August 2005

NASA/TM-2005-213527

Mars Tumbleweed: FY2003 Conceptual Design

Assessment

Jeffrey Antol, Philip C. Calhoun, John J. Flick, Gregory A. Hajos, Jennifer P. Keyes, and

Frederic H. Stillwagen

NASA Langley Research Center, Hampton, Virginia

Shawn A. Krizan

Analytical Mechanics Associates, Inc., Hampton, Virginia

Christopher V. Strickland

Swales Aerospace, Hampton, Virginia

Rachel Owens

Virginia Polytechnic Institute and State University, Blacksburg, Virginia

Langley Aerospace Research Summer Scholar (LARSS) Program, 2003

Michael Wisniewski

University of Michigan, Ann Arbor, Michigan

Langley Aerospace Research Summer Scholar (LARSS) Program, 2003
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August 2005

NASA/TM-2005-213527

Mars Tumbleweed: FY2003 Conceptual Design

Assessment

Jeffrey Antol, Philip C. Calhoun, John J. Flick, Gregory A. Hajos, Jennifer P. Keyes, and

Frederic H. Stillwagen

NASA Langley Research Center, Hampton, Virginia

Shawn A. Krizan

Analytical Mechanics Associates, Inc., Hampton, Virginia

Christopher V. Strickland

Swales Aerospace, Hampton, Virginia

Rachel Owens

Virginia Polytechnic Institute and State University, Blacksburg, Virginia

Langley Aerospace Research Summer Scholar (LARSS) Program, 2003

Michael Wisniewski

University of Michigan, Ann Arbor, Michigan

Langley Aerospace Research Summer Scholar (LARSS) Program, 2003
Available from:

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7121 Standard Drive 5285 Port Royal Road

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Acknowledgments

Special thanks to Dennis Bushnell, LaRC Chief Scientist, for his support of this work through the LaRC Creativity

and Innovation (C&I) initiative.

Thanks to the members of the LaRC Tumbleweed team and their contributions to this report:

Jeff Antol – 1.0, 2.0, 3.5, 4.1, 4.2.1, 4.2.3, 6.0, 8.0

Philip Calhoun – 5.1, 5.4, 5.5

John Flick – 5.1, 5.2, 5.3 (with input by Matt Toniolo (AMA) and Steve Stalos (AMA)), 5.5

Gregory Hajos – 7.0

Jennifer Keyes – 4.2.2

Fred Stillwagen – 3.4

Bob Evangelista (AMA) – Figure 2

Shawn Krizan (AMA) – 3.3, Figure 1

Jeff Murch (AMA) – Figures 1, 5, and 6

Chris Strickland (Swales) – 3.2, Figures 15 and 16

Rachel Owens (LARSS) – 5.5

Michael Wisniewski (LARSS) – 3.3

Thanks also to the LaRC researchers and engineers who took time to provide information and recommendations:

Rob Andrews, Dr. Keith Belvin, Ingred Carlberg, Julio Chu, Neil Coffey, Dr. John Connell, Dr. Bill Cook, Charles

Dunton, Kevin Earle (NASA Co-op), Steve Harris, Lana Hicks-Olsen, Nancy Holloway, Luther Jenkins, Warren

Kelliher, Rob Kline (AMA), Renji Kumar (AMA), Dr. Joel Levine, Dan Norfolk, John Quagliano, Steve Ruggles,

Kurt Severance, Dr. Walt Silva, Ralph Stevens, Dr. Ji Su, Adam Thomas (LARSS), Carolyn Thomas, Judith

Watson, Gary Wainwright, Rich White, Steve Nevins, Washito Sasamoto, Dr. Stan Woodard, Tian-Bing Xu, and

Tom Yager.

Special thanks go to the Library and Media Services Branch of the Langley Research Center who published this

document, with special acknowledgment to the NCI Technical Publications Support Group at Langley: Susan Hurd,

editing; Dee Bullock, illustration and image preparation; Patricia Gottschall, desktop publishing; and Mary Edwards,

proofing.

The use of trademarks or names of manufacturers in the report is for accurate reporting and does not

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the

National Aeronautics and Space Administration.
1. Introduction

1.1. Background

NASA Langley Research Center (LaRC) is studying concepts for a new type of “rover” vehicle that

would be propelled by the wind (fig. 1). Known as a Mars “Tumbleweed,” it would derive mobility

through use of the Martian surface winds. Tumbleweeds could conceivably travel greater distances, cover

larger areas of the surface, and provide access to areas inaccessible by conventional rovers. They would

be lightweight and relatively inexpensive, allowing a multiple vehicle network to be deployed on a single

mission. Tumbleweed rovers would be equipped with microelectronic sensors for conducting science and

serve as scouts—searching broad areas to identify specific locations for follow-on investigation by other

explorers (fig. 2).

Figure 1. NASA LaRC Tumbleweed concepts (clockwise from top left) Box Kite, Tumblecup, Wedges, and

Dandelion.

A deployed Tumbleweed would be approximately 4 to 6 m in diameter and have a mass of approximately 10 to 20 kg, including the science instrument complement and supporting subsystems of

structures, power, communication, and navigation.

Figure 2. Artist depictions of the Wedges and Dandelion concepts on Mars.
2

1.2. Goals of 2003 Study

Preliminary assessments of LaRC Tumbleweed concepts were conducted in Fiscal Year (FY) 2002

(ref. 1). An extensive investigation was planned for FY03, with the following goals:

 Refine the science objectives and develop a candidate mission scenario.

 Define supporting subsystem capabilities and assess technologies.

– Structures/Materials

– Power

– Communications

 Conduct wind tunnel testing to determine drag characteristics of concepts.

 Develop a dynamic simulation capability to assess rolling, bouncing characteristics.

 Collaborate with universities to study Tumbleweed concepts.

 Provide outreach presentations to the general public.

The following sections provide a summary of the accomplishments within these areas.

2. Science Objectives

2.1. Review of MEPAG Goals

The goals and objectives defined by the Mars Exploration Program Advisory Group (MEPAG) (ref. 2)

were examined in the FY02 Tumbleweed study:

 Determine whether life ever arose on Mars.

 Determine climate on Mars.

 Determine the evolution of the surface and interior of Mars—“geology.”

A subset of the MEPAG objectives (within these goals) that a Tumbleweed rover potentially could

perform was identified. These objectives were tabulated along with the associated measurements and

related instruments. The level of controllability (e.g., stop/start) needed by the Tumbleweed to accomplish

the measurements was also assessed and identified.

In FY03, the subset of MEPAG science objectives was reexamined, including an extensive literature

review and discussions with scientists and engineers at NASA LaRC and elsewhere that are involved in

instrumentation development for Mars exploration. As a result, the “Search for Life” goal was identified

as the most scientifically interesting and challenging mission for the Tumbleweed concept. In this role,

Tumbleweed rovers would serve as in situ scouts, surveying vast areas of the planet’s surface to locate

areas of interest for follow-on, comprehensive surveys by future landers, rovers, or perhaps human

explorers.
3

2.2. Tumbleweed Science Objectives Based on MEPAG Goals

Table 1 summarizes the objectives, investigations, and measurements within the MEPAG “Search for

Life” goal that Mars Tumbleweed rovers could potentially conduct. The table also includes associated

instrumentation. A detailed table in appendix A provides mass, power, and data estimates of existing or

planned near-term instruments that correspond to the example instrumentation. This table only provides a

representative instrumentation set and is not intended to be all-inclusive. Specific science mission objectives, along with an associated instrument complement, will need to be identified by a science definition

team.

Table 1. MEPAG Search for Life Goal:

Objectives, Investigations, Measurements, and Example Instruments for Tumbleweed

MEPAG objective/

investigation MEPAG measurement Example instrumentation

Determine whether life Water vapor sensors

exists today—Carry out

in situ exploration of areas

suspected of harboring

liquid water.

For at least 20 stations at 4 targeted

sites, conduct in situ geophysical

and chemical searches for

subsurface water and other

volatiles over km2 surface.

Gas chromatograph/mass

spectrometer

X-ray diffraction

X-ray fluorescence

Laser Raman spectrometer

Mossbauer spectrometer

In situ/mobile platforms deployed

to at least 3 well-characterized and

diverse sites to assess the

mineralogy,... Mini-thermal emission spectrometer

(TES)

Alpha proton X-ray spectrometer geochemistry, and ... Mini-Mass spectrometer

Gas chromatograph/mass

spectrometer

Assess the extent of

prebiotic organic chemical

evolution—Search for

complex organic molecules

in rocks and soils.

organic materials

Laser desorption spectroscopy

Visual observations for Panoramic stereo camera

life. Imaging Imager for Mars Pathfinder

2.3. Preliminary Mission Concept

The current Mars program strategy in the search for life is to “follow the water”; however, many sites

on Mars that may harbor water and are scientifically interesting are not accessible by conventional robotic

systems, partially due to the high winds associated with these areas. Mark Richardson, a Caltech Mars

meteorologist, is quoted in the March 2, 2003 edition of the Los Angeles Times, “The planet’s most

interesting places tend to be associated with scary winds.”

An area of particular scientific interest is the gullies that have been observed in photographs (fig. 3)

taken by the Mars Global Surveyor (ref. 3). There is much speculation regarding the origin of these features, including subsurface aquifers, melting snow packs, carbon dioxide outbursts, and dry dust flows

(ref. 4). Current conventional rover designs would have difficulty accessing these features; however, a

Tumbleweed vehicle could potentially be made to roll down the gullies or examine regions below the

gullies within canyons and craters.
4

Figure 3. Gullies in Newton Basin (NASA/JPL/Malin Space Science Systems).

To investigate gullies, Tumbleweed rovers could be deployed at the entrance to a valley or canyon that

exhibits these features and allowed to be driven into the region by the wind. Alternatively, Tumbleweeds

could be deployed near the top of a valley, canyon, or crater and be blown by the wind down the slopes

into the area of interest. The Tumbleweed would serve as a scout in these areas, searching for water

vapor and traces of biogenic gases.

3. System Definition

3.1. Overview of Design Challenges

Development of a lightweight system is important for a robust Tumbleweed capable of operating in

the low density atmosphere of Mars. Lightweight structures and materials will be needed as well as

lightweight subsystems for power, communications, and deployment.

3.2. Structures and Materials

The structural system for the Mars Tumbleweed must provide the core support and maintain the correct shape for roving while supporting the other subsystems and protecting the science instrument core

from permanent damage. The entire tumbleweed rover must be packaged such that it could be carried as

a secondary payload on a Mars mission but still deploy into the final shape large enough to propel itself in

the Martian winds. It will serve as the main frame to attach the wind-capturing devices and hold the correct shape to offer better rolling and maneuverability. Science instruments will be carried in an inner core

that must be protected by the structure during expected high-speed impacts. Portions of the structure may

be considered multifunctional, as they may carry out other system operations such as propulsion, communication, or other subsystems (as may be designed). Material selection for the tumbleweed structural

systems must also allow for the high thermal gradients expected during the mission time frame and the

many other environmental effects of Mars. Some environmental considerations are the rock impact tolerance and the ability to stop rips in thin films that may be used. The materials will also have to be capable

of having high packing-to-deployment ratios.
5

To understand and design the structure for a Mars Tumbleweed rover, a two-dimensional (2-D) representation of the rover was created and simulated during high loads. The 2-D model was chosen to best

characterize the behavior and be able to more easily compare the various Tumbleweed concepts. The

present designs were modeled as a central core payload mass in the center of a hoop connected by either

tension or compression elements. The highest expected loads occur during impact, either from deployment or from tumbling across the Martian surface. The nonlinear impact analysis program, LS-DYNA3D, was used to study the impacts of the rover concepts. Dr. Chris White from JPL created the models

and presented the results by assuming a 20-m/s impact speed into a rigid planar surface at a 45° angle.

Trend lines were created by assuming different payload masses and hoop and spoke sizes. The tension

spoke cases provided the best results, while the compression spokes were very dependent on orientation

of the spoke to the ground. Different methods were used to better quantify the beam spoke analysis, but

the coupling of the orientation and deflection proved to be difficult to overcome. The results do show

promising trends when using the tension spoke and give insight into how the beam spoke versions will

behave under impact. Additional analyses will be conducted in FY04.

3.3. Power

A preliminary review of power source considerations for Mars Tumbleweed was conducted and several potential power systems were compared (table 2). The advantages and disadvantages of the power

system concepts were defined in relation to system drivers and mission parameters; however, a detailed

trade study is needed to identify the best power source for each tumbleweed design configuration. The

results of this preliminary assessment indicate that while solar arrays are a very reliable power source for

conventional rover designs, radioisotope power sources provide a greater range of power capabilities and

continuous power over a long period of time. Thus, they may be more applicable to a Tumbleweed vehicle. The technologies in table 2 are discussed in the following sections.

Table 2. Power System Options

Technology Power Advantage Disadvantage

Photovoltaic solar

arrays

>10 W - Low Mass

- High TRL

- Durability

- Limited range (day cycles,

shadowing effects, etc.)

