Solving the expendable lander and MAV trap
by John Strickland
|Paradoxically, the current plans for the MAV (Mars ascent Vehicle) as described in an Aviation Week article contain some technically progressive features.|
Unfortunately, this does not seem to be the case, based on both the very recent releases of the Planetary Society’s report on human Mars missions (see “Orbiting first: a reasonable strategy for a sustainable Mars program”, The Space Review, October 5, 2015) and multiple new reports on NASA’s Mars Ascent Vehicle (MAV) design, details of which have not publicly been released. Those recent developments demonstrate continued support for antiquated, fully-expendable architectures. Although Congress is currently starving NASA of funding for both commercial crew and beyond low Earth orbit hardware development, it is not clear where the very effective opposition to reusability ultimately comes from. This is certainly not related to the effort that goes into designing the expendable vehicles, since no one should fault the engineers doing excellent and detailed design work on them. They do not control policy. There is, nevertheless, a very large gap in thinking and planning between the current official Mars plans, derived ultimately from the Apollo mission model and based on all-expendable boosters and spacecraft, and the newer, technically progressive concepts which focus on reusable, commercially-derived vehicles.
Paradoxically, the current plans for the MAV (Mars ascent Vehicle) as described in an Aviation Week article, contain some technically progressive features, such as use of in situ resources for propellant production and ingenious engineering designs for payload packaging. However, the thing that really counts, reusability, is still missing. Worse, the overall mission plan seems to leave nothing behind, either in Mars orbit or on the surface, for the next crew to use, so there is no practical material advancement from mission to mission. When all of the expendable vehicles have been used up, such a program would probably end, just like Apollo did.
What keeps the current Mars vehicle designs from being reusable? (I am focusing on the MAV for this example of extreme expendable design.) A MAV is an expendable crew vehicle that takes the crew on a one-way trip back to Mars orbit, where it docks with an Earth return vehicle and is then thrown away, since there is no way it can be used again. One main difference from a reusable system is the multi-stage design, with one expendable descent stage to land the empty MAV and two expendable stages to return to orbit, excluding the separate vehicle used to land the crew.
The two-stage MAV would be flown to Mars, and then carried down to the surface on top of a descent vehicle, which does nothing besides land the MAV on the surface, as shown in the NASA image above. The MAV would then sit on the surface of Mars for over two years before the crew arrives. When it finally returns to orbit with crewmembers on board, it leaves the descent vehicle behind, then it throws away its first stage, which crashes onto Mars, so only the smaller second stage with the crew cabin actually reaches Mars orbit. Since the crew stage cannot land or take off by itself, it is useless once it reaches and docks with the Earth return vehicle.
Thus, to use the MAV for a return to orbit, three separate vehicles are needed, (1) a larger lander to bring the MAV down to the surface, (2) the MAV first stage, and (3) the MAV’s second stage with crew cabin. Once each stage has completed its task, it cannot practically be used again, even if it is not destroyed. This also means that the crew must fly on a series of vehicles that were never flight-tested, and there is only one vehicle of each type, thus there is no redundancy.
Looking at the image, it is obvious that one composite vehicle is stacked on top of another. All three vehicles have fuel tanks, rocket engines, controls, and structure. About the only features not duplicated in the other vehicles are the crew cabin and the landing legs. It is more than achingly obvious that if all four vehicles (now including the vehicle that lands the crew) could be combined into one, the total mass would be greatly reduced. That mass could then be used to provide a larger crew vehicle, duplicate vehicles for redundancy, or more propellant to carry more supplies.
|A better concept would have both the oxygen and methane created using Mars ice as a source of hydrogen. Even better than that, the system could skip all of the more complex synthesis of oxygen and methane from carbon dioxide and hydrogen, and make both oxygen and hydrogen directly.|
The other main difference is where the MAV’s fuel comes from. The MAV uses stored (liquid) methane propellant from Earth, but would have no LOX in its tanks before landing to save mass. The liquid oxygen for the ascent would be created on Mars from carbon dioxide. Such a design would already require a cryogenic propellant storage capacity with solar powered cryo-coolers to reliably maintain the zero boil-off state for several years to preserve the methane. Cryogenic liquid methane must be kept below -260°F to prevent it from boiling away. The logic for this plan is that while oxygen can be extracted directly from the carbon dioxide in the Mars atmosphere, to make the methane fuel would require a supply of hydrogen as well as carbon. However, if the MAV were simply supplied with more cryo-coolers, the hydrogen needed to make methane could be brought along from Earth, just as Robert Zubrin proposed to do in the Mars Direct plan 25 years ago.
