To reduce aerodynamic drag. A short Book.

(3) SILL LONGERON LOWER ARM POSITIONING UNIT MECHANISM ROTARY ACTUATOR ACCESS DOORS UPPER MAIN HONEYCOMB FULL-DEPTH HONEYCOMB TRAILING EDGE CLOSEOUT RIBS (HONEY - COMB) LOWER MAIN HONEYCOMB PANEL FORWARD LOWER COVER EQUIPMENT BAY FRAMES Figure 4-11.— Orbiter passive thermal control system. 4-31 4. ORBITER SYSTEMS Propulsion ORBITAL MANEUVERING SYSTEM/ REACTION CONTROL SYSTEM.