Plugged motors creation

[Bryshereyy]

I was browsing old archived websites the other day. One of them spoke of plugging booster motors with epoxy. It actually made sense to me. But what does the hive mind say? Would it be safe?

 

[heada]

You're going to get lots of opinions on if it is valid or if it is a modification and thus turns it into an EX motor. In the end, talk to the RSO of your local club because their opinion is the one that really matters most.

My opinion is worth exactly as much as you paid for it....and that is taking a reload and removing the ejection charge and filling the well with grease is no different that adding epoxy to the end of a BP motor, both should be allowed and fine.

 

[Rocketjunkie]

Black powder motors. Carefully dig out the clay and ejection charge. Leave the delay charge in place. Cover with glue and let dry. Safe to fly, yes. Certified, no. Composite motors, remove ejection cap and remove the BP. Then replace the cap. Manufacturers allow this and it is certified. Reloads, assemble normally. Just don't put the BP in the ejection well. Also certified.

 

[lakeroadster]

I was browsing old archived websites the other day. One of them spoke of plugging booster motors with epoxy. It actually made sense to me. But what does the hive mind say? Would it be safe?

NAR Model Rocket Safety Code
2. 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.

Is it allowed at a NAR event?
It is deemed tampering with the motor, unless there is a written procedure, provided by the manufacturer, for whatever you are doing to the motor. If there is no written procedure by the manufacturer, it is not allowed, per NAR.

Is it safe?
That's why the NAR specifies that there must be a written procedure by the manufacturer.

Hive mind
When it comes to "modifying a motor".... plugging a booster motor with epoxy is about as safe as it gets, in my personal opinion. But just be aware an RSO may have an issue with it, and if there is an accident, your NAR insurance may not cover that accident.

 

[Back_at_it]

Lake makes a good point on the NAR rules and the RSO. If it were me, I would not fly them at a club event. That is a pretty good way to get yourself in hot water. What you do with your friends in a random field somewhere is up to you and your "adult" judgment.

Is it safe to plug one. I don't know...... But I can say that I have plugged motors tons of times and flown them in the various gravity recovery rockets with only one issue. I had one motor that I think had a "hot" ejection charge and it blew the insides out the rear of the motor. I mean it totally gutted the motor case from the ejection reward. All others worked as expected.

When I was doing it I didn't have Epoxy money so I simply used whatever glue I had laying around. Elmers Glue-All, Titebond, etc. They all worked fine. Just don't glob it in there all at once and expect it to dry. Two or three small layers were enough to prevent the ejection from blowing through the top.

Again. Not telling you to do this. You need to use your own judgement.

 

[lakeroadster]

Lake makes a good point on the NAR rules and the RSO. If it were me, I would not fly them at a club event. That is a pretty good way to get yourself in hot water. What you do with your friends in a random field somewhere is up to you and your "adult" judgment.

Is it safe to plug one. I don't know...... But I can say that I have plugged motors tons of times and flown them in the various gravity recovery rockets with only one issue. I had one motor that I think had a "hot" ejection charge and it blew the insides out the rear of the motor. I mean it totally gutted the motor case from the ejection reward. All others worked as expected.

When I was doing it I didn't have Epoxy money so I simply used whatever glue I had laying around. Elmers Glue-All, Titebond, etc. They all worked fine. Just don't glob it in there all at once and expect it to dry. Two or three small layers were enough to prevent the ejection from blowing through the top.

Again. Not telling you to do this. You need to use your own judgement.

If you're going to plug it, epoxy is probably the best. It cures from the heat it generates, so a thick plug will cure properly in the manufacturers specified timeframe.​

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I mixed up some partially thinned down Gorilla Wood Glue to paper the fins on my P-40 project. After I was done papering the fins there was about 3/8" thick of the mixture in the bottom of the yogurt cup I put it in. 1 weeks later I popped that glue out of the yogurt cup, it still wasn't dry. A week later, it was dry. That's 2 weeks later... in a climate with humidity levels that are typically in the teens or single digits.​

 

[BABAR]

Black powder motors. Carefully dig out the clay and ejection charge. Leave the delay charge in place. Cover with glue and let dry. Safe to fly, yes. Certified, no. Composite motors, remove ejection cap and remove the BP. Then replace the cap. Manufacturers allow this and it is certified. Reloads, assemble normally. Just don't put the BP in the ejection well. Also certified.

