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Elthy

How dangerous is a NERVA during its lifetime?

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What are you talking about? People store hydrogen long term all the time. It's really easy. You just have it in a sealed metal tank. It must be completely sealed, with the valves exiting the tank the kind that don't leak. (I'm not an expert on valve composition, you could just solder the valve itself if you had to)

Hydrogen likes to leak but it can't go through a solid metal wall. Then, you mount a cryocooler - NASA has tested some on the ground - on the tank and power it with solar panels, RTGs, or in the case of a NERVA rocket, you have a way to generate power using the same nuclear reactor you can use as an engine. (there are heat exchanger loops that go through the reactor core). The cryocooler recondenses the hydrogen vapor so the pressure inside the tank doesn't rise to the point it explodes.

Your monthly losses will be vanishingly close to zero. Why hasn't NASA flown something like this? Actually, they have, how do you think IR telescopes work?

Hydrogen can diffuse through solid walls. This can be mitigated with cryocoolers as you mentioned, hence the "zero boil-off", but even there there are some limitations. For one thing there are non-condensible effects problems due to thermal stratification, Marangoni convection and insulation from the Knudsen layer that are unique to microgravity.

Alas zero boil-off technologies are still at low TRL and are a big part of the reason NASA probably isn't going to be flying any missions with NTR in the near future. The old studies for NERVA in the 1970's handwaved the problem of long-term storage of cryogenics in space away. These days we understand a bit more clearly that the problem is not a super easy one. The IR telescopes you mention use 30-80K cryocoolers, or helium vapor cooling (not cold enough to condense hydrogen at realistic tank pressures for NTR). The current state-of-the-art 18K space rated crycooler can only remove something like 2W, which is not nearly enough. The first real technology demonstrator mission for zero boil-off, CRYOTE, was not funded by congress in 2014. This is why NASA is moving in the direction of hypergolics/SEP (and away from anything requiring long-term storage of cryogenics) for future manned exploration missions beyond LEO.

Edited by architeuthis

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Hypergolics for a manned mission to mars? They will be flying up the fuel for years until they have enough...

If i remember it right you need about 6km/s to get from LEO to low Mars orbit without aerobraking. With an hypergolic engine with an ISP of 316 (Shuttle OMS) you would need 6 times the payload as fuel, with H/Lox only 3 times. Using a nuclear thermal rocket with an ISP of 900 it would reduce to to only once, you would need as much hydrogen as your dry wheight.

Im realy interested how SpaceX mars colonial transporter tries to slove this problem. Even a quick, reusable launcher couldnt get enough hypergolics into orbit to allow for more than a single exploration trip.

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How exactly would you get tritium in the exhaust ? Superheated hydrogen isn't exactly a very good at capturing thermal neutrons...

Good point. There would most likely be slightly elevated levels of tritium, and possibly some bits of the fuel, but not much else.

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What are you talking about? People store hydrogen long term all the time.

and it leaks.

It's really easy. You just have it in a sealed metal tank. It must be completely sealed, with the valves exiting the tank the kind that don't leak. (I'm not an expert on valve composition, you could just solder the valve itself if you had to)

Hydrogen likes to leak but it can't go through a solid metal wall.

Are you sure you aren't thinking about some other gas? Hydrogen not only leaks through solid metal, it turns most solid metal walls brittle. It might not leak very fast, but it will be a few years before the planets are aligned for a return trip.

Then, you mount a cryocooler - NASA has tested some on the ground - on the tank and power it with solar panels, RTGs, or in the case of a NERVA rocket, you have a way to generate power using the same nuclear reactor you can use as an engine. (there are heat exchanger loops that go through the reactor core). The cryocooler recondenses the hydrogen vapor so the pressure inside the tank doesn't rise to the point it explodes.

Your monthly losses will be vanishingly close to zero. Why hasn't NASA flown something like this? Actually, they have, how do you think IR telescopes work?

