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Short Lifetime For Nuclear Thermal Rockets In Space?


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What about the residue heat problem? The more the nuclear thermal engine runs the more nuclear waste builds up in it and the longer it takes to shut off and more fuel needs to be wasted to cool it down. 

It is not hard to calculate how much work one can get. 1 kg of plutonium can produce 83,610 GJ of thermal energy, I calculate that is roughly equal to boiling ~8500 tons of water to ~3500 K (~10 MJ/kg of water), hot enough to get over 450 s in Isp. Of course at least half of that energy is lost as inefficiency of a thermal engine, plus more to run pumps, lost radiation to space, etc, I would say certainly more then 3000 tons of water to >450 s Isp per Kg of Pu should be feasible. 

For hydrogen it much higher heat capacity per mass means you will "only" get 1671 tons of hydrogen to 3500 K at 100% efficiency, but that is an Isp of ~1050 s.    

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43 minutes ago, RuBisCO said:

What about the residue heat problem? The more the nuclear thermal engine runs the more nuclear waste builds up in it and the longer it takes to shut off and more fuel needs to be wasted to cool it down. 

It is not hard to calculate how much work one can get. 1 kg of plutonium can produce 83,610 GJ of thermal energy, I calculate that is roughly equal to boiling ~8500 tons of water to ~3500 K (~10 MJ/kg of water), hot enough to get over 450 s in Isp. Of course at least half of that energy is lost as inefficiency of a thermal engine, plus more to run pumps, lost radiation to space, etc, I would say certainly more then 3000 tons of water to >450 s Isp per Kg of Pu should be feasible. 

For hydrogen it much higher heat capacity per mass means you will "only" get 1671 tons of hydrogen to 3500 K at 100% efficiency, but that is an Isp of ~1050 s.    

 

For what it's worth, if nuclear can run for 24 hours without reprocessing, that is ALREADY longer than the amount of propellant it could reasonably carry anyway. Unless it runs even longer at low g acceleration...does it?

These are not torchship constant high thrust efficient engines, so the amount of propellant required to do that could only be manifest with lots of throwaway staging rockets.

 

But nonetheless it is very interesting to see that nuclear although it has its's merits, also has it's challenges....perhaps ones not previously considered much until now.

 

I certainly was not aware of all this...just a good guess...as I know IRL physics is perpetually on 'hard mode' LOL.

 

No free lunch. Have to work for every advantage gained and accept the challenges and overcome them however we can best do so to meet the challenge.

Edited by Spacescifi
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21 minutes ago, Spacescifi said:

For what it's worth, if nuclear can run for 24 hours without reprocessing, that is ALREADY longer than the amount of propellant it could reasonably carry anyway. Unless it runs even longer at low g acceleration...does it?

These are not torchship constant high thrust efficient engines, so the amount of propellant required to do that could only be manifest with lots of throwaway staging rockets.

But nonetheless it is very interesting to see that nuclear although it has its's merits, also has it's challenges....perhaps ones not previously considered much until now.

I certainly was not aware of all this...just a good guess...as I know IRL physics is perpetually on 'hard mode' LOL.

No free lunch. Have to work for every advantage gained and accept the challenges and overcome them however we can best do so to meet the challenge.

Lets say we have  nuclear rocket engine of 100 kN of thrust, at 480 s of Isp it would go through 21.2 kg of water per second. Assuming ~3500 tons of water efficiency previously calculated, it would take 45.8 hours to go through all that water for 1 kg of plutonium. Lets assume a 4000 tons wet weight, thus 500 ton dry weight so nearly 10 km/s of change in velocity, but at a pitiful average of 0.06 m/s^2  acceleration. Now is two days of continuous thrust a torchship, no, nor is going through that much propellant.  The advantage though is that said nuclear rocket engine could go through ALOT of propellant on very little fission fuel. Lets say we have a 20 ton Lunar fuel shuttle with said 400 kN worth of nuclear rocket engines, It can lift 100 tons of water up, bring 40 tons to low lunar orbit and land again having consumed 60 tons. It could do all that ~29 times on just one kg of plutonium, bringing  1166 tons of water to lunar orbit. So a nuclear thermal rocket might be best for when you need to shuttle stuff from gravity wells that lower thrust higher Isp engines can't do. 

