Nukemobiles on Mars

Terraformer · 2016-01-24 16:20:57

It strikes me that the most effective option might be to stage from orbit for a few months and develop a way to extract oxygen from the atmosphere for the flight back to orbit. Drop a homing beacon and ISPP plant, wait until you have enough oxygen, and potentially fuel, to get back to orbit, and send the away team down. Do that over several sites, until you've identified a suitable location for a base, and then send all your surface equipment down. If you've got water there, then you can produce as much fuel as you want and explore a lot more.

kbd512 · 2016-01-24 17:07:20

JoshNH4H wrote:

Critical mass of various isotopes in kg:
U-233: 15.8
U-235: 46.7
Pu-239: 10.0
Am-242[sic]: 8.8

Within reason, it's not critical mass we're interested in, it's specific heat.

JoshNH4H wrote:

Other isotopes are listed too, if anyone is interested. Critical mass is determined by nuclear cross section and by the number of neurons produced per fission event, so even if the cross section is high that doesn't necessarily mean that the critical mass will be low. For what it's worth, there are isotopes on this table with lower critical masses than Am-242m, but they're even harder to produce. While it's true that lower critical mass implies lower shielding mass, it's also true that any non-nuclear power source requires no mass to shield from radiation.

The critical mass chart in the link provided did not indicate what isotope of Am242 data was provided for.

JoshNH4H wrote:

What drove you to 20 tonnes of landed mass? Let's say a Falcon Heavy can put 10 tonnes on the surface of mars. Each FH launch costs ~125 mn. That's not so much compared to the mission cost, so I don't think that it makes sense to put a hard limit anywhere. Instead, once the mission's purpose has been decided, we need to decide whether spending money on something makes sense compared to the value of the stated purpose of the mission.

The 20t landed mass requirement was what I calculated one FH could deliver using relatively near-term SEP technology and ADEPT. It also happens to coincide with initial landed mass targets from NASA and Boeing.

Unless you just want to spend a lot of money shipping solar panels and batteries to Mars, reactors make more sense and can provide power for several missions.

JoshNH4H wrote:

Relative to a chemical rover, a nuclear one will cost more and take more time to develop. On the other hand, it gives you unlimited range (or rather, range limited by your provisions). But how much good does unlimited range do you when you only have 550 days of surface time and a handful of crew? A circle 1000 km in diameter is huge, after all. It's bigger than Texas or France and roughly comparable to the entire northeast United States.

How do you know how long it would take to develop a chemical powered rover versus a nuclear rover?

If a nuclear powered rover moves at 40km/h, it can circle the planet several times in a single 500 day surface stay. It only takes a few weeks to circle the planet at 40 km/h. The astronauts might want to make a trip to the poles to see how much water we can get our hands on to produce rocket fuel and fuel for chemical powered rovers. There's nowhere on the planet that's more than two weeks driving distance.

JoshNH4H wrote:

I would have initial missions be focused on proving resource availability and finding a spot for an initial colony location. Once the colony is there it would increasingly seek to produce everything locally, with the seed of machines sent from Earth to help them do so.

NASA's mandate is to explore space, not colonize planets. Even if it was, nuclear powered rovers are still better for finding resources.

JoshNH4H wrote:

I'm not opposed to nuclear power in general. I agree that vibrations from a rolling vehicle are a concern but aren't prohibitive. I agree that a solar electric vehicle would probably have lackluster performance. I'm unsure on whether a solar electric tug or chemical propulsion is better, although I may try to run some numbers on it.

With near-term SEP technology and FH rockets, we should be capable of 20t landed payloads. I estimate payload delivery costs, not development, at ~$200M for the FH/SEP/ADEPT hardware. Each rover would be a $50M-$75M. Most of that cost is reactor, electronics, and CL-ECLSS. If we launch four rovers and a lander with consumables and spare parts, we're looking at launch costs approaching or slightly exceeding the cost of one SLS flight.

JoshNH4H wrote:

My challenge to you is to consider everything as a cost/benefit optimization and make decisions accordingly, recognizing that we have yet to lock ourselves into any design choice, and also recognizing that NASA has virtually no pull over Congress. When you say this:
At some point in the very near future, a NASA administrator is going to have to tell Congress and the President to either educate themselves about what space exploration involves, with respect to power requirements for deep space exploration or to simply shut down the program. The most ignorant people in the room can't continually dictate what NASA develops for space exploration.
What you're really saying is that there will be no Mars mission in the foreseeable future. We can do better than that.

I have thought quite a bit about the cost-benefit analysis of nuclear power for Mars. My conclusion is that batteries and/or super capacitors need to be substantially lighter and smaller than they presently are and solar panels need 50% greater power density to simply obtain the benefit of their use that we obtain here on Earth. I honestly believe batteries and solar panels will only continue to improve, but the improvements required to compete with a nuclear reactor optimized for power density will probably take as long or longer than it would take to simply develop the reactor.

There won't be any Mars exploration for the foreseeable future because NASA has permitted the Congress and the President to dictate solutions to problems they know nothing about. And no, we can't do any better than that until we have a NASA administrator with a backbone.

SpaceNut · 2016-01-24 19:32:16

Maybe to make the RV'ing around mars more probable we would need to figure out how long an ICE driven vehicle can go before being out of fuel when run 24/7 mode so as to figure out the spacing of the refueling stations that would need to be preloaded before going to mars with a human crew.

Also being in a continous run mode means we are not doing eva's to put our feet on the planet unless we plan on stopping locations preplanned before aligning the stations for fuel refills as once we get the bug we will wnat to be outside even in a suit as much as possible.

I am also wondering if we want a smaller RV to make the nuclear power plant of a smaller size to fit possibly for 2 crew members rather than the full crew of 4 or more. We need to pin down the real power consumption for life support versus that of driving to solve the sizing issue....

kbd512 · 2016-01-24 20:57:04

SpaceNut wrote:

Maybe to make the RV'ing around mars more probable we would need to figure out how long an ICE driven vehicle can go before being out of fuel when run 24/7 mode so as to figure out the spacing of the refueling stations that would need to be preloaded before going to mars with a human crew.

I don't think it's possible to carry enough fuel to idle the vehicle 24/7.

SpaceNut wrote:

Also being in a continous run mode means we are not doing eva's to put our feet on the planet unless we plan on stopping locations preplanned before aligning the stations for fuel refills as once we get the bug we will wnat to be outside even in a suit as much as possible.

If it's possible for reasonably sized solar panels and batteries to provide power for ECLSS and to heat the vehicle, then you don't need to run the vehicle's methalox engine 24/7.

SpaceNut wrote:

I am also wondering if we want a smaller RV to make the nuclear power plant of a smaller size to fit possibly for 2 crew members rather than the full crew of 4 or more. We need to pin down the real power consumption for life support versus that of driving to solve the sizing issue....

As previously stated, the MTVL is roughly the size of a 2016 Chevrolet Suburban. The MTVL is 2" shorter and 18" wider than the Suburban, with length being nearly identical. The vehicle is already dimensionally smaller than NASA's Chariot and ATHLETE (with habitat module) vehicles except in length (it's just shy of 6M long, compared to Chariot's 4.5M and ATHLETE's 8.4M).

Each MTVL was intended to accommodate two crew members for 250 days, so four MTVL's are required for four crew members.

There's no reason why a fifth MTVL could not be added to the mission to serve as a fuel carrier for the methalox powered rovers. The methalox plant would be stored in the cargo area. Each MTVL could be powered by a very light and compact 100hp methalox engine and relatively small tanks to hold LOX and LCH4. Everything explosive would be forward of the driver's compartment pressure hull. Each manned MTVL would instantly be 2t lighter (8t less mass between four vehicles), but the fuel carrier would still need a reactor to provide power.

JoshNH4H · 2016-01-24 22:38:36

Within reason, it's not critical mass we're interested in, it's specific heat.

What? I can't possibly imagine what importance this has for a nuclear reactor, especially because fuel rods are not made of metal, but instead from any of a variety of solid ceramic compounds which would have different properties from the pure metal. For example, Uranium is used as UO2, U3O8, U3N4, UC, as well as molten fluoride salts.

Re: Whether the critical mass is for Am-242, Am-242m, or Am-242m2, I wondered the same thing. So I headed over to the Lund/LBNL Table of Radioisotopes. Am-242 does not decay by Spontaneous Fission at all and is therefore not fissile. Am-242m2 has a half-life of 0.014 s and therefore is unlikely to have been considered as the material for any kind of fission reactor.

