POST2 increases the trajectory simulation capability of the original POST computer code and provides a state-of-the-art software tool. POST2 contains many basic models (such as atmosphere, gravity, propulsion and navigation system models) that are used to simulate a wide variety of launch, orbital, and entry missions. As indicated above, POST2 can support multiple vehicles in a single simulation, each with independently defined environments, vehicle and attracting body characteristics. Thus, each vehicle can have its own guidance, navigation, and control system for completely independent, onboard autonomy. Conversely, effects of multi-body and interaction forces that depend on the relationship of one vehicle to another can be included.

Additionally, POST2 can support three degree of freedom (3DOF) and six degree of freedom (6DOF) trajectories within the same simulation; not only can each vehicle trajectory support different degrees-of-freedom, but also each trajectory segment within a given simulation can be either 3DOF or 6DOF. The internal structure for variable value storage was adjusted in POST2 to permit efficient storage of multiple vehicle/simulation information as well as update coding structure to current standards. This usage of variable structures also increases POST2 portability to other platforms. Variable structures (and substructures) also provide code efficiency by allowing the same FORTRAN routines to be used for engineering calculations (e.g., aerothermodynamic heating, aerodynamic forces, propellant flowrates, attitude angles, etc.) of all vehicles by simply exchanging the data of one vehicle for another during the simulation. New input enhancements provide N-dimensional tables, multiple criterion to identify trajectory phase completion, Boolean logic for these multiple criterion, and the ability to use variable (instead of constant) values to trigger events, as well as increase code portability between various computer platforms and operating systems.

Several features related to the multiple vehicle capability of POST2 enable trajectory simulations not previously possible in a single POST run. POST2 provides the ability to initialize multiple vehicles from the state of another during the simulation. That is, an additional vehicle can be initialized by providing input deltas to another vehicle’s state (position, velocity, attitude, and attitude rate) at any event in the simulation. This capability also includes a mechanism for transferring some or all of the parent vehicle’s angular momentum to the children vehicles. Also, the ability to activate and/or deactivate any number of vehicles at any event in the simulation is included. The standard POST2 input options are available to initialize the state of any vehicle being activated. POST2 maintains and increases the user’s ability to modify certain subroutines for specific applications. The software is constructed such that user provided code can be included to provide vehicle aerodynamic data, atmosphere model, and even optimization capability. While POST2 provides very adequate models for including these data and functions, the user has substantial flexibility to include mission specific models as well as company proprietary representations and functionality. Additionally, support for statistical analysis approaches (such as Monte Carlo dispersion analyses) is also provided.

POST2 has been compiled and run on the following operating system: PC-Linux, PC-WINDOWS, and MAC OSX.