Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!tut.cis.ohio-state.edu!att!cbnews!military From: willner@cfa.harvard.edu (Steve Willner) Newsgroups: sci.military Subject: Re: Particle Beam Gun Message-ID: <10621@cbnews.ATT.COM> Date: 26 Oct 89 01:28:04 GMT References: <10329@cbnews.ATT.COM> Sender: military@cbnews.ATT.COM Lines: 34 Approved: military@att.att.com From: willner@cfa.harvard.edu (Steve Willner) >> : of usable size. Trouble is, air resistance goes up with velocity, so the > Actually the viscous force (e.g. air resistance on a projectile) is > proportional to the velocity squared. Even more of a problem. Drag forces are confusing, and neither statement is entirely right. The drag force depends on two dimensionless numbers, the Reynolds number and the Mach number. The latter is fairly familiar and is just the ratio of object speed to sound speed. The former may be unfamiliar: it is essentially the ratio of "impact drag" to "viscous drag." In practical weapons and most vehicles, the Reynolds number is large and viscous drag can be neglected. Exceptions include water vehicles at slow speeds and conditions within the boundary layer, where the Reynolds number is small and viscous drag dominates. (Another example is microscopic organisms, where the Reynolds number is very tiny indeed!) At _low Mach numbers_: Viscous drag increases linearly with speed. Impact drag increases as the square of speed. As noted, impact drag is generally the only drag component that matters. High Mach numbers - applicable to the original suggestion of extreme velocity projectiles - are more complicated, and I don't know of simple formulas. I _think_ the speed dependence may even differ for objects of different shapes, but I'm surely no expert on this subject. ------------------------------------------------------------------------- Steve Willner Phone 617-495-7123 Bitnet: willner@cfa 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Internet: willner@cfa.harvard.edu