Two Hobbyline CATOs ????

[Handeman]

Last weekend I flew two G64W-10 29/40-120 reloads in my rocket. Both motors lit and lifted the rocket about 15 - 20 feet above the pad (This tells me the igniters didn't clog the nozzles or it would have over-pressurized and never left the pad.. At that point the ejection charges went off blowing the nose cone and chute off. The rocket went unstable and spun in place for a second and when the thrust ended, it fell to the ground with a 4 ft flame coming out the BT.

The powder well was melted off the forward closures of both motors.


Disassembly showed the aft o-ring was between the nozzle and the aft closure, as it should be when assembled IAW step 13, Fig-9 of the instructions.


Removing the forward enclosure showed the o-ring was stuck to the forward closure indicating it was installed correctly IAW step 9, Fig-5 of the instructions.


The forward insulator was still attached to the Liner tube as usually happens with these reloads.


There was nothing left inside the forward closure as the delay grain, delay insulator and o-ring were all burned away.


I've assembled over a 100 of these and have never had a CATO in 11 years. I don't believe I errored in assembling the delay grain. I greased the o-ring with the other two. The o-ring was inserted into the forward closure (I pour the parts onto a tray for assembly and didn't have any parts left over.) These had 10 second delays so there was no delay spacer to insert and possible get on the wrong end of the delay. I chamfered/deburred the delay insulator with my thumbnail like I do for all the AT reloads I do. The delay grain was inserted into the delay insulator and that was inserted into the forward closure and stuck out about 1/16" IAW step 6 Fig-3 of the instructions.

Both motors were from my latest order, but both were from different batches.

Can anyone think of anything that would have caused the flame to get past the delay grains? It happened about a half second after ignition since the rocket lifted 15 - 20 ft above the 6' pad, so things held for a while. The ejection charges didn't go off until failure when the rocket was 15 - 20 feet up. I don't think it was an over pressurization because that should have blown the forward or aft closures off, but....

Anyway, any ideas would be greatly appreciated.

 

[tmacklin]

How many times have the motor casings been used? Perhaps they have eroded or experienced metal fatigue over time? Just a thought.

 

[Handeman]

How many times have the motor casings been used? Perhaps they have eroded or experienced metal fatigue over time? Just a thought.

One was 11 years old, the other about 3. It wasn't a metal failure. The flame and pressure bypassed the delay grain and the hole in the powder well in the forward closure worked as a nozzle. Of course being aluminum they eroded away by the time the propellant was burned up.

 

[hball55]

Next time I flew one of those reloads I would add some masking tape around the delay to make an extra tight fit. Either that or add more grease around the outside of the delay. What grease are you using, super lube? Dow 111?

 

[bill_s]

Gotta be something. I know you know what you're doing so this concerns me. Drill to vigorous or wrong side? It was really cold. I built my G64 for that day at home.

I'm relieved I called Hobbylinc and told them to go on and ship the -7's pronto, back before the originally scheduled launch date . However that -10 is the needed delay for my new rocket and a nice motor, I assume plenty of G64-10W motors have been flown successfully?



P.S. did you intend to tighten the closures before flight?

 

[DizWolf]

Two widely used reloads, built properly by someone who knows what he's doing, from different batches, in cases of differing age, both blow at the same time?

Wish I had anything of a clue.....but that's just -weird- Did you forget to sacrifice a live chicken to the rocket gods prior to launch and anger them?

 

[cavecentral]

I have never had an AT fail either. I have notices that when the delays switched to the plastic sheath instead of the cardboard tube, they felt loose to me. I always wrapped them in masking tape. Still no issues.

I had some 24mm SU F30s fail and contacted AT. While I was at it, I asked about the loose delays on some reloads. The person wasn't aware of any issues from that. I still used tape anyways.

 

[Handeman]

Next time I flew one of those reloads I would add some masking tape around the delay to make an extra tight fit. Either that or add more grease around the outside of the delay. What grease are you using, super lube? Dow 111?

I've been using Vaseline for 11 years now, as per reload instruction step 1.

 

[Handeman]

Gotta be something. I know you know what you're doing so this concerns me. Drill to vigorous or wrong side? It was really cold. I built my G64 for that day at home.

