Plywood Thrust Plate for High Power Rocket

[ghuber]

A question for the experts on this forum: Is there good guidance / experience on the motor impulse / rocket diameter for which a plywood thrust ring is suitable rather than needing a metal one?

The motivation for this question is a potential L3 design that is a 4" Fiberglass Body Tube with tube fins. I'd like to potentially build a removable motor mount so I can fly it on both 54 and 75mm motors. Because the design is tube fins (like my L1 and L2 designs) there are no through the wall fins to secure the centering rings and transfer the load to the body tube, and so I believe it would be wise to transfer the force of the motor directly from the motor to the body tube using a thrust plate.

For smaller rockets using F/G class motors I've built plywood thrust plates that sit on the bottom of stacks of interior centering rings (usually the stack is kept together with screws and/or epoxy). But when one gets to 4" and larger body tubes and higher impulse motors, it seems to me that the possibility that the plywood flexes/deforms and therefore allows the motor to pass through the thrust ring is too great. I could imagine cutting a thrust plate from 1/4" plywood and stacking on top 2 1/4" plywood centering rings to prevent deformation, but I'm not sure if others have guidance/experience here. Thank you.

 

[OverTheTop]

What about using an Aeropack-style motor retainer at the front, and taking all the thrust through that? That way the rear plate need to take very little "thrust" and can become mainly cosmetic. The front mount must, of course, be able to take the expected maximum load (peak) for the largest motor you will fly. This mount is positioned at the location where you will get the longest motor you will want to fly this on. Shorter motors are accommodated using spacers that go between the front of the motor and these mounts. Just make sure the rear thrust ring on the motor doesn't push on the rear plate when you screw it in.

If you are concerned about the strength you can make one up out of plywood and apply the full expected load, or a little more, to prove the strength. Make sure there is enough surface area to take the required force, calculated using the shear specification for the glue. I would use marine ply so you don't get any 'surprise" inclusions in the part. If you design with a greater safety factor this will allow you to use lower quality wood also.

FYI, this is how I design my HPR rockets. It actually moves the thrust much further forward in the rocket and reduces the buckling load on the airframe.

Motor ejection will complicate things using this method!

 

[ghuber]

Thank you. I had considered that option, but wasn't sure I wanted to do that for two reasons. First, I tend to fly DMS motors and wanted to be able to use the included motor ejection charges as a backup. Second, I wasn't sure if there Aeropack front-retainers could be used with casings/motors of different diameters.

 

[OverTheTop]

I have made spacers and used different sized motors in the past. The longitudinal spacer needs to be strong enough not to buckle, and the spacers for the motor to motor mount are effectively just like centering rings. They can be quit thin as they won't need to take any thrust. You could even just make some out of plywood and friction fit them . Only need one ring at the front and one at the rear.

Here is one I did a few years back that is quite light and just centers the 38mm motor in the 54mm bore.

 

[G_T]

The picture is of a 6" rocket, 83# on the pad when loaded with a small O motor. The plywood is a double layer of 1/2", so 1" total. There is structure on the inside to transfer load up through the tube and the fins. The trimmed fender washers on the bottom are for motor retention. When flying smaller motors, I slid a centering ring and allthread cage in with a base plate of quarter or 3/8" plywood, I forget. Bottom was painted with grill paint and needed refreshing after each flight depending on motor size. 114mm O, no problems. 88mm N, and the convection cooks off a layer of plywood. 75mm full M and smaller, not really a problem.

Plywood can work fine up to quite a bit larger than this! You just have to design for the loads, that's all. Make a structure that can withstand the max expected thrust with a healthy safety margin of 1.5 or even 2. And as always, inspect after each flight.

Plywood has been used in supersonic jets. Don't worry about it for our applications; just design accordingly.

You don't have to do it as complicated as I did! Mine was completely disassemblable.

Gerald

 

[kalsow]

Even though your fins don't use them, you could always put in the equivalent of 3-4 fin tabs that span the gap from motor mount to airframe tube. Even with a moderately long motor mount you can get quite a lot of contact surface to transfer forces.

Plywood is strong stuff.

 

[ghuber]

Thank you both for you advice. I plan to post a 3D mockup of a potential design to get feedback on. I would like the whole setup to be removable, but that may dreaming rather than reality.

 

[G_T]

Beyond a certain point, it is much less work to make two lower rocket sections than one section that can be disassembled.

Gerald

 

[AHansom]

I made a thrust plate for my 6" tube fin from a 1/4" centering ring and a 1/2" centering ring for a 7.5" rocket. First glued the front centering ring motor mount tube and the rear ring flush with the end of the tube with about a 1/2" of motor mount sticking out. Then glued the 7.5" centering ring to the flush centering ring. When the epoxy dried I used a router trim bit to finish it to the same outside diameter as the rocket. Thrust ring held up to an M2020. Tube fins not so much

 

[user 30299]

The pictures show the plywood thrust plate and centering ring assembly for my L3 rocket. That's a 5.5" body tube and 75mm motor tube with the Aero Pack flange-mount retainer. And I have the Aero Pack motor adapter so that I can also use a 54mm motor case. Sadly this has not flown yet because of the Covid restrictions. I'm using two 1/4" thick centering rings, and a 1/4" thick thrust plate. The rings and plate are glued together with Titebond wood glue, and then mounted to the body and motor tube with epoxy.