At what point are wood engine mount rings a good idea?

[Bill S]

I'm contemplating upgrading a kit I will be working on, and wanted to ask if it would be a good idea to upgrade the engine mount. The rocket is one of the Estes Longship (a new kit), that is set up for up to E12 motors, and has 3 cardstock motor mount rings. I may be using either Aerotech E30s, or possibly an F44, and I'm not sure if the existing mount would be able to take the increased thrust of the Aerotech motors. What would be an reccommended upgrade to ensure that the mount doesn't rip free of the tube? The tube looks to be an BT-55, and I wasn't sure if using the typical thick cardboard green rings would be okay or if I needed to use something stronger.

Thoughts?

 

[prfesser]

Hi Bill,
I think it would be ok with just the stock rings, but an easy improvement if you're concerned would be to cut rectangular "struts" or gussets that are as long as the distance between rings, and as wide as the space between MMT and airframe. You have three rings, so I'd cut eight gussets, four between rings 1 and 2, and four between 2 and 3. 1/16" balsa or reasonably thick cardboard will do just fine. Glue each gusset to the rings and to the MMT.

FWIW the motor mount in Apogee's 1/70 Saturn V uses just cardboard rings and gussets like this. It flies on a G80, the MMT is 29 mm and the airframe is 5.5" in diameter, so there's a large span between airframe and MMT. And it's a heavy rocket.

Best -- Terry

 

[neil_w]

The tube looks to be an BT-55, and I wasn't sure if using the typical thick cardboard green rings would be okay or if I needed to use something stronger.

The green cardboard rings are ridiculously strong. It looks like the ones in the kit, though, are regular laser-cut cardstock.

Are they strong enough for an F44? I don't know. However, since I have always have a pretty good supply of 5055 plywood rings around, I'd probably make the swap. In fact, if I were really planning to fly on motors like that, I might even sub in a piece of BT50H for the motor mount tube (would have to see if the nozzle could be made to work).

For my own taste, though, I don't think an F44 would be most pleasing engine for that kit. I think an E15/E20 would be perfect (I really like those, what can I say). I would like to actually see it launch.

 

[scsager]

I like to think of the Aerotech F44 motor as an E+ motor. A "full" E motor is 40 newton seconds total thrust. The F44 is an "F" class motor because it exceeds 40 NS total thrust. The F44 has 41.45 NS total thrust. This is a "baby" F-motor.

Don't Let the "F" scare you into thinking this is "crazy" powerful.

One other thing is the 24mm motor mount in a BT55 body tube. The strongest rings will be the stock rings. Good construction techniques and a liberal amount of glue are your friend. Wood rings will NOT be any stronger in this application.

Wood glue will be plenty strong - but tends to "GRAB" on motor mounts like this. You might consider substituting hobby-store epoxy for the motor mount to avoid the "Grabbing" problem. 5-minute or 15-minute epoxy will work fine.

 

[Bill S]

Neil, you may be right that the E20 might be a better option. I failed to notice that the E30s have a shorter burn time. I had been looking at them because of the Blue Thunder propellant - I was under the impression that the White Lightning E20s might be harder to ignite. I am just going on what I have read here, as I haven't worked with Aerotech composite motors yet (I have some D10/D21 motors that I haven't used yet).

Prfesser, I do like the idea of using ribs between the motor mount rings. I'd have to weigh everything first though - I would prefer to keep the weight of the modified motor mount and Aerotech motor lower than the weight of the original motor mount and Estes E12. Which looking at the weights involved, it should work out okay.

 

[neil_w]

Neil, you may be right that the E20 might be a better option. I failed to notice that the E30s have a shorter burn time. I had been looking at them because of the Blue Thunder propellant - I was under the impression that the White Lightning E20s might be harder to ignite. I am just going on what I have read here, as I haven't worked with Aerotech composite motors yet (I have some D10/D21 motors that I haven't used yet).

I haven't yet had a problem lighting an E15/E20, but I've only tried to fly them fairly new. I now have a few that are several years old, and I'm going to be watching them closely. To be sure, the E30 should be easier to light. I have some in my box waiting to fly... one day.

