Mars Direct Rethought - Fixing the potholes in Zubrin's plan

turbo · 2002-08-21 09:12:37

About the greenhouse floor, does that have to be inflatable as well? I'm thinking of the polypropelyne (spell?) plastic shipping pallets used by the US Postal Service for air cargo transport. The pallets are light, sturdy, already manufactured in quantity. Find some air cargo agents around the Winnepeg airport and they should be able to show you one of these things (they end up everywhere).

Pallets laid out for the floor size desired, some "foamcore" (thin styrofoam sheets with paper coverings similar to gypsum-board aka drywall) for insulation, and maybe some vinyl floor runners inside for better traction. The inflatable portions I could only guess at how to make.

From Florida,
turbo

Bill White · 2002-08-21 09:38:25

So who is interested?

I am very interested yet I lack any of the scientific qualifications now needed. Perhaps I could assist with marketing and distribution once a project gets going.

*IF* a viable Mars soil simulant is created and produce grown in that simulant, such produce could be sold at a premium similiar to the "certified organic" marketing label.

Grow grapes or olives and then make wine or olive oil to sell under a "certified Mars" label.

This might help raise funds to build a test bed greenhouse.

Phobos · 2002-08-21 21:34:03

White LEDs:
I'm in favour of using a transparent inflatable greenhouse. That is far simpler than any mirror or electrically illuminated design. As for light details, I'll let the agronomists argue.

All of these methods have their pros and cons and it might turn out best to use them in various combinations. One of the positive features about mirrors is that they require no powersource other than natural light from the sun and I think if we plan to grow huge fields of crops sometime in the future on Mars it will turn out advantageous to use mirrors since plants are very power hungry critters. I think it all boils down to a matter of scale, it might be best to use white LEDs on a small scale and mirrors on a big scale or a combination of the two.

RobS · 2002-08-22 09:57:12

Robert, it occurs to me that Winnepeg in winter may very well get about the same amount of sunlight as the Martian equator. Have you looked up that information somewhere?

As for mirrors, the design I have in mind (I am no engineer) is extremely simple. If a hemicylinder is laid on the ground with its axis north-south, one curved side faces the east and one faces the west. Many commercial greenhouses are hemicylinders already. Each curved surface is a quarter of a circle. Cover the outside of the greenhouse with a thermal blanket that has a silvered inside. In the morning, remove the thermal blanket covering the east side of the greenhouse. Sunlight enters and hits the plants; the light that passes over the heads of the plants hits the silvered blanket covering the western wall and is reflected not only eastward but also downward (because of the wall's curvature) by the silvered surface. This gives the plants a source of illumination from the west as well as from the east, and the thermal blanket covering the western side reduces heat loss. In the Canadian prairies the wind is often out of the west, so reducing morning heat loss helps get the interior temperature up from its dawn low.

In the afternoon the process is reversed, with the sun shining in the western side and reflecting off a thermal blanket covering the circular eastern side.

On Mars, a simple system of electric motors could raise or lower a silvered thermal blanket. In Manitoba that might not work because of ice and snow buildup, and it would not be practical for schools to raise and lower the insulation by hand all the time. But it would be a useful experiment in improving greenhouse functioning.

You may want to talk to professional greenhouse operators and get some idea of how much heat the sun makes inside a greenhouse. My impression is that an airtight greenhouse can easily be 40 degrees Fahrenheit above the outside temperature. This impression comes from walking inside a greenhouse on a chilly March day; they had the exterior exhaust fan on to pull out the hot air.

This is a fascinating experiment and a very clever idea; it could be good for the Mars Society as well, since it has obvious terrestrial applications. Good luck with it!

-- RobS

Nirgal82 · 2002-08-22 10:02:38

Well mirrors require no power source other than that is used in their construction. Orbital mirrors will be very expensive...
And if you are talking smaller scale mirrors, you need a power source to swivel them around the greenhouse for maximum insolation. Unless it turns out to be cheaper and none to much trouble to produce more mirrors and just to send out a couple of people to rearrange them during the course of the day, however that is a chore that is bound to cut into other things over the course of an M-year...

But back to the main issue. Other potholes, heh
What about that tether that is to be used to spin the Hab?
Whats it made out of, and more importantly, how feasible is it?

Secondly, I'd like to start discussion here on what types of experiments and other "things to do" can we pack into the Hab. All the equipment for such things is bound to weigh alot, and the hab has serious space restrictions.
Maybe some redesign to the Hab can be made, if only we knew what rocket would eventually lift it up there.

