Vertical Lift – Not Just For Terrestrial Flight

Larry A. YoungArmy/NASA Rotorcraft Division

Ames Research CenterMoffett Field, CA

Abstract

Autonomous vertical lift vehicles hold considerable potential for supporting planetaryscience and exploration missions. This paper discusses several technical aspects ofvertical lift planetary aerial vehicles in general, and specifically addresses technicalchallenges and work to date examining notional vertical lift vehicles for Mars, Titan, andVenus exploration.

Introduction

The next few years promise a uniqueconvergence of NASA aeronautics andspace programs. NASA planetaryscience missions are becomingincreasingly more sophisticated.Manned and robotic exploration of thesolar system planets would be greatlyenhanced through the development anduse of robotic aerial vehicles. Since the1970’s a number of Mars (fixed-wing)Airplane concepts have been proposedfor Mars exploration.

The Army/NASA Rotorcraft Division --in collaboration with the Center for MarsExploration -- at NASA Ames has beenperforming initial conceptual designstudies of Martian autonomous rotorcraft AHS/AIAA/RaeS/SAE International PoweredLift Conference, Arlington, VA, October 30-November 1, 2000.

for planetary exploration and sciencemissions (fig. 1). Initial results havebeen quite promising. As a result of thisearly work, the utility of rotorcraft,VTOL vehicles, and hybrid airships forMars exploration and planetary sciencemissions as a whole can be technicallyjustified as follows.

Why vertical lift vehicles for planetaryexploration? For the same reasons whythese vehicles are such flexible aerialplatforms for terrestrial exploration andtransportation: the ability to hover andfly at low-speeds and to take-off andland at unprepared remote sites. Further,autonomous vertical lift planetary aerialvehicles (PAVs) would have thefollowing specific advantages andcapabilities for planetary exploration:

• Hover and low-speed flightcapability would enable detailed

https://ntrs.nasa.gov/search.jsp?R=20090023136 2018-04-14T14:28:56+00:00Z

and panoramic survey of remotesites;

• Vertical lift configurations wouldenable remote-site sample returnto lander platforms, and/orprecision placement of scientificprobes;

• Soft landing capability forvehicle reuse (i.e. landerrefueling and multiple flights)and remote-site monitoring;

• Hover/soft landing are good fail-safe ‘hold’ modes forautonomous operation of PAVs;

• Vertical lift PAVs would providegreater range and speed than asurface rover while performingdetailed surveys;

• Vertical lift PAVs would providegreater resolution of surfacedetails, or observation ofatmospheric phenomena, than anorbiter;

• Vertical lift vehicles wouldprovide greater access tohazardous terrain than a lander orrover.

In addition to the potential science andtechnology benefits resulting from thedevelopment and use of vertical liftplanetary aerial vehicles, there aresubstantial opportunities for technologytransfer from a vertical lift PAVdevelopment effort. These technologytransfer opportunities include: advancedhigh-efficiency propeller or proprotordesigns; precision guidance, navigationand control at low altitudes and near-terrain obstacles; adaptive (inner-loop)

flight control; autonomous systems workbased on vertical lift vehicleapplications; high-frequency open- andclosed-loop smart structures/actuators.

Figure 1 – Vertical Lift Planetary AerialVehicles as ‘Astronaut Agents’

Ultimately, the objective of this paper isto inspire the vertical flight researchcommunity to consider and to embracethe concept of vertical lift planetaryaerial vehicles and to participate in theirultimate development and use. Specificopportunities for vertical life PAVs inplanetary science and some of the PAVdesign challenges are presented in thispaper. Ongoing work, including that inacademia, is also described.

Opportunities

As noted earlier, work is being pursuedat the Ames Research Center’sArmy/NASA Rotorcraft Division on aMartian autonomous rotorcraft forscientific exploration of Mars. Why

not, though, a Venusian hybrid-airship?Or, a Titan VTOL? Or, alternatively,why not any number of vertical conceptsthat could provide unique missioncapability for planetary science?

This paper examines the question of thefeasibility of vertical lift planetary aerialvehicles. In particular, discussion in thepaper will be directed at the threeplanetary bodies in our solar systemwhere vertical lift vehicles might provefeasible. Table 1 is a summary of thekey surface atmospheric properties forMars, Titan, Venus, and Earth. Thisinformation will be used to examine thegeneral aerodynamic attributes ofvertical lift PAVs.

Table 1 – Summary of PlanetaryDescriptions (Ref. 1)

MeanRadius(km)

Gravity!

(m/s2)Mean

SurfaceAtmos.Temp.(o K)

MeanSurfaceAtmos.Pressure

(Pa)

MeanSurfaceAtmos.Density(kg/m3)

Atmos.Gases

Venus 6052 8.87 735.3 9.21x106 64.79 CO2

96%N2 3.5%

Earth 6371 9.82 288.2 101,300 1.23 N2 78%O2 21%

Mars 3390 3.71 214 636 1.55x10-2 CO295%

N2 2.7%Ar 1.6%O2 0.1%

Titan(Saturnmoon)

2575 1.354 94 149,526 5.55 N2 65-98%

Ar<25%CH4 2-

10%

! Mean values noted for planet radii and gravity to accountfor the oblateness of the planet. Mars surface temperature, pressure, and density varies

significantly spatially and temporally; surface temperaturerange of 140-300oK; surface pressure 636±240 Pa. SeasonalCO2 sublimation and condensation at the polar caps(particularly at the southern polar cap) is the chief reason forthe atmospheric pressure and density variations.

Figures 3 and 4 are approximateestimates of the speed of sound andkinematic viscosity for various differentplanetary bodies in the solar system.These estimates were derived inreference 2.

