Estes ejection charges causing CATOs in low tube volume rockets!

[RedMax]

About 2 weeks ago, I decided to go out to the field with my little sister and put some rockets in the air. She's an avid rocketeer as well, so she brought down her Custom Rockets Galileo. The Galileo has 4 pretty large vent holes cut in the tube since it's a small rocket with nearly no empty volume. The Galileo goes out to the pad for what should have been its 5th successful flight on a 1/2A6-2- zooms up to Very High™, and then a deafening bang like a rifle rings out! The whole thing had exploded, we could only find 1 1/2 legs, bits of paper from the shroud, a shard of the balsa nosecone(!), and the offending engine. No burn-through, and upon soaking the casing in the sink and unwinding it I determined that the problem was not a propellant CATO, it was merely an insanely powerful ejection charge. We were both upset, but hey, those are the breaks!
Fast forward to yesterday: We go out to the field again to fly my Stratocruiser with 808 camera onboard, and she tags along with another of her rockets. This time it was the N-1, a little minimum diameter scratch build about 8" long running on an A8-5. I decide to let her have the inaugural launch for the day, and she scampers down to the pad with the rocket. It goes up to around 600 feet, and at ejection the SAME THING HAPPENS. Blam, tube popped like a biscuit can, plastic NC shattered, and the tiny 6" chute melted to a little wad. These ejection charges are too strong, especially for a little 1/2 A/A8 engine! We're never going to pressurize a BT-80 with the ejection charge of a 1/2A , so give us a break, Estes!

Anyway, there's my rant. What can I do to prevent this in the future?

 

[Area66]

.....and the tiny 6" chute melted to a little wad.

Are you using wadding pad ?

 

[shreadvector]

You probably had a catastrophic failure which can occur at any time from ignition to ejection (or lack of ejection).
If the motor was too tight, then it might not have been a cato, but if there were clear vent holes then the ejection charge really was too strong and you did have a cato.

Fill out a MESS form online and also notify Estes. You will need the date code on that motor casing or the others in the pack.

I have no idea why you would soak an empty motor casing in water since that would not produce any hint or clue about what happened. You only soak loaded black powder motors in water to destroy them

 

[RedMax]

Shread, I've always soaked my old engine casings in water. That makes it easier to unwind the paper making up the casing, which allows me to see if there are any partial burn-throughs.
Area66, of course!

 

[RedMax]

Sorry for the terse reply- had to go do something urgent.
Anyway, both engines were friction-fitted with masking tape. Both failures also occurred well after apogee, so I'm sure it was the ejection charge. I've already filled out MESS reports for both engines.
A bit more on the soaking: I'm honestly surprised more people don't do it. Soaking the expended casings in a bowl of water makes the paper swell and the glue disintegrate making it easy to unwind the paper that the casing is made of. Usually the first two layers of paper touching the propellant are burned through or severely blackened. This is often a sure indicator of an abnormal motor: any motor that burns through more than the first two layers or does not burn through to blacken the second is likely funky. It also allows you to see exactly how much propellant is in the engine, and therefore the engine class, by how much of the casing is burnt. I once had a B6-4 that was filled to the brim, and when used was consistent with the performance of a C6. Sure enough- it was filled to the propellant level of a C6.
Plus, it also allows me to see exactly how many engines I've burnt per year without saving sulphurous-smelling cases. I cut out the part of the soaked casing with the label on it and save it. Last year I burned through 12 A8-3, 27 B6-4, 2 B6-0, 2 A8-7, and 15 C6-5.

 

[kyle]

I always look at the fore end of my engines just to see how far they're filled. A supposed B filled to the level of a C should be fairly obvious (halfway filled casing vs. fully filled casing). If you've a B filled to the brim, best not to launch it and report it as the hazard it is. To do otherwise then complain about it later is just silly.
Maybe it's a a temp or age thing, though? How old are the engines? Still not going to affect the obviously misconstructed engines, but might help figure out what happened.

 

[shreadvector]

If the motor was friction fit TIGHT and there were no large vent holes, then there was no cato, it was simply a case of not understanding how a rocket motor works and that the ejection charge HUGE volume of gas has to go somewhere. if the motor is tight and the gas has nowhere to go. it will either overcome the tight friction fit and blow the motor out the back like a bullet or it will blow the body tube apart.

For a rocket that ejects the motor, you want it to be just tight enough that it will not fall out by itself, but loose enough to easily eject.

I have never soaked my casings to unroll them to see how much cardboard has been burned and I've only launched a few tens of thousands of motors since 1970.

 

[455 Buick]

Same day CATO on 2 different engines is unusual. Any chance that the chute and wad packing that day caused the issue?

