Problem with CTI G33 Motor

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Richard Dierking

Well-Known Member
TRF Supporter
I used one of the seven (7) 29 mm CTI G33's I had in a static test stand at FAR on Saturday (09-19-20). I wanted to test the 3-D printed TVC gimbal to assure it would hold up before using the G33 in my rocket. One concern I had was the spike shown on the graph obtained from ThrustCurve (shown below). What is this initial spike, and wonder what the pressure was at that point?

So, the motor I used failed 0.013 seconds from ignition (immediately). Both the forward and aft failed with the guts shooting out both ends including propellent grains and the delay grain. I've attached a couple frames from a GoPro 8 I had going at 240 frames/second. The other GoPro camera I had with video overhead was completely destroyed. So, yeah, I'm angry that I lost a camera and enclosure, about $275. Plus of course the TVC gimbal with the servos! This is a commercial motor using the igniter supplied with the motor, and it shouldn't blow up. :-( Now, what am I going to do with the 6 motors I have left? The case and aft closure was messed up and I don't trust these motors. Steve Shannon Well-Known Member TRF Supporter For future reference, put a mirror above the motor and point the camera at the mirror so the camera isn’t directly inline with the case. Motors are designed to fail in that direction if they must fail. Also, please file a MESS report. It’s easy and helps us all. Www.motorcato.org. While you’re there, look to see if anyone else has reported similar issues. Also, take the wrecked case and closure to your dealer for warranty replacement. I believe that spike is mostly the compressed black powder starter pellet burning and possibly some surface roughness that exposes extra surface area of the APCP. I’m sorry this happened. Last edited: Charles_McG Ciderwright My thread was already linked (The Ute Tomahawk), but I'll add that I later recovered the rest of the motor, stuffed it in a new casing and flew it in an upscale Blender. CATO'd 2 seconds into the burn. That's evidence against it being a bad starter pellet, since it didn't have one any longer. It means I lost a second case though :-(. Didn't claim the second CATO as warranty - after all, it had already failed once. Richard Dierking Well-Known Member TRF Supporter I appreciate your responses. I do. If you've been in rocketry for awhile, you know stuff like this happens. Still, it stings a bit; I put a lot of work into that TVC gimbal. Looking on the glass half full side, really glad it wasn't in the rocket with all the electronics! And the rocket has a camera too! So, please let's get to the bottom of this. If there's a problem, get it fixed before someone else's stuff gets messed up. Again, I have good video and it's at 239.2 frames/second. The igniter wire is still in the nozzle when the motor over-pressurizes. I bought the motors from two different vendors. All by mail because of the current situation. Spent over$100 on Hazmat fees already. Who would pay to return them? (I have a really bad feeling about the answer for that one.) I think I might just ask for a replacement case, closure, and another motor.

Since FAR is a research facility, I might try and "fix" the motors myself. I didn't have small enough calipers to measure the nozzle throat but was able to get a good approximation using drill bit sizes before firing the motor. I got 2.70 mm (0.106"). I provided the motor info to a friend that builds motors, and he calculated like 900 psi for the pressure. Seemed high already, and doesn't include an igniter in the nozzle.
So, guess what, the igniter partially plugs the nozzle during ignition and crowd pleaser. Math probably works out nicely for the motor without considering the affect of the igniter.

Another important thing for everyone to consider is to be careful even when using commercial motors. Spectators were a safe distance in bunkers when I fired this motor and I was behind my truck. The force of the failure was enough to completely destroy a camera in a protective enclosure and propel parts about 100' away.

Shows the forward part of the motor with the delay grain and charge well.
Why you should always take a photo before pushing the button so you can remember how nice it is was. LOL

Rocketjunkie

I've flown 6 of them with no issues. Did you use the supplied ematch (with tiny head)? If you used a dipped igniter then you will have problems. The spike in the thrust curve is the spitting out of the igniter.

