...So I made this rocket over the past few months, all while procrastinating on joining this forum.
Inspired by a finishing move from the anime Tengen Toppa Gurren Lagann, I decided to make an oddroc with the form of a spinning conical 'drill' which would be powered by a pair of canted green Aerotech G76 motors. It would have a hole through the center, allowing it to spin about a 1/2" rod when launched, and I wasn't sure how to recover it.
I figured this would be horribly unsafe, so I shelved the idea for a month until I realized that I could write a simulation for it, and even use that as a final project for one of my college classes. So, I started going through designs, trying to figure out how to get the rocket to eject a parachute large enough to support it, without risking tangles caused by the separation of two spinning parts.
I eventually selected rear-ejection: the motors sit diagonally in a 4-inch phenolic coupler tube that is 7 inches long, which contains a baffle system to prevent burning particles from poking holes in the parachute. You can see the internal structure here:
If you're wondering why the spin induced by the motors is in the opposite direction of the flutes on the drill, don't complain to me: they animated it that way. Also, on its first flight, I put some dog barf in just in case, and I plan to keep doing that. Better safe than sorry!
This slides into the body of the rocket, which is underpinned by an 11-inch-long piece of 4" tube, and has "centering rings" (I don't know if you can call them that) around it creating the conical profile of the rocket. At the front, inside a centering ring, is a 29mm phenolic tube that seals with the aluminum tube from the motor canister, preventing ejection gases from exhausting through the launch lugs in the top and the bottom. That centering ring has another pair of eyebolts matching the ones on the motor canister, to one of which the shock cord is tied. (Both are there for balance).
That leaves a donut-shaped space around the 29mm tube for a parachute to go, so I purchased a 48" Spherachute, which fit perfectly into the space available along with Kevlar shock cord, and matched the expected 5.5 pound weight of the rocket.
At the nose, I have a piece of nylon with a brass tube (.5 inch ID for a launch lug) press-fit into a hole bored through it, and the nylon was turned on a lathe to fit into the 29mm phenolic tube and have a conical surface lining up with the rest of the conical shape of the rocket.
For the skin, I epoxied two pieces of posterboard around the rocket, forming a conical surface. On top of this, I put a layer of 4-oz fiberglass, 2mm aero-mat (http://www.acpsales.com/Aero-Mat.html) foam for thickness, then two more layers of fiberglass. This I painted green and silver to match the Giga Drill Breaker concept that inspired the whole deal, and was finally finished.
Obligatory non-standard "dumb rocket picture":
I launched it at the February 2012 ROC launch, which was terribly windy, but I would not be stopped from launching my new rocket.
I've been working on how to implement that. One huge concern is rotational symmetry: it would need two sets of batteries and light arrangements. A second concern is that I would have to drill into the bottom and make some firm mounts, lest the electronics get flung off by centripetal acceleration! But if I can do that, I can also put some instrumentation on it, letting me measure the rotation rates and stuff.
If I find some good approximately clear green-glowing phosphorescent paint, I will paint the spirals with it and "charge" the rocket with a UV flashlight before launching. That would make for awesome on-pad shots, but it would get drowned out by the glare from the motors. And maybe tape green performance glowsticks (the kind that last 5 minutes but are really bright) to the bottom, or something.
It's definitely not the safest rocket to launch at night, though. I think I'll stick to evening launches at the latest. It'll still be spectacular because the bottom of the rocket is pretty light-colored and reflects the light from the exhaust fairly well.
Welcome to TRF.
Thats pretty darn cool dude.
X2 on that!
The only concern I would have with adding electronics is the rotational balance of the rocket itself. That rocket looks like it would be spinning at a fair rate and if the static balance is off by a few grams, it may shake itself apart 1/2 way up it's flight.
I like the Drill. It's unique, and CarVac certainly has a welcoming place here.
Plays with wood, cardboard, and carpenters glue at home. :caffeine:
"We have done the impossible and that makes us mighty"
-rocketeers vs. BATF
Ego intentio parumper perficio fuga , tamen Ego peto a perficio reverto.
