Feasibility of Hybrid Nuclear Thermal Jet and Nuclear Thermal Rocket?

[jfull]

Someone may have brought this up before, but how easy/difficult would it be to create an engine that could function as both a Project Pluto style nuclear ramjet and an NTR?

I've been thinkng about it, and I can't see anything preventing such a design from being built, but I'm not an expert on either technology.

The only issues I could see would be that the nuclear jet mode probably wouldn't be able to reach the same speeds or altitude as the jet engine mode of a Sabre-type engine. But as long as it could get the craft above a fair bit of atmosphere, the NTR mode should be able to get it the rest of the way to orbit. (IRL NTRs not being as low thrust or as impractical in atmosphere as those in KSP).

One thing I want to make clear: the idea of putting live nuclear reactors in the air or into orbit is controversial and I understand all the reasons why. I'd be concerned about the safety of a craft using engines like these myself, despite the benefits.

But lets try to avoid any heated discussion over the ethics of nuclear propulsion.

[Scotius]

Yes. That old horse was already pummeled to death. Many times. Project Pluto sounds awesome at first sight. At second sight, you will notice inconveniences...like need to shovel mountains of money into developing necessary technologies, murderous (and continuous) sonic boom, safety concerns (think 11.09...but with nuclear reactor on board). And of course unavoidable backlash from the environmentalists all over the world - and in this case they would be totally justified.

[jfull]

Yes. That old horse was already pummeled to death. Many times. Project Pluto sounds awesome at first sight. At second sight, you will notice inconveniences...like need to shovel mountains of money into developing necessary technologies, murderous (and continuous) sonic boom, safety concerns (think 11.09...but with nuclear reactor on board). And of course unavoidable backlash from the environmentalists all over the world - and in this case they would be totally justified.

But lets try to avoid any heated discussion over the ethics of nuclear propulsion.

This is kinda the stuff I'd rather not have brought up

What I want to know is if the technology could work, since we have built prototypes of NTRs and Nuclear ramjets years ago

Edited December 28, 2014 by jfull

[shynung]

Well, both nuclear thermal jets and rockets have been studied in Project Pluto and NERVA, respectively. The SABRE engine from the Skylon project is a study of a hybrid jet/rocket engine, even though it hasn't gone very far as of now. Though they did get the precooler part running.

Basically, a hybrid nuclear thermal jet/rocket is an amalgamation of these technologies, which is to say that the concept is feasible.

[B787_300]

Oh the concept is entirely feasible. Now the issue is that you dont want to run those things here on earth for the most part. Radiation is a terrible thing, and while the benefits are large (great ISP, decent Thrust) the radiation is a huge NO in that category. and even if you want to use in in Space there are issues (there were protests and suits brought against NASA for launching RTGs) so while the tech is there, using it is more difficult.

[shynung]

I don't see why the radiation would be a huge concern. Barring an open-cycle reactor, all nuclear fuels and reaction products stays in the reactor. The propellants (either ambient atmosphere or liquid rocket propellant) are merely heated, and spewed out the nozzle. And it doesn't have to be direct heating, either; an intermediary coolant can take the thermal energy from the reactor and subsequently heat the propellant with that energy.

Protests against RTGs and nuclear technologies in general are mostly political in nature. As in, people simply don't like anything with the word 'nuclear' in it, regardless of its actual usefulness or safety. Let's just leave it at that, shall we? I don't want anymore pro/anti nuclear argufights.

Edited December 28, 2014 by shynung

[jfull]

You could significantly reduce radiation concerns by launching from the middle of the desert, and of course making the craft an unmanned cargo ssto

[shynung]

Or use a closed-cycle nuclear thermal rocket. It can be designed to minimize radioactive isotopes in the outgoing exhaust.

Though, I have to say that making a single-stage-to-orbit craft is far more difficult than it may seem.

Edited December 28, 2014 by shynung

[morat242]

I don't see why the radiation would be a huge concern. Barring an open-cycle reactor, all nuclear fuels and reaction products stays in the reactor. The propellants (either ambient atmosphere or liquid rocket propellant) are merely heated, and spewed out the nozzle. And it doesn't have to be direct heating, either; an intermediary coolant can take the thermal energy from the reactor and subsequently heat the propellant with that energy.

