De laval nozzles are optimized for a specific altitude... So what?

Sloppy researcher ain't you?

What's your Imaginary Qualification in?

De Laval nozzles will only accelerate gases above mach 1 at the specific back-pressure they are designed for.

See the problem here?

So if they're designed to work best at sea level they will stop accelerating gases above mach 1 within a couple of thousand feet.

By the time they're up to, say, 50,000 feet they'll be practically useless.

And good luck getting to teh munn at subsonic velocity!

Like Twelve Steps said; it's not rocket science...

nope, u got your facts wrong.

the de laval nozzle will have mach=1 in the throat, so critical conditions, when combustion chamber pressure Pc and exit pressure at the nozzle Pe have a particular ratio.

considering a mono-dimensional isentropic flow that ratio for sonic flow at the throat of the nozzle is

Pe/Pc=(1+(gamma-1)/2)^(-(gamma/(gamma-1)))

where gamma is a property of the fluid you're considering, for example ideal gasses have a gamma of 1.4 and a ratio of 0.528.

whenever the ratio Pe/Pc is below the one given by the formula above u have sonic condition at the throat of the nozzle.

(if u're not used to the terminology throat is the section between the convergent and divergent part of the nozzle)

optimizing a nozzle means that at the exit of the nozzle u have the outside pressure, this is optimization cause the thrust is obtained by giving an increment of velocity to the fluid and if u convert all the pressure and thermal energy in kinetic energy u've optimized the nozzle cause u cannot give the fluid a higher velocity.

if one optimizes a nozzle at sea level u have that the pressure at the exit of the nozzle Pe is equal to the ambient pressure Pa but as u climbing the exit pressure remains the same(cause u're nozzle has been designed to expand up to a certain point) but theambient pressure decreases so u're not using all the energy of the fluid and u're not efficient.

if u on the other end optimize a nozzle for very high altitude and u use it at sea level u have a much worse scenario, cause u're actually over expanding u're flow and it creates a system of shock wave to return to the ambient pressure, and this reduces a lot your trhust.

btw rocket do not accelerate external fluid like jet engines so the thrust is given only by the the flow rate and velocity exiting the nozzle(plus a term dependet on presssure difference between exiting condition and ambient pressure if the nozzle is not optimize) so it really doesn't matter if u have an subsonico o supersonic flow, thrust is always produced, of course since the trhust depend directly on the speed of exiting fluid the higher the better.

morevoer u know speed of sound do depend on temperature right?? the higher the temperature the higher the speed of sound so even a subsonic flow at very high temperature has a speed greater than the speed of sound at ambient temperature.