Lastly what is the current Hab/aerosheild weight limit?

your friendly neighborhood Martian...
-Matt

RobertDyck · 2002-08-22 15:33:01

I did a rough calculation of sunshine to find latitude equivalent to Mars. If you ignore absorption by atmosphere, the calculation is simple trigonometry. Mars is an average of 227,940,000 km from the sun, and Earth is an average of 149,600,000 km away. Using the inverse square rule, light intensity per unit area on Mars will be 43.075% that of Earth. Taking the arccosine will give you 64.485 degrees, so Fairbanks Alaska during the spring or autumn equinox on a clear day would have equivalent light intensity as the Mars equator on its equinox. At the summer solstice add the Earth's tilt of its axis (23.4?) to the latitude to get 87.9?; Devon Island is 75?22'N so it should get an equivalent light level on May 10 and August 2. In winter subtract tilt to get 41.1?. Winnipeg is at 49?54'N latitude, so it should get an equivalent light level on October 26 and February 14.

You're right; Winnipeg in winter is a good equivalent to Mars.

RobS · 2002-08-23 10:16:24

I just ran across a very interesting study about moon and Mars exploration from the International Astronautics Society (based in Paris). The 101-page report can be seen in pdf format at http://www.iaanet.org/p_papers/cs52.pdf.

The report analyses moon and Mars programs with an eye for identifying common elements and developmental programs. It looks at five Mars scenarios: a single mission to Mars, multiple missions, an outpost (6-12 people, permanent), a laboratory (24 people), and a base (100 people). It looks at a similar range of lunar scenarios: temporary lunar outpost, permanent lunar outpost, lunar laboratory, lunar factory, and lunar settlement. It concludes that the most cost effective combination is a lunar laboratory and a Martian outpost (at least over 30 years; over a longer term, one can expect both to grow). It projects a cost for the combination of about $80 billion, comparable to the cost of the International Space Station. Either project alone is cheaper, but the combination is about $3 billion cheaper than their combined costs and generates far more benefits, so on a cost/benefit ratio they favor the combination.

The report has a lot of features of interest to Mars Society members. If you are curious about alternatives to Mars Direct, it is an example of one. It is weaker than Mars Direct in terms of diagrams and engineering information, but is stronger in terms of breakdown down cost information: there is a chart for each facility showing how much it costs to develop and build reactors, rovers, habs, life support, science equipment, etc., how much each weighs, how many are needed, the total mass needed, and the total cost. It projects the masses for the lunar facility for up to 2,500 people, so you can get some idea of the costs and masses involved for Mars.

On the other hand, the authors were quite conservative, in terms of technology. I felt the ghost of Werner von Braun working behind the scenes. Initial flights to Mars would be zero-gee; later there would be 1/3 gee after the midcourse correction; there would be zero gee coming home. The first Mars flights would use solar power on the Martian surface. There would be no in-situ resource utilization for making fuel until later flights, which means the first flight has to be very big and heavy (an advantage: heat shields and landing equipment developed for the landing of a fully-fueled Mars ascent vehicle are later used for cargo flights to the surface). The mission involves three vehicles instead of two: a hab, a Mars ascent vehicle that goes to Mars orbit only, and an Earth return vehicle that stays in orbit until needed to take the astronauts home. (Another advantage of sorts: the MAV can be upgraded eventually to become a reusable Mars shuttle. This is not so easy for the Mars Direct ERV, which is a two-stage vehicle.) The entire flight profile involves chemical propellants (hydrogen-oxygen for the Earth and moon; methane-oxygen for Mars); no nuclear, solar-thermal, or solar-electric options are included. Most surprising of all, the entire scenario depends on spending $17 billion to develop a gigantic, 6,000 ton reusable heavy lift vehicle capable of putting 375 tons in low earth orbit. It would have fly-back first and second stages. This is about twice the launch weight and capacity of a Saturn V. If one of them exploded on the launch pad, it would probably break all the windows out to Orlando. I am not sure, but it would have a power getting close to the Hiroshima bomb, it seems to me.

The masses of the various components struck me as very conservative as well. Eventually Mars would get four 160-kilowatt nuclear reactors weighing 12 tons each (the Mars Direct 100 kilowatt reactors weigh 3.5 tonnes; the Duke et all plan to make hydrogen-oxygen fuel on the moon would use a 25 kilowatt reactor weighing 1 tonne). This seems like old calculations to me; indeed, new thermionic conversion appears to be 15% efficient instead of 5%, so if it can be developed for reactors, the Mars Direct 3.5 tonne reactor would make 300 kilowatts instead.

I was particularly surprised to see lip service paid to the ices at the lunar south pole; the plan called for putting an oxygen-making unit on the moon right away which would extract the oxygen from lunar rocks, and postponed the harvesting of lunar ice for later. This strikes me as crazy; everything I’ve read suggests that extracting oxygen from the lunar rocks takes a lot of energy and is complex. They could save a few hundred million putting their moon base at the lunar south pole, using the near-perpetual sunlight, and perfecting techniques to exploit lunar ice.

The report also virtually ignores Phobos and Deimos, even though they might save billions of dollars as sources of fuel in Mars orbit. The report acknowledges the value of doing science on them, but gives no details.

