled in 2013 due to cost overruns. Which is a shame, considering it posts more than four times the conversion efficiency of a classic thermocouple RTG. But hey, considering the work is very recent, it is probably very well electronically documented and can be picked up again at a later date.

[magnemoe]

^This. If it's to power something like VASIMR, or activities that require a lot of energy (say, ISRU refinery), then it's probably worth sending a reactor along. Though, if it's just for deltaV, a nuclear thermal rocket can do the job just as well.

An reactor and vasimr has higher isp but lower trust.

Almost any manned mars missions involve nerva or reactors. its also pretty required for an moon base.

Some larger unmanned missions also require reactors.

All of this is expensive projects,

[shynung]

An reactor and vasimr has higher isp but lower trust.

Almost any manned mars missions involve nerva or reactors. its also pretty required for an moon base.

Some larger unmanned missions also require reactors.

All of this is expensive projects,

To be fair, going to space in the first place is already an expensive business. Most of the costs would sink into lifting all that equipment, reactor, fuel rods, and everything else out of Terra's gravity well. That, and maybe R&D.

[Requia]

There's no technological reason a perfectly well shielded nuclear reactor couldn't be flown in space. Powering some sort of "ion drive" it would be more efficient and probably safer than a nuclear thermal rocket, and better for an interplanetary trip too because ion drive propellant doesn't boil off like liquid hydrogen. In my view it's probably the way to get people to Mars and back.

The barriers are political, which means I'm not allowed to discuss them on Squad's forums.

There are no real political barriers, except to the extent that politicians screw with NASA in general, Prometheus (a full project to create a 200kWe nuclear spacecraft) was killed in order to fund other NASA projects, not because people objected to the reactor. It's quite a bit safer than RTGs, so anybody who tried to object would look like a fool once its pointed out that if the reactor crashes into the ocean, exactly nothing would happen if it hasn't been turned on yet.

[Nothalogh]

Hello, I was thinking about all these controversies related to things like VASIMR's or PPT's and their quite ludicrous power usages. Would having a small nuclear reactor onboard provide the power necessary? How much I know that all the radiation shielding on a manned craft, combined with the low thrust on these kinds of engines render them impractical, but surely on an unmanned craft, none of that is strictly necessary - a small shielding would be necessary to protect rom micrometeorites, but that's it, right? Do safety issues or guidelines prohibit an unshielded nuclear reactor from being in Earth orbit, from fear of it deorbiting and spreading radiation?

So long as you and your nation have the stones to tell the oxygen thieves to go get bent, it's only a question of thrust

[jwenting]

There are no real political barriers, except to the extent that politicians screw with NASA in general, Prometheus (a full project to create a 200kWe nuclear spacecraft) was killed in order to fund other NASA projects, not because people objected to the reactor. It's quite a bit safer than RTGs, so anybody who tried to object would look like a fool once its pointed out that if the reactor crashes into the ocean, exactly nothing would happen if it hasn't been turned on yet.

wrong. It's "nuclear", therefore the environmental and anti-nuclear lobby are going to go ape and bombard the population with doomsday stories about nuclear explosions from failing rockets, which is going to cost anyone approving the funding to lose a lot of votes, which means it never gets funding.

And if a private company were to attempt for a launch permit you'd get the same thing, and thus no launch permit.

We may know it's nonsense, (most of) the politicians and activists may know it's nonsense, but that doesn't mean it's not going to happen (as we know, it's happened numerous times in the past).

[magnemoe]

To be fair, going to space in the first place is already an expensive business. Most of the costs would sink into lifting all that equipment, reactor, fuel rods, and everything else out of Terra's gravity well. That, and maybe R&D.

it it, and like in KSP you only need nerva for larger payloads, same goes for vasimr and reactors.

Development costs is also an factor, if you have one mission who need the reactor you have to bundle the reactor development cost into that mission.

[cryogen]

The Russian Topaz reactor massed 320 kilograms and produced 10 kW of power. I don't know what their shielding was like, but considering radiation is an issue for electronics and they needed to be fitted onto a rocket on the ground I'd be surprised if they were totally unshielded.

I think that's just the reactor core. The total system mass was 980 kg, if this source is accurate:

https://books.google.com/books?id=0D7OBQAAQBAJ&pg=PA425&lpg=PA425

Shielding only needs to cover a tiny angle, in the direction of the satellite payload.

[Streetwind]

Ooooh, interesting. I always wondered what radiators would weigh.

So this is what... just over four kg of radiator mass per kW of waste heat? Yes, that number includes the shielding, but for simplicty's sake, let's call it 4 kg per kW of waste heat.

That would put NASA's SAFE-400 test reactor at 1,200 kg radiator mass since it has 300 kW waste heat to reject. And a total system mass of ~1,750 kg, which presumably includes shielding already, as I didn't bother subtracting it above. Since it produces 100 kW of power, that would put it at 57.14 W/kg, or if you prefer, 17.5 kg/kW.

What does solar offer by comparison? Well, a wikipedia quote from around 2005 says that arrays with 300 W/kg are available. A convenient data point, since the SAFE-400 is also from the early 2000's. That's rather puzzling, isn't it? It suggests that, as long as physical size is not an issue, solar cells from ten years ago will significantly outperform our best attempt at a space grade nuclear reactor even as far out as Mars, where roughly 43% of sunlight is available (~130 W/kg with the mentioned arrays). It outperforms it by such a large margin that even if modern technology flat-out halved the weight of the radiators compared to the TOPAZ system, the reactor would still not be anywhere near competitive at Mars.

Considering how nuclear reactors are always touted as the wonder solution to electric propulsion, I always end up really puzzled whenever I look at the numbers like this, and I wonder what the rest of you are thinking. Am I missing something here?

[Darnok]

wrong. It's "nuclear", therefore the environmental and anti-nuclear lobby are going to go ape and bombard the population with doomsday stories about nuclear explosions from failing rockets, which is going to cost anyone approving the funding to lose a lot of votes, which means it never gets funding.

And if a private company were to attempt for a launch permit you'd get the same thing, and thus no launch permit.

We may know it's nonsense, (most of) the politicians and activists may know it's nonsense, but that doesn't mean it's not going to happen (as we know, it's happened numerous times in the past).

What? We can't send it in two parts?

First send reactor and second fuel, just like we are sending crew with safe capsule and abort systems, and then put fuel into reactor in safe orbit?