2020's nuclear thermal propulsion efforts

[Robotbeat]

Complete waste of money, Solar Electric propulsion is superior for virtually every conceivable use scenario, particularly the High Delta-V missions. 

For cis-lunar space where transfers to-from the lunar surface and various lunar and high-Earth orbits are often ~2km/s, the ISP of a NTP simply isn't high enough to produce a propellant fraction saving with would outweigh the heavy mass of the engine and the bulky low density hydrogen tanks.

For example

2.5km/s DeltaV with HydroLox 450 ISP, propellant fraction 44%
2.5km/s DeltaV with NTP 900 ISP, propellant fraction 25%

Savings 19% propellant fraction, but now all propellant mass is H2 so bulk density drops to 71 kg/m^3, about 1/4th that of a Hydro-LOX mix, so tank sizes actually increases by a factor of 2.2.  That will eat up several percent dry mass and then the engine the rest.

With SEP at 5k ISP and same DeltaV, propellant fraction is 5%, at high density, and much of your engine mass is actual a solar array that can power the mission hardware when you reach the destination.

There's no magic rule about nuclear engines that says that you can only use hydrogen as your propellant. Throw methane in there if you want better bulk density and more delta-V.

I mean... The bulk density of methane is barely better than optimized hydrolox and the Isp isn't really any better (500-600s-ish?). With the super heavy reactor giving you really high dry mass and/or really low burn acceleration (which means gravity losses), I sincerely doubt you'd actually achieve better stage performance than just hydrolox.

So the magic rule that you can only use hydrogen in nuclear engines is because it's really the only thing worth it. Anything heavier, and you might as well just use the chemical energy within the propellant itself and save yourself a TON of money.

[Robotbeat]

Why not both? The private sector can develop SEP and ISRU so that the government can marshal its resources to tackle nuclear propulsion and space-rated nuclear power. Just about any kind of propulsion or propellant system except for nuclear is now within the capability of commercial spaceflight. For the government to completely foot the bill for anything other than nuclear propulsion would be the waste of money.

Nope, most SEP research is still government funded for one, just because commercial interests can also contribute doesn't mean government effort should cease.  NASA still dose huge amounts of Aeronautics research which is then farmed out to the industry to make it help make it competitive on the world market.

NTP and especially nuclear-power in space are a waste because neither civilians nor governments will have any use for such a device regardless of who were to develop it.

I think kilopower is useful. Even if just for deep space probes and stuff. A nice upgrade from Pu238 in terms of available power (mainly because Pu238 is in limited supply).

[RotoSequence]

I mean... The bulk density of methane is barely better than optimized hydrolox and the Isp isn't really any better (500-600s-ish?). With the super heavy reactor giving you really high dry mass and/or really low burn acceleration (which means gravity losses), I sincerely doubt you'd actually achieve better stage performance than just hydrolox.

So the magic rule that you can only use hydrogen in nuclear engines is because it's really the only thing worth it. Anything heavier, and you might as well just use the chemical energy within the propellant itself and save yourself a TON of money.

Who's talking about using it in a rocket stage with gravity losses? I'd only suggest it for a deep space cruise engine for cislunar patrols with a large delta-V budget, with the option for high thrust transits in an emergency.

[Lodrig]

I mean... The bulk density of methane is barely better than optimized hydrolox and the Isp isn't really any better (500-600s-ish?). With the super heavy reactor giving you really high dry mass and/or really low burn acceleration (which means gravity losses), I sincerely doubt you'd actually achieve better stage performance than just hydrolox.

So the magic rule that you can only use hydrogen in nuclear engines is because it's really the only thing worth it. Anything heavier, and you might as well just use the chemical energy within the propellant itself and save yourself a TON of money.

Who's talking about using it in a rocket stage with gravity losses? I'd only suggest it for a deep space cruise engine for cislunar patrols with a large delta-V budget, with the option for high thrust transits in an emergency.

Robobeat is right a NTR gets its ISP benefits ONLY from the property of it's propellants because the temperature it raises it's propellants to is no hotter then that of a normal chemical engine.  This is because while a nuclear fuels could reach almost unlimited temperatures if we let them, they would melt themselves if we let them do that.  And the propellants heated by those nuclear fuels still need to flow through a nozzle without melting it either, so you end up with around 3500K after we apply every bit of our know material science and active cooling to keep the engine from destroying itself.

[Lodrig]

Why not both? The private sector can develop SEP and ISRU so that the government can marshal its resources to tackle nuclear propulsion and space-rated nuclear power. Just about any kind of propulsion or propellant system except for nuclear is now within the capability of commercial spaceflight. For the government to completely foot the bill for anything other than nuclear propulsion would be the waste of money.

