Rotorcraft as Mars Scouts

L.A. Young V. GulickE.W. Aiken R. Mancinelli

Army/NASA Rotorcraft Division SETI InstituteG.A. Briggs

Center for Mars Exploration (CMEX)

NASA Ames Research CenterMoffett Field, CA 94035

650-604-4022layoung@mail.arc.nasa.gov

Abstract—A new approach for the robotic exploration ofMars is detailed in this paper: the use of small, ultra-lightweight, autonomous rotary-wing aerial platforms.Missions based on robotic rotorcraft could make excellentcandidates for NASA Mars Scout program. The paperdetails the work to date and future planning required forthe development of such ‘Mars rotorcraft.’

INTRODUCTION

The need for mobility has been recognized by scientistsfor decades to be fundamental to the exploration of thehighly varied and very rugged martian surface. Suchmobility would allow in situ investigations of the manysites -- already identified using orbital data -- that hold thesecrets of martian geologic, climatological and (potential)biologic history. At present, however, we are frustrated inour ambition to reach these sites by the size of landingerror ellipses and the difficulty of maneuvering a surfacerover among the many obstacles that litter the surface.Thus, presently we must land in “safe” sites (i.e. relativelysmooth over large distances) and accept the fact that therange of our surface rovers is greatly insufficient to reachour priority targets.

In future, the size of landing error ellipses will decreaseand terminal guidance will allow landings in the morerugged terrains that typify targets of real interest. In suchcases the modest mobility of surface rovers will allowsamples to be gathered and in situ measurements to bemade in some exciting sites. Surface travel will, however,always remain limited in capability. Indeed, many of themost interesting geological features on Mars lie in terrainsthat are essentially unreachable by wheeled vehicles andcurrent landing systems. Examples include theheadwaters of the newly discovered small martian gulliesand the layered cliff faces along the walls of VallesMarineris. Yet exploration of these features is critical tounderstanding their formation and the role of water inMars' present and past climate.

Presented at the 2002 IEEE Aerospace Conference, Big Sky, MT,March 9-16, 2002. U.S. Government work not protected by U.S.copyright.

Mobility provided by aerial vehicles promises to take ourexploratory capability to a new level by obviating the needto maneuver around obstacles or to be enormously large todrive straight across boulder-strewn terrain. A planetaryaerial vehicle would ideally have the flexibility to take offfrom its landing site, transit to, then hover over andexamine high priority science targets. Such an idealvehicle would also be capable of landing at any chosensite. Likewise the vehicle would be capable of returningto the more comprehensively instrumented lander fromwhich it was deployed. Our ideal vehicle would alsooffer the opportunity to perform multiple flights byrecharging at the lander.

This landing and take-off requirement for in situ scienceinvestigations makes a fixed wing aircraft unsuitable toreplace a surface rover though well suited for long range,ultra-high- resolution remote sensing. Correspondingly,balloons trailing flexible tethers offer interestingpossibilities for martian exploration including some in situscience but they lack the control necessary for reachingspecific targets and for returning to a fixed lander.

Mobile vehicles able to carry out the kind of in situinvestigations that scientists most ardently desire must becapable of both precise control and of vertical landingsand take-off. This narrows the field to rocket-poweredhoppers and rotorcraft. The former surely can be made tofunction (as demonstrated by the 3 meter hop made byLunar Surveyor VI in 1967) but have been studied only alittle. Rotorcraft for Mars exploration have been seriouslystudied in the last few years and have shown real promise.

In the near term a rotorcraft-equipped lander is viewed asa very attractive candidate for a Scout-class missioncapable of high resolution remote sensing surveys, limitedin situ science, and the return of samples to the parentlander for detailed analysis. Such a Scout-class missionwould be targeted to a high priority science site that likelywould be in rugged terrain. In the longer term rotorcraftcould become essential elements on MSR missions andcould support eventual human exploration missions inmany ways. This paper considers the first steps in thedevelopment of Mars rotorcraft technology and itsapplication to a first Scout-class mission (Fig. 1).

The Army/NASA Rotorcraft Division -- in collaborationwith the Center for Mars Exploration -- at NASA Ameshave been studying the design challenges andopportunities for martian autonomous rotorcraft for pastseveral years. The feasibility of vertical flight in themartian atmosphere has been established by design studiesby NASA Ames Research Center and independentanalyses performed by several university teams [1-11].Work on the Mars rotorcraft concept is moving on frompreliminary system analysis to proof-of-concept testarticle design, fabrication, & assessment and fundamentalexperimental investigations of the unique aerodynamics ofthese vehicles. In particular, an isolated rotorconfiguration -- designed to constraints compatible withflight in the martian atmosphere -- has been designed andfabricated and is currently undergoing pre-test preparationfor hover testing in a NASA Ames environmentalchamber. Complementary work is also under wayexamining autonomous system technology and othercritical enabling technologies for vertical lift planetaryaerial vehicles.

Mars Scout missions, included in NASA’s new Marsexploration strategy, are intended to be competitivelyselected small-scale projects that complement the baselineMars program of remote sensing orbiters and complexsample return missions. An initial solicitation has beencirculated for Mars Scout concept studies. A formalAnnouncement of Opportunity is expected by midcalendar year 2002. Both the baseline Mars ExplorationProgram and the Mars Scout missions are intended tomeet the specific goals documented by the planetaryscience community’s Mars Exploration Payload AnalysisGroup (MEPAG) [12]. A key feature of many of theMEPAG objectives is the requirement for multiple anddiverse site investigations and sampling missions, one forwhich a Mars rotorcraft/scout would represent anexcellent solution.

