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American Institute of Aeronautics and Astronautics
1
A Wind -powered Rover for a Low-Cost Venus Mission
Gina Benigno1
Towson University, Towson, MD 21252
Kathleen Hoza1
Massachusetts Institute of Technology, Cambridge, MA 02139
Samira Motiwala1,2
Stanford University, Stanford, CA 94305
Geoffrey A. Landis,3
NASA John H. Glenn Research Center, Cleveland, OH 44135
Anthony J. Colozza4
Vantage Partners LLC, Cleveland, OH 44135
Venus, with a surface temperature of 450°C and an atmospheric pressure 90 times higher
than that of the Earth, is a difficult target for exploration. However, high-temperature
electronics and power systems now being developed make it possible that future missions
may be able to operate in the Venus environment. Powering such a rover within the scope of
a Discovery class mission will be difficult, but harnessing Venus’ surface winds provides a
possible way to keep a powered rover small and light. This project scopes out the feasibility
of a wind-powered rover for Venus surface missions. Two rover concepts, a landsailing rover
and a wind-turbine-powered rover, were considered. The turbine-powered rover design is
selected as being a low-risk and low-cost strategy. Turbine detailed analysis and design
shows that the turbine can meet mission requirements across the desired range of wind
speeds by utilizing three constant voltage generators at fixed gear ratios.
Nomenclature
Arotor = Rotor area
Ablades = Blade area
Asail = Blade area
AR = Aspect ratio
b = Sail luff height
CD = Drag coefficient
CL = Lift coefficient
Cr = Rolling resistance
CP = Center of pressure
Fp = Propulsive force
Fs = Sail force
g = Gravitational acceleration
h = Height
1 NASA Space Academy 2012, NASA Glenn Research Center, Cleveland OH
2 AIAA Student member
3 NASA John Glenn Research Center, mailstop 302-1, 21000 Brookpark Road, Cleveland OH 44135. AIAA
Associate Fellow
4 Vantage Partners, LLC, John Glenn Research Center, mailstop 309-1, 21000 Brookpark Road, Cleveland OH
44135.
American Institute of Aeronautics and Astronautics
2
l = Sail length
L = Characteristic length of rotor blade
m = Rover mass
mblades = Rotor blade mass
mtower = Turbine tower mass
n = number of rotor blades
P = Power
Pmax = Maximum available power
Pkinetic = Kinetic power
r = Rotor radius
R = Radius of Venus, 6051.2 km
Re = Reynolds number
S = Sail surface area
t = Time taken to complete one full orbit
TSR = Tip speed ratio
Vapp = Apparent wind speed
Vblade = rotor blade speed
VC = Circular velocity
Vwind = True wind speed
α = atmospheric absorption
γ = Viewing angle from center of Venus
λ = radio wavelength (cm)
µ = Standard gravitational parameter
µv = kinematic viscosity of Venus atmosphere
ρ = Atmospheric density
I. Introduction
Venus is one of the more scientifically interesting planets in our solar system, yet is relatively unexplored, and
much is still unknown about the planet. While similar in size, density, and surface gravity, Venus is also quite
different from our own planet, Earth. It has a surface temperature of 464°C, and an atmosphere mainly composed of
Carbon Dioxide (96.5%) and Nitrogen (3.5%) at surface pressure of 92 Bar.1
These surface conditions make design of a spacecraft to penetrate the Venusian atmosphere difficult. Previous
missions to the Venusian surface have been quickly destroyed by the extreme surface temperature and pressure. The
longest-lasting spacecraft on the Venusian surface to date is the Russian Venera 13 lander, which managed to
survive Venus’s surface conditions for 127 minutes before losing its connection with Earth.
Most electronics designed for use on Earth are not able to withstand surface temperatures upwards of 400°C;
however, several institutions are successfully developing high-temperature electronic components, using silicon
carbide and other wide-bandgap semiconductor based electronics.2 Currently, they have successfully tested silicon
carbide based electronics at 500°C 3 for extended periods of time. The further developments of these silicon-carbide
based electronics will be invaluable for future missions to Venus.4 They will contribute to not only the scientific
instruments, but also communications systems, as well as potential mechanical systems for a Venus rover.5
Venus’s opaque atmosphere makes it difficult to study the surface from space, except by radar images. Magellan
was able to successfully map out 98% of Venus’s surface,6 however, only so much can be discovered about a planet
by images taken from orbit; detailed exploration requires “ground truth” to be established from surface missions.
Whereas short-lived probes have successfully landed on the surface of Venus, a rover has yet to be sent there.
Recent Mars missions have shown the value of rovers on planetary surfaces, allowing the ability to do observations
at multiple locations, rather than being stuck to just the landing location. A rover may also have the ability to scrape
under the surface of Venus; it is likely that the rocks and soil are chemically altered through interaction with the
atmosphere of Venus. Being able to study underneath the soil and the insides of rocks could provide us with entirely
new information by accessing samples unaltered by contact with the Venusian atmosphere.
With the use of high-temperature electronics, the goal of this rover mission is to be able to survive for many
days. A longer lifetime means a much longer record of atmospheric and surface conditions can be taken, covering a
large part, if not all, of a Venus solar day.
Fig. 1 illustrates an artist’s conception of this conceptual design for the Venus rover.
