Rocket science propelled to new heights by software
Accurate ‘virtual validation’ of complex rocket designs enables scientists to create more efficient propulsion systems
A new CFD (computational fluid dynamics) software package is helping rocket scientists to develop higher performance propulsions with the ability to do more accurate testing, all in a virtual world.
With detailed combustion data hard to obtain in the harsh environment and very little validation data possible, simulating rocket design has been a difficult science until now.
Now US-based ORBITEC, a subsidiary of Sierra Nevada Corporation, has enlisted the expertise of CFD code developers Convergent Science, to begin a new era of rocket engine development and the upscaling of its rockets to offer competitive options for access to space.
Like most combusting systems, rocket engines present some challenges in simulation. The modelling of cryogenic and supercritical propellants, the combustion of kerosene-based fuels and the patented vortex swirling flow can prove difficult, previously increasing the need to build more prototypes. “In the past we’ve not really been able to use CFD in a predictive way,” says ORBITEC 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 but struggled to get momentum from the data. We needed a better strategy with which to insert the numerical analysis.”
The cost of physical testing during the development cycle has motivated ORBITEC to embrace CFD. Whilst small scale, experimental tests are reasonable and can be investigated on the thrust stand, as scale increases, so the costs rapidly rise. “When we were first developing our vortex engine, most of the engines we were testing could be held in one hand – 50lbfor 100lbfthrust,” says ORBITEC’s Propulsion Lead Dr. Marty Chiaverini. “We could do a lot of the prototyping in-house or outsource it at relatively low cost, so it wasn’t really an issue if we needed to run hundreds of tests. Now the engines are much bigger, up to around 30,000 lbfof thrust, excessive iteration becomes prohibitively expensive.”
According to ORBITEC’s Dr. Chiaverini, the primary challenge is optimising the fuel injector design to yield high performance at a larger scale. He hopes that addressing this with Converge CFD will illuminate the physical phenomena and facilitate new engine development. The software company’s CONVERGE CFD code, originally developed for and proven on internal combustion engines, has some unique properties to enable ORBITEC to make a step change in the efficacy of the analysis. Instead of using a static, manually generated grid for the duration of the simulation, CONVERGE automates and fully couples the mesh generation at each time step. Uniquely, this process is coupled to both the flow and chemistry solvers. Known as Adaptive Mesh Refinement (AMR), this allows the simulation to automatically refine the mesh density around areas of specific interest, such as turbulent zones or large temperature gradients. These schemes drastically reduce preparation and computation time by up to five times.
For ORBITEC, the cut-cell and automatic meshing approaches applied in CONVERGE mean starting iterating in a day rather than a week. “From an engineering point of view, not having to mesh manually is huge,” says Dr. Coil. “It’s not that you don’t have to think about the mesh – it still succeeds or fails on your choices – but you don’t have to spend hours perfecting the mesh to avoid skewness like we did with the package we used previously.”
The engineers at ORBITEC have begun experimenting with CONVERGE and already have seen good results from the package on a number of smaller projects such as external flows over rockets, wind tunnels and preburners. Bolstered by this success, they are now planning to model a new 30,000lb thrust rocket engine. The hope now is to integrate CFD into the design processes for all of their new engines.
“Our CONVERGE software is already a powerful and trusted development tool in the automotive sector and this shows it can help engineers, even in areas where CFD was not deemed effective enough to be embraced,” says Convergent Science’s marketing director Rob Kaczmarek. “The savings possible in both time and costs through using CONVERGE are valid in any combustion application.”
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About Convergent Science
Founded in 1997 in Madison, Wisconsin, Convergent Science, Inc. is a global leader in computational fluid dynamics (CFD) software. Its customers include leading automotive and commercial vehicle manufacturers, tier one suppliers and professional motorsport teams.
Its flagship product, CONVERGE, includes groundbreaking technology that eliminates the user-defined mesh, fully couples the automated mesh and the solver at runtime and automatically refines the mesh when and where it is needed.
CONVERGE is revolutionising the CFD industry and shifting the paradigm towards predictive CFD.
Headquartered in Madison, WI, ORBITEC was founded in 1988. In the early days, the company was a small, aerospace R&D company, thriving on the US government’s Small Business Initiative Research programs. Now a wholly-owned subsidiary of Sierra Nevada Corporation, ORBITEC is offering more mature products to the aerospace industry. Although broad in its capabilities, ORBITEC has two primary specialties: controlled environments for space and propulsion systems. ORBITEC Propulsion is known for its vortex cold-wall engines that utilize a swirling oxidizer flow to protect the walls of the combustion chamber from the harsh combustion. The liquid and hybrid versions of these engines were evaluated extensively at a small demonstration scale. Fruitful partnerships with government agencies (DoD, NASA) and commercial space enterprises have enabled tailoring the vortex engines for specified applications and near-term implementation. The path to advanced development has presented a new set of challenges. The first is scaling up to much larger thrust levels (~30,000 lbf). Moving to larger engines moves some of the physical phenomena into different regimes and increases complexity and cost in hardware development, propellants, and testing facilities. ORBITEC has increased the size of its test facilities to accommodate the new, larger family of engines. Introduction of different propellant combinations typically requires significant redesign, but the vortex engine has been successful with propane & nitrous oxide, LH2 & LOX, and kerosene & LOX with only minor tweaks to the design. Moving to marketable flight systems has also required the development of entire propulsion systems that include turbopumps, propellant feeds, and the like rather than simply thrust chamber assemblies. The short-duration, demonstration tests of early vortex engines used heat-sink nozzles that were basically large hunks of copper, but the new engines feature cooled nozzles integrated into the propellant feed system. The entire program has also shifted to flight-weight assemblies. Thus the success of the vortex engine and the near-term insertion of it into deployed propulsion systems has offered ORBITEC engineers a new set of challenges. Implement CONVERGE CFD is one of the ways in which we are refining our methods to meet the new demands.