MITEE - A Miniature Nuclear Propulsion Engine

New WOrlds COM ^ | FR Post 4-26-02 | Editorial Staff

[vannrox]

MITEE - A Miniature Nuclear Propulsion Engine

Chemical rockets have reached their limits. The hydrogen/oxygen chemical rocket can only deliver a specific impulse of about 450 seconds. The specific impulse is a measure of the thrust delivered by the rocket per unit mass of propellant consumed. Increasing the specific impulse to 1000 seconds, which is possible with Nuclear Thermal Propulsion, enables the spacecraft velocity to be over 2 times greater than that for a hydrogen/oxygen rocket, for the same mass of propellant consumed.

Sidebar 1 compares the IMLEO (Initial Mass in Low Earth Orbit) for hydrogen/oxygen rockets with those for nuclear rockets as a function of the velocity increase (Delta-V) imparted to the spacecraft, starting from its initial location in Low Earth Orbit. The IMLEO increases exponentially with Delta-V, causing the practical limit on Delta-V to be ~10 kilometer per second for hydrogen/oxygen rockets, and ~22 kilometers per second for nuclear rockets. The greater Delta-V capability of nuclear rockets allows space missions, particularly to the Outer Planets (see The MITEE Express to Jupiter and Beyond) to be carried out in much shorter time with much smaller launch vehicles. The spacecraft trajectories are direct, and do not require multiple gravity assists from flybys of Jupiter and other planets.

In addition to faster, cheaper mission, nuclear rockets enable unique new missions not possible with chemical rockets, such as sample return from Europa and Pluto, a Pluto orbiter, etc.


Plus Ultra has developed a new and unique, very small, very lightweight nuclear rocket concept, termed MITEE (MInature ReacTor EnginE). MITEE is described in detail in 3 reports: MITEE-1, the initial design; MITEE-2, are advanced design; and MITEE-3, a report on the NASA-SBIR program carried out on MITEE.

As illustrated in Sidebar 2, the MITEE nuclear engine consists of a close-packed assembly (typically 37) beryllium pressure tubes. Each pressure tube contains an outer annular cylinder of 7LiH moderator, and an inner annular rod of perforated tungsten 235UO2 metal matrix composite fuel sheets. Cold hydrogen propellant flows downwards at ~100 K along the outer surface of the 7LiH moderator, then radially inwards through the moderator and the tungsten - UO2 fuel sheets. The hydrogen propellant emerges from the final fuel sheet at 3000 K, and then flows longitudinally down through a central hot gas channel to the exit nozzle at the end of the pressure tube.


The MITEE engine is similar to the Particle Bed Reactor (PBR) engine which underwent development for defense applications (see Nuclear Thermal Propulsion), except that it is smaller and lighter, uses multiple pressure tube construction instead of a single pressure vessel, and tungsten -UO2 metal matrix fuel sheets instead of a packed bed of small HTGR type fuel particles. The main features of MITEE are summarized in Sidebar 3.

The mass of the MITEE reactor is very low, ~100 kg, and the weight of the complete engine only about 140 kg. This order of magnitude reduction in mass over previous engines like NERVA makes MITEE very attractive for planetary science missions where high V performance and very low engine mass are critically important to mission success, and to keeping mission cost within acceptable limits.


MITEE has a very strong technology base. Its tungsten -UO2 fuel was demonstrated to have >3000K lifetime capabilities in the 710 reactor program in the 1960's. Fuel region power density capability of 30 megawatts per liter was demonstrated in the PBR program, 3 times greater than the value called for in MITEE.

[jimkress]

Nuclear thermal rockets are thus not useful (or politically possible) as boosters launching from the ground.

Don't think that is the point here. As I read it, the MITEE would be used only in space to propel missions from earth orbit (or Lunar bases) to other planetary (or stellar) locations.

[fourdeuce82d]

As a rocket scientist type guy, could you comment on the following? Also, what's your take on Zubrin in general?

