Author Topic: High end electric/ion engines - current status?  (Read 18976 times)

Offline mong'

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Re: High end electric/ion engines - current status?
« Reply #40 on: 02/15/2007 05:26 PM »
if you want both high thrust AND high specific impulse and have nothing against nuclear power, then the answer is Nuclear Salt Water Rocket (NSWR),
theoretically it can produce about 10MN (1000 tons) of thrust with 6000 sec ISP. the ISP could be improved by using more or less water in the nuclear reaction.
the exhaust is going to be rather nasty and the tanks quite heavy (since they need to be neutron absorbers and keep water liquid), but the performance is there and it can be done today. it will take a lot of R&D but it's feasible

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #41 on: 02/15/2007 06:37 PM »
I'll settle for high thrust OR high Isp, and I won't insist they be on the same ship, but I do want enough total impulse on whatever craft that I can get a reasonable payload to where I want to go and get there in reasonable time.

And using chemical propellants to get to Mars doesn't meet my needs.

The 100,000 sec systems are of some interest long term, but I doubt we'll be building manned craft for the Oort cloud before we're swarming all over the space between here and the asteroid belt, and the systems of a few thousand seconds and a reasonble fraction of a g probably should be higher priority.

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #42 on: 02/15/2007 07:02 PM »
Nuclear Salt Water Rocket is going to be a very finicky animal to control: if you use a thick reaction chamber lined with a suitable neutron reflector to create the critical mass when you squirt the fissile salt solution into the chamber--this is entirely analogous to a continuous nuclear explosion. The neutron flux coming from the reaction chamber will be high enough that some will leak passed your reflector (because it isn't 100% efficient in reflection!) and diffuse into your fuel-solution storage tanks. These neutrons will interact--guaranteed--with fuel atoms there. The additional moderation offered by the water in the solution will likely ensure that this will quickly become an uncontrolled chain reaction in the storage tanks, and your vehicle will blow up.

Fissile isotopes are tricky to handle in quantity. Fissile isotopes in solution are even trickier when the aggregate quantity of fissile material inside a storage vessel begins to approach a critical mass.  If you propose storing the 'fuel solution' in seperate containers, and mechanically injecting them one at a time, it would probably be safer and more reliable just to use the fissile material to make metal core pulse units, and go with a nuclear pulse Orion ship.

Also, like a gas core reactor, you have issues with controlling containment of the fissioning plasma without sacrificing too much efficiency; you also have coupling of massive amounts of electromagnetic radiation (here HARD UV and soft X-Rays) to the walls of the reaction vessel. In a highly ionized uranium plasma there is enough electrogmangnetic radiant coupling that literally several tens of gigwatts of power can be directly coupled to the reaction chamber walls--your reactor will vaporize unless you pump a lot of hydrogen through it to cool it!




Offline mong'

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Re: High end electric/ion engines - current status?
« Reply #43 on: 02/15/2007 07:23 PM »
like I said, it's going to need a lot of R&D just to keep it from blowing up. but the problems can be solved.
and it's much more practical than an Orion since you don't have this slightly annoying issue of mass producing nukes, it also has higher performance

Offline vda

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Re: High end electric/ion engines - current status?
« Reply #44 on: 02/15/2007 09:21 PM »
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TyMoore - 14/2/2007  9:02 PM
Nuclear Salt Water Rocket is going to be a very finicky animal to control: if you use a thick reaction chamber lined with a suitable neutron reflector to create the critical mass when you squirt the fissile salt solution into the chamber--this is entirely analogous to a continuous nuclear explosion.

I think in NSWR salt solution goes supercritical and explodes JUST OUTSIDE the ship. The trick is to make solution flow fast enough in the center of the "tube" so that it "shifts" out of the ship just milliseconds before neutron production hits the peak.

IIRC in nuclear bomb neutron production goes thru ~50 "generations" (one "generation" is ~ mean time of neutron absorbtion/fission event), and last four of them produce ~99% of all power.

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #45 on: 02/16/2007 03:06 AM »
O.K., but I'm skeptical on that too. Fast neutrons (ones with kinetic energies in the MeV range) move at nearly the speed of light, and these are the ones that will be causing most of the fissions. Thermalization of neutron flux comes later after multiple collisions with moderating substance, like hydrogen (in water) or carbon (in graphite or polyethylene.) I still think its going to blow up...

