Plugging Booster Motors For Cluster Applications

[lakeroadster]

For clustered motor applications it has been stated here on the forum that it is advantageous to plug booster motors to prevent lighting off the adjacent motors delay charge prematurely.

So what have you used and recommend for plugging the booster motor?

Wadding and tape,

Wadding and a poured / cured epoxy plug,

or ???

What Say Ye?

 

[KennB]

Go with your first choice; wadding and tape. Vendor wadding or dog barf will do.

Plugging with epoxy would constitute modifying the motor and is not allowed.

Have multiple fun with your cluster.

 

[MClark]

What kind of motor are you plugging?
Estes, reloadable, single use HPR?

 

[Dugway]

On my Cluster Duck I use aluminum tape with no wadding. So far, so good.

 

[kuririn]

Lake, best solution short of using all 3 engines with delays and ejection charges is to plug the motor tubes. (I would opt for using all three motors with delays because if one or two don't light you still would have an ejection of the recovery device Also -x motors cost the same as -0 motors). The NCR Cluster Duck has the top of the booster tubes sealed by the centering ring and the blowby gases are vented via a punched hole into the recess between the core tube and booster tubes and out the back end of the rocket. The Semroc Hydra 7 has a wooden plug just in front of the engine in the booster tubes. Although you are a lone wolf this way you would comply by design should you want to launch at a NAR sanctioned meet later on and not have to modify (epoxy) any motors every time you launch. Also, as I understand it, some clubs/meets allow motor eject and some do not. I'm guessing that the risk of fire hazard plays a big part in that decision. Cheers.

 

[lakeroadster]

What kind of motor are you plugging?
Estes, reloadable, single use HPR?

Estes 24mm's

 

[jmuck78]

Go with your first choice; wadding and tape. Vendor wadding or dog barf will do.

Plugging with epoxy would constitute modifying the motor and is not allowed.

Have multiple fun with your cluster.

What's the difference between plugging the motor ejection with epoxy and plugging it with tape and wadding? I presume we are talking about black powder motors here since the aerotech single use motors have the removable red plug that allows you to remove the ejection charge without modifying the motor.

 

[vcp]

On 18mm motors, I 3D printed little 'top hat' shaped plugs that friction-fit into the motors. Besides the friction-fit, the brim of the hat is clamped between the motor and the motor mount. A small tab hanging 'below' the hat allows fingertip removal.

 

[KennB]

What's the difference between plugging the motor ejection with epoxy and plugging it with tape and wadding? I presume we are talking about black powder motors here since the aerotech single use motors have the removable red plug that allows you to remove the ejection charge without modifying the motor.

Paragraph 2 of the Model Rocket Safety Code states, "Motors. I will use only certified, commercially-made model rocket motors, and will not tamper with these motors or use them for any purposes except those recommended by the manufacturer." NAR S&T considers plugging with epoxy as tampering with the motor. The wadding or dog barf and tape option is reversible, if necessary, so the motor can be returned to it's "as manufactured" state.

This has been discussed in many threads on this forum with most everyone in agreement with this interpretation. It's always best to not give an insurance claims agent a reason to deny policy coverage.

 

[jimzcatz]

Plug mine with epoxy regularly

 

[BABAR]

It's always best to not give an insurance claims agent a reason to deny policy coverage.

THAT part is hard to argue with!

You get fecal turbine interaction with a plugged motor you are on your own for the damages!

 

[jmuck78]

Paragraph 2 of the Model Rocket Safety Code states, "Motors. I will use only certified, commercially-made model rocket motors, and will not tamper with these motors or use them for any purposes except those recommended by the manufacturer." NAR S&T considers plugging with epoxy as tampering with the motor. The wadding or dog barf and tape option is reversible, if necessary, so the motor can be returned to it's "as manufactured" state.

This has been discussed in many threads on this forum with most everyone in agreement with this interpretation. It's always best to not give an insurance claims agent a reason to deny policy coverage.