Protests against RTGs and nuclear technologies in general are mostly political in nature. As in, people simply don't like anything with the word 'nuclear' in it, regardless of its actual usefulness or safety. Let's just leave it at that, shall we? I don't want anymore pro/anti nuclear argufights.

Except that they were never able to lift enough shielding mass to produce an aircraft capable of carrying a useful payload. They only ever managed to shield the flight crew from the reactor, and only ever pulled the control rods when it was already in flight. Who wants to service an aircraft that is spraying neutrons at you? Who wants to live under a flightline with a lightly shielded reactor flying overhead every half hour? What would reactors at full takeoff power do to an airport?

And note, GE's design for the X-6 powerplant was 5 tons of reactor, 48 tons of shielding, 9 tons of engine, and 20 more tons to actually hook everything up. And that was for a reactor that was A) not fully shielded and directly heating the air, leading to radioactive exhaust as well as the expected spray direct from the reactor. P&W's indirect cycle engine was never anywhere close to ready, and would have been substantially heavier if it had been.

Edited December 28, 2014 by morat242

[MBobrik]

Except that they were never able to lift enough shielding mass to produce an aircraft capable of carrying a useful payload. They only ever managed to shield the flight crew from the reactor, and only ever pulled the control rods when it was already in flight. Who wants to service an aircraft that is spraying neutrons at you? Who wants to live under a flightline with a lightly shielded reactor flying overhead every half hour? What would reactors at full takeoff power do to an airport?

A hydrogen tank and all other machinery between the reactor and the crew/ payload would provide more than adequate shielding for the crew. Ground facilities could be protected by an *external* shield. And the runway would have to be from materials that are resistant to secondary neutron activation, there should be no human nor animal outside within several miles distance from the airport while this thing is taking off. But once in air, and over ocean, the inverse square law will offer enough safety. Surely it would leave a trail of ozone, nitrogen oxides, and traces of carbon 14 in its wake but if the traffic is sufficiently low this would not be a serious environmental problem. If it were to crash, reactors running on high enriched uranium are small and opposed to all other reactors uses, run briefly. They therefore won't accumulate large amounts fission products. the worst thing that would happen is a local spot of radiated water that dissipates quickly into irrelevance.

Concerning construction and operation, it would presumably operate like SR 71 except it could fly higher because its engines don't need oxygen. Then, at some point, external air would be replaced with hydrogen, and it would continue nerva-style.

[jfull]

I really wouldn't like to see this kind of engine used on a crewed vehicle.

The vision I had for it was a Skylon (or perhaps more like HOTOL, with the engine at the back) type unmanned craft, but with increased cargo capacity and lower propellant requirement.

Basically just an effective way to get hardware into orbit, not people

[Nuke]

if you just want a launch vehicle it would be better to keep the reactor on the ground to make fuel for a chemical launch vehicle. but if you want something you can take off from earth, fly to mars, and land there, and make the return trip all with one engine and no propellant usage while in an atmosphere, for a general go everywhere ship, then that would be totally awesome.

i think a molten salt type reactor would work best here, you have your closed fuel loop isolated from your coolant loop. where the coolant loop is open, taking atmosphere from an intake or hydrogen from a cryotank and spewing it out the tail pipe, or to a closed radiator loop for offline power generation. that is a lot of heat exchangers which are heavy, not to mention the shielding for the fuel loop. you would need a high temperature version of the sabre precooler to provide the hundreds of megawatts in heat transfer. but then you get the problem of embrittlement caused by neutron damage in all those tiny tubes to deal with, so you might want another coolant loop in there or compromise with a less efficient, more robust heat exchanger. either way its going to cost you thermal power. i have a feeling it would be very difficult to get an engine with a twr > 1.

bring in a small fusion reactor with direct conversion, maybe you can power electric turbine engines in the atmosphere and mpd engines in space, but that is probibly centuries away and just as theoretical as the fission hybrid.

Edited December 28, 2014 by Nuke

[NERVAfan]

Even on purely technical (not political) grounds, I'm not sure it would be all that awesome given the reactor dry mass and shielding mass, etc. (It might be, but it's not necessarily an automatic win - you'd have to do a trade study vs. chemical options.)