Re: 20 tonnes, I'll say again that the cost of sending a second FH is not really that high in the context of a Mars mission. I don't know if you're planning to have a second Falcon Heavy, but regardless additional launch mass is simply not that expensive - if it's worth it.

Dollar figures are pretty much impossible to give for a nuclear vs. chemically powered vehicle. But I will say that I have a chemically powered vehicle with a range of 700 km. Most Americans do. What I'm proposing wouldn't be that different from every internal combustion engine in the world. In fact, and M-113 uses an internal combustion engine. ISPP will cost money to develop and more importantly to prove its reliability. But ISPP also has cobenefits, namely that you do save a bunch of money on sending fuel to the surface of Mars, which is the largest mass associated with a Mars mission. If you're using solar power, ISPP+an Internal Combustion Engine is also a potential answer to the question "What do you do if there's a 3-month long dust storm?"

I tend to agree that a nuclear reactor is a superior technology than solar. in addition to the obvious benefits related to constant power, it probably is lower mass and almost definitely requires less setup. I think there's legitimate concern to be had when you're using it as a power source for a moving vehicle (which will tend to have people walking around in the non-shielded area). But it has drawbacks. For a private mission, nuclear technology is quite heavily regulated. There have been no new nuclear reactors in the US used for power generation since the 1970s, after all. For NASA, the politics are pretty bad. Sure, a nuclear mission is probably a better one, but it's not the only possible mission.

I'm not particularly ideological on the question of nuclear vs. solar for mission power. But I'm holding my ground on the belief that a radioactive car is a bad idea.

RobertDyck · 2016-01-25 00:36:14

kbd512 wrote:

Rob,
You can't run a space exploration program off of nostalgia. NASA needs to decide what it wants to do and then focus all effort and funding on that goal.

Yes, but that's been the problem since Nixon cancelled Apollo.

kbd512 wrote:

If the goal is to get humans to Mars, then it must be accepted that there are certain aspects of the Martian surface environment that don't work in favor of solar and battery technology, namely the extreme temperatures and distance from the Sun. As SpaceNut pointed out, for solar panels to work as efficiently as they do on Earth on Mars, a 40% power density improvement would be required. The mass of the solar panels is only part of the problem.

Yes, but Mars Direct requires a launch vehicle the size of SLS block 2 or 2B. In fact, Ares V and SLS block 2 were based on the Ares launch vehicle from Mars Direct. So we're getting what we need. And the Earth Return Vehicle requires a capsule, just like Dragon. So it may not be focused, but we're getting what we need.

As for solar, sunlight that reaches Mars is 47% as intense. But Mars has no ozone layer, and no clouds. That means it's always a bright, clear, cloudless day, so always perfect sunshine. And UV is actually more intense than what reaches the surface of Earth, solar panels designed for space already use that UV, so that compensates for some of the light loss. And solar panels available for home owners is not as highly efficient as space cells. Solar cells available in the 1970s only converted 4% of light to electricity. Solar panels available for your cottage convert 14%. The yellow roll-out solar array first used for Hubble converted 12%. Solar arrays for space today convert 29.5%. So they're a lot more efficient. 47% * 29.5% = 13.865% so current space solar would provide the same amount of power as panels you can get at Home Depot, Lowes, Rona in Canada, or some place like that. We're good.

kbd512 wrote:

Apart from the mass and volume of solar arrays that would provide sufficient power for manned applications, the most significant problem is the weight of the batteries required to store that energy. The array mass and volume are trivial problems compared to the battery mass and volume problems.

I already posted links to manufacturers of batteries that can be used in space. Lithium-ion, so they're safe and high density. And car manufacturers wanted a battery with 200 watt hours per kg before they would develop an electric car. That was their demand in the 1980s. But physics is a hard limit, you aren't going to get better. You could consider ultra-capacitors, but the best chemical battery will always be lithium-ion, and the best energy density ever achieved was 150 watt-hours per kg. Notice cells available today are 99 Wh/kg, but built to handle realistic temperature swings.

RobertDyck · 2016-01-25 00:54:47

I should also point out, Congress has conflicting objectives. They want to please the most people, so they get enough votes to get elected. That means they want to support all those jobs for contractors who work for NASA. But they also need to keep cost down. They're aware Old Space contractors gouged NASA, and at times NASA themselves were not as responsible with taxpayer money as Congress would like. How to deal with this? The traditional way is competition. Get multiple suppliers to compete, so they drive prices down. So Congress wants enough commercial contractors working for NASA to do this. SpaceX appears to be applying pressure on ULA, so it's working. Blue Horizon is applying pressure on SpaceX; they aren't getting funding from NASA, but I'm sure Congress is cheering them on.

Currently commercial suppliers for ISS have 2 cargo ships: Dragon and Cygnus. And 2 crew ships: Dragon v2 and CST-100. Orion isn't commercial. That's the bare minimum for competition. Dream Chaser didn't get a contract for crew, but now has one for cargo. Additional competition should drive prices down. And it's really designed for crew, so will continue to nip at the heals of SpaceX Dragon, Orbital Sciences Cygnus, and Boeing CST-100. Besides, Dream Chaser satisfy those voters who want a shuttle back.

Meanwhile, many voters want to go back to the Moon. Orion is built to do that. Notice I said voters. That's what Congress cares about. And many voters as well as Congressmen want US dominance in space. So demonstrating American can still do the Moon proves it hasn't lost it.

Notice this isn't all about Mars. Many in the Mars Society would like it to be, but it isn't. A station in LEO and the Moon are goals too.

kbd512 · 2016-01-25 04:22:35

JoshNH4H wrote:

What? I can't possibly imagine what importance this has for a nuclear reactor, especially because fuel rods are not made of metal, but instead from any of a variety of solid ceramic compounds which would have different properties from the pure metal. For example, Uranium is used as UO2, U3O8, U3N4, UC, as well as molten fluoride salts.

I know I'm going to do a poor job of explaining this, but bear with me:

I want a fissile material that generates a lot of heat and has a half life long enough to be useful for power generation. If critical mass was the only consideration for fuel selection for a fission reactor, then we'd simply select the hottest isotope we could create. Unfortunately, that wouldn't work very well for power generation because such an isotope would only exist for a few seconds or less and transmute into various other elements. Based on everything I've read, Am242m has one the highest cross sections of any common isotope and one of the lowest critical masses required for fission. Like Plutonium, Americium is hot, but not so hot as to be impractical for power generation.

As far as non-technical reasons for using Am242m are concerned, Americium is not considered to be nuclear weapons material. I'm relatively certain that launching rovers with reactors fueled with Pu239 would be portrayed as launching nuclear weapons into space by the media and the politicians of other countries and this country. Am242m is not some miracle fuel I'm counting on to save the day, it's merely a usable fuel, on par with Pu239, that does not carry Plutonium's stigma with it. The goal here is to develop a micro reactor that generates 100kWe.

I'm not unaware or unconcerned with the general aversion to nuclear power, but a space exploration program that intends to put humans on Mars must have realistic power generation options available. If I thought using Pu239 as fuel would be politically feasible at all, I'd use readily available Pu239 and call it a day. Who knows, maybe I could sell the idea as a socially responsible way of getting rid of weapons grade material.

JoshNH4H wrote:

Re: Whether the critical mass is for Am-242, Am-242m, or Am-242m2, I wondered the same thing. So I headed over to the Lund/LBNL Table of Radioisotopes. Am-242 does not decay by Spontaneous Fission at all and is therefore not fissile. Am-242m2 has a half-life of 0.014 s and therefore is unlikely to have been considered as the material for any kind of fission reactor.

The half life of Am242m/Am242m1 is a bit over four hundred years.

JoshNH4H wrote:

Re: 20 tonnes, I'll say again that the cost of sending a second FH is not really that high in the context of a Mars mission. I don't know if you're planning to have a second Falcon Heavy, but regardless additional launch mass is simply not that expensive - if it's worth it.

If we expend enough of those "low-cost" $125M rockets to send ISPP plants to Mars, using the cost of the rockets as justification for not developing portable nuclear reactors, we'll have paid for the development program for a very small reactor. This hypothetical Am242m fueled reactor is a more compact SAFE-400.

JoshNH4H wrote:

Dollar figures are pretty much impossible to give for a nuclear vs. chemically powered vehicle. But I will say that I have a chemically powered vehicle with a range of 700 km. Most Americans do. What I'm proposing wouldn't be that different from every internal combustion engine in the world. In fact, and M-113 uses an internal combustion engine. ISPP will cost money to develop and more importantly to prove its reliability. But ISPP also has cobenefits, namely that you do save a bunch of money on sending fuel to the surface of Mars, which is the largest mass associated with a Mars mission. If you're using solar power, ISPP+an Internal Combustion Engine is also a potential answer to the question "What do you do if there's a 3-month long dust storm?"

