Hypersonic commentary

Solving the multi-physics challenges of hypersonic rocketsBy Yuya Ando, Bobby Cook and Makoto Shibahara, MSC Software

Once hypersonic flight was the sole preserve of American and Russian space programmes during the middle to later parts of the 20th century. Now, with an exponential increase in satellites being launched to support telecommunication and Internet of Things advances, plus a renewed interest in humans landing on the Moon and even probes from earth landing onto asteroids, many other nations have started to develop an interest in high-speed manoeuvrable rockets for both civil and military applications. Private billionaire entrepreneurs such as Elon Musk, Jeff Bezos and Richard Branson are publicly proclaiming to offer affordable high speed and sub-orbital commercial flights for paying passengers. And there are ambitious private industry plans to go to Mars and the moon again this decade which means that hypersonic flying has never been more relevant. Indeed, in the civil aviation sector alone if affordable hypersonic

flight could be achieved, it would be possible to travel from London to Sydney in 4 hours or from Los Angeles to Tokyo in 2 hours. The world is truly entering a new era of space and high-speed aerospace travel (1).

The physics of hypersonic flows around rockets, and more importantly the associated multi-physics involved is very complex indeed, involving complex interactions that are incredibly demanding for both Computational Fluid Dynamics (CFD) and Computer-Aided Engineering (CAE) software codes to simulate and produce accurate virtual predictions. Hypersonic vehicles experience multiple phenomena simultaneously including aerodynamic shocks, aero-thermal heating effects and aero-structural stresses. In addition, due to the ionisation of gases around the surface of rockets all sorts of electromagnetic effects impact the rocket’s electronics control systems.

Figure 1 illustrates both classic Mach number fluid flow characterisations relative to the Speed of Sound for various flow regimes from subsonic (Mach number less than one) to hypersonic (Mach Number greater than 5) according to NASA (2). During hypersonic flight, surfaces of rockets are exposed to temperatures of many thousands of Kelvins, non-equilibrium thermal/chemical reaction dynamics, high temperature gradients, thermal-structural deformations, and ablation – the burning of the rocket’s surface such it gets physically consumed and the nose shape in particular changes and even develops pitting. Increasingly, hypersonic rockets such as those of the Space X Falcon 9 launch rocket need to have manoeuvrable surfaces and grid type fins to guide the rocket’s trajectory and to recover it for reuse in subsequent missions thus reducing costs. Hence, surface deformation due to aero/thermal loads & ablation combined with flight control systems,

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and separation of booster rockets are important multiphysics phenomena that need simultaneous co-simulations. Moreover, rocket surfaces are being made of exotic composites and metal-ceramic matrices to withstand the excessive temperatures and pressures of hypersonic flight well. Such materials do not have standard properties or orthogonal properties and increasingly require Integrated Computational Materials Engineering (ICME) simulations to truly represent the complex material properties associated with such novel surface materials. MSC Software’s ICME solution can simulate CMCs (Ceramic Matrix Composites) and C/Cs (Carbon Composites) as a low cost, high speed screening tool to help minimise the time, cost and risk associated with high temperature materials development for production at scale. CMCs and C/Cs have good high strength-to-weight ratios and excellent heat handling capabilities.

Software Cradle was one of the first commercial CFD companies in the world to offer computational fluid dynamics software based out of Osaka, Japan. It’s CFD technology and business was acquired by MSC Software in 2016 and MSC itself was acquired by Hexagon AB in 2017. Just over 5 years ago, Cradle released

one of the most modern general purpose CFD codes in the world, scFLOW, with its powerful geometry handling and meshing capabilities and special density-based solver formulation developed by Prof. Hiroaki Nishikawa from the National Institute of Aerospace, NASA Langley and Old Dominion University, USA,

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Figure 1: Definition of rocket speeds relative to Mach 1.0 by NASA (2), left, and typical ionisation phenomena of the fluid the rocket passes through, right

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the pioneer of the FUN3D aerospace solver. Figure 2 shows a typical benchmark hypersonic validation of an unmanned suborbital winged rocket in scFLOW compared to aerodynamic characteristics of a Winged Reusable Rocket (WIRES) by Kyushu Institute of Technology (Kyutech) in Japan (3). In addition, Figure 3 shows another hypersonic validation of a classic rocket application requirement: transient separation of an orbiter body from a booster rocket flying at Mach 8.1 for a Two-Stage-To-Orbit (TSTO) Launch Vehicle with scFLOW’s powerful overset meshing

technique (4).The Nishikawa solver allows scFLOW to deliver robust, accurate and computationally quick density-varying CFD simulations for Mach Numbers up to 8.0, and, when coupled with MSC Software’s tried and proven co-simulation capabilities with well respected structural analysis codes like MSC Nastran and Marc, we are very much pioneering multi-physics co-simulation couplings for hypersonic flows. Such couplings allow for simultaneous aerodynamic and structural loads to be computed along with aerothermal heating of the rocket’s surface.

