The Atlas V has been modified for human spaceflight. Human-rating required new computers to monitor performance and trigger an abort when necessary, data links between the rocket and spacecraft, and other changes. Crewed flights will include a mechanism to allow astronauts to manually abort. The Atlas V has already carried the Boeing CST-100 Starliner on its first unmanned spaceflight. The Atlas V that carried the Starliner was equipped with two SRBs from Aerojet Rocketdyne. This was the only Atlas V ever to fly without a payload fairing. With the Starliner on top, the rocket is 172 feet tall.