The inefficiency and low general reliability of internal combustion engine vehicles is acceptable here on Earth because there are lots of roads, gas stations, repair facilities, and other humans to assist you when you have problems. If a vehicle runs out of gas or breaks down on Mars, the problem is far more serious. If there is any significant problem with the only fuel refinery on Mars, the rovers aren't going anywhere.

Regarding the M113's internal combustion engine, look at the size of the engine and transmission compartment. Then take a look at the hybrid-electric Earth-bound MTVL variant of the M113 and notice how much more space there is in the vehicle.

JoshNH4H wrote:

I tend to agree that a nuclear reactor is a superior technology than solar. in addition to the obvious benefits related to constant power, it probably is lower mass and almost definitely requires less setup. I think there's legitimate concern to be had when you're using it as a power source for a moving vehicle (which will tend to have people walking around in the non-shielded area). But it has drawbacks. For a private mission, nuclear technology is quite heavily regulated. There have been no new nuclear reactors in the US used for power generation since the 1970s, after all. For NASA, the politics are pretty bad. Sure, a nuclear mission is probably a better one, but it's not the only possible mission.

With the nuclear reactor, we can throw away 75% of the thermal energy generated by fission and still run the vehicle around the planet continuously until the tracks or road wheels are worn out. The electric power conversion efficiency of the solars panels is lacking and the power density of the internal combustion engine is also decidedly lacking, but gas mileage is just an itty bit better.

JoshNH4H wrote:

I'm not particularly ideological on the question of nuclear vs. solar for mission power. But I'm holding my ground on the belief that a radioactive car is a bad idea.

If Mars was closer to the Sun or had a denser atmosphere, I'd look at other ways to power the rovers. I don't care if the reactor is placed in another vehicle and it's preferable if it is. But I'm holding my ground on the belief that loss of power equates to loss of crew and loss of mission.

kbd512 · 2016-01-25 12:09:53

RobertDyck wrote:

Yes, but that's been the problem since Nixon cancelled Apollo.

Therefore, the real paradigm shift that Josh said needed to happen for Mars exploration to occur is to stop living in the past and to stop trying to make marginally functional solutions work.

The STS hardware was workable for the purpose for which it was intended. It was far more expensive than it needed to be, but it worked. Now we're going to repurpose extremely expensive but reusable STS hardware for the expendable SLS and somehow this launch vehicle will be inherently more reliable and cost less than STS.

The mega-rockets and mega-spacecraft of the past aren't particularly helpful for space exploration because the cost is so high that there's no funding left for the types of payloads that make them useful.

A 26t capsule isn't required for space exploration. On the other hand, the 28t-45t deep space habitat that'll receive a fraction of the funding that Orion receives is required. Capsules are inadequate lifeboats and exceptionally poor habitats.

RobertDyck wrote:

Yes, but Mars Direct requires a launch vehicle the size of SLS block 2 or 2B. In fact, Ares V and SLS block 2 were based on the Ares launch vehicle from Mars Direct. So we're getting what we need. And the Earth Return Vehicle requires a capsule, just like Dragon. So it may not be focused, but we're getting what we need.

Yeah, it's not focused at all. More living in the past. A SLS type vehicle made some sense when STS type vehicles were in active use. You design a rocket for a payload in a certain mass range. If you need a 125t to 150t lift capability, then that's what you design your rocket to lift.

RobertDyck wrote:

As for solar, sunlight that reaches Mars is 47% as intense. But Mars has no ozone layer, and no clouds. That means it's always a bright, clear, cloudless day, so always perfect sunshine. And UV is actually more intense than what reaches the surface of Earth, solar panels designed for space already use that UV, so that compensates for some of the light loss. And solar panels available for home owners is not as highly efficient as space cells. Solar cells available in the 1970s only converted 4% of light to electricity. Solar panels available for your cottage convert 14%. The yellow roll-out solar array first used for Hubble converted 12%. Solar arrays for space today convert 29.5%. So they're a lot more efficient. 47% * 29.5% = 13.865% so current space solar would provide the same amount of power as panels you can get at Home Depot, Lowes, Rona in Canada, or some place like that. We're good.

If we are good, then why was Curiosity nuclear powered? The requirement for 125We and batteries was a problem serious enough to warrant the use of nuclear power, but that problem improves when 100kWe is the requirement?

RobertDyck wrote:

I already posted links to manufacturers of batteries that can be used in space. Lithium-ion, so they're safe and high density. And car manufacturers wanted a battery with 200 watt hours per kg before they would develop an electric car. That was their demand in the 1980s. But physics is a hard limit, you aren't going to get better. You could consider ultra-capacitors, but the best chemical battery will always be lithium-ion, and the best energy density ever achieved was 150 watt-hours per kg. Notice cells available today are 99 Wh/kg, but built to handle realistic temperature swings.

Let's hope this new lithium ion battery technology is suitable for use in space:

New Lithium Ion Battery

Assuming it will be suitable for space applications, we'd have a battery technology that makes solar power a practical option.

JoshNH4H · 2016-01-25 23:10:31

kbd512-

I want a fissile material that generates a lot of heat and has a half life long enough to be useful for power generation. If critical mass was the only consideration for fuel selection for a fission reactor, then we'd simply select the hottest isotope we could create. Unfortunately, that wouldn't work very well for power generation because such an isotope would only exist for a few seconds or less and transmute into various other elements. Based on everything I've read, Am242m has one the highest cross sections of any common isotope and one of the lowest critical masses required for fission. Like Plutonium, Americium is hot, but not so hot as to be impractical for power generation.

Per the same table from my previous post, the half-life is 141 years. I don't know if you mean hot as in radioactive or temperature or what, but this explanation still doesn't make any sense to me. Heat capacity is a measure of the amount of heat required to raise the temperature of a particular substance. Do you mean specific power? What relation do you think this has to nuclear cross section?

While we're on the topic of isotope choice, I actually really question if critical mass is the determining factor in core size. After all, a 10 kg sphere of any fissile element would be about 1 L in volume. I don't know where you got this number, but if the core is indeed the size of a 55 gallon drum (Where does that come from, by the way?), that means it's 99.5% not fissile material. The limiting factor is heat transfer, probably. 400 kWt is a whole lot after all, and to keep your fuel rods from melting you need more surface area. In fact, many nuclear reactors cut U-235 with U-238 or even alloy it inside a fuel rod that is mostly something else, mostly for safety and heat transfer reasons.

Per the next part, I would like to compare two statements you've made:

On 1/24, you wrote:

NASA has permitted the Congress and the President to dictate solutions to problems they know nothing about. And no, we can't do any better than that until we have a NASA administrator with a backbone

And then today, 1/25, you wrote:

I'm not unaware or unconcerned with the general aversion to nuclear power [...] If I thought using Pu239 as fuel would be politically feasible at all, I'd use readily available Pu239 and call it a day.

One day, the administrator of NASA needs to grow a backbone and the next day you're saying to develop a new nuclear cycle that promises to be more expensive than any existing now for political expediency. Do you really think that anyone with the power to allocate millions or billions of dollars of funding knows the difference between Uranium, Plutonium, and Americium? It seems like you rejected this argument when it works against you, only to invoke it when it supports the idea you're pushing. It sounds like these nuclear M-113s are part of your vision for Mars exploration. It's important to have a vision - without vision, we never would have climbed down from the trees. But reconciling your vision and the tangible world is the hard part, sometimes.

On design, internal combustion engines are incredibly reliable. If properly maintenanced, you can buy a car today and expect it to drive for 200,000 miles. That's 15 times around Mars, if you were wondering. How far you can drive depends on how big your tank is, and with two years of prep time even low-powered solar panels could generate a good amount of food. I'm thinking to distribute a few stationary "gas stations" within your area of interest. The fuel cache would be verified before you go, of course. Solar panels aren't going to be as economical on Mars as Earth, but they're still possible and could be worth it.

RobertDyck · 2016-01-26 03:27:57

kbd512 wrote:

The STS hardware was workable for the purpose for which it was intended. It was far more expensive than it needed to be, but it worked. Now we're going to repurpose extremely expensive but reusable STS hardware for the expendable SLS and somehow this launch vehicle will be inherently more reliable and cost less than STS.