Nope, most SEP research is still government funded for one, just because commercial interests can also contribute doesn't mean government effort should cease.  NASA still dose huge amounts of Aeronautics research which is then farmed out to the industry to make it help make it competitive on the world market.

NTP and especially nuclear-power in space are a waste because neither civilians nor governments will have any use for such a device regardless of who were to develop it.

That'a a rather broad statement; every RTG-powered probe we've sent and the Curiosity and Perseverance rovers would choose to disagree. The idea behind low-enriched Uranium-based reactors is about working with safer, less-controlled fuel with an eye to broadening the use of such power sources. See Kilopower.

Your right that was overtly broad, I did not mean to include RTG or the promising Sterling-Engine based uses of Nuclear power as these have excellent usage in outer solar system probes.

I had been assuming that butters term 'space-rated nuclear power' was his way of saying PWR (Pressurized water reactor), a technology that's a big dead-end due to the heat-rejection in a vacuum problem.  Sorry to him if I misinterpreted that.

[Genial Precis]

My back-of-the-napkin calculation says that if I'm allowed to radiate heat at 500 C, I can run a 1 GWth nuclear generator with a 5E4 m^2 radiator array. If that array is composed of 0.5 mm wall thickness 6 mm diameter stainless steel pipe, for example, it will mass 200 tons. If I want a 1000 ton vehicle to achieve 6 km/s deltaV on 100 tons propellant (leaving 200 tons for the reactor itself and 500 tons payload/other components), it will need engines with a specific impulse of 6000 s. This is clearly an electric thruster of some sort.

Since the heat must be rejected at such a high temperature, the reactor itself has to be molten salt, and I don't like the odds of graphite moderator surviving such high hot-end temperatures (I pick 1000 C), so it would be a molten salt fast reactor.

Assuming 10% thermal efficiency, the thrusters have up to 100 MW available, so with an Isp of 6000 s they would produce 1.7 kN thrust, for an acceleration of 1.7e-3 m/s^2 acceleration, for 21 days of acceleration up to 3 km/s deltaV at each end.

The role of such a vehicle would be shipping things from orbit to orbit, which it would do quite well, minimizing the requirement to ship propellant into orbit at either end of its route. However, it doesn't appear to have enough deltaV to make trips to and from Mars more than once per synod, so if you spend a billion dollars on your reactor and try to amortize the cost over 20 synods, it's not so attractive.

Nuclear thermal propulsion has all these problems but worse, I think. If you could use it as a booster, that would be another thing, but...

[RonM]

My back-of-the-napkin calculation says that if I'm allowed to radiate heat at 500 C, I can run a 1 GWth nuclear generator with a 5E4 m^2 radiator array. If that array is composed of 0.5 mm wall thickness 6 mm diameter stainless steel pipe, for example, it will mass 200 tons. If I want a 1000 ton vehicle to achieve 6 km/s deltaV on 100 tons propellant (leaving 200 tons for the reactor itself and 500 tons payload/other components), it will need engines with a specific impulse of 6000 s. This is clearly an electric thruster of some sort.

Since the heat must be rejected at such a high temperature, the reactor itself has to be molten salt, and I don't like the odds of graphite moderator surviving such high hot-end temperatures (I pick 1000 C), so it would be a molten salt fast reactor.

Assuming 10% thermal efficiency, the thrusters have up to 100 MW available, so with an Isp of 6000 s they would produce 1.7 kN thrust, for an acceleration of 1.7e-3 m/s^2 acceleration, for 21 days of acceleration up to 3 km/s deltaV at each end.

The role of such a vehicle would be shipping things from orbit to orbit, which it would do quite well, minimizing the requirement to ship propellant into orbit at either end of its route. However, it doesn't appear to have enough deltaV to make trips to and from Mars more than once per synod, so if you spend a billion dollars on your reactor and try to amortize the cost over 20 synods, it's not so attractive.

Nuclear thermal propulsion has all these problems but worse, I think. If you could use it as a booster, that would be another thing, but...

Nuclear thermal propulsion doesn't need large radiators. Like a chemical rocket, most of the heat is carried away by the propellant exhaust.

[volker2020]

I wonder, if anybody planed a reactor, not just creating heat, but to create electric energy, maybe even a plasma, that would accelerated the fuel even further. Kind of a nuclear pumped vasmir drive.

[Welsh Dragon]

Nuclear thermal propulsion doesn't need large radiators. Like a chemical rocket, most of the heat is carried away by the propellant exhaust.

And then you stop your burn. You will still need cooling, unlike a chemical engine, there is still a potent source of heat (radioactive decay of fission products) after you end your burn.

[RonM]

Nuclear thermal propulsion doesn't need large radiators. Like a chemical rocket, most of the heat is carried away by the propellant exhaust.

And then you stop your burn. You will still need cooling, unlike a chemical engine, there is still a potent source of heat (radioactive decay of fission products) after you end your burn.