A quick googling of

"IR telescope hydrogen cooling" and "IR telescope helium cooling" seems to imply they typically use helium (and your above comments apply better to helium). The Hershel Space Observatory used helium for just this use, except that it ran out of He after 4 years. The James Webb project looks like it will run with hydrogen cooling, but not so much before then (and appears to have moved to H2 due to insufficient rare He isotopes). Reading a paper on its cooling makes keeping a H2 tank cool in Martian orbit seem considerably easier: three parasols will keep the tank at 40K (at 1AU, hopefully a bit less out at Mars) and presumably you would only need a peltier to cool it down somewhere between 22-33K (the James Webb telescope needs sub-Kelvin. NERVA hardly does). Nevertheless, it involves a lot of dry mass to haul to Mars. And it will still leak.

http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pdf [paper on James Webb cooling]

http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf [NASA 2009 paper that details plans for NERVA trip to Mars. Admits that no zero-boiloff H2 containers existed in 2009.]

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Hypergolics for a manned mission to mars? They will be flying up the fuel for years until they have enough...

If i remember it right you need about 6km/s to get from LEO to low Mars orbit without aerobraking. With an hypergolic engine with an ISP of 316 (Shuttle OMS) you would need 6 times the payload as fuel, with H/Lox only 3 times. Using a nuclear thermal rocket with an ISP of 900 it would reduce to to only once, you would need as much hydrogen as your dry wheight.

Im realy interested how SpaceX mars colonial transporter tries to slove this problem. Even a quick, reusable launcher couldnt get enough hypergolics into orbit to allow for more than a single exploration trip.

It's a hybrid hypergolic/SEP system as I said. Most of the delta-v is provided by SEP. I saw several presentations on this from folks at NASA GRC, Langley, and JPL at AIAA's 2015 Space conference. This is basically the current thinking on what's achievable in terms of manned Mars exploration with the realistic constraints of no budget increases, and only 2 SLS flights per year on average.

If you're interested here is some more information:

https://www.nasa.gov/sites/default/files/files/20150408-NAC-Crusan-EMC-v7a.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150009466.pdf

Edited by architeuthis

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One dangerous thing about NERVA is that while it can take you to distant planets, nobody really knows how to get it to bring you home.

NERVA uses hydrogen. It has to, otherwise you don't get enough exaust velocity to bring your ISP up to sufficient reasons to use NERVA (presumably helium *should* work, although much less efficiently: but I think you would need to crank up the temperature to chemical rocket levels. My guess is that things [i.e. the nuke parts] would get *really* dangerous at that point.) Hydrogen leaks. Hydrogen also typically boils off: the X15 + B52 carried 2 and a half tanks worth of H2 for the X15. One tank to fly with and the other tank and a half to refill the tank as it boiled off. Current NASA plans to Mars include a "zero boiloff" H2 tank, but so far no such thing exists (presumably you "just" need to keep the tank under 33K (H2's critical point)). Even if you do that, expect to lose at least 1% of your H2 each month. That Isp of 800 looks at lot less good when you suddenly have a lot less fuel.

Solar shields would help quite a bit.

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and it leaks.

Are you sure you aren't thinking about some other gas? Hydrogen not only leaks through solid metal, it turns most solid metal walls brittle. It might not leak very fast, but it will be a few years before the planets are aligned for a return trip.

A quick googling of

"IR telescope hydrogen cooling" and "IR telescope helium cooling" seems to imply they typically use helium (and your above comments apply better to helium). The Hershel Space Observatory used helium for just this use, except that it ran out of He after 4 years. The James Webb project looks like it will run with hydrogen cooling, but not so much before then (and appears to have moved to H2 due to insufficient rare He isotopes). Reading a paper on its cooling makes keeping a H2 tank cool in Martian orbit seem considerably easier: three parasols will keep the tank at 40K (at 1AU, hopefully a bit less out at Mars) and presumably you would only need a peltier to cool it down somewhere between 22-33K (the James Webb telescope needs sub-Kelvin. NERVA hardly does). Nevertheless, it involves a lot of dry mass to haul to Mars. And it will still leak.

http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pdf [paper on James Webb cooling]

http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf [NASA 2009 paper that details plans for NERVA trip to Mars. Admits that no zero-boiloff H2 containers existed in 2009.]