Not really. STS was supposed to be fully reusable, and purpose built to service ISS. It ended up a compromise that did everything, but nothing well. They cut the development cost, but the result was excessively high per-flight cost. And the flight rate was far too low; facilities built for 50 flights per year continued to have fixed overhead cost, but with 6 flights per year could not justify that cost. When you amortize the overhead cost to flights, that increased the per-flight cost even further.

kbd512 wrote:

The mega-rockets and mega-spacecraft of the past aren't particularly helpful for space exploration because the cost is so high that there's no funding left for the types of payloads that make them useful.

Actually, you're wrong. We need a rocket that can lift a spacecraft the size of a Mars Direct habitat or ERV to Mars. In fact, SLS block 1 and even block 1B aren't big enough. We need block 2 or 2B. And NASA is trying. An expendable stages require light-weight propellant tanks, and engines. They built the core of SLS from the lightest weight tank they know how to build. And used the most advanced technology and infrastructure they have. The result is the SLS core. For initial boost, they used solid rockets. The philosophy is solid rockets are cheap. They have low Isp, so on their own can't get anything into orbit. But they're a very cheap means for the first stage boost. Notice what Saturn V used: RP-1 (kerosene) and LOX for the first stage, then LH2/LOX for upper stages. The idea of using solids for initial boost, then a sustainer with LOX/LH2, follows that same philosophy. Cost and safety of those solids are an issue, but that was the idea.

The problem is their obsession with extreme Isp. You're right, that did make sense for a vehicle that reused the engines. Rocketdyne developed the SSME. They were bought by Boeing, and developed RS-68 for Delta IV. That engine was purpose built to be expendable. It had lower Isp, but reduced cost to manufacture the engine. Cost savings for the engine was greater than cost increase for more propellant and larger tank, so total launch cost was optimized. Salesmen for Boeing convinced NASA to use RS-68 for Ares V because of this. However, the engine guys at NASA wanted to build an SSME with 50% more thrust for Ares V. They demanded they "man rate" RS-68, then proceeded to redesign it. They added all the features of SSME missing from RS-68, effectively building the 50% larger SSME that they wanted. This was blatant violation of orders from NASA management. They got away with it for a while. Then Obama got elected, and cancelled Constellation entirely. Then Congress resurrected it, but chose SSME without modification instead of RS-68. So that was a way to stop the engine guys from building a new engine. However, they are now modifying SSME. It's being made cheaper for expendable use. So it isn't 50% more powerful, it isn't as cheap as RS-68, but at least it's better than what they were doing.

Orion: Then there's Orion. NASA was originally going to man-rate an EELV: Atlas V or Delta IV. The purpose was a smaller rocket that can lift a capsule into LEO. But salesmen from ATK convinced NASA to use one of their solid rockets instead. They claimed their solid rocket is already man-rated, so doesn't require the expensive work they others needed. Then they proceeded to man-rate it. (Yes, expensive work, and they got the contract on the basis this wouldn't have to be done at all.) They quickly discovered their rocket has a vibration problem when lifting a small payload. So they build the capsule to be as big and heavy as possible, to dampen those vibrations. When you have to make your spacecraft heavy to compensate for a failure of the launch vehicle, it should be obvious you have a major problem. But they didn't, they continued. Orion is "Apollo on steroids!" It can carry 6 astronauts instead of just 3. Well, that's what their marketing said, but actually that's not true. Apollo could carry 3 to lunar orbit, or 5 to Skylab. Orion could carry 4 to lunar orbit, or 6 to ISS. So it's only 1 more crew member for each destination. And they claimed Orion still has a lower total launch mass! Well, again not actually true. The service module only has enough propellant to return from lunar orbit, not enter lunar orbit. Apollo did both, and when it entered lunar orbit it had a LM attached. The Orion capsule alone actually has greater mass than the Apollo Command Module alone. For Orion to work, the upper stage of SLS must remain attached for Lunar Orbit Insertion (LOI). SLS Block 1 will launch Orion on a lunar flyby only, the upper stage doesn't have enough propellant for LOI. SLS Block 1B has the larger upper stage, able to enter lunar orbit. But it isn't big enough for TLI alone with an LM attached, much less also performing LOI. My understanding is neither Block 2 nor 2B are big enough for both TLI and LOI with an LM attached. Block 2 won't be built anyway. It isn't that SLS is too small, the issue is Orion is too heavy, and its service module too under-powered.

Another frustration was Boeing ran out of money when developing Orion. One part of the bid for Orion, they key part different than Boeing's bid, was liquid methane / LOX for the service module, instead of hypergolics. But when they ran out of money, they tried to switch to hypergolic. Then completely ran out of money. They made a swap deal with the European Space Agency, so they get free service modules from them. It uses hypergolic propellant, and has slightly less performance than even the hypergolic service module Lockheed-Martin tried to design.

Question: Could SLS Block 2B insert Orion and an Apollo size LM into lunar orbit if Orion had a LCH4/LOX service module, using carbon fibre epoxy propellant tanks to reduce mass? That would still use the Exploration Upper Stage of SLS Block 2B for both TLI and LOI. It uses LH2/LOX so higher Isp. Hopefully minimal boil-off during transit to the Moon.

kbd512 wrote:

A 26t capsule isn't required for space exploration. On the other hand, the 28t-45t deep space habitat that'll receive a fraction of the funding that Orion receives is required. Capsules are inadequate lifeboats and exceptionally poor habitats.

Capsules are good enough for the Moon. And there are still people at NASA thinking of the Moon, and not Mars or an asteroid.

kbd512 wrote:

If you need a 125t to 150t lift capability, then that's what you design your rocket to lift.

That's what they need, and that's what they're building. However, they're using the same contractors they've worked with since the '60s. Those companies have gone through a series of mergers, resulting in monopoly and collusion where there is still more than one. That has resulted in price gouging.

kbd512 wrote:

If we are good, then why was Curiosity nuclear powered? The requirement for 125We and batteries was a problem serious enough to warrant the use of nuclear power, but that problem improves when 100kWe is the requirement?

Note Mars Direct uses a nuclear reactor with much more power than the RTG of Curiosity. But that's for the ERV, launched without crew, and a robotic rover separates the reactor before starting it. And crew do not stick around while the reactor is operational, the reactor does its work while crew are still on Earth. When crew do enter the ERV, they immediately leave, and the reactor stays behind. Practically paranoid considerations for radiation. The hab doesn't use nuclear, it uses solar.

Current high efficiency space solar cells are good enough. However, I keep talking about a new technology in a paper I read in year 2000. Initial work was done by the Los Alamos National Laboratory. They wanted someone to make one for them, to verify efficiency is as high as their theoretical calculations predicted. So they got U.C. Berkeley to do it. This is gallium-indium-nitride. Note current space solar cells are triple junction, and the top junction is gallium-indium-phosphate. So this new one uses the same semiconductor, just a different dopant. All junctions of the new one are the same chemistry, just a different concentration of nitrogen for each junction. Current cells use a completely different chemistry for each junction. So the new one is not only much more efficient (70.2% conversion instead of 29.5%), but should be cheaper to manufacture. But no one is working on them. Corporate greed. However, these new cells are "nice to have", but not absolutely required. We can make do with current cells.

kbd512 · 2016-01-26 06:07:25

JoshNH4H wrote:

Per the same table from my previous post, the half-life is 141 years. I don't know if you mean hot as in radioactive or temperature or what, but this explanation still doesn't make any sense to me. Heat capacity is a measure of the amount of heat required to raise the temperature of a particular substance. Do you mean specific power? What relation do you think this has to nuclear cross section?

I should have just said that both isotopes are suitable for fission and fairly radioactive instead of saying "hot". Regarding specific heat, the proper terminology would be energy density. I think of the nuclear fuel in terms of how much of it I need to raise its temperature to transfer heat from fission to a working fluid to produce electricity. If we can design a reactor that produces 400kWt using 5kg of fissile material instead of 10kg of fissile material, then as long as the reactor loaded with 5kg of fissile material produces the required thermal output over its intended design life and doesn't melt from the heat it produces or transmute the materials that serve as containment with gamma and neutron radiation, then that's what we want. Or so I would think. The thermal neutron cross section and neutron flux determines the reactor's thermal output.

Plutonium melts at around 639C and Americium melts at around 1176C. Any Pu239 fueled reactor with similar thermal output to SAFE-400 (max design core temp is 1020C, I think) would alloy Pu with Zr to raise its melting temp. That increases the core dimensions and weight. For fissile materials with lower cross sections, better reflectors are required to sustain fission in a subcritical mass. That also increases core dimensions and weight. With Am242m, you can use a thinner reflector and still sustain fission due to the higher neutron cross section. It should follow that dimensionally larger cores necessitate heavier shielding. Lighter and smaller power packs are generally preferable to heavier and bulkier power packs for mobile applications.

