With a practically infinite amount of regolith lying around on the martian surface, it should be pretty easy to shield habitats deployed on Mars to Earth radiation standards.

For interplanetary journeys, however, I would dump the water shields for the twin artificial gravity habitats before the final trajectory burns into orbit, which would substantially lower fuel requirements.

Once in high Mars orbit, a pre-deployed propellant producing water depot (filled with water originating from the Moon) would supply the interplanetary vehicle with both water shielding and LOX/LH2 for the return trip to cis-lunar space. Eventually, the martian moons could be the source of water for return trips to cis-lunar space.

For interplanetary missions requiring several months aboard ship, I think it would be essential for the psychological health of the crew for the habitat modules to be reasonably comfortable and spacious. That’s why I propose using the– uber lite– habitats derived from SLS propellant tank technology.

SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots

http://newpapyrusmagazine.blogspot.com/2014/05/sls-fuel-tank-derived-artificial.html

Utilizing Lunar Water Resources for Human Voyages to Mars

http://newpapyrusmagazine.blogspot.com/2015/01/utilizing-lunar-water-resources-for.html

What you don’t understand is that young females will be at greater risk than middle age males no matter how much you play with “adjusting” their exposure. There is a reason for that “legal” exposure limit. Playing the exposure game is a dead end. It is ethically flawed that astronauts have to volunteer for missions that permanently damage their health and will kill a certain percentage of them with cancer. You have to realize this is a part of Human Space Flight that is not adjustable. Standards for workers on Earth are not based on an adjustable death rate for certain occupations. Legally and ethically there is no solution except providing a base line Earth environment. Astronauts are radiation workers. NASA, by setting exposure limits so far above what is allowed on Earth, is setting up Human Space Flight for failure. It is bad enough that leaving Earth orbit means an unavoidable radiation bath going through the Van Allen belts. The rest IS avoidable and trading off lifespan for less expensive spacecraft is THE showstopper.

If human beings are going to go into space for years at a time the “Parker minimum” is the reality and “adjusting exposure” is the showstopper. It is happening- objections to exposing astronauts to these health hazards without providing them lifetime medical care and several flat pronouncements that radiation makes deep space missions impossible.

Near sea-level radiation and Earth gravity, the “Parker minimum” is the prerequisite that will enable enable long duration human space flight beyond low Earth orbit. Failing to provide this minimum environment means no human space flight. There is no cheap.

But I would add an additional rule that would limit exposure during an entire round trip mission to less than 50 Rem so that a young female astronaut could make an additional deep space voyage during her career. One trip to Mars is like limiting Columbus to one trip to the Americas– even though he thought he was in South Asia:-)

30 centimeters of water should be enough to protect astronauts from heavy nuclei and major solar events during an interplanetary journey while also exposing them to less than 25 Rem per year of cosmic radiation. This is about 20 centimeters less than what I usually advocate. But 18 months (9 months to Mars and 9 months back) through interplanetary space with a 30 centimeter water shield would still only expose astronauts to about 38 Rem during solar minimum conditions (the worse conditions).

Since astronauts would probably reside in regolith shielded habitats on the martian surface that would probably expose them to less than 5 Rem per year during solar minimum conditions, less than 10 Rem of additional radiation exposure would be added to the mission. But that would still expose astronauts to less than 50 Rem in total during the entire mission. Working and exploring on the martian surface for a few hours every week would only add a couple of Rem exposure during the mission.

Marcel

My thinking on bomb propulsion concerning this has evolved over the last five years or so from the firm belief that only a metal disk massing several thousand tons will work to the reality that a “soft” interim will be necessary. I imagine this craft would begin it’s journey from a lunar “frozen” polar orbit after filling up it’s radiation shield with water.

The Medusa concept, which uses a several thousand foot diameter parachute and utilizes kilometer long spooled cables to reduce acceleration, is the most likely candidate for a interplanetary spacecraft that could be available in the next quarter century. There is also the option of lifting a metal plate up a slice at a time and welding it together in space. I am guessing the “slice at a time” would require a much larger launch vehicle than SLS. Perhaps both methods will be used while developing the lunar manufacturing capability to produce the really huge plates necessary for efficient pulse propulsion.

I also doubt the first true spaceship mission outbound would be going to Mars. Ceres is a better destination. It can land and utilize the ice and explore the possible ocean with mini-subs.

