Motor cases...how many times can they be reloaded before they fail?

We have a few 29/120 cases that we have been using for motor testing, and we intend to fire them alot. I am curious about how many times you can reload and fire a motor case before you have to start worrying about case fatigue-related failures?

Thanks

Jason

F3Victor7 said:

We have a few 29/120 cases that we have been using for motor testing, and we intend to fire them alot. I am curious about how many times you can reload and fire a motor case before you have to start worrying about case fatigue-related failures?

Thanks

Jason

Click to expand...

For small motors like those, I would put the number at at least a hundred.

Just do a through inspection before you fire them and they should last indefinitely. Sometimes its hard to diagnose a motor failure, but most of the time it is propellant defect or user error.

I've got a 29/40-120 case I've used a lot since 2003. A very conservative estimate would be 15 flight per year, so that puts it north of 150 flights. I see no reason it won't last another 150 or more.

The usual response is, "three, four times at most. Send me your old cases to be properly recycled..."

Worsaer said:

The usual response is, "three, four times at most. Send me your old cases to be properly recycled..."

Click to expand...

Once, to be safe. I pay $5.00 for your empties....

Handeman said:

I've got a 29/40-120 case I've used a lot since 2003. A very conservative estimate would be 15 flight per year, so that puts it north of 150 flights. I see no reason it won't last another 150 or more.

Click to expand...

And 2 with blown end closures, right?

There isn't so much a predictable number as much as "until something happens" -- lost rocket might be #1.

bill_s said:

And 2 with blown end closures, right?

There isn't so much a predictable number as much as "until something happens" -- lost rocket might be #1.

Click to expand...

True, but that wasn't the cases/closures fault. I got new forward closures and have continued flying the same cases.

fyrwrxz said:

Once, to be safe. I pay $5.00 for your empties....

Click to expand...

I pay 6
- J

How many flights can you get from a motor case?
One less than the actual number of flights before failure.
This can be expressed mathematically as TFbf +1 = Oh Crap!!

But seriously folks; has anybody ever performed enough actual testing to say with any authority that a case should be retired after X number of flights or a total of X amount of Newton seconds impulse?

Hopefully I asked that last part of the question correctly.

Deformation around the snap ring grooves would be bad. Discoloration of anodizing would be bad. Changes in case dimensions would be bad. Serious nicks or major scratches would be bad as they weaken local areas of the case. I'd consider excessive corrosion bad as well as that also weakens the case.

Essentially, if a case doesn't look really good, look at it really closely. If in doubt, get someone else with experience to look at it as well. Smaller cases are inexpensive enough that it is not worth risking a rocket for that one more flight.

The bigger the case, the higher the pressure, and the longer the burn, the more all of this matters. Commercial cases are designed with a considerable safety margin, but that is based on some assumptions. Get a case hot enough to destroy the T6 temper rendering it T1, and flying it immediately, would be begging for a failure. Wait a week and it ages to T2, and it is still asking for it. Anyway that's all the recovery you'll naturally get from aluminum. Chances are a small case would be difficult to get hot enough to be an issue. It can be done though with large ones!

The smallest cases probably have the biggest safety margins.

Gerald