NSEG was developed to perform rapid aircraft mission analyses. It is based upon the use of approximate equations of motion whose form varies with the type of flight segment. Flight segments considered are takeoff, accelerations, climbs, cruises, descents, decelerations, and landings. Layered atmosphere options are available. The program can also be used for flight envelope mapping. NSEG provides the capability to analyze aircraft missions from low to hypersonic speeds.
Realistic and detailed vehicle characteristics are input to NSEG to permit accurate mission analysis. NSEG allows engine scaling so as to fit the design under analysis. NSEG contains several approximate flight path optimization capabilities based on Rutowski energy-like criteria for considering minimum time or fuel flight segments and maximum range segments during climb or descent. Takeoff and landing analysis is based on the Air Force Flight Dynamics Laboratory DATCOM method of high lift aerodynamic modeling. There are three main atmosphere options available; the 1962 U.S. Standard atmosphere, a stratified atmosphere model, and an external atmosphere model supplied by the user. The stratified atmosphere model requires the input of the number of layers (maximum 25), altitudes, temperatures, and pressures. The mission specification is open-ended in that the upper limit on the number of flight segments to be included in a mission profile can be increased with a simple program change. Input consists of vehicle characteristic data, data to specify details of the mission, and selection of program options. (Aerophysics Research Corp. for NASA Langley)
This program was released by NASA through COSMIC as LAR-12299. The italicized text above is from the official NASA release.
Unfortunately, the source code is mixed Fortran and Control Data 6600 assembler. I may be able to produce Fortran equivalents for the assembler code, or perhaps some wonderful person could help me out.