🚀 go-pugleaf

-From: stick@lopez.marquette.MI.US (Stick,CommoSigop)
-Subject: Re: Micro-g in KC-135
-Date: 15 Dec 92 22:19:31 GMT
-Organization: Great White North/UPLink

-In <Bz7v1M.6DC@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:

->In article <1gfti1INNaqj@rave.larc.nasa.gov> claudio@nmsb.larc.nasa.gov (Claudio Egalon) writes:
->>What causes the microgravity in the KC-135, the centripetal 
->>acceleration at the top of the parabola, which may cancel the gravity 
->>acceleration,  or something else???

->You don't "cancel" the gravitational acceleration, you fall with it.  The
->KC-135 flies the exact trajectory that it would follow if it were falling
->free in a vacuum.

->That trajectory isn't exactly a parabola; it is in fact a segment of an
->elliptical orbit (one that intersects the Earth's surface).  It's very
->close to being a parabola.  It would *be* a parabola if the Earth were
->flat and gravity did not diminish with altitude.

-   Interesting.  The way I see it, the -135, after nosing up, pushes over
-in such a manner that its acceleration towards the Earth's surface exactly
-matches the acceleration of gravity.   And, I don't know for sure if its
-flight path would be that of a parabola, it doesn't maintain its lateral
-velocity vector.  The speed vector parallel to the Earth's surface would be
-at its greatest when the aircraft was in level flight.

The horizontal velocity vector *has* to remain essentially the same, or
you won't get zero-G.

While the trajectory might technically be an ellipse, I think it's more
practical to use the approximation that the Earth is flat, the gravitational
field is constant, and the trajectory is a parabola. After all, over the
course of a 30-second zero-G trajectory, the local intensity of the Earth's
gravitational field should show an altitude-related variation of only about
one part in 3000, and the Earth beneath the airplane should curve by only
about 1/16 of a degree. The trajectory of the plane isn't precise enough
to reflect such small differences.

Approximations of this type are useful in many fields of engineering. For
instance, among those who build their own amateur telescopes, a 4-inch f/10
primary mirror used to be one of the popular choices, in part because for
that size mirror, a spherical curvature (much easier to grind than a parabolic
curvature) is sufficiently close to a parabola that further figuring isn't
really needed.

John Roberts
roberts@cmr.ncsl.nist.gov