Liquid fission rocket

RobertDyck · 2002-09-19 04:22:38

Is it possible to use principles of continuous combustion to create an efficient nuclear fission jet engine? The Orion was based on nuclear bombs exploded behind the spacecraft, with a heavy concussion plate to absorb the portion of the shockwave directed to the spacecraft, and shock absorbers to smooth the acceleration. Most of the force of explosion would be lost to space. That is highly inefficient; but it was still supposed have an Isp up to 2500 seconds. Internal combustion in car engines is a contained explosion that pushes a piston. Air breathing jet engines burn similar fuel but with continuous combustion. Could nuclear fission provide thrust on the scale of an atomic bomb but with continuous combustion, and be contained so all of the thrust is applied to the spacecraft?

Jet and rocket engines generate thrust by gas expansion. To ensure maximum expansion and low pressure propellant thanks, this is done through a phase change: solid or liquid to gas. Fission of U235 involves absorbing a moderated (slowed) neutron to become U236. That breaks down in a fraction of a second into Krypton Kr89, Barium Ba144, and 3 high-speed neutrons. Krypton is a gas above -153.35?C, and barium boils at +1640?C (+2984?F). The NERVA engine had an exhaust temperature of +5500?F. Uranium dissolves in hydrochloric and nitric acids, so it can be liquefied. Could that solvent, or a liquid uranium compound, be mixed with a liquid moderator in the combustion chamber? The moderator in a fission bomb permits a rapid reaction. Could a rapid, continuous reaction be sustained in a rocket engine? You wouldn't want the solvent itself to be the moderator; radiation from the engine could trigger the fuel tank to become a giant atomic bomb. Without anything to moderate the neutrons, uranium can't explode. A 2-part fuel is for safety. The nuclear thermal rocket was supposed to have a radiation reflector for the combustion chamber wall. That permits a nuclear reaction with sub-critical mass.

Tyr · 2002-09-19 11:36:28

Molten core fission reactors for rocket engines are theoretically possible.

Mark S · 2002-09-20 08:52:42

The debate over Orion's efficiency rages. Over the years, bomb designers have worked on shaping the blast created by nuclear weapons, and I'm guessing that the Orion pulse units would throw the bulk of their force against the pusher plate.

Gas-core reactors have also been proposed for rockets. A Uranium-hydrogen plasma would be contained within a vortex, and the hydrogen would somehow be ejected while the Uranium plasma stayed contained within the reactor. It sounds like a far-term technology, but it promises an ISp greater than 5,000 seconds.

RobertDyck · 2002-09-21 01:35:48

Orion is based on bombs. The international community will never allow nuclear bombs in space. Orion will never fly for that reason alone. Add to that the inherent inefficiency of Orion. If bomb designers were able to perfectly shape the bomb so the force was perfectly 1 dimensional, it would still require as much aft pressure as forward to push a shock wave toward the pusher plate. That means a perfect shape would have at most 50% efficiency. Such perfection is impossible. The pressure of the shock wave itself will cause expansion into the vacuum of space. Expect less than 1% efficiency. But that low efficiency can produce 2500 second Isp, then what would you get from completely containing the nuclear combustion?

Gas-core reactors are based on a nuclear reactor that uses a slow reaction to produces heat alone. That heat is used to boil liquid hydrogen into super heated gas. Liquid hydrogen is injected into the top center of the vortex, and hydrogen gas is expelled from the bottom center. Most of the uranium gas would be pushed away from the center of the vortex by hydrogen gas expansion. Since the exhaust aperture only permits central gas of the vortex to escape, the uranium is recirculated. The US military has been using uranium vortexes for years to separate isotopes for enrichment, so the physics of the vortex is well known.

