Motor Tubes - How Short is too Short?

I am building an upscale Solar Warrior. The body tube is going to be 34" long and I will be using TTW fins.

Here is the situation-I have an 19" piece of 29mm motor tube. I am thinking about cutting it in half for two 9.5" motor tubes. (I have another body tube for another rocket and I am trying to stretch my resources).

Is there any rule about how long a motor tube needs to be? The 9.5" should accomodate the G80s I plan on using for motors.

Tony,

Having the motor case extend past the top of the mount is not a problem. What I use as a rule of thumb is, with thru-the-wall fins, as long as I can put a centering ring on the bottom and top of the fins to sandwich them, it will work. If the fins have a short root cord, a 3rd centering ring at the top of the mount tube is a good idea. 9" is plenty for a motor mount for 29mm, IMHO.

Carl

There is no specific rule about how long the motor tube should be. Something to keep in mind: the shorter the motor mount, the more internal volume the ejection charge must fill. You might notice that a G80 pushes the nose cone off a Lil Nuke with alot more force than it does off of something like an EZI. The less space inside it has to fill, the faster the recovery system deploys.

Out of personal preference, I'd probably use most of that 19 inch tube.

Based on my experience, 9 1/2" will be more than enough as long as the mount is strong. TTW fins and plywood CRs should ensure this for a G80 and even larger. There is no particular need for a tube that is longer than the motor and the motor can even extend further. I guess pressurization should be considered as Gothique pointed out. However, a G80 is robust enough for most (many?) rockets up to 4" unless it will be an upscale Mean Machine or the like. Even with 34" of 4" tube you should be fine. Is that length of the pressurized section? You have to remember the mmt, NC shoulder, etc all take away from the area to be pressurized.

Originally posted by rstaff3
However, a G80 is robust enough for most (many?) rockets up to 4" unless it will be an upscale Mean Machine or the like.

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Don't tell me you did an upscale Mean Machine... I beefed my Mean Machine up to take Es and Fs but on one hot summer day, the sun got to the paint and the thin tubes and now it looks like a black banana. Rock on!!!

Thanks for the info. I guess I will decide after I cut out my fins and make a final determination then so that I get my centering rings to butt up against the fins.

Nope, no upscale Mean Machine. Over reacted to point out that it might be possible to build a rocket that needs more than a G80 for ejection.

I've flown my stock Machine on E15s. A beefed up one would be cool on F's !!!

GO TERPS!

You might keep it full length. I've had motor tubes just barely below the nose cone shoulder. That way, the chute can go around the motor tube, and no wadding is required. (This works well, assuming that there is sufficient difference in the diameter of motor tube and body tube.)