On the other hand, the report answers a few questions I’ve seen people ask on this list. It gives some reasons for doing the moon and Mars, rather than bypassing the former in favor of the latter. Right now space suits at the International Space Station are designed to be used only five times; it is easier to haul up new suits than design longer lasting ones. A moon base would need longer lasting suits, just like Mars. The isolation of the moon forces the creation of longer lasting, more redundant, systems; right now the International Space Station is designed so that in a disaster allows it can be abandoned in a matter of minutes (which is why it has only three occupants; a Soyuz lifeboat can’t hold any more than that). Some tasks, such as long-term recycling of waste products, surface exploration, utilization of regolith ices, manufacture of concrete, plastic, and metals, gravitied greenhouses, long-term maintenance, and testing multiple-use vehicles, can be done on the moon quite well and would develop useful experience for Mars.

An interesting aspect of the report is its ability to categorize and quantify benefits to life on Earth for the combined moon-Mars mission, even in “soft” areas like improving quality of life. It used a technique involving surveying the public. The report also calls for establishment of an international moon-Mars organization for the effort.

The report also gives a glimpse “beyond" Mars Direct. It envisions a reusable transportation system (though no Cyclers). It talks about 10,000 kilometers of traverses across Mars. It talks about the development of low earth orbit facilities through use of lunar materials and even some Mars materials (though not much, and the kinds are not specified).

The timetable is interesting as well. It would get the ball rolling in 2005, when the International Space Station is finished; politically one could not get it rolling any earlier anyway. It envisions the first moon landing about ten years later (as early as 2012 would be possible) and the first Mars landing in the mid 2020s (though 2018 would be possible). The $70 billion spent over 30 years would involve peak spending of about $7 or $8 billion per year in the early teens, then the maintenance and slow expansion of the facilities would run about $2 billion per year, total. That’s really not bad!

Anyway, take a look. It offers much food for thought.

-- RobS

RobertDyck · 2002-08-23 11:30:32

I haven't read the article yet, but exploiting lunar ice for rocket fuel is not a good idea. Yes, extracting oxygen from lunar rocks is complex and energy intensive, but there just isn't much water ice on the Moon. If you want to build a colony or just a permanent base on the Moon, water ice has to be reserved for recycling applications such as a greenhouse. While Lunar Prospector was sending back results, NASA reported water ice at the bottom of plar craters that was only 1 cup over an area the size of a football field. And this is the dense concentration. It would be a crime to waste what little lunar water exists on rocket fuel.

Phobos · 2002-08-23 19:25:15

It projects a cost for the combination of about $80 billion, comparable to the cost of the International Space Station. Either project alone is cheaper, but the combination is about $3 billion cheaper than their combined costs and generates far more benefits, so on a cost/benefit ratio they favor the combination.

Damn Rob S, you've left me speechless with such encouraging news! The ISS is a boring and barely productive piece of crap. I think we should deorbit it and start sinking some money into these other projects. The public could get a lot more excited over the prospects of a Moon base than the ISS especially if the maintenance costs are similiar! If those numbers hold up (they do seem a bit optimistic to me but what do I know) it'd be a real tragedy to keep sinking money into the ISS. A smaller and cheaper spacestation would probably do just fine for zero-grav research.

Merp · 2002-08-23 23:57:53

Hi all. This is my first post... I thought I would cut my teeth replying to Matt

Firstly, Zubrin clearly states that a tether be used to connect the hab/aeroshell combo to the spent rocket stage that pushed the whole thing on the way to Mars.
What is this tether going to be made of?
I've heard that tethers have problems with oscillations.
Can mid-course corrections be made using precision computer controlled propellant firings?

You could use carbon nanotubes as the tether material. They are mentioned in a recent Space.com article about current groups with designs on a terrestrial space elevator. I imagine the same issues would be applicable to both applications, except for scale.

On this issue of mid-course corrections... I think it would be quite possible. After all, nearly every correction must be well timed anyway. I would advocate carrying some small amounts of reserve propellent and hydrazine just in case rotation or course needs further refinement (owing to the lack of experimental data on such).

Next, the whole plan rests on in-situ production of rocket fuel for the return trip. How effective and fail proof is the storage process for keeping the fuel for the duration of the wait for the crew as well as the duration of the proposed 500 day stay?

What risk factors do you imagine? Storage of a propellent that doesn't need cryogenic storage should be relatively simple - or at least a lot simpler than imported alternatives. In fact I think a bigger risk factor would be the transit and storage of the small amount of H2 feedstock that Zubrin specifies.

Speaking of fuel production, the power source suggested by Zubrin is a small nuclear powerplant. How likely is this?
And what reasonable alternatives are there? Also, keep in mind that it also has to power the Hab reliably for the 500 day stay. (unless the Hab comes with its own power)

The hab, according to Zubrin's fitout, has an independent 5kWe Solar power source. Presumably this includes batteries and is operable on the surface (pg 94 TCFM certainly talks like it is).

