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General Discussion => Advanced Concepts => Topic started by: vda on 02/12/2007 06:18 am

Title: High end electric/ion engines - current status?
Post by: vda on 02/12/2007 06:18 am
Hi,

So far almost all Solar System missions were using chemical engines, and frankly, they are far too inefficient for the task. Nine years to Pluto? Come on... Exploring/settling solar system using this technology will take centuries...

DS1 was a successful technology demonstrator for higher-ISP engine. What's next? Are there any plans to use something "bigger"?
Huh, probably my question should be separated into two:

1. What are theoretical performance limits of electric/ion engines (including powerplant)? For example,
 1a. If one puts 25 ton Pluto bound space probe in LEO, what reactor should be there (mass, type, etc). Propellant? Ion engine (which type)?
 1b. What are current estimates for maximum achievable T/W for "big" ion engine? Are they practical for relatively high-thrust tasks (e.g. orbit insertion at Neptune)? Imagine that you have "unlimited" amount of electricity (many megawatts). What type of ion engine will perform best? What propellant is best? (Re xenon: IIUC it's best for "small" probes like DS1 because it is using electricity most efficiently, but resulting ISP is rather low I guess?)

2. Is there any plans to actually build and fly something more advanced ("bigger") than DS1 in next 10-15 years?
Title: RE: High end electric/ion engines - current status?
Post by: lambda0 on 02/12/2007 08:29 am
Gridded ion engines, such as the one used on Deep Space 1, have some physical limitations that prevent to use them with high power : the fact that the plasma is not neutral between the grids introduce a limitation on the thrust density (about 1 N/mē). The gridded ion thruster of DS1 had a power of about 2 kW.
The Hall effect thruster is better from this point of view, it could be used for higher power, maybe up to a few hundreds of kW  (NASA is testing a 150 kW Hall thruster). Initially, gridded ion engines had a higher specific impulse (5000 s) than Hall thrusters, but the difference is shrinking.
Sending "big" probes to the outer planets require at least 100 kW : it was the level required for JIMO. This amount of power can be supplied by a nuclear generator; like the SAFE-400. JIMO was designed with gridded ion thrusters, using xenon. There are also some propositions to send smaller orbiters to Neptune, based on gridded thrusters, or Hall thrusters, with a power in the order of 10 kW, but the payload is less than 1 ton, and a flight time of about 15 years.

For megawatts level, both gridded ion thruster and Hall thrusters are not adapted : thrusters that accelerate a neutral plasma are superior  and allow high thrust density. This category includes the VASIMR, the MPDT (magnetoplasmadynamic thruster), the LiLFA (Lithium Lorentz force accelerator), but they require more R&D.
According to Chang-Diaz, from Ad Astra Rocket, a small 200 kW VASIMR should be ready for flight test in 2010. For the MPDT, lab tests have been done at high power : up to 30 MW, which is the level required for interplanetary manned flight.

Most of those development are still at lab level, but I think that the next ten years will be very interesting, as some of those concepts emerge as credible solutions, and also as a high power electric generators become available. However, I don't think that very high power electric propulsion required for manned flight (10 to 100 MW) will be available before 2030-2040. The applications in the next 10 years are mostly for probes, and maybe for sending cargo to the moon on slow economical trajectories.

Title: RE: High end electric/ion engines - current status?
Post by: mong' on 02/12/2007 11:08 am
Quote
vda - 12/2/2007  8:18 AM

So far almost all Solar System missions were using chemical engines, and frankly, they are far too inefficient for the task. Nine years to Pluto? Come on... Exploring/settling solar system using this technology will take centuries...

problem with elctric propulsion is thrust, it is very low, a few milinewtons, it would take forever to accelerate any decent spacecraft (let' say a few tons)to earth escape velocity. and if you plan to send one to pluto it can't use solar power. that leaves only a small nuclear reactor as the power source, and those are heavy, reducing the acceleration of the spacecraft even more.

