The incredible, expendable Mars mission
An analysis of NASA’s 2009 Mars Design Reference Architecture 5.0
The massively expendable mission to Mars component vehicle list
Here is a list of all of the vehicular components of a single DRM 5.0 Mars mission and their disposition. This is one-third of the individual vehicle components needed for a three-mission decadal program.
Total number of expendable vehicles and major components per single mission: 19
The most glaring deficiencies in the plan compared to current technically “progressive” plans are the lack of any reusable components as shown above, and of any in-space refueling of the in-space stages at a logistics base. The fundamental decision to not use propellant depots in orbit forces the decision to have all propulsion components expendable. The use of an entire special module just to keep each mission vehicle’s components in orbit while they are being assembled over a period of months is particularly strange, since if the rockets must be assembled, you would think they would be assembled at a logistics base that has its own capacity to maintain orbit. There is no description as to how the components are assembled and no mention of whether the current space station could be used for this purpose. Some of the expendable in-space propulsion stages used in the mission seem to be very small when fewer, larger stages could do the job much more efficiently.
Since the current station is in a relatively high inclination low Earth orbit, which takes extra fuel to reach and where any stored components are at risk of impacts from space debris, it would make much more sense to assemble a Mars expedition at a point much further up Earth’s gravity well. The regular delivery of propellant over a period of about a year, to depots in a location such as one at or near the Earth-Moon L1 point, would mean that much less propellant would be needed for the actual departure of the expedition. The high location would also allow the propellant to be stored with very low risk of loss from space debris impacts. Up to several thousand metric tons of propellant would be required for an effective Mars expedition. For the 2009 mission, this would lower the fuel requirements by about 500 tons, transferring that to reusable tanker runs. Such runs from LEO to L1 to deliver propellant would be routine and there is no major problem if a single shipments fail to arrive. Once propellant is available, stored in propellant depots at such a base, it can be loaded directly into the stages needed for the Mars departure at that time, and each stage would thus not need its own cryocooler and thermal protection. If a stage is going to retain propellant for use many hours or even months later, then having a cryocooler would make sense. No cryogenic level thermal protection seems to be clearly shown in any of the images of the vehicle propulsion modules, where all of the propellant tanks seem to be exposed to the sunlight with no sunshields.
For booster stages used in launches from the surface, some people are starting to see the rationale behind reuse, now that at least one company is close to demonstrating such reuse. But most people still think of an upper stage in space as one that has to be thrown away when it is empty. However, if a booster stage can return to a launch site, land, and be reused, an in-space stage can be reused by just returning to a logistics base and docking with it. It does not have to endure the stress of reentry or landing, it only needs enough fuel to reverse course and dock. The stage is worth far more than the small amount of extra fuel needed to recover it, especially since it is already in orbit. Of course, this assumes you have a logistics base with propellant depots.
The next major issue is the complete lack of redundancy. There are several obvious kinds of redundancy. For vehicles operating in space, redundancy primarily means having more than one vehicle of each type. This logic would dictate using two crew habitat vehicles during the transit to Mars and back, each capable of supporting the entire crew. Adhering to the current belief that the extremely high launch costs of the last 50 years will still be around by 2030, relatively low mission masses are assumed. This greatly limits the number of vehicles, thus greatly limiting redundancy. Thus there is only one kind of each vehicle sent to Mars on each mission.
For vehicles that take off from or land on a planetary surface, one kind of redundancy is engine-out capability. With that capability, chances of vehicle failure are greatly reduced. Even if rocket engines were now as reliable as jet engines, we would still want one-engine-out capability for space vehicles. The other kind of redundancy related to takeoffs and landings is providing self-rescue for the crew by designing each crew vehicle’s cabin to also serve as an independent escape vehicle during an emergency. The chances of two separate vehicles failing, one right after the other, are very low. This capability was effectively present in the US lunar module during the descent, but not during the ascent. Since parachutes do not work effectively on the Moon or Mars, rocket power is the only way to survive if a primary crew vehicle fails. This logic is similar to the emergency escape rockets used during launches of crews in capsules from the Earth. The escape cabin/capsule can abort either to orbit or to the surface, depending on where the failure occurs.
To get the crew down to the surface and back into Mars orbit for the 2009 plan, essentially four separate vehicles are needed: the two huge aeroshells and the MAV and the crew habitat-lander inside them. Yet there is only a single one of each of these on the mission, and all must work perfectly to get the crew back into orbit. Each of these vehicles need structure, attitude control, propellants, rocket engines, fuel tanks, and so on. Thus with four separate vehicles, there is still zero redundancy.
