Bob Clark

Neutron production in the lunar subsurface from alpha particles in galactic cosmic rays

http://www.terrapub.co.jp/journals/EPS/pdf/2011/6301/63010025.pdf

“While GCR nuclei comprise only about 1% of GCR particles, they contain more than 10% of the nucleons in the GCR…..the neutron production in the lunar subsurface by those particles is 2% of that by protons and alpha particles. We can there- fore conclude that neutron production from the GCR heavy components (Z > 2) is negligibly small.

Heavy ion interaction plays an essential role in terms of space dosimetry. According to Hayatsu et al. (2008), who considered only GCR as incident particles, secondary neutron and GCR heavy components (Z ≥ 2) contribute about 9 and 84%, respectively, to the ambient dose equivalent on the lunar surface. Therefore, it is of importance that the transport and interactions of heavy ions are preformed with accuracy.”

Regolith Biological Shield for a Lunar Outpost from High Energy Solar Protons

http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=21049474

“As the shield thickness increases, the overall dose within the habitat decreases, though the primary and secondary protons remain the principal contributors to the estimated dose. The high energy solar protons with > 200 Mev penetrate deep into the shield and dominate the dose estimates with shield thickness up to 100 g/cm (Table. 3). With a smaller shield thickness, the dose from the secondary neutrons is less than that from the primary and secondary protons. Since the neutrons’ primary mode of energy loss is by collision with the shield molecules, their penetration into the shield material is greater than charged particles of the same energy. Thus, as the shield thickness increases, the contribution of the secondary neutrons to the overall dose in the habitat increases, to ~ 50% at shield thickness of 100 g/cm^, and exceeds that of the protons at higher shield thickness.
In order to reduce the total dose inside the habitat below that of the crew 30 days limit of 25 Rem, a shield thickness of ~30 g/cm is needed, on top of the 10 cm aluminum support structure (Table 3). The primary and secondary protons contribute ~ 15 Rem (0.15 Sv) to the total dose, while the secondary neutrons contribute ~ 7 Rem (0.07 Sv) to the total dose. In future outposts, astronauts would not be the only inhabitants. As travel to the lunar surface become more affordable, tourists and associated workers will be among the inhabitants of such mesmerizing facilities.
Though realistic radiation exposure limits need to be established, until then, the radiation dose of 5 Rem for radiation workers is likely the preferred limit. In this case, additional shielding around the lunar habitat above 30 g/cm^ will be required. In order to reduce the dose in the habitat from the February 1956 like solar flare event to below 5 Rem, an additional shield thickness of ~ 120 g/cm^, above that needed for the crew, would be needed; raising the habitat total shield thickness to approximately 150 g/cm^ (Table 3). This shield thickness translates to a dimensional thickness of approximately 0.56 m and 1.24m of aluminum and lunar regolith, respectively. It is important to note that for the 5 Rem dose limit, the secondary neutrons are the dominant contributor.”

Marcel

The work done on the ISS is not routine maintenance. No way. Some astronauts have lost fingernails because the gloves are so hard to work in.

And as for the 10 centimeters of regolith stopping heavy nuclei: it takes 14 feet of water or plastic to stop secondary radiation at a level equal to 18,000 feet above sea level.

If you are going to give people a radiation bath it should be made clear and not hidden behind false promises and present radiation exposure limits for astronauts. Those limits are not acceptable for people living on the Moon for any length of time.

At some point I am going to have to stop debating you on this; your mind is made up. And I think that time is now.

What the fluid management system they are developing and have built a couple small thrusters and a small generator as prototypes for is this:

It substitutes components with moving parts for heavier components that did not have moving parts. It integrates the thrusters and electrical system and propellant managment system into one system. It reduces the boil off by having the boil off burned in low thrust ullage thrusters constantly and this keeps the liquid mass concentrated at the back of the tank thus reducing the heat affecting it.

