Author Topic: High end electric/ion engines - current status?  (Read 19124 times)

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #20 on: 02/13/2007 12:48 pm »
Here's a pretty nice work on VASIMR with a Vapor Core Reactor from the University of Florida:

http://www.inspi.ufl.edu/gcr.pdf

In my time as a board moderator for the Nuclearspace.com website, quite a few nuclear industry folks posted there...and the basic idea of reactor scaling laws that I came away is that above a certain threshold, reactor scaling is very non-linear. Operating with highly-enriched uranium as a fuel that a solid-state reactor running at say 1000 MW (in an NTR application) will be only slightly smaller than a reactor running at 4000 MW.  Of course there are all kinds of things that can and would be done to tailor the power density and temperature profile of such a reactor, but reactors running in an NTR (nuclear thermal rocket) mode are fairly different  from a reactor designed with electrical energy production in mind. Any kind of thermal to electrical power conversion system is going to need a radiator--and the size of that radiator is of course governed by the Stefan-Boltzman Law:

P=A*e*sigma*(T2^4-T1^4) where P=power radiated in watts, A is the area of the radiating surface (which if its a double back radiator you get twice the radiating surface of a single back radiator!) e=the emissivity of the radiator (usually if it is painted or anodized flat black this will be very close to 0.9-1.0;) sigma=the Stefan-Boltzman constant (5.671*10^-8 W/(m^2*K^4) T2=the temperature of the radiator surface in Kelvin; T1-temperature of the background (which for space is 3.7 Kelvin, but this is so small we can neglect it and just say it's zero!)

The size of the radiator needed varies directly with the power to be dissipated, but varies with the 4th power of the temperature. If using a Brayton-Cycle gas turbine power conversion system, one can take a hit on efficiency, go with a lower efficiency with a higher heat rejection temperature, and save a bundle on the size of the radiator. Doubling the temperature of heat rejection results in a radiator that is 1/2^4 = 1/16 the size! So even fairly small increases in heat rejection temperature can result in big savings in radiator mass...

If I had access to neutron diffusion transport codes for modelling nuclear reactor cores, it may be possible to rough out a design for a 600MWt reactor cooled with helium gas, running a Brayton-Cycle gas turbine producing 200 MWe. One would have to do a detailed thermodynamic analysis of the cycle to determine the operating temperatures so that a radiator size can be suitably selected. It would of course be beneficial to size the radiator for the full 600MWt power output to allow gas turbine rundown (and to remove decay heat after nuclear shutdown) when your electric propulsion system shuts down...

If we allowed peak radiator temperature of 1000 K, dissipating 600 MWt (full reactor power,) would require a total of 10,580 m^2 of radiating surface. If we radiate from both sides we can halve this, and if we use 4 identical panels arranged in a cruciform  pattern on a boom (near the reactor power conversion systems) then we can divide this value by 4 again, so we get: 1 of 4 radiator panels with area of 1323 m^2 or a square about 36.4 m (119 ft) on a side.  If we turn on the Brayton conversion system and the thermal power radiated drops to 400 MWt instead of 600 MWt, then the radiator temperature will drop to about: 904 K (1170 degrees Fahrenheit.) This seems reasonable.  Of course I haven't done ANY mass estimates, which is "kind of" important, but I have to get ready for work now. Sorry!

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #21 on: 02/13/2007 01:19 pm »
We've discussed this in some detail in the electric booster topic and the Interesting Fusion Talk at Google topic, but evidently it bears some repeating here.

I worked for R. W. Bussard for over 5 years on his fusion project, which he feels he got running well enough in late 2005 to be a concept validation.  He believes he can get net power from p=B11 fusion by scaling up his design so that the key element has a 2 meter radius.  The p-B11 reaction produces almost all of its energy as three alpha particles, and so can, in principle, use direct conversion to produce high voltage DC, with very little waste heat.  His spacecraft designs based on this reactor system get by with much less radiator than other nuclear-electric designs, and the performance is boosted accordingly.  He has a dozen or so papers on the subject, most dating to the 1990's. References are given in my posts in one of the other topics.

