**The Retelling of**

**"Romancing the Rover (How Sojourner Came to Be in the Late 1980s and
Its Journey to Mars)"**

*JPL Stories, September 28, 2000*

*Presenter*: Don Bickler

*Retelle*r: Marilyn Morgan with Don Bickler

The sixth in the Library's series of JPL Stories attracted a roomful of
enthusiasts, including some of the presenter's team members "to see," he
asserted, "if I make a mistake." The room buzzed with anticipation, and
even before Don Bickler began his talk, we could perceive a familiar JPL
gestalt --- the energetic demeanor, the ebullient manner, the general
look of someone who is about to share something that is really
terrifically exciting.

Sojourner had gone to Mars, fulfilled all expectations, captured the
public imagination, even became a toy, and finally emerged as an icon of
JPL ingenuity. We were about to hear from someone who was present at the
creation --- and not only that, he was wielding the wrench and
screwdriver.

I'm convinced that engineers, despite their sober and serious
reputation, have just about all the fun there is to be had in the world.
They grab onto a problem and address, define, analyze, and solve it
intellectually. That's exhilarating in itself, but then they get to
*build things*: structures, bridges, vehicles, rockets. Rocketry
engineers have even more fun: they get to build things and then fire
them into the sky or (accidentally of course) blow them up. If you have
seen the old films of Robert Goddard and his crew jubilantly
experimenting with liquid-fueled rockets in the New Mexico desert, you
know what I'm talking about.

Engineers who build robotic vehicles also have a bit of that maniacal
gleam in the eye. They experience the joyfulness of creating things that
scurry, scoot, roll, reach, climb, scramble, pivot, and peregrinate,
while carrying out instructions on command or, even better, these
neoteric critters decide things on their own through software created by
their human designers. Like symphonies, sophisticated robots are art
forms. But the robots that seem to operate so easily and beautifully are
not born so much as bred, through generations of robots, some of which
never get beyond the drawing board, and some of which are magnificent
failures. Sojourner, which seems the perfect realization of a compact
robotic rover, evolved at JPL over quite a few years. Her distant
lumbering ancestor is the Surface Lunar Roving Vehicle, circa 1967.

**The Bogie Man**

Don Bickler was introduced by Barbara Amago of the JPL Library
storytelling team. We learned that Don graduated from Northwestern
University in 1956 with a degree in mechanical engineering, and
eventually ended up at JPL in 1975. He is now supervisor of the Advanced
Mechanical Systems Group in the Mechanical Engineering Section (352).
Bickler's invention of the rocker-bogie suspension system earned him a
nickname --- The Bogie Man.

(An aside --- According to the Oxford English Dictionary, a golfing
"bogey" derives from a popular 1899 song describing a dreaded person,
devil, or goblin, but a mechanical "bogie" originated in 19th-century
northern English dialect, describing a low truck on four small wheels
used by masons to move large stones. Eventually "bogie" came to describe
a truck running on two or more pairs of wheels supporting the forepart
of a locomotive engine on which it swivels freely in passing curves.)

**Rough-Terrain Vehicles**

A before-talk exchange between Don and an audience member confirmed that
the bogie design is commonly used in tandem-wheeled trucks, military
trucks, and cranes to provide stability and mobility on uneven terrain.
Could Sojourner, so refined looking, be related to these rough
characters?

In the 1960s, we were told, the military developed several types of
articulated vehicles. Don showed photographs of these strange vehicles,
including a six-wheeled "bent jeep" that was supposed to be able to
climb over low obstacles, and the Lockheed Twister, bearing the
appearance of several vehicles in a state of post-collision. Another,
the sturdy-looking Gamma Goat, was classed as a 1-1/2-ton truck and
sported articulated six-wheel drive. All these were designed to traverse
rough terrain such as might be encountered in war-fighting situations or
military logistical enterprises. Early rover designers studied these
types of vehicles for clues to creating mobile machines that would be
successful in planetary excursions.

**Lunar and Planetary Roving Vehicles**

Sojourner's pedigree begins with a Surface Lunar Roving Vehicle (SLRV)
delivered to JPL about 1967. The segmented vehicle was supposed to fold
up into an "S" shape, and could climb obstacles 1.5 times its wheel
diameter. The SLRV was never launched, but provided a study object for
early JPL rover engineers. The vehicle was invented by Mieczyslaw G.
Bekker, whose team designed the Lunar Roving Vehicle used in the Apollo
15, 16, and 17 missions (1971--1972). Bekker's first book on the theory
of land locomotion and terrain--vehicle interactions ("terramechanics")
was published in 1956, and by 1963 he was proposing Moon rover designs.
Bekker tried lots of things in his designs, including variations on
Archimedes' screw.

