Vertical Lift – Not Just For Terrestrial Flight

Larry A. Young Army/NASA Division Ames Research Center Moffett Field, CA

Abstract

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

for planetary exploration and science

Introduction missions (fig. 1). Initial results have been quite promising. As a result of this The next few years promise a unique early work, the utility of rotorcraft, convergence of NASA aeronautics and VTOL vehicles, and hybrid for space programs. NASA planetary Mars exploration and planetary science science missions are becoming missions as a whole can be technically increasingly more sophisticated. justified as follows. Manned and robotic exploration of the solar system planets would be greatly Why vertical lift vehicles for planetary enhanced through the development and exploration? For the same reasons why use of robotic aerial vehicles. Since the these vehicles are such flexible aerial 1970’s a number of Mars (fixed-wing) platforms for terrestrial exploration and concepts have been proposed transportation: the ability to hover and for Mars exploration. fly at low-speeds and to take-off and land at unprepared remote sites. Further, The Army/NASA Rotorcraft Division -- autonomous vertical lift planetary aerial in collaboration with the Center for Mars vehicles (PAVs) would have the Exploration -- at NASA Ames has been following specific advantages and performing initial conceptual design capabilities for planetary exploration: studies of Martian autonomous rotorcraft • Hover and low-speed flight

AHS/AIAA/RaeS/SAE International Powered capability would enable detailed Lift Conference, Arlington, VA, October 30- November 1, 2000. and panoramic survey of remote flight control; autonomous systems work sites; based on vertical lift vehicle applications; high-frequency open- and • Vertical lift configurations would closed-loop smart structures/actuators. enable remote-site sample return to lander platforms, and/or precision placement of scientific probes;

• Soft landing capability for vehicle reuse (i.e. lander refueling and multiple flights) and remote-site monitoring;

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

• Vertical lift PAVs would provide greater range and speed than a surface rover while performing detailed surveys; Figure 1 – Vertical Lift Planetary Aerial • Vertical lift PAVs would provide Vehicles as ‘Astronaut Agents’ greater resolution of surface details, or observation of atmospheric phenomena, than an Ultimately, the objective of this paper is orbiter; to inspire the vertical flight research community to consider and to embrace • Vertical lift vehicles would the concept of vertical lift planetary provide greater access to aerial vehicles and to participate in their hazardous terrain than a lander or ultimate development and use. Specific rover. opportunities for vertical life PAVs in planetary science and some of the PAV In addition to the potential science and design challenges are presented in this technology benefits resulting from the paper. Ongoing work, including that in development and use of vertical lift academia, is also described. planetary aerial vehicles, there are substantial opportunities for technology transfer from a vertical lift PAV development effort. These technology Opportunities transfer opportunities include: advanced high-efficiency propeller or As noted earlier, work is being pursued designs; precision guidance, navigation at the Ames Research Center’s and control at low altitudes and near- Army/NASA Rotorcraft Division on a terrain obstacles; adaptive (inner-loop) Martian autonomous rotorcraft for scientific exploration of Mars. Why not, though, a Venusian hybrid-? Or, a Titan VTOL? Or, alternatively, Figures 3 and 4 are approximate why not any number of vertical concepts estimates of the speed of sound and that could provide unique mission kinematic viscosity for various different capability for planetary science? planetary bodies in the solar system. These estimates were derived in This paper examines the question of the reference 2. feasibility of vertical lift planetary aerial vehicles. In particular, discussion in the paper will be directed at the three planetary bodies in our solar system Neptune where vertical lift vehicles might prove Uranus feasible. Table 1 is a summary of the Saturn key surface atmospheric properties for Mars, Titan, Venus, and Earth. This Jupiter Titan y y information will be used to examine the y y general aerodynamic attributes of Mars vertical lift PAVs. Earth

Venus

0 200 400 600 800 1000 1200 Table 1 – Summary of Planetary Speed of Sound (m/sec) Descriptions (Ref. 1)

Mean Gravity! Mean Mean Mean Atmos. Fig. 2 – Estimates for the Speed of Radius (m/s2) Surface Surface Surface Gases Sound (km) Atmos. Atmos. Atmos. Temp. Pressure Density (o K) (Pa) (kg/m3)

6 Venus 6052 8.87 735.3 9.21x10 64.79 CO2 Neptune 96% Uranus N2 3.5% Earth 6371 9.82 288.2 101,300 1.23 N2 78% Saturn O2 21% -2 Mars 3390 3.71 214 636 1.55x10 CO2 Jupiter 95% N2 2.7% Titan Ar 1.6% O2 0.1% Mars Titan 2575 1.354 94 149,526 5.55 N2 65- (Saturn 98% Earth moon) Ar<25% Venus CH4 2-

10% 1E-07 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 Kinematic Viscosity (m^2/sec)

