This was my first attempt at a
"pure" tube rocket. The tube is a plastic flourescent light protector
tube, available at large hardware or lighting stores. They come in 4
and 8 foot lengths, and I decided to go for all the gusto and make
one as long as I could. I christened it "The Skewer."
These tubes can hold amazing pressures, I tested one to 140 psi a while back, and I'm sure that's not the limit (though it was the limit for my pump!). As far as I can tell, the plastic they're made of is the same as the bottle plastic, PETE.
The ends of the tube were plugged with bottle necks that were shrunk down over a custom-made form. There's a separate page with the construction details if you're interested. The rocket failed its first pressure test, so was actually about 2 inches shorter than full length (I had to cut off the failed joint and redo it).
The picture at the top right gives you a good idea of the scale: I'm six feet tall. We had to tilt the rocket (and launcher) over to get the water in (forgot to bring a step ladder). It was filled from the top through the bottle neck, then the cap was screwed on and the nose cone slipped over it and taped in place.
I spritzed some yellow and black paint on to help with visibility (but managed to resist calling it "The Stinger"). It was *still* barely visible during flight, even with the high contrast stripes. Without the paint I'm quite sure I wouldn't have been able to see it at all. (By the way, Graingers sells these tubes in other colors besides clear: red and yellow if I remember right.)
Here's some data about the rocket and the launch, for you analytical types.
The rocket landed only 15.5 meters
from the launch point! Note that there was no recovery system, so the
rocket destroyed itself when it crashed down, as you can see to the
right here. That's the nose cone and front bottle neck near the top,
the tube broke just below the joint.
The altitude was measured using an inclinometer (Alti-Trak, made by Estes), so is probably good within 10% or so. I was very careful about the measurement along the ground, using a trundle wheel (visible at my feet in the picture at above right) to pace it off in the straightest line I could, so I think this is a pretty good figure. I ran some simulations before the launch (using Clifford Heath's on-line simulator, you can also download Bruce Berggren's simulation spreadsheet in both Excel and SYLK formats) and was expecting it to get a little higher, about 200 meters, but I underestimated the Cd (coefficient of drag). All the surface area on this large rocket causes lots of drag.
I had two people timing with stopwatches, and they agreed in both cases within a few hundredths of a second of each other. The above times are the averages.
It was a nice, calm, sunny morning, perfect for rockets, and we launched at a great big empty field near my house here in San Francisco. The launch and flight were beautiful: straight up, pause, straight down.
I was the altitude measuring guy, so was 150 meters away, and because I was trying to be very careful with the inclinometer, holding it steady at the apogee point to let the pendulum settle, I lost sight of the rocket and missed the crashdown. Witnesses, however, assure me that it was satisfyingly dramatic: pure vertical approach, whistling loudly down to a crunching impact. The simulations predict an impact velocity of about 90 mph. Amazingly, the nose cone (Estes) survived the impact, but the front six inches or so of the tube literally shattered, and about 3 feet of it was bent at crazy angles. I'll cut it off and make a smaller 4 or 5 foot rocket from the remains.
I ran more simulations after the launch, entering the hard data and letting the Cd vary between .5 and 2. If I match the flight time, I get a Cd of about .7 but an altitude of 250 meters, not a good match. If I match the observed altitude, the Cd is around 1.5-1.6, but the flight time is shorter than observed, about 12 seconds. I suspect the latter is close to the truth. The simulation doesn't try to account for the "turnover" of the rocket, when there's significant drag for a moment or two as the rocket turns and re-orients itself in the first part of its descent. This would account for some portion of the slightly longer observed flight time.
All in all, this little experiment (BIG experiment actually) was great fun! Now I need to build another one, but this time with a recovery system so I can launch it again...
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