Aerotech RMS fail. What did I do wrong?!

[JSW]

Trying to understand what I did wrong.

Areotech 24/40 RMS.

It seemed to ignite. The rocket made it off the rail. Then fell backward. See video.

The main casing got pushed out the back of the rocket, followed by the main propellant grain and delay grain. Neither had ignited. The red vent cap on the rear nozzle was still on. The forward closure and red ejection charge cap was stuck up in the motor block. I got it out, and found that the red ejection charge cap was still on and intact. Ejection charge powder was gone.



This was the 2nd RMS I've ever done,... I was in a super hurry and had a house full of (other peoples) kids screaming about when we could leave to launch rockets. I couldn't find the lube that came with, so I made due with a tiny bit of Vaseline on the o-rings. I assume that was the problem,... Vaseline = flammable. Somehow the Vaseline on the forward o-ring ignited the ejection charge (but somehow didn't ignite the delay or propellant?!)

Does that make sense? Any thoughts?

 

[rharshberger]

Vaseline was not an issue, many others have used it in the past.

Pictures of all the parts would help diagnose the issue (fuel, casing, everything).

 

[Steve Shannon]

There’s nothing wrong with using Vaseline. You had a Cato, a high pressure event that caused the forward closure to be blown off. There are several reasons this can happen and some of them might not be your fault.
One cause is to have more propellant exposed. The pressure within a rocket motor is proportional to the area of fuel that’s burning. It’s usually progressive, meaning that the burn rate goes up exponentially with pressure, leading to even more pressure.
But one thing that never helps is to get in a hurry or be distracted while assembling a motor.

 

[Buzzard]

Was the delay element assembly too loose in the forward closure? I has to be a snug fit, otherwise hot gas could leak past and ignite the ejection charge. Aerotech recommends using tape to make sure the fit is snug.
I have seen this happen.

Chas

 

[Steve Shannon]

Was the delay element assembly too loose in the forward closure? I has to be a snug fit, otherwise hot gas could leak past and ignite the ejection charge. Aerotech recommends using tape to make sure the fit is snug.
I have seen this happen.

Chas

The red cap was still on the forward closure, so the powder didn’t ignite in the ejection charge well.
You’re right that the delay grain must seal tightly against the forward closure.

 

[Steve Shannon]

I would urge the OP to contact Aerotech customer service and submit this as a MESS report at motorcato.org. Aerotech customer service may be able to help him figure out what went wrong. They’ll replace the motor under warranty if it wasn’t his fault and help him learn what to avoid if it was.

 

[DabCat]

What motor and when and where did you buy the casing? A little while back, Aerotech had a bad batch of 24/40 casings where something was wrong with the threads and they were prone to CATOs. I've seen tons from this batch CATO. 3 of mine have.

 

[jmasterj]

Perhaps the forward closure wasn't threaded all the way on? Worth checking the threads for damage, though that probably won't confirm anything. Sounds like a CATO to me.

 

[waltr]

Which load was used?
The 24/40 case has 8 different loads available.

Also some close up photos of the fwd case end and the fwd closure can help determine cause.
These can also be submitted to the MESS report.

 

[AeroTech]

Without more information it’s way too soon to be advising what the flyer should or shouldn’t do, other than contact Karl at [email protected].

 

[Getmore]

Was the igniter fully installed to the top?

 

[4regt4]

What motor and when and where did you buy the casing? A little while back, Aerotech had a bad batch of 24/40 casings where something was wrong with the threads and they were prone to CATOs. I've seen tons from this batch CATO. 3 of mine have.

Is there a way to identify the suspect 24/40 casings? I have an unused one. Would like to know before I use it.

Hans.

 

[3stoogesrocketry]

Trying to understand what I did wrong.

Areotech 24/40 RMS.

It seemed to ignite. The rocket made it off the rail. Then fell backward. See video.

The main casing got pushed out the back of the rocket, followed by the main propellant grain and delay grain. Neither had ignited. The red vent cap on the rear nozzle was still on. The forward closure and red ejection charge cap was stuck up in the motor block. I got it out, and found that the red ejection charge cap was still on and intact. Ejection charge powder was gone.

