Building the Semroc Sprite (part 1)

For its winter project, the Syracuse Rocket Club is making Sprites. This isn’t a group build of a single rocket but individual builds of a single design: the K-15 Estes Sprite, or Astron Sprite as it was first known when introduced in 1964. Looks like its first catalog appearance was 1966.

I suppose if any club members have an unbuilt original Estes Sprite kit they could build that, but otherwise we’ll be doing either builds of kits like the Semroc Sprite (a reproduction of the K-15), Fliskits Tumbleweed (a BT-5 more-or-less downscale), or 3D Rocketry Circulus 2 or Circulus 3 (2.26″ or 3″ kind-of upscales), or scratch built clones, downscales, or upscales. I think at least one MMX downscale is planned. There will be upscales, but presumably none quite as large as Mark Lloyd’s O motor powered one.

My plan is to build two Sprites: first the Semroc kit, and then a scratch built BT-80 upscale.

Here’s the kit.

Mostly pretty self explanatory: Instructions, body tube, nose cone, launch lug, motor hook, ring fin, flat fins, marking guide, decals. The little piece of Tyvek covers the hook on the outside of the body tube. The fins are a surprising ⅛” thick.

The thick walled short pieces of tubing are maybe a bit of a puzzle. The Sprite was designed for tumble recovery: the ejection charge pushes the motor backward, but not loose. That shifts the center of gravity to presumably close to the center of pressure, destabilizing the rocket so it tumbles to earth rather than coming in ballistic. It used the “Series III” motors which were shorter than the original Series I. My understanding is that Mabel, the Estes motor making machine, could make motors only in one physical size, 18 mm by 2.75 inches. Low impulse motors didn’t need that much volume so ended up with an empty section at the front. For the Series III motors, they just cut them back to 1.75 inches.

Later Estes introduced 13 mm motors, 1.75″ long, which replaced the Series IIIs. The Semroc Sprite uses 13 mm motors, but they couldn’t revise the design for a 13 mm motor mount, because the 13 mm motors are too light to bring the center of gravity far enough back. Instead they supply these tubes, 13 mm inside diameter and 18 mm outside. You’re supposed to glue or friction fit a 13 mm motor into one of these tubes to make, essentially, a Series III motor out of it.

So that’s the kit. Skill level 1? I guess I can do this.