A two-body design has enabled an instrumented penetrator projectile to function with increased tolerance to initial misalignment among (a) the longitudinal axis of the penetrator, (b) the velocity of impact on the ground, and (c) the local perpendicular to the surface at the point of impact. In the original application, the two-body penetrator will be launched from a spacecraft to impinge on Mars, where it will sample the soil. The two-body penetrator design is easily adaptable to instrumented penetrators for sampling subsurface materials in sand, soil, mud, snow, or ice in hostile or inaccessible environments on Earth.
Older instrumented-penetrator designs feature unitary bodies that are much longer than they are wide. In a typical case, if the longitudinal axis of the body deviates from impact velocity direction by an angle of more than about 6°, then the penetrator buckles and/or fails to penetrate.
The two-body penetrator (see figure) includes a forebody and an afterbody. The forebody contains the instrumentation and machinery required to penetrate the ground; the forebody has a hemispherical tip and is shorter and stubbier, in comparison to a typical older unitary penetrator. The afterbody contains batteries, radio-communication circuitry, and those sensors (e.g., a Sun sensor and an atmospheric-pressure sensor) that must not penetrate the ground. Prior to impact, the forebody is stowed in a longitudinal cylindrical recess in the afterbody.
Upon impact, a flange on the bottom of the afterbody becomes braked upon contact with the ground. The braking action is such that in the presence of misalignment, the afterbody turns so that the recess holding the forebody becomes aligned more nearly perpendicularly to the ground surface. As the afterbody decelerates, the forebody slides out of the recess, penetrating the ground underneath the afterbody. The forebody and afterbody remain connected by a flexible cable that pays out from the forebody during impact.
The concept has been tested in experiments in which two-body penetrators were fired from an air gun into sand, soil, and cement-mix targets at speeds from 168 to 208 m/s. In one test in which a penetrator was fired into the ground at an angle of 75° with the surface, the forebody was found to have traveled into the soil at an angle of 83° with the surface; that is, more nearly perpendicularly to the surface, as intended.
This work was done by Donald Bickler and Tommaso Rivellini of Caltech for NASA's Jet Propulsion Laboratory. NPO-20295