View attachment 534851

This was the 2nd RMS I've ever done,... I was in a super hurry and had a house full of (other peoples) kids screaming about when we could leave to launch rockets. I couldn't find the lube that came with, so I made due with a tiny bit of Vaseline on the o-rings. I assume that was the problem,... Vaseline = flammable. Somehow the Vaseline on the forward o-ring ignited the ejection charge (but somehow didn't ignite the delay or propellant?!)

Does that make sense? Any thoughts?

This failure is a interesting one . As you mentioned, both red caps remained on there respective parts. With out any more information , or advanced pictures , it sounds like the initiator plugged the nozzle and since the internal pressure of a motor is higher towards the front , it blew the forward closure out first. It is possible it was a case failure , but we need to see the case to make that call.

 

[JSW]

Thanks for all of the comments! (I was expecting to get made fun of for using Vaseline.)

The 24/40 case and 3x F24-7W motors were purchased (as a set) from BuyRocketMotors.com in May 2022.

I cleaned up the case and closures. Case seems fine. Threads are look fine. Closures thread onto the case like normal. Forward closure looks a tiny bit beat up, but still looks ok. Pictures attached.



Now that I think about it,... I had a hard time connecting the aft closure ring to the case. The combined length of (forward o-ring) + (forward washer) + (propellant grain) + (rear o-ring) + (rear nozzle) was too long to allow the threads of the rear closure ring to engage to the case. So I loosened up the forward closure a couple threads, got the rear closure ring on, and tightened both down.

I just tried again using a 2nd set of loads to see if it was any different. It's the same. Even with firm pressure, the rear case threads aren't exposed enough for the rear closure ring to engage at all. (The 4th picture shows the idea,... you can see a couple case threads,... but it's not enough to engage with the ring.)



So maybe I put so much pressure on those threads that they popped apart during ignition?

Appreciate you all helping with this analysis.

I'll reach out to Aerotech and see what they think.

Other notes:

• Ignitor was pushed all the way up and touching the delay.
• The red cap on the aft end of the motor had a vent hole cut in it. Can't remember if it was still attached.
• The delay was snug up in the forward closure.

 

[mikec]

Are you sure you're putting the aft O-ring where it's supposed to go? I don't see it in your picture 4 and should be able to.

 

[DigBaddy]

Thanks for all of the comments! (I was expecting to get made fun of for using Vaseline.)

The 24/40 case and 3x F24-7W motors were purchased (as a set) from BuyRocketMotors.com in May 2022.

I cleaned up the case and closures. Case seems fine. Threads are look fine. Closures thread onto the case like normal. Forward closure looks a tiny bit beat up, but still looks ok. Pictures attached.

View attachment 535043View attachment 535042View attachment 535044

Now that I think about it,... I had a hard time connecting the aft closure ring to the case. The combined length of (forward o-ring) + (forward washer) + (propellant grain) + (rear o-ring) + (rear nozzle) was too long to allow the threads of the rear closure ring to engage to the case. So I loosened up the forward closure a couple threads, got the rear closure ring on, and tightened both down.

I just tried again using a 2nd set of loads to see if it was any different. It's the same. Even with firm pressure, the rear case threads aren't exposed enough for the rear closure ring to engage at all. (The 4th picture shows the idea,... you can see a couple case threads,... but it's not enough to engage with the ring.)

View attachment 535046

So maybe I put so much pressure on those threads that they popped apart during ignition?

Appreciate you all helping with this analysis.

I'll reach out to Aerotech and see what they think.

Other notes:

• Ignitor was pushed all the way up and touching the delay.
• The red cap on the aft end of the motor had a vent hole cut in it. Can't remember if it was still attached.
• The delay was snug up in the forward closure.

I think you may have a situation I found loading up a F24-7, the delay spacer is too long and needs to be sanded thinner.

I have yet to burn the motor, but once sanded it went together like one would expect. Without sanding the threads wouldn't engage.

 

[DabCat]

Is there a way to identify the suspect 24/40 casings? I have an unused one. Would like to know before I use it.

Hans.

I'm not 100% sure how they could be identified. All my cases and the ones I've seen have been replaced with new ones from Aerotech (warranty). None of my CATOs caused a ton of damage, so I would say just fly it as normal. I remember having them replaced at NSL when it was hosted at Lucerne (2019 I think?). So I bought them at least a year prior, maybe longer. I wouldn't be too worried. I don't think there are many bad ones out there. Aerotech is pretty good with stuff like that.