Once "throttled down" by reinserting the control rods let the propellent flow for a short time to help cool the engine. Only need modest radiators during idle unless it's a bimodal engine that can be used to generate electricity when not generating thrust.

[Lodrig]

When a reactor is suddenly shutdown their will be residual heat production from the spontaneous decay of fissile products, in other words this heat is not dependent on any neutron flux of chain reaction.  The decay heat itself decays rapidly on a predictable trend line.

https://en.wikipedia.org/wiki/Decay_heat#/media/File:Decay_heat_illustration2.PNG

It should be noted that these assessments of decay heat assume a reactor has been running long enough to reach a steady state operation in which it's creating fissile products at the same rate they are decaying at.  This will not be the case in a NTR which has only had a few minutes of activation to an orbital maneuver.  So while the exponential decay of the decay heat should be the same, it should be less as a percentage of the engines peak heat output, by how much I can't say.

The problem is the long tail of just enough heat to be problematic if it's not handled.  Using additional propellant flow through the engine for even an hour might be feasible and might even yield some usable thrust, but it would likely be at lower then ideal ISP.  For a planned burn like Trans-Mars-Injection this residual thrust could be taken into account and made part of the insertion burn but again at a cost to ISP.

How to deal with days worth of low level heat really presents a different problem that is likely better solved with a radiator and closed loop coolant vs continually losing propellant.

[Asteroza]

I wonder, if anybody planed a reactor, not just creating heat, but to create electric energy, maybe even a plasma, that would accelerated the fuel even further. Kind of a nuclear pumped vasmir drive.

A bimodal reactor variant of ESA'S NTER design could function like that, using the turboinductor in the last segment to get hydrogen to near plasma conditions, as a possible replacement for a VASIMR helicon which is used for ionizing hydrogen in the front end of the VASIMR engine, assuming a secondary turbobrayton electric power cycle powers the VASIMR drive itself. Your cooling flow goes from VASIMR internals, to nozzle, to chamber, to reactor exit chamber, to turboinductor, to main reactor wall, to reactor injection head, then reactor, turboinductor, chamber, VASIMR accelerator, and magnetic nozzle.

The problem there, is that a turboinductor is functionally a heavy tungsten heat exchanger, with the associated mass penalty. While there might be some benefit to nuclear preheating of input hydrogen for a VASIMR (since one presumably needs nuclear power to provide electricity for the VASIMR, one wouldn't expect a NTR variant for that, rather a boosted closed cycle nuclear powerplant (with radiators and turbines in a brayton power cycle) that functionally can switch between low power "hotel load" with a full closed cycle, and a semi-open cycle high power mode where hydrogen "auxiliary coolant" is "exhausted" into a convenient secondary system that is also a high power consumer.

[edzieba]

I wonder, if anybody planed a reactor, not just creating heat, but to create electric energy, maybe even a plasma, that would accelerated the fuel even further. Kind of a nuclear pumped vasmir drive.

That would be the Serpent engine.
More info here from the BIS Scorpio craft design.

[RonM]

When a reactor is suddenly shutdown their will be residual heat production from the spontaneous decay of fissile products, in other words this heat is not dependent on any neutron flux of chain reaction.  The decay heat itself decays rapidly on a predictable trend line.

https://en.wikipedia.org/wiki/Decay_heat#/media/File:Decay_heat_illustration2.PNG

It should be noted that these assessments of decay heat assume a reactor has been running long enough to reach a steady state operation in which it's creating fissile products at the same rate they are decaying at.  This will not be the case in a NTR which has only had a few minutes of activation to an orbital maneuver.  So while the exponential decay of the decay heat should be the same, it should be less as a percentage of the engines peak heat output, by how much I can't say.

The problem is the long tail of just enough heat to be problematic if it's not handled.  Using additional propellant flow through the engine for even an hour might be feasible and might even yield some usable thrust, but it would likely be at lower then ideal ISP.  For a planned burn like Trans-Mars-Injection this residual thrust could be taken into account and made part of the insertion burn but again at a cost to ISP.

How to deal with days worth of low level heat really presents a different problem that is likely better solved with a radiator and closed loop coolant vs continually losing propellant.

How about looking at NERVA engine designs. You won't see large radiator systems. I'm sure the engineers took heat into account.

[Robotbeat]

I mean... The bulk density of methane is barely better than optimized hydrolox and the Isp isn't really any better (500-600s-ish?). With the super heavy reactor giving you really high dry mass and/or really low burn acceleration (which means gravity losses), I sincerely doubt you'd actually achieve better stage performance than just hydrolox.

So the magic rule that you can only use hydrogen in nuclear engines is because it's really the only thing worth it. Anything heavier, and you might as well just use the chemical energy within the propellant itself and save yourself a TON of money.

