Nuclear rocket and aircraft engines

[b0ss]

I've recently been super inspired by the Interstellar Extended mod for KSP, but it's been causing me some trouble lately. I have no idea what kind of engines are safe to use in Earth's vicinity! As of late I've been especially conflicted by the thermonuclear turbine, some say it's safe for use in atmosphere and others say that all the air that flows past the reactors becomes irradiated. I know that the "Lightbulb" closed cycle gas core concept are safe to use in the atmosphere but beyond that I have no idea how to determine whether a type of nuclear propulsion is safe to use on or near Earth

[p1t1o]

21 minutes ago, b0ss said:

I've recently been super inspired by the Interstellar Extended mod for KSP, but it's been causing me some trouble lately. I have no idea what kind of engines are safe to use in Earth's vicinity! As of late I've been especially conflicted by the thermonuclear turbine, some say it's safe for use in atmosphere and others say that all the air that flows past the reactors becomes irradiated. I know that the "Lightbulb" closed cycle gas core concept are safe to use in the atmosphere but beyond that I have no idea how to determine whether a type of nuclear propulsion is safe to use on or near Earth

Well "safe" is a fluid concept. Simply put, anything "closed cycle" is "safe" in that it wont spew particles of reactor fuel out of the exhaust (merely irradiated air is not necessarily harmful).

On the other hand, even a closed-cycle gas-core engine is severely radioactive in its own right. In space, you can get away with just putting shielding between the engine and crew, on Earth perhaps not so much, and shielding is very heavy.

A nuclear turbine can be closed or open cycle.

 

Edited March 29, 2018 by p1t1o

[ment18]

Generally, any engine that doesn't use up nuclear material is safe.  For example, if an engine converts enriched uranium to depleted uranium its ok, but if the uranium just disappears then it probably spews nuclear material and is considered unsafe.  Nuclear material can be a variety of things such as deuterium, tritium, plutonium, uranium, thorium etc.

[wumpus]

Even Orion was believed to only cause low levels of damage to the Earth, and it flew by exploding nuclear weapons.  In practice, the calculations were dreadfully flawed (the radioactive material would be collected by the magnetosphere and returned to Earth) for most launch sites, but you could still get the "low radiation result" by lifting off of Antartica.

[Nuke]

also the chance of the plane crashing is non-zero.

[sevenperforce]

In practice, there's a total moratorium of in-atmosphere tests on any NTRs, because in theory some of the liquid hydrogen will mutate by neutron capture and turn radioactive.

Even though there's NO actual risk of harm. From that part, at least.

[magnemoe]

15 hours ago, Nuke said:

also the chance of the plane crashing is non-zero.

This is an major issue, and its not an issue in space, yes an engine fail might kill the crew but this is an danger with any engine fail in deep space. 
An crash with an nuclear powered plane would probably release radioactivity. 
Note that nuclear powered planes was dropped early also by USSR

[Nefrums]

On 3/30/2018 at 3:18 PM, sevenperforce said:

In practice, there's a total moratorium of in-atmosphere tests on any NTRs, because in theory some of the liquid hydrogen will mutate by neutron capture and turn radioactive.

Even though there's NO actual risk of harm. From that part, at least.

Mutate?  Neutron capture turns hydrogen into deuterium, that is stable not radioactive.

[sevenperforce]

On 3/31/2018 at 12:15 PM, Nefrums said:

Mutate?  Neutron capture turns hydrogen into deuterium, that is stable not radioactive.

Transmutation, technically. 

Neutron capture turns protium into deuterium and deuterium into tritium. The odds of any protium atom experiencing multiple neutron capture events while being pushed through a NERVA core are vanishingly low, but since it's technically possible, the exhaust is classified as potentially radioactive.

[kerbiloid]

Laser (or magnetic)-initiated aneutronic fusion of non-cryogenic fuel.

it stores no radioactives,
it produces only helium (can inflate colored balloons!),
it radiates no neutrons,  only charged particles which can be held inside a magnetic trap until getting slow,
it requires no rare materials (boron, nitrogen, hydrogen or so)
it is highly scalable,
it is implemented in KSPI-E

The only disadvantage: irl we stiil have to open that TechTree node.

