🚀 go-pugleaf

On Date: Mon, 4 Jan 1993 13:44:26 GMT, "Herity D." <dherity@cs.tcd.ie>
writes :
>As I understand it, the exhaust is confined between the spike and the
>external airflow. So how can it work in a vacuum ? Can it ?

The exhaust plume boundary expands as the ambient pressure decreases in 
*both* bell and aerospike nozzles (this can be seen very well during 
night launches).  High performance over most of a rockets' operational 
profile (low pressures and in a vacuum) is achieved by having a high 
exit/throat area ratio which allows large plume expansion.  Doing this 
with a bell nozzle can result in flow separation at the walls of the 
nozzle near the exit when operating at low altitudes (launch), which 
leads to loss of performance and possible structural failure of the 
nozzle due to dynamic loads [flow separation is responsible for the large 
nozzle motion on the SSMEs during startup - watch closely next launch if 
you can get NASA Select].  Therefore a compromise altitude must be used 
for the design point of a bell nozzle.

The advantage of an aerospike is that it can achieve the desired high 
exit/throat area ratio at high altitude while still being efficient at 
lower altitudes due to the automatic compensation.

As I noted earlier, an excellent basic description of nozzle design at 
aerospike engines can be found in Rocketdyne's "Threshold" magazine, 
Number 8, Spring 1992. (write to Rocketdyne Division/Rockwell 
International, 6633 Canoga Avenue, Mail Code AB57, Canoga Park, CA, USA, 
91304 or call (818) 568-2380 to get a copy).

Disclaimer: Opinions stated are solely my own (unless I change my mind).
Ben Muniz, Rocketdyne, SSF Dynamics | "Man will not fly for fifty years"
munizb@rocket.rdyne.rockwell.com    |     Wilbur to Orville Wright, 1901