One reason why temperatures approaching that of the Sun's surface are hard to duplicate is that most structural materials will melt away if exposed to such temperatures for an appreciable length of time. Longduration hypersonic facilities operating at more moderate temperatures (approximately 3000 ° F) are practicable, but this is not true with hypervelocity facilities. The reentry vehicle designer must settle for intermittent testing-perhaps a fraction of a second duration-to preserve the structural integrity of the facility. Even so, full duplication of reentry temperatures, pressures, and velocities is beyond the reach of terrestrial test equipment, and partial simulation becomes a fact of life. Assessing the adequacy of the simulation and designing experiments to minimize the uncertainties become major challenges in providing reliable test results for the spacecraft designer to go on.