Scaled Apollo Capsule Heat Flux Measurements in the X3 Expansion Tube
Planetary entry thermal protection systems are a critical and mission limiting technology for space exploration. Of special importance for super orbital Earth return, are the high velocity, high Reynolds number regions of the trajectory where extreme turbulent heating occurs. This paper reports on preliminary heat flux measurements performed over a 6.4% scale Apollo capsule model in the X3 expansion tube at the University of Queensland. Experiments were performed over a stagnation enthalpy range from 10.7 to 30 MJ/kg with related diameter-based freestream Reynolds numbers from 0.5 to 0.07 million. Two of these conditions were similar to the highest enthalpy reflected shock tunnel experiments which have ever been performed with a scaled Apollo capsule model. Additionally, another two conditions were at higher enthalpies than it is possible to achieve in reflected shock tunnels, allowing flow environments which can only be generated in expansion tubes to be studied. Evidence of the onset of transition was seen for all test conditions, even in cases where natural transition did not occur in reflected shock tunnels. The differences seen in this work between expansion tube and reflected shock tunnel experiments is motivation for performing further planetary entry transition experiments in the future to understand why differences are seen between different types of test facilities and to allow these difficult to generate high enthalpy conditions to be studied further.