Comment by adastra22

> Chemists were concerned that rod logic knobs touching each other would form chemical bonds and remain stuck together, rather than disengaging for the next clock cycle.

The rod & shaft designs are passivated. This kind of reaction wouldn't happen unless you drove the system to way higher energies than were ever considered.

> Someone calculated the power density of one of Drexler's early proposals and found that it exceeded the power density of high explosives during detonation

I think this is a (persistent) misunderstanding. His original work involved rods moving at mere millimeters per second. There are a number of reasons for this, of which heat dissipation is one. However all the molecular mechanics simulations done operate at close to the speed of sound in the material, simply because they would otherwise be incalculable. There is sadly a maximum step size for MD simulations that is orders of magnitude lower than what you would need to run at realistic speeds.

> You could just run them many orders of magnitude slower, but Merkle tackled the issue instead by designing reversible logic families which can dissipate arbitrarily little power per logic operation, only dissipating energy to erase stored bits.

The rod logic stuff is supposed to be reversible too. Turns out it isn't though. But it could be close enough if operated at very low speeds or very low temperatures.

The rod logic stuff is WAY smaller than the rotational logic gates in TFA. For some applications that matters, a lot.

If you are going to go for the scale of these rotational systems, you might as well go electronic.