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#1 2016-06-13 21:49:13

Tom Kalbfus
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Registered: 2006-08-16
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Nerva Rocket for Planetary transportation

This idea is a simple rocket hopper, includes a nuclear reactor. First you compress and liquefy carbon-dioxide from the atmosphere, then that serves as the reaction mass for the Nerva rocket. You can fly from one point on the planet's surface to another very quickly, and certain designs might also achieve orbit.

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#2 2016-09-13 05:43:21

elderflower
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Re: Nerva Rocket for Planetary transportation

It probably would be easier and make for a more simple vehicle to put the reactor near the base generating power(which will be needed in quantity) and manufacture propellant locally, either using imported hydrogen, or by using hydrogen extracted from local ice resources.

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#3 2016-09-14 23:10:29

kbd512
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Re: Nerva Rocket for Planetary transportation

Just like H2O, CO2 is a pretty poor propellant choice for a NTR.  The advantage, of course, is that the propellant only has to be sucked out of the Martian atmosphere and liquefied.  The real issue is the durability and maintainability of the NTR, the turbopumps, and the propellant storage tanks.

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#4 2016-09-15 00:15:04

RobertDyck
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From: Winnipeg, Canada
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Re: Nerva Rocket for Planetary transportation

Start with the correct location. You need a glacier or better. Produce LCH4/LOX using Robert Zubrin's ISPP. Mars atmospheric conditions mean little heat loss for a soft cyrogen, so little boil-off. Keep the reactor at base to power ISPP.

A Mars hopper would look like this...
66a.jpg

or this...
spacex-grasshopper-test-xmas-2012-lg.jpg

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#5 2016-09-15 12:10:16

GW Johnson
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From: McGregor, Texas USA
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Re: Nerva Rocket for Planetary transportation

Why not just use chemical propulsion for your hopper?  Fly it from fuel stop to fuel stop.  You'll be making propellants there,  use them.  And avoid the radiation dose and heavy shielding issues of a nuclear engine. 

LOX-LCH4 seems to be what everybody thinks they can make on Mars.  Although the LOX requires water.  And it's where you get your hydrogen for the LCH4.  You must be an ice miner to do this effectively.  Which means locating your base or station adjacent to a massive ice deposit is a gilt-edge priority.  And that makes ground truth about such ice a necessity. 

Nuclear can't be scaled down very much from the Phoebus/Kiwi/NERVA range of sizes.  Chemical can.  Start with a small 1 or 2 man craft and try it out.  If you really like it,  bigger birds can always be built. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#6 2016-09-15 13:39:06

elderflower
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Re: Nerva Rocket for Planetary transportation

A lot of reactors use carbon as a moderator. I suspect that CO2 would contribute to this and might give problems of reactor control.
Also there are big heat lags in fission reactors so you have to give them a lot of notice of a shutdown. You cant just cut off the coolant when you want the thrust to stop. For this reason nuclear thermal devices are really only suitable for interplanetary or interstellar operations.

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#7 2016-09-20 16:15:47

kbd512
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Re: Nerva Rocket for Planetary transportation

elderflower wrote:

A lot of reactors use carbon as a moderator. I suspect that CO2 would contribute to this and might give problems of reactor control.

Why would CO2 make reactor control an issue?  There were numerous operational plants that used CO2 as a working fluid and there have been several proposals for using supercritical CO2 reactors.

elderflower wrote:

Also there are big heat lags in fission reactors so you have to give them a lot of notice of a shutdown. You cant just cut off the coolant when you want the thrust to stop. For this reason nuclear thermal devices are really only suitable for interplanetary or interstellar operations.

Perhaps a nuclear powered ground vehicle would be a more realistic use of nuclear power for transportation on Mars than an aerospace vehicle.

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#8 2016-09-26 05:36:27

elderflower
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Re: Nerva Rocket for Planetary transportation

My opinion is that nuclear power would be best as a static installation at a main base and rovers/hoppers should be powered with tanks of chemicals which can be prepared there, just as earth rovers  are, except that oxidants will have to be taken along as well.
As to graphite moderated, CO2 cooled reactors, some of these have failed dangerously. You don't want that if you only have two situated close to your major infrastructure.

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#9 2016-09-29 12:28:31

kbd512
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Re: Nerva Rocket for Planetary transportation

elderflower wrote:

My opinion is that nuclear power would be best as a static installation at a main base and rovers/hoppers should be powered with tanks of chemicals which can be prepared there, just as earth rovers  are, except that oxidants will have to be taken along as well.

