How About A Nuclear Space Shuttle

John · Senior Member

How About A Nuclear Space Shuttle

GoogleNaut · Guru

Interesting idea...we've looked into this before. The primary problem (aside from protection of the crew from radiation) is thrust-to-weight ratio. Nuclear engines do not generate quite as much thrust to weight ratio as a chemical engine. The reason for this, is that if one factors in the relatively heavy (dense) radiation shadow shielding which sits between the reactor pressure vessel and the pumping machinery, then an NTR engine will always have a thrust-to-weight ratio less than an 'equivalent' chemical engine. A Wikipedia search on nuclear thermal rocket engines yielded an article:
http://en.wikipedia.org/wiki/NERVA

One article:
http://en.wikipedia.org/wiki/Nuclear_thermal_rocket
seemed to suggest that an NTR would produce a thrust to weight ratio of about 1:1, while the first article above seemed to indicate a 4:1 thrust-weight ratio. I think a 4:1 is not entirely out of the question, although this is dismal compared to a modern high performance chemical engine which can aproach 55 to 60 to one (a space shuttle SSME is about 55:1 thrust to weight ratio.) So this makes NTR an attractive upper stage, but would be difficult to utilize as a first stage based solely upon the thrust to weight ratio.

Another complicating factor would be the necessity of placing the engine as far back as possible (to keep it far away from crew and payload) within the fuseluge. This shifts the center of gravity way back which will be a definite design issue when trying to design an aerodynamically stable airframe. Examining the configuration of the space shuttle, one can see the influence of the weight of the three SSME's and the presence of the dual OMS pods (with propellants) shifting the CG so far back, that the wing must also slide back. This results in the characterisitic "V" shape of the shuttle's wing--most of the lifting surface is at the rear where much of the weight is. A single stage to orbit NTR powered 'shuttle' will probably be an even more extreme lifting body.

Other than that, I would imagine that NTRs would work just fine. As Jaro has pointed out many times to me and to others, since NTRs tend to operate for short periods of time (minutes at a time, versus a stationary power plant which may operate for years at a time,) the amount of fuel burnup is low and the accumulated fission products is also correspondingly low. So decay heat is fairly low, as will the residual radiation after shut down. A short cooldown period should be sufficient for swapout. I'm not sure how short, though. Perhaps days or a week--Jaro would know better than I. Shorter period swapouts not requiring exposure of maintenance crews might be accomplished by using a special 'hot cell' with servoed remotes. Engineering leak-proof quick connects for propellant and electrical couplings could be a trick though--I don't think that's ever been done for any engine.

-- Edited by GoogleNaut at 06:02, 2006-02-26

G R L Cowan · Veteran Member

GoogleNaut wrote:

Interesting idea...we've looked into this before. The primary problem (aside from protection of the crew from radiation) is thrust-to-weight ratio. Nuclear engines do not generate quite as much thrust to weight ratio as a chemical engine...

There's a conceivable way of alleviating the thrust-to-weight and radiation shielding problems, at the cost of imposing some tight scheduling constraints: get rid of the neutrons. Use really hot beta-active isotopes and ride to orbit, and maybe back down if you do the retroburn right away, on a giant, very short-lived RTG.

Since writing that I have realized that instead of having trillions of watt-days per day from the stationary reactor, one might equip it with a large molten salt reservoir -- (Li Na K)F is kind of keen -- and run it at very high power for the brief time it takes to activate the beta-decayers, then tap the heat at a more usual power-plant sort of rate.

--- Graham Cowan, former hydrogen fan
Boron: internal combustion, nuclear cachet

john fraz · Veteran Member

RE: Nuclear Space Shuttle

John · Senior Member

RE: How About A Nuclear Space Shuttle

Martin Schwingenheuer · Member

Dear Members.
A Nuclear Shuttle could be similar like Ezechiel Shuttle mentioned by Josef Blumrich.What are you thinking about this?
Or a compact dome shaped vehicle with a dozend clean NTR Nerva derivates on the rear ,therefore the ship could start without chemical axillary power.
Best Regards:

Martin

GoogleNaut · Guru

Sure you could put a lot of parallel engines together. But you'd have to be careful about cross coupling the neutron flux from the engines---not taking this into account could create nasty surprises, as the whole engine array could begin to behave like one large reactor. This is one reason why most multi engined NTR's (such as LANTR) are depicted with a cluster of at most 3 engines, usually spaced in a circular array. This makes sense in the thrust distribution view--a circle distributes thrust loads more directly to the tankage which is the primary structural backbone of most liquid rocket systems. Also, the radiation environment around the reactors should be more symmetrical, making it easier to compensate and control 'coupled' neutron fluxes from the individual NTRs.

