NEW RESULTS FROM APOLLO PROGRAM SCIENCE

Apollo lunar missions remain the most ambitious manned spaceflights in history, five decades after their conclusion. Apollo free-return mission design atmospheric reentry speeds reached 11.1 km/s, or 98.66% of Earth escape kinetic energy. Heat shield testing at such conditions was a major challenge, requiring high-reliability science. The 1960s solution involved analytic equations with numeric simulation, both verified by hypervelocity plasma arc jet lab testing, a triple standard for reliable design.

Researchers at NASA Ames used tektite specimens for the task. This mostly silicate cosmic impact ejecta melt had been blasted into space, vacuum quenched during an extended loft, and naturally ablated from a cold initial state during atmospheric entry. After carefully reproducing Australasian tektite glass, spherical samples were exposed to arc jet conditions simulating near-escape speeds and upper-atmospheric conditions. The NASA scientists were able to control ablative effects at those extreme speeds to exactly match condition-sensitive features found on the natural tektites, including anterior face ring waves and flange flattening in addition to overall mass loss.

Reproducing morphologic features of naturally ablated tektite via hundreds of tests over several years allowed D. R. Chapman to tune coefficients of the heat transfer equation describing hypervelocity transit from the free molecular to the collisional regime of the upper atmospheric column. The NASA Ames team determined ablated tektite reentry speeds and vertical flight path angles, and the ‘Chapman equation’ is still used for modern spacecraft heat shield design, a win-win for NASA’s Apollo program.

In an error of omission, the same team never accounted for longitude shift due to 3.5 to 11.5 hours of Australasian tektite loft at those reentry speeds in order to determine regions of possible terrestrial origin. Instead, they promoted lunar origin. Terrestrial origin of tektites became clear after lunar sample return, while lunar origin of tektites led to mistrust of Chapman’s tektite reentry conditions, a case of throwing away the baby with the ‘lunar origin’ bathwater. Today, dynamically correct assessment of the A-given-B suborbital problem reveals the Australasian tektite source as roughly coincident with the N. American Great Lakes.