Note that liquid oxygen (LOX) must be kept below -297°F, only slightly colder than liquid methane. With additional cryo-coolers, power, and good insulation, liquid oxygen and even liquid hydrogen (LH2) can also be kept as liquids indefinitely without loss. The oxidizer and fuel combination of LOX and LH2 is much more powerful than LOX and methane, making a single-stage-to-orbit liftoff from Mars much easier and simpler. While liquid hydrogen does have some peculiarities, the operational problems these cause are more than made up by its much greater performance. The US has been using hydrogen for rockets for more than 50 years.
A better concept would have both the oxygen and methane created using Mars ice as a source of hydrogen. That ice is available just below the surface in many locations. Even better than that, the system could skip all of the more complex synthesis of oxygen and methane from carbon dioxide and hydrogen, and make both oxygen and hydrogen directly via electrolysis from the melted ice. A miniature fuel production plant would have over two years to make the fuel needed, and would only need to make a single metric ton of fuel per month, or about 30 kilograms a day. The mass of the plant would be a fraction of the mass of the 30 metric tons of propellant it would create, which could then be replaced with additional supplies and equipment for the mission.
With the minimal MAV design described in the Aviation Week article, the design limits to save mass for the Mars mission are so stringent that the MAV would only be able to be used for two days before its life support capacity was depleted, and it would not even have an airlock to save structural mass. The astronauts would have to leave their bulky Mars suits behind in a rover and walk into the MAV through an expendable tunnel, and then don lighter flight suits. One almost gets the impression that the management is struggling to maintain expendability, in the face of engineering logic, resulting in almost bizarre solutions, such as the tunnel. All of the intensive mass-saving effort shows just how limited the crew would be in their vehicles, equipment, food, stores, and scientific instruments, greatly increasing the risk of a failure and loss of crew due to equipment failure, an unexpected delay, or a simple mistake.
So let us assume for a magical moment that mission and vehicle designers could be set free from the current restrictions against reusability, but with most of the current mass restrictions. With reusable vehicles, the separate crew lander and the MAV would be combined into a single larger and more capable vehicle, a “Mars ferry” that can make multiple trips down and up again. The fuel plant would be initially brought down as cargo on a ferry, and left on the surface to produce propellant. Due to the lower gravity and lower orbital velocity of Mars, and the fact that a landing vehicle can get rid of over two-thirds of its velocity during entry, propellant requirements for landing are about one fifth of those for the return to orbit, and a lot of cargo can be brought down with only a small amount of fuel.
|How then can we enable a truly robust Mars mission? The answer is by designing for a high-mass expedition.|
However, when such a ferry vehicle first lands, there would be no fuel available to take off immediately. To keep things simple, a combination excavator, rover, and fuel production system landed by the ferry could dig a hole, extract and melt ice, turn it into hydrogen and oxygen rocket propellant with electrolysis, and then drive over to the parked ferry and pump the stored fuel into the ferry, where it would be kept cold. The mobile plant would do this repeatedly until the ferry’s tanks are full. To reach orbit again, a ferry would need to use about five times as much fuel as it takes to land, but there is little need to take cargo up, just crew. When the first ferry’s tanks are full, the crew can land in another ferry and begin surface operations.
Assuming a landing site has been chosen for a series of expeditions to use, it would have been scouted previously by a rover and also by orbiters. The rover would also “prove out” that the site had a suitable ice deposit, just below the surface and accessible by just a small excavator. A prototype ice-to-propellant system would have already been landed at the site and could have provided propellant for a sample return mission. Orbiters have been detecting ice deposits closer and closer to the equator of Mars, so landing sites would not be restricted to areas above latitude 30 degrees.
This is of course only a description of how to “modernize” a minimal Mars expedition, only slightly more capable, but safer, than the ones currently being planned. As pointed out above, extreme mass limits on an expedition greatly limit what a crew can actually do on the surface. How then can we enable a truly robust Mars mission?
The answer is by designing for a high-mass expedition. For this, use of expendable boosters like the Space Launch System would be far too expensive. By designing the space vehicles to be launched into low Earth orbit by commercial fully reusable boosters, the cost of launching the expedition would be cut by about a factor of ten or more. By relying on private companies to design, build, and operate the boosters, NASA would avoid the development and construction costs, and would pay less for the actual launches than it would with its own boosters. This means more mass into orbit at a lot lower price, and a lot faster. We would thus get a much more capable and larger expedition at a much lower cost.