On the nuclear issue, the NRC's recent SSE survey highlighted the requirement for nuclear power generation in space. Also featuring prominently is the development of advanced RTGs:

The solution to the power and propulsions problems is development of advanced nuclear power sources... Advanced radioisotope thermoelectric generators (RTGs) are required to replace the depleted inventory of first-generation RTGs. Advanced RTGs are required for both spacecraft power and for early low-power versions of in-space nuclear electric propulsion (NEP)... Finally, a compact and efficient (high thrust to mass ratio) flightqualified nuclear-fission reactor should be developed... The SSE Survey is highly supportive of NASA?s nuclear power aand in-space nuclear propulsion initiative. The committee believes that this program can produce advanced flight-qualified RTGs in the second half of this decade that could be flown on the Europa Geophysical Explorer and Jupiter Polar Orbiter with Probes, and on the Mars Smart Lander/Mobile Surface Laboratory.

Onward and upward...

Is there actually enough to do during the long 500 day stay?
I was wondering if you could pack an M-year's worth of experiments into the tunacan that is the Hab while still meeting wieght requirments.

Once again, Zubrin's weight allowance for science and field equipment is 1 tonne. I don't know how much equipment this buys, but it is probably important to remember that the astronauts can use the same equipment to do the same experiments across their entire accessible range. Boring for the astronauts, maybe, but probably very valuable science-wise. In comparison to robotic visits, 1 tonne is a huge amount, and it is leveraged by the fact that humans can use such equipment a lot more flexibly, and in different situations than a robot.

Also, remember that time will be taken up by travelling (in the rover), day to day maintainence, R&R and communications and reports to Earth. If the next Hab were deployed elsewhere on the planet, the explorers would only 500 days to explore as much as possible of it. This doesn't seem like a lot to me.

I hope we can fix these little problems. Perhaps these are some of the reasons why NASA, ESA, or Russia hasn't picked up this obviously ingenious plan.

I know.. I keep thinking that there must be some pretty serious flaws in the design for the agencies to ignore it like they have. Does anyone know the rationale behind Nasa's Mars Semi-Direct? Perhaps if we can work out why they changed the architecture we could discern what they perceived its faults to be?

My guess is that if Mars Direct is perceived to have flaws they could be:

1) that the weight allowances might be way off
2) that maintainence issues might be under-represented - What are the consequences of dust contamination of the hab and vital machinery, etc?

Cheers.

RobertDyck · 2002-08-24 04:28:08

I tried to address inefficiencies in Mars Direct to improve the architecture, and ended up with something similar to NASA's Mars Semi-Direct. I could post what I came up with, but it is rather long. There are two points in NASA's plan I disagree with, points that show fear and an inability to truly lead space technology. The first is expansion of the crew from 4 to 6 and insistence that the crew include a medical doctor with no duties other than healing the crew. The other is avoidance of ISPP for the return trip. The NASA Design Reference Mission (DRM), nick-named Semi-Direct, uses ISPP for the Mars Ascent Vehicle that travels to Mars orbit, but it still carries all the way from Earth the propellant for the return to Earth. In fact, NASA is afraid to use ISPP for a robotic sample return mission. Robert Zubrin's company received a NASA grant to produce a laboratory prototype ISPP plant that already proved the concept. The only next step to prove ISPP is to use it on Mars; that means sample return.

Merp · 2002-08-24 05:04:04

I would very much like to hear about the inefficiencies of Mars Direct, and how they might be addressed.

Cheers!

RobS · 2002-08-24 11:06:12

Regarding the low density of water ice at the lunar poles, Robert, do you recall where you found that information? I thought I had seen information suggesting the regolith was several percent or more water. Of course, there is conflicting science about the amount of water at the lunar poles, with more recent data suggesting greater quantities. Interestingly enough, the radar data suggests more water at Mercury's poles than the moon's! Possibly we will be doing in situ resource utilization of regolith water there before we do it on the moon; assuming we can overcome the huge delta-vee problem of sending people to Mercury, of course.

Regarding carbon nanotubes mentioned in a more recent posting, it is not yet clear that they will be developed in useful quantities and reasonable prices by the time people go to Mars, as they are still a laboratory phenomenon.

-- RobS

RobS · 2002-08-24 11:52:55

I just started writing a lengthy comment about redesigning Mars Direct, and it disappeared into the ether! Here's a shorter version. I can see several ways the Mars Direct possibly could be improved.

