Problem with CTI G33 Motor

[asheriff]

Burned some G33's today to verify any problems, 8 in total. Biggest thing I niticed with the first 5 motors was the loud pop from the pellet, seemingly pressurizing the motor far more than needed, then the drop in presure snuffing out the motor before it can light (similar to the VMax issue snuffing out the delay).

Just today, one of the flyers in our club (MASA Minnesota) experienced the same thing. The G33 pellet popped, lifting the Estes Leviathan about 2 feet, but the motor didn't ignite. No damage to the case or rocket. He was able to successfully ignite the now pellet-less G33 with a pyrogen igniter (AT First Fire I believe).

I have a video I can post if anyone is interseted.

 

[Richard Dierking]

Finished the remaining 5 motors that were dated APR 15 2019. I got a bit faster at the end but still 3 of the 5 pellets broke when I trimmed them down. And, I was taking my time. Also, I removed the BP charge in all the motors because they are either going in the test stand or flying with electronic deployment.

Again, it was a bit of a hassle doing this and I don't recommend that any student teams like TARC use motors that have to be modified. I had a good work area (photo) wore eye protection and careful about prevention of static charges. I have experience using pyrodex pellets from air starting 2nd stage motors. If you had to do a bunch of these, it would be a drag. Again, you must have tools like the dowel that will fit and go real slow with things like removing the grains and pellet, and cutting it down. Best done before any launch.

The APR 15 2019 reloads are supplied with solid blue igniters. The NOV 22 2018 reloads had blue-white igniters.

The nozzles for all the APR 15 2019 were very similar to the NOV 22 2018 motors. 7/64" (2.78 mm) drill bit would only fit partially through the nozzle throat. I ran this bit through all the motor nozzles to make them a uniform size. The igniter measurements follow.

Igniters measured at the solder connection to the head.
1. 2.40 x 1.98 mm
2. 2.56 x 1.98 mm
3. 2.53 x 2.14 mm
4. 2.40 x 1.84 mm
5. 2.54 x 2.00 mm

Basically, you are feeding a igniter (e-match) 2.48 mm (greatest width) through a nozzle throat that's 2.78 mm. Not much room.

 

[Charles_McG]

I think motorcato.org would confirm it, but I think the <=3G Blue Streaks have the same issue. I've had 1 pop, and witnessed others. Also a really small nozzle.

I've done the pellet cut on 1G 38mm motors. Not easy, not really a field job.

 

[UhClem]

Rocket Propulsion Elements has an equation relating the mass of pyrogen to the internal volume of a motor: 0.12 * Vf^0.7 (Free volume in cubic inches and mass in grams.This is from the seventh edition as the sixth used 0.5 instead of 0.12. Why? I don't know and the text doesn't say.) But the data used to fit that is from very large motors with the smallest having a free volume of ~1,000 cubic inches.

But the take away is that the mass of pyrogen should vary with motor size. Using the same mass over a range of sizes can cause problems. Either slow or no ignition in the largest, or smaller ones that go BOOM!

As for modifying a G33, that is still prohibited by NFPA 1122.

 

[Richard Dierking]

As for modifying a G33, that is still prohibited by NFPA 1122.

I think drilling out the nozzle throat so it's uniform size wouldn't be modifying the motor. I hope this is correct?

But, frankly, I don't like having to modify the pellet which is part of the motor design and you shouldn't have to dink with a commercial reload.
I'm testing a couple more before trying in my rocket. I just dismantled my damaged GoPro to remove the LiPo for disposal. I'm not happy and thinking about just moving on if there's another failure.

When I started this thread, I knew that it would be difficult to dismiss a static failure using the supplied igniter and videoed the whole thing.
I hope some good comes out of all this.

 

[Bamraam]

OK. For recycling, I was just thinking about the prior posts where people had motors that didn't go on the first try. Any official procedure should address this potential situation. If it doesn't, people will start resorting to all kinds of solutions, and keep in mind that most of these motors are used at commercial launches.

I think it's important to let people know that if the suppled igniter doesn't fit through the nozzle when inserting it at the pad (standard procedure), it's not a good idea to remove the nozzle and pull the leads of the igniter through the nozzle throat.
Again, how would this affect the warranty since they are using the supplied igniter and haven't modified anything.