Helium is colder than liquid hydrogen, although it has been proposed to use "hydrogen slush" (14 kelvin), which is still warmer than the temperature of liquid helium.

I don't see how hydrogen slush can diffuse through metal walls as there is no gas phase.

The case of NERVA, you will have hundreds of kilowatts, even megawatts of power available. You can afford to run some very energy hungry cryocoolers that NASA hasn't bothered experimenting with. I don't perceive this to be a problem. (I mean, it's an engineering requirement, but if you have the resources to build an interplanetary NERVA rocket in orbit you can probably pay the people smart and skilled enough to solve it)

Edited by SomeGuy12

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Im not sure about the SEP plans of Nasa. They need several tons of Xenon to get to Mars, but Xenon is the rarest non radioactive element on earth. This may work for a few missions, but not for something like the frequent mars transfers SpaceX plans to do.

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I just read Voyage by Steven Baxter. (FANTASTIC book BTW, highly recommend it. Not the best Alt History/Mars mission book, but still amazing, and I feel as if it's under-rated.) And there are two things that kind of confuse me.

1.) When testing the engine in the beginning he said you shouldn't cluster multiple engines. Is that true? Granted most the Saturn Nerva designs I've seen have one engine on the S-IVB. But still.

2.) If the reactor core ruptures, would it be a danger to the crew (Granted I know that anyone who is close to a reactor core would be in danger but how badly and->)? Because how would it get in?

1.) Yup, neutrons produced on one reactor would spill into the others nearby, throwing a big spanner in the works of a carefully controlled chain reaction based on controlling neutron flux.

2.) Really hot nuclear waste would be spilled. As soon as it left the shadow of the shield, it would start frying all susceptible materials (people, computers) without anything to stop it. I.E: real bad. Fortunately, other than a deliberate explosion/blockage of the hydrogen lines, I don't see how that would happen, if the reactor overheated it should melt, not explode, and any fragments spewed out the back, where they are still in the shadow on the shield until distance makes the shield redundant. Flux goes down with 3/(4*Pi*r^3), so at ten times the distance you get a thousand times less radiation.

One dangerous thing about NERVA is that while it can take you to distant planets, nobody really knows how to get it to bring you home.

NERVA uses hydrogen. It has to, otherwise you don't get enough exaust velocity to bring your ISP up to sufficient reasons to use NERVA (presumably helium *should* work, although much less efficiently: but I think you would need to crank up the temperature to chemical rocket levels. My guess is that things [i.e. the nuke parts] would get *really* dangerous at that point.) Hydrogen leaks. Hydrogen also typically boils off: the X15 + B52 carried 2 and a half tanks worth of H2 for the X15. One tank to fly with and the other tank and a half to refill the tank as it boiled off. Current NASA plans to Mars include a "zero boiloff" H2 tank, but so far no such thing exists (presumably you "just" need to keep the tank under 33K (H2's critical point)). Even if you do that, expect to lose at least 1% of your H2 each month. That Isp of 800 looks at lot less good when you suddenly have a lot less fuel.

NERVA can run on methane or ammonia without issues, and it actually did in several tests. Even with plain water it would give out a higher Isp than the best chemical engine... although not by much (550s IIRC). With ammonia or methane you can get Isps that actually matter a lot... remember the Discovery in 2001 (the novel of course), run on ammonia-fueled NTR's. You don't need to go to 1000s (an NTR performance if built today, and we stop taking a first-generation engine as the only word in NTRs) to achieve things like Mars and back in a single stage, after all.

You know, I feel like flying these things around is like pointing a gun at everything in the vicinity from the way you talk about it. Can any of these ships even pass by each other? Docking is going to be a much more nerve wrecking procedure than it already is now that accidently pointing your craft to the wrong place can kill a bunch of people.

Maybe instead of a shadow shield we have to bite the bullet and shield it more?

