Humans Fast, Machines Slow

Rand Simberg talks about impedance matching. So I’d like to make a post of my comment there (I’ve always wondered why this obvious alternative gets mentioned so little…)

What to do when you arrive at Mars or Earth with your solar electric propelled vessel?

So, the problem with most low fuel demand velocity change schemes is that they only give slow accelerations. Low fuel high velocity change means solar or nuclear electric propulsion and aerocapture mainly.

High delta vee aerobraking is hard to do in one pass – it gets dangerous because of atmospheric variability and potentially other reasons.

Simple: detach a small capsule with the humans that goes directly to the surface (with only days of life support) and leave the untended craft to do multi-pass aerobraking. Hitting van Allen belts a few more times or taking a long time doesn’t matter that much with no humans onboard.

You could also potentially ultimately leave the long distance craft at some Lagrange point instead of LEO. (Cue some clever and complex maneuvers to save fuel – maneuvers that take long.)

Something similar could also be done when a long distance stack is assembled in LEO: send the humans there only after it’s through the belts. They can go with a smallish capsule again. Potentially at some Lagrange point, or in space without any fixed reference, just along the way. It could be dangerous though if the capsule doesn’t have much life support.

Many of these things have potential delta vee penalties as well as timing inflexibilities, but they could have enough other benefits that they should be considered.

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Far Away, Yet So Close

He who controls the [Earth-Moon Lagrange points/Phobos/Deimos/Lunar North Pole], controls the solar system.

Why?

Because in space, it is not the tyranny of distance that sets the rules – it’s delta vee instead.

Since there’s no resistance, traveling large distances just takes longer, but doesn’t necessarily require more propellant. Unmanned craft can take this long trip time just fine. This is completely different from the implicit mental models of everyday life or historical exploration, travels and colonization. Even places that are far away in distance can be close in delta vee, and vice versa.

The Earth-Moon Lagrange (EML) points have really low energy trajectories to all the other places, including low Earth orbit (or Earth re-entry). They’re the crossroads. They’re probably not controllable though, like you can’t control low Earth orbit either, it’s just a figure of speech* to stress their significance.

For example, Phobos and Deimos have really low delta vee needs from EML2. And they have really low gravity. This means that it’s cheap to send stuff to them, but perhaps more importantly, it’s cheap to bring stuff from them. Since a lot of space faring is limited by mass that can be brought to locations, a low energy source of material is a real paradigm changer.

The Lunar north pole’s peaks of eternal light are much closer to Earth, but the Moon is so heavy that it takes quite a lot of propellant to descend to and ascend from the surface. The good constant sunlight is an asset though. The area is limited so this is the best incentive so far for a “race”, though I’m skeptical of that.

This post was written partly inspired by Paul Spudis’ and Clark Lindsey’s talking about the importance of the Moon as an enabler for other stuff – I am somewhat less certain. (On VASIMR and JIMO I can refer to Kirk Sorensen who has good reasons for skepticality – the power to mass ratio needed is huge and that’s the really hard part, yet it’s rarely talked about. Space reactors are much harder than Earth ones because of the cooling problem.)

We must dismiss analogies that do not work, since space is a different medium. We must use completely different planning than for exploration on land or the seas, because of the completely different role distance plays. And we must also plan on advancing from exploration ultimately to infrastructure, colonization and self sufficiency.

*: From Frank Herbert’s Dune of course.

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Dual Propellant Expander

Or what you are going to call it, an unrealized proposal from Aerojet around 1984. PDF Found on NTRS.

The idea was to have two turbopumps (like on SSME), but instead operate on the expander cycle. Two heat exchangers, two turbines, two pumps. One for each propellant.

 

Both propellants go through a heat exchanger and an expander driving a pump

 

This is a LOX-hydrogen engine. Also this means that since there is the same propellant on both sides of the axle, in the turbine and in the pump, no elaborate seals are needed. Original intent for these engines was for in-space reusable stuff, that needs to be operated many times and for a long time without maintenance. Size was in the RL10 class, about 70 kN. (RL10 has grown though.)

