Relative utility and necessity of motor mount tubes

[bad_idea]

On another thread, I posed a question about how necessary motor mount tubes really are. I'm starting a new thread here rather than continue to derail that one.

It's drifting off topic a bit, but I have wondered about this for a while. Why do most rockets (aside from min. di.) use motor mounts? I could see where it would brace the inside of the centering rings, but I wonder how important that is on lower power rockets, or even if the centering ring fits a larger motor tightly enough. Is it just hobby-wide inertia, or is there a logical reason?

I've thought about dispensing with the motor mount in a low/mid power build as an experiment to see if I learn anything the hard way, but that's a good way from the top of my project list at this point.

@Steve Shannon and @GlueckAuf posted thoughtful replies as follows:

People have just used centering rings, without a motor mount tube, for high power rockets. Personally I like having the fin tabs epoxied to the motor mount tube on TTW construction. Then the fin tabs, centering rings, and MMT all form a pretty strong structure to withstand thrust, but an argument could probably be made that the upper and lower centering ring on either side of the fin tabs makes a pretty strong structure.
Try it. That’s not a bad experiment.

In non-minimum diameter rockets, the motor tube distributes the motor's thrust evenly along its full length to all the centering rings (I use 3 here), to the fin tabs, and in turn to the slots in the air frame.

View attachment 489855

Remove the motor tube (sizing the centering rings accordingly) and the motor's thrust ring concentrates thrust against the aft-most centering ring alone, unless another manner of better distributing that thrust is thought up and applied.

View attachment 489856

Thus, the motor tube adds great strength to the rocket at little cost in added weight. It helps to seal off the aft end of the rocket so that ejection charge pressure isn't bled off inefficiently. And, as I did with the design illustrated here, a longer-than-usual version of it can be employed as a stuffer tube to allow reducing the rocket's pressurized volume, and hence the quantity of ejection charge media that would otherwise be required to adequately pressurize the air frame to exit the recovery gear.

Some talented designers and engineers have developed safe, workable alternatives to the traditional way our sport has adopted the motor tube to elegantly solve the problem of distributing thrust as efficiently as possible to the entire rocket. But the motor tube seems to perform this task pretty efficiently, and the parts makers provide us a wide variety of sizes of centering rings, tubes, and related hardware like motor retainers to make it an easy method to employ.

I agree with you both. Motor mount tubes are a useful solution to several problems at once, I've just wondered if they're necessary as often as they're used. Especially since most rocketeers (myself included) have a tendency to overbuild.

In a rocket that doesn't have a lot of empty volume forward of the motor for instance, the stuffer tube approach isn't needed as much. For a low-thrust motor or one that ends with a full diameter plate behind the body tube instead of inside it, thrust distribution isn't as important a function. For a slower or slower accelerating rocket, boxing all parts to make the strongest possible structure may also not be a priority. And no doubt there are other reasons to use them or not use them.

I'd guess that most rockets have some use for them, but I'd also guess that 99% of rockets that aren't minimum diameter have them, which has made me wonder. To this point, I have used them when not going min. di., but thinking about it some more, some experimenting may be working its way up the aforementioned project list.

Thank you both for the detailed replies!

 

[kuririn]

99% of low power rockets are symmetrical and the thrust line is centered along the vertical axis.
The motor mount keeps the motor centered and prevents it from moving forward during ascent (thrust stage) and moving backward during ejection firing.
The cheapest and simplest way to accomplish this is with a paper tube, paper centering rings and a paper thrust ring.
It's economics, good fellow.
Ta ta.

 

[cwbullet]

Motor mount tubes are not needed and can be removed. You just have to shrink the centering rings and reinforce them.

 

[bobbyg23]

Just curious as to why you would want to do away with the mm tube?

 

[BigMacDaddy]

I am 3D printing motor mounts w/ ABS plastic for most of my scratch builds so no tubes here (that would just be extra weight if printed). I usually print a centering ring/engine block for top side of engine and incorporate a quarter-twist retainer cap in the bottom of body tube (I use the retainer cap to hold / center the motor on the bottom). I am just using Estes engines so having a top engine block is convenient.

To glue the top mount in place I wrap a bit of plastic wrap around an engine (so glue does not stick to engine accidentally), load engine into the top motor mount, put some wood glue inside tube per normal mounting approach for centering rings, slide top mount on motor into the body tube, and assemble the whole motor assembly with the motor inside to get spacing correct. Need to wait for glue to dry before you remove the engine. I know wood glue is not the best for adhering plastic to cardboard but I like that the wood glue creates a ridge on top of the mount.

 

[bad_idea]

Just curious as to why you would want to do away with the mm tube?

It's less about doing away with it than seeking to understand how and why it is used to understand when it is or isn't needed.

To use another common part as an example, I don't usually put motor blocks in rockets (all low- and mid-power to this point), choosing instead to add tape thrust rings to Estes motors yet maintain the ability to use longer motors at will. An upcoming build will use 13mm Estes motors. As all the 13mm motors are the same size, there is no reason to go to the trouble of tape thrust rings, so this rocket will use motor blocks.