Edited April 2, 2018 by kerbiloid

[AeroGav]

On 30/03/2018 at 1:10 AM, Nuke said:

also the chance of the plane crashing is non-zero.

Actually, in air crashes it is very rare for the casing of a jet engine to rupture.   The rest of the aircraft structure, fuel tanks, passenger cabins, people,  become crumple zone for the relatively sturdy engine.  Unfortunately the debris thus liberated gets sucked into the engine and the turbine blades, spinning at near supersonic speed, don't do well.

I presume a reactor core would be even tougher and less likely to get damaged, especially if measures are taken to put it in a less vulnerable location.

Whilst the core might be intact, the crash will certainly destroy the coolant loop.      Unless its a design that can survive that (pebble bed?), or unless you only crash on water, you'll now have a melt down on your hands.

[sevenperforce]

4 hours ago, AeroGav said:

I presume a reactor core would be even tougher and less likely to get damaged, especially if measures are taken to put it in a less vulnerable location.

Whilst the core might be intact, the crash will certainly destroy the coolant loop.      Unless its a design that can survive that (pebble bed?), or unless you only crash on water, you'll now have a melt down on your hands.

Meltdowns are not as easy to achieve as people often think. There are numerous ways to scram a reactor.

[Bill Phil]

On 4/2/2018 at 7:05 AM, sevenperforce said:

Transmutation, technically. 

Neutron capture turns protium into deuterium and deuterium into tritium. The odds of any protium atom experiencing multiple neutron capture events while being pushed through a NERVA core are vanishingly low, but since it's technically possible, the exhaust is classified as potentially radioactive.

Seeing that tritium is a beta emitter... the risk is extremely low. Well, assuming the engine is working as intended, that is.

Tritium is much more dangerous when ingested. 

I honestly wouldn't be surprised if NERVA exhaust is less radioactive than coal plant exhaust.

[sevenperforce]

8 minutes ago, Bill Phil said:

Seeing that tritium is a beta emitter... the risk is extremely low. Well, assuming the engine is working as intended, that is.

Tritium is much more dangerous when ingested. 

I honestly wouldn't be surprised if NERVA exhaust is less radioactive than coal plant exhaust.

A reasonably well-designed solid-core NTR is absolutely less radioactive (in terms of its exhaust) than a coal plant. No question. Orders-of-magnitude difference.

[p1t1o]

37 minutes ago, sevenperforce said:

Meltdowns are not as easy to achieve as people often think. There are numerous ways to scram a reactor.

But to scram one that has crashed catastrophically out of the sky? It doesnt have to meltdown to be a radiological disaster.

[sevenperforce]

3 minutes ago, p1t1o said:

But to scram one that has crashed catastrophically out of the sky? It doesnt have to meltdown to be a radiological disaster.

Automatically scramming it, either via a deadman power system or some other failsafe, is pretty standard.

But yes, if the casing ruptures, you do end up with a radiological spill.

[AeroGav]

7 minutes ago, p1t1o said:

But to scram one that has crashed catastrophically out of the sky? It doesnt have to meltdown to be a radiological disaster.

Achieving Scram - reactor shut down - is easy,  and most meltdowns happened well after the core was scrammed.

The problem is ,  the chain reaction doesn't neatly   break down  semi stable large plutonium atoms (that only decay when struck by neutrons from other fission reactions) into smaller atoms that are stable in one neat step.

Fission fuels break down into smaller atoms which are actually less stable ,  due to having a non-optimal proton neutron ratio.    Eventually these decay into stable species by kicking out excess neutrons or protons,  but that reaction happens regardless of whether the control rods have been put in to stop the chain reaction or not.      After shutting down a reactor,   it will continue to produce "decay heat"  of about 2% of its peak output for a few weeks.         When the NERV, or nuclear turbojet is firing, the propellant,  or air flow from the turbines does the cooling.

Plane crashes, propellant flow stops,  turbines disintegrate.    Reactor scrams, but this 2% decay heat is not getting removed, so within a few hours the core melts and breaches containment, seeps into the water supply, other fun stuff.

[p1t1o]

Just now, AeroGav said:

..most meltdowns happened well after the core was scrammed.

Yes this was something that occurred to me too, scramming does not equal instant safety.

To be honest, in the end I think that nuclear reactors are too heavy to make them the best choice for in-atmosphere travel. Not only are they very dense, but they miss out on all that lovely free oxidising potential in the air.

