Plugged motors question.

ThirstyBarbarian said:

Does a D12-0 really have any "bang powder?" Is there an ejection charge at all? My understanding has always been that it does not have an exploding ejection charge --- all it has is a single fuel grain that provides thrust and burns through at the end, allowing hot gas forward. There isn't an explosion, just a burn through with a couple seconds of flame and pressure. It seems like if the zero motor were secured tightly against a flame-proof bulkhead, that would be adequate. You would NOT want to retain a motor with an ejection charge against a bulkhead! BAM! But I think a zero-delay booster motor would work fine that way. The pressure would continue to vent out the nozzle, just like before the burn through.

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As an end burning black powder motor with an ejection charge burns and provides thrust for propulsion, the grain diminishes in length until it comes in contact with and ignites a different composition known as a delay grain and the burn then continues at a much slower rate until it ultimately contacts the ejection charge which then deflagrates and bursts the clay cap, pressurizing the airframe and ejecting the recovery system.

As an end burning black powder booster motor burns and provides thrust for propulsion, the grain diminishes in length until it becomes so thin that a structural failure of the propellant grain occurs, breaking the remaining grain into smaller burning particles which take the path of least resistance which is suddenly forward and away from the nozzle. If this forward pressure is sufficiently retained, the booster casing will most likely be ejected backward out of the motor mount. However, if the cavity at the head of a booster motor is plugged with any material that is both non-flammable and sufficiently bonded to both the casing and the propellant grain, the motor will simply burn out and all of the propellant's gaseous products will be forced out of the nozzle.

My videos at post number 5 clearly illustrate what I just described.

tmacklin said:

As an end burning black powder motor with an ejection charge burns and provides thrust for propulsion, the grain diminishes in length until it comes in contact with and ignites a different composition known as a delay grain and the burn then continues at a much slower rate until it ultimately contacts the ejection charge which then deflagrates and bursts the clay cap, pressurizing the airframe and ejecting the recovery system.

As an end burning black powder booster motor burns and provides thrust for propulsion, the grain diminishes in length until it becomes so thin that a structural failure of the propellant grain occurs, breaking the remaining grain into smaller burning particles which take the path of least resistance which is suddenly forward and away from the nozzle. If this forward pressure is sufficiently retained, the booster casing will most likely be ejected backward out of the motor mount. However, if the cavity at the head of a booster motor is plugged with any material that is both non-flammable and sufficiently bonded to both the casing and the propellant grain, the motor will simply burn out and all of the propellant's gaseous products will be forced out of the nozzle.

My videos at post number 5 clearly illustrate what I just described.

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I watched the videos, and it seems like if the zero delay booster motor is "plugged," then the motor just seems to burn out at the end. I ma guessing that how this actually plays out would have a lot to do with how it is plugged. For example, if it is plugged with clay that forms a cap directly agains the propellant grain, like it would be in a -P motor (or if epoxy were applied to the forward end of the grain after market in accordance with all relevant laws, codes, regulations, rules, customs, conventions, treaties, etc.), then the plug would support the grain, and it would not necessarily have the structural failure you described, and would simply burn out.

But if the plug were not sealed to the grain, like maybe a solid bulkhead that acted as the motor block and had an air gap between the end of the grain and the bulkhead, then the unreinforced end of the grain would have the structural failure you described, and you would have all of the smaller burning particles, and that would mean a pressure spike as the remaining particles burned more quickly than an intact grain.

Do you have any idea of how much pressure that burn through would cause, and could it damage a motor tube with a sealed bulkhead? I'm thinking in terms of a motor tube with a wooden bulkhead or plug in place of a motor block, and the forward end of the motor would be held tight right against the bulkhead/plug with a screw-type retainer like the Estes screw-on retainer. The only expansion space would be inside the casing between the end of the grain and the bulkhead. The brunt of the burn through would be against the bulkhead and some small amount of pressure might leak between that bulkhead and the end of the casing to contact the motor tube.

Thirsty;

In the first video I friction fitted a 1/2 inch diameter hardwood dowel into the booster cavity without any adhesive of any kind. It may or may not have fully contacted the propellant grain and there obviously was a pressure spike near the end of the burn as evidenced by the flash, sound and reverse ejection of the spent casing.