I’ll let my ignorance show.

Assuming at least 1/4” of internal body tube Above the clay, why take the clay and the ejection charge out? Why not just fill that 1/4 inch with epoxy?.

Would the ejection charge left inside be powerful enough to either blow off your epoxy or blow up the cardboard thick walled motor casing?

Would it make that much difference with the ejection charge firing and venting out the nozzle? you would have a millisecond of thrust at the end of your boost but given you’re not using that thrust anyway.

also, having been too timid to “fold, spindle, or mutilate” rocket motors, while i have seen pictures of cut motors and of course the diagrams and actual models, when working from on end of an intact motor is it that easy to separate the ejection charge from the delay charge?

my solution to the “plugged motor” issue has usually been somewhat outside the box, don’t plug the motor, plug the tube, lightly friction fit the motor just enough (may not need any tape at all) that it doesn’t fall out on the way to the pad, and eject the motor. Before the naysayers jump in, motor eject is NOT violation of the safety code although it IS prohibited in COMPETITION. i DO recommend avoiding it when fire risk is high, as the falling casing it a bit hot and at least theoretically could cause a fire. You also need to follow field rules (some fields prohibit the practice, examples are sod farms where the casing may damage mowers or other equipment.). It is good practice to TRY to retrieve ejected casings, although given they are small and biodegradable I am not certain that if you happen to miss one it is of any great consequence.

another option is to eject them with streamers. Go up a tube size (for 18 mm go from BT-20 to 50). Tape a FLAMEPROOF crepe paper streamer (they are all SUPPOSED to be flameproof, I’d get a lighter and make sure) anyhoo, tape the streamer to the side of the motor. fold a length in 1/2, then 1/2 again, then 1/2 again until you get a length that will wrap around the motor at least once and not more than twice, and that will fit snugly enough that it won’t fall out on the walk to the pad but loose enough that it can eject. You will need to play with different lengths until you get it right. The streamer allows the motor a bit more time to cool on descent, more importantly it makes it easy to find and retrieve.

Do NOT roll the motor in the crepe paper, the drag forces of a falling casing are not favorable to UNROLLING a rolled up streamer, whereas they are VERY favorable to UNFOLDING a streamer.

do NOT try a parachute. I know this because I stupidly built a long gap stage with a chute in a pod that was to deploy at separation to ease the long gap booster to the ground (the challenge with non-electronic long gap staging is NOT getting the sustainer to light, that’s easy. The problem is keeping the booster from retiurning ballistic and core sampling.) The quick thinking readers will likely immediately recognize that deploying a parachute while the rocket is at or near maxV (At least maxV for the whole stack) does NOT end well. One of 8 shroud lines remained intact. Streamers, especially folded ones, are much more forgiving.

As has previously been said, consulting the planned RSO, field rules, and the landowner BEFORE bringing The rocket (or building it at all, if you are limited to a single field) is always a good plan.

 

[lakeroadster]

do NOT try a parachute. I know this because I stupidly built a long gap stage with a chute in a pod that was to deploy at separation to ease the long gap booster to the ground (the challenge with non-electronic long gap staging is NOT getting the sustainer to light, that’s easy. The problem is keeping the booster from returning ballistic and core sampling.) The quick-thinking readers will likely immediately recognize that deploying a parachute while the rocket is at or near maxV (At least maxV for the whole stack) does NOT end well. One of 8 shroud lines remained intact. Streamers, especially folded ones, are much more forgiving.

I wonder how over the top shroud lines would fair for the parachute? You turned me on to those, turnabout is fair play.

​

 

[BABAR]

I wonder how over the top shroud lines would fair for the parachute? You turned me on to those, turnabout is fair play.

View attachment 538855 View attachment 538856​

Kind of like the F4, if you put enough motor in it a brick will fly.

you can certainly make a chute and shroud lines that will handle the force, but the force is going to be extreme and has to be transmitted “somewhere”. while I am sure it isn’t EXACTLY instantaneous, the velocity change (literally “the shock”j from current rate to minV of chute drag effect I theeeeeenk is a lot more rapid with a chute than a streamer. Possibilities are shredded chute, broken shroud lines, broken shock cord, broken shock cord attachment, or complete success. A long and strong elastic shock cord segment might also work. My personal feeling since at my prior field ejecting motor cases by themselves wasn’t an issue at all, that a streamer on a motor casing up to say a D is plenty, a chute is overkill in my opinion.

the equivalent is a very early or very late chute deployment, something that doesn’t usually occur with electronics but unfortunately does happen more Frequently than we would prefer with BP motors, most commonly due to poor motor choices (delay OR impulse), but also weathercocking and motor to motor variation (recent two stage rocket @BEC launched with a 4 second delay sustainer motor, I think his altimeter document it was a fraction of a second. Fortunately recovered fine.). So I will stand my ground on this one, chutes are not a good choice of recovery for ejected motors.

