Aviation Fuels

[Mossie]

Rocket engines can't be run on the same fuel as other engines, as it's the reaction of the fuels to another component, rather than their combustion that creates thrust.  In fact, the design of a rocket has to be very specific to the fuel your using, swapping fuels from one type to another would be either useless or potentially disastrous.

Well, yes and no. Chemical rockets do use combustion to create thrust.
You can build a rocket that runs on e.g. kerosene; in fact, the first stage of the Saturn V did just that. If you use kerosene, the other 'half' of the fuel is liquid oxygen. The Spectre and other engines used by the British ran on a mix of HTP (hydrogen peroxide) and kerosene.
You're correct that rocket engines are fuel-specific, mainly because the combustion chamber needs to be designed for the fuel used.

Thanks for clearing me up on that Hobbes!

  So ...
I want to build an aircraft powered by a piston engine and a turbojet, using the same fuel for both.
What is the best option for piston engine: petrol/gasoline or diesel?

Could Methanol be used?  I know it can be used in piston engines, I have no idea about it's application for jet engines.

[dy031101]

I revisited the BSP entry on rocket fighters recently and recall seeing descriptions to the effect of Spectre being a "cold" rocket but employing a nasty chemical for fuel (which I assume the book to mean HTP).

It is said that Saro SR.53 was originally meant to have some prototypes that mounted Spectre and some using its competition Screamer for the sake of comparison.  The chart at the end of the chapter lists Spectre as its powerplant (as well as that of its subsequent development the P.177).

What, in layman's terms, would the operational difference be between fuel used by chemical rockets (which I assume Screamer would be) and that used by "cold" rockets?  Is one necessarily safer to handle than the other?

[MartG]

What, in layman's terms, would the operational difference be between fuel used by chemical rockets (which I assume Screamer would be) and that used by "cold" rockets?  Is one necessarily safer to handle than the other?

A 'cold' rocket in this context ( i.e. Spectre and its siblings ) is one that uses Hydrogen Peroxide ( aka High Test Peroxide/HTP ) as a monopropellant, using a catalyst in the engine which chemically disassociates the HTP into its component molecules. A 'hot' rocket simply adds kerosene to this, using the Oxygen liberated by the breakdown of the HTP as its oxidiser, to increase thrust. An example of a hot HTP rocket is the Stentor used in the Blue Steel missile

[dy031101]

Thanks for the clarification.

Screamer is said to use liquid oxygen and methanol.  How does it compare to HTP in terms of performance and safety?

[Hobbes]

Don't know about performance, but LOX and methanol are a lot less volatile. Liquid oxygen is very cold, so it has its own problems, but nothing like the corrosiveness and toxicity of HTP.

[PR19_Kit]

There's a term used in rocketry called 'Specific Impulse' which is a measure of the efficiency of the fuell and the combustion process.

It's also applied to other engine types as well, but seems to be used more in rocketry. There's a good, but quite mathematical, Wikipedia page on it at http://en.wikipedia.org/wiki/Specific_impulse that doesn't talk too much about the various fuels but it does talk about how it's measured etc.

[I KNEW all that poring over textbooks during my HND course at Cranfield would come in handy one day!  ]

[Hobbes]

Here's a list of rocket fuels: http://www.braeunig.us/space/propel.htm

[PR19_Kit]

Nice one Hobbes! 

The tables at the bottom show the Specific Impulse clearly.

[rallymodeller]

The best bet right now, and one the US military is testing, is biodiesel. It's chemically similar enough to standard diesel (which is in itself close to kerosene) that it doesn't take much to convert existing aircraft. A little more refinming and chemistry and you get bio-kerosene. Remember, the first German jets ran on basically straight diesel oil, so it's not that far fetched.

Going to gasified fuels (LNG, CNG, hydrogen, etc.) would entail far too much conversion, an entirely new infrastructure and would be far too expensive in the end, with not enough bang for the buck. Taking a look at that Tu-155 cutaway shows exactly what would be lost.

However, if gasified fuels were used on existing military aircraft, expect to see a profusion of drop tanks and so forth.

[famvburg]

     I don't think so. They had separate tanks for the jet fuel they carried as a tanker, but their jet engines burned avgas like the R-4360s.

But piston (gasoline) and turbojet? I assume they cannot.

yep, Boeing KC-97s had separates tanks for their jets and pistons engines.  

[famvburg]

    I'm pretty sure I read that the Me 262 burned avgas.

What fuel did Me-262s run on?

[famvburg]

    They can also burn 'high octane' avgas, such as 100/130, 100LL & in the good ol' days when the US military were flying big piston a/c that had jets for extra boost, they burned 115/145.

I know for a fact that at least some jet engines can run quite well on low octane (80/87) avgas. One of my first days ramp ratting at Buttonville we had a visiting Gazelle helicopter ask for a top-up and I told him we didn't have jet fuel (A few months later we did). "No problem" he replied, "80 octane is fine." In it went, up he went, and several more times in fact, before we had an MU-2 based there and jet fuel became a regular commodity. Whether it could be done all the time is something I'm not sure about though, but maybe Evan could help us here?

[famvburg]

     I know the Douglas F3D Skyknight (Skynight for some who can't spell) used avgas. Strange I've always thought, but.... I don't know the details on how jet engines on avgas burners are set up to burn avagas, but they do burn avgas & don't carry 2 separate fuels. Regarding using avgas in a turbine, some engines are prohibited from anything but jet fuel. Others, like Garretts & PT-6s can burn a certain amount of avgas over a certain period of their TBO. Performance suffers on avgas due to the higher operating temps, i.e., the engine reaches max temps well before it reached its max power output. As for other fuels, I remember in the late '70s just as Rockwell Thrush cropdusters were being fitted with PT-6s, the first one I saw had placarded on the fuel filler openeing all the fuels it was approved for & I know it ranged from Jet Fuel in its various incarnations, Diesel Fuel, Kerosene as well as mentioning AvGas, at the time probaly 80/87 & 100/130, but I know it was almost a joke that it didn't matter what you put in it, it would burn it.

I know for a fact that at least some jet engines can run quite well on low octane (80/87) avgas. One of my first days ramp ratting at Buttonville we had a visiting Gazelle helicopter ask for a top-up and I told him we didn't have jet fuel (A few months later we did). "No problem" he replied, "80 octane is fine." In it went, up he went, and several more times in fact, before we had an MU-2 based there and jet fuel became a regular commodity. Whether it could be done all the time is something I'm not sure about though, but maybe Evan could help us here?

[Ed S]

A couple on interesting antedotes along this line.

When I flew T-33's we could use avgas as an alternate fuel.  But you had to add so-many quarts of motor oil per so-many gallons of avgas.  (I don't remember the actual ratio.)  In general you had a 10-20% decrease in range when using avgas.

The OV-10 had a dial on the side of the engine that you could adjust depending on the density of the fuel you were using.  It helped prevent overtemps when burning anything other than jet fuel. Basically if it was liquid and flammable, you could run it through a turbine engine.

Ed

[rallymodeller]

The B-36 used avgas in the podded jets as well. There were tabs in the engine exhausts to regulate EGT, as avgas burns much hotter than Jet A.

I'll have to find it, but somewhere I have an early B-52 pilot's manual (B-52E, I believe) that has instructions on how to run the engines on a variety of different fuels in an emergency situation -- i.e. your base got nuked and the only thing at the airfield you used as your alternate is avgas, or diesel, or cooking oil, or whatever.