Assistance requested diagnosing what might have happened with a bunring up at launch

All -

I had a burn-up at launch that's a bit of a mystery and was wondering if anyone could help me figure out what might have happened. I had a Cessaroni 38mm 3-grain I212 in a Cessaroni casing in a Madcow Super DX3 rocket. When I launched it though, the rocket never left the pad, the top came off pretty quickly, and flames came out the top. I originally thought I forgot to put the casing in, however I confirmed the casing was in the rocket, though it meted somewhat (see picture, though note the top was screwed in all the way I believe). Any idea what might have happened?

Many thanks!
- Adam

rear closure failure.

The pro38 design has what I'd call a flaw. The plastic rear closure is easy to mis or cross thread. when that happens....this happens.

The best way to avoid it is to remove the closure from the load, push the liner and load into the casing, and then use a flipped 38mm spacer to make sure the liner is fully seated, then screw in the rear closure.

DizWolf said:

rear closure failure.

The pro38 design has what I'd call a flaw. The plastic rear closure is easy to mis or cross thread. when that happens....this happens.

The best way to avoid it is to remove the closure from the load, push the liner and load into the casing, and then use a flipped 38mm spacer to make sure the liner is fully seated, then screw in the rear closure.

Click to expand...

Wow Dave,,,,
You da man...............lol...
I didn't know that at all about CTI 's

Teddy

This also looks very similar to an igniter that might not have been all the way to the top. If a motor lights in the middle, it can cause a 2 way road flare type burn.

When those failures happen (cross threading or partially inserted igniter) how do the delay grain and forward closure burn through? Is it a matter of enough heat and pressure in the wrong direction?

If you look closely at the picture, there are four intact threads at the lower part of the frame (which is correct for 38mm nozzle), but only two threads are seen at the top part of the frame. Two threads are gone on one side of the nozzle.

At the same time, the plastic did not melt - so it was not exposed to exhaust temperatures, i.e. The nozzle remained in the case for entire burn duration.



I suspect cross-threading of the nozzle into the case. Upon ignition, one side of the nozzle popped a bit out. This opened space on the top of the motor, enough for pressurized gases to escape. Ejection charge got ignited from top side and blew the rocket open, etc.

Let me guess, the top of the case melted like a candle wax?

Sorry for your loss! I had the same thing happen to my rocket too...

Thanks all, for the info! I attached a couple of other pictures of the nozzle, and upon inspecting it physically, it looks like there are 4 threads all the way around, but they are kind of flattened-out/fused together in at least one place. As for the top of the case, it is actually intact. The top part of the motor is down around 1-2 inches and seems quite stuck/melted into the case.

Does any of that help?

A couple other questions come to mind. What motor is it, and what size case? How many, if any, spacers were installed?

38mm 3G I212 SS motor in a 3G case (so no spacers).

The fore end of the motor should run all the way to the end of the case, you say there's a one to two inch gap?

Do you recall if the paper cap on the forward end of the motor was flush with the top of the case when you built it?

and dumb question I know, but when it burned, was it smokey?

The threads on the nozzle looks OK but that doesn't mean the case threads were good. It's probably too late now for a good lookie loo.

I have had the nozzle mis thread when loading it in the casing. It results in a well cooked rocket.

cwbullet said:

I have had the nozzle mis thread when loading it in the casing. It results in a well cooked rocket.

Click to expand...

Yea, I think we're chasing around in circles and it's just gonna come back to this. The upper part of the liner and bulkhead probably just dropped down during the cook-off.

I had a similar incident with a 38 mm J595 Blue Streak. Motor did not light, just blew off rear closure and JB Welded Aeropack retainer. JB Weld and Retainer held, but paper tube underneath gave way and spit out entire motor grains in chunks, unburned. Jeroen thought that it was due to possibly purchasing an old motor that was damaged. Left in photo shows rear closure, right is blown off Aeropack retainer, upper is aft end of 38 mm case. You can see melted closure in casing. Past that is intact liner. Maybe was cross threaded closure. Would have destroyed my rocket had not the paper tube given way.

DizWolf - I'm pretty sure the paper cap was flush with the top of the case. And yes, it did seem pretty smoky when it burned.

afischer said:

DizWolf - I'm pretty sure the paper cap was flush with the top of the case. And yes, it did seem pretty smoky when it burned.

Click to expand...

Did you drill the delay grain at all - if so was there definitely a delay grain in the forward closure?

Yes I did drill a delay and then put it back in

afischer said:

Yes I did drill a delay and then put it back in

Click to expand...

When you did that:

1.did any grains fall out or get taken out of liner? If so did you make sure the grain with the pellet was placed back on the top, next to delay?

2. If they did come out.....one end of each grain is flat & the other is concave. They MUST go back in with a flat surface mating to a concave....never flat to flat or concave to concave.

DizWolf said:

rear closure failure.

The pro38 design has what I'd call a flaw. The plastic rear closure is easy to mis or cross thread. when that happens....this happens.

The best way to avoid it is to remove the closure from the load, push the liner and load into the casing, and then use a flipped 38mm spacer to make sure the liner is fully seated, then screw in the rear closure.

Click to expand...

I was 'helping' a guy do his L2 with hia spanking new 5g pink and prepping my 5g long burn pro38 and could not get either load to seat the final 1/8 inch to properly start the nozzle thread.

Bandman came over and helped us wrazzle both loads seat a hair further..but we were digging in the trash for a spent liner to push with..neither one of us thought to grab a spacer !

So thanks for the tip!

Adam sorry that your DX3 burnt up.. some would say spread some happy horse droppings and say this is an opprotunity to build a better one ..but that isnt me. Let me know if there is some thing I can help with when you rebuild or replace..

Kenny