American Institute of Aeronautics and Astronautics
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Figure 1. Artist’s Conception of Venus Rover
II. Venus Rover Concept
High surface temperature conditions on Venus poses a significant challenge to designing a Venus power
systems. Previously proposed rover designs7,8 implemented the use of radioisotope power systems, which produce
power by converting the heat from radioactive decay into electricity, using either thermoelectric conversion
(“Radioisotope Thermoelectric Generators,” or “RTG”)9 or Stirling10 conversion technology. Although they have
been proposed for use on Venus,11 radioisotope power systems are expensive and heavy,12 and also may have
difficulties operating in the corrosive, high-temperature, high-pressure environment of Venus, which may makes
them unsuitable for a Venus mission. The cost and lack of availability of the plutonium isotope make them a poor
choice for a low-cost, Discovery class mission.
In this design study, we analyze an alternative method for generating power: harnessing the wind on Venus as a
power source for a rover. Wind speeds on the surface of Venus have been directly measured only at four sites on the
Venus surface, at which the Venera probes found13,14 speeds in a range of 0.3 to 0.9 m/s. Although the absolute
speed is slow compared to winds on Earth, the density of the Venus atmosphere is high enough that this wind can be
used to generate force.
This study analyzed two main rover concepts: a “landsail” rover and a wind turbine-powered rover. These
concepts rely on surface winds to generate power for the scientific instruments and for mobility. Design assumptions
for these concepts include wind speeds of 0.3 m/s (worst-case scenario) and 0.6 m/s (most likely or average case
scenario), a rover base mass of 70 kg, rolling resistance of 0.05 (the equivalent of a stagecoach on a dirt road),
power requirements of 30 Watts for scientific instruments, and total energy of 233 W-h per science stop.
A. Landsailer Design
The Venus landsailer concept is in essence a sailboat on wheels, as depicted in Fig. 2.15 The sail can be treated
as an airfoil, which develops aerodynamic forces as air flows past the sail. These forces may be decomposed into
components of lift and drag, perpendicular and parallel to the airflow, respectively.
The apparent wind speed is the speed relative to the moving vehicle, and is the vector sum of the true wind
speed (wind speed relative to the ground) and the speed of the vehicle relative to the ground.16 However, for the
Venus landsailer, the velocity of the vehicle will be low compared to the wind speed, and hence the apparent wind
speed can be assumed to be equal to the true wind speed, Vwind.
American Institute of Aeronautics and Astronautics
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Figure 2. Venus Landsail Concept (from ref. 15)
Thus, the force on the sail can be calculated by the lift equation:
!=!!
!!!!!
!"#$
!!!!"#$!! (1)
where ρ is the air density on Venus (67 kg/m3), Vwind is the wind speed, Asail is the sail surface area, and CL is the
aerodynamic coefficient. Although the atmospheric density is high compared to that of Earth, the wind speeds are
much lower, resulting in a similar Reynolds number for the Venus rover in comparison to typical terrestrial sails.
The force required to propel a vehicle depends on the rolling resistance, which is a function of both the terrain
and the wheel type and size:
!=!!!!!! (2)
where Cr is the coefficient of rolling friction, m is the rover body mass, and g is the gravitational acceleration on
Venus (8.87 m/s2). The minimum size of the sail needed to move the rover for assumed parameters can be computed
by setting the two forces equal to each other. Fig. 4 shows a graph of the minimum sail area as a function of mass
for rolling resistance coefficients ranging between 0.02 (rolled new gravel) and 0.08 (medium hard soil), since the
rolling resistance of the rover on Venus is just an assumed parameter for an unknown environment and wheel
design.
The star on Fig. 3 indicates the sail area size for the assumed parameters of 70 kg rover mass and 0.05 rolling
resistance, which yields a minimum sail area of 10.64 m2 needed to move the vehicle.
The sail itself can be dimensionalized by considering different aspect ratios (AR) of the sail. The aspect ratio is
defined as:
!" =!!!
!!"#$
(3)
where b is the length of the luff and Asail is the surface area of the sail. The sail may be estimated as a simple
geometric triangle with height b and length l, as shown in Fig. 4.
CP represents the center of pressure, which is estimated as 1/3 the height from the base of the triangle.
Aspect ratios determine the efficiency of the sail, because higher aspect ratios yield lower lift-induced drag. Typical
aspect ratios for terrestrial sails range between 1 and 6; higher aspect ratios generate more lift but experience stall at
high angles of attack whereas lower aspect ratios generate greater drag.16 A polar diagram by Marchaj17 showing lift
versus drag with plotted data for sails for different aspect ratios show that the most favorable aspect ratios for ocean
cruising and maximum performance are between 2.5 and 3.5.
Table 2 shows different aspect ratios and their corresponding lengths and the center of pressure location for the
sail area size determined. Packaging and center of pressure create challenges to sail design, so the lower aspect ratio
of 2 was chosen to keep the height of the mast lower. This results in a triangular sail with a 45° base angle.
