Nuclear Transfer Vehicle Design

John Creighton · 2004-11-29 18:51:30

This thread is created to discuss the methodology of nuclear vehicle design. It is my understanding that a nuclear transfer vehicle has a number of systems some optional and some essential. For instance it has a coolant system. This system has two objectives. The first is to keep the reactor bellow 5000 K. The hotter the reactor runs the greater the efficiency at which electricity can be produced. So if we are using electric power we want to keep the reactor as hot as possible. The other is to keep the systems outside of the reactor to a suitable temperature. The higher the trust level the more of the cooling can be taken care of by the fuel flow and consequently the smaller the cooling system needs to be in terms of a working fluid and radiators. The propellant is heated when it flows by the reactor. Depending on the amount of fluid that flows by the reactor it could be in a gas or a plasma state, further heating can be accomplished by afterburners as is done in the Triton case. If the fuel is heated to a plasma it may be possible to accelerate electrically which I think is done in VASMIR (if I understand correctly) or by plasma pressure through a magnetic nozzle. Otherwise the gas can be ionized and then accelerated by several methods.

John Creighton · 2004-11-29 18:58:03

Arcjets
http://sci.esa.int/science-e/www/object … ...id=1537

For electrothermal propulsion systems to achieve exhaust velocities much higher than 10 000 m s-1, portions of the propellant flow must attain temperatures in excess of 10 000 K while being kept out of contact with the engine component walls. Arcjets accomplish this by passing the propellant through an electric arc that heats it before it expands through a nozzle.
Core arc temperatures of 10 000 to 20 000 K mean that exhaust velocities of 5000 - 6000 m s-1 (Isp = 500 - 600 s) at efficiencies of around 40% are possible using catalytically decomposed hydrazine.
Arcjet thrusters entered commercial use for north-south station keeping on the Telstar-4 series of geostationary communication satellites in 1993. Higher power arcjets providing sufficient thrust for orbit transfer or primary propulsion manoeuvres have been demonstrated on test flights but problems with electrode erosion and availability of sufficient electric power have delayed their introduction for operational missions.

These engines seem to deliver a lot of thrust and have a fairly high ISP. Couldn’t the electrode erosion problem partly be solved by having redundant electrodes. To me I would think that would significantly increase the life space of the vessel. For higher ISP couldn’t the plasma be further accelerated. I think this ship has some potential.

Euler · 2004-11-29 19:21:26

If you are going to use nuclear power, it makes no sense to generate electricity to power an arcjet. If you use the heat generated by the reactor directly in a nuclear thermal rocket, you can achieve an Isp of around 900s, and you will get a higher thrust than with any electric system. NEP systems really only come into consideration if you need Isp values higher than can be generated by an NTR.

John Creighton · 2004-11-29 19:24:01

This site is pretty dood as far as engines go:
http://www.vectorsite.net/tarokt3.html][3.0] Advanced Space Rocket Propulsion Systems (2)

This is fairly extensive
http://www.princeton.edu/~globsec/publi … Background on Space Nuclear Powerhttp://www.google.ca/search?q=cache:k8t-aJ2C6AwJ:www.princeton.edu/~globsec/publications/pdf/1_1-2Aftergood.pdf+%22space+nuclear+reactors%22%2Btypes%2Bweight%2Btable&hl=en](HTML)

Steven Aftergooda
This paper introduces the technology of space nuclear power, reviews the history of its deployment, provides background information on current development programs, and examines the proposed applications of space nuclear power systems.
SPACE POWER CONCEPTUAL DESIGN SUMMARY A space nuclear power system converts the energy from a nuclear heat source into electricity to power a particular load or application. The elements of this process may be outlined as follows:

Table 1: Solar and Chemical Power Equivalen1s of Two Space Reactors
Nominal Chemical Solar panel Solar panel
projected equivalent equivalent equivalent
mass mass mass area
kg kg kg m' (1 kWhr/kg) (lOW/kg) (130W/m2)
SP-l00 -5.000 -6 x 10^6' 10.000 750
700 kWe. 7 years
Multi-megawatt -50.000 -9 x 10^7 1 x 10^6' 75.003
70 MWe. 7 year

I want to keep the program affordable. The goal of the first people to try to achieve the maximum results with as few people as possible. Why? Because robots are much cheaper to support then people. When we start to move large amounts of people to mars I want to have a lot of processes already up and running and a lot of the kinks worked out. I really believe that the initial goal should be research both in the fiends of science and technology. The cost of a large settlement can’t be justified until it achieves a high degree of independence. I believe in an incremental approach to space building on what was done and trying to gain the maximum value out of the human missions as possible thorough automation and tellerobotics. Self sufficiency

For how long. Anyway the longer they stay the greater their celebrity status should be when they get back right? The only reason to bring the back early is if no crew as qualified wants to stay any longer or if it is deemed important to further investigate the medical effects of the trip on earth.

John Creighton · 2004-11-29 19:55:15

If you are going to use nuclear power, it makes no sense to generate electricity to power an arcjet. If you use the heat generated by the reactor directly in a nuclear thermal rocket, you can achieve an Isp of around 900s, and you will get a higher thrust than with any electric system. NEP systems really only come into consideration if you need Isp values higher than can be generated by an NTR.

Then why do arcjets only get an Isp of 500. I thought the reactor temperature couldn't exceed 5000 k but the arc in the arcjet get up to 10 000 k to 20 000 k. Shouldn't arc jets have a higher ISP. Anyway if the propellant reaches 5000 k from the reactor there should still be quite a heat difference that can be used for electric power generation.

John Creighton · 2004-11-29 19:58:03

NEP systems really only come into consideration if you need Isp values higher than can be generated by an NTR.

We should look at the thrust to ISP curves it will better help us figure out the economics. See my post on:
"Plotting the Trust, Specific Impulse Economic Viability Boundary "
in:
http://www.newmars.com/forums/viewtopic … 7]Resource Transport from asteroids to GEO (Technology and Economics)

Euler · 2004-11-29 20:32:04

Then why do arcjets only get an Isp of 500. I thought the reactor temperature couldn't exceed 5000 k but the arc in the arcjet get up to 10 000 k to 20 000 k. Shouldn't arc jets have a higher ISP. Anyway if the propellant reaches 5000 k from the reactor there should still be quite a heat difference that can be used for electric power generation.

The Isp is proportional to the square root of the temperature divided by the square root of the molecular mass. NTRs use hydrogen as fuel because it forms the lightest molecules and can therefore have the highest velocity. Arcjets usually use hydrazine, so they can end up with a lower Isp despite having higher temperatures.

John Creighton · 2004-11-30 08:42:49

The Isp is proportional to the square root of the temperature divided by the square root of the molecular mass. NTRs use hydrogen as fuel because it forms the lightest molecules and can therefore have the highest velocity. Arcjets usually use hydrazine, so they can end up with a lower Isp despite having higher temperatures.

What would be the drawback of using hydrogen for an arcjet?

John Creighton · 2004-11-30 09:23:46

I was thinking last night that the ISP should depend on more then just temperature. If you heat a reservoir and fire it out a nose then there is a clear limit on how much ISP you can get out of it. However if the reservoir is already moving when it is heated to its maximum temperature shouldn’t you be able to get more ISP out of it. Also notice when gas expands it cools off. The expanding gas will have to move faster to keep the same mass flow. Add more thermal energy to the expanding gas shouldn’t we be able to get it moving faster. What is the ISP limit of a thermal system. Maybe it is higher then 900 s.