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Mars

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y yyy

Speed of Sound (m/sec)Speed of Sound (m/sec)Speed of Sound (m/sec)Speed of Sound (m/sec)

Fig. 2 – Estimates for the Speed ofSound

1E-07 0.000001 0.00001 0.0001 0.001 0.01 0.1 1

Venus

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Jupiter

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Neptune

Kinematic Viscosity (m^2/sec)Kinematic Viscosity (m^2/sec)Kinematic Viscosity (m^2/sec)Kinematic Viscosity (m^2/sec)

Fig. 3 – Estimates of KinematicViscosity

Additional data related to planetaryatmospheric properties can be found, forexample, in Ref. 3-6.

Despite the considerable amount of datarelated to planetary atmospheres, muchmore data can, and must, be discoveredduring the course of the developmentand general application of PAVs.

In establishing the feasibility of verticallift (and other) PAVs, it is not sufficientto merely question whether or not flightin extraterrestrial atmospheres istheoretically possible. Clearly definedplanetary science goals and opportunitiesare also required so that vertical lift PAVdesigns can be optimized. Table 2summarizes a partial list of planetaryscience goals/opportunities in whichvertical lift, or rotary-wing, platformscould contribute.

Table 2 – Planetary ScienceOpportunities (A Partial List Only)

Science/Exploration Opportunities

Mars • Search for water or past signs of water(characterize global distribution)

• Search for life or evidence of past life• Understand the atmospheric and geological

evolution of Mars; perform comparativeanalyses of the Mars planetaryevolutionary process with the otherterrestrial-type planets in our solar system

• Survey for resources that would expandexploration capability and support for anextended human presence on Mars

Titan • Search for the precursor biochemicalcomponents of life

• Perform atmospheric science studies tounderstand the unique nature of the Titanatmosphere (i.e. its high density/pressure)

• Survey for chemical resources/volatiles thatcould enable in-situ propellant and fuelproduction; resulting propellant could beused for sample return missions to Earthand expanded surveys of the otherSaturnian moons

Venus • Correlate space-based cartographic andinferred geological data with detailedsurveys in targeted areas using vertical liftPAVs.

• Acquire adequate data to understand thefundamental atmospheric and geological

evolutionary processes that led our ‘sister’planet to be radically different from Earth

• Determine if planetary-scale ‘green-house’effects can be halted and/or reversed

Finally, in addition to the scientificbenefits resulting from the employmentof vertical lift vehicles, considerableenthusiasm and support from theAmerican public could be generated forboth the demonstration of extraterrestrialatmospheric flight.

Mars, Titan, and Venus

Mars, Titan, and Venus are as differentfrom each as they are with respect toEarth. Each planetary body -- eachmajor science/exploration mission, infact -- will entail radically differentaerial vehicle design challenges. Thiswill be highlighted in the discussion tofollow which outlines some thoughtsregarding notional vehicleconfigurations and design considerationsfor each of the three planetary bodies inour solar system where a vertical liftcapability might theoretically makesense for exploration/scientificinvestigation.

Mars

Most of the work to date investigatingthe feasibility of vertical lift planetaryaerial vehicles has focused of rotary-wing configurations for Marsexploration. Mars, of all the planetarybodies in the solar system, holds thegreatest interest for NASA researchers.Both the Offices of Space Science and

Space Flight actively promote/directresearch and engineering effort for therobotic and, ultimately, humanexploration of Mars (fig. 4). Reference7 details NASA' strategic plan for theHuman Exploration and Development ofSpace (HEDS) -- which clearlyemphasizes the importance of Marsmission planning.

Fig.4 – Mars (Image from Hubble SpaceTelescope (HST))

Martian autonomous rotorcraft will havelarge lifting-surfaces and will berequired to have ultra-lightweightconstruction (refer to figure 5 forisolated rotor sizing for hover in Martianatmosphere). This in turn will pose achallenge in making them sufficientlyrobust to operate in the Martianenvironment. Some early work anddiscussion on Martian vertical liftvehicles can be found in Ref. 8 -- 13.

(Disk Loading = 4 N/m^2)

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Fig. 5 – Martian AutonomousRotorcraft: Large, Ultra-lightweight,

Fragile-Looking…

Early conceptual design study work atNASA Ames focused on a Mars tiltrotorconfiguration (Fig 6a-b and Ref. 2).This configuration reflected an aerialplatform that potentially maximizedoverall mission flexibility. Assumingthe use of an Akkerman hydrazinereciprocating engine (Ref. 14), a small(10 kg) Mars tiltrotor was shown topotentially have a range capability on theorder of 150-250 kilometers (assuming alimited amount of hovering and verticaltake-off and landing operation).Alternate propulsion systems were notexamined in this initial work.

This early Mars tiltrotor work at NASAAmes illustrated the promise of verticallift planetary aerial vehicles. However,it was also clear from this work thatdeployment of even a small Marstiltrotor requires human assistance invehicle assembly. Alternative vehicleconfigurations needed to be examinedfor early robotic missions that did notrequire human presence on Mars.

(a)

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Fig. 6 -- A Mars Tiltrotor: (a) helicopter-mode in vertical climb over Valles

Marineris; (b) airplane-mode

Recent work at Ames has focused on acoaxial helicopter configuration for earlyMars exploration missions (fig. 7).

Fig. 7 -- A Coaxial HelicopterConfiguration for Mars Exploration

(‘Search for Water’)

Figure 8 shows first-order estimates ofthe forward-flight performance of a 10kg

coaxial helicopter configuration. Theperformance estimates for this smallcoaxial helicopter assumes that the rotortip Mach number is 0.65, the diskloading is 4 N/m2 and the rotordiameters are 2.44 meters. A veryconservative set of induced powerconstant and mean blade profile dragcoefficient were used for the rotorperformance estimates. Theseconservative performance coefficientswere selected to account for the highprofile drag seen for low Reynoldsnumber airfoils, as well as the effect of avery large blade root cutout to allow forrotor fold and telescoping fortransport/deployment, and the high tiplosses for large aspect ratio rotor blades.References 15-16 provide general designanalysis guidance for this first-orderperformance assessment.

