How Long does a motor mount/tube need to be?

[KilroySmith]

So the booster on the fiberglass rocket I'm building came with a 305 mm long, 54 mm motor tube. This appears to be great for any rational motor. Now, if I get irrational and want to see this rocket come down in a lot more pieces than it went up in, and decide to put an L1000 in it, is that enough support? The L1000 is twice as long, at 635 mm long. How much lateral and/or rotational support does the motor tube need to provide? Any thoughts?

 

[rharshberger]

The stock motor tube is long enough for a L1000.

 

[KilroySmith]

How short would be too short? Or does it really not matter because there simply isn't much rotational force on the motor? From a strength standpioint, there probably isn't enough length to put a third centering ring on it. Which brings up the question, how often do motor mounts fail under motor thrust?

 

[K'Tesh]

How short would be too short? Or does it really not matter because there simply isn't much rotational force on the motor? From a strength standpioint, there probably isn't enough length to put a third centering ring on it. Which brings up the question, how often do motor mounts fail under motor thrust?

Basically, it's there to hold the motor in place. If you don't have an internally mounted thrust ring, then it doesn't have to be long at all.

The answer to the mount failing under thrust question depends on a lot of factors, namely how well the builder constructs it. If it has TTW fins that are glued to the MMT tube, it's really hard for the mount to fail though. This is presuming, of course, that we're talking about normal launch situations and not a booster's cato damaging the sustainer.

 

[cerving]

A longer motor mount does have a couple of advantages, it acts as a stuffer tube and reduces the amount of volume in the body tube that the ejection charge has to pressurize. I like to use a motor mount that's slightly longer than the largest motor that I'm going to use, just for that reason. It also makes it easier to do internal fillets on the fins if you use three centering rings, one in front, one just ahead of the fin roots, and one at the back (which you glue in with the motor retainer AFTER you fillet the fins).

 

[NateLowrie]

A longer motor mount does have a couple of advantages, it acts as a stuffer tube and reduces the amount of volume in the body tube that the ejection charge has to pressurize. I like to use a motor mount that's slightly longer than the largest motor that I'm going to use, just for that reason. It also makes it easier to do internal fillets on the fins if you use three centering rings, one in front, one just ahead of the fin roots, and one at the back (which you glue in with the motor retainer AFTER you fillet the fins).

+1 for the longer motor tube that acts like a stuffer tube, IF you are using that compartment for housing recovery gear. A lot of the kits have you tie the shock cord to an eye bolt on the centering ring right above the TTW fins. This can create a packing problem with a longer motor where the parachute or recovery harness becomes stuck between the motor and the outer airframe. A longer tube with a third top centering ring can alleviate the issue by moving that attchment point above the motor. Of course, this isn't an issue don't use the space for recovery gear.

As Cris mentions it's also easier to do the internal fillets on the fins if because you can leave the back CR off and still keep the MMT centered. I still tend to prefer epoxy injection, especially in tight spaces, but that method works.

 

[KilroySmith]

Thanks for the info, guys. I appreciate it.

I hadn't considered the problem of recovery gear getting stuck between the motor and airframe; haven't had that problem personally, but I can see it happening under 30G of acceleration...

This is another step on my quest to understand how strong the various parts of a rocket NEED to be, as opposed to how strong (and heavy) I can make them. With the wide range of construction quality I've seen, if motor mount failures are uncommon, especially with TTW fins, that gives me some insight on the stress v. strength of that area.

 

[NateLowrie]

Thanks for the info, guys. I appreciate it.

I hadn't considered the problem of recovery gear getting stuck between the motor and airframe; haven't had that problem personally, but I can see it happening under 30G of acceleration...

This is another step on my quest to understand how strong the various parts of a rocket NEED to be, as opposed to how strong (and heavy) I can make them. With the wide range of construction quality I've seen, if motor mount failures are uncommon, especially with TTW fins, that gives me some insight on the stress v. strength of that area.

Your main source of strength with the motor mount and the fins on a TTW design is going to be on the attachment of the fins to the motor tube. It generally has 2 functions: 1) stiffen the fin to reduce flutter and 2) strengthen the motor mount attachment. The most important thing about TTW fin mounting is the internal fillets on the fin to motor mount connection. Be sure the fin is resting solidly against the motor mount along the entire length and you have a good fillet along that edge. You can also use some 1/4" or milled fiberglass in the epoxy to strengthen that joint further.

 

[ksaves2]

A longer motor mount does have a couple of advantages, it acts as a stuffer tube and reduces the amount of volume in the body tube that the ejection charge has to pressurize. I like to use a motor mount that's slightly longer than the largest motor that I'm going to use, just for that reason. It also makes it easier to do internal fillets on the fins if you use three centering rings, one in front, one just ahead of the fin roots, and one at the back (which you glue in with the motor retainer AFTER you fillet the fins).

