Nuclear Thermal Rocket - startup/shutdown and throttling.

walterscientist
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Posts: 36

Nuclear Thermal Rocket - startup/shutdown and throttling. Jun 5, 2019 14:12:20 GMT

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Post by walterscientist on Jun 5, 2019 14:12:20 GMT

While watching the finale of the Chernobyl miniseries yesterday a question came to my mind. An NTR is type of a nuclear reactor which means it cannot be just turned on/off like (some) chemical rockets. I looked up articles about NTR propulsion, but found little specific about startup/operation/throttling/shutdown. Presumably because how quickly can the engine adjust its thrust generally wouldn't be relevant for long-term missions. Mostly I found out the engine can explode if shut down abruptly.

My question comes from wondering if it is realistic to expect a combat spacecraft to be able to rapidly change thrust of its NTR from minimal to full in order to evade (dodge) enemy attacks. If the engines cannot do that, it would be probably necessary to run them constantly at (nearly) full thrust and use other means that can be operated quickly to facilitate rapid maneuvers. Most likely taking advantage of thrust vectoring and changing facing of the craft. This would be however rather wasteful on fuel, especially for smaller ships.

If anyone has any information I would be really grateful.

airc777
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Posts: 408

Nuclear Thermal Rocket - startup/shutdown and throttling. Jun 5, 2019 15:26:50 GMT

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Post by airc777 on Jun 5, 2019 15:26:50 GMT

Speculation:

If throttle response takes priority over the systems thrust to mass ratio then design a system that instead of throttling up and down switches between open cycle (spending reaction mass to generate thrust) and closed cycle (internally cycling the reaction mass as coolant and radiating the waste heat) very quickly while maintaining 100% reactor power. Adds the mass of the radiator and a really powerful series of valves, works around the throttle response time by just not throttling the reactor.

bigbombr
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Posts: 1,165

Nuclear Thermal Rocket - startup/shutdown and throttling. Jun 5, 2019 17:34:44 GMT

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Post by bigbombr on Jun 5, 2019 17:34:44 GMT

Jun 5, 2019 14:12:20 GMT walterscientist said:

While watching the finale of the Chernobyl miniseries yesterday a question came to my mind. An NTR is type of a nuclear reactor which means it cannot be just turned on/off like (some) chemical rockets. I looked up articles about NTR propulsion, but found little specific about startup/operation/throttling/shutdown. Presumably because how quickly can the engine adjust its thrust generally wouldn't be relevant for long-term missions. Mostly I found out the engine can explode if shut down abruptly.

My question comes from wondering if it is realistic to expect a combat spacecraft to be able to rapidly change thrust of its NTR from minimal to full in order to evade (dodge) enemy attacks. If the engines cannot do that, it would be probably necessary to run them constantly at (nearly) full thrust and use other means that can be operated quickly to facilitate rapid maneuvers. Most likely taking advantage of thrust vectoring and changing facing of the craft. This would be however rather wasteful on fuel, especially for smaller ships.

If anyone has any information I would be really grateful.

Reactors can be turned on as fast as the structure can handle the thermal expansion stress. When you turn the reactor 'off', it indeed won't shut off fully instantly, rather after shutdown it will still produce about 5-6% of nominal power output right after shutdown. So this should be dealt with using radiative cooling, though using the cooling loop through the nozzle (the one normally used for regenerative cooling) in a closed loop fashion could potentially provide enough cooling to not need an additional radiator.

For example, considering an NTR that during full throttle produces 1 GW of thermal power, it will produce around 50 MW of thermal power right after shutdown.
The Stefan-Boltzmann law for thermal radiation goes like this: radiated power = area*emissivity*5.67*10^-8*temperature^4

Since emissivity can be close to one, we'll drop this factor and rearrange the formula like this:
Area = P/(T^4*5.67*10^-8) to find the radiating area needed to cool our reactor post shutdown. Assuming we're willing to let our surface heat up to 3000 K we get the following:
Area = 50*10^6 W/(3000^4*5.67*10^-8) = 10.89 m² of radiative surface, which is quite a lot, but this cooling requirement drops further as more time after the burn passes.

Alternatively, since we tend to run our NTR's a lot hotter we might be willing to accept a hotter radiating surface, for example at 3500 K.
In that case we need only 5.88 m² of radiative surface.

Alternatively, open cycle cooling could be used to supplement radiative cooling until the reactor core has 'cooled down' a bit.

