Mike wrote:
>>>>>
I think that the 750 RPM was after orbital insertion. "The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute."
Recalling from memory I can't see any way that it was doing 12.5 RPS at launch. I recall something on the order or 1-3 RPS at launch.
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That sounds strange to me too, as I noted. But, there was no other means to have made the satellite 4th stage spin any faster that I know of, than the initial spin during launch.
One thing that occurred to me about looking at video footage again. Video is effectively 30 FPS (technically 29.97 FPS). Movie film, IIRC, is usually shot at 24 FPS. So the video conversion mucks up things somewhat, but at least that would add an inter-frame about every 4 frames. But if you watch online, on say YouTube, it might be downconverted to something like 15 FPS, or even 12 FPS, from 30 FPS, having been originally converted from 24 FPS.
So, it may be messy to work out the rotation rate from video.
Whoa, no, it's not messy at all. I stopped writing this and happened to Google this video:
https://www.jpl.nasa.gov/jplhistory/videos/explorer1-launch.php
Download the large 12 MB movie to your hard drive, it is better than the small one that plays on that page.
At 35 seconds into it, they show start-up of rotation of a Jupiter-C flight (it has a test warhead nose on it, not the Satellite, though it is shown in the movie as though it is Explorer-1). It gets to spinning.... REALLY fast. So now I do believe it did spin as fast as 12.5 RPS, or 750 RPM.
Interestingly, it is seen rotating from left to right (counter-clockwise top view), but the launch footage of Explorer-1 looks like it rotates the opposite way. But, that may be a stroboscopic effect of the movie (and/or video) frame rate, or possibly the footage might have been reversed from the start-up footage since Explorer-1's rocket does have "UE" on it and not a reversed "EU". It would be just plain weird if it actually rotated one way for one flight then the opposite way for another.
Actually, in looking at the spin-up footage again, definitely it is stroboscopic effect. Later when that sucker gets to spinning so fast, it looks like it rotates from right to left as far as the vertical stripes are concerned. But you know from having seen it start up, that it was rotating left to right. From a longer distance, as in the launch footage, you could not notice the speed-blur of the stroboscopic effect as you can from the start-up close-up. And take note how it seems to "slowly" rotate from right to left (stroboscopic effect), but again you know it is spinning like mad from left to right.
- George
