4 ms·
It's actually a very neat trick, kind of similar to the way ice dancers speed up or slow down their rotation. Why aren't we using this instead of (or in additio
by Udo 14y ago
It's actually a very neat trick, kind of similar to the way ice dancers speed up or slow down their rotation. Why aren't we using this instead of (or in addition to) flywheels for attitude control in spacecraft? It seems a cat-like mechanism of shifting angular momentum around could help reduce fuel costs by lessening the need to load/unload energy through maneuvering thrusters.
- photon137 14y agoI worked on designing such a thing while at university using differential angular momentum - kind of like a cube with a high-speed rotating wheel embedded in each face. The wheels on opposite faces cancelled out each other's angular momentum - but if you vary the speed of one of the wheels slightly, it would give a small amount of angular momentum about any desired axis.
- pvarangot 14y agoDifferential control of spinning wheels is used to control attitude in spacecraft. Wertz's book describes the attitude control of the SEASAT satellites that used two canted reaction wheels in the pitch/yaw axis for momentum bias, and actuated them differentially to correct pointing errors. Read the chapter on momentum bias.
- andrewcooke 14y agowell flywheels are the same physics. but i think what you're asking is why do we have fairly compact flywheels rather than longer, more extended "tails"? and the answer is probably that it makes more engineering sense. you can increase the angular momentum stored in something by making it "bigger" or by spinning it faster. i imagine it's a lot easier to spin a small flywheel to thousands of rpm than to make a "tail" (spinning at a very low speed) thousands of times larger. also - unless i missed it - this video doesn't really explain all of it. by itself a cat's tail isn't nearly large enough or rotating fast enough to spin a cat's body. the cat is also very "intelligently" (presumably "designed" through natural selection) doing much of the rotation while in a U shape. that reduces the net angular momentum considerably (the two "arms" of the U effectively cancel). and, from what i remember elsewhere, they also reduce the amount of rotation needed by rotating to only get one pair of legs perfectly aligned. as long as those touch down first there is a little time (i admit i am not sure it is sufficient) to then rotate the other legs while "holding on" to the ground.
- photon137 14y agoOh, I can chip in here regarding big vs small flywheels. If you look at the purpose of a flywheel - ie storing kinetic energy - a bigger/smaller wheel can serve the same purpose. The reason we don't have "large" flywheels is twofold: 1. size (obvious) 2. Torsional/shear stress - most materials are bad at tolerating shear stress. A bigger flywheel would require immense amounts of torque to get it moving. Generating a higher torque would impose huge amounts of torsional stress on the shaft driving the wheel.
- shardling 14y ago>also - unless i missed it - this video doesn't really explain all of it. by itself a cat's tail isn't nearly large enough or rotating fast enough to spin a cat's body. There's a whole section where they discuss how it's not the tail at all, and even bobtail cats can land on their feet. So you definitely missed it!
- andrewcooke 14y agouff sorry. unfortunately i can no longer edit that post.