Simplicity and margin were claimed

Think for example if you let a fired turbopump sit in space for a long time. Will some fuel leak to the oxidizer side through the seals? This could avoid that. (You can use helium purges too though but then you’ve got one more fluids you need to tank.)

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Reality and Future

Jeff Greason is a rational person who simply gets it. It is mind boggling how completely opposite from someone like Mike Griffin he is.

See Jeff’s presentation with the Augustine Panel.

Paraphrasing, “we could go to Mars with Ares V but we shouldn’t – cause we couldn’t stay anyway”. Exactly. That’s the problem with NASA. (or the major one)

I bet he will be ignored completely.

Also, I would like to work for that guy. Too bad because of ITAR I couldn’t work in the USA.

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Heavy Lift Unnecessary

There are a lot of implicit assumptions that heavy lifters of this or that throw weight must be used for future exploration beyond low Earth orbit.

These “needs” have never been logically derived from anything.

Yet space policy and exploration architectures must be based on rationality above all. There is no excuse whatsoever to do things on a whim. Hundreds of billions of dollars, and the future of humanity’s spacefaring are at stake.

There is no foreseeable need to launch over 25 tonne monolithic payloads to low Earth orbit in lunar exploration, and probably even that number could be seriously diminished with some more thorough planning. Orion, EDS, LSAM, all are below that weight, if they are refueled using a depot in space.

If the huge development and operational estimated costs for a heavy lifter rocket go away, then that money is freed for real exploration work. In-space hardware development, more launches, more missions and operations.

Flight rate is _the_ most important way of reducing launch costs, the single largest impediment for advancement of spacefaring, and the propellant depot enables a higher launch rate. Multi-launch scenarios with a propellant depot also enable competition, redundancy and flexibility, all very good things, ensuring safety, robustness and progress.

I repeat as a summary how

1) Solutions for space exploration, like any large endeavour, must be rationally justified. No baseless assumptions should remain.

2) The need of heavy lift is a baseless assumption. It can be one of the alternative ways of execution, but it can not be a starting point or an axiom.

3) The current architecture is heavily based on the implicit assumption of heavy lift. Hence a rational space exploration architecture would examine things from the ground up. It could end up with some radically different conclusions.

4) Propellant depots is one alternative way of executing space exploration beyond LEO, and it does not need heavy lift.

5) Propellant depots can, if executed correctly, increase launch rates many fold, and thus enable lower costs, progress, reliability, redundancy, robustness – all the things that the space shuttle promised but failed to do because it was a sole solution that could not sustain a high enough launch rate and was too costly.

6) NASA at the same time should keep on working with fundamental research, to enable continuous progress trends in space technology.

7) Space exploration should look as different from Apollo as possible – there should continuity and continuous improvement possibilities, robustness and progress. The architecture should be affordable as well.

8) New space technology, like cheaper launchers, should be demonstrated at a smaller, humble scale first. That way many things can be tried and progress is faster, for the same price and effort. One failure also will not be as critical.

9) There seem to be impediments for information flow inside NASA, and many professionally acknowledged things like propellant depots, EML2 rendezvous or space tethers are never even mentioned in NASA high level planning. This is not rational.

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Lunar Bases Must Be Buried

Concept art of lunar bases tends to show spherical or cylindrical structures, but they suffer from one problem: radiation. (Both of the gamma / particle and heat kinds). The lunar environment has lots of solar and cosmic radiation. Nights also last for two weeks, during which a badly insulated thing will freeze.

If you bury your moon base under a layer of regolith, you can avoid both of these problems. You don’t have to bring heavy shielded modules from earth, or heat during the night with nuclear batteries. Regolith is thermally well insulating.

NASA seems to catch onto this a little in some clearly low budget “alternative configuration” posted at Nasawatch, but it only goes halfway, putting some shallow “berms” around cylindrical structures, for shielding.

In reality, lunar bases (if crews are to spend many lunar nights there) would probably be completely buried.