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24 minutes ago, RuBisCO said:

Lets say we have  nuclear rocket engine of 100 kN of thrust, at 480 s of Isp it would go through 21.2 kg of water per second. Assuming ~3500 tons of water efficiency previously calculated, it would take 45.8 hours to go through all that water for 1 kg of plutonium. Lets assume a 4000 tons wet weight, thus 500 ton dry weight so nearly 10 km/s of change in velocity, but at a pitiful average of 0.06 m/s^2  acceleration. Now is two days of continuous thrust a torchship, no, nor is going through that much propellant.  The advantage though is that said nuclear rocket engine could go through ALOT of propellant on very little fission fuel. Lets say we have a 20 ton Lunar fuel shuttle with said 400 kN worth of nuclear rocket engines, It can lift 100 tons of water up, bring 40 tons to low lunar orbit and land again having consumed 60 tons. It could do all that ~29 times on just one kg of plutonium, bringing  1166 tons of water to lunar orbit. So a nuclear thermal rocket might be best for when you need to shuttle stuff from gravity wells that lower thrust higher Isp engines can't do. 

 

Good points.

 

Nuclear is just one tool of many, good for specific jobs to be sure.

Is it worth the cost for manned flight?

I would say yeah so long we have not better 'tools'.

Is it even better for robo-ships? Yes.

The only real cost I see is reprocessing.

The public will cry foul if a nuclear vessel does reentry solely for reprocessing. No one wants that risk.

Which is why space infrastructure already in place would be more public friendly.

Hopefully the initial nuclear fleet can build a reprocessing lunar plant base for all future nuclear Earth vessels to visit.

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10 hours ago, Spacescifi said:

Good points.

Nuclear is just one tool of many, good for specific jobs to be sure.

Is it worth the cost for manned flight?

I would say yeah so long we have not better 'tools'.

Is it even better for robo-ships? Yes.

The only real cost I see is reprocessing.

The public will cry foul if a nuclear vessel does reentry solely for reprocessing. No one wants that risk.

Which is why space infrastructure already in place would be more public friendly.

Hopefully the initial nuclear fleet can build a reprocessing lunar plant base for all future nuclear Earth vessels to visit.

It may be worth the cost in that it can use water, radically reducing the processing needs of ISRU propulsion manufacturing: no need to crack water into hydrogen and oxygen, no cryogenic and deep cryogenic coolers and storage.  Heck on Mars CO2 could simply be compressed and used directly although an Isp above 250 would be unlikely, that and carbon migration and coking might be a problem (at insane temperatures CO2 breaks down to C and O2, and thus carbon soot could build up in the engine as well as migrate through the hot metal changing its physical properties for the worse) but propellant straight from the air is a sweet offer. 

Machines have less radiation concerns, though putting the nuclear engines far behind with a shade shield would provide good protection for crew, except when landing as radiation will reflect off the ground. Certainly to test nuclear thermal in space it should be on unmanned ships at first. 

Yes I don't see reprocessing as happening for this, they will be used until they are used up and then buried somewhere, more fissile fuel or just fresh engines would be brought up from earth, the argument could be made to the public that each one required the cannibalizations of nuclear bombs.  I have a scene play out in my head of thousands of years in the future people on terraformed Mars who have forgotten the past come upon an mountain surrounding by giant titanium pillars with warnings written all over and images of skeletons and trifoils on them, and they have to figure out what it means "Perhaps the ancient ones are warning us of something terrible being buried here?"    

