Here is another video (from 11 March 2013, and only 1631 views) explaining the trimethyl aluminium theory.
Now we know why there's a gap... something about oxygen supply through the engine.
There are so many problems with the theory proposed in this video. Where to start?
It's hard to say if they thought up this cockamamie idea by themselves or got taken for gullibility by someone who figured out a way to make it sound plausible to someone who might know a little but not enough. Why they wouldn't run it past someone who does have some knowledge of gas turbine combustion chamber design is a mystery to me. That would have saved them a lot of trouble making a worthless joke of a video.
The short story is that they are saying that since a gas turbine burns fuel very efficiently, there is not enough air left in the combustion chamber to support combustion of trimethyl aluminum, and so it passes through the hot section intact, and doesn't burn until it "seeks out" air in the wake of the airframe. That is their explanation for the gap behind the engine before the visible contrail is seen. [In reality the gap is actually there because the water vapor (formed during combustion by combining hydrogen in the fuel with oxygen) is invisible until it cools to condensation temp and subsequently freezes into ice crystals.]
The big mistake they make is equating fuel combustion efficiency with a lack of air in the combustion chamber, as if all the air is depleted by the combustion itself. That is not true! I shall now explain why. My explanation is based on intimate knowledge gained as a 1978 graduate of the General Electric Company's Field Engineering Technical Center in Schenectady, New York, and further field experience with gas turbine maintenance. My specialty was industrial scale gas turbine engines in the 15-30 megawatt range. The description below relates to gas turbines with can style liners. The most modern aviation turbine combustion systems use similar designs, but all use about 25% of the axial compressor air for combustion and cooling/dilution of combustion gases in the combustion chamber area.
The gas turbine does not have an enclosed combustion chamber like most internal combustion engines[diesel or gasoline]. It is an open area inside a can-shaped combustion liner. The liners are made of very high strength and temperature alloy steel and are very thin. They are thin to allow them to stay cool rather than be burned out by the flame of combustion. The liners are also surrounded by flowing air which helps to cool them, and are perforated or slotted to allow even more cooling air along their length.
Inside the liner, beginning at one end, there is a fuel nozzle which sprays a continuous flow of fuel down the center of the can. The flame is first exposed to a swirling vortex of low velocity air which also "guides" the flame holding it in the center of the liner. Further down the liner the flame intensity decreases and progressively larger holes allow even more compressed air to enter and cool the flame down before it enters a transition piece from the tubular form of the combustion liner into an annular [ring-like] shape guiding the mixture of combustion gases and cooling air towards the annulus of the [ring shaped] nozzle itself.
Actually, only a very small portion of the air which enters the gas turbine combustion chamber is used for combustion. The VAST majority of the air is used for swirling and mixing the fuel and air for combustion, guiding and holding the flame, cooling the liner itself, and lastly diluting the combustion gas tempering it for its subsequent expansion through the nozzle and turbine blades themselves.
Bottom line, there is FAR more compressed air available even after combustion to allow any combustible fuel similar to trimethylaluminum(TMA) to obtain COMPLETE combustion within a gas turbine combustion chamber. If TMA were in the fuel or put into any part of a gas turbine, and I mean ANY part, it will burst into bright white flame and the end product will be highly erosive aluminum oxide particles able to quickly destroy any hot gas path parts in encounters.
If anyone tells you different, they are morons who don't know diddley squat about gas turbines, and you can tell them Jay Reynolds told you so!
For a detailed look at a variety of industrial gas turbine combustion liners, see: