Young AHS Design2000

7/24/2019 Young AHS Design2000

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Design Opportunities and Challenges in the Development of VerticalLift Planetary Aerial Vehicles

Larry A. Young

Robert T.N. ChenEdwin W. Aiken

Army/NASA Rotorcraft Division

Geoffrey A. BriggsCenter for Mars Exploration

NASA Ames Research CenterMoffett Field, CA

Abstract

The next few years promise a unique convergence of NASA aeronautics and spaceprograms. NASA planetary science missions are becoming increasingly moresophisticated. This will ultimately culminate, in part, in the development of planetaryaerial vehicles (PAVs). Early work in this area has principally focused on conceptualdesign of fixed-wing aircraft configurations for Mars exploration. However, autonomousvertical lift vehicles hold considerable potential for supporting planetary science andexploration missions. This paper discusses in a general sense the technical opportunitiesand challenges in developing autonomous vertical lift PAVs. Through this discussion avision for using PAVs in planetary exploration is presented.

Introduction

Manned and robotic exploration of theSolar system planets would be greatlyenhanced through the development anduse of robotic aerial vehicles. Since the1970s a number of Mars (fixed-wing)

Airplane concepts have been proposedfor Mars exploration.

Presented at the American Helicopter Society

International Vertical Lift Aircraft Design SpecialistsMeeting, San Francisco, CA, January 19-21, 2000.Copyright 2000 by the American Helicopter Society, Inc.All rights reserved.

The Army/NASA Rotorcraft Division --in collaboration with the Center for MarsExploration -- at NASA Ames has beenperforming initial conceptual designstudies over the past year of a Martianautonomous rotorcraft for planetaryexploration and science missions (fig. 1).Initial results have been quite promising.

As a result of this early work, theauthors have generalized their thoughtsregarding the utility of rotorcraft, VTOLvehicles, and hybrid airships for Marsexploration and planetary sciencemissions as a whole.


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Why vertical lift vehicles for planetaryexploration? For the same reason thatthese vehicles are such flexible aerialplatforms for terrestrial exploration andtransportation: the ability to hover and

fly at low-speeds and to take-off andland at unprepared remote sites.Further, autonomous vertical liftplanetary aerial vehicles (PAVs) wouldhave the following specificadvantages/capabilities for planetaryexploration:

Hover and low-speed flightcapability would enable detailedand panoramic survey of remote

sites;

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

Soft landing capability for vehiclereuse (i.e. lander refueling andmultiple sorties) 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 to

hazardous terrain than a lander orrover.

Further, even if a planetary aerial vehicleis not a vertical lift aircraft or rotorcraft,

there are several rotary-wingtechnologies that will nonetheless have aprofound influence on PAVdevelopment. These technologiesinclude: high-efficiency propeller orproprotor design; precision guidance,navigation and control at low altitudesand near-terrain obstacles; adaptive(inner-loop) flight control; autonomoussystems work based on vertical liftvehicle applications; high-frequency

open- and closed-loop smartstructures/actuators.

Figure 1 Vertical Lift Planetary AerialVehicles as Astronaut Agents

The objective of this paper is to inspirethe vertical flight research community toconsider and to embrace the concept ofvertical lift planetary aerial vehicles andto participate in their ultimatedevelopment and use.


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State of the Art in Planetary Science

Over the past forty years planetaryscience has made incredible advances by

means of robotic missions carried out byspacecraft from our planet. Fly-byprobes, orbiters, landers, hard-probes/penetrators, rovers, and aerostatshave been launched, successfullycompleted their missions, and providedus invaluable data to expand ourunderstanding of the solar system (fig.2). Today, planetary science is poisedto make further advances using roboticplanetary aerial vehicles to conduct

scientific investigations.

The development or evolution ofplanetary aerial vehicles will likelyparallel the evolution of terrestrial flyingvehicles: first will come balloons,followed by airships and/or fixed-wingaircraft, and finally rotary-wing orvertical lift vehicles. Balloons, oraerostats, have already been flown inVenus upper atmosphere (jointly by the

Soviet Union and France) on the Vega 1and 2 missions in December 1984. Soonother types of PAVs will be developed,launched, and used to conduct planetaryscience missions.

The development of PAVs will posenew and exciting challenges foraeronauticalengineers.

Opportunities

As noted earlier, work is being pursuedat the Ames Research CentersArmy/NASA Rotorcraft Division on aMartian autonomous rotorcraft. Whynot, though, as a next step, a VenusianVTOL? Or a Jovian flyer? Or, even, a

Titan rotary-wing aircraft? Applicationof vertical lift and rotary-wingtechnologies to the development ofplanetary aerial vehicles would beextremely beneficial to the United States

long-term planetary exploration effort.

Fig. 2 Our Solar System

This paper poses -- and makes an initialstart in addressing -- the question of thefeasibility of vertical lift PAVs, as wellas the more general question of the

applicability of rotary-wing technologiesto planetary science and exploration.Table 1 is a summary of the key surfaceatmospheric properties for variousplanets in our solar system. This tablehas been divided into three parts: adescription of terrestrial type planetsand moons; outer, or gas-giant, planets;


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planets and moons with tenuous, ornonexistent, atmospheres. Later in thepaper this information will be used toexamine the general aerodynamicattributes of vertical lift, and other,

planetary vehicles.

Table 1 Summary of PlanetaryDescriptions (Ref. 1)

MeanRadius(km)

Gravity!

(m/s2)

MeanSurfaceAtmos.Temp.(

oK)

MeanSurfaceAtmos.

