Science in the driving seat

Lunar exploration came of age with the J-class missions of Apollos 15, 16 and 17. Traverses on foot to single points of interest gave way to wide-ranging sorties that could visit multiple destinations. The crews were carefully coached on the skills of field geology and on the need for strict documented sampling of the rocks and soil. A powerful illustration of NASA’s move towards a science-based justification for the flights was the successful lobbying of the geology community to have a professional scientist included on the final Apollo mission.

To accommodate this expansion of Apollo’s science role, engineers made a series of improvements to both the Saturn V launch vehicle and the Apollo spacecraft. The payload of the launcher was increased by small improvements in the E-l engines and by the deletion of some of the ullage and retrorocket motors that pulled the first and second stages apart at staging. Minor improvements were made to the loading and utilisation of its propellants to ensure greater depletion at cut-off. Changes to the launch trajectory included using a more easterly launch azimuth to Lake maximum advantage of Earth’s rotation and a lower parking orbit because a rocket that did not have to lift so high could carry more weight.

Changes to the spacecraft included an extra hydrogen tank in the service module to supply more power and w’ater. Another oxygen tank had already been added in the light of the Apollo 13 incident. The LM was endowed with larger propellant tanks and additional tanks for water and oxygen along with an extra battery in the descent stage. This increased the time on the Moon from two to three days. The thrust of the LM’s descent engine was increased merely by extending the length of its nozzle to direct the expanding exhaust gases along the thrust axis before they are released to space. The limiting factor was the nozzle’s clearance from the lunar surface. The increased ability to take mass to the Moon was exploited to greatest effect by one additional piece of equipment housed in an empty bay of the descent stage; the lunar rover.

Wheels on the Moon

The extra mobility afforded by the lunar roving vehicle (LRV) had a profound effect on the scientific harvest that was gained from the Apollo J-missions and there were many reasons for this. It could take crews to diverse sites for study and sampling. It gave them a little physical rest as they drove between planned stops. It could carry a substantial load of tools, cameras and ultimately rock samples. And also by virtue of a remotely-controlled TV camera, each geology stop could be supported by the eyes and knowledge of the scientists and engineers on Earth.

The rover was an ingenious device that managed to fulfil a wide range of tasks within extremely narrow constraints. It had to be light in weight and foldable in order to be carried aboard the LM. It had to withstand the rigours of launch and the passage across cislunar space as well as being able to operate in the extremes of dust, vacuum and temperature on the Moon’s surface. Beyond the basic task of

John Young works at Apollo 16’s rover. (NASA)

David Scott works with engineers on a checkout of the deployment of Apollo 15’s rover from the side of LM Falcon. (NASA)

Motor controller Diagram of the layout of the lunar roving vehicle. (Redrawn from NASA source.)

transporting two crewmen and their tools across the Moon, the rover had to help them navigate while out of sight of the LM and it had to support a demanding suite of communications functions between the Moon and Earth, including live television. This unusual and highly successful contraption was designed, built and tested within 18 months of being given the go-ahead.

Л large square panel formed the central part of the chassis upon which were mounted simple foldable seats, an instrument panel and a T-shaped control stick. Two smaller chassis panels were attached at the ends, with hinges so the)’ could be folded against the central panel during flight. Each end panel held a pair of wheels which themselves were folded over the central chassis to allow the whole contraption to fit within one of the wedge-shaped bays of the descent stage. To deploy it. a crewman would pull on a lanyard to lower the rover from its bay. First the rear, then the forward chassis panels came free, both spring-loaded to fold out and engage in place. Likewise, the wheels w:ere spring-loaded to swing into their correct position and lock. Once the rover was on the ground, it was straightforward for the crewmen to lift it off its deployment hinge and begin to load it up with the tools, cameras and other equipment they would need for their traverses. The front chassis carried a pair of batteries that were installed in a fully charged state by technicians on the launch pad. In total, these could supply over eight kilowatt-hours of electrical power. Electronic packages were mounted nearby and these used the batteries as heatsinks. Engineers then arranged that when the rover came to a stop, the crew would lift dust covers to expose a series of radiators to deep space in order to release the accumulated heat.

The problem of surface navigation was solved by use of a directional gyro and by the measurement of distance based on pulses that marked the rotation of each wheel. The system’s logic was clever enough to select the third-fastest wheel so that slipping wheels would be ignored. By processing this information, the bearing and distance to an initialisation point could be displayed on the instrument panel. Normally, of course, this initialisation was done near the LM at the start of each day’s drive and it included an alignment of the gyro.

’’Okay, Bob, let me give you some numbers." said Gene Cernan to Robert Parker in Houston after he and. lack Schmitt had deployed their science station at Taurus – Littrow. They were ready to go on their first drive, but first Cernan had to initialise the rover’s nav system. He Look readings from a tilt meter that gave pitch and roll angles for the vehicle. The yaw angle could be worked out from a foldout sundial that indicated the angle of its centreline relative to the Sun.

"Sun shadow is zero. I am rolled right four degrees. I am pitch zero. I can’t be rolled right four degrees. That indicator can’t be right. I question that.’’ Perhaps the low’er gravity and the alien landscape were playing tricks with his sense of orientation. "I might be rolled left a couple of degrees. Are you happy with that. Bob? Roll indicator is indicating… Make it three degrees right.’’

"Okay, and I copy."

Almost instantly, a flight controller turned the attitude angles into a heading with respect to north w’hich Parker relayed to the Moon. "Okay, torque to 279." Cernan then slewed a heading indicator to show 279. The rover was aimed slightly north of due w’est. As they moved across the surface, the indicator would display their heading, and their route back would be shown by two numerical displays for bearing and distance.

Each wheel had its own 180-watt electric motor that was sealed into a pressurised unit along with gearing and a brake. A clever arrangement called a harmonic gear
stepped the motor’s rotation down by a ratio of 80:1 by having the motor turn an elliptical rotor inside a flexible cylinder. The cylinder had gear teeth on its outside which meshed with gear teeth on the inside of an outer ring, but only at the two ends of the ellipse. This arrangement meant that a complete turn on the inner rotor would move the cylinder by only a small number of teeth. The braking system was quite conven­tional, being operated by cable linkages which actuated drum brakes. All the control functions of the rover; steering, forward, reverse and braking; were brought into a centrally mounted T-handle that was accessible to both crewman though no LMP ever got to drive a rover on the Moon. Each wheel had a fender and these had pull-out extensions which were deployed to contain dust raised while driving. Seats, armrests and footrests were unfolded to their final positions, as was the console with its T-handle. At this point, the commander could get on, power it up and take it for the short drive to where their gear had been stored in the other stowage quads on the LM. The rubber tyres of the MET were discarded in favour of a mesh design made from piano wire that could withstand a large amount of deformation. An inner tyre of metal bands provided additional protection against hard impacts with rocks. The rover could clear a rock that protruded up to 30 centimetres. In planning a mission, there was usually some worry about whether the chosen site would be navigable to a rover. Pre-mission photography tended to lack the resolution to show small boulders that would reduce what NASA called ‘trafficability’; the rover’s ability to get about the site without being impeded by excessive numbers of rocks too large to drive over.