KOHLHASE: “Yes, it was early 1960. I’d been at JPL
for about eight months when my supervisor asked
me to design a trajectory to go from Earth to Mars at
the next opportunity in the fall of 1962. He wanted to
know the most favourable launch and arrival dates,
and asked me to describe what Mars would look
like if we planned a safe fly-by on the generally sunlit
side of the red planet. By “safe,” he meant not too
close to risk impact, given navigation uncertainties.
“So, I walked into this small conference room in
1960, mentally rehearsed what I planned to say and
draw on the blackboard concerning a Hohmann
transfer ellipse to Mars. The door opened and in
walked Dr. William Pickering (Director of JPL) and
Dr. Wernher von Braun. They sat down next to me,
Pickering on my left, von Braun on my right.
“My supervisor called the meeting to order and
introduced me. I struggled on wobbly legs to reach the
blackboard which, by now, was blurry and indistinct.
I can still remember raising my hand to draw the Sun
surrounded by the relevant orbits of Earth and Mars. I can
usually make pretty good free hand circles and ellipses,
but I suspect these were erratic. Pickering and von Braun
followed with serious faces, eyes and ears trained on
my modest delivery. I rambled on for some 20 minutes,
a few questions were asked, and the two super-brains
finally excused themselves.
I have no idea how I came
across, but I will never
forget the experience.”
RS: Before the Mars
missions, you were part
of the team designing
missions aimed at
going to the Moon?
firing its engines to correct the trajectory. The young
engineer gave the spacecraft team a series of turns
and a burn to execute. However, these instructions
took Ranger further from the Moon rather than closer
because there had not been a firm agreement
on the sign convention (minus or plus) to use in the
calculations between the navigation team and the
spacecraft team. That shook up the people at the lab
and they didn’t ever want that to happen again.
“For that reason, they asked me and Dave
Curkendall to design an analogue device that could
check all the Ranger manoeuvres from then on and
make sure they didn’t turn in the wrong direction.
We would take all the Ranger orbit determination
estimates from then on and actually rotate our device,
like a slide-rule, to check them with the computer
programme to see if they agreed. That was probably
my main contribution to Ranger, making sure we
never made another mid-co urse correction error.”
RS: You mention the computer programme. What
was the computer capability back then in the
early 60s? Could you pre-programme much of
what happened on board the spacecraft?
KOHLHASE: (Laughs). “Well, even much later on, in the
Voyager spacecraft, the onboard computers have a
total memory of only 8
KB. (Note: a cell-phone
today has over 100,000
times more memory than
a Voyager spacecraft!)
For Ranger you could
programme its simple
‘computer’ so that at a
certain time in the future
the spacecraft would turn
or change course, you
could turn the camera
on, simple commands like
that, but it wasn’t nearly as
sophisticated as Voyager.
It couldn’t carry out
onboard fault protection
or correct itself. You had
to know in advance
what you wanted to do
as a function of time.”
KOHLHASE: “Yes. There
were nine Ranger missions
to the Moon (between
August 1961 and March
1965). Several of the early
Rangers failed and the
Lab was greatly criticised
for that. Although many of
those were mistakes by the
launch vehicle contractor,
and the later Rangers
were generally successful,
RS: Computing power was
it was felt that the Lab
only one of the challenges
The Ranger Manoeuvre Model, designed by Kohlhase and Curkendall, used
had too academic an
you faced working in this
to check mid-course corrections on early unmanned flights to the Moon.
attitude. Soon this resulted
Credit: Charles Kohlhase
totally new field of robotic
in Deputy Directors coming
planetary exploration?
in from the military to get
us away from a perceived ‘loose’ Caltech-JPL attitude.
KOHLHASE: “There were many. But the secret was
“In that regard, a young engineer was doing the
that once I did a job on any particular programme,
manoeuvre analysis for Ranger. This involved tracking
whether it was Ranger, Mariner, looking at trajectories,
the spacecraft after launch to determine its orbit
navigation or targeting, as soon as I’d developed that
and, if it was not on target for the Moon, making a
particular job skill, and the supervisor said ‘That’s right,
mid-course correction by rotating the spacecraft and
that’s what we want’, then I added a new tool to my
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