Author Topic: NEP AG transit to Mars  (Read 53524 times)

Offline TyMoore

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RE: NEP AG transit to Mars
« Reply #40 on: 11/21/2006 09:00 pm »
Personally, I've looked at a concept that puts most of the nuclear fuel, or even possibly the reactor on a stripped down verson of a CEV. Use a stripped down capsule that has no lifesupport system, but does have a LES tower and parachute recovery system, ablative heat shield, and of course a recovery beacon. Inside, mount the reactor or nuclear-fuel core in a cask (assuming that part is actually small enough to fit!) inside the capsule. Launch the whole thing on something bigger than the "Stick" so that it can carry enough propellant for a rendezvous with your assembly area. Sure, it's a definate cost, weight, and complexity penalty, but you can use the CaLV to orbit almost all the rest of the vehicle. By using a clever application of a 'boilerplate' CEV for the nuclear fuel transport, you can minimize (almost completely eliminate) any possibility of the fuel core even being damaged in a launch abort situation--let alone resulting in dispersion of radionucleides in the air or water.

I think a dedicated CEV derived "Hazardous Payload Capsule" makes sense.

Of course, this means that whatever reactor is ultimately designed must either be fueled in space by remote, or you've got to send a crew up there to insert a fuel core into the pressure vessel of an NTR or some other power plant. This has never been done before and this would entail a lot of 'firsts' that will admitedly add expense, risk, and complexity to an otherwise expensive, risky, and complex mission....Hmmm...

Offline kfsorensen

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RE: NEP AG transit to Mars
« Reply #41 on: 11/21/2006 09:08 pm »
Space reactors are mandated by federal law to utilize highly-enriched uranium (>93% U-235) as fuel.  The half-life of uranium-235 is hundreds of millions of years, the half-life of the remaining U-238 is 4.5 billion years.  Which means their level of radioactivity is exceedingly small.  In the event of total core vaporization, a couple of hundred kilograms of uranium would be returned to the ocean, to mix with the millions of tonnes of uranium already dissolved there.

You may be confusing reactor fuel (enriched uranium) with the radioactive isotope plutonium-238 that they use in RTGs.  Pu-238 is much more radioactive (with a 78 year half-life) and should not be released to the environment.  The vaporization and dispersion of an entire uranium core (before any fission reactions have taken place) is not a cause for great concern environmentally.

Offline TyMoore

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RE: NEP AG transit to Mars
« Reply #42 on: 11/21/2006 09:35 pm »
I know that before activation of the core, the radionucleide loading is quite small. And personally, I wouldn't lose any sleep over a core burning up in the atmosphere (unless I was working on the project and the loss of a powerplant set my schedule way back!)


As long as you are not trying something really silly like bring back an expended core so that the spent nuclear fuel could be placed in Yucca Mountain (I guess I shouldn't give them any ideas!)  then NTR or NEP are completely safe as far as I am concerned.

Still, it was an idea worth exploring. The effort may either placate or atleast reassure some nuclearphobes who are otherwise supporters of space exploration.

And yes Pu-238 as used in RTGs is a lot 'hotter' (decay activity level wise) than Pu-239. U-235 is actually quite mild compared to Pu-239...

I didn't know about the Federal Law Mandating highly enriched cores--I just assumed that the use of 90-95% Enriched Uranium was more engineering necessity to get the most watts/kg of mass than the Law.

Interesting...

Offline kfsorensen

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RE: NEP AG transit to Mars
« Reply #43 on: 11/21/2006 10:07 pm »
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TyMoore - 21/11/2006  4:18 PM

I didn't know about the Federal Law Mandating highly enriched cores--I just assumed that the use of 90-95% Enriched Uranium was more engineering necessity to get the most watts/kg of mass than the Law.

Interesting...

The reason for the HEU mandate (as opposed to launching cores that went critical on Pu-239, LEU, or U-233) is that an HEU core will generate less Pu-239 through neutron absorption than an LEU core.  Pu-239 (and other transuranic) levels in the core need to be kept to a minimum so that in the event that a core experiences a failure as it "climbs out" of the gravity well the orbital decay lifetime of the spacecraft (and its "hot" core) is sufficient to allow nearly all of the fission products in the core to decay to stability.

Offline Avron

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RE: NEP AG transit to Mars
« Reply #44 on: 11/22/2006 03:57 am »
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Marcus - 21/11/2006  3:59 PM

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Avron - 11/1/2006  7:29 PM

I have two issue, that someone could provide some insights...



