Expanding nozzle

While the expanding nozzle is the least technically advanced and simplest to understand from a modeling point of view, it also appears to be the most difficult design to build.

Engine bells must be cooled to avoid damage from the hot rocket exhaust, and this has presented problems in expanding nozzle designs.

In the case of liquid hydrogen, the fluid also has the disadvantage of being highly reactive chemically, making a variety of common flexible materials unsuitable for use in this role.

For the aforementioned reasons modern designs (e. g. NK-33-1, RL-10A-4, and RL-10B-2) feature radiatively cooled reinforced carbon–carbon nozzle extensions needing no coolant plumbing at all.

Since these engines are fired from the point of liftoff into extra-atmospheric space flight, any sort of altitude compensation could dramatically improve their overall performance.