Pressure

(Pa)

MeanSurfaceAtmos.Density(kg/m

3)

Atmos.Gases

Terres-

trialType

Planets&Moons

Venus 6052 8.87 735.3 9.21x106

64.79 CO296%

N23.5%

Earth 6371 9.82 288.2 101,300 1.23 N278%O221%

Mars 3390 3.71 214 636 1.55x10-2 CO295%

N22.7%Ar 1.6%O20.1%

Titan(Saturn

moon)

2575 1.354 94 149,526 5.55 N265-98%

Ar


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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; performcomparative analyses of the Mars

planetary evolutionary process with theother terrestrial-type planets in our solarsystem

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

Titan Search for life or the precursorbiochemical components of life

Perform atmospheric science studies tounderstand the unique nature of the Titanatmosphere (a high density/pressureatmosphere)

Survey for chemical resources/volatilesthat could enable in-situ propellant andfuel production at the lander site;

propellant could be used for samplereturn missions to Earth, expandedsurveys of the Saturnian moons including expanded vertical lift planetaryaerial vehicle surveys of Titan

Venus Correlate space-based cartographic andinferred geological data with detailedsurveys in targeted areas using verticallift PAVs.

Acquire adequate data to understand thefundamental atmospheric and geologicalevolutionary processes that led oursister planet to be radically differentfrom Earth

Determine if planetary-scale green-house effects can be halted and/orreversed

Jupiter Understand the atmosphericscience/physics of outer, gas-giant

planets Use outer planet atmospheric data as a

comparative benchmark database torefine atmospheric and meteorologymodeling potentially leading to newinsights and improvements inmeteorology and climatology predictionsfor Earth

Understand the planetary-scalethermodynamics of outer, gas-giant

planets where a net positive heatgeneration is maintained

Europa Search for life or the precursorbiochemical components of life

Survey for chemical resources/volatilesfor expanded mission/science potential

Acquire data to understand the geophysicsunderlaying the existence and

preservation of a hypothetical Europansub-surface ocean

Examine fault lines or other potential

weaknesses in the ice crust (initiallyidentified from radar-mapping from anorbiter) for siting of icebots and/orother drilling equipment to break throughto hypothetical underlying ocean, where

possibly hydrobots could be releasedand explore

The Moon Continue to acquire data (particularlythrough deep-core drilling) to understandthe formation process of the Moon

Continue/expand upon search for waterice at the Moon poles; existence of thisresource will be critical to the extent andmagnitude of lunar exploration

Perform a comprehensive mineralogicalsurvey for the Moon to identify potentialresources required for a sustainedhuman presence

Use the Moon as a staging area forcontinued exploration of the solar system(and through far-side observatories) andthe Universe

Asteroids Perform cartography and geo-chemical

analyses to understand planetesimalformation, and, through extrapolation,planetary/solar system formation.

Perform geological and mineralogicalsurvey of asteroids (particularly near-Earth asteroids) and determine ifeconomically valid resources could beextracted from asteroids and transportedto Earth or manned space facilities

Table 3 Contributions of Vertical Lift

Technology to Planetary Science

Potential Vertical Lift Contribution

Venus, Mars,Titan

Vertical lift vehicles (aided by, or solelyusing, rotors as the means of propulsion)can be developed and flown to support

both proof-of-concept, extended roboticscience missions, and in the case ofMars support human exploration of the

planet. Almost all of vertical lift and/orrotary-wing multi-discipline knowledgeand technologies would have applicationto vehicle development and missionexecution for planetary science missionsto these planets/moons.

Jupiter,Saturn,Uranus,

Neptune

Vertical lift capability is not required forany PAVs to be used for scientificinvestigations of the gas-giant, outer solarsystem, planets. However, rotary-wingtechnologies such as rotor aeromechanics(for propeller design), etc., would beapplicable for vehicle development forthese planets.

Mercury,Pluto, the

The tenuous or nonexistent atmospheres ofthese planets, moons, and other planetary


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Moon,Europa, andother moons,asteroids, andcomets

bodies prohibit the application of rotary-wing propulsion. Instead, vehiclesemploying chemical or electrical

propulsion (rockets or ion-engines) will beused for ballistic and/or low-level flightand take-off and landing to explore these

planetary bodies. However, even underthese circumstances, the rotorcraft and

vertical lift technical communities cancontribute. In particular, guidance,navigation, and control technologiesdeveloped for hover and nap of the earthlow-speed flight can still be successfullyapplied to rocket/ion-engine propulsionvehicles for low-level flight/exploration.

Considerable enthusiasm and supportfrom the American public could begenerated for both the demonstration of -and the science returned from -

extraterrestrial atmospheric flight.

General Aerodynamic Attributes for

Extraterrestrial Aerial Flight

A comparative first-order aerodynamicanalysis will now be presented forvertical lift (and other) PAVs. Analysisresults for terrestrial aerial vehicles willbe used as baselines for the vehiclessized for other planetary bodies. Thiscomparative analysis will be a first steptowards understanding the opportunitiesand challenges of the vertical liftplanetary aerial design.

Figures 3 and 4 are approximateestimates of the speed of sound andkinematic viscosity for various differentplanetary bodies in the solar system.The estimates were made based on datafrom Table 1, Ref. 1, real gas data fromreference 6, and using the Maxwell-

Rayleigh power law, (m~m0(T/T0)n,

where m is dynamic viscosity and m0,

T0, and n are real gas constants for theprimary atmospheric constituent for

each of the planets), and the ideal-gaslaw. Improved thermodynamicequations of state can be used for refinedanalyses of the atmosphericcharacteristics of the various planets in

the solar system.

