I'm glad you were able to convince yourself that you might have been
wrong, since no one else can.
On Fri, Dec 15, 2017 at 3:05 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
On Thu, 14 Dec 2017, Ben Brockert wrote:
And to first order, that's correct. The rule-of-thumb first-guess Cd for
satellites is 2.2, and most satellites that don't have a lot of area
parallel to the flow (end-forward chocolate-bar shapes like GRACE, or
edge-on solar sails, are exceptions) do indeed have a Cd in that
vicinity.
Yes. So if you make a satellite with a bunch of fin area where each fin
plane contains the vector of orbital direction it will have significantly
more drag than is proportional to its frontal area, which cuts down its
impact sweep area in the direction of its velocity.
Yes, surfaces edge-on to the velocity vector don't have as much drag, but
they do have some -- a first rule of thumb is that they're equivalent to
10-20% that much frontal area. The random motion of air molecules is a
modest fraction of orbital velocity, so they come in from a cone centered on
dead ahead, not just from dead ahead, and some of them do hit edge-on
surfaces.
Unfortunately, the impact hazard isn't mostly from straight ahead. I saw a
plot once for (I think it was) Space Station Freedom, in a 28.5deg orbit, of
impact hazard vs. direction. The peaks were at 30-40deg to left and right
of the velocity vector, if I recall correctly. (It's somewhat a function of
orbit inclination, because debris isn't uniformly spread -- there are
clusters at specific inclinations.) So edge-on fins do still count as
impact sweep area.
*However*...
The impact hazard is almost all in the horizontal plane. LEO is just barely
clear of (most of) the atmosphere; anything coming from much of an angle up
or down would have an orbit that intersects the thick part of the
atmosphere, i.e. a negligible orbital life, and so almost nothing does.
If you could ensure that your fins stayed not only edge-on, but also
*horizontal*, perhaps by adding a gravity-gradient stabilization boom (and
the associated damper), *that* would give you drag area that wasn't
significantly exposed to impacts. Not nearly as light and simple, alas.
Henry