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Fig. 8 – Mars Coaxial HelicopterPerformance Estimates

Figure 8 shows first-order estimates forvehicle range as a function offuel/energy-source weight fraction forthe 10kg vehicle. Three families ofcurves are shown in the figure: range

estimates using battery technology,estimates for fuel cells, and propulsionfrom a hydrazine-based Akkermanengine. These range estimates are forforward-flight power levels only (for avehicle velocity of 40 m/s or an advanceratio of 0.26) and do not account for theenergy/fuel increment for vertical take-off, hovering, and landing. Drive trainand transmission efficiencies are takeninto account in the range estimates.Note that additional battery or fuel cellcapacity is required for scientificinstrumentation and mission/flight-control power.

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) Hydrazine Akkerman Engine(SFC=1kg/MJ), Breguet Eq.

Lithium Ion Battery (150Wh/kg)

Fuel Cell (300 Wh/kg)

Fig. 9 – Mars Coaxial Helicopter RangeEstimates

A clear trade-off can be seen from thecoaxial helicopter range estimates. TheMartian rotorcraft could either usehydrazine reciprocating enginetechnology that uses a non-replenishablesupply of fuel, or battery or fuel celltechnology which could be rechargedusing almost inexhaustible (lander-based) energy sources such as solar cellarrays and/or nuclear radioisotopethermoelectric generators (RTG’s).

Exciting advances are being made inbattery and fuel cell technology. Theenergy density numbers used in therange estimates of figure 9 areconservative with respect to today’stechnology. Future advances may havea considerable impact on missioncapability of planetary aerial vehicles.On the other hand, reciprocating enginesdriven by hot gases from mono-propellants (such as hydrazine) or bi-propellants have undergone considerableanalysis in the past and should not bediscounted for their use in planetaryaerial vehicle propulsion.

The flight dynamics of a Martianrotorcraft will likely be quite unique ascompared to its terrestrial counterparts.The rotor(s) for a Martian rotorcraft willhave very low Locke numbers and willhave correspondingly have very lowaerodynamic damping. The blades willalso likely have relatively low values oftorsion and bending stiffness because oftheir large blade planform area and ultra-lightweight structure.

The key to the successful developmentof Martian autonomous rotorcraft will bethe development of ultra-lightweightstructures and equipment for suchvehicles. Though a considerable bodyof statistical and semi-empirical weightprediction tools exist for rotorcraft (see,for example, Ref. 17-21), none aredirectly applicable to the unique designchallenges of Martian rotorcraft. Thesetools need to be modified/refined toaccommodate the innovations inmaterials and structural components toarrive at acceptable preliminary designmethodologies for planetary aerialvehicles that have acceptableengineering accuracies.

Titan

Titan, Saturn’s largest moon (fig. 10), isunique in the solar system in that it is theonly moon that has a substantialatmosphere. Titan’s atmosphere iscomprised primarily of nitrogen, argon,methane – and may have similarproperties to Earth’s early atmosphere,before life began. The Voyager 1 and 2‘Grand Tour’ missions provided asubstantial amount of information aboutTitan. Nevertheless, Titan’s surface isshrouded by a thick atmospheric hazeand little is known about it. RecentHubble Space Telescope and ground-based telescope astronomicalobservations relying on new infra-redtechniques are starting to provide someinsight into the surface features of Titan,but only a faint hint of what may lie onTitan’s surface can be discerned fromthe existing available data. By 2004,the joint NASA, ESA, and Italian spaceagency Cassini space mission will reachSaturn’s orbit and release the Huygensprobe (descending via parachute) intoTitan’s atmosphere. The Huygensatmospheric probe and thecomplementary Cassini observationswill provide invaluable insights into theatmospheric chemistry/properties ofTitan. It is unclear whether the Huygensprobe will be able to soft-land on Titan’ssurface and successfully communicatedata back to Earth (this might bepossible, but is outside the officialmission scope). This accomplishmentwill likely come from future missionspost-Cassini/Huygens.

The use of vertical lift planetary aerialvehicles to explore Titan would be atremendous enabler of scientificinvestigations of one of the solarsystem’s more mysterious planetary

bodies. References 22–23 provide someimportant insights into the potential forvertical lift aerial exploration of Titan.

Fig. 10 – Titan (Image from HST)

With the arrival of the Cassini/Huygensspacecraft to Saturn and Titan in 2004 --and the anticipated science and outreachbonanza from this mission -- there maybe an opportunity to take advantage ofthe excitement underlying this adventureto advocate possible follow-on missions.Among these possible follow-onmissions is an exploration of Titanemploying small robotic vertical liftaerial vehicles.

A key consideration in the developmentof a Titan vertical lift aerial vehicle isthe robustness of the platform, the abilityto execute multiple flights, whileminimizing overall vehicle mass.Transport of such a vehicle from Earthto Titan will be an expensiveundertaking. Maximizing science returnand overall mission duration will becrucial given the expense of theenterprise.

Because of the thin atmosphere of Mars– and, therefore, the large rotor diameterand blade planform area required for

vertical flight -- ducted fan vertical liftvehicles are impractical for Marsexploration. The opposite is true foraerial vehicles for Titan. Ducted fanconfigurations such as tilt-nacelleaircraft are perhaps ideally suited forTitan (fig. 11). Ducted fan aerialvehicles would inherently be morerobust operating at low-altitudes in anunknown, potentially hazardousenvironment, than conventional rotors.A variety of ducted fan VTOL conceptshave been tested on Earth, both inmodel- and full-scale, wind tunnel andflight test. Among these ducted fanvehicles are the Doak VZ-4, theGrumman 698 (tilt-nacelle) and the BellX-22A (quad fan) aircraft (see Ref. 24-27). For low-speed, short ranges,configurations analogous to the SikorskyAircraft Cypher ducted coaxial-rotorUAV could also potentially beapplicable to Titan exploration.