I need to inspect and do some measurements this afternoon and I will consider what you said.

To be clear, is it correct that modifying the pellet doesn't void the warranty?

You're right, it's not a good idea. If the igniter doesn't fit through the nozzle and you remove the nozzle to feed the wire through it will certainly block it at ignition and likely cause the motor to over pressurize.

 

[Bamraam]

@Bamraam , I’m planning a flight as early as tomorrow with an H53MY. The 5 grain motor has more length/volume to disperse that initial pop. Have you had any reports that would suggest this motor needs a trimmed starter, too?

No, so far just the 1-3 grain motors have had issues. I am planning on testing a few others next week just to be sure.

 

[Bamraam]

I apologize for any interruption in the questions being asked. I'm going to be as quick as possible with this. I wanted to post about the NOV 22 2018 motor first. Then, I'm planning on going pretty quickly through the 5 APR 15 2019 motors I have.
I have the procedure for trimming the pellet. It's 7 pages long. Just saying, so the vendor is going to sell someone a motor and direct them to this document. Verses, telling them that they could buy a AT G with no modifications necessary. And, if the buyer doesn't feel comfortable doing it themselves, the vendor is going to stop what they are doing and tend to this. Good luck with that.

Again, these are 29 mm, 3 grain, G33
Date Code: NOV 22 2018
Igniter: 2.39 x 2.56 mm at the solder connection to the head.
Nozzle: 2.78 mm at aft end of nozzle. It's smaller diameter at the forward end. (Photos)
Does the igniter pass thru the nozzle: Yes
It's very difficult to remove the grains from the liner. I used a 7/8" diameter wood dowel. This would be difficult to do in the field if you didn't have something like this dowel without damaging the grains.
Started to score the pellet after removal with 1/4" aluminum rod and it broke almost immediately. However, I was able to trim in down and re-insert it. Again, this would be difficult to do in the field. I'm in my shop with all my tools and materials, with no wind.
I drilled out the nozzle throat with a 7/64 drill bit so it was uniform. This motor will be static tested on October 3. May the Force be with me.

View attachment 433008View attachment 433009View attachment 433010View attachment 433011View attachment 433012

Yes the grains are a tight fit, might be better to try and remove the pellet while the grains are still in the liner. Good luck with your tests.

 

[Charles_McG]

No, so far just the 1-3 grain motors have had issues. I am planning on testing a few others next week just to be sure.

Update: a beautiful flight.

 

[Richard Dierking]

Just today, one of the flyers in our club (MASA Minnesota) experienced the same thing. The G33 pellet popped, lifting the Estes Leviathan about 2 feet, but the motor didn't ignite. No damage to the case or rocket. He was able to successfully ignite the now pellet-less G33 with a pyrogen igniter (AT First Fire I believe).

Probably a First Fire Jr. I had a couple 3 packs of them and tried inserting through the nozzles of the G33's I had. Again, keep in mind that I previously drilled the nozzles with a 7/64" bit to make them uniform in size. (Initially, the bit would not go all the way through the nozzle.) The First Fire Jr. igniters fit with only slight resistance going through the nozzle.

I'll see how it goes with the two motors I'm firing on Saturday. Just removing the pellet and using a First Fire Jr. may be a good option; it may even be the best option. Although I'm thinking that CTI would probably not embrace that procedure.

 

[Charles_McG]

I won't clutter up this thread with more info on my H53 flight - if you're interested, there's more here:
https://www.rocketryforum.com/threads/tuning-a-sim-to-flight-data.161546/post-2048620
That said, that day my son had a problem with a Pro24 3G with the igniter not fitting through the nozzle. An F51CL. @Bamraam , I can get the date code if desired. He spoke with Wildman Tim, and got a new starter for it. Flew fine.

 

[Evolvant]

Yes the nozzle throat on smaller motors produces challenges to find the right igniter. If the igniter didn't fit in the nozzle then that's our fault. We check each igniter to ensure that they fit in the smallest nozzles, but manufacturing tolerances can still cause the odd igniter to not fit. Hopefully the new igniter we're looking into will soon be approved. Did you apply for warranty on your two cases?