Richard Dierking

Well-Known Member
TRF Supporter
I've flown 6 of them with no issues. Did you use the supplied ematch (with tiny head)? If you used a dipped igniter then you will have problems. The spike in the thrust curve is the spitting out of the igniter.
Yes, I used the small match for ignition with no modifications. It went in with no problems however, I did remark that I thought there wasn't much room.
If the spike is the igniter being ejected, then it's not accounted for in the design of the motor. At least, I don't think someone would reduce the area of the throat at ignition in their formulas for pressure and thrust.

Just thinking about some possibilities. Perhaps the motor didn't change since it was certified but there's a batch of the small igniter that is slightly larger. Or the nozzle spec allowed for variance and there's some that are too small and the pressure spike (that we already know exists) becomes too large. BTW, I did take a close look at the nozzle when I was estimating it's size and it was clean.

Other reports of failure indicate immediate over-pressurization. But, they didn't have a video camera capturing how quickly this happened.

If I'm convinced to keep the other G33's I have, I will get some measurements and post the results. But, any motor modifications will be posted to the research motor section of the forum.
I guess we could compare the igniters supplied with the motors here. I have one motor from the same date stamp and 5 with another.

Rocketjunkie

I guess we could compare the igniters supplied with the motors here. I have one motor from the same date stamp and 5 with another.
They should be bare ematches. G33's have a BP pellet pressed into the top grain. You will see this when you remove the forward closure to adjust the delay.

Richard Dierking

Well-Known Member
TRF Supporter
They should be bare ematches. G33's have a BP pellet pressed into the top grain. You will see this when you remove the forward closure to adjust the delay.
Yes, it was a bare e-match. I usually don't call them e-matches any more (long story there). But, yes, nothing except the match head. I also checked the forward end of the motor for the pellet. Are you sure it's BP and not pyrodex?

Anyway, I did check the motor components before using it.

And, it was a new case and closure.

I don't know how many motors are usually tested for certification. If it's not many, perhaps the motor I used was on the sad end of marginal. To be honest, I just don't know much about how motors are spec and made. But, I'm learning.
I'm still interested to hear what the pressure would be at the spike. With the same motor dimensions, if it was a G63 what would the pressure be? At what pressure would the motor fail?

Also, again being frank about it, I'm kind of done with any 29 mm motors. I know people that make motors and they usually don't make anything under 38 mm because it's a hassle that isn't worth it. Now, I'm beginning to appreciate that.

I'm waiting to hear about my report and claim. Depending on the response, I might start measuring stuff like the nozzle throats and igniters.

Richard Dierking

Well-Known Member
TRF Supporter
BTW, I just noticed that one of the other G33 failure reports on the Motorcato database, was the same date code as my failure: NOV 22 2018.

So...I say we put the one I have left in a rocket and fly it! Anyone? Anyone? LOL

cerving

Owner, Eggtimer Rocketry
TRF Supporter
I lost my upgraded Estes Saturn V to a G33 last year at NSL... virtually the same failure. It basically blew the rocket in half.

Richard Dierking

Well-Known Member
TRF Supporter
For future reference, put a mirror above the motor and point the camera at the mirror so the camera isn’t directly inline with the case. Motors are designed to fail in that direction if they must fail.
Thank you Steve. The small make-up mirrors available at stores like Walmart would be great for this. They are small, so easy to mount, and not expensive.
I really didn't anticipate the motor failing. In all these years, I've only had one other commercial motor fail, the J1999. Remember that one?

I lost my upgraded Estes Saturn V to a G33 last year at NSL... virtually the same failure. It basically blew the rocket in half.
Yes, I believe you completed a CATO report and I saw that. If so, the motors had the same manufacturing date, NOV 22 2018. Really sorry about your Saturn V Cris.

Deliberating on what I'm going to do with the motors I have left. Considering opening up the nozzle and static testing again; not with a TVC mount and GoPro right above the motor!
On the other hand, perhaps CTI will want the motor back with the NOV 22 2018 for evaluation. I enjoy testing stuff and if I were them, I would really want to get my hands on this motor.