(a)

(b)

Fig. 11 -- A Titan Tilt-Nacelle VTOL:(a) take-off; (b) cruise.

Figure 12 shows a first-order estimate ofhover total shaft power for a notionalTitan tilt-nacelle VTOL vehicle havingtwo ducted fans that can pivot at thewing tips (similar in configuration to theVZ-4). A conservative shroud thrustfraction of 0.3 (i.e., 30% of the totalthrust is provided by the duct/nacelleaerodynamics in hover, see Ref. 28-29)is used in the hover performanceestimate. The hover performance andfan sizing estimates are for a diskloading of 600 N/m2, a fan blade tipMach number of 0.7, and a fan solidityof 0.25. This corresponds to a meanfan blade lift coefficient of 0.75, whichshould be reasonable for the airfoilReynolds numbers estimated for theTitan ducted fan vehicle. A nacellecenterbody fairing radius of 20% of thefan blade span is assumed in theanalysis. Fan airfoil Reynolds numbersare greater than 106 at the fan blade tip.(Compare that to the rotor blade tipReynolds numbers for Martian rotorcraftwhich are estimated to be less than 105.)A Titan VTOL’s ducted fans will bevery small and consume very littlepower as a result of the very low gravityfield for Titan. As the atmosphericdensity near Titan’s surface is quite highcompared to Earth, forward-flight profileand parasite power will becorrespondingly quite high and willrestrict the maximum velocity of thevehicle to relatively low speeds.

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Fig. 12 – Ducted Fan HoverPerformance for Titan Vehicle

Figure 13 shows range estimates for a50kg Titan twin tilt-nacelle/ducted-fanVTOL vehicle, assuming powermatching between the hover and cruisedesign points. The range estimates arebased on the estimated power fromfigure 12 with reasonable drive train andelectric motor efficiencies applied. Thecruise speed is assumed to be 50 m/sec.These range estimates do not include theimpact of take-off, landing, and hover onpower availability.

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Fig. 13 – Range Estimates for a TwinDucted Fan Titan VTOL

Venus

Of the three planetary bodies besidesEarth where it theoretically might befeasible to design and fly vertical liftaerial vehicles, Venus (fig. 14) willlikely pose the greatest challenge. Thisis particularly ironic as, to date, onlyVenus can lay claim to having had aerialvehicles fly within its atmosphere(discounting, of course, entry/descentparachutes for probes and landers). Theformer Soviet Union with its Vega 1 and2 missions traversed across the upperatmosphere of Venus with two balloons.These balloons drifted with high-altitude(~50-55 km) winds almost across 30%of the circumference of the planet.Though many other planetary missionshave been proposed usingballoons/aerobots and fixed-wingaircraft, this accomplishment was thefirst and so far only demonstration ofaerial flight in an extraterrestrialenvironment.

Fig.14 -- Venus (Image Based on RadarMap from Magellan Spacecraft)

The extremely high atmosphericdensities near Venus' surface (plus the

near-Earth-magnitude of its gravitationalfield) would suggest that a buoyant, orsemi-buoyant, vehicle might representthe most practical design for explorationof Venus (fig. 15). In fact, Venus'surface atmospheric density is so greatthat such a semi-buoyant vehicle wouldin some ways likely have attributes morein kind with an underwater submersiblethan a terrestrial airship. The airframe ofa Venusian hybrid-airship would be arigid hull, which would have to be ableto sustain substantial pressuredifferentials across the external/internalsurfaces of that hull.

Venus’ high surface temperatures alsopose tremendous challenges for aerialvehicle design. Power usage must bekept to a minimum for propelling thevehicle so as to minimize waste heatgeneration and build-up from the vehiclepropulsion system and electronics.Though active and passive technologiesexist for thermal management ofplanetary science hardware, extendedoperation of such hardware near Venus’surface is currently problematic withtoday’s technology. This will thereforemean that the lift required for take-offand landing will need to be kept to anabsolute minimum (thus necessitatingbuoyancy fractions greater than 75%). Itis also likely that electric propulsion willbe required to maximized overallpropulsion efficiency. The use of high-temperature batteries (such as NaSbatteries) or fuel cell systems will alsobe required.

Fig.15 -- A Notional Venusian HybridAirship with Twin Hulls and Tandem

Tilting Propellers and Wings

A Venus vertical lift vehicle mission’sduration and science return couldperhaps be maximized by designing it tohave two phases/stages. The first phasewould entail low-altitude powered flightwith the vehicle acting as a semi-buoyant hybrid-airship capable of take-off and landing on the Venusian surface.This phase would be comprised of a fewhours of powered flight at most, givenlikely limitations in power availability.Then, when vehicle power andtemperature reach critical levels, ballastin the form of drained batteries (andpotentially unnecessary surface scienceinstruments) could be released and alonger duration high-altitude(unpowered) flight phase could beexecuted with the vehicle acting purelyas a balloon. This two stage missionapproach could potentially maximize thescience return and overall investment ofa Venus vertical lift vehicle byoptimizing overall flight endurance andvehicle and scientific instrumentationoperation.

Figure 16 shows a first-order estimate ofa notional Venus hybrid-airship’s hullsize. This hull volumetric sizingestimate is consistent with the abovedescribed two stage (combination of

powered and unpowered flight) missionapproach. This hull size estimateparallels in general the analysis given inreference 30 for a low-altitude (~10km)Venus balloon. The results shown inthis figure assumes a hybrid-airshipbuoyancy fraction of 0.9, a propulsionenergy-source (batteries, fuel cells, etc.)weight fraction of 0.25, and anunpowered balloon altitude (aftercompleting low-altitude poweredvertical lift flight and then dropping thepropulsion energy-source as ballast) of3.1km. Helium is assumed as thehybrid-airship lifting gas in the figure 16hull volumetric size trend. A thin skinof titanium alloy is assumed for the hull.Hull skin thickness using titanium alloysranges from 0.5 to 1mm thick for vehiclemass from 10 to 50 kg. This skinthickness is derived such that skinstresses are less than the material yieldstrength, with a small margin of safety(Ref. 31). The hull was modeled as anellipsoid with a fineness ration of 3.More rigorous follow-on analyses willneed to consider thin wall pressurevessel elastic buckling effects in moreaccurately defining the hull crushpressures and the hull geometry and skinthickness. The proposed thin titaniumalloy hull skins will be very hard toform/bond and will be subject to easydamage. Advanced types of high-temperature and high-strength materialsshould also be considered for the hullskins.

