From: Angel Oliveira, of Madrid, Spain
To: Kelly Beck, flight director

Question: How does the shuttle, Solid Rocket Boosters and External Tank do the roll, pitch and yaw maneuver after launch, and which aerosurfaces make it possible?

Answer: Angel,

The shuttle completes the roll maneuver after liftoff using hydraulic gimbals mounted on each of the Solid Rocket Boosters and all three of the Main Engines. Two gimbals are mounted on each of the Solid Rocket Booster nozzles to control the direction of the booster's thrust in the pitch and yaw directions. There are also two gimbals mounted on each of the three Main Engines to control the main engine's thrust in the pitch and yaw directions. All 10 gimbals are commanded by the flight control computers to work together to point the shuttle in the desired direction. Since we only have pitch and yaw gimbals, we have to command the gimbals in different directions to roll the shuttle. For example, to complete the roll after liftoff, the left Solid Rocket Booster commands a pitch up at the same time the right Solid Rocket Booster commands a pitch down. This combination of pitch gimbal commands results in a left roll of the shuttle.

The aerosurfaces are not used during the launch to control the vehicle. They are only used to alleviate stresses on the structure caused by dynamic pressure at the lower altitudes. In addition, the aerosurfaces would be ineffective at the very high altitudes due to the lack of airflow over the wings.

Thanks for the question!

Michael L. Sarafin
STS-111 Orbit 1 GNC Officer (Guidance, Navigation, and Flight Control)