1. Using a Mars Asecent Vehicle (MAV) to get to orbit and an Earth Return Vehicle (ERV) to go from Mars orbit to Earth.
Disadvantages of the change: you have to design three vehicles and leave one unmanned in orbit several years, where something could go wrong. Probably not a good idea for the first few missions.
Disadvantage: You have to fly more mass to Mars because the return fuel for the ERV has to come from Earth.
Possible advantage: If in-situ resource utilization is not possible because of the lack of a reactor on the Mars surface, this is the way to go. It reduces the weight flying back from the surface to the absolute minimun.
Possible advantage: The MAV could be a single stage vehicle; the designs even call for it to be a single stage vehicle. It could be made reusable relatively easily, allowing flights from the Martian surface to Phobos and Deimos and back, or allowing repeat flights to the ERV. If the ERV were "stored" on Phobos while the crew were on Mars, flights up every six months would allow routine maintenance of the ERV, exploration of Phobos, and possible experiments with in-situ resource utilization on the moon.

2. I'd aim to explore use of Phobos as early as possible. It is 5.5 km/sec from Mars and 1.9 km/sec from Earth. It's probably carbonaceous chondrite and may be as much as 10% water by mass. If our moon really has too little water for making rocket fuel, Phobos is the next closest object in terms of delta-vee (except for earth-crossing asteroids). Phobos may have an important role to play in supplying water to low earth orbit.

3. I'd explore use of solar thermal rocket engines for pushing Mars Direct 80% of the way to Mars (that is, almost to escape velocity from Earth). Solar thermal engines are under development by NASA engineers. They are still small, but have the potential to generate a lot more thrust than ion engines. Their specific impulse is about 800 seconds; almost as good as nuclear. Perigee kicks would raise the apogee and allow a final perigee burn by a chemical rocket. The crew woiuld fly out to the Mars Direct vehicle just before the final trans Mars injection and would take as much time to get to Mars as before. The equipment would take six months to get into position for the crew to fly up (with Soyuz spacecraft, as Robert suggests). Payload is increased 40-50%, allowing a smaller heavy lift rocket.

4. If spent fuel tanks can be converted into increased habitation for the flight to and from Mars--creating the beginnings of a Cycler--the astronauts would have something to do and there would be a gradual improvement in housing on the flight out. This would require a few tonnes of equipment and solar panels, though.

5. Mars Direct was designed before NASA had developed the inflatable hab concept. Now, that concept may require rethinking the Mars Direct plan. It may be better to combine the ERV and hab together into a single larger ERV that would have the small crew quarters of the original ERV and a cargo hold capable of carrying an inflatable hab, a greenhouse, and a pressurized rover. Rather than fly one vehicle to Mars, one could fly two ERV/habs, each with three or four crew; they could rescue each other at any stage of the trip. On the surface the inflatable hab would be set up and covered with regolith at some distance from the ERVs (or maybe they'd be MAVs). Possibly for the flight back, the ERV's empy cargo bay could be set up as partial housing.

Those are a few possibilities.

-- RobS

RobertDyck · 2002-08-24 12:06:14

I heard the concentration of water ice on the moon from the NASA web coverage for Lunar Propsector. This was data directly from the source as the data came in. When I looked up the Lunar Prospector to link to this message, it does describe concentrated pockets: 4.6% on the north pole and 3.0% on the south. I didn't see that at the time they stopped live web coverage, but it is still a low.

Merp · 2002-08-24 19:14:57

Thanks RobS

One thing I have always wondered about cyclers is how easy would it be to 'jump on' as it went past? Additionally, if the maneuver for whatever reason failed, what abort options would be available?

I'm not sure I like the idea of separating the MAV and ERV since it introduces two significant complications - the need for an orbital rendevous and the maintenance issues you mentioned, including long term storage of cryogenic fuels in orbit. The reusability of a MAV you mention is very promising but would require an integrated landing capability that would drive up the mass.

On the other hand, For both ERV and Hab missions I really get the feeling that we could do with more space. The MAV/ERV should have enough space for the reactor, a chemical plant (of greater flexibility and adaptability than Zubrins' ) and other missions supplies for both scientific and engineering experiments.

The pressurised Rover should definately arrive with the Hab, to provide the mobility required in the case the landing places the crew far away from the ERV. Apart from this I think the weights Zubrin quotes are bit optimistic.

All of which suggests we should use alternate propulsion to increase the mass delivered to the surface. This isn't as big a deal as it was when Zubrin wrote The Case for Mars.

I wonder if we could modularize certain pieces of equipment to make them common between both the ERV / Hab? eg If both the Hab and ERV have three 'stages'.

Hab - standard Hab section(HAB), mission section (MIS), and Lander sections (LND) (Re-entry rockets and struts on the bottom, aeroshield, parachute, solar energy and communications gear on the top).

ERV - standard Hab section(HAB), standard Lander section (LND) and earth return section (ERN) (delivers the Hab to mars orbit or earth)

This obviously isn't a perfect concept (where does the chem plant go?), but I think investing in designs with common elements would be worthwhile, saving money, simplifying production and allowing flexibility in mission by swapping out sections.

