Launch Rod Clamp

UPDATE 2022-11-18:  I added a third rod notch to the clamp design, for added flexibility, and I have also uploaded slightly modified versions of both clamp and knob for metric M6 (10mm head) hex-head machine screws.  I found that M6-1.0x25mm worked best for me, as 20mm long screws didn't seem to be quite enough.

One of the minor annoyances of flying model rockets is trying to support a rocket on the launch rod.  You usually need to do this so the micro clips don't short against the blast deflector, though my stands all use ceramic deflectors these days, but there are other good reasons to want the rocket held gently and in a non-binding way on the rod prior to ignition.

And I've tried all kinds of things… clothespins, spring clamps, a broad variety of “utility” clips picked up at the dollar store.  Nothing really works quite right though.

So it occurred to me after a recent launch session that maybe I should engineer something for this specific purpose.  Something that would fit right, hold the rocket with a minimum of interference, and be easily installed, moved, and removed.  But I didn't have any really good ideas how to do this, so I trolled Printables for ideas, using the search “clip.”

I scrolled down… clip, clip, clip, clip, clip… clamp.  CLAMP.  Instead of trying to work out some sort of spring-powered retention, I could use a screw to secure and release my holder.

The idea gelled in my mind quickly now.  I pulled parts out of previous projects, including a knob based on Frugal Shave's design (of which this is tagged as a remix), and a hinge borrowed and modified from anewsome's “Buckle Box” found on Thingiverse here: https://www.thingiverse.com/thing:82620

The first prototype worked perfectly (and I don't get to say that very often) but there was still room for improvement.  The version shared here is the fourth version; all the prototypes worked, so since I have a four-rod stand I'll be throwing them all in the range box.

The clamp and knob are both set up for the 7/16" inch heads of standard ¼-20 hardware.  A ¾ inch long ¼-20 hex head machine screw pressed or pulled (with a nut on the other side) into the shortened knob results in a perfect fit in the clamp body, with nothing significant sticking out the back side.

Now you're thinking, this is overengineered.  Way bigger than it needs to be.  You'd be right, of course.  Shrinking the hinge more than I did would make the tolerances too tight, so that became the key factor in scaling the rest of the design.  Since I already had quite a bit of ¼-20 hardware, and had already printed and knew I liked Frugal Shave's knob design, I chose that for the retention device.  All that was left was to design the two-position 1/8 inch rod notches and print it.

When using this clamp, the intention is that you use the furthest-out position when supporting a rocket by its launch lug, and the next position when holding it by the bottom of the sustainer.  I also recommend making sure there's an air gap between the clamp and the blast deflector in case it gets hot.  Tighten the knob just until the clamp holds the rocket without slipping down… don't overtighten as you might break it.