As he expects the reactors to be up in the several-gigawatt range, he has strong reservations about being able to use anything remotely resembling a gridded ion engine.  His preferred means of putting power into reaction mass for shorter ranges and low Isp is to heat the reaction mass using relativistic electron beams, achieving Isp in the 1200 to 5000 second range, depending on the application.  For longer missions to the outer planets to as far as the Oort cloud, he expects to use "diluted fusion product" thrust, adding reaction mass to the particle stream produced by the fusion engine.  He expects an excellent power to weight ratio with either approach, and one of his papers explores the impact of the resulting ability to move payload on colonization efforts ... quite dramatic.

Sounds pie-in-the-sky, except that I've seen his experimental results from November 2005, I understand the design due to having worked on it, and I think the thing is going to work.  The machine is definitely capable of making fusion, and his scaling laws give him confidence that it will hit breakeven at a reasonable size.  If he's right, the solar system is ours in a decade or two.

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #22 on: 02/13/2007 01:31 pm »
Here's a partial list of references from my 1998 article on the subject of Inertial Electrostatic Confinement fusion:

9.  R. W. Bussard, "Fusion as Electric Propulsion," Journal of Propulsion and Power, v 6, no 5, Sept-Oct 1990, pps 567-574.
10.  R. W. Bussard and L. W. Jameson, "From SSTO to Saturn's Moons:  Superperformance Fusion Propulsion for Practical Spaceflight,"  30th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 27-29 June, 1994, AIAA 94-3269.
11.  Inertial-Electrostatic-Fusion Propulsion Spectrum:  Air-Breathing to Interstellar Flight, R. W. Bussard and L. W. Jameson, Journal of Propulsion and Power, v. 11, no. 2, pps 365-372.
12.  R. W. Bussard, "System Technical and Economic Features of QED-Engine-Driven Space Transportation,"  33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 6-9 July, 1997, AIAA 97-3071.

The Bussard paper from fall 2006 can be found here:

http://www.askmar.com/ConferenceNotes/2006-9%20IAC%20Paper.pdf

My Analog article on the subject:

http://fusor.net/newbie/files/Ligon-QED-IE.pdf

Offline vda

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Re: High end electric/ion engines - current status?
« Reply #23 on: 02/13/2007 01:36 pm »
Quote
TyMoore - 12/2/2007  2:48 PM
If we allowed peak radiator temperature of 1000 K, dissipating 600 MWt (full reactor power,) would require a total of 10,580 m^2 of radiating surface.

Why radiate full reactor power? Aren't we planning to run our reactors for electricity and/or direct production of exhaust - which takes part of power output?

Offline vda

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Re: High end electric/ion engines - current status?
« Reply #24 on: 02/13/2007 01:51 pm »
Quote
lambda0 - 12/2/2007  1:30 PM
Quote
...
 we use MPD. ISP is ten times lower, so exhaust mass should go up x100 in order to use the same amount of energy.
Reactor power: 1MW
Reactor mass: ~500kg
Exhaust, m/s: 100000 (ISP ~10000)
Exhaust, kg per sec: 0.0002 (200 mg)
Thrust: 20 kg*m/s^2 (20N)
Acceleration: 0.04 m/s (40mm/s)
T/W ratio: 0.004

Above numbers are incorrect because (a) conversion efficiency is not taken into account and (b) MPD thruster's own mass is assumed to be zero. I suppose I should divide those numbers at least by four in order to compare it to the first set of numbers (remembering that realistic numbers for both "engines" are even lower).

So far it doesn't look too bad - hey, 1 centimeter/second acceleration is not so feeble. In one month it gives you ~25 km/s delta-v (and will consume 500 kg of propellant, so... less than 25km/s :) , but still...).
Where am I wrong? How much lower are "realistic" numbers?

This reactor mass is not realistic, I think it should be at least 10 or 20 times higher with current technology. And you cannot compare directly the mass of nuclear reactor that generates electricity to the mass of a nuclear thermal thruster (such as the fission fragments reactor): the first one is much heavier because of all the equipment necessary to generate electricity.

Fission fragment is not "thermal" in a sense that it does not thermalize radiation but tries to let it fly right away, creating very high ISP exhaust. But I digress. If fission fragment reactor is going to be lighter - that's even better!

And anyway a significant fraction of such reactor's power will still be converted to heat, it's unavoidable. Which should be used in some way - converted to electricity, I guess? We may end up with combined fission fragment / MPD engine.

