The Falcon 9 is a two-stage rocket. it is powered by LOX and RP-1. Both stages have Merlin 1D rocket engines. The first stage has nine that are adapted for use at sea-level. The second stage has one that is adapted for use in a vacuum. The engines uses a pyrophoric mixture of triethylaluminum-triethylborane (TEA-TEB) to ignite the engine. The first stage engines are set in a form SpaceX calls "Octaweb." Many cores have four landing legs at the base of the Octaweb. To control the core while it falls through the atmosphere, SpaceX uses grid fins. They deploy from the vehicle after separation. The legs deploy moments before landing.