Minimum diameter of ejection charge passage thru BT20/BT5 balsa transition

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Dotini

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I've built and launched a small rocket with a BT20/BT5 transition section, but was afraid to bore a hole thru the section because I didn't know how small a hole I could get away with. Nor did I trust thinning out the balsa with a big hole for fear of resulting structural weakness. Anyone with experience in this area? I'd appreciate some guidance here.
 
Drilling through a balsa transition is an unusual step. I'd probably be reluctant to do it. More commonly, you would tie the shock cord to the bottom of the transition and have the rocket separate there. That way you'd be packing the streamer into a BT20 rather than a BT5, which would be easier.
 
I'm building another rocket with a BT50 to BT20 balsa transition. I carefully bored a passage through the balsa with the same diameter as the engine casing inside diameter. I coated the passage and all exposed surfaces with epoxy. With this, I'm confident of structural integrity and successful ejection of the recovery devise. But, as always, I could be wrong! I'll let you know what happens after I launch it.DSC00019.jpg
Ringtail rocket with BT20/BT5 balsa transition
 
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I'm building another rocket with a BT50 to BT20 balsa transition. I carefully bored a passage through the balsa with the same diameter as the engine casing inside diameter. I coated the passage and all exposed surfaces with epoxy. With this, I'm confident of structural integrity and successful ejection of the recovery devise. But, as always, I could be wrong! I'll let you know what happens after I launch it.View attachment 455227
Ringtail rocket with BT20/BT5 balsa transition
Ah, well that explains why you don't want the rocket to separate below the transition. :)

What I would try to do is drill the whole *carefully* and then insert a body tube in the whole (make the hole just big enough a snug fit). For a 20-50 transition, you'd embed a BT5. For a 5-20 transition, you'd embed a.... eh, I'm not sure what the next size down would be equivalent to the inner diameter of a 13mm motor. BT3 maybe?

In a previous build I had a custom-turned transition (by Gordon Agnello, natch) with an embedded BT50. Gordon drilled the hole for me and shipped it to me with the tube embedded to keep it well supported, so it saved me the trouble. The resulting piece was quite strong with the tube inside. For that particular piece I'm quite sure I would not have been able to drill that hole myself. For your smaller transitions it might be possible.

Certainly you will have to be very careful and gentle drilling through a 5-20 transition. I'd be sure to put a short piece of body tube onto each shoulder while drilling to hold it together.
 
I think you should be fine. Diameter greater than the internal diameter of your motor casing is likely sufficiemt (make extra sure you have good positive motor retention, a smaller diameter tube will increase back pressure and subsequent chance of spitting a poorly secured motor.). A second concern is heat resistance.

in my own experience, I have gotten in trouble with using stuffer tubes (mainly for pop pod gliders like clones of Apogee Darkbird) where I downsized one tube size from the motor mount (saves weight and provides much more room for pod streamer or chute.

the downside of a smaller tube is that the flame from the ejection charge is concentrate more, particularly at the place just forward of the engine block, and tended to burn through.

I suspect your epoxy may be sufficient. I have gone to placing a rolled up piece of aluminum can just forward of the motor block, with good results. YMMV
 
When i have worries about tube strength due to ejection pressure or heat, i just double up on the tube. Slice the tube lengthwise all the way down one side and slip it around the outside (or inside) of the tube that will contain the ejection gases. If you slip it on the outside of the tube you can then put epoxy on the outside seam for strength.....as the seam will be very small. Inside, make sure you glue the end or use an engine block just in case. I have never had the inside shoot out, but better to be safe than sorry. Just my 2 cents worth :)
 
I am disappointed by this thread. I thought it was going to be about the MINIMUM diameter. I thought some wise voice of experience would recommend something like a minimum diameter of twice the nozzle throat. I would find that info much more interesting.

As developed, it is not a problem. I have relevant experience from NAR Payload competition. Even light contest balsa is very strong. Make the wall thickness ad thin as you dare , say 1/16". It will stand up to the kick from a piston launched A10, and the ejection charge. If you want it stand up to many flights, coat the inside of the balsa with epoxy or some other protective layer.
 