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Fig. 16 – Hull(s) Size Estimate

Figure 17 shows how drift altitude(altitude for unpowered neutral flotation)varies with respect to ballast (propulsionenergy-source) weight fraction. Thisanalysis assumes that there is thermaland pressure equilibrium at the driftaltitude across the external and internalsurfaces of the hybrid-airship hull(s).

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Fig. 17 -- ‘Drift’ Altitude of UnpoweredVehicle After Ballast Drop

Figure 18 shows a first-order estimate ofthe hover performance and sizing of atandem propeller/tiltwing combination(sandwiched between twin airship hulls)that could be used to take-off and landfrom Venus’s surface. The performance

and sizing estimates shown in the figureassume the airship buoyancy fraction of0.9 (therefore, the propellers have to liftonly 10% of vehicle weight in hover), atip Mach number of 0.1, a 200 N/m2 diskloading, and a solidity of 0.4 for thepropellers. These propellercharacteristics are more in common withsubmersible propulsors (see, forexample, Ref. 32) than the conventionalterrestrial rotary-wing platform.Adopting a twin hull (side-by-side)configuration for a Venusian hybrid-airship is perhaps reasonable so as toprotect the propellers of such a vehiclefrom hazardous terrain/obstacles duringtake-off and landing. The obvioustradeoff for such a configuration is thehandling characteristics of the vehicle insideslip in forward flight, potentialsubstantial nonuniform flow field effectson the propeller performance due to thepresence of the twin hulls, and increasedoverall complexity of the vehicle.These issues will need to be examined incloser detail in future design studies ofthis concept. References 33-34 providesadditional insight into airship designconsiderations.

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Sizing Estimates

Only limited information exists for theoperation of mechanical and electroniccomponents at the extreme temperaturesexpected at Venus’s surface.References 30 and 35 go into limiteddiscussion of this topic, but it is clearthat a substantial amount of researchremains to be performed in this areabefore extended duration exploration ofVenus’ surfaces or low altitudeatmosphere can be effected.

Technical Challenges andOpportunities

Autonomous vertical lift PAVs will behigh-risk and high-payoff developmentventures. Though an impressive – andever-expanding -- amount of data existsfor the planetary bodies in our solarsystem, nonetheless, these data arebarely adequate for the purposes ofdesigning and building PAVs. Suchvehicles will need to be highly adaptive(from a controls and structuresperspective), have conservativeperformance margins, and will requirehigh degrees of mission/flight autonomyto adequately deal with correspondinglevels of uncertainty in the mission andflight environment.

Rotary-Wing Aeromechanics

Several rotary-wing aeromechanicschallenges exist for the development ofvertical lift planetary aerial vehicles.First of all, in many cases, inadequateplanetary atmospheric data and/ormodeling may exist to design vehicleswith required performance margins.Further, very limited empirical data

exists for vehicle and control/liftingsurface aerodynamics for such extremeenvironments – including the low-Reynolds number, compressible flowrequired for flight in the Martianatmosphere. This will inevitably resultin the reliance on analytical tools withlimited validation to predict vehicleaerodynamics for flight in otherplanetary atmospheres. Correspondingly,limited data exist for the high disk-loading, high solidity, large aspect ratioblades, rotor designs required forexploration of Venus and Titan. It isespecially critical to acquire airfoil androtor performance databases consistentwith these planetary environmentextremes to validate design and analysistools. Achieving aeroelastic stability forrotors and/or wings will be challenging,given ultra-light weight structuresrequired for most PAV configurations.As can be seen from the discussion sofar, a mixed fleet of vehicle types islikely needed to comprehensiveplanetary science missions.

Vertical lift PAVs are not likely to beall-weather vehicles. Season, location,existence of atmospheric disturbances ofa certain magnitude, and even time ofday may dictate whether a PAV missioncan be initiated or not. For example,Mars undergoes seasonal extremes ofatmospheric mass due to sublimationand condensation of CO2 at the polarcaps. Further, seasonal 300-500 km/hrplanetary-wide storm fronts (or‘sandstorms’) also exist. It is unlikelythat PAV missions can be sustainedduring these seasonal storms.Accordingly, preservation of flightvehicle (and other) assets in the face ofthese weather extremes will be a keyconsideration for human exploration ofMars. Rotor blade ‘icing’ will take on a

whole new meaning for flight on Titan.And, finally, consider the environmentaleffects of corrosive atmosphericchemical compounds on vehicleperformance and reliability for flight inVenus’ atmosphere.

Autonomous System Capability

It is currently beyond the demonstratedautonomous system technology state-of-the-art to enable vertical lift flight in anextraterrestrial environment. NASAcurrently has several initiativesunderway investigating autonomousflight control of spacecraft and planetaryscience platforms, as well as terrestrialaircraft. Though extraterrestrial rotary-wing platforms will pose uniquechallenges for autonomous systemtechnology, it should be hoped that therewill be a general applicability of thethese automated reasoning andautonomous flight control efforts to thevertical lift PAV problem. The problemis not just software-related, but light-weight, low-power, computationallyintensive, reliable (radiation tolerant, forexample) flight control and missioncomputer systems capable of meetingvertical lift PAV requirements will alsoneed to be demonstrated. It is crucial toinitiate proof-of-concept demonstrationsfor key hardware/software componentsfor autonomous flight of terrestrialplatforms from which vertical lift PAVmission performance can beextrapolated. A key assumptionunderlying any autonomous systemtechnology development effort forvertical lift PAV should be that limiteduse of lander or orbiter assets should beassumed for guidance, navigation, andcontrol (GNC) and mission/flightsupport. A complete onboard package

of sensors and autonomous system flightcontrol capability should be assumed forvertical lift PAV -- although, suchcomplete vehicle system autonomy hasnot been demonstrated.