ThirstyBarbarian

Well-Known Member
TRF Supporter
I think that initial thrust spike is the pyrodex pellet burning. And right after the spike, the rounded hump in the thrust curve is the slow-burning mellow yellow propellant coming up to pressure. The nozzles on the mellow yellow reloads are very narrow, and most likely it just clogged with the igniter. That pyrodex pellet is similar to what you might find in a small-caliber ammunition cartridge, so if the nozzle clogs, the pressure generated by the pellet alone could be comparable to what you might see in a pistol with a clogged barrel. BOOM!

Unfortunately a lot of long-burn motors have designs that make them more prone to nozzle clogs and catos. Narrow nozzles for one thing. And offset cores on the moon burner style can also lead to igniters clogging the nozzle.

jqavins

Joseph Avins
TRF Supporter
Gosh, Richard, above and beyond all else, damn, that sucks!
My thread was already linked (The Ute Tomahawk), but I'll add that I later recovered the rest of the motor, stuffed it in a new casing and flew it in an upscale Blender. CATO'd 2 seconds into the burn. That's evidence against it being a bad starter pellet, since it didn't have one any longer. It means I lost a second case though :-(. Didn't claim the second CATO as warranty - after all, it had already failed once.
Reusing grains that have been through a CATO? Sounds like a bad idea to me. Alright, I have very little experience with these, yet I would expect the chance of a damaged grain to be pretty high. If the flame front gets into a crack where it has a lot of surface area then BOOM!

Since FAR is a research facility, I might try and "fix" the motors myself.
As I said, and acknowledging that I can't call myself any kind of expert, I really wouldn't do that. At the least, talk to Cesaroni before you do.

If the spike is the igniter being ejected, then it's not accounted for in the design of the motor. At least, I don't think someone would reduce the area of the throat at ignition in their formulas for pressure and thrust.
Since the spike is seen in the published thrust curve, it obviously is acknowledged in the testing. I'd be very surprised if a competent engineering staff didn't take that data and and do some analysis to verify that it shouldn't break anything. But it quite likely did break something. Which would mean the spike had to be anomalously large, assuming that's the actual cause. (The failure still could be due to something else, but probably not.)

Just thinking about some possibilities. Perhaps the motor didn't change since it was certified but there's a batch of the small igniter that is slightly larger. Or the nozzle spec allowed for variance and there's some that are too small and the pressure spike (that we already know exists) becomes too large. BTW, I did take a close look at the nozzle when I was estimating it's size and it was clean.
Or the pellet was made from a batch of BP that burns too fast. Or the nozzle was out of spec. Of one of the grains had a crack. Or... Or... Or... The best thing to do is give CTI all the information there is and let them conduct the research and analysis. It seems like a recall of that date's G33s might be in order.

I'd ask CTI if they want that one from the same date back (and if they'll pay the shipping). And if they don't, I'd pitch it.

UhClem

Well-Known Member
I am reminded of a similar problem with the short (single grain) Pro38 motors a while back that was the result of an unexpected change in ignition pellet burn rate. CTI published an ignition pellet modification for that.

November 2018 is within the time frame of higher burn rates. http://www.pro38.com/news.php

Richard Dierking

Well-Known Member
TRF Supporter
As I said, and acknowledging that I can't call myself any kind of expert, I really wouldn't do that. At the least, talk to Cesaroni before you do.
Definitely would do that Joe. It's difficult for me not to start looking at the motors I have and start measuring stuff but I'm trying to preserve everything in unopened packages until I hear back.
There's good supervision at FAR with research activities and I would obtain the advice of competent others before attempting anything like modifying the reloads.

The grain that was ejected from the case was not burning and the ones I can see inside show no indication of ignition.

Thank you for the link David. Yes, that info is interesting in light of what I experienced. At commercial launches, I would not be a big fan of any field modifications. Adjusting a delay grain is one thing, starting to mess with the grains and ignition pellets is another. My opinion. And, what happens if the motor doesn't ignite? These are special igniters, and what if someone resorts to another type of igniter?