the only thing ion engine are good at is orbital manoeuvering. a couple of ion thrusters powered by a small nuclear power source would do wonders for an orbiter, especially around saturn /jupiter, it could slowly alter its orbit to get closer to every moon in those systems. the nuclear power source could also power more capable instruments and higher datarate communications.
but this probe should use good old chemical propulsion and planetary flybys to get to its target, it may take a few years but it will always be shorter(and safer since you'd have to turn-on your reactor for the whole duration of the flight) than with ion thrusters only.

there are more powerful forms of ion propulsion with higher ISP and moderate thrust, like the magnetoplama rockets but they need much bigger power source, several tens of megawatts. the mass of such reactors compared with their low to moderate thrust (10-100N) also plagues them with very low accleration
Title: Re: High end electric/ion engines - current status?
Post by: Jim on 02/12/2007 11:18 am
The Dawn spacecraft is using an ion thruster
Title: Re: High end electric/ion engines - current status?
Post by: meiza on 02/12/2007 12:12 pm
The biggest problem with ion engines is the power source.

If you look at JIMO, most of the mass was those huge radiators which are needed to cool the working fluid to extract energy from the reactor. That was partly because the operating temperature of the reactor was quite low.

Ion engines with ISP of 20,000 s have been lab demonstrated, but the high ISP only hurts (for now) because it takes more energy to accelerate to same speed. (The lower fuel consumption doesn't help as the fuel is a negligible mass anyway). Also the acceleration is slow.

It'd take a mission that used decades to accelerate to huge speeds, which only would have that high isp thruster as the optimal solution.

It is not sensible to analyze the propulsion systems with an assumption of "infinite power", when the choice of a propulsion system is rigidly linked to the power source. It's the same as comparing a ferrari and a lorry - either one is useless for the other's job, and you can't say which one is better without any context.
Title: Re: High end electric/ion engines - current status?
Post by: vda on 02/12/2007 01:47 pm
Quote
meiza - 11/2/2007  2:12 PM
It is not sensible to analyze the propulsion systems with an assumption of "infinite power", when the choice of a propulsion system is rigidly linked to the power source. It's the same as comparing a ferrari and a lorry - either one is useless for the other's job, and you can't say which one is better without any context.

Ok, I can rephrase it. I am interested in high-power ion engines, ones which are capable of orbit insertion job. DS1 class engine is clearly far too feeble for that. Ok, so let's presume we have a ~1MW reactor (i.e. a lot of electricity). What type of engine can use that much electric current? What are expected thrust figures?

Of course this info will not be applicable to crafts with 10kW power source, I understand that.
Title: Re: High end electric/ion engines - current status?
Post by: mong' on 02/12/2007 02:30 pm
I assume you mean 1 megawatt. that's low end MPD thrusters, and the thrust is pretty low, 100 newtons (22 lbs) at the most. a 1 MW reactor is going to weigh at least 30 tons.
it could perform interplanetary transfer manoeuvers albeit very slow ones (several years to reach mars)
Title: Re: High end electric/ion engines - current status?
Post by: meiza on 02/12/2007 02:30 pm
It depends on the mass of the 1 MW (?) reactor and the power conversion equipment plus the required mission acceleration and delta vee and the size of the required payload. You could bolt on a lots of ion engines etc...
If you want to conduct a mission to the Oort cloud and need 100 km/s delta vee then you have no choice but to use high isp ion engines and it'll take a decade to just accelerate to the required velocity. Or something like that. Depends on power source a lot.
If you just want to go to Mars perhaps use vasimr or if to the moon then hall thrusters. Depends on the power source mass again.

For short trips and when in a hurry, use low ISP. For long trips or if you have a lot of time, use high ISP. If you have huge amounts of cheap and light power, use higher isp.
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon on 02/12/2007 09:34 pm
Just for grins, converting 150 kW, that's 201 horsepower.  So that puts the present "big" ion engine at a power comparable to a Cessna 182.  Which is a big improvement over the last one I heard of, which was an Air Force experiment from the 1990's that was doing orbital maneuvers with less power than a Piper Cub.