American Institute of Aeronautics and Astronautics
5
Figure 3. Sail Area vs. Mass for Various Rolling Resistances
Figure 4. Sail Geometric Shape
Table 2. Sail Dimensions for Various Aspect Ratios
Asail = 10.64 m2
AR
b (m)
l (m)
CP (m)
1
3.26
6.52
1.09
2
4.61
4.61
1.54
3
5.65
3.77
1.88
4
6.52
3.26
2.17
5
7.29
2.92
2.43
6
7.99
2.66
2.66
American Institute of Aeronautics and Astronautics
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Given the center of pressure, the base of the rover needs to be sized so that the force generated on the sail
(approximately 31 Newtons) does not cause the rover to tip and be unable to recover. A base width of 0.35 meters is
required to overcome this force; the width is designed for 0.5 meters to include margin.
Power can generated from two main sources for this conceptual design: work done on the sail from the
aerodynamic force (e.g., acting on a generator attached to the wheels) or from solar cells placed on the sail surface
or on the rover body. The power requirements for the mission are 30 Watts for science instruments and an additional
30 Watts for steering and mechanisms (which need not be operated simultaneously). Wind imparting kinetic energy
onto the sail cannot transfer all of its energy to the sail. The Betz limit dictates the maximum power available for a
wind turbine:16
!
!"# =!"
!"
!
!"#$%"& =!"
!"
!
!
!!!!
!"#$
!! (4)
However, sails are typically only one-fourth as effective as wind turbines in extracting the wind’s energy, and
the maximum power achieved occurs when the rover speed is 1/3 of the true wind speed, where the wind does more
work on the relatively slow-moving vehicle because it is required to “push” harder.16. For this case
!
!"# =!
!"
!
!
!!!!
!"#$
!!!"#$!! (5)
where CD is the drag coefficient on the sail.
Solar cells operate poorly at the Venus surface due to both the low light levels and the high temperature.
However, photovoltaic solar cells have been calculated to have some performance on the surface of Venus. Dual-
junction PV solar cells have about 0.2% efficiency at the surface and require an estimated 1.6 m2 of surface area to
generate 1 Watt of power.18 Table 3 summarizes the total power generated from the solar cells (sail and rover) and
from wind energy (propulsion).
Table 3. Available Power
Power Source
Power Generated
(W)
Sail
13.65
Rover
0.07
Propulsion
1.8
TOTAL
15.56
Power generated from solar cells can be stored in batteries as charged energy for a long period of time in order to
meet power requirements. If 24 hours are allowed for charging, approximately 330 W-h of energy can be stored and
used for almost 11 hours of either science missions or mobility.
B. Wind Turbine Design
The turbine-powered rover (Figure 1) uses a two-bladed rotor to capture kinetic energy in Venus surface winds.
This energy is converted to electrical energy via a high temperature generator. While Venus surface conditions
present serious challenges for generator function, high-temperature motors suitable for operation at Venus surface
conditions have been developed.19 This design could be modified to run backwards as a generator. Electrical energy
produced would be stored in sodium-sulfur batteries, which can operate at Venus surface temperatures.20 This
energy would then be used to power science instruments and drive the wheels via a high temperature motor.
The power generation of the turbine will be dependent on the power of the air incident upon the turbine disk
area. The total kinetic power of this air is given as:
!=!!
!!!!!
!"#$
!! (6)
For a sail, the theoretically extractable power is considerably less than the total kinetic energy of the air, as given
by the Betz limit seen in Eq. (4). Furthermore, unavoidable inefficiencies in the rotor will significantly reduce
actual power production.
Rotor efficiency will have a large impact on the power output of the rover, and it is necessary to estimate the
efficiency of a turbine operating in Venus surface conditions. It is not immediately obvious how well performance
analyses for large commercial turbines operating on Earth will apply to a small wind turbine operating in a high-
density atmosphere. However, in terrestrial wind turbines, efficiency is approximately linear with lift/drag ratio,21
and lift/drag ratio is correlated to the turbine’s Reynolds number, as seen in Fig. 5.
American Institute of Aeronautics and Astronautics
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Figure 5. Lift/Drag Ratio vs. Reynolds Number for Terrestrial Wind Turbines
The Reynolds number gives the ratio of inertial to viscous forces, and is defined as:
!" =!!
!""!
! (7)
Since the rotor may be moving much faster than the wind speed, we can no longer approximate the apparent
wind speed as equal to the wind speed. Apparent wind velocity Vapp and characteristic length L are both design
variables of the rotor.21 The apparent wind velocity depends on tip speed ratio, which is defined as:
!"# =!!"#$%
!
!"#$
(8)
Tip speed ratio is tied to rotor efficiency,22 as shown in Fig. 6:
Figure 6. Efficiency as a Function of Tip Speed Ratio (TSR) for a Two-Bladed Rotor
Peak efficiency is achieved when TSR is approximately 6.3. Using a design wind speed of 0.6 m/s and design
TSR of 6.3 gives a design apparent wind speed of 3.8 m/s
American Institute of Aeronautics and Astronautics
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The characteristic length L for the rotor blades will depend on solidity, defined as:
!"#$%$&' =!!"#$%&
!!"#"!!!"#$$!!"#$%&'
(11)
For a two-bladed configuration, terrestrial wind turbines have efficiency optimized when solidity is ~0.05. 21
Setting solidity equal to 0.05 and using the relationships detailed above reduces turbine efficiency to a function of
rotor area. This allows for calculations of energy stored for a given wind speed as seen in Fig. 8.
