Phase 1 of the Advanced Development Program for a High Performance Oxygen/Hydrogen Rocket Engine, was an evaluation of the critical technology associated with the staged-combustion bell nozzle engine system. Experimental evaluation was conducted in the areas of preburner, main chamber, nozzle, turbopump bearings, and engine controls. In addition, engine system (module) preliminary design and applications studies were conducted. In the Module Design Study, a system cycle balance, Steady-state off-design analyses, transient analyses, component and system design studies, a weight study, and a parametric engine study were completed. The Applications Study was completed, and a separate final report, AFRPL-TR-67-270, was issued. Under the Cooling Investigation, 50K staged combustion tests were conducted that demonstrated high impulse efficiency and the two-position translating nozzle. Under the Turbopump Components investigation, endurance testing of the hydrogen turbopump bearing demonstrated long life is feasible, but roller skewing remains a significant problem. Under the Module Control System investigation, the oxidizer flow divider valve, the mixture ratio valve, and ignition systems were designed, manufactured, and tested with the preburner and main burner. Continued development is needed to improve seal performance. The Preburner Demonstration investigation was completed, and ignition, control, and dynamic combustion stability were demonstrated; however, additional development is required to reduce the hot gas temperature profile. Under the 250K Staged Combustion investigation, the 250K main chamber was tested.