In attempting to look up the power of a Saturn V first stage, I found "Its energy output was equivalent to that of 85 Hoover Dams, and its roar at liftoff equaled that of 8 million hi-fi sets (over 200 million watts). "

http://edition.cnn.com/interactive/space/9907/apollo11.saturn/sic.html

Come on, Dr. Bussard!  We need those 10 gigawatt reactors!
Title: Re: High end electric/ion engines - current status?
Post by: meiza on 02/12/2007 09:49 pm
It's not the ion engine that produces the power. In a sense, it doesn't "have" any power, it only "uses" that 150 kW. It always needs an external power source. The ion engine could rather be seen not as a Cessna engine but as a Cessna propeller.

The very real questions are ~always about the power sources and their masses.
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon on 02/12/2007 10:11 pm
Exactly.  Actually, I think ion motor is the correct term, for that reason.  But the overall system, including a reactor to provide the power, would qualify as an engine by the definition I'm familiar with.

In the early jet age, people always insisted on knowing the equivalent horsepower of jet engines since that was the traditional figure of merit for piston/prop aircraft.  Expressions of jet horsepower were always impressively higher, but the designers themselves preferred to stick to thrust.  That also persisted thru chemical rockets.

Now, with solar or nuclear power sources, we're getting back to converting electrical or thermal power and a reaction mass flow to thrust.

I find occasional lapses to horsepower to be useful just to gain some perspective.  150 kW, or even 1 MW, is pretty pathetic compared to what we really want for Flash Gordon space travel.  Gigawatts start to get useful.

I once "designed" an air-breathing 4-seat jet called a Sunfire for a story.  I modeled it as having a 40 MWe fusion power source, based on a particular subsonic jet fighter's power output (55,000 HP).  By SF magic and some ion-propulsion hand-waving, this little aircraft was the same weight as the fighter, and hit the same speed at the operating altitude of the original jet.  But this hypothetical machine could continue to operate in rarified atmosphere to the fringes of space, then switch to on-board water to continue to LEO and circularize.  I constructed a thrust-lift-drag-weight model of this which ran on a Radio Shack Model III (this was back in the distant past when computers used Roman numerals).  The little jet could reach orbital velocity and 75 miles of altitude after going about 3/4 of the way around the equator.  Acceleration was pretty pathetic toward the end, though.
Title: Re: High end electric/ion engines - current status?
Post by: mong' on 02/12/2007 10:26 pm
Quote
meiza - 12/2/2007  11:49 PM

It's not the ion engine that produces the power. In a sense, it doesn't "have" any power, it only "uses" that 150 kW. It always needs an external power source. The ion engine could rather be seen not as a Cessna engine but as a Cessna propeller.

The very real questions are ~always about the power sources and their masses.

the way I see it personnally is that chemical rocket engines are both an engine and a power source combined in one device, the power source being the propellant itself. in the strict sense of the term, the engine (i.e: the device that transform energy into thrust for motion) is actually the nozzle, just like the ion engine transforms electricity into thrust (using propellant)
Title: RE: High end electric/ion engines - current status?
Post by: scienceguy on 02/13/2007 02:40 am
Let's just speculate that sometime in the future we have super efficient helium-3 fusion and the protons produced can be used to generate electricity directly.

Could such an apparatus be light enough and yet generate enough power for something like a VASIMR?
Title: Re: High end electric/ion engines - current status?
Post by: wingod on 02/13/2007 04:33 am
Quote
vda - 12/2/2007  8:47 AM

Quote
meiza - 11/2/2007  2:12 PM
It is not sensible to analyze the propulsion systems with an assumption of "infinite power", when the choice of a propulsion system is rigidly linked to the power source. It's the same as comparing a ferrari and a lorry - either one is useless for the other's job, and you can't say which one is better without any context.

