I've been experimenting with a new (10/97) rocket configuration that shows promise: good flight characteristics, and not hard to build. The basic idea is this:
You have your "motor", consisting of some number of bottles and associated releases, etc. You screw on a long tail made from that flourescent tube protector, with small fins down at the end. At the mouth of the bottle, up inside the top of the tail tube, is a quick-connect nipple.
The launcher is a long PVC tube with the female end of the quick-connect at the top. You fill your motor with water, screw on the long tail, and slide the tail down over the launch tube. The quick-connect locks it down.
Here's a not-very-clear picture of the rocket lying next to the launch rod:
So what you have is a rocket body (just a 1-liter bottle above), a long tail with small fins at the end, and a long launch tube to get it started in the right direction. Because the fins are so far behind the motor (where all the weight is) this is a very stable configuration. The thrust is all down the center of the tube. and it's a reduced nozzle, which makes the lauches look really cool :-)
There's one big wrinkle: when I first tried this, with a tail tube about 2 feet long, my maiden launch attained a stunning altitude of 12 feet. All the thrust was used up creating foamy turbulence in the long tail tube. I started cutting it down shorter, and at about 14 inches I started getting reasonable altitudes of over 100 feet. Even shorter was better, at about 10" I seemed to get all the thrust there was.
But a shorter tube loses some of the stability advantages of the long tail. So I tried cutting vents in the tube to "blow off" the excess pressure at the start, and that worked great! A vent or two cut into the tube just below the nozzle prevents the formation of turbulence, and the thrust does its job.
The only tricky part to build is the fitting that screws onto the bottle. It has a quick-connect nipple, and fits closely in the fluorescent tube. But it turns out to be pretty easy: you build the nipple into a bottle cap in the usual quick-connect adapter way, then you dig around in your PVC collection, get a short piece of that thin-walled PVC that just fits over a bottle cap, glue that into one side of a 1" coupler (which fits closely in the flourescent tube) with PVC cement, then saw off a ring about 1/2" - 3/4" (1-1.5 cm?) of the "double thick" PVC. Voila! Here are the two completed parts, seen from both sides:
The bottle cap (with nipple) press fits into the PVC ring like this:
(You'd want to glue it on a "real" rocket, but it hangs on well enough for launching). The PVC ring fits closely inside the tail tube. You glue (or tape, for experimental versions) the PVC Ring/Bottle Cap/Nipple assembly into one end of the tube, like this:
and put some fins on the other end.
The launcher part is mostly simple PVC stuff, exactly like the previously discussed Bigfoot Quick-connect adapter. The only difference is that I fastened the bike cable to the sleeve of the quick-connect with strapping tape, instead of hose clamps (which don't fit up inside the tube). Here's the business end:
Finally, here it is on the pad, ready to go (Sorry the picture's so bad, but hopefully by now you know what you're looking at):
(By the way, at the bottom there you see my "Littlefoot" launcher, basically a big metal pipe flange on a piece of plywood with screw-on PVC legs, one of which has the valve on it to connect a pump. It packs down small, but accepts the wonderful trigger mechanism on my bigfoot launcher. Perfect for those launches far from home...)
There are lots of unknowns about these tailed rockets still, like the best shape and location for the anti-turbulence vent, and what the losses are in the tube and how to minimize them (I'm sure there are still losses there, but I think they're minimal). Give it a try, and let me know how it works!
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