It reduces the boil-off rate by about half. The sunshade under development and extreme isolation design and a couple other tricks like vapor path cooling can supposedly reduce it more; the goal being enough to perform a Mars mission. The fly in the ointment is the spacecraft are the standard zero-G unshielded designs. IMO any long duration missions require artificial gravity and hundreds of tons of shielding against heavy nuclei; and that will require nuclear propulsion (and that requires a Moon base from which to assemble, test, and launch such missions).

For going to the Moon it is all very clever- except you lose the redundancy of the former separate systems and if any part of it, such as the piston engine generator, accumulator systems, or the low thrust thrusters fail then the system does not work. What was batteries and tanks for helium and hypergolics is now stuff that has alot of moving parts. In addition the maneuvering thrusters are no longer hypergolic and lose the extreme simplicity and reliability. It might work for a LH2 Lunar Shuttle but that….aint gonna happen without a hangar. I might also add that it uses the only LH2 Earth departure stage there is- the Centaur. That vehicle is extremely fragile and has never been man-rated.

You also included the robot refueling in your citation of what has “been done” on the ISS.
Sorry but that does not fly.

Since a single reusable lunar shuttle would probably only be used ten times over the course of perhaps five years before its engines might be replaced, astronauts at a lunar outpost are going to have plenty of time on their hands for maintenance and inspections. Plus they’ll have plenty of advice from engineers from Earth if they have any problems. But being able to maintain reusable spacecraft, lunar habitats, and machinery on the lunar surface is part of the pioneering effort. And astronauts preparing for long muti-year expeditions to the Martian surface are going to have to be able to maintain things and to fix things while they are there. The following are articles on the ULA’s reusable cryogenic technologies:

An Integrated Vehicle Propulsion and Power System for Long Duration Cryogenic Spaceflight

http://www.ulalaunch.com/site/docs/publications/Integrated%20Vehicle%20Propulsion%20and%20Power%20System%20for%20Long%20Duration%20Cyrogenic%20Spaceflight%202011.pdf

Development Status of an Integrated Propulsion and Power System for Long Duration Cryogenic Spaceflight

http://www.ulalaunch.com/site/docs/publications/IVF-Space-2012.pdf

A STUDY OF CRYOGENIC PROPULSIVE STAGES FOR HUMAN EXPLORATION BEYOND LOW EARTH ORBIT

http://www.sei.aero/eng/papers/uploads/archive/GLEX-2012-05-1-4×12564-CPS-Study-revC.pdf

“You cannot do serious maintenance in a spacesuit- I have read interviews with astronauts describing spacesuit work. Impossible to maintain a wing of landers on the Lunar surface. As for “automatically” fueling a spacecraft with liquid hydrogen and oxygen……..science fiction. Filling a tank with liquid hydrogen is a complicated process and keeping the “cryo depot” stable is also time consuming and maintenance intensive”

Yes you can. Its been done on the ISS. Plus lunar astronauts will probably also have mobile robots with human-like manual dexterity to assist them.

“Secondary radiation from heavy nuclei is a problem. It is actually THE problem. That is wrong.”

Only 10 centimeters of lunar regolith are required to stop heavy nuclei. And secondary particles would be significantly mitigated by the other 1.4 meters of additional regolith.

Lunar Station Protection: Lunar Regolith Shielding

http://www.spaceagepub.com/pdfs/Lindsey.pdf

Marcel F. Williams

Never seen a thing except for a graphic of a tin can with a sunshade. Enormous? More proof please. Then I might be more optimistic.

2) “-that’s exactly what you need for a reusable vehicle-”

No, you need a little more than an ignition system. The valves and plumbing all have to be periodically purged, inspected, and op-checked just for starters. The tanks have to be inspected for cracks and embrittlement because cryogenics are notoriously hard on any lightweight materials. Liquid helium has to be available to pre-cool both the lander systems and the connections for draining or filling. Any dust getting into the guts will have to be periodically cleaned out. Everything has to be made so it can be inspected and replaced and the maintenance procedures to go along with that have to be verified. Cold welding, radiation degradation, damage from both loading and unloading cargo, landing cycle stresses on the airframe, and on and on. And I am just a mechanic- an engineer probably has a list several hundred pages long of what has to be addressed to make a lander reusable. With an expendable vehicle you don’t have to worry about any of it. You wanna play you gotta pay.