The mass is estimated by summing the estimated mass of the turbine tower, blades, and generators. For a given
height, the mass of a tower varies with the square of tower height. Based on known values for terrestrial turbine
tower masses, the approximate mass of the mast for a Venus wind turbine can be interpolated: 23
!!"#$% =3.64ℎ! (9)
To leave room for blade clearance of science instruments while keeping the center of pressure low, the tower is
assumed to have a height 30 cm greater than the rotor blade length.
Rotor blade mass scales approximately with the radius raised to the 2.3 power.24 Using a scaling factor of 1.63
based on terrestrial wind turbine data,24 blade mass can be approximated as:
!!"#$%& =!∗1.63!!.! (10)
Generators are estimated to weigh 0.88 kg each, based on the mass of a demonstrated high-temperature motor.25
When combined, these three components will compose the bulk of the turbine weight and allow for estimation of
turbine mass as shown in the blue curve in Fig. 7.
Figure 7. Turbine Sizing for wind speed 0.6 m/s
Power production is directly dependent on turbine area and a large-sized turbine has the obvious advantage of
providing more power to the rover. However, constraints on mass and packaging make it necessary to keep turbine
size relatively small. Using the methods detailed above to estimate rover size, it seems likely that 5 m2 turbine area
will provide the appropriate balance. At this baseline area of 5 m2 and give 0.6 m/s winds, the turbine will generate
5.2 Watts. Thus, it will take 80 seconds to store enough energy to move 5 meters, and 45 hours to store enough
energy to do one science run.
Assuming wind speed on the Venus surface to lie in the (relatively wide) range of 0.3 to 0.9 m/s, we perform
the analysis varying wind speed while holding area constant at 5 m2, to give the results in Fig. 8.
At the low end of the wind speed range, the turbine will produce 0.59 W, take about 12 minutes to store enough
energy to move, and take ~17 days to store enough energy for one science run. This shows that the 5 m2 turbine will
provide enough power for mission success even in the lowest wind speeds considered. Equally importantly, the
turbine will not tip the rover even at the highest wind speeds.
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Figure 8. Analysis of a 5 m2 Turbine
III. Trade Studies and Down-Select
Both Venus rover concepts have their advantages and disadvantages. The landsailer is simpler – it has fewer
moving parts and is overall less massive – and it has greater aesthetic appeal. However, the motor actuating the mast
would lack precision control and the rover would be difficult to package; the mast would need to be deployable. In
addition, if the unpredictable wind speeds on the Venusian surface causes the vehicle to tip, the mission is
compromised.
The wind turbine rover is much easier to control with finer precision motors, easier to pack, takes advantage of
higher wind speeds to generate a greater amount of power, and is less likely to tip. Its main disadvantage is the
higher complexity required (more moving parts). It is also more massive than a landsailer and would require a
longer charging time for lowest expected wind speeds (on the order of weeks).
Due to the lower risk, the wind turbine was selected as the Venus rover design concept for this study.
IV. Candidate Science Payload
The designed mission for the Venus rover is to carry a payload of science instrumentation to the surface, and the
detailed design of the mission will depend on the science payload selected. Although designing scientific
instruments for the Venus surface is not part of this study, a notional payload is required to enable the rover design
requirements to be established. Thus, the science package listing here should not be considered the “final” payload
selection, but rather a notional set of payload instruments representative in overall function to what would be
selected for an actual mission. The instrument set here is based on past instruments flown to Mars, with the objective
that the capabilities of this mission would be similar in scope to the scientific mission performed by the MER rovers.
The amount of power available is limited. To be able to get the most out of Venus’s wind speed, a low-mass,
low-power payload is necessary for the rover. This was a significant factor in the instrument set selected here. The
candidate scientific instrument list is shown in Table 1. Several other instruments were considered, but were
removed from the list due to higher mass or power than the design goal required. The resulting instrument list
contains instruments that image the site, measure the chemistry and mineralogy of the surface, and grind or drill
samples that will be taken from Venus’s surface, as well as measure temperature, pressure, and wind speed and
direction.
American Institute of Aeronautics and Astronautics
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Table 1. Candidate Instrument List
Instrument
Mass
(kg)5
Power (W)5
Size (cm)
Bits per
Measurement
Operating
Temperature (°C)
Alpha Particle X-Ray
Spectrometer (APXS)
0.5
2.5
8.5 long,
5 diameter
12,288
-40 to 5
Calibration Target
0.1
0
10 x 10
-
-
X-ray Diffraction (XRD)
5
15
30 x 20 x 10
2,793,600
Optimal: -60
Microscopic Imager
0.2
5
12 x 7.5 x 5
8,388,608
Operating: -55 to 20
Non-Operating: -110 to 50
Thermal Emission
Spectrometer (TES)
2.4
5.6
23.5 x 16.3 x
15.5
10,567,680
-45 to 50
Navigation Cameras
.2 x 4
2
5 x 5 x 10
8,388,608
CCD: -55 to 20
PanCam
.27 x 2
3
50 x 60 x 110
8,388,608
Operating: -55 to 20
Non-Operating: -110 to 50
Radar Reflector
1
0
50 x 50 x 20
-
-
Radiant Heater (Mini-TES)
2.3
45
10 x 10 x 10
0
-
Rock Abrasion Tool (RAT)
- ARM (IDD)
0.7
10
10 long,
7 diameter
3,600
0 - 500
Rover Environmental
Monitoring Station
1.2
50 hrs
10 x 10 x 10
300
-130 to 70
A significant constraint on the instrument design for the rover is the high surface temperature. The candidate
instrument listed are primarily designed to work at temperatures of 20°C and below. Since thus rover is not designed
with a cooling system, it is assumed that by the time this mission is ready to launch, high-temperature electronics
will be available, allowing instruments of similar capability to withstand the extreme Venusian surface temperatures.
