Map of cislunar space — the logical next arena of human spaceflight.
There is an on-going study effort at the National Academy on the “value proposition” for human spaceflight. This study was requested by the Congress in its last Authorization Act for NASA. The committee has requested input from the public on their thoughts regarding human spaceflight. Although I have doubts about the value of this study, I still felt compelled to submit my thoughts on this topic. Frequent readers of this blog will recognize most of these points, but many on the committee may not know these arguments and I think it is valuable to recollect and collate them in one place to clarify our thinking. A PDF version of this white paper can be downloaded at the NAS committee web site or here at my main web site.
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ABSTRACT
The valuable partnership of humans and robots in space was demonstrated with the building of the ISS and the servicing of the Hubble Space Telescope. Machines alone cannot perform many of the intellectual and manual activities required for space utilization and development. Humans have proven themselves capable and often indispensable in advancing space objectives. With the completion of the International Space Station, human missions beyond low Earth orbit are the next logical steps. Of all possible destinations beyond LEO, the Moon provides the optimum location for the development of new space faring capabilities and the best opportunity to maintain a permanent presence in space. The lunar surface contains the material and energy resources required to develop a permanent, extensible space transportation system. By using an incremental, scalable architecture, our civil space program can realistically achieve two components vital for success – affordability and sustainability. This system will provide access to the lunar surface and all points within cislunar space, permitting the construction of large, distributed satellite systems. A permanent transportation system in cislunar space – fueled by propellant made from lunar water – creates an extensible, maintainable space faring capability.
The United States and its civil space program stand at a critical juncture. Due to strategic confusion and loss of direction, our nation is in imminent danger of permanently destroying this once-great capability. It is crucial that this committee understands that our nation’s technical competitiveness and economic health, along with that of other countries who value and champion individual liberty through the power of free markets and democratic pluralism, is at stake.
As requested in your call for papers, I organize my thoughts around your suggested questions.
1. What are the important benefits provided to the United States and other countries by human spaceflight endeavors?
Humans are needed in space to do those important and critical tasks and activities that only people can accomplish. These types of jobs break down into three main categories.
To explore, discover and learn. While scientific exploration is often thought to be central to the robotic space program, the Apollo and Space Shuttle programs demonstrated the critical role humans play in this area. Field science is interacting with an exotic environment such that specific and relevant questions are both posed and answered in real time. Beyond sample collection and the deployment of equipment, human creativity is crucial. The spontaneous interaction of human cognition and intelligence, skills honed through highly interpretive knowledge, innate manual dexterity and real-time adaptation in the event of difficulties and/or opportunities, all offer important advantages over purely robotic spaceflight.
To create, build and maintain. This category of activities takes advantage of the proven ability of people and machines working together to build and maintain large distributed space systems on orbit. The most complex spacecraft ever built, the International Space Station (ISS), was constructed this way; this platform could not have been built and launched en masse in a single launch vehicle – it had to be assembled on site. In a similar manner, space systems constructed in the higher orbits of cislunar space (e.g., geosynchronous orbit – GEO) could likewise be made to possess unprecedented degrees of power and capability; an ISS-sized communications complex constructed in GEO with global coverage and virtually unlimited bandwidth would render the current terrestrial cell phone network obsolete. For this practical reason alone, routine movement of people and machines throughout cislunar space should be a priority goal of human spaceflight. Once built, such a space transportation system will serve as our means to travel into the Solar System beyond the Moon.
To preserve, protect and survive. It has been posited by some that the movement of humanity into the cosmos is the “ultimate” long-term goal of human spaceflight. A species present in multiple locations faces better survival odds than a single-planet species in the event of a planetary catastrophe. Although this may be an ultimate rationale for human spaceflight, it is not a viable objective for a federal civil space program. Before we can settle space, we must develop the ability to get there. Once there, we must create the ability to stay there. These latter two tasks constitute the development of a space faring capability, a sine qua non to space settlement and human expansion into the universe. Developing such skills is an appropriate task for our national space program. Instead of fixating on Mars, human space exploration should focus on becoming space faring – a goal that includes Mars, but one far richer in possibilities, both exploratory and practical, and one whereby we create wealth rather than simply consume it.
In short, we desire to develop the ability to go wherever we want or need to in space, with people and machines, to accomplish whatever goals and objectives we may desire. By accepting this as our overall goal and stating at the outset what our endpoint is, the development of a strategic approach and tactical path to attain this ability becomes possible.
2. What are the greatest challenges to sustaining a U.S. government program in human spaceflight?
Although it is tempting to ascribe the cause of our current space malaise to a lack of funding, in fact the problem is more fundamental – it is a failure to fully understand exactly what we are trying to accomplish and why. I discuss the “why” in the section above; if these objectives are granted as valid for the sake of argument, how then might we best begin to establish such capabilities?
