Hypervelocity Re-entry Research Articles

Technique Development for Investigating Axisymmetric Ablation Models in Hypersonic Impulse Facilities

Arcjet facilities have been used extensively for ablation and materials characterization but arcjets do not correctly simulate the hypervelocity flow during reentry. Impulse facilities can simulate aspects of hypervelocity reentry flows, but cannot duplicate the extended-duration thermal environments. To address such deficiencies in short-duration testing environments, a novel plasma technique for preheating axisymmetric samples of heat-shield materials has been developed. The heating technique has been demonstrated using a short-duration blow-down arrangement at the University of Southern Queensland. A 50-mm-diameter graphite disk sample with a uniform thickness of 2 mm was heated from the downstream side with plasma to approximately 2500 K, and then it was exposed to a cold Mach 4.5 flow. The disk was mounted on an experimental probe that was very similar to the European standard probe, normal to the flow. Future applications for the technique are expected to be found in expansion tube facilities that can simulate the true flow energy under reentry conditions.

DSMC modeling of rarefied ionization reactions and applications to hypervelocity spacecraft reentry flows

The DSMC modeling is developed to simulate three-dimensional (3D) rarefied ionization flows and numerically forecast the communication blackout around spacecraft during hypervelocity reentry. A new weighting factor scheme for rare species is introduced, whose key point is to modify the corresponding chemical reaction coefficients involving electrons, meanwhile reproduce the rare species in resultants and preserve/delete common species in reactants according to the weighting factors. The resulting DSMC method is highly efficient in simulating weakly inhomogeneous flows including the Couette shear flow and controlling statistical fluctuation with high resolution. The accurate reliability of the present DSMC modeling is also validated by the comparison with a series of experimental measurements of the Shenzhou reentry capsule tested in a low-density wind tunnel from the HAI of CARDC. The obtained electron number density distribution for the RAM-C II vehicle agrees well with the flight experiment data, while the electron density contours for the Stardust hypervelocity reentry match the reference data completely. In addition, the present 3D DSMC algorithm can capture distribution of the electron, N+ and O+ number densities better than the axis-symmetric DSMC model. The introduction of rare species weighting factor scheme can significantly improve the smoothness of the number density contours of rare species, especially for that of electron in weak ionization case, while it has negligible effect on the macroscopic flow parameters. The ionization characteristics of the Chinese lunar capsule reentry process are numerically analyzed and forecasted in the rarefied transitional flow regime at the flying altitudes between 80 and 97 km, and the simulations predict communication blackout altitudes which are in good agreement with the actual reentry flight data. For the spacecraft reentry with hypervelocity larger than the second cosmic speed, it is forecasted and verified by the present DSMC modeling that ionization reactions will cover the windward capsule surface, leading to reentry communication blackout, and the communication interruption must be considered in the communication design during reentry in rarefied flow regimes.

Hafnium Diboride as a saturable absorber for Q-switched lasers

ABSTRACTHafnium diboride (HfB2) is used in hypervelocity re-entry vehicles (such as intercontinental ballistic missile (ICBM) or ICBM heat shields), nuclear reactors and aerodynamic leading edges because of its strong tensile strength and high thermal resistance. HfB2 is an ultra-high-temperature ceramic (UHTC) with a melting point of 3250°C. In this paper, we study the performance of HfB2 as a saturable absorber in Q-switched lasers. HfB2 has a non-saturable absorptance of 2.5 dB/µm, a saturable absorptance of 0.8 dB/µm and a saturation fluence of 12 µJ/cm2. In addition, the saturation lifetime is estimated as 989.3 ps, while the recovery lifetimes are estimated as 254.9 and 22.1 ps. As a saturable absorber in our Q-switched laser, we observed pulses with durations between 880 and 2000 ns. With the addition of an acousto-optic modulator, we have observed Q-switching mode-locking, with pulses as low as 250 ns. HfB2 can potentially work at very high power, since its damage fluence is no less than 361 mJ/cm2.

