The MAV will be an SSTO (Single Stage To Orbit) VTOL (Vertical Take-Off and Landing) vehicle with powerful LCH4/LOX engines, capable of pinpoint landing on Mars, refuelling from local Martian resources, and carrying up six astronauts to Mars orbit for transfer to the MTV. Ideally it will also be capable of then returning to Mars surface.
The MAV has two main sections:
- The lower portion is the ascent/descent stage, which includes:
- the engines
- fuel tanks (LCH4 and LOX)
- a sophisticated GNC system
- solar panels
- ISPP plant (electrolysis unit and Sabatier reactor)
- a water-mining robot called AWESOM (Autonomous Water Extraction from Surface Of Mars)
- The upper portion is a DragonRider (Pern-2) with trunk containing additional solar panels.
The MAV’s engines will need to be LCH4/LOX (liquid methane/liquid oxygen), in order that they can be refuelled from local Martian resources (see below). Several types of engines that burn methane fuel have been developed, however, SpaceX are currently developing a new LCH4/LOX engine called “Raptor” that is likely to be the preferred choice.
Based on SpaceX’s record, the Raptor design will presumably be more modern and advanced than existing methane fuelled rocket engines. It will also have a high degree of interoperability with other SpaceX hardware used in the mission, such as the Dragon. Furthermore, since SpaceX will already be involved, using more hardware from the same company should, in theory, reduce costs, if for no other reasons that a fewer number of engineers need to be paid, since engineers who understand the Dragon and Falcon are also probably the same people who understand the Raptor engine.
The engines must be powerful enough to carry the MAV from Mars orbit to surface, and back. In theory, if the MAV is capable of refuelling itself at Mars Surface, it should be able to do this repeatably (thus making it a SSTO VTOL RLS).
The MAV will have an integrated ISPP plant comprised of:
- water mining robot (AWESOM)
- electrolysis unit
- Sabatier reactor
- cryogenic propellant storage
The ISPP process currently envisaged for the MAV closely parallels that described in the Mars Direct architecture (1990). In Mars Direct, hydrogen (which represents the lightest fraction of the propellant yet also the hardest to obtain from local Martian resources) is carried from Earth, and reacted with CO2 obtained from the Martian atmosphere in order to produce methane (CH4) and oxygen (O2), which are liquified and stored cryogenically. Additional oxygen is obtained from carbon dioxide via the reverse water gas shift reaction, which produces water (H2O) that is then electrolysed to hydrogen (H2) and oxygen (O2).
The primary difference in Blue Dragon is that hydrogen is not carried from Earth, but is obtained by mining water from the surrounding regolith, and electrolysing it into hydrogen and oxygen. Every kilogram of Martian regolith is estimated to contain up to 40g of water (Slosberg), and 80% of this can be liberated from the top [x]cm of regolith using microwave radiation.
The optimal stoichiometric ratio for LOX/LCH4 bipropellant is 7:2. Mars Direct specifies a fuel requirement of 24 tonnes of CH4. Taking that as a nominal value (for now), the amount of LOX required is therefore 84 tonnes.
The process is as follows:
Step 1: Carbon dioxide is obtained from the Martian atmosphere by filtering out dust, removing water via zeolite adsorption, compressing the remaining gas mix to 700 kPa, and allowing it to equilibrate to ambient Martian temperatures (about 210K). The CO2 will condense, enabling it to be separated from the remaining gases.
Step 2: The AWESOM rover traverses back and forth across a patch of ground, microwaving the regolith below and collecting released water. When the rover’s tank is full, it returns to the MAV to deliver its payload of water, then returns to work. This step requires some form of LPS (Local Positioning System) so the rover knows which ground it has covered.
Step 3: Water is separated into hydrogen and oxygen gas via electrolysis:
2 H2O(l) → 2 H2(g) + O2(g)
Step 4: Oxygen produced in step 3 is stored cryogenically as LOX.
Step 5: Methane is produced by reacting carbon dioxide (CO2) obtained from the Martian atmosphere with hydrogen (H2) produced in step 3, via the Sabatier reaction:
CO2(g) + 4 H2(g) → CH4(g) + 2 H2O(v)
The Sabatier reaction occurs at high temperatures, optimally around 600K. Water produced by the reaction is cycled back into the electrolysis unit to produce more hydrogen and oxygen.