From other answers I gather that 1MW is actually too small (impossible to build a 1MW reactor with 2kW/kg). Ok. Since 25 tons at LEO seems to be a higher end of available launch vehicles... can 10 ton reactor deliver 10-20MW?

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #25 on: 02/13/2007 11:37 pm »
Quote
vda - 13/2/2007  6:36 AM

Quote
TyMoore - 12/2/2007  2:48 PM
If we allowed peak radiator temperature of 1000 K, dissipating 600 MWt (full reactor power,) would require a total of 10,580 m^2 of radiating surface.

Why radiate full reactor power? Aren't we planning to run our reactors for electricity and/or direct production of exhaust - which takes part of power output?


I was thinking in terms of when propulsion or other loads were in transition (ion engines shut down when thrust phase is terminated, or during menuvers, or some other electrical load change during cruise, or at destination...) It would seem to be prudent to overdesign the radiator system to handle the full rated thermal power--just incase the primary energy conversion system sheds its load somewhere along the line. No point in having a melt down just because thrust phase is terminated or a circuit breaker pops. Besides having a little extra thermal capacity also allows dealing with some of the decay heat that will be generated by the reactor core owing to accumulation of radioactive fission products....

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #26 on: 02/14/2007 01:02 am »
For most fluid thermo cycles, the radiators would have to cool the fluid effectively for the cycle to be efficient.  For simply dumping waste heat when you shut off the load, they could run hotter.

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #27 on: 02/14/2007 03:39 am »
Exactly. For the case with the 600MWt reactor, generating about 200MWe with a Brayton-Cycle turboalternator and rejecting about 400MWt of primary heat, I get a steady state operating temperature of about: 880 K or about 1123 degrees F. Those are rough numbers, but it gives an idea of what needs to be done.  The radiators must be designed to handle the higher heat load (and higher temperature.) Else, this would not be responsible engineering, especially where a nuclear power system is concerned.

Still, I must confess that regardless of the size of the reactor, 200MWe of Brayton Cycle turbogenerator isn't exactly small pickings either. This is a big chunk of machinery. A large GE90-85B Turbofan engine, very similar to that used on Boeing's 747, generates about 84,700 hp, which is about the mechanical equivalent of 63 MW. So 200 MWe will translate to about 211 MW of mechanical power if the generator is 95% efficient. 211 MW mechanical translates to about 283,000 hp, or 3.3 times the GE turbofan. Even though a Brayton cycle system is not the same as an open cycle aircraft turbofan (The Brayton is going to be heavier by far,) and even though the Brayton might be using a different working fluid like helium-with some xenon burnable poison, this loose comparison still gives me some idea of how much machinery is necessary to process this power--and from a spacecraft point of view, this is going to be pretty heavy.

Some general specs on the GE90-85B can be found at:

http://www.geae.com/engines/commercial/comparison_turbofan.html

Offline lambda0

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Re: High end electric/ion engines - current status?
« Reply #28 on: 02/14/2007 08:15 am »

Here is a study for a high-power space nuclear reactor for electric propulsion :
"Ultra-High Power Space Nuclear Power System Design and Development."
NASA, CR-2001-210767
http://myfreefilehosting.com/f/dce782f78e_9.28MB
(135 pages, 9 Mbytes)

This document is a study of a 200 MWe space nuclear generator, based on a Rankine cycle, that could be build with current technologies.  It describes also a small 10 MWe version. This includes the radiators, the shielding, the conversion systems, etc.
The 10 MWe version weights "only" 40 tons, and could be reduced to 32 tons.
The mass of the 200 MWe version is about 485 tons, that could be reduced to about 392 tons...
(even Ares V will not be able to launch this !)
However, the conversion efficiency is quite low (below 20%), and it is based on technologies available in 2001. I suppose it should be possible to do better with some R&D, but I am not very optimistic with this kind of reactor.
Vapor core reactor with MHD conversion is a far better concept, but it also requires much more research.



Offline vda

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Re: High end electric/ion engines - current status?
« Reply #29 on: 02/14/2007 08:15 am »
Quote
Tom Ligon - 12/2/2007  3:19 PM
The p-B11 reaction produces almost all of its energy as three alpha particles, and so can, in principle, use direct conversion to produce high voltage DC, with very little waste heat.