Human spaceflight requires specific destinations to be programmatically viable. The idea that we should develop technical systems before we decide where we will go takes us nowhere. The clear next step for human spaceflight in the post-ISS era is beyond low Earth orbit. The Moon offers the best near-term, sustainable destination to practice and accomplish those space faring goals mentioned above. The reasons are three-fold:
1. The Moon is close. Only a few days away, the Moon is constantly accessible for launch at any time. Its proximity permits remote control of machines by operators on Earth, allowing us to perform many menial and preparatory tasks by teleoperated robots prior to human arrival. The closeness of the Moon makes our early venture beyond LEO safer, as mission aborts are easier and opportunities occur more frequently than for trips of interplanetary dimensions. Milestones and capabilities for lunar return can be set and met in achievable timeframes.
2. The Moon is interesting. Etched in the Moon’s surface is the geological record of its history, as well as the history of the Earth-Moon system and that of the Sun and galaxy. A small rocky planet of surprising complexity and richness, the processes that operated on the Moon also operate on all of the other terrestrial planets. The Moon uniquely retains a record of the early impact bombardment history of the Earth, something no other space destination offers. A record of the ancient Sun and its output through time is recorded in the dusty regolith of the Moon’s surface, information vital to the reconstruction of Earth’s climate history. The airless, seismically quiet lunar surface is a superb platform to observe the universe by constructing astrophysical instruments of unprecedented sensitivity and capability. The Moon’s extreme vacuum and thermal environment permits scientific experiments difficult or impossible to attempt elsewhere in space or on Earth.
3. The Moon is useful. The Moon possesses the material and energy resources needed to create new spaceflight capabilities. Areas near the lunar poles contain abundant water (billions of tons) and receive near-continuous sunlight. These properties permit sustained human presence on the Moon through the use of sunlight to generate electrical power and the harvesting of water ice to both support human life and make rocket propellant through electrolysis of water into liquid oxygen and hydrogen. The Moon’s surface materials provide feedstock for the production of metals, ceramics and useful objects, allowing us to construct new space systems derived from sources other than Earth, the deepest gravity well in the inner Solar System. The utilization of the Moon’s material and energy resources permit us to build a permanent, reusable and extensible space transportation system, a system that not only permits access to and from the lunar surface, but to all other points in cislunar space.
All of the Earth-orbiting satellite assets upon which modern technical civilization depends reside in the volume of space between Earth and Moon (cislunar space). Currently, custom designed satellites are launched on expendable vehicles, used for a time and eventually abandoned. Because our options are limited to satellites that can fit on the largest launch vehicles, and since we cannot get people and machines to high Earth orbit (e.g., MEO, GEO and the L-points) to build, service and repair space systems, our satellite assets are mass- and power-limited and thus, capability-limited. By creating a system that can routinely access cislunar space (using rocket fuel made from lunar polar water), we are able to transport people and equipment to any point to service existing assets and build new and powerful distributed systems. Once we break free from the expense and restrictions of hauling everything out of Earth’s gravity well, we become capability UN-limited in space.
To be politically sustainable over many years and decades, it is important to construct a program that is affordable and is seen to accomplish recognizable milestones at frequent intervals. One way to achieve this is to implement an architecture that consists of incremental steps, each one small enough to be affordable under reasonable funding scenarios, yet capable over time of being operated collectively as a large, distributed system. Because the Moon is close (round trip time for a radio signal is 3 seconds), we can control robots via teleoperation from Earth and prepare an outpost and manufacture rocket propellant and other useful materials prior to human arrival. People can then move into a turn-key outpost on the lunar surface, emplaced by robotic machines, with the ability to refuel a reusable lander for trips to and from the lunar surface. Such an architecture has been developed (see bibliography) with estimated costs that fit under a NASA budgetary envelope of less than one-half of one percent of the federal budget.
3. What are the ramifications and what would the nation and world lose if the United States terminated NASA’s human spaceflight program?
America is not the only entity interested in space. Other nations, corporations and non-government entities have already shown that they plan to be present in LEO, in cislunar space, and on the Moon. If the United States as a collective entity (i.e., the nation as represented by the federal government) is not present on the new frontier of space, what other country will promote, advance and protect our political and economic values in this area?
We depend upon a technical industrial infrastructure for our national economy and security. That industrial base has significantly deteriorated in the years since the end the Cold War. The great advances in consumer products in the last 20 years do not fully develop all of the technical capability needed for national security purposes and vice versa. A vigorous civil space program has proven to be an excellent means to develop and maintain this capability, one that we may need at any time on very short notice. Thus, civil space occupies a critical niche in the American national defense posture, regardless of our avowed peaceful intentions in space.
In an early speech defending the Apollo program, President John F. Kennedy laid out the reasons that America had to go the Moon. Among the many ideas that he articulated, one stood out. He said, “whatever men shall undertake, free men must fully share.” This was a classic expression of American exceptionalism, the idea that we explore new frontiers not to establish an empire, but to ensure that our political and economic system prevails (or at the least, is represented). That system has encouraged and defended basic human freedoms and put new wealth into the hands of the greatest number of people in the history of the world.
