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AIAA ADS ConferenceMay, 20-22, 2003

Monterey, Ca.

The Mars Exploration Rovers Entry Descent and Landing and the Use of AerodynamicDecelerators

Adam Steltzner*, Prasun Desai+, Wayne Leeφ, Robin Brunoλ

NASA Jet Propulsion Laboratory+NASA Langley Research Center

ABSTRACT∗+φλ

The Mars Exploration Rovers (MER) project, thenext United States mission to the surface of Mars,uses aerodynamic decelerators in during its entry,descent and landing (EDL) phase. These twoidentical missions (MER-A and MER-B), whichdeliver NASA’s largest mobile science suite to dateto the surface of Mars, employ hypersonic entry withan ablative energy dissipating aeroshell, asupersonic/subsonic disk-gap-band parachute and anairbag landing system within EDL. This paper givesan overview of the MER EDL system and speaks tosome of the challenges faced by the variousaerodynamic decelerators.

INTRODUCTION

The MER-A and MER-B missions will deliver theidentical rovers to the surface using an EDL scenariobased on Mars Pathfinder heritage.1 The overallchallenge of the EDL phase to MER is quite great.MER is exploiting very similar EDL architecture tothat of MPF but is delivering ~80% more payloadmass (445 kg vs. 250 kg). This increased deliveryperformance of the EDL system has requiredextensive redesign of the aerodynamic deceleratorsused in the system.

The vehicles are headed to the equatorial regionof Mars with MER-A targeted to land at the in theGusev crater and MER-B scheduled for landing inthe Meridiani plains. They are to be launched in Juneof 2003. Each lander will carry a rover that willexplore the surface of Mars making in-situmeasurements. However, unlike the Mars PathfinderSojourner rover, these rovers are larger

∗ Senior Staff, Mechanical Engineering+ Aerospace Engineer, Exploration Branchφ Senior Staff, System Engineeringλ Senior Staff, Mechanical Engineering

(approximately 1.5 m by 1 m) and more capableaccommodating an increased suite of scienceinstruments. In addition, the rovers will be able totraverse greater distances (approximately 1 km)during surface operations.

The landers will decelerate with the aid of anaeroshell, a supersonic parachute, solid propellantretrorockets, and air bags for safely landing on thesurface. Fortuitous hyperbolic approach conditions inthe 2003 opportunity enable the use of the MPFarchitecture for EDL. An overview of the EDLhardware is shown in Figure 1.

Unlike MPF, however, there is no base station onthe MER landers. The more capable MER roverscarry all equipment (e.g., science instruments,communications, etc.) necessary for surfaceoperations. Hence, once the rovers drive off thelanders after landing, they are self-sufficient (Fig. 2).The MER landers act functionally as landing vehiclesthat only delivery a payload (the rovers) to thesurface.

The MER mission relies on Mars Pathfinder“heritage”. However, to meet the increased surfacepayload requirements, the EDL performance needshave driven some significant changes to the MPFEDL design. First, the parachute has been increasedin size by 40% in area and its construction has beenextensively modified to deliver higher strength toweight. Additional sets of steering rockets andadditional sensing capacity has been added to thevehicle to counter the threat of increased wind shearbrought about in part by a requirement to land duringthe daylight hours. Finally, extensive redesign of theairbag subsystem was required to meet the finalimpactrequirements.

17th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar19-22 May 2003, Monterey, California

AIAA 2003-2125

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

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Figure 1. EDL Hardware Overiew

Figure 2. Mars Exploration Rover On Surface

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.ENTRY CONDITIONS

The MER landers will arrive at Mars in Januaryand February 2004. There are a number ofdifferences between the MER and MPF entryconditions. In particular, the entry mass, entry localtime, and landing site altitude are different. All ofwhich drive changes to the EDL design from that ofMPF. The entry conditions for MER and MPF aresummarized in Table 1.

The MER entry mass of 835 kg is much higherthan the 585 kg of MPF. The local entry time is in themiddle of the afternoon, leading to a less denseatmosphere profile than the pre-dawn entry of MPF.In addition, there is a requirement to be capable ofreaching a higher landing site altitude than MPF.These three differences produce a higher terminalvelocity and less time for performing all the EDLevents due to the reduced deceleration during theentry. Therefore, the EDL systems are beingmodified to accommodate these issues. An overviewdescribing the specific changes to each EDL flightsubsystem is presented in the following sections.