Fission combustion would be different than a nuclear thermal rocket. It would use uranium as propellant rather than just an energy source. There are two ways to increase propulsive force of a rocket: increase exhaust velocity or increase exhaust mass. Increasing the mass means increasing propellant mass as well, so that tends not to translate into increased Isp. Nuclear combustion would produce a great deal of heat, as well as doubling the number of atoms by transmuting uranium into krypton and barium, and changing propellant phase from liquid to gas. All this translates to extreme combustion gas pressure, so extreme exhaust velocity. Hypersonic exhaust velocity can produce wear on engine parts, but these gasses have very high mass therefore relatively lower velocity for given force. Temperatures involved would be extremely high, but gas expansion reduces temperature. Heat has 4 modes: translational, rotational, vibrational, and electronic. After a nuclear reaction it may take time for the modes to even out. Only translational heat produces gas expansion, so the gas may continue to produce more translational heat as it expands. That heat will produce more expansion.

A nuclear combustion engine would have a radiation reflector instead of a pusher plate. The pusher plate and shock absorbers would be heavy. That means lower fixed mass. Both the Orion and a liquid fission rocket would have uranium fuel between the engine and living compartment of the spacecraft, so a great deal of mass to block radiation.

The by-products of uranium fission are not stable isotopes; they will continue to decay. They have a half-life, which means half of the material will decay in that time. Krypton Kr89 has a half-life of 3.15 minutes to become Rubidium Ru89. Ru89 half-life is 15.15 minutes to become Strontium Sr89. Sr89 half-life is 50.53 days to become yttrium Y89, which is stable. The other product of uranium fission was barium Ba144. Ba144 half-life is 11.5 seconds to become lanthanum La144. La144 half-life is 40.8 seconds to become cerium Ce144. Ce144 half-life is 284.893 days to become praseodymium Pr144. Pr144 half-life is 17.28 minutes to become neodymium Nd144. Nd144 half-life is 2.29 quadrillion years (10^15 years) to become cerium Ce140, which is stable. All of these decay steps emit a beta particle (electron), except the last one. Nd144 emits an alpha particle (helium nucleus) to become Ce140.

The initial by-products of uranium fission, however, have a half-life of 3.15 minutes or 11.5 seconds. Exhaust gas will be expelled in a fraction of a second, so there would be no significant decay while inside the engine.

The temperature of these reactions would still be very hot. That requires cryogenic propellant to cool the engine, such as liquid hydrogen. Heavy water reactors use heavy water as both the coolant and moderator. What makes heavy water heavy is that the hydrogen is an isotope called deuterium. Would liquid deuterium act as a moderator?

Shaun Barrett · 2002-09-22 01:50:34

Hi Robert!
I'm a little rusty on my nuclear physics and wondered whether you might humour me by clarifying something?

You state: "Without anything to moderate the neutrons, uranium can't explode."

I thought the moderator was designed to harmlessly absorb neutrons before they can collide with another uranium nucleus and produce more neutrons. Hence, I thought that removing the moderator would allow more 'cascading' of the fission process and thus release more energy. From this, it seems to me that, without anything to moderate the neutrons, the risk of an explosion is increased .... or, at least, the reactor's heat output is increased.
From what you've said, I suspect I must have an oversimplified view of the controlled fission process. Where am I going wrong here?

Incidentally, a few months ago, I noticed an article about an Israeli physicist who advocated using an Americium based rocket engine. He was impressed at how a useful level of fission could be maintained in even quite thin strips of Americium. So he envisaged a core of long thin plates of it separated by spaces through which a steady stream of propellant could be passed. In the course of its passage between the Americium plates, the propellant would be heated to extremely high temperatures before being expelled.
Thus a simple, high-efficiency, continuous-thrust nuclear engine is created. I don't recall the ISp figures, unfortunately.
I recognise this is not the 'fission combustion' you postulate because the propellant and the fissile material are separate, but it sounds like the kind of continuous and efficient propulsion you're looking for.
Anticipating your objections(! ), the Americium-based engine's ISp is probably not much more than 1000 seconds, and I suppose you're looking for something closer to 10 times that figure(? ). Also, I suppose Americium is too rare and too expensive.

I like your idea for 'fission combustion', if it could be shown to be a valid means of employing fission's potential. If it could be made to work, it might be the answer to our prayers!