Ummmmm, that too plus a long motor tube gives one the chance to salvage a cardboard rocket from a mishap with an intact fincan. I have two rebuilds on a cardboard 4" rocket. First one was single deploy with ejection charge failure. On the death dive, the recovery laundry fell down the tube and pushed the nosecone off while about 100 feet in the air. Zipper of course occurred but the rocket was knocked sideways and landed flat. Fincan was intact, cut it off and made a DD rocket out of it.
Flew the heck out of it until this happened: [video=youtube;hMvT9BKzbUg]https://www.youtube.com/watch?v=hMvT9BKzbUg[/video] (Posted before)
Surprisingly, since I had a very long stuffer tube, even though the sustainer hit nose first, plenty of fincan left to do another coupler repair. The fins survived intact. I'm going to use some light glass cloth to re-enforce the replacement tube and
perhaps avoid a future blowout. Frame by frame is cool to watch. Kurt

 

[KilroySmith]

Your main source of strength with the motor mount and the fins on a TTW design is going to be on the attachment of the fins to the motor tube. It generally has 2 functions: 1) stiffen the fin to reduce flutter and 2) strengthen the motor mount attachment. The most important thing about TTW fin mounting is the internal fillets on the fin to motor mount connection. Be sure the fin is resting solidly against the motor mount along the entire length and you have a good fillet along that edge. You can also use some 1/4" or milled fiberglass in the epoxy to strengthen that joint further.

There are a lot of interesting dynamics and forces in the whole motor->motor tube->(CR + TTW fin)->body tube system. I don't have the background to calculate them, which is kinda why I'm asking, hoping that someone has some good information. For example, in a design with CR's but no TTW, the glue joint between motor tube/CR, and CR/body tube, has both a shear force as well as a peel force (due to bending of the CR) happening simultaneously. With TTW, the peel force is essentially removed (because there won't be CR bending), and the shear force is reduced due to having much more area transferring motor force to the body tube.

Fortunately, in most designs, it's easy to add a huge amount of strength to the whole fin can assembly. Filleting both sides of CRs, gluing/filleting fins in 5 locations (external body tube, internal body tube, motor tube, upper CR, lower CR), supporting engine retainer by mounting it flush against the rear CR, etc, all add huge amounts of strength, but can also add a lot of weight. How much of this is really necessary, and how much is there just to give us warm fuzzy feeling about building it bulletproof? I know there isn't an absolute answer here, but I'm trying to gain a lot of wisdom without having to go through the pain of the bad experiences...

 

[markkoelsch]

Realistically, I think we could build our rockets without a motor tube. Fin tabs cut to length to but against the motor.

Certain types of construction I.e. Injectable fillets would be out, but there would be likely be weight savings.

 

[MClark]

I built a few with no mmt and the motor held by the centering rings.
Notch the rings to lock the fins.
Done to save weight

M

 

[NateLowrie]

Realistically, I think we could build our rockets without a motor tube. Fin tabs cut to length to but against the motor.

Certain types of construction I.e. Injectable fillets would be out, but there would be likely be weight savings.

I think you definitely could do that. I would use an aluminum thrust plate or similar stepped centering ring that directly transfers the load to the outside airframe.

 

[markkoelsch]

I think you definitely could do that. I would use an aluminum thrust plate or similar stepped centering ring that directly transfers the load to the outside airframe.

Not my thought. Centering ring over the fin tabs. Motor abuts the centering ring and transfers load to centering ring and fin tabs.

See Dave Trianno/ Shadow Aerospace Trinity rocket.

 

[rharshberger]

I think you definitely could do that. I would use an aluminum thrust plate or similar stepped centering ring that directly transfers the load to the outside airframe.

+1 this, use a thrust plate to transfer thrust directly to the airframe then the fillets in the mmt can be lighter, and the stresses on the motor tube fillets and centering ring fillets is greatly reduced.

 

[Nick@JET]

At the bottom of the airframe you can see the thrust plate that they are referring to (Anodized red alum)


Also this motor tube is 21" but can accept a 6xl which is 48" long

 

[KilroySmith]

This is great stuff, guys. Has anyone ever (that's a long time, I know, but I've only been doing this a couple of years) measured the strength of well-built CR->Motor tube joints? There's a huge number of variables there, but I think even one set of measurements would be excellent. We already have a lot of heuristics in this hobby; it'd be nice to have one of "MM Tube->CR strength is about yy N / mm for cardboard, xx N/mm for epoxy/fiberglass" to answer this kind of question. Yes, I could do the work - don't have time at the moment - but if someone else has already done it, I'd love to use their efforts. /frank

 

[bobkrech]

from the webarchives. https://web.archive.org/web/20130916191702/https://www.rocketmaterials.org/index.html

Real rocket material test data from a now defunct website....

 

[KilroySmith]

That's great info. For my specific question, the strength of fin cans varies all over the place depending on the builder. I don't think you could come up with the heuristic that I was hoping for. Oh well, that probably explains why there isn't one...

Thanks for that link.

/frank

 

[NateLowrie]

That's great info. For my specific question, the strength of fin cans varies all over the place depending on the builder. I don't think you could come up with the heuristic that I was hoping for. Oh well, that probably explains why there isn't one...

Thanks for that link.

/frank

Also note that the strength of the fin can is highly dependent on how the builder processes the epoxy the an composites. The same fin can can have dramatically different strength properties depending on surface preparation, epoxy and material handling. https://www.rocketryforum.com/showt...what-your-government-doesn-t-want-you-to-know is a good resource on properly bonding composites.