To account for the fact that these instruments will be of different design, a growth factor of 150% was assumed on
the mass and power values shown in table 1.
Atmospheric science requirements include data on surface pressure, temperature, and wind speed and direction.
These will be measured using a Rover Environmental Monitoring Station (REMS) instrument similar to that used on
the MSL mission to Mars, incorporating a series of booms containing different sensors to measure meteorological
data.26
The Alpha Particle X-Ray Spectrometer (APXS) is typical of instruments to measure the elemental composition
of samples. This is done by analyzing the energy spectrum of the characteristic fluorescent X-rays emitted when a
sample is irritated with alpha particles from radioactive source.27,28,29 While existing sensors for Energy-dispersive
X-ray detectors used in an instrument such as this achieve low noise levels only when operating at low temperature,
the noise floor in a detector is an exponential function of the temperature divided by the bandgap of the
semiconductor used for the detector (typically a p-i-n diode operating at high reverse bias). Thus, the basic physics
implies that by using a wide-bandgap semiconductor for the detector, an acceptable noise level can be achieved at
Venus temperature. Although such high-bandgap avalanche photodiodes are not currently being manufactured,
there is no fundamental reason that they could not be developed.
A Thermal Emission Spectrometer (TES) is typical of a non-contact measurement. TES is a Fourier Transform
Spectrometer that will provide remote measurements of mineralogical as well as thermophysical properties of its
surface surroundings. An example of the effective operation of thermal emission spectroscopy on a planetary rover
is the use of the miniTES instrument on the Mars Exploration Rovers.30 Infrared detectors (bolometers) can operate
at the required temperature, however, to function, the instrument needs to be at a lower temperature than the sample
being analyzed. Due to Venus’s greenhouse effect, objects in equilibrium at the surface are at nearly the same
temperature, meaning the sample will be about the same temperature as the instrument. For the Venus instrument, a
radiant heater will be used instead to heat the sample for the necessary temperature difference. The radiant heater
5Adjusted.
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will be a simple metal halide lamp similar to those used on submersible vehicles.8 These are rated to a depth of
2,000 meters under water, corresponding to a pressure of roughly twice the Venus surface. The heated sample will
emit a unique thermal infrared spectrum, characteristic of the different minerals contained in the sample.30
A third composition and mineralogy tool that could be used on such a rover is an X-ray diffraction tool. An
example of an X-ray diffraction instrument is the CheMin package used on the Mars Science Laboratory to analyze
the crystallographic composition of samples.31 The CheMin expose a beam of X-rays to a sample of rock, and the x-
ray diffraction pattern reveals the crystal structure of the sample. Each mineral has a specific diffraction pattern
which is identified by the angular dependence of the diffracted beams.
One of the desired objectives of a surface science investigation is that the soil and surrounding rocks be studied.
A Rock Abrasion Tool (RAT)32 will be used so that the analysis tools can be used to investigate the chemical
composition and mineralogy of not only surface samples, but also soil that is not immediately exposed to the
Venusian atmosphere, as well as the insides of rocks. The RAT will position itself against a rock or soil surface
using the rover’s arm it is positioned on, and use a grinding mechanism to dig into the rock to expose a fresh sample
that has not been exposed to the Venusian atmosphere. Once pristine surface is exposed, the APXS, mini-TES. and
X-ray diffraction instruments can be employed to determine the mineralogy of samples not immediately exposed to
the Venusian atmosphere. Adaptation of the Rock Abrasion Tool to operation on Venus requires use of high-
temperature motors; an abrasion and coring tool for Venus is currently being developed,33 and a drill to has been
developed and demonstrated in operation on the surface of Venus by previous Russian missions.5
Imaging investigations are also important. A Microscopic Imager (MI)34 will be used to take close-up, high-
resolution images of rocks and surface soil. As it is placed on the rovers arm, used in conjunction with the RAT, the
MI will have the capability to take images of subsurface samples as well. Close up images of the Venusian surface
will be useful for studying the microstructure of rocks and soil. The Panoramic Camera, or PanCam for short, will
take high-resolution panoramic images of the surface.35 Images taken by PanCam would be useful in identifying
potentially interesting sampling sites, while the Navigation Cameras (“NavCams”)36 will take wider field-of-view
images to be used to direct the rover driving. Images provided from NavCam as well as PanCam will also provide
data for investigative studies of surface morphology, rock and soil distribution, as well as the general surface
geology. A calibration target will also be placed on the surface of the rover in view of the cameras, to provide a
reference for the cameras to calibrate images to the Venus illumination spectrum, so that the data taken on Venus
can be compared to data taken on Earth. Finally, the radar reflector is an unpowered instrument that will be used to
allow the rover to be located, either from a radar in an orbiting satellite, or directly from Earth.
