Page:NASA Launch Vehicle Handbook.pdf/9

 Flight Programs

For nominal flights, the pitch gyro of the Scout will be torqued at rates which will produce a zero lift; that is, gravity turn, trajectory. This is done by first approximating as accurately as possible the pitch rate history associated with a desired controls-locked, no disturbance, zero-lift trajectory and then modifying this slightly to account for an inherent system lag. The basic pitch program is in reality a series of step functions of such magnitude and duration that the total area formed by them is equal to the total area under the pitch attitude rate curve, θ, of the expected trajectory. The magnitude and length of the step attitude rate functions are determined from a series of straight line slope approximations to the desired pitch attitude curve. Accuracy of the program is affected by winds, thrust misalignment, and inherent control system lags during the first stage and droop associated with the characteristic deadband of the second and third stage "on-off" control systems. Generally however the difference between the programmed attitude and the desired flight-path angle are small except at launch with these differences tending to diminish asymptotically with time as the velocity vector gradually tends to align itself with the thrust vector. No attempt is made to adjust the pitch program for winds and thrust misalignment at this time.

In programming a Scout trajectory, certain restrictions must be adhered to. First, the Scout must be launched at elevation angles of 78° or greater if the aerodynamic heating encountered is not to become too severe. Secondly, the vehicle cannot be programmed to fly a trajectory much different from a nominal zero-lift trajectory due to structural limitations during first-stage flight and to the fact that the maximum available control power imposes certain limitations on the permissible deviations from a zero-lift flight path. At present no provision is made for roll or yaw maneuvers although such maneuvers do seem feasible with only nominal changes to the guidance system.

During actual flights, Scouts A, B, and C rocket motors will be fired according to the following sequences. The first-stage rocket motor will be fired from the ground level. After its burnout, the first-stage motor will remain attached to the vehicle until an altitude of 130,0OO feet is reached. The coast to 130,000 feet is done to effectively cancel the aerodynamic instability of the remaining stages and to relieve heating loads that could be incurred by igniting the second stage at lower altitudes. The second stage is ignited by means of a programmer and the first stage is immediately blast-separated. Following second-stage burnout, the vehicle coasts for a nominal period of 5 seconds after which time the programmer ignites the third stage and the second-stage burned out motor is blast separated. At this time the