Another example from the same design: having lost a few motors to retroejection, I don't normally trust friction fit motors so use some other means of motor retention*. This rocket will use a cluster of 13mm A10-PT motors around a larger central motor that will provide motor ejection. Being plugged, the A10-PT won't have a strong tendency to bail out backwards, so I will likely simplify retention and go with friction fit.

There are plenty of cases where a motor mount makes sense. In better understanding the edge cases where it doesn't, I hope to better my future designs. To return to my examples, if I didn't have an understanding of how and why engine blocks and friction fit were used, the design of the rocket would be very different.

* When my neighborhood rocket field reopens in the spring, I intend to test tape retention until I get it right, but my current experience with it is insufficient.

 

[Jay Rairigh]

It's less about doing away with it than seeking to understand how and why it is used to understand when it is or isn't needed.

It is never really 'needed'. Hell, you can glue your motor into your rocket if you want to.

A typical motor mount assembly consisting of: centering rings, some form of thrust ring, some type of motor retention, AND a tube to hold all of these things makes it very simple to put in a motor, fly it, remove spent motor, add a new motor and fly again.

I am making something very, very simple sound way too complicated. If you have a better way, then just do it.

 

[BigMacDaddy]

Couple of thoughts about possible benefits of motor mount tube:

• Makes it easier to build your whole motor mount outside body tube (e.g., spaces front centering ring and engine block correctly).
• Can provide structure / strengthen fin cans for through-body fin tab designs (also assures fin tabs do not interfere with engine insertion).
• Makes it easier to insert motor (guides motor through front centering ring.

 

[bad_idea]

I know that some find this sort of question silly, so I appreciate all of the thoughtful replies. There are a few things here I'd not thought about nor seen written (e.g., ease of insertion past the upper centering ring).

 

[jqavins]

Couple of thoughts about possible benefits of motor mount tube:

• Makes it easier to build your whole motor mount outside body tube (e.g., spaces front centering ring and engine block correctly).
• Can provide structure / strengthen fin cans for through-body fin tab designs (also assures fin tabs do not interfere with engine insertion).
• Makes it easier to insert motor (guides motor through front centering ring.

Beat me to some of this. The first and last bullets especially. Expanding a bit on the first, you can make sure the centering rings are parallel to one another, which is good even if it's not critical. The tube makes a convenient handle for inserting the rings, although yes, there are other ways. If you're using an engine block, where else you gonna put it?

And finally, there is surely an element of inertial to it; centering rings that take standard tube sides in their holes are off the shelf products.

 

[bad_idea]

centering rings that take standard tube sides in their holes are off the shelf products.

That last one is a big one. Had to make two different sizes of centering rings this weekend (both for motor mounts, as it happened). Had I been building without a motor mount, I'd have even more custom dimensions to get right.

 

[bjphoenix]

It's less about doing away with it than seeking to understand how and why it is used to understand when it is or isn't needed.

Also remember that in the beginning motors were retained via friction fit, so the friction was between the motor and the inside surface of the motor mount tube. If you use a different retention method such as metal clip or screw-on retainer these are most easily mounted to a tube. All of this can be done other ways, even if you use a tube for a motor the surrounding structure can be done different ways.

 

[RocketRev]

To use another common part as an example, I don't usually put motor blocks in rockets (all low- and mid-power to this point), choosing instead to add tape thrust rings to Estes motors yet maintain the ability to use longer motors at will. An upcoming build will use 13mm Estes motors. As all the 13mm motors are the same size, there is no reason to go to the trouble of tape thrust rings, so this rocket will use motor blocks.
* When my neighborhood rocket field reopens in the spring, I intend to test tape retention until I get it right, but my current experience with it is insufficient.

At one time, I think that there were a bunch of different length motors available in the 13mm motor diameter category. Many of them were designed primarily for contest use and who knows when the motor manufacturers might bring them back. So you might want to revise your thinking on "all the 13mm motors are the same size." They may 'currently' be all the same size. I don't claim to know, but they didn't used to be and who knows the future?

Brad

 

[RocketRev]

Another example from the same design: having lost a few motors to retroejection, I don't normally trust friction fit motors so use some other means of motor retention*. This rocket will use a cluster of 13mm A10-PT motors around a larger central motor that will provide motor ejection. Being plugged, the A10-PT won't have a strong tendency to bail out backwards, so I will likely simplify retention and go with friction fit.

* When my neighborhood rocket field reopens in the spring, I intend to test tape retention until I get it right, but my current experience with it is insufficient.

If you're going with a cluster, all the more reason for using MMT's. Plus the fact that friction fitting motors can work very well with an MMT, but I can't see how to do friction fitting without an MMT.

Brad

 

[bad_idea]

If you're going with a cluster, all the more reason for using MMT's. Plus the fact that friction fitting motors can work very well with an MMT, but I can't see how to do friction fitting without an MMT.

Everything I have on the drawing board or under construction now uses motor mounts. I was simply using friction fit vs. external retention as an example of things that might not have been obvious without some understanding of how and why parts are used.