 

[BABAR]

Back to plugged motors, suspect even the nicest RSO (is that an oxymoron) may disallow modified plugged motors because if fecal turbine interaction does occur, even if for completely unrelated reasons, it may invalidate NAR insurance coverage because it violates the letter of the safety code. Same thing for personal launches.

 

[vcp]

On 18mm booster motors, I've 3D printed little 'top hat' shaped plugs that friction-fit into the motors. Besides the friction-fit, the thin brim of the hat is clamped between the motor and the motor mount. A small tab hanging 'below' the hat allows fingertip removal. Since it's readily removable, I can't imagine it would be considered a modification.

 

[teepot]

Ok. I have a question. I built a 2 stage rocket. The sustainer has two pods. I was/am going to put a parachute in one of the pods. And a equal weight of paper or something in the other pod. My concern is will the heat of the sustainer motor fry the parachute? The chute will have a Nomex blanket around it. I figured if I use a long shock cord the two stages will be far enough apart not to fry the chute. I am also wondering that if the sustainer motor only burns for a second that the chute would survive given that the shock cord is long enough. The booster is to heavy for tumble recovery. Comments are welcome.

 

[lakeroadster]

Ok. I have a question. I built a 2 stage rocket. The sustainer has two pods. I was/am going to put a parachute in one of the pods. And a equal weight of paper or something in the other pod. My concern is will the heat of the sustainer motor fry the parachute? The chute will have a Nomex blanket around it. I figured if I use a long shock cord the two stages will be far enough apart not to fry the chute. I am also wondering that if the sustainer motor only burns for a second that the chute would survive given that the shock cord is long enough. The booster is to heavy for tumble recovery. Comments are welcome.

So the sustainer carries the parachute for the booster, and at stage separation the booster pulls out the parachute? Interesting design.

Everything is at close proximity so the chute may get burnt. Have you considered a flameproof streamer, instead of a parachute?

Maybe posting a photo would better help us to visualize the design.

 

[FredA]

So the sustainer carries the parachute for the booster, and at stage separation the booster pulls out the parachute? Interesting design.

The pair are moving pretty fast at separation - this is asking for a zipper and/or stripped chute.

 

[lakeroadster]

The pair are moving pretty fast at separation - this is asking for a zipper and/or stripped chute.

Perhaps. Over top shroud lines could be used and a zipper assumes an internal chord mount. In this case the Kevlar could be glued into the fin fillet.

Without seeing the design... we're just spit balling.

 

[BABAR]

Ok. I have a question. I built a 2 stage rocket. The sustainer has two pods. I was/am going to put a parachute in one of the pods. And a equal weight of paper or something in the other pod. My concern is will the heat of the sustainer motor fry the parachute? The chute will have a Nomex blanket around it. I figured if I use a long shock cord the two stages will be far enough apart not to fry the chute. I am also wondering that if the sustainer motor only burns for a second that the chute would survive given that the shock cord is long enough. The booster is to heavy for tumble recovery. Comments are welcome.

This has been my go-to solution for long gap stage rockets, where the booster is long (and somewhat Heavy due to length), main problem is the booster for long gap is STILL stable post staging and will recover ballistically, resulting in a core sample for soft earth or a crumpled tube for a hard impact. So I needed a non-electronic means of deploying a chute or streamer AFTER sustainer separation.

in any case, my solution requires a CLUSTER on the booster (not sure if you were planning that anyway.). The MAIN motor is a zero delay heavy lifter (d12-0 is typical, can go with c11-0, only if very light TOTAL stack do I go with C6-0. Man on man do we need Estes to release a C5-0!). The booster has attached a pod, basically another body tube and motor mount AND nose cone AND parachute. For safety reasons I recommend using a very low impulse motor, like a 1/4A3-3T or 1/2A3-2T or A10-3T if you want to go BT-5 (you need a longer tube if you go this route), or if you need more space go BT-20 with a A8-3 or 1/2 A6-2. Actually if you don’t go minimum diameter you can use a larger body tube and an appropriate motor mount (say BT-20 for a 13 mm motor or BT-50 for 18 mm motor) to get more space. In any case keep delay under 3 seconds.

