Are tripropellant jet engines a viable way to increase fuel efficiency for large commercial airplanes?

Question

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.

Answer 1

First of all: jets are not "n-propellant". 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 "boil-off". Twice as heavy as kerosene tank is a huge understatement IMHO, as in modern jet liners the tanks - "wet wing" - 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.

Answer 2

Yes, you can reduce takeoff mass by exchanging kerosene with hydrogen, but only above a minimum volume where the increased weight of the tank does not cancel out this advantage. But any manufacturer introducing such an aircraft today would immediately hit the problem of fuel availability - there is simply no infrastructure for hydrogen at airports.

Historically, the military paved the way for infrastructure improvements, and with aerial refueling, satellite technology and the vanishing importance of long-range platforms (except for unmanned reconnaissance maybe) there is no pressing need today to roll out the hydrogen infrastructure.

Note that project Suntan (which became the Lockheed CL400) invested into the liquid hydrogen infrastructure that enabled the use of cryogenic rockets in the Apollo program. Maybe a future military hypersonic vehicle will do the same for hydrogen in civilian air transport, but I am skeptical that this will happen in my lifetime.