0 200 400 600 800 1000 1200

Venus

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Planetary

Bodies

Speed of Sound (m/sec)

Fig. 3 Speed of Sound for DifferentPlanetary Atmospheres

1E-07 0.000001 0.00001 0.0001 0.001 0.01 0.1 1

Venus

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Planetary

Bodies

Kinematic Viscosity (m^2/sec)

Fig. 4 Estimates of KinematicViscosity for Different Planetary

Atmospheres

Figures 5a-d is a set of bar charts fordifferent rotors sized (using simple rotor


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momentum theory analysis) for hover indifferent planetary surface atmospheres,

assuming constant solidity (s=0.1),

mean lift coefficient (CL=0.4), and tipmach number (Mtip=0.7). Results for

three different vertical lift planetaryaerial vehicle masses (10, 25, and 50 kg)are shown in the figures.

0 1 2 3 4 5

Venus

Titan

Earth

Mars

Planetary

Bodies

Rotor Radius (m)

PAV Mass = 50kg

PAV Mass = 25kg

PAV Mass = 10kg

Fig. 5a Single Main Rotor Radius Sized

for Hover

0 1000000 2000000 3000000 4000000

Venus

Titan

Earth

Mars

Planetary

Bodies

Reynolds Number

PAV Mass = 50kg

PAV Mass = 25kg

Four-bladed rotor, PAVMass = 10kg

Fig. 5b -- Rotor Blade Tip ReynoldsNumber

1 10 100 1000 10000 100000

Venus

Titan

Earth

Mars

Planetary

Bodies

Disk Loading (N/m^2)

Fig. 5c Disk Loading of Single MainRotors

0 2000 4000 6000 8000 10000

Venus

Titan

Earth

Mars

Planetary

Bodies

Ideal Hover Power (Watts)

50kg

25kg

PAV Mass = 10kg

Fig. 5d Ideal Hover Power of SingleMain Rotors in Different Planetary

Atmospheres

Figures 5a-d are presented forcomparative purposes only; nooptimization of the single main rotors

has been performed. Figure 5a showsthe range of rotor sizes required forhover on each of the four planetarybodies (Mars, Venus, Titan, and Earth)where it might be beneficial to havevertical lift capability. Figure 5b showsthe relative extremes of blade tipReynolds number for rotors on various


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planets. Figure 5c shows the range ofrotor disk loading for vertical lift PAVs.Figure 5d shows the ideal rotor hoverpower required.

As shown in Fig. 5c, a rotary-wingvertical lift planetary aerial vehicle forexploration of the Venusian surfacewould have disk loading approachingthat of terrestrial marine screwpropellers. Alternatively, a Mars rotordisk loading would be similar to human-powered helicopter rotors (Ref. 7). Inboth cases, though, no data exist forcompressible flow at these density andlow Reynolds number ranges. In the

case of the Venus rotor, given theunrealistically high disk loading, a tipMach number of 0.7 is much too high forflight near the surface of Venus. A morerealistic tip Mach number range wouldlikely be Mtip


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Figure 6a compares fixed-wing planformarea for various PAVs in airplane-modeforward-flight cruise. The wingplanform area was sized for the designpoint condition of CL = 0.8 at the Vmin

Mach number = 0.1.

0.01 0.1 1 10 100

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Wing Area (m^2)

50 kg

25 kg

PAV Mass = 10 kg

Fig. 6a Wing Planform Area for Fixed-Wing PAVs for Various Planets

Collectively, PAVs for the outer, gas-

giant planets require roughly the samegeneral range of wing planform area.Further, the outer, gas-giant PAVscompare fairly well to the wing arearequired for equivalent terrestrial fixed-wing aircraft (when sizing is based onvehicle mass, not weight). The Titan andMars fixed-wing aerial vehicles are atopposite extremes with respect to wingplanform area requirements.

0.E+00 1.E+06 2.E+06 3.E+06 4.E+06 5.E+06 6.E+06 7.E+06 8.E+06

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Reynolds Number

50kg

Mass = 25 kg

Vehicle Mass = 10 kg

Fig. 6b -- Wing Reynolds Number

Figure 6b shows the wing Reynoldsnumber (based on mean chord length) forthe various sized PAVs. Only the MarsPAV clearly has a wing Reynoldsnumber in the laminar flow range. Thewing profile drag coefficient is, as aconsequence, generally higher for theMars fixed-wing PAV than for thevehicles sized for the other planetarybodies. The lack of data (particularly forthe compressible flow region) for lowReynolds number airfoils, in the range ofthe Mars PAV airfoils, is one of thedesign challenges for developing an aerialvehicle for Mars exploration.


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0 0.2 0.4 0.6 0.8 1

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Rotor Radius (m)

50kg

Mass = 25 kg

Vehicle Mass = 10 kg

Fig. 6c Propeller Sizing Comparisonfor Various Planets

Figure 6c-d compare the rotor radius anddisk loading of fixed-wing PAVpropellers sized for different planetaryatmospheres. Collectively, theterrestrial baseline and the outer, gas-giant planet propellers compare closelyto each other in terms of size and diskloading. The Mars fixed-wing PAVpropeller is much larger, and has a

significantly lower disk loading, than theother vehicles. The propeller/rotor andwing size required for Mars PAVs willpose significant weight and deploymentdesignchallenges.