Burying might actually be easy: a small automated/remotely controlled snow blower style rover vehicle might be able to do it slowly with the help of just solar power. Since there is no air, tiny amounts of regolith can be thrown large distances. A thin wheel with whiskers spinning rapidly would throw the sand to the wanted direction. A lunar day is 336 Earth hours. Even if the “regolith lobber” robot can not survive the night and is expendable, it could manage to move significant amounts of regolith. A sub-MER size rover with large solar cells could throw perhaps 20 grams of regolith per second, or 72 kg in an hour. That’s 7 tons if there is 100 hours of efficient sand throwing time.

Say, landing at 50 hours from dawn, setting up 50 hours (survey area, lower rover from lander, unfold solar cells etc), operating for 150 hours (which includes maneuvering 50 hours and sand lobbing 100 hours), and finally stopping at a low sun angle 50 hours before dusk.

Such concepts are probably unlikely to work in an atmosphere, though I would be happy if proved wrong. The 2009 lunar regolith excavation challenge is coming up, after all…

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What NASA Should Do

And partly what this blog is about (I realized in the middle that I’m typing like in a slide show, so I changed it into bullet points, as it’s an overview and not a deep text). I present my vision that should be aimed for:

What should NASA do?

• In the near term, NASA should change to EELV:s (Atlas V, Delta 4) and COTS (Falcon 9, Taurus 2) as launchers for the ISS and lunar programs.
• At the same time, NASA should do basic research and cheap small tech demonstrators for space technologies that give more for less.
• This should move humanity closer towards spacefaring.

Spacefaring? Spacefaring is making space operations routine.

• Space faring requires that space access is cheap, reliable and hassle free.
• Launch is only part of the spacefaring,
• But only from that point on can the better in-space technologies (tethers, ballutes, sails, ISRU, slings, whatnot) be developed.
• Hence launch improvements are absolutely crucial for spacefaring

How can cheap and reliable space access be reached? There must be:

• Many independent providers of space access.
  
• It is done largely with well reusable vehicles.
  
• The architecture – more of a market – is multi-faceted and the launchers can be improved, new ones can enter the market and old ones can be scrapped
  

This coal can be reached, in the next few decades.

Things to avoid:

Technically unrealistic choices at the highest level:

• In the NASP program, the early performance numbers were fudged and there were unacceptable internal politics meaning no real independent technical criticism would be heard at the top
• In the “Safe Simple Soon” Ares rockets vs the already flown EELV:s debacle, OMB has lacked the expertise to keep NASA on a leash so they are a “loose cannon” controlled too much by the whims of a leadership that fires all who disagree
• Countless other examples…

Program mentality:

• Apollo was ended since it was just a short unsustainable program with a specific stunt style goal, not fitting in any overarching smart picture as a sustained capability
• STS has been an unimprovable yet critical massive monolith, barely sustainable, for various reasons
• Danger of having yet another single solution launcher (or two) just for a definite program

Lack of motivation:

• Has NASA become too big and corrupt by internal politics to really do technical or economic choices? Has it just become pure politics and internal struggles for personal or group benefits? (ESMD) There are great and talented people working there, but does it make a difference?
• What does the whole agency exist for anymore anyway? Or its current lunar program? Is it just a relic from Apollo?
• How much actually flying a few people to space every year conflicts and directs efforts away from the goal of reaching real spacefaring?

Summarized, NASA’s goal should be a spacefaring humanity in the future, not having a narrow minded program after another.

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FLEX – Flexible Exploration Architecture

I wrote this architecture proposal, FLEX, a few years ago. It analyzes NASA’s approach that the ESAS study picked and notices how most of the mass in a lunar exploration stack in LEO is actually liquid oxygen. By using a propellant depot, the LOX can be lifted with tankers and any launchers imaginable (I wouldn’t use a Pegasus though). The rest of the stack is also naturally divided into about 20 ton chunks: EDS with its hydrogen, the CEV crew vehicle (Orion) and the LSAM lander (Altair).

No new heavy lifters need to be developed, there is enough US, nevermind world launch capability to support a moon exploration program. Launchers can also be improved on the run, because they are not tied to the single use, nor is the use dependant on the single launcher, and because they can fly often, hence improvements are worth the investment. This all could be achieved much sooner and cheaper than the current approach, and is much more robust for the future.

