To meet the needs of NTP engine system design analysis, NASA has developed an accurate, versatile program, known as the Nuclear Engine System Simulation (NESS) to support ongoing and future engine system and stage design study efforts. To accomplish this, Science Applications International Corporation's (SAIC) NTP version of the Expanded Liquid Engine Simulation (ELES) program was modified extensively to include both Westinghouse Electric Corporation's near-term and next generation solid-core reactor design models, ENABLER I and ENABLER II, respectively. The Westinghouse reactor design models are based on the near-term and upgraded version of the solid-core ENABLER NTP reactor design concept. The ENABLER I model provides a near-term solid-core reactor design based on the NERVA reactor. ENABLER II provides a more advanced reactor design, with flow paths and scaled fuel, reflecting state-of-the-art technology, and yielding reactor designs with higher power densities and lower weights.

The NESS program is used for rapid, preliminary detailed design analysis of both the reactor and key engine systems. The code designs the reactor, turbomachinery, tankage, nozzle, lines, and valves in terms of both weight and performance/operating characteristics. NESS is capable of modeling expander, gas generator, and bleed cycles, along with multiredundant propellant pump feed systems. Engine systems may be designed and evaluated for both pump-out and normal operating conditions, with an option available for automated iteration of pump design to satisfy both operating conditions. Turbopump design options include the efficient axial multistage pump along with the traditional centrifugal pump. Key code outputs include reactor operating characteristics and weights, as well as engine system parameters such as performance, weights, dimensions, pressures, temperatures, mass flows, and turbopump operating characteristics for both design and off-design operating conditions. All cycles use hydrogen as the propellant, with oxygen used as needed for the gas generator cycle. NESS is easy to use, runs quickly, and is flexible enough to efficiently address a wide variety of solid-core NTP engine system design options. Because of the modular nature of NESS, it has great potential for further upgrades in its design/technology option and analysis capabilities.

Through an initial validation effort, the NESS program has been deemed accurate enough to support preliminary engine and vehicle system design and mission analysis efforts. The development of NESS is considered to be one of the key steps required to support NTP design. It can be a valuable design tool element when integrated into an advanced NTP engine system design workstation.