Ok, I can rephrase it. I am interested in high-power ion engines, ones which are capable of orbit insertion job. DS1 class engine is clearly far too feeble for that. Ok, so let's presume we have a ~1MW reactor (i.e. a lot of electricity). What type of engine can use that much electric current? What are expected thrust figures?

Of course this info will not be applicable to crafts with 10kW power source, I understand that.

When we did our 500 kW Hall thruster system (about 21-25 newtons) the dry mass was right at 10 metric tons.

We looked at a hybrid system for Jimo that would have used a Solar Electric Hall Thruster powered tug to get Jimo out to Mars and then let it go on to Jupiter.  I would have worked much better than jimo by itself.  In the end JIMO was its own worst enemy.

Title: Re: High end electric/ion engines - current status?
Post by: lambda0 on 02/13/2007 09:08 am
Quote
When we did our 500 kW Hall thruster system (about 21-25 newtons) the dry mass was right at 10 metric tons.

Hello

I didn't know that a 500 kW Hall thruster have been developed. Was it the power of a single thruster or for an array ? I thought that the most powerful Hall thruster being tested now was 150 kW (the NASA-1000M at Glenn Research Center).
Do you have some doc about this 500 kW thruster ?

Title: Re: High end electric/ion engines - current status?
Post by: vda on 02/13/2007 10:54 am
Quote
meiza - 11/2/2007  4:30 PM
It depends on the mass of the 1 MW (?) reactor and the power conversion equipment plus the required mission acceleration and delta vee and the size of the required payload. You could bolt on a lots of ion engines etc...

Trying to collect some data on Internet and do basic math:
We can build fission reactors at 2.2kW per kilogram of reactor mass [src: http://en.wikipedia.org/wiki/Nuclear_photonic_rocket]
Probably the most energy-efficient use is to directly expell fission fragments. [http://en.wikipedia.org/wiki/Fission-fragment_rocket]

Reactor power: 1MW
Reactor mass: ~500kg
Exhaust, m/s: 1000000 (ISP ~100000)
Exhaust, kg per sec: 0.000002 (2 mg)
Thrust: 2N
Acceleration: 0.004 m/s^2 (4mm/s^2)
T/W ratio: 0.0004

Doesn't look that bad at all. Questions:
1. How far is this stretched from really achievable parameters? (really achievable ISP? how much of energy will be absorbed as heat instead of being expelled as exhaust? do we need radiators and how much they will weigh? etc)
2. Fission fragment rocket engine was never built IIRC, even as a small scale prototype. We only ever tried other, less 'direct' nuclear-to-exhaust convertors - I mean, reactor -> electricity -> ion engine.

[http://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster]
Let's say 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: 20N
Acceleration: 0.04 m/s^2 (40mm/s^2)
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?
Title: Re: High end electric/ion engines - current status?
Post by: lambda0 on 02/13/2007 11:30 am
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.
However, with a high power electronuclear reactor (100-200 MW), with gaseous core and advanced MHD conversion system, it might be possible to obtain specific power of 2 or 3 kW/kg. With a 200 MW reactor of this kind, a VASIMR or MPD thruster could send a spacecraft to Mars in 2 months, even less.
But all this remains quite speculative, I don't think that this nuclear reactor exists for the moment.

