Motor Tube Length Question

[Co-leader of the LRP]

I have been wondering about the length of motors vs the length of the motor tube,e question being can you fly a motor that it shorter than the tube in it? e.g. Estes 24mm motors have 2 sizes, the D12 being 7cm long and the E12 being 95mm long. If I had a motor mount with 96mm between the engine hook and the thrust ring, could I only fly the E12s in it, or could I fly the D12s in it as well. Could there be some sort of adapter tube that is put at the end of the D12 between it and the thrust ring? Keep in mind that I'm just using these motors as an example, and am not asking specifically about these ones for your answers. I'm just asking about motors in general.

Edited because I was incorrect about the sizes of the motors and so I changed that.

 

[rcktnut]

You can do that, it's been done for years and years that way. You could use a spent motor cut to size, or the tube that comes with the kit to place the motor block /thrust ring in the correct spot, if they do it that way yet?? Haven't built one of them in years.

 

[KilroySmith]

Yes. In fact, it's very common when using 24mm "D" engines (The Estes D12 is about 7 cm long; the Estes E12 is about 9.7 cm long) to install a spacer above the engine so that the nozzle ends up in the right place.

 

[Co-leader of the LRP]

You can do that, it's been done for years and years that way.

Well, I didn't know, and that's why I'm asking. I only got into rockets last august and I've only ever flown 18mm A and B motors which are all the same length.

 

[afadeev]

I have been wondering about the length of motors vs the length of the motor tube,e question being can you fly a motor that it shorter than the tube in it? E. G Estes 29mm motors have 2 sizes, the E12 being 95mm long and the E16s and F15s being 114 mm long. If I had a motor mount with 115 mm between the engine hook and the thrust ring, could I only fly the E16s and F15s in it, or could I fly the E12s in it as well. Could there be some sort of adapter tube that is put at the end of the E12 between it and the thrust ring?

Yes, and there are at least two (2) ways of dealing with this situation.

The length of the motor mount tube (MMT) is not really an issue.
The key is how you transmit the motor's thrust to the rocket, and secure the motor to the MMT.

1. Traditional low-/mid-power Estes instructions call for gluing a "thrust ring" inside the motor mount, and you would glue it far enough to accommodate the longest motors. Then add spacers for shorter motors (usually included in kits), to keep them from moving too far forward and burning the aft end of the motor mount.
2. A more versatile approach, and one that works with all Estes and non-Estes motors, is to omit the "thrust ring" altogether. Instead, apply a 1/8-1/4" wide layer of tape to the aft end of the motor. Insert the motor, and observe how the tape "ring" transmits the motor's thrust to the aft end of the motor mount. This is how all single-use and re-loadable Aerotech / CTI / Loki motors are constructed. This will allow you to fly motors of all lengths, shapes, and impulses in the future.

I always go with option #2, and pair it with a quality motor retainer, similar to those that can be found here:
https://www.rocketarium.com/Build/Motor-Retainers

A proper retainer does a much better job of keeping the spent motor in the MMT during firing of the ejection charge. It is also easier to manage then an Estes hook, and looks way better.

In the early days, I had lost a few smaller re-loadable motors that were not retained securely enough by Estes hooks. Over-energetic ejection charge events would blow the motors out of the tail end of the rocket. That's neither safe, nor inexpensive.

Good luck!

 

[KilroySmith]

Well, I didn't know, and that's why I'm asking. I only got into rockets last august and I've only ever flown 18mm A and B motors which are all the same length.

No problem; easy questions get easy answers. Welcome.

I've always been partial to the Estes Big Daddy as a first D or E engine rocket. Here's a picture:
https://estesrockets.com/product/002162-big-daddy/
If you scroll down just a little bit, there's a photo of the kit contents. There are two small tubes (the motor mount tube (brown) and the thrust ring installation tool (yellow) ), and right next to those are the green thrust ring, and the orange spacer for D sized engines.

 

[kuririn]

Most Estes kits that can use either the 24mm C,D, or E engine have the orange spacer enclosed in the kit.
Also very easy to make your own by cutting off a 1" section of a spent 24mm motor casing.

 

[rcktnut]

If you have built kits already with motor hooks or prefer using the motor hooks a piece of masking tape wrapped around the motor and the hook will keep the motor from ejecting from the mount. Never lost a motor yet using that method.