EDL SEQUENCE

Both MER landers will enter the Mars atmospheredirectly from their interplanetary transfer trajectories.The MER EDL sequence is illustrated in Fig 3. UponMars arrival, the landers will be separated from theirrespective cruise stages 30 minutes prior toatmospheric entry. After approximately 240 s fromentry interface, the parachute is deployed (altitude of~8.9 km), followed by heatshield release 20 s later.The lander descent along its bridle is initiated 10 slater. At an altitude of 2.4 km above ground level(AGL), the radar altimeter acquires the ground.Altimeter data is used by the on-board flight softwareto determine the times for airbag inflation,retrorocket motor ignition, and bridle cut. Nominally,airbag inflation will occur 4 s prior to retrorocketmotor ignition (~160 m AGL). At an altitude of ~15m AGL, the bridle will be cut, and the inflatedairbag/lander configuration freefalls to the surface.Sufficient impulse remains in the retrorocket motorsto carry the backshell and parachute to a safe distanceaway from the lander.

Figure 3. MER EDL Sequence of Events

• Lander Separation: E+271s, L-72s

• Heatshield Separation: E+261s, L-82s

• Parachute Deployment: E+241s, L-102s ,8.6 km ,430 m/s

• Peak Heating / Peak Deceleration E+122s, 6.3 earth g

• Cruise Stage Separation:

• Radar Ground Acquisition: 2.4 km AGL

• Start Airbag Inflation: E+335s, L-8s

• Bridle Cut: E+340s, L-3s, 13 m

• RAD/TIRS Rocket Firing: L-6s

• Landing: E+343s

• Entry Turn Starts:

• Entry: E-0 s, L-343s, 128 km, 5.4 km/s surface relative

• Bridle Descent Complete: E+281s, L-62s

• Bounces, Rolls Up to 1 km

• DIMES Images Acquisition: 2.0 km AGL

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ENTRY

The maximum inertial entry velocity (across MER-Aand MER-B) of 5.8 km/s is 20% less than that ofMPF. This slower entry velocity coupled withutilizing a shallower entry flight-path angle allowsthe accommodation of the higher MER mass of 835kg during the entry. The nominal entry flight-pathangle selected for MER is –11.5 deg. Forcomparison, the MPF entry flight-path angle wassteeper having a value of –14.2 deg. The MER entryangle was chosen to be as shallow as possible toaccommodate the entry mass, while still satisfyingthe requirement of maintaining a 1.0 deg marginbetween skip-out and the 3-σ shallow entry. Thenavigation uncertainty on the entry flight-path angleis ±0.25 deg for both missions. This navigationcapability may require the execution of a trajectorycorrection maneuver as late as 12 hours prior toentry. For comparison, MPF planned for a latemaneuver, but its execution was not necessary due tofortuitous navigation. The uncertainties listed are forarrival to Northern Latitude landing sites. As thelanding sites shift towards Southern Latitudes, theseuncertainties decrease approximately by half.

Hypersonic deceleration is accomplishedutilizing an aeroshell. The MER aeroshell is based onthe MPF design with only minor changes to increaseinside volume2. The aeroshell consists of a forebodyheatshield and an aftbody backshell. The forebodyshape is a Viking heritage 70 deg half-angle spherecone. The forebody material is a lightweight ablator(SLA-561), while the backshell is protected with aspray-on version of the SLA-561 material. Theheatshield is also based on MPF heritage with aminor change to the thermal protection system (TPS)thickness. The MER heatshield is sized to 1.63 cminstead of the 1.91 cm of MPF. Despite the heavierentry mass, the combination of a slower entryvelocity and shallower entry flight-path angleproduces a less severe entry environment ascompared to MPF. The resulting performance marginallows for a reduction in the TPS thickness. Theentry environment is described in Table 2.

INITIAL DESCENT

Due to the higher entry mass, less dense atmosphere,and higher landing site altitude, the time fromparachute deployment to retrorocket initiation isreduced as compared to MPF. In order to providesufficient time for performing all EDL events duringdescent, a combination of modifications is made tothe MER entry. The altitude at parachute deploymentis raised, the parachute deployment algorithm ismodified, and the size of the parachute is increasedfrom that of MPF.

