Is there a practical carbon-free jet fuel?

Question

For example, when I want to use a small turbojet or low bypass turbofan as both the APU and bleed air supplier for blown flaps, ailerons, elevators, (like the ShinMaywa US-2 amphibious plane) and rudders, (and hot wing deicing) there's a lot of internal ducting, and a large number of them are quite narrow, so having soot in them is like having thrombus in capillaries, so it's best that it doesn't contain so much as a single carbon atom in it.

EDIT: OK, the small engine mentioned above is used as a gas generator, not for thrust, that's the main engines' job. For the sake of the argument, let us assume the main engines are not available for bleed air like in real-life large airliners because they are say mounted on gimbals in a tilt-turboprop. If this sounds artificial, how about having a complete set of blown surfaces so the tilt-turboprop can have control authorities in pitch, roll, and yaw at airspeed 0 knots, i.e. when it's taking off vertically. Being a tilt-turboprop means it can not have helicopter-like variable pitch propellers like the V22 Osprey and has to rely on external measures for attitude control.

EDIT: It doesn't matter how weak the alternative fuel is as long as it can support the small jet engine's N1 RPM. It is only for the small engine.

Answer 1

A different fuel is not necessary. Even the most extreme experiment in diverted jet engine output, the Hunting H.126 did not divert its exhaust flow, it just tapped off compressed air. Its takeoff speed was 52 km/h. All aircraft need forward thrust, and with modern high-bypass turbofans you can get massive amounts of compressed air that's more than enough for blowing any surface you want. The BAe Harrier is a good example; the front nozzles of the Pegasus blow compressed air and match the thrust of the jet exhaust in the rear nozzles.

Fuels roughly fall into 3 categories:

1. hydrocarbons (everything from methane to jet fuel and diesel). They all contain carbon. Some leave less soot than others: methane is relatively clean. But it's a gas at room temperature, which makes it difficult to store (you need heavy high-pressure tanks).
2. Hydrogen. Clean, but very difficult to store (needing even heavier high-pressure tanks than methane), and explosive in a wide range of circumstances.
3. Oddballs like hydrazine and hypergolics. They're all nasty, toxic fuels that require specialized handling which make them unsuitable for civilian use. Hypergolics are two-part fuels that spontaneously combust when in contact with their counterpart.

Some more details on hydrazine:

Hydrazine is highly toxic and dangerously unstable

Anyone involved in fueling a hydrazine-powered vehicle has to wear a full Hazmat suit. That alone makes it difficult to use a hydrazine-powered aircraft on most airfields as it'd disrupt normal operations.

Hydrazine is also inefficient. For jet fuel, you can pull the oxidizer from the surrounding air. Hydrazine has a specific impulse of 220 s, while jet fuel has a specific impulse in the region of 350 s. So you have to carry 350/220 is 1.5 times more fuel to do the same work.

The exhaust products of hydrazine are extremely flammable (hydrogen). You could burn that in a jet engine, which would significantly increase the specific impulse. Hydrogen is a bit finicky, but hydrogen-fueled aircraft have been tested.

Answer 2

This question is based on a fundamental misunderstanding: by definition, NO bleed air system has combustion byproducts in it. None.

Bleed air is tapped, or "bled", from the compressor section before fuel is introduced in the combustion chamber. Which makes sense when considering that bleed air is generally used to run Air Conditioning packs... you don't really want exhaust fumes making their way into the pressurized cabin!

Any system that IS taking exhaust gasses & doing something with them, is by definition NOT a "bleed air" system.

The solution for the underlying question is similar to what's used in the 737 NG: an APU that burns standard, plentiful, reliable jet fuel, and which turns a generator, and a load compressor. That compressor provides the bleed air to the jet's pneumatic systems, and plenty of it, and quite efficiently. When you put a bleed air load on that APU, the EGT rises, but not because you're stealing cooling air, but because the APU is now burning more fuel to do more work. Great system, works well.

But at the end of the day, combustion byproducts are always going to be undesirable to have in a pneumatic system, even if it's "only" a constant supply of water vapor. No fuel will do what the OP envisions, but a different approach toward producing the high-pressure air will work just fine.

Answer 3

To answer the question as stated in the header, hydrogen powered aircraft have been tried with some success (they fly since about 1989). Many problems need to be solved there but seems less issues than with hydrazine. Burning hydrogen produces just water, so the fuel should be carbon neutral. However not all technologies used to obtain the hydrogen are carbon neutral.

Some burning metals like magnesium "slurry fuel" have been tried in about 1950, with the claims it may produce more thrust. Looks like these works did not advance till the flyable prototype.

Electric aircraft can be carbon neutral, depending on how was the electricity obtained. They also already fly, from as early as from 1917, but even as of 2019 mostly just as technology demonstrators.

Nuclear powered aircraft have been thought about in the past but none were built.