It should be noted that nuclear reactors in general have a lot of void fuel, fuel needed to maintain criticality, that is once the reactor uses ~10% of its fuel it can't hold criticality anymore, so the other ~90% goes to waste.  So yes reprocessing would be good in recycling engines, but the infrastructure required is rather large, it is not a easy task as the fission products come in just about every element. 

I would hope for fusion "torch" ships running of He3 or B11+H1 would do most of the work of orbital running and nuclear thermal would be more limited to landing and taking off from moons and Mars. Unmanned Solar sails would be best for asteroid mining as they can be manufactured from the asteroids out of cheap metals (magnesium and aluminum) that may be by-products of refining precious metals.    

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4 hours ago, RuBisCO said:

It may be worth the cost in that it can use water...

You want to run water over ... a nuclear reactor? Pretty sure fluorine is only slightly worse than superheated steam. I will watch your reactor disintegrate from my fortified observatory here on Earth, thank you. :lol:

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41 minutes ago, SOXBLOX said:

You want to run water over ... a nuclear reactor? Pretty sure fluorine is only slightly worse than superheated steam. I will watch your reactor disintegrate from my fortified observatory here on Earth, thank you. :lol:

 

Hmmm...so you really can't throw any propellant down the reactor and call it good?

I suspected as much.

 

You really do have to process your propellant into LH,   and LOX I reckon may also wreck it because of oxidation.

 

So we are talking a finite amount of propellant choices we can actually ISRU off apparently if you are right. Mostly of the nonreactive types like hydrogem, helium etc.

 

Has anyone tested water propellant on a nuclear reactor and watched to see if the reactor was damaged or not?

Edited by Spacescifi
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IDK if it's been tested, but it really doesn't need to be. Just heat some metal and dunk it in cold water. If you have it even fairly hot, you'll get stress fractures. That, plus neutron embrittlement, plus the chemical action of steam, would make sure the rod casings are destroyed sooner rather than later.

Then again, I could have a totally skewed perspective, and be wrong.

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32 minutes ago, SOXBLOX said:

IDK if it's been tested, but it really doesn't need to be. Just heat some metal and dunk it in cold water. If you have it even fairly hot, you'll get stress fractures. That, plus neutron embrittlement, plus the chemical action of steam, would make sure the rod casings are destroyed sooner rather than later.

Then again, I could have a totally skewed perspective, and be wrong.

 

Probably has to do with the oxygen in water. Since O2 reacts with virtually everything.

 

LH works fine.

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On 5/23/2021 at 3:00 PM, SOXBLOX said:

You want to run water over ... a nuclear reactor? Pretty sure fluorine is only slightly worse than superheated steam. I will watch your reactor disintegrate from my fortified observatory here on Earth, thank you. :lol:

In theory a rotory bed reactor would minimize how many parts are touching the steam and allow for superheating in the void above the fuel elements as well as film cooling along the exit throat and return to pump throat. At 3500K most of the water is splitting back and forth from O and H radicals and back so yes incredible corrosive. 

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On 5/23/2021 at 3:41 PM, Spacescifi said:

 

Hmmm...so you really can't throw any propellant down the reactor and call it good?

I suspected as much.

 

You really do have to process your propellant into LH,   and LOX I reckon may also wreck it because of oxidation.

 

So we are talking a finite amount of propellant choices we can actually ISRU off apparently if you are right. Mostly of the nonreactive types like hydrogem, helium etc.

 

Has anyone tested water propellant on a nuclear reactor and watched to see if the reactor was damaged or not?

No a reactor can be designed to minimize or prevent superhot corrosive propellant from contact with it, remember existing H2+O2 chemical engines are also shooting out superheated steam and not disintegrating. An H2/O2 is probably a safer bet technologically, also it solve the reaction control problem, for to have nuclear reaction control you would need a axillary nuclear engine running at lower temperatures to push steam  down pipes and out of nozzles for 150~200 Isp out RCS. Also H2/O2 engines are likely more easily throttleable then nuclear. 

