The injection (INJ) subsystem has been in a design freeze since last semester (Fall 2012). This subsystem includes the injector (shown below) and the injector housing used to hold it in place. The injector plate is made of titanium since it is one of the few materials that can withstand the temperatures seen in the combustion chamber with the material properties that are sought after (lightweight and strong). The injector plate is used to atomize the liquid oxidizer (nitrous oxide) so that it reacts and burns with our solid fuel grain.
While the injector plate itself is not too massive, the injector housing is very overdesigned (with a safety factor higher than 10). While we are not currently working on a redesign to reduce this, it will certainly have to be redesigned to save mass for our able-to-launch system.
See fuel update for a look at what the injection system looks like.