As some of you know from the "Let's talk about the Romulan BOP
" thread, I've been working on calculations for theoretical power outputs of different fusion reactions of different fusion fuels.
The first thing I found, and I won't spend as much time on this as I could, is that deuterium (heavy hydrogen) is a far superior fuel in almost every way to protium (light hydrogen). Deuterium has five reactions to choose from and they delineate well from relatively easy to near impossible. Protium only has three and the easiest is very difficult and complicated. (The hardest protium reaction is equally near impossible as the hardest deuterium reaction.) Under the same tankage deuterium has twice the density of protium and since fusion energy is all about the change in mass from fuel to exhaust, this is important.
There are two drawbacks to deuterium:
1) Biology incorporates protium. Deuterated chemicals do not react quite the same and though they are not quite poisonous, neither are they useful. IE, drink enough heavy water and you will die because your chemical biology will not be able to incorporate the slightly different chemestry of deuterium. If you were to use protium as your fuel then you could store some of your fuel in your food and water, allowing for a larger tankage without increasing your fuel tanks. This is not possible with deuterium, as such....Of course, replicators would change this equation somewhat.
2) The main of the interstellar medium is protium. The ability to augment your fuel supply by scooping up the interstellar medium while in flight would mean an increased range. Indeed, if enough could be scooped up, you could have a nearly infinite range. (I argued this in the above Romulan BOP thread.) Again, replicators or anti-xeno-effect chambers could negate this disadvantage.
Ok. But this is not what I came her to write about.
It is perfectly possible to store your fusion fuel --deuterium-- in a chemical compound in order to either greatly decrease the size of the tankage or to greatly increase the energy density per volume. Not only that, but by storing it in a chemical that is easier to store --less cryogenic, for example-- you also greatly decrease the mass of the tanks. However, in doing so you will decrease the energy density per kilogram of the fuel. This is an engineering trade off that is quite familiar in cars and airplanes: hydrogen has much more chemical energy per kilogram than gasoline, but is harder to store and has far less energy per liter. In cars and airplanes, the advantages of having a small, simple tank outweigh the disadvantage of lugging around more mass for the same energy output.
Since it's reasonable to assume Star Fleet engineers could easily realize the possibility of chemical storage of deuterium but choose to store it as a liquid instead, it seems to me, for some technical reason, energy density per kilogram is more important than energy density per liter.
Theoretically, this is true for rocket propulsion, which is what I usually assume impulse engines to be: With the same wet-to-dry mass ratio, liquid deuterium has more delta_V (change in velocity) than any of the hydrocarbons I've studied. However, practically speaking, ease of tankage and better per-volume density translates into a higher wet-to-dry mass ratio, making deuterated propane a higher performing fuel for rocket propulsion even with its considerably lower mass-to-enegy conversion. (I can back up these statements with theoretical calculations.)
This leaves warp drive as the primary reason to use liquid deuterium. I deduce from these observations that, every thing else being equal, pushing around more mass is a heavier burden than making a larger warp field. IE, warp power requirements scale more quickly with increased mass than with increased volume.
Let me say that again. I deduce from fusion fuel storage methodology that: Power requirements for warp fields increase more quickly with increased mass than with increased volume.
IMHO, this is an important datum for us tech trekies.