In his paper describing the concept, Zubrin considers using a Nuclear Salt Water Rocket for a round trip mission to Titan, Saturn's largest moon. The NSWR would be fueled by 20% enriched uranium in the chemical form of a soluble salt (uranium tetra-bromide) dissolved in ordinary water at about the same atom number concentration as the salt in sea water.

Fissionable isotopes in such concentrations can easily produce great heat from fission reactions or even a nuclear explosion. An uninterrupted volume of this liquid massing a few dozen kilograms would reach critical mass, massively fission in a sustained chain reaction, and explode. In Zubrin's scheme 41,000 kilograms (41 tonnes) of the salt water fuel are stored in a neutron-absorbing fuel tank. The fuel tank would be made from long tubes of boron carbonate, a strong structural material that strongly absorbs thermal neutrons, preventing the fission chain reaction that would otherwise occur in the fuel. The liquid fuel is pumped from the storage tank into a absorber-free cylindrical reaction chamber which allows buildup of neutron flux to the critical point where sustained nuclear fission can occur.

In a nuclear rocket the reaction chamber presents a severe materials problem because no conceivable mechanical structure could sustain the force of a nuclear explosion. However, Zubrin uses a very clever trick. He has used a simplified model to show that the distribution of fission-inducing thermal neutrons in the reaction chamber depends critically on the velocity of the liquid fuel as it passes through the reaction chamber. This dependence occurs because the moving salt water fuel is also the medium in which the neutrons are slowed. If the liquid is at rest, the maximum flux occurs at the center of the cylinder, but if the moderating fuel liquid is in motion, the point of maximum flux is skewed downstream and also rises to a much higher maximum. If the right fuel velocity is chosen, the thermal neutron flux (and therefore the site of maximum fission energy release) can be made to peak very sharply just outside the exit end of the cylindrical reaction chamber.

In other words, one can produce a continuous controlled nuclear explosion in the region just behind the nuclear rocket. At this point the water of the fuel liquid flashes to very high temperature steam, expelling reaction mass with an estimated exhaust velocity of 66,000 meters per second (as compared with perhaps 4,500 m/s for a chemical rocket). The NSWR engine is calculated to produce a thrust of almost 3 million pounds (1.3 x 107 N) and to have a power output of 427 gigawatts. With this kind of performance, the mission to Titan could be launched from low earth orbit with an acceleration of almost 4 g's and could, in principle, be carried out with low launch mass, low cost and high efficiency.

Zubrin also considers how a NSWR might be used in a more ambitious 120 year one-way probe mission to Alpha Centauri. He envisions a 300 tonne spacecraft carrying 2700 tonnes of salt water fuel containing 90% enriched uranium. This highly enriched fuel would be burned in a high efficiency engine to produce an exhaust velocity of 4,700,000 m/s, permitting the spacecraft to achieve a velocity that is 3.63 % of the velocity of light. He proposes to use most of the fuel for acceleration and to use a magnetic sail (see Analog, May-'92) for deceleration by creating drag against the interstellar medium.

What Zubrin has described, therefore, is a high-energy space propulsion technology suitable for deep space and interstellar missions that could be implemented with fairly modest extensions of current technology. Moreover, the end of the cold war has left in its wake considerable stockpiles of fissionable materials (239Pu and highly enriched 235U) from decommissioned nuclear weapons that can be regarded as a source of cheap fuel for such projects. Zubrin also points out that, despite the highly radioactive exhaust of the NSWR, the engine itself need not be radioactive to any significant degree. The fuel has only low-level alpha activity, the fission products from the consumed fuel are vented into space, and the induced activity from the large neutron flux produced by the fission burning can be minimized by constructing the engine from such low activation materials as graphite and silicon carbide. Once the engine is turned off, therefore, there should be no significant radioactive inventory present to endanger the crew of a manned mission.

