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November 28, 2007

Vasimr engines plus 200 MW of nuclear "batteries" = 39 days to Mars

A proposed portable nuclear reactor (simplified solid core) is the size of a hot tub and will be able to generate 27MW. It is in funded development. A 200 KW version of the Vasimr engine is being ground tested in 2008 and a flight version is being readied for 2010. Seven of the nuclear generators would provide 200 MW of power to enable 39 day one way trips to Mars. Two technologies that are both in funded development and with no major feasibility questions could revolutionize space travel.

LATEST UPDATE - Clarification about what is novel about this design and what should be the same as other nuclear reactor and nuclear propulsion systems:

The reactor does not exist yet. Therefore, it is not space rated. They have just announced that they are working on it. They are talking 2012 for the first one to get finished for some ground application.

However, it is just another solid core nuclear reactor. I do not see why other nuclear reactor designs and solid core rockets would work and this would not. There have been other nuclear thermal spacecraft designs using similar technology. I am just choosing to pair the reactor with the Vasimr plasma drive instead of using direct nuclear propulsion systems. It is nuclear electric powering a vasimr drive.

The patent indicated that if they used thorium hydrate then the reactor would run at about 1900 degrees, which could be better for nuclear power system for a rocket. However, they are first working on uranium hydrate.

I have not done any detailed design for this system, but there is not that much about it that is that novel compared to other nuclear reactor for space rocket designs. The main novelties - no people needed to tend the nuclear reactor - it keeps a constant temperature by itself - it is simple and presumably easier to build and maintain - less waste than many other systems. Heat piping, radiation shielding, heat radiators, conversion of the steady state heat to electricity are all things that can be tweaked based on the application and which can be cribbed from past nuclear rocket designs. Some other nice things are that they are talking about mass factory production and low costs.

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.

UPDATING AGAIN:
There are some basic sources online to perform the rough estimates of heat pipe weighting and heat radiators.

Some other component weights for other nuclear rockets.

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 and the physics of dealing with the heat is the same. The Hyperion reactor's main advantage is that it self-regulates to whatever temperature range it is designed for based on different metal hydrates that are used. Dealing with heat and electricity conversion is not changed from other solid core nuclear reactors.

I have a new article that discusses the state of thermoelectronics, which has improved a lot since the mid-90s and which has a lot of money going into improving them to help deal with high oil prices. The goal of the DOE EERE is to raise diesel engine efficiency by over 50% by 2014 with addon systems to utilize waste heat.

ANOTHER UPDATE: I exchanged email with hyperion power systems (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.


The proposed nuclear "battery" reactor

UPDATE: For a space craft we would want to eliminate extra weight and especially any dirt radiation shield. There are several approaches. Research has been looking at using lightweight electric and electrostatic fields for radiation shields. We would only need to concentrate shielding on the crew quarters. The crew quarters would probably be in the front with the 600 tons of fuel in fuel tanks between them and the reactors. Shorter trip times mean less exposure to low gravity and cosmic rays.

Spacecraft designers may also use a ship's own cryogenic fluids as a radiation screen by arranging the cargo tanks containing them around crew compartments.

"In most [mission] scenarios, you need liquid hydrogen for fuel and you need water," explained Richard Wilkins, director of NASA's Center for Applied Radiation Research at Prairie View A & M University in Texas, conducting one study into liquid shield approaches. "And these are all considered materials that are particularly good for cosmic ray shielding."


Here is a 12 page pdf on electrostatic radiation shielding



Another issue is converting heat from the nuclear plant to electricity. There has been a great deal of progress on thermoelectronics. Thermoelectronics are electronics that convert heat into electricity. The thermoelectric effect is discussed at wikipedia

One company Powerchips claims to be able to achieve 70-80% carnot efficiency. This would mean that at 70% carnot efficiency if the hot side was 500 degrees celsius and the cold side was 0 then 45% of the heat would be captured as electricity. If the cold side could get down to -80 or -90 then the effiency would be 54%. The cold side might get that cold or colder in space if it was shaded.