Go read it if you haven’t.

There are some comments at an old Nasaspaceflight.com thread that deal with a lot of the common questions about it.

I really don’t have the faintest idea of the background knowledge level of the readership here so I don’t know how much basics I should give, so feel free to ask in the comments if anything is unclear.

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Thorium MSR Design Possibilities

There are a huge array of design possibilities for a Thorium molten salt reactor / liquid fluoride thorium reactor, but this post takes a very simplified approach to map a small part of the fascinating and diverse landscape with a little rough drafting. We concentrate on thermal spectrum designs. I’m strictly an amateur in these matters so everything I write should be taken with a grain of salt and checked more carefully with people who actually know what they are talking about. 🙂

My references are Kirk Sorensen’s energyfromthorium.com and David LeBlanc’s excellent slide set. Highly recommended!

One or Two Fluids?

One choice is having two fluids (core and blanket) or just a single one (Thorium-232 and Uranium-233 salts mixed in the same vessel).

The single fluid reactor seems like a much simpler alternative. There are problems though that when Thorium-232 absorbs a neutron, it becomes Protactinium-233 (Pa) before becoming Uranium-233. The half-life of Pa is about 30 days, and if it catches a neutron, it becomes U-234, which is not a fissile material. So the neutron was wasted. This is pretty bad since Pa’s neutron cross section is over twice that of Th’s so it wants to catch those neutrons. But there are ways around this.

Protactinium Extraction?

For one, you can perform protactinium removal from the single fluid core. This involves liquid bismuth and I’m not very familiar with the process, but it is more complex than U or FP separation (see below). The separated Pa-233 is put into tanks to wait for its decay into U-233 before it is put back into the core. This also enables pure U-233 extraction which is somewhat problematic in a proliferation sense (I’m not very knowledgeable in this direction).

In a two fluid reactor, you can probably get by without Pa removal by just making the blanket bigger, so there is much more Thorium than Pa at any given time, giving a better statistical chance for a given neutron to hit a Thorium and not a Protactinium atom. This can also involve fancy geometries, since the core stays the same size. For example there can be a long empty gap between the core and the blanket through which the neutrons fly.

Uranium Extraction?

In a single fluid design the new Uranium is already in the core after it is produced from the Thorium, so this step is not needed. But in a two fluid design it is practically mandatory. The ThF4 and UF4 mix is bubbled with Fluorine gas, and the UF4 turns into UF6, which is a gas. The gas is separated from the liquid. UF6 handling is already a known 60 year old industry from nuclear fuel processing. The UF6 is changed back to UF4, which is then inserted into the core. The ThF4 is inserted back into the blanket of course.

Moderation, How?

The most traditional way would be to moderate the reactor with solid graphite rods. There are thermal spectrum spreading and since the fuel is in liquid form, expansion and convection effects that mostly help with control here, but single fluid reactors still can have situations of positive power coefficients. Graphite has issues of swelling and shrinking under radiation and a limited lifetime. Regularly changed moderator pieces would be radioactive waste as well. Loose graphite rods or even pebbles could be ways to prevent stress cracking.

There have been designs without any solid moderators as well. The salts moderate some anyway, and some reactors run simply at faster spectrums, the latter again bringing control problems.

And lastly, there is the heavy water moderated MSR, of course loved by many Canadians because of their CANDU expertise, the current heavy water moderated solid fuel reactors. The heavy water would not act as a coolant at the same time, but would be separated in thermos style vacuum jacketed pipes. Any breach would boil the moderator away, reducing reactivity. This jacketing would enable easy low pressure vessel construction, like for the rest of the reactor.

Geometry?

The first MSR designs were naturally small and spherical. A number of geometry proposals have surfaced: pipes coming from many directions into a bundle of criticality or a cylindrical core with a cylindrical blanket around it. There are many conflicting requirements like maintaining criticality, preventing overcriticality, minimizing neutron waste, minimizing inventory size, coping with thermal expansion, passive safety, ease of construction and transportation, scalability…

Inventory Size?

A small fissile inventory has a number of benefits. The reactor is simply smaller for a given power ratio, making it cheaper. Also less U-233 for the starting load needs to be produced elsewhere. If the breeding ratio is low and the fissile inventory large, it takes long (perhaps twenty years) for an MSR to produce enough U-233 to start another new MSR, probably requiring big production of U-233 elsewhere, changing overarching infrastructure plans significantly.

Fission Product Removal?

This is a very common feature of MSR designs (some once-through no-refuel systems for ships and spacecraft can take this out), but how often it is done, varies. Vacuum distillation means reducing the pressure until the most volatile stuff starts boiling. Only UF4 fuel salt should be left after this. With a single fluid salt there is a problem that the Thorium Fluoride salt boils as easily as some of the fission product fluoride salts and hence something else would need to be done, or just some Thorium be “thrown away” together with the fission products. On the other hand, some fission products are gases like Xenon, which boil already in the core. Many of the fission products are also valuable rare elements by themselves and can be sold. The MSR naturally separates fission products from fuel, unlike current solid fueled reactors, making this line of technology look unexplored and interesting. Since there is less waste sucking up neutrons in the reactor, the neutron efficiency of an MSR can be high and it can get by with little fuel and a small physical size.

Power Extraction, How?

This is more of an “auxiliary” function. The most common design is connection to a helium heat exchanger probably through another salt loop to reduce radiation loads, and the helium would drive a heat engine. High temperatures (compared to solid fueled traditional nuclear reactors) are achievable and all kinds of thermal processes become attractive as well. Since the molten salt solidifies at a certain temperature, the heat exchanger can not go below that. Coupled with the high efficiency and high top temperature, you get small radiators and cooling towers and small water usage compared to traditional reactors.

Control, How?

An ideal MSR is extremely easy to control because of the natural negative feedback of the reactor – increase power usage by increasing coolant flow, the reactor cools and increases reactivity and gets to a steady level. As far as I know, the original aircraft reactor experiment was not throttled in any other way. There simply are no control rods. An MSR would automatically operate in a designed, relatively narrow temperature region of a few tens of Kelvins. In the event of a significant overheating accident, a freeze plug at the bottom of the reactor would melt and the salts would drain into tanks below for slowly cooling down.

Waste Handling

First the most valuable materials like Xenon, Tritium and various other substances could be sold. Since the waste would by majority be fission products and barely no heavy transuranics, they could be stored on-site until they quickly lost enough radioactivity and cooled for final storage of a few hundred years inside rock.

So, What Is The Best Configuration?

As a personal preference, my best guess for the best of all worlds is a two fluid design, with a cylindrical “LeBlanc” geometry. Adding power would simply involve making a longer cylinder. No Pa removal, but the blanket could be oversized. Remember that all the vessels would be at low pressure, and the blanket isn’t even that hot and is only filled with Thorium, which is cheap. Initial core load U-233 amount would still be low.

Next to the reactor would be the fluoridation tower for taking U from the blanket and putting it to the core. Hydrogen would be needed to change the UF6 back to UF4. (Generating chemically nasty HF at the same time.) A vacuum still would remove the fission products from the core, and this would be located close by too.

Since there would be no high pressure vessels, the reactor core and blanket could be quite light and probably road transportable as a whole. The material of choice for the high temperature salt sections would be Hastelloy.

Moderation could either be nonexistent, by loose graphite profiles inside the core, or Hastelloy pipes containing a vacuum walled heavy water pipe. This is an area that needs lots of innovation to minimize maintenance and waste.

According to calculations made by David Leblanc, a 1 m diameter 6 m long “pipe style” reactor (easily truck transportable for construction!) could produce 400 Megawatts of electricity (900 MW thermal) with only a few hundred kilos of fissile inventory! Most of the salt mass would be “filler” (Fluorine, Lithium, Beryllium) and Thorium.

This is a very stark contrast to the current nuclear plants with their huge high pressure vessels, currently probably only manufacturable in one steel plant in Japan, requiring many years of ahead time for ordering, and lots of welding on the build site as well.