Guidance, Navigation, and Control

Unlike terrestrial UAVs, PAVs can notrely on GPS systems for guidance andnavigation (Ref. 36). Further, high-precision digital maps will not likelyexist either for GNC (development ofsuch maps is instead a goal/mission ofPAVs). Onboard navigation sensors,appropriate for an extraterrestrialenvironmental for a highly mobilerobotic vehicle, have yet to bedemonstrated. In particular, planetaryatmospheres such as Venus and Titancould be nearly opaque to light in thevisual range; therefore, non-opticalsensors might be required for GNC.Exotic (as compared to terrestrial UAVs)types of control actuators or controlstrategies may need to be developed tominimize vehicle weight, to operateunder severe environmental conditions,and to minimize flight control processorworkload.

Structures and Materials

Ultra-light weight structures will beessential for vertical lift PAVs –particularly for vehicles for Marsexploration. Structures and materialswill be subjected to incredible extremesof temperature and pressure as well asbeing subjected to poorly understoodlevels of atmospheric turbulence,varying weather conditions, and multi-component and multiphase (and

potentially corrosive) chemicalconstituents.

Propulsion

Outside of Earth, there is very little freeoxygen in other planetary atmospheres.Therefore, new propulsion systems willhave to be devised that do not rely onoxygen (or provide for the onboardstorage of oxygen that has either aterrestrial origin or was generated bychemical in-situ production from alander/main base). Reliability issuesmust be taken into account (includingauxiliary systems for start/restart) forcurrent implementations of mono-propellant engines such as the Akkermanhydrazine engine (Ref. 14). Solar fluxavailability is greatly diminished forother planets (in the case of Venusbecause of cloud/haze cover, and in thecase of Mars, Titan and the outer planetsbecause of distance from the Sun) forsolar energy based propulsion systems.Average Mars solar flux is only ~43 %of Earth’s (Ref. 1). Nuclear-energy-based (for example, using RTGs (Ref.37-38)) electric motor propulsion ispossible, but a significant weight penaltywould be associated with this approach.Advanced battery and fuel-celltechnology are propulsion systempossibilities (Ref. 39), but still need tobe matured for space systems.

For flight in Titan’s atmosphere, in-situmethane may be extracted from themoon’s atmosphere and combusted withoxygen/oxidizer from a terrestrialsource. Electric power generation on alander platform could recharge anonboard battery or fuel cell on a PAVbetween missions.

Deployment

Planetary aerial vehicles will besubjected to considerable constraintsregarding mass and volume. This willpose challenges for all vehicledevelopment disciplines, but willparticularly affect the means andsystems involved in the vehicledeployment. Vehicle assembly,configuring for flight, and deployment ofPAVs pose unique challenges comparedto terrestrial aerospace vehicles. Newdesign approaches, mechanical systems,and structures will need to be developedfor PAVs. The advantage of vertical liftPAVs (over other types of PAVs) is thatthey can be assembled (if need be),configured for flight, and launched froma lander, with adequate time fordeployment; they will not have to relyon deployment during entry into aplanet’s atmosphere. Reelable (Ref. 40-41), foldable, or telescoping variable-diameter rotor blades are all possiblecandidates for achieving minimumvehicle volume (in stowed/packageform) for integration into launch andentry vehicles.

Telecommunication

Telecommunication poses a considerablechallenge for robotic planetary sciencevehicles. Communication delays aresubstantial to and from Earth to otherplanetary bodies, particularly whentaking into account relative orbitalrotation of those bodies with respect toEarth, the location of the aerial vehicleon the planetary surface, and delays insatellite overflight with respect to theaerial vehicle (or delays until vehicle

return to the lander). Further, high-bandwidth signals for data-intensivescience missions dictate even tighterconstraints in communication optionsand data transfer opportunities to Earth.

Ongoing Work

Work to date within the NASA AmesRotorcraft Division has focused onvehicle conceptual design studies. As aresult, reference 2 provided an initialdiscussion of the technical challengesand opportunities of vertical lift PAVs.This conceptual design work continuesand focuses on not only alternate vehicleconfigurations for Mars exploration buthas begun to consider vehicle conceptsfor other planetary bodies.

In addition to vehicle configurationstudies, a university grant with CarnegieMellon University developed a baselineconceptual design of a mission/flightcontrol computer architecture for anotional Martian autonomous rotorcraft.This initial mission/flight control workhas focused on the use of visual cueingsystems to provide for vehicle guidanceand navigation. Onboard visual systemsfor GNC for vertical lift aerial vehiclesare potentially an ideal solution forautonomous extraterrestrial flight.Impressive gains have been made in thisfield but a considerable amount of workremains to be accomplished in this area.

Ongoing work on vertical lift planetaryaerial vehicles within NASA Amescontinues to focus on the design andanalysis of Martian autonomousrotorcraft for science (MARS)configurations (fig. 19). This effort

includes initiating development of low-cost proof-of-concept test articles fordemonstrating critical MARStechnologies – including thedevelopment of a hover test stand fortesting full-scale rotors at Marsatmospheric densities and a tetheredhover flight demonstrator (Fig. 20a-b).An initial baseline Mars rotor is in thefinal stages of fabrication. This rotor isa nonoptimized configuration butreflects many of the design constraintsrequired for an actual flight article. Theblades are composed of lightweightRohacell foam hollowed out internallywith a leading edge fiberglass layup forprotection and chordwise mass balance.The blades are dynamically tailored tominimize hover ground resonance, buthave not yet been optimized for forward-flight blade/hub loading. The blade rootcut-out for the rotor simulates theunfaired blade span required for theblade fold/telescoping needed for vehicletransport and deployment.