Presently there is only one revenue producing space industry: satellites. In my view the only construct sharing all the available technology and resources is that classic icon of the 50’s, the von Braun wheel. Assembling wheels in lunar orbit out of wet workshops and filling them with water lifted from polar ice deposits, then transiting them back to GEO, is my solution. I am a troubleshooter by trade and the flow chart says this is the only fix.

The 100 billion dollar plus yearly revenues from the satellite industry can be captured by replacing the present GEO junkyard with far more efficient and capable human crewed space stations. But nobody is saying a word about this either.

Because it is a politically incorrect subject nobody will touch it- so the only thing left to discuss is manufacturing rocket propellent from lunar resources. It is a confusing and ultimately self-defeating situation. It seems even more hopeless when a further extremely unpopular fact comes to light: there is only one form of nuclear propulsion that will work- bombs. Go ahead and eye-roll, I know you cannot help it. But as someone once said, “the universe is not only queerer than we suppose….”

It is not that manufacturing this propellent is pointless; it is only pointless for travel beyond cislunar space. For operations involving transits from GEO to the Moon and back it is of course the only way to create any infrastructure. But since Mars is the present public relations hook, the “horizon goal”, lunar resources must be talked about in a Mars context. Since a cislunar infrastructure would be required to build the atomic spaceships capable of interplanetary travel then it is not exactly a deception- more of a facade.

Square one is radiation. It is why the Human in Human Space Flight comes first. Slowly but surely it is being admitted that massive shielding is required for long duration missions. Dragging space advocates kicking and screaming to this door still does not mean they will open it. The NewSpace philosophy is directly opposed to what such massive shielding entails- equally massive state-run projects. But it is unavoidable and makes the previously stated inconvenient nuclear truth unavoidable. Only nuclear energy can push a cosmic ray water shield massing in the neighborhood of a thousand tons around the solar system. This is a conservative figure considering it takes 500 tons to shield a small capsule. For multi-year missions a great deal more space will be needed to prevent psychotic astronauts.

Space radiation and the consequential requirement for nuclear propulsion inescapably takes us to the Moon. The Moon is no distraction, it is the prerequisite. A thousand tons of tap water is not going to be lifted out of the gravity well of Earth. Nuclear devices are not going to be tested anywhere in the magnetosphere.

But nobody is saying a word about any of this in the media. They would be crucified.

And you’re still going to need a substantial amount of water (probably over 150 tonnes) for drinking, eating, washing, the production of air, and for radiation mass shielding. Supplying that water from the Moon’s low gravity well would be a lot cheaper than supplying that from the Earth– especially if the vehicle is used again for future crewed missions.

Marcel

At least, it keeps me from having to say the same thing.

Very well said Dr. Spudis! And a great article in general.

My preference for using liquid hydrogen over liquid methane for reusable propellant depots, interplanetary rockets, and reusable shuttles are as follows:

1. Hydrogen and oxygen propellant can almost indefinitely be stored as water at orbiting propellant depots. When LOX and LH2 are needed to fuel an interplanetary journey, the propellant can be produced through solar or nuclear powered electrolysis and cryocooling just a few weeks or months ahead of time.

2. Solar powered cryocoolers can also be used to re-liquify ullage gasses on LOX/LH2 interplanetary orbital transfer vehicles for zero boil-off of hydrogen and oxygen.

3. Propellant manufacturing– water depots– can also provide the habit modules of interplanetary vehicles with water for drinking, washing, oxygen production and for mass shielding the habitat from deleterious levels of cosmic radiation and major solar events.

While liquid methane has the advantages of smaller tank volume and reduced boil-off relative to liquid hydrogen– it also has the disadvantages– of having a much lower specific impulse plus the need for an additional (none methane) ignition source of fuel in order to start the engine.

Even on the surface of Mars, it would be simpler to manufacture and fuel a shuttle vehicle with LOX/LH2 from martian sources of water rather than trying to produce methane and oxygen from CO2 and martian water– plus the ignition fuel.

A methane depot program would still require the deployment of a water depot– possibly even for the production of oxygen. But a propellant (LOX/LH2) manufacturing water depot, on the other hand, would only be one simple solar or nuclear powered machine that could do it all!

Marcel