We make the Moon the first destination for humans beyond LEO because it has the material and energy resources needed to create a true space faring system. With both abundant water and energy available near the lunar poles, we return to the Moon to learn how to extract and use those resources to create a permanent transportation system, one that opens up cislunar space and allows routine access with machines and people. Such a system is the logical next step in space security and commerce. Cislunar development is fiscally prudent and ensures that our civil space program is relevant to important national interests of security, technology and economy, as well as advancing scientific understanding and knowledge.
Space should be more than a sanctuary for science and PR stunts. Space needs to be a true frontier, beaconing scientists and pilots as well as miners, technicians, construction workers, entrepreneurs and settlers. Decisions made now may decide humanity’s fate for generations.
Bibliography
A Rationale for Cislunar Space http://blogs.airspacemag.com/moon/2011/04/a-rationale-for-cislunar-space/
The New Space Race http://www.spaceref.com/news/viewnews.html?id=1376
Develop Cislunar Space Next http://spudislunarresources.nss.org/Bibliography/a/a40.pdf
Spudis testimony to House Space Subcommittee, May 21 2013 http://spudislunarresources.nss.org/Opinion_Editorial/testimony2013.pdf
Using the resources of the Moon to create a permanent, cislunar space faring system
As usual, well done, sir. I wish I could add something of value to this conversation, but the best I can do is spread the word on Facebook and Google+. You express all the things I have supported for years, yet I always felt like I was shouting into a hurricane and no one could hear me.
Gary,
Thanks. I know what you mean about shouting into a hurricane — now, there are two of us that can’t be heard.
What is unfortunate is that this logical path is avoided in favor of Dr. Aldrin’s more costly “plan” for Mars (which I’m sure is what Obama’s inspiration for his ridiculous notions).
Couldn’t the word be spread to others, so that we may eventually be heard over the storm?
I’m pretty sure Phil Plait would agree with these points that you make here, for starters.
By all means, please help spread the good word. I’m peddling as hard as I can.
Looking for reactions here….
Let me propose an alternative: The US should fund a much larger civil manned space program aimed at extending human societies into the solar system. This would include exploration, settllement, exploitation of non-terrestrial resources, major expansion of our technological capabilities, and economic development. Technologies highlighted for development would include terraforming, nuclear rocketry, space-based industrialization, and others. The goal, to be achieved before the end of the century, would be self-sustaining and self-governing colonies with tens of thousands of inhabitants on the Moon, Mars, and elsewhere in the solar system.
The suggested funding, at the federal level. is 1% of GNP — roughly 150 billion dollars at present, rising as the Anerican economy grows. This would be increased, of course, by commercial investments and parallel space spending by other nations. The sum is substantial but afffordable — 150 billion dollars is roughly one quarter of what the US currently spends on social security, roughly one quarter of medicare costs, roughly one fifth of military and homeland defense spending; it is about one sixteenth of our public and private spending for medical care. And while it is difficult to make precise comparisons, 1% of GNP appears to be in line historically with spending on exploration and colonization in classical Greek city-states and Elizabethan England.
The objection will be raised that space can be colonized at a slower pace, with far fewer people, for much smaller sums — that a more politically acceptable human space program can be implemented, if only the right selling points can be found, if only the boundless idealism of the voters is successfully stirred, if only the self-interest of motivated entrepeneurs can be provoked by the promise of future profit., Fifty years of history demonstrates these approaches don’t work.
The blunt truth is that colonizing the solar system is not going to be simple, easy, or cheap. We will need technologies and materials which do not yet exist. We may need social innovations which cannot yet be envisioned. We will need decades of time-consuming and expensive infrastructure building before humans will thrive in space — times and costs which would bankrupt most commercial firms and many nations.
And yet, these apparant obstacles are virtues. The US economy is growing sluggishly, workers’ wages have risen almost imperceptibly for fourty years, unemployment is at a far-from-optimal 7.5% even while the labor participation rate drops, and economists grumble about technological stagnation and lack of business investment, while corporations amass stockpiles of unspent wealth because they can find nothing profitable in which to invest. We have more college graduates than jobs for graduates, with fewer and fewer businesses willing to hire inexperienced graduates, with outsourcing and increasingly sophisticated computer software steadily winnowing the ranks of blue and white collar workers alike. And we are in better shape than most nations.
We need new industries. Not just Americans, really, but al of us — all nations, all creeds, all manner of men and women, all humanity. We need large new industries, spending and making trillions of dollars, providing worthwhile employment for millions of workers. We need new ideas and new machinery and new technological processes. We need room for expansion and places where people can build better lives for themselves and and opportunities for profit that transcend coupon clipping and rent seeking, and endless scope for the expansion of human intellecti and spiritual well being. We need a frontier for the brave and restless and ambitious, and it will benefit even those of us who stay at home and only dream.
It seems worth doing. Perhaps future historians will approve of us. Perhaps our posterity will. And perhaps we’ll come to appreciate what we’ve made possible outselves.
Mike,
Your vision of a greatly expanded effort in space is very attractive to many of us, but we live in a society in which collective decisions are made through the political process and there simply isn’t any inclination to spend much more on space than the amounts that we currently do. One could wish for more, but that’s not a likely success strategy.
