Nuclear light bulb space craft "Liberty ship"

[Moontanman]

What we really need is a real space ship to not only explore the solar system but to lift huge payload to earth orbit and to bring those same payloads back. This space ship should be reusable and durable enough to be used many times. I envision this space craft not only being used for Earth to orbit and back but as a interplanetary space craft. What I am talking about is described in this article.

 

BRUCE BEHRHORST ARTICLE LIST

[Roadam]

I read the article a while back and the only serious thing left to figure out is how to contain the fuel in the glass bulb whilst not melting it. I guess that electromagnetic is the way to go but for that we need high temperature superconductors with the ability to carry really strong magnetic fields.

 

I was thinking about acoustic levitation, but then I realised that we have plasma in the core.

 

But otherwise its a very good article and I actually have that parent site under bookmarks.

[Moontanman]

I read the article a while back and the only serious thing left to figure out is how to contain the fuel in the glass bulb whilst not melting it. I guess that electromagnetic is the way to go but for that we need high temperature superconductors with the ability to carry really strong magnetic fields.

 

I was thinking about acoustic levitation, but then I realised that we have plasma in the core.

 

But otherwise its a very good article and I actually have that parent site under bookmarks.

 

I think the glass bulb is cooled by the liquid hydrogen reaction mass. I have tried to pin down the method used to contain the uranium hexafluoride gas, it would seem the best method would be electromagnetic but most sources seem to skip over that like it's a given.

[Roadam]

If one wants to contain all the radioactive material in the bulb, then cool hydrogen shouldnt even get into the bulb, but if not then the superhot core could easily touch the walls and melt them. The temperatures stated are about 25k K for core and I think fused quarz melts at about 1-2K K.

 

Nothing is hard, just a little bit of a superconductor and a duct tape, and you get a reactor. But its surely fascinating to think about what can be done if it would be possible to build a rocket like that.

[Moontanman]

If one wants to contain all the radioactive material in the bulb, then cool hydrogen shouldnt even get into the bulb, but if not then the superhot core could easily touch the walls and melt them. The temperatures stated are about 25k K for core and I think fused quarz melts at about 1-2K K.

 

Nothing is hard, just a little bit of a superconductor and a duct tape, and you get a reactor. :hyper: But its surely fascinating to think about what can be done if it would be possible to build a rocket like that.

 

Actually the hydrogen doesn't flow into the bulb to cool it it flows around the bulb to cool it.

[Roadam]

Thats ideally, but that glass has 20k on the one side and it has to have no more than 1500k on the other. I didnt yet go into the details, but I dont think that you could easily cool the bulb without it melting on the inside.

 

Well, if the magnetic containment would be good enough to keep the hot core away from the walls, then I guess everything would be good.

[CraigD]

I’ve read everything I’ve been able to find via nuclearspace.com about the “nuclear lightbulb” gas core nuclear fission rocket motor and the “Liberty Ship” heavy lifter to be built of 7 of them, and didn’t find much serious engineering detail or less than wildly optimistic speculation.

 

I’ll try to outline, in a somewhat rambling narrative, the basics of the nuclear lightbulb engine:

 

Like the simpler “open cycle” motor, coolant (hydrogen, which in an open cycle motor is also propellant) is flowed inside the reactor vessel (which is transparent glass), in order to “swirl around” the hot fissile material containing gas to prevent it from contacting the vessel walls. Magnetic fields may also be used to keep the hot gas away from the vessel walls. This allows the gas to reach a very high temperature (most sources say something like 20000 – 25000 C, though my Wein’s law calculations put it around 18000) without melting the vessel.

 

Unlike the open cycle motor, the heating of the hydrogen within the vessel is not the heating of the propellant. Where the usual open cycle motor is surrounded by neutron reflector and a gamma photon (30 MeV) absorbing shell (typically of beryllium oxide), which also absorbs the much lower energy ultra violet photons (10 eV). The ultraviolet photons, which were created by the hot gas inside the vessel, potentially carry many times more energy (as much as 5-6 times) that the neutrons and gamma photons, because they are produced by collision of the much more massive atoms (mostly uranium, its lighter fission products, such as krypton and barium, and “carrier” elements such as fluorine) that carry most of the energy of nuclear fission. (nuclear physics source: wikipedia article “nuclear fission”)

 

Hydrogen absorbs UV photons well, so the liquid and gas hydrogen inside the outer vessel and outside the inner should absorb nearly all of the reaction energy. The hydrogen inside the inner vessel absorbed UV photons, too, but because it is a much thinner layer than the outer, not as much – otherwise, the motor would just be an open cycle motor. What a NLE does with its heated core hydrogen, I didn’t notice any reference to – apparently either it’s stored, or refrigerated and recycled.