Of course Mars makes CH4/O2 viable which is easier to store and work with then H2. 

On 5/23/2021 at 8:18 PM, Spacescifi said:

 

Probably has to do with the oxygen in water. Since O2 reacts with virtually everything.

 

LH works fine.

Hydrogen from ISRU would most likely mean you are stuck with O2 as waste product (if manufacture from water) for ever 1 ton of hydrogen one gets 8 tons of oxygen waste! Per ton of water required a nuclear water engine of 450 ISP outperforms as nuclear hydrogen of 1000 ISP out of 15 km/s of delta-v, because you have to go though 8 times as much water to fuel up the nuclear hydrogen. There is another option though which is to use oxygen in a ion engine and hydrogen for landing purposes.

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40 minutes ago, RuBisCO said:

...remember existing H2+O2 chemical engines are also shooting out superheated steam and not disintegrating. 

Well, they also don't contain brittle nuclear fuel rods.

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4 hours ago, SOXBLOX said:

Well, they also don't contain brittle nuclear fuel rods.

I was thinking nuclear fuel pellets, grains even, sand size, in a rotary nuclear rocket engine, held in place by centrifugal force alone (or perhaps sintering  when cooled to low temperatures when the engine is off):

main-qimg-8ba464ef8783109b3d97c0eed08335 

Edited by RuBisCO
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6 hours ago, RuBisCO said:

 

No a reactor can be designed to minimize or prevent superhot corrosive propellant from contact with it, remember existing H2+O2 chemical engines are also shooting out superheated steam and not disintegrating. An H2/O2 is probably a safer bet technologically, also it solve the reaction control problem, for to have nuclear reaction control you would need a axillary nuclear engine running at lower temperatures to push steam  down pipes and out of nozzles for 150~200 Isp out RCS. Also H2/O2 engines are likely more easily throttleable then nuclear. 

Of course Mars makes CH4/O2 viable which is easier to store and work with then H2. 

Hydrogen from ISRU would most likely mean you are stuck with O2 as waste product (if manufacture from water) for ever 1 ton of hydrogen one gets 8 tons of oxygen waste! Per ton of water required a nuclear water engine of 450 ISP outperforms as nuclear hydrogen of 1000 ISP out of 15 km/s of delta-v, because you have to go though 8 times as much water to fuel up the nuclear hydrogen. There is another option though which is to use oxygen in a ion engine and hydrogen for landing purposes.

 

Thank you.

 

Well I am not above fiction. A scifi filter device that filters out liquids into their constitutent gases would be really nice.

From there it would only be a matter of chilling the said gases till they liquify and separating them into the necessarry refrigerated tankage.

Which would save the normal days or weeks it would take to fill up, refine, and boost back to space again.

 

The device need not be overpowered either especially if it requires occasional maintenance or can only run constantly for so long or simply wears out a part regularly and needs regular spares.

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54 minutes ago, Spacescifi said:

 

Thank you.

 

Well I am not above fiction. A scifi filter device that filters out liquids into their constitutent gases would be really nice.

From there it would only be a matter of chilling the said gases till they liquify and separating them into the necessarry refrigerated tankage.

Which would save the normal days or weeks it would take to fill up, refine, and boost back to space again.

 

The device need not be overpowered either especially if it requires occasional maintenance or can only run constantly for so long or simply wears out a part regularly and needs regular spares.

Well that not scifi, we have the technology to crack water to H2 and O2, separate and cryoliquify, it just takes a lot of power and equipment mass. Nuclear-water would skip all that but at the cost of needing very new technology that has only be proposed and never tested at any scale, that being extremely high temperature steam in a nuclear thermal rocket.    

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14 minutes ago, RuBisCO said:

Well that not scifi, we have the technology to crack water to H2 and O2, separate and cryoliquify, it just takes a lot of power and equipment mass. Nuclear-water would skip all that but at the cost of needing very new technology that has only be proposed and never tested at any scale, that being extremely high temperature steam in a nuclear thermal rocket.    