I tend to agree, except that the Isp requirements for going to and from the surface of Mars outstrips current chemical rocket technology without refueling.  If the only refueling and refurbishment events can take place on the surface of a planetary body, that substantially simplifies engine / reactor maintenance, fuel storage, and infrastructure requirements.

A tri-modal LOX Augmented NTR, such as the Triton concept, would be an ideal TMI, TEI, and Mars orbital transfer propulsion system.  I think a 50t delivered payload should be the design reference point.  I can't think of what required utility a more massive landed payload capability would provide for a startup colony.

On Mars, you really need nuclear power for colonies because the amount of infrastructure required for solar or geothermal to function is a bit beyond what we can easily provide.

I think Mr. Musk has the right idea, with respect to a fully reusable liquid-only booster.  However, I also think his engineers have a really difficult engineering challenge ahead of them, with respect to a fully reusable spacecraft capable of surviving repeated reentry events at Earth and Mars without refurbishment.  If the in-space propulsion system is separable from the interplanetary spacecraft, the engineering challenges of the design will likely be substantially lessened.

elderflower wrote:

As to graphite moderated, CO2 cooled reactors, some of these have failed dangerously. You don't want that if you only have two situated close to your major infrastructure.

Although I haven't followed GCFR development and implementation very closely, the only accidents with this technology that I'm aware of come from nuclear weapons production and the 1967 Chapelcross incident, which was admittedly pretty bad but also resulted in no loss of containment. Can you provide any documentation on the dangerous failures you noted?

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#10 2016-10-01 11:44:09

JoshNH4H
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From: New York, NY, USA, Earth, Sol
Registered: 2007-07-15
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Re: Nerva Rocket for Planetary transportation

I think it's worth considering what a hopper would actually be used for when there are other, more efficient options (land rovers, specifically).  Hoppers will be more expensive but faster than rovers, and also will be better able to traverse difficult terrain.  For example, Valles Marineris might at first be very difficult to cross by car if you need to go around instead of through it.  In the case of an emergency evacuation hoppers might be instrumental in transferring people quickly from the failing habitat to another more viable one. 

In general, people's time is not going to be all that valuable in the context of a colony.  The costs of having and using a rocket will probably be higher than the cost of supporting a person in a slower car for a longer amount of time.

Having said that, there will be some need for fast transport.  What will that look like?

Hoppers, by their nature, hop.  It doesn't seem unreasonable to have a network of fueling sites every 1000 km or so to refuel at.  A 1000 km hop requires a delta-V of about 3,850 m/s* (Half that if a crash landing is okay!). 

In my mind, there are four fuel combinations worth discussion for this mission profile:

  • H2/NTR

  • CO2/NTR

  • CH4/LOX

  • CO/LOX

With pros and cons as follows:

H2/NTR:

Pros:  Highest Isp.  Assuming an Isp of 900 s, the hopper would only need to be 35% propellant by mass.  H2 can be electrolyzed from water found in a glacier.

Cons: Nuclear reactor requires heavy shielding.  NTR technology doesn't really exist yet.  The NTR engines that have been designed have been low-thrust.  Hydrogen is a deep cryogenic.  Hydrogen is not dense.  While the propellant fraction may only be 0.35,for each kilogram of dry mass you need 7.6 L of liquid Hydrogen.  A fuel that requires water will also require a bigger refueling center because it will need to be able to mine a glacier for water in addition to other activities.

CO2/NTR:

Pros: CO2 can be extracted from the atmosphere and liquified with almost no additional processing.  An NTR could feasibly be bimodal and have a unit to extract and liquify atmospheric CO2, thus giving this rocket an almost unlimited range.  With an Isp of 300 s, the propellant fraction will be 0.72.  However, because of the high density of liquid CO2 each kilogram of dry mass requires just 1.7 L of CO2.

Cons:  Nuclear reactor requires heavy shielding.  NTR technology doesn't really exist yet.  The NTR engines that have been designed have been low-thrust.  CO2 requires pressurization to remain liquid.  CO2 will likely dissociate into CO and O in the engine, meaning that you will need to design it to be resistant to attack by both reduction and oxidation at high temperatures.  This is a nearly impossible feat.