Primary batteries >10 W - Low mass

- No need for thermal control

- Easy to package

- High TRL

- Durability

- Low life time

- Limited in power (without

impacting mass)

(RTPV) Radioisotope

Thermophotovoltaic

10 to 75 W - Attractive alpha - Radioactive material

- High radiator temps

- Need unobstructed view

(for cooling)

- Low efficiency

- Low TRL

RHU milliwatt

radioisotope

40 mW at

4-percent

efficiency

- Low mass

- Durability

- Radioactive material

Thermoelectric 1 to 20 W - Can be achieved within the

mission time frame

- Manageable thermal control

- Durability

- Radioactive material
6

3.3.1. Solar Power

The use of solar power as a potential power source for Tumbleweed is a primary option. Solar arrays

have been a reliable source of power for the recent Mars surface exploration missions, Mars Pathfinder,

and Mars Exploration Rovers. Although the design and structure of Tumbleweed have yet to be determined, several configurations being explored could easily accommodate solar array power, including the

Wedges and Box Kite designs, which offer sufficient surface area to accommodate solar panels. Configurations such as the Dandelion or Tumblecup do not easily lend themselves to solar array placement. For

the use of solar arrays on such designs, complex deployable systems may be necessary, but would be an

undesired complexity that could greatly affect anticipated mass requirements and overall performance.

An overall disadvantage of solar arrays on Tumbleweed results from the rolling motion of Tumbleweed. Conventional Mars rovers use solar power because the Sun easily illuminates the fixed arrays.

The rolling Tumbleweed, based on the spherical shape, has, at a maximum, only one half its potential

illumination area available at any one time. Solar arrays would therefore need to encompass the entire

Tumbleweed and be well positioned for maximum solar radiation. Additional mass associated with this

“blanket” of solar cells may be detrimental to mass constraints. Additionally, while the Box Kite design

provides greater surface area to accommodate the arrays, the orthogonal panels may also cause unwanted

shading of panels, which would further decrease available energy based on solar zenith angles.

Further disadvantages exist because of the accumulated dust in the atmosphere. The dusty environment of Mars decreases solar flux and also covers solar array panels. For this reason, prior missions were

planned to last only 90 days due to dust accumulation on solar panels and the resultant lack of power.

Data from previous missions show an average decrease in available power of 0.3 percent per day, which

equates to a 25-percent power deficiency after only 96 days. Although the Tumbleweed is expected to

roll, it is unknown whether this rolling will remove or possibly increase dust accumulation on the solar

panels. And finally, disadvantages exist for potential exploration sites. As discussed in section 2.0, many

scientifically interesting sites may be located in canyons and craters. These environments would prevent

direct solar flux for a large part of the day, significantly reducing the available energy.

3.3.2. Primary Batteries

Nonrechargeable batteries are flight proven and highly reliable for short mission lifetimes. Primary

batteries are typically used for high power but short duration operations. Although battery technologies,

including thin-film batteries and lithium chemistries, have continually improved, the Tumbleweed long

duration mission objectives do not support the use of primary batteries as the main power source. Primary batteries offer high specific energies, but power requirements for long missions increase system

mass above mission constraints. The expectations for a multiyear mission for Tumbleweed greatly

accommodate reusable forms of energy or nuclear power sources, which have much longer mission capabilities than primary batteries. Primary battery technology cannot provide the necessary power within

Tumbleweed mass constraints to satisfy expected power requirements.

Although primary batteries are not viable solutions to Tumbleweed’s main power needs, they may be

useful during the landing sequence and/or deployment. The large size of Tumbleweed requires a deployment prior to or after landing on Mars. This deployment may require power and could be initiated by

primary batteries. These batteries could supply the required power for the short time segment and have

little impact on the overall mass of the Tumbleweed. Thus, although primary batteries are not recognized

as a chief power option, they may prove valuable for other vital aspects of the mission.
7

3.3.3. Radioisotope Thermophotovoltaic Systems

Radioisotope Thermophotovoltaic (RTPV) systems convert heat from General Purpose Heat Source

(GPHS) modules into electrical energy by using thermophotovoltaic cells. GPHS modules have been

used on radioisotope thermoelectric generators (RTG) for previous space missions including Galileo,

Cassini, and Ulysses and have served as a proven continuous power source independent of solar energy.

The Galileo RTG consisted of 18 GPHS modules designed for an electrical output of 285 W. GPHS

modules contain plutonium (Pu-238) fuel that releases heat upon the nuclear decay of the radioisotope.

This heat is converted to electrical energy for a total specific energy of over 5 W/kg.

Studies sponsored by the Department of Energy (DOE) have investigated the use of singular and fractional GPHS modules for the design of small RTPV generators. These RTPV systems would produce

20 W of electrical energy or less and could be a main power source for smaller spacecraft. Simply put,

the low power RTPV systems are scaled versions of RTGs. However, photovoltaic (PV) cells of the

RTPV system convert the infrared heat from nuclear decay into electrical energy as compared to the

thermocouples of RTGs. Another difference for RTPV systems is the need of larger radiator fins for a

lower cell temperature, which increases power output of PV cells; so a minimal cell temperature is

desired. However, larger radiators needed to produce these lower temperatures may greatly increase system mass.

RTPV systems may be easily integrated into the core of several tumbleweed designs because the

RTPV system power source is very compact. The inner core, which houses the radioisotope, as well as

necessary shielding, is a cube with 7.6-cm sides for the 10-W design. The large radiator fins necessary

for excess heat dissipation could potentially be incorporated into the Tumbleweed structure to save mass.

These experimental RTPV designs present power outputs that adequately address preliminary

Tumbleweed system requirements within an acceptable mass range and offer continuous long life power,

thus providing a potential viable power source for Tumbleweed. RTPV units are also more rugged and

resistant to damage than solar arrays. Disadvantages of RTPV systems include cost, difficulties associated with launching nuclear power sources into space, and a low technology readiness level of 5. However, future advancements within nuclear programs, such as NASA’s Prometheus project, may alleviate

some of these disadvantages.

3.3.4. Milliwatt Radioisotope Power

Milliwatt Radioisotope Power Sources use Radioisotope Heater Units (RHU) as a source of thermal

energy. The energy emitted from this small radioisotope source is converted to electrical energy by a

small thermoelectric generator. RHUs are very small and lightweight and produce 1 W of thermal

energy. They have been used in many spacecraft, including three aboard Mars Pathfinder Sojourner

rover, to provide localized heating to electronics and instruments. The future Mars Scout mission

PASCAL will use milliwatt radioisotope power to collect atmospheric data.

The experimental results examined the use of one and two RHUs per thermoelectric module. Efficiency and power output significantly increased with two RHUs, but increased thermoelectric properties

of Mars PASCAL provide greatest efficiency at the cost of added mass. These milliwatt radioisotope

power sources offer similar advantages to RPTV systems, including ruggedness and continuous power,

but power output may be lower than required. Selection of milliwatt radioisotope power sources will be

primarily dependent on payload power requirements.
8

3.3.5. Microthermoelectric Generators Systems

Similar to previously mentioned radioisotope power sources, microthermoelectric generators convert

thermal energy into electrical energy. A temperature difference between the hot and cold surfaces of the

thermoelectric generator causes a flow of current with power output proportional to the thermal gradient.

Due to the presence of thermal gradients on Mars at night, thermoelectric generators may be a sufficient

source of power for Tumbleweed. These thermal gradients exist near the surface (~3 m), lower than the

expected Tumbleweed diameter, and can reach greater than 20 °K.

A disadvantage of thermoelectric generators is that heat pipes would be required to create hot and cold

surfaces for maximum power output. These pipes must extend to the extremes of the thermal gradient

layers and may reach 3 m, increasing the power system mass. Furthermore, the thermal gradients may not

provide enough energy to exceed specific power output of other power sources. Additionally, the motion

of Tumbleweed must be controllable to position thermoelectric modules for highest efficiency. Purely

passive Tumbleweeds would greatly hinder the use of thermal gradients on Mars as a source of power.

Many units would be required, and the scalability and networking integrations would be costly.

3.3.6. Advanced Power Systems

Several advanced power generation systems were also investigated, including piezoelectric materials,

kinetic devices, flywheels, and wind turbines. Piezoelectric devices use kinetic motion to induce pressure

on lead zirconate titanate (PZT) films to produce electrical energy. While they display a promising

future, their current low power output and efficiency hinder near term use for a Tumbleweed power

source. Kinetic devices, such as kinetic dynamos found in certain flashlights and watches, would likewise

convert the kinetic motion of the Tumbleweed into useable electrical power. However, due to problems

associated with the magnetic fields they generate and issues pertaining to scalable technologies, kinetic

power systems do not appear feasible in the near term for Tumbleweed. Flywheels store kinetic energy in

a rotating disk and offer high specific power for energy storage; however, the uncontrolled nature of a

passive Tumbleweed would not provide adequate energy for efficient storage by flywheels. Wind turbines, including system implementation to ensure perpendicular wind flow to the turbine, would also

be costly. Since Tumbleweed motion is dependent on surface winds, the potential of using wind turbines

for power generation may also exist. However, the wind turbine would have to be independent of

Tumbleweed motion to enable power storage. Thus, while advanced technologies may provide power

generation while offering a mass savings, they are currently not flight proven and would require considerable development before they could be used on the Martian surface.

3.4. Communication

Communications for the Tumbleweed mission will rely on existing orbiting assets in Mars orbit(s) to

provide the relays necessary for communications (fig. 4). Using Mars relay assets, the Tumbleweed

communications will consist of Ultra High Frequency (UHF) communications from the surface vehicle(s)

to orbiting spacecraft and will use existing formats and protocols already established for the Mars

communications relays (ref. 5). The basic requirements trades for Tumbleweed communications are as

follows:

 Daylight communications versus nighttime communications

 Minutes per pass; passes per day or per week

 Data rate (raw telemetry versus processed telemetry)
9

 Mission duration

 Multiple coverage to multiple ground vehicles

 Surface location

 Surface vehicle antenna selection (e.g., patch, dipole, helical)

 Transmit power

Mars Ne tw ork Assets

Ground relay (?)

Tumbleweed(s)

DTE (?)

Supplied Asset

Baseline

Trade

Figure 4. Mars Tumbleweed communications concept.

Each Tumbleweed vehicle will use UHF communications to transmit data to the orbiting assets when a

communications opportunity exists. The use of an acknowledge and receipt protocol ensures that the data

will be transmitted when a Tumbleweed establishes a link to an orbiting asset. Basic characteristics

should be these:

 Radio Protocol: CCSDS Prox-1 Space link

 Full duplex, half duplex, simplex

Transmit

 437.1 MHz; transmit power of 1 to 10 W minimum

 Data rates: Support for 2, 8, 32 kbps, minimum

 Modulation: Uncoded convolutional (k = 7, R = 1/2)

 Electra transceiver compatible

Receive

 405.585625 MHz

 Data rates: 1 to 1024 kbps, mod 2 steps

 Modulation: k = 7, R = 1/2; Reed-Solomon (204/188 or 255/239)

 Electra transceiver compatible
10

3.5. Deployment Concepts

Five concepts for deployment of Tumbleweeds onto the Martian surface have been developed (figs. 5

and 6):

 Secondary payload on aeroshell

 Secondary payload on lander

 Secondary payload on cruise stage

 Primary payload on dedicated Tumbleweed mission—self landing

 Primary payload on dedicated Tumbleweed mission—airbag landing

The first three deployment options (fig. 5) are concepts whereby the Tumbleweed is a secondary payload, flown as a “piggyback” to a previously planned mission with available mass and volume margin. In

the first deployment concept (fig. 5(a)), a Tumbleweed rover is packaged as ballast on the backshell of the

entry system of a primary lander mission to provide a relatively inexpensive ride to Mars. In this scenario, the Tumbleweed is deployed at the proper time after the lander separates and is clear from the

backshell. One disadvantage to this approach is that the ballast location and mass requirement for the

primary mission may not match the mass properties and packaged volume of a Tumbleweed vehicle. The

next deployment option (fig. 5(b)) also presents a Tumbleweed vehicle carried as a secondary payload on

a lander mission; however, the Tumbleweed is attached to the lander itself rather than the backshell and is

deployed after a successful touchdown and activation of lander. As with the first concept, this option also

provides a relatively inexpensive “piggyback” ride to Mars; however, because of potential risks to the

primary mission from a deployable device attached to it, this option may not be an acceptable approach to

the Mars program. The third deployment option (fig. 5(c)) provides an option for a secondary payload on

the cruise stage. The Tumbleweed vehicles would be packaged in their own aeroshells attached to the

cruise stage, similar to the Deep Space 2 (DS-2) probes that flew with the Mars Polar Lander. The aeroshells containing the Tumbleweeds would be deployed from the cruise stage at the proper time following

separation of the primary lander from the cruise stage. This option allows some flexibility for deployment to a different location from the primary mission; however, it is also anticipated to be more expensive

than the previous two options due to the dedicated Tumbleweed aeroshells.

The last two options (fig. 6) present deployment scenarios in which Tumbleweeds are primary payloads rather than secondary “piggyback” payloads. This approach provides the advantage of deploying

multiple Tumbleweed rovers in particular locations at the particular time desired; however, these concepts

represent high cost missions due to dedication of the entire mission to a Tumbleweed payload. The first

of the primary payload options (fig. 6(a)) presents an option similar to figure 5(a), in which the Tumbleweed is deployed from the backshell; however, in this case, there is no primary lander payload in the

aeroshell, only Tumbleweed vehicles. This approach provides the option of being able to disperse a

group of Tumbleweeds over a large area while the backshell is descending. The second primary payload

option (fig. 6(b)) provides a concept with high heritage and proven success. The Tumbleweeds would be

packaged within an airbag landing system similar to that used for Pathfinder and the Mars Exploration

Rovers (MER). After successful landing and deflation of the airbags, Tumbleweed vehicles could then be

deployed one at a time when the wind conditions are favorable or blowing in a particular direction.
11

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12

(a) Primary payload—self landing.

(b) Primary payload airbag landing.

