Engines of Our Ingenuity

No. 47:
MOMENT OF INERTIA

by John H. Lienhard

Click here for audio of Episode 47.

Today, we learn how government secrecy sabotaged an early satellite. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Peter Likins, the President of Lehigh University, tells an intriguing tale about the first satellites. The first three satellites launched were the Russian Sputniks I and II, in October and November 1957, and the American Explorer I in January 1958.

Explorer I was long and narrow like a pencil. It was supposed to rotate around its own centerline, like a pencil spinning about an axis along its lead -- spinning with the least inertia. It was definitely not supposed to rotate about an axis perpendicular to its centerline, with its ends describing circles -- in the maximum inertia mode.

The radio astronomer Ronald Bracewell at Stanford University tracked the first Sputnik and determined that it was spinning in its maximum inertia mode. He knew about the dynamics of bodies that spin and consume a little of their own spin energy while they do, because that's how galaxies behave. What Bracewell knew, and what the Explorer engineers didn't know, was that the minimum inertia rotation of Explorer I would be unstable -- that it would soon flip over and start windmilling through space.

So Bracewell called the engineers at the Jet Propulsion Laboratory to warn them. But people in charge of secrecy wouldn't let him talk with the engineers. He had to get the word out by publishing a paper in the open literature. It came out seven months after Explorer I was launched -- seven months after Explorer I made just one earth orbit and then flipped over to windmill from then on.

Actually, in 1957, another engineer named Landon described that kind of instability in laboratory notes at RCA. But he didn't publish them, nor was he aware of the Explorer problem. Information that isn't made public can't do anyone much good.

There can, I suppose, be good reasons for secrecy in technology; but make no mistake, secrecy is an enemy of progress. Creativity, freedom, and openness are natural bedfellows.

A Russian engineer recently pointed out why the United States stays ahead of Russia in computer development. Once we established a lead, he says, Russia tried to keep up by copying what we'd already done. They have plenty of ways to break through our security. But, being forced by their system to play that game, instead being allowed to trust their own inventive genius, they're trapped in a technology that's doomed to stay one step behind us.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Likens, P., Spacecraft Attitude Dynamics and Control-- A Personal Perspective on Early Developments. Invited Lecture preprint, American Institute of Aeronautics and Astronautics, 1985.

Bracewell, R. N., and Garriot, O. K., Rotation of Artificial Earth Satellites. Nature, Vol. 182, Sept. 20, 1958, pp. 760 et seq.

This Episode has been revised as Episode 1332. I am grateful to James Casey, University of California at Berkeley, for suggesting this topic, and for considerable counsel on the subject.

This episode is one of two that Vladimir Shtern rendered into Russian in response to astronaut Andrew Thomas's request that we send a set of Engines episodes up to the Mir Space Station. (The other one was Episode 1284.) For the text in Russian, click on:

Page 1 of Russian Text
Page 2 of Russian Text



The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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