Fig.1 – A Rotary-Wing Mars Scout

Specifically, a Mars Scout mission would entail landingon the martian surface a suite of science instruments tostudy the geology and organic chemistry of martianstratigraphic outcrops, rock fragments, soil and dust todetermine its past water history and biological potential.The lander would likely be a variant of the 1998 MarsPathfinder lander (also the basis for the 2003 landermissions) and would carry a rotorcraft (in the place ofPathfinder’s Sojourner rover) to image and obtain spectraldata of key geological sites, and to acquire samples fromup to 10 km or so distance from the lander.

The ultra-lightweight rotorcraft will operate largelyautonomously and will be targeted to sites of interestidentified from available orbital imaging and spectral dataafter the actual landing site is accurately determined. Therotorcraft will acquire high-resolution imaging andspectral data and return small samples of soil and rockfragments from the designated sites. The instrumentationcarried by the lander will include an optical microscope,an Infrared (IR) spectrometer and a Gas ChromatographMass Spectrometer (GCMS), capitalizing oninstrumentation that has already been developed.

A NOTIONAL MARS ROTORCRAFT MISSION

Science Goals and Objectives:

Determining the mineralogy of the martian surfacematerial is the first step in understanding martiangeochemistry. In situ analyses of the martian surfacematerial can determine the mineral and volatile content ofmartian surface material. Knowing the mineralogy of asample of the martian surface material provides data onthe environment under which it was formed and can beused to better define the early environment of Marsespecially with respect to the history of water. Forexample, clays and evaporitic salts require the presence ofwater for their formation; as a consequence, if they formpart of the martian surface material their presence wouldbe evidence for water to have been on the martian surfacefor some length of time. Acquisition of samples fromseveral locations in the region around the lander toprovide a definitive characterization of the site is a keygoal of a Mars rotorcraft mission. The three-dimensionalmobility provided by a Mars rotorcraft would allow forexploration and science missions well beyond lander(accuracy as well as hazard avoidance) and rovercapabilities (range/speed limitations and limited access tohazardous terrain). Because of the enhanced mobilityrepresented by the vertical lift aerial vehicles, a lander canstill land in relatively benign terrain but with a Marsrotorcraft providing mission support research could beconducted within surface areas that no other roboticexplorer (or astronaut) could safely reach (Fig. 2).

Fig. 2 – Vertical Flight: Surpassing the Limitations ofLanders and Rovers (Mars Rotorcraft Flight Path, Outlined in Red,

Over Terrain Map of Mars Surface; EDLS Error Ellipse Also Shown)

After take-off a Scout rotorcraft would follow a specificflight plan over interesting terrain, for example the courseof a small gully or along a specific cliff face selected fromorbital images. Forward and aft mounted cameras wouldprovide target-specific views (at a resolution of a few cm)unobtainable by fixed-wing aircraft or rovers. Therotorcraft will land at the chosen site, using imaging datato orient itself and touch down safely. Landing-legmounted instruments would include a microscopic imagerfor measurement of grain characteristics and sizes. Asample-collecting scoop would be integrated into onelanding leg to collect soil samples at the remote site thatcan be transported back to the lander for further analysis.

Sites well suited to rotorcraft exploration include (Fig. 3):• The layered walls of, and mesas within, the Valles

Marineris• Young gullies on steep crater walls• Headwaters of outflow channels and valley networks• Basal scarp surrounding volcanoes, e.g. Apollinaris

Patera, to search for hydrothermal spring deposits andexplore sapping valleys.

Fig. 3 – ‘Search for Water’

Mission Description:

Prime Mission 10 to 15 Sols (a Sol is one martian ‘day’)devoted to acquisition, and in-situ analysis, of soil andsmall rock samples immediately adjacent to the lander(using a robotic arm); 5-10 Sols for the set-up (again usingthe lander’s robotic arm) and checkout of the ultra-lightweight rotorcraft; 1 sol to demonstrate the ability totake-off from the lander and land back on its pad, all ofthe flight taking place within lander line of sight (e.g. to~100 m radius); 1 sol to demonstrate a remote landing andtake-off within line of site of the lander; then 20-30 Solsto carry out a series of flights to survey the landing site toa radius of several kilometers. All power would beprovided by the lander solar array panels. The rotorcraftwould be recharged between flights by the solar arraypanels (4-6 Sols between aerial survey flights and 6-10Sols for time between sample return flights).

Extended Mission 20 to 40 Sols devoted to up to 4remote-site soil/rock sampling mission flights to adistance of several kilometers from the lander. Eachsortie would be accomplished (largely autonomously)within a ~ 6 hour period to avoid the need for therotorcraft to survive the night sitting directly on themartian surface. The science analyses of the returned

samples would take place on the lander and results wouldbe transmitted to Earth during the time that the rotorcraftwas refueling. (Note that overall mission duration may besignificantly affected by which of the two primarypropulsion systems options are chosen for the rotorcraft.)

