How do you launch a spacecraft with a 12-meter-diameter antenna via a 3.6-meter-diameter launch vehicle? How does a single launch vehicle support different spacecraft firmly enough to withstand launch forces, but gently enough to drop them into their correct orbits with a rotational speed no more than one-sixth of a clock's second hand? Moving assemblies—or mechanisms—accomplish critical tasks such as these.
During launch, mechanisms help the launch vehicle shed excess weight. For example, many vehicles carry external solid-rocket motors that must be discarded once their fuel is used up. Release mechanisms and hydraulic actuators propel them away from the core vehicle.
Similarly, a launch vehicle's internal fuel tanks are shed when their fuel is depleted. An explosive charge breaks the connection between a rocket's stages, then compressed springs and guides help separate the spent stage.
Further into flight, as the atmosphere becomes negligible, the protective payload fairing becomes unnecessary, so it gets jettisoned. A continuous explosive charge or discrete pyrotechnic bolts break the connections between fairing segments. Then, hydraulic actuators team with open hinges, cams, or other mechanical apparatus to move the segments away from the launch vehicle without recontact.
The final separation is the payload release. One method for this is similar to a stage separation, where separation nuts release structural bolts joining the payload and rocket. Another method uses a clamped-band system, in which both bodies possess similar interface rings, and a band is tightened around their flanges. In this case, a bolt cutter or separation nut releases the band, thus allowing compression springs to impart relative velocity between the bodies.
Critical mechanisms such as these require stringent analysis to ensure proper design and operation. The Aerospace Corporation has developed specialized tools for static, kinematic, and dynamic analyses to predict positions, velocities, accelerations, and reaction forces for multiple rigid and flexible bodies. These computer-aided simulations model complex, three-dimensional separation mechanisms and events to verify that mechanisms will perform as intended.