The focus of this initial proof-of-concepthover testing is the assessment of overallrotor hover performance. The proof-of-concept rotor blades will have constantchord and will use the Eppler 387 (Ref.42-43) airfoil. Recent unpublishedresults from NASA Langley wouldsuggest that the Eppler 387 has fairlyhigh lift coefficients (and low pitchingmoment cofficients) for the Reynoldsand Mach number ranges of interest.Future Mars rotor test articles will likelysee the use of optimized airfoils, asignificant evolution in blade/rotorgeometry, and improved dynamiccharacteristics.

Fig. 19 – Inhouse Analysis

(a)

(b)

Fig. 20– Proof-of-Concept Test ArticleDevelopment: (a) Baseline rotor design;

(b) Proof-Blade Fabrication

In addition to the inhouse research anddevelopment efforts, a considerableamount of emphasis has been placed onpublic and educational outreach for theproject.

Public and Educational Outreach

Educational outreach in the early stagesof this endeavor is vitally important for anumber of reasons. First, the successfuldevelopment of planetary aerial vehicleswill be by necessity a highlycollaborative, multidiscipline effortincluding universities as well as NASAand the rotorcraft industry. Second, anearly introduction of this new concept totoday's students will hopefully prove tobe an important inspirational catalyst toa founding/pioneering generation ofextraterrestrial aviators and planetaryaerial vehicle designers.

In this regards the Year 2000 AmericanHelicopter Society Student DesignCompetition was successfully initiatedon the design topic of a Martianautonomous rotorcraft (Ref. 44). Afollow-on NASA-sponsored studentdesign competition for the conceptualdesign of a Titan vertical lift aerialvehicle is currently being planned.

Proposals for the AHS competition(undergraduate and graduate level) weresolicited for three design areas: vehicleconfiguration, propulsion system, andmission/flight-control computerarchitecture. Briefly summarized, thegeneral mission/design requirements forthe AHS student design competition fora Martian rotorcraft were:

Assume a Mars Mission landing date2005. A Martian autonomousrotorcraft will be deployed from alander on the surface. The mission ofthis Martian autonomous rotorcraftwould be threefold: a proof-of-

concept demonstration of rotary-wing flight in the Martianatmosphere, a limited aerial survey(with photographic telemetry) whilein flight, and successful soft-landingon the Martian surface.

Required Mission Elements include:Deployment from Mars landerSystem CheckoutStart / Warm-upHoverCruise /Maneuver / Send

TelemetryReturn to specified locationHoverLandShutdownOptional Enhancement:

RestartHoverReposition small distanceHoverLandShutdown

Or, more specifically, for the vehiclestudy, the design requirements were:

• Vehicle 'Gross Weight' mass not toexceed 50 Kg

• Minimum sustained 'controlled-flight' duration of no less than onehalf-hour is required. Range is ofsecondary concern; ideally, rangeshould be greater than 25 km.

• Maximum cruise altitude (AGL) to100 meters (low-level flight).

• Vehicle is capable of hovering /soft-landing on Martian surfaceafter controlled-flight has beendemonstrated. It is a desiredobjective to demonstrate a restart

and second takeoff and landingfollowing the required soft landing.

• Photographic images taken inflight and post-soft-landing will betransferred via vehicle telemetry toa lander or an orbiter for storageand transfer to Earth GroundControl. Flight profile and vehiclestatus telemetry should also betransferred from the Martianautonomous rotorcraft to the landeror an orbiter.

• Flight/Mission Package 'Avionics'including camera, sensors andtelemetry shall be assumed to beno more than 10% of vehicle mass

• Martian autonomous rotorcraft willbe capable of sustainingcontinuous full sensor and datarelay power consumption (firstorder power consumption estimateto be made as a part of vehicledesign) for four (4) hours afterseparation from the lander and 31/2 hours after demonstration ofsoft-landing.

• The air vehicle must beautonomously deployed from theMars lander. Complete vehicleautonomy must be demonstratedafter release from the lander; onlypassive telemetry will be receivedfrom the Martian autonomousrotorcraft.

• Auxiliary (nonflight) systems onthe lander can be used toassemble/deploy the Martianautonomous rotorcraft and/or fuel,power, or spin-up the rotor(s).

• The vehicle must be capable ofsustained hovering flight for noless than one minute duration.

• Maximum Mars entry accelerationto be assumed to be 100 m/sec2

Exceptional design study papers fromthe participating universities werereceived. The winning teams of thiscompetition will be announced at the57th Annual Forum of the AmericanHelicopter Society in Washington, DC.

A follow-on NASA-sponsored studentdesign competition is currently in theplanning stage and will focus on avertical lift vehicle for Titan. It is likelythat the following nominal mission anddesign requirements will be proposed tothe competing student teams:

Assume it is the year 2016. Twelveyears after the successful explorationof the upper atmosphere of Titan bythe Cassini/Huygens mission -- sixyears of engineering developmentfollowed by six years of transit toSaturn from Earth -- an orbiter satelliteand aeroshell entry vehicle arrive atTitan for the long awaited follow-upmission. The orbiter will executedetailed lidar/radar mapping of Titan.The lander besides carrying its owncomplement of sophisticatedinstruments will also act as a transportcarrier, launch platform, and homebase for a small robotic vertical liftplanetary aerial vehicle. This aerialplatform will be used on a numbermissions/flights over several weeks tocomplete a detailed survey of terrain(both solid and, potentially, liquid(methane) surfaces) measured in areaover several hundred square

kilometers. Communication betweenthe aerial vehicle and the lander andEarth is maintained by the orbitersatellite.