For about the same amount of vehicle mass delivered to Mars orbit from Earth (about 200 tons), you could have three reusable Mars lander-ferries, each of which could be used several times to land and take off again to bring down more cargo. This is possible even if the ferries use the much less powerful oxygen-methane fuel mix advocated in the document, assuming you have some extra propellant available in orbit for each new trip down. With hydrogen-oxygen propellant produced at the surface base from Mars ice, all transport needs at Mars could be supplied locally. Each ferry should be able to take half of the crew back to orbit if one ferry is disabled. This provides a spare ferry to take the last crew to return to orbit, and of course, a reusable ferry could also go back down again for another trip up. Bringing a propellant depot to Mars orbit is no harder than bringing the individual vehicles with their propellant.
The amount of equipment and supplies the crew can take down to the surface is extremely limited. Only two vehicles land, where the total mass of both before the landing is about 206 tons. Most of that is vehicle and propellant mass. Each of the aeroshell vehicles weighs about as much as the lander vehicle inside it. There are also problems with the lander vehicles. The crew habitat is part of the crew lander and so cannot be buried to provide radiation protection. A major part of the MAV is the lander stage. The actual landed payload for each of the two landing mission is about 40 tons, so out of the 80 landed tons, 20 is the ascent vehicle, about another 20 for the crew habitat (mounted on the lander), about 10 tons for one pressurized long distance rover, and 16 tons for two small reactors. That leaves about 14 tons for everything else, including scientific equipment. Presumably the largest amount of equipment and supplies consists of food, oxygen, and the ISRU equipment to create oxygen for the MAV. This leaves very little room for any scientific equipment and no room for a spare long-distance rover, needed for a rescue if the single one they have breaks down far from the landing site.
The MAV/lander has to carry its own ascent fuel (methane) down with it, but it creates its own oxygen for the ascent out of Mars carbon dioxide. Since the mission carries no excavator to dig up Mars ice to create both hydrogen and oxygen propellants, it cannot bury the nuclear reactors, and so must also carry three kilometers of heavy power cable to power the MAV, the lander/habitat, and to recharge the rovers. It thus cannot produce methane fuel for the MAV, since the only local source of hydrogen would be water ice. If a small excavator could be carried down, the reactor could be safely buried and both fuel and oxidizer could be created directly from Mars materials, allowing much more cargo to be brought down to the surface.
Once the crew returns to orbit for the return trip to Earth, the effects of the totally expendable mission become even more apparent. The crew transfers back to the crew habitat, and the MAV is abandoned. When the habitat is thrust back into a trajectory toward Earth, there is nothing usable left in Mars orbit for any future crew to use, even though the plan calls for three missions over a decade. The resulting shrinkage of usable hardware as the mission progresses is reminiscent of an Apollo mission, which starts with a 3,000-ton rocket and ends with a tiny capsule which is never reused. Of course, this mission is like 12 Apollo missions.
The amazing, shrinking Mars mission:
Assuming at some point there will be a version 6 of this report, what are the most important lessons to be learned from the 2009 mission design, and issues to be addressed, based on the new knowledge and technology gained since that report was issued? Clearly future planning for Mars missions should be based on the use of reusable boosters and spacecraft to the largest extent reasonably possible. Missions should have a minimum of two of each module critical to crew survival, all critical propulsion should have engine-out capability, and all crew vehicles doing landings and/or takeoffs need crew escape capsules in case of vehicle failure during flight. Use of such emergency capsules should be designed to be feasible at any time during a flight, up or down. Mars missions should be mounted from a logistics base near the top of the Earth’s gravity well, such as at Earth-Moon L1. Scale tests of supersonic retropropulsion vehicles should be accomplished on sounding rockets to validate the method and to avoid the use of giant expendable aeroshells. Cryogenic propellant depots need to be perfected by advances in both cryocooler technology and in the transfer of cryogenic liquids in micro-gravity. Existing technology to turn dirty ice into liquid hydrogen and oxygen needs to be engineered to operate on Mars and the Moon. Mars expeditions need to carry much more equipment including scientific equipment to make a Mars mission worthwhile. With the probable participation of the private sector, Mars is a very enticing destination for the global space community. Let’s make it happen.