Astrobotic is developing lightweight robotic excavators for the Moon to recover the rich volatiles at the poles. Recent data indicate that not all of these volatiles exist only in hard-to-reach dark crater floors -- some may be found in regions that periodically receive sunlight, but covered by an insulating layer of soil. These Astrobotic machines enable removal of the insulating dry soil to mine the water ice and other volatiles, with solar-powered rovers. (Going into permanently dark crater floors may require nuclear isotope power, which a commercial operation may have difficulty obtaining and getting approval to launch.) Polar volatiles can be transformed into rocket propellant to refuel spacecraft for their return to Earth, thus making future space exploration sustainable by enabling explorers to start "living off the land." Low mass machines in low gravity environments can only produce limited traction with which to resist excavation forces. Bucket-wheels keep excavation resistance low by taking only small bites of regolith. They achieve high production by taking a large number of these small bites repeatedly. Mounting a bucket-wheel transverse to an excavator’s driving direction further reduces load in the tractive direction by making use of lateral wheel forces. Experimental apparatus has been built (see image above) for measuring excavation forces and production rates of bucket-wheels digging in lunar simulant. The effects of bucket-wheel orientation on excavation forces will be fully characterized, and a bucket-wheel design and configuration resulting in high productivity and low excavation resistance will be distinguished for excavator development.