V. Turbine-Powered Rover Detailed Design
A. Mission Requirements
To this date, spacecraft that have been sent to Venus include orbiters, atmospheric probes, stationary landers, and
balloons, but a rover has yet to be sent to explore Venus. The goal of this project is to analyze a conceptual design
for a rover to be sent to, and survive, the extreme conditions of the Venus surface. Due to these extreme surface
conditions, the longest a lander or probe has ever lasted is the Venera 13 for 127 minutes. The short mission times
mean that not much has been able to be studied on the surface first hand. This mission is designed to verify
assumptions based off the small amounts of data gathered from previous probe or lander missions, as well as
uncover entirely new data. Though the past probes to reach the surface of Venus have made advances over what was
known before them, a rover would be next level of discovery for Venus. Not only would one spot be able to be
studied, but several meters of area surrounding the landing site.
It is possible that the top layers of soil and outside of rocks are affected by the chemicals in the atmosphere. By
studying the lower layers of soil and the insides of rocks, the rover will be able to study samples that have been
untouched by the Venusian atmosphere for hundreds, or potentially even thousands of years. This will probe the
geological history of Venus, and determine the state of alteration of the mineralogy due to surface/atmospheric
interaction, and possible even shed light on the intriguing question of whether Venus ever had water on its surface,
and when (or if) the global climate had altered to its current state as a planet of extreme surface conditions.
B. Concept of Operations
Atmospheric entry and descent through the Venus atmosphere have been previously done by NASA with the
Pioneer Venus mission atmospheric probes.37 This mission assumes planetary entry will be performed with a direct
atmospheric entry using a 4.5 diameter aeroshell containing a heat shield on the bottom and a parachute system in
American Institute of Aeronautics and Astronautics
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the top compartment. Upon atmospheric entry and deceleration, the heat shield will drop and the spacecraft will
slow down through the deployment of a drogue parachute followed by another high-temperature parachute.
The rover is assumed to land directly on its wheels, similar to the design in a previous study;8 however, landing
analysis or simulations were not conducted to validate the feasibility of the suspension system’s ability to absorb the
shock upon landing. There is no “typical” surface terrain to select on Venus; its diverse surface features include,
“mountain ranges, craters, continent-spanning sinuous hills, and chaotic terrain.” However, it is expected that a
highly sloped or extremely rugged landing site can be avoided and that the terrain will be relatively flat upon
landing. Surface roughness may be estimated from images obtained by the Russian Venera landers,38 as shown in
Fig. 9.
Figure 9. The surface of Venus, as viewed from Venera-9 (top) and Venera-13 (bottom) landers
The Venusian solar day lasts approximately 116.75 Earth days,39 so about 58 days of sunlight can be achieved
per day. Although the rover does not require sunlight for operation, since it is wind and not solar powered,
illumination is required for vision, and hence the mission will be designed to operate only during the daylit portion
of the Venus day. The mission duration was assumed to be 55 days, during with time all scientific operation and
data transmission back to Earth is to be accomplished. Under worst-case expected conditions of 0.3 m/s wind
speeds, the rover will need to charge for approximately 17 days between science runs, enabling for a total of 4
science runs. Average-case conditions of 0.6 m/s wind speeds enables for 19 mission science runs with two days of
charging time in between each run.
C. Wind Turbine Detailed Design
Initial analysis assumed that the rotor and generator would both operate at their maximum efficiencies;
however, because of the uncertainty regarding Venus surface conditions as well as some variation in wind speed
being likely, this is unworkable without the use of a variable gear ratio or voltage control of the generator. Both of
these techniques would require significant addition of moving parts and therefore complexity to the rover, which
would increase the risk of mission failure. To avoid this risk, three different generators will be used, each of which
will have a fixed gear ratio and voltage. Relationships between torque, RPM, and efficiency are based on fixed gear,
constant voltage assumptions40 and values for the high-temperature motor25 and can be seen in Fig. 10.
By switching between these generators, the rover will be able to maintain a tip speed ratio, torque, and RPM,
which will keep both the rotor and generator within acceptable operating limits over the required range of wind
speeds.
American Institute of Aeronautics and Astronautics
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Figure 10. RPM and Efficiency as Functions of Torque for a Constant Voltage Generator
Figure 11. Analysis of efficiency, power stored, tip speed ratio, and time to store energy for Generator #1 (low wind
speed)
American Institute of Aeronautics and Astronautics
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Figure 12: Analysis of efficiency, power stored, tip speed ratio, and time to store energy for Generator #2
(intermediate wind speed)
Figure 13: Analysis of efficiency, power stored, tip speed ratio, and time to store energy for Generator #3 (high wind
speed)
American Institute of Aeronautics and Astronautics
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VI. Rover Mechanical and Subsystem Design
A. Four-Wheel vs. Six-Wheel Design
Trade studies were conducted to determine the most feasible design for the rover body configuration. The
ability of the rover’s mobility system to traverse obstacles and its steering mechanism are major elements that
characterize the rover. Two types of suspension systems in particular were investigated for this mission: four-wheel
suspension and six-wheel rocker-bogie suspension systems.
Four-wheeled suspension systems use differential steering to change direction and require few motors for
actuation. The rocker-bogie suspension systems consist of two links, a main rocker, and a forward bogie on each
side, as illustrated in Fig. 14 (right). A wheel and steering mechanism is attached to one end of the main rocker
while the opposite end is connected to the forward bogie through a passive pivot joint. Weight is distributed on the
wheels by defining rocker and bogie lengths as well as pivot joint positions.41
Rovers capable of surmounting large obstacles in comparison to wheel size have a high degree of mobility.
