DynaSoar
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Here's how I made a positve motor retention system for my 29mm mach buster. It's also very suitable as a replacement for Aerotech's huge stiff engine hook.
1 Estes BT55 tube coupler
3 Estes 2.75" engine hooks (with the "Z" shaped bends in the bottom)
Thin CyA
A BT55 couple sides very nicely over a 29mm heavy wall body/motor tube with just enough slack. If you don't have a coupler, an inch or so of BT55, sliced down the side with a sliver removed so the result doesn't overlap itself when wrapped around the main tube is just fine (Probably better in fact, because it'd be lighter and thinner).
Mark the main tube 1/4" from the aft end. This will be where the aft end of the coupler goes.
Bend the top bend of the engine hooks backwards until they snap off. Flatten the end with pliers or a hammer.
Slide the coupler into place over the main tube.
Slide an engine hook between the coupler and tube, and push it up so that 1/4" is below the aft end of the tube (1/2" below the coupler). Mark the hook where it comes out of the forward end of the coupler. Do this for all three hooks.
Take the coupler off the main tube. Bend the hooks over 180 degrees at the mark, so they'll lay against the main tube, and bend OVER the coupler. Go ahead and pinch the bend down flat.
Slide a hook into place on the coupler (stick it through the coupler and pull the new bent over tab over the outside). Mark where it goes with a pen and remove it. Put a drop of thin CyA between the marks and pull the hook back into place. Repeat for the other hooks, 120 degrees apart. Once dry, the retainer is done.
Slide the retainer onto the main tube to check the fit. If too tight, take off, sand the main tube, and replace. Mark the main tube at the forward end of the retainer. Remove, paint the main tube with CyA, and replace. Paint CyA along the fore and aft joints of the coupler and main tube, all the way around. You're done. The diameter difference is so slight that an even fillet would be difficult, and unnecessary. I tried, with Fix-It epoxy clay. Wasn't worth the effort.
If you want to use epoxy for the main tube cementing (due to the reasonable worry that the CyA may grab before you get the coupler to the right spot), use only enough to paint a film on the cementing region, so there's not a glob pushed up when you slide the retainer on.
If you have the Estes engine hooks with the "Z" style bottom end, be aware that the bend that holds the motor and the bottom most bend in the hook are not the same; there's a 1/8" difference. Just make sure you're measuring things based on the part of the hook that'll hold the motor. Also, you can bend the very end of the finger tab down towards the aft, or even cut off the end with diagonal cutters, so it's more streamlined.
Weight: betwen 2 and 3 grams including all the CyA. Maybe 4 to 5 grams if you use epoxy.
Cost: If you bought it all from say, Balsa machining, $2.00 total.
50 cents for the coupler. If you don't have any surplus Estes 2.75" engine hooks laying around to use for free, I'm betting you could find someone who did. Same with a scrap of BT55 if you don't have a coupler.
Size considerations: This takes a bit more tube length, but no more diameter, than a Slimline retainer. It fits very well over an AT 29mm motor tube. It also fits very nicely between said AT motor tube and a PML "1.1" (29mm motor tube size) urethane tail cone. My money say this will handle any ejection charge the AT monster hook will handle, and is a lot easier to use.
Note: some places (like BMS) sell 2.75" engine hooks that are thinner than the Estes style with the "Z" shaped bottom. Their bottom bend is just a 90 degree angle bend. Given the tiny weight savings, I prefer the Estes hooks for this, because the "Z" bend makes it much easier to push an engine in, and because the greater amount of hook under the aft end of the motor makes it less likely that the engine could slip out. The "Z" bend would act as a spring and push back on the motor as it pushed down; the 90 degree bend hooks would only catch it. And be careful handling the hooks after you break/cut them: the broken end is sharp.
Also, this requires you to hold back three hooks simultaneously to get the motor out. It takes practice, or help. It shouldn't be easy, or it wouldn't work, but it's still easier than that stiff AT hook. And if you replace the AT hook with this, be aware your only engine block for thrust will be the thrust ring inside the motor tube. You might want to give it an extra epoxy fillet.