Sitting on top of any rocket is freaking insane. At least NERVAs don't have failure modes that involve all of the contained energy in a stage being release at once... say like in a chemical rocket. [shudders]. Have you seen videos of what happens when a rocket blows up? Imagine that next to a fuel depot... Nope, any movement anywhere near another ship is done on dependable RCS. And you hope both hypergolic fuel lines don't choose to rupture at the same time...

A NTR, on the other hand, is intentionally a subcritical lump of enriched material that you make undergo fission with neutron reflectors. If you lose cooling for too long, it fails of course, but if so it basically melts and losses criticality fast, and without the mirrors the reaction stops immediately. The lumps of radioactive material won't have any sudden impulse to fling themselves around like they are some cheap solid rocket casing failing... though they will form a pretty nasty radioactive cloud you will want as far away from you as possible. And I hope you make sure that if the turbopump fails the pieces don't go through the core... moving parts have a habit of failing.

Rune. Handle with care, and use when and only when it makes sense.

Edited by Rune

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If the core ruptured, the crew likely wouldn't be in any danger due to radiation from the core material. Alpha and beta radiation would be stopped by a single sheet of tin foil. Gamma would be the real danger, but adding gamma radiation to space is like tossing a bucket of water in the ocean. The real danger is how are you getting home now that your engine has crapped the bed.

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Yeah. I think being stranded in space without power is a lot more dangerous than the radiation. And the problems with neutron shielding between NERVA cores, etc, makes it inconvenient to have much redundancy.

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NERVA can run on methane or ammonia without issues, and it actually did in several tests. Even with plain water it would give out a higher Isp than the best chemical engine... although not by much (550s IIRC). With ammonia or methane you can get Isps that actually matter a lot... remember the Discovery in 2001 (the novel of course), run on ammonia-fueled NTR's. You don't need to go to 1000s (an NTR performance if built today, and we stop taking a first-generation engine as the only word in NTRs) to achieve things like Mars and back in a single stage, after all.

Not sure about the mass of the other "fuels", but plain water won't give a higher ISP than hydrogen and oxygen. The reason is that both work by heating water vapor as hot as they can and then using that energy as exhaust velocity. Last I heard, not only did NTRs use lower temperatures than chemical rockets, the chemical rockets were limited by their temperature. No known metalurgy could handle higher temperatures. Obviously, there are densities between hydrogen and water, but expect it to be hard to match chemical ISPs with non-hydrogen NTRs.

Having a NTR come apart would be bad. First, assuming you still have RCS thrusters, max thrust away from the radioactive material flying in formation with you (it presumably failed well out of any atmosphere and will stay in motion...). Hopefully you can get far enough away (note that if the radioactive bits are flying away sideways that isn't much better. You have shielding facing back and much less shielding to the sides). After that, you aren't going home (at least not on your own). Possible rescue missions.

From start of Earth-Mars transfer but before Mars capture burn: probably the easiest rescue. Ship will be coming "back" to Earth's orbit (around the Sun). Rescue ship launches to a highly eccentric orbit around the Sun along the orbit. Flight might take months, but delta-V issues won't be much worse than the Moon. You will get in your Dragon (or Orion) capsule and you will be thankful.

From start of Mars capture but not completing it: Nasty case. All the problems and delta-v needed to get to Mars in the first place, without the Obereth effect to help in capture. If the ship winds up in Mar's orbit (around the Sun, but not orbiting Mars) I'm not even sure this is possible (without exorbitant delta-v. We simply can't do exorbitant delta-v).

From leaving Mars but not landing on Earth: life support failure and certain death. It might work in KSP, but only as long as you don't have life support mods. Pretty much any of these rescues will take months (or years) and only work if you use the mission's food/air/water to live. If you already used them, you die. Note that ignoring even life-support issues, if your plan included aerocapture back at Earth that means this is your last burn. In other words you won't even get a course close to Earth until the very last seconds of the burn. Plot a rescue ship going almost all the way to Mars orbit and then doing a rendezvous without Obereth. Maybe you might get the two transfer orbits to sync up like the first case, but I doubt it. Probably no way you could scrape up the delta-v for this one.