Fig. 3 – Estimates of Kinematic

! Viscosity Mean values noted for planet radii and gravity to account for the oblateness of the planet. Mars surface temperature, pressure, and density varies significantly spatially and temporally; surface temperature range of 140-300oK; surface pressure 636±240 Pa. Seasonal Additional data related to planetary CO2 sublimation and condensation at the polar caps atmospheric properties can be found, for (particularly at the southern polar cap) is the chief reason for the atmospheric pressure and density variations. example, in Ref. 3-6. evolutionary processes that led our ‘sister’ planet to be radically different from Earth Despite the considerable amount of data • Determine if planetary-scale ‘green-house’ related to planetary atmospheres, much effects can be halted and/or reversed more data can, and must, be discovered during the course of the development and general application of PAVs. Finally, in addition to the scientific benefits resulting from the employment In establishing the feasibility of vertical of vertical lift vehicles, considerable lift (and other) PAVs, it is not sufficient enthusiasm and support from the to merely question whether or not flight American public could be generated for in extraterrestrial atmospheres is both the demonstration of extraterrestrial theoretically possible. Clearly defined atmospheric flight. planetary science goals and opportunities are also required so that vertical lift PAV designs can be optimized. Table 2 summarizes a partial list of planetary science goals/opportunities in which Mars, Titan, and Venus vertical lift, or rotary-wing, platforms could contribute. Mars, Titan, and Venus are as different from each as they are with respect to Earth. Each planetary body -- each Table 2 – Planetary Science major science/exploration mission, in Opportunities (A Partial List Only) fact -- will entail radically different

Science/Exploration Opportunities aerial vehicle design challenges. This will be highlighted in the discussion to Mars • Search for water or past signs of water follow which outlines some thoughts (characterize global distribution) • Search for life or evidence of past life regarding notional vehicle • Understand the atmospheric and geological configurations and design considerations evolution of Mars; perform comparative analyses of the Mars planetary for each of the three planetary bodies in evolutionary process with the other our solar system where a vertical lift terrestrial-type planets in our solar system • Survey for resources that would expand capability might theoretically make exploration capability and support for an sense for exploration/scientific extended human presence on Mars investigation. Titan • Search for the precursor biochemical components of life • Perform atmospheric science studies to understand the unique nature of the Titan Mars atmosphere (i.e. its high density/pressure) • Survey for chemical resources/volatiles that could enable in-situ propellant and fuel Most of the work to date investigating production; resulting propellant could be used for sample return missions to Earth the feasibility of vertical lift planetary and expanded surveys of the other aerial vehicles has focused of rotary- Saturnian moons wing configurations for Mars Venus • Correlate space-based cartographic and exploration. Mars, of all the planetary inferred geological data with detailed bodies in the solar system, holds the surveys in targeted areas using vertical lift PAVs. greatest interest for NASA researchers. • Acquire adequate data to understand the Both the Offices of Space Science and fundamental atmospheric and geological Space Flight actively promote/direct research and engineering effort for the 3500 3.5 (Disk Loading = 4 N/m^2) robotic and, ultimately, human 3000 3

exploration of Mars (fig. 4). Reference 2500 2.5 ) m 7 details NASA' strategic plan for the ( Human Exploration and Development of 2000 2 1500 1.5 Space (HEDS) -- which clearly (Watts) emphasizes the importance of Mars 1000 1 Rotor Radii mission planning. 500 Power (Hover Figure of Merit = 0.4) 0.5 Rotor Radius 0 0 5 1015202530 Vehicle Mass (kg)

Fig. 5 – Martian Autonomous Rotorcraft: Large, Ultra-lightweight, Fragile-Looking…

Early conceptual design study work at NASA Ames focused on a Mars configuration (Fig 6a-b and Ref. 2). This configuration reflected an aerial platform that potentially maximized Fig.4 – Mars (Image from Hubble Space overall mission flexibility. Assuming Telescope (HST)) the use of an Akkerman hydrazine reciprocating engine (Ref. 14), a small (10 kg) Mars tiltrotor was shown to Martian autonomous rotorcraft will have potentially have a range capability on the large lifting-surfaces and will be order of 150-250 kilometers (assuming a required to have ultra-lightweight limited amount of hovering and vertical construction (refer to figure 5 for take-off and landing operation). isolated rotor sizing for hover in Martian Alternate propulsion systems were not atmosphere). This in turn will pose a examined in this initial work. challenge in making them sufficiently robust to operate in the Martian This early Mars tiltrotor work at NASA environment. Some early work and Ames illustrated the promise of vertical discussion on Martian vertical lift lift planetary aerial vehicles. However, vehicles can be found in Ref. 8 -- 13. it was also clear from this work that deployment of even a small Mars tiltrotor requires human assistance in vehicle assembly. Alternative vehicle configurations needed to be examined for early robotic missions that did not require human presence on Mars. coaxial configuration. The performance estimates for this small coaxial helicopter assumes that the rotor tip Mach number is 0.65, the disk loading is 4 N/m2 and the rotor diameters are 2.44 meters. A very conservative set of induced power constant and mean blade profile drag (a) coefficient were used for the rotor performance estimates. These conservative performance coefficients were selected to account for the high profile drag seen for low Reynolds number airfoils, as well as the effect of a very large blade root cutout to allow for rotor fold and telescoping for transport/deployment, and the high tip losses for large aspect ratio rotor blades. (b) References 15-16 provide general design analysis guidance for this first-order Fig. 6 -- A Mars Tiltrotor: (a) helicopter- performance assessment. mode in vertical climb over Valles Marineris; (b) airplane-mode 9000 Vehicle Mass = 5kg 10kg 8000 25kg 50kg Recent work at Ames has focused on a 7000 coaxial helicopter configuration for early Mars exploration missions (fig. 7). 6000 5000

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2000 Total Shaft Power (Watts) Total Shaft Power (Watts) Total Shaft Power (Watts) Total Shaft Power (Watts) 1000

0 0 102030405060 Forward-Flight Speed (m/sec)