I'm relieved I called Hobbylinc and told them to go on and ship the -7's pronto, back before the originally scheduled launch date . However that -10 is the needed delay for my new rocket and a nice motor, I assume plenty of G64-10W motors have been flown successfully?



P.S. did you intend to tighten the closures before flight?

I actually drilled one delay to about 7 seconds (3/32") and made sure the drilled face was towards the propellant grain. I don't think that had anything to do with it since the drilled and non-drilled both failed in the same way at the same time.

Both loads were assembled at the field, one at a time. I used my closure tool to tighten both aft closures like I've been doing for years. Both closures were tight afterwards. The large 1/16" o-rings and propellant grains worked fine. It was the delay o-ring or delay grains that failed somehow.

 

[Handeman]

I have never had an AT fail either. I have notices that when the delays switched to the plastic sheath instead of the cardboard tube, they felt loose to me. I always wrapped them in masking tape. Still no issues.

I had some 24mm SU F30s fail and contacted AT. While I was at it, I asked about the loose delays on some reloads. The person wasn't aware of any issues from that. I still used tape anyways.

Has anyone else had issues with loose grains?

I thought they were pretty loose also, in fact, when I picked up one of the forward closures during assembly and tilted the threaded end down, the delay grain fell out of the delay insulator and I had to reassemble it before screwing it onto the case.

Could the temp of 25 deg F. have an effect on the delay grain and made it too loose to seal against the delay insulator? Maybe that is where the flame bypassed the delay grain, between the grain and the insulator. If that is the case, the delay o-ring wouldn't have failed either.

 

[cavecentral]

Has anyone else had issues with loose grains?

I thought they were pretty loose also, in fact, when I picked up one of the forward closures during assembly and tilted the threaded end down, the delay grain fell out of the delay insulator and I had to reassemble it before screwing it onto the case.

Could the temp of 25 deg F. have an effect on the delay grain and made it too loose to seal against the delay insulator? Maybe that is where the flame bypassed the delay grain, between the grain and the insulator. If that is the case, the delay o-ring wouldn't have failed either.

I had one just about fall out too. I assembled them in the basement at ~65 degrees F. Seemed like a problem to me, but I never heard of any failures.

 

[blackjack2564]

Yes, I had the same issue, but lucked out. Even though I had blow by or early delay burn, the closure just opened touch hole a bit & it was still usable. Now I slather grease around the delay insulator to hold in place, then install into closure. Wipe off excess grease & put delay in last, to keep grease off the delay. Yes it was a G-64.

Did burn up the rocket, was a Cheeta. Doubt temp had anything to do with it.....was 85 here when it happened.

 

[Worsaer]

Has anyone else had issues with loose grains?

Could the temp of 25 deg F. have an effect on the delay grain and made it too loose to seal against the delay insulator? Maybe that is where the flame bypassed the delay grain, between the grain and the insulator. If that is the case, the delay o-ring wouldn't have failed either.

Are you saying it was below freezing at time of launch? O-ring failure due to sub-freezing temps sounds eerily familiar...

"Feynman's investigation eventually suggested to him that the cause of the Challenger disaster was the very part to which NASA management so mistakenly assigned a safety factor. The O-rings were rubber rings designed to form a seal in the shuttle's solid rocket boosters, preventing the rockets' hot gas from escaping and damaging other parts of the vehicle. Feynman suspected that despite NASA's claims, the O-rings were unsuitable at low temperatures and lost their resilience when cold, thus failing to maintain a tight seal when rocket pressure distorted the structure of the solid fuel booster. Feynman's suspicions were corroborated by General Kutyna, also on the commission, who cunningly[6] provided Feynman with a broad hint by asking about the effect of cold on O-ring seals after mentioning that the temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28 to 29 °F (−2.2 to −1.7 °C); previously, the coldest launch had been at 53 °F (12 °C)."

 

[CarVac]

Are you saying it was below freezing at time of launch? O-ring failure due to sub-freezing temps sounds eerily familiar...