 

It takes power, equipment, and time.

 

I am talking about a box like device with several partitions. You pour water into it. Close it off, the device, while partition walls heat up. The wall below the water allows hydrogen gas through, while the top area is taken up by the remaining oxygen gas.

 

All of this would occur relatively quickly, however long it took to pour and repour after eatch batch of gas gained and stored. Faster than IRL. The lower partition would only be permeable to hydrogen atoms, so if saltwater, the salt would collect above the partition and likely liquify too from laying atop the hot partition that only allows hydogen gas to pass through.

 

 

The steam nuclear rockets seem rather like an accident waiting to happen though. It would be nice but I think even if we used antimatter the steam would still corrode the nozzle via disassociation heat.

 

Better to just separate the LH  and LOX. Especially if you have an easy scifi way to do so that is fast.

Edited by Spacescifi
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On 5/25/2021 at 6:46 PM, Spacescifi said:

I am talking about a box like device with several partitions. You pour water into it. Close it off, the device, while partition walls heat up. The wall below the water allows hydrogen gas through, while the top area is taken up by the remaining oxygen gas.

Sounds like  a standard electrolysis cell to me:

pem_electrolyzer.png

Above you can see the hydrogen and oxygen are separated into separate pure gas streams by a proton-exchange membrane. Higher efficiency can be achieve using hydroxyl exchange membranes (alkaline fuel cell) the best I have seen and theoretically possible is 75%, but that is with electricity, which means means you have to count the inefficiency of making electricity. Lets say you use a nuclear reactor with a very efficient closed cycle helium baryon cycle of 50%, so your water electrolysis is going to be at best 37.5% total efficient.  What is desirable is to have a more direct heat process, for example a sulfur-iodine cycle could do up to 50% efficiency from heat alone, no conversion to electricity.

Edited by RuBisCO
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On 5/25/2021 at 7:46 PM, Spacescifi said:

Better to just separate the LH  and LOX. Especially if you have an easy scifi way to do so that is fast.

I mean a LOX afterburning NTR is a good way to get some extra thrust albeit with a big efficiency penalty.

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1 minute ago, Spaceman.Spiff said:

I mean a LOX afterburning NTR is a good way to get some extra thrust albeit with a big efficiency penalty.

That sounds like the worse of both options, you need to develop NTR and you need to make LH2 and LO2 via ISRU. Either have H2/O2 in space fuel economy with existing tech, or have nuclear-water with technology that has been proposed for decades but never developed. 

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5 minutes ago, RuBisCO said:

That sounds like the worse of both options, you need to develop NTR and you need to make LH2 and LO2 via ISRU. Either have H2/O2 in space fuel economy with existing tech, or have nuclear-water with technology that has been proposed for decades but never developed. 

Obviously you don't use it all the time. That's why it's an afterburner. You don't see fighter jets running an afterburner for entire flights. It would be used to achieve a higher TWR temporarily. 

After you expend all the LOX, (perhaps on an orbital insertion) you would still have LH2 left over and you'd have a lower mass as a result of LOX's greater density.

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On 5/25/2021 at 8:59 AM, RuBisCO said:

 

No a reactor can be designed to minimize or prevent superhot corrosive propellant from contact with it, remember existing H2+O2 chemical engines are also shooting out superheated steam and not disintegrating. An H2/O2 is probably a safer bet technologically, also it solve the reaction control problem, for to have nuclear reaction control you would need a axillary nuclear engine running at lower temperatures to push steam  down pipes and out of nozzles for 150~200 Isp out RCS. Also H2/O2 engines are likely more easily throttleable then nuclear. 

Of course Mars makes CH4/O2 viable which is easier to store and work with then H2. 