JoshNH4H wrote:

While we're on the topic of isotope choice, I actually really question if critical mass is the determining factor in core size. After all, a 10 kg sphere of any fissile element would be about 1 L in volume. I don't know where you got this number, but if the core is indeed the size of a 55 gallon drum (Where does that come from, by the way?), that means it's 99.5% not fissile material. The limiting factor is heat transfer, probably. 400 kWt is a whole lot after all, and to keep your fuel rods from melting you need more surface area. In fact, many nuclear reactors cut U-235 with U-238 or even alloy it inside a fuel rod that is mostly something else, mostly for safety and heat transfer reasons.

Critical mass is not the only factor that determines core dimensions. There are lots of other factors that determine critical mass (reflector, isotope selection, fuel density, fuel geometry, etc). I'm focused on the mass and dimensions of the reflector required to sustain fission. SAFE-400's reflector diameter is 51CM and the hexagonal core is 25CM across. Wouldn't it be great if that 13CM thick Be reflector could be 5MM, due to Am's high cross section, and still produce 400kWt? Wouldn't it be great if the remaining 12.5CM could be replaced with shielding?

For space nuclear power applications where weight and size have design implications for whatever the reactor is mounted in, is it better to have a thicker reflector (and subsequently heavier and bulkier radiation shielding) to enable the use of cheap fuel (Plutonium or Uranium) or is it better to have a minimum diameter reflector and radiation shielding enabled through the use of more expensive fuel (Americium) with properties that make thicker reflectors unnecessary?

JoshNH4H wrote:

One day, the administrator of NASA needs to grow a backbone and the next day you're saying to develop a new nuclear cycle that promises to be more expensive than any existing now for political expediency. Do you really think that anyone with the power to allocate millions or billions of dollars of funding knows the difference between Uranium, Plutonium, and Americium? It seems like you rejected this argument when it works against you, only to invoke it when it supports the idea you're pushing. It sounds like these nuclear M-113s are part of your vision for Mars exploration. It's important to have a vision - without vision, we never would have climbed down from the trees. But reconciling your vision and the tangible world is the hard part, sometimes.

Whenever someone says the "N" word, any and every possible excuse will be made to ignore the fact that some technologies are more suitable than others for specific applications. There are no gas stations on Mars, Mars is 50% further from the Sun than Earth is, and there are dust storms that can last for days, weeks, or even months. Nuclear reactors are totally unaffected by any of that, but let's try to design a solar powered rover that uses an internal combustion engine with fuel and a refinery from Earth.

JoshNH4H wrote:

On design, internal combustion engines are incredibly reliable. If properly maintenanced, you can buy a car today and expect it to drive for 200,000 miles. That's 15 times around Mars, if you were wondering. How far you can drive depends on how big your tank is, and with two years of prep time even low-powered solar panels could generate a good amount of food. I'm thinking to distribute a few stationary "gas stations" within your area of interest. The fuel cache would be verified before you go, of course. Solar panels aren't going to be as economical on Mars as Earth, but they're still possible and could be worth it.

How many times have you successfully started your ICE car when it was -50C outside?

GW Johnson · 2016-01-26 10:40:23

I'm from, and in, Texas, but I did spend two record-setting winters in Minnesota about 20 years ago. Never had a place to plug in a block heater, so I just had to learn to start dead cold. That took an enormous battery and very thin lube oil. I did this routinely at -10 to -30 F, which is -23 to -34 C.

For water-cooled engines, there is a limit to what antifreeze can do for you: ethylene glycol at 63%-37% proportions with water is good to -60 F (-51 C), but no further. Leave it running if it gets colder than that, or else store it heated when not running. Liquid cooling is not a very good idea for the severe cold.

An air-cooled 1957 VW beetle did well at Little America station in the 1957-1959 Geophysical Year expedition at winter temperatures far below -80 F (-62C). It used gasoline as fuel, kerosene as lube oil, and 20 or 30 weight oil thinned with kerosene as its gear lube in the transmission. They could use it when the snow cat would not run at all.

There's usually ways to cope with cold when using machinery, at least down to around -100 F (-73 C), based on South Pole station experience. But jet fuel and diesel won't work as fuels. Gasoline does, though. That's how they measured -130 F (-90 C) temperatures in flight above the south pole region long ago, when airplanes burned gasoline, and had only mechanically-operated flight controls.

GW

Last edited by GW Johnson (2016-01-26 10:44:40)

JoshNH4H · 2016-01-26 21:28:03

Isotope choice:

Energy density is not a function of neutron cross section, and nor is power density. The energy released per fission is not a related to whether or not (or to what extent) an isotope is radioactive. It increases slightly as the atomic number goes up, but because the energy content of nuclear fuel is already so high that's not a big concern. The big concern for core design is heat transfer. Heat transfer in a nuclear core is typically convective. Convection is generally modelled with the following equation:

P=h*A*ΔT

Where:

P is the power of convection (Energy per unit time transferred)
A is the area
ΔT is the temperature difference between the hot object and the cold fluid (or cold object and hot fluid, I suppose)
h is the "convection coefficient", which is a function of all sorts of things like the fluid speed and properties, the shape of the solid object, etc.

h in reactors is already designed to be as high as possible in order to save on the amount of nuclear fuel (and therefore nuclear waste), so I would be surprised if there were anything that could be done using presently-existing designs to do better. Perhaps a better way to say this is that h is the limiting factor for core size, so decreasing the size of the core is dependent mostly on getting a higher h-value rather than a more fissile fuel. The ΔT is driven by material constraints. The hotter the fluid you're transferring heat to, the more efficient your heat engine will be. A colder coolant is generally pretty wasteful. This is why it's necessary to have as large a surface area as possible, and also is why having an isotope with a lower critical mass is probably not going to reduce the size of the nuclear core.

The savings on neutron reflectors is probably illusory, too. It turns out that the materials which make good neutron reflectors are exactly the same ones which make good neutron shields. Neutron shielding is important, because over time a big enough neutron flux can actually make the materials you're using as shields radioactive. I would call that tertiary radiation, but I'm not sure if that's the technical term. And again I would reiterate that the critical mass of U-235 is much higher than any other isotope being discussed. See the table below with the bare-sphere critical mass of U-235 nondimensionalized to 1.

U-235 - 1.000
U-233 - 0.338
Pu-239 - 0.214
Am-242m - 0.188
Cm-247 - 0.148
Cf-249 - 0.126
Cf-251 - 0.118

I'm putting this here for two reasons. Firstly is that, even if it were true that core size scaled with critical mass (Which I still don't believe it does), the critical mass of Americium-242m and Plutonium-239 are very similar, especially when compared with Uranium-235. The only real difference is that Plutonium is way more readily available.

The second point I would like to make here is that there's a continuum going from higher critical mass/easier to produce (U-235, which is separated using physical processes and not nuclear ones) all the way down to much lower critical mass/much harder to produce (Californium-251, which is produced in the same way as Am-242m only along a longer chain.). To put it even more pointedly: What makes you so sure that the additional cost and investment over using plutonium is worth a 12% decrease in critical mass while seeming to lack interest in a further 38% reduction in critical mass that you would get by using Cf-251 instead?

It doesn't look like you've done any parametric or design studies to get to the option you picked. It seems more like it's held your interest and that's why you're supporting it. And again, a nuclear powered tank driving around on Mars would be hella cool. But that's not enough.

You wrote:

Whenever someone says the "N" word, any and every possible excuse will be made to ignore the fact that some technologies are more suitable than others for specific applications.

I wrote:

Solar panels aren't going to be as economical on Mars as Earth, but they're still possible and could be worth it.

I wrote:

additional launch mass is simply not that expensive - if it's worth it.

I wrote:

I tend to agree that a nuclear reactor is a superior technology than solar [...] but it has drawbacks

I wrote:

Sure, a nuclear mission is probably a better one, but it's not the only possible mission.

I wrote:

I'm not particularly ideological on the question of nuclear vs. solar for mission power. But I'm holding my ground on the belief that a radioactive car is a bad idea.

I wrote:

Relative to a chemical rover, a nuclear one will cost more and take more time to develop. On the other hand, it gives you unlimited range (or rather, range limited by your provisions).

I wrote:

I'm not opposed to nuclear power in general. I agree that vibrations from a rolling vehicle are a concern but aren't prohibitive. I agree that a solar electric vehicle would probably have lackluster performance. I'm unsure on whether a solar electric tug or chemical propulsion is better, although I may try to run some numbers on it.

