Normally they're set off by the shockwave of the explosive nearby
going off, which is to say that sudden large pressure rises set them
off. A heartily supersonic bullet can make a large sudden pressure
spike, a 300 mph impact not so much.
On Mon, Aug 3, 2015 at 7:02 PM, Dave McMillan <skyefire@xxxxxxxxxxxx> wrote:
The recent PEPCON discussionk, along with a recent MythBusters rerun,
got me to thinking on this subject. The Mythbusters ep in question is the
one where they demonstrate that a Hollwood SFX explosive can be set off by a
high-powered rifle bullet (~1400mph, IIRC), but then tried to set off a
car-trunk load of the stuff by slamming a truck into it at ~300 mph (using
the University of New Mexico's rocket sled), and failed. A quick BOTE
suggests that the truck should have had more than 200x the impact energy of
the bullet, so I got to thinking: just what are the key factors in play
here? Total impact energy obviously isn't it. Is it energy *concentration*
-- joules/unit volume, or even joules/impact area? Or is it the impact
*velocity*, as relative to the shockwave propagation rate in the explosive?
Or is this one of those questions where the answer is "it depends"? :)