Fig. 8 – Mars Coaxial Helicopter Performance Estimates Fig. 7 -- A Coaxial Helicopter Configuration for Mars Exploration (‘Search for Water’) Figure 8 shows first-order estimates for vehicle range as a function of fuel/energy-source weight fraction for Figure 8 shows first-order estimates of the 10kg vehicle. Three families of the forward-flight performance of a 10kg curves are shown in the figure: range estimates using battery technology, Exciting advances are being made in estimates for fuel cells, and propulsion battery and fuel cell technology. The from a hydrazine-based Akkerman energy density numbers used in the engine. These range estimates are for range estimates of figure 9 are forward-flight power levels only (for a conservative with respect to today’s vehicle velocity of 40 m/s or an advance technology. Future advances may have ratio of 0.26) and do not account for the a considerable impact on mission energy/fuel increment for vertical take- capability of planetary aerial vehicles. off, hovering, and landing. Drive train On the other hand, reciprocating engines and transmission efficiencies are taken driven by hot gases from mono- into account in the range estimates. propellants (such as hydrazine) or bi- Note that additional battery or fuel cell propellants have undergone considerable capacity is required for scientific analysis in the past and should not be instrumentation and mission/flight- discounted for their use in planetary control power. aerial vehicle propulsion.

400 The flight dynamics of a Martian Hydrazine Akkerman Engine (SFC=1kg/MJ), Breguet Eq. rotorcraft will likely be quite unique as 350 Lithium Ion Battery (150 compared to its terrestrial counterparts. Wh/kg) 300 Fuel Cell (300 Wh/kg) The rotor(s) for a Martian rotorcraft will

250 have very low Locke numbers and will have correspondingly have very low 200 aerodynamic damping. The blades will 150 also likely have relatively low values of

100 torsion and bending stiffness because of their large blade planform area and ultra- 50

Forward-Flight Range, (km) lightweight structure. 0 0 0.1 0.2 0.3 0.4 The key to the successful development Fuel/Energy-Source Weight Fraction of Martian autonomous rotorcraft will be the development of ultra-lightweight Fig. 9 – Mars Coaxial Helicopter Range structures and equipment for such Estimates vehicles. Though a considerable body of statistical and semi-empirical weight prediction tools exist for rotorcraft (see, A clear trade-off can be seen from the for example, Ref. 17-21), none are coaxial helicopter range estimates. The directly applicable to the unique design Martian rotorcraft could either use challenges of Martian rotorcraft. These hydrazine reciprocating engine tools need to be modified/refined to technology that uses a non-replenishable accommodate the innovations in supply of fuel, or battery or fuel cell materials and structural components to technology which could be recharged arrive at acceptable preliminary design using almost inexhaustible (lander- methodologies for planetary aerial based) energy sources such as solar cell vehicles that have acceptable arrays and/or nuclear radioisotope engineering accuracies. thermoelectric generators (RTG’s). Titan bodies. References 22–23 provide some important insights into the potential for Titan, Saturn’s largest moon (fig. 10), is vertical lift aerial exploration of Titan. unique in the solar system in that it is the only moon that has a substantial atmosphere. Titan’s atmosphere is comprised primarily of nitrogen, argon, methane – and may have similar properties to Earth’s early atmosphere, before life began. The Voyager 1 and 2 ‘Grand Tour’ missions provided a substantial amount of information about Titan. Nevertheless, Titan’s surface is shrouded by a thick atmospheric haze and little is known about it. Recent Hubble Space Telescope and ground- based telescope astronomical Fig. 10 – Titan (Image from HST) observations relying on new infra-red techniques are starting to provide some insight into the surface features of Titan, With the arrival of the Cassini/Huygens but only a faint hint of what may lie on spacecraft to Saturn and Titan in 2004 -- Titan’s surface can be discerned from and the anticipated science and outreach the existing available data. By 2004, bonanza from this mission -- there may the joint NASA, ESA, and Italian space be an opportunity to take advantage of agency Cassini space mission will reach the excitement underlying this adventure Saturn’s orbit and release the Huygens to advocate possible follow-on missions. probe (descending via parachute) into Among these possible follow-on Titan’s atmosphere. The Huygens missions is an exploration of Titan atmospheric probe and the employing small robotic vertical lift complementary Cassini observations aerial vehicles. will provide invaluable insights into the atmospheric chemistry/properties of A key consideration in the development Titan. It is unclear whether the Huygens of a Titan vertical lift aerial vehicle is probe will be able to soft-land on Titan’s the robustness of the platform, the ability surface and successfully communicate to execute multiple flights, while data back to Earth (this might be minimizing overall vehicle mass. possible, but is outside the official Transport of such a vehicle from Earth mission scope). This accomplishment to Titan will be an expensive will likely come from future missions undertaking. Maximizing science return post-Cassini/Huygens. and overall mission duration will be crucial given the expense of the The use of vertical lift planetary aerial enterprise. vehicles to explore Titan would be a tremendous enabler of scientific Because of the thin atmosphere of Mars investigations of one of the solar – and, therefore, the large rotor diameter system’s more mysterious planetary and blade planform area required for vertical flight -- ducted fan vertical lift vehicles are impractical for Mars exploration. The opposite is true for Figure 12 shows a first-order estimate of aerial vehicles for Titan. Ducted fan hover total shaft power for a notional configurations such as tilt-nacelle Titan tilt-nacelle VTOL vehicle having are perhaps ideally suited for two ducted fans that can pivot at the Titan (fig. 11). Ducted fan aerial wing tips (similar in configuration to the vehicles would inherently be more VZ-4). A conservative shroud thrust robust operating at low-altitudes in an fraction of 0.3 (i.e., 30% of the total unknown, potentially hazardous thrust is provided by the duct/nacelle environment, than conventional rotors. aerodynamics in hover, see Ref. 28-29) A variety of ducted fan VTOL concepts is used in the hover performance have been tested on Earth, both in estimate. The hover performance and model- and full-scale, wind tunnel and fan sizing estimates are for a disk flight test. Among these ducted fan loading of 600 N/m2, a fan blade tip vehicles are the Doak VZ-4, the Mach number of 0.7, and a fan solidity Grumman 698 (tilt-nacelle) and the Bell of 0.25. This corresponds to a mean X-22A (quad fan) aircraft (see Ref. 24- fan blade lift coefficient of 0.75, which 27). For low-speed, short ranges, should be reasonable for the airfoil configurations analogous to the Sikorsky Reynolds numbers estimated for the Aircraft Cypher ducted coaxial-rotor Titan ducted fan vehicle. A nacelle UAV could also potentially be centerbody fairing radius of 20% of the applicable to Titan exploration. fan blade span is assumed in the analysis. Fan airfoil Reynolds numbers are greater than 106 at the fan blade tip. (Compare that to the rotor blade tip Reynolds numbers for Martian rotorcraft which are estimated to be less than 105.) A Titan VTOL’s ducted fans will be very small and consume very little power as a result of the very low gravity field for Titan. As the atmospheric (a) density near Titan’s surface is quite high compared to Earth, forward-flight profile and parasite power will be correspondingly quite high and will restrict the maximum velocity of the vehicle to relatively low speeds.