•Science Payload

Lander Instruments Lander instruments would includeas a minimum the following: microscopic imager; IRSpectrometer or Raman Spectrometer; GasChromatograph Mass Spectrometer (GCMS); wide-fieldoptical camera for documenting/tracking Mars rotorcrafttake-off and landing, and also used to guide lander roboticarm positioning for soil/rock sample transfer from therotorcraft to the lander and to aid in the aerial vehicle set-up and recharging.

Vertical Lift Aerial Vehicle Instruments The aerialvehicle instruments would include: forward- and aft-mounted optical cameras for Guidance/Navigation andaerial survey images; sun tracker; atmospherictemperature and pressure sensors for flight readiness anddocumenting remote-site climatology; landing-leg-mounted camera for soil/rock sample identification andleg-integrated sample probe/scoop positioning; severalvehicle health and flight safety, navigation and controltransducers; IMU and assorted accelerometers for flightcontrol.

General Lander and Rotorcraft Description

A lander carrier with solar array petals similar inconfiguration of the 2003 MER and Mars Pathfinderlanders [13-14]; an in-situ instrument science module forprocessing and analyzing soil and small rock samples; arobotic arm for sampling/transferring rock samples andfurther, assisting set-up, handling, and usage of the Marsrotorcraft; the vertical lift aerial vehicle itself, with atransport frame and auxiliary support equipment; landermission computer and communication package. Fig. 4a-esummarizes the vehicle deployment from the lander.

(a)

(b)

(c)

(d)

(e)Fig. 4a-e – Mars Rotorcraft Deployment

Primary Mission Objectives

•Examine mineralogical and biochemical characteristics ofsoil and small rock samples..

•Perform low-altitude, high-resolution aerial surveys inhazardous or otherwise inaccessible terrain; identifyremote-sites for follow-on sampling mission flights

•Perform a technology/flight demonstration of anautonomous vertical lift planetary aerial vehicle tosupport infrastructure development of a class of‘astronaut agents’ that could enhance mission sciencereturn for human exploration of Mars.

Extended Mission Objectives

• Perform in situ science at, and return of soil samplesfrom, up to four sites of special geological interest. Thesesamples will be analyzed by instrumentation on thelander. The rotorcraft would fly, in a matter of minutes,to a site up to 10 km distant. Prior to landing it wouldhover to record high resolution, multi-spectral images tocharacterize the site and to orient the rotorcraft forlanding. After landing a sampling probe – such as ascoop – would acquire soil and rock fragments for return.

The rotorcraft would then return by a direct path and setdown on its pad. The samples would be transferred to thelander instruments by means of the lander’s articulatedarm and the Mars rotorcraft would be hooked up tolander auxiliary systems for recharging (Fig. 5).

>10 kmRadius

VTOL w.1 min. Hover

>30 min.Flight Time

Fig. 5 – Mission Objectives and Flight Requirements(Background Photo Courtesy of USGS)

Implementation

Crucial to the success of any Mars Scout/Rotorcraftmission will be the formation of a strong project team thatprovides the critical multi-disciplined expertise andtechnology. Research and technical communities thatheretofore have not interacted with each other will have toform close, efficient working partnerships. This processof opening communication and team building has begunbetween planetary scientists, spacecraft designers andmission developers, and the rotorcraft researchcommunity.

Mission and Flight System Architecture

To minimize overall real and perceived risk, a Mars Scoutrotorcraft mission must use as much ‘heritage’ technology(i.e. previously demonstrated with flight hardware) aspossible. Therefore, a Mars Scout rotorcraft mission willlikely model itself on the Mars Pathfinder mission,substituting the rotorcraft for the Sojourner rover. Thetechnology development will focus on the rotorcraft: itsaerodynamic properties, its propulsion system and itsautonomous operation.

The baseline Mars rotorcraft vehicle mass design target is20 kg, but tradeoff studies should be made, varying thevehicle mass from 10 to 20 kg, to examine the impact onmission performance versus risk (Table 1). The vehicleneeds to be capable of sustaining at least 30 minutes offlight in addition to 2 take-off and landings – at the landerand at the chosen distant site. The ability torecharge/refuel back at the lander will be an essentialmission feature. The larger the vehicle the more payload(in the form of science instrumentation and soil/rocksamples) can be carried by the Mars rotorcraft, but, forexample, the greater the mission cost and overall powerrequirements.

Table 1. Mars Rotorcraft (Coaxial Helicopter) Sizing

Vehicle Mass (kg) = 10 20

# of Rotors 2 2# of Blades per Rotor 4 4Rotor Radius (m) 1.22 1.72Disk Loading (N/m2) 4.0 4.0Mean Blade Lift Coefficient 0.4 0.4Blade Solidity 0.19 0.19Blade Tip Mach # 0.65 0.65Forward Mean Cruise Speed (m/sec) 40 40Maximum Power (Watts) 1550 3380Total Range (km), 25% fuel fraction,electric propulsion w. fuel cell

~50 ~50

Two different propulsion systems (which include thelander’s power subsystem) will need to be examined inparallel in the conceptual and preliminary design stages ofa Mars Scout/Rotorcraft effort: regenerative fuel-cell-based electric propulsion versus Akkerman hydrazineengine. Both propulsion technologies have their relativeadvantages and disadvantages. But, both types ofpropulsion are capable of meeting the primary andextended mission objectives outlined for the notional MarsScout mission. Nonetheless, power will be a crucialconstraint on Mars missions. Advanced solar cell arraysystems will likely need to be developed in order to meetthe significant power demands inherent for future Marssurface missions (Fig. 6a-b).