Hi @Bamraam,

I did. I also had a J410 CATO on me last year. It burned right though the plastic liner and then right though the case. I filled out all the paper work for the J410, 386G case, the G33's and the 2x 29mm cases. That was over a year ago, and I still have not received any warranty HW or Motors. I'll admit, I haven't pushed extremely hard but have followed up a couple times. No need to call anyone out here. The CTI dealers I've purchased from have all treated me really well. The warranty process though has failed me so far. ;-)

 

[Richard Dierking]

Sometimes deviations from the original topic can be very useful. It's unfortunate actually that the "thread" has to be linear. I think 2 and even 3 dimensional web conversations would be cool.

So, anyway, sometimes it's good to stand back and consider the whole situation and really evaluate what's happening. I suggest everyone that's interested do that for this and comment if you would like.

For me, I see a disproportionately large pellet being used in a motor. Why that pellet was used, I can imagine. Why even use the pellet, again I can imagine. The motor nozzles are small for the desired pressures. At least you need the pressures to get the desired thrust and duration of the burn. You have to create a small (special) igniter because you have the small nozzle. Then, you must create even a smaller e-match. It has to be big enough to light the large pellet (well, the pellet has been cut down now), but if the igniter is too large (even the leads, so may need small igniter with small leads) the motor is going to immediately over-pressurize. Keeping in mind that the motor is already designed to produce high pressure, and there is an initial spike that has been recorded even on the certification tests.

So, announcements and procedures are created. Then, others are created, and so on. And, consumer confidence goes down. But, it's really not going to deter most of the people I know in this hobby. Nothing probably could.

Manufacturers of commercial motors are going to need to loose some sleep and not make mistakes. We know what happens when we make mistakes, right? We seldom learn much from our mistakes other than learning not to make mistakes. I know this sounds rough. Hey, rocketry is rough on us all.

 

[Bamraam]

I won't clutter up this thread with more info on my H53 flight - if you're interested, there's more here:
https://www.rocketryforum.com/threads/tuning-a-sim-to-flight-data.161546/post-2048620
That said, that day my son had a problem with a Pro24 3G with the igniter not fitting through the nozzle. An F51CL. @Bamraam , I can get the date code if desired. He spoke with Wildman Tim, and got a new starter for it. Flew fine.

Thanks, that won't be necessary.

 

[Bamraam]

Hi @Bamraam,

I did. I also had a J410 CATO on me last year. It burned right though the plastic liner and then right though the case. I filled out all the paper work for the J410, 386G case, the G33's and the 2x 29mm cases. That was over a year ago, and I still have not received any warranty HW or Motors. I'll admit, I haven't pushed extremely hard but have followed up a couple times. No need to call anyone out here. The CTI dealers I've purchased from have all treated me really well. The warranty process though has failed me so far. ;-)

Send me a PM, I'll look into your warranty

 

[Richard Dierking]

I static tested two G33's on Saturday. I wasn't able to obtain data from the load cell on my static test stand because of an issue with my laptop but figured determining the successful firing of these motors was the most important thing.

I tested the NOV 22 2018 date code first. Then, I fired the APR 15 2019 date code. Both were nominal ignition and burn. The NOV 22 2018 date code is the same as the one that failed and started this thread.

I referenced the Pro38 Bulletin, 1G Failures, April 15, 2019 and followed the procedure outlined in the Instruction For Pro38 Pellet Trimming for these motors. If you follow this procedure, I think you will not have a problem using these motors. I suggest following all safety precautions including wearing safety glasses and doing this before the launch.

I took GoPro video of the static firings. The whole video is a bit boring frankly so I will not be making a Youtube of that. It's much more exciting for the motor to fail.
Anyway, here are some frames from the video taken at 239.5 frames/second.


At first indication of ignition (just a small puff of smoke from the hole in the yellow cap).
Time = 0.000 sewconds

Cap blown off at 0.137 seconds.


Motor appears to pressure up at 0.162 seconds.

Anyway, I hope this provides some reassurance to people that have purchased these motors.
I haven't heard anything about my warranty claim on the failed motor that started this thread and it's been over a month.