In those days --- the heady Apollo days --- NASA was thinking big, and
cost was not a problem. The Apollo LRV was about 10 feet long by 6 feet
wide (to wheel centers). Each spring-steel, wire-mesh wheel was driven
by a 1/4-hp electric motor, and the front and rear wheels had separate
steering systems. Either of the front or rear wheels could be turned
while the other pair was locked straight, or both could be used
simultaneously, so the rover could turn completely around within its own
length. It could climb over obstacles 1 foot high and negotiate
25-degree slopes. All these were desirable characteristics in a roving
vehicle. Additionally, the Apollo LRV had to fit into the Lunar Descent
Module --- for stowage, all four wheels folded inward and the vehicle
folded double --- and be easily deployed on arrival.

Another idea for a lunar rover was the MOLAB or mobile laboratory: a
3-ton, closed-cabin vehicle with a range of about 100 kilometers. The
MOLAB was an articulated vehicle with independent suspension and
1.5-meter-diameter wheels, but was far too large for what NASA
ultimately required for Apollo.

**Big Mars Rover Concepts**

In the 1980s, interest in Mars spurred new thinking at JPL about what we
would want in a robotic rover, should the opportunity arise for a
journey. The SLRV, the baseline vehicle, morphed into the Little Blue
Rover. The Blue Rover had an elastic (spring) chassis and could clamber
over 1-1/2-wheel-diameter obstacles. Don showed us a vintage-1986
drawing of a rover concept that illustrated a six-wheeled articulated
vehicle of three sections: a forward portion with two science-instrument
arms, a middle portion with camera, and an aft portion that seemed to be
solely RTGs on two wheels. (The shape of the vehicle wheels resembled
narrow automobile tires, and this became an issue when different types
of terrain were considered.) Bickler began thinking about bogie linkages
to overcome the undesirable characteristics of using springs.

The next illustration Don showed us was "Fig. 3" from his 1989 patent
(no. 4,840,394) for an articulated, springless suspension that maintains
uniform distribution of weight and traction on the wheels. Fig. 3 showed
a six-wheeled vehicle with front and rear linkages coupling the front
and rear wheels to the middle wheel. It was assumed that equal weight on
each wheel is best, but the trick was to make three linkages work ("like
clapping with three hands"). The design has a master link on each side,
but there was a problem --- you can't steer the wheels. The next drawing
was a wire frame of a six-wheeled vehicle based on a pantograph design.
(A pantograph is the drawing instrument made of four rigid bars joined
in parallelogram form to maintain scale while copying a drawing.) The
linkages allow steerable struts. Now we're getting somewhere.

In a JPL review discussing rover designs, a CAD of the pantograph design
drew pointed criticism from John Casani --- "too complicated," "we don't
know if it will work," and "why not four wheels?" That shout from the
back of the room was Bickler's: "How about a model?!" Don's 1/8-scale
table-top model for the pantograph was favorably received at the next
review.

In the search for balance among the trade-offs, considerations of weight
on the wheels, size of the wheels, number of wheels, type of wheels,
steering and articulation, surface speed, ground clearance, and how high
the vehicle could climb (size of the obstacle to overcome) were all part
of the picture. Four wheels were not enough; six was the optimum number
of wheels for stability and obstacle-crossing capability. The idea was
that a rover would have to be able to climb over obstacles 1.5 meters
high --- almost 5 feet. Objects of this size could be seen from orbit.
Don's photograph of a person holding a 1.5-meter-long measuring stick
showed us that such a rover would have to be really large, and indeed,
the next photo showed an experimenter in the east Mojave Desert,
standing, for scale, next to wheels of the requisite size: huge, 1-meter
wheels on a single axis. These experimental wheels were narrow in shape.
Wheel shape is something to think about; you don't want your rover to
start off on a Martian traverse and get ignominiously stuck in a ravine
into which the wheels precisely fall. Don recalled riding a bicycle as a
kid in Chicago along streetcar tracks and realizing that "if you get
stuck in the tracks, you're dead." This insight, he declared, "really
made an impression on me."

And now, Don said, "everybody tried to get into the act" --- from
universities to the Russians, the race was on to build the best rover.
To climb a 1.5-meter obstacle, inflatable segmented tires or cone-shaped
wheels were considered. But venting and pumping inflatables is not
desirable in the thin Martian atmosphere, cone-shaped wheels gyrate, and
eggbeater-shaped wheels deteriorate and are "too wiggly." JPL's 1990
Robby Rover was a six-wheeled, rubber-tired, articulated rover testbed
13 feet long by 6.5 feet wide, resembling an oversized toy model of a
Monster Truck --- hardly the sleek, efficient design we would hope for.
Robby was also encumbered with a multibody design. Multiple bodies
require connecting cables: a key thermal issue is trying to keep several
bodies warm. As Don remarked, "the thermal world is a big one" --- a
whole overlay of engineering trade space.