Figure 6. Primary payload deployment options.
13

4. Aerodynamic Testing

4.1. Overview of Testing Strategy

The overall strategy for aerodynamic testing of Tumbleweed concepts is threefold:

 Measure the aerodynamic properties of Tumbleweed concepts

- Static testing in free-stream flow

- Dynamic testing (rotating models) with surface effects

 Investigate boundary layer surface effects

- Static testing in an Atmospheric Boundary Layer (ABL) wind tunnel

 Examine concepts in a relevant Martian environment

- Testing in relevant temperatures, pressures, and atmospheric composition

The FY03 work encompassed static testing in free-stream flow and atmospheric boundary layers.

4.2. Wind Tunnel Test Results

4.2.1. North Carolina State University Subsonic Wind Tunnel

Undergraduate students from the North Carolina State University (NCSU) researched, designed, and

analyzed Tumbleweed rover concepts (fig. 7) as part of a senior aerospace spacecraft design class (ref. 6).

The NCSU team also designed and built a variety of Tumbleweed wind tunnel test models based on the

concepts (fig. 8).

Figure 7. NCSU Mars Tumbleweed concepts. Figure 8. NCSU wind tunnel models.

The models (20 to 23 cm in diameter) were tested in the NCSU Subsonic Wind Tunnel (fig. 9) at 3.8

to 8.3 m/s wind speed, in order to match the Reynolds number to those that would be experienced on

Mars (Re = 50000 to 150000 based on 6-m spherical Tumbleweed in Martian winds of 7 to 15 m/s). The

results showed that the Box Kite had the greatest drag coefficient (Cd) (an average of ~1.1) over the other

Tumbleweed concept models (fig. 10). However, it is important to note that angle-of-attack dependencies

were not examined in the NCSU tests. The Box Kite model was oriented with one disk perpendicular to

the flow, approximating the characteristics of a flat plate. Additional tests are necessary in order to

understand how different orientations may change these results.

Tumbl Box
14

Figure 9. NCSU Tumblecup model in test section of NCSU Subsonic Wind Tunnel.

Reynolds number

Cd

0 50000 100000 150000 200000 0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

Baseline sphere

Tumble cups

Box Kite

Wedges

Dandelion

Figure 10. NCSU Subsonic Wind Tunnel results.
15

4.2.2. NASA LaRC Basic Aerodynamic Research Tunnel (BART)

The main objective of the LaRC Basic Aerodynamic Research Tunnel (BART) testing was to provide

additional preliminary data on the aerodynamic characteristics of the various Tumbleweed rover concepts

in a static free-stream flow environment for input to the dynamic simulations. An additional objective of

the testing was to assist with the down-selection of Tumbleweed concepts for further development, testing, and analysis. As with the NCSU tests, the LaRC tests did not seek to duplicate a Mars relevant environment, but instead to obtain measurements at Reynolds numbers similar to those that would be experienced on Mars (~50000 to ~125000). To accomplish these objectives, two test methodologies were used:

sting-mounted force measurements and smoke visualization. The goal of the sting-mounted tests was to

acquire numerical data on the basic aerodynamic properties (lift, drag, moments of inertia, and so on) of

the Tumbleweed concepts. However, to complement these measurements, smoke visualization was used

to discern whether any particular concept created visible disturbances to the airflow around it that could

then be attributed to better drag and aerodynamic qualities.

Four Tumbleweed concepts were tested in the LaRC BART: a “Wedges” model made of Styrofoam

balls attached to a central spherical core; a “Box Kite” model using a commercially available watercraft

radar reflector; and “Tumblecup” and “Dandelion” models made of stereolithography parts that could be

assembled in 12, 24, or 36 cup/stem configurations. A 4-in. diameter sphere (the same core used for the

Wedges, Tumblecup, and Dandelion models) was also tested to provide a baseline measurement of the

well-defined aerodynamic properties of a sphere. A medium sized tumbleweed plant, roughly 14 to 20 in.

in diameter, was purchased from the Kansas Prairie Tumbleweed Farm in Garden City, KS and also tested

in the BART to study the aerodynamic properties from a biomimetics perspective (fig. 11).

Figure 11. Tumblecup (top left), Tumbleweed (top right), Box Kite (lower left), and Dandelion (lower right) wind

tunnel models in BART.
16

Several critical goals were defined for the stereolithography model design process. First, the models

had to take the shape of the previously designed Tumbleweed concepts, specifically those of the

Tumblecup and Dandelion. Second, the models had to be modular to allow for multiple configurations of

both concepts as well as for disassembly for easy transport and storage. Third, the final assembled designs were each to be 12 in. in diameter so that comparative drag data could be obtained. All these goals

were met. The NASA LaRC Stereolithography Lab was tasked to produce the model parts, including

three central cores (one with 12 holes, one with 24, and the third with 36, outfitted with threaded inserts),

the cup attachments, and the dandelion petals. The connections between these elements were accomplished through use of nuts, bolts, and threaded rod cut to appropriate lengths. Thus, the testing of three

differing configurations of both Dandelion and Tumblecup models was enabled.

The Wedges design also benefited from this stereolithography work as the Styrofoamballs were

attached with the same threaded rod used for the Dandelion models to the 12-hole center core. A small

watercraft radar reflector was used for the Box Kite model and included the added benefit of already

including a sting adapter attachment mechanism. Finally, it was decided that because the various concepts were modeled after the Tumbleweed, the actual Tumbleweed plant should hold a distinct place in

the testing as well.

The BART is an open-circuit, closed-test section tunnel located in Building 1214 at LaRC. The test

section has dimensions of 28 in. high by 40 in. wide by 10 ft long and has a maximum velocity of

185 ft/s, which translates to a Re/ft of 1.14 M. Furthermore, the turbulence intensity ranges from

0.04 percent to 0.09 percent. There are a variety of flow visualization techniques available from relatively simple smoke flow (that the Tumbleweed Team used) to off-body visualization using a laser light

sheet. Additional capabilities include transition detection by means of sublimating chemicals, pressure

determination through the Electronic Scanning Pressure System (ESP), aerodynamic loads with one of the

many internal strain gauge balances, as well as laser velocimetry and digital stereo particle image velocimetry. The BART Facility Coordinator and lead technician is Mr. Richard White and the Facility

Research lead is Mr. Luther Jenkins (ref. 7).

The balance chosen for the tests was strain gauge balance 733. The BART personnel had initially

suggested the HH19a because it is a lighter and more sensitive model; however, because all test models

exceeded the moment arm limit of the HH19a balance, we used the 733. The characteristics of the 733

balance are displayed in appendix B. In addition, the moment center is 0.673 in. behind the centerline of

the forward dowel pin. The hole in which this dowel pin is inserted can be seen in appendix B as part of

the sting attachment piece that was machined for the testing. The balance excitation is 5.000 V and the

delta W is 2.2706E-2 lb. This balance was last calibrated on July 9, 2002, by Engineer McWithey

(ref. 8).

A test matrix was developed for both the smoke visualization and the balance tests. See table 3 for the

final order and characteristics of the tests. The 12-in-diameter sphere used in the smoke tests was not

used in the balance tests because its mass exceeded the force limits on the balance. Instead, the 4-indiameter sphere was used as the baseline for the balance tests. Initially, changes in the angle-of-attack

(AOA) sweep resulted from trouble with the LabViewsoftware being used to control the arc sector.

Mr. White consulted another experienced programmer and the anomaly was resolved, allowing the full

range, 5° to +30°, to be achieved.
17

Table 3. BART Test Matrix

Run no. Model Test type Velocities tested, ft/s AOA sweep, deg

 1 12-in. sphere Smoke 11, 20, 40

 2 36 Dandelion Smoke 11, 20, 40

 3 12 Tumblecup Smoke 11, 20, 40

 4 24 Dandelion Smoke 11, 20, 40

 5 12 Dandelion Smoke 11, 20, 40

 6 36 Tumblecup Smoke 11, 20, 40

 7 12 Dandelion inverted petals Smoke 11, 20, 40

 8 24 Dandelion inverted petals Smoke 11, 20, 40

 9 Box Kite on floor Smoke 11, 20, 40

10 Box Kite raised with plate parallel to flow Smoke 11, 20, 40

11 Box Kite raised with section parallel to flow Smoke 11, 20, 40

12 Box Kite with center hole covered Smoke 11, 20, 40

13 12 Dande-cup Smoke 11, 20, 40

14 4-in. sphere Smoke 11, 20, 40

15 Wedges Smoke 11, 20, 40

16 36 Dandelion inverted petals Smoke 11, 20, 40

17 Tumbleweed plant Smoke 11, 20, 40

18 4-in. sphere Balance 20, 40 5 to 10

19 36 Dandelion Balance 20, 40 5 to 10

20 12 Tumblecup Balance 20, 40 5 to 12

21 24 Tumblecup Balance 20, 40 5 to 12

22 12 Dandelion Balance 20, 40 5 to 30

23 Box Kite with center hole covered Balance 20, 40 5 to 30

24 36 Tumblecup Balance 20, 40 5 to 30

25 24 Dandelion Balance 20, 40 5 to 30

26 Wedges Balance 20, 40 5 to 30

27 24 Dande-straw-stem Balance 20, 40 5 to 30

28 12 Tumblecup Balance 20, 40 5 to 30

29 Box Kite with center hole open Balance 20, 40 5 to 30

30 Tumbleweed plant Balance 20, 40 5 to 30

31 4-in. sphere Balance 20, 40 5 to 30

The test procedures varied for the smoke visualization and the balance tests; however, both began with

integration of the appropriate model for the first run, followed by installation of the model into the test

section by the BART technician. During the smoke flow visualization tests, a standard theatrical smoke/

fogger machine was positioned at the front of the tunnel before the first screen. The tunnel fans were then

turned on and brought up to speed, the smoke machine was activated, and finally the two video cameras

focusing on the test section started recording. As the recording commenced, the time stamp displayed by

the cameras was manually noted to aid post-test data reduction. After approximately 30 s had elapsed at

one wind velocity, Mr. White was signaled to increase the fan speed to the next desired test point and the

time stamp on the cameras was again noted. Throughout this process, still photographs were taken of the

models from different angles with a digital camera, while in the control room, the next model was being

configured for the next test run. After all desired velocities were achieved and videotaped, the fan was
18

slowed and stopped, the model was removed from the test section, the next model was installed, and the

process was repeated. Time was allotted between test runs to allow for the smoke to clear from the

BART facility, providing a better video and photographic environment for the next test run.

The smoke visualization tests provided a unique opportunity to view the flow over the Tumbleweed

concept models. Several phenomena were observed, including flow separation off various Tumbleweed

structures, which was particularly apparent with all the Dandelion models and the 12 Tumblecup model

(fig. 12(a)). Boundary layers were evident across a number of the models, as a darker region would form

between the outer edge of the model and the smoke flow (fig. 12(b)). The Box Kite model exhibited

some particularly interesting flow dynamics, one image of which is seen in figure 13(a). Smoke was seen

turning at nearly right angles as it flowed into a quadrant of the model, impacted a plate, and then turned

to escape the corner and continue downstream. The Tumbleweed plant also provided an intriguing result

(fig. 13(b)) as it appeared to catch most of the smoke and then allowed small, low velocity “puffs” to

emerge from the backside of the plant.

(a) 12 cup Tumblecup, 11 fps. (b) 36 cup Tumblecup, 20 fps.

Figure 12. Tumblecup smoke flow visualization.

(a) Box Kite, 20 fps. (b) Tumbleweed plant, 11 fps.

Figure 13. Box Kite and Tumbleweed plant smoke flow visualization.
19

Test section

floor

Tumbleweed model

Rotating

arc

sector

α

Figure 14. Test configuration for force measurements.

After all smoke visualization test runs were completed, the team stood-down for approximately onehalf day to allow for installation of the arc sector, which provides the capability for the sting-mounted

models to be swept through a range of AOA. A diagram of the arc sector-balance-sting setup within the

test section is seen in figure 14. The symbol alpha () denotes the model AOA.

Once the appropriate model was installed in the tunnel, a “tare” (zeroing of the balance) was conducted by sweeping the arc through all AOA tests without the fan operating to create loads, and by taking

a reading to establish the starting or zero point. The model was then returned to 0° AOA and the tunnel’s

fan was brought up to speed. As soon as the flow appeared stable at the appropriate velocity, the arc sector began the sweep of AOA starting at 5° and ending at 30°, in 1° increments from 5° to 10°, and in 2°

increments from 10° to 30°. After a full AOA sweep was conducted, a second and third sweep was conducted to gather three full sets of data that could be averaged. When the third sweep at the first velocity

was complete, the fan speed was increased to the second velocity and the procedure of three AOA sweeps

was repeated. When all six sets of data for a single model were gathered, the fan was decelerated and

stopped, the model in the test section was removed, and the next model was installed.