Just a thought

PS. Does anyone know where I can find out information on using the TransHAB in a Mars mission?

--Merp

RobS · 2002-08-24 23:25:52

Thanks, Robert, for the website. Six billion tons of ice at concentrations of up to 3% probably can be harvested to make chemical rocket fuel more cheaply than hauling fuel up from the earth's surface, though the challenges of handling regolith at 100 Kelvin will take some time to overcome, and one would need a pretty big investment to figure out how to do it. I suppose I'd look at the six billion tons (if the figure is accurate; all the publications admit to large error margins) rather like the way we view coal and petroleum on Earth: a valuable resource to exploit now and replace with something else later. One would start by finding the richest deposits, because they'd be the cheapest to harvest, and work one's way to the poorer concentrations. It would take centuries to use up six billion tons of water; maybe millennia. It is difficult to imagine demand for lunar water rising to, say, 10,000 tons a year by 2100; it would start at hundreds of tons per year. But even 100,000 tons per year for a thousand years is only a hundred million tons of water, a tiny fraction of the estimated reserve. Once it's gone, your technology may be good enough to haul water from elsewhere in the solar system, even from Earth!

But, of course, the estimates may be high and the moon may have much less. We'll have to await better data.

-- RobS

RobertDyck · 2002-08-25 00:54:48

The Mars Direct plan includes landing the ERV on Mars Surface. That includes the heat shield for Earth atmospheric entry, but it is not the heat shield used to land on Mars. The ERV has to include parachutes for Earth entry, high-temperature tolerant hull, landing or splash-down equipment, as well as food and life support for the 6 month trip back to Earth. None of this has any use during surface operations on Mars. The obvious question is why you would drop all of that into the gravity well of a planet just to lift it out again? Furthermore, Mars has an atmosphere. That may make aerocapture, aerobraking and landing easier, but it adds aerodynamic drag to anything leaving Mars. The Mars Habitat may have plenty of room for astronauts on the trip to Mars and on the surface, but the ERV is cramped for the trip back.

Mars Direct made excellent use of latest technology available when it was written, about 1990, but has not been updated to include developments since. For example, the hab to be constructed of weldalite; that is aluminum-lithium alloy. Space inflatables including TransHAB have been developed since. An inflatable hab optimized for Mars surface would not require the micrometeorite shield of TransHAB because Mars has an atmosphere. The surface hab would only need protection from wind-blown dust and sand, including high-speed wind storms, and from corrosion from alkali, salty, and super-oxidized dust.

Mars mission plan - Mars orbit rendezvous

The start of the mission would launch a Mars Ascent Vehicle, similar to the ERV of Mars Direct, but with just a single ascent stage, extra large propellant tanks, and a crew cabin barely large enough to accommodate the astronauts in spacesuits and their samples. The ascent cabin would not even have life support; astronauts would live on suit support during ascent. The MAV would be launched into LEO, then spiral out from the Earth by a TMI stage that uses solar-electric propulsion. The MAV would gain gravity assist from the Moon then slowly traverse to Mars unmanned. It would enter Mars atmosphere by direct entry to avoid propellant use, land automatically and start producing propellant for the return trip. Total trip time from launch to Mars surface would be about 2 years.

At the same time, a second unmanned launch would send a cargo spacecraft. This would also be lifted into LEO and use solar-electric propulsion and direct entry. It would also take about 2 years, but it would follow the MAV closely and use an active beacon from the MAV to land within walking distance. The cargo craft would carry a pressurized rover, an unpressurized 2-man rover, garage tent, inflatable greenhouse, and inflatable laboratory together with laboratory equipment. For examination of the MAV, the cargo craft could carry an automated inspection rover roughly the size of Sojourner.

After the MAV propellant tanks are full and ready to return to Earth, then the manned mission would be launched. But the manned spacecraft would consist of two parts: a dedicated zero-G space habitat and a surface habitat. The space habitat would be a space inflatable with micrometeorite shield constructed using TransHAB technology. Life support would use a reusable CO2 sorbent and electrolysis tank like that used on ISS, but it would be augmented with a Sabatier reactor to prevent consumption of water. Water recycling would be the high efficiency system NASA already developed. TMI for the manned mission would be chemical propulsion with the two habitats docked. Trip time for men to Mars would be about 6 months. Upon reaching Mars, the spacecraft pair would aerocapture into highly elliptical, high orbit; that is, barely captured into orbit. The crew would then transfer into the surface hab. At this point the surface hab would just be a capsule large enough to accommodate the astronauts; the rest of the hab would be collapsed and stowed. The surface hab would then detach from the space hab, taking the heat shield with it. The surface hab would use the heat shield to enter the atmosphere at a trajectory so fast it is almost direct entry. The surface hab would follow the beacon of the MAV, but the cargo lander would also have a beacon as a backup.

