Bouncing off the atmosphere

Jens Schweikhardt wrote
in :
# Snidely wrote
# in :
# ...
# # Isn't part of the point that the atmosphere, tenous as it is at the
# # reentry interface, is being compressed by the speed of the returning
# # capsule, to the point where it offers signficant resistance and lift?
#
# But where does the lift come from? The Apollo CMs are blunt cones, a
# shape very far from a lift providing shape like a wing. The pressure
# builds up in front of the blunt end, which moves almost horizontally
# and the decelaration force acts horizontally. For "bouncing off" a
# strong vertically acting force would be required, no?
#
# I wonder if Gene Kranz was simplifying things when he said "If it comes
# in too shallow its going to richochet off the Earth's atmosphere just
# like a stone does off water." (Source:
# http://www.youtube.com/watch?v=kdz6Zi3sJd8 at 7:20) I believe he just
# refers to the altitude-over-time plot, not the actual physical process
# of momentum exchange (and thus upward force).

Oh well, who am I to suspect Gene Kranz would simplify things.

# For me it boils down to the question: why does the altitude raise again?
# Possible answers:
# a) Newton 1 = Law of Inertia, i.e. no deceleration below orbital speed
# so the capsule keeps an elliptical orbit (though with reduced apogee)
# b) Aerodynamic effects providing lift
# c) Something else
# d) A combination of the above
#
# My gut feeling as a physicist tells me a). But my knowledge of orbital
# mechanics and aerodynamics is limited at best.

The answer is in this awesome NASA educational video, titled
"Apollo Atmospheric Entry Phase NASA":
http://www.youtube.com/watch?v=qpL7VaugDa8
which leaves no question unanswered. For the curious: my gut feeling
was wrong (how clever of me to have appended some weasel words :-)
If you ever wondered how they computed when it's time for TEI and
all the bits of where they come down, this is a must see!

Regards,

Jens
--
Jens Schweikhardt http://www.schweikhardt.net/
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