A truly high-mass Mars expedition would include a whole fleet of vehicles, not just a few. The fleet would provide a high degree of safety and redundancy for the crew and would allow a much larger amount of cargo to be landed on Mars. There would be several ferries to move crewmembers between Mars orbit and the surface. The ferries would be wide and low, to provide maximum deceleration during entry and also make it hard to tip over on landing. Each ferry would have a crew cabin with powered self-rescue capability. There would be a logistics base in Mars orbit with a fuel depot and a crew habitat. Oxygen and hydrogen in bulk would be brought from the Earth or Moon to the Mars orbit base to start operations, and that fuel would gradually be replaced by extra fuel brought up with each ferry flight to orbit. The rest of the fuel brought up would be sufficient to land the ferry again with another cargo or crew, making such a ferry a single-stage-to-orbit-and-return (SSTOAR) vehicle.
There would also be more than one Earth return vehicle in orbit. There would be enough food and supplies delivered to the surface so that a crew could even wait on the surface for another expedition to arrive before it left. This level of multiple redundancies makes it very hard to lose a crew, and thus reduces the chance of a program being cancelled after an accident.
|To go to Mars we need: reusable rockets and spacecraft; a public-private partnership for space transportation and missions; logistics bases in LEO, near the Moon, and in Mars orbit; and a willingness to fund cost-effective but also high-mass and robust lunar and Mars missions.|
Outside of official sources, a consensus seems to be building in support of lunar and Mars expeditions based on commercial boosters and spacecraft. The same combination of a base and crew refuge in space and a surface base that supplies fuel to the space base is very efficient and safe; what works on Mars will also work on the Moon. A base at L1 and a base near a lunar pole mirror the base in Mars orbit and a Mars surface base. Both surface bases would need to have ice deposits and equipment to process the ice. Round trips between each pair of bases would each need about 5.2 kilometers per second in delta-V (change in velocity), and thus a roughly equal amount of propellant.
Unfortunately, the official sources are still adamant that both expendable rockets (like the SLS) and expendable spacecraft (like the MAV) must be used for both lunar and Mars expeditions. They let people talk about reusability but they will not allow it to actually be implemented in hardware.
This is starting to create the impression of a huge technological and policy rift, where the NASA leadership and the private companies will no longer be in sync with each other. Rick Tumlinson, in a September editorial in SpaceNews, sums up the current state of affairs very well. He then points out how the rift between the public and private sectors could eventually reach a point where governments might prevent upstart companies from landing on Mars at all. There are plenty of reasonable-sounding grounds for this, such as preventing contamination of Mars by bacteria-laden human explorers for as long as possible. The question is, how long could such a moratorium against landings last? Other countries would then surely fill the gap and land first. Tumlinson then proposes a solution: a partnership between the private and public sectors, with the private sector providing the transportation and the public sector developing space infrastructure and conducting a more intense search for good landing sites.
In the same vein, Lori Garver, former deputy administrator of NASA, points out in an interview with Alan Boyle that “giving the private sector more of a role in space activities is the best way for NASA to accelerate the pace of space exploration and exploitation”. This includes Mars missions. In a related interview with Boyle, she said, “Getting the costs down to get to space. That’s key, that’s been a barrier, and that is happening.”
The recent article here about The Planetary Society’s Mars plans shows how it closely adheres to current NASA thinking. What is being proposed is a sacrifice of the space station program on the altar of the SLS. This would then be a space program without any infrastructure in space at all. There would be no L1/L2 logistics base to launch efficient Mars missions from via an Oberth Earth flyby. There wouldn't even be a space station in LEO to act as a transfer and fueling station to reach L1/L2. There would be no logistics base in Mars orbit to serve as a crew refuge and to support landings. There would be no reusable spacecraft, period, nor would there be any infrastructure left behind anywhere in space for the next crew to use. All would be accomplished via the unaffordable SLS launches.
Finally, researchers at several NASA centers have proposed a COTS-like model for access to cislunar space and the moon, with the direct intent to extend it to allow access to Mars. Such an approach would actually fit within the NASA budget. Its continuing limitations have stopped other such programs dead in their tracks. It is significant that it takes only about 75 tons of LOX and LH2 propellant to bring 25 tons of the same propellant from the lunar surface to L1 or L2, including a round trip back to the Moon for the tanker. To bring the same amount of propellant to L1 from the Earth would take at least 1,000 tons (assuming the bulk of it is first stage methane-oxygen). This means that it would take at least 13 times more energy to get the fuel for a Mars mission to the logical starting point, underscoring the importance of infrastructure in cost reduction.
So the following points are clear. To go to Mars we need: reusable rockets and spacecraft; a public-private partnership for space transportation and missions; logistics bases in LEO, near the Moon, and in Mars orbit; and a willingness to fund cost-effective but also high-mass and robust lunar and Mars missions. If at least some of these points are not met, it will be many decades before we will go to Mars, if at all.