I was just reading about a tripropellent rocket engine that apparently passed all tests and was ready for flight before the program was cancelled in the early 90's. The engine was the RD-701 for the MAKS spaceplane program. It used kerosene and liquid oxygen before switching to liquid hydrogen.
So I'm now thinking about tripropellant jet engines. If we had a small tank of liquid hydrogen to either mix with the regular jet fuel or replace it for a short while, would this viably improve fuel economy for large commercial airplanes?
(I've heard that jet engines can easily run on almost anything that burns, so presumably it would be easy to get a jet running on hydrogen. Correct me if I'm wrong.)
There are many types of efficiencies, so let me be clear about which one I'm interested in: Fuel economy, as in miles per passenger-gallon.
Let's ignore any logistics on the ground about obtaining and storing the hydrogen. However, storage aboard the airplane itself should certainly be considered and might be a very detrimental factor, as might flammability. I'm pretty sure we would at least need double-walled, vacuum insulated tanks to let us store that hydrogen throughout the flight. They might be twice as heavy as normal tanks.
If you know anything about hydrogen, its chemical energy (per mass) is greater than gasoline....by a factor of 3 IIRC. The Lower Heating Values are around 120 versus 40. So we might all ask why not just use liquid hydrogen in everything and what is taking so long? Well apart from cryogenic considerations, it's not very dense at all. We would need a huge tank maybe even bigger than the airplane itself to store the necessary hydrogen, even tho that huge volume of hydrogen would still weigh less than the necessary jet fuel.
I think the idea of tripropellant engines is to allow some hydrogen with a not-so-overwhelming tank. The tank would only be as big as we can reasonably handle. Therefore we use some hydrogen to get a bonus in fuel economy.
P.S. I'm not really asking this from a "green" perspective. Hydrogen and oxygen burn to form pure water exhaust, which sounds so harmless, but water vapor is actually a very potent greenhouse gas, much more potent than CO2 molecule-for-molecule---especially when released at high altitudes. I'm asking this purely from a fuel-economy standpoint.