Yes, I've seen that thread. Idea looks promising, but seems to need at least two R&D iterations (first demostrate sustained fusion in subscale model, then sustained net energy production in full-scale powerplant). Maybe more. Only after that it will make sense to actually try to put it into spacecraft.

I was thinking more along the lines of fission reactors since those are a much more familiar technology now; and more specifically, what we can actually hope to build & launch in next 10-20 years ("current status").

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #30 on: 02/14/2007 01:23 pm »
NERVA was supposedly not far away from being able to fly, and all the stuff is stored under a mountain in Nevada.  That's a hydrogen-cooled fission reactor, with about 860 sec of Isp.  That technology should be usable for Mars missions, and could acquire reaction mass any place you find hydrogen.  I don't think that design requires radiators as it's not a closed cycle.  The design had chronic problems with plate errosion and vibrations due to the high gas velocity, and tends not to have a very high burn time.

We'd all like a lot higher performance, but I think NERVA at least gives a benchmark to work from.  Any closed cycle nuclear electric system, including reactor, power conversion, radiators, and thrusters, needs a net performance significantly better than NERVA to be worthwhile.

I am not up on state of the art of thermionic power systems, but NASA seems to like 'em.  
http://ntrs.nasa.gov/search.jsp?N=4294927215

The little ones used for deep space power run a long time with little fuss.  I'm wondering what one could do marrying that concept to a fission reactor, to get the power density up.  Could a fission reactor be used to excite charged particle emissions and use that to produce electric power directly?  Ah, wrap the reactor core in those rare earths I want to mine from the asteroids? ;-)

It probably won't work ... fission reactors don't like high temperatures, so it is still going to need cooling.  Building one that does like to run hot tends to make it prompt-critical when cold ... that's a problem with NERVA style, as well.

Dr. Bussard told me a tale about an early test of KIWI-A, the original Rover engine that led to NERVA.  They had it out in the desert, exhaust nozzle pointed to the sky, when a rain storm approached.  Picture, if you will, a cold reactor (maximum reactivity), designed for high reactivity at high temperature, graphite moderated, fully fueled, with a funnel pointed at a source of additional moderator about to pour from the sky!  I've seen pictures on the web of a test in which they deliberately blew up one of the later engines ... unlike modern nuclear powerplants, the NERVA-style engines were quite capable of blowing up.

Offline meiza

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Re: High end electric/ion engines - current status?
« Reply #31 on: 02/14/2007 01:29 pm »
It's better to have hot radiators in space. That lowers thermal to electric conversion efficiency, but saves weight, as the radiators don't need to be so big. Even if the reactor has to be bigger, it's still small compared to the radiators.
Radiated power from the radiator is relative to the temperature to the power of four, so if you double your turbine input temperature in kelvins, you only need 1/16 the area to radiate the same energy out.
The optimal point of course is dependent on the specific masses of the reactor and turbine (or what ever power converter is used) vs the radiators.

That's also why you want high temperature reactors in space since you can raise the high temperature and thus raise the low temperature and still get good efficiency and small radiators.

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #32 on: 02/14/2007 02:09 pm »
The trouble is, fission plants don't like high temperatures.  They can't run at normal combusion temperatures due to material limitations and the fact they lose reactivity when hot.  I've seen the schematics for Virginia's Surrey plant, for example, and they actually use oil-fired superheaters to heat the steam after the reactor has had its input.

And a closed thermodynamic cycle'e efficiency is limited by how cold you get the working fluid at the bottom end.  Nukes are more sensitive to this than fossil fuel plants as they can't get as hot.  Hence the huge cooling towers.  Due to their sheer size and mass, those cost more than the reactors in a typical nuclear plant.

If the radiators have to run hot, overall efficiency will suffer.  But big radiators are a serious mass penalty.  The trade-off will probably be to use an exotic working fluid, build a high-reactivity core, and live with a system that is not as intrinsically safe as a terrestrial power plant.  The engineers are going to be in a tight squeeze trying to optimize it, and the performance will be less than folks might imagine if they're not familiar with the technology.