(a)

(b)

Fig 6a-b – (a) Mars Pathfinder Type Solar ArrayArrangement; (b) Potential Advanced Array Layout

The ability to carry out multiple flights will be an essentialfor Mars rotorcraft scout missions. More analysis anddevelopmental work needs to be performed before a finalpropulsion downselect can be made. Many other factorswill need to be taken into account in making that decision– including reliability, toxicity of materials, and potentialenvironmental contamination of the Mars survey sites.

Design studies and experimental investigations shouldalso continue throughout the early stages of the Marsrotorcraft development effort to continuously benchmarkcoaxial helicopter configurations against quad-rotor

vehicle designs (Fig. 7) – the two leading vehicleconfiguration candidates for early Mars rotorcraftmissions. Both vehicle configurations have considerablemerit/potential for early robotic missions to Mars [9-11].By pursuing parallel investigation of both aerial vehicletypes in the early stages of a Mars Scout developmenteffort, a strong final mission candidate design will likelyemerge.

Fig. 7 – Coaxial versus Quad-Rotors

It is crucial to recognize that rotorcraft are not merelycandidates for Mars Scout missions but are an essentialenabling technology for Mars exploration effort –including, ultimately, human exploration of the planet(Fig. 8). Astronaut is going to a paramount issue withregards to the exploration of Mars. Competing with safetyconsiderations would be the need to survey large areas ofthe planetary surface – and/or gain access to inhospitableterrain – so as to conduct mission critical research. Acompromise solution would be to rely to on roboticexplorers – particularly vertical lift aerial vehicles – to aidin the human exploration of the red planet.

Fig. 8 – Robotic Rotorcraft as ‘Astronaut Agents’(Background illustration courtesy of JPL)

Table 2 is a preliminary ‘Science to Mission TraceabilityMatrix’ for the notional Mars Scout rotorcraft missionoutlined in this paper. Information contained in this tablecan be used by science team peers and reviewers, andmission planners, to aid in assessing whether or not amission candidate concept can meet its identified goalsand objectives. Rationale is provided within the matrix asthe proposed science instrumentation for the notionalMars Scout mission, and mission features, that will meet acritical subset of the MEPAG Mars exploration scienceobjectives.

Table 2. Science-To-Mission Traceability Matrix

Science Driver InstrumentRequirement

Mission Requirement Flight SystemRequirement

Comm. and Ground Data SystemRequirement

Mission OperationsRequirement

Technology Requirement

1. MEPAG Goal I,Objective A,“Determine if LifeExists Today,”Investigation 2,3.,5, 6

GCMS (GasChromato-graph MassSpectrometer)

MicroscopicImager

1. Perform low-altitude,low-speed aerial surveyand select remote-siteswhere geologic formationswould suggest water wasonce existent;2. Acquire at multiple sitessoil and small samples toassess existence of clays,hemotites, and/orsedimentary rocks throughspectrometry;3. Through use of GCMS,assess potential of soilsample for containingorganic compounds and/orlevels of oxidants

EDL must be capableof delivering to themartian surface a 20kgaerial vehicle; 20 kg ofscience analysispackage/station; and atetrahedral solar array‘petals’ for power; arobotic arm andsupport frame for set-up and recharging

1. Aerial survey digitalimages will comprise the largestfraction (~75%) of data transmittal toEarth; aerial and remote-site (near-and far-field) images will need to betransmitted throughout missionduration in order to provide thescientific community the contextualbackground to accompany the soiland rock sample analyses;2. Sophisticated softwarefor science analysis, dataprioritization and communication,and mission planning will berequired for both the lander sciencestation and the aerial vehicle.

1. Singleoperations shiftrequired forEarth/Landercommunication;2. Two-three‘off-days’ betweencomplete data setdownlink and initiationof next aerial vehicleflight required forscience teampreliminary analysisand planning;

A. Heritage Instrumentation

B. Development of a ‘Mars Rotorcraft’

C. Develop In-Situ Handling &Processing Tools for the Lander SciencePackage/Station.

D. From an overall Mars program riskmanagement perspective, it wouldprobably be best to couple a ‘low risk’and a ‘high risk’ (such as one employinga Mars rotorcraft) during the same Marstransit window opportunity.

2. Goal I, Obj. B,“Determine if LifeExisted in thePast,” Investig. 1& 2

IR (Infra-Red)Spectrometer

MicroscopicImager

Through use ofmicroscopic imager androckpreparation/processingtools (grinding/slicing)assess rock samples forpaleobiology potential.

Sample handling andprocessing techniquesneed to be developedto transfer samplesfrom rotorcraft tolander science module.

--- ---

A. Microscopic imager and IRSpectrometer will be heritage from2003 MER missions.

B. Robotic arm will have partialheritage from Mars Polar Landerhardware

3. Goal I, Obj. C,“Assess Pre-BioticOrganicChemistry,”Investig. 1

GCMS

---

Cross-contaminationbetween samples mustbe minimized. Propercataloging, archiving,and/or disposition ofsamples must beprovided for.

