Planar Laser Induced Iodine Fluorescence for the Investigation of the Aerodynamics of Reaction Control System Jets on Mars-Entry Aeroshells

In order to improve landing accuracies on Mars in preparation for future manned mission, effort has been made in improving control of the vehicle through the use of reaction control system jets under rarefied conditions where normal control surfaces (ailerons, rudders, etc.) are ineffective. Despite their use on Apollo and Viking landers, as well as the space shuttle, firing the reaction control system (RCS) jets can have unanticipated effects on the aeroshell such as augmented heating and induced adverse forces and moments. In order to better understand the aerodynamics of the interactions of reaction control system jets with the aerodynamics of the spacecraft aeroshell in high-speed flow, a qualitative and quantitative study using an experimental method termed Planar Laser Induced Iodine Fluorescence was conducted.
Planar Laser Induced Iodine Fluorescence (PLIIF) is a non-intrusive optical diagnostic technique that utilizes the principles of fluorescence spectroscopy in order to obtain high-resolution planar qualitative flowfield images that clearly exhibit shocks and interactions, as well as quantitative planar velocity and temperature flowfield measurements. The PLIIF method is the only diagnostic techniques that can produce planar velocity and temperature measurements in the mixed continuum and rarefied flowfield conditions that exist for the experiment reported herein.
Using a low-pressure chamber, a model was subjected to Mach 12 freestream flow in order to simulate entry conditions at high speeds. For this research, two RCS jet configurations – a jet issuing transverse to the freestream and a jet issuing parallel to the freestream - were investigated at several thrust coefficients. It was found that the jet issuing transverse to the freestream had strong interaction with the bow shock that formed off the model, resulting in the bow shock being pushed farther away from the model as well as an inhibited RCS jet expansion. This RCS jet/bow shock interaction can be supposed to have a significant effect on the surface pressures on the model to induce forces and moments. The jet issuing parallel to the freestream indicated little interaction with the model bow shock. For both jet configurations, recirculation regions near the RCS jets with low velocities and high temperatures suggests that flow around the aeroshell in a Mars entry flight would have total temperature recovery in these regions, inducing localized heating.
Qualitative results for the parallel jet configuration at a jet thrust coefficient of 0.5 and quantitative results for both configurations at a jet thrust coefficient of 1.0 were compared to CFD/LeMANS calculations completed at the University of Michigan. The LeMANS computations were computed with consideration for the experimental setup in order to produce high fidelity for conditions with the experiment for comparisons. The results showed agreement in the model bow shock structure on the forebody of the model but exhibited a difference in shock structure and RCS jet structure elsewhere. The velocity and temperature comparisons along the jet centerlines of both configurations showed good agreement. Once compared to the experimental results, computational results can be considered to give predictions of RCS performance. A study of control gain by the University of Michigan indicated that the parallel jet results in near ideal control of the aeroshell while the transverse jet results in diminished control effectiveness. Such measurements have not previously been possible and are due to the unique capability for the PLIIF technique in mixed continuum/rarefied flows.