RobertDyck · 2002-09-22 11:29:41

The 3 neutrons from cracking a uranium atom fly outward so fast that they cannot be absorbed by another uranium atom. Unless you slow the neutrons down, they will just bounce off. Light water reactors blend a moderator such as graphite with the fuel rods so it is all mixed together. Radiation does not have to travel far to be absorbed by the next uranium atom. Control in a light water reactor is with control rods that absorb radiation, controlling raction rate by reducing radiation density. This can "turn off" the reactor by reducing radiation density below the critical level. A heavy water reactor uses heavy water as the moderator instead of graphite (or some other material) blended with the uranium. Radiation has to pass through coolant water to be absorbed in the next fuel rod. However, that also means if a heavy water reactor has a coolant leak it doesn't melt down, it just stops.

The Americium-based engine does sound interesting. I would like to hear if anyone has done any work on it.

Preston · 2002-09-22 13:41:39

The textbook Introduction to Nuclear Engineering by Lamarsh and Baratta (2001) claims that "the estimated mass of a chemical rocket required for a manned mission from a stationary parking orbit to an orbit around Mars is approximately 4,100,000 kg. The mass of a nuclear rocket for the same mission is estimated to be only 430,000 kg." Whatever! This is in the introduction as one of the reasons why nuclear engineering is useful. With Mars Direct, of course, the vehicle that carries the astronauts is more like 130 tonnes (?). Maybe they assumed some opposition class monstrosity?

Shaun Barrett · 2002-09-22 18:29:09

Thanks Robert!
I think I understand this 'moderator' thing a little better now.

Actually, my memory is getting as bad as my grasp of nuclear physics!
I checked up on the Americium-based rocket concept at
this site and discovered it's closer to your fission combustion idea than I had thought!
In fact, the fission products themselves are used as reaction mass.
It's really just a more efficient kind of ion engine, apparently.
Sorry if I've caused a lot of confusion!
???

RobertDyck · 2002-09-23 01:11:14

Hey, Preston. The 4,100 tonne spacecraft is probably the "Battlestar Galactica" design that came out of the 90-day report. They key points Mars Direct used to reduce mass were production of fuel on Mars for the return trip, and aerocapture. That is a hell of a lot less fuel you have to take with you. I believe "Battlestar Galactica" also included an onboard greenhouse to grow food. Mars Direct used a chemical-mechanical recycling system to recycle air and water, and brought packaged food.

Since you are studying nuclear engineering, could you tell us whether deuterium gas could be used as a moderator for a gas core nuclear reactor? After cooling the engine, liquid deuterium would be gas when injected into the combustion chamber. Fission combustion would require a reaction a lot faster than a reactor, combustion would require a reaction almost as fast as a bomb. What moderator is required to do this? What is the critical mass of uranium? Nerva used a radiation reflector to encase the reactor, that should permit lower critical mass. What material is available for a reflector, how effective is it, and what does that reduce critical mass to? What was used as a moderator for the gas-core nuclear reactor?

We could choke the exhaust to maintain critical mass in the combustion chamber. To ensure uranium doesn't exhaust before reaction, we could use the same vortex as the gas core nuclear thermal rocket. To use deuterium as a moderator, we would have to mix it with uranium gas rather than injecting it down the center. The keys to success are low uranium mass in the combustion chamber, and rapid reaction.

RobertDyck · 2002-09-23 23:42:59

I just got spam from a company in China trying to sell fire proof ceiling tiles. I've never seen spam from China before. This makes me think that posting details like critical mass on a public form is a very bad idea. Numbers really do make the difference between an engine that works, and one that doesn't. However, numbers than could be applied to weapons should not be published here. I would like a paper study to see if this idea has merit, but could you contact me privately Preston? Thanks.

Preston · 2002-09-26 17:23:57

Worry not, I don't know enough about critical mass for that to matter! My nuclear engineering experience is about one month in an introductory class so far. However, I also took a freshman research project class, which was just writing a paper under the supervision of the Fusion Professor, and I wrote this paper. I was just a freshman, so don't blame me if the model is a little sketchy. I'm doing physics, nuclear engineering is just a minor. As for your questions, all I can say is, deuterium is kind of expensive as I understand it, so as long as you don't need a large fuel tank full of it, it would be ok.