It could even be possible to create a 'fission fragment sail' by coating a thin sheet of material with thin layer of fissile isotopes. Bombarding this layer with neutrons will cause fissions near the surface of the layer. Fission fragments will be stopped by the layer--which transfer momentum to the sail. Fission fragments that are ejected will also transfer momentum to the sail. Ejected fission fragments ought to provide high Isp but low thrust...

I think that the best use of nuclear materials is to use them in a reactor for power conversion. Either create electricity from heat (as in NEP,) use the heat directly (as in NTR) or some combination of both (Pratt and Whitney Triton Bi/Trimodal NTR with LOX afterburning and Brayton Cycle auxilary power generation.)

Offline vda

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Re: High end electric/ion engines - current status?
« Reply #46 on: 02/16/2007 11:53 AM »
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TyMoore - 15/2/2007  5:06 AM
It could even be possible to create a 'fission fragment sail' by coating a thin sheet of material with thin layer of fissile isotopes. Bombarding this layer with neutrons will cause fissions near the surface of the layer. Fission fragments will be stopped by the layer--which transfer momentum to the sail. Fission fragments that are ejected will also transfer momentum to the sail. Ejected fission fragments ought to provide high Isp but low thrust...

Imagine a flat surface at the back of spacecraft with the outermost layers composed of highly fissile materials. Like: layer of plutonium, with curium-247 or even californium-251 at the very top. (Cf-251 half-life is ~900 years and it is way more fissile than Pu-239, Cm-247 half-life is 15 million years and it is slightly less fissile than Cf-251). With underlying layer of neuthron reflector or hopefully some form of direct convertor of radiation to electricity. (I want to reduce amount of power which is converted into heat as that one eventually requires more mass for radiators etc).

This assembly is build so that it is subcritical (indeed, it is probably hard/impossible to make thin layer of anything to go critical anyway). Compact neutron sources are placed above this surface  - essentially, small (in tens of kilograms) nuclear reactors designed to leak large fraction of their neutrons. By moving them closer to the surface their added neutron flux makes entire assembly critical and (I hope it's possible) a lot of fission fragments are not trapped in the material but escape, creating thrust.

I am not an engineer, especially nuclear engineer, so it's quite possible I'm speaking gibberish here.

Offline mong'

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Re: High end electric/ion engines - current status?
« Reply #47 on: 02/16/2007 12:11 PM »
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TyMoore - 16/2/2007  5:06 AM

It could even be possible to create a 'fission fragment sail' by coating a thin sheet of material with thin layer of fissile isotopes. Bombarding this layer with neutrons will cause fissions near the surface of the layer. Fission fragments will be stopped by the layer--which transfer momentum to the sail. Fission fragments that are ejected will also transfer momentum to the sail. Ejected fission fragments ought to provide high Isp but low thrust...

that was a proposal for antimatter propulsion, fire antiprotons at a U235 coated sail to produce thrust, but it's still very low thrust/high ISP

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #48 on: 02/16/2007 01:35 PM »
Yep--this is about the only way to directly use the kinetic energy of the fission fragments for propulsion that I can think of--and I am not an engineer either. Over at nuclearspace.com we explored a lot of different nuclear power and propulsion options, and some of the folks there were nuclear engineers but not aerospace engineers. A fission fragment sail was one of those things that seemed to make sense--although I am not very partial to it, because the thrust is so small that seems impractical to try to send humans for interplanetary journeys with them. So something more indirect, but higher thrust is needed. And this indicates to me some kind of power conversion step--ergo, probably a nuclear reactor operating either as an NTR or as a power system for NEP.

There are several very good ion engines out there: the Hall Effect Thruster, the Electrostatic Thruster; Magneto Plasma Thruster; and I think VASIMR will be a real contendor in 15 years.

I think that Solar Photovoltaics is a contendor for power sources, especially some new technologies which are almost unbelievably thin, foil like solar cells. A Solar Electric Propulsion vehicle may yet be a real possibility.



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