All told, gas-cycle power generation is an awfully complex system for space propulsion.  We'll use something else if we can.  But we have a lot of experience with it for terrestrial power plants, including sub propulsion, so we know it CAN work if that's the best we can do.

Offline lambda0

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Re: High end electric/ion engines - current status?
« Reply #33 on: 02/14/2007 02:15 pm »
Quote
...
We'd all like a lot higher performance, but I think NERVA at least gives a benchmark to work from.  Any closed cycle nuclear electric system, including reactor, power conversion, radiators, and thrusters, needs a net performance significantly better than NERVA to be worthwhile.
I agree with that. I think that electric propulsion will be used for manned flights only if it allow a flight time to Mars not higher than 100-110 days, because in this case, a round-trip mission in less than 1 year is possible, instead of almost  3 years with chemical propulsion. If it is not possible, there will be no clear advantage compared to "traditional solutions" such as Mars semi-direct mission.
However, solar electric propulsion at 1-10 MW level could be interesting for sending cargo to Mars, because the IMLEO could be up to 2 times smaller than with chemical propulsion : flight time will be comparable, or higher, but in this case, one tries to optimize the mass, not the flight time as for manned mission.
It is worthwhile trying to develop high power electric propulsion, with solar or nuclear generator, because even if it does not reach the performances level required for fast manned interplanetary travels, it has other interesting applications.

Quote
I am not up on state of the art of thermionic power systems, but NASA seems to like 'em.  
http://ntrs.nasa.gov/search.jsp?N=4294927215
Those references are 30 years old...
Thermionic conversion is also an interesting solution for some applications, it has been used on soviet spy satellites (reactor Topaz), but the efficiency is low and I am not sure that it is adapted to high power, in the range > 100 MW. If someone find a study of such a reactor, I am interested. There are also some problem of life time, due to the use of corrosive elements (cesium at high temperature).
However, it's seems that in Russia some people are working on a 1 MW reactor with thermionic conversion :
http://www.rssi.ru/IPPE/General/spacer.html
But anyway, that's not enough : we need at least tens of megawatt for manned flight with electric propulsion.



Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #34 on: 02/14/2007 06:38 pm »
Looking over some of the old Project Orion stuff (the idea of using nuclear explosives for propulsion, not the modern craft of the same name), I noticed that they tended to coat both the bombs and pusher plates with "reaction mass".  The chosen material was typically polyethylene for the bombs, graphite for the pusher plates.  Both are excellent neutron moderators, and I have to believe a large part of their purpose was to soak up the kinetic energy of fast neutrons.

Someone here, I think, suggested Bussard's present machine, even if it could not hit breakeven, might be a good neutron source for causing controlled fission in, for example, U235 or plutonium, or perhaps even U238.   This has me wondering, if it were possible to use some strong neutron source to conduct this reaction, and if fast or at least epithermal neutrons were the goal, reactor temperatures could be substantially higher.

And it gets me wondering, if one could do a reaction intended to have a high output of fast neutrons that were not required to be thermalized to continue the reaction, might it be possible to use those to heat gas for propulsion?

Offline vda

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Re: High end electric/ion engines - current status?
« Reply #35 on: 02/14/2007 09:27 pm »
Quote
Tom Ligon - 13/2/2007  3:23 PM
Could a fission reactor be used to excite charged particle emissions and use that to produce electric power directly?

It can be better. Build a reactor in which fragments of fissioned uranium are largely fly away and produce thrust. Isp of said fragments is on the order of 100000.

http://www.batse.msfc.nasa.gov/colloquia/abstracts_summer05/rsheldon2.html

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #36 on: 02/14/2007 09:58 pm »
Well, heck, why not?  The Rover program was originally envisioned as a means of launching ICBM's.  The nasty habit it had of spewing radioactive crud into the environment was objectionable, but in a nuclear war?  But obviously not a good peacetime LV.  So they figured it would be for space propulsion.

And once you're out in space, where radiation is everywhere, why the heck worry about spewing fission products?  Suddenly it is a virtue.

That link didn't say much.  Nice Isp, but what kind of acceleration?  In-system, it would sure be nice to hit 1/3 gee or so.