(standard disclaimer: Since writing this paper I realized that a liquid hydrogen propellant is unnecessary and it is far more efficient to just increase the thickness of the shell. This greatly increases the thrust while only partly decreasing the specific impulse because the increased shell thickness allows for more fusion to take place. This effect is illustrated in one of the graphs at the bottom of the paper. Also reflected in another graph is a correction to the calculated optimal specific impulse for tin (to about 7.2 x 10^5 s at a shell thickness of 0.027 cm and f=27%). The other metals also have correspondingly higher specific impulses. These changes along with a slower Mars mission would lower the helium-3 requirement significantly.)

Number04 · 2002-09-26 20:11:44

damn... you guys are smart.

I feel very out of my field here. Of course, if am in my first year of electronics electrical engineering, so, I don't have much of a field.

sethmckiness · 2002-09-27 00:10:41

Well number 04 I am in the same shoes as you. I am a journeyman level Sat Communications Maintainer in the Air Force and(equal to a few years of EE with Communications training in UHF and SHF) I also have a few years of Geology too. so I know a little about lot and a lot about very little... unfortunatly.. Will try to work towards a physics degree soon though..

dickbill · 2002-10-09 16:05:05

Hi Robert,

You seem pretty good at physic. I posted a question in another thread about combustion-fission. I wanted to know if a chemically drive fission was possible. Is it wrong to think that some fission of few uranium atoms could help a rocket propellant mixture to deliver its power ? Very very few fission I mean, more like a catalyzer for the propellant reaction.

Also, do you think that "cold fusion" has a future in propulsion ? I read somthing like a cavitation of bubble inside an acetone solution delivers more power, in addition to some neutrons, than it consoms energy. In a 2001 Science paper I think.
best,
dickbill

RobertDyck · 2002-10-09 23:56:08

I am always leery of chemical/nuclear combinations. Chemical reactions require more propellant mass than nuclear, so that reduces the effectiveness of nuclear drive. Nuclear reactions require a certain critical mass and the reactor typically requires a very heavy fissile fuel such as uranium, so that increases the fixed mass. A nuclear drive requires a very high specific impulse (low propellant mass) to justify the high fixed mass. If you do use very few uranium atoms to help a chemical rocket, how do you build the radiation density necessary to sustain fission? The usual means to maintain the radiation density is to increase fissile fuel mass to the point where natural decay produces the required radiation density. Once enough fuel mass is accumulated so that radiation from natural decay is sufficient to support a fission chain reaction, it is called critical mass. There are tricks to maintain radiation density with lower fuel mass, but I don't see how to do it with the "few uranium atoms" you imply.

"Cold fusion" was a term originally applied to a discovery a few years ago that later proved to not be fusion at all. The researcher did discover a means to unbalance the modes of heat. That is, briefly he increased the translational mode of heat while decreasing the other modes (vibrational, rotational, and electronic). Since most thermometers measure the translational mode only, that appeared to produce more heat that it really did. This is a useful technique because it can extract more heat in useful form; translational heat is what conducts from coolant to a heat exchanger. It could permit a coal or nuclear power generating plant to produce more electricity from the same amount of fuel. Unfortunately his premature announcement that this was "cold fusion" discredited his research and very little work has been done since.

On the other hand, the "cavitation of bubble inside an acetone solution" is real fusion. The solution was not normal acetone, it was deuterium acetone. This technique was first described to me as "sonoluminescence". It uses highly focused sound waves in water. The tight focus heats a small spot of water so it not only boils to a bubble of steam, it actually becomes super-heated plasma. That is where the glow comes from. The plasma bubble is very small, but very hot. The focused sound waves also greatly increase pressure inside the tiny bubble. Using deuterium acetone instead of water, the deuterium actually undergoes nuclear fusion. Helium-3 and tritium have been confirmed in the solution after the reaction. The only way to create an element is via nuclear reactions, so this confirms nuclear fusion has occurred.

The friend who first described sonoluminescence to me suggested this as a means to initiate a sustained fusion reaction. He thought a mini-fusion explosion could be triggered which would produce a shock wave in the solution that could be directed back to the focal point; thus initiating another reaction. The result would be a self-sustaining pulse fusion reactor into which you would feed deuterium bearing liquid fuel. Could it be scaled up to a commercial reactor? I don't know. That would require very carefully reflecting each shock wave to focus it into another pulse. It also requires controlling each pulse so it is neither too strong nor too weak.

