mh9162013
Well-Known Member
Does that use generic/aftermarket blades or proprietary blades?
Why am I making centering rings?
- Thread starter Duncan.Byers
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lakeroadster
When in doubt... build hell-for-stout!
Here is a motor mount that I built over 50 years ago. This rocket used a BT-55 and had an 18mm mount. We bought some FSI 21mm motors so I tore out the old mount that was standard tube and centering rings, I rolled a new tube of the appropriate diameter, and put it in with balsa strips. In the photo that's the tube that is in between the smallest tube and largest tube. Then a couple of years ago I decided I would fly it again so I put a piece of BT-20 in there with an engine hook and some small balsa strips, same concept but 50 years later.
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Concur that centering boards are a great solution, with the caveat that except for boosters, you need SOMETHING that will block the ejection gases from escaping out the back.
this will likely be a ring, but a piece of foam board or heavy cardstock would work fine, so you WILL need at least one ring. But if it is placed in front of your boards, it doesn’t need to be that strong, the boards will keep it in place.
what surprises me is that nobody to my knowledge had used this to help secure through-the-wall fins. Seems to me if you carefully position your boards so they are next to the slots, it gives you a great surface area to glue the tabs of the TTW Fins, seems like a heck of a lot easier than trying to do those internal fillets, and likely as strong if not stronger.
for low power rockets, I have used foam board to make centering rings up to BT-20 to BT-80. I cut the internal diameter a little small intentionally, and sand it so it fits snugly on the inside tube. Knock wood, but haven’t had a failure with low power motors yet.
for small diameter differences like BT-20 to BT-50, I take a roll of 3/4” masking tape and using a blade divide the roll into three sections, just rolling the blade AROUND the tape roll (you only need to cut a few mm, you don’t need to do the whole roll at once.). This gives me 1/4” wide strips as long as I need. I wrap the around the inner tube where I need the rings until I get the right diameter. I then add some thin CA to lock it (if you don’t, the motor heat can soften the tape adhesive.
works BT-20 to BT-50, or BT-5 to BT-20. Any wider than that gap it isn’t practical.
this will likely be a ring, but a piece of foam board or heavy cardstock would work fine, so you WILL need at least one ring. But if it is placed in front of your boards, it doesn’t need to be that strong, the boards will keep it in place.
what surprises me is that nobody to my knowledge had used this to help secure through-the-wall fins. Seems to me if you carefully position your boards so they are next to the slots, it gives you a great surface area to glue the tabs of the TTW Fins, seems like a heck of a lot easier than trying to do those internal fillets, and likely as strong if not stronger.
for low power rockets, I have used foam board to make centering rings up to BT-20 to BT-80. I cut the internal diameter a little small intentionally, and sand it so it fits snugly on the inside tube. Knock wood, but haven’t had a failure with low power motors yet.
for small diameter differences like BT-20 to BT-50, I take a roll of 3/4” masking tape and using a blade divide the roll into three sections, just rolling the blade AROUND the tape roll (you only need to cut a few mm, you don’t need to do the whole roll at once.). This gives me 1/4” wide strips as long as I need. I wrap the around the inner tube where I need the rings until I get the right diameter. I then add some thin CA to lock it (if you don’t, the motor heat can soften the tape adhesive.
works BT-20 to BT-50, or BT-5 to BT-20. Any wider than that gap it isn’t practical.
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I like it .... what does the "pressure chamber" for ejection look likeAfter cutting gobs of custom centering rings for re-sized tubes, I stopped and went. Why circles and not straight lines to do this? (It's sloppy work, I know....)
View attachment 519573
Any one of the thousands of you who are far smarter than me have a good reason to keep cutting rings??? This is far far easier.....
lakeroadster
When in doubt... build hell-for-stout!
BT-50 0.976
BT-20 0.736
0.24/2 = 0.12 gap
BT-55 1.325
BT-50 0.976
0.349/2 = 0.1745 gap
I just built a couple BT-55 based rockets with TTW fins and 24 mm motor tubes. The masking tape centering rings worked well. I use wood glue to make fillets on both sides to the masking tape cr's. That locks them in place and prevents any future unraveling.