**Little Rovers and Linkages**

JPL was starting to envision small, cost-efficient, capable rovers for
planetary missions. But the pantograph design, Don decided, had too many
links. What was desired was a stable platform, no springs, and a single
body. The challenge was to accomplish this with fewer links. The JPL
family of vehicles has these features in common (the list from Don's
viewgraph):

• There are no springs or elastic members other than those used with the
tires.

• There is a single rigid body.

• The rover has very high ground clearance.

• All wheels are capable of being steered.

• The body is differentially connected between the left and right sides
on a transverse (pitch axis) shaft.

The question to be answered through the engineers' art was --- How
simple can it be made and still have the desired performance?

M.G. Bekker --- whose name became associated with the Apollo lunar
roving vehicles --- mentioned a series of equations in his 1969 book,
*Introduction to Terrain--Vehicle Systems*. The equations, representing
the behavior of wheels over obstacles, had been developed by one
Frederick Jindra. Jindra had written a paper that appeared in the
"Journal of Terramechanics" in 1966, wherein he showed that it is
"easier" for a six-wheeled tractor-trailer vehicle to negotiate a *step*
compared with negotiating a *bump*. (In this scenario, the tractor has
one set of wheels at the front and one set at the rear; the trailer,
hitched to the tractor, has one set of wheels at its rear.) A step is
defined as a rectangular object that raises the entire vehicle, whereas
bumps can fit in between the wheels. As the trailer is pulled by the
tractor, the trailer's wheels rise vertically in attempting to negotiate
the bump. But the trailer's wheels lock up and are restricted from going
forward; at the same time, they pull the tractor backwards. The tractor
wants to keep going, and its second set of wheels have a tendency to
drive the vehicle up under itself, raising the front wheels. This was
nicknamed "popping a wheelie."

The problems for a rover, expected to meet plenty of bumps on Mars, were
evident. If you could maintain the wheel load distribution without
resorting to the use of an oscillation-producing spring suspension, you
would increase the stability and terrain-negotiating performance of the
vehicle. Through optimizing the geometry of a bogie system, Bickler
produced the (now-legendary) rocker-bogie suspension design. No one is
exactly sure who named this bit of engineering history: a six-wheeled
chassis with a springless, articulated system that maintained even
weight and traction on all the wheels --- the simplest mechanism to
satisfy the requirements without pantograph linkages

**The Iterations of Rocky Rover**

Don showed us photographs of the iterations of Rocky, as the robotics
team experimented with electronics, batteries, materials, and loads.
They even went to a farm equipment show to learn about struts. Rocky I,
with 9.25-centimeter (3.66-inch) diameter wheels, could traverse steps
and gaps over two wheel diameters, and climb over rocks about twice its
size. The Rocky II design was abandoned before realized in hardware, but
Rocky III was another improvement on the rocker-bogie design, with more
mobility and lower surface pressures. Rocky III had 13-centimeter
(5-inch) wheels, the same dimensions as the 1/8-scale table model of
yore. (Bigger wheels work better in sand; smaller wheels sink.) Don
showed us a photograph of a "dramatically overloaded" Rocky III being
tested in the Mojave Desert, and here we observed the rover surrounded
by robotics team members bearing the bemused expressions one sometimes
sees on parents' faces as they watch a tottering but determined
offspring engage the uneven terrain of the world. The vehicle, with all
its apparatus, was so heavy that the metal wheels turned in the rubber
tires --- the experimenters had to use Crazy Glue to attach the tires to
the wheels.

**Sojourner Rover**

The slimmed-down, sophisticated Rocky IV was a flight prototype version
of Rocky III, with all-metal (rubber is not a desirable material on the
cold surface of Mars) flexible wheels to get around better in sand and
other soft materials. But the slotted wheels had a tendency to break and
protrude, locking against the struts, so the next wheel design was solid
metal. An iteration or two more, including Rocky VII, a close-coupled
design intended to have one motor drive two wheels, and finally Rocky
VIII --- the elegant little rover that became Sojourner.

