Assembled RMS motors

[Yoehahn]

I assembled some motors the night before a launch (cold fingers dont help assembling on field) and the launch got scrubbed. So how long can I leave them assembled or should i disassemble and put into ziplock bags right away?

 

[dixontj93060]

Loosen the closures a bit and store them in a dry area. I've kept some over a year.

 

[babp1011]

Loosen the closures a bit and store them in a dry area. I've kept some over a year.

I second this post. I called AT one time to see what their thoughts were on the matter and they said the same thing. They also mentioned to place a plastic bag over the nozzle end to limit the amount of air that getting to the grains to reduce oxidation. I don't know if this is a real issue, someone here might now better than me.

 

[Cyril's Scribe]

I wonder if it would be even better to put a desiccant baggy from a beef jerky bag or some such in with it, especially if you can find a fresh one. I imagine it would help keep the ersatz container more free of oxygen.

 

[gldknght]

I wonder if it would be even better to put a desiccant baggy from a beef jerky bag or some such in with it, especially if you can find a fresh one. I imagine it would help keep the ersatz container more free of oxygen.

Actually desiccant won't do anything but dry out the air in whatever container it's placed in.

 

[Zeus-cat]

AP propellant will both oxidize and absorb water. Both are bad for your motor. Oxidation will make it more difficult to ignite the propellant as it will always oxidize on the exposed faces, one of which is where the igniter touches it. I have sanded the slots on old motors and it helps to remove the oxidation.

If it absorbs too much water it will swell and may bulge out of the cardboard cases. If the motor is assembled this will not happen, but it has happened to motors that I have in storage. The real problem for an assembled motor is that the slot can swell shut and trap the igniter (if installed) or make it very difficult to install the igniter if not installed.

Put it in an air tight container with dessicant if you think it will be sitting for a while or you are in a humid environment.

 

[bill2654]

I have a couple motors that I assembled last November and launch was scrubbed. Loosened the closures and sealed em in a ziploc bag. Do that and they will be fine.

 

[dave carver]

You have to consider that SU motors sit around for years and fire just fine. Some have the same formulations for fuel as the reloads. I flew a 10 year old I132 with no problems and G55 24mm motors that were up to 12 years old with no anomalies. I do however loosen up the enclosures on loaded reloads to keep the O rings from compressing and taking a set. I have a D15 that's been loaded for 3 years. Problem is if there's a problem what is to blame. The D15 has a bad reputation in the first place, bad one to test with. I'll be fireing it this spring. I'll let you know what happens:shock:

 

[rcktnut]

If you have any BP charges in your motors you might want to remove that first before losening your closures. I like preassembling my motors also but never tighten down the closures or add BP untill I know they are going to go. I've had some and have had freinds that had some sitting for along time assembled ( 1year plus) and worked just fine.

 

[cavecentral]

I had some assembled for several months and had no issues. Mostly hobbyline and 29-38mm HPR.

If you loosen the closures, remove the BP first! Otherwise, you risk it entering the motor around the delay area. That can result in a CATO!

 

[MarkII]

Why the need to loosen the closures at all? The assembly process has compressed the O-rings and has created tight seals everywhere, so why mess with it? It isn't as if the rings are going to shrink or anything, especially not while they are compressed. And if they did shrink with age, then loosening the closures wouldn't help anything. I always assemble my motors during the evening before the launch.

Also, would covering the end of the nozzle with tape be just as effective as sealing the motor in an airtight bag?

 

[dixontj93060]

Why the need to loosen the closures at all? The assembly process has compressed the O-rings and has created tight seals everywhere, so why mess with it? It isn't as if the rings are going to shrink or anything, especially not while they are compressed. And if they did shrink with age, then loosening the closures wouldn't help anything. I always assemble my motors during the evening before the launch.

Also, would covering the end of the nozzle with tape be just as effective as sealing the motor in an airtight bag?

From an O-Ring design materials guide:

DEFORMATION
Rubber products should be stored in a relaxed condition, free from tension, compression, or other deformation which can lead to cracking or permanent shape change. Large O-rings and seals should not be stored on pegs.

 

[Zeus-cat]

From an O-Ring design materials guide:

DEFORMATION
Rubber products should be stored in a relaxed condition, free from tension, compression, or other deformation which can lead to cracking or permanent shape change. Large O-rings and seals should not be stored on pegs.

I'm not sure that applies as the O-rings have been installled; they are no longer being stored.

 

[MarkII]

I'm not sure that applies as the O-rings have been installled; they are no longer being stored.

Exactly; at that point they are already deformed (as they are designed to be) to seal the motor sections.

 

[new2hpr]

I have flown motors that were assembled approximately 5 years prior to me getting them. No issues on any of the flights. These were all Aerotech Blue Thunder propellant. If they were Redline (or any other that has had issues with delay oxidation), I would have used electronic deployment only and not risked the delay. In general, I won't trust the delay or ejection charge of an old motor, but if it was stored properly, I will fly it.
-Ken

 

[dixontj93060]

I'm not sure that applies as the O-rings have been installled; they are no longer being stored.

?!?!?

How are they not being stored? They are sitting on a shelf (just happen to be between a piece of metal and a piece of cardboard or AP) ready to be used in a high stress environment.

Me; I'd rather follow the recommendation of the O-ring and motor manufacturer.

 

[crossfire]

I had a AT K5550 loaded for five years. No problem flying at all. The fight went perfect.
gp

 

[MarkII]

?!?!?

How are they not being stored? They are sitting on a shelf (just happen to be between a piece of metal and a piece of cardboard or AP) ready to be used in a high stress environment.

Me; I'd rather follow the recommendation of the O-ring and motor manufacturer.

Yes but once they are installed in the motor during assembly, they are no longer "stored," but now are being put to use for their intended purpose. They are used to create tight internal seals inside the motor. My question had to do with loosening the motor closures for storage. Wouldn't that compromise the seals? If so, then why do it? My feeling is that once you have created those tight seals during your assembly of the motor, you should avoid subsequently messing with them by doing things like loosening the closures. How is backing off of the O-rings and compromising the seals that they have created, beneficial for storage purposes? And if loosening the closures isn't meant to back off on the seals, then what is it meant to do?

 

[dixontj93060]

Yes but once they are installed in the motor during assembly, they are no longer "stored," but now are being put to use for their intended purpose. They are used to create tight internal seals inside the motor. My question had to do with loosening the motor closures for storage. Wouldn't that compromise the seals? If so, then why do it? My feeling is that once you have created those tight seals during your assembly of the motor, you should avoid subsequently messing with them by doing things like loosening the closures. How is backing off of the O-rings and compromising the seals that they have created, beneficial for storage purposes? And if loosening the closures isn't meant to back off on the seals, then wwhat is it meant to do?

You achieve a good seal during firing because the O-ring provides outward force (providing a pressure seal) at the time it is put in use. If you have a long-term depression at the seal point the rubber conforms/deforms and no longer provides the pressure at the exact point needed. An analogy: you have a nice Mercedes sports car with firm conforming racing bucket seats. When you sit in it it cradles your arse like nothing else; you feel confident. Now go get an anvil and lay it in the seat for a week; then sit in it--no longer a snug fit. Even you sitting in it for a week (same ultimate shape) will depress the support and it won't provide the snug fit (outward/upward pressure as before when freshly depressed) and the confidence you want when you take turns at 90 mph.

Ultimately it is the same with an O-ring--you want the maximum (or at least the designed) amount of force applied at the critical points right at the time of use.

 

[jd2cylman]

O-rings by most accounts are not auto seats. O-rings are designed to deform to do their job. That is how they seal. I've pulled apart 50 year old tractors that still had the original o-rings installed and doing their job. O-rings will be just fine in a motor case. They are not really under pressure, because the motor has not been lit. In the industrial/agricultural world, o-rings are everywhere, holding back thousands of psi of oil pressure. These o-rings are not there to hold that pressure for a few seconds, days or even weeks. Granted they will not see the same temperatures as our reloads, but they will go through some wide temperature swings.The only caveat will be o-ring quality and material. I'm pretty sure AT, CTI and all the other mfgs use high quality o-rings. Leave them tight and fly them when you can! Hey! That sounds like a good sig!
Adrian

 

[5x7]

I think the only issue about loosing a seal is in the hobbyline 24-40 and 29 40-120. In those, the delay oring is compressed by the delay stack which is pushed on by a fiber washer. I have seen this washer deform (sag) under the pressure of the compression over time which makes it a remote possibility the delay rings could loose the seal. However, I do know a friend that fired a 29 40-120 motor that had been built for over a year and it fired fine.

 

[dixontj93060]

O-rings by most accounts are not auto seats. O-rings are designed to deform to do their job. That is how they seal.

Of course not, as stated it was an analogy.

They are not really under pressure, because the motor has not been lit.

Wrong, they are under compression if you don't loosen the closures.

Look, I'm not spending any more time on this. You guys store your assembled motors the way you want. The fact is O-rings experience deformation over time. This effect is called relaxation or creep (Ref: The Physics of Rubber, see esp. section When a constant load... and succeeding two sections). There are standardized tests to isolate this and people spend many cycles in research trying to improve this effect (e.g., Improving the Long-term Performance...). I used to know this stuff cold when working with Bell Labs to design membrane switches for phone systems but that was 30 years ago and I don't have the time (at least this week) to resurrect that research. But again, probably makes no difference as you guys seem pretty "set in your ways"; I expect I'd be wasting my time and energy in the end.