But yes, this is typically better than most chemical rocket failure modes. Apollo 13 was unbelievably lucky to survive long enough to tell Houston they had a problem (a chemical fuel tank exploded. Luckily it wasn't hypergolic ("just" hydrogen and oxygen) and didn't ignite).

[stan Love Astronautsplains Rocket Science (and why Mars is hard). Includes claim about current rockets hitting maximum ISP for water vapor]

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2.) Really hot nuclear waste would be spilled. As soon as it left the shadow of the shield, it would start frying all susceptible materials (people, computers) without anything to stop it. I.E: real bad. Fortunately, other than a deliberate explosion/blockage of the hydrogen lines, I don't see how that would happen, if the reactor overheated it should melt, not explode, and any fragments spewed out the back, where they are still in the shadow on the shield until distance makes the shield redundant. Flux goes down with 3/(4*Pi*r^3), so at ten times the distance you get a thousand times less radiation.

Well, I was thinking of safety requirements and possible accidents, and I believe that a NERVA is one of the few times that a positive void coefficient of reactivity is a really good idea. In an NTR, you are squirting liquid hydrogen slurry (or other hydrogen rich light materials) into the reactor, really cold and pure hydrogen makes a great neutron moderator, as you doubtless know. Should your reactor be designed with a negative void coefficient of reactivity for the coolant/propellant the reactor would experience a potentially quite pronounced spike in the neutron multiplication factor probably resulting in undue thermal stresses and possibly bringing it into the realm of prompt-criticality when you start pumping in the coolant after the unit has heated up. Whereas if the propellant were to say, absorb neutrons already moderated by graphite thus increasing the non-fuel absorption probability when the propellant is pumped in, you would have the reactor go subcritical and the reactor would cool slightly, but assuming you do use graphite, the thermal inertia of the core should mean that the drop is not that large. The reactor could be then slowly and safely brought to critical through control rod removal within a few minutes of starting the burn, generally lagging slightly behind the insertion of hydrogen coolant. It would also be advisable that the positive void coefficient of reactivity not be that large at all, else you might have some really big problems if a turbopump fails.

You might also consider having a negative void coefficient of reactivity and simply inserting the control rods to what amounts to a scram before inserting the propellant, however that also has a problem as it means that you will have to be a LOT more careful with the startup, where you want to ramp up reactor power at fine increments, but can have very little problem with shutdown, where fine control does not really matter.

So in other words, if you did not design it like a RBMK, you might have a safety issue. (Now that is one sentence I never thought I would say) Though, I wonder, would it be possible to conduct basic criticality testing on the core once in orbit before a burn to predict if a certain degree of reactivity will achieve criticality with the quantity of propellant flow through the core,

A NTR, on the other hand, is intentionally a subcritical lump of enriched material that you make undergo fission with neutron reflectors. If you lose cooling for too long, it fails of course, but if so it basically melts and losses criticality fast, and without the mirrors the reaction stops immediately. The lumps of radioactive material won't have any sudden impulse to fling themselves around like they are some cheap solid rocket casing failing... though they will form a pretty nasty radioactive cloud you will want as far away from you as possible. And I hope you make sure that if the turbopump fails the pieces don't go through the core... moving parts have a habit of failing.

Rune. Handle with care, and use when and only when it makes sense.

I do not get this part, I am not criticizing you, but I do not know what you mean that having a series of metal bits going through the core would be a bad thing beyond the traditional "damn, SCRAM the reactor" kind of response. Please explain it more as doubtless you have a reason as you know more than me on this matter.

Oh, and I have a question, considering your various statements on this matter, are you by any chance in a nuclear engineering field that involves reactor design or operation? If you are, can you tell me, in detail (if reasonable), of how wrong/right I was in my analysis of what kind reactor you would want for a NTR? Heck, even if you are not, tell me how wrong/right I was.

Edited by NuclearNut

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