On 26/07/2018 11:38, Bert Coules wrote:
Tim,

Many thanks for your thoughts.Â* The collapse has to be pretty much
instant, so any release of pressure on an inflatable dummy would need to
take immediate effect or nearly so.

snip

Inflate the 'balloon' with ammonia gas. To deflate quickly, squirt some
water in - ammonia dissolves in water very well and rapidly.

(Ok, maybe not, a bit dangerous, but it might provoke some other ideas.)

Cheers
--
Clive