(b)

Fig. 11 -- A Titan Tilt-Nacelle VTOL: (a) take-off; (b) cruise. 700 0.1 Venus

0.09 600 0.08 Of the three planetary bodies besides

500 0.07 Earth where it theoretically might be

0.06 feasible to design and fly vertical lift 400 0.05 aerial vehicles, Venus (fig. 14) will 300 0.04 likely pose the greatest challenge. This Fans), Watts is particularly ironic as, to date, only 200 0.03 Fan Blade Radius, m Venus can lay claim to having had aerial 0.02 100 vehicles fly within its atmosphere Total Shaft Power (Both Ducted Hover Power 0.01 Fan Blade Radius (discounting, of course, entry/descent 0 0 10 20 30 40 50 parachutes for probes and landers). The Vehicle Mass, kg former Soviet Union with its Vega 1 and Fig. 12 – Ducted Fan Hover 2 missions traversed across the upper Performance for Titan Vehicle atmosphere of Venus with two balloons. These balloons drifted with high-altitude (~50-55 km) winds almost across 30% Figure 13 shows range estimates for a of the circumference of the planet. 50kg Titan twin tilt-nacelle/ducted-fan Though many other planetary missions VTOL vehicle, assuming power have been proposed using matching between the hover and cruise balloons/aerobots and fixed-wing design points. The range estimates are aircraft, this accomplishment was the based on the estimated power from first and so far only demonstration of figure 12 with reasonable drive train and aerial flight in an extraterrestrial electric motor efficiencies applied. The environment. cruise speed is assumed to be 50 m/sec. These range estimates do not include the impact of take-off, landing, and hover on power availability.

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100 Battery (150 Wh/kg) Fuel Cell (300 Wh/kg) 0 Fig.14 -- Venus (Image Based on Radar 0 0.1 0.2 0.3 0.4 Map from Magellan Spacecraft) Energy Source Weight Fraction Fig. 13 – Range Estimates for a Twin Ducted Fan Titan VTOL The extremely high atmospheric densities near Venus' surface (plus the near-Earth-magnitude of its gravitational field) would suggest that a buoyant, or semi-buoyant, vehicle might represent the most practical design for exploration of Venus (fig. 15). In fact, Venus' surface atmospheric density is so great that such a semi-buoyant vehicle would in some ways likely have attributes more in kind with an underwater submersible than a terrestrial airship. The airframe of Fig.15 -- A Notional Venusian Hybrid a Venusian hybrid-airship would be a Airship with Twin Hulls and Tandem rigid hull, which would have to be able Tilting Propellers and Wings to sustain substantial pressure differentials across the external/internal surfaces of that hull. A Venus vertical lift vehicle mission’s duration and science return could Venus’ high surface temperatures also perhaps be maximized by designing it to pose tremendous challenges for aerial have two phases/stages. The first phase vehicle design. Power usage must be would entail low-altitude powered flight kept to a minimum for propelling the with the vehicle acting as a semi- vehicle so as to minimize waste heat buoyant hybrid-airship capable of take- generation and build-up from the vehicle off and landing on the Venusian surface. propulsion system and electronics. This phase would be comprised of a few Though active and passive technologies hours of powered flight at most, given exist for thermal management of likely limitations in power availability. planetary science hardware, extended Then, when vehicle power and operation of such hardware near Venus’ temperature reach critical levels, ballast surface is currently problematic with in the form of drained batteries (and today’s technology. This will therefore potentially unnecessary surface science mean that the lift required for take-off instruments) could be released and a and landing will need to be kept to an longer duration high-altitude absolute minimum (thus necessitating (unpowered) flight phase could be buoyancy fractions greater than 75%). It executed with the vehicle acting purely is also likely that electric propulsion will as a . This two stage mission be required to maximized overall approach could potentially maximize the propulsion efficiency. The use of high- science return and overall investment of temperature batteries (such as NaS a Venus vertical lift vehicle by batteries) or fuel cell systems will also optimizing overall flight endurance and be required. vehicle and scientific instrumentation operation.

