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The Generalized Fluid System Simulation Program (GFSSP) is a general-purpose computer program for analyzing steady state and time-dependent flowrates, pressures, temperatures, and concentrations in a complex flow network. The program is capable of modeling phase changes, compressibility, mixture thermodynamics, and external body forces such as gravity and centrifugal forces. GFSSP was developed at Marshall Space Flight Center for flow analysis of Rocket Engine Turbopumps and Propulsion Systems. GFSSP was the recipient of the NASA Software of the Year Award in 2001.

Seen here, is a nighttime rendering of the evolved SLS Block 1B Crew variant positioned on the mobile launcher.  This configuration of the rocket, with the Exploration Upper Stage, will provide in-space propulsion to send astronauts in NASA’s Orion spacecraft and heavy cargo on a precise trajectory to the Moon. The evolution of the rocket to SLS Block 1B configuration with EUS enables SLS to launch 40% more cargo to the Moon along with the crew. Manufacturing both the core stage and Exploration Upper Stage is a collaborative effort between NASA and Boeing, the lead contractor for EUS and the SLS core stage. SLS is the only rocket that can send Orion, astronauts, and supplies to the Moon in a single mission. The SLS rocket, NASA’s Orion spacecraft, Gateway, and human landing system are part of NASA’s backbone for deep space exploration. Under the Artemis program, NASA is working to land the first woman and the next man on the Moon to pave the way for sustainable exploration at the Moon and future missions to Mars. (NASA)

GFSSP is a network flow solver. The thermo-fluid system to be analyzed is discretized into a network of nodes connected by branches. Conservation of mass and energy is solved in the nodes to calculate fluid pressures and temperatures. The momentum equation is solved in the branches for flow rates. Conjugate Heat Transfer may be added to the model through the use of solid nodes connected by conductors. A graphical user interface, MIG (Modeling Interface for GFSSP), allows users to build models by pointing and clicking. Customized branch options or new model logic may be added to the code through Fortran user subroutines.

Properties for common fluids in propulsion applications are provided by the GASP/WASP and GASPAK property packages. Users can also supply fluid property interpolation tables. A library of common fluid components such as pipes, orifices, and valves is provided. Built-in advanced options include pressure and flow regulators, valve opening/closing, and propellant tank pressurization/draining. Thirty-two example problems demonstrate the use of the various capabilities of the code.

How to Get a Copy of GFSSP

NASA Civil Servants and On-Site Contractors

GFSSP is free for NASA civil servants and on-site contractors (on the NASA network), who can request a copy of GFSSP from Andre LeClair (

Other U.S. Government Agencies and Contractors

Employees of other U.S. government agencies and companies/institutions with contracts with the U.S. federal government may request GFSSP from the Tech Transfer Office.  It is recommended that the applicant also request Winplot at the same time.  Winplot can be used to make plots of the results of transient GFSSP models.




Universities in the U.S. may request the educational version of GFSSP for instructional use (for example, in engineering thermal design classes).  The educational version does not support Fortran user subroutines.

GFSSP Educational Version:

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