The sting-mounted tests led to several definitive results. The values of Cd, as well as the smoothness

of the data collected, varied with wind speed (and thus Reynolds number). The flow through BART was

very unsettled at 20 ft/s, but smoothed considerably at 40 ft/s. The majority of the Tumbleweed concepts

tested have drag characteristics that are independent of the AOA, based on the plots of drag coefficient

(Cd) versus AOA, as seen in figures 15 and 16. The main exception to this finding is the Box Kite, which

is greatly AOA dependent as illustrated by the highly sloped line in both figures. However, the Cd for

Box Kite is higher than those of the other concepts for over a significant portion of the tested AOA and

has a Cd in the range of the other concepts throughout the rest of the AOA. A rank ordering of the concepts was completed using the measured Cd values with the Box Kite being first, followed closely by the

24 and 36 Tumblecups, then the 12 Tumblecup, along with the Dandelion concepts and the Wedges concept falling last, just slightly above the 4-in. sphere. Also interesting to note from figures 15 and 16 is

that most of the Tumbleweed concepts have Cd values very close to each other. Additionally, the ranking

of a particular concept remained basically unchanged, regardless of the wind speed; however, only a limited number of wind speeds were run (20 ft/s and 40 ft/s). The drag coefficients (Cd) were calculated by

using the projected area of the models, which is not necessarily the most appropriate method; follow-on

data reduction efforts in FY04 will examine several alternative methods of calculating the Cd.
20

BART Test 74

20 ft/s

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

-5 0 5 10 15 20 25

Angle of attack, deg.

Tumblecup 12 Tumblecup 24 Tumblecup 36

Dandelion 12 Dandelion 24 Dandelion 36

Wedge Box Kite 4-in. sphere

Box Kite

(1.10)

Wedge 

(0.63)

4-in. sphere 

(0.59)

Dandelion 12

(0.92)

Dandelion 24

(1.04)

Tumblecup 36

 (1.15)

Tumblecup 24 

(1.24)

Tumblecup 12

(1.06)

Drag coefficient

Figure 15. Tumbleweed test results at 20 fps (Re = 128000).

BART Test 74

40 ft/s

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

-5 0 5 10 15 20 25

Angle of attack, deg.

Drag coefficient

Tumblecup 12 Tumblecup 24 Tumblecup 36

Dandelion 12 Dandelion 24 Dandelion 36

Wedge Box Kite 4-in. sphere

Box Kite

Wedge 

4-in. sphere (0.54)

(0.46)

Dandelion 12

(0.81)

Dandelion 24

(0.91)

Dandelion 36 

(0.91)

Tumblecup 36 

Tumblecup 24 

(1.02)

Tumblecup 12

(0.87)

Figure 16. Tumbleweed test results at 40 fps (Re = 256000).
21

(a) 24 stem Dandelion at 0° AOA. (b) Wedges at positive AOA.

Figure 17. Sting-based tests.

Figure 17 shows the test setup for the sting-based tests, with a 0° AOA case (fig. 17(a)) and a positive

AOA pitch (fig. 17(b)).

Therefore, based on the data obtained, reduced, and analyzed from the Tumbleweed wind tunnel testing conducted in the BART, several conclusions and recommendations for future testing opportunities can

be made. First, the baseline 4-in. sphere should be a part of each test conducted in the future, regardless

of the venue, to allow for baseline comparisons between datasets. Second, the “Wedges” concept, as it

was tested, needs to be dismissed from consideration for the final Tumbleweed design due to poor aerodynamic performance. Third, the top three models for continued testing are the Box Kite, the 24 Tumblecup, and the 24 Dandelion (with inverted pads). As discussed above, each of these models exhibited

unique and intriguing phenomenon and appear to be the best aerodynamically suited concepts. These

three models, configured just as they were for the FY03 BART tests, should be the subjects of future

tests: (1) to determine behavior in the desired Reynolds number environment (~25000 to ~125000); (2) in

a simulated Mars atmospheric boundary layer (ABL); and (3) in a Mars relevant environment that simulates the density, pressure, and temperature on Mars. Further tests of these models in a facility such as the

BART would be best suited to configuration refinement with some suggested areas of study shown below. However, the following list of testing options is neither exhaustive of all possibilities nor firmly set:

 Box Kite model

- Open and closed center hole

- Recessed sails

- Additional structural rings

 Tumblecup 24 model

- Varying cup sizes

- Varying cup/cone angles (wider cups versus narrower cups)

 Dandelion 24 inverted model

- Smaller radius of curvature on inverted petals

- Larger diameter petals

- 36 stems (rather than 24)
22

Fourth, the remaining models that were investigated in the FY03 BART testing (the 24 Dandelion

Straw Stem, the 12 Tumblecup, the 36 Dandelion, and the 24 Dandelion) should not be completely eliminated from consideration for the final Tumbleweed design as the specific science or instrumentation goals

of the mission may require or best be suited to one of these designs.

Several concerns were raised during and after the wind tunnel testing concluded. First, the wake rake

placed in the test section downstream of the model may have been too far behind the trailing edge of the

model to accurately record the differences in the flow characteristics between the different Tumbleweed

concepts. Despite reassurances that the overall turbulence, size, and speed are not affected by the downstream distance, it would be interesting to compare these data with those resulting from the rake being

placed within 0.33 m of the rear of the models to determine whether individual flow characteristics of

each model type could be observed. Second, there was a concern regarding the steadiness of the flow

through the BART test section at lower speeds. Testing at the desired Mars relevant Reynolds numbers

(~25000 to 125000) could not be performed in the BART using the current models and the 733 balance.

Additional tests are planned in the BART for FY04 which may be able to achieve the desired Reynolds

numbers by using smaller, lightweight models and a more sensitive balance.

4.2.3. Texas Technical University Boundary Layer Wind Tunnel

Dr. Alan Barhorst and Dr. Darryl James of the Texas Technical University (TTU) are preparing to test

Tumbleweed concepts in a simulated Mars ABL based on data from the Mars-GRAM 2001 atmospheric

model (fig. 18). The testing will determine the drag coefficient of various Tumbleweed concepts in

the ABL for comparison with drag coefficients obtained from a uniform flow. Uniform flow tests are

currently being conducted on TTU Tumbleweed concepts (fig. 19), with the ABL tests planned for completion in the spring/summer semesters of 2004.

Grid, 2-ft fence, spire test 1

y = 0.0086e 11.288x

R2 = 0.9226

0

50

100

150

200

250

300

350

400

450

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Velocity, m/s

Height, mm

Figure 18. Predicted parabolic boundary layer profile scaled to earth test conditions.
23

(a) TTU Tumbleweed uniform flow test setup. (b) Smoke flow visualization.

Figure 19. TTU Tumbleweed wind tunnel tests.

4.3. Summary

The aerodynamic testing opportunities for FY03 provided valuable data to the LaRC Tumbleweed

team. The NCSU tests provided data for similar Tumbleweed designs to those tested by the LaRC team,

at lower Reynolds numbers. Additionally, while the testing planned by Texas Tech University in the

ABL tunnel was delayed, the analytical work matching the Mars boundary layer profile was completed in

preparation for testing in FY04. The LaRC BART tests provided the team with a greatly enhanced understanding of the basic aerodynamic characteristics of the main Tumbleweed concepts under consideration

as well as considerable experience in designing and running a test. As a result of this work, the direction

of future testing is being defined and an additional entry into the BART has been secured for summer

FY04.

5. Dynamic Analysis and Testing

5.1. Summary of Analysis Plan

Approach: Develop simplified math models to continue investigation of key dynamic characteristics of

the Tumbleweed rover. Dynamics simulation development will be split into two separate efforts. One

effort will focus on the development and validation of the basic physics-based models of rolling, sliding,

and bouncing dynamics of a generic flexible Tumbleweed structure. The other effort will focus on development of simulation tool architectures for providing end-to-end Monte Carlo dynamics simulations

using representative Martian terrain. These two efforts will serve as a basis for future development of

Tumbleweed 6 DOF Monte Carlo simulator used to model motion over simulated Mars terrain.

5.2. Continuation of Feasibility Studies

Additional feasibility studies were performed that built on the results of the initial feasibility studies

performed in FY 2002 (ref. 1). The studies performed continued to explore the necessary Tumbleweed

rover properties for impending motion in a variety of worst-case situations. In FY 2003 there were two

situations that these additional studies focused on: static analysis of a Tumbleweed rover trapped on a

rocky slope and a simplified dynamics analysis of a Tumbleweed rover impacting a rock. These feasibility studies have helped build a fundamental understanding of the problem and guide future analysis
24

efforts. Additionally, they indicate, from a basic physics standpoint, that the Tumbleweed rover concept

is feasible and thus continues to be a worthwhile research venture.

5.2.1. Static Analysis of Tumbleweed Rover Trapped on Rocky Slope

On the Martian surface, wind speeds and directions vary constantly. Wind speeds vary from 2 to

>25 m/s depending on the season, time of day, and weather conditions. Winds typically cease all together

at night. Consider a Tumbleweed rover traveling along and being pushed by a 7-m/s afternoon breeze.

The Sun will eventually set, the winds will die down, and the rover will probably stop moving until the

winds pick up again the next day. As a worst-case scenario, the Tumbleweed could possibly stop on a

slope and become lodged between several rocks. What wind speed will then be required to nudge that

Tumbleweed out of its resting place? Is a wind of this speed one the Tumbleweed is likely to encounter?

What slopes and rock sizes can a particular Tumbleweed rover overcome? Will the Tumbleweed rover be

able to continue its mission up the slope after becoming trapped as described previously? This feasibility

study attempts to answer these questions.

Figure 20. Free body diagram of Tumbleweed rover trapped on sloped surface.

Figure 20 shows a free body diagram of a Tumbleweed rover trapped on a slope of angle S. The

Tumbleweed rover is assumed to be a nonrigid sphere. At rest with zero wind, the Tumbleweed’s weight

would act entirely through point G on the slope face. As the wind force (Fw) exerted on the Tumbleweed

increases, the weight that was exclusively distributed through point G at rest begins to shift over to point

P on the rock face. This motion continues through increasing wind velocities (U) until finally the Tumbleweed’s entire weight is distributed through point P. At this point in the transition, any increase in U

will enable the Tumbleweed to overcome the rock and continue upslope. Therefore, when summing

moments about point P, the following equation defines when Fw is sufficient to push the rover past the

upslope rock:

MFW> MW + MNO (1)

MFw is the moment about point P exerted on the Tumbleweed as a result of the wind force Fw and is

defined by

MFW = CDArefqsin 90 ( )  R 

 ( ) Rt  c (2)
25

where CD is the drag coefficient of a given Tumbleweed vehicle, Aref is the cross-sectional area, R is the

rock angle, Rt is the radius of the Tumbleweed, c is the deformation distance of the rover’s structure

under loading (given by 2μr

2Rt where μr is the coefficient of rolling resistance defined by the ratio a/Rt

(ref. 1)), and q is the dynamic pressure, given by

q = 1

2

U

2 (3)

with being the atmospheric density and U the free-stream velocity.

MW is the moment about point P pulling the Tumbleweed downslope and is defined by

MW = WX ( ) Rt  c (4)

where WX is the Tumbleweed weight component in the X direction given by W sin( ) R + S with R as

the rock angle and S as the slope angle. For typical aircraft tires, ranges for μr can be anywhere from

0.02 to 0.3, depending on the flexibility of the tire and the rolling surface conditions (ref. 9). The range

considered in this study was 0 to 0.15.

For this analysis, it is assumed that the wind force acts through the Tumbleweed’s center of gravity.

In reality, the center of pressure would be offset somewhat due to boundary layer effects. However, presently the significance of these boundary layer effects is unclear and is something that future analyses will

need to address. An effort is being made to characterize the Martian boundary layer, and is discussed in

section 4.2.3.

MNO is the moment about point P caused by the flexible Tumbleweed’s resistance to rolling and can

be accounted for by assuming an offset normal force.

MNO = NOa (5)

NO is the offset normal force, which is contributed to by the Y direction components of the Tumbleweed

weight and wind force. It then follows that this offset normal force is given by

NO = W cos( ) R + S 

 + CD Arefq cos( ) R + S   (6)

The distance a is the amount of normal force offset from the contact point P and is given by μrRt.

Figures 21 and 22 show wind speed versus rock height for a particular Tumbleweed rover configuration on flat ground and on a 30° slope, respectively. The Tumbleweed’s radius was varied from 1 to 4 m,

while the mass, drag coefficient, and coefficient of rolling resistance were all held constant. However,

any of the Tumbleweed design parameters described previously can be varied within the analysis codes

developed for this study. The drag coefficient of 1.0 was selected based on values obtained from the wind

tunnel testing discussed in section 4.

From figure 21, consider a 3-m radius Tumbleweed. On flat ground, a 16-m/s wind will be required to

overcome a 1.0-m high rock. As expected, for a 4-m radius Tumbleweed, a slower wind, roughly 10 m/s,

will be needed to overcome the same 1.0-m rock. We also see that the 1- and 2-m radius Tumbleweeds

are at best only capable of overcoming relatively small rocks at very high wind speeds.
26

Figure 21. Wind speed versus rock height for Tumbleweed on flat ground; Cd = 1, Mt = 110kg, Cr = 0.15.

Figure 22. Wind speed versus rock height for Tumbleweed on sloped surface; S = 30°, Cd = 1, Mt = 10kg,

Cr = 0.15.
27

Now consider the same 3- and 4-m Tumbleweeds from figure 22, where there is now a 30° slope to

overcome as well. For lower wind speeds, the 3- and 4-m Tumbleweeds now require a 17-m/s and 12-m/s

second wind, respectively, to overcome that same 1.0-m high rock. However, by comparing these two

charts, one notices that the wind speed requirements for conquering a rock on flat ground and conquering

a rock on a sloped surface become similar as the velocity increases. In other words, the 4-m Tumbleweed, regardless of whether it is on a flat or sloped surface, requires a 20-m/s wind to overcome a 2.5-m

high rock. The same trend is apparent for the 3-m Tumbleweed and also when other design parameters

such as mass and drag coefficient are varied. This result implies that, for slopes 30°, as the wind speed

approaches 20 m/s the moment created by the wind force becomes dominant, eventually negating the

effects of gravity pulling the Tumbleweed back down the slope. For slopes >30° this effect would eventually surface as well, but at higher wind speeds.