Figure 16 shows a first-order estimate of a notional Venus hybrid-airship’s hull size. This hull volumetric sizing estimate is consistent with the above described two stage (combination of powered and unpowered flight) mission 8 approach. This hull size estimate 7 parallels in general the analysis given in reference 30 for a low-altitude (~10km) 6 Venus balloon. The results shown in 5 this figure assumes a hybrid-airship 4 buoyancy fraction of 0.9, a propulsion 3 energy-source (batteries, fuel cells, etc.) 2 weight fraction of 0.25, and an 1 unpowered balloon altitude (after 0 completing low-altitude powered 0204060 vertical lift flight and then dropping the Vehicle Mass, kg propulsion energy-source as ballast) of 3.1km. Helium is assumed as the hybrid-airship lifting gas in the figure 16 Fig. 16 – Hull(s) Size Estimate hull volumetric size trend. A thin skin of titanium alloy is assumed for the hull. Hull skin thickness using titanium alloys Figure 17 shows how drift altitude ranges from 0.5 to 1mm thick for vehicle (altitude for unpowered neutral flotation) mass from 10 to 50 kg. This skin varies with respect to ballast (propulsion thickness is derived such that skin energy-source) weight fraction. This stresses are less than the material yield analysis assumes that there is thermal strength, with a small margin of safety and pressure equilibrium at the drift (Ref. 31). The hull was modeled as an altitude across the external and internal ellipsoid with a fineness ration of 3. surfaces of the hybrid-airship hull(s). More rigorous follow-on analyses will need to consider thin wall pressure 5 vessel elastic buckling effects in more 4.5 accurately defining the hull crush 4 pressures and the hull geometry and skin 3.5 thickness. The proposed thin titanium 3 2.5 alloy hull skins will be very hard to 2 form/bond and will be subject to easy 1.5 damage. Advanced types of high- 1 temperature and high-strength materials 0.15 0.2 0.25 0.3 0.35 Ballast (Propulsion Energy-Source) should also be considered for the hull Weight Fraction skins. Fig. 17 -- ‘Drift’ Altitude of Unpowered Vehicle After Ballast Drop

Figure 18 shows a first-order estimate of the hover performance and sizing of a tandem propeller/tiltwing combination (sandwiched between twin airship hulls) that could be used to take-off and land from Venus’s surface. The performance and sizing estimates shown in the figure assume the airship buoyancy fraction of Only limited information exists for the 0.9 (therefore, the propellers have to lift operation of mechanical and electronic only 10% of vehicle weight in hover), a components at the extreme temperatures tip Mach number of 0.1, a 200 N/m2 disk expected at Venus’s surface. loading, and a solidity of 0.4 for the References 30 and 35 go into limited propellers. These propeller discussion of this topic, but it is clear characteristics are more in common with that a substantial amount of research submersible propulsors (see, for remains to be performed in this area example, Ref. 32) than the conventional before extended duration exploration of terrestrial rotary-wing platform. Venus’ surfaces or low altitude Adopting a twin hull (side-by-side) atmosphere can be effected. configuration for a Venusian hybrid- airship is perhaps reasonable so as to protect the propellers of such a vehicle from hazardous terrain/obstacles during Technical Challenges and take-off and landing. The obvious Opportunities tradeoff for such a configuration is the handling characteristics of the vehicle in Autonomous vertical lift PAVs will be sideslip in forward flight, potential high-risk and high-payoff development substantial nonuniform flow field effects ventures. Though an impressive – and on the propeller performance due to the ever-expanding -- amount of data exists presence of the twin hulls, and increased for the planetary bodies in our solar overall complexity of the vehicle. system, nonetheless, these data are These issues will need to be examined in barely adequate for the purposes of closer detail in future design studies of designing and building PAVs. Such this concept. References 33-34 provides vehicles will need to be highly adaptive additional insight into airship design (from a controls and structures considerations. perspective), have conservative performance margins, and will require

1200 0.16 high degrees of mission/flight autonomy

0.14 to adequately deal with corresponding 1000 levels of uncertainty in the mission and 0.12 800 flight environment. 0.1