 

[rcktnut]

You guys store your assembled motors the way you want.

To bad when someone asks a question and the best answer ends up is what you state up above. Maybe it does not make a difference whether the closures are losened or not, but if the manufacturer suggests losening them that is what I would do. I don't worry about losening them because when I assemble my motors previous to the launch date I just snug the closures leave out the BP and take care of that right before I'm going to use that motor.

Different opinions/answers typical on forums, ultimately up to yourself to decide what to do. If it sounds good to you, do it!!!

 

[dixontj93060]

To bad when someone asks a question and the best answer ends up is what you state up above.

After giving answers three times with three different lines of reasoning and/or third-party support (BTW, first answer I gave was yours, regarding manufacturer's recommendation) and getting responses that amount to nothing but opinion or arbitrary isolated observations I refuse to continue to :bang:

 

[bobkrech]

You should follow the directions from AT regarding the storage of assembled reloadable motors for long periods of time.

Also you might want to download the O-ring Bible aka the Parker O-ring Handbook to see how o-rings work.

In a rigid assembly, compressed o-ring seals work for decades, however these o-rings are compressed against hard immovable surfaces. In many reloadable AT motors, the o-rings seal against cardboard tubes and disks which are not completely rigid. If too much grease is used, the cardboard can soften with time, the cardboard can bend and the seals my fail upon ignition. I'm assuming this is the reason why AT recommends unloading the o-rings by loosening a closure.

Also you need to know that the grease does not make the seal tight. It's the o-ring compression again hard smooth surfaces that makes the seal. The grease is strictly a lubricant that prevents the o-ring from twisting and ripping during installation and tightening. You need to wipe off 99% of the grease before you install the o-rings in a motor or risk inhibiting the surfaces of the propellant or delay grains.

Bob

 

[dixontj93060]

In many reloadable AT motors, the o-rings seal against cardboard tubes and disks which are not completely rigid. If too much grease is used, the cardboard can soften with time, the cardboard can bend and the seals my fail upon ignition. I'm assuming this is the reason why AT recommends unloading the o-rings by loosening a closure.

Although AT's recommendation applies whether the O-ring seats against a fiber washer, a relatively hard phenolic nozzle, or even a more rigid aluminum seal disk, so I doubt if the reason applies solely to softening of fiber/cardboard components.

 

[UPscaler]

loosen the closures, and put it in a ziploc bag! I flew a G33 that had been in the case for about 5 years!


(also flew and H123 that was almost as old as I am, but it hadn't been loaded)





Braden

 

[bobkrech]

Although AT's recommendation applies whether the O-ring seats against a fiber washer, a relatively hard phenolic nozzle, or even a more rigid aluminum seal disk, so I doubt if the reason applies solely to softening of fiber/cardboard components.

Jim

I've been designing and building laboratory apparatus with o-rings for 41 years.

• Static o-ring seals for a high pressure electric arc-driven shock tube driver operating up to 60,000 psi (4,000 atm.)
• Dynamic o-ring seals for a high pressure water pump for hydrostatically testing laboratory apparatus and motor casings to 10,000 psi. (680 atm.)
• Dynamic c=ring seals for rotary flanges in ultrahigh vacuum systems with vacuum levels below 10-8 torr (10-11 atm.)
• Static o-ring seals in ultrahigh vacuum system with vacuum levels below 10-10 torr (10-13 atm.)
• Cryogenic temperatures below liquid nitrogen 77 K (-321 F)
• Hot system up to 300 C (570 F)

O-rings in systems operating near the high temperature limit of the elastomer will eventually need replacement, however o-rings operating near room temperature last for decades in static applications.

You could ask 10 designers how they would design a reloadable motor system, and you'd probably get 10 different designs. I've never seen a rationale for the AT o-ring seal designs used in their reloadable motors, but it has worked for 3 decades if you follow the AT directions. There's always a cost/performance trade-off in any mechanical design, and using less expensive but more flexible cardboard insulators, casting tubes, and washers instead of more expensive but stiffer phenolic or metal parts was certainly one of the trade-offs. Hobby folks always complain about price and the manufacturers are sensitive to this. Using cardboard is less expensive and it performs adequately. I believe that the AT directions for loosening the closures is a result of this trade-off since cardboard isn't particularly stiff and is permanently deformable, especially if wetted, under compression for long time periods.

That's my rationale for the directions. Yours may be different, but the manufacturer knows more the either of us, so I'd follow their directions.

Bob

 

[dixontj93060]

...the manufacturer knows more the either of us, so I'd follow their directions.

Agreed.