>>1586749I understand they've set it up like that for the sake of simplicity, but it simply won't work. The carbonation of the cola is at the heart of the reaction, the mentos simply acts to release it. The mass of the coke being pushed out of the rear is what generates most of the thrust. So, unless someone keeps pouring coke into the reaction chamber, most of the mentos is simply dead weight, and that puny coke bottle isn't going to provide ejectable mass for any meaningful amount of thrust.
Not to speak of that without a proper valve at the entry point, roughly half of the pressure is going to vent out without producing any effective thrust.
A simple valveless solution to this would be to have a tower which feeds cola into the chamber. The depth of the cola would with gravity provide pressure, which would keep "fuel" flowing into the chamber as long as the pressure of the chamber is lower than that of the cola. Keeping the tube topped off, you'll get an amount of thrust relative to the depth of cola. The tube might suffer a blow-out, but that's just a sign it's not tall enough or that there's not enough liquid to keep the pressure for keeping stuff down.
This can be solved with a one-way valve between the tube and the bottle. This stops blowouts, but the overall maximum thrust is still limited by the pressure by which fuel is being fed to the chamber. As before, if chamber pressure exceeds fuel pressure, fuel is unable to enter the chamber, and the reaction slows down. As it slows, pressure drops, and fuel can once again flow in, leading to a repeating cycle of rising and dropping thrust with a peak rising a bit above cola pressure.
Assuming the bottle doesn't burst, of course.
The only true solution is acquiring an actual pump to feed cola into the bottle at high pressures at an optimized rate (considering the strength of the chamber/bottle) and design a bell nozzle optimized for the expected range of engine pressures and local atmospheric pressures and...
