Notes on the mechanical design of the Sojourner rover

Nicolas Raynaud, February 2025

Wanting to build a fun robot mower, I studied the Sojourner rover as a base for my design. Looking at online 3D models, I fell into the rabbit hole of accuracy of historical artefact description. My goal in this document is to show the smart that JPL put in this little rover.

Marie Curie

The Marie Curie rover is a double for Sojourner, two rovers were built at the same time and the best one was selected to be sent to space. Today the Marie Curie rover is exposed at the National Air and Space Museum in DC. Most high resolution photos available are from this rover since the other one left èarth in 1996, fong before we had hight resolution digital cameras in our pockets.

Differences with Sojourner

the antenna hook has been removed
the Wheel Abrasion Experiment is missing
the Material Adhesion Experiment is missing (but the notch is here)
the front solar panel bumper has been cut and re-painted without respecting the 10cm pattern

Document search

A lot of scientific publications have clues about the inner working of the rover. Some of them are so old that the pictures are very bad, but sometimes the publisher has a better copy in print.

JPL archives were not forthcoming with high res pictures, they had contentions with a documentation precise enough to be able to build a replica of the rover, it is US Federal IP and they don't want to encourage copies. I didn't push the discussion further to avoid wasting my correspondent's time, they didn't give me any references about archives policies.

The old JPL website is still available on the Wayback Machine: https://web.archive.org/web/20160118204242/http://mars.jpl.nasa.gov/MPF/rovercom/pixt.html.

Follow-up missions used the same building block as Sojourner, in particular, the cancelled '01 Surveyor mission and the INSIGHT mission¹.

The most important documents are the very high resolution pictures (8688×5792px) of Marie Curie taken the Air and Space Museum in DC, they are available in Open Acces

The rover telemetry is available online.

Off-the-shelf parts

Commercial parts can help make sense of scale on the pictures.

Most screws are assumed to be metric².
The motors are Maxon RE16³ (sometimes "RE016") with a single output shaft⁴, whose documentation is still available online.
The potentiometers are BI precision 6173⁵ (identified by the part number on pictures), whose documentation is also available online.
The gearboxes are described as "Globe Motors" 5 stages planetary gearboxes, but sadly this company has disappeared and I couldn't identify the gearbox

Documented dimensions

They are generally ambiguous, I have color coded the various instances of the same dimension.

$${\color{red}630 \space mm}$$ long (Bickler 98)
cleats that protrude 10 millimeters (Bickler 98)
$${\color{magenta}79-millimeter}$$-wide wheels (Bickler 98)
Each wheel of the rover is $${\color{cyan}13 \space cm}$$ in diameter and $${\color{magenta}6 \space cm}$$ wide (Matijevic 97, ignoring the width)
the rover has a 74cm turning diameter (Matijevic 97)
The rover is $${\color{red}65 \space cm}$$ in length, $${\color{green}48 \space cm}$$ wide and $${\color{yellow}30 \space cm}$$ tall in its deployed configuration (Matijevic 97)
In this stowed position, the rover height is reduced to $${\color{orange}19 \space cm}$$. (Matijevic 97)
In the deployed configuration, the rover has ground clearance of 15cm. (Matijevic 97)
solar panel comprised of 13 strings of 18, GaAs cells each of size 2cm by 4cm. (Matijevic 97)
wheel radius $${\color{cyan}(6.5 \space cm)}$$ (Matijevic 97)
(APXS) a 0.076 m diameter bumper (Blomquist 1995)
$${\color{red}68 \space cm}$$ long by $${\color{green}48 \space cm}$$ wide by $${\color{yellow}28 \space cm}$$ high (Cooper)
Deployed Dimensions, cm: Length $${\color{red}62}$$, Width $${\color{green}47}$$, Height $${\color{yellow}32}$$ (The Rover Team 1997)
Wheels (Six), cm: Diameter $${\color{cyan}13}$$, Width $${\color{magenta}6}$$ (The Rover Team 1997)
Forward camera separation 12.56 cm (The Rover Team 1997)
Approximate (camera) height above surface 26 cm (The Rover Team 1997)
The rover stands $${\color{yellow}300 \space mm}$$ tall but stows to $${\color{orange}180 \space mm}$$ (Eisen 1997)
Sojourner, when stowed, is $${\color{red}650 \space mm}$$ long and $${\color{green}480 \space mm}$$ wide with $${\color{cyan}130 \space mm}$$ diameter wheels, $${\color{magenta}80 \space mm}$$ wide. (Eisen 1997)
The wheel cleats protrude 10 millimeters (THE MARS ROVER MOBILITY SYSTEM)
The wheels are $${\color{magenta}79 \space millimeters}$$ wide (THE MARS ROVER MOBILITY SYSTEM)

"The [Instrument Deployment Arm (IDA)] originated as the robotic arm built for the Jet Propulsion Laboratory (JPL) for the cancelled Mars 2001 Surveyor mission in 1998. The IDA is slated for refurbishment" Fleischner, R., “InSight Instrument Deployment Arm”, in 15th European Space Mechanisms and Tribology Symposium, 2013, vol. 718, Art. no. 14. https://esmats.eu/esmatspapers/pastpapers/pdfs/2013/fleischner.pdf ↩
"With few exceptions, all Pathfinder’s parts are metric sized, from its nuts and bolts to its 10-kilogram (22-pound) robotic rover" https://www.latimes.com/archives/la-xpm-1995-02-21-mn-34480-story.html ↩
"the Maxon RE016 motor which had been used successfully on the Mars Sojourner Rover" Braun, David & Noon, Don. (1998). "Long life" DC brush motor for use on the Mars surveyor program. https://www.researchgate.net/publication/23604427_Long_life_DC_brush_motor_for_use_on_the_Mars_surveyor_program ↩
"in the 2 years between the Sojourner and Robotic Arm programs, Maxon began offering ball bearings and a double-ended shaft on the RE016" ibid. ↩
"Steering position was read directly via a conductive plastic Beckman potentiometer" Eisen, Howard & Buck, Carl & Gillis-Smith, Greg & Umland, Jeffrey. (1997). Mechanical Design of the Mars Pathfinder Mission. ↩

nraynaud/Notes Sojourner.md