Zero G

[ChristiaanJ]

I suppose most of you know the "zero G" aircraft like the NASA KC-135, the A300 and now the 727.

When Concorde stopped flying, it was suggested, tongue in cheek, they should keep one for zero-g flights because it would allow zero g for two to three times longer.
But then I was told by a NASA engineer (who actually flew on the NASA "Vomit Comet" KC-135) that somewhere the speed of the aircraft dropped out of the equation that defined the duration of the zero g: the speed didn't matter, just the availablity of a nice big cabin to work in.

I just now got involved in a discussion about this (I think that discussion was triggered originally by the zero-g dog video on YouTube you all must have seen.... never mind ....), and tried to re-do the maths.
And according to my maths, assuming the same procedure: pull-up to 45°, push-over at zero g, until pitch-down at 30° (or 45° for simplicity of the maths), the duration of the zero-g state is proportional to the speed of entry of the parabola at 45°.

I would like a second opinion :)

[OutOfRunWay]

Sounds correct to me, Concorde would shoot up higher and take longer to come down.. I wonder how the Concordes complex Centre of Gravity Management would interfere with this process, though?

regards

[Slasher]

CJ your forgettin the Conk has (had) a delta wing.

Asuming a aft CG that would mean a pull-up at M.98 to only
30 deg, push over at 0 G until pitch down 25 deg (not exact
figures as Im not Conk endorsed)

I think overall youd be lookin at only a slight increase in the
O G duration.

[Mark 1]

I believe the Virgin Spaceship 1 managed about 3 minutes of zero G parabola flight starting with a vertical speed somewhere over 2000 MPH.

Unfortunately it takes a bit longer to repeat the exercise.

[chornedsnorkack]

Asuming a aft CG that would mean a pull-up at M.98 to only
30 deg, push over at 0 G until pitch down 25 deg (not exact
figures as Im not Conk endorsed)

But this was not a point.

What about a hard pull-up at M 2,00?

[pilotmike]

ChristiaanJ is completely right - the faster you start the 'zero G parabola', the longer it lasts. But to get maximum time from it, you would have to maximise the vertical component, much as Mark 1 and chornedsnorkak have implied.

It would appear that your NASA engineer was wrong, or you misunderstood him, as initial vertical speed is EVERYTHING! Shoot a projectile upwards at high speed, and I promise you that the faster you shoot it, the longer it remains at zero G. In the extreme case, send a projectile around the earth at the sufficient speed, and it will remain in zero G orbit indefinitely.

The shape of the wing and the CG position as brought in by Slasher, including random figures of M.98, 30 degrees and 25 degrees are complete red herrings, which inevitably lead to an incorrect conclusion. Even a stone or an Aeronautics text book achieve zero G every time you throw them (provided you never open the book!), regardless of their shape or C of G position.

If anyone were to be serious about CJ's proposal, many other limitiations would need to be considered, eg wind loading, max altitude limitations etc. Of these, not least would be the fact that there are no airworthy Concordes available, and it is doubtless too small to be able to do anything useful with whilst weightless anyway!

[stevehudd]

You know when you throw that book or stone like you say, wouldn't the pushing force of throwing them mean a +G Force, Like If I Pushes down on the yoke too hard I would go past 0G and go into -G So IF I did start apitch down from a high VS I couldnt slowly push it forward at any rate could i as I'll either be -G or +G It has tobe just right :)

[pilotmike]

Whereas there would be considerable acceleration, and therefore 'G force', as the stone is thrown, lasting for just a tiny fraction of a second, I can assure you that there is no residual G force whilst the body is in freefall, with the exception of the tiny air friction force.

Your further observations about controlling the vertical 'pitch' in an aircraft to achieve zero G are basically correct, as far as I could decipher. It is a delicate art to achieve the perfect pitch to maintain zero G, neither positive nor negative, and to stop the freefalling bodies inside from hitting the structure. Remember seeing things floating about in video of freefall inside those aircraft? They do move about a bit - in addition to any velocity imparted by pushing etc, and this is a visual realisation of the imperfection of the attempted parabolic trajectory.

[stevehudd]

I remember seeing someone do zero g in a light aeroplane but with a dog in the back :p

[ChristiaanJ]

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.