I understand what the Model Rocket Safety Code says, and I agree with it. What I have yet to see is 1) an explanation as to why preventing motor ejection with tape and wadding is any more or less risky than plugging the motor with epoxy and 2) how an external application of epoxy (still talking about Estes-style black powder motors) is a "modification" to the motor. If the original intent of the model rocket safety code section on motor modification covers epoxying the ejection charge on -x or the burn through on -0 motors, than it would logically apply to tape and recovery wadding over the forward end of motors since the intent and effect is the same. The only difference being the tape and wadding is much less determinative since tape and wadding may not necessarily seal the forward end of the motor. The argument that the motor can be returned to is't as-manufactured state is a non-sequiter since the flyer always has the option of discarding the motor if the epoxy is deemed unsafe, especially given that the flyer cannot return the motor to its as-manufactured state after it has flown. We aren't talking about scraping out the clay plugs, adding black powder or other chemicals, widening the nozzle, or adding other holes to the motor - those types of things are what's covered by the model rocket safety code.

Separately, on the Aerotech single-use motors, I have also seen people argue that removing the red plugs and emptying the black powder ejection charge qualifies as "modifying" the motor. If the test is reversibility, however, removing or replacing the black powder on Aerotech Motors would pass the reversibility test. I have received Aerotech motors where a red plug has come loose during shipment, and I have been told by the manufacturer to simply pour the loose black powder back in and replace the removable red cap.

This is not an academic debate - and I understand the desire to keep the insurers happy and not give anyone an excuse to interfere with the hobby. At the same time, we need to make sure that we don't interpret safety guidelines so broadly that there could be unintended consequences. For example, high power flyers regularly use ignition pellets on the igniters - are we going to call that a modification as well? Safety is the first, second, and third goals here, so if there's an argument as to why epoxying the forward end of a black powder motor makes the motor unsafe to use, then I'm curious what it is - both so I can avoid doing something unsafe and so that when I parrot this interpretation at the RSO table, I have solid logic to support it.

 

[shreadvector]

I understand what the Model Rocket Safety Code says, and I agree with it. What I have yet to see is 1) an explanation as to why preventing motor ejection with tape and wadding is any more or less risky than plugging the motor with epoxy and 2) how an external application of epoxy (still talking about Estes-style black powder motors) is a "modification" to the motor. If the original intent of the model rocket safety code section on motor modification covers epoxying the ejection charge on -x or the burn through on -0 motors, than it would logically apply to tape and recovery wadding over the forward end of motors since the intent and effect is the same. The only difference being the tape and wadding is much less determinative since tape and wadding may not necessarily seal the forward end of the motor. The argument that the motor can be returned to is't as-manufactured state is a non-sequiter since the flyer always has the option of discarding the motor if the epoxy is deemed unsafe, especially given that the flyer cannot return the motor to its as-manufactured state after it has flown. We aren't talking about scraping out the clay plugs, adding black powder or other chemicals, widening the nozzle, or adding other holes to the motor - those types of things are what's covered by the model rocket safety code.

Separately, on the Aerotech single-use motors, I have also seen people argue that removing the red plugs and emptying the black powder ejection charge qualifies as "modifying" the motor. If the test is reversibility, however, removing or replacing the black powder on Aerotech Motors would pass the reversibility test. I have received Aerotech motors where a red plug has come loose during shipment, and I have been told by the manufacturer to simply pour the loose black powder back in and replace the removable red cap.

This is not an academic debate - and I understand the desire to keep the insurers happy and not give anyone an excuse to interfere with the hobby. At the same time, we need to make sure that we don't interpret safety guidelines so broadly that there could be unintended consequences. For example, high power flyers regularly use ignition pellets on the igniters - are we going to call that a modification as well? Safety is the first, second, and third goals here, so if there's an argument as to why epoxying the forward end of a black powder motor makes the motor unsafe to use, then I'm curious what it is - both so I can avoid doing something unsafe and so that when I parrot this interpretation at the RSO table, I have solid logic to support it.

BP motors and Composite motors are different.

Different manufacturers have issues specific rulings/instructions for adding epoxy plugs or removing ejection charges.

You cannot remove the ejection charge from an Estes motor without scraping it out. Forbidden by Estes.
If you try to plug a booster motor, you run the risk of a HUGE overpressure when the flame reaches the forward end. If the user modifies the motor with too little epoxy, it will bow under pressure resulting in cracked thin membrane of propellant with greatly increased burning surface area and blow out the top. Simply puttine adding and tape over the top of a booster motor may not hold in the pressure when it burns through the front. For a motor with an ejection charge, trying to seal it with tape or epoxy will created an exploding firework-type event.

If Aerotech has instructions on their website for removing the red cap and pouring out the BP ejection charge and filling the well with wadding or grease, then it is allowed by the manufacturer.

 

[jimzcatz]

BP motors and Composite motors are different.