Combining the reactions in step 3 and 5, the overall result is:
CO2(g) + 2 H2O(g) → CH4(g) + 2 O2(g)
66 tonnes of carbon dioxide combined with 54 tonnes of water (a total of 120 tonnes) will produce 24 tonnes of methane and 96 tonnes of oxygen. This gives us a surplus of 12 tonnes of oxygen that may be used to supplement air supplies for the Hab or the Rover.
Obtaining 66 tonnes of carbon dioxide requires processing about 70 tonnes of Martian atmosphere. The density of the atmosphere at the surface of Mars is about 0.02 kg/m3, which means 3.5 million cubic metres of atmosphere
If our goal is for the MAV to be fully fuelled within 20 months (the time difference between the arrival of the MAV at Mars and the departure of the crew from Earth), that means processing about 117kg (5830 m3) of atmosphere per day, or 80 grams (4 m3) minute. It remains to be seen how achievable this is. However, there is 44 months between the arrival and departure of the MAV, therefore, a slower processing rate may still be acceptable, although the goal of ensuring that the MAV is fully fuelled before the crew leaves Earth would not be met.
The ability to make use of Martian water is fundamental to the Blue Dragon architecture; therefore, it’s preferable to locate Marsopolis in a region where the water content is reasonably high, while still aiming for lower latitudes in order to simplify landing and to maximise solar power.
The diagram below shows the lower limit of water concentration across Mars.
The proposed location for Marsopolis (discussed in the Location section) is in Arcadia Planitia, a region to the north-west of Olypmus Mons. The water content at the landing site should be at least 10% by mass, and the presence of grooves and ridges in the region are indicative of ground ice in the near surface. Furthermore, Arcadia Planitia is extremely flat, which will support autonomous water mining by a mobile robot.
If we estimate that the average water concentration of the regolith is 10% by mass [ref], and that 80% can be extracted from the top decimetre of regolith by microwave radiation [ref], 675 tonnes of regolith will need to be processed.
675 tonnes of regolith = 67.5 tonnes water + 607.5 tonnes dry soil
67.5 tonnes of water * 80% captured = 54 tonnes
The density of dry Martian soil is about 1.4 g/cm3 = 1400 kg/m3 [ref]. If the regolith is 10% water, its density will be about:
10% * 1000 kg/m3 + 90% * 1400 kg/m3 = 1360 kg/m3
The volume of regolith to process is therefore:
675,000 kg / 1360 kg/m3 = 496 m3
Assuming we can only extract water from the top decimetre of regolith by microwave radiation, the area that must be covered by the rover is about:
496 m3 / 0.1 m = 4960 m2
If the AWESOM rover has a 1 metre wide catchment, it must therefore traverse a 100 metre long strip about 50 times (or cover a square about 70 metres on a side).
Specifying the same 20 month time constraint, AWESOM must collect 90 kg of water per day, which requires covering about 8.3 m2 of terrain per day on average. This seems like it should be easy. It will be preferable to collect the water as rapidly as possible, for peace of mind, and also because H2 is needed before CH4 can be produced. The design goal will be to build the robot to be capable of carrying up to 100 kg of water; it should therefore return to the MAV to unload at least once per day.
It may be that a single AWESOM is capable of collecting enough water for the MAV as well as for the Hab; although, considering the importance of water, having two at the site will still be preferable. Note that there’s no reason why the AWESOM must be delivered to Mars inside the MAV. It could be delivered separately in one of the cargo modules, and drive itself over to the MAV.
It’s true there there would be a benefit if the MAV and AWESOM were an integrated package (i.e. the AWESOM robot is carried inside the MAV), because then the MAV could refuel anywhere on Mars; it would simply land and deploy its AWESOM to collect more water. However, for our early MAV designs it will be preferable to reduce mass as much as possible.
It may even be preferable to keep all ISRU hardware separate, in, for example, a Green Dragon capsule, and transfer propellant to the MAV via hoses. There are several benefits to doing this, such as a more efficient integrated ISRU design, and reduction in the landed mass of both the MAV and Hab. However, connecting up hoses may be difficult to do autonomously, and it will be better if the MAV is fully fuelled before the crew leaves Earth.
