From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.tech Subject: Re: Heat Sink Heat Shields References: <5dcb47db.0310011151.51d744ce@posting.google.com> <5dcb47db.0310030723.3b2688dc@posting.google.com> <9186edb5.0310042111.41efb20c@posting.google.com> <9186edb5.0310050942.4862c9be@posting.google.com> NNTP-Posting-Host: 209.88.68.230 henry@spsystems.net (Henry Spencer) wrote in message news:... > In article <9186edb5.0310050942.4862c9be@posting.google.com>, > Vincent Cate wrote: > >So one big question I have is what is the right CD (coefficient of drag) > >for an Apollo style capsule on the way down? Henry, do you know? > > Unfortunately, no -- a browse through references doesn't give me that > number, although it could probably be worked out from some of the things > they do give. If I find some time I'll have a try at that... On page 133 of "Re-entry and Planetary Entry Physics and Technology I" by W.H.T. Loh it says that CDmax is about 1.7 for newtonian hypersonic flow for this type of thing (not Apollo specifically). Trying to get an intuition for the Cd I note that: drag = Cd * dragArea * 0.5 * density * v*v We can break out: massPerSecond = dragArea * density * v And if we were bring all the molecules in our path up to our speed then: momentumChange = massPerSecond * (velocityChange = v) So in this case: drag = Cd * 0.5 * momentumChange F = M * A = M * deltaV/deltaT = momentumChange From this I think that a CD of 2 means you bring the average molecule in your path to your speed (or to a stop relative to you). Is this right? It feels right that something like Apollo would be a bit under 2 since the air is still moving some as it goes past. I found on page 142 of SMAD 2nd edition (section 6.2.3) that a typical satellite CD is 2.2. Also, on page 207 they have a table 8-3 with CD values for different sats that go from 2 to 4. This would make sense because in free molecular flow you could bounce a lot of molecules back in the direction they came from, depending on your sat's shape. So CD=4 would mean bouncing everything back. -- Vince