Are tripropellant jet engines a viable way to increase fuel efficiency for large commercial airplanes?
First of all: jets are not &quot;n-propellant&quot;. Rockets are bipropellant. That's because rocket needs to carry fuel and oxidizer, while air-breathing jet gets the oxidizer (plus huge amounts of reaction mass) for free from the air. So jets carry only one propellant (which doesn't make them monopropellant either). The tripropellant rockets were imagined to overcome problems with specific impulse. The most complicated and impractical rocket engine could go as high as 542s - while average jet operates around 3000s easily. So specific impulse is the problem jets never had in the first place.
The biggest advantage of RD-701 was to use the kerosene-hydrogen mix on low altitude (where kerosene is more efficient) and then switch to pure hydrogen in space (where hydrogen is more efficient). They started with a hydrogen engine (RD-0120) and then added kerosene to increase efficiency by reducing the amount of hydrogen needed - so the exactly opposite reasoning. The main problem with your idea here is that jets never leave the kerosene zone.
The main problem of liquid hydrogen is that the fuel is lighter, but the tanks are so heavy it completely obliterates any savings. Rockets here get it easy - rocket working lifetime is measured in minutes (Space Shuttle burn: 8m30s Soyuz burn: bit over 10m50s). A plane on the other hand would need to store the hydrogen for a dozen of hours = even more weight. There is no weight-efficient way of keeping the cryogenic fluid on the go. All you can do is to let some of it &quot;boil-off&quot;. Twice as heavy as kerosene tank is a huge understatement IMHO, as in modern jet liners the tanks - &quot;wet wing&quot; - are almost free (in terms of mass). Can't do that with hydrogen - you'll need to sacrifice cargo space for dedicated LH2 tank.
Another issue is that hydrogen burns in higher temperature. We're already having problems with making the turbine survive kerosene temperatures, adding hydrogen would make the engine design even more difficult. From the green perspective it's not all roses either, because the higher combustion temperature, the more NOx created.
I have to agree with others here - hydrogen planes won't happen unless we'll run out of easier options. Having 2 kinds of fuel is the most difficult of them all.