In the second video, the dowel was adhered to the casing with epoxy and the motor burn simply ended when the fuel was expended. I believe that a booster motor that is plugged with a solid fill of liquid epoxy and allowed adequate time to fully cure would be every bit as safe as a factory plugged motor using a compressed clay-grog mixture. Care must be taken to insure that the epoxy does not cure too rapidly, overheat and crack, and the inside of the casing should be cleaned and roughened to increase the bond.

Is this a "motor modification" as defined by NAR rules? Ask the rulers.

ThirstyBarbarian said:

But if the plug were not sealed to the grain, like maybe a solid bulkhead that acted as the motor block and had an air gap between the end of the grain and the bulkhead, then the unreinforced end of the grain would have the structural failure you described, and you would have all of the smaller burning particles, and that would mean a pressure spike as the remaining particles burned more quickly than an intact grain.

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I believes this illustrates that:



The removable plug was at the end of the casing and the epoxy plug was on the propellant.

billspad said:

I believes this illustrates that:

View attachment 258465

The removable plug was at the end of the casing and the epoxy plug was on the propellant.

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That's an interesting graph! It shows pretty much what I was visualizing, including the spike with the removable plug. One interesting thing is it looks like you get close to another .2 seconds of thrust by plugging the motor. Thanks for posting this.

ThirstyBarbarian said:

That's an interesting graph! It shows pretty much what I was visualizing, including the spike with the removable plug. One interesting thing is it looks like you get close to another .2 seconds of thrust by plugging the motor. Thanks for posting this.

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Dittos! I think that the extra two-tenths of thrust results from the "capture" of the gaseous portion of the burn which is normally directed forward to the staging process when the grain fails at the end of the burn. I'm certain that the propellant grain of a given booster is longer than the propellant portion of the grain in a regular motor.

tmacklin said:

I'm certain that the propellant grain of a given booster is longer than the propellant portion of the grain in a regular motor.

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Strangely enough, not always. There are variations in black powder and the amount of propellant is adjusted to give the advertised total impulse.

tmacklin said:

Thirsty;

In the first video I friction fitted a 1/2 inch diameter hardwood dowel into the booster cavity without any adhesive of any kind. It may or may not have fully contacted the propellant grain and there obviously was a pressure spike near the end of the burn as evidenced by the flash, sound and reverse ejection of the spent casing.

In the second video, the dowel was adhered to the casing with epoxy and the motor burn simply ended when the fuel was expended. I believe that a booster motor that is plugged with a solid fill of liquid epoxy and allowed adequate time to fully cure would be every bit as safe as a factory plugged motor using a compressed clay-grog mixture. Care must be taken to insure that the epoxy does not cure too rapidly, overheat and crack, and the inside of the casing should be cleaned and roughened to increase the bond.

Is this a "motor modification" as defined by NAR rules? Ask the rulers.

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You got your 2 types of rockets, those you fly with the group and those you don't. I will continue to epoxy plug motors for my private launches. My clusters are for my enjoyment I guess. That and the crowd of kids it always draws

billspad said:

Strangely enough, not always. There are variations in black powder and the amount of propellant is adjusted to give the advertised total impulse.

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I'm sure you are correct that there are variations in black powder, both in formulation and unintentional manufacturing varience. However, the only way to know for sure would be to either pose such questions to Estes or Quest, or dissect a standard motor such as a C6-5, measure the propellant portion of the grain and compare the numbers with the propellant portion of a booster motor.

Out of curiosity, I just measured the length from the top edges of spent Estes C6-0 and C6-7 motors down inside to the clay nozzle and the length was essentially identical at 2.375 inches. An unfired C6-0 has a cavity at its top of 0.6875 inches whereas a C6-7 has a recess to the clay cap of 0.0625 inches, a difference of 0.625 inches and the booster has a total grain length of 2.375 - 0.6875 = 1.6875 inches. The actual lengths/thicknesses of the various compositions within the C6-7 motor are hidden and thus unknown.

I also learned a long time ago that it's never a good idea to bet against a plumber. :wink:

tmacklin said:

I'm sure you are correct that there are variations in black powder, both in formulation and unintentional manufacturing varience. However, the only way to know for sure would be to either pose such questions to Estes or Quest, or dissect a standard motor such as a C6-5, measure the propellant portion of the grain and compare the numbers with the propellant portion of a booster motor.