Again we are back to pips http://www.newmars.com/forums/viewtopic.php?t=2504](see: Space Elevators and Pipelines). I realized the effect of viscosity. Viscosity creates friction which adds temperature but it increases entropy. It is a net loss but it is not totally bad. Clearly their will be many factors that influence the maximum ISP for a thermal engine besides the maximum possible temperature and the molecular mass.

Others include the viscosity of the fluid and the compressibility of fluid. I think a nuclear thermal rocket should have several reactors each one adding more heat to compensate for thermal expansion. So to keep too much energy from being lost by radiation the waste heat can be captured and used to preheat the incoming fluid. I actually think that with zero viscosity fluids (I know they don’t exist for such a temperature range) you could get nearly 100% efficiency. Why? Because entropy is created by lost work over a temperature boundary. If the temperature boundary is kept small and does work by heating the fluid as it crosses each boundary the efficiency will be very high. There is one fluid with zero viscosity (http://www.fluidmech.net/msc/super/super-f.htm]Helium II) unfortunately it is only a supper fluid at near zero Kelvin. No god for a thermal engine that must operate at 5000K.

PurduesUSAFguy · 2004-11-30 09:32:31

The only advantage of using a nuclear reactor to power an arc jet instead of a direct nuclear thermal rocket is that you run into the issue of ablation in a NTR so you get a mildly radioactive wake. An arc jet would side step that problem but it seems like it would be possible to engineer a reactor to get around the ablation issue.

GCNRevenger · 2004-11-30 10:08:22

"However if the reservoir is already moving when it is heated to its maximum temperature shouldn’t you be able to get more ISP out of it."

No, you can't. That ignores the microscopic/kenetic behavior of gasses. The inceased velocity will just distribute through all the molecules' random motion.

You overstate the problem of viscosity I think since the flow path is so short. Specific impulse is still almost entirely controlled by the temperature and molar mass of the exhaust gasses.

John Creighton · 2004-11-30 10:21:59

No, you can't. That ignores the microscopic/kenetic behavior of gasses. The inceased velocity will just distribute through all the molecules' random motion.

The velocity increase to the thermal expansion will be ordered, the velocity increase due to temperature will be disordered. However the thermal expansion due to this increase temperature will do work on the gas which will translate into ordered velocity.

You overstate the problem of viscosity I think since the flow path is so short. Specific impulse is still almost entirely controlled by the temperature and molar mass of the exhaust gasses.

Well I am envisioning several pipes wrapping around the reactor getting closer and closer trying to capture every last ounce of thermal energy. The pipes will rap the long way around a cylinder. If the path is too short increase the length of the cylinder and the number of nuclear reactors along the axis of the cylinder. Granted the amount of pipes will have to be balanced against their weight. There will be a tradeoff.

John Creighton · 2004-11-30 10:23:22

No, you can't. That ignores the microscopic/kenetic behavior of gasses. The inceased velocity will just distribute through all the molecules' random motion.

This will happen over time due the viscous action of the fluid. It doesn't happen instantaneously.

John Creighton · 2004-11-30 10:49:53

A better way to look at it. For incompressible flows work is equal to force times distance so if you increase the distance the fluid travels you increase the distance but reduce the force there is no advantage gained. However for a compressible gas the expanding of the gas does work on the fluid. If the gas doesn’t have time to expand this energy will be lost.

GCNRevenger · 2004-11-30 11:44:36

No, you can't. That ignores the microscopic/kenetic behavior of gasses. The inceased velocity will just distribute through all the molecules' random motion.
This will happen over time due the viscous action of the fluid. It doesn't happen instantaneously.

Then how long does it take?

You are also trying to address the wrong problem I think John, the supply of energy isn't the problem, the reactor weight isn't a huge factor, and whatever small gains of a few meters per second you can come up with is inconsequential versus the normal exhaust velocity.

I will also invoke the concept that everyone loves to hate:

If it were practical and that bennefical, someone would have thought about it already. We only tinkerd with nuclear engines for over thirty years is all. If several decades of hard work by civilian and USAF engineers and about ten billion dollars of funding didn't suggest that such an arrangement would be a good idea, then it probobly isn't. They can't all be incompetant and overlooking some fundimental physical insight that (only) you want to apply.

