Start Stop Transients Engine start - thrust buildup: 1.5 - 2 sec if engine is warm .5 second longer if engine is cold It takes 11 1/2 seconds to start total - 3 1/2 sec to get engine out of section and 8 sec prestart. Seconds count - 15lb of payload is lost for every second of wait. Digression - they burn the Hydrazine at the end to lift heavier payloads. Start Stop Transients Must consider - loads on vehicle - loads on space craft Axial impact loads dictated by start stop transients. Questions regarding start-stop transients: What loads are produced? How are they amplified? What will they do to payload? 0 - full thrust of 22,500 lb in 2 seconds propose 0 - 50% (11,000lb) then sit momentarily, and 2 sec increase to 22,000lb Shut Down - Spring back effect - 2-3g acceleration. When thrust is cut - puts loads on payload. New proposal - 1-2 seconds at 50% level then shutdown. Same argument - all parts are designed for worst case. Why make it easier for: - components - payload Start Sequence: 1. Vehicle supplied He pressure is created by activating the Oxidizer and fuel prestart solenoids 2. Both inlet valves (O2 and H2) are opened allowing oxidizer and fuel to flow 3. Vent valves close 4. Bypass (for prestart flow) on Oxidizer Flow Control and purge check valve is closed. Enough O2 flows through the system to start the engine. The rest of the Oxidizer Flow Control valve opens as LOX pressure comes up(~ as turbine starts pump?) opening spring-loaded closed valve. 5. Gaseous H2 drives the turbopump. Moderate chamber pressure during the start transient. As O2 comes up, opens valve (as stated above). This valve varies O2 flow to adjust (modulate) the mixture.