 

[jqavins]

At one time, I think that there were a bunch of different length motors available in the 13mm motor diameter category. Many of them were designed primarily for contest use...

Hmm, I'd like to read more about that. I do remember that when small motors came put, Estes went with 18 mm by, um, something shorter than 70, and Centuri went with the 13 by 45 mm that's in use now; I guess Estes switched over when they bought Centuri.

 

[bad_idea]

Thrustcurve shows a few OoP 13mm motors in longer lengths, and no doubt there were others. The 13x83mm Apogee C6 must have been a fun one to fly.

I'm not too concerned about longer motors coming along in 13mm. If they do, that will just be a good excuse to build another rocket.

 

[jqavins]

The 13x83mm Apogee C6 must have been a fun one to fly.

Wow, you said it, brother. That is was one hot little engine!

 

[Funkworks]

... how necessary motor mount tubes really are...

It's easier to slip a motor in a tube than in a series of rings. I can imagine how annoying it could be to target the 2nd ring once the motor is halfway through the 1st ring. The first customers were kids barely old enough to hammer a nail.

 

[neil_w]

It's easier to slip a motor in a tube than in a series of rings. I can imagine how annoying it could be to target the 2nd ring once the motor is halfway through the 1st ring. The first customers were kids barely old enough to hammer a nail.

I think the bottom line is that a motor mount tube solves a bunch of small problems and/or irritations in one very simple step, at extremely low cost in dollars or weight. All those little problems can be solved other ways, but except in unusual situations why bother?

 

[RocketRev]

Thrustcurve shows a few OoP 13mm motors in longer lengths, and no doubt there were others. The 13x83mm Apogee C6 must have been a fun one to fly.

I'm not too concerned about longer motors coming along in 13mm. If they do, that will just be a good excuse to build another rocket.

Good point. There never are enough excuses for building more rockets! LOL!

Brad

 

[mbeels]

Already some good answers here, but my 2 cents:

It allows for a variety of motor retention methods such as: spring clip (LPR), Aeropack, friction fit (any)

It lets you put the centering rings in convenient places (such as for through the wall fins) and not worry about motor length. For example, your upper centering ring could be 10" from the bottom of the rocket, but you might want to use a 5" long motor.

 

[tjkopena]

One point that I don't think has come up in this thread is that an MMT provides some heat insulation. For a counter-example, my R6 kit does not have a traditional MMT. Instead there's a 3D printed ABS part that serves as motor tube, motor block, shock cord anchor, launch lug, and fin alignment jig all in one. This works well for a quick-build micro-rocket. But even for the tiny MMX motors, that hunk of ABS expands and doesn't quite settle back to its original dimensions. Takes more flights to be noticeable than I'd expect most people to fly a single MMX rocket. We've only really noticed it on a pair of rockets that we've flown quite a lot, one specifically doing repeats to test a launch system a friend's been developing. Not a big deal, the motors just get a bit tighter to insert & remove over time, and a quick scrape with a file restores the fit. But I'd expect similar materials with larger motors to be more impacted. Having an MMT in there would insulate those components.


R6 kit. Fin canister in red.


Many (most? all?) of the Estes ready-to-fly or almost-ready-to-fly kits also do not use MMTs. Those that I've seen have a forward centering ring and some combination of a rear ring or short tube and the retainer cap. They solve the problem of aligning the motor to enter the forward ring during insertion with small ramps spaced around the ring opening, which also serve to strengthen it.


Estes Moon Dog. The retainer cap, not shown here, provides aft centering.


Estes Spirit. Aft centering is via a short tube/thick ring that is also where the retainer cap screws on.


Another approach that I've played with but not had reason to use are lengthwise centering tabs instead of rings. No motor tube and no centering rings, just a couple ribs running inside the body tube the length of the motor. Slightly more friction for fit & retention will be generated against such lengthwise supports than a couple centering rings. They also address the problem of aligning the motor to go into a forward support by never having it unguided. A closely related example to visualize this are Dynasoar's gliders, all of which that I've seen use centering tabs instead of rings. They utilize an MMT but a similar design could hold the motor directly, probably with different materials. I'm assuming part of the role of the MMT here is heat insulation protecting the foam, similar to the point above, in addition to being easier to assemble outside the body tube, etc.. Check out the instructions for their Aurora Clipper, from which this pic is taken:


Motor mount for Dynasoar's Aurora Clipper.

 

[cerving]

Just curious as to why you would want to do away with the mm tube?

It saves weight, in the part of the rocket that you usually don't want excess weight. For "some" rockets that may be a big deal... I'm thinking some of the small Mach 1 FG rockets, where the motor mount tube is a significant fraction of the total weight. For a 4" rocket with a 75mm motor mount, it probably doesn't matter very much... and it gives you something to attach the fin roots and motor mount to.

 

[Rocketjunkie]

Wow, you said it, brother. That is was one hot little engine!

I've flown a few of these I have also made 13 mm C20-12 and 13 mm D40-13 motors. Picture is a D40 and contents.
Top casing, liner. Next row forward, rear closures, nozzle, O-rings. Bottom, 4 1" long propellant grains and a 3/4" delay grain, same propellant.