Brief:
This is a brief review of Giant Leap Rocketry's 38mm snap-ring style SLIMLINE Motor Retainer. SLIMLINE retainers are also available for 29mm, 54mm, 76, and 98mm motor tubes. There are also motor adapters, boat-tail retainers, and a newly released threaded SLIMLINE retainer system.

Construction:
The SLIMLINE retainer system includes a sleeve, a snap ring, and a spacer washer. The instructions come on a single 5" x 8" sheet and include diagrams. The sleeve is black anodized aircraft-grade aluminum (6061-T6), and the two rings are stainless steel.

The main feature is the snap ring that pops into a groove on the inside of the sleeve. This ring is easily visible in the attached photo. Unlike the typical snap rings used on the Kosdon style motor hardware, no special tools are required to install or remove this ring. One end is notched and a small screwdriver is all that's required. This internally mounted ring allows the outer diameter to be thinner than other quick-release retainers do, which is important to reduce aerodynamic drag on minimum diameter rockets.

To install the SLIMLINE, you mount the motor tube so that its protruding behind the aft centering ring. The sleeve is made to fit all motor tubes and should be test-fit for both tightness and spacing. If you don't test fit it with your motor hardware, you may not have enough room between the motor tube and the snap ring to accommodate the motor's thrust ring - this would be a show-stopper! If it's too tight, you sand the tube a bit. If it's too loose, you add small slivers of tape 120 degrees apart. It fit my LOC motor mount tubing just right. As with most bonding applications, the tube should be roughed up. No processing is required on the inside of the sleeve since it is grooved. To glue it on, you slather some heat resistant epoxy such as JB-Weld onto the exposed tube, and twist the retainer on.

The retainer is advertised to work with AT, Kosdon, Ellis, and hybrid motors. For motors with integral retaining rings, you just slide them in, insert the notched end of the snap ring, and then press it in with a circular motion. For motors that don't have a rear thrust ring, the instructions say to glue in a section of coupler tubing at the depth of the longest motor, and then cut additional sections of coupler to form spacers when using shorter motors. You then insert the motor and spacer washer (really more of a retaining ring), followed by the snap ring.

A note on spacing: I was concerned that I might mess up and make it so the snap ring wouldn't fit once there was a motor installed. In addition to checking and double-checking my work, I also inserted the spacer washer, which isn't required for AT hardware. This provided provide a small safety margin.

Construction Rating: 4 out of 5

Flight:
I used this retainer on my Upscale Big Brute. Although I have only flown it once, I have inserted and removed AT casings several times and have formed my opinion: this retainer is pretty neat, but I like the screw type (i.e. Aero Pack) retainers better. The SLIMLINE is a easier to use than "Kaplow" style retainers (I re-use a couple of sets from rocket to rocket, so I have to install and reinstall these every time). The snap ring is very easy to remove, but I'm all thumbs and have to work at getting it on. I don't mean to imply this is really tough, but is not as easy as screwing on a cap. Maybe I will improve with more practice.

Flight Rating: 4 out of 5

Summary:
In summary, the SLIMLINE is a just a tad more difficult to install and use than an Aero Pack retainer. Still, if you're building a minimum diameter model and want to minimize drag, this is the way to go. The final factor is cost. The SLIMLINE currently goes for $20, with its competitor costing $28. However, you can reuse the latter's cap and buy additional housings for $15. You can do the math for your application and needs. I probably will not buy many of any of these machined units, as they are all more expensive than my homegrown or reused clips. But, those boat tail adapters sure look cool...

Overall Rating: 4 out of 5