[AeroGav]

1 minute ago, p1t1o said:

Yes this was something that occurred to me too, scramming does not equal instant safety.

To be honest, in the end I think that nuclear reactors are too heavy to make them the best choice for in-atmosphere travel. Not only are they very dense, but they miss out on all that lovely free oxidising potential in the air.

Weight is even more of an issue in space.  You solve that by only shielding the side of the reactor that faces the crew (called a shadow shield).  The other sides, while sealed to keep radioactive materials within,  will not block the X Rays and Gamma rays it produces while running, so you don't want to get closer than a few miles.    Also after shutdown, it remains to a lesser extent harmful to be on that side for some time after, because of the "decay heat" reactions i mentioned in my previous post.

I actually think nuclear space planes are quite elegant, and build them in KSP too.     In the lower atmosphere , it operates like a turbojet, with an intake and compressor turbine, but instead of a combustion chamber injecting Kerosene, you heat the air with the reactor.   Why carry chemical fuel when you've already got a perfectly good heat source on the ship? Then expand the heated air through a turbine stage (to drive the compressor) and then the nozzle (to provide thrust).   

When air breathing becomes impossible, you could in theory use your reactor to heat liquid hydrogen or liquid ammonia from onboard storage instead and act like a conventional nuclear thermal rocket.

Achieving good TWR is harder with a nuclear power plant than a chemical one, which is why this tech seems better suited to space planes ..  thanks to lift, you can get by with a TWR of less than one.

Such an engine offers a lot of possible operating modes.   Will it make sense to use all of them ?

For example, at low altitude, when acting as an air-breathing turbojet,  engine output would probably be limited by the heat output of the reactor.     But you could use some of your liquid hydrogen/ammonia in an afterburner to get extra thrust for takeoff/crossing mach 1.

At higher altitudes when the air is thinner,  the limiting factor on air breathing power will be the ability of the turbine material to cope with heat.    The reactor will not be able to run at its max thermal output as the airflow diminishes, or it will heat that air too hot for the turbine to handle it.    However,  the reactor core materials can handle significantly higher temperatures than the turbine blades, so is it worth making a reheat cycle to heat the air again downstream of the turbine and get some extra thrust from this surplus heat.

Also, you could again add hydrogen or other closed cycle propellant to burn downstream of the reheater, to raise the exhaust temp/velocity still further.   

If the propellant of choice is cryogenic, you could use it to precool the intake charge like on the SABRE./Skylon /RAPIER and extend the air breathing envelope further.

Finally, how is the "close cycle" transition handled ?  Abrupt?    Or do you gradually throttle down one while diverting more heat into the NERV cycle?

 

Tech issues to overcome -

1. Are you pumping the air / propellant directly through the core, or are you using an intermediate cooling loop ?

Some of the possible modes described above require an intermediate, which adds weight and complexity.  Probably lower exhaust temps/isp too,  especially given that pumping superheated fluids is not a known tech

If going direct, like the classic NERVA,    the coating material of your fuel rods needs to cope with oxidizing agent at high temps (air) and reducing agent at high temps (closed cycle mode - hydrogen / methane / ammonia )

 

2.  Maintaining positive lift drag ratio over an enormous speed range.    Conventionally understood supersonic aerodynamics won't give you that.    Compression lift wave rider aerodynamics have promise,  but are poorly understood because wind tunnels cannot simulate the conditions.   Now that the military has taken an interest in hypersonic weapons, this may change.

 

3.  Servicing the damn thing between flights.            OK, this might just be a NASA cultural thing, but look at the huge army of very well paid techs that had to crawl over the Space Shuttle for months to ascertain it was safe to re-use and put humans on.   It would almost have been cheaper to build a new Shuttle for every flight.     Now imagine doing the same with a recently shut down nuclear reactor  that's still undergoing decay reactions and isn't shielded on all sides, how long will the maintenance take now and how much will it cost.

Perhaps you could use a robotic vehicle to attach lead plates to the unshielded side of the reactor before sending the techs in.   Answers on postcard, please.

 

[wumpus]

1 hour ago, AeroGav said:

Achieving Scram - reactor shut down - is easy,  and most meltdowns happened well after the core was scrammed.

