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  1. #1
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    Protection from heat/force of ejection charge?

    Has anyone found a way to protect their engine pod from this kind of damage?

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    I'm really a fan of RGs but after just 2 flights I'm experiencing this kind of charring and ejection charge damage in my models.

    I've tried coating the insides with CA and high heat aluminum paint and adding additional vent holes but the short engine pods used with RGs don't stand up too well to the heat and pressure of even 13mm motors like the 1/2A3-2T, A3-4T, or A10-3, which also seem to have far too large/violent ejection charges.

    NAR# 103899

  2. #2
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    Wow, that looks pretty severe. I'm sorry, I have no ideas.

    NAR 91107, Level 2

    I really, really hate bugs.

  3. #3
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    You might slip a piece of coupler in there and change it whenever needed.
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  4. #4
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    A little bit of wadding in the "hollow" end of the motor will mitigate this but not solve it completely.
    Take Care,

    Duane Wilkey
    NAR 63416/TRA 4295 - L3
    Webcoder for ISOAR - TRA Prefecture 21
    Former Member NAR L3CC
    TARC Mentor

  5. #5
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    Quote Originally Posted by Steve Shannon View Post
    You might slip a piece of coupler in there and change it whenever needed.
    Although I'm concerned about the added weight on an already built model, sleeving it sounds like a great idea
    considering the alternative. The idea becomes even more appealing if I design with it in mind on a scratch build.
    Thanks for both a practical and rather brilliant solution Steve. Guess my BAR-ness is showing, but I'm getting there.
    NAR# 103899

  6. #6
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    Quote Originally Posted by DuaneW View Post
    A little bit of wadding in the "hollow" end of the motor will mitigate this but not solve it completely.

    Thanks Duane, combined with Steve's idea I'm a lot more optimistic about not having to retire my newly built gliders after only a couple of flights.
    I have a few scratch builds in mind for the winter so I'll let you all know how these fixes are working.
    NAR# 103899

  7. #7
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    Quote Originally Posted by Rktman View Post
    Has anyone found a way to protect their engine pod from this kind of damage?

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    I'm really a fan of RGs but after just 2 flights I'm experiencing this kind of charring and ejection charge damage in my models.

    I've tried coating the insides with CA and high heat aluminum paint and adding additional vent holes but the short engine pods used with RGs don't stand up too well to the heat and pressure of even 13mm motors like the 1/2A3-2T, A3-4T, or A10-3, which also seem to have far too large/violent ejection charges.
    Wow, that’s a lot of char.
    I have had this problem with pop pods for rear eject boost gliders when I downsize the tube of the pod core tube (allows more room for recovery device) and with chimneys on two stager boosters. If you watch a video of an engine firing on test stand you will see a decent flame for a bit after the ejection fires. I think this is why hooking a shock cord to an engine mount on BP motors sometime results in burn through of the cord (even Kevlar)after a few flights.

    I have used a rolled up aluminum can side, when rolled tightly will “unroll” to fit inside your body tube. A little tricky to line up the holes (I recommend equal size holes on both sides of the tube to equalize the venting, not sure it makes a difference but can’t hurt.). Shouldn’t much of a challenge for you, I have seen your builds.

    The aluminum roll if sized right will function both as an engine block and an inner tube protector.

    Good luck!
    It is amazing what you can do when you don't have a choice.

    Smart people learn from their mistakes.
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  8. #8
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    Not sure if this will work, but another option might be to trim the model to be engine eject rather than just engine burn the string. An ejected engine won’t have as much burn time to barbecue the mount. Would not be an option for a model in rocket glide competition.
    It is amazing what you can do when you don't have a choice.

    Smart people learn from their mistakes.
    REALLY SMART PEOPLE learn from OTHERS' mistakes.

  9. #9
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    Quote Originally Posted by BABAR View Post
    Not sure if this will work, but another option might be to trim the model to be engine eject rather than just engine burn the string. An ejected engine won’t have as much burn time to barbecue the mount. Would not be an option for a model in rocket glide competition.
    What do you think of using a booster engine with no ejection charge? I'm thinking it's the concussive force of the ejection charge that does most of the damage (i.e. destroying nose cones and engine pods, eroding vent holes) so doing away with it may ameliorate the situation??
    Out of frustration I've seriously considered spitting engines but want to be a "good citizen" as far as not littering a flying field with casings so would like to try exhausting every reasonable option first.