600 0.08

0.06 400 Rotary-Wing Aeromechanics 0.04 Propellers), Watts 200 Propeller Radius, m 0.02 Several rotary-wing aeromechanics Shaft Power Radius 0 0 challenges exist for the development of 10 20 30 40 50 vertical lift planetary aerial vehicles. Vehicle Mass, kg First of all, in many cases, inadequate planetary atmospheric data and/or Fig 18 – Hybrid-Airship Tandem modeling may exist to design vehicles Propellers Hover Performance and with required performance margins. Sizing Estimates Further, very limited empirical data exists for vehicle and control/lifting whole new meaning for flight on Titan. surface aerodynamics for such extreme And, finally, consider the environmental environments – including the low- effects of corrosive atmospheric Reynolds number, compressible flow chemical compounds on vehicle required for flight in the Martian performance and reliability for flight in atmosphere. This will inevitably result Venus’ atmosphere. in the reliance on analytical tools with limited validation to predict vehicle aerodynamics for flight in other Autonomous System Capability planetary atmospheres. Correspondingly, limited data exist for the high disk- It is currently beyond the demonstrated loading, high solidity, large aspect ratio autonomous system technology state-of- blades, rotor designs required for the-art to enable vertical lift flight in an exploration of Venus and Titan. It is extraterrestrial environment. NASA especially critical to acquire airfoil and currently has several initiatives rotor performance databases consistent underway investigating autonomous with these planetary environment flight control of spacecraft and planetary extremes to validate design and analysis science platforms, as well as terrestrial tools. Achieving aeroelastic stability for aircraft. Though extraterrestrial rotary- rotors and/or wings will be challenging, wing platforms will pose unique given ultra-light weight structures challenges for autonomous system required for most PAV configurations. technology, it should be hoped that there As can be seen from the discussion so will be a general applicability of the far, a mixed fleet of vehicle types is these automated reasoning and likely needed to comprehensive autonomous flight control efforts to the planetary science missions. vertical lift PAV problem. The problem is not just software-related, but light- Vertical lift PAVs are not likely to be weight, low-power, computationally all-weather vehicles. Season, location, intensive, reliable (radiation tolerant, for existence of atmospheric disturbances of example) flight control and mission a certain magnitude, and even time of computer systems capable of meeting day may dictate whether a PAV mission vertical lift PAV requirements will also can be initiated or not. For example, need to be demonstrated. It is crucial to Mars undergoes seasonal extremes of initiate proof-of-concept demonstrations atmospheric mass due to sublimation for key hardware/software components and condensation of CO2 at the polar for autonomous flight of terrestrial caps. Further, seasonal 300-500 km/hr platforms from which vertical lift PAV planetary-wide storm fronts (or mission performance can be ‘sandstorms’) also exist. It is unlikely extrapolated. A key assumption that PAV missions can be sustained underlying any autonomous system during these seasonal storms. technology development effort for Accordingly, preservation of flight vertical lift PAV should be that limited vehicle (and other) assets in the face of use of lander or orbiter assets should be these weather extremes will be a key assumed for guidance, navigation, and consideration for human exploration of control (GNC) and mission/flight Mars. Rotor blade ‘icing’ will take on a support. A complete onboard package of sensors and autonomous system flight potentially corrosive) chemical control capability should be assumed for constituents. vertical lift PAV -- although, such complete vehicle system autonomy has not been demonstrated. Propulsion