Sophisticated data management toolswill be required to optimize ‘datafusion’ between the in-situ analysisresults for soil and rock samples andthe sample ‘context’ informationderived from the aerial survey andremote-site imagery.

--GCMS will have heritage dating back tothe Viking lander missions.

4. Goal III, Obj. A,“DeterminePresent State,Distribution, andCycling of Water,”Investig. 2

GCMS

MicroscopicImager

APXS (AlphaProton X-RaySpectrometer)

Through use of the APXSassess the morphology ofsmall rock samples fororigin (volcanic versussedimentary)

--- -----

APXS will have heritage technologydating from the Mars Pathfinder mission.

Requirements on Notional Mission

Orbiter Not required; will utilize pre-existing communication assetsand/or lander-based directcommunication with Earth

Launch Vehicle Delta II 7925-9.5Launch Date ~ June 2007Mission duration 90 Sols (upon landing)Flight System Elements Cruise stage; Entry, Descent,

Landing System (EDLS):Pathfinder/MER-styletetrahedron with inflatableairbags

Requirements on Spacecraft Flight System

Control method Spin stabilized; 2 rpm cruisestage.

Instrument Power Minimum instrumentation (andpower requirements) fortrajectory corrections andspacecraft health monitoring;no spacecraft scienceinstrumentation per se.

Special protection: Mars rotorcraft will becomposed of materials andsub-systems that will need tobe assessed for theirenvironmental compatibilitywith spacecraft cruise stage.

Radiation environment No RTGs required; solar andbattery power only.

EDL Maneuvering: None required beyondmatching MER or PathfinderError Ellipses.

Requirements on Communications & Data System

Data Volume (Mbytes per day): ~100 Megabytes(per flight)

Number of data downlinks per day: 1Real time requirements: None

CRITICALITY OF IN-SITU ANALYSIS CAPABILITY

It will be essential, in order to accomplish the ambitiousnotional Mars Scout outlined above, to not onlyemphasize the development of rotary-wing technologiesbut to also develop science instrumentation and tools toenable sophisticated in-situ analysis (on the lander) of soiland rock samples. A major area of investigation is theproper handling, processing, and archiving the soil androck samples – both on the Mars rotorcraft and the lander-based in-situ analysis science system.

A variety of tools and robotic devices will be needed toeffectively use a Mars rotorcraft as a sampling device fora lander-based system of in-situ analysis.

Mars Rotorcraft

Sponsons would be mounted to the rotorcraft to supportrobotic actuators/effectors. A robotic arm with severaldifferent types of end effectors (grippers, scoops, etc.)would be used to collect soil and rock samples in theimmediate vicinity of the rotorcraft, while it is at rest onthe ground. Short hops (of a few meters) with therotorcraft might be necessary in order to acquire certainselect specimens. A tethered spring-loaded ‘harpoon’ forcollecting samples beyond the reach of a robotic armmight also be used. Such a ‘harpoon’ device could alsohave a camera attached to it so as to get panoramicphotographs of the remote-site location, with possibly theMars rotorcraft in the foreground. The Mars rotorcraftsponson would also support sample collection boxes fortransport of soil/rock sample back to the lander.

Lander

The lander would need to have a large robotic arm to aidin the initial deployment of the Mars rotorcraft.Additionally, the lander robotic arm would be used toattach power cable to Mars rotorcraft for electricrecharging, or a line for refueling vehicle propellant.Finally, the lander robotic arm would have to transfersoil/rock samples from the Mars rotorcraft collectionboxes to the lander in-situ analysis science module.

Lander in-situ analysis science module would have tohave a in-box hopper for transfer of the soil/rock samplesto an internal array of handling and processing tools; suchtools would include specimen fixtures/clamps, grindingand cutting wheels, and drills for studying the interior ofrocks and generating fine particulate for chemicalanalysis. Internal mechanisms would also have to existinside the lander in-situ analysis science module totransfer specimens to individual scientificapparatus/instrumentation. Lander would also require aset of sample storage bins for cataloging/archivingspecimens, with the possible capability of withdrawingthe sources from storage and retesting as needed/justified.Avoiding cross-contamination of soil/rock specimensduring the handling and analysis process will be essentialin ensuring data quality.

This whole process – sample collection and in-situanalysis -- would have to be fully automated, includingthe data acquisition, processing, and transmitting ofinformation back to mission scientists on Earth.Scientists could adjust, as need be, mission planningbetween Mars rotorcraft flights, subject to preliminaryresults derived from the lander in-situ analysis.

DEVELOPMENT OF REQUIRED ROTARY-WINGTECHNOLOGIES

Heritage systems and technology would be used as muchas possible in this notional Mars Scout mission, and willinclude as a minimum: all lander-based scienceinstrumentation, the lander and aeroshell/entry vehicleconfigurations, and the spacecraft system. Newtechnology for this notional Mars Scout mission willprimarily be in the form of the Mars rotorcraft.

Analytical assessments have been made of the Marsrotorcraft concept over the past two years both withinNASA and other institutions [4-11].