The rocket itself is not perfectly balanced, but that doesn't matter. It's much stronger than necessary for surviving flight, since it's built for the tumbling landings that happen in windy conditions like its first launch.
It doesn't actually spin very quickly: my simulations predicted that it would spin at around 30 revolutions per second, and it actually did that: at burnout it completes about a half turn per frame of video at 60FPS. You can't see that since YouTube drops half of the frames before reencoding it. I bet the rocket could easily survive spinning four times faster than it does, especially since the weight is concentrated in the center of the rocket and not near the edges.
Hopefully the next time I launch it, both motors will ignite simultaneously and I'll get a vertical flight. I did use FirstFire Jr's instead of the included Copperheads, but if G76's don't do it, I'll drop 40 bucks and fly it on a pair of G138's, since Blue Thunder supposedly ignites really consistently and quickly.
Incidentally, those and maybe the G64 are the only motors I think I can safely fly this rocket on. The motor tubes cannot fit anything longer than the shoulder on a 29/40-120, and you need a lot of impulse and liftoff thrust to get a 5-pound, 13" diameter rocket moving.
It is spinning fast, I suppose, but it's not forced to rotate around the launch rod when it's going that fast. It is free to spin around its own CG, which is probably within a few hundredths of an inch from the center.
The thick foam-core fiberglass shell is fine, since it's plenty strong in tension. The plywood centering rings have no problem, and everything else is close enough to the center of the rocket for the forces to not be very strong.
It's landings that are the problem: I built the rocket's skin strong enough to take a beating on the corners when tumbling like crazy across the hard lakebed.
The result was that the flight was absolutely perfectly straight up....and tailstood because of the spin stabilization. That was bad, since it's rear ejection, and the chute got tangled in the shock cord.
The damage isn't bad; the aft centering ring is slightly dislodged with dirt crammed in the gap between it and the skin. I could probably fly it as is, but I kinda want to make sure it's fixed before doing so. Only problem is that I can't figure out how exactly to do it without adding too much weight.
Next time I do fly it, I'll angle it away from the flight line and shorten the ejection time so that it still has some forward velocity at deployment, which evidently worked the first time. Plus, angling it away gives the crowd a good view of the exhaust jets.
...when I have the time and patience to do so, maybe I'll upscale it and fly it on I245G's... and use chute cannons. A cluster of parachutes a la Apollo capsules would be cool, eh? Or maybe do the same size, but packing more motor.
Inside the cone there is the 4-inch tube. At the front, around the launch lug tube, is the parachute. Behind that is the motor assembly. When the black powder goes off, the motors fly backwards with enough momentum to pull the parachute out of the 4-inch tube.
To prep the rocket, I 5-way Z-fold the 48" chute and roll it, wrapping the shroud lines around, and then curl it into a crescent shape and insert it around the launch lug tube.
When the motors eject, momentum causes the motor assembly to yank the chute out of the tube, where it then is able to unroll. The crescent shape doesn't unroll while it's in the tube, so there is almost no chance of tangling occurring inside of the rocket.
What I believe was the point of failure on the last flight is that once the chute was out, the rocket fell back onto it, preventing the chute from blowing clear of the shock cord. On its next flight (two days from now) I plan to insert as much shock cord as possible before inserting the chute, and I will launch the rocket at a 15 degree angle from vertical to make sure it has some forward velocity while ejecting, like it did on its first flight.
Speaking of flying it again, I heavily sanded the seam around the back and reinforced it with chopped kevlar in Aeropoxy 6209. It's not as rigid as it once was (when I tap it, the pitch is still lower where it got damaged), but it should be fine.
Successful launch! I launched it at a 15 degree angle from vertical, and packed the shock cord in before the parachute, and everything worked great. Interestingly, even though I drilled both motors' delays to 5 seconds right next to each other, one went off at around 6 seconds and the other at about 9 seconds. Luckily, the first one was easily able to blow out the motor tube and pull out the chute; you can see the puff of smoke from the second black powder charge a little later. Good thing there are redundant charges!
Interestingly, I forgot to tighten the closures on the motors, since I had stored them assembled for a week and loosened them to avoid crushing the o-rings, but nothing bad happened. Yay for safety factors!