As for propulsion, I don't see how to apply sonoluminescence to propulsion. I suppose you could use it as a trigger to ignite nuclear fusion in a cryogenic liquid deuterium medium, but the result would either be too weak to be useful, or a fusion explosion of the entire liquid deuterium medium. The trick is controlling the reaction to keep it sustainable, yet small enough to be containable with a rocket engine.

nebob2 · 2002-10-10 00:40:32

To answer the origonal question, yes I think you can create a continuous fission reaction. The fuel would have to be laced with fissionable components so if allowed to freely react, it would go critical. This is the idea behing the nuclear salt-water rocket designed by Robert Zubrin in the early 1990's. Such a concept needs far more then afew atoms to work, but would probly require an extensive magnetic nozzle to prevent the explosion from destroying the ship.

Most of the articles I have read by sonoluminescence researchers have been sceptical if the process can be scaled up to any sort of power producing levels.

dickbill · 2002-10-10 06:49:14

Thanks to reply Robert,

So OK, forget about fission or fusion. Then, are we going to be stuck forever with hydrogen and oxygen ? Those huge H2 tanks seem very incovenient. Is there no new molecule combination better or equivalent to H2/O2, which would require just small external tanks like those used for fighters ?
I am talking about chemistry here, for example, is there any useful application for the carbon cages everybody's talking about ?

nebob2 · 2002-10-10 10:45:32

There are several propellants which give off more energy the oxygen-hydrogen ones so common today. The problem with many of these is that the reaction product, or a signifigent portion, is not a gas, but a solid. Both NASA and the USAF experimented with adding hydrogen a working fluid in addition to the fuel and oxidizer in these higher energy fuels, such as Be-O2/H2 and Li-F2/H2. Ideal Isp for either fuel is around 700s. The tests were far from conclusive, as the Be-based fuel had low combustion efficiency and the Li-based fuel worked best for short missions, with F2/H2 being more efficient for longer ones. For boosters, tripropellent engines burn hydrocarbon fuel at low altitudes to minimize drag, and switch to hydrogen at higher altitudes to increase efficiency.

To get down to the size of a drop tank to power a booster or interplanetary craft, chemical propulsion just can't do it. Nuclear or plasma is the only way to do that in large craft, chemical combustion will never reach the effiency of either.

Phobos · 2002-10-10 20:10:59

You seem pretty good at physic. I posted a question in another thread about combustion-fission. I wanted to know if a chemically drive fission was possible. Is it wrong to think that some fission of few uranium atoms could help a rocket propellant mixture to deliver its power ? Very very few fission I mean, more like a catalyzer for the propellant reaction.

Your idea sounds similiar to the aimstar engine that would use very small amounts of anti-matter to ignite a fusion reaction in a fusion rocket. With the recent developments in trapping anti-matter (if those discoveries are genuine) concepts like ICAN which would use very very small amounts of antimatter as an aid in propulsion might not be that far off.

Preston · 2002-10-11 17:40:06

Robert, I think the power levels of sonoluminescence fusion are far too miniscule to allow for refocusing of the shockwave. The energy delivered by the sound is of much higher energy. Even if the energy from the fusion could be boosted, and we use inertial confinement fusion as an analagous model, the necessary focusing would be far too imprecise to cause collapses perfect enough to get more fusion. In inertial confinement fusion, the confinement is about 3 nanoseconds - so the lasers have to be timed much better than that to collapse the fuel uniformly.

Experiments were done with sonoluminescence in a vomit comet (KC-139 or something), so that the bubble shape was not distorted by gravity -- this boosted the intensity slightly because it collapsed more uniformly. But there's no way to avoid the fact that you're collapsing something as 'crude' as a bubble in liquid.

Interesting side note: If you've seen Chain Reaction with Keeanu Reeves, the fusion reactor was based on sonoluminescence.

Seems to me that a chemical fuel can burn itself quite well and to completion without fission to help it along.

dickbill · 2002-10-11 18:13:50

Hi all,

It's not that "chemical" is that bad, it's just that the concept of wasting engines, turbopomps, tanks and basically 90% of the expensive hardware to send 10% in orbit is deceptive. The space shuttle, yes, but the boosters and the big tank are lost. In addition, the space shuttle's weight is 90 tons or so, that would be better to put something else than the space shuttle itself on top of the big boosters and its powerful engines. The idea seems obvious, why not to put a full load instead of the space shuttle, like a an interplanetary space ship, nuclear/plasma or ion propulsed . Just keep the boosters and the tank. This has never been proposed ?

ArianeV has been proposed originally to launch a small navet, but the concept has been abandonned. It seemed a good launcher for that purpose however. Same for the russian Energia. With no goals to go further, or no big easy money at the end, such projects are simply abandonned. But there is a goal to go further: the Mars and space exploration and colonization.

nebob2 · 2002-10-11 19:14:58

It all comes down to the need for a BDB. I think the designers of Sea Dragon may have had the right idea, reduce the cost of the booster by excepting sub-optimal proformance. By using cheaper materials, deadweight was increased, but the vehicle could always be scaled up to compinsate. Once it becomes cheap to place large payloads in orbit, expansion into space is inevitible.

Shuttle C was very similar to what you are describing, essentialy a partialy reusible cargo pod which would replace the shuttle for cargo capibility. The engines and boosters were reused, but the rest was expended.

Austin Stanley · 2002-10-13 02:12:36

Regardless of weather or not liquid fission is possible or not, I bet the key limiting issue for these types of enginges will be heat. Whatever you intend to push these very hot gasses agianst is also going to get very hot. And there are limits to what amount of heat any substance can take. As I understand it you could use a nuclear reaction (be it controled as in a reactor or runnaway as in a bomb) to heat your propelent up practicly as much as you could desire (well maybe not but you could make it realy, realy hot). But you have to find a way to hold onto this hot gas long enough for it to give you some thrust. You can refrigerate you plate (like they do on most modern chemical engines), make it realy thick (like with car engines), give it a high melting point (jet engines), let it radiate is some how, or whatever. But you've got to deal with that waste heat somehow. Or do I misunderstand things entirely?

Unrelated and even contradictory to what I just said. It ocurs to me that you could heat up chemical propelents before you combust them (although combustion maybe more difficult at high temperatures). Modern engines already do this, but not on a very large scale. You might be able to, for example, heat you LOX/LH up with a nuclear reactor and then combust them. Assuming combustion is possible, I don't see any theoretical reason why the temperature (and thus velocity) wouldn't add up. Practicly there might, of course, be some difficulties (like what I mentioned above). What do you think?

dickbill · 2002-10-13 12:30:34

Can you describe shuttle-C nebob2 ? I 've never heard about it. If this is what you said, the shuttle is replaced by a "cargo pod", the total mass of this cargo pod should then be around 100 tons. 2 or 3 like that, assembled into orbit, could constitute the first space ship designed for interplanetary trip, and never designed to land, wherever is the place to land. That ship should just have to come back to low earth orbit to refuel after its trip and would only go from low earth orbit to a low planet orbit.
The safest fuel to propulse this ship, according to what I've read in this thread, is still a chemical H2/O2 (or hydrocarbure/O2) for the orbital insertion where you need plenty of power quickly available, associated, maybe, whith a nulear or ionic reactor for the long term trip.

It seems to me that a fleet of Shuttle + Shuttle-C makes sense and makes the long term spatial exploration possible. Since the fleet of space shuttle already exist, why NASA never invested in that shuttle C ?, it's certainly not more expensive than the ISS.

nebob2 · 2002-10-13 15:34:17

I doubt a pysical nozzle would survive the reation. It sin't heat so much as ablation caused by the plasma. That is why I suggested using a magnetic nozzle, eliminating the need for the plasma to touch the nozzle's surface.

I could take up more space describing it but this site will give you the details. If I remember right a demonstator was actually built, although it no longer exists. 77 tonnes to LEO with virtually no new development costs.

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#1 2002-09-19 04:22:38

Re: Liquid fission rocket

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