Different manufacturers have issues specific rulings/instructions for adding epoxy plugs or removing ejection charges.

You cannot remove the ejection charge from an Estes motor without scraping it out. Forbidden by Estes.
If you try to plug a booster motor, you run the risk of a HUGE overpressure when the flame reaches the forward end. If the user modifies the motor with too little epoxy, it will bow under pressure resulting in cracked thin membrane of propellant with greatly increased burning surface area and blow out the top. Simply puttine adding and tape over the top of a booster motor may not hold in the pressure when it burns through the front. For a motor with an ejection charge, trying to seal it with tape or epoxy will created an exploding firework-type event.

If Aerotech has instructions on their website for removing the red cap and pouring out the BP ejection charge and filling the well with wadding or grease, then it is allowed by the manufacturer.

Man are you wrong. Estes can't forbid anything once something is sold. They can choose not to honor a warranty if they want. Plugging a booster motor is as safe as not plugging it Booster motors don't have a core. I don't see how plugging them is more dangerous than a standard motor. You have the same chance of a cato. Just luck. Your physics make no sense

 

[dhbarr]

Man are you wrong. Estes can't forbid anything once something is sold. They can choose not to honor a warranty if they want. Plugging a booster motor is as safe as not plugging it Booster motors don't have a core. I don't see how plugging them is more dangerous than a standard motor. You have the same chance of a cato. Just luck. Your physics make no sense

This conflates physics with bureaucracy, which is easy to do. One of them is provable, rational, and consistent.

The other one often governs what we're allowed to do and often depends on vague interpretations of murky corner cases.

 

[jmuck78]

BP motors and Composite motors are different.

Different manufacturers have issues specific rulings/instructions for adding epoxy plugs or removing ejection charges.

You cannot remove the ejection charge from an Estes motor without scraping it out. Forbidden by Estes.
If you try to plug a booster motor, you run the risk of a HUGE overpressure when the flame reaches the forward end. If the user modifies the motor with too little epoxy, it will bow under pressure resulting in cracked thin membrane of propellant with greatly increased burning surface area and blow out the top. Simply puttine adding and tape over the top of a booster motor may not hold in the pressure when it burns through the front. For a motor with an ejection charge, trying to seal it with tape or epoxy will created an exploding firework-type event.

If Aerotech has instructions on their website for removing the red cap and pouring out the BP ejection charge and filling the well with wadding or grease, then it is allowed by the manufacturer.

1) I agree that scraping out an ejection charge is forbidden, stipulated.

2) Thank you for identifying a specific risk for plugging the booster motors, good point.

3) The booster motors work by igniting the second stage only after the motor has burnt through almost all of its propellant. So, by the time the flame reaches the plugged end of the booster motor, the risk of a "huge overpressure" is almost nil because almost all of the propellant has been expended.

4) Stipulating that the huge overpressure is a real probability, how is that any different than the normal risk with Estes Black Powder motors? When black powder motors CATO, they tend to pop and it's over. If the motor cato's at the end of its burn, what's the risk?

5) "For a motor with an ejection charge, trying to seal it with tape or epoxy will created an exploding firework-type event." I don't think so - again, all of the motor propellant has already burned. I think plugging the forward ejection on a black powder motor would more likely blow the charge out of the aft nozzle of the motor, that's what it was designed to do in the first place. But we could always test this and find out.

 

[jimzcatz]

Everybody knows, or at least should know that if you wish to plug an Estes motor that has an ejection charge just remove the ejection charge Don't really have to plug it. And saying that it's forbidden? Bulls.... Estes has no power to forbid anything once I own something.

 

[jmuck78]

Everybody knows, or at least should know that if you wish to plug an Estes motor that has an ejection charge just remove the ejection charge Don't really have to plug it. And saying that it's forbidden? Bulls.... Estes has no power to forbid anything once I own something.

I don't think he meant forbidden by Estes - I thought we were discussing what constitutes modification of a motor according to the model rocket safety guidelines, and therefore, what would be accepted/rejected by an RSO.

 

[lakeroadster]

Thanks for all the discussion fella, much appreciated,

Follow Up...

I emailed Estes and asked, in regard to plugging their booster motors, what is their opinion in regard to:

A) the epoxy plug method was modifying the motor, and,
B) if using their wadding and a tape plug was modifying.​

This was their reply:

Our Estes model rocket engines have been designed and manufactured for the past 60 years to function as either a timed ejection charged engine or a 0-ejection charge booster. We do not recommend any kind of modification of our model rocket engines.
​

I've since decided for my 3 motor cluster that (1) D12-5 and (2) D12-7 motors are my best option. That way if one or two motors don't fire at launch, or malfunction, It'll still spit out the laundry.