In conclusion, this study shows that Tumbleweed rovers of reasonable size and mass can overcome

substantial rock impedances on relatively extreme slopes. A 30o slope on Mars is not common over long

distances and is a very conservative estimate of a worst-case condition. Similarly, the large rocks considered in this study (1.0 to 2.5 m) are not common features on the Martian surface and would also be very

conservative, worst-case condition assumptions. The wind speeds required to dislodge a Tumbleweed

rover in this situation (approximately 10 to 20 m/s, depending on the slope angle, rock size, and Tumbleweed configuration) are not common. However, they are not unheard of. Wind speeds average from 2 to

5 m/s during the day with relatively strong gusts of 10 to 20 m/s. Seasonal dust storms can produce winds

exceeding 25 m/s; therefore, a lodged Tumbleweed rover may have to wait for a sufficiently strong wind

to push it out of its entrapment, but across most areas of Mars it will be hard pressed to render itself

permanently immobile.

5.2.2. Simplified Dynamic Analysis of a Tumbleweed Rover Impacting a Rock

An additional feasibility study was undertaken to examine, on a fundamental preliminary level, the

dynamics of a Tumbleweed rover impacting a rock. Consider a Tumbleweed rover rolling on a flat surface at a given velocity exposed to a constant wind. Somewhere along the way, the Tumbleweed hits a

rock and bounces up into the free stream. What is the significance of wind pushing the Tumbleweed

downrange compared to the same scenario in a zero wind environment? What pre-impact velocity is

required to enable the rover to bounce up and over the rock and not back and in the opposite direction

from which it came? This feasibility study attempts to answer these questions.

Figure 23 shows the free body diagram of a Tumbleweed rover impacting a rock. The Tumbleweed

and the rock are assumed to be rigid structures. Collisions between the Tumbleweed and the rock and the

Tumbleweed and the ground were modeled as “billiard ball” collisions with assumed coefficients of restitution. Before impact with the rock, the Tumbleweed has some initial velocity Vi. After impact with the

rock, the Tumbleweed will have some velocity Vf in the direction T from the horizontal. The rock is

fixed to the ground and does not acquire any velocity after the collision. The initial velocity vector can be

broken up into components that are parallel (t direction) and normal (n direction) to the point of impact P.

It then follows that the post and pre-impact velocities are related as follows:

Vft =Vi

t (7)

Vfn = eVin (8)
28

Figure 23. Free body diagram of Tumbleweed rover impacting rock.

Vft and Vi

t are the post- and pre-impact velocities of the Tumbleweed in the tangential (t) direction,

respectively. Vfn and Vin are the post- and pre-impact velocities of the Tumbleweed in the normal (n)

direction. The coefficient of restitution is defined by e and is arbitrarily assumed to be 0.7 for both rock

and ground Tumbleweed impacts. It is necessary to note that assumptions regarding structural properties

of the Tumbleweed, such as flexibility and coefficient of restitution, are arbitrary at this point in the project. Further development of the rover’s structural design parameters, such as mass, size, and construction materials, will be required to move to higher fidelity dynamics analyses.

The final, post-impact velocity of the Tumbleweed is then given by

Vf = Vft

2 +Vfn2 (9)

and the post-impact trajectory angle T is given by

T = R + tan1 Vfn

Vft















 (10)

where R is the rock angle defining the size of the rock. From this final velocity and trajectory angle, it is

then possible to compute the Tumbleweed’s post-impact, normal projectile trajectory. Going one step

further, the contribution of the free-stream wind to the Tumbleweed’s downrange motion can also be

approximated by accounting for the relative wind velocity acting on the Tumbleweed.

Figure 24 shows the trajectory of a particular Tumbleweed configuration impacting a rock defined by

a 30o rock angle (R), which translates to a rock approximately 0.4 m high. The red plot shows the postimpact trajectory of the Tumbleweed if there were no wind contributing to downrange motion, the motion

of a normal projectile. The green plot shows the post-impact trajectory of the Tumbleweed accounting

for the downrange contribution of the free-stream wind push. In a 6-m/s wind, the wind contribution is

significant, doubling the achieved downrange motion in the same number of bounces.
29

Figure 24. Tumbleweed trajectory after rigid body collision with a rock. RT = 3m, m = 10kg, Cd = 0.5, e = 0.7,

R = 30°, Vi = 2.5 m/s, U = 6 m/s.

Figure 24 also shows that the initial velocity of the Tumbleweed was sufficient enough to allow the

Tumbleweed to bounce high enough, approximately 0.45 m, to clear the top of the 0.4-m rock. Additionally, it can be seen that the trajectory angle, T, is less than 90°, which maintains the forward momentum

of the Tumbleweed, enabling it to progress over the rock instead of bouncing off in the opposite direction

from which it originally came. From this simple analysis, rough approximations can be made for initial

Tumbleweed velocity requirements necessary to dynamically conquer rocks of various sizes.

5.3. Commercial off the Shelf (COTS) Analysis Tool Study

In addition to the ongoing development of the 2-D simulation test bed, several COTS software packages were studied to evaluate their dynamics analysis capabilities and potential application to the

Tumbleweed dynamics problem. More specifically, the main objective of the study was to evaluate the

fidelity of each COTS package when it was used for modeling sphere-to-surface interactions to simulate a

Mars Tumbleweed rover. If a capable COTS software package were found, it would be a valuable contribution to the ongoing development of the in-house, 2-Dimensional Simulation Test Bed, as well as to the

dynamics analysis activity as a whole, by serving as an additional independent modeling tool. The three

software packages that became the focus of the study were Maya(Alias/Wavefront), ADAMS(MSC

Software), and Vortex(CM-Labs).

Due to the nonlinearities and complexities of the full-scale simulation of a Tumbleweed rover in

motion, it is difficult to determine the accuracy of any analysis output directly. Instead, several test cases

that have well-known and easy-to-determine analytical solutions were designed (e.g., translation, rotation,

friction, gravity, and so on). These test cases were then assembled within each software package, and

their output results were compared to the known analytical solutions. While passing each of these tests

does not ensure fidelity in the full-scale simulation, failing one or more of the tests clearly indicates

weaknesses in the tool’s analysis abilities. A basic description of these test cases follows, along with the

associated requirement for each.
30

Translation

The translational motion of an object must obey

F = ma (11)

where F is the force acting on the Tumbleweed, m is mass, and a is acceleration.

Rotation

The rotation motion of an object must obey

T = I (12)

where T is torque acting on the Tumbleweed, I is the mass moment of inertia, and  is the angular

acceleration.

Friction

It must be possible to model static and dynamic friction. Static friction must allow specification of a

constant force that must be overcome before sliding motion starts. Dynamic friction must obey

FF = μN (13)

where FF is the friction force, μ is the coefficient of friction, and N is the normal force. For a rolling

body it must be possible to specify an initial rolling resistance, which must be overcome before the body

will move, and a dynamic rolling resistance separate from the dynamic friction that opposes the sliding of

the rolling body.

It is necessary to note here that dynamic friction is not the same as rolling resistance. Dynamic friction is the result of one surface moving over another and does not occur for a body rolling without slipping. Rolling resistance arises due to other effects, mainly flexibility of the rolling object, which leads to

deformations and energy dissipation. Static friction does not prevent a rolling body from moving; it

causes it to roll without slipping.

Gravity

It must be possible to model the effects of gravity and to be able to specify the field strength to that of

Mars.

Wind

It must be possible to model a constant wind force and to be able to introduce random gusts. The wind

force of an object must obey

F = CDAref q (14)

Recalling equation (3) from section 5.2.1, it is important to note that for a moving Tumbleweed, the freestream velocity equates to the relative velocity between the wind and the Tumbleweed.

Collisions

It must be possible to model the collision of a moving object with a static object of arbitrary shape. It

must also be possible to specify the coefficient of restitution between two surfaces.
31

Terrain

It must be possible to import a representation of terrain via a terrain database and model collisions of a

moving object with the imported terrain.

Summary of Activity and Results

Table 4 summarizes the results of the COTS analysis tool study discussed in this section. Each entry

labeled “Pass” indicates that the test results agreed with the known analytical solutions and that little or

no special measures were needed to make the software work properly. Entries labeled “Fail” indicate that

the software was incapable of reproducing the known analytical solutions or that it was impractical to

model the problem correctly within the tool for that particular test case. Entries labeled “Marginal” indicate that the software only worked on some levels for a particular test case.

Table 4. COTS Tool Analysis Results

Translation Rotation Friction Gravity Wind Collisions Terrain Slide-to-roll

MAYA Pass Fail Pass Pass Fail Marginal Pass N/A

ADAMS Pass Pass Pass Pass Pass Fail Marginal Pass

Vortex Pass Pass Marginal Pass Pass Pass Pass Pass

MAYA Shortcomings

 Rotation test: Fail

Moments of inertia cannot be specified. Furthermore, the default values for an object’s moments

of inertia are very inaccurate. For a uniform density sphere and a thin walled hollow sphere, the

MAYA default moments of inertia are off by factors of 3.36 and 2.0, respectively.

 Wind test: Fail

The net force acting on the object is treated as being linearly proportional to the relative velocity

of the object. This statement is incorrect. The net force of the wind acting on an object should be

proportional to the square of the relative velocity.

 Collisions test: Marginal

Coefficient of restitution not properly defined.

ADAMS Shortcomings

 Collisions test: Fail

Proper trajectory of a bouncing ball with a specified coefficient of restitution could not be

reproduced.

 Terrain test: Marginal

Without the ADAMS/Exchange module, the ADAMS/Solver will only except computer-aided

design (CAD) files of a Parasolid format (from Unigraphics). In addition, ADAMS exhibited

some instabilities regarding certain Parasolid files.

Vortex Shortcomings

 Friction test: Marginal

Vortex only uses a single coefficient of friction and does not model static friction separately from

dynamic friction.

As a result of this study, for testing purposes, the Vortex software package was identified as a good

choice for analysis of the Tumbleweed rover dynamics problem. A developmental analysis tool was
32

developed around the Vortex software and is discussed in section 5.4.1. However, it should be noted

that since the development of the Vortex application, several prior issues with the ADAMS software

have been resolved. For various reasons the ADAMS software could provide a high fidelity analysis

capability. See Mars Tumbleweed Rover COTS Analysis Report (ref. 10) for further details regarding the

COTS analysis tool study.

5.3.1. Evaluation of Mars Tumbleweed Rover Monte Carlo Simulator

Using Visual C++ 6.0, a wrapper application called the Mars Tumbleweed Monte Carlo Simulator, or

just Tumbleweed simulator for short, was created to drive the Vortex simulation software. The current

developmental version of the Tumbleweed simulator allows the user to perform preliminary Monte Carlo

simulations for the purpose of evaluating the statistical distributions of Tumbleweed rovers over a Mars

relevant terrain. In other words, a virtual environment can be created within the tool to examine the performance of various Tumbleweed rover configurations.

A comprehensive evaluation of the developmental Tumbleweed simulator was performed to determine

the current analysis capabilities of the tool and to identify areas for future improvement. Several different

Tumbleweed rover configurations and a Mars relevant terrain were modeled within the Tumbleweed

simulator. The evaluation was structured to answer three basic questions. Each question is addressed

below and is accompanied by the relevant identified improvement areas. Improvement of the tool is a

goal of the FY2004 activities.

Question 1. Can a realistic Mars environment be modeled within the simulator?

The Tumbleweed simulator allows the user to define terrain size, rectangular rock dimensions for an

upper bound large rock and a lower bound small rock, the number of rocks at each boundary that will be

included on the terrain, and a number of rectangular rocks sized randomly in between the specified upper

and lower bounds. The current version of the simulator includes one other terrain option that allows the

user to enter in a specified terrain profile, which in general would consist of a 2-D array of vertical

heights. This option could be used to create riverbeds, craters, or other sloping surfaces. However,

simulating a relevant Mars rock field is the most difficult terrain-modeling challenge, as well as the

toughest to analyze. Therefore, for this evaluation, the capability of the simulator to model a rocky

Martian plain was the focus.

At the time of this analysis, detailed, high-resolution terrain data for Mars were limited to the landing

locations of the Viking and Pathfinder missions. For this evaluation, the Viking 1 landing site was arbitrarily chosen. Figure 25 shows the cumulative number of rocks per square meter as a function of rock

diameter (ref. 11). The curves on this figure were plotted from mathematical expressions derived from

stereo measurements at the Viking 1 landing site.

As discussed previously, the current version of the Tumbleweed simulator only provides the user with

control over the number and dimensions of the upper and lower bound rocks. If the user then wishes to

place rocks of different dimensions onto the terrain, their only option is to specify a number of rocks to be

randomly sized somewhere in between the dimensions of the already specified upper and lower bound

rocks. Working within this restriction, a number of large rocks (2-m cubes) and small rocks (1-m cubes)

were chosen for the terrain. These sizes were chosen to create a rock field that would be difficult for the

Tumbleweed rover to navigate. The idea was that the difficult terrain would challenge the capabilities of

the Tumbleweed simulator while at the same time offer an interesting environment to observe the dynamics of the Tumbleweed rover.
33

Figure 25. Cumulative number of rocks versus diameter. (Golembek & Rapp: Size Frequency Distributions of

Rocks on Mars, ref. 11).