In fact, we can begin to pursue this path without an extremely large influx of money. Seventeen billion dollars per year is not chicken-feed and one can craft a program that makes measurable progress (perhaps not a rate that we would ideally desire, but progress nonetheless) toward the goals you seek. As I state in my white paper, before we can settle space, we must be able to sail upon it. Let’s focus on developing that capability first. Settlement can (and in my opinion, will) come in its own good time.
Dr. Spudis,
Four out of five days, I’d agree with you. Today however ….
The Academy has raised the issue of what a desirable manned space program might be in normal circumstances — i.e., with a negligible federal deficit, with “normal” levels of investment and employment and inflation and other economic variables.
That isn’t the world in which we find ourselves. It isn’t a world we have seen for five years now, and economic predictions by Congressional Budget Office and other organizations suggest a return to normal conditions is unlikely for another four or five years — or longer. Some economists even express fears that we will NEVER return to the high growth rates and low unemployment of the late 1990s.
We’ve got an Economy problem, as well as a Spaceflight problem, I’m trying to say. Thus the notion of an ambitous space program which automatically alleviates some of those economic problems. It’s frequently argued that spaceflight must wait until ordinary people with no interest in it can see that it somehow provides a profit. Restoring a productive economy with low unemployment would seem to qualify.
That would be my major point in an essay. A second point I might raise (but didn’t in the first comment) is that most Americans would like to see market-based economic systems in future space settlements, but there’s little understanding of how such economic systems might develop. By themselves, corporations are not going to invest tens of billions of dollars per year on risky investments that will not begin to pay off for decades. We’re not going to make space settlement affordable until access cost is reduced by two or three orders of magnitude — and nobody’s likely to solve that problem until there are existing settlements which provide a market for cheaper methods of accessing space, whether those be better rockets or space elevators or Cavorite. A large federal space program could completely change this picture. Suddenly there would need for large industrial firms to manage billion dollar spacecraft production lines (we might even find ways to reward the companies which used their own money for this!). But there would also be scope for individuals or small businesses who grow vegetables on site for a buck a pound, rather than import them from earth at $20,000 per pound. As with railroads and highways, once we’ve built the connections, the entrepreneurs can appear, But government has to fill its role first.
So my second point is that a large space program is much likelier to lead to succesful colonies with thriving economies. Slow-but-steady settlement plans strike me as leading to the lunar equivalent of Antarctic bases — living conditions inprove with time, the number of year-round workers goes up bit by bit, but no one raises kids at the South Pole, no one retires there, no one becomes rich, and no one — yet — has written the Great Antarctic Novel. It’d be a real pity if we get to the end of the century and our notion of Luna City boils down to four Air Force officers and a dozen civilian contractors.
Thanks for your thoughts.
In effect, you are arguing for a civil space program as a gigantic federal economic “stimulus” project. Considering the success level of such efforts to date, I think that you might find that a hard sell.
More or less, though it would differ in important respects from what we saw in 2008-09. 150 billion bucks is much less than 800 billion, for example. On the other hand, I’m thinking of a spending program which would essentially run on forever, rather than being terminated after a few years of recovery. An even more important difference is that I’m not suggesting space as an immediate cure for our current “Great Recession” but for the long term malaise Tyler Cowen has dubbed The Great Stagnation. So I’d bill this as more of a 21st Century Hiighway Program than pure stimulus.
I concede it’d be a tough sell — the professional economists blogging on the ‘net are convincing too many folks in Congress or the White House on the value of their bright ideas either. But the idea does have some attractions, so it struck me as worthy of some exposure,
“The blunt truth is that colonizing the solar system is not going to be simple, easy, or cheap. We will need technologies and materials which do not yet exist.”
I agree there is no cheap or easy- but the rest I have to throw the flag on; we have everything we need to do it right now. IMO the path to accomplishing the first colony rests with DOD money for planetary protection and survival colonies. But Dr. Spudis and his telecom angle may work just as well or better.
By the way, you write really well Mike.
Mr. Spudis,
It’s always such an immense pleasure to read your blog and your opinions on spacy policy! So many times, it seems that you’re a voice of reason in the wilderness. I can’t overstate how much I agree with your views! Thank you, and keep up the good work!
With sincere and kind regards,
Leonidas
Leonidas,
Thank you for your kind words. I sometimes despair that I am simply wasting time repeating myself. I shall re-read your post in the future when I get that feeling again.
Good things need repeating. It is my understanding, that the return-to the-Moon-and-settlement-there mindset is shared by many (probably most) within the space community and also by Congress itself, because they all understand the value of this. So there’s maybe reason for someone to be cautiously optimistic.
Even with the decline of the Vision for Space Exploration (which was exactly the right kind of long-term space policy the US needed IMHO), Congress has shown constant bipartisan support through the years for the Moon and the development of cislunar space infrastructure, as shown in the NASA Authorisation Acts of 2005, 2008, 2010 and as it seems in the 2013 also. Here’s a draft of the proposed House version of the bill, I read about today, here:
http://www.spacenews.com/article/civil-space/35799draft-nasa-authorization-bill-nixes-asteroid-retrieval-mission#.UbwwV5zhfI4
The NASA administrators should take note of the fact that if even the NASA rank and file are opposed to the asteroid retrieval mission then something’s wrong.