1 1 0 100 1000

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Disk Loading (N/m^2)

Fig. 6d Propeller Disk Loading

0 5000 10000 15000 20000 25000

Earth

Mars

Titan

Jupiter

Saturn

Uranus

Neptune

Propeller Power Estimate (Watts)

50 kg

Mass = 25 kg

PAV Mass = 10 kg

Fig. 6e Power Estimate for PAV

Propellers

Figure 6e compares the propeller shaftpower cruise requirements for thevarious propeller-driven, fixed-wingPAVs. These estimates are based on themethodology of Ref. 13. The Mars andTitan propeller power requirements aremuch lower than the power requirementsfor the outer, gas-giant PAVs. Providing

adequate power for propulsion for thegas-giant PAVs will be a key designchallenge for their development.

A Venus fixed-wing, propeller-drivenplanetary aerial vehicle is not shown inFig. 6a-e since the parasite drag will bevery large for a Venus PAV that fliesclose to the planets surface. Such avehicle would have to be substantiallyslower than the other vehicles. (A Venus

PAV would have a forward-flight speedof approximately M=0.05 -- versusM=0.2 as assumed for the other vehicles-- in order to have roughly the sameoverall vehicle drag.)

Vertical lift planetary aerial vehicles donot make sense for gas-giant planets, as


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they do not have surfaces to take-off andland from, or hover over, in theconventional sense. Propeller-drivenfixed-wing PAVs, on the other hand, domake sense. Because of the high power

requirements for higher speeds, themaximum cruise speed has been limitedin this sizing comparison to a relativelylow Mach number of 0.2. Oneconsequence of limiting the maximumcruise speed to this relatively low Machnumber is that the PAV will not be ableto make headway against the very strongJovian (and likely other gas-giant)tailwinds; therefore the vehicle flight-path will be partially dependent upon

the upper atmospheric wind-patterns.

A quick review of Fig. 6a-e reveals thatvehicles sized for the outer, gas-giantplanets will likely look similar to small,moderate aspect ratio, terrestrial fixed-wing unmanned air vehicles (UAVs),with respect to overall propeller size,wing area, and operating Reynoldsnumbers range. Packaging anddeployment of Titan and outer, gas-giant

PAVs will be somewhat more tractablethan a similar vehicle for Mars and,yet, power requirements for a MarsPAV will be significantly lower than theother planetary bodies.

Considering more exotic vehicles (fromthe rotorcraft community perspective),one can envision rotary-wingtechnologies being applied to other typesof planetary aerial vehicles, including

hybrid airships (Refs. 14-15) andchemical, or electrical, propulsionballistic hoppers (Refs. 16-18).Hybrid airships, particularly conceptsrelying on propellers for not onlyprimary propulsion but low-speedhandling/flight as well, can directlybenefit from the application of rotary-

wing technologies. Even rocket, or ion-engine, ballistic hoppers can benefitfrom guidance, navigation, and controlwork developed by the vertical lift androtorcraft communities. Further

autonomous system technologydeveloped for terrestrial vertical liftaircraft can be directly applied tohoppers that would be used forplanetary bodies with tenuous ornonexistent atmospheres.

Martian Aviators

As noted earlier, balloons/aerostats (Ref.19) and Mars Airplanes (Refs. 20-24)have been proposed for some time forMars exploration. Only recently havevertical lift configurations beenconsidered.

In the near future, vertical lift PAVs willlikely focus on Mars - robotic andmanned exploration (fig. 7). Thissection briefly discusses Mars mission

options, and provides some thoughts ona notional early robotic mission.

Assume for the moment a Mars Missionlanding date of 2005 or 2007. Assumefurther that a Martian AutonomousRotorcraft for Science (MARS) will bedeployed from a lander on the surface.The mission for such a Martianautonomous rotorcraft would bethreefold: a proof-of-concept

demonstration for rotary-wing flight inthe Martian atmosphere, a limited aerialsurvey (with photographic imagetelemetry) while in flight, and asuccessful soft-landing on the Martiansurface.


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Fig. 7 - Mars

Maximum vehicle mass would be quite

small, ~10-20 kg. Minimum sustainedcontrolled-flight duration would be for ashort period of time approximately aminimum of a half-hour flight wouldlikely be required. Range would be ofsecondary concern. Ideally, range shouldbe greater than 10 km. Maximum cruisealtitude would be quite low approximately 100 to 300 meters. Thevehicle would have to be capable ofdemonstrating vertical lift capability -

and, therefore, should be capable of (atminimum) soft-landing on Martiansurface after controlled-flight has beendemonstrated.

The Martian atmosphere is 95% CO2with the remaining 5% comprised of N2and other trace gases (see Table 1).Further, the atmosphere of Mars isextremely cold and thin (approximately1/100th of Earths sea-level atmospheric

density). This is roughly equivalent toflying an aerial vehicle at an altitude of100,000 feet in the Earths atmosphere.Further, a variation of approximately20% for density and pressure will beseen on Mars for both changes in surfaceelevation and planetary atmosphericmass (a consequence of polar CO2

condensation and sublimation whichoccurs on a seasonal basis). Given thethin, carbon-dioxide-based Martianatmosphere, developing a rotorcraftdesign that can fly in that planetary

environment will be very challenging.Despite the low density of the Martianatmosphere, rarefaction effects likelyneed not be considered in Mars PAVaerodynamic analysis for low-level flightfor Reynolds numbers Re>104M2, Mbeing the Mach number (Ref. 10).