Even if SSTO was a requirement, chemical SSTO is entirely possible with off-the-shelf technology IIRC, just not especially practical in terms of payload fraction.

And if Skylon gets developed....

EDIT: Well, I guess on purely technical grounds you could do an unmanned launcher with much less shielding, as jfull says. Still, many payloads probably won't like massive radiation doses.

EDIT x2: Don't get me wrong, I'm a huge fan of nuclear propulsion (hence my username) but I don't think solid-core ones are really worth it for ground launch -- they make awesome upper stages where specific impulse is most important.

If you had a "Nuclear Lightbulb" (closed-cycle gas-core) that might change, and Orion (external nuclear-pulse) is awesome for ground launch... assuming you ignore those political/environmental concerns

Edited December 28, 2014 by NERVAfan

[morat242]

A hydrogen tank and all other machinery between the reactor and the crew/ payload would provide more than adequate shielding for the crew. Ground facilities could be protected by an *external* shield. And the runway would have to be from materials that are resistant to secondary neutron activation, there should be no human nor animal outside within several miles distance from the airport while this thing is taking off. But once in air, and over ocean, the inverse square law will offer enough safety. Surely it would leave a trail of ozone, nitrogen oxides, and traces of carbon 14 in its wake but if the traffic is sufficiently low this would not be a serious environmental problem. If it were to crash, reactors running on high enriched uranium are small and opposed to all other reactors uses, run briefly. They therefore won't accumulate large amounts fission products. the worst thing that would happen is a local spot of radiated water that dissipates quickly into irrelevance.

Concerning construction and operation, it would presumably operate like SR 71 except it could fly higher because its engines don't need oxygen. Then, at some point, external air would be replaced with hydrogen, and it would continue nerva-style.

Dude, they tried it in a B-36. The 48 tons of shielding deemed necessary for an operational aircraft was *with* shadow shielding the crew cabin and using the aircraft structure and fuel tanks as additional protection. With the experimental aircraft, they used the water coolant tanks for the reactor as well (as the X-6 was to have been air-cooled).

And that was with no payload and only a partly shielded reactor. To actually have a useful payload, you need a massively larger aircraft (shielding being roughly proportional to the square root of reactor power). Like, even without any intentions of going to space, you need an 747-8 sized aircraft at minimum. Even then, you'll be taking out most of the passengers.

You're talking about shielding the runway and the airport buildings, that's great. How the heck you can ensure that everything within several miles of an airport is devoid of life, I don't know, but whatever. What happens when the aircraft needs maintenance?

And in the end, we get, what, the ability to save on kerosene and liquid oxygen? If you could guarantee the massive space payload demand required to make this even vaguely feasible, you could easily justify a laser launch system...and then you could leave the nuclear reactors on the ground in full containment domes. Or not have them in the first place. What problem does this solve that there isn't an easier, cheaper, and safer solution for already?

[K^2]

I would strongly urge running it as closed cycle reactor + ETR/Scram, rather than open cycle NERVA style. Any dust passing through NERVA heat exchanger can become secondary source of radiation. Bad mojo. But having electric exchanger eliminates such possibility.

Otherwise, only danger is if the thing crashes. Even then, there are reactor designs that would minimize risk of contamination.

Bigger question is cost efficiency. Such SSTO would be huge, but have small payload. Fuel costs might be low, but operational and maintenance costs will probably be way higher than conventional rocket.

[MBobrik]

Dude, they tried it in a B-36. The 48 tons of shielding deemed necessary for an operational aircraft was *with* shadow shielding the crew cabin and using the aircraft structure and fuel tanks as additional protection. With the experimental aircraft, they used the water coolant tanks for the reactor as well (as the X-6 was to have been air-cooled).

Actually, It was only 12 ton and its design was far from optimal ...