Bob Clark
“So many times, it seems that you’re a voice of reason in the wilderness.”
The only person on this planet with a plan pointed in the right direction. In a sideways way it reminds me of the few voices who spoke out about commercial nuclear energy in the beginning; it was not the environment they were concerned about- it was proliferation. We know how that turned out- they were right.
We have a few scientists (actually only one in Dr. Spudis that has any exposure) saying the Moon is where we have to go next and not going amounts to a profound failure.
I hope it turns out different this time.
The nuclear aside comes from my belief that nuclear energy is to the space age as steam was to the industrial revolution. It might not be a good idea on Earth but space is made for it- and this connects to that capsule and escape tower on the SLS as the best way to transport fissionables to the Moon.
My thoughts also!
Nuclear is the way to go in space. Chemical rocket propulsion has reached its limits so many years ago, and despite what many are saying, I don’t think that chemical rockets can become much more cheaper and affordable than they are today, so that they can become ‘revolutionary’. Nuclear propulsion might not be needed for going to the Moon -it’s a nice 3-day ride with chemical propulsion already- but for interplanetary space, it will be inevitable in the long run.
And what better testbed for nuclear energy and propulsion than the Moon? And not going back to the Moon, saying ‘been there, done that’? I find that a tragic failure of gigantic proportions, on all levels.
The biggest obstacle of course towards nuclear propulsion isn’t technical, but political (oh, this dreaded ‘n’ word!).
“Nuclear propulsion might not be needed for going to the Moon -it’s a nice 3-day ride with chemical propulsion already-”
The only fissionables we want in the Earth’s magnetosphere are specially packaged to survive any mishap and on their way to the Moon.
Any nuclear acitivity near Earth is unacceptable- which is why the Moon is absolutely necessary as a staging area.
I think saying “nuclear” in regards to space is more acceptable now after that multi-megaton explosion over Russian not long ago. Even the most rabid Earth First tree-hugging fanatic would be happy to push the detonate button to avoid an extinction level event.
“I think saying “nuclear” in regards to space is more acceptable now after that multi-megaton explosion over Russian not long ago.”
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.
Excellent article and thanks for the interesting links to the Committee on Human Spaceflight papers!
Its obvious that America needs a water and fuel producing lunar outpost at one of the lunar poles! And its also obvious that this should be NASA’s top priority for a serious pioneering manned space program that intends to someday move humans permanently beyond the Earth and eventually even beyond cis-lunar space.
The Obama administration’s argument that any NASA effort to return humans to the Moon would hurt manned efforts to land people on Mars is ludicrous. The opposite its true since a water and fuel producing outpost within the low gravity well of the Moon would actually make it substantially easier to fuel ships for Mars while also providing mass shielding for such vessels for protection against cosmic radiation.
Fortunately, both Democrats and Republicans in Congress have pretty much ignored the Obama administration’s lack of vision and lack of interest in NASA’s manned space program by imposing their own manned program (SLS and MPCV).
Now its time for Congress to take the next logical step to get America back to the Moon by shifting manned spaceflight funds towards the development of a reusable single stage LOX/LH2 lunar lander. Full funding for the development of such a single stage lunar cargo and crew vehicle, IMO, should start in 2015 so that it can be ready by 2022 when the SLS is fully operational.
Marcel F. Williams
Thanks Marcel. By the way, I urge you and all other readers to prepare and submit their own responses to the NAS committee. The more people we have advocating the right path, the better chance we have of actually following it.
“-development of a reusable single stage LOX/LH2 lunar lander-”
Better make it a hypergolic lander designed to be modified to a LOX/LH2 lander later; cannot store cryogenics for the journey to the Moon- have to use storables to start with. It might sound doable to just accept some boil-off for the journey but the stuff does not store well and it is asking for trouble. As for “reusing” the lander- the only practical way to refuel and perform maintenance is in an underground lunar hangar. This does not rule out transferring storable propellent in space but it must be realized nitrogen tetroxide and whatever variant of hydrazine is no picnic either; extremely toxic.
The realization of a manned geostationary telecom platform may require hypergolic fuel to be sent to these stations from the Moon at intervals.
The whole concept of transferring fuel in space is really oversimplified to sway the public. It will cost a tremendous amount of time and money to make it work (if it can be made to work), will cause inevitable accidents, and was wisely avoided in past programs.
It can wait.
A cryogenic “lander” operated from an underground lunar hanger can launch and intercept payloads (or passengers) on the classic free return and bring them down to a Moon base in a short sortie. A little gravity and a underground tank facility on the Moon makes this possible; a cryogenic depot in space is IMO not practical.
The reusable LOX/LH2 shuttle that I envision would only have to store its cryogenic fuel for a maximum of a couple of weeks in order to avoid significant boil-off. But briefer turn arounds (only a few days on the lunar surface) could occur if a lunar shuttle originally fueled on Earth was sent to the lunar surface only to exchange crews for the lunar outpost.
