Qualman Rocketry

Handmade Engine Clips - Qualman Clips
by David Qualman, owner Qualman Rocketry
Once I started scratch building rockets, a problem I encountered was the expense, and availability, of engine clips.  They are a basic component, but are the one piece of metal that goes into a low power model rocket.  So, I couldn't just fabricate one from cardstock or plastic.  This became an even greater concern to me once I started teaching model rocketry in the grade schools.  I made the kits for the class to keep the costs low, and I found that the cost of the engine clips were too high for the price point I wanted.

So I cogitated and experimented to find a way to make my own low-cost engine clips.

I had several goals in the design:
  • Suitable for 4th and 5th graders - so no exposed sharp edges once assembled.
  • Low cost
  • Made from easily accessible materials - OK, easily accesible at a local hobby store, even if not from a craft store.
  • Easily made with tools I already had - no special shears, forming tools, or sheet metal brake allowed.  Needle-nose pliers and wire cutters were OK.
  • It would automatically stay aligned with the body tube. It would not rock side to side or twist, because it needs to hold the engine in place.
Here's what I came up with:
Since I've never seen an engine clip designed as I have, I've taken the liberty to call them "Qualman clips" because, heck, we should all be able to have something named after us.
The clips are made from piano wire I get from my local hobby store.  I use .025" for 13mm engine mounts, and 0.039" for 18mm engine mounts, and differing larger sizes for higher power rockets. The piano wire comes in bundles of 3 pieces, each 3 feet long, for about $2.50 per bundle.  Since an engine clip requires about 7" of wire each, I can get about 15 clips, so each clip costs under 20 cents each.  That meets the low cost requirement.  Since the piano wire can be shaped with my needle-nose pilers, and cut with my wire cutters, that meets the tools requirement.  I should note that some wire cutters are only strong enough to cut copper wire, and are actually softer than piano wire.  I started by using the cutter section of my drop forged Vise-Grips (i.e. locking pliers), and eventually bought a wire cutter with hardened steel.
Note the upper wire.  It is about 1" long, give or take.  That compares to the upper arm of a standard engine clip, such as is made by Estes or Quest, which might only be 1/8" long.  There's a reason for that.  As many in the rocket hobby have heard, it is a common mistake to assume that the upper arm on a standard engine clip is used to keep the engine from going up into the rocket.  As we know, that's what the engine block is for.  Well, not with this clip.  On a Qualman Clip, the upper arm actually is used to hold the engine in place, and to keep it from going up into the rocket.  It crosses over the top if the engine, holding it in place, so that an engine block isn't even needed for most low power rockets.  That's an even greater savings, since the engine block could be left out of the kit.
If the rocket is heavy, or a larger engine is used, like a D engine, I'll also mount an engine block as extra reinforcement.  I've never had the upper arm bend and give way, but I have had the larger engines dig into the wire a bit.  
An additional requirement was met by extending the upper arm all the way across the body tube.  It won't twist around like it would if it were inserted into one just side of the engine mount. (In order to give full disclosure, that was actually the original reason for the long upper arm.  Later, it was found to be sufficient to act as an engine block, as described below.)
The retention hook is made by wrapping the clip around needle-nose pliers.
Cutting the thin piano wire can leave a sharp burr that would be unwelcome to little kids' fingers.  For this reason, the remaining wire is left a bit long, so that it can be tucked back inside of the engine mount.  For safety, the end of the wire can be wrapped with tape, which can be left on during assembly, as shown in the photo above.  This way, after assembly, all of the pointy edges are hidden inside the rocket, for safe launching.
But, there's still a problem, because the narrow wire could tear upward through the engine mount, breaking free, and giving a bad, bad experience.  But, that's solved with a piece that is already included in the rocket - the upper centering ring.  The upper centering ring is mounted so that it is flush with the upper arm of the Qualman Clip.

When ignited, the engine pushes up on the upper arm of the Qualman Clip, which pushes on the upper centering ring, which is attached to the body of the rocket, so the entire rocket goes up, and up, and up!

Also, this is a good time to mount a piece of fire-resistant string to the engine mount, below the centering ring, as a place to connect the shock cord.  For an extra secure grip, put it below the Qualman Clip.
The lower centering ring is used to keep the engine mount from rocking side to side.  The small divet in the center hole, and the glue fillet around the centering ring, hold the clip securely.  The masking tape is wrapped around the mount to hold the clip during assembly. Once glued together, we've got a completed engine mount using a Qualman Clip.