Closeup, slow motion of ~ Space Shuttle Main Engine startup.
* The “sparks” are igniters meant to burn off excess hydrogen gas during engine start but before mainstage (full power) operation. Without these, a large cloud of explosive hydrogen and air could form under the Shuttle. If it exploded, it could damage the Shuttle structure or knock off tiles. These sparks DO NOT “ignite” the rocket engines. Engine ignition is accomplished by an internal “blowtorch” of fuel and oxidizer in each engine, which is ignited with sparkplugs!* Notice that the engines start in a sequence which is about 120 milliseconds (a touch more than 1/10 second) apart. They do not start at the exact same time.
* At engine start, the engines are moved (gimballed) away from each other because they jump around during start. If they were too close, they might collide.
* The engine steering hardware is hydraulically “disconnected” from the engines during start so that the engines can bounce around without breaking the “steering linkages”.
* During engine start, before full power is reached, the exhaust (flames) disconnect or separate from the nozzle interior, causing violent thrust vector movements and misalignments. This is what makes the engines jump and wobble during startup.
* After the engines are fully started and running, the hydraulic steering is re enabled and the engines are steered toward proper liftoff angles.
* Once the engines are started, the Space Shuttle tips forward several feet, then springs back. When it is sprung back to true vertical (and if the 3 engines are OK), the two large solid rockets are started, hold down nuts are blown off with explosives and it’s LIFTOFF!
* For comparison, all THREE Space Shuttle Main Engines (not counting the two large, tall tubular solid rockets) generate about as much thrust as only ONE first stage moon rocket engine (the F1) did.
* The propellants for the Space Shuttle Main Engines are all contained in the big central “external tank”. Oxygen on the top 1/4 and hydrogen on the bottom 3/4. The fuel (hydrogen) is so cold it would freeze AIR into a solid “ice” which means fuel lines must be free of all air (they are purged with helium).