Humans at a lunar outpost would then have to wait for the arrival of the next lunar shuttle and MPCV from Earth if they wanted to return to Earth. Both scenarios would mean throwing away the lunar shuttle after each mission since it would be left in lunar orbit, too deficient in remaining fuel to return to the lunar surface.
But once the outpost can produce its own fuel, then it can fuel the reusable lunar shuttles on the Moon for round trips to lunar orbit and back to the lunar surface to be refueled again. That would mean only the MPCV would have to be launched from Earth to lunar orbit– instead of the MPCV and a lunar shuttle– for crew exchanges. This should substantially reduce recurring cost.
However, the reductions in recurring cost will depend on how many times the lunar shuttle can be reused which– I would guess– would be at least ten times before its RL-10 derived engines would have to be replaced. Dr. Spudis and Lavoie has proposed conveniently replacing lunar shuttle engines with engines removed from unmanned lunar heavy cargo missions. After engine replacement, the lunar shuttle can perhaps be used another ten times before the vehicle itself would have to be replaced by a brand new lunar shuttle launched from Earth.
Marcel F. Williams
Paul,
Your presentation was a good representation of your (and if I may be allowed to say it for a number of us who post here) and our position.
I went to the Academy studies link and looked through several (not all) of the other listed postings and noted a number of similarities (again this is based on looking at a randomly selected sample):
– All of those presentations seem to be supporting the current “plan” (i.e. Asteroid then Mars).
– All of those presentations seem content light compared to yours
– All of those presentations seem elaborate GGI illustration heavy compared to yours (lots of pretty pictures and many of those GGI illustrations seem to have come from internal NASA reviews I have previously seen).
Not sure how the final Academy recommendation will come out, but it is a shame the Academy did not seem allow a voting block for non-presenters to show how they feel about the various concepts presented.
Joe,
Having been previously involved in National Academy studies, I know that submissions from outside the committee hold little weight and I expect that this effort will be no different. I noted a similar attitude from the 2009 Augustine committee, who had apparently decided what their conclusions were going to be well in advance of receiving outside input.
However, I wanted at least to get on the record that not everyone in the space business has a) lost their minds; or b) fallen for the snake-oil fantasies of the New Space fanboys.
“I noted a similar attitude from the 2009 Augustine committee, who had apparently decided what their conclusions were going to be well in advance of receiving outside input.”
Yes, while I was not directly involved I noted the same thing, Sigh.
“However, I wanted at least to get on the record that not everyone in the space business has a) lost their minds; or b) fallen for the snake-oil fantasies of the New Space fanboys.”
Good idea. On a more positive note:
http://www.spacenews.com/article/civil-space/35799draft-nasa-authorization-bill-nixes-asteroid-retrieval-mission#.Ubzzt6Eo7IW
The Congress appears poised to scuttle the Asteroid Retrieval Mission and reassert a Return to the Moon (and a backup to commercial crew for ISS Support) as the primary goal for the program:
“Continued commitment to develop the Space Launch System and Orion Crew Vehicle to return to the Moon and beyond, but no funding for an asteroid rendezvous mission. Reiterates Congressional direction that Orion be a backup system to support the Space Station if necessary.”
“-Congress appears poised to scuttle the Asteroid Retrieval Mission and reassert a Return to the Moon-”
We have an HLV and a capsule with an escape tower to put on top of the stack- but we need a lander. That is a problem because of the time required to get one in production.
Maybe the Chinese could build one for us faster; they certainly have their hypergolic technology working for them. I do not favor the Russians for this for a very simple reason; about 1 out of 4 rubles paid to their space program goes to the Russian Mafia.
One option might be like the original Apollo lander concept which had the reentry capsule mounted on the lander. The Chinese might be able to build the lander for us and we could start using it to land cargo (and rovers) almost immediately and then have it land the Orion capsule separate from the service module as soon as that could be worked out.
It just goes to show you, what a huge waste it was to throw away all the Saturn V/Apollo infrastructure in the ’70s, Now you have to build it from scratch for the second time.
As for the Chinese taking a part in all of this-not likely. As the political climate stands today, I think that the Chinese and US space programs will just follow parallel, separate courses for many years to come.
And even though I aknowledge the terrific international cooperation achieved with the ISS, I find the recent attitude of the Russians offensive and it makes me to dislike them a lot. Even though the Cold War is decades behind us, not a moment after the US retires the Shuttle, and the Russians turn to their usual propaganda of how the ‘Soyuz era’ is rising, leaving the era of the ‘untrustworthy’ Shuttle behind, and they go on rising their prices arduously for leasing Soyuz seats to the Americans all the time. With friends like these, who needs enemies?
http://www.nbcnews.com/technology/google-begins-launching-internet-beaming-balloons-6C10331940
The future of telecommunications- which is intertwined with human space flight- may move from geostationary satellites to high altitude airships. Far cheaper than rockets. That might be the end of any kind of space flight. While the line of sight is much shorter than space, it would still allow a certain number of airships hovering at around 100,000 feet to effect a global network.
