Avoiding excess hydrogen or carbon

We could avoid excess hydrogen or carbon if instead of using a single feedstock molecule to provide both, we used two feedstock molecules -- one of which was hydrogen-rich and the other carbon-rich. Provided that the actual ratio of carbon to hydrogen in a hydrocarbon assembler was between the ratios of these two feedstock molecules, we could avoid any excess of either hydrogen or carbon.

The most hydrogen-rich feedstock molecule is hydrogen gas. For various reasons (discussed earlier) hydrogen gas might not be desirable as a feedstock molecule. An alternative hydrogen-rich feedstock molecule would be methane, which has a sufficiently high ratio of hydrogen to carbon that it seems unlikely that useful hydrocarbon structures would have a higher ratio.

The most carbon-rich feedstock molecule of relatively small size would be some type of buckyball. C₆₀ is the best known. Polyynes terminated by hydrogen are smaller and have a good hydrogen to carbon ratio, but become increasingly unstable as they are made longer.

For the present proposals, we will assume that the 2:1 ratio of carbon to hydrogen in butadiyne is approximately the same as the ratio of these elements in a hydrocarbon assembler. Any excess of either element will be dealt with by building hydrogen-rich or carbon-rich structures, as appropriate, and retaining these structures in the assembler (zero residue).