Many thanks all for your "second opinions" ! You confirmed my initial reasoning.
And the math turns out to be childishly simple....
For zero G the vertical speed component has to decrease by 10m/sec (9.8msec if you want to be finicky) each second. So the duration to the top of the parabola (where the vertical speed becomes zero) in seconds is (initial vertical speed in m/sec) / 10.
Plug in speed and angle for a typical zero-G aircraft and you get 10 to 12 seconds. Plug in the 2150 mph and 85° of SpaceShipOne, and you get about 90 seconds.
Total zero-g duration will be double that, if you start pulling up at the same height that you started, a bit less if you pull up a bit earlier.
pilotmike,
Thanks for your posts.
"My" NASA engineer was quite positive about it, and since he was a regular customer on the KC-135, I believed him without re-doing the math......
Mark1,
Thanks for reminding me of SpaceShipOne, which is of course a perfect example.
And again thanks to everybody,
The suggestion about using Concorde was made totally "tongue in cheek" on another forum, late 2003, at the time of the end-of-service.
For any sort of significant gain you'd want to start at Mach 2.0, i.e. 50,000ft and do a 3G pull-up (certified limit). The resulting zoom climb would take you way over the certified ceiling.... And all that for another 20 seconds in a cabin that, nice at it was, would not have been very useful for zero-g experiments (as pilotmike said).
For certification purposes, Concorde has done zero-g flights, and even -1g flights, but not with quite the 45° pull-ups and pull-outs used in the dedicated Zero-G aircraft.