Author Topic: Power Supply for VASIMR  (Read 34471 times)

Offline wingod

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Power Supply for VASIMR
« on: 11/29/2007 04:38 am »
I thought about this when I saw the article yesterday but someone has gone for it and written one about using this new "Nuclear Battery" technology to power a VASIMR engine

Really really REALLY interesting.

http://advancednano.blogspot.com/2007/11/vasimr-engines-plus-200-mw-of-nuclear.html



Offline A_M_Swallow

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Re: Power Supply for VASIMR
« Reply #1 on: 11/29/2007 05:50 am »
Seven nuclear power "batteries" to get to Mars in 39 days sounds like the manned version.   The cargo version will probably take 100 days using 2 nuclear "batteries" or possibly use 50 MW solar arrays to power the VASIMR.  The trips are sufficiently short that the spacecraft can be reused.

At these sort of speeds we no longer need to use the shortest route.  I wonder how long the trip will take at maximum separation between the Earth and Mars?  All year round trips to Mars will make a Mars colony much more viable.

If a small VASIMR SEP is going to the Moon in 2010 then a version can be sent to Mars in say 2012.

Offline pippin

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Re: Power Supply for VASIMR
« Reply #2 on: 11/29/2007 08:59 am »
Just 10 ARES V flights to assemble one... Sounds a bit big to me.
Oh, and burying the "battery" is probably also not an option for spaceflight ;-)

Offline meiza

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Re: Power Supply for VASIMR
« Reply #3 on: 11/29/2007 01:15 pm »
Physics!!

How does the "nuclear bathtub" work? I've heard very sceptical mentions from nuclear engineers, saying how reactors have to have a big surface area to move the heat out - something that the nuclear bathtub doesn't have.
And you need a temperature difference (if you don't have a direct conversion system), something which requires big big radiators in space. When you see a picture of JIMO, most of that stuff is *radiators*.

So you can't just take "hey 200 MW, let's stick it on a spacecraft".

Gah.

Offline neviden

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Re: Power Supply for VASIMR
« Reply #4 on: 11/29/2007 03:20 pm »
It's all about cooling.

Getting power in space is easy. You can get 100 MW of thermal power to one spot with big lightweight mirror or hot reactor.

The hard part is using 100 MW of thermal power but at high isp. If you just use the heat to directly heat the propellant you get low isp in the range of 800s (NTP). Once you calculate the cost and weight of the whole ship (reactor, shields, plumbing,..) you are no better then the simple chemical rocket. VASIMR is good since you can get very high isp. The problem is that you need electrical power. You can’t use thermal power. And to get it you need to run some kind of thermal to electric conversion. And that basically means that to get your 100 MW VASIMR running, you must reject 200 MW of thermal power (very optimistically) in what is big thermos bottle (vacuum).

VASIMR is useful only when you have lightweight electric power generator (with the radiators) so that it can actually make a difference. Unless of course the VASIMR is only part of the fusion drive in that case it’s not really VASIMR, is it..

Offline khallow

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Re: Power Supply for VASIMR
« Reply #5 on: 11/30/2007 02:44 am »
Even if dumping heat directly to propellant isn't that useful, it's still a way to dump heat. One could use that as a way to preheat propellant that is to be ionized (the assumption being that heating will help lower the energy threshhold for ionization).
Karl Hallowell

Offline Lampyridae

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Re: Power Supply for VASIMR
« Reply #6 on: 11/30/2007 03:29 am »
I've come across one white paper that proposed used a gasdynamic laser as a radiator, which could then be used to heat propellant. Well, actually it wasn't the gasdynamic laser doing the heating - that had a negative temperature, and it fires back into the working gas, resulting in a standard blackbody radiation (albeit very bright and in a small space) that could be dumped through an optical window. A mirror could focus that radiation into a propellant. At least that's what I gathered from reading the thing. Apparently it doesn't violate entropy laws, either - but the idea's gotten no farther than that white paper.

Liquid droplet radiators could dump the heat a lot better, but that's still mass and bulk that needs on-orbit deployment and assembly...

Offline neviden

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Re: Power Supply for VASIMR
« Reply #7 on: 11/30/2007 07:04 am »
Quote
khallow - 30/11/2007  4:44 AM
Even if dumping heat directly to propellant isn't that useful, it's still a way to dump heat. One could use that as a way to preheat propellant that is to be ionized (the assumption being that heating will help lower the energy threshhold for ionization).
Heating propellant directly is useful to get to about 1000s isp. Anything above that is not possible since your support structure will melt. While it is a good boost it doesn’t really matter if your target isp is in the thousands of seconds. That is why you need electric power. At high isp levels the flow rate of propellant is measured in grams per second. Not something that would be enough to cool it. It would be good if you had low isp and it is already used that way (chemical rocket) but once you go to high isp (VASIMR) then it doesn’t really help that much.