(a)

(b)

Fig. 9a-b – (a) University of Maryland MARV; (b)Georgia Institute of Technology GTMARS

Further, through the co-sponsorship of Sikorsky Aircraftand NASA Ames, the American Helicopter Society,International conducted its Year 2000 university studentdesign competition on Mars rotorcraft (Fig. 9a-b). Thesehighly detailed design studies of the Mars rotorcraftconcept – based on a common set of design requirementsvery much consistent with the notional Mars Scoutmission outlined in this paper – effectively constitutes aset of independent reviews/assessments of the feasibilityof the concept by academic institutions [9-11]. In allcases, these academic AHS design competition

participants analytically verified the feasibility of theMars rotorcraft concept. Additionally, funding from theNASA Institute of Advanced Concepts has been providedto university researchers [3] for complementary work on avery small rotary-wing platform which has Marsexploration potential, among other applications.

Isolated Rotor Hover Performance Experimental & CFDInvestigations

A hover test stand, and a baseline proof-of-concept rotor(see Fig. 10 and Table 3), have been fabricated and arenearly ready for testing in a large environmental chamber– which can simulate Mars surface atmosphericconditions. This proof-of-concept rotor, though not as yetan optimized design, has been designed and fabricated tomany of the exacting requirements dictated for a flightvehicle – including ultra-lightweight construction andblade dynamic tuning for low structural loads andvibration. The rotor airfoil used for this proof-of-conceptrotor is the Eppler 387, a well-known low Reynoldsairfoil. Recent unpublished two-dimensional airfoil testdata in compressible, near transonic, test conditions atNASA Langley has been acquired for this airfoil,demonstrating moderately high lift coefficient values (R.Campbell - private communication). An advantage ofrotorcraft, versus any other aerial vehicle proposed forMars exploration, is the ability to conduct hover testing inexisting ground-test facilities; additionally, it is also theunique advantage of the Mars rotorcraft concept thattypically the most severe aerodynamic performanceoperating condition is in hover rather than forward-flight.

Fig. 10 – Mars Rotor Hover Test Stand

Table 3. Proof-of-Concept Mars Rotor Description

Number of Blades 4Rotor Diameter 2.438mBlade Root Cut-Out(To simulate bladetelescoping requiredfor storage/transport)

40% blade span

Disk Loading(Nominal ‘1G’)

4 N/m2

Tip Mach Number 0.65Blade Tip Reynolds # 54,855Thrust Coefficient, CT(Nominal ‘1G’)

0.0108

Mean Blade LiftCoefficient

0.4

Blade Chord 0.3048m (constant) from 40% radialstation outward

Rotor Solidity 0.191Blade Linear TwistRate

0 deg. out to 40% span;+2.4 to –2.4 deg. from 40 to 100%span.

Blade Weight 0.35 kg per bladeFirst FundamentalElastic Modes

1.264 per rev – first flap mode;1.118 per rev – first lag mode;2.310 per rev – first torsion

Outer Blade SpanAirfoil Section

Eppler 387

Spar Section Circular tube with chordwise flat platestiffener (30% chord)

Blade Construction Milled foam airfoil fairings withinternal cavities & graphite LE cap;Circular graphite tube spar acrosscomplete span of blade;45 deg. graphite chordwise flat platestiffeners from 5% to 40% station.

Rotor HubConfiguration

Rigid/cantilevered hub, withtension/torsion straps, dry contact pitchbearings, & pitch arms at 5% station

The analytical tools used to date in assessing the aerialvehicle performance will be significantly upgraded in thenear future by applying very sophisticated rotorcraftmodeling tools to perform comprehensive analyses inforward-flight (Fig. 11a-c) and Navier-Stokes CFDpredictions of the Mars rotorcraft in hover. Confidence inthese CFD predictions will be gained through validationagainst the experimental data resulting from the proposedproof-of-concept hover testing. Subsequent to the initialisolated rotor hover testing and the CFD work, a tethered‘flight’ of a stripped down proof-of-concept vehicle in theAmes environment chamber will be pursued. Thisvehicle, by necessity because of Earth’s higher gravity,will have to be powered by ground-based power sourcesand flight controllers (among other things) but willrepresent a major step ahead in the development of a Marsrotorcraft.

(a)

(b)

(c)

Fig. 11a-c– Advanced Computational Analyses; (a)comprehensive aeromechanics analysis; (b) Navier-Stokes

CFD (OVERFLOW-D) grids; (c) OVERFLOW rotorwake vorticity prediction

Coaxial Rotor/Vehicle Hover Test

There is a considerable body of experimental data andanalysis tools for coaxial helicopter hover performance(for terrestrial vehicles). There is no such data orvalidated tools for a coaxial helicopter designed to operateunder martian environmental conditions. As a part ofpreliminary test preparation prior to tethered hover flight,a proof-of-concept coaxial configuration (the MartianAutonomous Rotorcraft Test Article, or MARTA) is

being developed to test coaxial rotor performance in- andout-of-ground effect. Performance measurements are tobe made by means of load cells mounted to the coaxialhelicopter’s main sponsons/landing-gear (Fig. 12). Therotor blade sets for the coaxial helicopter ground test willbe identical to the rotor blade set used in the isolated rotorhover test.

Fig. 12 – Mars Coaxial Rotor Hover/Ground Test

Coaxial Helicopter Tethered Flight (Hover)Demonstration

Upon completion of the MARTA hover/groundaerodynamic performance testing, the model will bemodified and used as a tethered hover flight demonstrator(Fig. 13).