Temperature is everything... my favorite, the GCNR engine can theoretically reach ~50,000K, which is quite a bit more then the 3000-4000K of solid-core NTRs. The VASIMR engine can access temperatures close to 1,000,000K, albeit at low thrust levels and extreme electrical requirements.

John Creighton · 2004-11-30 12:15:50

Temperature is everything... my favorite, the GCNR engine can theoretically reach ~50,000K

Do you think we can build a GCNR engine in a reasonable time frame or should we stick to solid core for now? How long do you think it will take to develop this technology? Anyway as far as development goes I think we should use the triton as the first ITV. Is the trip to mars shorter or longer with a triton then a chemical rocket. Clearly the triton is just a stepping stone rocket and large advance can be made in nuclear thermal rockets beyond the triton without too much development. But if we keep waiting for better technology we will never leave. I agree a GCNR should put everything else to shame, we should be able to joy ride around the solar system at our please. With such a rocket asteroid mining will become more then feasible. But I don’t think GCNR is a near term proposition.

John Creighton · 2004-11-30 12:20:24

it were practical and that bennefical, someone would have thought about it already. We only tinkerd with nuclear engines for over thirty years is all. If several decades of hard work by civilian and USAF engineers and about ten billion dollars of funding didn't suggest that such an arrangement would be a good idea, then it probobly isn't. They can't all be incompetant and overlooking some fundimental physical insight that (only) you want to apply.

Maybe people have thought about it already but they didn’t want to go through the work of trying to optimize all the fluid flow and heat exchange. Keep in mind afterburners in planes use a similar concept. That is provide more then one stage of heating. Also think of the triton it uses hydrogen to provide further heating after the engine left the core. Now what percentage of the work is done by temperature/pressure and what percentage by the expansion of the gas I am not sure.

John Creighton · 2004-11-30 13:10:21

I'm having troble finding thermodynamic data tables. I found this expert system program but it is not free.

http://pdpa.uam.mx/~subrata/testcenter/ … plets.html

There seems to be alot of databses on this site.
http://www.nist.gov/srd/]http://www.nist.gov/srd/

NIST Standard Reference Database 103
NIST ThermoData Engine Version 1.0
....
Price: $3,790.00

Not on a students budget.

This is free but it is just for air and I don't know what range of temperatures and pressures it is good for:
http://www.aoe.vt.edu/~devenpor/tgas/]h … npor/tgas/

GCNRevenger · 2004-11-30 13:13:14

I very much doubt that they wouldn't ignore some multi-stage engine plan if it would have offerd superior performance because they "didn’t want to go through the work."

Afterburners nor the Triton's LOX mode do any such thing. Yes there is more heat energy available from the chemical combustion, but they produce more thrust ultimatly by throwing more mass into the exhaust. More heat energy, but more mass too.

The optimum configuration for a solid-core NTR engine, where the highest practical exhaust temperatures are achieved for the lowest reactor mass is already pretty well known.

John Creighton · 2004-11-30 14:05:28

I very much doubt that they wouldn't ignore some multi-stage engine plan if it would have offerd superior performance because they "didn’t want to go through the work."

I disagree. Since there of yet doesn’t exist any nuclear thermal rocket in space I think the approach would be to get something simple working first. After that they can devote more research to optimizing the design. Look how big JIMO is the reactors on JIMO are the size of a bucket. Other reactors are used for redundancy and not further heating. If you put the reactors in series along the exhaust path then some of the redundancy benefits of the nuclear reactors would be lost. Moreover, JIMO barely fits into currant rockets. Imagine if we made the thing bigger.