Who's talking about using it in a rocket stage with gravity losses? I'd only suggest it for a deep space cruise engine for cislunar patrols with a large delta-V budget, with the option for high thrust transits in an emergency.

Assuming we're talking about Nuclear Thermal Rockets, there are still lots of gravity losses. Basically, the burn takes so long (sometimes between 30 minutes and nearly an hour) that you can no longer treat it as impulsive and so you lose much of the benefit of the Oberth Effect since your burn is not occurring as deep within the local gravity well.

This is much the same reason why electric propulsion (whether nuclear or solar) isn't quite as useful as we'd like. With low thrust propulsion, the required delta-v is roughly doubled. NTR would be somewhere between that and chemical rockets. It's hard to incorporate this effect in back-of-the-envelope calculations, but it's very real and seriously hampers a lot of the benefit of NTR.

EDIT: Like electric-propulsion, it is possible to mitigate this somewhat by using multiple burns instead of one long burn, but this only works for achieving escape velocity. Once escape velocity is achieved, you have just one shot. Also, it means you have multiple passes through the Van Allen belts, longer mission times as you wait for another pass, etc.

[john smith 19]

Assuming we're talking about Nuclear Thermal Rockets, there are still lots of gravity losses. Basically, the burn takes so long (sometimes between 30 minutes and nearly an hour) that you can no longer treat it as impulsive and so you lose much of the benefit of the Oberth Effect since your burn is not occurring as deep within the local gravity well.

Doesn't  that really depend on relative size of the engine thrust to the payload size?

[Asteroza]

How about looking at NERVA engine designs. You won't see large radiator systems. I'm sure the engineers took heat into account.

I'm pretty sure NERVA derivatives baseline low flow propellant flushing for decay heat mitigation, AKA dribbling propellant through the reactor after the main burn. Functionally simple, but wasteful of propellant, and messes with your trajectory a little.

[Lodrig]

How about looking at NERVA engine designs. You won't see large radiator systems. I'm sure the engineers took heat into account.

The lack of any apparent radiator system for a system that was tested in atmosphere is not conclusive of what the system would have needed in space, this goes for EVERY device put into the vacuum of space, not just an engine.

Further more the expected use of NTR at the time would generally be single firing, TLI from LEO in which the engine along with the whole stage it was part of is just discarded as their was at the time or mentality of that day no such thing as reuse, refueling or any way to keep H2 liquid long enough to be relevant.  In this scenario decay-heat is irreverent and the whole stage can melt to slag for all you care.  After all many conventional engines were not restart-able so their was little demand for the issue to be solved, I have no doubt the issue and many others COULD be solved but it doesn't mean they been yet.

This pattern seems to come up frequently when people talk about NTR, they say it's all solved and we just need to build it with little or no development cost.  And when you point out that the performance isn't enough to justify it's use they point to advanced theoretical designs that were never tested.  Their is a consistent tendency to try to have the best of both and to compare what NTR's might be after BILLIONS in development to the present state of competitor techs while ignoring what those techs could become with that same spending.

These currently funded developments look to me as nothing more then congress (or even specific Senators) trying to be an engineer, or a not so subtle way to subsidize research for the nuclear power industry.

[john smith 19]

These currently funded developments look to me as nothing more then congress (or even specific Senators) trying to be an engineer, or a not so subtle way to subsidize research for the nuclear power industry.

As opposed to subsidizing say the coal or oil industries instead?

[RonM]

How about looking at NERVA engine designs. You won't see large radiator systems. I'm sure the engineers took heat into account.

The lack of any apparent radiator system for a system that was tested in atmosphere is not conclusive of what the system would have needed in space, this goes for EVERY device put into the vacuum of space, not just an engine.

Further more the expected use of NTR at the time would generally be single firing, TLI from LEO in which the engine along with the whole stage it was part of is just discarded as their was at the time or mentality of that day no such thing as reuse, refueling or any way to keep H2 liquid long enough to be relevant.  In this scenario decay-heat is irreverent and the whole stage can melt to slag for all you care.  After all many conventional engines were not restart-able so their was little demand for the issue to be solved, I have no doubt the issue and many others COULD be solved but it doesn't mean they been yet.

This pattern seems to come up frequently when people talk about NTR, they say it's all solved and we just need to build it with little or no development cost.  And when you point out that the performance isn't enough to justify it's use they point to advanced theoretical designs that were never tested.  Their is a consistent tendency to try to have the best of both and to compare what NTR's might be after BILLIONS in development to the present state of competitor techs while ignoring what those techs could become with that same spending.

These currently funded developments look to me as nothing more then congress (or even specific Senators) trying to be an engineer, or a not so subtle way to subsidize research for the nuclear power industry.

You have a lot of misconceptions about NTR designs. Do some research.