Robon, the Final Engine

Whenever a creature you control crews Robon,
the Final Engine, you get {e}.
Crew 3 (Tap any number of creatures you control
with total power 3 or more: This Vehicle becomes an
artifact creature until end of turn.)
Whenever Robon attacks, you may pay {e}{e}
{e}. If you do, it gains flying and double strike
until end of turn.