This section is geared those who have a background in aerospace engineering:
The concept of multiple rocket exhaust areas was based upon the Strutjet concept, which used multiple rockets in an individual air duct. The mixing effects of multiple rockets entrained more air for combustion, leading to greater efficiency. The team wanted to examine having multiple rocket exhaust ares without multiple heavy thrust chambers, so they built a annular nozzle with three major circular arcs and small circular air entrainment tubes in between each arc. For comparison, the team also used a circular nozzle.
To enable accurate comparison, the mass flow and Mach number was kept constant across the two nozzles. The experiment was also set up have the maximum amount of entrained air at the duct exit. Pressure sensors were arranged around the nozzle and exit plane of the duct. To replicate high speed environments, ambient air was injected into the duct at high pressures, replicating up to Mach 2 speeds (after expansion of the flow). No fuel was injected.
Initial results showed the annular nozzle entrained more air than the circular nozzle at lower pressures. The pressures taken at the duct exit plane also showed that the air pressures were more uniform in the annular nozzle configuration, suggesting more mixing of the air had taken place. These results showed an average Mach number 58% higher in the annular configuration than the circular configuration.
Please see the article “Experimental Investigation of an Alternative Rocket Configuration for Rocket Based Combined Cycle Engines” in the July-August edition of the AIAA Journal of Propulsion and Power for more details.