This is a good (although a bit painful) reminder to expect the unexpected in our hobby. I'm not trying to bash CTI, just help to determine the problem and maybe spare others some grief.

Steve Shannon

Well-Known Member
TRF Supporter
Definitely would do that Joe. It's difficult for me not to start looking at the motors I have and start measuring stuff but I'm trying to preserve everything in unopened packages until I hear back.
There's good supervision at FAR with research activities and I would obtain the advice of competent others before attempting anything like modifying the reloads.

The grain that was ejected from the case was not burning and the ones I can see inside show no indication of ignition.

Thank you for the link David. Yes, that info is interesting in light of what I experienced. At commercial launches, I would not be a big fan of any field modifications. Adjusting a delay grain is one thing, starting to mess with the grains and ignition pellets is another. My opinion. And, what happens if the motor doesn't ignite? These are special igniters, and what if someone resorts to another type of igniter?

This is a good (although a bit painful) reminder to expect the unexpected in our hobby. I'm not trying to bash CTI, just help to determine the problem and maybe spare others some grief.
A regular igniter, instead of an electric match, can be used with a CTI motor if the ignition pellet has been removed. The reason for the pellet is to allow the use of electric matches which transfer much less heat than an igniter.

The ignition pellet modification required by CTI is simple, but at least so far has only been recommended by CTI for the Pro38 1 and 2 grain motors. The pellets used in the Pro29s I would expect to already be a smaller pellet. CTI warranty work is handled by their dealers.

If the motor doesn’t light, wait at least a few minutes before approaching, then replace the ematch and try again, same as any other motor. I don’t recall seeing CTI motors with the pellets that haven’t ignited. That’s why they’re a favorite for staging.

Richard Dierking

Well-Known Member
TRF Supporter
But, a regular e-match or First Fire would not fit in the G33 nozzle; not even close. I know this because I took out a regular e-match and tried it at FAR. Just for comparison, a regular e-match is about twice as large as the igniter supplied with the motor.
Yes, you could remove the nozzle, place the igniter leads through the nozzle, then re-assemble the motor. But, I don't wish to go down that path. It's not the procedure I use. I install the igniter at the pad when the rocket is vertical.
And, if you are thinking about igniters in my multistage rockets, I follow a strict written procedure, and I'm very careful.

And, they supply a blue-white igniter, so it's not a higher energy e-match. But, for these motors, I guess you don't have to worry about recycling the igniter. :-/
Sorry, being a smarty pants.

It's beginning to sound like a out of spec pyrodex pellet combined with a small nozzle throat further restricted by the igniter leads. Too much pressure too early in the ignition.
We'll see.

Regarding the warranty claim:
I got the first two G33's from AMW (Date Code NOV 22 2018) and I haven't received any response from them yet. Also, I got the case but not the aft closure from them.
However, I got 5 from Wildman along with the closure and they responded almost immediately with the CTI claim form. I submitted the completed form to them.

Returning the motors, including the failed one, if that's what is requested, would be interesting currently. Other than perhaps FAR, there are no launches in California. And, I might not see AMW ever again.

UhClem

Well-Known Member
These are special igniters, and what if someone resorts to another type of igniter?
There is nothing special about them. Their only job is to ignite that pellet.

You can use a regular igniter but you must be careful to insert it all the way. Otherwise there is a risk of igniting the propellant in a way that does not propagate up to the forward end. Resulting in a long low thrust burn and in extreme cases, a melted motor case. (Replicated by CTI once long ago.)

jqavins

Joseph Avins
TRF Supporter
The grain that was ejected from the case was not burning and the ones I can see inside show no indication of ignition.
Just in case it wasn't completely clear (it probably was, but I like making sure) it's not combustion I'm concerned about, but latent (or otherwise unnoticed) physical damage. A grain with a crack where you can't see it, it seems to me, could be a bomb. And since these grains have been subject to some very rough handling, in my mind they'd never be trustworthy unless CTI tells you that I'm wrong.

Richard Dierking

Well-Known Member
TRF Supporter
There is nothing special about them. Their only job is to ignite that pellet.