Example of a high power vapor core reactor for electric propulsion :
http://www.inspi.ufl.edu/gcr.pdf
http://adsabs.harvard.edu/abs/2004AIPC..699..379K
Title: Re: High end electric/ion engines - current status?
Post by: meiza on 02/13/2007 12:21 pm
Yeah. Nuclear reactors in space are not easy for generating electricity since you need big radiators. You don't need that on earth or if you use nuclear thermal rockets.
With current solar electric we're talking perhaps in the very rough ballpark of 100 W/kg.
There are a few threads already about these kinds of things

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1139&start=1
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1218&start=1
Title: Re: High end electric/ion engines - current status?
Post by: meiza on 02/13/2007 12:24 pm
Oh and this of course about VASIMR:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1382&posts=60&start=1
Title: Re: High end electric/ion engines - current status?
Post by: lambda0 on 02/13/2007 12:45 pm
This should be interesting :
http://www.hapcos.org/DOCS/download.php?id=81&type=pdf

In fact, there may be another possibility : a few months ago, I made a few calculations for a manned Mars mission based on a MPD propulsion and a solar power plant. A solar power plant is generally considered to be more massive than the equivalent nuclear reactor, but some recent developments on thin films may change this and allow to build a very low mass power plant.
In this simulation, the specific mass of the power plant+PPU is 0.9 kg/kW, the thruster is a LiLFA which is a variant of the MPD that uses lithium as propellant : easier to store than hydrogen, that contributes also to reduce the structure mass of the spacecraft.
With a 100 MW power plant, I found that it may be possible to send a 40 t payload to Mars in about 110 days.
This calculation takes into account the efficiency, the mass of the thrusters,...
http://img504.imageshack.us/my.php?image=mpd5ug.gif
(it's in french, sorry, I didn't have time to translate).

That's very approximate calculations, not optimized, to be confirmed by more serious studies, but I suspect that there might exist "non-nuclear" solutions for short manned Mars missions : if the flight time can be reduced to no more than 100-110 days, it is possible to come back to Earth after a 1-2 months stay on Mars (which implies a 8-9 months mission), instead of waiting for the next conjonction (which implies an almost 3 years mission).
And of course, all that depends on the real performances of  those solar thin films...

Title: Re: High end electric/ion engines - current status?
Post by: TyMoore 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!
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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.
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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
Title: Re: High end electric/ion engines - current status?
Post by: vda 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?
Title: Re: High end electric/ion engines - current status?
Post by: vda 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?
Title: Re: High end electric/ion engines - current status?
Post by: TyMoore on 02/13/2007 11:37 pm
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vda - 13/2/2007  6:36 AM

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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....
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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.
Title: Re: High end electric/ion engines - current status?
Post by: TyMoore 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
Title: Re: High end electric/ion engines - current status?
Post by: lambda0 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.


Title: Re: High end electric/ion engines - current status?
Post by: vda on 02/14/2007 08:15 am
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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").
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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.
Title: Re: High end electric/ion engines - current status?
Post by: meiza 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.
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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.
Title: Re: High end electric/ion engines - current status?
Post by: lambda0 on 02/14/2007 02:15 pm
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...
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.


Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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?
Title: Re: High end electric/ion engines - current status?
Post by: vda on 02/14/2007 09:27 pm
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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
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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.
Title: Re: High end electric/ion engines - current status?
Post by: lambda0 on 02/15/2007 02:34 pm
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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...

Title: Re: High end electric/ion engines - current status?
Post by: vda on 02/15/2007 03:05 pm
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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?).
Title: Re: High end electric/ion engines - current status?
Post by: TyMoore 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.
Title: Re: High end electric/ion engines - current status?
Post by: mong' 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
Title: Re: High end electric/ion engines - current status?
Post by: Tom Ligon 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.
Title: Re: High end electric/ion engines - current status?
Post by: TyMoore 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!



Title: Re: High end electric/ion engines - current status?
Post by: mong' 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
Title: Re: High end electric/ion engines - current status?
Post by: vda 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.
Title: Re: High end electric/ion engines - current status?
Post by: TyMoore 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.)
Title: Re: High end electric/ion engines - current status?
Post by: vda 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.
Title: Re: High end electric/ion engines - current status?
Post by: mong' on 02/16/2007 12:11 pm
Quote
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
Title: Re: High end electric/ion engines - current status?
Post by: TyMoore 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.