inertial initiator , acceleration problem 1

[sunthorn]

I have been asked to work on a inertial initiator for an artillery shell that would work at the following coditions. Its been a while since i've had to do any serious mechanical design calculations so i am unsure of how to approach calculations. Although I have no problems drafting the design idea in Acad or Pro/E.

It must initiate an all fire condition when a 950g +/- 150g acceleration of 15 milliseconds or more is applied and must no fire when a 2000g load of 5 milliseconds or less is applied.

The design i'm working on calls for a brass pistion of a known weight (60 grams or so) mounted in a SS sleave/housing. The piston would be pinned using a wire that would fail in double shear when the all fire condition exists.

In order to determine the correct sizing of a wire made from a brass alloy with known properies. Should i determine the kenetic energy of the piston generated during the firing, convert that to a force and then apply that force to the wire and work backwards to obtain a wire size that will shear at the 950g shock and not the 2000g shock.


Any suggestions would help me greatly.

Sunthorn

 

[Barry1961]

At a glance it would seem you would need some type of damping. Maybe a rubber housing that would not deflect far enough under the short high G load to shear the wire.

Barry1961

 

[sunthorn]

We have thought that we may need some type of damping system, but space wise we are constrained. This entire initiator assym. must fit in a .75" diam x .75" tall cylinder.



The initiator will activate a chemical process via pyrotechnics in the a joining volume. This chemical process (600 C to 700 C) is very sensitive to organic materials decomposing.

 

[Barry1961]

If I understand the application you may only need damping in one direction. It would seem like you could use a Belleville washer on the side where you want it to stroke.

If you just use the brass wire it will become weaker with each hit as the repeated small hits cut thru it a bit at a time. Like bouncing down a rough road.

I would be wary of urethane or rubber washers after thinking about it. The spring back time may be to long.

You can get a Belleville washer that is .75" OD by .070" high that has a 432lb load rating at .007" deflection. You are going to have to use up about 207 ft-lb of energy so you may want to decrease the length of the piston and stack some washers to get the stroke you need. The washer above is one of the stiffer ones.

I would go for a much lighter piston with a long stroke and a shear hole oversized enough that the wire would not be touched by it during a false hit.

Barry1961

 

[Barry1961]

Something else to consider is going to be how they are going to test it. I have never done ordinance but for equipment they simulate a life time of use in as short of a time as possible. If they put this in a shaker and turn it on the spring is going to get hot quick. That much energy will have to be dissipated over some minimum time between hits.
The people at the testing grounds will work with you if you let them know up front what you need. Don't wait until after they build a special machine to test this and it fails before telling them.
Also remember if the test you want done takes 6 months your accounting dept. will lynch you.


Barry1961

 

[sunthorn]

you are correct the damping would only need to be in the axial direction of the initiator although the 2000g shock can occur in any axis.

i didn't even think of heating of a spring if used as a damper when enviromental test are done.

 

[Cockroach]

Yes Sunthorn, I would apply dynamic load over and above static conditions. You may also what to use the properties of the wire at "static thermal conditions" that would exist after repeated fire, that would give you worst case scenario.

Military application, love it. Hope the Noble Prize in engineering is on the horizon.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Barry1961]

If this is a old type rifled barrel you might be able to trigger it with the spin. I did a couple of wags and came up with about 6 in-lb of torque.
If you can use the spin it would solve a lot of the problems you would have in testing with heat. It should also be easy to make it immune to any linear forces.

Barry1961

 

[sreid]

Does anyone know how shell arming devices are currently implemented and their range of go/no-go?