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How does a parabolic arc work during a 'weightless' flight (like NASA's KC-135 'vomit comet'? A previous post stated (incorrectly I believe) that weightlessness comes when the plane is 'nose-down', falling from the crest to the trough of the curve. On NASA's web site however (, it shows that the weightlessness actually comes at the top of the curve. How is this possible?

I have flown in the C-9a (successor to the KC-135) Weightless Laboratory (the "weightless wonder"). Weightlessness is the lack of support force. The flight that I was on had a weightless portion of the parabola begin just before hitting the peak and ending on the down slope. The graph in the web site you quote is close, with perhaps a bit more time on the descent. On our flight, we ranged about 25 - 32 seconds per parabola. The graphic quoted shows 25 seconds. Was each of the 40 parabolas I experienced exactly like the graphic? No, but it was a close approximation. Toss a ball. The entire time it is in the air it is weightless. The trajectory of the plane is designed so that it is like a tossed ball. The "non-powered" part of the trajectory begins before going over the top.

---Nathan A. Unterman

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