WHY the low impulse? Clusters are a bit complicated. With a two motor cluster on the booster you have three possibilities.

1. Optimal. Both main (0-delay) and pod (short delay) light. The design needs to be such that the main alone can safely lift the stack to the end of the rod/rail and get enough velocity that stack is stable. The main burns out and the burn through (0-delay motors have no ejection charge or clay cap, the hot gases light the sustainer) ignites the sustainer. Even AFTER separation, inertia will keep the BOOSTER going UP for a short time. Meanwhile, the BOOSTER POD propellant has probably already burn out, but the delay SHOULD easily allow the sections to separate PLUS 1-2 seconds before the pod ejection charge fires and deploys the chute. The delay time serves two purposes, first it allows the sustainer to separate so the sustainer motor doesn’t fry your booster chute. Second and nearly as important, at the moment of separation BOTH booster and sustainer are hauling butt fast, probably at Vmax for booster. This is a really BAD time to deploy a chute, and there is a good chance of shredded chute, broken shroud lines, and/or broken shock cord, zipper, or detachment of shock cord. The delay allows the booster to slow down a bit before chute deployment.

2. Suboptimal A. Booster MAIN doesn’t light, Booster POD does. Because you have a very low impulse motor in the pod, with a full stack, the stack may not clear the rod (0 risk of fire or injury or damage) and if it does, it will not go far, particularly if you go with 1/4A3-3T. Even if it drops to the ground, it won‘t landshark very far. As titled, suboptimal but pretty safe. Assuming a safe field with the area around the pad clear of fire hazards, it’s pretty low risk with a LOW IMPULSE POD MOTOR, compared to say having a D motor in the pod (really bad idea) that may get the rocket off the pad and either landshark a good distance or head upward but NOT IGNITE the sustainer. This is bad.

3. Suboptimal B. Booster MAIN lights, Booster POD does NOT. assuming design is such that the booster MAIN alone gets stack safely up and off the rod, staging occurs and the booster tumbles down or core samples if it is long and goes ballistic. The leading edge is the open tube, NOT the mose cone of the pod, so even if it comes in ballistic the tube is going to crumple and is unlikely to hurt anyone or anything it hits.

DESIGN for SUCCESS, but be prepared to FAILURE.

Remember, the purpose of the pod motors(s) is solely to provide a means of deploying your chute or streamer at the optimal time. While the pod motor(s) may assist in and augment the boost capability, the MAIN motor should be of sufficient impulse to do the job ALONE.

my experience has been about 95% success, I have had at least 1, maybe 2 type A and B failures without significant problems and with no safety issues. Type A Failures are usually a bit embarrassing bit amusing, I had one that just cleared the rod, then dropped to the ground leaning upright against the sawhorse of the launch rod at a club launch. The type B failure had no damage to the booster. It did weathercock severely and I never found the sustainer.

hope you get three straight vertical trails!

oh yeah, if you have TWO pods on your booster, put chutes and cones and 1/4A3-3T motors in both, you have now reduced Probability of a type B failure by 50%. Just make sure you have 12 volt ignition source. Also, with such small motors, even a type A failure likely will not even clear the rod, so hazard (fire risk and personal or property injury) is close to zero.

so if you go this route, wish you FOUR straight vertical trails.

 

[teepot]

This has been my go-to solution for long gap stage rockets, where the booster is long (and somewhat Heavy due to length), main problem is the booster for long gap is STILL stable post staging and will recover ballistically, resulting in a core sample for soft earth or a crumpled tube for a hard impact. So I needed a non-electronic means of deploying a chute or streamer AFTER sustainer separation.

in any case, my solution requires a CLUSTER on the booster (not sure if you were planning that anyway.). The MAIN motor is a zero delay heavy lifter (d12-0 is typical, can go with c11-0, only if very light TOTAL stack do I go with C6-0. Man on man do we need Estes to release a C5-0!). The booster has attached a pod, basically another body tube and motor mount AND nose cone AND parachute. For safety reasons I recommend using a very low impulse motor, like a 1/4A3-3T or 1/2A3-2T or A10-3T if you want to go BT-5 (you need a longer tube if you go this route), or if you need more space go BT-20 with a A8-3 or 1/2 A6-2. Actually if you don’t go minimum diameter you can use a larger body tube and an appropriate motor mount (say BT-20 for a 13 mm motor or BT-50 for 18 mm motor) to get more space. In any case keep delay under 3 seconds.