You can use a regular igniter but you must be careful to insert it all the way. Otherwise there is a risk of igniting the propellant in a way that does not propagate up to the forward end. Resulting in a long low thrust burn and in extreme cases, a melted motor case. (Replicated by CTI once long ago.)
Sorry if there is a misunderstanding, but I meant they are special because they are smaller than typical e-matches. But, I don't have recent experience with smaller motors (under 38 mm). They are like Quest igniter size. All the other igniters I have would not fit through the nozzle throat of the G33.

Anyway, the thing that puts me on solid ground with my warranty claim is that I didn't modify or substitute anything when I used this motor. There's not even any announcement about potential problems with this motor and how to resolve them.

I am strict following procedures for commercial motors and flights. Some may think that I'm unreasonable with my expectations for commercial motors, but I think you should use them according to the directions without modifications and they shouldn't fail or blow up.
And, if a motor is defective in it's present form, it should be recalled.

I spent a lot of money on these motors and shipping fees. My camera and the TVC mount was about \$400. What happened is not my idea of having fun "blowing" a lot of money. And, Cris lost his Saturn V.

And, Joe, you are right.
Now, get your mind out of the gutter and let me float by.

jqavins

Joseph Avins
TRF Supporter
Some may think that I'm unreasonable with my expectations for commercial motors, but I think you should use them according to the directions without modifications and they shouldn't fail or blow up.
And, if a motor is defective in it's present form, it should be recalled.
I don't think anyone, including motor/engine manufacturers, disagrees with you.

Evolvant

Active Member
I had 2x G33's with May 30, 2018 fail on me last year. Both on the pad spitting nozzles out. Gave up on them after that.

cerving

Owner, Eggtimer Rocketry
TRF Supporter
A regular igniter, instead of an electric match, can be used with a CTI motor if the ignition pellet has been removed. The reason for the pellet is to allow the use of electric matches which transfer much less heat than an igniter.

The ignition pellet modification required by CTI is simple, but at least so far has only been recommended by CTI for the Pro38 1 and 2 grain motors. The pellets used in the Pro29s I would expect to already be a smaller pellet. CTI warranty work is handled by their dealers.

If the motor doesn’t light, wait at least a few minutes before approaching, then replace the ematch and try again, same as any other motor. I don’t recall seeing CTI motors with the pellets that haven’t ignited. That’s why they’re a favorite for staging.
Is removing the pellet a "manufacturer approved" mod? I thought only cutting down the pellet was approved by CTI...

Richard Dierking

Well-Known Member
TRF Supporter
I don't think anyone, including motor/engine manufacturers, disagrees with you.
Thank you Joe. And, I apologize if this was part of my rant. Anger is one of the steps of grief from rocket disaster, and at the last step, the person either a) moves on to another hobby, or b) buys more rocket stuff.
I purchased a 2.26" Tomahawk from LOC (38 mm motor mount thank you), and the healing process is proceeding nicely.

One problem I see with the pellet modification is introducing the possibility of more operator error. And, someone might think they can take their own initiative and fix problems themselves; Procedures and actions outside the scope of commercial motor use. For example, should I modify the pellets in the motors I have? No, just because it sounds very similar and there's a procedure for other motors it doesn't mean that's the problem with the G33. Or, should I remove the pellet and ignite with a First Fire Jr? Result may be just as bad. Again, I believe that all significant problems need to be handled by the authorized distributor and manufacturer.

Also, how many motors need to fail before action is taken? That is, what's the process of failure evaluation? btw, the Motorcato website Steve posted is great! Anyone that experiences a failure should post their info.

Charles_McG

Ciderwright
Also, how many motors need to fail before action is taken? That is, what's the process of failure evaluation? btw, the Motorcato website Steve posted is great! Anyone that experiences a failure should post their info.
I think you'll find the motor manufacturers have a higher action threshold than you do.

And the updated motorcato site is indeed awesome.