    Of course the other thing is that I have a fascination for all things variable geometry. Having a glider or heli unfold successfully is still personally more satisfying than watching the launch of a mega-engined HPR rocket. I have yet to figure out how to actuate those changes without a burn string or some other sort of simple mechanical means.
    NAR# 103899

  10. #10
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    For one, the vent holes could be a bit bigger. Adding a SHORT piece of tube coupler also helps. Big thing I like to do is to apply thin CA, which while not burn-proof, does make cardboard and balsa more burn-resistant. I always add some thin CA to vent holes in tubes, and let it run inside of the tube a bit, but avoid letting it get into the engine area. If I use a full nose cone, then I coat the base of the nose cone with thick CA.

    But the biggest trick I use with most Rocket Gliders, is to split a balsa nose cone in half lengthwise. So rather than side vent holes, the ejection charge blows out of the front, where the missing half of the nose is. Adds a bit of drag but not much and I like the tradeoff (been doing most burn-thread R/G's that way since the late 1970's). Also can use one nose cone for two gliders...... or have a spare half to use to replace the first nose half later as eventually it will get roasted.

    Before gluing the nose half in, I first cut a shallow v-notch into the nose half, along the "base" area and the "flat side" where it was split, in part to make up a sort of ramp to guide some of the ejection charge that would be 50% blocked by the base (on some occasions I have gotten fancy, wrapped sandpaper around a suitable diameter of dowel, and sanded a curved "elliptical notch" into the nose half). But also, I have used a notch to give a bit of a hollow area for the burn thread to run across, so that hot gases fully engulf the thread. I have not documented that in a drawing, though.

    In any case, I apply thick CA to the nose base, flat "half", and the V-notch, to inhibit burn damage. After gluing the nose half in, I cut out a 180 degree arc of tube coupler, the length of the nose base, and glue that to the area of the engine mount tube where the other half of the nose cone would have been, and apply a little bit of thin CA inside. That helps to reduce burn/char damage.

    Below is a plan shown an example of what I mean (except the V-notch is not shown).

    Also see the "Stiletto" series of Rocket Gliders on my website:
    http://georgesrockets.com/GRP/CONTEST/ContestPlans.htm


    Last edited by georgegassaway; 29th November 2017 at 06:00 AM.

    Contest flying, Sport flying, it's all good.....
    2016 Bike Mileage total: 1843 miles. 5 Miles a day for the year!

  11. #11
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    Quote Originally Posted by georgegassaway View Post
    For one, the vent holes could be a bit bigger. Adding a SHORT price of tube coupler also helps. Big thing I like to do is to apply thin CA, which while not burn-proof, does make cardboard and balsa more burn-resistant. I always add some thin CA to vent holes in tubes, and let it run inside of the tube a bit, but avoid letting it get into the engine area. If I use a full nose cone, then I coat the base of the nose cone with thick CA.

    But the biggest trick I use with most Rocket Gliders, is to split a balsa nose cone in half lengthwise. So rather than side vent holes, the ejection charge blows out of the front, where the missing half of the nose is. Adds a bit of drag but not much and I like the tradeoff (been doing most burn-thread R/G's that way since the late 1970's). Also can use one nose cone for two gliders...... or have a spare half to use to replace the first nose half later as eventually it will get roasted.

    Before gluing the nose half in, I first cut a shallow v-notch into the nose half, along the "base" area and the "flat side" where it was split, in part to make up a sort of ramp to guide some of the ejection charge that would be 50% blocked by the base (on some occasions I have gotten fancy, wrapped sandpaper around a suitable diameter of dowel, and sanded a curved "elliptical notch" into the nose half). But also, I have used a notch to give a bit of a hollow area for the burn thread to run across, so that hot gases fully engulf the thread. I have not documented that in a drawing, though.