Prfesser, I do like the idea of using ribs between the motor mount rings. I'd have to weigh everything first though - I would prefer to keep the weight of the modified motor mount and Aerotech motor lower than the weight of the original motor mount and Estes E12. Which looking at the weights involved, it should work out okay.

Almost anything is lighter than an E12. Motor mount ribs would weigh almost nothing.

 

[David Schwantz]

As Terry said, they are strong. But 1/8th" lite ply is a good choice. You can always cut lightning holes also.

 

[caveduck]

You can just order some of the fat 0.25" long BT-50 to BT-55 rings from Semroc/eRockets: https://www.erockets.biz/semroc-centering-rings-bt-50-to-bt-55-split-kraft-wound-2pk-cr-50-55s/. For another upgrade while you're at it, you might consider some of the Semroc .021 heavy wall motor mount tube BT-50mf https://www.erockets.biz/semroc-body-tube-t-50mf-8-1pk-sem-bt-50mf-8/ . If you do that you'll need different rings for BT-50H to BT-55 size, which Semroc also has - laminate two with 5-min epoxy.

 

[jqavins]

Hi Bill,
I think it would be ok with just the stock rings, but an easy improvement if you're concerned would be to cut rectangular "struts" or gussets that are as long as the distance between rings, and as wide as the space between MMT and airframe. You have three rings, so I'd cut eight gussets, four between rings 1 and 2, and four between 2 and 3. 1/16" balsa or reasonably thick cardboard will do just fine. Glue each gusset to the rings and to the MMT.

Interesting. I've never heard of that one before.

I like to think of the Aerotech F44 motor as an E+ motor. A "full" E motor is 40 newton seconds total thrust. The F44 is an "F" class motor because it exceeds 40 NS total thrust. The F44 has 41.45 NS total thrust. This is a "baby" F-motor.

Don't Let the "F" scare you into thinking this is "crazy" powerful.

It's not the F that might be cause for concern, it's the 44 Even it were a full F, that would only mean 44 N average thrust for a longer time; it's force that (might possibly) break a motor mount, not impulse. Even the average thrust is less important than the peak thrust, so one should look that up if one is concerned.

And here, I wouldn't be concerned.

In general, I'd be more inclined to worry about increasing the thrust when the rings cover a large span. BT50 to BT55 is so small that I don't think you have thing one to worry about. If it were BT50 to BT80 I'd think twice.

 

[Alan15578]

You should be fine with the cardboard centering rings. The motor thrust is not the real issue. However, you could tear up the rings when friction fitting or removing a stuck motor. If you have a cato or hard landing, you could also damage the rings. If you are using shear pins to keep the nose on, make sure that it blows off before the engine mount lets go. If you just want to make the rocket more robust and you don't care about the weight, use those heavy CR rings or plywood.

 

[Bill S]

I ended up putting in 8 3/32" gussets in between the centering rings, and will be gluing it in place with 15 min epoxy. Here is what I ended up with:


That mount, if I get some in the tube where the gussets are, shouldn't go anywhere, I would think.

 

[jqavins]

Good God, man! Are you trying to go LP to the moon?

 

[Bill S]

Nah, but I do plan to try out some Aerotech E20s, maybe something a little more powerful. I already had one rocket (Estes Vapor) fall on its tail twice now due to parachute hanging up problems, and the second time the motor mount came loose - it was with a similar setup (3 cardboard rings in an BT-60 tube). I don't expect the Longship to have that happen to it, but the extra insurance won't hurt any.

 

[lakeroadster]

Don't forget to run a simulation so you know how much nose weight to add.......

In regard to the motor mount... simply making the lower ring out of 1/8" basswood ply would have sufficed. That could have been slid over the existing mount as a retrofit. The plywood bulkead transfers the thrust loading into the BT-60, at which point the only purpose of the other centering rings is... well... centering.

 

[o1d_dude]

All cardboard. BT60 tubing.

Scratch-built Larry Brand tubefin.

Boosts on E30’s and gets out Dodge in a hurry.