Richard Dierking

Well-Known Member
TRF Supporter
I think you'll find the motor manufacturers have a higher action threshold than you do.
I bet if they started to take responsibility for all the damages that would change. Like replacing a motor and case vs having to replace an entire rocket and all the work that went into it.

Charles_McG

Ciderwright
I bet if they started to take responsibility for all the damages that would change. Like replacing a motor and case vs having to replace an entire rocket and all the work that went into it.
I bet it would. Like a switch to single use only, or an exit from the market.

I'm really not belittling you. You've seen my threads, you know I share your pain. I've had the CTI 38mm-1G cato. I've had the CTI small Blue Streak cato. I've had the CTI G33 cato. (Speaking of which, I'm prepping an H54 - I should look at the nozzle diameter on that one. I think the ickle nozzle throat is a challenge.) I've had the Estes E9/E12 cato. (On a C11, too.)

Sometimes motor issues do get their attention - like the Pro38-1G starter pellet issue. The Vmax snuffing got someone's attention, but I think that was more about rockets coming in ballistic than rockets being damaged on the pad.

I'm surprised motor instructions aren't filled with fine print about fliers assuming all but very limited liability.
Oh, wait... http://www.pro38.com/pdfs/Pro29_instructions.pdf

Steve Shannon

Well-Known Member
TRF Supporter
I bet it would. Like a switch to single use only, or an exit from the market.

I'm really not belittling you. You've seen my threads, you know I share your pain. I've had the CTI 38mm-1G cato. I've had the CTI small Blue Streak cato. I've had the CTI G33 cato. (Speaking of which, I'm prepping an H54 - I should look at the nozzle diameter on that one. I think the ickle nozzle throat is a challenge.) I've had the Estes E9/E12 cato. (On a C11, too.)

Sometimes motor issues do get their attention - like the Pro38-1G starter pellet issue. The Vmax snuffing got someone's attention, but I think that was more about rockets coming in ballistic than rockets being damaged on the pad.

I'm surprised motor instructions aren't filled with fine print about fliers assuming all but very limited liability.
Oh, wait... http://www.pro38.com/pdfs/Pro29_instructions.pdf
When I first noticed the problems with one grain Pro38s, it was nearly an accident and it was directly related to seeing several reports here on TRF in a short period. When I wrote to CTI about the Pro38 one grain they responded that they hadn’t received reports of problems.
I think sometimes that our ways of looking for problems doesn’t allow us to see patterns. As Alan Whitmore points out repeatedly, we don’t know the denominators. We don’t know how many G33 motors have been produced and sold. Catos caused by excess gas generated by a black powder formulation that has a higher specific impulse are distributed over a wide range of different motors, not specific to a single motor, such as we expect with E9 motors.
I’m extremely grateful to John Coker for redesigning the Motorcato.org website. That will only help us spot these kinds of problems. But our most daunting problem still is getting people to use it. Just the other day a friend of mine repeated the remark that the MESS report system is a black hole. Somehow we have to get the word out that we have changed that completely and that it’s now a tool for all of us.
The other thing to do is to let us know when you think you see a pattern. The Vmax problem was spotted by flyers who witnessed extremely high numbers of them failing during a drag race and brought it to the attention of TMT.

Charles_McG

Ciderwright
The other thing to do is to let us know when you think you see a pattern. The Vmax problem was spotted by flyers who witnessed extremely high numbers of them failing during a drag race and brought it to the attention of TMT.
Steve, I almost tagged you when I made the Vmax remark. Glad to see I didn't need to

I've heard the comment that CTI says they haven't had any reports, sometimes in the same weekend that Wildman Tim hands me the warranty replacement and comments that it's the fourth one that shipment. it makes me wonder if there is a gap on the CTI side. That warranty replacement info, or the request forms, aren't getting to the right place.

And I still don't buy the 'we don't have the denominator' argument. You don't have to know how deep the ocean is to tell that next wave is going to swamp you. Barring a sudden fad for flying a certain kind of motor, or a new motor, a change in reporting rates is still a signal. If you put it on a control chart, you wouldn't need to know the scale of the Y axis to determine a background variability and spot a change from that.