On 24 Jun 2004 01:35:58 GMT, (Regnirps) wrote:

(Steve VanSickle)

This particular *design* won't work, yes, but why not the "method" (i.e.
moving surfaces to make for a "hands off" reentry)?

I suppose it can be dome somehow, but you are talking 18,000 mph instead of
3,000. If there is a way to skim along and slowly loose energy I'd love to find
it. But as it now stands, as you lose energy you start to drop into more
atmosphere and more drag and loose it faster and drop faster and more heat
and....

Anyway, an ninformed quick calculation. Kinetic energy is proportional to the
square of the velocity. So, 18,000 mph is six times faster than 3,000 mph but
you will have 36 times as much kinetic energy, which will become heat (mostly I
think).

-- Charlie Springer

You've captured a large portion of the problem.

First let me say that Hypersonics and Reentry are not my field, so
what I say is necessarily of a general nature and may miss the
details.

The higher you go the more potential energy you've got to get rid of
in re-entry. As you move from sub-orbital through low earth orbit
(Shuttle) to high earth orbit to lunar return and beyond you end up
with more and more energy to get rid of. I believe it's significantly
worse than just the additional potential energy because of increasing
orbital velocity. I defer to our resident orbit wonk on the point,
but I seem to remember that the higher your circular orbit the higher
your orbital velocity. I don't even want to think about elliptic
orbits, they make my head hurt. But the higher you go you gain
kinetic energy as well as additional potential energy. All of this
has to go to heat if you want to end up at zero velocity by the time
you reach the dirt.

Reentry trajectories are often depicted on velocity-altitude plots.
These have velocity across the x-axis and altitude going up. A
vehicle arrives at the upper edge of the atmosphere with more velocity
the higher it comes from. so it comes in the top of the plot at a
greater entry speed. If you plot the trajectories we have data for
you see that Mercury and Gemini and Apollo move progressively to the
right on the plots. They begin with more and more energy. Apollo is
the case we have with the highest entry speed and the worst
atmospheric heating problem.

As you move to the right (move from sub orbital, to higher orbits,
etc) you move into regions where the thermochemistry get more and more
difficult. As you move to the right you enter regions where Oxygen
disassociates, then Nitrogen disassociates, then ionization occurs.
SS1's Mach 3 peak velocity avoids nearly all of that. At Mach 3 the
regime is arguably not Hypersonic at all. (The boundary between
supersonic and hypersonic is not a well defined line.)

As you move faster than SS1 and into the Hypersonic regime you end up
being far more concerned with heating problems than aerodynamic ones.
As a case in point, when the shuttle burned up over Texas the peak
dynamic pressure was 75 lbs per square foot. Not much. All the
trouble came from the viscous heating at those Mach numbers.

One additional factor which bears on the shuttlecock concept is that
in the hypersonic regime the heating problem gets worse as the leading
radius of curvature gets smaller. This is why vehicles come back
blunt side forward. Never say never, but I suspect that unless they
are made of pure unobtaneum the leading edges of the shuttlecock's
"fletching" would burn right off once you got up past Mach 5 or so.

With enough ablative material you might be able to counteract this,
but as you move up in speed other techniques of attitude control will
begin to look more attractive. The ESA is planning a vehicle which
will test aerodynamic control of attitude. It's called Cheops or
something like that, and they were testing models of it in the Mach 6
tunnel while I was at von Karman last year.

--
David Munday -
Webpage: http://www.ase.uc.edu/~munday
"Adopt, Adapt, and Improve" -- Motto of the Round Table