 

[JSW]

Are you sure you're putting the aft O-ring where it's supposed to go? I don't see it in your picture 4 and should be able to.

good eye. that might be it.

i put together again, but with the o-ring on top of the nozzle,... and can get the threads on the rear closure ring to engage like i'd expect. hmmm.

 

[waltr]

Looking at your photos- The threads on the aft closure and inside the case do not look good, they have a white marks that look like damage.
A way to check is thread on partially and see how much play, in-out and back-forth, there is Since you have a 'new' case do the same and compare. If there is excess play then either the caes or closure is no good and should not be used. Just checked my case and there is no play in the threads.

I just assemble an F24W load. The 1/16" (thinner) O-ring goes between insulator and fwd closure. The 3/32" (fatter) O'ring between Nozzle and aft closure. The aft closure threads barely engaged but pushing and turning they did. Then needed the aft closure wrench to fully tighten. This is how all the loads in the 24/40 case has gone together.
The easiest thing to do wrong is put then two O-rings in the wrong places. I always have to double check which one goes where. And I always follow the instruction for the load I am using since a different load may be built differently.

Are you sure you're putting the aft O-ring where it's supposed to go? I don't see it in your picture 4 and should be able to.

Good catch Mike

 

[ksaves2]

There’s nothing wrong with using Vaseline. You had a Cato, a high pressure event that caused the forward closure to be blown off. There are several reasons this can happen and some of them might not be your fault.
One cause is to have more propellant exposed. The pressure within a rocket motor is proportional to the area of fuel that’s burning. It’s usually progressive, meaning that the burn rate goes up exponentially with pressure, leading to even more pressure.
But one thing that never helps is to get in a hurry or be distracted while assembling a motor.

Yup,

A lot of folks here have fastidiously built commercial motors in accordance with the manufacturer's instructions and have had CATO's. It's a thing about rocketry. Though usually commercial motors have a less CATO rate than experimental motors, if one refines a mix they can get reliable motors that work just about every time. I have a two grain 38mm H160'ish motor that works every time using AT hardware and nozzle. (that I opened up a little bit)

Kurt

 

[Tom Flint]

Both closures must thread on all the way. I had this same failure but I left the aft closure loose by a couple threads and it blew the aft closure off. The closure feels like it won't go on all the way but you have to use extra muscle and maybe pliers. It'll go all the way on. Your casing has probably been damaged beyond further use by the explosion, even if it still seems to thread together ok.

 

[rharshberger]

Both closures must thread on all the way. I had this same failure but I left the aft closure loose by a couple threads and it blew the aft closure off. The closure feels like it won't go on all the way but you have to use extra muscle and maybe pliers. It'll go all the way on. Your casing has probably been damaged beyond further use by the explosion, even if it still seems to thread together ok.

Some loads the rear closure will NOT thread in all the way, and doing so will damage the liner. In the instructions AT will call out if a max gap is allowed. Pliers on cases/closures is bad, in the Hobbyline cases the rear closure wrench is all that will ever be needed normally....unless you cant unscrew a closure, then its handy to have a small strap wrench so as not to mar the casing or closures. I have flown many reloadable motors 18mm to 75mm and never needed more than a strap wrench on a really tight closure closure. Sometimes waiting until the case is completely cool will allow the tight closure to be removed easier, or warming the case and spraying a bit of "canned air/duster" on the closure (turn can upside down to get the supercold liquid) will cause the closure to shrink enough to loosen it for removal.

 

[beeblebrox]

If the nozzle was wrong, IE too small a throat for that reload, it would cause an overpressure.
One last thing, Is the case exactly 2 3/4"? If it is too short due to manufacturing error that would be a problem...

I happen to have a case that is too long...But that is easier to deal with...Longer insulator tube fixes that.

 

[jmasterj]

A rag is also a great option for screwing/unscrewing those little 24mm closures.

 

[jimzcatz]

Both closures must thread on all the way. I had this same failure but I left the aft closure loose by a couple threads and it blew the aft closure off. The closure feels like it won't go on all the way but you have to use extra muscle and maybe pliers. It'll go all the way on. Your casing has probably been damaged beyond further use by the explosion, even if it still seems to thread together ok.

No they dont. It's absolutely imperative that the forward closure be seated completely,not so much for the rear. This coming directly from Gary a kajillion years ago at LDRS in Orangeburg. Granted we were discussing random RMS loads and not the 24/40 in particular.

