Motor mount design

[Azamiryou]

What are the design considerations for a LPR motor mount? What determines the length of the tube and placement of centering rings? I notice in kits the tube usually extends well past the thrust ring. Any reason for that? (I'm not talking about stuffer tubes, just an extra long motor tube.) Would moving the front centering ring to the front of the tube cause any problems?

Example from Estes Crossfire ISX. Why is the front CR 3/8” ahead of the thrust ring, and why is there another 1/2" of tube ahead of the CR?

 

[Pinetree]

In response to your original question, I don't think there's any harm in having the front centering ring near the top of the motor tube.

In response to the edited version of your question, I'm guessing that the rear ring is placed there so that there's enough room to move the motor mount clip out of the way to attach the motor.

Either way, it's not rocket science.

 

[Azamiryou]

I'm guessing that the rear ring is placed there so that there's enough room to move the motor mount clip out of the way to attach the motor.

I'm talking about the front ring.

Thanks for the reply. I like putting the ring at the front because I like baffles and it makes it easier to get crud out. But then I got to wondering why it's almost never at the front in kit instructions...

 

[Pinetree]

Once you get into the bigger kits, for example the Der Big Red Max, it is at the front.

 

[neil_w]

I usually put the front centering ring a bit back from the front of the tube because it makes it easy to fillet the front of the ring. Sometimes the front ring goes over the end of the motor hook to secure it. Also, having a little extra tubing at the front gives a bit of margin for error during the build.

But honestly, for LPR motor mounts it simply doesn't matter very much either way. If there's a specific reason why the Crossfire has that much tubing in front of the centering ring it eludes me.

 

[Pinetree]

Generally speaking, once you get into the higher power motors, you're going to have the recovery attachment point on the front centering ring, hence the need to move it toward the top of the motor tube.

I can't speak for the design of the Estes kits, but I really don't see how it matters a whole lot.

 

[Azamiryou]

Sounds like going forward I could just put the front centering ring the same place as the thrust ring, and cut off any extra tube sticking out in front of that. (Talking about the small rockets I usually fly, that is. I have a few with shock cords attached to motor mounts, but since I like baffles, I usually have a baffle to attach the shock cord to.)

 

[goose_in_co]

But honestly, for LPR motor mounts it simply doesn't matter very much either way. If there's a specific reason why the Crossfire has that much tubing in front of the centering ring it eludes me.

The only reason that I have seen for specific placement of centering rings on a LPR rocket is IF there are through the wall fin tabs that extend to the motor mount tube. In that case, they need to be out of the way of the fin tab for the fin tab to seat properly.

Otherwise I agree with Neil.

I have seen three centering rings before, one on the front of the tube, one in the middle to hold the motor clip, and one at the aft end with a slot so that the motor clip can move. But, it is cheaper to supply 2 centering rings instead of 3.

 

[bjphoenix]

For lpr I think the best compromise is to put the front ring right at the thrust ring. Any tube in front of that isn't doing anything. The motor mount has to resist motor thrust, it has to resist any force that happens when the rocket lands and the motor hits the ground first, it has to resist any force trying to pull the motor out if it is stuck, and it has to resist any backwards force from the ejection charge. I had a tube crumple during landing from impact force on the end of the motor, I've also had a motor mount pull out when I was trying to remove a stuck motor. Both of these are somewhat unusual situations, otherwise I don't think it matters a whole lot where the rings are placed.

 

[jflis]

Bingo! The further this forward ring is from the engine block, the greater the amount of unsupported motor tube is exposed which can result in damage/crimping on landing. If this tube crimps, but is not noticeable with visual inspection then you have a weakened, possibly bent motor tube for your next launch…

 

[Back_at_it]

I typically leave about 1/16" of tube exposed above the top ring. This creates a nice fillet at the top and help prevent glue from running over the tube and down into the motor tube.

Remember that Estes has kids in mind when they write these instructions and kids tend to get carried away with glue. And by kids, I mean ME

 

[mh9162013]

One thing I like about lower-power rockets is that you have more freedom in how you build your rockets. So if you want to use a baffle and push your top/front centering ring closer to the end of the motor tube, you should be fine here.

As for why the instructions were written as they were, I don't know. Could be a cut and paste job from another kit's instructions. So if the top/front centering ring placement isn't critical, might as well save time by cutting and pasting so you can start drafting/drawing the next step of the instructions.

 

[lakeroadster]

What are the design considerations for a LPR motor mount? What determines the length of the tube and placement of centering rings? I notice in kits the tube usually extends well past the thrust ring. Any reason for that? (I'm not talking about stuffer tubes, just an extra long motor tube.) Would moving the front centering ring to the front of the tube cause any problems?

Example from Estes Crossfire ISX. Why is the front CR 3/8” ahead of the thrust ring, and why is there another 1/2" of tube ahead of the CR?

It's standardization of motor mount tubes. From a company standpoint, minimizing the P/N's for motor mount tubes just makes sense.

But from a scratch build perspective, the entire top end of the tube could be removed.

 

[bjphoenix]

Bingo! The further this forward ring is from the engine block, the greater the amount of unsupported motor tube is exposed which can result in damage/crimping on landing. If this tube crimps, but is not noticeable with visual inspection then you have a weakened, possibly bent motor tube for your next launch…

Last year I built an 18-24 adapter so I could fly rockets like HiFlier XL on C5 motors in conditions of lower visibility. I used an 18mm tube, long enough to fit in an E mount, put a motor hook on it, and made some centering rings. It fit and worked well but after a couple of launches it crimped at the front. I built it because I wanted to, I didn't have to buy anything, and I didn't have to remember to use the orange spacers. I thought it had crimped because of thrust from the "super C" motor but later realized it was probably from the back end of the rocket impacting on landing. I was going to rebuild it, stronger, but I've decided I probably should just buy a stock of C11 motors instead.

I'm building an upscaled Sprint right now using the nose cone and tail cone from one of Estes BT60 nose cone packs. Since it has a tail cone the rear centering ring will be pretty far forward from the back end of the motor. I don't want the front ring too close to the rear ring so I'm putting it maybe an inch in front of the front end of the motor. I cut a 3/4" long slice of D12 motor casing to use as an engine block so the front centering ring is just a little bit off of the front of the engine block.