 

About this time, I was asking myself “so why not just have an open cycle motor?” There are a couple of obvious answers:

 

Even if the LH2 does a perfect job of keeping them from melting the vessel, fission fuel (uranium) and products will get mixed into the coolant. If this is also your propellant, as it is in an open cycle motor, your rocket exhaust is full or dangerous radioactive waste.

 

Because you’ve got to have a big exhaust hole in an OCM, it’s presumably harder to keep it swirling in the proper pattern and at the proper pressure to protect the vessel than with a closed inner vessel.

 

After learning this, I next wondered how difficult it is to make a gas core reaction of any kind. The answer is pretty daunting – though it’s been studied for more than 50 years, nobody has built even a small, lower-power gas core test reactor.

 

Now, pretty smart technologists as far back as the 1960s seemed to think that not only could GCNRs be built, but be very low mass, high power, and flown in spacecraft. At first glance, this seems like unreasonable optimism, but a factor supporting such optimism is that much of the difficulty of juggling a core of 20000 C fissioning gas with swirling cold H2 and magnetic fields is lessened if the vessel is not experiencing much acceleration, as an Earthbound research reactor (which is, of course, under a constant 1 g of acceleration) would be. The rocket motors they had in mind produces very small accelerations, on the order of 0.01 g, and would be fired only in space, where their radioactive exhausts wouldn’t be a problem. In other words, it’s easier to operate a GCNRM in space than on Earth.

 

All this leads me to what I think are a couple of big problems with the whole Liberty ship idea:

 

Because it will be subject to varying accelerations of 0 to 3 or more gs, stabilizing the fissioning gas in the reactor will be very hard. Because the glass inner vessel wall is thin (if you make it thick, it will be increasingly opaque to UV, defeating the main reason for it being glass), even a brief failure that allows the 20000 C gas to touch the 1650 C melting-point vessel will burn through it.

 

A very fast reacting feedback system might be able to meet this challenge, but it’s a scary, intrinsically unstable juggling act.

 

A nuclear lightbulb reactor emits a lot of gamma photons. One the same mass and size of a NERVA 2 (which has actually been built and test run), will emit a gamma photon flux proportional to its power, about 10 times the NERVA 2. The 12000 kg NERVA 2, including a 240 kg shield, gave a total radiation dose of about 130 grays to its payload, 26 times the human fatal dose. Because they have minimal shielding, nuclear rocket motors also release an extraordinary number of neutrons, many in the low-velocity range prone to absorption by atomic nuclei, so they transmute their external materials into radioactive isotopes.

 

Extra shielding for sensitive or living payloads, and operating the rockets only in remote locations, such as ocean launch sites, as Anthony Tate’s “Opening the Next Frontier” proposes, could address these issues, but describing these or other nuclear rocket motors as “clean” is, while in a ecological sense true, common sensically deceptive. In other words, I wouldn’t want one in my back yard. More seriously, I fear their high neutron fluxes limit the reusability of the proposed motors and entire vehicles – though, given their potential for tremendously economical payload launching, this is, IMHO, more of an aesthetic flaw than a practical one. A rocket that can orbit a million kg could be single use only, and still be a tremendous advance in spaceflight.

 

While I think “Opening the Next Frontier” is technically underweight, and understates the challenges of nuclear fission rocket motors, I strongly agree that it’s a shame there’s been so little study and actual prototyping of NFRMs since the early 1970s. While there’s not been a complete absence of research (eg: the Pratt & Whitney TRITON solid core nuclear rocket motor), I agree Tate’s feeling that, had the level of engineering R&D effort of the 1950s-70s been sustained, nuclear rockets might currently exist, and have dramatically improved spaceflight technology.

[Roadam]

Very nice summary CraigD. Phew, that took some time to read. ;)

 

Well I was thinking about that containment thing, and I think that any strong containment would deflect fuel much stronger than a force of 3g would pull. After some time I came across Dr. Bussards last project. The Polywell, its actually inertial fusion containment device. But as it basically contain ions in the very center, I was thinking about using it for containment of nuclear fission. Although I am not sure how dense would the fuel have to be to become critical. Silly, but practical if whole fusion doesn't come out as its theorized.

Polywell - Wikipedia, the free encyclopedia

 

So I am imagining. Six metal toruses with superconducting wires inside would be cooled with liquid hydrogen. It would flow trough eight tubes which would also suspend the magnets at the corners. Also at one of these corners the ions would be injected. So if these ions would be of sufficient density in the middle, the spontaneous radioactive process would generate enough neutrons to split additional uranium cores and so igniting the core. Whole thing would be placed inside glass sphere so that electrons and ions circulating on the field lines just wouldn't strike it.

 

As the temperature of about 20k and associated speed of individual particles is much smaller than that required for fusion, containment would be even better, provided that particles would actually be ionized.

 

Alas it would be nicer if fusion itself would work in the same setting.