Ablation in hypersonic rocket flights

Engineers designing hypersonic rockets require an extensive knowledge of the thermal and structural behavior of the materials used particularly related to the body’s shape and its dynamics which help ensure its reliability for optimising the deployment of its payload. MSC Software has extensive experience in modelling structural ablation for solid rocket motors with the MSC Marc code being used for complex thermo-chemical-aerodynamic processes by

Figure 3: Hypersonic Two-Stage-To-Orbit (TSTO) Launch Vehicle Validation experimental image (left) versus Cradle overset mesh (centre) and CFD prediction (right) for separation of an orbiter from a booster rocket flying at Mach 8.1 (4)

Figure 2: Aerodynamic characteristics of a Winged Reusable Rocket: Cradle CFD vs experiment for flows of Mach 0.9, Mach 1.3 and Mach 4.0 (3)

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customers like Safran in France (5,6). During these rocket launches, the combustion of solid fuel propellant generates intense heat, often reaching 3,600 K. This results in thermal decomposition of the combustion chamber’s housing and the motor’s nozzle due to pyrolysis, and the ablation/erosion of these surfaces due to thermal, chemical, and mechanical processes. The thermal strains and externally applied loads in the nozzle can result in large stresses that may have negative consequences on the rocket’s structural integrity. Work done with MSC Marc software has focused on the numerical simulation of the structures undergoing these phenomena. Marc allows for radiation issues to be simulated using Monte Carlo techniques and pixel-based modified hemi-cubes and it accommodates the effects of ablation changing the geometry of the FEA

Figure 4: Schematic representation of pyrolysis and ablation physics for a Snecma solid fuel rocket (left) and MSC Marc ablation simulation of the surface with red areas being the predicted ablated zones (right)

Figure 5: Adams-scFLOW co-simulation of a rocket fin deployment transient fluid-multibody analysis (7)

simulation with high gradients being captured by activate adaptive meshing and large displacements being addressed – see Figure 4.

Simulating manoeuvrability in hypersonic rockets

Increasingly, hypersonic rockets are being developed with flaps and fins that deploy to aid in its control and manoeuvrability especially as some companies like SpaceX seek to recover booster rockets. The transient aerodynamics of rocket manoeuvres can be incredibly complicated. scFLOW from MSC Software has had its CFD capabilities coupled with the market leading Adams multibody dynamics solver to simulate the deployment of fins on a rocket shape and their subsequent impact on the resulting rocket flow field during their transient deployment – see Figure 5 (7).

Multibody dynamics

Measuring surface changes in hypersonic rockets with Hexagon’s CMM machines

Hexagon Manufacturing Intelligence’s CMM scanners have a track record of measuring to a high level of accuracy surfaces associated with the Ariane Booster Rocket in Europe (8). Safran who make these exotic rocket composites have applications for surface measurement and defect analysis for rocket surfaces – see Figure 6. Data on composite surface topography, roughness and erosion can be obtained from Stationary CMMs, Portable Arms with Scanners, and Laser Trackers with Scanners. In addition, Hexagon’s industrial CT-scanners can also provide data inside ablated surface regions of rockets to quantify changes in shape and geometry.

Summary

Hypersonic flights are becoming more and more common in civil and military applications across the aerospace, space and defence sectors. Multi-physics simulations and especially CAE co-simulations for a variety of physics types are becoming essential and a cost-effective way to understand the underlying physical phenomena of such vehicles before and during flight testing. CAE simulation physics types such as structures, fluids, materials, multibody dynamics and acoustics all need to be coupled to understand the full impact of such hypersonic phenomena.

MSC Software has a long and rich aerospace history and offers a wide portfolio of point and coupled solutions via MSCOne tokens to yield unique co-simulations allowing for highly accurate, fast and robust CFD

focused simulations at hypersonic speeds as well as aero-thermal-structural interactions to be modelled with MSC Nastran, the de facto aerospace structures standard, and Marc for highly non-linear structural deformations. Moreover, Marc has specialist ablation modelling capabilities for hypersonic rocket surface scenarios, plus rocket control flaps and fins can be modelled very accurately using MSC Adams in association with scFLOW. Finally, Digimat and MaterialCenter offer state-of-the-art ICME solutions with 10X productivity gains in hypersonic rocket applications. When these are all coupled with Hexagon’s powerful optical, laser and CT-scanning capabilities, a unique simulation and testing portfolio helps companies to develop hypersonic rockets and missiles for the future.

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Figure 6: Safran rocket composite being scanned by a Hexagon CMM machine (8)

References

(1) “Rival powers jockey for the lead in hypersonic aircraft” by M. Dempsey, BBC Website, September 1st 2020: https://www.bbc.co.uk/news/business-53598874(2) NASA website: https://www.grc.nasa.gov/www/k-12/rocket/mach.html(3) “Development and Flight Test of Winged Rocket,” Yonemoto, K., Shidooka, T. and Okuda, K., 27th International Symposium on Space Technology and Science, ISTS 2009-g-16, Tsukuba, Japan, July 5-12, 2009(4) “Aerodynamic Interaction between Delta Wing and Hemisphere-Cylinder in Hypersonic Flow,” Nishino, A. et. al., J. of Japan Society for Aeronautical and Space Sciences, 2004 (5) “Thermal Decomposition Analysis of Rocket Motors and other Thermal Protection Systems using MSC.Marc-ATAS” by T. B. Wertheimer and F. Laturelle, NASA TFAWS, 2003:https://tfaws.nasa.gov/TFAWS03/Data/Aerothermal%20and%20CFD%20Session/Wertheimer.pdf(6) “Thermal Stress Analysis of TPS using Marc” by T. B. Wertheimer and F. Laturelle, NASA TFAWS, 2008: https://tfaws.nasa.gov/TFAWS08/Proceedings/Papers/TFAWS-08-1024.pdf(7) Contravolts Missile Cosimulation with Adams and scFLOW, Posted Mar 29, 2019 https://www.youtube.com/watch?v=xUBoJqMpV2M(8) CMM Scan of a Safran rocket composite:https://www.youtube.com/watch?v=wW_AnZ1Cdiw%C2%A0&feature=youtu.be

For more information contact: Bobbycook@hexagon.com