From figure 25, it can be seen that on a 1-km2 terrain, there should be approximately 130000 1-m in

diameter rocks and 4500 2-m in diameter rocks. The distribution neglecting the crater rim rocks was

selected because it allows for higher numbers of large rocks and was well suited for the difficult terrain

desired. Computer memory restrictions only allowed a total of 10000 rocks to be included on the 1-km2

terrain. This memory consideration is a fundamental issue with the way Vortex is set up and will not

allow the modeling of large terrains with large numbers of rocks. As a result, the terrain was restricted to

4500 rocks of 2-m diameter and 5500 rocks of 1-m diameter. Figure 26 shows a comparison between a

photograph of the actual Viking 1 landing site and the virtual terrain generated within the Tumbleweed

simulator.

Winds were modeled at a constant velocity and direction. The velocity chosen was a typical Martian

average velocity of 7 m/s. The current version of the Tumbleweed simulator does not account for boundary layer effects.

Figure 26. Left, photograph taken by Viking 1 lander. Right, virtual terrain within Tumbleweed simulator modeled

after Viking 1 landing site.
34

5.3.2. Environment Modeling: Identified Improvement Areas

The memory problem present in the current version of the Tumbleweed simulator when generating

large terrains with large numbers of rocks is a major restriction to modeling a realistic Mars terrain. The

idea of the Tumbleweed concept is to traverse vast and possibly highly rugged areas of the Martian surface. Any mission could potentially cover hundreds, if not thousands, of kilometers. Alternative methods

for terrain modeling within the tool will have to be explored. For example, one possible alternative may

be to generate only small patches of the terrain as a Tumbleweed rover travels through it. This option

would free up memory resources because only small terrain areas would need to be stored at any given

time.

Currently, there are limited data describing wind conditions on the Martian surface. Detailed models

exist for replicating the environment of the upper atmosphere (Mars-Global Reference Atmospheric

Model). The Viking and Pathfinder missions both performed some wind experiments on the surface, but,

in general, little is known about the wind profile and boundary layer effects on the surface.

Question 2. Can a realistic Tumbleweed rover be modeled within the simulator?

Tumbleweed rovers were modeled within the simulator for this evaluation. Two different configurations were examined: a 3-m and a 5-m radius rover, both with a mass of 20 kg. Figure 27 shows the

comparison between an artist’s rendition of one particular Tumbleweed configuration and the virtual

Tumbleweed modeled with the Tumbleweed simulator.

Figure 27. Left, LaRC artists rendition of Tumbleweed rover. Right, virtual Tumbleweed rover modeled within

Tumbleweed simulator.

5.3.3. Tumbleweed Modeling: Identified Improvement Areas

The major obstacle to modeling a Tumbleweed rover within the simulator is correctly modeling a

flexible structure, or at least the dynamic losses that are associated with a flexible structure impacting and

rolling on a nonrigid surface. The current version of the Tumbleweed simulator models the Tumbleweed

rover as a rigid body. All impacts are treated as rigid body collisions. The inherent flexibility the structure would actually have is accounted for somewhat by using of a coefficient of restitution. However,

dynamic losses due to structural rolling resistance, which is identical to the rolling resistance discussed in

section 5.2.1, are not accounted for. To truly replicate the motion of a Tumbleweed rover, these dynamic

losses will need to be accounted for in future analysis tools. High fidelity, finite element tools exist that

can accurately model impacts. However, these tools are highly taxing on computational resources and are
35

simply impractical for Monte Carlo type simulations. Alternative methods will need to be explored that

can approximate the dynamic behavior of a Tumbleweed rover. One possibility might be to use a simplified modeling approach, which incorporates simple physics relationships in parallel with empirical test

data to apply correctional “fudge factors” to the tool.

Additionally, there are two other areas that the simulator should improve in regarding Tumbleweed

modeling. First, the coefficient of drag of the Tumbleweed is hard coded into the current version as that

of a sphere at low Reynolds numbers (CD = 0.5). As already discussed in the aerodynamics section,

many of the current designs have much higher coefficients of drag (CD > 1.0). Being able to vary the

drag coefficient of the rover within the Tumbleweed simulator will be required to analyze these other

configurations.

Question 3. Can anything be learned about the dynamic behavior of the Tumbleweed rover from

the tool in its current state of development?

With the terrain and Tumbleweed rover modeled within the simulator to the best of its current ability,

the next step in the evaluation was to set up and run a series of Monte Carlo simulations to take a preliminary look at the dynamic behavior of a Tumbleweed rover. The two rovers (3- and 5-m radius) were run

simultaneously over the simulated terrain, with an initial separation of approximately 300 m to avoid

Tumbleweed-to-Tumbleweed collisions. Their initial positions were slightly varied with each Monte

Carlo iteration so that the path each rover took across the terrain would be different for each run. The

simulation consisted of 2000 iterations.

Figure 28 shows the path taken across the simulated Viking terrain by one Tumbleweed rover over a

120-s time interval. The blown-up area shows a segment of the Tumbleweed’s path where there was an

impact with a rock. The rover is thrown in the opposite direction as a result of the collision until eventually it resumes its movement in the downwind direction.

Rover Path Over the Simulated Terrain (R = 5m, m = 20 kg)

Figure 28. Tumbleweed rover path from one Monte Carlo run, 120 s.
36

Figures 29 and 30 show the distribution of the 2000 Tumbleweed runs over the simulated terrain during a 120-s time interval for the 3- and 5-m radius rovers, respectively. Each point represents the location

of the rover on the terrain at the end of 120 s. In the case of the 3-m radius Tumbleweed, the data are

fairly uniformly scattered, with most of the rovers being located 100- to 250-m downrange. For the 5-m

radius Tumbleweed, one would expect the scatter to look similar to that of the 3-m Tumbleweed, with the

only major difference being that the larger diameter rovers would, on average, be able to move further

downrange in the same 120 s. Looking at figure 30, this is seen to some degree, but not as much as would

be expected. Additionally, it can be seen that there are some strange concentrations of stuck Tumbleweeds in the first 150-m downrange.

Figure 29. Tumbleweed distribution over simulated terrain (3-m radius).

Figure 30. Tumbleweed distribution over simulated terrain (5-m radius). Concentrations are within the circled

regions.
37

Figure 31 displays histograms of the distributions over the simulated terrain for the 3- and 5-m radius

rovers. The concentrations of the 5-m radius rovers discussed previously become even more apparent

when looking at the histograms. For the 3-m radius Tumbleweed, approximately 360 of the 2000

(18 percent) iterations performed show the rover to be located 0 to 100 m downrange after 120 s. For the

5-m radius Tumbleweed, approximately 890 of the 2000 (45 percent) iterations are located in the same

downrange region over the same time interval.

Figure 31. Tumbleweed rover distribution histograms.

One possible hypothesis for this discrepancy is directly related to the parameters selected for the

simulated terrain within the Tumbleweed simulator. Recall from question 1 that the terrain defined was 1

km2 with a total of 10000 randomly placed 1- or 2-m cubic rocks. On average, it works out to be approximately 1 rock every 10 m. Coincidentally, 10 m is also the diameter of the larger Tumbleweed

rover.

There are two basic ways in which a Tumbleweed rover can navigate past a rock: one is to go around

it, and the other is to go over it. From a static analysis viewpoint, the 7-m/s wind used in the simulation is

not sufficient to push either of the Tumbleweed rovers over any of the 1- or 2-m rocks. In other words,

recalling equation (1) from section 5.2.1, for 3- and 5-m radius Tumbleweeds in a 7-m/s wind, MFw will

always be less than the sum of MW and MNo . As a result, assuming the Tumbleweed is starting from

rest, to navigate through the simulated terrain the rovers will have to go around the rocks. As can be seen

in figure 32, the 3-m radius Tumbleweed has a distinct advantage due to its smaller size for navigating

around and between the rocks of this particular terrain. From a dynamic standpoint, the initial velocity

(Vi) of the rover, its radius, and the size of the rock it is impacting will determine whether or not the collision will result in the rover’s bouncing up and over the rock or up and in the opposite direction from

which it originally came. More specifically, for the rover to bounce up and over the rock T, the post

impact trajectory angle of the Tumbleweed (fig. 23) would have to be less than 90°, and the post impact

bounce height would have to be higher than the height of the impacted rock.
38

Figure 32. Static perspective showing how 3-m Tumbleweed can navigate simulated terrain better than the 5-m

Tumbleweed. Inertias assume a uniform density sphere.

Using the assumptions and equations discussed in section 5.2.2 regarding impact analysis, the dynamic

conditions required for going over the top of a given rock can be approximated. The calculations were

carried out for the rock sizes and Tumbleweed rover configurations used in this evaluation and are summarized in table 5.

Table 5. Requirements To Overcome 1- and 2-m Cubic Rocks Dynamically

3-m radius Tumbleweed 5-m radius Tumbleweed

Rock size, m

T, deg Vi, m/s T Vi, m/s

1 x 1 x 1 93 NA 71 ~3

2 x 2 x 2 140 NA 103 NA

From table 5, it can be seen that of the 3- and 5-m radius Tumbleweeds, the 5-m rover is the only one

of the two that can dynamically conquer the rocks on the simulated terrain, but only the 1-m rocks. The

5-m Tumbleweed would need an initial velocity of approximately 3 m/s to bounce up high enough to go

over top of the 1-m rock. For the other cases, T is always greater than 90°. As a result, it does not matter

what initial velocity they have before the collision, the rovers in these cases will always bounce back in

the opposite direction from which they came; thus, the velocity for these cases is defined as “not applicable” (NA) in table 5.

From the preliminary analysis discussed previously, a hypothesis can be generated that would suggest

that because of the properties of this particular simulated terrain and the chosen configuration of the

Tumbleweeds, the 3-m radius rovers are more efficient at navigating through the rock field than the 5-m

radius rovers. In the first 100- to 150-m downrange, the 5-m radius Tumbleweeds might not have sufficient time to accelerate to the 3 m/s necessary to bounce up and over the 1-m rocks. As a result, they will

bounce backward and eventually become lodged, as shown in figure 32, much more frequently than the

3-m radius rovers. Even though the 3-m radius rovers cannot ever bounce up and over any of the rocks in

this terrain (because the relationship between the dimensions of the rocks and the rover will never allow

T to be greater than 90°), they will always have an easier time going around the rocks laterally simply

because of their smaller diameter.

It is important to reiterate that this analysis is preliminary and effects of rock sizes on Tumbleweed

navigability will obviously need to be looked at in more detail. However, this analysis has yielded an

interesting result that might not have been noticed unless a Monte Carlo type simulation was studied. A
39

preliminary conclusion can be drawn about the relationship between Tumbleweed size and terrain navigability that, at the very least, challenges the intuitive assumption that bigger Tumbleweeds with higher

surface areas are always better at achieving maximum downrange distance. In fact, this analysis shows

that on a preliminary level, it may actually turn out that the optimal Tumbleweed size is highly dependent

on the properties of the terrain it is intended to traverse.

5.4. Development of Two-Dimensional (2-D) Simulation

Development continues on an in-house dynamics analysis tool, a MATLAB/Simulink physics-based

simulation. The simulation incorporates translational and rotational dynamics, wind forces acting on the

Tumbleweed rover, a simplified mass/spring flexible structure model, and ground contact effects, such as

friction and rolling resistance (fig. 33). The current model considers two-dimensional planar motion;

development efforts in FY2004 will focus on refinement of the current modeling assumptions and extension of the model to three-dimensional motion. This simulation will provide an understanding of the

range of Tumbleweed design parameters that are important for mobility over Martian terrain. The inhouse simulation tool has been developed with an emphasis on providing ease of model customization to

perform dynamic mobility studies of novel concepts for both passive and actively controlled Tumbleweed

rovers.

Empirical tests were performed to evaluate modeling assumptions used in the MATLAB/Simulink

physics-based simulation. The model demonstrated the ability to predict rolling motion on an inclined

plane and vertical bouncing, including structural damping effects (see section 5.5). Work is ongoing to

refine the model to better predict the sliding-to-rolling transition during structural deformations that result

from impact during bouncing. The planar motion simulation tool has been used to perform some initial

dynamics studies.

Normal force

Drag force

Wind

Friction

Weight

KS

Figure 33. Tumbleweed on sloped terrain.

5.4.1. Rolling Motion Along Sloped Terrain

The planar motion simulator described in section 5.4 was used to perform a study of the steady-state

rolling velocity of a flexible Tumbleweed on a sloped terrain. A range of Tumbleweed design parameters

was studied, including mass, size, drag coefficient, and rolling resistance. Results of steady-state rolling

speed versus wind speed for a range of terrain slope conditions were determined by using a representative

Mars gravity constant (3.7 m/sec2) and surface atmospheric density (0.0155 kg/m3). Figures 34 and 35

demonstrate the effect of a rolling resistance coefficient on the steady-state rolling velocity for a set of
40

representative Tumbleweed design parameters. The Tumbleweed mass (10 kg) and overall radius (3 m)

were selected based on estimates from previous feasibility studies (ref. 1). The aerodynamic drag coefficient was set equal to an approximate value for a smooth sphere (Cd = 0.5). This value serves as a lower

bound on the range of drag coefficients obtained from wind tunnel tests in the BART facility of representative Tumbleweed configurations (see section 4.2.2). The range of rolling resistance coefficients was

based on values for tires with low inflation pressures over a range of surface conditions. Figure 34 shows

the steady-state rolling velocity versus wind speed for a rolling resistance of 0.05, which is an approximate lower bound for a medium hard road surface. Figure 35 shows the rolling velocity for a rolling

resistance of 0.15, which is an approximate lower bound for loose sandy surfaces. These values were

considered within the range that would be expected over various Martian surface conditions.