I wrote:

My challenge to you is to consider everything as a cost/benefit optimization and make decisions accordingly

I will say it again, if I haven't before: I am not anti-nuclear

I love nuclear power. I think it makes a lot of things a lot easier. I think it has absolutely enormous potential. But it's also something we may have to live without. If you'll allow me to leave aside for a moment the notion of a nuclear-electric land vehicle (Which is a bad idea for other reasons), I will reiterate that I think a nuclear-powered mission would be more capable and lighter than a solar powered one.

Money spent on development is real money, and political capital has real value. It could perhaps be better spent on something else than nuclear power when alternatives exist. Solar irradiation on Mars is roughly comparable to Iceland. It's not ideal, but neither is it impossible, and even when dust storms are taken into account Mars is a better environment for solar power than Earth in some ways. I think In Situ Propellant Production is a prerequisite for a chemically powered rover, but I also think that's a very worthwhile technology to be investing in to begin with.

When it comes to ICEs on Mars, there are a few options, including heating the ICE with waste heat from the crew cabin, finding a different coolant fluid that won't freeze (methanol/water mix?) or designing the engine to freeze gracefully. Heating the engine with waste heat is probably the quickest, and building an engine with cold-tolerant fuels, coolants, and materials, is probably the most worthwhile long term.

SpaceNut · 2016-01-26 22:16:36

I would agree In Situ Propellant Production is a very worthwhile technology to be investing in to begin with as it would solve my own homes need for energy, clean water and some types of burnable waste removal not to meantion it can be sustainable. Doing these things for here on earth will make a design robust for use elsewhere.....

The other nuclear topic Gw and I did the same basic power blackbody thermal temperature transfer for how to radiate the heat from a SAFE-400 for other reactors knowing that Mars surface temperature swing makes sizing the radiator area for a mobile unit problematic unless we can modulate the reactors reactions down during the day to allow for a more constant temperature of disapation providing we can shield a crew within the mass envelope to what we can land.

The ICE vehicle works well if the insitu propellant plants are landed spaced appart for conveint refueling stops resizing what we can land to making of fuel is part of the problem for how far we can travel before refueling the rover.

Antius · 2016-01-27 06:39:06

A nuclear rover would be an enormous asset to a moon base. There is far less potential for insitu propellant production on the moon. Reactor heat could be used to bake volatiles out of the regolith. A properly designed coolant system might use regolith as a heat sink - picking it up, heating it and dumping it again. There is even potential to use heat from the reactor to produce sintered blocks that can then be stacked into domes. Sintered blocks would have sufficient compressive strength to produce domes up to kilometres in diameter on the moon.

kbd512 · 2016-01-27 09:37:30

JoshNH4H wrote:

Isotope choice:
Energy density is not a function of neutron cross section, and nor is power density. The energy released per fission is not a related to whether or not (or to what extent) an isotope is radioactive. It increases slightly as the atomic number goes up, but because the energy content of nuclear fuel is already so high that's not a big concern. The big concern for core design is heat transfer. Heat transfer in a nuclear core is typically convective. Convection is generally modelled with the following equation:
P=h*A*ΔT
Where:
P is the power of convection (Energy per unit time transferred)
A is the area
ΔT is the temperature difference between the hot object and the cold fluid (or cold object and hot fluid, I suppose)
h is the "convection coefficient", which is a function of all sorts of things like the fluid speed and properties, the shape of the solid object, etc.

When you have two materials with very similar weight and volume, but one material has a much higher neutron cross section than the other material, then the volume and therefore weight of all the non-fissile materials required to reliably sustain fission increases when you decide to use the material with the lower neutron cross section. This particular application of nuclear power requires extreme power density as a function of the complete design.

Put another way, if the Pu239 requires a .5t reflector and the Am242m requires a .1t reflector of lesser diameter and both require some specific density and volume of shielding over the core to absorb neutron and gamma radiation, then which design would weigh less and occupy less volume?

Even if the core dimensions of the Pu239 fueled reactor is only 9" x 12", and the core dimensions of the Am242m is 6" x 12", the Pu239 fueled reactor has a core volume that has increased by more than 100%. The shielding has to envelop the reactor. It's mass and volume will also increase. It's easy to see why core dimensions matter so greatly for mobile applications of nuclear power.

Focusing solely on the power density of the fissile material and ignoring every other aspect of the reactor's design will not lead to a compact and lightweight power pack.

You can build a reactor that weighs 5t that produces the same thermal energy as a reactor that weighs 10t and use the exact same materials in its construction in different ways.

I liken this to the argument made that we have solar panels that can produce 175We/kg. If we immediately consume all that energy and don't have to store it for future use then we can ignore the weight of batteries required to store the energy. If we do have to store energy, as we would if we needed to power a vehicle 24/7 on a rotating planetary body that doesn't face the sun at all times, then we have to take the weight of the batteries into account.

JoshNH4H wrote:

h in reactors is already designed to be as high as possible in order to save on the amount of nuclear fuel (and therefore nuclear waste), so I would be surprised if there were anything that could be done using presently-existing designs to do better. Perhaps a better way to say this is that h is the limiting factor for core size, so decreasing the size of the core is dependent mostly on getting a higher h-value rather than a more fissile fuel. The ΔT is driven by material constraints. The hotter the fluid you're transferring heat to, the more efficient your heat engine will be. A colder coolant is generally pretty wasteful. This is why it's necessary to have as large a surface area as possible, and also is why having an isotope with a lower critical mass is probably not going to reduce the size of the nuclear core.

Can you increase surface area using smaller heat pipes?

JoshNH4H wrote:

The savings on neutron reflectors is probably illusory, too. It turns out that the materials which make good neutron reflectors are exactly the same ones which make good neutron shields. Neutron shielding is important, because over time a big enough neutron flux can actually make the materials you're using as shields radioactive. I would call that tertiary radiation, but I'm not sure if that's the technical term. And again I would reiterate that the critical mass of U-235 is much higher than any other isotope being discussed. See the table below with the bare-sphere critical mass of U-235 nondimensionalized to 1.

As core dimensions increase, surface area to volume decreases, so the neutron reflection is lessened for dimensionally larger cores.

Be is not even close to an ideal neutron absorber. It's primary purpose in SAFE-400 is reflection to sustain fission. Even so, there's quite a bit of neutron leakage.

JoshNH4H wrote:

I'm putting this here for two reasons. Firstly is that, even if it were true that core size scaled with critical mass (Which I still don't believe it does), the critical mass of Americium-242m and Plutonium-239 are very similar, especially when compared with Uranium-235. The only real difference is that Plutonium is way more readily available.

To a point, there's no direct correlation between core dimensions and critical mass. I've never claimed that there was. I do claim that decreasing the core dimensions, assuming you can still generate the same amount of heat required to move the coolant through the heat pipes to generate electricity, makes the reactor lighter and smaller. You seem to be fixated solely on critical mass. I'm focused on determining what other materials (alloying metal in the case of Pu to keep it from melting at the temperatures that we'd run this reactor at and reflector dimensions for Pu and Am) are required to reliably sustain fission because that affects core dimensions and, subsequently, the weight of the shielded reactor.

JoshNH4H wrote:

The second point I would like to make here is that there's a continuum going from higher critical mass/easier to produce (U-235, which is separated using physical processes and not nuclear ones) all the way down to much lower critical mass/much harder to produce (Californium-251, which is produced in the same way as Am-242m only along a longer chain.). To put it even more pointedly: What makes you so sure that the additional cost and investment over using plutonium is worth a 12% decrease in critical mass while seeming to lack interest in a further 38% reduction in critical mass that you would get by using Cf-251 instead?

The overall dimensions of the reactor, including the shielding, is what I'm focused on. The properties of Am242m appear to make the larger neutron reflector less critical to sustaining fission than for Pu239 or U235.

With respect to cost, should there be any great issue with building just one Am242m fueled reactor, if only to further our knowledge of that material and what it could mean to the space program if it does everything it appears to do for reactor design?

JoshNH4H wrote:

It doesn't look like you've done any parametric or design studies to get to the option you picked. It seems more like it's held your interest and that's why you're supporting it. And again, a nuclear powered tank driving around on Mars would be hella cool. But that's not enough.

Would the basic research required to do a design study significantly detract from any other program?

The MTVL is an aluminum box on tracks that's slightly wider than a Chevrolet Suburban. The fact that it uses tracks does not make it a tank. It happens to be dimensionally large enough to store half of the consumables required for a Mars surface stay of 500 days for two crew members.

