# Rover mission analysis and design

## Objective

This rover design document is a collection of Mars rover design relationships. For deep space missions, there has been little published work of design rules-of-thumb, such as can be found for Earth orbiters[1]. Space Mission Analysis and Design [2] (SMAD) is a comprehensive resource for rules of thumb, empirical formulas, and algorithms for the design of low-Earth orbit, unmanned satellites. Some design guidelines provided in SMAD are broad enough to be applicable to deep space probes as well. Human Space Mission Analysis and Design (HSMAD) [3] dedicates a few pages to the design of crewed lunar rovers; in particular, it provides a design algorithm to rapidly obtain order-of-magnitude estimates of mass and power requirements for a pressurized rover. The intent of the rover design document is to provide similar resources for the particular case of Mars robotic rover systems. The design document provides design rules of thumb and also references to articles relevant to the field of Mars rover design. The document is a collection of parametric relationships that help design and evaluate rover properties and performance (e.g. speed), both at the system and subsystem level, based on broadly defined scientific mission objectives.

## Target Audience

This document is intended for students who desire to understand the high level scaling laws of rover systems and science payload designers who need to measure the impact of their payload on a rover vehicle. This document can also serve as the starting point for the development of more elaborate rover system design tools in the fashion of MSE.

## System level design

Wilson et al.[4] provide a database of eight Team X[5] rover designs which are used to derive mass fraction relationships.

Inputs Instrument mass

Outputs Rover total mass

Based on Team X designs, Sojourner and MER, the payload mass fraction ${\displaystyle a_{science}}$ of a rover is between 8% and 16% of the total rover mass and it is on average 12%. Therefore, for a given scientific payload mass ${\displaystyle M_{science}}$, including instruments and acquisition tools, the expected total rover mass ${\displaystyle M_{rover}}$ is provided by the following relationship.

### Cost

#### Operations Cost

According to the NASA budget request for 2005 [6] and to a NASA press release about the MER operations extension [7] the average operations cost rate of a rover is approximately \$1.25M per month.

### Exploration performance

Inputs wheel diameter

Outputs Rock clearance and obstacle density

#### Rock Clearance

A rover equipped with a rocker-bogie suspension is able to drive over rocks whose size is less than one and a half times the wheel diameter.

#### Rock abundance

The number of rocks that are larger than a given size is derived from the rock abundance model developed by Golombek [8].

## Subsystem level design

### Rover Hardware Subsystem Decomposition

Subsystem decomposition of a planetary rover

The remaining of this page is organized according to a subsystem decomposition of a rover system, as illustrated in the figure. Each subsystem section provides sizing and scaling relationships.

### Power

Design rules that govern a rover power system are no different from those that apply to a regular spacecraft power system. SMAD provides guidelines for the design of spacecraft power systems.

Two types of power system have so far been used on Mars: solar power (MPF, MER) and radioisotope power (Viking landers). The MSL rover is expected to be the first Mars rover to use a radioisotope power system (RPS).

#### Solar power

The solar power system of surface vehicle differs from that of a satellite due to interactions with planetary dust. Over time, the deposition of dust on solar panels contributes to the degradation of power output.

Experience with the first 300 sols of Spirit operations shows that dust deposition losses reach a maximum of 30% over time [9]. The experimental curve of loss due to dust (${\displaystyle L_{dust}}$) as a function of mission duration can be approximated by the following function:

${\displaystyle L_{dust}=0.7+0.3*exp(-T/100)}$

where T is the mission duration measured in sols. Loss due to dust is combined with losses (${\displaystyle L_{i}}$) due to solar array inherent degradation. The resulting formula for end-of-life power (${\displaystyle P_{EOL}}$) as a function of beginning of life power (${\displaystyle P_{BOL}}$) is:

${\displaystyle P_{EOL}=L_{i}*L_{dust}*P_{BOL}}$

A MMRTG generator is about 0.64 meters in diameter (fin tip to fin tip) by 0.66 meters long and weighs about 43 kilograms [10]. A MMRTG generates 125 We at beginning of life; the output power diminishes over time at a rate of 1.6% per year [11].

A MMRTG uses 4kg of Pu${\displaystyle ^{238}}$; one gram of Pu${\displaystyle ^{238}}$ costs approximately \$2000 [12].

### Mobility

Subsystem decomposition of a planetary rover

The rover mobility subsystem includes hardware aspects (e.g. suspension, wheels, motors) but also performance aspects (e.g. ground clearance, mechanical speed).

#### Mobility hardware

As a reference, the mass of the MER mobility subsystem is 34.5 kg [13][14].

#### Mechnical speed

The figure shows the mechanical speed of various Mars rovers and Earth testbed rovers (References: MER [15]; Sojourner and MSL [16]; ExoMars [17]; Marsokhod 75 [18]; FIDO [19]; Nomad [20]; IARES-L and KWM [21]

## JPL Rovers

### Sojourner

Sojourner references: [22] , [23]

The mass of Sojourner is 11.5kg of which 9kg go into the vehicle. The rover's dimensions are 65cm in length, 48cm in width, and 30cm in height.

