Deep Throttle of a Nitrous Oxide and HTPB Hybrid Rocket Motor

Stephen Whitmore, Utah State University
W. Z. Peterson
S. D. Eilers

Originally published by AIAA in AIAA Journal of Propulsion and Power.


Deep-throttle static test results from an N2O and HTPB hybrid rocket motor are presented. The nominal 800 N thrust level was turned down to less than 12 N while still maintaining stable and controlled combustion. This 67:1 turn down was accomplished using a commercial-off-the shelf throttle valve and a solid rocket motor case adapted for hybrid rocket testing. During throttled motor tests the pressure ratio across the injector grows from a nominal value of 2.0 to greater than 3.0. This feature contrasts with the observed behavior of liquid rockets where injector pressure ratio drops significantly during deep throttle. This characteristic likely supports the observed hybrid burn stability during deep throttle. Data comparisons with a physics-based, throttled, hybrid rocket burn model accurately match for combustor pressure, thrust, and propellant consumption. At throttle levels approaching 20% of nominal, the N2O exiting the throttle valve is entirely in a vapor state. The vapor chokes the injector and eliminates feed system coupling. This two-phase effect is another likely reason for the unexpected combustion stability observed at very deep throttle levels. The ability to throttle deeply opens the possibility that hybrid thrusters could be simultaneously deployed for both main spacecraft propulsion and reaction control.