Enough traction is required from the rear wheels to thrust the forward wheels with enough force to overcome an
obstacle. Four-wheeled rovers are typically incapable of traversing obstacles larger than the wheel radius due to lack
of traction while six-wheeled rovers are able to surmount these obstacles head-on because the extra wheels provide
greater traction and reduce the normal force on each wheel. Rocker-Bogie suspension systems can also perform
multiple types of steering (Ackerman, crabbing, differential, and zero radius) for greater mobility.41
Although the limited amount of traction is an issue for a four-wheeled rover design, the rover is still able to
overcome obstacles if the obstacle is approached at an angle and traversed one wheel at a time. This enables the
four-wheel design to have similar capabilities to the six-wheel design but with fewer control system requirements,
less suspension mass, and less actuation motors, which greatly reduces the weight and complexity of the system and
fits better within the scope of a Discovery-class mission. Furthermore, the rocker-bogie suspension system carries
the risk of jamming an obstacle between the tandem wheels of the bogie.
Figure 14. Four-Wheeled rover (left) and Six-wheeled Sojourner rover with rocker-bogie suspension (right)
B. Wheel Sizing and Drive Train Configuration
The diameter of the wheel is proportional to the size of the obstacle, and for a four-wheeled vehicle, it can be
estimated that the rover can overcome an obstacle roughly 18% of its wheel diameter. If an average obstacle size of
15 centimeters tall is assumed on the surface of Venus, the minimum wheel diameter required to traverse these
obstacles is approximately 0.833 meters.
Two different drive train configurations were investigated for this concept: a dual-output angle drive and a
standard four-wheel drive, as shown in Fig. 15. The dual-angle output drive concept is simpler and uses only two
motors for mobility, whereas the standard four-wheel drive would have a motor to power each wheel. However, the
simpler drive-train configuration can only accommodate one mode of steering (skid steering)41 and therefore not
selected for the design.
American Institute of Aeronautics and Astronautics
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Figure 15. Dual-Output angle drive (left, courtesy University of Oklahoma) and standard four-wheel drive (right)
C. Rover Dimensions
Figures 16 and 17 shows the dimensions of the conceptual rover, which has a base width of 1.5 meters, a length
of 2 meters and a height of 2.75 meters. The relatively small size enables the rover to fit into a designed aeroshell
3.35 meters high and 4.5 meters in diameter without folding, as shown in Fig. 17, which simplifies the design and
avoids the need for complex folding mechanisms. The 4.5 meter diameter aeroshell is based on the Pioneer-Venus
atmospheric-entry aeroshell;37 the bottom of the aeroshell is heat shield, and the top compartment will store the
parachute system (not shown). Detailed analysis of aeroshell packing and entry, descent, and landing analysis was
not conducted in this study.
Figure 16. Dimensioned Rover (Side View)
American Institute of Aeronautics and Astronautics
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Figure 17. Dimensioned Rover (Top View)
Figure 18. Rover fit in the Pioneer-Venus Large Probe aeroshell: Side View (Left) and Top View (Right)
D. Communications Design
The first step for the communications system was a trade study to decide between communication via an orbiting
relay satellite to connect to Earth, or a direct to Earth transmission. Although use of a relay increases the mission
cost by adding a second, orbiting, spacecraft, the antenna size and power required for a direct-to-Earth data link was
prohibitive. To minimize the transmission power required, a design in which the rover communicates through an
orbiting satellite was chosen to relay the data to Earth.
To determine the optimum orbiter altitude, the viewing angle from the center of Venus is calculated first. With
an assumed viewing angle from the surface of 45°, the orbital angle from the center of Venus, γ, is:
!=tan!!!
!!! (11)
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Figure 19. Angle definitions:
viewing angle (from the surface)
and orbital angle (from center of
Venus)
where h is the height from the surface, and the radius of Venus R is 6051.8
km. The range of orbital altitude considered ranged from 200 km above the
surface, barely above the Venusian atmosphere, to 10,000 km above the
surface. As the altitude of the relay satellite increased, the viewing angle from
the center of the Venus (and hence the fraction of the orbit that the relay is in
view) increased accordingly, as shown in Fig. 19. The final calculation used
the fraction of the orbit to calculate and find the highest fraction of power,
including both the view angle and the 1/r2 power law, to find the altitude at
which the highest bit rate to transfer data to and from Earth is achieved. The
analysis showed that that the highest fraction of power received by the orbiter
occurs with the lowest Venus orbit. Hence to get the highest amount of bits in
a given period of time, the lowest Venus orbit is optimal.
The orbital speed as a function of communication orbiter’s altitude is:
!
!=!
!!! (12)
where
µ
is Venus’ standard gravitational parameter, 324,859 km3/s2. The
orbiting velocity was then used to find the overhead time, in which the
satellite was within the 45° viewing angle. The time it took to take one full
orbit around Venus was found using Kepler’s law:
!=!!!!!
! (13)
The amount of time the orbiter is in view of the lander (within 45° of the zenith) is shown in Fig. 20.
