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. Space Mission Analysis and Design (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) 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. provide a database of eight Team X rover designs which are used to derive mass fraction relationships.

Payload mass fraction
Inputs Instrument mass

Outputs Rover total mass

Based on Team X designs, Sojourner and MER, the payload mass fraction $$a_{science}$$ 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 $$M_{science}$$, including instruments and acquisition tools, the expected total rover mass $$M_{rover}$$ is provided by the following relationship.

Operations Cost
According to the NASA budget request for 2005 and to a NASA press release about the MER operations extension 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.

Rover Hardware Subsystem Decomposition
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 . The experimental curve of loss due to dust ($$L_{dust}$$) as a function of mission duration can be approximated by the following function:

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

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

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

Multi-mission radioisotpe thermoelectric generator
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. A MMRTG generates 125 We at beginning of life; the output power diminishes over time at a rate of 1.6% per year.

A MMRTG uses 4kg of Pu$$^{238}$$; one gram of Pu$$^{238}$$ costs approximately $2000.

Mobility
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.

Mechnical speed
The figure shows the mechanical speed of various Mars rovers and Earth testbed rovers (References: MER ; Sojourner and MSL ; ExoMars ; Marsokhod 75 ; FIDO ; Nomad ; IARES-L and KWM

Sojourner
Sojourner references: ,

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.