End Note

There is real small MSR hardware that has been demonstrated to work, and there are good conceptual designs for real powerplants that could change the world energy generation picture completely. I am not aware of any other energy production technology which is so based on reality and still potentially revolutionary at the same time. This thing has a very good chance of working well. It does not require significant new scientific developments or a fifty year timetable, like fusion. It is not as limited in availability and dependability like wind or solar power. It is good that those alternatives are pursued too, but the MSR is a bigger answer to a bigger problem.

-the Molten Salt Reactor, Real and Revolutionary!

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Bringer of the Second Nuclear Age

Thorium Molten Salt Reactors – A Really Short Intro

Thorium is about thrice as abundant compared to Uranium. And countless times more abundant than the U-235 Uranium isotope that is used mostly by modern nuclear reactors.

You can use Thorium, Th-232, as the sole fuel in a special nuclear reactor, which first turns the Thorium into Uranium, namely, U-233, and then splits it in fission. The good thing is that very little Thorium is needed (under 1000 kg per gigawatt year), and no isotopic enrichment (that Uranium needs to separate U-235 from U-238 ) has to be done, and there are practically no transuranic products from the reactor (that’s the long-lived radioactive stuff). The reactor is called the Molten Salt Reactor – MSR, or the Liquid Fluoride Thorium Reactor – LFTR. The trick is to not use any fuel rods, but have the nuclear fuel in a molten salt. More on why this is done below.

A U-233 atom receiving a neutron splits into fission products, giving energy, and also gives off two neutrons (statistically, a bit more).  One neutron is used to hit another U-233 so the reaction continues, while the other is used to turn Th-232 into U-233. This is called breeding, where enough neutrons are produced so they can, in addition to keeping the chain reaction going on (like in all current nuclear reactors), also “breed” new nuclear fuel.

Now, there are some breeder research reactors working with U-238, the more common Uranium isotope, being bred to Plutonium Pu-239, but with Plutonium the problem is that you have to use fast spectrum (otherwise there won’t be enough neutrons), which again often brings some controllability issues. Also, nuclear bomb material can be produced because Pu-239 is easy to separate chemically compared to the different isotopes of Uranium which are chemically similar. U-233 from a Thorium reactor on the other hand is not very suitable for bomb material, and there are options to denature it while in the reactor so it’s unusable.

The Th-232 / U-233 MSR is unique in that it can be a breeder reactor in a thermal spectrum. This is achieved with keeping both the fertile and fissile material in a molten salt, Thorium and Uranium Tetrafluoride. (ThF4 / UF4.) The reactor core is filled with Uranium while there is a blanket of Thorium around. When a Thorium atom changes into Uranium, it is taken out of the blanket by bubbling the blanket it with Fluorine gas which turns the UF4 into UF6, a gas. It is again reduced to UF4 with hydrogen and then put into the core. This is the critical idea of the molten salt reactor, continuous reprocessing. Also, the fission product waste can be removed during continuous reprocessing (vacuum distillation). The molten salt also enables harmful fission products like Xenon to bubble out of the molten salt immediately so they don’t worsen the neutron economy by capturing neutrons themselves (a significant problem in current reactors, limiting their fuel use percentage.)

Thorium MSR Simplified Schematic

The MSR has been demonstrated in the US Oak Ridge National Laboratory in the sixties and seventies with multiple prototypes, but the development stopped since the fast uranium breeder was the way chosen instead (for a variety of reasons, many of which are not valid today).

Many think that it could be a good solution to the energy crisis. It is relatively known technology yet changes the nuclear equation hugely, by reducing fuel needs AND waste immensely, multiple ten fold. 1000 kg per year for a gigawatt sized plant would mean you could run 30 years with one container of Thorium. The waste, of similar mass, could be relatively safe after 300 years, since it would not have much transuranic elements.

Sadly, most nuclear engineers have not even heard about MSR / LFTR. There is a lot more material on Kirk Sorensen’s site, energyfromthorium.com, most highly recommended.

As Alvin Weinberg said, we are still living in the First Nuclear Age. The Molten Salt Reactors could bring the next one.

I’ve been planning a few posts about molten salt reactors so this is an introductory post to the matter. Please, ask anything in the comments!

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