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Spacefraft Aerodynamics - my speculations.



Being a structural engineer and not an aerospace engineer my thoughs on this
subject are only my speculation, but never the less, this is my two cents
worth.

    I agree about the spacing of the atoms/ions/molecules as being quite
significant. The principle effect of this is that compression waves (like
sound) would propagate very slowly if at all and so the drag/lift parameters
would be better modeled by Mark effects (supersonic) with Renolds effects
(subsonic) being insignificant.(I agree, "laminar" and turbulent" flow ideas
are insignificant - they relate to the Renolds or "subsonic' effects)

    Based in the idea that atoms/ions/molecules are an average of 25 mm
apart and have an average molecular mass of 28 (Nitrongen molecule) then the
density would be 2.98E-18 kg/m3 and at a velocity of 9929 m/s (velocity at
perigee at 320 km altitude), the stagnation pressure (presure due to the
difference in kinetic energy) would be 1.47E-10 Pa (Newtons per square
metre).

    If the gas particles were to simply accumulate in a pressure wave where
they contact the spacecraft the "bleed away" due to the pressure differences
between the front of the space craft and the side of the space craft (like
normal supersonic pressure waves) then I can't see a drag coefficent of 2 or
2.2 being relavant. I would speculate that it would be about 1.0 (1.0 is the
coffecient of drag of the front of a round flat plate at 90 degrees to a
fluid flow). Since negative absolute pressures are not possible and the
absolute pressure is almost zero then the back of the plate, or in this
case, the space craft would contribute basicly nothing to the drag forces (I
agree with the stream of particles idea).

    I think the drag force (N) on the space craft would be the area of the
frontal silhoutte (m2) x stagnation pressure (Pa) x the Coefficent of Drag
(I think 1.0 would be pretty close to the mark).

    Any one else have some ideas on this?

Regards,
Murray Peterson
VK2KGM

----- Original Message -----
From: "Ken Ernandes" <n2wwd@mindspring.com>
To: "Margaret Leber" <maggie@voicenet.com>; "Douglas Braun & Nadia
Papakonstantinou" <doug@dougbraun.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Friday, April 06, 2001 1:23 AM
Subject: Re: [amsat-bb] Spacefraft Aerodynamics [was:Re:AO-40: Plans for
April-June 2001]


> ----- Original Message -----
> From: Douglas Braun & Nadia Papakonstantinou <doug@dougbraun.com>
> To: Margaret Leber <maggie@voicenet.com>
> Cc: <amsat-bb@AMSAT.Org>
> Sent: Thursday, April 05, 2001 11:07 AM
> Subject: Re: [amsat-bb] Spacefraft Aerodynamics [was:Re:AO-40: Plans for
> April-June 2001]
>
>
> > At the atmospheric pressures where the satellite is, the classic
concepts
> > of "laminar" and turbulent" flow do not hold.  The reason is that the
> > gas molecules are able to travel a significant distance (several cm?)
> before
> > colliding with another gas molecule.  Thus, the aerodynamic forces on
> > the S/C can better be modeled by stream of individual particles than
> > by fluid flow.  I believe that the drag would be almost exactly
> > proportional to the frontal area (i.e. silhouette) of the S/C.
> >
> > Perhaps somebody who has studied this subject more recently
> > than me (i.e. in the last decade or two) can give a more detailed
> > and accurate explanation.
>
> Generally your coefficient of drag (CD) is somewhere around 2.2 for a
> spacecraft, which is a tad worse than a normally-oriented flat plate (CD =
> 2.0), which would represent the frontal area silhoutte.
>
> 73, Ken N2WWD
>
>
> ----
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