Outside of Earth, there is very little free Guidance, Navigation, and Control oxygen in other planetary atmospheres. Therefore, new propulsion systems will Unlike terrestrial UAVs, PAVs can not have to be devised that do not rely on rely on GPS systems for guidance and oxygen (or provide for the onboard navigation (Ref. 36). Further, high- storage of oxygen that has either a precision digital maps will not likely terrestrial origin or was generated by exist either for GNC (development of chemical in-situ production from a such maps is instead a goal/mission of lander/main base). Reliability issues PAVs). Onboard navigation sensors, must be taken into account (including appropriate for an extraterrestrial auxiliary systems for start/restart) for environmental for a highly mobile current implementations of mono- robotic vehicle, have yet to be propellant engines such as the Akkerman demonstrated. In particular, planetary hydrazine engine (Ref. 14). Solar flux atmospheres such as Venus and Titan availability is greatly diminished for could be nearly opaque to light in the other planets (in the case of Venus visual range; therefore, non-optical because of cloud/haze cover, and in the sensors might be required for GNC. case of Mars, Titan and the outer planets Exotic (as compared to terrestrial UAVs) because of distance from the Sun) for types of control actuators or control solar energy based propulsion systems. strategies may need to be developed to Average Mars solar flux is only ~43 % minimize vehicle weight, to operate of Earth’s (Ref. 1). Nuclear-energy- under severe environmental conditions, based (for example, using RTGs (Ref. and to minimize flight control processor 37-38)) electric motor propulsion is workload. possible, but a significant weight penalty would be associated with this approach. Advanced battery and fuel-cell Structures and Materials technology are propulsion system possibilities (Ref. 39), but still need to Ultra-light weight structures will be be matured for space systems. essential for vertical lift PAVs – particularly for vehicles for Mars For flight in Titan’s atmosphere, in-situ exploration. Structures and materials methane may be extracted from the will be subjected to incredible extremes moon’s atmosphere and combusted with of temperature and pressure as well as oxygen/oxidizer from a terrestrial being subjected to poorly understood source. Electric power generation on a levels of atmospheric turbulence, lander platform could recharge an varying weather conditions, and multi- onboard battery or fuel cell on a PAV component and multiphase (and between missions. return to the lander). Further, high- bandwidth signals for data-intensive Deployment science missions dictate even tighter constraints in communication options Planetary aerial vehicles will be and data transfer opportunities to Earth. subjected to considerable constraints regarding mass and volume. This will pose challenges for all vehicle development disciplines, but will particularly affect the means and Ongoing Work systems involved in the vehicle deployment. Vehicle assembly, Work to date within the NASA Ames configuring for flight, and deployment of Rotorcraft Division has focused on PAVs pose unique challenges compared vehicle conceptual design studies. As a to terrestrial aerospace vehicles. New result, reference 2 provided an initial design approaches, mechanical systems, discussion of the technical challenges and structures will need to be developed and opportunities of vertical lift PAVs. for PAVs. The advantage of vertical lift This conceptual design work continues PAVs (over other types of PAVs) is that and focuses on not only alternate vehicle they can be assembled (if need be), configurations for Mars exploration but configured for flight, and launched from has begun to consider vehicle concepts a lander, with adequate time for for other planetary bodies. deployment; they will not have to rely on deployment during entry into a In addition to vehicle configuration planet’s atmosphere. Reelable (Ref. 40- studies, a university grant with Carnegie 41), foldable, or telescoping variable- Mellon University developed a baseline diameter rotor blades are all possible conceptual design of a mission/flight candidates for achieving minimum control computer architecture for a vehicle volume (in stowed/package notional Martian autonomous rotorcraft. form) for integration into launch and This initial mission/flight control work entry vehicles. has focused on the use of visual cueing systems to provide for vehicle guidance and navigation. Onboard visual systems Telecommunication for GNC for vertical lift aerial vehicles are potentially an ideal solution for Telecommunication poses a considerable autonomous extraterrestrial flight. challenge for robotic planetary science Impressive gains have been made in this vehicles. Communication delays are field but a considerable amount of work substantial to and from Earth to other remains to be accomplished in this area. planetary bodies, particularly when taking into account relative orbital Ongoing work on vertical lift planetary rotation of those bodies with respect to aerial vehicles within NASA Ames Earth, the location of the aerial vehicle continues to focus on the design and on the planetary surface, and delays in analysis of Martian autonomous satellite overflight with respect to the rotorcraft for science (MARS) aerial vehicle (or delays until vehicle configurations (fig. 19). This effort includes initiating development of low- cost proof-of-concept test articles for demonstrating critical MARS technologies – including the development of a hover test stand for testing full-scale rotors at Mars atmospheric densities and a tethered hover flight demonstrator (Fig. 20a-b). An initial baseline Mars rotor is in the final stages of fabrication. This rotor is Fig. 19 – Inhouse Analysis a nonoptimized configuration but reflects many of the design constraints required for an actual flight article. The blades are composed of lightweight Rohacell foam hollowed out internally with a leading edge fiberglass layup for protection and chordwise mass balance. The blades are dynamically tailored to minimize hover ground resonance, but have not yet been optimized for forward- flight blade/hub loading. The blade root cut-out for the rotor simulates the unfaired blade span required for the blade fold/telescoping needed for vehicle transport and deployment. (a) The focus of this initial proof-of-concept hover testing is the assessment of overall rotor hover performance. The proof-of- concept rotor blades will have constant chord and will use the Eppler 387 (Ref. 42-43) airfoil. Recent unpublished results from NASA Langley would suggest that the Eppler 387 has fairly (b) high lift coefficients (and low pitching moment cofficients) for the Reynolds Fig. 20– Proof-of-Concept Test Article and Mach number ranges of interest. Development: (a) Baseline rotor design; Future Mars rotor test articles will likely (b) Proof-Blade Fabrication see the use of optimized airfoils, a significant evolution in blade/rotor geometry, and improved dynamic In addition to the inhouse research and characteristics. development efforts, a considerable amount of emphasis has been placed on public and educational outreach for the project. concept demonstration of rotary- wing flight in the Martian Public and Educational Outreach atmosphere, a limited aerial survey (with photographic telemetry) while in flight, and successful soft-landing Educational outreach in the early stages on the Martian surface. of this endeavor is vitally important for a number of reasons. First, the successful Required Mission Elements include: development of planetary aerial vehicles Deployment from Mars lander will be by necessity a highly System Checkout collaborative, multidiscipline effort Start / Warm-up including universities as well as NASA Hover and the rotorcraft industry. Second, an Cruise /Maneuver / Send early introduction of this new concept to Telemetry today's students will hopefully prove to Return to specified location be an important inspirational catalyst to Hover a founding/pioneering generation of Land extraterrestrial aviators and planetary Shutdown aerial vehicle designers. Optional Enhancement: Restart In this regards the Year 2000 American Hover Helicopter Society Student Design Reposition small distance Competition was successfully initiated Hover on the design topic of a Martian Land autonomous rotorcraft (Ref. 44). A Shutdown follow-on NASA-sponsored student design competition for the conceptual Or, more specifically, for the vehicle design of a Titan vertical lift aerial study, the design requirements were: vehicle is currently being planned. • Vehicle 'Gross Weight' mass not to Proposals for the AHS competition exceed 50 Kg (undergraduate and graduate level) were solicited for three design areas: vehicle • Minimum sustained 'controlled- configuration, propulsion system, and flight' duration of no less than one mission/flight-control computer half-hour is required. Range is of architecture. Briefly summarized, the secondary concern; ideally, range general mission/design requirements for should be greater than 25 km. the AHS student design competition for a Martian rotorcraft were: • Maximum cruise altitude (AGL) to 100 meters (low-level flight). Assume a Mars Mission landing date 2005. A Martian autonomous • Vehicle is capable of hovering / rotorcraft will be deployed from a soft-landing on Martian surface lander on the surface. The mission of after controlled-flight has been this Martian autonomous rotorcraft demonstrated. It is a desired would be threefold: a proof-of- objective to demonstrate a restart and second takeoff and landing • The vehicle must be capable of following the required soft landing. sustained hovering flight for no less than one minute duration. • Photographic images taken in flight and post-soft-landing will be • Maximum Mars entry acceleration transferred via vehicle telemetry to to be assumed to be 100 m/sec2 a lander or an orbiter for storage and transfer to Earth Ground Control. Flight profile and vehicle Exceptional design study papers from status telemetry should also be the participating universities were transferred from the Martian received. The winning teams of this autonomous rotorcraft to the lander competition will be announced at the or an orbiter. 57th Annual Forum of the American Helicopter Society in Washington, DC. • Flight/Mission Package 'Avionics' including camera, sensors and A follow-on NASA-sponsored student telemetry shall be assumed to be design competition is currently in the no more than 10% of vehicle mass planning stage and will focus on a vertical lift vehicle for Titan. It is likely • Martian autonomous rotorcraft will that the following nominal mission and be capable of sustaining design requirements will be proposed to continuous full sensor and data the competing student teams: relay power consumption (first order power consumption estimate Assume it is the year 2016. Twelve to be made as a part of vehicle years after the successful exploration design) for four (4) hours after of the upper atmosphere of Titan by separation from the lander and 3 the Cassini/Huygens mission -- six 1/2 hours after demonstration of years of engineering development soft-landing. followed by six years of transit to Saturn from Earth -- an orbiter satellite • The air vehicle must be and aeroshell entry vehicle arrive at autonomously deployed from the Titan for the long awaited follow-up Mars lander. Complete vehicle mission. The orbiter will execute autonomy must be demonstrated detailed lidar/radar mapping of Titan. after release from the lander; only The lander besides carrying its own passive telemetry will be received complement of sophisticated from the Martian autonomous instruments will also act as a transport rotorcraft. carrier, launch platform, and home base for a small robotic vertical lift • Auxiliary (nonflight) systems on planetary aerial vehicle. This aerial the lander can be used to platform will be used on a number assemble/deploy the Martian missions/flights over several weeks to autonomous rotorcraft and/or fuel, complete a detailed survey of terrain power, or spin-up the rotor(s). (both solid and, potentially, liquid (methane) surfaces) measured in area over several hundred square kilometers. Communication between • There is a requirement for multiple the aerial vehicle and the lander and flights/missions with the aerial Earth is maintained by the orbiter vehicle. Therefore, the vehicle satellite. must be capable of returning to the lander and refueling or recharging Specific design requirements will likely for subsequent flights/missions consist of the following: • Maximum vehicle gross weight to • A minimum total vehicle range of be less than 100kg (mass). 300 km while carrying a 10% payload fraction is required. • Vehicle should be capable of landing on both solid, uneven (icy) • Maximum cruise altitude is 2km; surfaces and liquid (methane) high-altitude cruise leg of mission pools; this will impact concepts for will comprise less 50% of total vehicle landing gear design. mission range; remaining 50% of Assume that the solid surface for flight is at low altitude (less than vehicle remote-site landing will 500 meters) and low-speed. include surface debris of 0.03 cubic meters. Vehicle will hover • Vertical take-off and landing over and land on a lander platform capability is required for the when returning post-mission/flight. vehicle design. • The vehicle must be capable of • A mid-mission hover out-of- autonomous flight and take-off and ground effect for one minute is landing required, followed by a mid- mission vertical landing and take- • The aerial vehicle must be capable off. of successful enduring a maximum aeroshell entry deceleration of 100 • Assume a maximum of 5m/sec m/sec2. gusts in hover and low-speed flight. • Flight/Mission Package 'Avionics' including computer, sensors and • There is no maximum speed telemetry shall be assumed to be requirement. Range and payload no more than 10% of vehicle are more critical design goals. • Auxiliary (nonflight) systems on • Vehicle should be capable of the lander can be used to propulsion system shutdown, and assemble/deploy the Titan vertical restart, upon landing mid-mission. lift aerial vehicle and/or fuel, Auxiliary power source should be power, or spin-up the capable of supporting vehicle rotor(s)/propulsion system. stand-by flight systems and science payload for four hours at the mid- mission remote site. Program advocacy will be as important an element for the successful development of vertical lift planetary aerial vehicles as any given technical accomplishment. Therefore, public and educational outreach efforts are crucial to the ultimate viability of planetary aerial vehicles.