Nearly 10 mths to get a answer.. thank-you Marcus

Offline SteveMick

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Re: NEP AG transit to Mars
« Reply #45 on: 11/22/2006 05:33 pm »
I mistakenly said "Solarex" when I should have said "Spectrolab" - a division of Boeing. They make a variety of cells for space applications and their web address is logically enough www.spectrolab.com
Steve

Offline James (Lockheed)

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Re: NEP AG transit to Mars
« Reply #46 on: 11/27/2006 11:47 pm »
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Carl G - 6/11/2006  7:30 PM

Whoa, old thread, but this is a good one. This would be an amazing ship to build.

Sure, but those days of this being possible, due to budgets, are over.

Offline kfsorensen

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Re: NEP AG transit to Mars
« Reply #47 on: 11/28/2006 01:26 am »
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James (Lockheed) - 27/11/2006  6:30 PM

Sure, but those days of this being possible, due to budgets, are over.

If the AG-NEP vehicle comes out to be a lower overall cost than a chemical or a nuclear thermal option, then I don't see why that would have to be the case.  Such an total cost analysis should also include the costs of microgravity adaptation research and countermeasures that would not need to be done if the crew could get the gravity they need for health.

Offline To The Stars

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Re: NEP AG transit to Mars
« Reply #48 on: 11/28/2006 03:28 am »
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vanilla - 27/11/2006  8:09 PM

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James (Lockheed) - 27/11/2006  6:30 PM

Sure, but those days of this being possible, due to budgets, are over.

If the AG-NEP vehicle comes out to be a lower overall cost than a chemical or a nuclear thermal option, then I don't see why that would have to be the case.  Such an total cost analysis should also include the costs of microgravity adaptation research and countermeasures that would not need to be done if the crew could get the gravity they need for health.

I'll be a happy man if and when that day comes where this sort of vehicle becomes a reality.

Offline wingod

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RE: NEP AG transit to Mars
« Reply #49 on: 11/29/2006 04:56 pm »
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Chris Bergin - 9/1/2006  10:45 AM

We're placing another Powerpoint presentation on to the download section - this is from slightly earlier (2002) but has good explantatory background.

58 pages.

I like this one from Dr. Stuhlinger from 1956


Offline wingod

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RE: NEP AG transit to Mars
« Reply #50 on: 11/29/2006 05:04 pm »
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SteveMick - 21/11/2006  2:15 PM

The numbers you used for solar cell power density are way off. Triple junction PV for use with concentrated sunlight made by the "Solarex" co. currently approach 1KW/kg. The concentrator can easily have a factor of 10KW/kg. and as a result, mass goes up very little as sunlight intensity drops as Mars is approached. Since this tech is developed and at least two orders of magnitude cheaper, I am puzzled as to why NEP would ever be considered for this role.
 Also, a solar electric rocket can operate as a solar thermal rocket to acheive Earth escape from LEO much faster. It really is the best of both worlds and has other advantages besides. The concentrator mirrors can double as communication and/or radar antennas and the intense heat at the concentrator's focus can be used for direct ISRU.
 Please use this as the "competition" for NEP and not a straw man system. I think you'll come to agree that NEP is impractical or at least inferior for Mars transit.
Steve

When we put our system together for Langley last year we achieved about 75w/kg for the total system using fairly conservative 33% solar cells.


Offline SteveMick

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RE: NEP AG transit to Mars
« Reply #51 on: 11/29/2006 08:04 pm »
Were you using concentrator type cells from Spectrolab and if so, at what concentration factor?
 What kind of concentrator did you use - one from L'Garde?
 No one argues that traditional non-concentrating type arrays have a mass that is closer to NEP they are so relatively massive.
 Concentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage.
 Steve

Offline kfsorensen

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RE: NEP AG transit to Mars
« Reply #52 on: 11/30/2006 12:08 am »
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SteveMick - 29/11/2006  2:47 PM

Concentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage.