Fig. 8 Notional Martian Tiltrotor(Some Assembly Required)

A Mars tiltrotor is a particularlyattractive configuration option (fig. 8).

A tiltrotor represents a goodcompromise between hover performanceand cruise range/endurance bothattributes are extremely important forMars exploration. Figures 9a-d presentsome initial sizing/performance estimatesfor a small (10 kg) autonomous Marstiltrotor configuration. One of the


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biggest issues for the Mars tiltrotorconfiguration is that the deployment of atiltrotor from even the surface of Marswill be fairly complicated, and willrequire astronaut-assisted assembly or an

autonomous assembly process on thelander platform.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.1 0.2 0.3 0.4 0.5 0.6

Rotor Mean Lift Coefficient

Ma

rsTiltrotorRadii(m) Mtip = 0.5

0.6

0.7

Design Point

Fig. 9a Mars Tiltrotor Rotor Radii SizeEstimates (Vehicle Mass = 10 kg)

Figure 9a shows the trend of rotor size

as a function of rotor mean liftcoefficient and tip Mach number. Anotional rotor design point of Mtip =0.7and CL=0.4 is noted on the figure. Theresulting rotors are quite large and willnecessitate special considerations instowage (in the aeroshell entry vehicle)and deployment on the Martian surface.This result points to the necessity offolding and/or telescoping rotor blades.

0

200

400

600

0.4 0.5 0.6 0.7 0.8 0.9

Hover Figure of Merit

Rotor

Shaft

Power

(Watts)

Design Point

Fig. 9b Rotor Shaft Power (per rotor)

Figure 9b shows rotor shaft powerversus figure of merit (FM). From apractical standpoint, because of thecompressible low-Reynolds numberregime in which the Mars tiltrotor bladesmust operate, a low figure of merit wasintentionally chosen as a design point(0.6 versus the FM~0.8 noted forterrestrial tiltrotors).

0

5

10

15

20

25

0.4 0.6 0.8 1 1.2

Maximum Wing Lift Coefficient

WingPlanform

Area(m^2)

End of ConversionMach # = 0.1

Mach # = 0.15

Mach # = 0.2

Design Point

Fig. 9c Mars Tiltrotor Wing Area SizeEstimates (Vehicle Mass = 10 kg)


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Figure 9c shows wing planform area as afunction of maximum wing lift coefficientand the end of conversion Mach number(airspeed at which the wing, versus therotors, carries all the vehicle lift). Three

considerations constrain the wing sizingeffort: first, rotors will have a maximumadvance ratio to which the rotors can flyedgewise in helicopter-mode (because ofhigh vibratory loads); second, wingmaximum lift coefficient is significantlylower for wing/control surfaces in thelow Reynolds number regime; third,aeroelastic stability (particularly forultra-light weight structures)considerations will limit the maximum

cruise speed below that of conventionalterrestrial tiltrotors. Unfortunately, it isbeyond the scope of this paper toaddress these design considerations inother than a qualitative sense. Thedesign point noted in Fig. 9c reflectsthese design considerations/constraints.

Figure 9d shows preliminary rangeestimates (using the Breguet rangeequation) of the 10 kg Mars tiltrotor

configuration, assuming propulsionprovided by an Akkerman hydrazinepiston engine (Ref. 25), for variousvehicle L/Ds and fuel fractions. Thespecific fuel consumption (SFC)constant used for the Akkermanhydrazine piston engine is 1.0 kg/MJ.An Akkerman engine is amonopropellent-based propulsionsystem and, therefore, should operatesatisfactorily in the carbon-dioxide-

dominated atmosphere of Mars. It hasbeen successfully used on high-altitude,long endurance terrestrial experimentalaircraft. A typical L/D for conventionalterrestrial tiltrotor aircraft XV-15 andV-22 is L/D~7 (Refs. 26-27). HigherL/D values might be possible for anoptimized Mars tiltrotor configuration.

In particular, lower parasite drag (ascompared to terrestrial aircraft) mightmake it possible to design more efficientMars tiltrotor configurations.

As shown in figure 9d, a Mars tiltrotorusing hydrazine piston enginepropulsion will be a short- to medium-range planetary aerial vehicle. In order toimprove vehicle range, in addition toimproving L/D efficiency of the aircraft,the propulsion system SFC must beimproved. This will necessitatedeveloping alternate propulsion systems.

0

100

200

300

400

500

600

0.15 0.2 0.25 0.3 0.35 0.4 0.45

Fuel Fraction

Range(Km)

L /D = 6 L/D = 8

L /D = 10 L/D = 12

Fig. 9d Breguet Range Estimates for aMars Tiltrotor

Throughout this early Mars tiltrotorconceptual/preliminary design work, aone parameter vehicle weight equationwas used to estimate the Mars tiltrotorempty weight mass: m = 0.4S3/2, where

S is the wing planform area. This oneparameter empty weight equation wasderived using empirical tiltrotor andfixed-wing high altitude long endurance(HALE) vehicle weight data. Estimatesusing this empty weight equation for the10 kg Mars tiltrotor yield a usable fuelweight fraction of approximately 25%.


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Further preliminary design analysis ofthe Mars tiltrotor concept must besubstantiated by detailed componentweight estimates.

Another Martian autonomous rotorcraftconcept being explored a configurationmore conducive to an early Mission date(because of packaging, assembly, anddeployment considerations) is a coaxialhelicopter configuration. This coaxialMartian helicopter should have foldedand telescoping rotor blades to minimizevolume during launch, transit, andentry/landing on the Martian surface.The primary disadvantage of a coaxial

helicopter is that it would be a slowervehicle with considerably less range thana Mars tiltrotor configuration.Nonetheless, a coaxial Mars helicopterwould make a good vehicle configurationfor early proof-of-concept roboticmissions to Mars.