Specific design requirements will likelyconsist of the following:

• A minimum total vehicle range of300 km while carrying a 10%payload fraction is required.

• Maximum cruise altitude is 2km;high-altitude cruise leg of missionwill comprise less 50% of totalmission range; remaining 50% offlight is at low altitude (less than500 meters) and low-speed.

• Vertical take-off and landingcapability is required for thevehicle design.

• A mid-mission hover out-of-ground effect for one minute isrequired, followed by a mid-mission vertical landing and take-off.

• Assume a maximum of 5m/secgusts in hover and low-speedflight.

• There is no maximum speedrequirement. Range and payloadare more critical design goals.

• Vehicle should be capable ofpropulsion system shutdown, andrestart, upon landing mid-mission.Auxiliary power source should becapable of supporting vehiclestand-by flight systems and sciencepayload for four hours at the mid-mission remote site.

• There is a requirement for multipleflights/missions with the aerialvehicle. Therefore, the vehiclemust be capable of returning to thelander and refueling or rechargingfor subsequent flights/missions

• Maximum vehicle gross weight tobe less than 100kg (mass).

• Vehicle should be capable oflanding on both solid, uneven (icy)surfaces and liquid (methane)pools; this will impact concepts forvehicle landing gear design.Assume that the solid surface forvehicle remote-site landing willinclude surface debris of 0.03cubic meters. Vehicle will hoverover and land on a lander platformwhen returning post-mission/flight.

• The vehicle must be capable ofautonomous flight and take-off andlanding

• The aerial vehicle must be capableof successful enduring a maximumaeroshell entry deceleration of 100m/sec2.

• Flight/Mission Package 'Avionics'including computer, sensors andtelemetry shall be assumed to beno more than 10% of vehicle

• Auxiliary (nonflight) systems onthe lander can be used toassemble/deploy the Titan verticallift aerial vehicle and/or fuel,power, or spin-up therotor(s)/propulsion system.

Program advocacy will be as importantan element for the successfuldevelopment of vertical lift planetaryaerial vehicles as any given technicalaccomplishment. Therefore, public andeducational outreach efforts are crucialto the ultimate viability of planetaryaerial vehicles.

Back On Earth (Technology TransferOpportunities)

Why should the vertical flightcommunity be interested in promotingand participating in the study and,perhaps, the ultimate development ofvertical lift planetary aerial vehicles?There are several reasons. First of all,PAV advanced autonomoussoftware/hardware technology would beapplicable to terrestrial UAV’s (fig. 21).Technologies developed for PAVs –including microelectronics/sensors andlightweight power sources/systems suchas fuel cells and advanced batteries --could be applicable to Micro AirVehicles (MAVs).

Programmatically, vertical lift PAVdevelopment will promote a strongworking relationship between NASAAeronautics, Space, and InformationTechnology programs. And finally, aspreviously noted, the development anduse of vertical lift planetary aerialvehicles for Mars, Titan, and Venusexploration would considerably enhancepublic and educational outreach andpositive awareness for the rotary-wingcommunity and terrestrial rotorcraftapplications/missions.

Fig. 21 – Planet Earth (RealizingTechnology Benefits Back Home)

Concluding Remarks

Initial work to date suggests that verticallift planetary aerial vehicles arepotentially feasible. Such vertical liftplanetary aerial vehicles havetremendous potential to support NASAplanetary science and explorationprograms. Vertical lift planetary aerialvehicles – if ultimately proven to befeasible -- will be employed in bothpurely robotic missions, or as 'astronautagents' for manned planetaryexpeditions. In particular, Martianautonomous rotorcraft being used in‘scout’ and/or utility roles would enablefundamental scientific quests such as the‘hunt for water’ and the search for life.With research into and development ofsuch vehicles there are tremendousoutreach possibilities for the rotorcraftcommunity. Further, there aresignificant potential technology transfer

opportunities for terrestrial rotorcraftapplications.

Three notional vehicle concepts werebriefly examined in this paper. Thesevehicle concepts reflect the breadth ofpowered/vertical lift and rotary-wingtechnologies that could be applied tosupport planetary science missions. Thisinsight is particularly appropriate tohighlight during the Year 2000International Powered Lift Conference.

In conclusion, autonomous vertical liftplanetary aerial vehicles can potentiallyplay a vital future role in the explorationof the solar system. A considerableamount of work lies ahead to developsuch vehicles. A modest level of effortcontinues to be sustained within theRotorcraft Division at NASA AmesResearch Center to define and developthe technologies necessary for verticallift planetary aerial vehicles.

Acknowledgments

The support of the NASA AmesAerospace Directorate and the CenterDirector's Discretionary Fund isgratefully acknowledged. Thanks mustalso be given to Mr. George Price andMr. Christopher Van Buiten of SikorskyAircraft for their efforts on behalf of theAHS International’s Student DesignCompetition on the topic of a Martianautonomous rotorcraft. Finally, thestrong support and advocacy of Mr.Edwin W. Aiken, of NASA AmesResearch Center, for the investigationinto and development of vertical lift

planetary aerial vehicle technology isalso gratefully acknowledged.

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4. TITLE AND SUBTITLE Vertical Lift - Not Just For Terrestrial Flight

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Army/NASA Rotorcraft Division,Army Aviation and MissileCommand,Aeroflightdynamics Directorate (AMRDEC), Ames ResearchCenter,Moffett Field,CA,94035

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13. SUPPLEMENTARY NOTES AHS/AIAA/RaeS/SAE International Powered Lift Conference, Arlington, VA, October 30-November 1, 2000

14. ABSTRACT Autonomous vertical lift vehicles hold considerable potential for supporting planetary science andexploration missions. This paper discusses several technical aspects of vertical lift planetary aerial vehiclesin general, and specifically addresses technical challenges and work to date examining notional vertical liftvehicles for Mars, Titan, and Venus exploration.

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