Power subsystem:

Solar arrays use GaAs cells; they weigh 0.340 kg and have an area of 0.22 m2. The nominal peak power is 16.4W. Primary batteries weigh 1.24 kg.

Thermal subsystem:

Three RHUs were installed in the axle of the rover inside the WEB.

## References

• Wertz, James R.; Wiley J. Larson (1999). Space Mission Analysis and Design (3rd ed.). Kluwer Academic Publishers. ISBN 1-881883-10-8.
1. E.S. Lamassoure, S.D. Wall, R.W. Easter, "Model-Based Engineering Design for Trade Space Exploration throughout the Design Cycle", AIAA Space 2004 Conference and Exhibit, 2003, Long Beach, CA
2. J.R. Wertz, W.J. Larson (1999). Space Mission Analysis and Design, 3rd Ed., Kluwer Academic Publishers. ISBN 1-881883-10-8
3. Larson, W.J. and L.K. Pranke, Human Spaceflight Mission Analysis and Design (HSMAD), McGraw-Hill, Inc.
4. G.R. Wilson et al., "Mars Surface Mobility: Comparison of Past, Present, and Future Rover systems", 36th Annual Lunar and Planetary Science Conference, March 14-18, 2005, League City, Texas
5. J.L. Smith,“Concurrent Engineering in the JPL Project Design Center,” Society of Automotive Engineers, Paper 98AMTC-83, 1998.
6. NASA 2005 Budget Request for Mars Exploration, p. 3-13
7. NASA, "NASA extends Mars Rovers' Mission", press release, April 08, 2004, http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040408a.html
8. M. Golombek et al.,"Assessment of Mars Exploration Rover Landing Site Predictions", Nature, 436(7047), pp. 44-48, July 2005
9. P.M. Stella, R.C. Ewell, J.J. Hoskin, "Design and performance of the MER (Mars Exploration Rovers) solar arrays", Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, p. 626 - 630, 3-7 Jan. 2005
10. Department of Energy, "Space Radioisotope Power Systems, Multi-Mission Radioisotope Thermoelectric Generator", September 2006
11. Balint, T.S., Jordan, J.F., “RPS Strategies to Enable NASA’s Next Decade Robotic Mars Missions”, 56th International Astronautical Congress 2005, Paper # IAC -05-A5.2.03, 17 - 2l October, 2005, Fukuoka, Japan
12. Balint, T.S., Jordan, J.F., “RPS Strategies to Enable NASA’s Next Decade Robotic Mars Missions”, 56th International Astronautical Congress 2005, Paper # IAC -05-A5.2.03, 17 - 2l October, 2005, Fukuoka, Japan
13. Mars Exploration Rover Technical Data, The Mars Exploration Rover Maintenance Manual, Compiled and maintained by Rupert Scammell[1]
14. D.S. Lee, "Mars Exploration Rover Primary Payload Design and Verification", Spacecraft & Launch Vehicle Dynamics Environment Workshop Program, June 17, 2003
15. Roncoli R.B., Ludwinski J.M., "Mission Design Overview for the Mars Exploration Rover Mission", AIAA/AAS Astrodynamics Specialist Conference, August 5-8, 2002, Monterey, CA
16. Muirhead, B., "Mars Rovers, Past and Future", Proc. of the IEEE Aerospace Conference 2004, Vol 1, p134, Big Sky, MT, March 2004.
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18. Centre National d’Etudes Spatiales, editor, Missions, technologies et conception des vehicules mobiles planetaires / Missions, technologies and design of planetary mobile vehicles, Cepadues, Toulouse, September 1992.
19. Volpe R., Baumgartner E., Schenker P., Hayati S., "Technology Development and Testing for Enhanced Mars Rover Sample Return Operations", Proc. of the IEEE Aerospace Conference 2000, Big Sky, MT.
20. Zakrajsek J J., McKissock D.B., Woytach J. M., Zakrajsek J. F., Oswald F.B., McEntire K.J., Hill G.M., Abel P., Eichenberg D. J., Goodnight T.W., "Exploration Rover Concepts and Development Challenges", AIAA-2005-2525, NASA/TM-2005-213555, 1st Space Exploration Conference: Continuing the Voyage of Discovery, AIAA, Orlando, FL, January 30-February 1, 2005.
21. Russian Mobile Vehicle Engineering Institute, Specimens of Space Technology, Earth-Based Demonstrators of Planetary Rovers, Running Mock-Ups, 2002, Saint-Petersburg, Russia.
22. D.L. Shirley, "Mars Pathfinder Microrover Flight Experiment - a paradigm for very low-cost spacecraft", Acta Astronautica, vol 35, pp. 355, 1995