WHY the low impulse? Clusters are a bit complicated. With a two motor cluster on the booster you have three possibilities.

1. Optimal. Both main (0-delay) and pod (short delay) light. The design needs to be such that the main alone can safely lift the stack to the end of the rod/rail and get enough velocity that stack is stable. The main burns out and the burn through (0-delay motors have no ejection charge or clay cap, the hot gases light the sustainer) ignites the sustainer. Even AFTER separation, inertia will keep the BOOSTER going UP for a short time. Meanwhile, the BOOSTER POD propellant has probably already burn out, but the delay SHOULD easily allow the sections to separate PLUS 1-2 seconds before the pod ejection charge fires and deploys the chute. The delay time serves two purposes, first it allows the sustainer to separate so the sustainer motor doesn’t fry your booster chute. Second and nearly as important, at the moment of separation BOTH booster and sustainer are hauling butt fast, probably at Vmax for booster. This is a really BAD time to deploy a chute, and there is a good chance of shredded chute, broken shroud lines, and/or broken shock cord, zipper, or detachment of shock cord. The delay allows the booster to slow down a bit before chute deployment.

2. Suboptimal A. Booster MAIN doesn’t light, Booster POD does. Because you have a very low impulse motor in the pod, with a full stack, the stack may not clear the rod (0 risk of fire or injury or damage) and if it does, it will not go far, particularly if you go with 1/4A3-3T. Even if it drops to the ground, it won‘t landshark very far. As titled, suboptimal but pretty safe. Assuming a safe field with the area around the pad clear of fire hazards, it’s pretty low risk with a LOW IMPULSE POD MOTOR, compared to say having a D motor in the pod (really bad idea) that may get the rocket off the pad and either landshark a good distance or head upward but NOT IGNITE the sustainer. This is bad.

3. Suboptimal B. Booster MAIN lights, Booster POD does NOT. assuming design is such that the booster MAIN alone gets stack safely up and off the rod, staging occurs and the booster tumbles down or core samples if it is long and goes ballistic. The leading edge is the open tube, NOT the mose cone of the pod, so even if it comes in ballistic the tube is going to crumple and is unlikely to hurt anyone or anything it hits.

DESIGN for SUCCESS, but be prepared to FAILURE.

Remember, the purpose of the pod motors(s) is solely to provide a means of deploying your chute or streamer at the optimal time. While the pod motor(s) may assist in and augment the boost capability, the MAIN motor should be of sufficient impulse to do the job ALONE.

my experience has been about 95% success, I have had at least 1, maybe 2 type A and B failures without significant problems and with no safety issues. Type A Failures are usually a bit embarrassing bit amusing, I had one that just cleared the rod, then dropped to the ground leaning upright against the sawhorse of the launch rod at a club launch. The type B failure had no damage to the booster. It did weathercock severely and I never found the sustainer.

hope you get three straight vertical trails!

oh yeah, if you have TWO pods on your booster, put chutes and cones and 1/4A3-3T motors in both, you have now reduced Probability of a type B failure by 50%. Just make sure you have 12 volt ignition source. Also, with such small motors, even a type A failure likely will not even clear the rod, so hazard (fire risk and personal or property injury) is close to zero.

so if you go this route, wish you FOUR straight vertical trails.

I have several rockets that have strap on boosters. The outboards won't move the rocket. The core has to light for them to fly. The last time I tried to fly a small one only the outboards lite. It was very funny. The C6-0's burned and the rocket never moved. The nose cones popped, the chutes came out and the boosters fell to the ground. Everyone had a good laugh. We fly off a dry lake bed so no danger of fire. Thank you for the advice. I hadn't thought of that method. I'll have to come up with a design that will us it.