The naming of the rover, Don explained, was Donna Shirley's idea, and
"it had to be female." (Donna Shirley was manager of the JPL Mars
Exploration Program.) When informed that the rover would be named
thusly, Don impishly proposed the name "Trixie." Donna affixed him with
a look (and one can only imagine what that gaze conveyed), and
proclaimed, "You've got nothing to do with it, Bickler!" A playful name
was not to be; instead, the rover was eventually named in honor of the
remarkable 19th-century American abolitionist and champion for women's
rights, Sojourner Truth.

**Q&A**

During the question and answer portion of the storytelling, someone
asked about optimum wheel size: how does one get there? It turns out
that the wheel size is selected through exploring the solution space
using a blend of human judgment and computer analyses. As Don pointed
out, "a computer programmed to overlook flotation in soft sand would say
that zero-diameter wheels are best." As to whether a Sojourner could be
created in the "faster-better-cheaper" environment, Don's answer was
"Absolutely!" It's engineering interest, not the prosaic limits of
40-hour weeks, that drives the process, and "we have a very big LEGO
kit" for experimentation.

Another questioner asked about the biggest threat to the rover, and
that, as might be surmised, is a situation where the rover somersaults.
The rover allows for more than a wheel diameter of obstacle traversal,
and can be symmetrical in forward/backward motion capability. A
hazard-detection system is designed to prevent an overtipping condition
by sensing pitch and roll; if pre-set limits are exceeded, the rover
stops. Sojourner can actually climb a 45-degree slope (if the soil can
be piled that steep), and can scale a 20-centimeter-high rock. If
Sojourner were somehow flipped over, though, it could not right itself.
Information on traversable obstacles (and the allowed traversability
numbers are conservative) is stored on board, and if Sojourner
encounters a non-traversable object (a rock too high), it is supposed to
turn away and find another path. An interesting example of Sojourner
meeting and evaluating an obstacle on Mars is captured in a brief movie
at \<<http://mars.jpl.nasa.gov/MPF/ops/rvrmovie.html>\>. Open up the
first item on the list, \<rover_movie_sol24_S0050Q.gif\>, and you'll see
the rover bump a pyramid-shaped rock, turn, rotate 35 degrees, back up,
and then blithely move forward as its right set of wheels clambers right
over the rock. (Sojourner is capable of climbing over rocks twice that
size.) The movie is a neat little example of the rover's climbing
abilities, and provides a good view of how the suspension system and
steering work.

Then there was the sand issue --- could Sojourner get stuck? Don
gleefully told the story. The team had tested the rover in sand created
to "match Houston's recipe" for lunar soil, so they knew the rover was
capable of traversing the fluffy stuff. A (nameless) challenger, citing
M.G. Bekker's equations, maintained that the JPL rover was such a poor
design that it could not handle a downhill slope. The challenger was
politely invited to come to JPL, where a hole was dug in the simulant
for the rover. Sojourner, of course, promptly climbed right out.

At the end of the too-short hour, Barbara Amago thanked Don and
presented him with a small crystal globe, the customary gift that
represents the jewels of insight brought to us by the JPL storytellers.

Eyeing it appreciatively, The Bogie Man pronounced it "one heck of a
marble."

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**Web Sites of Interest**

[http://robotics.jpl.nasa.gov/tasks/scirover/genealogy/homepage.html](http://robotics.jpl.nsa.gov/tasks/scirover/genealogy/homepage.html)
--- Timeline of JPL Mars rover development.

<http://mars.jpl.nasa.gov/MPF/rovercom/pixt.html> --- Mars Microrover
photo gallery. Here are many pictures of various rovers as well as
photos from the Pathfinder mission. Includes a composite image comparing
the size of Sojourner with the Apollo Lunar Roving Vehicle.

[http://www.delphion.com](http://www.delphion.com/) --- The 1989
rocker-bogie patent. Type in "Bickler" and a list of patents comes up,
including no. 4,840,394 for the 1989 articulated suspension system.

<http://mpfwww.jpl.nasa.gov/rovercom/rovintro.html> --- Information on
Sojourner (Mars Microrover).

<http://www.bchip.com/mars/rover/descrip.html> --- Detailed description
of Sojourner rover.

<http://mars.jpl.nasa.gov/MPF/ops/rvrmovie.html> --- "Movies" of
Sojourner on Mars.

<http://www.bchip.com/mars/rover/name.html> --- How Sojourner rover got
its name.

<http://www.sojournertruth.org/> --- The Sojourner Truth Institute Web
site.

<http://www.digitalsojourn.org/speech.html> --- Sojourner Truth's famous
1851 speech.

<http://www.osa.com.au/~cjh/lego/> --- An Australian hobbyist's
palm-sized Sojourner model built of LEGO blocks, with a miniature
rocker-bogie system.

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