At the other extreme is the manned geostationary telecom platform- launched from the Moon. This would use Lunar Resources to fill a spacestation partially with water and thus shield a crew of technicians from radiation in geostationary orbit. In this scenario a small number of these thousand plus ton stations could practically run the planet. As Dr. Spudis inferred, we might not even need cell towers anymore.
If I was not a space advocate and regarded space as critical to the continuation of our species then I might just say let the dream end. Especially if there was alot of money to be made.
But being able to deflect asteroids and comets and having an isolated population invulnerable to infection is my goal- not making money off cell phones. And I think the DOD has the money to make it happen. Doing this and building pseudo-spaceships that can run the world from orbit- and also dock with propulsion modules and head outbound- is all things to all people. That is what I would write a “white paper” on.
The Moon is where it can all happen- and it will take human beings there to make it so. I just finished listening to Buzz Aldrin on public radio talking about peeing on the Moon and supporting space tourism. Then they asked his opinion on “mooning.” This is how far we have fallen.
“-recurring cost will depend on how many times the lunar shuttle can be reused which– I would guess– would be at least ten times before its RL-10 derived engines would have to be replaced. Dr. Spudis and Lavoie has proposed conveniently replacing lunar shuttle engines with engines removed from unmanned lunar heavy cargo missions. After engine replacement, the lunar shuttle can perhaps be used another ten times before the vehicle itself would have to be replaced by a brand new lunar shuttle launched from Earth.”
I worked on and flew on all weather rescue helicopters for a decade; perhaps a few things in common with the mythical machine we are pontificating on. I have also read some on the original lander. The ascent engine- the really important part of the module- was a pressure fed hypergolic with an ablative thrust bell. It was as simple as it could possibly be made and Armstrong wanted the very basic electrical system replaced with a mechanical manual firing lever- but was overuled for some reason. IMO they should have listened to the guy that would have to fly it.
When you start talking about variable thrust turbo-pump driven cryogenic rocket engines with all the bells and whistles you are talking about something very different. And when you start talking about reusing it you are talking about yet another level of complexity. And when you talk about replacing it on the Moon…….
Before I was in aviation I was in armor- and I changed a few tank powerpacks (engine/transmission units) on the sides of mountains in Korea in the middle of the night with the temp around zero and the wind-a-howling. In Alaska we did not even try to work on helicopters outside- if you have ever been to Alaska you would know why.
Nothing compared to the difficulty of working on the surface of the Moon. If a helicopter goes down it can auto-rotate (unless it is a V-22) and you can do a survivable controlled crash. There is no controlled crash in a vacuum. There is no surviving most equipment failures in space. It is IMO impossible to do the things you are matter of factly discussing.
What is needed is an underground hangar- heated and pressurized. That is where you can-very carefully- refuel, and perform maintenance and do engine changes or whatever you want. It aint gonna happen any other way. So until that underground base is operational it is going to have to be KISS.
When you start talking about variable thrust turbo-pump driven cryogenic rocket engines with all the bells and whistles you are talking about something very different. And when you start talking about reusing it you are talking about yet another level of complexity. And when you talk about replacing it on the Moon…….
If any of this were easy, it would have already been done.
None of what Marcel proposes violates physics and it is the direction in which we need to move anyway. Yes, boil-off is a problem with low temperature cryogens, but it’s one that needs to be addressed and solved, so why not get on with it?
I reiterate: if we continue to rely on everything brought up into space being launched from Earth’s surface, we are mass- and power-limited in space and thus, forever capability limited. Learning new skills (e.g., ISRU, cryo depots, engine change-out) is critical. In fact, it IS the mission on the Moon.
“-why not get on with it?”
Because what he envisions will not work Dr. Spudis. To reuse or refuel or change engines the first requirement is a place to work outside of a spacesuit- and considering the radiation problem that means underground- or at least in a shielded area like a crater with a water roof.
It may work out using liquid hydrogen and liquid oxygen eventually- or it may be that some form of storable produced from local resources will be the standard. But before any trials begin the first requirement is a place where people can work safely and comfortably. To get there is going to require what we know works; an expendable hypergolic lander. Repurposing these lander components on site is one thing- but expecting them to do more than land and take-off once to start with is a big mistake IMO.
Start with cargo landers that land once. Then landers that land and take-off once. When a basic facility to “turn it around” is available then a lander that can be fueled and fly up and bring things down a certain number of times.
When there is a large base with everything needed THEN it is time to start playing with liquid hydrogen and oxygen and high performance engines.
A high performance lunar shuttle would be a big enabler for making a Moon base self-sufficient by bringing down all the hardware required to get the chemical plants and foundries and machine shops and everything else going- but not if it turns out like the other high performance shuttle we all know about.
I reiterate: storable propellants manufactured from local resources may work well and cryogenics may not. In either case starting with cryogenics is not practical IMO.
Von Braun and Silverstein had disagreements over propellants as an excerpt from this NASA history document show.
