A NASA engineer saw our image of the space shuttle Discovery's engines and thought it wasn't good enough. She sent us these exclusive pictures looking right inside the nozzle. Surprise: It looks exactly like a shower head. Zoom in!
I always thought it was going to be one single hole. Instead, it has several little holes set on a flat metal surface, like in a shower head.
Indeed, it looks like a shower head. It's actually the injector. The LH2 comes out, in gaseous form, through the smaller holes around the whole circumference and the LOX comes out, in liquid form, through the small center hole. The picture without the injector shows the igniter. This igniter is turned on for about 3 seconds when the SSMEs are fired right before lift off. After the 3 seconds it shuts off as once the fuel starts burning it becomes self-sufficient. The injector cap (shower head) was designed by Pratt and Whitney. The SSMEs were designed and built by Rocketdyne.