Sample return missions have the ability to vastly increase scientific understanding of the origin, history, current status, and resource potential of solar system objects including asteroids, comets, Mars, and the Moon. However, to make further progress in understanding such bodies, detailed analyses of samples are needed from as many bodies as possible. A standoff sample collection system concept has been developed that would quickly obtain a sample from environments as varied as comets, asteroids, and permanently shadowed craters on the Moon, using vehicles ranging from traditional planetary spacecraft to platforms such as hovering rotorcraft or balloons on Mars, Venus, or Titan. The depth of penetration for this harpoon- based hollow collector was experimentally determined to be proportional to the momentum of the penetrator in agreement with earlier work on the penetration of solid projectiles. A release mechanism for the internal, removable sample cartridge was tested, as was an automatic closure system for the sample canister.
A harpoon test facility has been designed and built in order to benchmark various harpoon and collector designs. The test facility uses a spring-loaded steel cable to launch test harpoons into drums of dry sand, or into mixtures of sand and gravel, ice, snow, or gravel and snow/ice mixtures. The cable is drawn back by a remotely operated electric winch to a predetermined tension, and then is fired remotely via solenoid activation of the trigger mechanism. Considerable thought has gone into the design of what is essentially a modern version of a medieval ballista to ensure that this system can be operated as safely as possible. The current design uses dual six-spring steel custom automobile leaf springs coupled to a 1/2-in. (≈1.3-cm) steel cable to power the test stand. Tension on the bow string is measured by a 10,000-pound (≈44-kN) strain gauge coupled between the winch and the trigger mechanism. The tension is read remotely via a readout unit attached to the gauge at the end of a 20-foot-long (≈6-m) cable. The system can fire harpoons into any of four target media held in drums beneath the ballista: sand, pebbles, rock salt, and vermiculite. Other targets can be added as appropriate.
The data collected using this testbed clearly demonstrated that the penetration depth of the harpoons varied with the momentum. This has several important implications. First, it means that it will not be possible to lightweight this system as more massive harpoons at the same velocity will penetrate deeper into any given target. While this is similar to the behavior determined for solid projectiles, these harpoons are hollow and it is uncertain if the compression of the target material in the gullet of the harpoon would significantly change the behavior of the harpoon. A surprising result of these tests was that the tip geometry of the harpoon had no measurable effect on the penetration depth for any of the media tested.
This work was done by Joseph Nuth, Donald Wegel, Lloyd Purves, and Edward Amatucci of Goddard Space Flight Center. GSC-16706-1