    In any case, I apply thick CA to the nose base, flat "half", and the V-notch, to inhibit burn damage. After gluing the nose half in, I cut out a 180 degree arc of tube coupler, the length of the nose base, and glue that to the area of the engine mount tube where the other half of the nose cone would have been, and apply a little bit of thin CA inside. That helps to reduce burn/char damage.

    Below is a plan shown an example of what I mean (except the V-notch is not shown).

    Also see the "Stiletto" series of Rocket Gliders on my website:
    http://georgesrockets.com/GRP/CONTEST/ContestPlans.htm



    Much thanks George, looking forward to getting more than a flight or two out of my RGs, especially the mini-engined ones.
    Will definitely incorporate these changes and other tips from this thread in all future scratch builds.
    NAR# 103899

  12. #12
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    Quote Originally Posted by Rktman View Post
    What do you think of using a booster engine with no ejection charge? I'm thinking it's the concussive force of the ejection charge that does most of the damage (i.e. destroying nose cones and engine pods, eroding vent holes) so doing away with it may ameliorate the situation??
    Out of frustration I've seriously considered spitting engines but want to be a "good citizen" as far as not littering a flying field with casings so would like to try exhausting every reasonable option first.

    Of course the other thing is that I have a fascination for all things variable geometry. Having a glider or heli unfold successfully is still personally more satisfying than watching the launch of a mega-engined HPR rocket. I have yet to figure out how to actuate those changes without a burn string or some other sort of simple mechanical means.
    Booster engine would likely not work as there is no delay by definition . So your deployment would be far too early and with your rear end hinge design the premature opening while the Rocket is still at high velocity would probably rip the rotors right off the rocket

    Concussive force from the Estes shotgun ejection charge can be a problem particularly with Boost gliderss like the Semroc hawk also known as the Estes Astron falcon. I wonder if in part this may be a problem due to an asymmetrical vent hole only on one side with sudden lateral force breaking the spar. I don’t know how much it helps but I tend to put one vent hole on each side to equalize the ejection Force and hopefully bminimize any lateral motion at deployment.

    Regarding good citizenship certainly if you find a spit out motor it’s good to pick it up but given everything in the rocket motor is biodegradable I don’t think this really is a big issue. Nobody is going to get hurt by getting hit on the head with a tumbling A B or C size engine and I don’t even think D is a problem. I am biased as probably 30% of my Rockets actually use this technique.

    I share your enthusiasm for variable geometry Rockets and I find it a fun engineering challenge. My rockets tend to have four sides because using this I can use two rubber bands to hold the rotors in and hold the engines in. In fact some of my rocket do not even have a motor mount tube. The method I use is similar to the whopper flopper chopper thread previously posted.
    It is amazing what you can do when you don't have a choice.

    Smart people learn from their mistakes.
    REALLY SMART PEOPLE learn from OTHERS' mistakes.

  13. #13
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    Quote Originally Posted by BABAR View Post
    Booster engine would likely not work as there is no delay by definition . So your deployment would be far too early and with your rear end hinge design the premature opening while the Rocket is still at high velocity would probably rip the rotors right off the rocket

    Concussive force from the Estes shotgun ejection charge can be a problem particularly with Boost gliderss like the Semroc hawk also known as the Estes Astron falcon. I wonder if in part this may be a problem due to an asymmetrical vent hole only on one side with sudden lateral force breaking the spar. I don’t know how much it helps but I tend to put one vent hole on each side to equalize the ejection Force and hopefully bminimize any lateral motion at deployment.

    Regarding good citizenship certainly if you find a spit out motor it’s good to pick it up but given everything in the rocket motor is biodegradable I don’t think this really is a big issue. Nobody is going to get hurt by getting hit on the head with a tumbling A B or C size engine and I don’t even think D is a problem. I am biased as probably 30% of my Rockets actually use this technique.

    I share your enthusiasm for variable geometry Rockets and I find it a fun engineering challenge. My rockets tend to have four sides because using this I can use two rubber bands to hold the rotors in and hold the engines in. In fact some of my rocket do not even have a motor mount tube. The method I use is similar to the whopper flopper chopper thread previously posted.
    On my gliders I use at least 2 vent holes—one on either side across each other—and lately a 3rd up top and forward a bit. Hasn't helped with the damage. So on all future scratch builds I'll use a combo of the suggestions from this thread (coating with epoxy, "sleeving" the area forward of the engine with a coupler, slicing the nose cone in half to allow ejection venting from the front of the engine tube).

    I've not had any problems with my helis, the 3 vent holes are large enough and far enough from the engine to allow a good degree of cooling and dissipation of force so that's all good. I just didn't anticipate this kind of problem with RGs since this is my first foray into them and I had no idea the relatively small engine pods would suffer this kind of damage. But I have to say it's been a valuable learning experience that can only benefit my future builds.

    I'm glad you brought up the point that booster engines have no delay, I'd forgotten about that little detail. While the engine pod might benefit from no ejection blast I'm sure the sudden deployment of wings while the glider was still accelerating upward would at the very least strip the wings off. Not a good tradeoff. I'm far more optimistic now that I can at the very least ameliorate the problem.
    NAR# 103899

  14. #14
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    Interesting that you don’t have the problem with the vent holes on the helis. Are the vent holes on the helis further forward than the vents on the BGs? Is there a bulkhead on the helis that “forces” the engine flame to go out the holes?

    Here is a video of a rocket burn
    https://m.youtube.com/watch?v=cxuDT1yewYQ

    Notice at the end a persistent flame come out the forward end. Maybe on the BGs the chamber forward of the engines is so small that all the flame is concentrated right on your holes?

    From a flight standpoint unless you are in Rocket Glider competition (where I think rules don’t allow ejection of engine or pop pod or anything, as opposed to boost glider comp where you can eject anything you want, only the glider matter) the engine is just dead weight for the glider. Makes sense to dump it at apogee. Only other case I can think retaining it is helpfulis where local conditions are high fire risk, I believe clubs may ban sparky motors and engine ejection.

    I may play around and check weight of aluminum can sleeve vs a coupler. The can sleeve may be lighter AND more durable.
    Last edited by BABAR; 29th November 2017 at 04:57 AM.
    It is amazing what you can do when you don't have a choice.

    Smart people learn from their mistakes.
    REALLY SMART PEOPLE learn from OTHERS' mistakes.

  15. #15
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    I am leery of using a booster motor for models that need to burn a thread.

    The thread may not burn.

    Contest flying, Sport flying, it's all good.....
    2016 Bike Mileage total: 1843 miles. 5 Miles a day for the year!

  16. #16
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    Quote Originally Posted by BABAR View Post
    Interesting that you don’t have the problem with the vent holes on the helis. Are the vent holes on the helis further forward than the vents on the BGs? Is there a bulkhead on the helis that “forces” the engine flame to go out the holes?

    Here is a video of a rocket burn
    https://m.youtube.com/watch?v=cxuDT1yewYQ

    Notice at the end a persistent flame come out the forward end. Maybe on the BGs the chamber forward of the engines is so small that all the flame is concentrated right on your holes?

    From a flight standpoint unless you are in Rocket Glider competition (where I think rules don’t allow ejection of engine or pop pod or anything, as opposed to boost glider comp where you can eject anything you want, only the glider matter) the engine is just dead weight for the glider. Makes sense to dump it at apogee. Only other case I can think retaining it is helpfulis where local conditions are high fire risk, I believe clubs may ban sparky motors and engine ejection.

    I may play around and check weight of aluminum can sleeve vs a coupler. The can sleeve may be lighter AND more durable.

    My heli vent holes are at the front end so apparently the heat and force of the ejection charge dissipates sufficiently to not cause any charring or physical damage.
    You're right that RG engine pods are so small (to save weight) the ejection charge can cause significant damage. I was shocked to see flame pouring out the vent holes
    after the initial ejection charge went off. I was static testing a scratch build to make sure the engine pylon was high enough for the exhaust to clear the glider
    body and that poor pod looked like a butane lighter with an inch long flame flickering out for almost a full second.