 

[BABAR]

I have several rockets that have strap on boosters. The outboards won't move the rocket. The core has to light for them to fly. The last time I tried to fly a small one only the outboards lite. It was very funny. The C6-0's burned and the rocket never moved. The nose cones popped, the chutes came out and the boosters fell to the ground. Everyone had a good laugh. We fly off a dry lake bed so no danger of fire. Thank you for the advice. I hadn't thought of that method. I'll have to come up with a design that will us it.

When there is no fire risk and a dry lake bed where a misplaced casing is not a major faux pas, motor eject is also an option, although it won’t release a chute

 

[dwinings]

Black powder motors. Carefully dig out the clay and ejection charge. Leave the delay charge in place. Cover with glue and let dry. Safe to fly, yes. Certified, no. Composite motors, remove ejection cap and remove the BP. Then replace the cap. Manufacturers allow this and it is certified. Reloads, assemble normally. Just don't put the BP in the ejection well. Also certified.

When I was practicing for S8 RC rocketglider for the World Championships I would take D12-3 and scrap out the ejection charge, no glue needed. They worked fine that way, but yes it is not recommended by the manufacturer so not certified.

 

[Rocketjunkie]

I cover the exposed delay material with something, even if just masking tape. I had a cluster, 1 motor didn't light. When the dual deploy apogee charge fired, it back lit the unfired motor. The rocket recovered OK but the booster was in flames. (Viper 4, usually flown with motor ejection if flying on 24 mm BP motors.) This flight was 2 CTI F30 and 2 Estes E9.

 

[green dragon]

I've always just plugged top with epoxy.
Leave ejection charge in - at ejection it usually blows the nozzle out ( same clay as the forward cap )

 

[rcktnut]

I don't understand this thread. OP was asking about plugging BOOSTER motors, Last I heard BOOSTER motors don't have an ejection charge. What's with digging out the clay cap and ejection charge? I don't think that is a BOOSTER motor.

On 18mm booster motors, I've 3D printed little 'top hat' shaped plugs that friction-fit into the motors. Besides the friction-fit, the thin brim of the hat is clamped between the motor and the motor mount. A small tab hanging 'below' the hat allows fingertip removal. Since it's readily removable, I can't imagine it would be considered a modification.

That or "flame" proof the MMT with a bulkhead inside the MMT that the top of the booster motor rests up tight against.

 

[Daddyisabar]

I have seen a guy fly a three motor cluster Estes SR 71 where he tried to epoxy plug two C6 5 motors in the pods without removing anything. Result - two blown up pods. If you get a shotgun ejection motor yer in big trouble. A wimpy one might work. Roll the dice baby! Daddy needs new shoes!

To insure a good bond I will score (cross check) the paper just above the propellant, you can rough up the rest with course grit sand paper. A good dolop of 5 min epoxy. Give it a twirl to completely coat the inside of the casing, let the rest settle and cure.

I never dig out or attempt to plug a green or purple label motor, way too Dark Side and would severely chastise my inner RSO.

Plugged booster red lables will give a bit more thrust from a complete, fully incased burn
Love it when you can see the sun light shining through the nozzle when looking through the epoxy plug after use. I show them to the Top Men saying "Sure burned all that propellant, maximum efficiency, nice and clean!"

 

[shockie]

Black powder motors. Carefully dig out the clay and ejection charge. Leave the delay charge in place. Cover with glue and let dry. Safe to fly, yes. Certified, no. Composite motors, remove ejection cap and remove the BP. Then replace the cap. Manufacturers allow this and it is certified. Reloads, assemble normally. Just don't put the BP in the ejection well. Also certified.

A few drops of plain old water will essentially dissolve the clay cap that has some embedded black power grains from the ejection charge

The remainder can be scraped out using a piece of shape plastic.

Never use anything metal.

And you don't really need an epoxy cap.

Stuffing it full of wadding and taping the end will usually suffice. Or a simple wood plug taped in.

I've done the above on 1/4A to C size BP motors.

 

[Daddyisabar]

A few drops of plain old water will essentially dissolve the clay cap that has some embedded black power grains from the ejection charge

The remainder can be scraped out using a piece of shape plastic.

Never use anything metal.

And you don't really need an epoxy cap.

Stuffing it full of wadding and taping the end will usually suffice. Or a simple wood plug taped in.

I've done the above on 1/4A to C size BP motors.

"Usually" was a word you Never used with our club's old RSO!