The Path to Passivation
By S. Petitot, S. Casalino and B. Vieille, French National Center for Space Studies (CNES), Evry, France, and B. Lazaro and E. Gonzalez, SENER, Universidad Politecnica Madrid, Spain
 Diagram of the Ariane 5 ECA cryogenic upper stage (ESC) |
The ARIANE 5 ECA is an expendable launch system. This means that after it delivers its payload, which is typically satellites into geostationary transfer orbit, none of its parts will be reused. For these missions, the cryogenic upper stage (ESC) is designed to achieve a stable status so that it can remain in orbit, structurally intact, for 25 years. A sequence of operations has been developed to make this outcome problem-free. An important step in the sequence involves the removal of any residual liquid propellant in the tanks. During this so-called passivation phase, the tanks are depressurized by safety valve openings and the liquid is allowed to evaporate. The passivation phase is very important to study, because the evaporation occurs at a time when the ESC is subjected to linear and angular accelerations, and the induced liquid motion can give rise to forces and moments that can strongly impact the dynamic behavior of the ESC module.
 Surface grid for the LOX tank (center, bottom) and the LH2 tank (outer, above) |
A study has been conducted using FLUENT to simulate the liquid motion in the ESC in the period leading up to and during the passivation phase. When the simulation begins, the ESC has forward axial motion, and a constant torque is applied to make it start spinning about this axis. After 150 seconds,it is spinning with a rotation speed of 45 degrees/s and the torque is stopped.The axial speed decreases throughout this period, and after a total of 400seconds, the valves open and the passivation process begins. After 300 more seconds, the ESC is accelerated abruptly and torques about the other axes are applied. In the final stage, which lasts for another 50 minutes or so, the ESC undergoes a tumbling motion as it gradually decelerates.
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The chronology of forces and moments applied to the ESC before and during the passivation phase |
The ESC has two cryogenic tanks, which contain liquid oxygen (LOX) and liquid hydrogen (LH2). Both tanks are included in the FLUENT simulation,using a total of 513,000 hexahedral cells. The fluid properties for the isothermal calculation are defined using CNES software. The coupled motion of the ESC and contained liquids is captured using a six degrees-of-freedom (6DOF)model, implemented through user-defined functions (UDFs). Viscous stresses are included through the use of the RNG k-ε turbulence model. The volume of fluid (VOF) model with the geo-reconstruct scheme is used to capture the evolution of the free surface. Three fluids are defined: helium gas (the primary phase), liquid hydrogen, and liquid oxygen, and surface tension is taken into account. The external forces and torques described above are prescribed in a UDF, and are modified by the forces and torques that result from the fluid sloshing.
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The algorithm describing one time step of the coupled calculation |
At CNES, a turbulent approach was used, and the results were compared to laminar calculations done by SENER, a Spanish consulting firm with scientific advisers from the University of Madrid. To achieve a comparable angular speed after 150 seconds, the torque in the laminar case had to be stopped10 seconds early, at 140 seconds. Turbulent effects make a full 150 second spin-up period necessary, but the axial angular acceleration remains non-zero following this time as it takes time to decay. The difference in behavior is due to the increased viscous dissipation in the turbulent case. When a turbulent approach is used, the liquid phase forms a nearly uniform layer that rotates along the outer wall. The laminar calculation, on the other hand, predicts that the fluid forms a rotating localized mass instead. The movement of this mass back and forth inside the vessel gives rise to angular accelerations in the non-axial directions, and these grow in amplitude, causing an instability inside the vessel. For the turbulent case, small off-axis oscillations develop only after 700 seconds, when additional thrusters of the attitude control system are activated. This initiation time of oscillation development is very close to the time seen during the first flight of the ESC-A stage in February2005. The amplitudes of these turbulent oscillations remain considerably less than when the laminar approach is used.
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Angular accelerations of the fluid in response to the torques and forces applied by the thrusters; the large amplitude oscillations predicted by the laminar model (the z-angular acceleration is similar to the y-component) correspond to bulk rotation of the liquid |
An examination of the liquid volume fraction in the tanks helps explain the difference in the laminar and turbulent behavior. After the spin-up period has ended, at 150 seconds, a side-view of the tanks shows that the turbulent approach predicts a cohesive liquid domain, especially in the LH2 tank. When laminar conditions are assumed, large pockets of liquid separate and flow independently in the tank, reducing the off-axis fluid force components and increasing the liquid moments. Thus turbulence acts to improve the kinetics of fluid movement along the side walls under spin conditions. For both tanks,the turbulent approach predicts a smoothly rotating mass of liquid at the wall. For the laminar simulation, the perturbations early on develop into an unstable rotation of a localized liquid mass inside the vessel. Essentially, the laminar approach delays the time needed by the fluid to reach a smooth rotation at the side walls under spin conditions, especially in the LH2 tank where a huge fragmentation of the liquid occurs. By contrast, the effective viscosity of the turbulent approach allows a well-behaved fluid mass to rotate along the LH2 tank perimeter.
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| Liquid volume fraction in the LH2 (top) and LOX (bottom) tanks at the end of the spin-up cycle, computed using a turbulent (left) and laminar (right) approach |
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Laminar (left) and turbulent (right) predictions of the liquid surface, colored by liquid velocity, after 150 seconds |
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