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Question:
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 (http://jsc-aircraft-ops.jsc.nasa.gov/Reduced_Gravity/trajectory.html), it shows that the weightlessness actually comes at the top of the curve. How is this possible?



Replies:
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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