Model Rockets

[gabe]

Btw, I think someone mentioned it, buying engines. Due to the way the post office works shipping engines can be expensive. The best place to buy them is from whatever hobby shop in your area sells them to the school kids. The vast vast majority of rockets flown today are done with elementary schools. There you won’t have to pay shipping or anything.

Also, many chain stores carry rockets for Xmas or for the beginning of summer and then they clearance them out for nothing between those times. Because of this you can get B-6 engines really cheap. I think it’s B-6, maybe B-4.

100% on this. Not only are the engines hard to ship, it really is good to support your local hobby store. Pretty much all hobby stores now are locally owned small businesses and with fewer people getting into hobbies they are going out of business left and right. I'm big on just paying that extra dollar or two and knowing it's staying in the community and ensuring that store will be there in the future when I need something.

[Sniperfan]

The way I did the stability calculation was: find 25%. So I drew my proposed rocket out on paper to scale, and measured the projected side area including two of the four fins. Then I calculated where the 25% line of the area was with 25% front/top, and 75% bottom/rear, which was about a foot below the nose cone/body tube join line. I had to add a little weight IIRC to the nose cone to get to the 25% line, but it flew, and even flew successfully when one engine failed to ignite, so I figure I got it right. So, short version: get the rocket to balance on the 25% line.

That rocket was somewhat heavy, so if I had paid any attention to weight savings it probably would have been stable on it's own, and I probably could have use smaller fins as well. If I was to build a new one I'd use blue foam for the nose cone and just glass it with a layer of 3/4oz glass, as balsa isn't so cheap these days. Two D engines is impressive on lift off for sure.

I left all of my rockets with my friend Doug, who died of lymphoma in 2004 at 39 years old. His family had him cremated, and pushed all of his pictures, papers and model rockets into a pile at their ranch in nor cal and burned it all.

[Rainmaker]

If you can model your rocket in a CAD program, one that can tell you center of mass, it can help you determine your rocket's stability.
- Find the center of mass of your rocket = Center of Gravity (don't forget to model it as hollow in the right places, and with engine in place, etc.)
- Make a thin extrusion of the cross section of the rocket and find the center of mass of that extrusion = Center of Pressure.



If I remember right, you want your center of pressure to be below your center of gravity.

[SignOfZeta]

There are some pretty nice rocket simulator programs these days where you can even plug in popular rockets and available engines to determine max ceiling, how far away it will drift, etc. https://www.apogeerockets.com/RockSi...e?pg=quickside

[Sniperfan]

Well, this was in the dark ages, probably 1989. I took what I knew about model airplanes and applied it.

I had: Big bertha, V-Maxx, Honest John, the c-engined V-2, the c-engined Saturn -V, The christmas-wrapping-paper tube thing, and I also made a twin c-engined Big bertha test bed to see what lighting off dual rocket motors was like. The dual-C Big Bertha was not as stable, and if only one motor lit it would take 20 degree turn at the top of the launch rod. I climbed a few fences getting that one back.

Doug had: The Recruiter, the Cam-Roc, whatever the 6-foot long black rocket was called, a mosquito that disappeared on it's first flight, and a couple others that didn't las too long.

It was fun while it lasted.

makes you wonder what getting motors will be like as hobby shops disappear. You'd probably have to order in bulk to make the haz-mat fee less difficult to swallow.

[flyweightnate]

See, I thought Estes had their premade motors declared nonhazardous for shipping, somehow.