I've also ensured the design is "robust" just in the unlikely event (2) motor ejection charges do fire instantaneously.

 

[jmuck78]

I emailed Estes and asked, in regard to plugging their booster motors, what is their opinion in regard to:

A) the epoxy plug method was modifying the motor, and,
B) if using their wadding and a tape plug was modifying.​

This was their reply:

Our Estes model rocket engines have been designed and manufactured for the past 60 years to function as either a timed ejection charged engine or a 0-ejection charge booster. We do not recommend any kind of modification of our model rocket engines.
​

I've since decided for my 3 motor cluster that (1) D12-5 and (2) D12-7 motors are my best option. That way if one or two motors don't fire at launch, or malfunction, It'll still spit out the laundry.

I've also ensured the design is "robust" just in the unlikely event (2) motor ejection charges do fire instantaneously.

I'm sorry, but that's the stock, knee-jerk reaction you would get from any manufacturer. Estes instructions also say that they don't recommend making any changes to the assembly process as it could result in an unsafe rocket.

 

[rharshberger]

As one of my clubs RSO's if you show up at a launch with an epoxy plugged -0 Estes or Quest motor it will ONLY be allowed to fly as a TRA Research motor (as long as I am aware said motor has been modified). Estes has offered -P motors in the past, the reason for my opinion is that neither the club or the manufacturer can be held responisble for the action of modifying a motor and as NAR does not allow research motors it could not be flown at a NAR launch.
I believe other clubs would concur that epoxy plugging a -0 is a modifiction, where as tape and dog barf have no real structural integrity and serve solely to prevent the heat and flame of the -0 motors burn through from damaging the motor mount of the rocket.

 

[jmuck78]

As one of my clubs RSO's if you show up at a launch with an epoxy plugged -0 Estes or Quest motor it will ONLY be allowed to fly as a TRA Research motor (as long as I am aware said motor has been modified). Estes has offered -P motors in the past, the reason for my opinion is that neither the club or the manufacturer can be held responisble for the action of modifying a motor and as NAR does not allow research motors it could not be flown at a NAR launch.
I believe other clubs would concur that epoxy plugging a -0 is a modifiction, where as tape and dog barf have no real structural integrity and serve solely to prevent the heat and flame of the -0 motors burn through from damaging the motor mount of the rocket.

I don't disagree that many RSO's would only allow a plugged Estes motor to be flown as Research. I'm asking what the risk is. I'm trying to get to the "why", I'm not asking to fly a plugged Estes motor. And the "why" has to be more than "it's a motor modification" - the why needs to be "because we are afraid of X happening". What is X?

 

[rharshberger]

I don't disagree that many RSO's would only allow a plugged Estes motor to be flown as Research. I'm asking what the risk is. I'm trying to get to the "why", I'm not asking to fly a plugged Estes motor. And the "why" has to be more than "it's a motor modification" - the why needs to be "because we are afraid of X happening". What is X?

One word...Liability

 

[shreadvector]

1) I agree that scraping out an ejection charge is forbidden, stipulated.

2) Thank you for identifying a specific risk for plugging the booster motors, good point.

3) The booster motors work by igniting the second stage only after the motor has burnt through almost all of its propellant. So, by the time the flame reaches the plugged end of the booster motor, the risk of a "huge overpressure" is almost nil because almost all of the propellant has been expended.

4) Stipulating that the huge overpressure is a real probability, how is that any different than the normal risk with Estes Black Powder motors? When black powder motors CATO, they tend to pop and it's over. If the motor cato's at the end of its burn, what's the risk?

5) "For a motor with an ejection charge, trying to seal it with tape or epoxy will created an exploding firework-type event." I don't think so - again, all of the motor propellant has already burned. I think plugging the forward ejection on a black powder motor would more likely blow the charge out of the aft nozzle of the motor, that's what it was designed to do in the first place. But we could always test this and find out.