3) “Radiation on the Moon is not a problem.”

Secondary radiation from heavy nuclei is a problem. It is actually THE problem. That is wrong.

4) “Refueling a lunar shuttle will probably be done automatically with mobile tankers. But even if some lunar astronauts had to spend ten percent of their time outside of the protective cocoon of their shielded habitats (2.4 hours a day/ nearly 17 hours per week) maintaining a lunar shuttle-”

You cannot do serious maintenance in a spacesuit- I have read interviews with astronauts describing spacesuit work. Impossible to maintain a wing of landers on the Lunar surface. As for “automatically” fueling a spacecraft with liquid hydrogen and oxygen……..science fiction. Filling a tank with liquid hydrogen is a complicated process and keeping the “cryo depot” stable is also time consuming and maintenance intensive.

I am on board with the LH2 Lunar Shuttle; I am ready to start turning wrenches on it right now- but you are not describing the reality of what it will take to make it work.

I will just keep saying- It aint gonna happen without a hangar.

1. Boeing, Lockheed-Martin, and the ULA are investing an enormous amount of resources developing the cryogenic refueling of space craft: their LOX/LH2 ACES program. This by the way is the technology that most anti-SLS advocates wanted to utilize for lunar exploration.

2. The latest RL-10 engines are supposed to have the ability of at least 50 restarts. And that’s exactly what you need for a reusable vehicle. So there’s no need to change the engines to operate a reusable space vehicle. But if you could eventually replace the engines, then recurring cost would fall substantially more.

3. Radiation on the Moon is not a problem. Simply surrounding a habitat module with less than 1.5 meters (less than 5 feet) of lunar regolith would fully protect astronauts from major solar events while also reducing annual cosmic radiation exposure to less than that legally required for radiation workers on Earth (less then 5 Rem/year). And there’s obviously no shortage of lunar regolith on the surface of the Moon.

4. Refueling a lunar shuttle will probably be done automatically with mobile tankers. But even if some lunar astronauts had to spend ten percent of their time outside of the protective cocoon of their shielded habitats (2.4 hours a day/ nearly 17 hours per week) maintaining a lunar shuttle and other stuff like exploring, they would still only be exposed to an additional 2.5 Rem/year. NASA’s annual limit of radiation exposure to astronauts is 50 Rem/year.

Marcel F. Williams

Manning a space station in orbit around Mars or outpost on the moons of Mars with a dozen or more people would seem to be essential for safe human landings on the Martian surface because of the significant time delay communicating between Earth and Mars.

NASA could test the use of small extraterrestrial launch operations teams for launching manned and unmanned reusable vehicles to and from the lunar surface with a small team of people located at a lunar outpost or from a Lagrange point outpost.

So a lunar outpost program with its own ground operations team would once again serve as an essential precursor program for ground or orbital operations for a future Mars outpost program.

Marcel F. Williams

Hmm, I don’t know. I think that we have a long way to go on this one still. Most people have Fukushima still fresh in their minds (for better or for worse). Add to that, the ‘Why do we spend money up there’ tree-hugging fanatics and you can see that nuclear+space together, aren’t exactly popular topics.

The most foundamental issue IMHO that should be addressed first, is making the public realise that space isn’t just a ‘hobby’ for ‘nerds’, a handout for a special community of scientists (as many see it today as such), but that it’s the future. That the solutions that everyone so desperately seeks, about the environment, the economy, society etc, will come from space. Space is the future.

Until then, the general public will just react emotionally to topics like ‘nuclear’ and ‘space settlement’, rather than think logically and critically.