Re: Why are rocket exhaust areas broad ?

Current rocket systems have nozzles with a fixed exhaust area.  Optimum 
expansion occurs when the exhaust pressure equals the ambient pressure.  
Since the nozzle is fixed in area, optimum expansion is fixed for a 
particular altitude.  Therefore, upon ignition, the exhaust plume has a 
visible tail.  As the rocket reaches increasing altitudes, the tail widens 
and eventually disappears.  Once the plume passes optimum expansion 
(nozzle exit pressure equals ambient pressure), the plume widens beyond 
the diameter of the nozzle.  Once the rocket leaves the atmosphere, the 
plume seems to disappear, as the ambient pressure drops to near zero.

-Dennis