Answers to 3, 4 and 5:

3) When the dome of burning gets near the thin web of remaining propellant, it ruptures the top of that thin web and the result is a HUGE amount of burning surface. With the top of the booster open (normal) that jet of hot flaming stuff goes up and out. See video from NARAM R&D report and the D12-0 in particular. AVI Gold Series motors had a smaller casing glued into the top of a booster motor. The smaller casing bit has the delay and ejection charge. When they reached the end of the propellant burn, they would often 'blow an aneurysm' (fail the casing out the side or just blow the delay/ejection module out the top) because of the huge overpressure. The nozzle is not sized to handle that much pressure (it ain't a B14 or F100 nozzle).

4) If the plugged booster blows at the end of burn, the rocket is moving fast and 'bad things' can happen to the rocket (parts blow off, like fins, rocket goes unstable, parts catch fire, dogs and cats sleeping together, etc.)

5) Ejection charge has a huge burning area and if it is contained it is like a skyrocket "report".

 

[lakeroadster]

I'm sorry, but that's the stock, knee-jerk reaction you would get from any manufacturer. Estes instructions also say that they don't recommend making any changes to the assembly process as it could result in an unsafe rocket.

So jmuck78 knows more than the motor manufacturer? Point being.. at some point somebody has to make the call. We are talking in circles... and it is getting us nowhere.

 

[jmuck78]

So jmuck78 knows more than the motor manufacturer? Point being.. at some point somebody has to make the call. We are talking in circles... and it is getting us nowhere.

Obviously not, and that's not what I said. I do, however, know a non-engineering response when I see one. Asking that question was the equivalent of asking Ford if you recommend driving their Mustang above the speed limit - you know the answer before you ask and you know which department is going to give you that answer.

 

[BABAR]

And the "why" has to be more than "it's a motor modification" - the why needs to be "because we are afraid of X happening". What is X?

Actually, the why DOESN’T Need to be “because we afraid of X Happening.”

Insurers will use any and every excuse to deny coverage. I strongly suspect that if something bad happens just the fact that you modified a motor may be sufficient for NAR insurers to deny a claim. Estes itself (themselves?) also will not back you up, that is clear.

Laws don’t need to be logical or rational.

But hey, I’m still back on “just eject the silly 18mm engine casings and be done with it!” Hopefully rstaff3 has his Clifton Tracking Station Hat of Death on so he doesn’t get hit by one.

But if all you are trying to do is keep a forward ejection charge of one motor from lighting off other motors from their FORWARD engine end, heck just stick a little wadding (might use the tissue type instead of barf, then you don’t need tape) in there and call it done. Plugging it seems like overkill. In fact, using wadding instead of a plug provides redundancy in case the ejection charge of the one engine you DIDN’T plug has a problem. The others will chip in and kick out your chute.

While you CAN “backlight” a 0 delay booster (forward burn through of one booster lighting the exposed forward end propellant of ANOTHER booster that didn’t light at launch) I am not sure you even CAN do the same thing with a standard NON booster engine like a C6-5. The clay cap should prevent it.

John, I think your staggered delays should also work. Not convinced it’s NEEDED, but should work.

I am curious though, would 3 Cs firing ejection simultaneously be that big a deal? Would it be that much more than a single D (or E or F)?

 

[rharshberger]

I am curious though, would 3 Cs firing ejection simultaneously be that big a deal? Would it be that much more than a single D (or E or F)?

Check out post #8 in the first thread below, yes two charges going off at the same time can damage a LPR tube, the first three pictures show the damage. The upper and lower of the three were after the booster was cut open to get access to the central mmt, the center photo is from the rear looking up the central motor tube prior to cutting apart. This may or may not happen when several charges go off, the second thread has a post by Carl McLawhorn (Semroc) that Estes 18mm motors contain .5grams of BP for the ejection charge, so 1.5 grams going off simultaneously COULD do pretty serious damage to LPR tubes, a lightly fit nose cone that is easily pushed out could probably keep the tube from being damaged, but that much heat might also cook the tube pretty quickly.

https://www.rocketryforum.com/threads/sa-3-goa-semi-scale-2-stage-with-cluster-booster.70827/

https://www.rocketryforum.com/threads/how-much-bp-in-estes-ejection-charges.449/

 

[burkefj]

Actually can you fly a bp research motor? I thought not, so a modified bp motor would not be considered a certified motor and would not be allowed at a research launch right?

 

[rharshberger]

Actually can you fly a bp research motor? I thought not, so a modified bp motor would not be considered a certified motor and would not be allowed at a research launch right?

I believe you are correct Frank, BP is one of the propellants not allowed for TRA Research motors. In that case they wouldn't be legal to fly except at a personal launch.