Let me put it another way. Space shuttle has 12 GW engines at liftoff with isp in the area of 400 s. It is 95% propellant, burns its engines for few minutes and gets the maximum of 10 km/s delta-v. If we had one in orbit we could easily get to Mars orbit and back. The problem is, that we don’t have free 2,000,000 kg propellant depot in LEO so that we could refill our Space Shuttle once in orbit.

If you could get engines with higher isp you would not have to carry that much propellant in the first place even if you had to burn them for longer. And while you do need that much power to actually get from the ground (to fight the gravity), that power isn’t needed once you are in freefall (in orbit). Sure it would be mightily helpful if you would have 12 GW thrust engines in orbit. If you had isp that would be high enough so that you could actually carry enough propellant (that means you would not burn it in few minutes) you would be able to use it to get to Mars and back in few days. Even better would be if you had 12 GW thrust engines with high isp, but once you look how big that ship would be you get to understand the problem..

Offline advancednano

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RE: Power Supply for VASIMR
« Reply #8 on: 11/30/2007 03:47 pm »
I exchanged email with Hyperion Power Generation (the maker of the new power generator. They indicate that the Sante Fe reporter made a mistake. The output is about 25-17 MW ELECTRIC [This statement was also consistent with the patent which talked about tens of MW in electricity.] They also said that the containment vessel will be dense enough that no radiation will escape even if it is not buried in the ground.

So in addition to the regular electric generation there would be probably double that amount of thermal power. Which could be partially converted to electricity using thermoelectronics. 30-66% with better technology like powerchips

Lighter radiation shields are possible.

I have asked them about weight of the reactor and different heat pipe configuration, and they are considering whether to answer me about it. I believe that they may have more heat piping and just are not showing it in the diagrams.

Offline meiza

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Re: Power Supply for VASIMR
« Reply #9 on: 11/30/2007 04:58 pm »
I still don't see the power extraction mechanism.
Current reactors have fuel in thin rods and water circulates around them, extracting the heat. Then it's turned into electricity in a steam turbine.


Offline neviden

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RE: Power Supply for VASIMR
« Reply #11 on: 11/30/2007 05:20 pm »
Quote
advancednano - 30/11/2007  5:47 PM
I have asked them about weight of the reactor and different heat pipe configuration, and they are considering whether to answer me about it. I believe that they may have more heat piping and just are not showing it in the diagrams.
That is the most critical point. How much would everything weigh on a spaceship. Radiators, shielding, structure, reactor, piping, electrical conversions for specific thrusters (if they need different voltage for example), cables, controls..

You take the weight, divide it by how much power you get and you get nice KW/kg number. That is the most important thing that you need to know if you want to make any comparison. Higher it gets, faster, lighter and more capable your spaceship will be.

VASIMR itself is useless without some very capable electrical system to power it.

Edit: Oh I see I have missed that magical 70% - 80% lightweight 200 MW “powerchip” conversion. That is why there is no need for radiators that you would otherwise need to dump 500+ MW of heat in vacuum..

Without them your spaceship would look something like this (only bigger, since this one is only 10 MWe):
http://gltrs.grc.nasa.gov/reports/2003/TM-2003-212349.pdf

Oh, yeah, that company that makes those “powerchips” will be big, better buy it now. ;)

Offline advancednano

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RE: Power Supply for VASIMR
« Reply #12 on: 11/30/2007 06:42 pm »
It is only the size of a hot tub. 8 cubic meters. Even at max avg density of 8. That would be 64 tons. There is only about 600-1200 kg of uranium hydrate.

I believe that for a space ship lighter radiation shielding could be used.

I discuss the recent thermoelectric advances which are part of a quite well funded program to raise the efficiency of cars and trucks. This could be used in place of heavier power conversion. Should be ready in 2014 (actually the first versions will be in BMW car in 2010. Crude versions are already used for beer refridgerators and are used for car seat warmers). Could boost the electric power conversion by 60%. Pushing electrical power from 25MW to 40MW per unit. Only 5 units would be needed to achieve 200MW.

With electrostatic radiation shielding and advanced thermoelectronic heat to electricity conversion then I believe each unit could be brought down to ten tons. Most of the weight is the tons of fuel that the Vasimr would be using as reaction mass.

Vehicle efficiency technology gets about 176 million per year The new thermoelectronics is not magic tech.