The highly radioactive exhaust, of course, constitutes a major disadvantage for the NSWR scheme. The prospect of contaminating space with radioactive waste is certain to draw strong opposition from the same environmental and anti-nuclear groups that have opposed the use of nuclear power sources in NASA's deep space missions. Zubrin argues, however, that the NSWR's exhaust velocity of 66 km/sec far exceeds the escape velocity of any planet, and that as long as the exhaust vector does not intersect the Earth "the amount of contaminant reaching the Earth could be insignificant" even in an NSWR launch from low earth orbit. In fact, since the atoms of exhaust gas have sufficient velocity that they are not bound by the Sun's gravity well, the expelled exhaust will dissipate rapidly and will soon leave the Solar System altogether. Zubrin also points out that since the NSWR is not a weapon or a bomb, its testing and use does not violate the 1968 Test Ban Treaty. Therefore, unlike the Orion scheme, its use as a space propulsion system is legal.

There would, of course, be some very demanding technical challenges in designing a safe and reliable NSWR. The extremely high exhaust temperature and velocity of the device present a particular challenge in designing an exhaust nozzle for the NSWR that will not be severely eroded during a brief period. Zubrin suggests that a continuous flow of normal (unsalted) water along the surface of the reaction chamber and nozzle could provide cooling and extra reaction mass, but this remains to be demonstrated. A full design would also have to consider possible failure modes, including the possibility of a fuel pump failure that could cause a fuel detonation within rather than behind the reaction chamber. These appear to be solvable problems, but they would have to be addressed.

In summary, the NSWR appears to be a radical but feasible solution to the problem of mounting an interstellar mission with essentially existing technology. An unmanned Alpha Centauri probe of the type that Zubrin suggests could be built starting today and at a cost that I would guess would be much smaller than the growing price tag of NASA's troubled Space Station Freedom project. The anticipated mission time of 120 years is a long time. But the sooner we start, the sooner we (or our descendants) will get a closeup look at our neighboring star systems.

[Willie Green]

Please, Freeper Engineers, explain this mess!

It's a high-tech Rube Goldberg project: an entertaining exhibition of complex technology with little practical application.

[Lazamataz]

It's a high-tech Rube Goldberg project: an entertaining exhibition of complex technology with little practical application.

Quitchyer bitchin'. These nuclear engines will be the power source for your high-speed trains!

[Willie Green]

Not likely.

[Lazamataz]

Well, then, these miniature nuclear reactors will provide jobs in the high-tech arena, and people can take your high-speed trains to work!

[Ernest_at_the_Beach]

Thanks for the ping!

To find all articles tagged or indexed using tech_index

Click here: tech_index

[GlesenerL]

Ion engine worked great.

But its for after you've left the atmosphere.

[RightOnline]

"I got tiredhead in the first sentence. Please, Freeper Engineers, explain this mess!"

Why, certainly.

First, you take yer basic nuclear propulsion engine. Then............you miniaturize it.

Ya want I should continue?

[aruanan]

The prospect of contaminating space with radioactive waste is certain to draw strong opposition from the same environmental and anti-nuclear groups that have opposed the use of nuclear power sources in NASA's deep space missions.

Ha ha ha ha ha ha ha ha ha ha ha ha--breath--ha ha ha ha ha ha.....

Space, the biggest "thing" around, populated by an almost uncountable number of stars, all of which are constantly spewing vast quantities of radiation far and wide, and someone is worried about contaminating space with the output of a totally insignificant amount of radiation from a nuclear rocket engine? Anyone who is needs to have his head examined for the nearly perfect vacuum it contains.

[martin gibson]

I suppose the dilythium crystals just weren't GOOD ENOUGH for the propeller beanies at NASA.