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Done both. That's how I know.

I also learned a long time ago that it's never a good idea to bet against a plumber. :wink:

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A lesson you should never forget.

billspad said:

Done both. That's how I know.


A lesson you should never forget.

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LOL!

Okay, Bill, but tell me this. The Estes model rocket engine chart in my paper edition of their 2012 catalog shows the propellant weights for the B6-0, B6-4 and B6-6 engines and the propellant weights for the C6-0, C6-3, C6-5 and C6-7 engines (AKA motors) and their thrust characteristics to be identical, respectively. The casings are also identical as are the nozzle throat diameters for each engine thrust level. Since the nozzles, propellant weights and maximum thrusts are identical, where is the variable possibly taking place other than in the additional mass of the delay grain, ejection charge and clay cap of the standard motors as compared with the booster versions?

While I've never had the privilege of visiting with the manufacturers, I have disassembled a motor just to see for my self what lurks inside. The very dark gray black powder propellant transitions to a very light gray delay composition, which indicates to me that it is of a much different composition and burns at a much slower rate. This is followed by a very small amount of granular black powder which is topped off with the famous clay plug which retains the granular powder and provides the compression necessary for an "ejection event".

If I have misstated anything here, please enlighten me.

tmacklin said:

LOL!

Okay, Bill, but tell me this. The Estes model rocket engine chart in my paper edition of their 2012 catalog shows the propellant weights for the B6-0, B6-4 and B6-6 engines and the propellant weights for the C6-0, C6-3, C6-5 and C6-7 engines (AKA motors) and their thrust characteristics to be identical, respectively. The casings are also identical as are the nozzle throat diameters for each engine thrust level. Since the nozzles, propellant weights and maximum thrusts are identical, where is the variable possibly taking place other than in the additional mass of the delay grain, ejection charge and clay cap of the standard motors as compared with the booster versions?

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The propellant weights listed there and on the NAR S & T engine data sheets are the numbers from when the motors were certified. Again, each lot of propellant is different and the manufacturers (or at least one that I know of) adjusts the amount of propellant to maintain the advertised total impulse. If you can get the search engine on this forum to work better than I can, go back to 2008 and look for a discussion about this very same topic.

While I've never had the privilege of visiting with the manufacturers, I have disassembled a motor just to see for my self what lurks inside. The very dark gray black powder propellant transitions to a very light gray delay composition, which indicates to me that it is of a much different composition and burns at a much slower rate. This is followed by a very small amount of granular black powder which is topped off with the famous clay plug which retains the granular powder and provides the compression necessary for an "ejection event".

If I have misstated anything here, please enlighten me.

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That is correct but doesn't have a lot to do with what we're talking about. Two engines from the same batch of bp will have the same amount of propellant. Two made months or years apart most likely will not. I don't know how I can make it any clearer.

billspad said:

The propellant weights listed there and on the NAR S & T engine data sheets are the numbers from when the motors were certified. Again, each lot of propellant is different and the manufacturers (or at least one that I know of) adjusts the amount of propellant to maintain the advertised total impulse. If you can get the search engine on this forum to work better than I can, go back to 2008 and look for a discussion about this very same topic.

Thanks for the information Bill, but I'll pass on searching the bowels this forum. I have no doubt that you have given me the "straight poop".




That is correct but doesn't have a lot to do with what we're talking about. Two engines from the same batch of bp will have the same amount of propellant. Two made months or years apart most likely will not. I don't know how I can make it any clearer.

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I once weighed three E9-6 motors all from the same new package in order to determine their total mass in a triple 24mm cluster rocket I was building. As I recall these motors varied in weight from 66 to 70 grams. These motors were made at the same time and were from the same batch. Whether this observation is of any value I know not. Thanks again Bill, you have made yourself perfectly clear.

tmacklin said:

I once weighed three E9-6 motors all from the same new package in order to determine their total mass in a triple 24mm cluster rocket I was building. As I recall these motors varied in weight from 66 to 70 grams. These motors were made at the same time and were from the same batch. Whether this observation is of any value I know not. Thanks again Bill, you have made yourself perfectly clear.