After landing, the first order of business would be deploying the inflatable surface hab. This hab would have a recycling life support system similar to the space hab. However, it would also have a box capable of accepting chunks of Martian permafrost, melting them and filtering the water with a reverse osmosis filter. In case the recycling system breaks down, this permits the crew to replace lost water. Since the air recycling system is based on electrolysis of water, this would also replace lost oxygen. The surface hab would land with food, life support, and spacesuits for all crew members for the entire surface stay. It would not include a rover, or any laboratory facilities. Since the cargo lander would be delivered to Mars immediately following the MAV, the laboratory would be confirmed on Mars surface before the astronauts left Earth. This keeps the surface hab very small.

Surface operations would include a suitcase style PLSS on the unpressurized rover. The suits would have a 30 minute backup, and astronauts would be restricted to a 30 minute walk from the rover. This permits exploration for the full duration of the suits. In case of suit failure and rover failure, astronauts could be rescued with the other rover. The suit PLSS would use a reusable sorbent, microwave regenerable in the hab. The backup would be a closed-loop system that uses LiOH sorbent, a second oxygen tank, and an airbladder over the chest, neoprene air dam around the face, and one-way valves routing air from the helmet to the chest airbladder, from the airbladder to the sorbent, then past an oxygen regulator back to the helmet. Backup oxygen would be pressure regulated only. This permits the backup system to operate without electricity, so it works in case of battery failure. The primary sorbent cartridge would be field replaceable in Mars atmosphere, as would the battery. The primary oxygen tank would be field refillable, and the PLSS would include hoses and fittings for buddy breathing. The suitcase style PLSS would have the same duration as the backpack PLSS, include another 30 minute backup, and use the same systems as the backpack PLSS so parts are interchangeable.

Departure from Mars would involve astronauts loading all their samples onto the MAV, then getting into the cabin in their spacesuits. The MAV would ascend without dropping any stages, rendezvous with the space hab and dock. Yes, this involves Mars orbit rendezvous, and in a highly eccentric orbit at that, but orbit rendezvous and docking was developed by Gemini, and used routinely by Apollo, Skylab, Mir, and ISS. I would call it a mature technology. After docking, the assembly would thrust toward Earth. The MAV would be the Trans-Earth Injection stage using propellant from ISPP. The MAV would remain docked to the space habitat all the way to Earth. All life support including food stores for the return trip to Earth would be in the space hab. This means that if a free return to Earth is necessary, the space hab the astronauts rode to Mars would be their intended ride back to Earth, so it is already stocked for the trip.

Upon reaching Earth, the astronauts would climb into the Earth reentry capsule. This would be separate from the MAV cabin since a free return would not have the MAV docked. This provides the added advantage that the heat shield, parachute, and other landing or splashdown equipment does not have to be landed on Mars. In the case of a normal mission, it would be parked in Mars orbit until return to Earth. The MAV cabin, in fact, would not be pressurized; it would barely be an aeroshell optimized for the ascent. Docking the MAV would involve making a solid attachment for thrust, not a pressure seal. The MAV "docking hatch" would be nothing more than an access door to line-up with the outer pressure hatch of the space hab's airlock. Obviously the landing legs, ISPP plant and power plant of the MAV would be left on Mars surface.

The Earth entry capsule could be based on the descent module of the Russian Soyuz spacecraft. The "headlight and windshield" design provides the minimum exterior surface area with maximum interior volume. The Russians designed a Soyuz for direct entry from lunar return trajectory, but we would have to scale it up for 4 astronauts plus sample containers from over a year of field work. If development of such a capsule was not available, we could use the X-38; but I question whether its heat shield could withstand direct entry at interplanetary velocity. I favour the capsule because it may not be reusable, but it has lower mass than a lifting body.

As an additional backup mode, the cargo lander could include a duplicate set of food for the surface stay. The inflatable laboratory in the cargo lander could be used as backup habitat, and the pressurized rover's life support system could supply the laboratory in case the surface hab's life support failed.

This mission has another advantage: In case we don't have an HLLV available, the manned vehicle is already composed of separate components docked together. It consists of the space hab, Earth reentry capsule, surface hab, and TMI stage. Since the space hab would be based on TransHAB technology, it could be launched within a payload fairing narrow enough for Energia, or even Space Shuttle, Proton, Angara 5A, Atlas V, or Delta IV Large. In the case where Earth orbit assembly is required, the TMI stage could be launched last so long term on-orbit storage of cryogenic propellant is not required.

This may sound similar to the NASA DRM, but the primary difference is use of the MAV as the TEI stage. This enables ISPP for the return to Earth. It also eliminates storage of cryogenic propellant in Mars orbit.