Back On Earth (Technology Transfer Opportunities)

Why should the vertical flight community be interested in promoting Fig. 21 – Planet Earth (Realizing and participating in the study and, Technology Benefits Back Home) perhaps, the ultimate development of vertical lift planetary aerial vehicles? There are several reasons. First of all, PAV advanced autonomous software/hardware technology would be Concluding Remarks applicable to terrestrial UAV’s (fig. 21). Technologies developed for PAVs – Initial work to date suggests that vertical including microelectronics/sensors and lift planetary aerial vehicles are lightweight power sources/systems such potentially feasible. Such vertical lift as fuel cells and advanced batteries -- planetary aerial vehicles have could be applicable to Micro Air tremendous potential to support NASA Vehicles (MAVs). planetary science and exploration programs. Vertical lift planetary aerial Programmatically, vertical lift PAV vehicles – if ultimately proven to be development will promote a strong feasible -- will be employed in both working relationship between NASA purely robotic missions, or as 'astronaut Aeronautics, Space, and Information agents' for manned planetary Technology programs. And finally, as expeditions. In particular, Martian previously noted, the development and autonomous rotorcraft being used in use of vertical lift planetary aerial ‘scout’ and/or utility roles would enable vehicles for Mars, Titan, and Venus fundamental scientific quests such as the exploration would considerably enhance ‘hunt for water’ and the search for life. public and educational outreach and With research into and development of positive awareness for the rotary-wing such vehicles there are tremendous community and terrestrial rotorcraft outreach possibilities for the rotorcraft applications/missions. community. Further, there are significant potential technology transfer opportunities for terrestrial rotorcraft planetary aerial vehicle technology is applications. also gratefully acknowledged.

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