The parachute system/deployment process ismodified in three ways to provide sufficient time fordescent. First, the parachute deployment altitude israised by setting the deployment dynamic pressurefor MER to a mean value of 725 N/m2, slightlyhigher than the 600 N/m2 used by MPF. Thiscondition corresponds to an altitude of approximately8.9 km. Second, a more accurate parachutedeployment algorithm is implemented based ondynamic pressure rather than a deceleration valuewhich was used on MPF. A dynamic pressure triggerminimizes the variation in the deployment dynamicpressure resulting from off-nominal conditions. Themaximum dynamic pressure observed inapproximately 800 N/m2, which is a ~15% higherthan 703 N/m2 expected by MPF. Overpressure testsconducted during MPF indicate feasibility ofexceeding 730 N/m2.

Lastly, in order to accommodate the higher entrymass, the MER parachute was modified form theMPF design. The area was increased 22% and theeffective drag area was increased by 28%. Theincrease in drag area come from an increase in Cdwhich was accomplished by a reduction of 5% in theband length while keeping the reference areaconstant. This resulted in a Cd of 0.43 vs. Cd of 0.41.The larger drag area provides greater deceleration,thereby increasing the descent time. The MERparachute is based on the MPF design, which was amodified Viking heritage disk-gap-band design.Additionally, the parachute trailing distance of 9.4body diameters is identical to MPF, which avoidswake interference issues. This increase in chute sizecoupled with the changes to parachute deployment,

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allow for sufficient time for performing all EDLevents.

TERMINAL DESCENT

After deploying the parachute, the vehicle will takeapproximately 120 s to descend to the surface.However, simulations indicate that the statisticalvariations in atmospheric density and parachute dragperformance predictions may result in descent timesas short as 65 s. The predicted upper-bound terminalvelocity of 85 m/s is 32% greater than thatexperienced by MPF. Table 3 lists the majorcontributions to the differences in terminal velocitybetween MER and MPF.

The higher velocity results in the need for largerretrorockets with 70% more impulse than those usedon MPF. These solid rocket subsystem, named theRocket Assisted Deceleration Subsystem or RAD, arearranged in a symmetric cluster of three along theinside surface of the backshell (see Fig. 1), and willprovide a total system impulse of up to 95,000 N-s.During the descent, the flight-computer will utilizeradar altimeter data to ignite the retrorockets at analtitude of approximately 160 m. The goal is to zerothe vertical velocity 13 m above the ground. At thattime, the parachute and backshell will be released bysevering the bridle, after which the lander freefalls tothe ground.

In order to tolerate higher near surface winds andwind shears, an additional set of steering rockets havebeen added to MERs EDL system. These rockets,known as the Transverse Impulse Rocket Subsystemor TIRS, were not present in the MPF EDL scheme.The TIRS is made up of three 2000 N-s impulse solidrocket motors. These motors are arrayedcircumferentially at 120 degrees intervals around thebackshell. They are canted a 85 degrees from theRAD rocket thrusts vector. On MER a descentcamera takes three pictures separated by about 5seconds each, these are used to determine thevehicles transverse velocity with respect to thesurface. This measurement and data taken by thevehicle’s IMU are used to determine if the TIRS willbe used.

Monte Carlo analysis of the terminal descentperformance indicates a non-trivial (~10%) numberof cases with impact velocities in the 20 m/s to 30m/s range which reduces the margin on airbagcapability described below. These cases result whenthe vehicle is hit with wind shears near the time ofretrorocket ignition. The shears cause pendulummotion in the terminal descent configuration. Thiscauses the backshell to swing off vertical at angles ofup to 20 deg, resulting in a self-induced horizontalvelocity component from firing the retrorockets witha net off-vertical thrust vector. In order to improvethe performance reliability of the EDL system, thethree small “steering” rockets (1200 N-s each) wereadded. These rockets can be fired at the time ofretrorocket ignition to steer the net rocket impulsevector to a more verticle position.

LANDING

Impact with the Martian surface will be cushioned bya set of airbags similar in outward appearance butsignificantly changed in design to those used onMPF. The airbag system actually consists of fourseparate interconnected airbags arranged in atetrahedron shaped structure. Each of the bagsconsists of six spherical-shaped lobes and is madefrom the material Vectran. The significantmodification in the airbag design comes from the useof a double bladder system. The MPF airbags hadsingle bladders and four layers of “abrasionprotection” in the form of 100 denier vectran cut with~6% fullness and attached atop the bladder. Testinghas shown that a double bladder system in which theinner gas sealing membrain is cut with fullness,removes the gas membrain from the pressure vesselinduced stresses (PR/t stresses). This keeps a gasseal in cases where the Pressure related stresseslocally overwhelm the material strength. Recent droptests performed at the NASA Glenn Research Center,Plum Brook Station facility, indicate an impactvelocity absorption capability of at least 26 m/s witha MER landed mass of 550 kg. For comparison, theairbags were qualified to 26 m/s with a landed massof 410 kg during the MPF testing campaign.