I tried to take a picture to post, but the cheap disposable camera won't focus well close enough to see the details. If I can find someone with a decent camera, I'll get some pics to post.
1 Estes BT55 tube coupler
3 Estes 2.75" engine hooks (with the "Z" shaped bends in the bottom)
Thin CyA
A BT55 couple sides very nicely over a 29mm heavy wall body/motor tube with just enough slack. If you don't have a coupler, an inch or so of BT55, sliced down the side with a sliver removed so the result doesn't overlap itself when wrapped around the main tube is just fine (Probably better in fact, because it'd be lighter and thinner).
Mark the main tube 1/4" from the aft end. This will be where the aft end of the coupler goes.
Bend the top bend of the engine hooks backwards until they snap off. Flatten the end with pliers or a hammer.
Slide the coupler into place over the main tube.
Slide an engine hook between the coupler and tube, and push it up so that 1/4" is below the aft end of the tube (1/2" below the coupler). Mark the hook where it comes out of the forward end of the coupler. Do this for all three hooks.
Take the coupler off the main tube. Bend the hooks over 180 degrees at the mark, so they'll lay against the main tube, and bend OVER the coupler. Go ahead and pinch the bend down flat.
Slide a hook into place on the coupler (stick it through the coupler and pull the new bent over tab over the outside). Mark where it goes with a pen and remove it. Put a drop of thin CyA between the marks and pull the hook back into place. Repeat for the other hooks, 120 degrees apart. Once dry, the retainer is done.
Slide the retainer onto the main tube to check the fit. If too tight, take off, sand the main tube, and replace. Mark the main tube at the forward end of the retainer. Remove, paint the main tube with CyA, and replace. Paint CyA along the fore and aft joints of the coupler and main tube, all the way around. You're done. The diameter difference is so slight that an even fillet would be difficult, and unnecessary. I tried, with Fix-It epoxy clay. Wasn't worth the effort.
If you want to use epoxy for the main tube cementing (due to the reasonable worry that the CyA may grab before you get the coupler to the right spot), use only enough to paint a film on the cementing region, so there's not a glob pushed up when you slide the retainer on.
If you have the Estes engine hooks with the "Z" style bottom end, be aware that the bend that holds the motor and the bottom most bend in the hook are not the same; there's a 1/8" difference. Just make sure you're measuring things based on the part of the hook that'll hold the motor. Also, you can bend the very end of the finger tab down towards the aft, or even cut off the end with diagonal cutters, so it's more streamlined.
Weight: betwen 2 and 3 grams including all the CyA. Maybe 4 to 5 grams if you use epoxy.
Cost: If you bought it all from say, Balsa machining, $2.00 total.
50 cents for the coupler. If you don't have any surplus Estes 2.75" engine hooks laying around to use for free, I'm betting you could find someone who did. Same with a scrap of BT55 if you don't have a coupler.
Size considerations: This takes a bit more tube length, but no more diameter, than a Slimline retainer. It fits very well over an AT 29mm motor tube. It also fits very nicely between said AT motor tube and a PML "1.1" (29mm motor tube size) urethane tail cone. My money say this will handle any ejection charge the AT monster hook will handle, and is a lot easier to use.
Note: some places (like BMS) sell 2.75" engine hooks that are thinner than the Estes style with the "Z" shaped bottom. Their bottom bend is just a 90 degree angle bend. Given the tiny weight savings, I prefer the Estes hooks for this, because the "Z" bend makes it much easier to push an engine in, and because the greater amount of hook under the aft end of the motor makes it less likely that the engine could slip out. The "Z" bend would act as a spring and push back on the motor as it pushed down; the 90 degree bend hooks would only catch it. And be careful handling the hooks after you break/cut them: the broken end is sharp.
Also, this requires you to hold back three hooks simultaneously to get the motor out. It takes practice, or help. It shouldn't be easy, or it wouldn't work, but it's still easier than that stiff AT hook. And if you replace the AT hook with this, be aware your only engine block for thrust will be the thrust ring inside the motor tube. You might want to give it an extra epoxy fillet.
I tried to take a picture to post, but the cheap disposable camera won't focus well close enough to see the details. If I can find someone with a decent camera, I'll get some pics to post.