RobertDyck · 2016-01-27 11:08:17

JoshNH4H wrote:

It doesn't look like you've done any parametric or design studies to get to the option you picked. It seems more like it's held your interest and that's why you're supporting it.

Design study? This is NERVA, design study began in the 1950s, serious work in the 1960s and early 1970s. Completed in 1971. It was further updated, NERVA 2 completed 1974. In 1991, a study updated the design. That was revised in detail in 2005.
Astronautix: NERVA

Gross mass: 158,400 kg (349,200 lb)
Unfuelled mass: 27,000 kg (59,000 lb)
Thrust: 333.00 kN (74,861 lbf)
Specific impulse: 925 s
Burn time: 3,575 s

Astronautix: Timberwind 45, further information provided on a forum, but source cited as Aviation Week and Space Technology, "Particle Bed Reactor Central to SDI Nuclear Rocket Project", 1991-04-08, page 18

Gross mass: 28,000 kg
Unfuelled mass: 7,500 kg
Thrust: 441.30 kN (99,208 lbf)
Specific impulse: 1,000 s
Burn time: 449 s

Higher Isp was achieved by increasing exhaust temperature. That had the problem of creating hot spots in fuel elements that melted the face of fuel elements facing each other. Those melted spots would fuse together. This agglomerated the fuel, making the engine non-restartable.

This was the smallest of the Timberwind engines. I chose this one for comparison because it has thrust most similar to NERVA. Yet it still has greater thrust, and much smaller engine mass. Cited mass is absolutely everything: reactor core, housing, propellant pumps, exhaust bell nozzle. Just add liquid hydrogen propellant tank.

I met some of the researchers reviewing NERVA. They came to the Mars Society convention in 2005. I met one at the 2002 convention. I pointed out the difference in engine mass between NERVA and Timberwind, and asked them to apply whatever Timberwind did to make their engine just as low mass. Since then others on this forum pointed out the major reason was nuclear fuel: AM-242m instead of U-235. NERVA already used 99.9% enriched U-235, according to the NERVA engineer I spoke with at the 2002 convention.

RobertDyck · 2016-01-27 17:52:42

I have a document that states critical mass for a bare sphere of fissionable material. Without a reflector. It gives units in kg. Considering what this is, I shouldn't post it on an internet forum. However, ratios are almost the same as the ones Josh posted. It only gives numbers for U-235, U-233, and Pu-239. To give you an idea, I saw a TV show that was a docu-drama about the Manhattan Project. At the beginning it shows one general enter a physicist's apartment (without permission, while he was in the bath tub) and grill him about technology. He said the amount of material required for a bomb was about the size of an orange. I did a quick check, if it's highly enriched U-235 then it's about the size of a large orange.

Most of the fuel mass listed for nuclear engines is other stuff, not fissionable material (nuclear fuel). Why? Physics dictates how much fissionable material we need. A neutron reflector can reduce that, but that's about it. Further reductions require reducing the other stuff. Why do we need the other stuff? The team who developed Timberwind did a great job of reducing total engine mass. Can we use their design? Or at least design elements?

SpaceNut · 2016-01-27 19:01:36

Antius wrote:

A nuclear rover would be an enormous asset to a moon base. There is far less potential for insitu propellant production on the moon. Reactor heat could be used to bake volatiles out of the regolith. A properly designed coolant system might use regolith as a heat sink - picking it up, heating it and dumping it again. There is even potential to use heat from the reactor to produce sintered blocks that can then be stacked into domes. Sintered blocks would have sufficient compressive strength to produce domes up to kilometres in diameter on the moon.

I am wondering how to place regolith onto a radiator plate without damaging the underlying radiator by placing to much onto it for a roving RV. Then there is the temperature sensing of it such that we unload it with some method that does not as well cause damage to this same radiator only to keep repeating until we turn the reactor off.

If I recall the lunar outpost was trying to use a burried reactor for creating power in the topic.
Will place link here once I find it.

http://www.newmars.com/forums/viewtopic … 72#p105572
http://www.newmars.com/forums/viewtopic … 38#p108538

The other power creation used solar hydrogen fuel cells to create continous power on Mars that was as welll posted in the same topic IIRC...

http://www.newmars.com/forums/viewtopic … 22#p103722

kbd512 · 2016-01-27 21:11:00

SpaceNut wrote:

I am wondering how to place regolith onto a radiator plate without damaging the underlying radiator by placing to much onto it for a roving RV. Then there is the temperature sensing of it such that we unload it with some method that does not as well cause damage to this same radiator only to keep repeating until we turn the reactor off.

Put a small box around the radiator plates to hold regolith. The edges of the plates should face an opening at the top of the bin. A hinged screen door will cover the opening to the plate bin to prevent rocks large enough to damage the radiator plates from entering during loading. Drive the rover forwards slowly to fill the plate bin with regolith. The reactor will be at the rear of the vehicle and the plate bin will be at the front of the vehicle to distribute weight evenly.

The setup will look something like a commercial garbage container, but with a screen door for a lid. It will be attached to the rover with a set of electrically powered arms similar to the arms that lift commercial garbage cans over the top of garbage trucks. When you've heated the regolith to the point where it's necessary to dredge a fresh box of the stuff, tilt the regolith container to dump the contents out and then driver forward again to collect more regolith. Obviously this works best in areas where the regolith isn't terribly rocky.

I'm sure there's some better way to do it that I haven't thought of, but that's my idea.

Antius · 2016-01-28 10:46:19

kbd512 wrote:

SpaceNut wrote:
I am wondering how to place regolith onto a radiator plate without damaging the underlying radiator by placing to much onto it for a roving RV. Then there is the temperature sensing of it such that we unload it with some method that does not as well cause damage to this same radiator only to keep repeating until we turn the reactor off.
Put a small box around the radiator plates to hold regolith. The edges of the plates should face an opening at the top of the bin. A hinged screen door will cover the opening to the plate bin to prevent rocks large enough to damage the radiator plates from entering during loading. Drive the rover forwards slowly to fill the plate bin with regolith. The reactor will be at the rear of the vehicle and the plate bin will be at the front of the vehicle to distribute weight evenly.
The setup will look something like a commercial garbage container, but with a screen door for a lid. It will be attached to the rover with a set of electrically powered arms similar to the arms that lift commercial garbage cans over the top of garbage trucks. When you've heated the regolith to the point where it's necessary to dredge a fresh box of the stuff, tilt the regolith container to dump the contents out and then driver forward again to collect more regolith. Obviously this works best in areas where the regolith isn't terribly rocky.
I'm sure there's some better way to do it that I haven't thought of, but that's my idea.

Sounds like a good plan. One stumbling 'block' I can see with the regolith heat sink idea is the poor thermal conductivity of regolith under vacuum conditions. On Mars, there is enough atmosphere between soil grains to to allow thermal conductivity to approach that of Earth soils. On the moon, bulk regolith is a superb insulator, as good or better than rockwool is here on Earth.

It would be advantageous to design the rover to process lunar material on a steady flow basis. That way, you can effectively build a reverse heat exchanger with the heat transfer fluid entering hot regolith and heating it to 900C and leaving cold regolith.

JoshNH4H · 2016-01-28 17:35:56

RobertDyck,

The critical masses of every fissile isotope are available to any person, for free, on the internet. I have already linked to them twice in this thread and in an earlier post transcribed a few to make a comparison. Seeing as the plans for the nuclear bombs dropped on Hiroshima and Nagasaki can be found on the internet, I see little if any reason to try to be cagey about it. Incidentally, the international Nuclear Non-Proliferation regime is functionally based on the difficulty of obtaining weapons-grade nuclear material, and not on knowledge about nuclear technology. Case in point, there's a list of critical masses on Wikipedia that only differs from what I've posted in that it's more expansive. Lighten up mate

There have been plenty of trade studies done on nuclear propulsion, but nuclear reactors for power generation are somewhat different. The case of a reactor for use in a land vehicle is a bit of a special case, too. I haven't looked into it too closely, but I'm not aware of any studies on the pros and cons of different fissile isotopes for use in space reactors. If you know of any I'd love to see.