When i have worries about tube strength due to ejection pressure or heat, i just double up on the tube. Slice the tube lengthwise all the way down one side and slip it around the outside (or inside) of the tube that will contain the ejection gases. If you slip it on the outside of the tube you can then put epoxy on the outside seam for strength.....as the seam will be very small. Inside, make sure you glue the end or use an engine block just in case. I have never had the inside shoot out, but better to be safe than sorry. Just my 2 cents worth :)
Balsa Machining sell 34" couplers. You could sleeve your tube rather than cut a tube.
 
When i have worries about tube strength due to ejection pressure or heat, i just double up on the tube. Slice the tube lengthwise all the way down one side and slip it around the outside (or inside) of the tube that will contain the ejection gases. If you slip it on the outside of the tube you can then put epoxy on the outside seam for strength.....as the seam will be very small. Inside, make sure you glue the end or use an engine block just in case. I have never had the inside shoot out, but better to be safe than sorry. Just my 2 cents worth :)
For pressure, this makes sense. For HEAT on the other hand, not so sure, depends a bit on how many flights you are likely to do.
Consider this video of an Estes motor test at around 1:45, burnout of the C motor with ejection charge. For a second or two there is an actual tongue of flame lazily burning from the front of the motor. I have always been pleasantly surprised that even minimum diameter cardboard rocket tubes don’t catch fire from the ejection charge. Shock cords, including KEVLAR shock cords, can and do burn through SOMETIMES (although they don’t to my knowledge catch fire) when they are mounted on the motor mount, I suspect because during packing they sometimes fold directly in front of the ejection charge. But minimum diameter tubing rarely, (interestingly, with the old Quest black powder motors, often the motor casing got so hot I would see “browning” of the white paint on the OUTSIDE of the body tube around the CASING. I have personally seen charring of a standard body tube when I went LESS THAN MINIMUM diameter with tubing just forward of the motor casing, and I think it is because the heat is concentrated over a smaller surface area. Doubling the tube thickness will almost certainly DELAY burn through for more flights, but it won’t prevent it. COATING it with another substance, white or wood glue, epoxy (JB Weld is often mentioned for its heat resistant properties

https://en.m.wikipedia.org/wiki/J-B_Weld#J-B_Weld_epoxy) may help, again the question is less, does the COATING hold up to the heat, vs does the intact coating sufficiently PROTECT/INSULATE the underlying surface?

aluminum itself is flammable, but aside from powder or shavings, seems like sheet aluminum holds up pretty well to multiple short exposures.

looking at the inside of a used Estes black powder motor, I think the inner layer of the paper DOES get physically charred. Since it is single use, it’s irrelevant. While homemade motor techniques are restricted to a certain section of this forum, if indeed the internal paper layer was intact, I would think the logical cheapest way to make motors would be to recycle used casings (tube and nozzle already there, would simplify at least one or two steps of the process.). I am thinking the reason it isn’t done (and yes I am reaching here, I neither have nor want the experience of building my own motors, it’s not my gift), anyhooo, the reason it isn’t done is I suspect because the used casing don’t have sufficient integrity for re-use, at least in part BECAUSE the inner layer is cooked.

perhaps those with more experience building motors can set me straight on this?
 
For pressure, this makes sense. For HEAT on the other hand, not so sure, depends a bit on how many flights you are likely to do.
Consider this video of an Estes motor test at around 1:45, burnout of the C motor with ejection charge. For a second or two there is an actual tongue of flame lazily burning from the front of the motor. I have always been pleasantly surprised that even minimum diameter cardboard rocket tubes don’t catch fire from the ejection charge. Shock cords, including KEVLAR shock cords, can and do burn through SOMETIMES (although they don’t to my knowledge catch fire) when they are mounted on the motor mount, I suspect because during packing they sometimes fold directly in front of the ejection charge. But minimum diameter tubing rarely, (interestingly, with the old Quest black powder motors, often the motor casing got so hot I would see “browning” of the white paint on the OUTSIDE of the body tube around the CASING. I have personally seen charring of a standard body tube when I went LESS THAN MINIMUM diameter with tubing just forward of the motor casing, and I think it is because the heat is concentrated over a smaller surface area. Doubling the tube thickness will almost certainly DELAY burn through for more flights, but it won’t prevent it. COATING it with another substance, white or wood glue, epoxy (JB Weld is often mentioned for its heat resistant properties

https://en.m.wikipedia.org/wiki/J-B_Weld#J-B_Weld_epoxy) may help, again the question is less, does the COATING hold up to the heat, vs does the intact coating sufficiently PROTECT/INSULATE the underlying surface?

