NPSS 3.3 Brings Hybrid Electric Propulsion and External Tool Integration to Aerospace Simulation

Propulsion simulation software rarely makes headlines outside the aerospace engineering community. But the tools that allow engineers to model gas turbines, rocket engines, and hybrid electric propulsion systems without building physical prototypes are foundational to modern aircraft and spacecraft development. NPSS - the Numerical Propulsion System Simulation software - has occupied this role since its origins at NASA's Glenn Research Center in the 1990s, and Southwest Research Institute has now released version 3.3 on behalf of the NPSS Consortium.

The update is not a rearchitecting of the software but a targeted set of capability additions that respond directly to where propulsion engineering is heading: toward electrification, toward tighter integration with external modeling tools, and toward more flexible data handling.

Electric Port Support: Modeling the Hybrid Future

The most significant new capability in NPSS 3.3 is electric port support. As the aerospace industry moves toward hybrid turbo-electric configurations - systems that combine gas turbines with electric motors and generators - simulation tools must model both the thermodynamic behavior of combustion systems and the electrical dynamics of motors, inverters, and batteries in the same computational environment.

NPSS 3.3 introduces dedicated electric port interfaces that allow users to build hybrid propulsion models, including electric vertical takeoff and landing (eVTOL) systems. Complex numerical functions support the mathematical representation of electrical components, enabling analysis of energy conversion efficiency, thermal loads, and system interactions across the full power train.

This capability matters because hybrid-electric architectures behave differently from purely mechanical systems in ways that are difficult to capture with separate models running independently. Control inputs on the electrical side affect compressor operating points; turbine shaft power splits between mechanical and electrical loads depending on flight condition. Modeling these interactions in an integrated environment provides a level of fidelity that isolated thermal or electrical models cannot achieve.

Functional Mock-Up Interface: Breaking Tool Silos

Engineering simulation exists across dozens of specialized software packages. A gas turbine performance model built in NPSS may need to interact with a structural dynamics model, a control system simulation, or an aircraft-level energy management tool - each running in different software from different vendors.

NPSS 3.3 incorporates the Functional Mock-up Interface (FMI) industry standard, which provides a common protocol for different simulation tools to exchange data and synchronize in time during co-simulation runs. A gas turbine model in NPSS can now communicate with an external engine control system model, allowing both to run simultaneously and interact as they would in the actual aircraft.

"A gas turbine performance model created in NPSS can now communicate with an external engine control model, enabling co-simulations that combine performance and external control models at the same time," said Griffin Beck, who oversees the NPSS Consortium at SwRI.

Foreign Function Interface: Custom Physics Integration

The foreign function interface (FFI) takes integration further at the level of individual calculations rather than full software packages. Where FMI allows two complete simulation environments to communicate, FFI allows NPSS to call specific external functions written in other languages or compiled from third-party code.

The practical application Beck highlighted is combustion modeling. Standard NPSS combustion representations make simplifying assumptions about flame dynamics and chemical kinetics. An organization with a high-fidelity combustion code - one that captures detailed chemistry at the cost of computation time - can now link that specific function into an NPSS model while leaving the rest of the simulation running in NPSS's efficient environment.

Context and Users

NPSS has been in continuous development since 2013 under SwRI's management and supports applications ranging from commercial turbofans to liquid rocket engines, refrigeration cycles, solar power systems, and industrial gas turbines. The Consortium model - where aerospace companies and government agencies collectively fund development and share access - has enabled the software to evolve with industry needs without the development costs falling to any single organization.

Version 3.3 also adds native CSV import and export, reducing the friction involved in handling engine performance data generated externally or passed downstream to other tools. This may seem minor relative to the FMI and FFI additions, but data format incompatibilities are a practical bottleneck in complex modeling workflows that consume significant engineering time.