Figure 19. Bit Rate as a Function of Orbiting Satellite Height6
After finding the total bits/measurement for each instrument, the numbers of bits/measurement were totaled. This
number was then used in conjunction with the overhead time to find the bit rate. For this part in the project, it was
assumed that each instrument would only run once, and that the total number of bits would be able to be transferred
to the orbiting satellite in one overhead pass. This way, the lowest bit rate per single overhead pass would be able to
be found. In the most strained, basic scenario, each instrument would be able to run at least once and be able to
transfer all of the data back to Earth in one overhead pass. These bit rates will inevitably change, depending on the
amount of power designated to data transfer. It will also change due to the fact that the data coming from some
instruments will require more frequent measurements than others. The NavCam, for example, will need to send a
great deal of data back to Earth, to allow navigation to support driving. Other instruments, such as the CheMIN will
need to send only one measurement back to Earth at a time. The frequency at which each instrument will take
measurements has not yet been determined.
6 Calculated using total number of bits/measurement.
0"
1000"
2000"
3000"
4000"
5000"
6000"
7000"
8000"
9000"
10000"
0" 100000" 200000" 300000" 400000" 500000" 600000" 700000"
Orbi%ng(Height,(km(
(
Bit(Rate,(bits/s(
American Institute of Aeronautics and Astronautics
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Figure 20. Satellite Overhead Time in Relation to Orbiting Height
E. Radio Subsystem Design
The radio subsystem design requires a trade-off between the efficiency of operation at high temperature, the
radio opacity of the Venus atmosphere, and the complexity of tracking.
A solid-state radio system operating at a frequency of 500 MHz was chosen. Despite Venus’s thick and opaque
atmosphere, a frequency of 500 MHz is able to penetrate through the atmosphere, letting the rover connect to the
orbiting satellite and send data back to Earth.
The transmission frequency was chosen to be one at which silicon-carbide power amplifiers have been
demonstrated, in order to allow a fully solid-state transmitter operating at Venus ambient. NASA has successfully
tested a differential oscillator operating at 500 MHz at 475°C,42 a higher temperature than the surface of Venus,
which demonstrates that the main power amplifier of the communications system can be designed to operate at the
required temperatures.
The opacity of the Venus atmosphere to radio waves is a function of wavelength. Radar measurements show that
the opacity in the microwave to VHF region can be modeled as:43
α = (3.8/λ)2 (14)
where α is the absorption, and λ the wavelength in cm. At our target frequency of 500 MHz (λ =60 cm), the
absorption of the radio signal by the atmosphere is too small to be significant. The chosen frequency lies between
those used by the Russian Venera surface-to-relay communications, 122.8 and 138.6 MHz, and the Pioneer
atmospheric probes transmission frequency, 2.3 GHz,44 two previous missions which both successfully
communicated from the surface of Venus to spacecraft overhead.
Omnidirectional antennas do not require pointing, but since they transmit in all directions, only a small fraction
of the energy radiated is directed toward the receiver. An antenna with higher directivity (i.e., a high gain antenna)
allows more of the transmitted energy to be received by the relay satellite and hence reduces the energy required per
bit communicated, but puts constraints on the wavelengths or else requires larger antenna size. A high antenna gain
also requires that the antenna be accurately pointed at the receiver. Antenna pointing adds to the complexity in the
system, requiring either mechanical components (gimbal, motor) or a phased-array for electronic steering, as well as
added computational complexity of understanding the rover orientation and tilt and calculating the satellite location
relative to the rover. The three-element Yagi antenna chosen is a compromise between the simplicity of an
omnidirectional antenna and the complexity of a dish. It provides a moderate gain of about 7.5 dB (compared to a
dipole). As a result, it will require pointing. To mitigate the complexity of tracking, it was assumed that four Yagi
antennas are mounted on the rover, and the transmitter output is switched to the appropriate antenna. The rover
orientation may also be adjusted to optimally place the antenna pattern over the orbiter track.
VII. Conclusion
The Curiosity mission to Mars has reminded the public of the value of rovers on extraterrestrial planets, and it is
time to extend this capability to another planet, Venus. Venus is of scientific interest at least as great as the interest
in Mars, and, the more can be found out about Venus by use of a rover mission, while the public is still excited and
willing to support extraterrestrial planetary rovers.
0"
2000"
4000"
6000"
8000"
10000"
0" 500" 1000" 1500" 2000" 2500" 3000" 3500" 4000"
Orbi%ng(Height,(km(
Orbiter(Overhead(Time,(s(
American Institute of Aeronautics and Astronautics
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The benefit of having a rover on Venus is the fact that it can conduct science in more than one location, rather
than being fixed at the landing spot. This, with the use of high temperature silicon-carbide electronics, may be
exactly what is needed to find out the geological history and present geology and mineralogy of Venus. While it is
difficult to design a rover to survive the surface conditions of Venus, that just means that this mission is that much
more important. If there is no challenge, and nothing new to learn, then there would be no point to science at all.
Venus has both of these components at large, making it one of the most scientifically interesting planets in our solar
system.
Acknowledgements
The student researchers on this project enjoyed the support of the NASA Space Academy 2012 program. We
would like to thank NASA Glenn artist Terence Condrich for his artist’s conception of our rover, Dr. Kankam and
Bernice Beznoska of NASA Glenn for their work hosting the NASA Space Academy, and our respective California
and Maryland Space Grant Consortiums for their funding this summer.
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