Bill White · 2002-08-25 15:21:12

The crew would then transfer into the surface hab. At this point the surface hab would just be a capsule large enough to accommodate the astronauts; the rest of the hab would be collapsed and stowed. Then surface hab would then detach from the space hab, taking the heat shield with it. The surface hab would use the heat shield to enter the atmosphere at a trajectory so fast it is almost direct entry. The surface hab would also follow the beacon of the MAV, but the cargo lander would also have a beacon as a backup.

Might you modify an X-38 to land on Mars, homing in on the MAV & surface hab beacons? Is the ground simple too rocky for an X-38 landing under parafoil?

I suppose you would need an upgraded X-38 with avionics and a pilot operated rudder - or parafoil controls.

$500 million for four (4) X-38s was to include all R&D - how much more would it cost per vehicle to build 4 more X-38(m) models. The heat shields should do fine given the thin atmosphere and 3/8th gravity.

Off the shelf, as much as possible?

RobS · 2002-08-25 20:07:34

Robert, you mention a gravity assist from the moon. Do you know how much delta vee this can produce? It seems to me I have seen a number of half a kilometer per second. That makes a substantial difference in a trip to Mars.

I am fascinated by your Mars Direct restructuring; thank you for it. I am still chewing on it.

-- RobS

RobertDyck · 2002-08-25 21:17:44

The X-38 is an intriguing idea. It doesn't have a regenerable life support system, and I don't think it has enough room to accomodate the inflatable habitat, powerplant for life suport, furniture, or equipment. But you could include the habitat with the cargo lander. Video of the X-38 landing shows it skids only about 3 feet, so Mars rocks should not be significantly different than dessert. The only questions are whether the heatshield is applicable to Mars, how the parachute has to be reconfigured, and whether the parachute can slow the craft enough in Mars atmosphere to land without landing rockets. The X-38 would not be large enough to aerocapture the space hab, so that would require a separate heatshield. Would separating the hab from crew lander produce lower total mission mass, or larger?

RobertDyck · 2002-08-26 10:51:39

Thanks for the Aerogel link. I didn't realize it is already available in transparent form, and blanket form. The blanket could be considered for spacesuits and the hab. The transparent form for a greenhouse. I'll see if my engineer friend from Montreal is willing to work on thermal anaylsis if the greenhouse. We can see if sheet plastic with spectrally selective cloating and argon between two layers is sufficient, or whether we should consider transparent aerogel.

RobS · 2002-08-26 15:19:21

I found on the web the answer to my question about the gravity assist the moon can give. It turns out the answer is quite complicated and depends on approach speed, approach angle, departure angle, and whether you have to move above or below the ecliptic or not (which rapidly eliminates any gravity assist). It is possible to get about 300 meters per second, it would appear, out of a lunar gravity assist on Mars trajectories. But it varies from year to year. I suspect that is not anywhere as much as it sounds, either. Escape velocity is 3.3 km/sec from a low Earth orbit and for a Hohmann orbit to Mars from low earth orbit it is 3.8, but you can't gain 0.3 of the 0.5 you need from the moon. If you leave low earth orbit at 3.8, you end up leaving Earth with a lot more than 0.5 km/sec because you get away from the Earth so quickly, it has less time to slow you down; you end up keeping several kilometers per second (remember, you are already whirling around Earth at 7.5 or so). If you fly slowly to the moon, it'll give you just 0.3 km/sec, and you need a lot more than that. So gravity assist by the moon doesn't seem to help very much.

Potentially, the moon can give a gravity assist equal to twice its rotation speed around the Earth (1 km/sec, so a 2 km/sec gravity assist is possible). But I guess not to go to Mars. Maybe for some asteroids, though.

-- RobS

RobertDyck · 2002-08-26 16:29:38

To benefit from gravity assist you have to depart in the direction the planet or moon is traveling. For our Moon, travel straight from Earth toward the Moon timed to pass the Moon on a full moon. That's when the Moon has greatest velocity in the direction of Earth's orbit about the sun. Change of angle will be about 90?. Mars is inside the ecliptic so that is not a concern. Departure velocity has to be more than Earth escape velocity, not necessarily enough for a Hohmann transfer orbit. Solar electric propulsion will continue to thrust the whole trip to Mars: 6 month spiral out of Earth orbit then 1.5 years to Mars. I get the time figures from the original planned path of Deep Space One. It was supposed to pass Mars to get a gravity assist on its way to one of the asteroids. Gravity assist from the Moon may not be much, but with solar electric propulsion every bit counts. Hohmann is the lowest energy transfer orbit, and for chemical rockets that translates to lowest propellant mass. However, solar electric propulsion collects solar energy as it travels so the longer the trip the more energy available. The lowest propellant for solar electric is continuous thrust to the destination at the highest specific impulse and highest thrust level the solar panels can supply. Spacecraft engineers balance solar panel mass vs propellant mass.

All this is applicable to unmanned vehicles.

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