[AR] Re: SpaceX F9 Launch/Update -- Live Link
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 22 Dec 2015 19:21:56 -0500
On Mon, Dec 21, 2015 at 09:08:46AM -0800, Paul Breed wrote:
I think for an optimum result you want some wind this carries
the downward en-trained air away from the vehicle making act more like a
ballistic object and reduces random areo variance.
IE with a 5 knot wind a helicopter can descend vertically....
With zero wind a helicopter can not really descend vertically and must
carry some forward speed into the hover near ground effect....
The reason for that, though, doesn't carry over to rockets. With
helicopters, when trying to hover in a calm, you are in essence trying
to re-accelerate the same air you've already accelerated. Say,
instead of accelerating air from zero to X, you're accelerating it
from X to 2X. That gives the same lift (assuming the same mass of air
is accelerated), but requires three times the power (4X^2 - X^2 vs
just X^2) -- and helicopters usually don't have a factor of three to
spare in power.
With rockets, though, you're just always accelerating propellant that
starts at speed zero relative to the rocket.
--
Norman Yarvin
http://yarchive.net/blog
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