Keeping water in a liquid form within the rover will require heat, perhaps more than can be reliably produced from solar cells. An RTG (Radioisotope Thermoelectric Generator), like the one used in Curiosity, will provide both heat and electrical energy.
One of the primary challenges associated with the MAV is the power system.
In Mars Direct, once the MAV (a.k.a. ERV) lands on Mars, a small robotic truck emerges with an nuclear reactor. The truck carries the reactor some way off, trailing electrical cable connecting it to the MAV in order to provide power to the ISPP plant.
The reason why it must be distanced from the MAV is because the reactor will be unshielded and the MAV is designed to carry people. Shielding is typically very heavy and would add a lot of mass to the MAV’s payload, making it much harder to land on Mars. If an unshielded reactor is operated inside or near the MAV, it would irradiate the vehicle, making it unsafe for carrying people. Therefore, it must be moved a safe distance away where radiation from the reactor will not reach the vehicle.
There are some issues with this design. If the reactor is unshielded, even if it’s moved some distance away from the MAV (and the Hab and other parts of the base), it will create a zone around it into which the astronauts must not venture. Perhaps the MAV will not be affected by radiation from the reactor, but the natural environment surrounding the reactor will. We should treat Mars a little more responsibly than this. What if the reactor is parked near something of scientific interest? What if the reactor needs attention – for example, the cable becomes loose?
If the reactor is shielded, it achieves two things. It means it does not need to be relocated from the MAV, thus eliminating the mass of the robotic truck and partially offsetting the mass of the shielding. It would also allow for the reactor to be kept contained within the MAV, enabling it relocate itself to almost anywhere on Mars and refuel (as long as there is sufficient water in the surrounding regolith). This would be an extremely useful capability, but not essential for the first H2M mission.
One of the advantages of using a nuclear reactor, in the Mars Direct design, is that it represents a reliable source of abundant energy that can be used to convert the hydrogen into methane as quickly as possible, in order to minimise boil-off and ensure that ample fuel will be manufactured.
However, if we commit to using local water as a hydrogen source, there will be no boil-off, and less reason for to manufacture all the propellant as quickly as possible. Considering this, as well as the difficulties associated with using a nuclear reactor to power the ISPP process, it will be better to use solar panels.
Solar panels represent a fluctuating energy source that will vary with the diurnal cycle, seasonal cycle, and atmospheric dust levels. As a rule of thumb, daytime solar energy on Mars is about half that of Earth. However, we have about 20 months between when the MAV arrives at Mars and when the crew leaves Earth at the subsequent launch opportunity, which should be plenty of time to make the necessary propellant if we wish to ensure that the MAV is fully fuelled before the crew leave Earth. Actually, there is about 44 months between when MAV’s arrival and departure, which should certainly be ample if that much time is needed.
The Dragon’s trunk contains solar panels, which can be used to provide some of the power necessary for ISPP, and the MAV’s ascent stage may include additional panels as required.
By choosing solar panels over a reactor for ISPP power, we eliminate the mass of the reactor, shielding, the robotic truck to relocate it, and the electrical cable that would connect the reactor to the MAV. We eliminate the risk of an unshielded reactor irradiating the MAV, or the surrounding terrain, or the crew.
If the AWESOM can be carried inside the MAV, using solar panels for power also provides for relocating and refuelling, because the solar panels can be folded up inside the vehicle before launch.
Are solar panels a reliable energy source on Mars? The Mars Exploration Rovers Spirit and Opportunity were powered by solar energy. Spirit was active for 6 years, and Opportunity has been going for 9.
Differences from Mars Direct ERV
- This is a much lighter vehicle. It does not include the mass of:
- 6 tonnes of hydrogen brought from Earth
- Nuclear reactor
- Light robotic truck for relocating reactor
- Electrical cable to connect MAV to reactor
- Instead, it includes:
- AWESOM water-mining robot (although this could be delivered separately)
- Solar panels
- No need for reverse water gas shift reactions or dissociation of CO2 to produce additional O2, as ample is obtained from water electrolysis.