The total critical mass is from 600-1200 kg. The total mass for the nuclear reactor is probably under 100 tons and possibly in the 10-20 ton range. A nuclear powered Vasimr rocket would enable one way trips to Mars in 39 days and delivering 22 tons of payload. Vasimr engines can get up to 50,000 ISP which is 1100 times more fuel efficient than the Space Shuttle. The nuclear space vehicle would weigh about 600-1500 tons fully fulled. So it would take several launches using chemical rockets to put the pieces in orbit for assembly. A slightly scaled back system with one or two nuclear reactors would still enable trip to Mars for 70-100 day trips to Mars.

There is no serious scientific question about whether these two technologies (improved nuclear fission and Vasimr plasma propulsion) will work. It is a matter of funding the work and doing the engineering development.

Combining this power source which is targeting 2012 operation with Vasimr plasma drive would then enable very good space transportation out to Mars or the asteriods.


Franklin Chang Diaz and his 200 kw Vasimr engine

This 28 slide presentation by Andrew Petro of NASA shows that using a Vasimr propulsion system with 200MW of nuclear power would enable a one way trip to Mars in 39 days.


Information on the 200MW Vasimr system and one way travel times to Mars

The VASIMR system is a high power, electrothermal plasma rocket featuring a very high specific impulse (Isp) and a variable exhaust. Its unique architecture allows inflight mission-optimization of thrust and Isp to enhance performance and reduce trip time. VASIMR consists of three major magnetic stages where plasma is respectively injected, heated and expanded in a magnetic nozzle. The magnetic configuration is called an asymmetric mirror. The 1st stage handles the main injection of propellant gas and the ionization subsystem; the 2nd stage acts as an amplifier to further heat the plasma. The 3rd stage is a magnetic nozzle which converts the plasma energy into directed momentum. The magnetic field insulates nearby structures from the high plasma temperature (>1,000,000 oK.) It is produced by high temperature superconductors cooled mainly by radiation to deep space. Some supplemental cooling from the cryogenic propellants ( hydrogen, deuterium, helium or mixtures of these) may also be used.

The system is capable of high power density, as the plasma energy is delivered by wave action, making it electrodeless and less susceptible to component erosion. Plasma production is done in the 1st stage by a helicon discharge, while additional plasma heating is accomplished in the 2nd stage by the process of ion cyclotron resonance.


Another paper analyzing vasimr engines

FURTHER READING
Another nuclear powered vehicle that we have the technology to start building now is the liberty ship a gaseous core nuclear design. It could launch 1000 tons into orbit in one trip and would not leak any nuclear material. This kind of design is needed to greatly improve launching from earth to orbit. The nuclear Vasimr only helps with getting from orbit to anywhere else.

Be sure to read my analysis of the patent for the nuclear "battery" a solid core uranium hydride reactor.

MIT interview with Franklin Chang Diaz, president and CEO of Ad Astra Rocket Company, who is working on the Vasimr propulsion system that could shorten trips in space and improve fuel efficiency.

For Mars and beyond, we will need to develop nuclear electric power. If we don't, we might as well quit. We're not going to get anywhere without it.

I also would not want to send people to Mars on a fragile and power-limited ship. If you send people that far, you have to give them a fighting chance to survive, and the only way you can do that is if you have ample supplies of power. Power is life in space.


Here is a 47 slide presentation by Tim Glover on a 12MW Vasimr system.


Tim Glover's presentation shows components that would be needed for high power Vasimr systems like this 4MW ICRF antenna


Size of parts for a 1MW Vasimr engine


2.8 MW RF power converter


The components and weights of a 2.5MW vasimr engine design


This pdf from 2003 surveyed various near term propulsion options for trips to Mars.

If we get fusion power working then we can do even better.

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9 comments:

M. Simon said...

Only one small problem for the "nuclear battery". Its output is 27 MWth.

How exactly are you going to convert that to electricity? How will you reject the waste heat? How much will your radiator weigh? How much is your shield going to weigh?

Here is a better bet, that has the potential for direct conversion from charged particles to electricity:

Bussard Fusion Reactor
Easy Low Cost No Radiation Fusion

It has been funded:

Bussard Reactor Funded

The above reactor can burn Deuterium which is very abundant and produces lots of neutrons or it can burn a mixture of Hydrogen and Boron 11 which does not.