Last Edit: Jun 5, 2019 17:54:07 GMT by bigbombr

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AtomHeartDragon
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Posts: 1,257

Nuclear Thermal Rocket - startup/shutdown and throttling. Jun 6, 2019 19:08:34 GMT

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Post by AtomHeartDragon on Jun 6, 2019 19:08:34 GMT

Jun 5, 2019 17:34:44 GMT bigbombr said:

Jun 5, 2019 14:12:20 GMT walterscientist said:

While watching the finale of the Chernobyl miniseries yesterday a question came to my mind. An NTR is type of a nuclear reactor which means it cannot be just turned on/off like (some) chemical rockets. I looked up articles about NTR propulsion, but found little specific about startup/operation/throttling/shutdown. Presumably because how quickly can the engine adjust its thrust generally wouldn't be relevant for long-term missions. Mostly I found out the engine can explode if shut down abruptly.

My question comes from wondering if it is realistic to expect a combat spacecraft to be able to rapidly change thrust of its NTR from minimal to full in order to evade (dodge) enemy attacks. If the engines cannot do that, it would be probably necessary to run them constantly at (nearly) full thrust and use other means that can be operated quickly to facilitate rapid maneuvers. Most likely taking advantage of thrust vectoring and changing facing of the craft. This would be however rather wasteful on fuel, especially for smaller ships.

If anyone has any information I would be really grateful.

Reactors can be turned on as fast as the structure can handle the thermal expansion stress. When you turn the reactor 'off', it indeed won't shut off fully instantly, rather after shutdown it will still produce about 5-6% of nominal power output right after shutdown. So this should be dealt with using radiative cooling, though using the cooling loop through the nozzle (the one normally used for regenerative cooling) in a closed loop fashion could potentially provide enough cooling to not need an additional radiator.

For example, considering an NTR that during full throttle produces 1 GW of thermal power, it will produce around 50 MW of thermal power right after shutdown.
The Stefan-Boltzmann law for thermal radiation goes like this: radiated power = area*emissivity*5.67*10^-8*temperature^4

Since emissivity can be close to one, we'll drop this factor and rearrange the formula like this:
Area = P/(T^4*5.67*10^-8) to find the radiating area needed to cool our reactor post shutdown. Assuming we're willing to let our surface heat up to 3000 K we get the following:
Area = 50*10^6 W/(3000^4*5.67*10^-8) = 10.89 m² of radiative surface, which is quite a lot, but this cooling requirement drops further as more time after the burn passes.

Alternatively, since we tend to run our NTR's a lot hotter we might be willing to accept a hotter radiating surface, for example at 3500 K.
In that case we need only 5.88 m² of radiative surface.

Alternatively, open cycle cooling could be used to supplement radiative cooling until the reactor core has 'cooled down' a bit.

Open cycle cooling with NTRs is commonly known as 'thrust'.

For gimballed main engines or any other method of thrust vectoring, keeping thrusting during combat might actually be quite reasonable solution. I wonder if using multiple small NTRs could produce much steeper response curves.

Last Edit: Jun 6, 2019 19:12:26 GMT by AtomHeartDragon

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feld
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Nuclear Thermal Rocket - startup/shutdown and throttling. Sept 18, 2019 15:07:54 GMT bigbombr likes this

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Post by feld on Sept 18, 2019 15:07:54 GMT

All above essentially correct. A few additions:

1. "Open cycle cooling with NTRs is commonly known as 'thrust'" - true. This has historically (in the literature and in actual testing) been done by putting low flow rates through the main engine nozzle. This is possible because the decay heat removal rates are much much lower than full power operation. The thrust/weight is, given the mass of these vehicles, typically trivial. Another historical solution (in the literature) has been a secondary system that uses multiple smaller nozzles and positions them for net zero momentum change.

2. Throttling an NTR over a wide range is tricky because the reactor needs to be able to change power relatively quickly. This was doable in ROVER/NERVA because the hydrogen propellant/coolant is an excellent neutron moderator so, when you inject more hydrogen it naturally increased the power output of the reactor while simultaneously increasing cooling flow. It is still much easier though to design a reactor which stays within a narrow power band (for many of the reasons listed above). I cannot find the historical NERVA number in my papers right now but I don't think that the program's throttling capabilities were noteworthy. My memory might fail me on that. One of the reasons that the LANTR (Lunox Augmented Nuclear Thermal Rocket) concept is interesting is that it allows throttling of an NTR while keeping the reactor at the same power level: you inject oxygen into the hydrogen exhaust in order to trade Isp for increasing thrust, allowing you a much greater throttle tradespace than NTR alone.

3. One of the stresses of particular concern on startup would be brittle fracture, not technically a purely thermal stress because it assumes a pre-existing flaw geometry to act as a focus. It happens when you bring very cold solids up to temperature too fast and is characterized by sudden catastrophic deformation of a metal with little or no plastic deformation (thus little or no warning).

Please let me know if this stuff is of any use, in a clearly well read group like this, it is hard for me to know if the stuff I am writing from my profession is already well known or actually elucidating.

Also don't want to be pedantic

v/r
feld

Last Edit: Sept 18, 2019 15:15:51 GMT by feld: Technical accuracy pass