You said one nose cone was wood, the other plastic, but any chance that humidity, cold weather, condensation, and swelling/shrinkage of balsa/cardboard had anything to do with tight nose cone fits that day to cause over-pressure...if the pre-launch prep was done far enough before the launch to not notice that post-assembly swelling had occurred?

One of the rockets was chute recovery (wood nose cone). Was the plastic nose cone one tumble/rear eject? I could see as Shreadvector said that a tight friction fit with a glued nose cone would cause over pressure.

 

[samb]

I can personaly testify to the fact that some Estes A8's have an energetic ejection charge (called "shotgun" in these parts) and a rather loud report. I don't regard them as defective. The usual result is shock cord separation, especially on models with the standard length Estes shock cord. It's also possible that a blowout could occur along a weak spot in the airframe when the wadding/recovery device is jammed in or the nose cone is unusually tight. So... what to do ? During construction use a longer shock cord and make sure the mount is secure. During flight prep make sure the the recovery wadding is not packed tight against the motor. Make sure the recovery device can move within the tube as well. Powdering a chute before packing will help ensure deployment from a tight roll. I usually will use a streamer for minimum diameter models that came with a parachute in the kit. The test I learned back in the day is to hold a flight ready model upside down give it a gentle shake. The nose cone and recovery device should slide out right out.

I have never soaked a burnt engine case and can't comment on the accuracy of the OP's analysis or his conclusion: "... often a sure indicator of an abnormal motor...".

 

[RedMax]

Plastic NC was chute, wood was tumble. Given the replies here, I suspect that a tight/swollen engine may have been to blame for the Galileo. It's really humid here in Louisiana and my sister likes to prep her engines well before launch.
Shread, both rockets were flown several times before on the same engines without ejecting the engine or blowing apart the tube. About the soaking- to each his own.
The rockets were flown on two different days. The engines were new.
Kyle, I decided to fly the malfunctioning B because it was the only engine I had at the time and I had to impress small children.

 

[hcmbanjo]

The Galileo has 4 pretty large vent holes cut in the tube since it's a small rocket with nearly no empty volume.

A BT-20 model with four vent holes?
I would think four vent holes might weaken a thin BT-20 tube.
Add a strong ejection charge and that might have led to it blowing apart.

Most featherweight models usually eject the engine with a loud pop.
I've had a strong ejection break the boom on a Semroc Hawk boost glider.

Estes ejection charges seem stronger than they were years ago.
It might be to insure a parachute deploy for first time flyers.
When you think about it, an 18mm engine has to blow out parachutes on BT-20 diameters through longer BT-60 diameter tubes.

 

[RedMax]

Big fan of your blog, Chris!
If 18mm ejection charges are that strong, are 24mm charges stronger?

 

[caveduck]

Over the years I've seen a number of over-energetic ejection charges, some that have burst a regular Estes tube to shreds. For whatever reason 13mm motors seem more prone, and there seem to have been more in recent years. It's hard to imagine that the volume of BP used would be that inconsistent, but the clay cap confinement could be. BP burn rate increases drastically with pressure, leading to explosive behavior with very small amounts of powder if tightly confined. It's hard to say for sure but my impression is that this was a rare to nonexistent problem back in the days of paper caps. IMO grossly excessive ejection energy, though uncommon, is indeed a malfunction.

 

[RedMax]

Do quest engines have this problem? May have to start buying those for my smaller rockets if they have a paper cap. I remember seeing one once with a bit of paper on the end with Chinese characters on it.

 

[MALBAR 70]

Quest motors are the opposite. Estes motors have a "shotgun" ejection,Quest motors have a "flea fart" ejection.

I have used both without a problem. But be careful Quest motors are a longer burning and will send a small/light rocket higher.

 

[hcmbanjo]

Malbar is right about the Quest Chinese made engines.

The A6-4 and B6-4 engines are okay.
Loud thrust and will leave some soot inside your model. I like them in shorter and stubby rockets.
They will eject a streamer or parachute in minimum diameter models that aren't tall without much problem.
Just don't pack the wadding too tight.
I wouldn't expect them to eject anything much out of a BT-60 Big Bertha style model.

The Chinese made C6 engines won't lift anything heavy, so stay with the A6-4 or B6-4.
It's too bad, the Quest C6 can be fun, it's a longer burn engine.

Sometimes you'll find the paper cap still in the top of the engine after a flight.
The cap just turns like it was hinged.

 

[dpower]

IIRC, the Galileo is intended to kick the motor, for featherweight/helicopter(ish) recovery. If it was too tight, that may have contributed to the damage.