"Feynman's investigation eventually suggested to him that the cause of the Challenger disaster was the very part to which NASA management so mistakenly assigned a safety factor. The O-rings were rubber rings designed to form a seal in the shuttle's solid rocket boosters, preventing the rockets' hot gas from escaping and damaging other parts of the vehicle. Feynman suspected that despite NASA's claims, the O-rings were unsuitable at low temperatures and lost their resilience when cold, thus failing to maintain a tight seal when rocket pressure distorted the structure of the solid fuel booster. Feynman's suspicions were corroborated by General Kutyna, also on the commission, who cunningly[6] provided Feynman with a broad hint by asking about the effect of cold on O-ring seals after mentioning that the temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28 to 29 °F (−2.2 to −1.7 °C); previously, the coldest launch had been at 53 °F (12 °C)."

They're not cork, so they should be fine. I've launched in very cold weather before, and the limit is finger dexterity not o-ring flexibility.

 

[o1d_dude]

I witnessed the same thing happen at our LUNAR launch yesterday.

I'm pretty sure the reload involved was a G64W also. The rocket had the 4' long flame which ignited the airframe. Afterward, we checked and Hobbyline casing had no powder well left either buy in this case there were jagged edges left so it most likely didn't melt away.

 

[bill_s]

Has anyone else had issues with loose grains?

I thought they were pretty loose also, in fact, when I picked up one of the forward closures during assembly and tilted the threaded end down, the delay grain fell out of the delay insulator and I had to reassemble it before screwing it onto the case.

Could the temp of 25 deg F. have an effect on the delay grain and made it too loose to seal against the delay insulator? Maybe that is where the flame bypassed the delay grain, between the grain and the insulator. If that is the case, the delay o-ring wouldn't have failed either.

That is odd. Always tight, to almost too tight. New production?

 

[Astronot]

I haven't had a whole lot of experience with these RMS motors, but my forward insulator always seems to stick to delay insulator like so and not the grain insulator. You say your delay insulator burned up completely, so yours probably had nothing left to stick to. It may be possible that if the delay was loose that some of the ejection charge powder trickled down into the forward closure's delay well. Outsdie of that. IDK.

 

[hball55]

If your delay grain is loose, regardless of any other factors, it has to be made to fit tightly . . . either by using masking tape or extra grease on the outside of the delay liner. I prefer masking tape, as extra grease has a habit of getting where you don't want it.

 

[Handeman]

If your delay grain is loose, regardless of any other factors, it has to be made to fit tightly . . . either by using masking tape or extra grease on the outside of the delay liner. I prefer masking tape, as extra grease has a habit of getting where you don't want it.

I'm not sure, but I don't think it got between the delay liner and the closure because that space is sealed with the delay liner squeezed between the forward insulator and the o-ring. I think it got through between the delay grain and the delay insulator.

These were the new plastic coated delay grains like the HPR loads. But unlike the HPR loads where the o-ring fits onto the delay grain, the delay insulator presses against the o-ring and there isn't really much to seal the forward end of the delay grain. I don't think the paper to plastic interface seals as well as the cardboard to cardboard did.

 

[CarVac]

I'm not sure, but I don't think it got between the delay liner and the closure because that space is sealed with the delay liner squeezed between the forward insulator and the o-ring. I think it got through between the delay grain and the delay insulator.

These were the new plastic coated delay grains like the HPR loads. But unlike the HPR loads where the o-ring fits onto the delay grain, the delay insulator presses against the o-ring and there isn't really much to seal the forward end of the delay grain. I don't think the paper to plastic interface seals as well as the cardboard to cardboard did.

It's the plastic to o-ring interface that seals, in both the HPR-style and the hobbyline.

 

[Handeman]

It's the plastic to o-ring interface that seals, in both the HPR-style and the hobbyline.

In the HPR, the delay grain extends above the insulator and the o-ring fits over the delay grain and against the insulator. The inside surface of the o-ring is sealed against the plastic of the delay grain while the lower face of the o-ring seals against the insulator.

In the hobbyline, the o-ring drops into the forward closure and the insulator and delay grain are the same length and push against the o-ring. As long as the gap between the insulator and the delay grain is pressed against the o-ring it will seal, but the way this is set up, both the insulator and the delay grain have to seal against the lower face of the o-ring.