Afterburners nor the Triton's LOX mode do any such thing. Yes there is more heat energy available from the chemical combustion, but they produce more thrust ultimatly by throwing more mass into the exhaust. More heat energy, but more mass too.

Maybe your right with regard to the triton. I suppose if the goal was exhaust velocity then all the oxygen would do is slow down the system, although the case might be different for airplanes.

The optimum configuration for a solid-core NTR engine, where the highest practical exhaust temperatures are achieved for the lowest reactor mass is already pretty well known.

Clearly we want the highest possible heating of the working fluid. Clearly we want to keep the reactor mass small. That says nothing about what architecture is used to take the heat from the reactor and transfer it into kinetic energy.

John Creighton · 2004-11-30 14:20:59

Another reason the engineers might of not thought the optimal solution initial is that computational fluid mechanics is very computationally extensive. It can take days to solve a single problem distributed over many computers. Moreover, the models my not apply well to these temperature ranges, pressures and exhaust velocity. They could collect data by building something. But if they have to build something why not build something that they can actually use. Why not build something that is simple and an improvement over current technology that they know will fly first.

P.S. Someone has to think of something first. I am not saying I ever had an original idea but maybe maybe I will implement it before the other guy...

GCNRevenger · 2004-11-30 14:33:03

"I think the approach would be to get something simple working first"

Perhaps you have heard about a little project that NASA ran called NERVA? Or the USAF's Timberwind project(s)? NASA has test fired many restartable NTR engines with peak Isp above 800sec with pretty high reliability. The USAF Timberwind engines that were dabbled with could match this or in the throw-away version could hit 1,000sec Isp for very small reactor mass. P&W's Triton engine is really just a modernized copy of NASA's NERVA engines that have been around since the 60's, and thats how they can do it for relativly little money.

I think you are also not understanding a few of the fundimental differences between power reactors and NTR engines. You can't compare JIMO's "100kwe Prometheous class" reactor to an NTR engine since they are designed to do different things, the power reactor designed to heat small amounts of circulated coolant for long periods of time, and the NTR engine designed to heat vast quantities of Hydrogen to extreme metal-killing temperatures all at once.

The reason that JIMO won't fit on a standard Delta-IV HLV is because it will need a TJI because it would be too heavy to reach a so-called "nuclear safe" orbit (>800km) where the reactor could be activated, its a safety measure. The reason that JIMO is so heavy is not because of the reactor, that is only a tonne or two, alot of the mass goes to the power conversion and cooling system (radiators) which an NTR engine does not need (but Triton will need too to some point).

A small NTR engine, somthing RL-10 or RL-60 sized like the Timberwind engines, if it is only needed to fire once like for TMI, would only weigh a few tonnes.

The arcitecture decided on is simple. For the NERVA/Triton style engine, a big block of Uranium and Graphite with holes in it is brought critical and LH2 is pumped in one end and hot H2 gas comes out the other. The Timberwind system either used a pebble bed or a perforated sheet fuel elements, designed to run at insane temperatures, but deliverd maximum Isp. Because of the heat, the core is spent in only one firing.

John Creighton · 2004-11-30 14:58:38

With 1,000sec Isp why even bother with electric propulsion except for deep space probes and station keeping?

Austin Stanley · 2004-11-30 15:07:01

I have to agree with GCN. It's not that the issues are talking about don't exists (viscosity, ect...) it's just that when you are dealing with gasses at this high a temperature, it becomes a negligable factor. The energy lose due to expansion, friction, whatever, is minescule in comparison to the energy the gasses already posses. In fact, I highly doubt you would be able to find a measurable diffrence if you were to test for them some how. Definetly not worth the extra mass you would introduce in trying to compinsate for these factors some how.

Austin Stanley · 2004-11-30 15:12:02

With 1,000sec Isp why even bother with electric propulsion except for deep space probes and station keeping?

Because it can only produce that ISP for a short period of time. The temperatures are to great to handled by conventional methods and the engine litteraly burns itself out.

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