Those look good and you put quite a bit into it. If I built a BT55 rocket with 24mm mount I might consider it as a "single use" rocket and not put so much time into it.
lakeroadster
When in doubt... build hell-for-stout!
Why "single use"?
My HiFlier XL on D12 power goes as high as I can see. I've launched it 3 times and not had to chase it too far. I'm guessing something like a Cherokee D would go correspondingly higher and be smaller at the same time. Maybe on a calm day it's not so bad, I did see a HiFlier XL launch on an E and it was recovered, but I'm usually not as lucky as other people.
lakeroadster
When in doubt... build hell-for-stout!
Luck has little to do with it. With some planning and follow through you'll meet your mission goals, whatever that may mean for you.
I run simulations on all my designs, and choose motors that will ensure flights typically between 800 to 1000 feet. And with rockets that are about a foot and a half long and about an inch or so in diameter, your odds are pretty good that you'll recover the rocket. Just so long as you follow NAR rules.
Good - Better - Best
I aim for "Best" every time, I don't always hit the mark, but I do aim for it every time.
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That was the whole earlier conversation - head slap by me - have to add at least one ring to contain the ejection pressure.
The Goblin is a BT-55 tube with a 24mm mount. Everybody knows it’s a bit of a hot rod on the Mighty Ds that were originally recommended for it, but it’s quite manageable on a C11-5. The stock dual streamer arrangement produces a more rapid descent and keeps it on the field.
I got sick of cracking fins on mine though, so I swapped the streamers for a 12-inch parachute and only fly it when it’s calm.
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I admire your work, but especially when in the midst of a build, I find “better” can be the enemy of “good.”
Then again, you are a craftsman AND an engineer, I have come to realize my talents are limited to the engineering side….
lakeroadster
When in doubt... build hell-for-stout!
I can't buy those around here so I tend to forget that they exist.
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Can't buy which? The C11-5s?
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Are those just multiple wraps of masking tape?
lakeroadster
When in doubt... build hell-for-stout!
Yes, I generally buy motors from Hobby Lobby. We have Hobby Town stores around but they have very limited inventory and what they do have is much more expensive than HL. I can buy C6 from HL but not C5 or C11.
I have been 3D printing a version of what you showed for a while. If all I need is a centering ring, I usually do them as a frame. For the top of a stuffer tube I do a solid disk (it also functions as a shock chord mount and I make it a bit stronger to work as a block to hold stuffer tube to body tube / airframe). For simplicity, I also use the same frame design as a parachute retainer if I need to hold parachute forward of engine. Easy enough to stuff dog barf or wadding through the holes and pack it where its needed.
I also started putting a frame in the bottom of nose cones so that parachute does not get jammed up in there. Extra shock chord mount as well (could probably just tie around the frame but those side elements seem a bit stronger).
I also started putting a frame in the bottom of nose cones so that parachute does not get jammed up in there. Extra shock chord mount as well (could probably just tie around the frame but those side elements seem a bit stronger).
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Buy from Amazon.
If you have Amazon Prime do they waive the hazmat shipping?
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I have yet to pay any shipping from Amazon with Prime.
With BP motors, Hazmat shipping only applies to E and larger.
This is my preferred source for Estes stuff. I find Amazon to be inconsistent with pricing and availability for hobby stuff. https://www.acsupplyco.com/
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You are correct about inconsistency but I try to get what I can with the free shipping. Even with E motors you can get free shipping.
If you ever have a big order, AC Supply gives free shipping for orders over $100, except for hazmat charges.
If you're getting BP E motors shipped to you without hazmat charges, I would be wary that they might be being shipped illegally.
Flight Sketch offers free shipping over $35.00
https://flightsketch.com/store/catalog/category/motors/by-impulse/c_17/
And I wouldn't trust any retailer who is willing to overlook regulations to sell $25 in rocket motors.
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The reason the BP distinction is important is because, e.g.
https://www.thrustcurve.org/motors/AeroTech/E30T/https://www.thrustcurve.org/motors/AeroTech/F30FJ/
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Hadn't thought about it much but will in future.
