BLOG | 30 August 2021


“Capable of fast transient and steady state solving times, Flownex affords users the ability of rapid system design iterations.”

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Space exploration and development in the 21st century

With the commercial interest in space exploration, from space tourism and mineral exploration to the need for space colonisation, it has become evident that the development of space craft and propulsion systems need to be done at a faster pace than ever before. Several rocket engine designs have been attempted with different types of fuels in the last decade with the main goals being:

  • increased efficiency,
  • reduced cost,
  • low operational risk,
  • and re-use.

“Flownex is the industry leader in 1D CFD software, allowing for the systems simulation of large interconnected thermal fluid systems.”

It has become Increasingly important to evaluate different cycles as fast as possible, however, when all the complexities of the rocket engine designs are taken into consideration, 3D CFD analysis on a system scale is not a viable option. Simulating the system components of the rocket engine in its entirety is important to understand how all these components will interact with each other and how the system will perform at different operating conditions. Parametric studies need to be performed to find optimal design conditions, but these can be tedious and time consuming.

This is where Flownex has gained popularity in the aerospace industry. Flownex is the industry leader in 1D CFD software, allowing for the systems simulation of large interconnected thermal fluid systems. Capable of fast transient and steady state solving times and introducing the ability for rapid system cycle design iterations, Flownex has been used extensively for R&D purposes where 3D CFD just cannot compete. Flownex uses fundamental physics and theory from leading research to solve some of the world’s most complex thermal system problems.

Modelling the RS-25 rocket engine

Today we look at the system simulation of one of the more complex rocket engines available, the Aerojet Rocketdyne RS-25. Used exclusively in the past as the 1st stage propulsion system for NASA’s Space Shuttle and now the Space Launch System (SLS), this rocket uses liquid cryogenic hydrogen and oxygen as the fuel and oxidiser. Rated at around 491,000 pounds of thrust at its nominal power level – it packs a punch.

“It is crucial then, to simulate the entire system as one to capture the interdependencies between components.”

Figure 1: RS-25 rocket engine (NASA: Sept 13, 2013).

Engine specifications

  • The RS-25 engine is a closed-loop engine utilising fuel-rich combustion in two pre-burners driving the turbopumps used to pump the hydrogen and oxygen sides.
  • Liquid cryogenic hydrogen is pumped through a low-pressure and high-pressure pump, then distributed through pipe networks in the nozzle and main combustion chamber.
  • The branches flowing around the nozzle removes heat from the nozzle material and is then directed to the pre-burners.
  • Another branch passing beside the main combustion chamber is also used for cooling and to then drive the low-pressure turbopump.
  • A portion of the liquid oxygen is branched to the pre-burners and the rest to the main combustion chamber where it mixes and combusts with the fuel rich products from the pre-burners.
  • The thrust is controlled using the two pre-burner oxygen valves. Adjusting these valves will alter the oxygen flow rate, changing the state of the combustion products, altering the power of the turbopumps which then changes the pumping rate of the fuel and oxidiser from the tanks and ultimately changes in thrust.

It is crucial then, to simulate the entire system as one to capture the interdependences between components.

Figure 2: RS-25 diagram (Wikipedia: RS-25).

Modelling the engine from scratch

To start modelling a thermal hydraulic system, it is best to work from a diagram or schematic of the network. This will structure the modelling process and ensure that the entire system and the necessary physics processes are covered. In Flownex, a background image can be imported onto the drawing page by simply right clicking on the clean canvas and selecting “edit”. From there, the “Style” can be selected using the three dotted button next to it and following the instructions:

Once the background image is selected, components can be dragged and dropped in from the components library.

The components library contains a large selection of components which can be used to simulate:

  • The piping system – realising pressure drops from primary and secondary losses,
  • turbopumps – with power matching capabilities,
  • convection and radiation heat transfer in the combustion chamber, nozzle, etc.

It is important to work from a point (such as the inlet of hydrogen) and model the components step-by-step while solving, as the model is progressed. This will ensure that any errors introduced during the modelling phase are picked up and rectified before carrying on.

Flownex also gives you the capability to import fluid properties for non-standard applications (such as cryogenic states) directly from the NIST database, REFPROP, with a level of detail chosen by the user. This is very useful especially if the fluid’s being used within the network are operating in ranges outside of those included in the standard fluids library.

The entire Flownex network of the RS-25 rocket engine is shown in the following image as overlay on the system diagram:

Some of the important Flownex library components used in the RS-25 model are explained here:


These are modelled using the basic centrifugal pump in combination with the simple turbine. The shaft component can be used to apply power matching between the pump and turbine – matching the power output of the turbine with the input power of the pump.

Heat transfer

The heat transfer from the combustion process in the nozzle and main combustion chamber can be realised using the composite heat transfer element. This element is capable to model the combined radiation, convection, and conduction heat transfer from the combustion process.

Combustion process

Flownex includes a CEA adiabatic flame component, which allows for combustion calculations and adiabatic flame temperatures to be obtained using NASA’s Chemical Equilibrium with Applications library. The combustion process in the pre-burners is fuel-rich and the excess hydrogen is burnt in the main combustion chamber after the addition of more oxygen. The exit thrust nozzle is used to calculate the thrust produced by the rocket, while the PID controller component controls the correct ratios in the branches.

“Flownex opens a world of opportunities for system level design, saving time and development cost.”

The RS-25 rocket engine is highly complex and the performance of different components influences each other. For this reason, such a system should be simulated as an integrated system of interdependent components.

Component optimisation

Flownex can be used to optimise individual system components and their interaction with the entire system.

Transient simulations

Flownex allows for transient simulations, giving the user the ability to analyse changes in the system over time including thermal inertia. Using the transient capabilities of Flownex, this network can be used to simulate the performance of the RS-25 during start-up, lift off and flight through the earth’s atmosphere.

Flight mission simulation

This model can be coupled to a full kinematic model to realise the drag force during flight (with changes to the drag as the density of air changes with elevation) and the mass change of the spacecraft as the fuel and oxidiser are consumed. The acceleration and velocity of the rocket can be calculated, and a full flight mission can be simulated for different scenarios.

The above example shows how the transient simulation capabilities of Flownex are used to analyse the system response when introducing changes in the system. For the above demonstration, the oxygen pre-burner valves are adjusted as well as the exit pressure. In this way, the engine performance can be monitored for different throttle percentages and different elevations (nozzle exit pressures).

Flownex opens a world of opportunities for system level design, saving time and development cost. A 1D CFD code can be used to solve complex thermal hydraulic systems beyond the reach of what can be done in a similar timeframe with 3D CFD, which is more appropriate for design of individual components and will compliment a 1D workflow. Flownex is used extensively in the aerospace industry to design and optimise systems and to gain a deeper understanding of the overall performance at different operating conditions.

Leander Kleyn

Leander Kleyn

Leander is a simulation design engineer at Flownex who specialises in Propulsion and Energy systems.


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