CO/LOX:

Pros:  CO2 can be extracted from the atmosphere and processed using the Reverse Water Gas Shift Reaction with Hydrogen more-or-less acting as a catalyst**.  Chemical rocket engines have high thrust which increases the payload.  CO/O2 are mild cryogenics but share a liquid range and so can be stored at the same temperature.  With an Isp of 280 s, the propellant fraction will be 0.75.  CO and O2 are both fairly dense, so each kilogram of dry mass requires 2.6 L of propellant.

Cons:  CO is a toxic gas, so it has to be handled with care.

Methlox:

Pros:  CH4 can be produced from atmospheric carbon dioxide and water using the sabatier reaction.  Chemical rocket engines have high thrust which increases the payload.  With an exhaust velocity of 365 s, this fuel requires a propellant fraction of 65%.  Methlox is relatively dense and so each kilogram of dry mass requires 2.2 L of propellant.  Chemical rockets tend to be high-thrust, which reduces required dry mass.

Cons:  A fuel that requires water will also require a bigger refueling center because it will need to be able to mine a glacier for water in addition to other activities.

For this mission profile, I think chemical is the way to go.  Its the simplest solution and is admirably suited to our needs.  CO/LOX (colox?) seems like a better choice for small scale, with methlox making more sense when this kind of transportation becomes more common.

The refueling centers are going to be big energy hogs.  For Colox, each kilogram of fuel will consume about 11 MJ (3 kWh) per kilogram of electrical energy.  For methlox, it's more like 20 MJ (5.5 kWh).  The relevant figures for H2/NTR and CO2/NTR are about 225 MJ/65 kWh and 1 MJ/0.3 kWh, respectively.  Per kilo of dry mass, that works out to 32 MJ/9 kWh for Colox, 37.5 MJ/10.5 kWh for methlox, 120 MJ/33 kWh for H2/NTR, and 2.6 MJ/0.7 kWh for CO2/NTR.

The following table sums up the numbers for each fuel.  For "Model Power Consumption", I assumed one hopper per day with a dry mass of 5 tonnes.

4r70ck.jpg

*For the purposes of this calculation I assumed Mars is flat.  I know that's not true but for distances under a few thousand km the difference is small.  This method overestimates the delta-V required to hop by a small amount but given gravity drag, air drag, and maneuvering this actually probably makes it more accurate rather than less.

**Reactions are as follows:
CO2 + H2 ⇌ CO + H2O (Reverse Water Gas Shift)
2 H2O → 2 H2 + O2 (Electrolysis)


-Josh

If you try to talk to me about cold fusion or propellantless drives I will ignore you.
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#11 2016-10-07 03:27:48

elderflower
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Re: Nerva Rocket for Planetary transportation

kbd512 wrote:

Although I haven't followed GCFR development and implementation very closely, the only accidents with this technology that I'm aware of come from nuclear weapons production and the 1967 Chapelcross incident, which was admittedly pretty bad but also resulted in no loss of containment. Can you provide any documentation on the dangerous failures you noted?

The failure at Chapel cross was of 2 out of 4 reactors. As I recall there was dimensional change in the graphite. In one there was damage to one or more fuel rods resulting in leakage of radioactive material. In reactor 4 no such leakage was reported, but the reactor was never restarted although it was supposed to have been repaired.
Other Magnox failures involved quite rapid corrosion by hot CO2. The reactors were derated by reducing operating temperature to avoid severe shortening of their lives.
I also recall reported failures of coolant circulators. This didn't cause reactor problems because of redundancy. On Mars there would be a problem of stand by power to these. If you have to shut down the reactor, how do you run the circulators to get rid of residual heat in the core?
On the positive side, these reactors could run with very low enrichment fuel.

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#12 2016-10-07 09:10:50

kbd512
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Re: Nerva Rocket for Planetary transportation

elderflower wrote:

The failure at Chapel cross was of 2 out of 4 reactors. As I recall there was dimensional change in the graphite. In one there was damage to one or more fuel rods resulting in leakage of radioactive material. In reactor 4 no such leakage was reported, but the reactor was never restarted although it was supposed to have been repaired.
Other Magnox failures involved quite rapid corrosion by hot CO2. The reactors were derated by reducing operating temperature to avoid severe shortening of their lives.
I also recall reported failures of coolant circulators. This didn't cause reactor problems because of redundancy. On Mars there would be a problem of stand by power to these. If you have to shut down the reactor, how do you run the circulators to get rid of residual heat in the core?
On the positive side, these reactors could run with very low enrichment fuel.