aluminum itself is flammable, but aside from powder or shavings, seems like sheet aluminum holds up pretty well to multiple short exposures.

looking at the inside of a used Estes black powder motor, I think the inner layer of the paper DOES get physically charred. Since it is single use, it’s irrelevant. While homemade motor techniques are restricted to a certain section of this forum, if indeed the internal paper layer was intact, I would think the logical cheapest way to make motors would be to recycle used casings (tube and nozzle already there, would simplify at least one or two steps of the process.). I am thinking the reason it isn’t done (and yes I am reaching here, I neither have nor want the experience of building my own motors, it’s not my gift), anyhooo, the reason it isn’t done is I suspect because the used casing don’t have sufficient integrity for re-use, at least in part BECAUSE the inner layer is cooked.

perhaps those with more experience building motors can set me straight on this?
OK, probably true. Just saying I have been doing this for anything over a D, E motor and have not had any burn through or brown marks on the outside rocket tube....I think the Estes lower power motor casings take most of the brunt of the heat and flame....just its a lot to hope in a stubbier rocket to take the heat without a long mount/engine tube to dissipate.
 
All manufacturers of baffles say to leave a distance of one major body tube diameter between the motor and the baffle. I have discussed this with one of the manufacturers. So in my rocket with BT20/BT50 small inside diameter (~0.45") balsa transition, I am leaving an additional 1" for the ejection charge to expand before encountering the baffle.

Thanks to all for replying to this thread.
 
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Another technique for drilling through transitions and similar things. First give them a good soak in Minwax Wood Hardener. Let dry fully. Now the balsa will be a bit harder and drill through with much lower chance of the fibers ripping.
 
Dotini, no advice to give but I just wanted to remark that your Ringtail rocket with BT20/BT5 balsa transition is beautiful. Very nice build.
 
Dotini, no advice to give but I just wanted to remark that your Ringtail rocket with BT20/BT5 balsa transition is beautiful. Very nice build.
Thank you ever so much for that high compliment! It means a lot to me. You may be interested in knowing this rocket has a name - the Animist - and is actually a semi-scale (right down to the 3 black rings) model of something 60 years ago. I am in the process of building another in a larger scale with a much more robust recovery system, but it will look very much like this one except slightly elongated.
 
I think you should be fine. Diameter greater than the internal diameter of your motor casing is likely sufficiemt (make extra sure you have good positive motor retention, a smaller diameter tube will increase back pressure and subsequent chance of spitting a poorly secured motor.). A second concern is heat resistance.

in my own experience, I have gotten in trouble with using stuffer tubes (mainly for pop pod gliders like clones of Apogee Darkbird) where I downsized one tube size from the motor mount (saves weight and provides much more room for pod streamer or chute.

the downside of a smaller tube is that the flame from the ejection charge is concentrate more, particularly at the place just forward of the engine block, and tended to burn through.

I suspect your epoxy may be sufficient. I have gone to placing a rolled up piece of aluminum can just forward of the motor block, with good results. YMMV
Back in the bad old days of my youth when I didn't know the rules, I made a 4 stage rocket with a 7-8" gap between each of the motors. I was nervous about reliable ignition, so I made flaming-particle concentrating cones at the top of the first 3 stages to ensure ignition of the subsequent stage. It worked quite well for two launches.

On the horizontal spin recovery Magnus effect rockets, all the ejection exhaust must initially escape through a 1/4" hole. I question if this is a potential problem - or if the considerable increase in tube length makes enough cushion in volume which allows the use of such a small hole?
 
Back in the bad old days of my youth when I didn't know the rules, I made a 4 stage rocket with a 7-8" gap between each of the motors. I was nervous about reliable ignition, so I made flaming-particle concentrating cones at the top of the first 3 stages to ensure ignition of the subsequent stage. It worked quite well for two launches.

On the horizontal spin recovery Magnus effect rockets, all the ejection exhaust must initially escape through a 1/4" hole. I question if this is a potential problem - or if the considerable increase in tube length makes enough cushion in volume which allows the use of such a small hole?
I think your model proved that your hole size worked quite well!

my max black powder gap stage is 53 inches
 
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