Fig. 13 – Coaxial Helicopter (MARTA) Demonstrator

This tethered ‘flight’ will be of a stripped-down versionof the proof-of-concept vehicle. Demonstration testingwill occur in the Building 242 vacuum/environmentalchamber at Mars representative atmospheric densities.The demonstration vehicle, by necessity because ofEarth’s higher gravity, will have to be powered via itstether cables by ground-based power sources and flightcontrollers. Key to the demonstration will be whether ornot hover out-of-ground effect is achieved.

Terrestrial-Analog Flight/Mission Demonstrations

It is essential that not only is the aeromechanics of rotorsand vehicles in simulated martian environments (usingvacuum/environment chambers) are studied during theearly stages of the concept development, but it is alsonecessary to perform terrestrial-analog demonstrations ofthe flight and mission characteristics of such vehicles.

A low-cost approach is being taken in developing acoaxial helicopter flight demonstrator for terrestrial-analog studies (Fig. 14). Such vehicles are designated asTerrestrial-Analog Mars Scouts (TAMS). A series ofsuch vehicles will be developed. The TAMS vehicles areconstructed primarily out of radio-controlled hobbyistelectric helicopter models. The TAMS vehicles have alsoacted as conceptual prototypes for the MARTA modeltested under simulated Mars atmospheric conditions.

Fig. 14 – Terrestrial-Analog (TAMS) Flight Demonstrator

The aerial survey potential for rotorcraft for Marsexploration is self-evident -- terrestrial rotorcraft havebeen used for this purpose from their earliest inception.But using rotorcraft as mobile ‘sampling’ devices to find,acquire, and return to lander-based in-situ analysisequipment will also be required for rotorcraft acting as‘Mars Scouts.’ How rotorcraft might be adapted andused for soil/rock sampling missions is still beingdefined/assessed. As a part of that assessment it isnecessary to develop a second TAMS vehicle thatfeatures/employs various types of robotic actuators andeffectors to validate the utility of such devices inrepresentative mission scenarios (Fig. 15).

Fig. 15 – Integration of Robotic Actuators/Effectors forSoil/Rock Sampling

Vehicle Autonomy

Unprecedented levels of vehicle autonomy will need to bedemonstrated to enable a Mars rotorcraft. Planetaryexploration is, in fact, perhaps the ultimate challenge forautonomous systems. The distances and communicationdelays between Earth and other planetary bodies are toogreat to allow for any sort direct flight/mission control ofrobotic aerial platforms. Further many state-of-artterrestrial aerial robotic systems rely heavily on GPSpositioning for navigation and control, an option notavailable for planetary aerial vehicles. Such vehicles willinstead have to rely upon more subtle devices/techniquesfor GNC. Several of these advanced techniques rely onthe emergent field of vision-based reasoning/processing.A study, resulting from a university grant issued byNASA Ames to Carnegie Mellon University, wasconducted examining from a conceptual designperspective the challenges and potential of using vision-based navigation systems for a Mars rotorcraft; thesepreliminary results were very encouraging. Further,planetary environments will have poorly understoodatmospheric characteristics and surface features.Adaptive control techniques coupled with contingencymission planning automated reasoning software will beessential for successful mission execution of thesevehicles. Fortunately, many of the above vehicleautonomy issues are currently active research areas withinrotorcraft and aerospace communities (Fig. 16).

Fig. 16 – Army/NASA Rotorcraft Division AutonomousRotorcraft Project

Ultimately, future generations of TAMS demonstratorswill need to embody and test increasingly higher levels ofautonomous system technology for overall risk reduction.

FUTURE INVESTIGATIONS & TRADE STUDIES

The work accomplished to date is only the beginning ofwhat is required to satisfactorily develop a rotary-wingplatform for the exploration of Mars. Many technicalissues remain to be explored and satisfactory solutions

derived. Proposing the use of a rotary-wing aerialplatform for a Mars Scout mission is not as mature atechnical approach as many other concepts likely to beadvocated for Mars Scout missions. And yet, the Marsrotorcraft concept offers such a tremendous potentialincrease in mobility for Mars exploration, with acorresponding near-order-of-magnitude increase inmission productivity, that a modest investment now, forthe future, should be justifiable.

Martian aerial scouts offer the potential to dramaticallyexpand the surface area of Mars that can be explored infuture missions. By flying over difficult topography,aerial vehicles are capable of covering much more areathan a rover in significantly less time. The 2003 missionMars Exploration Rovers will cover approximately 100meters per Sol; a Mars rotorcraft could cover over twentytimes that distance per flight (assuming a seven daybetween-flight cycle for vehicle recharging and dataanalysis/transmittal to Earth). By operating above theground surface, the potential line of sight of sensorsystems also greatly expands. A martian aerial scoutflying at 100m AGL would have a line of sight in excessof 25 km compared to the 5 km line of sight of a groundbased vehicle assuming flat terrain.

Powered-flight aerial vehicles are superior toballoons/aerostats in all respects, except maybe,simplicity. However, even with respect to theirconceptual simplicity, one has to acknowledge thatballoons, as represented by their terrestrial counterparts,are not without their own unique failure mechanisms (forexample, the early attempts to fly the erstwhile UltraLong Duration Balloon experiments). The ability toselect an area of interest on the martian surface, direct apowered aerial vehicle to that location, and to survey andconduct experiments as desired is essential for superiorscientific investigations of Mars. Having a balloonpassively, uncontrollably, skirt across the planet will be ofmodest benefit at best.

