Last month we examined several component options for our Mars rover. Here’s a link to that issue for a refresher. This month we’ll evaluate an open-verses-pressurized cabin, then select the elements we want to include in our vehicle.
     The Martian atmosphere is thin. Mars’s mean surface pressure is about 600 pascals (Pa), comparable to an Earth altitude of 44 km (27 mi). Astronauts must wear an EVA suit whenever outside.
     Most popular movies portray Martian rovers as enclosed vehicles. Let’s face it, Mark Watney in The Martian wouldn’t travel across Mars in an open cockpit to Schiaparelli Crater. The film would have been less compelling if his face had always been hidden behind a reflective visor.
     But as noted in Part 1, an open cabin neatly solves issues related to toxic dust contamination, which is excluded by the EVA suit itself. No inboard filtration system is needed.
     Heating the suit, critical for mid-winter days or mid-summer nights, can be facilitated by plugging it into a heat rejection system associated with a radioisotope thermoelectric generator. Heating a full cabin would require a larger, heavier RTG.
     But if we want a pressure vessel, carbon fiber composite is the material of choice. Offering high tensile strength and stiffness at a light weight, this material is extensively used in aerospace. Our cabin would employ a three-layer construction: an interior flat cross-fiber pressure vessel, a honeycombed or corrugated middle for insulation and additional sturdiness, and an exterior flat cross-fiber layer to resist impacts and abrasion.
     The question arises, should the cabin be windowless like a thermos bottle on wheels, or should it have windows? Using cameras and view screens makes sense from a structural integrity standpoint, especially if fore, aft, and side views are accommodated. Cameras have the advantage that they can be zoomed in to more closely examine features without an EVA.
     But there is precedent for windows in spacecraft. The ISS cupola's outer framework is made from forged aluminum, with an inner steel frame. Each window is composed of 4 separate layers, an outer debris pane, two 25 mm pressure panes, and an inner scratch pane. The pressure pane and debris pane are composed of fused silica glass, offering good transparency, heat retention, and high radiation resistivity. The panes can be replaced after an external pressure cover has been fitted.
     A pressurized cabin requires some form of an airlock. Let’s examine three basic ways to provide one, each with their own set of challenges: a “standard” airlock, a suitport, or a reduced cabin size.
      A standard airlock is the bulkiest option. It employs two hatches to allow ingress/egress from the rover interior to the airlock, then from the airlock to the outside. The airlock’s atmosphere will have to be compressed and stored to exit. Or it could simply be evacuated with the astronaut’s exit and replenished from a supply of compressed air when the astronaut returns. The large size could be mitigated if its walls are  inflatable, allowing the two hatches to rest against the side or back of the cabin when not in use.
     A suitport is a hollow horizontal tube with a small round hatch sealing it off from the rover cabin. The outside “door” is the bottom half of an EVA suit. An astronaut slides feet-first into the tube, wearing the top half of the suit, plus gloves and helmet. Once sealed into the tube, the top and bottom of the EVA suit are attached and seated. The interior gasses are pumped and stored or expelled when the user pushes out of the tube. The reverse steps allow reentry into the rover’s cabin. Compared to a standard airlock this reduces the pumped or replaced air volume.
      The downside is the system is clumsy. A fool-proof means of sealing and unsealing the two suit halves is critical. If not properly connected, an astronaut may not be able to retain consciousness long enough to correct the situation when outside. If the whole suit doesn’t release properly, she/he would be stuck inside the tube.
     A failure to liberate the suit from the exterior port would prevent an EVA. An inability to reconnect the suit to the port would strand the astronaut outside, or risk loss of interior cabin pressure when the hatch is opened. Suitports have been described in science fiction but may pose too many safety risks to be practical.
     The third option is to reduce the cabin size to fit a single occupant. Depending on how tight the dimensions are, an astronaut may be constrained to leave his/her EVA suit on, minus the helmet and gloves. Forcing an individual to remain suited for several days introduces nearly the same considerations—noted below—as an open cabin craft, but with more weight and certainly more expense.
     To recap, a pressurized rover would require a composite body. It would need a source of heat for the additional space, an air filtration system to minimize exposure to perchlorates and heavy metals (see Part 1). Visibility must be accommodated either via video screens or windows. Lastly it would have some form of airlock. But all these elements increase vehicle volume and mass, and therefore mission cost.

     Let’s assume we’ve convinced our billionaire to fund our Martian rover project. Our first task becomes specifying what the rover would be used for. If it’s the first missions to Mars, it will mostly move habitat modules into position for assembly, then transfer equipment from supply vessels. Once the base is assembled, it will be used to deliver scientific or communications equipment to, or collect samples from, nearby locations.
     An open cabin means wearing an EVA suit full-time. This entails wearing a diaper, or Maximum Absorbency Garment (MAG). These can absorb a thousand times the weight of the absorbent. But sooner or later, a colon must be voided. This gives a practical EVA duration limit of twenty-four hours. Having to wear a dirty MAG for several days could put a real crimp in astronaut comfort, let alone recruitment. For extended trips a closed cabin could offer a facility like a vacuum toilet used on the ISS. An open cabin is most feasible for single-day missions.
     Given the mass and volume constraints of early missions, including SpaceX’s Starship, and the minimal distances to be traversed on Mars, let’s opt for an open cabin rover. This means that missions will be restricted to single day affairs. But if we design our vehicle to travel 5 kph, our rover’s effective range would be 40 or 50 kilometers. That’s more than enough range to establish an enduring human presence on the Red Planet and perform research. A closed cabin model will have to wait until multiday trips become a necessity.
     Our power source will be a hydrogen fuel cell, with a small RTG to warm critical components and supplement EVA suit environmental systems. It would accept interchangeable mechanical arms, to support its missions like the Lunar Terrain Vehicle, and a trailer to carry spare parts, tools, mission equipment and samples. Our rover will use a tubular titanium frame with a rocker-bogie suspension and wheels similar to current Mars rovers, but capable of travelling at a blistering 5 kph.
     If anyone happens to know a billionaire willing to fund a prototype to demonstrate to NASA in ten years, have them reply to this email. But until then, expect future editions of this newsletter monthly. Plus, my next novel, Red Planet Lancers coming this February. Happy reading!

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Want a deeper dive? Check out these sources.

https://airandspace.si.edu/collection-objects/wheel-lunar-rover/nasm_A19750830000
https://www.nasa.gov/press-release/nasa-pursues-lunar-terrain-vehicle-services-for-artemis-missions
https://en.wikipedia.org/wiki/Crewed_Mars_rover#:~:text=Crewed%20Mars%20rovers%20%28also%20called%20manned%20Mars%20rovers%29,the%20crew%20to%20work%20without%20a%20space%20suit.
https://mars.nasa.gov/news/9474/nasas-oxygen-generating-experiment-moxie-completes-mars-mission/?ref=upstract.com
https://www.castrol.com/en_us/united-states/home/castrol-story/newsroom/features/keith-campbell-space-engineering.html
https://www.universityofcalifornia.edu/news/making-methane-mars#:~:text=It%20utilized%20a%20solar%20infrastructure,produce%20breathable%20oxygen%20from%20water.