Simple Mission Research Articles

Fatigue Life Prediction of Titanium Matrix Composites Under Mission Spectrum Loading

Fatigue life predictions are conducted for a high temperature composite for hypersonic vehicle structural applications. A SCS-6/Timetal® 21S [0/90]s composite is evaluated under thermomechanical fatigue (TMF) as well as under typical hypersonic mission profiles and segments of those missions. The fatigue life analysis is based on micromechanical stresses which, in turn, are input into a damage summation model. Spectrum loading is decomposed into a series of in-phase and out-of-phase TMF segments whose lives are predicted from a life fraction model which was developed for isothermal and TMF data. Analytical predictions for the mission spectrum and the simple mission profiles are compared with experimental data obtained here as well as in a previous study. The model is demonstrated to be able to predict fatigue life under mission spectrum loading based solely on calibration under simple isothermal fatigue and TMF conditions.

Tracking and data acquisition for space exploration

The tracking and data acquisition systems provide the key link between the remote spacecraft and the scientific experimenter on the ground. The operation of the space experiment takes place through the links of command, telemetry and tracking. The evolution from the early very simple spacecraft missions toward more complex and sophisticated missions has been paralleled by a similar evolution in the tracking and data acquisition systems. The early Minitrack interferometer tracking system still carries the major tracking workload for space missions; however greater tracking accuracy requirements for more recent missions, such as the Orbiting Geophysical Observatory and the Apollo mission, have brought about the development of unified tracking and data acquisition systems which utilize hybrid pseudo-random code/sidetone ranging techniques. The data acquisition has evolved from analog telemetry systems to the present day heavy use of PCM digital telemetry. Likewise the command systems have evolved from early simple ‘on/off’ command systems into PCM digital command data systems. The trend is toward greater real time control of more complex functions on board the spacecraft. Newer spacecraft are incorporating computer-type systems in the spacecraft which require programming and memory load through the ground command link. The most attractive concept for the next generation network for tracking and data acquisition is a network consisting of synchronous-orbit Tracking and Data Relay Satellites for covering launches and low-orbit earth satellites plus a few selected ground stations for supporting spacecraft in high earth orbit and lunar orbit.

Digital Simulation of Guidance and Control for Exo-Atmospheric Intercept

A mathematical model for various configurations of interceptor missiles operating outside the earth's atmosphere is described. The associated digital computer simulation program provides means for evaluating the performance of different guidance and control systems and for studying the influence of parameter variations on the overall mission effectiveness. The simulation program also provides a means for establishing dynamic and kinematic criteria for the variable mass interceptor configurations. A complete attack mission, from exo-atmospheric launch to kill, may be carried out in three-dimensional space with all six degrees-of-freedom of the interceptor accounted for in the dynamics. Several models of thrusting and passive tracking may be selected for guidance. The guidance law is based on biased proportional navigation. The program simulates the sensing of the target, control system functions, and interceptor dynamics, all in closed loop form. The control loops contain optional time lags, gains, limits, and shaping neetworks in addition to a White Gaussian noise generator capable of simulating both receiver noise and angular scintillation. The vehicle's physical characteristics, the control constants, and the intercept mission are entirely arbitrary. The target is a pre-programmed point mass whose trajectory may be described by tabular data, equations for constant acceleration, circular equations, or other options. Output generated consists of time histories of all significant functions and a series of plots generated on a 1403 printer. A simple mission used to demonstrate the capabilities of the program will be discussed.