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Same date code doesn't mean same batch. For what I assume are safety reasons, the engine machines don't hold a huge amount of black powder. When they're refilled the second batch of bp could be different than the first.

billspad said:

Same date code doesn't mean same batch. For what I assume are safety reasons, the engine machines don't hold a huge amount of black powder. When they're refilled the second batch of bp could be different than the first.

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Yes, that makes sense. I know from making my own black powder and black powder engines that getting all things consistent from batch to batch is often a daunting task. Humidity and species of wood used to make the charcoal are just a couple of variables and there are dozens of other factors as well. It is quite an accomplishment by Vernon Estes and the company he founded that his engines are of as uniform high quality as they are and remain so to this day.

Wow - seriously did not imagine how complicated this would turn out to be. I am going to go with a kind of hybrid.

The motor tube will only be about an inch longer than the motor. The inch or so of tube will be coated with epoxy and fitted with a flame retardant coating of aluminium foil and kevlar/nomex which, as a sandwich, should reduce heat effects. At the end of the motor tube will be a bulkhead that is similarly screened against heat. The motor will be friction fitted to allow any blast at the end of the main burn to push the motor out of the back end. On balance this seems the most practical and sensible course of action to keep everything as simple as possible.

I hadnt realised the flash/bang effect of a booster motor at the end of its burn was so vicious and the design of the booster doesn't really allow for any vent space unless I want to risk a paper/card booster being able to survive the flash and pressure effects of a D12-0 at burn out. The heat can be handled but the pressure cant in any simple way.

I was planning on showing a plan of this but unfortunately I got the flu and all of my rough designs are on a USB stick in my desk at work. I will try and get these back and uploaded this week to show a rough idea but I think on the whole just ejecting the D12 seems the most sensible and it also allows the overall weight to drop for recovery.

Astro-Baby said:

Wow - seriously did not imagine how complicated this would turn out to be. I am going to go with a kind of hybrid.

The motor tube will only be about an inch longer than the motor. The inch or so of tube will be coated with epoxy and fitted with a flame retardant coating of aluminium foil and kevlar/nomex which, as a sandwich, should reduce heat effects. At the end of the motor tube will be a bulkhead that is similarly screened against heat. The motor will be friction fitted to allow any blast at the end of the main burn to push the motor out of the back end. On balance this seems the most practical and sensible course of action to keep everything as simple as possible.

I hadnt realised the flash/bang effect of a booster motor at the end of its burn was so vicious and the design of the booster doesn't really allow for any vent space unless I want to risk a paper/card booster being able to survive the flash and pressure effects of a D12-0 at burn out. The heat can be handled but the pressure cant in any simple way.

I was planning on showing a plan of this but unfortunately I got the flu and all of my rough designs are on a USB stick in my desk at work. I will try and get these back and uploaded this week to show a rough idea but I think on the whole just ejecting the D12 seems the most sensible and it also allows the overall weight to drop for recovery.

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Actually, the engines are quite simple. It's the rules that give everyone fits! :wink: I look forward to your design ideas and a flight report. Good luck!

Heres a pic of how I planned the boosters.



The top one shows the original idea. The motor has butts up to a tube that runs the length of the booster body. This would be around 8" Long and around 1" diameter and may contain a baffle of some kind. The idea was at burn out the last remaining burn would be allowed to vent up this tube and, in my original thinking, this should provide a safe vent space. The tube would be heat protected as would be the upper part of the motor mount. In this scheme the motor would be held by a retainer clip.

Having looked at the videos and comments on this thread I seriously doubt that an 8" tube is enough to vent a C motor into so I am more minded to go with the bottom layout where the motor tube is actually blanked off at the bulkhead, the motor is not retained and any remaining blast is simply used to eject the motor from the booster.

The gap between the top of the motor and the bulkhead is about 1" but that could be reduced by having another bulkhead further forward to reduce the risk of the 1" of motor tube taking any pressure or heat.

Its taken a bit longer to get this posted than I had hoped - I have had the flu from hell which wiped me out totally for the last 10 days or so.

How about going with the bottom layout but adding a tube parallel to the motor tube and connected at the top? This would allow the gases and flaming particles to make a U turn and vent out the back and will eliminate motor casings raining out of the sky.