The results from this study can be used to estimate the effect of rolling resistance on mobility over a

range of Martian terrains and wind conditions. As can be deduced from figures 34 and 35, a lower value

for rolling resistance implies a relatively small range of wind conditions in which the Tumbleweed will

remain stationary on a given terrain slope. For rolling resistance of 0.05, the Tumbleweed point design

considered requires wind speeds >7 m/s to climb terrain and wind speeds >4 m/s to maintain position on a

slope of 5°. As the rolling resistance is increased to 0.15, the wind speed required for climbing the 5°

slope increases to 9 m/s, while a downslope motion (i.e., negative velocity) is prevented for any wind

with an upslope component (positive velocity).

The results from this type of simplified analysis can be used to determine Tumbleweed design sizing

requirements for accomplishing mobility objectives under the assumption that Tumbleweed mobility is

dominated by rolling. Local topography slopes and wind conditions that can be defined and/or bounded

for selected Mars sites may then be used to derive design requirements and site selection. Airborne

motion resulting from rock/terrain impacts may affect downrange mobility estimates as well as produce

the potential for crosswind trajectory dispersions. Impact dynamics during subsequent Tumbleweed

bouncing may also affect mobility estimates. Higher fidelity dynamics models should be developed in

follow-on studies to assess the effect of bouncing transients on Tumbleweed mobility estimates.

0 2 4 6 8 10 12 14 -10

-8

-6

-4

-2

0

2

4

6

8

10

12

Slope = 0 deg 

Slope = 5 deg 

Slope = 10 deg

Slope = 15 deg

Slope = 20 deg

0 2 4 6 8 10 12 14 -10

-8

-6

-4

-2

0

2

4

6

8

10

12

Wind speed, m/s

Slope = 0 deg 

Slope = 5 deg 

Slope = 10 deg

Slope = 15 deg

Slope = 20 deg

Velocity, m/s

Figure 34. Steady-state rolling velocity versus wind speed and terrain slope angle. Radius Tumbleweed = 3 m,

Cd = 0.5, Rolling resistance coefficient = 0.05.
41

0 2 4 6 8 10 12 14 -10

-8

-6

-4

-2

0

2

4

6

8

10

12

Slope = 0 deg 

Slope = 5 deg 

Slope = 10 deg

Slope = 15 deg

Slope = 20 deg

0 2 4 6 8 10 12 14 -10

-8

-6

-4

-2

0

2

4

6

8

10

12

Wind speed, m/s

Velocity, m/s

Slope = 0 deg 

Slope = 5 deg 

Slope = 10 deg

Slope = 15 deg

Slope = 20 deg

Figure 35. Steady-state rolling velocity versus wind speed and terrain slope angle. Radius Tumbleweed = 3 m,

Cd = 0.5, Rolling resistance coefficient = 0.15.

5.5. Using Simple Empirical Tests for Two-Dimensional (2-D) Simulator Validation

Empirical tests were performed by using spherically shaped objects of various sizes to evaluate the

modeling assumptions used in the MATLAB/Simulink physics-based simulation test bed (fig. 36). Based

on the test results, the model demonstrated the ability to predict rolling motion on an inclined plane and

vertical bouncing, including structural damping effects.

Figure 36. Test setup for simple empirical tests.

The primary objectives of the simple empirical tests were the following:

 Measure Tumbleweed model parameters:

1. Using known weights for loading and calipers for model deformation determine structural

stiffness, including range of linear response.

2. Contact patch under weights. Verify dynamics model assumptions relating to contact patch

size, Tumbleweed deformation, and stiffness.
42

3. Rolling resistance coefficient (from pending motion on inclined ramp). Determine

corresponding offset normal force location as a percentage of contact patch length.

 Conduct dynamics tests of model rolling and bouncing motion on inclined ramp:

1. Use video to capture dynamics.

2. Verify equilibrium rolling rate and velocity from simulation.

3. Determine dynamic rolling resistance. Compare with value obtained from test and values used

in simulation.

4. Determine corresponding offset normal force location as a percentage of contact patch length.

5. Measure position along ramp. Compare with simulation results.

6. Investigate dynamics during initial impact. Derive angular acceleration and rates from video

analysis. Determine structural damping.

7. Does rolling resistance dominate motion during initial impact?

8. Is sliding-to-rolling transition consistent with assumptions in simulation?

NO

mg

r

Fd

G

h

x

y

y x

Start

End

FBD

max

Figure 37. Flexible sphere rolling down ramp.

Figure 37 shows a free body diagram of the simple empirical tests, with the Tumbleweed approximated by a flexible sphere rolling down a ramp. The sum of the moments about point G is given by

equation (15):

 MG = 0  mgx (r y)+ I = mgsin (r y) x Fd (r y) (15)

where

x = sin (r  y) – (ax/g)[(7/5) r  y] (16)

and the Coefficient of Rolling Resistance (CRR) is defined as

CRR = x/r (17)

The torque produced by the drag force was neglected for simplicity. The general effects of rolling resistance can be gauged by evaluating the magnitude of the offset normal force (NO) torque. The significance of rolling resistance and stiffness/damping characteristics on a bouncing model can also be evaluated with this test configuration.
43

For the tests, a volleyball, bouncy ball, beach ball, Styrofoam ball, two (2) foam balls of different

densities, and a stereolithography (SLA) ball were used as the test models. A drag coefficient of 0.5 was

assumed for all balls because they were spherical.

First, basic structural parameters of the test balls were measured for incorporation in the simulation

model. A digital scale was used to measure each ball’s mass, and calipers were used to measure each

ball’s diameter. The percentage of each ball’s radius that it deformed under its own weight was determined by placing each ball on a flat glass surface and measuring the length of the contact patch. The

amount of deformation of the radius can then be derived from the contact patch measurement. This measurement was performed to estimate the ball’s stiffness and to verify the contact patch calculation used in

the simulation model. Equation (18) shows the relationship of Tumbleweed deformation to contact patch

length:

lc = [4(2Rt  )]1/2 (18)

where  is the deformation of the Tumbleweed measured along the radius Rt , and lc is the length of the

contact patch (note: lc is a sector length, assuming a circular shape for the Tumbleweed outer surface). In

a static test, each ball was placed on a flat glass surface. Known weights were added to each ball, and the

contact patch was measured. From this measurement, the deformation of the radius was calculated and

could be compared to the imposed loading to determine each ball’s stiffness.

Next, dynamic and static tests (fig. 38) were conducted to determine the rolling resistance and the relationship to the normal force offset distance from the Tumbleweed’s center of gravity. To determine the

offset normal force for a static condition and hence the rolling resistance, the balls were placed on a board

of known length. The board was raised until the balls began to roll (fig. 38(a)). To determine the offset

normal force in a dynamic condition, the balls were placed on a sloped board with a small initial velocity,

and the board was lowered until the balls stopped rolling. The vertical height of the board in both cases

was measured, and the angle of the slope at which the ball started to roll or stopped rolling was calculated. By using the following equation the distance to the offset normal force (Dn ) was calculated:

Dn = tan * R (19)

where is the angle at which the model starts to roll or stops, and R is the deformed radius of the

model.

(a) Determination of offset normal force. (b) Determination of damping ratio.

Figure 38. Simple empirical testing.
44

The damping ratio of each ball was determined from dropping the balls with no initial velocity from a

known height onto a flat surface (fig. 38(b)). Behind the balls was a grid to measure the height of each

bounce. A camera taking 30 frames per second videotaped all dynamics tests. The frame number for the

peak of each bounce was used to calculate the elapsed time between bounces. The parameters of each

ball and a 0° slope angle were input into the simulation test bed model. The damping ratio used in the

model was tuned until the model was able to match the height of the test bounces in the same amount of

time.

Finally, a series of dynamic tests of rolling and bouncing motion on the sloped ramp were conducted

for validation of model assumptions. Three sets of dynamics tests were conducted. Each of the dynamics

tests was conducted on a board 3 m long at inclinations of 5°, 7.5°, and 10°. The board was marked at

10-cm intervals down the length of the board, and at 1 cm intervals for the first 50 cm of the board.

Behind the first 50 cm of the board was a 1-in. square grid. In the first dynamics test, each ball was rolled

down the inclines. From the videotape, the time elapsed for each ball to roll down the length of the board

was determined and recorded at 20-cm intervals. In the second test, the balls were dropped from varying

heights in front of the grid. The time elapsed for each ball to roll down the board was determined and

recorded at 20-cm intervals as before. The vertical positions of each ball where also recorded before the

ball was dropped and when it crossed the 20- and 40-cm marks down the board. In the third test, the balls

were dropped at varying heights. The vertical and horizontal positions and the time elapsed were

recorded at the peak of each bounce and at each impact with the board until the ball had traveled 50 cm

down the board.

Measurement of the angle at which the balls began to roll and the angle at which the balls stopped

rolling when given a small initial velocity showed that the static rolling resistance was greater than the

dynamic rolling resistance. The rover model needs to be modified to include this effect. It currently

assumes that the rolling resistance at impending motion is the same as the rolling resistance when it is in a

dynamic condition.

The static test showed that the model assumption of linear stiffness appears valid. Results of the

deformation of each ball, with varying loads, are shown in figure 39. Several vertical drop tests were

conducted on a flat surface for each ball to determine damping, as described previously. Figure 40 shows

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

012345

Weight, kg

Volleyball

Styrofoam¤

Beach

Bouncy

OHR

SLA

Soccer

Deformed radius,

m

Figure 39. Deformation of radii of test balls under varying weights.
45

Figure 40. Bouncing height of volleyball.

a typical test result for the volleyball, as compared to the results from the simulation after tuning the

damping ratio. The circles represent the test data and the curves are the simulation results after tuning.

Since the model could predict the bounce height of the balls when dropped on a flat surface, the linear

damping assumption also appears to be valid.

For a rolling case, the model could predict the test balls’ downrange motion when the values for the

offset normal force were tuned. As the slope angle increased, the percent offset normal force generally

increased, which, in turn, increased the rolling resistance. Several rolling tests for each ball were

conducted on a sloped surface to determine the effective dynamic rolling resistance. Figure 41 shows a

Figure 41. Position of volleyballs on variable slopes.
46

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

4 6 8 10

Slope angle, deg.

Offset normal force, percent

Soccer

Volleyball

Styrofoam¤

Bouncy

OHR

Beach

Figure 42. Percent offset normal force versus slope angle.

typical result of the downrange position for the volleyball rolling test when compared to the simulation

results after tuning the rolling resistance for each slope angle. The circles represent the test data, and the

curves are the simulation results after tuning. Figure 42 shows the tuned values of offset normal force for

each ball for all tests. It was determined from these results that there seemed to be a near linear relationship between slope angle and the offset normal force. The tuned results are shown, along with a best linear fit for each ball in figure 42. This effect of increased offset normal force may result from deformations

of the Tumbleweed contact patch due to increased gravity loading along the direction parallel to the ramp

as the slope increases. This hypothesis seems to explain the results; however, further work is needed to

verify this assumption.

In a bouncing case, although the simulation model could predict the bounce height of the balls, it underpredicted the downrange motion because the rolling resistance had a dominant effect for each impact.

The test results showed that the downrange velocity of the test balls was not decreased by each impact,

which contradicted the simulation results. The vertical position versus downrange motion for selected

tests of the beach balls is shown in figure 43.

Figure 43. Trajectory of beach balls.
47

Further analysis needs to be conducted of the impact during bouncing to verify the assumption regarding the transition from sliding-to-rolling. The assumptions of linear stiffness and damping appear to

be valid. The model can predict downrange motion of the Tumbleweed when it is in a rolling condition

but not when it is bouncing. Further analysis of why the model cannot predict a bouncing trajectory also

needs to be conducted to correct this problem before the Tumbleweed simulation test bed can be extended

to six degrees of freedom and a terrain model can be added.

6. North Carolina State University Tumbleweed Prototype

6.1. Overview of the Box Kite Tumbleweed Earth Demonstrator

In addition to the wind tunnel testing described in section 4.2.1, undergraduate students from the

NCSU senior aerospace design class also researched, designed, and analyzed the Tumbleweed rover concepts shown in figure 7 and ranked the concepts based on the criteria shown in table 6. The Box Kite was

chosen as the most appropriate design for further development. The choice was based on these results

and was later modified by adding hoops to improve rolling characteristics (fig. 44).

Table 6. NCSU Selection Criteria

Name and score Drag Major parts Mass

Tumblecups 16.67 6.70 8.78 1.19

Box Kite 23.54 10.00 10.00 3.54

Balloons 19.27 6.23 10.00 3.03

Dandelion 23.36 6.63 6.72 10.00

Figure 44. NCSU Box Kite.

The NCSU students designed and built a Tumbleweed Earth Demonstrator (TED) of the Box Kite

concept (fig. 45) and presented their design, along with their associated research at NASA LaRC, as part

of a National Institute of Aerospace (NIA) Seminar (fig. 46). NCSU also involved 6th graders in an education outreach activity with Tumbleweed (see section 8.0). A mass breakdown of the TED vehicle is

provided in table 7.
48

Figure 45. TED debut at NCSU picnic May 9, 2003.

