NASA - National Aeronautics and Space Administration

"Nozzle side loads" are a major design consideration for rocket engine exhaust nozzles, and the J-2X is no exception. Side loads, or pressures exerted on the sides of the engine nozzle, are most severe during engine start as the rocket exhaust plume fills the nozzle, as well as at shutdown when the plume empties from the nozzle, pressing unevenly around the nozzle walls. Engineers must make the nozzle strong enough to withstand those uneven forces. Yet they must not risk over-designing it, which could add needless weight that detracts from the launch vehicle's overall payload capability. Testing at the Marshall Center's Nozzle Test Facility enables Pratt & Whitney Rocketdyne design engineers to characterize these side loads and apply this test data to computer analyses used to design the nozzle to withstand the side loads.

Subscale nozzle test articles were developed to simulate the J-2X nozzle -- both with and without the turbine exhaust gas injection. In the J-2X engine exhaust, gas from the turbines that pump propellants to the combustion chamber will be injected around the nozzle extension to provide a cooling barrier against the hotter combustion chamber exhaust, as well as provide additional engine thrust. This film coolant flow has a noticeable impact on the nozzle side loads. The recent tests simulated rocket exhaust flow, using warm air instead of hot rocket exhaust -- a process termed "cold flow" testing.

Since testing began in March 2008, more than 1,000 J-2X nozzle start simulations have been performed.

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