IIRC, there were a number of design characteristics and material selections made for those plants that were intended to reduce cost, but caused substantial problems with operations as a result of oxidation.  If we send something to Mars, it won't be made out of cheap materials.  These units would only output 100kWe to 1MWe and be of an improved (simplified) design.  There's no reason why the exact design features of Magnox should be replicated.

The reactor will be surrounded by CO2 on Mars, so it would be nice to be able to replenish coolant from the atmosphere.

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#13 2016-10-07 10:32:55

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Nerva Rocket for Planetary transportation

The big problems with Magnox were: (1) low power density, which really pushes up the capital cost of a nuclear reactor; (2) Graphite oxidation by hot CO2 gas, which was life limiting; (3) Poor fuel burn-up, which wacks up fuel manufacture and processing costs; (4) Low operating temperatures, resulting in low efficiency, which really magnifies all the other problems.  In terms of safety and operational reliability they were excellent, but were so poor on an economic basis that it really didn't matter.

With AGRs the UKAEA tried to get around these problems by enriching the fuel (which increases power density and burn-up) and increasing temperatures, which raised efficiency.  But it made the oxidation problem even worse and the solutions implemented to try and make oxidation manageable were so complex that build times were massively extended.  Factor into that the fact that all reactor designs were different and you have an economic travesty on your hands.  Instead of doing the sensible thing and buying developed LWRs from foreign vendors, the UKAEA tried to work around the limitations of an impractical technology.  They basically tried to polish a turd.

There is no way we would want to ship a graphite moderated reactor to Mars due to the sheer mass of the core.  An S-CO2 GCR might be a good option if it can operate as a fast reactor in direct cycle mode.  But the containment dome needed to mitigate the consequences of a coolant leak would be a high mass item and may need to be built on Mars.

Last edited by Antius (2016-10-07 10:35:22)

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#14 2016-10-07 11:36:42

kbd512
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Re: Nerva Rocket for Planetary transportation

Antius,

I'm trying to account for the fact that you could have a coolant leak with any reactor design.  If the coolant can't be replenished, then the reactor is going to melt down and there's very little a surface exploration team or even a colony could do about it.  We're not talking about a commercial power plant producing 100's of MWt.  We're talking about something smaller than many research reactors.

The LWR, PWR, and BWR designs are just too massive to ship.  The LFTR technologies are still largely in development since those technologies were defunded decades ago.  I suppose that no matter what design is selected, it will be relatively new and untested.

Given the choice and some realism regarding weight restrictions, what type of reactor would you send?

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#15 2016-10-07 14:26:22

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Nerva Rocket for Planetary transportation

kbd512 wrote:

Antius,

I'm trying to account for the fact that you could have a coolant leak with any reactor design.  If the coolant can't be replenished, then the reactor is going to melt down and there's very little a surface exploration team or even a colony could do about it.  We're not talking about a commercial power plant producing 100's of MWt.  We're talking about something smaller than many research reactors.

The LWR, PWR, and BWR designs are just too massive to ship.  The LFTR technologies are still largely in development since those technologies were defunded decades ago.  I suppose that no matter what design is selected, it will be relatively new and untested.

Given the choice and some realism regarding weight restrictions, what type of reactor would you send?

All true.  In terms of keeping weight to a minimum and providing a few hundred KW of electric power, I would recommend a sodium or lead cooled fast reactor.  The US dept of energy have plenty of of admittedly quite old materials data from the IFR project that could be used in the design of the reactor.  The coolant need not be pressurised, so coolant leaks are unlikely and a sterling engine provides a reliable heat engine.  Also, operating temperatures of 700C are achievable, making the reactor useful as a source of industrial heat.

A single loop BWR is probably workable as well.  A 100KWe reactor would need a radiator about 40m in diameter.  A PWR is probably not workable, due to the weight of the heat exchangers.  Remember, very little effort is made to minimise the weight of these items on Earth because it is not a design priority.  Naval nuclear reactors are volume constrained not weight constrained.  Civil reactors aren't really optimised for weight or volume.

Last edited by Antius (2016-10-07 14:33:44)

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#16 2016-10-08 04:40:24

elderflower
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Registered: 2016-06-19
Posts: 558

Re: Nerva Rocket for Planetary transportation

Given the much greater abundance of deuterated water on Mars, and that there will be a lot of electrolysis going on, which would further concentrate it, should we not consider using a heavy water reactor.

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