[jfull]

Orion (external nuclear-pulse) is awesome for ground launch... assuming you ignore those political/environmental concerns

I think the only situation we'd ever build a ground-launched nuclear-pulse craft would be if we needed to get a WHOLE LOT of people off planet very quickly (preservation of mankind in a doomsday scenario)

But the amount of payload you could launch would be mind-blowing. Plus I've seen a concept where the whole craft takes off from Earth, transfers to Mars, and lands (using its piston plate as a heat shield and giant landing foot no less) without staging.

It would be the ultimate way to shoot a colony to mars, as the whole craft could be dissembled for building structures.

Edited December 29, 2014 by jfull

[morat242]

Actually, It was only 12 ton and its design was far from optimal ...

The numbers I used were for the follow-on proposal for a functional nuclear-powered bomber (since the NB-36H's reactor filled the bomb bay) with a liquid metal cooled reactor instead of pressurized water to save weight. The reactor was to be partially shielded, probably to reduce the difficulties in ground handling.

[Merlin]

Does anyone remember the secret Nuclear engine tests around the 50's then they were banned.

Perhaps we can model the engine after that one.

I work in the Nuke field let me see if I can get the specs.

More to come.....

I Got the video

- - - Updated - - -

Nuclear Basics

http://www.world-nuclear.org/Nuclear-Basics/

Nuclear thermal rocket at Princton article

Nuclear Reactors and Radioisotopes for Space

http://www.world-nuclear.org/info/non-power-nuclear-applications/transport/nuclear-reactors-for-space/

- - - Updated - - -

Nuclear Basics

http://www.world-nuclear.org/Nuclear-Basics/

Nuclear thermal rocket at Princton article

https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Nuclear_thermal_rocket.html

Nuclear Reactors and Radioisotopes for Space

http://www.world-nuclear.org/info/non-power-nuclear-applications/transport/nuclear-reactors-for-space/

This to start.......

NERVA (Nuclear Engine for Rocket Vehicle Application)

http://www.daviddarling.info/encyclopedia/N/NERVA.html

These specs should be able to make one for Kerbal

http://www.daviddarling.info/images/NERVA_engine.jpg

I am not at that level maybe someone.....

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NERVA technology

The NERVA program started out with the following objectives:

multi-mission capability

man-rated

based on full-flow topping

minimum chamber temperature of 2360 K and a minimum chamber pressure of 450 psia

minimum 75,000 lbf thrust

endurance of 600 minutes and up to 60 cycles

capable of 85,000 lbf and 500 psia transients

incorporating adequate shielding for manned operations

storable for 5 years on the ground, 6 months on pad, and 3 years in space

transportable by land, sea, and air

To meet these objectives, NERVA consisted of two reactor projects: the Nuclear Reactor eXperiment (NRX) and the eXperimental flight Engine Prototype (XE-Prime).

During its lifetime the NERVA/Rover programs accomplished the following records:

highest power: 4500 megawatts thermal power

5,500°F exhaust temperature

250,000 pounds thrust

850 sec. of specific impulse

90 min. of burn time

thrust to weight ratios of 3 to 4

By the time the NERVA program was terminated, the NERVA-2 had been designed that would have met all of the program's objectives. Two of these engines would have been fitted to a NERVA stage capable of powering a manned interplanetary spacecraft.

Edited December 31, 2014 by Merlin

[Idobox]

A nuclear ramjet/NERVA wouldn't be ideal on Earth, for the same reason air-augmented rockets are not used: the air breathing part of the flight would be quite short, and wouldn't justify the extra-weight. Plus all the issues with nukes.

Now, for a Venus or Titan probe, it's a completely different story.

You could use the NERVA for the transfer and different orbital manoeuvres, then aerobrake, use the jet mode in the atmosphere to go around, land and take off again. Because the atmosphere on Venus is so thick, you get to use the jet mode a lot longer than on Earth.

[Merlin]

It is not a first stage rocket it is what you install in space for the inter planetary flights 250,000 ft/lb exhaust contains minor radiation best used in space.

- - - Updated - - -

layered use of various types of shielding including surrounding the entire ship in

a water tank outer shell and storage to slow down cosmic & other radiation down

followed inside that with Hydrogenous material (like white plastic pipes or blocks)

for interaction with neutron or proton radiation.

Then all you need to do is create a monster magnetic field around it all by the

design of the ship and some rotation for gravity ................