In May, NASA solicited a Lunar Terrain Vehicle (LTV) contract for service. The LTV will function like a cross between Apollo lunar and Mars uncrewed rovers. It will be driven by astronauts or remotely as an exploration platform. It must have capacity for two suited astronauts, accommodate a robotic arm or mechanism to support science missions, and survive the extreme temperatures at the South Pole. The press release is unclear if the Agency prefers a pressurized or open cabin concept. Award is scheduled for November 2023.
It looks like we’re too late to get in on this competition. NASA’s contract-for-services business model means we would need a developed vehicle, ready for the Agency to evaluate. Instead, let’s examine wheeled transport on the Red Planet. This gives us about a decade to consider the design parameters for our rover. That makes this a conceptual exercise, the substance of speculation. In other words, the stuff of science fiction!
Whether open or enclosed, a Mars vehicle must survive the intense cold. Mars’s equatorial Gale Crater sees average temperatures of summer highs of 36 degrees F and summertime lows of minus 105 degrees F. In winter the highs are minus 9 degrees F, and lows minus 126 degrees F. Winters at the south pole drop to a frigid minus 243 degrees F, frigid enough to maintain a frozen carbon dioxide ice cap.
Let’s start with lubricants, so vital for minimizing wear on moving parts. Here on Earth, arctic-rated lubricating greases operate down to minus 60 degrees F. Industrial graphite-based lubricants work down to minus 300 degrees F. The lubricant developed for NASA in the1960s lunar rovers is used to grease the Mars rover Curiosity, keeping it rolling for eleven-plus years. We’re covered here.
What about the cold’s impact on energy sources to propel our rover? Elon Musk and Space X intend to use methane for Starship’s fuel, making it a likely candidate. This fuel could be produced on the Red Planet using the Sabatier process, where an electrical power supply electrolyzes carbon dioxide, which, when mixed with water, produces methane.
Methane's boiling point is minus 260 degrees F, providing pressure to inject the gas into a combustion chamber. But a barrier to internal combustion engine (ICE) use on Mars will be the operating temperature of the motor oil. The best arctic-rated oils are only flowable down to minus 60 degrees F. If the block is not uniformly heated prior to start-up, piston rings could wear prematurely, accelerating oil loss. Uneven heating could also lead to gasket leaks or even a cracked block.
A second on-board energy source will have to keep the engine and crank case warm when the motor is not running, especially at stops away from the base and its electrical power supply. Another option for our rover is a hydrogen fuel cell (HFC).HFCs catalytically convert oxygen and hydrogen into water to produce an electric current, powering an electric motor. Water should be readily available since it’s a resource which will have to be present at any viable habitation.
Most HFCs operate at a limited temperature range, considerably warmer than Mars’s ambient temperatures. They too, will have to be heated. ICEs and HFCs have their challenges in the cold. What about batteries? Automotive lithium-ion batteries can be charged from 32degrees F to 113 degrees F and discharged from minus 4 degrees F to minus 140degrees F. The extreme cold of Mars would render them unchargeable and inoperable. Or worse, the water portion of the electrolyte would freeze, rupturing the casement.
Solid-State EV batteries can perform at temperatures as low as minus 22 degrees F and charge faster than lithium-ion. An improvement, but it’s obvious that batteries require a reliable supplemental heat source to function.
Radioisotope thermoelectric generators (RTGs) convert the heat of radioactive decay into electrical current. NASA already plans to use RTGs based on Uranium 238 to power lunar and Mars habitats. These power sources can warm themselves to maintain the thermocouple’s required operating temperature. Drawbacks are that they need radiation shielding, add weight to the transport space flight, and potentially risking astronaut health.
The Mars Curiosity and Perseverance rovers employ RTGs for power. They also warm internal components by conduction, electrify strategically placed electrical heaters, and enable the rover heat rejection system (HRS). The HRS uses fluid pumped through 60 m (200 ft)of tubing in the vehicle so that sensitive elements are kept at optimal temperatures.
