taking a cap-nap with the Epic rocket project, now with motor mounts on each fin, all part of this weekend’s build.
Scratchy started the day today with a nose dive into my coffee.
And yes, Vanita, you knew before we did that we just had to keep a couple of these feral kittens.
Happy Mother’s Day from all the kittens.
(and we have come to learn that Father’s Day is a bit more multiplexed here)
I could have guessed! I could tell these little fur ball had gotten into your heart. Thanks for giving them a proper home!
How well matched are the rocket motors? It would seem that the moment arms here will multiply any thrust differences and cause instability (turning, tumbling), no? Was there a motive for moving the motors away from the center line? I’m probably missing something? Cute kittens, btw. We always had one or two cats (and other critters) around when the kids were growing up.
heh…. The kittens seem to have fully grown purr boxes. They even purr when napping.
Each day, we find some reason to exclaim "kitties are so cute!". Yesterday, they mounted a successful attack on the hammock, flipping around. And Saturday, they saw a mirror for the first time, and Keeper locked up in an attack posture – for almost a minute before Scratchy entered the scene.
jerryfi & Halliday: Yeah. Could be zesty. Feedback and suggestions welcome. The motivation for having the motors farther apart is that the sparky plume of the intended motors can get quite wide, and so this will look like huge motor, a foot-wide.
We were going to wire all four igniters in parallel and hope launch control has enough amperage to fire them all.
The general plan is to have a powerful 54 motor doing most the work, so partial ignition of fin motors will be in the "noise" (so we tested a central CTI J360 and are planning G75 metalstorm motors for the fins). I have never seen a Cesaroni motor fail to light with the igniters they ship with their motors, so they seem pretty reliable.
One idea we are considering is a poor-man’s hold down clamp. I wonder if a rubber band between rocket and pad would be enough to hold it down if only G motors ignite for some reason. The midsize rubber band(s) would be wrapped over a screw in-line with and below the GLR rail guides, which should not induce a torque on the airframe)
To give more time for parallel ignition, we could use long wires taped to the body so they can lift a bit while still being in the proper position internally (to give a slow igniter a chance) but would pull away before leaving the pad (pre-measured length wrapped around an anchor point on the pad).
(We were also thinking about using a flight computer for launch detect, but give the current draw there, it might only make sense to use it to launch the central motor once the G motors kick in. So 3 g motors on launch with on board ignition of the central 54mm… but, I think that is less safe given any time delays as the rocket might clear the rail with lopsided G’s only, and then the big motor kicks in. Yikes. So this is why we are thinking about ways to keep it from lifting off if only 1 or 2 G motors are lit.
One way to cater for any thrust imbalance in the outboard fin-mounted motors would be to incline them slightly sideways so as to induce spin around the rocket’s longitudinal axis – aka "rifling" in artillery circles.
Any imbalance among the three will then result in a spiral wobble rather than lawn-darting. Add to this the gyro effect whereby the whole fuselage becomes a gyroscope contributing to directional stability. As added benefit, the "sparky plume" will appear even wider the more the motors deviate from vertical.
In fact if you incline the three subsidiary motors horizontally rather than vertically, you’ll lose any thrust benefit but you’d have an awesome Catherine Wheel.
Obligatory Disclaimer: I am not a rocket scientist – this may be a really bad idea.
LOL – I just had an inspiration that went something like this: you could add a noise-maker shape (essentially a whistle) oriented so that it is driven by spin rate… eg. the faster the spin the more noise (and greater the frequency) of the whistle. Audio recording of the launch would let you calculate the spin rate from the recorded whistle frequency. Yah, I know there are about five better ways to do this, but my mind tends to do a rapid right-hand-wall-walk through the whole possibility space. 🙂
Mad Idea of the Day: How about a nacelle-motor failure-detection system (laser diode plus photocell) that releases the fin motors to pivot by ~20 degree, thereby converting the vertical thrust to spin? I’ll bet you could figure out what offset angle is needed to compensate for one or even two motor failures. Simply pulling out a wire pin in the fin might achieve this… it would involve designing a new motor nacelle (one that pivots).
["He’s Mad I tell you! Mad!"]
Sometimes a test is worth 5 pages of analysis – almost as good as 20 powerpoint charts. 😉 Should be fun to watch… from a distance. Take good video.
Roughly, a 10% thrust difference in one G75 motor would produce ~ 10 in-lbs of torque, in pitch. The effect of that on flight path angle would depend on the longitudinal moment of inertia of the rocket body (which you can measure or calc). Aerodynamics comes into play also. Mounting the motors on the rocket body would reduce that torque by a factor of ~5. Does it matter? There’s nothing like a test, and it’s more fun;-) BTW, to induce a spin, it might be easier to put some angled fins on the body. And at this point, I’ll also invoke a "Jitze". 😉
Do you know of any high resolution 3D tracking systems for rocketry/similar application?
Why ARE you putting them out on the fins…?
Looks cool ?
Sensible spot is closest to main body…no?
Call me a spoil sport.
Exactly! A close cluster is more logical. The wide spiral cone of titanium sparks is meant to be a visual delight. And a good challenge (big motivation to get all motors to light)…
Or, in short, fire is cool.
Jerryfi – did I mention we will have a heavy strap-on video camera hanging off the rocket, to add to the asymmetry?….
Pieces of Eye: Yes. My son got a kick out of this one:
which can create some nice real-time overlays on Google Earth:
I did think you might have a camera on board. I _think_ the weight and drag of the camera will produce much lower torques than my assumed 10% thrust variation. 10 in-lbs is a pretty hefty torque. (That’s the equivalent of 10 lbs tugging at the side of rocket body) I don’t think you’ll see that from a camera mounted on the body (~1 inch off center). My uncertainty is the thrust variation between motors. 10% seems reasonable after looking at the thrust curves for some of these motors, but…. that’s why God invented the Darwin Awards.
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