Hydrogen from ISRU would most likely mean you are stuck with O2 as waste product (if manufacture from water) for ever 1 ton of hydrogen one gets 8 tons of oxygen waste! Per ton of water required a nuclear water engine of 450 ISP outperforms as nuclear hydrogen of 1000 ISP out of 15 km/s of delta-v, because you have to go though 8 times as much water to fuel up the nuclear hydrogen. There is another option though which is to use oxygen in a ion engine and hydrogen for landing purposes.

That is VERY important.

 

A propellant depot at the ISRU source would be ideal instead of every vessel doing it's own ISRU.

Easier to optimize space vessels for travel that way.

That's not the problem. The problem is the waste of all that oxygen!

The propellant depot only has so much storage space....they are not going to store and save much greater amounts of oxygen every time they mine 80 tons of LH from Europa!

More likely they would just pump it back into the ocean Europa has.

In cases where there is only ice and no liquid water, chances are the oxygen is wasted to vacuum by the propellant mining station since they cannot simply always afford to make the station bigger EVERYTIME they get extra O2.

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14 hours ago, Spacescifi said:

That is VERY important.

 

A propellant depot at the ISRU source would be ideal instead of every vessel doing it's own ISRU.

Easier to optimize space vessels for travel that way.

That's not the problem. The problem is the waste of all that oxygen!

The propellant depot only has so much storage space....they are not going to store and save much greater amounts of oxygen every time they mine 80 tons of LH from Europa!

More likely they would just pump it back into the ocean Europa has.

In cases where there is only ice and no liquid water, chances are the oxygen is wasted to vacuum by the propellant mining station since they cannot simply always afford to make the station bigger EVERYTIME they get extra O2.

Which is why I advocate for nuclear-water so that ISRU requirements are dramatically reduced, no need to crack water and cryoliquify hydrogen, also no waste oxygen. None the less I think with the present state of research H2/O2 is going to win out on the moon and CH4/O2 on Mars, nuclear only in nuclear power reactors to power ISRU at best. Human's are not Kerbals and as such I think there will be much political pressure against nuclear thermal rockets.  I mean if it was up to me we would go with Zubin's continuous nuclear exploration, nuclear salt water engines, regardless of the fact they would spew a beam of pure nuclear waste out the nozzel.  

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2 hours ago, RuBisCO said:

Which is why I advocate for nuclear-water so that ISRU requirements are dramatically reduced, no need to crack water and cryoliquify hydrogen, also no waste oxygen. None the less I think with the present state of research H2/O2 is going to win out on the moon and CH4/O2 on Mars, nuclear only in nuclear power reactors to power ISRU at best. Human's are not Kerbals and as such I think there will be much political pressure against nuclear thermal rockets.  I mean if it was up to me we would go with Zubin's continuous nuclear exploration, nuclear salt water engines, regardless of the fact they would spew a beam of pure nuclear waste out the nozzel.  

 

Thanks.

What I really want to know is how would the plume of a nuclear water steam rocket look?

Like steam?

Would it not have greater thrust but lower efficiency than an LH/LOX rocket?

The strangest thing will be take-off comparisons.

 

The standard LH/LOX rocket makes great flaming plumes with lots of steam.

The nuclear steam rocket? Would it produce any fire plume at all LOL?

I am picturing this steampunk rocket billowing out entire clouds of steam taking off at a notably faster rate than the flamey LH/LOX rocket we are used to.

The nuclear steam rocket nozzle will look glowing white I presume, or perhaps blue?

Yeah bluish likely, since water already glows blue around nuclear reactors.

 

So I imagine it would look like a very thrusty steam rocket with a light blue glow emanating fromm the nozzle and  the steam closest to it.

I imagine at high enough temperature you could get rocket fire plumes from water, but I also reckon that at temperatures that high the engine would melt all known materials anyway.

 

Clarifying anything I said is always welcome. As I expect you or someone likely has a correct answer.

:  )

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