This is a little tool I made. I made it to help me create perhaps a dozen or so little rockets and build them all as close to exactly the same as I could. It's just a used motor casing with a 1/4 inch length of BT-20 body tube glued around the base of it. A second 1/4 inch length of body tube is not glued on, but can slide on and off the motor casing.
See, I have a project I've been thinking about for this blog, which may never come to fruition, but it's been in the back of my mind for a long time. And for it, I need multiple rockets built as close to precisely the same as I can get. This tool is to help me build multiple copies of Flechette, a skinny, high-flying two-stage rocket I designed.
I flew - and lost - the prototype a couple seasons ago, but the proof of concept flight went well, and man, did it go high!
The booster stage of Flechette has a motor overhang - that is, the motor hangs out the back - by 1/4 inch, while the sustainer (upper stage) has an overhang of 1/2 inch. Since I wanted all my Flechette rockets to be the same, the tool I made is for pushing an engine block or thrust ring up into the tube a precise distance. With the basic pushing tool, I can get all my boosters to have the engine block set into the tube so all booster motors hang out 1/4 inch. If I slide the loose ring onto the pushing tool, now I can install a thrust ring that will leave 1/2 inch overhang. It'll be exactly the same every time.
I like precision, if I can manage it. I was never good at precision before I started building rockets. Making stuff was not something I was good at. So, while I don't have the knowledge of a maker who's been using tools or crafting things for a couple decades, I do my best to make things more precise if I can think of a way to do it.
Which is why I did this:
The Estes Nike-X instructions say to place the engine block inside the motor tube with a 3/8 inch overhang. My little pusher tool wouldn't work for that.
Look, friends, I know. I know... This is fussy and silly, and a totally unnecessary step. I'm not suggesting that you need to do this.
But wanted to, because I like precision, so I did it.
I measured the pusher tube supplied with the kit, marked 3/8 inch from the bottom, and wrapped it with tape. No need to put my thumbnail on the pencil mark - I could push the engine block precisely into place, and now that I have this in my tool box, I can use it for another build which uses the same measurement.
Look, don't make fun of me. Just use your thumbnail and you'll be fine.
Or, if you like this idea, use it.
Actually, this isn't a terrible idea if you're building with kids for the first time. More than one of my rocket camp kids pushed their tubes all the way into their rockets, leaving no overhang, because they forgot to keep their thumbnail on the mark. Your thumb is supposed to be what stops you from pushing the engine block too far into the rocket. Kids - or sloppy adult builders - might forget this and shove it all the way inside. You really need the motor to hang out the back a little bit, or you won't get it back out of the rocket after its first flight. If you're teaching a rocket class to kids, making a few of these with used casings might come in handy.
If you look at the motor mount for the Nike-X I'm building, you'll notice that the engine hook has no thumb tab.
I first learned that people sometimes cut this tab off from Chris Michielssen's Model Rocket Building blog. I didn't understand why you'd do that when I first started building rockets. I used to like the tab. It made it easy to pull the hook out of the way and insert an engine into the rocket.
But the more rockets I built, the more I realized that the tab can be in the way more often than not.
If you buy motor hooks from other vendors, for building your own, you usually won't have a thumb tab. And some companies' kits don't come with them - Semroc, for example.
You can cut the tab off, which involves using some wire cutters and a lot of force to score the bend in the hook where the tab starts, then wiggling it back and forth until it breaks off. I use needle nose pliers for this.
You can then either sand or file the rough end you've cut, or use that as the top end of the hook. The tabless end now becomes the bottom and holds the motor in.
I used to file the rough cut, but I found it tedious. I followed Chris' advice and simply turned the hook around on this one, and it works great.
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