Rocket Sentry is Engineer's killstreak.
Requires 5 kills to activate
When you activate it, the Engineer drops an orange sentry that fires homing rockets. These rockets do high damage and can kill enemies in one hit.
The Rocket Sentry is a small orange turret with 4 legs ( game's animation show only 3 ) and a rocket launcher extension that rises up briefly after it's activated.
Nathan uses this Killstreak ( only count Unique Heroes ).
Rocket Sentry was also a killstreak in Strike Force Heroes 2