Importance of an inner motor tube for high powered rocketry

Discussions on mid/high powered model rockets using F powered motors and above.

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Importance of an inner motor tube for high powered rocketry

Postby Shepherd » Sat Feb 02, 2019 12:48 pm

Hey guys,

Just wanted to get some clarification regarding the inner tube you see on most rockets (usually made of cardboard/phenolic) and i was wondering what the purpose of that actually was? Why not just slide the motor into the center rings and call it a day? Surely there'd be some minimal weight savings there, right?

Sorry if this seems like a stupid question.


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Re: Importance of an inner motor tube for high powered rocketry

Postby OverTheTop » Sat Feb 02, 2019 1:55 pm

Hi Shepherd. Welcome to the forum! I have pondered the same question myself.

In LPR I think it adds structural strength and makes it easy to mount a motor hook onto.

In HPR it can also add strength and spread the motor forces to the airframe more evenly. If you can get the forces to the airframe without failure and don't need the extra strength there then it seems reasonable to go without it and just use CRs to hold the motor. For example, if you used a 6" airframe and a 4" motor with an "Aeropack" style minimum diameter mount (to take the motor thrust at the forward end of the motor to the airframe, rather than the more traditional thrust ring at the rear) then you could just use centering rings to locate the motor.

I guess the succinct answer is "dinosaur genes". People build rockets without thinking where the forces are going and sometimes end up with vestigial parts in their rocketry anatomy. I am guilty of that too, and now try to consider what I can leave out of each build as long as the structure is not compromised.

Design it for the required forces and you are good to go!
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Re: Importance of an inner motor tube for high powered rocketry

Postby air.command » Mon Feb 04, 2019 7:58 am

It's a good question, and I think you could leave it out in some designs.

In addition to what OTT said, some other uses for the motor mount tube (MMT) include:
1. In LPR sometimes people will friction fit the motor, and this is easier with a tube than with centering rings. The tube will also commonly have the engine block/thrust ring attached. This is again easier inside the tube then with a centering rings.
2. Through-the-wall fins will often use the MMT as a second anchor point.
3. In HPR it makes it easier to fit / support different length motors, unless you have more centering rings.
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Re: Importance of an inner motor tube for high powered rocketry

Postby SpaceManMat » Mon Feb 04, 2019 11:48 am

If you don’t use a motor tube, keep in mind that your CR inner diameter will need to be smaller than standard.
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Re: Importance of an inner motor tube for high powered rocketry

Postby Wingnut » Mon Feb 04, 2019 2:00 pm

I think it's a great question.

I wouldn't think that the majority of rocket designs require the Motor Mount Tube (MMT) for structural purposes. Case-in-point are the many minimum diameter rockets, both LPR and HPR. The MMT seems more like was a simple and practical design philosophy which we just blindly follow.

However, thinking if over, I don't think you'd achieve any mass saving by omitting the MMT unless you're always going to use motor cases of the same length in the rocket.

LRP models use a "thrust ring" in the front end of the MMT to prevents the motor from travelling up the inside of the airframe. Removal of the MMT will require an alternate approach for the "thrust ring", likely requiring a part(s) larger than the simple centering ring of the same diameter as the motor; thus offsetting the mass saving from omitting the MMT.

Rocket motors of 29mm diameter, and larger, can be various lengths. The MMT (plus 2x centering rings) designed for the longest motor case you plan to use in the rocket will also hold any shorter motors. Omitting the MMT will likely require centering rings at various locations up the airframe to match the various length motor cases, offsetting the mass saving from omitting the MMT.

All that said, using a CTI's motor case with "spacers" means that you can fly a range of motors using a motor case of one length.


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