    I'm interested in sport flying—in short, for the fun of it—and as a sort of creative outlet. RGs require burn threads as a method of controlling deployment of the wings
    otherwise I'd eject my engines. I still haven't figured out a simple reliable alternative that wouldn't require heavy complicated mechanical linkages of some sort to achieve
    the same thing upon engine ejection.

    I like your thought of sleeving using aluminum but doesn't the NAR prohibit the use of metals in the immediate vicinity of the engine? (I could be mistaken since that's a memory from when I was a kid, and in the interim HPR engines that have aluminum casings have come on the scene so...).
    NAR# 103899

  17. #17
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    I like your thought of sleeving using aluminum but doesn't the NAR prohibit the use of metals in the immediate vicinity of the engine? (I could be mistaken since that's a memory from when I was a kid, and in the interim HPR engines that have aluminum casings have come on the scene so...).

    From the NAR website
    http://www.nar.org/safety-informatio...t-safety-code/
    Materials. I will use only lightweight, non-metal parts for the nose, body, and fins of my rocket.

    Doesn’t mean you can’t use metal in a rocket.
    Examples:
    Nose cone: Screw eyes, metal ballast (BBs, washers, lead fishing weights), also wires attached to nose cone to attach elastic bands for helis.
    Body tubes: screw fasteners
    Not sure what part you want to refer to attachment of a steel engine hook (which by the way is kind of pointy if you don’t bend it forward)

    Also see the following aluminum LOW POWER 24 mm engine casings from Apogee

    Aluminum 24 mm LOW POWER casing
    https://www.apogeerockets.com/Rocket...r125pu9hsovoq7

    and aluminum aft closure
    https://www.apogeerockets.com/Rocket...84r125pu9hsovo
    Clearly the safety code forbids use of metal in low power rocketry for the key structural parts (nose, tube, fins) but there is nothing forbidding use of metal completely, and as you can see from the above, sizeable uses of metal are allowed. Again, these are uses of metal in LOW POWER rockets.
    I guess there is some debate on this (I know Der MicroMeister disagrees with me on this, and he certainly is an expert not just on MicroMaxx but on competition, scale, heck just about everything!) but I would consider a protective aluminum sleeve a non-structural (non-weight bearing non-dynamic part, as opposed to a nose, tube, or fin) part of the motor mount. Just my take. I think it actually makes the rocket SAFER. So I use it for this specific purpose and don’t have any qualms about it.
    It is amazing what you can do when you don't have a choice.

    Smart people learn from their mistakes.
    REALLY SMART PEOPLE learn from OTHERS' mistakes.

  18. #18
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    Have you thought about putting a layer of high-temperature tape over the affected area? Kapton or other high-temperature tape is quite cheap these days, especially since 3D printers started using it sometimes on the print platform.

    You can pick it up on eBay or elsewhere quite cheaply.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  19. #19
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    Quote Originally Posted by BABAR View Post
    I like your thought of sleeving using aluminum but doesn't the NAR prohibit the use of metals in the immediate vicinity of the engine? (I could be mistaken since that's a memory from when I was a kid, and in the interim HPR engines that have aluminum casings have come on the scene so...).

    From the NAR website
    http://www.nar.org/safety-informatio...t-safety-code/
    Materials. I will use only lightweight, non-metal parts for the nose, body, and fins of my rocket.

    Doesn’t mean you can’t use metal in a rocket.
    Examples:
    Nose cone: Screw eyes, metal ballast (BBs, washers, lead fishing weights), also wires attached to nose cone to attach elastic bands for helis.
    Body tubes: screw fasteners
    Not sure what part you want to refer to attachment of a steel engine hook (which by the way is kind of pointy if you don’t bend it forward)

