Hi Ash

>Now that sounds like a great fun project :)

Not really but someone has to do it ;-]

>I'm working on a timer for use in Amateur Rockets, you can get 30Gs or more
>on the initial firing of a large motor..

I helped with an altimeter for a friends hi-powered "lawn dart" They are
getting those
things 10-15K off of the pad. It's amazing they get that high with only a
few seconds
burn time on the motors. I'm not sure what the actual G load is on these
guns but it should
be 100G plus. The altimeter was just thru hole with coating and it worked fine.

>I'm going to be using SMT components because of size and weight.. And
>because of F=MA, the smaller the mass, the less force acting on the
>component. However my timers are (hopefully) reused.. :)

The SMT seems to be the way to go.  It would cut the size and mass way down.

I'm not quite energetic enough to try and find something the size of a quarter
buried in snow somewhere on the side of a mountain ;-]

>Also, I've heard (but have no evidence) that crystals are much more
>sensitive to acceleration then resonators.. so be careful how you do your
>oscillator. Maybe use the internal R/C oscillator and a special pre-trim
>step prior to launch. Feed the PIC a known frequency so it can compute the
>appropriate calibration value.. This would have to be done at ambient
>temperature just before launch I would guess.

The hardest part about doing this is the "operators" in my case. It would
have to occur without user intervention. The guys using these are not techies,
are generally cold, wearing gloves and could care less (from what I've seen)
If I can figure out how to do this without them having to take off the
gloves they
would do it. ;-]

>  Perhaps the cheapest way to do this is with a
>thin wire loop through which a clip is inserted and a length of string ties
>the clip to the back of the barrel. This is assuming that your launcher is
>air powered..
The lanyard is how they want to do it. Its pretty simple, cheap and reliable.
All of this has to function reliably in some fairly harsh conditions.

>You also need to think very carefully about safely arming and disarming the
>device. You definitely don't what this going off in the gun or near you!
Well right now I'm working with 4 distinct steps to arm or disarm.
Reversing any of the
4 would stop the fuse easily. Even with one of the small charges (about
4lbs I think)
you have to run pretty damn fast to escape the blast radius. Currently they
are doing
this with burning fuses. so the only safety is to fire it or yank it apart.

Dave

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