 

[RocketRev]

I keep hearing people say that Vaseline is fine for lubing component parts in Aerotech reloadable motors. I beg to differ for two reasons. 1) Vaseline is flammable and it is a petroleum product. For that reason alone its not a good reason to use it. The other reason is that with not very much of an increase in temperature, anything over body temperature in fact and Vaseline loses its viscosity. It becomes thinner and starts to run and drip.

Yes, Yes, I know lots of folks have used it to no problems whatsoever. At least they are not aware of any problems. And some of you are going to moan and groan that anyone would dare to say otherwise. But this reloadable motor failure is undiagnosed and I haven't heard a good explanation yet. But you're sure "it couldn't be the Vaseline." Well the incredibly low viscosity of Vaseline under very minimal environmental temps, long before you put fire to the motor, leaves it flawed at best and dangerous at worst. Yes, I know its only a 29/40 motor, but practices have a way of continuing. The larger the motor you use Vaseline in, the more likely you will be to have a motor failure.

I'm sorry, but "man-up" and buy some real rocketry o-ring, liner, delay grain lubricant and leave the Vaseline at home in the bathroom where it belongs. I like Dow Corning 111 silicone grease. It is a high temp high viscosity high pressure lubricant made for o-rings in very high temp applications. One tube will last you for years in model rocketry.

Ignore me if you want to. That is your prerogative. But as both a TAP and an L3CC for a very long time now, I can tell you that if you came to me wanting to do an L-3 cert flight with vaseline in your motor, I would refuse to allow you to make your cert attempt until you disassembled your motor, cleaned off any vaseline residue and re-lube everything that is supposed to be lubed with a recommended high temp, high viscosity, high pressure, lube.

Brad

 

[jimzcatz]

Brad, with respect I beg to differ. It is well know that vaseline is indeed great for RMS motors. All you need is enough to make the rings shiny, this coming directly from the man himself Gary so many moons ago. Pretty much when ISP first introduced the reloadable motor. And your statement about denying a cert attempt because of vaseline is just plain wrong. the grease isn't there to make a seal, its just there for ease of assembly. Yes I quote Gary a lot. I have been in this hobby for more than a few minutes. Brad, I respect you immensely but your refusal to allow a cert attempt because of vaseline use in my opinion exceeds your TAP authority.And I would Never use DOW 111 on any liner,after having several liners glued to the case with that 111 snot. Its great for o'rings though. Would I use vaseline on a M motor, no but denying a flight for that reason is just not right.

 

[UhClem]

I keep hearing people say that Vaseline is fine for lubing component parts in Aerotech reloadable motors. I beg to differ for two reasons. 1) Vaseline is flammable and it is a petroleum product. For that reason alone its not a good reason to use it. The other reason is that with not very much of an increase in temperature, anything over body temperature in fact and Vaseline loses its viscosity. It becomes thinner and starts to run and drip.

I still remember watching Gary assemble motors at a 1996 Texas launch. Someone asked him why he was using that particular lubricant. The answer was, more or less, that he found the Slick 50 at Walmart before he got to the Crisco. Apparently there was a stop in Waco on the way to the field.

The only time when lubricant is critical is for hybrid motors in locations where it needs to be compatible with the NOX. (I loaned my case at that launch to be used in an Aerotech manufacturers demo. I forget which particular hybrid load.)

That said, I use Superlube. The spray version is very handy for coating the inside of motor cases. Much neater than trying to lube a liner.

But as for the 24/40 case. Sometimes the aft closure can be difficult to start. So I don't tighten the forward closure all the way until after I have the aft closure started. Then use cloth (blue jean pant leg in a pinch) to aid with the aft closure.

 

[carlsonmjc]

JSW I had a similar failure of a 24/40 motor back in 1996 or so. I was very experienced with proper assembly and to make a long story short it was a failure of the rear case threads. The threads are very fine, and this case let go of the rear closure. Same results you had. After cleaning, the two did screw together but under close inspection you could see the case threads were rounded off making it unusable. It was a Blue Thunder load.

Aerotech made me whole.

Mark C

 

[gldknght]

I've used vaseline on o-rings a LOT! And have had zero issues with it. If you are using enough vaseline for it to melt and drip with temperature changes, you are using way to much lube, which will cause other problems.