Mod policy is not to reveal the substance of the communications.
Understood, and I agree that is a good policy. However, we trust that if any potential individual liability is at stake, the proper disclosures will be made in an appropriate manner. Thanks for your help dealing with a legitimately vexing customer.
The word "jet" is not specific. It just means that you use a controlled explosion to generate thrust.
Or even just matter expelled with significant momentum. That's the bottom line for a "jet."
There are cold-gas methods of generating momentum. They occur, as you mention, by storing materials that can be mechanically or chemically (i.e., adiabatically) expanded and directed through proper orifices and ducts to generate momentum. We can decompose unstable materials rapidly by chemical means to achieve a dramatic change in volume. We can expel fluids stored under pressure. We can even accelerate materials to great velocity by electrostatic means. All these have real-world implementations in our engineering vocabulary.
But thermodynamic processes provide very attractive solutions. Thermal expansion is rapid, dramatic, and easily achievable through chemical processes. They add the advantage of combining the thermodynamic process reactants with the working fluid. We specifically seek low molecular weight compounds with small heat capacities so that the thermal energy of the chemical reaction translates more directly into mechanical expansion of the working fluid.
In what we commonly call a jet engine, we use ambient materials not only to sustain the combustion process that implements the thermodynamic component of the engine design, but also to provide the reaction mass. The oxygen sustains combustion, and a large portion of the heat thus produced is transferred to more inert elements of the working fluid. This minimizes the amount of reagents that must be carried in the vehicle. But further, most jet engines today are high-bypass turbofans, meaning that a large portion of the thrust is not created by thermodynamic means, but rather by ordinary aerodynamic means in the forward part of the engine. The ducted fan, driven by mechanical power generated from the hot gas in the engine, directs ambient fluid along the annulus of the engine.
Rocket engines, which are also "jets" in the sense that they rely on Newton's third law to produce thrust, obviously must carry all their reagents with them.
The layman's confusion occurs often when he misunderstands the coupling in the Newtonian engine. He wrongly believes that the thrust is produced as the high-velocity exhaust stream pushes against the ambient atmosphere or some solid surface, much as would occur if you sat in an office chair and pushed off from the wall with your feet. That's not how thrust works. Directing a jet of material in one direction produces a reaction in the opposite direction, with equal momentum. This is how momentum is conserved. It works even in a vacuum. Better, in fact, since the exhaust stream is not impended by an ambient.