Finally, as a point of reference, thefollowing derivatives are applicable toMars robotic planetary missions: 1 kg of

air vehicle adds 21.5 kg to the landerwhich adds 13 kg to the entry masswhich adds 20 kg to the launch mass.These incremental weight derivatives arederived from the 1998 Mars Pathfinderrover/lander mission (see Fig. 10). TheMars Pathfinder 16 kg rover andauxiliary equipment resulted in16*(1+21.5+13+20) = 890 kg of launchmass. Mars Mission costs can beapproximated using a fixed launch vehicle

cost of $70M and an incrementalMission development/flight cost of$180,000 per kg of launch mass (in 1997USD). Cost numbers based on MarsPathfinder: $70M plus 890kg*$180,000per kg = $230M.

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50 60

PAV & Aux. Equip Mass(kg)

MassEstimates(kg)

Lander Mass

Mars Entry M ass

Earth Launch Mass

Fig. 10 Estimates of Lander, Entry, &Launch Mass

Challenges

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 (at best) for the

purposes of designing and buildingPAVs. Such vehicles will need to behighly adaptive (from a controls andstructures perspective), haveconservative performance margins, andwill require high degrees of mission/flightautonomy to adequately deal withcorresponding levels of uncertainty inthe mission and flight environment. Alist of these and other technicalchallenges are summarized below.

Rotor Aeromechanics

Inadequate planetary atmospheric

data and/or modeling may exist todesign vehicles with requiredperformance.


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No empirical data exist for low-Reynolds number, compressible flow,so aerodynamic predictions may beinaccurate. Correspondingly, no dataexist for high tip Mach number, high

disk-loading rotor designs required forexploration of Venus and Titan. It isespecially critical to acquire airfoil androtor performance databasesconsistent with these planetaryenvironment extremes to validatedesign and analysis tools.

Achieving aeroelastic stability forrotors and/or wings will bechallenging, given ultra-light weightstructures required for most PAV

configurations. A single PAV platform design is

unlikely to address all conceivablemission requirements for any onegiven planet. A mixed fleet of vehiclesis likely needed to comprehensiveplanetary science missions.

Vertical lift PAVs are not likely to beall-weather vehicles. Season, location,existence of atmospheric disturbancesof a certain magnitude, and even timeof day may dictate whether a PAVmission can be initiated or not. Forexample, as noted earlier, Marsundergoes seasonal extremes ofatmospheric mass due to sublimationand condensation of CO2 at the polarcaps. Further, seasonal 300-500km/hr planetary-wide storm fronts (orsandstorms also exist. It is unlikelythat PAV missions can be sustainedduring these seasonal storms.Accordingly, preservation of flightvehicle (and other) assets in the faceof these weather extremes will be akey consideration for humanexploration of Mars. Further, in thecase of Jupiter, retrogradeatmospheric wind patterns (on the

order of 500-700 km per hour) maydictate the mission profile of anyPAV platform used).

Rotor blade icing will likely take on awhole new meaning for flight on Titan

or the outer, gas-giant planets.

Autonomous System Capability

It is currently beyond the

demonstrated autonomous systemtechnology state-of-the art to enablevertical lift flight in an extraterrestrialenvironment.

A light-weight, low-power,

computationally intensive, reliable(radiation tolerant, for example) flightcontrol and mission computer systemcapable of meeting vertical lift PAVrequirements has yet to bedemonstrated. It is crucial to initiateproof-of-concept demonstrations forkey hardware/software componentsfor autonomous flight of terrestrialplatforms from which vertical lift

PAV mission performance can beextrapolated.

Limited use of lander or orbiter assets

should be assumed for guidance,navigation, and control (GNC) andmission/flight support. Onboard-sensors and autonomous systemcapability should be assumed,although, such complete vehiclesystem autonomy has not beendemonstrated.

Guidance, Navigation, and Control

Unlike terrestrial UAVs, PAVs cannot rely on GPS systems for guidanceand navigation (Ref. 28).


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Further, high-precision digital mapswill not likely exist for GNC(development of such maps is insteada goal/mission of PAVs).

Onboard navigation sensors,

appropriate for an extraterrestrialenvironmental for a highly mobilerobotic vehicle, have yet to bedemonstrated. In particular, planetaryatmospheres such as Venus and Titancan be (nearly) opaque to light in thevisual range; therefore, non-opticalsensors would be required for GNC.

Exotic (as compared to terrestrialUAVs) types of control actuators orcontrol strategies may need to be

developed to minimize vehicle weight,to operate under severe environmentalconditions, and to minimize flightcontrol processor workload.

Structures and Materials

Ultra-light weight structures will beessential for vertical lift PAVs particularly for Mars exploration.

Structures and materials will besubjected to incredible extremes oftemperature and pressure as well asbeing subjected to poorly understoodlevels of atmospheric turbulence,weather conditions, and multi-component and multiphase (andpotentially corrosive) chemicalconstituents.

Propulsion

Outside of Earth, there is very littlefree oxygen in other planetaryatmospheres. Therefore, newpropulsion systems will have to bedevised that do not rely on oxygen (or

provide for the onboard storage ofoxygen that had either pre-launchterrestrial origin or was generated bychemical in-situ production from thelander/main base).

Reliability issues must be taken intoaccount (including auxiliary systemsfor start/restart) for currentimplementations of mono-propellantengines such as the Akkermanhydrazine engine (Ref. 25).

Solar flux availability is greatlydiminished for other planets (in thecase of Venus because of cloud/hazecover, and in the case of Mars, Titanand the outer planets because of

distance from the Sun) for solarenergy based propulsion systems.Average Mars solar flux is only ~43 %of Earths (Ref. 1).

Nuclear-energy-based (for example,using RTGs (Refs. 29-30)) electricmotor propulsion is possible, but asignificant weight penalty would beassociated with this approach.

Advanced battery and fuel-celltechnology are propulsion system

possibilities (Ref. 31), but still need tobe matured for space systems.

For the exploration of outer, gas-giantplanets, hydrogen could be drawn infrom the planetary atmosphere,compressed, mixed with vehicle-stored oxygen/oxidizer, and ignited inan internal combustion engine.

Two-stage systems may be apossibility. For example, electricpower generation on a lander platform

with recharging an onboard battery onthe PAV between missions.

Deployment

PAVs will be subjected toconsiderable constraints regarding


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mass and volume. This will posechallenges for all vehicle developmentdisciplines, but will particularly affectthe means and systems involved in thevehicle deployment

Vehicle assembly, configuring forflight, and deployment of PAVs poseunique challenges compared toterrestrial aerospace vehicles. Newdesign approaches, mechanicalsystems, and structures will need tobe developed for PAVs. Theadvantage of vertical lift PAVs (overother types of PAVs) is that they canbe assembled (if need be), configuredfor flight, and launched from a lander,

with adequate time for deployment;they will not have to rely ondeployment during entry into theplanets atmosphere.

Reelable, foldable, or telescopingvariable-diameter rotor blades are allpossible candidates for achievingminimum vehicle volume (instowed/package form) for integrationinto launch and entry vehicles.

An ambitious undertaking such as thedevelopment of PAVs will dictate awhole new design approach that musthave a high-degree of flexibility andaccuracy to analyze a broad class ofvehicle configurations and planetaryatmospheric model. Further this designmethodology must be predicated on theuse of standardized and integratedsoftware packages for preliminary design

and analysis, virtual environment, andflight control and autonomous vehiclesimulation tools that could be applied toa suite of planetary atmosphere models.The focus of the PAV conceptual designand simulation tools would primarily beon vertical take-off and landing (VTOL)

vehicles and hybrid airships forplanetary missions.

In order to minimize risk and maximizemission capability and probability of

mission success, it will be necessary todevelop design and simulation softwarethat will enable rigorous and timelyexamination of a large conceptual designspace for PAVs. As a first step, it willbe necessary to adapt existingconventional terrestrial rotorcraft,VTOL, and hybrid airship preliminarydesign and analysis tools to autonomousplanetary aerial vehicle design. Inaddition, developing specialized tools

tailored for PAVs will be necessary,since PAV configurations are likelyoutside the scope of conventionalrotorcraft, VTOL, and hybrid airshipempirical data. All analyses will have todraw alternatively on first principles andvarying degrees of analysis rigorousnessin the iterative design cycle process.

For conventional terrestrial rotorcraft,VTOL, and hybrid airships, mission

profiles and flight/operating conditionscan be defined in a relativelystraightforward manner; this will not bethe case for PAVs. Limited, andsometimes only through indirectmeasurements, data will be available forthe planetary environment in which thevehicles will fly, take-off, and land.PAV mission profiles will be partiallydefined by targets of opportunityidentified only during actual vehicle

operation/exploration in the planetaryenvironment. Therefore, research willneed to be conducted in integratingmission and virtual environmentsimulation software directly into thedesign and vehicle simulation softwarepackage. Vehicle simulation is only asgood as the underlying modeling


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employed. Uncertainties in missiondefinition, planetary environment, andthe vehicle design/analysis characteristicscould result in inaccurate simulationresults and mission feasibility

assessments. Acceptable vehicleperformance will have to be graded noton the basis of pilot ratings but othermission success criteria.

Unique design constraints exist withrespect to the package constraints for thePAV in the aeroshell, the effect on thevehicle due to the harsh environment ofspace (radiation, vacuum, temperatureextremes), and deployment issues from

the aeroshell (during descent) or lander(on the planetary surface). Further,considerable work is required to definerobust strategies and mechanisms forplanetary aerial vehicle deployment,whether it be for: a single integratedatmospheric entry and flight vehicle;PAV high-altitude atmospheric releasefrom an aeroshell; vertical lift PAVdeployment from a lander; vertical liftPAV autonomous assembly and

deployment from a lander; or astronaut-assisted assembly.

Figure 11 is a flow chart summarizing thepreliminary design process thatencompasses the multiple disciplines andunique design constraints/considerationsof vertical lift PAVs.

MissionRequirements

Science PayloadDefinition and Reqs.

I nitial VehicleConfiguration

Definition

Planetary AtmosphericModel

Airframe/VehicleAerodynamics

Rotor Perf ormance- Hover & Climb

- Propeller/Pr oprotor Airpla ne-Mode (if applicable)- Forward - Flight Helic opter-Mode

Lander Design &Weight Es timate

Deployment & Aeroshell Packaging

Vehicle Delta Performance & WeightPenalties

PAV Stability &

Control

Detailed WeightEstimates

Flight Control & Aut onomous System

Mission/Flight Computer Pe rform-ance, Power, and Weight

Detailed Design

Spacecraft Design

Orbital MechanicsAeroshell Design

Weight & Performance

Mission Cost

Mission

Reqs. Met?

