IPPW-9

1. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Obtaining atmospheric profiles during Mars entry Bart Van HoveÖzgürKaratekin Royal Observatory of BelgiumRinglaan 3, Brussel B-1180bartvh@observatory.be 9th International Planetary Probe Workshop Toulouse, France20 June 2012 IPPW-9 20 June 2012

2. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics EDL trajectory and atmospheric reconstruction Approach pioneered by Alvin Seiff in 1960’s Engineering return Validation of capsule and trajectory design tools, safe and accurate landing. Both for planetary entry and Earth re-entry. which environment is producing what we measure? Scientific return High vertical resolution versus remote observations… Atmospheric state ρ p T to constrain atmospheric models Atmospheric gravity waves and thermal tides [IPPW-9 F. Ferri et al. yesterday] Viking MER Pathfinder IPPW-9 20 June 2012

3. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Approaches to EDL atmospheric reconstruction • Classical approach: from the IMU’s • Inertial Measurement Unitswere installed on all Mars landers:accelerometers and sometimes gyroscopes • Densityfrom acceleration and drag coefficient • Pressure from hydrostatic equation • Temperature from ideal gas law • In general, CD depends on Mach, Reynolds numbers and aerodynamic flow angles. • Drag coefficient strongly limits accuracy IPPW-9 20 June 2012

4. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Approaches to EDL atmospheric reconstruction • Complementary data: from heat shield instrumentation • NASA Mars Science Laboratory(MSL) and ESA ExoMars EDL Demonstrator (EDM) heatshields are equipped with a grid of pressure and temperature sensors. • →Flush Air Data System (FADS) retrieves true relative flow direction (including winds) and free stream dynamic pressure and densityusing a heat shield pressure model MSL heat shield flush pressuresensors(MEADS) IPPW-9 20 June 2012

5. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Presentation overview Revisit IMU trajectory and atmospheric reconstruction for Mars Phoenix Ballistic entry with 5.6 km/s entry velocity at 146 km 3-axis accelerometers andgyroscopes at 200 Hz Atmospheric reconstruction from complementary heat shield instrumentation (pressures) Hypothetical pressure sensors on Phoenix heat shield Newtonian flow model to simulate wall pressures, calculate back using a least-squares solver Effects of atmospheric winds Artist rendition of Phoenix landing on Mars ← Phoenix descent observation by the MRO HIRISE camera orbiting Mars IPPW-9 20 June 2012

6. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Phoenix IMU trajectory reconstruction R Phoenix specialPDS: Withers et al. – Production of RDR for Phoenix ASE][2011 J. of S&R Phoenix special: Desai et al. – EDL performance][2011 J. of S&R Phoenix special: Blanchard et al. – Reconstruction] parachute opening Reconstructedtrajectory conditions at parachute opening (headingfrom gyroscopes) IPPW-9 20 June 2012

7. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics IMU atmospheric profiles reconstruction Results Temperature profile matches earlier results [2010 Withers et al.] Mars Analysis Correction Data Assimilation(MACDA) predicts profile fairly well Assuming a ballistic trajectory strongly affects atmospheric profiles, mainlythrough altitude in the hydrostatic pressure equation → IMU’s without gyroscopes don’t reconstruct atmospheric profiles well IPPW-9 20 June 2012

8. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Hypothetical Phoenix heat shield instrumentation Simulated heat shield pressures Simulate pressures at 7 Phoenix heat shield pressure taps, same layout as MEADS on MSL using modified Newtonian flow pressure model: with Phoenix trajectory reconstruction (gyroscopes)to represent “truth” values Reconstruct from simulated pressures Iterative least squares solver reconstructs flow direction (aerodynamic angles α and β) and static and dynamic pressures ptotandpfrom the simulated MEADSpressure taplay-out [nasa.gov] IPPW-9 20 June 2012

9. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Two ways to calculate density from heat shield instrumentation From the dynamic pressure From the static pressure Dynamic pressure from heat shield, velocity from trajectory reconstruction Hydrostatic equation in differential form Caution: differentiation is sensitive to noise and p is difficult to obtain accurately from FADS measurements as (fit 4th order polynomials at every point to calculate the derivative) IPPW-9 20 June 2012

10. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Comparison of density calculation methods from simulated pressures RMS uncertainty Uncertainties on all inputs were estimated, but zero uncertainty on pressure model! The simulated pwall have 3-σ uncertainty of ±0,5% through entire EDL, based onMonte Carlo analysis for MEADS MSL from q 850 Pa[2009, Karlgaard] Atmospheric density from the heat shield pressures is only slightly better above Mach 10 Differential method off the chart → very accurate pressure model required to improve density reconstruction in high hypersonic range IPPW-9 20 June 2012

11. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Wind modeling during entry phase MACDA wind profile (courtesy Luca Monatbone) • What is the impact of winds in the entry phase? • Can they be retrieved using the heat shield instrumentation? • Representative wind profiles from MACDA (Mars Analysis Correction Data Assimilation) • “know” the winds from a model • in the inertial planet frame,subtract wind from inertial velocity to obtain relative velocity: • subsequently, relative flow direction changes: • update simulated heatshield pressures ← IPPW-9 20 June 2012

12. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Change in flow angles due to modeled winds blue lines now represent the “truth” values from which heat shield pressures are simulated IPPW-9 20 June 2012

13. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Can we retrieve the winds from the instrumented heat shield? ← the change in angles of attack and sideslip can be resolved by the heat shield instrumentation Reconstructing the atmospheric winds Wind components reconstructed from heat shield pressures → A rough estimate of the winds might be obtained in the hypersonic phase IPPW-9 20 June 2012

14. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Conclusions • Good atmospheric reconstruction requires excellent trajectory reconstruction: every IMU should contain gyroscopes • Improving the density profile using an heat shield pressure instrumentation requires a very accurate pressure model in the high hypersonic range (above Mach 10) • A rough estimate of the winds might be obtained usingheat shield pressures in the hypersonic phase Acknowledgements Luca Monatbone(U.K. University of Oxford) for providing MACDA datahttp://badc.nerc.ac.uk WimVerbruggen (Royal Observatory of Belgium) for collecting Mars mission data IPPW-9 20 June 2012

15. Royal Observatory of Belgium Von Karman Institute for Fluid Dynamics Future work • Refine uncertainty estimates with Monte Carlo method • Evaluate different heat shield pressure tap lay-outs • Combine the IMU and heat shield instrumentation reconstructionsusing a 6-DOF extended Kalman filter • Trajectory simulation tool so that imposed winds take full effect • Perform an MSL case study when flight data becomes available • Perform an ExoMars EDM case study using a simulation tool • Additionally constrain trajectory with Doppler analysis ofradio communications IPPW-9 20 June 2012