Xref: utzoo sci.military:10690 rec.models.rockets:2315 Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!sun-barr!newstop!sun!amdcad!amdcad!military From: deichman@cod.nosc.mil (Shane D. Deichman) Newsgroups: sci.military,rec.models.rockets Subject: Re: Secant Ogive Message-ID: <1991Jun14.080300.20680@amd.com> Date: 13 Jun 91 16:29:40 GMT References: <1991Jun13.055308.22899@amd.com> Sender: military@amd.com Organization: Naval Ocean Systems Center, San Diego Lines: 55 Approved: military@amd.com From: deichman@cod.nosc.mil (Shane D. Deichman) [discussion of aerodynamics and ogive-secants deleted] John Bercovitz of LBL raises a very interesting point in his posting. This discussion, of how conical points have less drag than other types of projectiles, is similar to a recent discussion in sci.physics on the speed of golf balls. The point raised in the sci.phys postings was that the dimples on the golf ball actually increased the speed of the ball by decreasing the pressure drag created by the laminar flow. To illustrate (this figure is plagiarized from Jeff Berton at NASA Lewis Research Ctr :-) o o o --- o / o --- o / o --------- o / o Wake Flow o Phi / o -----> - - - - - - -o- - - - -/- - - - -o- - - - - - - o o o o o o --------- o o --- o o o --- According to Jeff, the theoretical value of phi (from noncompressible laminar boundary layer theory) is 109.6 degrees; experimentation shows a value of 112 degrees. Now, for a smooth surface, such as a billiard ball or a bullet, the flow is non-turbulent (i.e., laminar). This results in a lower value of phi, which in turn creates a wider wake. As this wake is wide, there is a greater pressure differential that creates a "pressure drag." This causes a reduction in velocity. The dimples on a golf ball perturb the smooth, laminar flow to create a more turbulent flow, thus increasing the value of phi. This in turn reduces the breadth of the wake and decreases the pressure drag -- result- ing in a higher velocity. Now, the question that comes to my mind is -- what are the drag character- istics of a smooth projectile such as a bullet or an ICBM's RV? These objects seem to be quite smooth, which (if the above discussion is correct) would imply that they suffer from pressure drag of biblical proportions. Not being an aeronautical engineer, I don't know of any means by which this problem is dealt with. Perhaps someone more insightful in Netland can clue us in... (Sorry to not answer your question, John, but I think you've provoked an interesting discussion!) -shane -- deichman@cod.nosc.mil