Ty Moore

Martin Schwingenheuer · Member

NTR engines isolation.
Maybe we could inerrogate the fuel like a neutron barrier,but the problem still exist when the propellant becoms low or empty.
An other way could be an arangement of the NTR´s circular around the fuselage,like mentioned in your discription.Due the high weight grapite or lead protection is nearly impossible.
A further difficult aspect is the fact,that the NTR´s uranium amount can´t be reduced for one flight only.It will contain the energy for hundreds of possible flights.
Martin

GoogleNaut · Guru

Typically, new reactors on startup have large excess reactivities--thust the integration of 'burnable' poisons are a must to control excess reactivity. "Burnable" poisons help reduce the activity level to a conrollable ammount. As the fuel is consumed, so is the consumable poison, so a much more even ouput can be achieved. I'm not sure how much 'burnable' poison might be needed for an NTR, where the power density is orders of magnitude higher than in conventional power reactors--probably a lot.

G R L Cowan · Veteran Member

GoogleNaut wrote:

Typically, new reactors on startup have large excess reactivities--thust the integration of 'burnable' poisons are a must to control excess reactivity. "Burnable" poisons help reduce the activity level to a conrollable ammount. As the fuel is consumed, so is the consumable poison, so a much more even ouput can be achieved. I'm not sure how much 'burnable' poison might be needed for an NTR, where the power density is orders of magnitude higher than in conventional power reactors--probably a lot.

I think the very high operating temperature, cf. www.osti.gov/energycitations.../828620.pdf, implies low burnup. That implies reactivity won't change much, and in turn this implies no need for burnable poison. Another factor favouring low burnup is how soon one runs out of hydrogen coolant.

Martin Schwingenheuer included:

A further difficult aspect is the fact,that the NTR´s uranium amount can´t be reduced for one flight only.It will contain the energy for hundreds of possible flights.

Leaving all the actinides on the ground, and having on board only, e.g., vanadium-52, solves that problem and some others.

--- Graham Cowan, former hydrogen fan
B: internal combustion, nuclear cachet

10kBq Jaro · Senior Member

G R L Cowan wrote:

There's a conceivable way of alleviating the thrust-to-weight and radiation shielding problems, at the cost of imposing some tight scheduling constraints: get rid of the neutrons. Use really hot beta-active isotopes and ride to orbit, .... on a giant, very short-lived RTG.

Graham,

I read your web page on very short-lived RTG.

Interesting concept, though very iffy, IMO.

Your power production arguments seem sound, but I'm dubious about the initial assumption of "Getting beta-ray power by adding a nuclear reactor's surplus neutrons to the nuclei of lighter, nonfissionable atoms requires, perhaps, ten fissions."

Where do you get the figure of ten fissions, to get one V52, while maintaining a functioning, critical nuclear reactor ?

Your V51 density would have to be quite high, compared to that of the fissile material, to get that kind of performance, I would think.

Here in Canada we use Co59 shim rods in our CANDU reactors to produce Co60 for commercial industrial and medical radiation source use. I have seen the rods sitting in a holding pool following irradiation -- the Cherenkov glow is much brighter than what you see in spent fuel pools.

But the quantity is small relative to the entire reactor core, and the Co60 production rate is slow, even though its 17 barn x-section is considerably better than the 4.8 barns for V51. You might think putting lots more Co59 into the reactor would increase production. But in fact it would only prevent the reactor from ever starting up. Increasing the fuel enrichment would help, but only a detailed numerical analysis could provide some limits....

The only idea similar to yours that I have seen, was the cold-war concept of creating a radioactive barrier against armoured invasion forces by spraying a strip of land with short-lived (some hours or days, at most), highly radioactive material created by neutron activation, as you suggest.

Naturally, the difficulties of quickly producing such large quantities of short-lived radionuclides were recognized -- chiefly due to fairly low neutron absorbtion x-section of the candidate base material, combined with the inadequate neutron flux of even large reactors.

So the solution proposed was to generate the required huge neutron flux using neutron bombs contained in some sort of engineered underground cavity, and distributing the product radioisotopes to the border region using a network of pipes.... The high flux is complemented by the high neutron energy from deuteron & triton fusion reactions (carrying some 80% of the total energy released - in contrast to the ~1% carried by fission neutrons), which puts the absorbtion x-section into a more favourable range, for many of the low x-section isotopes at thermal energies.

Of course the product material, dissolved in ordinary water, would be contaminated with some small amount of fission products, from the N-bomb trigger. That might be acceptable in a live-or-die war situation, but not for peacetime civilian rocket propulsion application. So I don't think this would be a viable alternative to reactor production of radioisotopes. And I seriously doubt that a reactor could do it

Taylorchef · Member

NASA did a study on several different types of nuclear powered Shuttles. They were SSTO, & Exo-atmospheric only & a couple in between.

I tried to Google for a link & couldn’t find anything. I know I have some stuff on a PDF file at home though (I can only access the internet at the library).

Contact me via e-mail, or I’ll try to remember it the next time I come here.

Taylorchef · Member

I found it!!! Here is the link to the NASA page that has the study done on a Nuclear Space Shuttle.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720015242_1972015242.pdf