Using bolts through Blue Tube/ cutting Blue Tube

[Colin Burfeind]

Hello, I'm in the process of designing an L1 rocket using 98mm*48in Blue Tube. I've designed a mechanical motor retention system for its ejection charge, but I'm also adhering my fins to both the motor mount tube and the body tube on the outside if possible. (is it somewhat reasonably possible to cut slits for fins into blue tube?)

Assuming yes, could I use either 4 or 8 bolts radially through the blue tube into the mount to hold the motor retainer for flight? This is somewhat structural, so I don't want to tear the bolts and motor out of the tubing at ejection. Basically, would the blue tube tear at the bolts or is it capable of withstanding a bit of force in this case.

I know this can be mathematically worked out for well-understood materials, but I doubt I could find data for blue tube like I could for aluminum or steel, so I'm going for knowledge by experience here.

Thanks

 

[markjos]

I think you’ll find that Blue Tube is pretty sturdy, relative to standard cardboard tube, and would almost certainly stand up to what you describe - though I’m not quite picturing the configuration. I think it will depend on what other forces are involved. Is it just the ejection charge, and resulting back pressure, or will that connection also be supporting the shock cord load, for example? Remember that the force applied by ejection charge is equal in all directions, so as long as what’s holding upper sections or the nose cone in place is weaker, then that’s what will give, first. You can certainly reinforce the holes with inserts, CA glue, wood backing, etc. All that said, there are lots of other motor retention options.

As for cutting fin slots - sure. A Dremel with a cutting disc (saw-toothed or abrasive grit) works very well for this. If you must, many passes with a sharp x-acto knife will do.

Blue tube is pretty versatile, sturdy, and easy to work with. Just keep it dry! (I’d wick CA glue into exposed edges to seal.). It will swell and delaminate at the edges if it gets wet.

 

[GlueckAuf]

I routinely use 3x, 1/4" x 20 oval weld nuts placed at 120 degrees and JB Weld'ed to the roughened, inside surface of a Blue Tube coupler to secure bulkheads and join sections with button-head screws. This approach has worked very well for me with no failures and multiple Mach 1 flights. The hardest part is achieving the precision to line up the holes, so I suggest drilling one hole at a time through the airframe/coupler and securing it with a weld nut and screw before drilling the others. (I now have a jig made for me by a club mate and machinist to do this with great precision.)

To drill smooth holes in Blue Tube, insert a tight-fitting coupler (wrapped with masking tape) into the airframe or a stack of plywood bulkheads joined with a center bolt first to prevent shredding the Blue Tube.



Dave Ebersole at Always Ready Rocketry machines my airframes for through-the-wall fins at a fair price and with CNC precision.


Good skies,

GlueckAuf

 

[mikec]

could I use either 4 or 8 bolts radially through the blue tube into the mount to hold the motor retainer for flight?

This would probably work, but frankly I think you'd be better off with a more conventional retainer bonded to the MMT or attached to the aft centering ring somehow.

Blue Tube is a fine material for sport flying, but it's somewhat brittle and once it starts delaminating, tends to disintegrate fairly spectacularly under extreme flight loads (at least in my experience.)

 

[GlueckAuf]

As far cutting lengths of Blue Tube goes (per your subject line) I have found no hand tool method better than using a professional tubing cutter. I use the American-made Ridgid model #31652, which handles tubing from 1-7/8" to 4-1/2" (or 50mm to 110mm). With careful set-up and patient, gradual operation, it provides a perfectly straight, perpendicular cut. As with drilling holes in Blue Tube, it's best to first snugly stuff the Blue Tube with a scrap length of coupler (built up with masking tape as needed) or stacked plywood bulkheads beneath the cutting line to get a clean cut.



Good skies,
GlueckAuf

 

[David Schwantz]

Cutting blue tube, I use a band saw. Drilling I use sharpened brass tubing with a backing block on the inside.