Abstract
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.