 

[CarVac]

In the HPR, the delay grain extends above the insulator and the o-ring fits over the delay grain and against the insulator. The inside surface of the o-ring is sealed against the plastic of the delay grain while the lower face of the o-ring seals against the insulator.

In the hobbyline, the o-ring drops into the forward closure and the insulator and delay grain are the same length and push against the o-ring. As long as the gap between the insulator and the delay grain is pressed against the o-ring it will seal, but the way this is set up, both the insulator and the delay grain have to seal against the lower face of the o-ring.

The way I see it, the insulator does two things: it keeps the closure from getting too hot during the delay grain burn (insulation), but it merely centers the delay grain itself. The plastic sheath of the delay grain pushes squarely against the thickest part of the o-ring.

The only thing I can see happening that would allow blow-by as in the OP's failure might be an o-ring failure of some sort: perhaps the o-ring crumpled, allowing gases by? It is resisting an inward force by the gases, so if it ever buckles then that's the end of that forward closure.

 

[Handeman]

The way I see it, the insulator does two things: it keeps the closure from getting too hot during the delay grain burn (insulation), but it merely centers the delay grain itself. The plastic sheath of the delay grain pushes squarely against the thickest part of the o-ring.

The only thing I can see happening that would allow blow-by as in the OP's failure might be an o-ring failure of some sort: perhaps the o-ring crumpled, allowing gases by? It is resisting an inward force by the gases, so if it ever buckles then that's the end of that forward closure.

I haven't looked at the insulators and the plastic coated delay grains so I don't know if the gap between the two push against the center of the o-ring. I would suspect they would design it to do that, but I can't say. Just from a gut feeling point of view, the HPR method would seem to be much more reliable then the Hobbyline method. I'm just wondering if the switch from paper tubes on the delay grains to plastic tubes could have cause a failure mode to develop that wasn't expected.

 

[CarVac]

I haven't looked at the insulators and the plastic coated delay grains so I don't know if the gap between the two push against the center of the o-ring. I would suspect they would design it to do that, but I can't say. Just from a gut feeling point of view, the HPR method would seem to be much more reliable then the Hobbyline method. I'm just wondering if the switch from paper tubes on the delay grains to plastic tubes could have cause a failure mode to develop that wasn't expected.

You don't have to do it in person:

https://aerotech-rocketry.com/uploa...1dc-791f5c173b2a_29-40_hp-g138t-14a _assy.pdf

I think it'd be quite the reverse: the plastic tubes would remove a potential flame tube in the spiral that could light the whole delay grain from the side.

 

[Handeman]

I didn't think of it that way. It really shouldn't matter, as long as the o-ring flattens out like the drawing shows, it would seal both the delay insulator and the delay grain and no flame would move through any spiral.

I just don't understand what happened with mine. Disassembly showed that the 1/16" o-rings, propellant grains, liner tube, fwd insulator were all assembled correctly. I know the delay o-rings went in right and the delay grain was the same length as the delay insulator because it was a G64W-10.

 

[cavecentral]

If there is any space between the delay and the insulator, could the heat melt the plastic and break the seal?

Just peeing in the wind with that guess.

 

[CarVac]

If there is any space between the delay and the insulator, could the heat melt the plastic and break the seal?

Just peeing in the wind with that guess.

I doubt that... you really need flow in order to get significant heat transfer even if there is some gap, and that would only happen if the o-ring had failed.

 

[cavecentral]

Yeah, didn't seem likely. Tough enough to get propellant grains to burn without space.

 

[Handeman]

I received two new fwd closures and reloads from Aerotech last week under warranty.

Flew another G64W-10 yesterday from the same batch that one of the two CATO motors was from. During assembly, I tilted the fwd closure over and the delay grain and delay insulator fell out of the closure. With it in the closure, if you shook it side to side, you could feel it rattling in the closure. Showed this to several people at the launch and they all agreed that a delay grain/insulator being that loose would result in blow by. I put a full wrap of masking tape around the delay insulator which gave it a snug fit. It flew without a problem.

My conclusion is that the blow by failures were caused by delay insulators that had too small of OD. Just saying.