Next one up (not literally) is going to be the Squirrel Works Callisto.
For the rocket club’s informal contest in June, ½A cluster duration, I’d found there’s no cluster kit on the market that’d be suitable. So I started trying to design a scratch build, but wasn’t feeling very inspired by what I was coming up with. Finally I decided the heck with it. For these contests, especially when it involves something I have little to no experience with — like clustering — I’m not particularly concerned with how well I compete anyway. If I have an entry and it flies and I get it back, that’s victory enough. So never mind a scratch design optimized for a single contest; I’m building something I’ll want to fly again.
I got onto the web sites of all the model rocket kit producers I could think of and looked for a single motor kit that:
- Has a BT-55 (or something close to that) body tube. Big enough for three 13mm motor tubes and a not too tiny parachute, but not much bigger than that, to keep weight and drag down.
- Has a recommended motor range from A to C. A because I’m looking to put it up on 3x½A, which is less total impulse than a full B. C because I’m looking for it to be stable on 3x½A (13 mm), which weighs roughly the same as a C (18 mm). (Initial mass of a ½A3-4T is 6.8 g; for a C6-5, 24.0 g.)
- Has low weight.
There were a couple of possibilities but the Callisto was the one that appealed to me. It has kind of a 1960s NASA vibe to it. (That’s a photoshopped Saturn V launch on the face card. Skylab. Okay, 1970s.) It has a BT-55 body tube reducing (via a balsa transition) to a BT-50 payload section and balsa nose cone. Stated weight is 1.6 oz and recommended motors are A6-4, A8-3, B6-4, B6-6, C6-5, C6-7. There’s a cardstock faux nozzle which will have to go, alas. Other than that I think it’ll be fine.
I’m going to be cutting my own centering rings, I guess.Until now I’ve preferred spending a couple bucks on a package of rings and letting someone else do the work, but that won’t be happening this time. Fliskits lists a 3×13 mm in BT-55 mount as a future part, but that’s about it for vendors. (They also have canted 3×13 mm in BT-55 as a current part, but that won’t work with a 4-fin rocket.)
I’m not sure even the future Fliskits part would be what I want, anyway.
I was thinking about motor retention — trying to decide whether to use hooks or friction fit, because I figured there wouldn’t be room enough between the little 13 mm motor tubes for a screw thread retainer like the one I used on the Ranger. (As demonstrated mathematically recently, a #4 screw just barely makes it between 18 mm motor tubes. Smaller diameter screws exist, but are hard to find (in more ways than one) and I’m not sure they’d be robust enough.) Took me way too long to realize that since three 13 mm tubes don’t take up the full diameter of a BT-55 airframe, they don’t have to touch each other; I could make space for a #4 screw. As a bonus, the screw should make it easy to use the nozzle after all — though for display only!
This Engine Centering Ring Generator is a handy tool. It doesn’t have a builtin capability to add space between the motor tubes, but by telling it I wanted a central “motor” 0.114″ in diameter I got what I wanted.
I now have preliminary OpenRocket sim files for both the standard and the cluster versions. Looks good. I don’t think any added nose weight will be necessary. I’ll post both here and submit them to rocketreviews.com once I’m done building and can finalize the weight overrides.