Whether we choose a closed or open cabin concept, an RTG-based HRS could heat a berth or plug into an EVA suit via a quick connect coupler to keep astronauts warm. Mars is more arid than any desert on Earth. Fortunately, the need for sealed bearings and bushings has largely been solved by current technology. So this is also settled.
But Martian dust contains a health hazard not commonly found here, perchlorate. When inhaled, this compound causes extreme respiratory inflammation. It clings to EVA suits, tools, or directly enters cabins through airlocks. Air filters and/or N95 or greater masks must be used for astronaut safety. Ironically, this argues for an open cabin design approach. Having astronauts travel in already sealed pressure suits adds no additional risk of contamination.
Any rover intended for long-term use requires provisions for maintenance and repair. There are four critical components that could result in mission failure without spare parts or an ability to service: wheels, suspension, powertrain, and pumps.
NASA has learned a great deal about wheels and wheel durability from Curiosity and Perseverance. They are hollow aluminum cylinders with cleats for traction and curved titanium spokes for shock absorbency. Given the increased payloads to be carried by future Mars rovers, they should arrive with a pair of spare wheels. Replacements required beyond that number can be delivered on subsequent missions.
The Agency’s unmanned Mars rovers utilize a rocker-bogie suspension system. This is an integration of tubular titanium frame and suspension elements. In essence, the cabin swivels about the frame between the front and middle pairs of wheels, maintaining its level position independent of the terrain. This design avoids shock absorbers though, increasing the risk of wear and tear. And obviating low-speed travel.
The best way to repair breaks is to utilize identical components for different areas of the frame. Employ similar couplers for joints and bends. Use similar tubing for frame components. In this way, a handful of spare parts could be used in a variety of locations, using identical fasteners. A single wrench would be all that’s required to service any frame and suspension failure.
Dissimilar power sources have differing maintenance and repair needs. A methane-burning ICE would be comparable to a car engine, but with a fuel-oxygen injection system. While the injector would isolate the interior from the dusty environment, oil would still burn away overtime. Astronauts won’t be able to head to the nearest parts store for a case of oil. A careful assessment of oil use verses travel time will be required to send enough to last several missions.
Moving parts like timing chains, cams, rocker arms and bearings will have to be designed for greater durability than engines here on Earth. A few smaller components like pumps, tubing, etc., can accompany the delivery flight.
If hydrogen fuel cells are used for propulsion, two points of failure must be addressed. The electrolytic membrane itself is susceptible to chemical erosion over time. Current research is leading to more durable nano-scale proton transfer structures.
At the other end of the power train lies the electric motor and gearhead. NASA uses one motor per wheel on the Perseverance and Curiosity rovers. This redundancy allows for continued operation should one drive motor fail. It simplifies overall design by eliminating transaxles to transmit torque to six wheels from a single motor. Electric motors are used to actuate drills, mechanical arms, etc. This also argues for standardization. Use identical motors for all wheels. One replacement motor could suit the needs of multiple missions.
The final components are pumps—for delivering heated fluids from an RTG, or for moving gases from one place to another like from an airlock to storage tanks. Pumps are simple devices, consisting of an electric motor and an impellor for liquids, or a piston and valves for gases. Assuming the motors are standardized, spare pumps could easily accompany rover deliveries.
That’s enough of a progress report for now. Next month, we’ll explore whether our rover should have a pressurized cabin or not, then decide on the final design elements. In the meantime, scroll down for an exclusive cover preview for my next novel, Red Planet Lancers, coming this Winter!
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Want a deeper dive? Check out these sources.
https://airandspace.si.edu/collection-objects/wheel-lunar-rover/nasm_A19750830000https://www.nasa.gov/press-release/nasa-pursues-lunar-terrain-vehicle-services-for-artemis-missionshttps://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.comhttps://www.castrol.com/en_us/united-states/home/castrol-story/newsroom/features/keith-campbell-space-engineering.htmlhttps://www.universityofcalifornia.edu/news/making-methane-mars#:~:text=It%20utilized%20a%20solar%20infrastructure,produce%20breathable%20oxygen%20from%20water.