Vertical lift aerial vehicles – including rotorcraft --combine the exploration area advantage described abovewith the ability to takeoff and land in unprepared sites ofscientific interest. Unlike “single shot” fixed wingaircraft concepts, a vertical lift aerial scout offers theopportunity to perform multiple mission sorties byrecharging at the lander site. A vertical lift aerial vehiclesolution enables sample return missions. Samples couldbe gathered from a wide radius to a lander/primary-base.As demonstrated on Earth, rotorcraft uniquely havesuperior low-speed handling qualities. Rotorcraft Marsscouts would enable low-speed, precise movement inthree dimensions allowing the craft to closely study cliffwalls or capture a 360° surface view of large objects.Highly sloped terrain, possibly resultant from erosion, canbe thoroughly studied. This terrain will remainunexplored by ground vehicles or fixed wing aircraftconcepts while a rotorcraft can fly low to the ground,

allowing great image detail. Low speed handlingqualities make takeoff and landing operations possible inunprepared terrain. Finally, fixed-wing aerial vehiclessuffer from substantial technical challenges in theirrelease from entry vehicles in descent, orlaunch/catapulting from ground-based assets. Evenhypersonic rocket-propelled ‘fixed-wing’ aerial vehicles -- that are both entry vehicle as well as aerial scout -- posesignificant technical challenges; such hypersonic aerialvehicles have very limited developmental heritage forterrestrial applications, let alone their readiness forplanetary exploration missions.

CONCLUDING REMARKS

The utility of rotary-wing aerial platforms for Mars Scoutmissions has been discussed in some detail in this paper.These ‘Mars rotorcraft’ provide unique missioncapabilities that no other aerial vehicles can provide.Further, Mars rotorcraft would significantly enhancemobility above that provided by rovers while at the sametime maximizing the science return of the mission.

Work is currently ongoing within NASA and its industrialand academic partners to address the critical technicalissues for Mars rotorcraft.

REFERENCES

[1] Savu, G. and Trifu, O. “Photovoltaic Rotorcraft forMars Missions,” AIAA-95-2644, 1995.

[2] Gundlach, J.F., “Unmanned Solar-Powered HybridAirships for Mars Exploration,” AIAA 99-0896, 37th

AIAA Aerospace Sciences Meeting and Exhibit, Reno,NV, January 11-14, 1999.

[3] Kroo, I., “Whirlybugs,” New Scientist, June 5, 1999.

[4] Young, L.A., et al, “Design Opportunities andChallenges in the Development of Vertical Lift PlanetaryAerial Vehicles,” American Helicopter Society (AHS)Vertical Lift Aircraft Design Conference, San Francisco,CA, January 2000.

[5] Young, L.A., et al, “Use of Vertical Lift PlanetaryAerial Vehicles for the Exploration of Mars,” NASAHeadquarters and Lunar and Planetary Institute Workshopon Mars Exploration Concepts, LPI Contribution # 1062,Houston, TX, July 18-20, 2000.

[6] Aiken, E.W., Ormiston, R.A., and Young, L.A.,“Future Directions in Rotorcraft Technology at AmesResearch Center,” 56th Annual Forum of the AmericanHelicopter Society, International, Virginia Beach, VA,May 2-4, 2000.

[7] Young, L.A., "Vertical Lift -- Not Just For TerrestrialFlight," AHS/AIAA/SAE/RaeS International PoweredLift Conference, Arlington, VA, October 30-November 1,2000.

[8] Young, L.A. and Aiken, E.W., “Vertical LiftPlanetary Aerial Vehicles: Three Planetary Bodies andFour Conceptual Design Cases,” 27th European RotorcraftForum, Moscow, Russia, .September 11-14, 2001

[9] Thompson, B., “Full Throttle to Mars,” Rotor &Wing, Phillips Business Information, LLC, Potomac, MD,March 2001.

[10]University of Maryland Design Proposalhttp://www.enae.umd.edu/AGRC/Design00/MARV.html.

[11]Georgia Institute of Technology Design Proposalhttp://www.ae.gatech.edu/research/controls/projects/mars/reports/index.html.

[12]NASA Mars Scout Program & Mars ExplorationProgram/Payload Analysis Group white paper:http://spacescience.nasa.gov/an/marsscoutsworkshop/mepag.pdf

[13]NASA Mars Pathfinder mission information:(http://www.jpl.nasa.gov/missions/past/marspathfinder.html)

[14]NASA 2003 Mars Exploration Rover missioninformation:(http://www.jpl.nasa.gov/missions/future/marsexplorationrovers.html)

BIOGRAPHY

Mr. Young has worked at NASA Ames Research Centerin the area of rotorcraft research for the past nineteenyears. He has worked on several large-scale rotorcraftexperimental programs in theNational Full-scale AerodynamicsComplex at NASA Ames. Mr.Young has served as the GroupLeader responsible for tiltrotortechnology investigations withinthe Aeromechanics Branch as wellas being the project managerresponsible for the development ofthe Tilt Rotor Aeroacoustic Model(TRAM) – a quarter-scale tiltrotortest stand. Mr. Young is currently leading severaladvanced rotorcraft technology efforts at NASA Ames,including the study of vertical lift planetary aerial vehiclesand Mars rotorcraft.