Dynamical uncertainty and demisability occurrence for the atmospheric re-entry of SOHO

Re-entry trajectories to the Earth have been recently considered as a valuable end-of-life option also for Sun–Earth Libration Point Orbits (LPO) missions. In this work, we investigate in detail the case corresponding to SOHO. On the one hand, we show how the main uncertainties associated with the problem affect the probability of re-entry and the corresponding point at the interface with the atmosphere. Monte Carlo propagations are applied to different cases of uncertainties. They correspond to the orbit determination, the efficiency of the maneuver required to target the Earth, and the characteristics of the spacecraft determining the solar radiation pressure effect. On the other hand, we provide a comparison between a classical re-entry from a LEO and a hypervelocity re-entry from a LPO, in terms of area affected by the re-entry and demise percentage.

Computational Analysis of Radiative Heat Loading on Hypervelocity Reentry Vehicles

Space exploration missions may include atmospheric entries at very high velocities (for example, sample return or entries from hyperbolic orbits) or into an atmosphere with highly radiative constituents (for example, Mars or Titan). The effects of gas radiation in the shock layer and wake flows may become significant at such flight conditions; hence, an accurate computational fluid dynamics analysis of these flows requires adequate modeling of the local absorption and emission properties and the energy transport by radiation. The wide range of numerical techniques for the integration of radiation in entry vehicle simulations includes Monte Carlo models, which enable accurate and computationally efficient predictions of radiative energy transport in participating (absorbing and emitting) media for complex geometries. Reentry type flow properties are characterized by a large variation of local absorption and emission properties in the spectral range. Therefore, and due to the absence of general averaging rules, a fine spectral discretization is needed for practical computations. Here, binning methods offer the potential to reduce the required resolution while maintaining a high level of accuracy. The application of a Monte Carlo radiation transport code using high spectral resolution and a loose coupling approach with the applied flow solver was found to give good agreement with the available heating rate measurements of the Second Flight Investigation of Reentry Environment flight experiment.

Hot-Wall Reentry Testing in Hypersonic Impulse Facilities

Hypersonic testing in impulse facilities replicates various flow characteristics accurately; however, because of the short test times, some phenomena cannot be fully simulated. Previous testing has been conducted with preheated walls up to 800 K; however, this falls well short of the typical temperatures characteristic of ablators in hypervelocity reentry (~2000–3000 K). The concept presented here is to use resistive heating on a carbon–carbon model to preheat the model immediately before conducting a test. This method has enabled testing at wall temperatures in excess of 2000 K in the X2 expansion tube at the University of Queensland. The model used was a short half-cylinder with a diameter-to-length ratio of 4.5 and was tested in a Earth-entry flow monitored by ultraviolet spectrometry. The testing campaign showed that there was a significant increase in the cyanogen formation off the surface of the model when it was heated to 2000 K. This testing demonstrates the viability of the presented method, which will enable a new range of testing to be conducted in impulse facilities with emphasis on hypervelocity flows, in particular those in which the real flight vehicle uses a carbon-based ablator as the thermal protection system.

Diagonal implicit scheme for computing flows with finite rate chemistry

A new algorithm for solving steady, finite-rate chemistry, flow problems is presented. The new scheme eliminates the expense of inverting large block matrices that arise when species conservation equations are introduced. The source Jacobian matrix is replaced by a diagonal matrix which is tailored to account for the fastest reactions in the chemical system. A point-implicit procedure is discussed and then the algorithm is included into the LU-SGS scheme. Solutions are presented for hypervelocity reentry and Hydrogen-Oxygen combustion. For the LU-SGS scheme a CFL number in excess of 10,000 has been achieved.

Thermal and ablative lag induced by a periodic heat input.

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An investigation of cylindrical starting flows

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Radiative and convective heating during hypervelocity re-entry

Radiative and convective heating during hypervelocity re-entry