The implication of it is that we will know in 6 to 9 months if the small reactors of that design are feasible.

If they are we could have fusion plants generating electricity in 10 years or less depending on how much we want to spend to compress the time frame.

BTW Bussard is not the only thing going on in IEC. There are a few government programs at Los Alamos National Laboratory, MIT, the University of Wisconsin and at the University of Illinois at Champaign-Urbana among others.

The Japanese and Australians also have programs.

al fin said...

Excellent post, Brian. All of us look forward to small scale fusion reactors for space travel, but in the meantime we have to do what we can.

Bussard looks good so far, but it's a long way from doing what it promises. We can't live and go to Mars on promises.

bw said...

hi M Simon

My article from last week as you know was about the space systems that would be possible if the Bussard fusion reactor technology is successful. I also had a link to it in this article at the end.

It is and has been at the end of the article, "If we get fusion power working then we can do even better."

I have added discussion of thermoelectronics for conversion of heat to electricity and I discuss a lightweight radiation shield and some simple design steps for radiation protection.

btw: I have lots of other articles on various space propulsion systems which would each have superior performance to what we currently have available. Mirrored Laser arrays, external pulse propulsion and others. My preference would be that the NASA budget and some of the military budget would go towards several of those approaches and whether funding continues depends upon successfully hitting various milestones.

bw said...

M simon,

I have confirmed with the company. The reporter made a mistake. The output is 17-25 MW of electricity. I would guess there would also be more than that amount in additional thermal energy (which with thermoelectronics could also be partially captured.

the reactor does not need to be buried to contain radiation.

qraal said...

Hi Brian

IS the reactor space capable? If it requires any gravity flows or convection flows then it has serious problems in microgravity.

Another thing you will need to allow for is the mass of radiators required for heat ejection - which tend to be rather hot for weight-efficiency, thus decreasing the system thermodynamic efficiency. Unless the reactor can run rather hot - say 1900 K with 600 K radiators, though then there are active cooling issues of the core itself that need to be addressed. NERVA, for example, could run really hot (2800 C) because it had a high coolant flow rate and direct ejection of the coolant-propellant.

bw said...

The reactor does not exist yet. They have just announced that they are working on it. They are talking 2012 for the first one to get finished.

However, it is just another solid core nuclear reactor. I do not see why other nuclear reactor designs and solid core rockets would work and this would not. There have been other nuclear thermal spacecraft designs using similar technology. I am just choosing to pair the reactor with the Vasimr plasma drive instead of using direct nuclear propulsion systems. It is nuclear electric powering a vasimr drive.

The patent indicated that if they used thorium hydrate then the reactor would run at about 1900 degrees, which could be better for nuclear power system for a rocket. However, they are first working on uranium hydrate.

I have not done any detailed design for this system, but there is not that much about it that is that novel compared to other nuclear reactor for space rocket designs. The main novelties - no people needed to tend the nuclear reactor - it keeps a constant temperature by itself - it is simple and presumably easier to build and maintain - less waste than many other systems. Heat piping, radiation shielding, heat radiators, conversion of the steady state heat to electricity are all things that can be tweaked based on the application and which can be cribbed from past nuclear rocket designs.

M. Simon said...

There could also be wear out problems with the hydrate similar to those in a lead acid battery.

The dehydrated uranium must be kept available for rehydration. It can't clump at the bottom of the vessel.

Electrostatic shields are wonderful things. Explain again how they will stop x-rays, gammas, and neutrons.

Using reaction mass as shielding is a good idea. If it was also a deliverable like water it could serve double duty.

bw said...


Electromagnetic shielding (different from electrostatic) seems likely to reduce then weight of radiation shielding against gammas, xrays by 80%


This combined with the water and reaction mass would stop the Neutrons as well with lower weight.

tom said...

Being so close to seeing all these technologies crossing together; there are still obstacles. I'm still on the fence as to do the first missions as un-manned autonomous systems or crewed? This is too high profile too risk a human tragedy, yet manned exploration raises the capabilities of these new systems. Best of luck that this should succeed.