[Brett66]

You laugh about the absurdity of people worried about contaminating space with radiation but these enviro-goofs do make that argument seriously. Anyone with half a brain knows better, but that's assuming the enviro-weenies have a brain at all.
Something I still can't figure out is why Michio Kaku, a brilliant physicist, spearheaded all of the protests against Cassini which was powered by a nuclear battery.
There was no scenario under which that battery was a threat to the Earth, even if it blew up and sprayed debris all over the US.
Kaku knew this, but there he was throwing in his hat with the brain dead enviro-weenies and their uninformed fearmongering.
I still can't figure out why he supported the insane ramblings of these freaks.
Maybe he's just a Marxist and all of the useful idiots generally served his purposes.

[HeadOn]

No way. Rube's machines have lots of moving parts. The only thing moving here is the hydrogen. But I do agree, it looks pretty impracticle right now.

Republic, to put it into everyday, kitchen physics, it's like a tea kettle. But instead of a kettle on top of a stove eye, picture a steel tube full of water (the kettle), surrounded by an electric heating element (the eye). Turn on the element, and steam shoots forcefully out one end (the kettle's whistle).

Now, instead of an electric element, substitute layers of radioactive materials which will generate a LOT of heat, just like our power plant reactors, and instead of water sitting in the tube, substitute cold hydrogen flowing through. Make the nozzle right, and the hydrogen "steam" gives you a LOT of thrust.

Simple, no?

[HeadOn]

As for that huge post #22, it's sort of like a bunsen burner, or a flame thrower, which uses pressure to get the violent activity (burning of gas, burning of petroleum product, fission reaction)out and away from the source of fuel which would destroy the device if it were "ignited".

[fourdeuce82d]

"Anyone who is (worried about radioactive contamination of space) needs to have his head examined for the nearly perfect vacuum it contains."

"opposition (because of contamination fears)from the same environmental and anti-nuclear groups that have opposed the use of nuclear power sources in NASA's deep space missions.

I agree with the sentiment, but examination is really superfluous.

[aruanan]

Something I still can't figure out is why Michio Kaku, a brilliant physicist, spearheaded all of the protests against Cassini which was powered by a nuclear battery.

Well, look at all the scientists who were leftists who spearheaded the "we're drowning in industrial carcinogens spewed by the greedy industrialist capitalist pigs and we're all going to get cancer and die" bugaboo of the late 60's, early 70's. Even though there was NOTHING in their science which prescribed the drastic measures they proposed, they still called for dramatic government controls over chemicals, "just to be safe". We have the current EPA, OSHA, and the Toxic Substances Control Act as a result. When it comes to choosing between their religion of socialism and science, they'll choose their religion almost every time.

[boris]

"So the thrust to weight ratio is still not sufficient to replace rocket boosters?"

Doubtful, even if the nutcases would let you launch.

--Boris

[boris]

"could you ever send a nuclear reactor into space with an acceptable level of safety?"

Sure. A nuclear reactor can be sent up "cold". If it is not critical (i.e., no chain reactions) you could walk up and kiss it.

Once in space, you "turn it on" by positioning reflectors and/or moderators to make the reaction "critical".

The Russians orbited enormous reactors to power sea-scanning radars. One of them fell on Canada several years back.

The containment of a space reactor would be very robust. I've seen high-speed film of containment vessels for RTGs being tested for integrity. Basically they are launched into a reinforced-concrete wall by a cannon or rocket. The wall loses.

--Boris

[boris]

My take on Zubrin is that he is very smart. Although very morose and lugubrious.

I had not heard of this concept; I'd like to see more on it. Do you have links?

A "conventional" nuclear-thermal rocket (NERVA or KIWI) simply uses a solid nuclear reactor which is cooled by hydrogen. It can get 800 seconds or possibly higher. As I mentioned, thrust-to-weight is a problem.

One question: if this concept relies on the fluid velocity to prevent an "explosion", how do you start it up...or shut it down?...

--Boris

[John Jamieson]

Unlike some other proposals in recent FR threads, this is real technology, with real promise and realistic goals. Great show recently on one of the Discovery channels about the work done in the '60's.