Half a croissant, on a plate, with a sign in front of it saying '50c'
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Reactionless Drive Comb

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This idea describes a reactionless drive which uses [Vernon]’s principle to make a much simpler solid-state vibrating machine.

It consists of two comb-shaped components.

Part A is made of a very stiff material with a very high internal speed of sound, so that forces and impacts propagate through it incredibly quickly. It has along its length a series of specially shaped teeth or steps, and at the right hand end has a kind of anvil surface. There is also a connecting-rod which connects the right hand end to a crankshaft, like this:

L_L_L_L_L_L_L_L_L_L ___D-----o

Part B is made from a dull material with a very low internal speed of sound, so that forces and impacts propagate through it rather slowly. It has along its length a series of specially shaped teeth or steps, and at the right hand end has a kind of anvil surface, like this:

1'''1'''1'''1'''1'''1'''1'' 1'''1'''1'''''''G

The two components are arranged so their motion is constrained to the left-right axis, and so their teeth mesh.

As component A is moved to the left, the anvil D strikes the anvil G, knocking component B to the left. As component A is moved to the right, the teeth L strike the teeth 1, knocking component B to the right.

Spin the crank up to 10,000rpm and watch the astonishing lack of reaction.

pocmloc, Mar 12 2011

Arrtrrrr Alternating_20Respo...0Radiate_20Reaction
Short and succinct outline of the principles of operation [pocmloc, Mar 12 2011]

THE Pendulum Test http://www.nemitz.net/vernon/Pendulum.gif
[FlyingToaster], this link has been part of the other page (misspelled "Arrtrrrr" above) for a long time. [Vernon, Mar 13 2011]

Speed of sound in rubber... http://www.ndt-ed.o...peedinmaterials.htm
... is 60 m/s acording to this scholarly citation. [pocmloc, Mar 13 2011]


       Now I get it. Neat, and actually closer to [Vernon]'s original description of a battering ram!
MaxwellBuchanan, Mar 12 2011

       Why thank you. Yes, ever since I read the superman-battering-ram analogy I have been trying to work out a way of directly mechanising it.
pocmloc, Mar 12 2011

       [pocmloc], the problem here is that if it works, you won't be able to tell if it works without more stuff than you have described. Remember that in the battering-ram thought-experiment, the sources of the applied forces do not have to be fixed in place; if the ram moves more in one direction than the other, the applicators-of-force can move along with it. You have not allowed for that in your description.   

       Also, in your design only Part B might have a chance of moving more in one direction than the other, in response to the forces applied to it. If it did, you have not offered any means of USING that!   

       I will confess that years ago I put a lot of thought into purely linear designs, which more closely resembled the battering ram thought-experiment than the two-loop artrr design that I described at the linked page. I THINK I have a solution that allows both Part A and Part B to move more in one direction (yes, the same direction) than the other. But it will cost a lot more to build than the two-loop design, so I see no reason to try it until after/IF the two-loop design--which is supposed to be a Proof Of Principle device--passes the Pendulum Test.
Vernon, Mar 13 2011

       [Vernon], to see if it works, you simply suspend the entire device from a string.
pocmloc, Mar 13 2011

       I'm gonna take for granted that this "pendulum experiment" everybody's talking about is to try to make a string point consistently in one non-directly-down direction, but could somebody provide a link or keyword for it ?
FlyingToaster, Mar 13 2011

       [pocmloc], perhaps that will suffice. How much will your motor and crankshaft weigh? All of that needs to be suspended, too. And the more that Part B is out-massed by the rest of the gadget, the less any overall unidirectional motion it exhibits will be able to affect the rest of the gadget (though of course longer suspending-strings will help, there).   

       The proper RPM for running it needs to be figured carefully. Let's think about Part A, and assume it is made of steel where the speed of sound is 5000 meters per second. Per Dr. Davis, a one-meter length would fully experience a force applied to one end in 2/5000 of a second (he says the force has to reach the far end and come back to the start). Since you DON'T want Part A to behave abnormally, you have to make sure it is never jerked faster than 2500 times per second.   

       Well, that's pretty easy. Note that for back-and-forth motion one full cycle will require 2 jerks, and the one-meter length of steel needs time to fully respond to both of them, so that's 2/2500 of a second per cycle, or 1250 jerks/sec in opposite directions (again, one full cycle is 2 jerks), and multiply by 60 to get the absolute max speed of 75,000RPM.   

       Now, what about Part B? I don't have a lot of data about the speed of sound in different materials, but it is my understanding that for most solids, the speed of sound is significantly faster than (exaggerating) the 400m/sec speed of sound in air. Here I just assume you can find a solid (hard rubber?) that has a speed of sound of 1000m/sec.   

       If we do the same type of calcs for this material as for steel, we get an "absolute max" of 15,000RPM. This, however, is the absolute max speed at which this material CAN fully respond to the applied force at one end. It is necessary to use a faster RPM, to reach the zone where this material starts to fail to fully respond to the applied-force-at-one-end. (It will, of course, still be able to fully respond to the multiple small forces applied via the comb-teeth.) You may therefore find it necessary to get close to the 75,000RPM maximum of Part A, to ensure that Part B has a good chance of behaving in a non-traditional manner.   

       In other words, running your gadget (if 1 meter long, or shorter) at a mere 10,000RPM will result in normal behavior such that its lack of reactionlessness would not be astonishing, at all!   

       Or, you can use a length rather longer than 1 meter, which will let you run it slower. See why my battering-ram thought-experiment stretches out for perhaps a kilometer ("two thousand handles")?!?! Meanwhile, my two-loop artrr is supposed to operate somewhere around 3kiloHertz, and its forces are NOT going to be mechanically applied!
Vernon, Mar 13 2011

       Given the actual figure for rubber, 60 m/s, my suggestion of 10,000rpm would be excessive to test this machine.   

       A motor and crank to wiggle a 1m bar of steel back and forth, say 1cm, at a few thousand rpm, plus a frame and bearings to allow both bars to wiggle back and forth at that speed, should not weigh too much.
pocmloc, Mar 13 2011

       Hmmmm...I wonder about what sort of rubber was measured there. When I said "hard rubber" I was thinking of the stuff they used to use to make bowling balls (they're all plastic or urethane or other stuff these days). I'd expect softer rubber to absorb sound more than transmit sound, and I'm pretty sure that absorbing force-waves in this gadget is more of a recipe for failure than for success. Well, there's only one way to find out, I suppose....
Vernon, Mar 13 2011

       [pocmloc], while the calcs above were for the maximum RPM at which the WHOLE length of material can respond to the force applied at one end, it may be necessary to do another calc for the divvied-up force (call the result Max2). (Simplistically, just multiply that RPM by the number of comb teeth.)   

       I didn't bother with that calc when assuming a 1000m/sec speed of sound, because with any comb having more than 5 teeth, you would run into the problem of exeeding the allowable maximum RPM for the steel, before you would pass Max2 for the second material.   

       However, if you can actually use something like rubber with a speed of sound of 60m/sec, you do need to compute Max2, because one cannot hope for this material to be able to respond to the divvied-up force if you exceed that RPM. For more info, search for "special range" in the annotations of the artrr page.
Vernon, Mar 13 2011

       Once the machine has been made, you can test it at any rpm you want.   

       If you are concerned about the internal damping of the rubber, you could use a spring for part B. Simply replace every section marked ''' with a spring.
pocmloc, Mar 13 2011


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