How The Lunar Rover Batteries Worked—And Nearly Failed—On The Moon

By 10 Min Read

When manufacturers develop new powertrains, they frequently test them in the most extreme conditions on Earth. For example, the all-electric Rolls Royce Spectre was subjected to temperatures as low as -40F (-40C)  and as high as 122F (50C), so engineers could determine how its range would hold up in extremes. This harsh-climate exposure is crucial for BEVs, as batteries lose a lot of efficiency in freezing or scorching temperatures.

There is no climate on Earth, however, that compares to the intensity of the Moon. 250 degrees F (120C) during the day, -200 degrees F (-130C) at night, and that’s in the Moon’s temperate zones. So how did NASA manage to build an electric Moon rover that could operate in such extreme conditions, all the way back in the 1970s?

A Very Expensive One Horsepower

Although a Moon mission vehicle was in the works from the very beginning of the Apollo program, the first Moon rover—officially known as the Lunar Roving Vehicle (LRV)—wasn’t launched until the fourth manned Moon landing, Apollo 15, in 1971. The end goals for the LRV were simple: increase the area astronauts could explore, and help carry Moon rocks back to the lunar lander. 

Spaceflight has a way of making simple demands challenging to meet, however.

The need to reduce the LRV’s weight required incredibly high-tech construction. Liberal use of aluminum and titanium got the LRV’s curb weight down to just 460 pounds, or roughly half that of a typical golf cart; Despite this, its payload was more than 1,000 pounds. It had to be foldable to stow on the lunar landing module; When launch-ready, it folded down to just 20 inches thick. It had zinc-plated, woven steel mesh wheels that could deform and spring back into shape, rather than pop or bend, to avoid blowouts on craggy rocks. Engineers gave it independent four-wheel-drive and four-wheel-steering for maximum traction and maneuverability.

Power demands were minimal, as the LRV would weigh just 76 pounds in the lighter gravitational field of the Moon. As such, it had just 1 total horsepower (.75 kilowatts), supplied via four quarter-horsepower series-wound DC motors (one at each wheel). Each motor had its own harmonic drive with an 80:1 gear reduction, to improve torque and response. The motor casings were pressurized to 7.5 PSI to prevent dust from entering the housings and ruining the brushes. Its official top speed was 8 MPH, although it cracked 11 MPH during Apollo 17 when astronaut Eugene Cernan floored it down a hill with a full payload of Moon rocks. 

Originally, the budget for the LRV was just $19 million; cost overruns put the final tab at $38 million, making it the worst dollar-per-horsepower deal in history.

NASA LRV (Lunar Roving Vehicle)

First Drive: The 1971 Boeing/Delco Lunar Roving Vehicle

Because the LRV was low-power, it demanded a light, compact battery pack, rather than a large high-capacity one. Since it was intended to be a single-use machine (all three LRVs ever taken to the Moon are still parked on it), the batteries also didn’t need to be rechargeable. As such, a pair of 36V, 4.1-kilowatt-hour silver-zinc batteries were used—a far cry from the 80+ kWh battery packs of modern cars. Silver zinc batteries are only minimally rechargeable, lasting anywhere from 10-50 cycles. However, they possessed the highest energy density of any battery in existence prior to the development of lithium cells, at around 220 Wh/kg (vs. modern lithium-ion’s 270 Wh/kg). For single-use applications, they were unbeatable in the 1970s. As a result, silver-zinc served as the gold standard for aerospace batteries in both the U.S. and the Soviet Union. 

Total usable range from the pair of batteries was rated at 57 miles during normal operation, with one battery powering the front wheels, and one powering the rear. The twin-battery design was built for redundancy, so if one battery died, the other could still power the entire LRV. This reduced safe usable range. NASA planners also wouldn’t allow astronauts to drive the LRV further away from the lander than they could walk, in case of a complete breakdown. This limited it to a six-mile radius of operations. The furthest an LRV was ever driven on a single mission was the final manned Moon landing, Apollo 17, where astronauts covered roughly 22 miles during the three-day expedition (including one continuous 12-mile drive). 

In a decades-later interview with Wired, astronaut (and rover driver) Charles Duke recalled that on Apollo 16, despite driving the LRV over 15 miles, “we didn’t even come close to running out of power.”

NASA LRV (Lunar Roving Vehicle) from the Lunar Rover Manual

Mirror, Mirror, On The LRV, Who’s The Hottest Of Them All

Temperature control was, unexpectedly, challenging. Silver zinc batteries perform poorly in cold temperatures, but the LRV was only intended for use during the 15-Earth-day-long lunar daytime, which meant that keeping them from overheating was the primary concern. The lunar surface itself can reach temperatures hotter than boiling water during these fortnight-long days, but since the Moon has no atmosphere, local temperatures vary wildly and are primarily determined by time spent directly in sunlight. The battery’s maximum operating temperature was rated at 125F (52C) and maximum survival temperature was 140F (60C), which it would rapidly exceed if exposed to broad daylight for too long.

This was easily solved during transit: the LRV was stored folded flat on the outside of the lunar lander. The lander would slowly rotate like a rotisserie chicken as it orbited the Moon, evenly distributing direct sunlight to each side, keeping temperatures manageable. On the surface of the Moon, however, there was no hiding from the rays of the sun, and the batteries would rapidly exceed their 140F limit. 

To solve this, both batteries were given passive radiators. Rather than using a traditional active liquid cooling system, engineers used fused-silica mirrors to reflect as much light—and remove as much heat—as physically possible. When the LRV was moving, the mirrors would be covered with a dust shield, to prevent the extremely fine lunar soil from coating the mirrors, which would ruin their ability to reflect light and radiate heat.

NASA LRV (Lunar Roving Vehicle)

Tom Hanks Tensely Driving A Moon Rover (or, Apollo 16)

This worked… to an extent. On Apollo 15, short trips and a low Sun angle kept the batteries within normal temperatures, but on Apollo 16, the landing location had higher temperatures. Making matters worse, an early accident meant the LRV lost a fender. A rooster-tail of fine dust followed the rover everywhere it went, and it coated everything—including the mirrors. The batteries refused to cool off in between drives, even after removing the heat shields and brushing off the mirrors, as the lunar soil retained heat too efficiently. The batteries could be slightly cooled by parking the LRV in the shade, but if it stayed in the shade for too long, the extreme cold would kill other electrical components. 

Mission Control had astronauts switch from one battery to the other for power supply, trying to let each one cool off for longer periods. Despite this, by the final excursion on the third day of Apollo 16, the LRV batteries exceeded their maximum survival temperature, reaching 143F (62C) by the end of the mission. Luckily, the LRV batteries worked fine despite the scorching temperatures, and no astronauts had to walk home. On Apollo 17, careful dust control and parking the LRV further away from the Lunar Landing Module—which radiated immense amounts of heat—kept battery temperatures within acceptable ranges. 

NASA LRV (Lunar Roving Vehicle)

To Artemis And Beyond

While NASA has planned humanity’s return to the Moon with the Artemis project, there’s no official word on which car is going up next, although there are many contenders—one of which is an Ultium-powered, GM/Lockheed joint project. While the future of Ultium is still in the air, it’s likely whatever vehicle traverses the moon next will use lithium-ion batteries just like terrestrial EVs, with similar temperature management strategies. It’s probable the new rover will have remote-control capabilities, as well, like the NASA VIPER Moon rover probe set to launch later this year.

The one thing that engineers will still need to contend with, no matter what? Dust… dust never changes.

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