The performance of the EDL system during thelanding event is a function of not only the impactvelocity state and the surface features with which the

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airbag landing system is interacting (rocks andslopes). A prediction of the maximum size for thelanding footprint for the arrival opportunity isdetermined by assuming a Northern Latitude landing(corresponding to largest navigation uncertainties).The resulting landing footprints for MER-A andMER-B are approximately 215 km by 15 km and 330

km by 20 km, respectively. The landing ellipses areshown in Fig. 5. As the landing site moves furthersouth, the size of the landing footprint decreases. Forcomparison, the predicted landing footprint for MPFwas 300 km by 50 km

Table 1

COMPARISON OF MER AND MPF ENTRY CONDITIONS

Parameter MER-A MER-B MPFArrival Date 4 Jan 2004 8 Feb 2004 4 Jul 1997Arrival Season Mid Winter Late Winter SummerEntry Velocity 5.7 km/s 5.8 km/s 7.26 km/sEntry Direction Posigrade Posigrade RetrogradeLocal Landing Time Afternoon Noon Pre DawnEntry mass 835 kg 835 kg 585 kgLanded mass 550 kg 550 kg 310 kgLanding site altitude –1.3 km –1.3 km –2.6 kmLanding Latitude 5° N to 15° S 10° N to 10° S 19° N

Table 2

COMPARISON OF MER AND MPF ENTRY ENVIRONMENTS

Metric Design Limit MPF MERPeak heating, W/cm2 41 50 105Total heat load, J/cm2 2900 3400 3900Peak deceleration, Earth g 8 10 20Peak stagnation pressure, kN/m2 12.2 25 24

Table 3

CONTRIBUTION OF ENTRY CONDITIONS ON TERMINAL VELOCITY

MER MPF Terminal Velocity ChangeHeaver Descent Mass (kg) 740 530 +20%Less Dense Atmosphere Mid-afternoon Pre-dawn +21%Higher Landing Site Altitude (km) –1.3 –2.6 +3%Larger Chute Drag Area (m2) 57 50 - 7%

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Figure 4. MER-A and MER-B Landing Footprints

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SUMMARY

This paper gives an overview of the MarsExploration Rover (MER) mission entry, descent,and landing (EDL) system design. This mission relieson Mars Pathfinder heritage. However, the entry,descent, and landing conditions and environments aredifferent from that of Mars Pathfinder. The entrymass will be higher. In addition, the local time ofentry is later in the day (early afternoon) which willresult in a less dense atmospheric profile.Furthermore, the landing site altitude is higher. Thesedifferences result in a higher terminal velocity andless time for performing all the EDL events ascompared to Mars Pathfinder. As a result of thesedifferences, modifications are made to a number ofthe EDL systems. The parachute size has beenincreased and its deployment algorithm has beenchanged to improve deployment accuracy. Inaddition, the size of the retrorocket motors isincreased to provide greater impulse. Thesemodifications are made to safely deliver the rovers tothe surface of Mars. The EDL system will continue toevolve as the mission approaches the launch date.

NOTATION

AGL Above Ground LevelEDL Entry, Descent, and LandingMER Mars Exploration RoverMPF Mars PathfinderTPS Thermal Protection SystemA AreaCD Drag Coefficient

ACKNOWLEDGEMENTS

The work described in this paper was carried outby the Jet Propulsion Laboratory, California Instituteof Technology (under a contract with NASA) and theNASA Langley Research Center. The authors wouldlike to extend their appreciation to all the members ofthe MER EDL design team for their contributions.

References

1. D. A. Spencer, R. C. Blanchard, R. D.Braun, P. H. Kallemeyn, S. W. Thurman,“Mars Pathfinder Entry, Descent, and

Landing Reconstruction,” Journal ofSpacecraft and Rockets, Vol.. 36, No. 3,May-June 1999, pp. 357-366.

2. Desai, P.D. and Lee.W.N, Mars ExplorationRovers Entry Descent and Landing System,AAS 01-021, 24th Annual AAS Guidanceand Control Conference, 1/13/2001,Breckenridge, Co.