Offline lambda0

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Re: High end electric/ion engines - current status?
« Reply #37 on: 02/15/2007 02:34 pm »
Quote
Tom Ligon - 14/2/2007  1:38 PM
...
Someone here, I think, suggested Bussard's present machine, even if it could not hit breakeven, might be a good neutron source for causing controlled fission in, for example, U235 or plutonium, or perhaps even U238.   This has me wondering, if it were possible to use some strong neutron source to conduct this reaction, and if fast or at least epithermal neutrons were the goal, reactor temperatures could be substantially higher.
...

http://en.wikipedia.org/wiki/Energy_amplifier
The idea is to replace the synchrotron and spallation process by an IEC reactor that burns D-D to produce neutrons : the system should be lighter and less expensive.
However, I'm a bit skeptical about the application for space propulsion. The main application would be for power plants.
I know that there are some researches to build a low cost and powerfull source of neutrons to drive the energy amplifier, and for the moment, a synchrotron is too costly to make this solution economically attractive.
Maybe that Dr Bussard can propose his system to Carlo Rubbia...


Offline vda

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Re: High end electric/ion engines - current status?
« Reply #38 on: 02/15/2007 03:05 pm »
Quote
Tom Ligon - 13/2/2007  11:58 PM
That link didn't say much.  Nice Isp, but what kind of acceleration?  In-system, it would sure be nice to hit 1/3 gee or so.

Ultra-high Isp engines usually don't have high thrust (unless you can somehow generate terawatts of power).

If you managed to build a reactor with 1kW/kg power density and also managed to make 100% of all fission fragments to fly away as exhaust at 1000000 m/s, then your acceleration will be 2 mm/s^2.

24*60*60 seconds of thrusting provide dv of 170 m/s, 365 days - 63 km/s, both with negligible 'propellant' use. IOW: suitable for missions to outer planets, not good for Mars trip.

However, real system will have some significant portion of power turned into heat/electricity, and this can be used for whatever other needs, including lower Isp, higher thrust ion engines.

As other posters noted, heat to electricity conversion is a problem because required machinery is heavy. We badly need more direct ways of converting fission energy to electric current.

Also I wonder whether reactor construction can be adjusted so that Isp go down (and therefore thrust go up). Most of the fission will obviously occur on the surface of fuel elements. Can they be coated with thin layer of low-Z material so that fission fragments knock off a few lighter atoms on their way out? (Other ideas?).

Offline TyMoore

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Re: High end electric/ion engines - current status?
« Reply #39 on: 02/15/2007 05:01 pm »
Well, a direct 'conversion' nuclear propulsion system with high specific impulse and relatively high thrust: I can think of the MiniMag Orion concept which is interesting because it uses electrically driven -Z-Pinch mechanism to electromagnetically implode an aluminum shell onto a small chunk of something like Cerium-141 I think to create a tiny super-critical mass. With about 1 pulse every second it seems that I remember reading an engine power of something like 260 GW (billion watts!) which gives the thrust of an SSME with several thousand seconds of specific impulse (like an ion engine)

It's an interesting concept--which Andrew's Aerospace is exploring:

http://www.andrews-space.com/content-main.php?subsection=MTA2

But the thrust and specific impulse issues are definately formidable: to get the thrust, you need lots of power. To get high Isp, with tiny thrust, you need lots of power; and to get both high thrust and high Isp, you need (lots of power)^2 !!

A very general relationship which is sometimes useful in comparing "Apples to Apples" is:

Pjet = 1/2 * F * Ve  or Pjet=actual 'jet' power developed by the engine in units of Watts; F = thrust of engine in Newtons, and Ve=exhaust speed in m/s or alternatively the Isp*g where Isp = specific impulse in seconds, and g = acceleration due to gravity on Earth, 9.80665 m/s^2

It is sometimes useful because it gives an 'order of magnitude' estimate of just how 'powerful' an engine we're talking about.

If F=1 N and Ve=g*3000 sec = 29420 m/s; then Pjet = 14.7 KW. So already we're talking about a major ion motor on a big satellite.

For a VASIMR style engine, let's say F=100 N, Ve=g*30,000 sec = 294,000 m/s; then Pjet=14.7 MW which is already in the realm of a very large solar array or a small nuclear reactor.

Incidently it also works with chemical rockets:

For the SSME: F=1.67*10^6 N; Ve=g*390 sec= 3825 m/s; Pjet=3190 MW.

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