Figure 46. TED presented at the LaRC Reid Conference Center May 14, 2003 (LaRC BART Jr. providing the wind).
49

Table 7. TED Mass Breakdown

Contingency

Components

Mass, g

current best

estimate Percent Mass, g

Total mass, g

Core 1752.60 5 87.63 1.19

Sails 488.58 2 9.77 3.54

Core bungee assembly 876.00 2 17.52 3.03

Ring structures 7200.00 5 360.00 10.00

Total for TED, kg 10.79

6.2. TED Instrumentation and Systems

The TED core (fig. 47) is a resin shell created by Fineline Prototyping, Inc. that protects the payload

from the environment and provides attachment points for the sails and outer rings and an access hatch for

the payload. The instrumentation payload consists of biaxial accelerometers, environmental sensors

(pressure and temperature transducers), an imager, and a Global Positioning System (GPS) tracking

device. The core also contains the data handling and communication system, which is a commercially

available unit for amateur rocketry.

Payload

Fineline prototyping resin outer shell

Access hatches

Figure 47. TED payload core.

7. Outreach

Outreach is a process that displays the accomplishments of NASA and inspires students to investigate

science and technical careers. The Tumbleweed rover concept catches the imagination of people of all

ages and educational levels.

The Mars Tumbleweed was featured in a number of outreach activities in FY03. The LaRC Public

Services Office, Office of External Affairs, requested a Mars Tumbleweed exhibit for the 2002 Virginia

State Fair. The Mars Tumbleweed display, with a simulated Mars rock field, was later used for other

outreach demonstrations, including the Jones Magnet School Planetarium, Langley Career Days (fig. 48),

LaRC Take Your Children to Work Day, and the Virginia Air and Space Center (VASC) Robotics Camp.
50

Figure 48. The Mars Tumbleweed display.

Jennifer Keyes spoke at the New Horizons Governor School about Mars Exploration on July 24, 2003

to two groups with about 40 students each, the first had 4th and 5th graders and the second 3rd and 4th

graders. The presentations consisted of a Microsoft PowerPoint presentation and an interactive activity in

which students were arranged into small groups and used materials ranging from toothpicks to Playdoh

to rocks. Each group designed its own rover. Each student received a certificate at the end of the event.

Student participation at LaRC for FY03 included Rachel Owens, a Langley Aerospace Research

Summer Scholar (LARSS) from Virginia Polytechnic Institute and State University, who performed

dynamics modeling and testing of Tumbleweed rovers. Michael Wisniewski from the University of

Michigan, Ann Arbor, was also a 2003 LARSS intern, researching electrical power system concepts for

the Tumbleweed rover. Additionally, Heidi Owens, a student at Bethel High School, Hampton, Virginia,

and one of the counselors at the Virginia Air and Space Center (VASC) Robotics Camp, joined the Mars

Tumbleweed Science team through the New Horizons Governor’s School for the Science and Technology

Mentorship Research Program. Her research project, “Instrumentation on a Mars Tumbleweed Rover to

Detect Astrobiological Life Forms,” concluded in May 2004.

The sixth grade science classes of Carnage Middle School in Raleigh, North Carolina participated in

an outreach program through their teacher Holly Hanrahan, a Kenan Fellow at NCSU. By working with

the Aerospace Engineering senior design class at NCSU (fig. 49), the Carnage students learned about

aerodynamics, structures, and other engineering disciplines needed to build planetary exploration systems.

Ms. Hanrahan and Dr. Fred DeJarnette presented their collaborative Tumbleweed Project at the International Workshop on Planetary Probe Atmospheric Entry and Descent Trajectory Analysis and Science in

Lisbon, Portugal on October 9, 2003 (refs. 12 and 13). Ms. Hanrahan also gave a lecture on the Tumbleweed at the North Carolina State Teachers Association conference on November 13, 2003.

The Mars Tumbleweed rover was featured extensively in the news media during 2003—The LaRC

Researcher News (5/23/03), the Richmond Times-Dispatch (5/17/03), Space News (5/26/03),

www.space.com, www.marstoday.com, and www.spaceref.com. Articles have also been included in the

NC State Extension and Engagement magazine, the Raleigh News and Observer, the Wake County (NC)

Public Schools newsletter, and the county’s magnet school newsletter. Holly Hanrahan and the students

participated in a live webcast to discuss their Tumbleweed activity. A television sequence for NASA’s

Destination Tomorrow, program 13, features the Mars Tumbleweed rover.
51

(a) Testing Tumbleweed designs in a wind

tunnel designed on the spot by the Carnage

students.

(b) Working with NCSU students on the TED.

Figure 49. Carnage Middle School students.

8. Concluding Remarks

8.1. Summary

NASA LaRC is studying concepts for a new type of rover known as a Mars “Tumbleweed,” that

would derive mobility through use of the Martian surface winds. The “Search for Life” goal was identified as the most scientifically interesting and challenging mission for the Tumbleweed concept. Significant progress was made in FY03 in defining power system options, structural materials, communication

architectures, and deployment options. The first wind tunnel tests of LaRC Tumbleweed concepts were

completed by using the Langley Research Center Basic Aerodynamic Research Tunnel (LaRC BART).

Progress was made on the development of a Tumbleweed simulation capability and an extensive test of

the Monte Carlo simulation capabilities was conducted. Empirical tests were also performed using

spherically shaped objects of various sizes to evaluate the modeling assumptions (e.g., rolling resistance)

used in the simulation. And finally, the North Carolina State University (NCSU) senior Aerospace design

class designed, built, and tested a Tumbleweed Earth Demonstrator (TED) of the Box Kite Tumbleweed

concept.

8.2. Plans

The study areas initiated in FY03 will continue to be addressed in FY04 at a greater level of detail:

 Science Mission Definition

 System Definition and Analysis

 Dynamic Modeling and Analysis

 Aerodynamic Modeling and Analysis

 Collaboration with universities and Public Outreach

Working with Mars program scientists, the science objectives will be refined and specific mission

scenarios will be defined. Areas where recent outflow of water may have occurred (e.g., canyons and
52

crater walls) and areas where water may have flowed in the distant past will be identified. Microinstrument technologies that are applicable to the search for life mission will be identified. Subsystem

technologies identified in FY03 will be applied in system trade studies to determine the advantages and

disadvantages of particular options for power, communications, structures and materials, and navigation.

The simulation test bed will be modified to incorporate additional analysis capabilities and move to a

Mars terrain-based simulation, incorporating terrain generated from Mars Orbiter Laser Altimeter

(MOLA) data. Emphasis will be on development of a 3-D test bed to simulate a virtual Tumbleweed

mission. Sophisticated empirical testing will be conducted, building upon the knowledge gained in the

simple empirical tests already conducted and which are necessary for further refinement of analytical

models and validation of modeling assumptions.

Analysis of BART test data will continue, including development of Reynolds number comparisons

and analysis of the effects of differing model frontal areas. Data from the TTU ABL tests will also be

analyzed, and results of all aerodynamic tests will be incorporated into the simulations. Options for

achieving lower Reynolds number testing will be investigated, including use of higher sensitivity balance

or University facilities. The Tumbleweed concepts will be refined for follow-on testing in the BART, and

the use of facilities capable of producing a Mars relevant environment will be investigated. Collaboration

efforts with NCSU and Texas Tech University (TTU) will continue, and additional collaborations will be

pursued, including the development of collaborative robotics/swarming algorithms with Case Western

Reserve University and the use of the Carnegie Mellon University inflatable robotic rover test bed for

dynamic testing.
53

9. References

1. Antol, J.; Calhoun, P.; Flick, J.; Hajos, G.; Kolacinski, R.; Minton, D.; Owens, R.; and Parker, J.: Low Cost

Mars Surface Exploration: The Mars Tumbleweed. NASA/TM-2003-212411, 2003.

2. Mars Exploration Program Advisory Group, Mars Exploration Program, Scientific Goals, Objectives,

Investigations, and Priorities, ed. R. Greely, March 2, 2001.

3. Malin, M. C.; and Edgett, K. S.: Evidence for Recent Groundwater Seepage and Surface Runoff on Mars.

Science, vol. 288, 2000, pp. 2330–2335.

4. Gullied Slopes on Mars. http://www.psrd.hawaii.edu/Aug03/MartianGullies.html Accessed October 7, 2004.

5. Mars Network User Guide: Relay Orbiters. http://mn.jpl.nasa.gov/relay.html Accessed October 26, 2004.

6. Minton, D.; Choe, D.; Davis, A.; Dougan, C.; Gregory, J.; McGrew, J.; Painter, B.; Pinto, A.; and Yarley, C.:

Mars Tumbleweed Preliminary Design Review. North Carolina State University, Department of Mechanical and

Aerospace Engineering, October 22, 2002.

7. Jenkins, L.: BART_Specs.ppt, NASA Langley Research Center.

8. Strain Gage Balance Calibration Results: Balance 733. Calibration Date 7/9/02, Calibrating Engineer McWithey,

NASA Langley Research Center.

9. Gillespie, T.: Fundamentals of Vehicle Dynamics. Society of Automotive Engineers, 1992.

10. Toniolo, M. D.; and Stalos, S.: Mars Tumbleweed Rover COTS Analysis Report. AMA Report No. 03-19,

Purchase Order No. L-71737D, 2003.

11. Golombek, M.; and Rapp, D.: Size-Frequency Distributions of Rocks on Mars and Earth Analog Sites:

Implications for Future Landed Missions. J. Geophys. Res., vol. 102, February 25, 1997, pp. 4117–4129.

12. Hanrahan, H.; Minton, D.; DeJarnette, F.; Camelier, I.; and Fleming, M.: Mars Tumbleweed, Planetary Probe

Atmosphere Entry and Descent Trajectory Analysis and Science Workshop, Lisbon, Portugal, October 6–9,

2003. (poster)

13. Hanrahan, H.; Minton, D.; DeJarnette, F.; Camelier, I.; and Fleming, M.: Conceptual Designs for a Mars

Tumbleweed: A New Way to Explore Mars, Planetary Probe Atmosphere Entry and Descent Trajectory

Analysis and Science Workshop, Lisbon, Portugal, October 6–9, 2003.

14. Evolved Water Experiment. http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=DEEPSP2&ex=1 Accessed

October 7, 2004.

15. Exobiology in the Solar System & The Search for Life on Mars. Report from the ESA Exobiology Team Study

1997–98, European Space Agency SP-1231, Sec. 11.7, Oct. 1999.

16. Discussion with Warren Kelliher, NASA LaRC. January 24, 2003.

17. Squyres, S.; et al.: The Athena Mars Rover Science Payload. The 29th Lunar and Planetary Science Conference,

#1101, Houston, Texas; March 16–20, 1998.

18. Christensen, P.; et al.: The Miniature Thermal Emission Spectrometer for the Mars Exploration Rovers. J.

Geophys. Res., vol. 108, 8064, 2003.

19. Mars Pathfinder. http://mars.jpl.nasa.gov/MPF/index1.html Accessed October 7, 2004.
54

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55

Appendix B. Balance and Sting Characteristics

Balance 733 Characteristics

Calibration load range Full scale

output Sensitivity constant Accuracy,

percent F. S. Component

(lb) or

(in-lb)

(N) or

(Nm) (mV/V) (lb/mV/V) or(in-lb/mV/V)

(N/mV/V) or

(Nm/mV/V)

(95 percent

Conf.)

Normal 35.0 155.688 1.351 25.9040 115.2269 0.12

35.0 155.688

Axial 10 44.482 1.250 8.0022 35.5955 0.11

 0 0

Pitch 50 5.649 1.495 33.4480 3.7791 0.09

50 5.649

Roll 15 1.695 1.877 7.9901 0.9028 0.09

15 1.695

Yaw 30 3.390 1.398 21.4517 2.4237 0.08

30 3.390

Side 10 44.482 1.171 8.5431 38.0016 0.12

10 44.482
56

Sting Adapter for Balance 733 Design
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

 4. TITLE AND SUBTITLE

Mars Tumbleweed: FY2003 Conceptual Design Assessment

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Antol, Jeffrey; Calhoun, Philip C.; Flick, John J.; Hajos, Gregory A.; 

Keyes, Jennifer P.; Stillwagen, Frederic H.; Krizan, Shawn A.; Strickland, 

Christopher V.; Owens, Rachel; and Wisniewski, Michael

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

Washington, DC 20546-0001

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 REPORT NUMBER

L-19067

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

Antol, Calhoun, Flick, Hajos, Keyes, and Stillwagen, Langley Research Center, Hampton, VA. Krizan, Analytical Mechanics Associates, 

Inc., Hampton, VA. Strickland, Swales Aerospace, Hampton, VA. Owens, VPI&SU, LARSS Program, 2003, Blacksburg, VA. 

Wisniewski, Univ. of Michigan, LARSS Program, 2003, Ann Arbor, MI. An electronic version can be found at http://ntrs.nasa.gov

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14. ABSTRACT

NASA LaRC is studying concepts for a new type of Mars exploration vehicle that would be propelled by the wind. Known as 

the Mars Tumbleweed, it would derive mobility through use of the Martian surface winds. Tumbleweeds could conceivably 

travel greater distances, cover larger areas of the surface, and provide access to areas inaccessible by conventional vehicles.

They would be lightweight and relatively inexpensive, allowing a multiple vehicle network to be deployed on a single mission. 

Tumbleweeds would be equipped with sensors for conducting science and serve as scouts—searching broad areas to identify 

specific locations for follow-on investigation by other explorers. An extensive assessment of LaRC Tumbleweed concepts was 

conducted in FY03, including refinement of science mission scenarios, definition of supporting subsystems (structures, power, 

communications), testing in wind tunnels, and development of a dynamic simulation capability.

15. SUBJECT TERMS

Mars; Mars rover; Tumbleweed; Wind driven; Surface exploration

18. NUMBER

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61

19b. TELEPHONE NUMBER (Include area code)

(301) 621-0390

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gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 08 - 2005