Yes

No

Fig. 11 Vertical Lift PAV PreliminaryDesign Cycle

Finally, perhaps the greatest challengefor the aeronautics and rotorcraftcommunities will be understanding thecultural background and technicalrequirements of a new type of customer:the planetary science community.


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Additional Considerations

In order to maximize the science returnfrom robotic PAVs it may also be

necessary to examine and implement thedevelopment of hybrid vehicles,symbiotic robotic systems, and/oroverall collections or communities ofrobots and astronauts.

It may be that hybrid vehicles will needto be developed to fully expediteplanetary exploration. One such hybridvehicle may combine flight with surfacelocomotion capability. Visionaries in the

early twentieth century proposed thedevelopment of vehicles that combinedthe features of airplanes and automobilesfor terrestrial personal transportation.There may be a greater need to developsuch hybrid (flight and surfacelocomotion) vehicles for extraterrestrialapplications. Another hybrid vehiclethat might deserve attention from PAVsdesigners is a vehicle that combinesrocket propulsion with rotary-wing lift

to optimize overall vehicle performance.Traditional concepts of rocket and aerialpropulsion may fall by the way-side bynecessity, as will the concepts ofindependent/separate surface-locomotionand aerial vehicles, for extraterrestrialapplications.

Symbiotic vehicles/robotic-systems (forexample, a rover transported by and/orlinked to a PAV) will likely require

development to efficiently expediteplanetary exploration. Basically, once avertical lift planetary aerial vehicle landsit has limited ability (less than a coupleof meters at most) to interact with theplanetary surface. PAV-mountedrobotic arms and similar devices will beinsufficient to meet the long-term needs

of the planetary scientist. Therefore,symbiotic robotic systems, with eitherfixed-based or mobile assets with surfacelocomotion capability, will be animportant capability to develop and

utilize for planetary science missions.For example, a large, long-range rovercould carry a small vertical lift PAV as ascout vehicle to map out both routes andstops for scientific research for the rover.Alternatively, a large vertical lift PAVcould sling load transport a small rover,or stationary sensor arrays, to targetedareas of interest. Finally, as a thirdexample, high-altitude, long endurance,fixed-wing (or hybrid airship) PAVs

could be linked to vertical lift PAVs toenable/support mission-planning,routing, coordination between multiplevehicles, and extended rangecommunication without having to relysolely on orbiters.

Planetary scientists and computerscience and robotics experts are alreadycollaborating to develop collectivecommunities of robots to be used for

extended periods of planetaryexploration. Of all the planetary bodiesin the solar system, the Moon and Marsare unique since one day a sustainedhuman presence will likely be establishedon them. Robotic systems employingrotary-wing-derived technologies couldact as a community of AstronautAgents for efficiently andcomprehensively conducting scientificexploration.

Potential for Leveraged, or Spin-Off,

Research and Technology

Why is the Army/Rotorcraft Divisioninterested in promoting and participating


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in the design study and, perhaps, theultimate development of vertical liftPAVs? There are several reasons:

PAV advanced autonomous

software/hardware technology is alsoapplicable for terrestrial UAVs;

Technology developed for PAVscould be applicable to Micro AirVehicles (MAVs);

PAV development will promote astrong working relationship betweenNASA Aeronautics, Space, andInformation Technology programs.

Where To Go From Here?

How can the rotorcraft communitycontribute to the realization of thisvision? First of all, the Army/NASARotorcraft Division at NASA AmesResearch Center intends to continue tosponsor/perform work in the area ofvertical lift planetary aerial vehicles.

Second, Sikorsky Aircraft is sponsoringthe Year 2000 AHS Student DesignCompetition where the topic is aMartian autonomous rotorcraft.Finally, all other members of therotorcraft community both industryand academia are encouraged to usetheir imagination and technical expertiseto expand upon the vision outlined inthis paper.

Concluding Remarks

Autonomous vertical lift PAVs canpotentially play a vital future role in theexploration of the solar system. Aconsiderable amount of work lies aheadto establish the feasibility of these

vehicles. This paper is just a first stepin that overall process.

Specifically, the preliminary discussionand analyses presented in this paper has

enabled the following initial conclusionsto be drawn:

Vertical lift planetary aerial vehiclescould potentially be developed forMars, Venus, and Titan;

Fixed-wing, propeller drivenplanetary aerial vehicles (leveragingrotary-wing technologies) couldpotentially be developed not only for

Mars, Venus, and Titan but also forthe outer, gas-giant planets;

Finally, even planetary bodies thathave tenuous, or nonexistent,atmospheres could benefit fromrotary-wing related technologies.

Vertical lift planetary aerial vehicles ifproven to be feasible -- will be employedin both purely robotic missions, or as

'astronaut agents' for manned planetaryexpeditions. There are, therefore, futureopportunities for the rotorcraft andvertical lift technical communities tocontribute to NASA solar systemexploration initiatives. In the course ofdeveloping planetary aerial vehicles,there is considerable spin-off potential toterrestrial rotorcraft.

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 Sikorsky


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Aircraft for their efforts on behalf of theAHS Internationals Student DesignCompetition on the topic of a MartianAutonomous Rotorcraft for Science(MARS). Finally, thanks in advance

should be extended to a future generationof Martian aviators and planetary aerialvehicle designers who will hopefully beinspired to make the vision described inthis paper a reality.

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