performance metrics this investigation focuses on the energy absorbing system of the mmeev and its...

Performance MetricsThis investigation focuses on the energy absorbing system of the MMEEV and its performance relative to two critical metrics: the payload’s peak acceleration and the absorber’s stroke length.

BackgroundThe Earth Entry Vehicle (EEV) was proposed as part of the Mars Sample Return (MSR) mission as a simple, reliable capsule that used a passive energy absorber instead of a more complex parachute system to safely return a payload to the Earth’s surface. The Multi-Mission Earth Entry Vehicle (MMEEV) uses a similar passive energy absorbing system meant to be robust and responsive to various sample return missions.

Model DescriptionTo investigate the absorber’s performance, a finite element model of the MMEEV was created and analyzed using the commercial solver LS-DYNA. To reduce finite element construction and analysis time, only those components related to the impact absorbing system of the MMEEV were modeled (payload, primary structure, foam, forebody, and the impact surface). All components were modeled after the baseline case: an impact into UTTR soil at 40.4 m/s with an aluminum forebody and Rohacell®  110WF foam.

Overview

Finite Element Model

Analysis

Conclusions

Design Variable Unit Min Max

Forebody density kg/m3 1744 19380Foam max stress kPa 2000 7970

Foam Young's modulus kPa 62280 332700

Foam strain at max stress -- 0.3 0.7

Ground density kg/m3 880.6 2092Impact velocity m/s -40.4 -30.3

Sensitivity AnalysisBy comparing the performance metrics from a baseline case to those obtained from independent variations of the design variables, the model was found to be sensitive to impact velocity, forebody density, soil density, and the foam’s stress-strain curve parameterizations (compressive strength, Young’s modulus, and strain corresponding the compressive strength).

Regression AnalysisThrough an automated process, a regression analysis on a full-factorial run matrix created from values in the adjacent table yields response surface equations (RSEs) that approximate the system response. The RSEs balance a maximum range of validity with minimal error. The polynomial R2 terms are 0.98 and 0.93 for stroke and acceleration respectively. Two types of model error are investigated : model fit error (MFE) and model representation error (MRE).

Design Variable Levelsforebody  (kg/m3) 1.94E+04 1.74E+03 1.06E+04 -- 

ground (kg/m3) 2.09E+03 8.81E+02 1.49E+03 --

max,foam (kPa) 4.40E+02 3.45E+03 7.97E+03 --

Efoam (kPa) 6.23E+04 1.67E+05 3.32E+05 --

2, foam 0.3 0.5 0.7 --

Impact Velocity (m/s) 40.4 30.3 32.8 35.5

Design Limitations Of the total 972 runs, 15% returned a non-physical solution where the impacting layer of foam collapsed on itself rather than compressing the surrounding foam elements. To reduce failures, max should be restricted to a lower bound of 2,000 kPa. This

boundary corresponds to Rohacell®  foams 110WF, 200WF, and 300WF as well as many other high density foams.Practical Use For a vehicle of similar size, geometry and material properties within limits of the adjacent table, the RSEs provide an approximation of the stroke with ±15% error and maximum acceleration with ±30% error. These approximations can be used in the preliminary design process for rapid performance analysis.

MMEEV Components

MMEEV Computational Grid Used for Structural Analysis

Model Fit Error of the RSE

Simulation Diagram Performance Metrics Sensitivities

Foam Constitutive Model Parameterization

Time Progression of a Nominal and a Non-Physical CaseDesign Variable Limits

MULTI-MISSION EARTH ENTRY VEHICLE IMPACT ANALYSIS Nicole C. Bauer / Brandon P. Smith / Christopher L. Tanner / David A. Spencer

Georgia Institute of Technology / Space Systems Design Lab / NASA Langley Research Center

Stress

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