    Also see the following aluminum LOW POWER 24 mm engine casings from Apogee

    Aluminum 24 mm LOW POWER casing
    https://www.apogeerockets.com/Rocket...r125pu9hsovoq7

    and aluminum aft closure
    https://www.apogeerockets.com/Rocket...84r125pu9hsovo
    Clearly the safety code forbids use of metal in low power rocketry for the key structural parts (nose, tube, fins) but there is nothing forbidding use of metal completely, and as you can see from the above, sizeable uses of metal are allowed. Again, these are uses of metal in LOW POWER rockets.
    I guess there is some debate on this (I know Der MicroMeister disagrees with me on this, and he certainly is an expert not just on MicroMaxx but on competition, scale, heck just about everything!) but I would consider a protective aluminum sleeve a non-structural (non-weight bearing non-dynamic part, as opposed to a nose, tube, or fin) part of the motor mount. Just my take. I think it actually makes the rocket SAFER. So I use it for this specific purpose and don’t have any qualms about it.
    I like your reasoning, and I now agree there's little reason NOT to sleeve it with aluminum since it'll be primarily forward of the motor casing and wouldn't fragment from a defective exploding low power motor. The sleeved section would only be an inch or less anyway and I'd be limiting it to just this one specific use (for rocket gliders) so I don't feel I'd be violating any safety issues in a recklessly widespread way.

    Besides in the interim I've tried coating the engine pod insides with CA, high heat paint, and even epoxy but it hasn't stopped the rather severe damage. I think this may be the definitive solution, along with using half a nose cone in lieu of using vent holes.
    NAR# 103899

  20. #20
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    Quote Originally Posted by OverTheTop View Post
    Have you thought about putting a layer of high-temperature tape over the affected area? Kapton or other high-temperature tape is quite cheap these days, especially since 3D printers started using it sometimes on the print platform.

    You can pick it up on eBay or elsewhere quite cheaply.
    Unfortunately I can't apply it to the insides of the 13mm or 18mm engine tubes where most of the heat damage occurs.
    NAR# 103899

  21. #21
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    Really good discussion, folks.
    "Lock S-foils in attack position....."

  22. #22
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    Quote Originally Posted by BABAR View Post
    I like your thought of sleeving using aluminum but doesn't the NAR prohibit the use of metals in the immediate vicinity of the engine? (I could be mistaken since that's a memory from when I was a kid, and in the interim HPR engines that have aluminum casings have come on the scene so...).

    From the NAR website
    http://www.nar.org/safety-informatio...t-safety-code/
    Materials. I will use only lightweight, non-metal parts for the nose, body, and fins of my rocket.

    Doesn’t mean you can’t use metal in a rocket.
    Examples:
    Nose cone: Screw eyes, metal ballast (BBs, washers, lead fishing weights), also wires attached to nose cone to attach elastic bands for helis.
    Body tubes: screw fasteners
    Not sure what part you want to refer to attachment of a steel engine hook (which by the way is kind of pointy if you don’t bend it forward)

    Also see the following aluminum LOW POWER 24 mm engine casings from Apogee

    Aluminum 24 mm LOW POWER casing
    https://www.apogeerockets.com/Rocket...r125pu9hsovoq7

    and aluminum aft closure
    https://www.apogeerockets.com/Rocket...84r125pu9hsovo
    Clearly the safety code forbids use of metal in low power rocketry for the key structural parts (nose, tube, fins) but there is nothing forbidding use of metal completely, and as you can see from the above, sizeable uses of metal are allowed. Again, these are uses of metal in LOW POWER rockets.
    I guess there is some debate on this (I know Der MicroMeister disagrees with me on this, and he certainly is an expert not just on MicroMaxx but on competition, scale, heck just about everything!) but I would consider a protective aluminum sleeve a non-structural (non-weight bearing non-dynamic part, as opposed to a nose, tube, or fin) part of the motor mount. Just my take. I think it actually makes the rocket SAFER. So I use it for this specific purpose and don’t have any qualms about it.
    Just discovered that the added benefit is that aluminum is significantly lighter than a tube coupler. A 1.5" length of both is too light to register on my postage scale but just by hand feel alone I'd say the aluminum is 50-65% lighter. Also far more durable and heat resistant than a paper tube coupler.

    NAR# 103899

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  5. Tube volume and force of ejection charge?
    By tdn in forum Propulsion
    Replies: 5
    Last Post: 5th October 2004, 08:44 PM

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