Descent Systems Study (DSS) (DSS-TASK)

​Landing human-scale payloads on Mars requires propulsive deceleration in the supersonic and subsonic flight regimes. The Descent Systems Study (DSS) project, aimed at CFD modeling efforts to compare with wind tunnel results, was active in the GCD Program for several years and held a Closeout Review in September 2024. The purpose of this effort is to thoroughly archive and document the Unitary Plan Wind Tunnel (UPWT) data from the Descent Systems Study (DSS) in the first 2 quarters of FY25. This effort will ensure that this critically valuable dataset will be available for future efforts in human Mars EDL. Any remaining funding should be applied to researching the next steps for propulsive deceleration maturation, including hot-fire supersonic wind tunnel testing and scaled flight testing at Earth, with the goal of delivering a report to the Deceleration Systems Capability and the Land Domain.​Testing was conducted in the NASA Langley Unitary Plan Wind Tunnel in order to investigate the aerodynamic interference of sub-scale versions of two Mars powered descent vehicle concepts at supersonic Mach numbers (2.4 and 3.5): a model based on a blunt hypersonic inflatable aerodynamic decelerator (HIAD) and the second representing a more slender rigid vehicle with body flaps (CobraMRV).  This paper covers computational flowfield predictions completed at wind tunnel conditions and comparisons to the test data.  On the blowing HIAD models, the time-averaged pressure inboard of the nozzles was generally well-predicted, especially if the nozzles are canted outward, or when the nozzles are located further from the nose.  At intermediate CobraMRV thrust coefficients, CFD pressures are more accurately predicted than they are for the HIAD models, largely due to the nozzle locations and pointing directions.  Overall, the CFD pressure coefficients were predicted within 0.2 of the steady pressure measurements for all blowing models, with smaller discrepancies at higher HIAD thrust, and larger discrepancies at higher CobraMRV thrust.  All HIAD models were predicted to have a gradually decreasing axial force coefficient as the total thrust increases, in agreement with available pressure sensitive paint data.  On models with canted nozzles or with nozzles further from the nose, the force coefficient was slightly higher for a given thrust.  On the CobraMRV model, the CFD also shows consistent results between solvers and follows trends revealed in the data; the aerodynamic force coefficient remains near the non-blowing value at a tunnel Mach number of 2.4 regardless of thrust, and increases above that level at a Mach number of 3.5, consistent with the discrete pressure data.  CFD analysis at tunnel and flight conditions will continue as flight system designs concepts mature.

Over the past several years, NASA studies of human-scale Mars entry, descent, and landing (EDL) have been focused on technologies to enhance or enable landing payloads on Mars that are much larger than those currently possible.  One of the enabling technologies is the use of retrorockets, starting at supersonic conditions, in place of a parachute.  All studies show that supersonic retropropulsion (SRP) is an enabling Mars descent technology for payloads larger than approximately five metric tons.  Supersonic parachutes have been used for all of NASA's successful scientific robotic missions to Mars, but parachutes are not scalable for human exploration payloads (20 metric tons and larger).  Powered flight has been successfully executed at Mars subsonic conditions, but it has never been needed at supersonic speeds.  The interactions between the retrorocket exhaust plumes and surrounding flowfield result in aerodynamic interference forces and moments that often are more difficult to predict than unpowered aerodynamics.  Also, the uncertainties in powered descent aerodynamics on entry vehicle stability, control, and aeroheating are not well understood.  Thus, predictive CFD uncertainties for SRP must be investigated in more detail than they have been to date, given how they will factor into assessing overall EDL risk and future mission success for landing humans on Mars.​  Towards that goal, the DSS projct partnered with the CFD as Surrogate for High-Supersonic Wind Tunnel Testing project to design, build, and test sub-scale SRP models for testing in the Langley Unitary Plan Wind Tunnel.  AETC provided the modl designers and facility time, and DSS executed a multi-year effort to document CFD analysis compared to the wind tunnel data.  The final results were summarized in AIAA papers:1. AIAA-2024-3970: Testing of Two Mars Powered Descent Vehicle Concepts in the Langley Unitary Plan Wind Tunnel, Karl T Edquist2. AIAA-2024-3971: Computational Analysis of Two Mars Powered Descent Vehicle Concepts Tested in the Langley Unitary Plan Wind Tunnel, Karl T Edquist

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