I was under the impression that the TimberWind engine was still Uranium fueled, but was able to achieve such high exhaust because it used Carbon-based containment in a "Pebble-Bed Reactor". The nuclear fuel was put in small "pebbles" which were made from Carbon and both moderated and contained the nuclear fuel. I imagine advances in high-temp ceramics also helped reduce the engine mass.

kbd512-

For the purposes of this discussion, I went and looked up the neutron absorption and scattering coefficients for the four isotopes we've been discussing. I found them here. Neutron cross sections as follows (measured in Barns, or 1e-28 square meters):

___Isotope____Scat. XS____Abs. XS___

U-233_________12.9_______575_____
U-235_________14.0_______681_____
Pu-239_________7.7_______1020____
Am-242m_______*__________*_____

*As it turns out, the nuclear cross-sections for Am-242m are not available on this page, which is normally quite exhaustive. Could you point me to your source for the claim that "Am242m has one the highest cross sections of any common isotope"? As your entire argument for Americium rests on this claim, and I can't find any evidence for it, our disagreement is moot if it's untrue or unverifiable.

As to the significance of critical mass, I've argued that it doesn't matter at all for the size of the reactor and the total shielding mass. You argued at first that it would make the core smaller, but are now arguing that a smaller critical mass reduces the required amount of moderator/reflector which would result in mass savings. I have three responses to that claim.

The first is that it probably doesn't matter all that much. You've given as an example a 400 kg reduction in reactor mass. If you'll accept my ballpark of $50,000/kg landed on Mars, that's a savings of $20,000,000 over a non-americium reactor. Especially because you've suggested that the vehicle would be reused, that amount of savings will likely be more than eaten up by the cost of developing Americium reactor technologies and actually producing any significant amount of Americium.

My second response is that critical mass (Or critical size) is still probably a better figure of merit than cross-section. The reason for this is that fission is a result of both free neutrons released per fission (n) and their likelihood of causing another fission event, (p). For a barely-critical mass:

n•p=1

p is a function of the neutron cross-section, the density and positioning of fissile material, and the probability than an absorbed neutron will cause a fission event. The critical mass takes all of these into account, while the neutron cross-section doesn't, and that's why i think that the critical mass is a better way at looking at the compactness of a reactor core.

My third response is that, for whatever reason, shielding is typically made from neutron reflectors and not from neutron absorbers. I can think of a couple possible reasons for it but it's just speculation. The first thing is that neutron absorbers will tend to become radioactive once they absorb more than one neutron, and by doing this add to the problem of tertiary radiation. Second is that when a nucleus absorbs a neutron, it normally emits a pretty high energy gamma ray, which is actually harder to shield against than a neutron. In any case, Hydrogen is a good neutron reflector but not a particularly good neutron absorber. I don't actually know how big of a problem neutrons vs x-rays and gammas are for shielding (Do you, and can you cite it?), but I do know that you were talking about using lighter elements and that seems to be the only logical reason why.

The shadow shield from X-rays is basically unaffected by the size of the moderator as X-rays come from the fissile material.

In principle, reducing the diameter of the fuel rods will increase the rate of heat transfer, but in practice I'm sure any design already takes this into account and makes the diameter as small as possible. My guess would be that the constraint on fuel rod diameter is a mechanical one, insofar as fuel rods that are too long and skinny are more likely to break and cause a containment breach. Strong fuel rods are particularly important in an application (Let's say a land vehicle) that will be subjecting the core to mechanical stresses while it's operating.

With respect to cost, should there be any great issue with building just one Am242m fueled reactor, if only to further our knowledge of that material and what it could mean to the space program if it does everything it appears to do for reactor design?

Neither the production of fissionable amounts of Americium nor the development of an Americium-fueled reactor is impossible. But it is something new! It's something that's never been done before in the history of our species. And that's where the cost comes in. Good design is expensive, and we need good design. Think about building a house. Any able-bodied person could do it, in theory. But in practice, making a building takes skill, tools, materials, and knowledge of how to use them. If I left you in a field with all the materials you needed to build a house but no construction experience, it would take time to figure out what you needed to do and how to do it. I don't know who you are or what your occupation is, but imagine how hard it would be to do it if you had to figure it out from scratch. That's where the cost comes in. And when it comes to nuclear technology, progress is typically impeded by a strong desire not to give anyone cancer or worse. Impossible? No. Expensive? Probably.

Would the basic research required to do a design study significantly detract from any other program?
The MTVL is an aluminum box on tracks that's slightly wider than a Chevrolet Suburban. The fact that it uses tracks does not make it a tank. It happens to be dimensionally large enough to store half of the consumables required for a Mars surface stay of 500 days for two crew members.

A design study on the matter would probably be pretty cheap and definitely be worth it. My point wasn't that the idea isn't worth studying, it's that the study hasn't been done and therefore the claims you are making are mostly baseless.

Wikipedia calls the M113 (after which you say this is based) an "armored personnel carrier". I suppose that's not a tank because it lacks a turret, but it sure looks like one.

Oh, and by the way:

Yesterday, 1/27, you wrote:

Even if the core dimensions of the Pu239 fueled reactor is only 9" x 12", and the core dimensions of the Am242m is 6" x 12", the Pu239 fueled reactor has a core volume that has increased by more than 100%. The shielding has to envelop the reactor. It's mass and volume will also increase. It's easy to see why core dimensions matter so greatly for mobile applications of nuclear power.

But two days before, on 1/25, you wrote:

If I thought using Pu239 as fuel would be politically feasible at all, I'd use readily available Pu239 and call it a day.

Why the discrepancy?

SpaceNut · 2016-01-28 21:50:04

The chain reaction of U-235 is shown in Figure 2.

The isotope 242mAm (half-life 141 years) has the largest cross sections for absorption of thermal neutrons (5,700 barns), that results in a small critical mass for a sustained nuclear chain reaction. The critical mass for a bare 242mAm sphere is about 9–14 kg (the uncertainty results from insufficient knowledge of its material properties). It can be lowered to 3–5 kg with a metal reflector and should become even smaller with a water reflector.

RobertDyck · 2016-01-28 22:54:17

JoshNH4H wrote:

RobertDyck,
...
I see little if any reason to try to be cagey about it.

I read the book "The Case for Mars" in spring 1998. I learned about the "Case for Mars" conferences in that book, but couldn't afford to make it that year. That turned out the be the inaugural convention of The Mars Society. I joined the Society in 1999. I also read about the Russian Energia rocket in that book. Many people did, and many members talked about going to Mars with that rocket. Boris Yeltsin was still president of Russia at that time. At the year 2000 Mars Society convention, I asked several people if anyone had actually asked the Russians. I didn't get an answer. Not even from Dr. Zubrin himself. His book quoted a study by Stanford University, he hadn't actually talked to the Russians. So in December 2000, I did. I phoned the then American subsidiary of the Russian corporation Energia. The person I spoke with told me some information, and gave me a fax number to their corporate head office in Korolev, a suburb of Moscow. It had an American phone number, but connected to a machine in their head office. I included my email address, and did get an answer by email a few months later. The answer was in Russian; it took a couple weeks using computer and internet tools to translate into English so I could read it. I posted my letter and the response, both the original and translated versions, on the local chapter website. Months later I got a response from a Mars Society member in Australia stating I had mis-translated one word. And a couple years later worked at a company here in a suburb of Winnipeg where I met a co-worker; he was from east Ukraine, spoke Russian, and got his degree in aerospace engineering from a university in Moscow. He was able to give the meaning of a few Russian acronyms.

In February 2002, I made another phone call. This time to KBKhA, manufacturer of the RD-0120 engine. That's the main engine for the core stage of Energia. I spoke to someone who spoke perfect English, even though this phone call was directly to Russia. He confirmed they still had the plans and jigs for that engine. It could be returned to production.

After all that I had mysterious clicks on my phone line. My mother complained about the clicks, claiming government spies were listening in. Considering I had contacted Russia, that was probable. My interest was just to find a way to get humans to Mars. Yet it appears my phone was tapped. And you wonder why I'm cagey about posting details about nuclear weapons? Yes, the only information I have is from the internet, but still!

I was worried that I had posted details how to make ammonium nitrate fertilizer from nothing but air, water, electricity, and equipment you can purchase from a local hardware store. Since the Oklahoma bombing, that fertilizer has been a controlled substance. The bomber was former US military special forces, specially trained to make improvised explosive devices from readily available materials, and special training to design a shaped charge, and special training to demolish large buildings such as government buildings. What he did is exactly what the military trained him to do. No one could do that without all that specialized training. I looked up how to make fertilizer for a greenhouse on Mars. Yet, government civil servants think this is the key ingredient to make a bomb. I'm worried "The Great Crash" was not equipment failure, but deliberate sabotage by government agents. You will notice that since the forum has been restored, I've posted details of how to produce nitrogen gas from Mars atmosphere, but I haven't posted details how to convert that into ammonium nitrate fertilizer.

Once burnt, twice shy.

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