“He was convinced that the use of LH2 in the upper Saturn stages was inherently sound, and his conviction was the major factor in swaying the whole committee, von Braun included, to accept LH2 boosters in the Saturn program. “Abe was on solid ground,” von Braun acknowledged later, “when he succeeded in persuading his committee to swallow its scruples about the risks of the new fuel.”47
An infinite budget and vast resources may have been on Abe’s side but it is not on Marcel’s in this argument Dr. Spudis.
Because what he envisions will not work
In your opinion. I have discussed this with several other engineers and they are of a different opinion. My point is that we are compelled by necessity to try to make it work. You cannot make make storable propellants (easily) from lunar reosurces and they are much less energetic than cryogens anyway.
Then I am also compelled by necessity and have changed my position on this.
LOX/LH2 it is.
Just to add a few details about the background for discussion of the proposed SSTO Lunar Vehicle.
– It’s engines would be direct derivatives of the RL-10 5A’s developed for the DC-x demonstrator vehicle for the Delta Clipper project some 20 years ago (the same engines were to be used for the descent stage of the Altair lander in Project Constellation).
– The DC-X flew with those engines (and they were reused) through some very challenging flight profiles.
– The ground crew to do the turnarounds (and it sometimes flew multiple times in the same day) was 12.
– While the Delta Clipper concept was controversial as to whether an Earth Surface to LEO SSTO was practical (bigger engines would have been required for the actual vehicle) I know of no one who does not believe that the RL-10 can be used for a Lunar Surface to orbit SSTO.
The concept has a lot of history (including practical testing) to support it.
If that is the path that will be taken then I am compelled to support it out of necessity. Your plan is the only game in town. I guess I will have to change my position (ouch!).
I would advise keeping an open mind though. I have seen engineers screw up some stuff royally and try to make obviously bad ideas work. I can give you a long list of examples if you wish.
There is also the option of easing the difficulties and accepting some lower ISP numbers by going with an alternate to hydrogen such as methane- which I believe the LR-10 has been tested with. Can methane be produced more easily than storables from lunar resources? I am not trying to demonize LH2; I love the stuff and there is no substitute in terms of performance- but it also has exotic properties that make it difficult to work with. Just saying “it won’t be easy but we have to do it” does not change that or lower the risk of failure.
The 12 man turnaround crew sounds pretty standard for a large aircraft. The point I am trying to make is that pre-flights and thru-flights and post-flights and scheduled maintenance- both calender and hourly- are what make things fly and not crash. You cannot do it in a spacesuit. You cannot change an engine in a spacesuit. You cannot keep a crew working all day in a radiation bath.
I do not want to write a book here but I have to make this case and the only way is with examples; Do you wonder why we lost so many troops to IED’s? You can take an obsolete tank model and strip the turret and turn it into a personell carrier and you have a 50 ton troop transport pretty much invulnerable to any IED. Tens of thousands of older tanks around the world sitting in tank parks that could be bought cheap. Why did’nt we do it? Why did we use vulnerable light wheeled vehicles? Maintenance. Tanks require a huge amount of back breaking labor to keep running because the tracks must be frequently torn apart and the transmissions wear out quick. It is why most tankers look like weight-lifters. They drink fuel gallons to the mile and tear up roads. But the Israelis have been using converted tanks for years because of IEDs. The point being you have to accept that going cheap costs lives. The powers that be decided that thousands of converted tanks roaring around Iraq as troop transports was not feasible. In the case we are discussing it is the expensive maintenance requirements that are really being discussed and those requirements start with a place to work that is not an irradiated vacuum.
I appreciate the LR-10 history. I cannot stress how important the basic reliability and maintainability of an engine is to keeping operations going. I have seen some real crummy engines in my time- and worked on one really good one; the General Electric T-700 turboshaft was a revelation to anyone who worked on it after junk like the Lycoming LTS-101 or the Garret ATF turbofan.
It is hard for upper echelon managers to wrap their head around but the success or failure of a program can depend on the orignial selection of any one component- once your machine is built it is too late to do anything about it- you failed long before when you made the choice and did not even know it. Managers accept what is and try and get the mission done with what they are given. But success or failure was often decided before they ever took on the job.
I hope the RL-10 is all you think it is. How exactly are you going to refuel this lander?
I was just thinking about the low ground crew numbers for the DC-X just the other day!
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
“Because what he envisions will not work Dr. Spudis. To reuse or refuel or change engines the first requirement is a place to work outside of a spacesuit- and considering the radiation problem that means underground- or at least in a shielded area like a crater with a water roof.It may work out using liquid hydrogen and liquid oxygen eventually- or it may be that some form of storable produced from local resources will be the standard. But before any trials begin the first requirement is a place where people can work safely and comfortably. ”
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
1) “-are investing an enormous amount of resources developing the cryogenic refueling of
space craft-”
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.
“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.”
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
Thanks for providing the info Marcel. I took a quick look.
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.
“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.”
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.
Here’s some more source material for you and others who want to learn more about regolith and water shielding:
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