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“Rocket science” is perhaps one of the largest misnomers in engineering. Broadly, if science is about understanding the physical world around us, engineering is about applying that understanding.
Likewise rocket science can be translated into engineering terminology as “the development of propulsion systems for spacecraft”. As in all areas of engineering, simulation software such as computational fluid dynamics (CFD) is being used to solve the most complex problems.
The application of CFD is not a new idea in propulsion, but detailed combustion chamber data is hard to obtain because of the harsh environment, and without validation data engineers are reluctant to trust predictions. However, more sophisticated software and more affordable computing power are pushing the boundaries of numerical simulation in rocket science more than ever before.
US aerospace group Orbitec is working with CFD code development company Convergent Science to help with the development of its latest Vortex rocket engine. The upper-stage engine will ignite at high altitudes and be used for orbital manoeuvring. The engines will also be used on small-to-medium-scale air- and ground-launch stage engines. Orbitec has demonstrated Vortex and is now poised to insert variants of the engine into flight systems.
Engineers dealing with the shift from R&D to product development at Orbitec are scaling up the engine design to larger sizes and producing entire flight-weight propulsion systems. At the small, conceptual scale, experimental costs are reasonable and different thrust permutations can be investigated on a stand. However, the size and higher cost of larger engines preclude trial-and-error type design. The company hopes to reduce costs by using CFD.
“In the past we’ve not really been able to use CFD in a predictive way,” says senior propulsion engineer Dr Millicent Coil. “Most of us are from experimental backgrounds, so we tended to model the rocket alongside our real-world testing, examine the results and then get no momentum from the data.”
The company is aiming towards using a fully predictive CFD simulation to calculate the thrust capability of new rocket designs. Dr Marty Chiaverini, propulsion lead at Orbitec, says: “We’d like to use the numerical tools to design a new engine instead of merely filling in missing information from test results.”
According to Chiaverini, the primary challenge is optimising the fuel-injector design to yield high performance at a larger scale. The fuel-injection hardware must be carefully designed to achieve optimum performance. Analyses have been introduced after the initial concepts are drawn and feedback is used to refine the design before any hardware is produced. This reduces the number of experiments required.
Engineers from Orbitec and Convergent Science are also partnering in areas such as turbulence and kinetics. Convergent’s CFD code was originally developed for internal combustion engines, and automatic mesh generation is coupled to both the flow and chemistry solvers. This allows the mesh density to be refined around specific areas of interest. Coil says: “Not having to mesh manually is huge. You don’t have to spend hours perfecting the mesh to avoid skewness.”
There are many challenges remaining before Orbitec can fully implement numerical modelling into its design process, such as the modelling of cryogenic and supercritical propellants, phase changes, and the combustion of kerosene. But engineers are confident that more work in the area will slash the number of physical prototypes needed. To them it truly is rocket science.