Offline meiza

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Re: Power Supply for VASIMR
« Reply #13 on: 11/30/2007 07:37 pm »
Note that the quantum dot graph has T_Cold = 50 C. I don't know how big radiators you need for that in space...
Pressurized water nuclear reactors run around 350 C on the hot side but have the advantage of being on earth where your cold side can be cooool. And they use high efficiency steam generators and end up being around 30% efficient.
So what we need:

1) hot side temp of the tub reactor system (or is there a lower limit for coolant coming in to the reactor? The Greenspan paper uses fancy metal coolants which mean the incoming coolant has to be hot too to stay molten, and thus the delta T is only 150 C)
2) mass of a certain tub size with certain power
3) we can calculate a reasonable cold side temp of a reasonable mass radiator
4) from these we get delta-T and can calculate the conversion efficiency

Only THEN can we estimate the mass of the system. That's a first order thing.

Offline advancednano

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Re: Power Supply for VASIMR
« Reply #14 on: 11/30/2007 08:44 pm »
The hyperion device runs at 400-800 degrees using uranium hydrates and can run at 1900 degrees using thorium hydrates.

We can look at other spacecraft designs that have heat radiators. Those are separate technologies from the main hyperion nuclear reactor.

Here is some info on other heat radiators for rockets

Here were some weights for a nuclear fission system.

BTW: on the topic of the people who are making light of the powerchip company, You can also mock general electric, general motors, john Deere, Delphi Automotive Systems, Dow Corning, Caterpiller, Argonne National Laboratory, and Oak Ridge National Laboratory and Virginia Polytechnic Institute and Ames National Laboratory and the other companies working on advanced thermoelectronics.

Offline meiza

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Re: Power Supply for VASIMR
« Reply #15 on: 11/30/2007 09:15 pm »
That is a remarkably efficient thermionic system and will change a huge amount of things on earth too.

Okay here is the calculation with a huge amount of assumptions:

-The coolant enters the tub at 400 C and leaves at 800 C.
-The thermionic quantum dot system has a ZT of 4.5 which means efficiency of 22% at 400 K delta T.
-For 200 MW electric power we need thus about 1000 MW thermal power.
-To radiate 1000-200 = 800 MW at 400 C = 700 K means
P = Re * 6E-8 * A * T^4
8E8 W = 1 * 6E-8 W/(m^2*K4) * A * (700 K )^4
A = 8E8 m^2 / (6E-8 * 700^4)
A = 55000 m^2.
Or a square with a side of over 200 meters.

On the other hand, if you accept a lower efficiency and lower delta T, you can run with a higher radiator temperature and that makes it smaller. If we want a 100 m square, the area drops to 1/4, T^4 grows to 4, T^2 to double and T to 700 K *(sqrt(2)) = 1000 K or 700 C.
Thus the delta T drops to 100 K and thermionic efficiency to about 10%. Meaning the whole system (reactor, converter, radiator area) must grow to 2X. That means the radiators are 1/4 * 2 = half the area, or about 140 m a side. There is a certain optimal low end temperature which depends on the specific radiator, converter and reactor masses.

Offline neviden

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Re: Power Supply for VASIMR
« Reply #16 on: 11/30/2007 09:45 pm »
You are missing the point. You don’t have to prove that NEP would work. It is a known and a good concept.

It is the matter of facts. How much would everything weigh and what could you do with it.. Even if you increase power conversion efficiency to 80% you still need to look at the whole picture. How heavy would reactor be? How heavy would that powerchip conversion be? What about structures? Tanks? Cables? Shields? Thrusters? Cryogenic stuff for your propellant?

What is the realistic kW/kg estimate. How big would everything be if you designed your ship to have and use 200 MWe thrusters? How much would it cost?

Offline meiza

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Re: Power Supply for VASIMR
« Reply #17 on: 11/30/2007 10:22 pm »
We simply don't have that data as the reactor concept is so vague as is the data about the thermiionic systems. I just showed that radiator area is pretty big even with very favorable assumptions.

Offline neviden

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Re: Power Supply for VASIMR
« Reply #18 on: 11/30/2007 10:53 pm »
I agree with meiza. But the whole NEP concept doesn't even need 200 MWe to be useful. Even 10 MWe with present technology would work almost as good as chemical/NTP and allow reusable spaceship capable of reaching Mars orbit and returning back.

Anything with more power and better kW/kg ratio would only shorten transit times, increase payloads and increase capabilities.

Offline advancednano

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Re: Power Supply for VASIMR
« Reply #19 on: 11/30/2007 11:22 pm »
I did not say that vasimr needed the Hyperion reactor to be useful. Vasimr is proceeding with its 200kw device.

However, the Hyperion reactor had a bunch of confused information online at slashdot and Sante fe reporter. I was providing more accurate info from the patent and from direct correspondence with the company.  I was also considering how it might help the Vasimr if it did get developed. I do not see any other relatively small solid core reactor nuclear projects with any funding.

The vasimr - hyperion reactor is a speculation based on limited info.

From the patent (section 58)

At the rate of power production assumed for the reactor, 50 to 100 W/cm**3.

If the density is 8. then it would seem to work out to 7-14KW per kg.

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