Using concentrators doesn't do anything about the basic problem of thermal rejection.  If you want to generate 4 MW of power, and you've got 33% efficient cells, assuming everything else is perfect, you need to collect and focus 12 MW of heat energy and then reject 8 MW of it to space.  The radiator will be sized according to the temperature at which you do this rejection, but as a general rule, solar cells don't like to get hot.  Let's assume you have some hot-shot cell that will go to 150 C and still run at 33% efficiency.  Even with a perfectly emissive radiator, you still need to reject 8 MW of heat at something less than 150 C.  But let's be kind and assume you've got a perfectly emissive radiator (e = 1.0) and you've got isothermal heat transfer from the back of your concentrator arrays to your radiator.  You're looking at 6600 square meters of radiator, or a square 80 meters on a side.  If you have to reject at a lower temperature, it gets worse with the fourth power of the temperature.

One of the basic advantages of a nuclear-electric power system is that it is a very dense heat source.  You could couple it to a power conversion system like a potassium-Rankine cycle and reject waste heat at much higher temperatures than solar cells like.

It is also conceivable that you could run a K-Rankine cycle off concentrated solar energy--for typical conversion efficiencies of 20% or so, you'll need 20 MW of input heat to drive the cycle at generate 4 MW of electricity.  That will require ~15000 m^2 of solar collection area (at a flux of 1300 W/m2 and perfect reflectivity) and 430 m2 of radiator area (assuming 16 MW of rejected heat at 900 K emission temperature and perfect emission).

Offline wingod

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RE: NEP AG transit to Mars
« Reply #53 on: 11/30/2006 02:46 am »
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SteveMick - 29/11/2006  2:47 PM

Were you using concentrator type cells from Spectrolab and if so, at what concentration factor?
 What kind of concentrator did you use - one from L'Garde?
 No one argues that traditional non-concentrating type arrays have a mass that is closer to NEP they are so relatively massive.
 Concentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage.
 Steve

Sorry I am not a believer in concentrator type systems.  They require precise pointing and the cells degrade faster in a radiation environment.  Also there is the issue of occlusion of the optics in a radiation environment such as going through the Van Allen belts.

I could be wrong but that is my position until someone gives me data otherwise.

Offline wingod

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RE: NEP AG transit to Mars
« Reply #54 on: 11/30/2006 02:47 am »
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vanilla - 29/11/2006  6:51 PM

Quote
SteveMick - 29/11/2006  2:47 PM

Concentrator cells are a real breakthrough and by using the concentrators for solar thermal and antenna duty magnifies this advantage.

Using concentrators doesn't do anything about the basic problem of thermal rejection.  If you want to generate 4 MW of power, and you've got 33% efficient cells, assuming everything else is perfect, you need to collect and focus 12 MW of heat energy and then reject 8 MW of it to space.  The radiator will be sized according to the temperature at which you do this rejection, but as a general rule, solar cells don't like to get hot.  Let's assume you have some hot-shot cell that will go to 150 C and still run at 33% efficiency.  Even with a perfectly emissive radiator, you still need to reject 8 MW of heat at something less than 150 C.  But let's be kind and assume you've got a perfectly emissive radiator (e = 1.0) and you've got isothermal heat transfer from the back of your concentrator arrays to your radiator.  You're looking at 6600 square meters of radiator, or a square 80 meters on a side.  If you have to reject at a lower temperature, it gets worse with the fourth power of the temperature.

One of the basic advantages of a nuclear-electric power system is that it is a very dense heat source.  You could couple it to a power conversion system like a potassium-Rankine cycle and reject waste heat at much higher temperatures than solar cells like.

It is also conceivable that you could run a K-Rankine cycle off concentrated solar energy--for typical conversion efficiencies of 20% or so, you'll need 20 MW of input heat to drive the cycle at generate 4 MW of electricity.  That will require ~15000 m^2 of solar collection area (at a flux of 1300 W/m2 and perfect reflectivity) and 430 m2 of radiator area (assuming 16 MW of rejected heat at 900 K emission temperature and perfect emission).

Or you use non concentrating cells and the equlibrium temperature is about 60-70c.


Offline Avron

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Re: NEP AG transit to Mars
« Reply #55 on: 11/30/2006 04:32 am »
Just wondering what the impact on movement out of the earth-moon system has on both concentrator type system efficiency and the cooling required, while I guess NEP is not impacted.

Offline kfsorensen

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RE: NEP AG transit to Mars
« Reply #56 on: 11/30/2006 04:48 am »
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wingod - 29/11/2006  9:30 PM

Or you use non concentrating cells and the equlibrium temperature is about 60-70c.

Yes but SteveMick was pretty emphatic that the concentrating cells are the ones that should be compared to NEP, and that beyond that even, that concentrating surfaces (inflatable mirrors and so forth) could offer even greater advantages.  I enjoy a good solar dynamic system just as much as the next guy, but I don't think it can credibly compete with NEP for a human Mars mission.

Offline wingod

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RE: NEP AG transit to Mars
« Reply #57 on: 12/01/2006 02:34 am »
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vanilla - 29/11/2006  11:31 PM

Quote
wingod - 29/11/2006  9:30 PM

Or you use non concentrating cells and the equlibrium temperature is about 60-70c.

Yes but SteveMick was pretty emphatic that the concentrating cells are the ones that should be compared to NEP, and that beyond that even, that concentrating surfaces (inflatable mirrors and so forth) could offer even greater advantages.  I enjoy a good solar dynamic system just as much as the next guy, but I don't think it can credibly compete with NEP for a human Mars mission.

Sorry I don't buy it.


Offline SteveMick

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RE: NEP AG transit to Mars
« Reply #58 on: 12/01/2006 04:51 pm »
Wow! I thought opinions were supposed to be based on reality and not feelings. If you disagree with the specific power reported by Spectrolab then please talk to them. On the other hand if you agree that the specific power of these cells is in the 1KW/kg range - an order of magnitude improvement over "regular" PV, then their superiority to any NEP system discussed here so far is beyond question.
 The other argument presented here is that current concentrating cells reject heat at a lower temperature and hence require a larger radiator. Since the concentrator support and backing structure is that radiator; there is a certain synergy that mitigates this problem to a great extent. Remember the huge radiators of the JIMO probe were only radiators. The solar radiator can be relatively larger since it doubles as structural support for concentrators that double as antennas and power solar thermal rockets as well.
 I want to emphasize the point that the ability to run in solar thermal mode means much greater acceleration rates and when kinematics favor thrust over Isp can be very advantageous. For LEO to escape a series of thrusts using the thermal rockets at perigee can work very efficiently by taking advantage of the relatively high velocity in this part of the orbit.
 Since the electric prop. sys.'s job in the case of solar thermal/electric propulsion(STEP)is merely to shorten transit time, it remains to be seen what amount of electric power production is optimum, but it need not be anywhere near as large as NEP which ONLY has electric propulsion.
 In addition, there is a high probability that carbon based nanotech cells can operate at far higher efficiencies and temperatures. One design patented in the Eighties used carbon diodes and antennas at nanoscale lengths to convert EM waves of light to elec. just like a radio receiver's antenna does. 60% efficiency or better was claimed in theory. Although no operating temp. was mentioned, you would expect it to be rather high.
 As for the lifetime of concentrator cell, you can carry ten sets of spares before they get as massive as current PV.
 Two to three orders of magnitude cheaper, much more versatile, faster transit times, faster communication, radio astronomy, radar investigations, regolith used as propellent(or just the volatiles) - yeah I guess it can in no way be considered "cedible competition". After all its so superior competition itself between NEP and STEP is not very credible.
Steve

Offline kfsorensen

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RE: NEP AG transit to Mars
« Reply #59 on: 12/01/2006 05:05 pm »
The backside of a concentrating mirror (such as would be used for solar thermal or solar dynamic) is not a suitable radiator, since the concentrator itself is designed to reflect the solar radiation to the receiver at the maximum possible efficiency (reflectivity = 1.0).  Theoretically, none of the heat would then end up at the concentrating surface.

At the receiver, on the other hand, the temperatures will get very high.  For a solar thermal engine, the hydrogen flow is the coolant, but if you desired to run a heat engine like a Brayton, Stirling, or Rankine, you still have to reject the waste heat, which will be between 70-80% of the heat load.  Conducting that heat from the receiver back to the concentrator and rejecting it from the backside will be problematic in many respects.  First of all, there is the sheer quantity of thermal energy to be moved, secondly, a hot back face on the concentrator could distort its optical surface, making its collection efficiency go down.  If the concentrator is an inflatable, then piping heat to it for rejection simply isn't a practical option.

If you use concentraing solar arrays, then the backface of the array can and will serve as the radiator, but you're back to the basic problem of the array area needed and the equilibrium temperature of the array (which in turn alters its efficiency).  To say nothing of pointing and attitude control concerns.

Solar thermal also can't be used to deliver enough of a "kick" at perigee to get you out of the Earth-Moon system at significiant (>2 km2/s2) C3.  Without that capability, much of the astrodynamic advantage of thrusting deep in the gravity well is lost.

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