The problem is ,  the chain reaction doesn't neatly   break down  semi stable large plutonium atoms (that only decay when struck by neutrons from other fission reactions) into smaller atoms that are stable in one neat step.

Fission fuels break down into smaller atoms which are actually less stable ,  due to having a non-optimal proton neutron ratio.    Eventually these decay into stable species by kicking out excess neutrons or protons,  but that reaction happens regardless of whether the control rods have been put in to stop the chain reaction or not.      After shutting down a reactor,   it will continue to produce "decay heat"  of about 2% of its peak output for a few weeks.         When the NERV, or nuclear turbojet is firing, the propellant,  or air flow from the turbines does the cooling.

Plane crashes, propellant flow stops,  turbines disintegrate.    Reactor scrams, but this 2% decay heat is not getting removed, so within a few hours the core melts and breaches containment, seeps into the water supply, other fun stuff.

This is one of my biggest problems with NTRs in regular use.  Expect to keep dumping hydrogen on the reactor long after pulling the rods and "stopping" the reaction.  Certainly, the immediate post-reacting cooling can likely give full thrust, but you still dump a lot of precious post-burn fuel with little thrust.  I've pointed out that simply dumping the core after each use is a likely system, and suspect that was the idea in the 1970s as well.

[Bill Phil]

41 minutes ago, wumpus said:

This is one of my biggest problems with NTRs in regular use.  Expect to keep dumping hydrogen on the reactor long after pulling the rods and "stopping" the reaction.  Certainly, the immediate post-reacting cooling can likely give full thrust, but you still dump a lot of precious post-burn fuel with little thrust.  I've pointed out that simply dumping the core after each use is a likely system, and suspect that was the idea in the 1970s as well.

Considering that they intended each NERVA to be used for 10 lunar flights, I doubt core ejection was ever an option.

It's possible that they had cooling systems in mind to cool the reactor after "burns", but multiple activation cycles would stress the system regardless. This led to the idea of a bimodal NTR, where you don't ever shut it down during the flight, but use it for power while not providing thrust. 

[Nuke]

16 hours ago, AeroGav said:

Actually, in air crashes it is very rare for the casing of a jet engine to rupture.   The rest of the aircraft structure, fuel tanks, passenger cabins, people,  become crumple zone for the relatively sturdy engine.  Unfortunately the debris thus liberated gets sucked into the engine and the turbine blades, spinning at near supersonic speed, don't do well.

I presume a reactor core would be even tougher and less likely to get damaged, especially if measures are taken to put it in a less vulnerable location.

Whilst the core might be intact, the crash will certainly destroy the coolant loop.      Unless its a design that can survive that (pebble bed?), or unless you only crash on water, you'll now have a melt down on your hands.

thing is 'very rare' is not never. even if its like one in a million thats still a non-zero chance. just takes one unlucky mishap to irradiate a lot of area. 

of course the only time you really want a nuclear powered aircraft is when you want to keep it up for weeks or months, so i doubt it would be viable as a passenger aircraft. its good if you need to keep a nuclear bomber on stand by. icbms obsoleted the aircraft reactor experiment, slam, etc. idk what other thing you would need a nuclear powered aircraft for, only things i can think of is research aircraft, perhaps as an atmospheric communications sat or a flying house for a rich person. 

[DDE]

On ‎05‎.‎04‎.‎2018 at 4:35 PM, AeroGav said:

3.  Servicing the damn thing between flights.            OK, this might just be a NASA cultural thing, but look at the huge army of very well paid techs that had to crawl over the Space Shuttle for months to ascertain it was safe to re-use and put humans on.   It would almost have been cheaper to build a new Shuttle for every flight.     Now imagine doing the same with a recently shut down nuclear reactor  that's still undergoing decay reactions and isn't shielded on all sides, how long will the maintenance take now and how much will it cost.

Perhaps you could use a robotic vehicle to attach lead plates to the unshielded side of the reactor before sending the techs in.   Answers on postcard, please.

Not sure about postcard, but here you go, purpose-built for Project PLUTO.

The Soviets built something remotely similar for the "dirty bomb" radiological warheads of the late 1940s.

[b0ss]

wow you guys are really smart. thanks so much for all those helpful answers!

[DDE]

@b0ss, we haven't even yet addressed the bear in the room: