An active flow control concept using counterflowing jets to significantly modify the external flowfields and strongly weaken or disperse the shock-waves of supersonic and hypersonic vehicles to reduce the aerothermal loads and wave drag was investigated. Experiments were conducted in a trisonic blow-down wind-tunnel, complemented by pre-test computational fluid dynamics (CFD) analysis of a 2.6% scale model of Apollo capsule, with and without counterflowing jets, in Mach 3.48 and 4.0 freestreams, to assess the potential aerothermal and aerodynamic benefits of this concept. The model was instrumented with heat flux gauges, thermocouples and pressure taps, and employed five counterflowing jet nozzles (three sonic and other two supersonic with design Mach numbers of 2.44 and 2.94) and nozzle exit diameters ranging from 0.25 to 0.5 inch. Schlieren data show that at low jet flow rates of 0.05 and 0.1 lb,/sec, the interactions result in a long penetration mode (LPM) jet, while the short penetration mode (SPM) jet is observed at flow rates greater than 0. Ilb,/sec., consistent with the pre-test CFD predictions. For the LPM, the jet appears to be nearly hlly-expanded, resulting in a very unsteady and oscillatory flow structure in which the bow shock becomes highly dispersed such that it is no longer discernable. Higher speed camera Schlieren data reveal the shock to be dispersed into striations of compression waves, which suddenly coalesce to a weaker bow shock with a larger standoff distance as the flow rate reached a critical value. The pronounced shock dispersion could significantly impact the aerodynamic performance (L/D) and heat flux reduction of spacecrafk in atmospheric entry and re-entry, and could also attenuate the entropy layer in hypersonic blunt body flows. For heat transfer, the results show significant reduction in heat flux, even giving negative heat flux for some of the SPM interactions, indicating that the flow wetting the model is cooling, instead of heating the model, which could significantly impact the requirements and design of thermal protection system. These findings strongly suggest that the application of counterflowing jets as active flow control could have strong impact on supersonic and hypersonic vehicle design and performance. IntroductionOne of the technical challenges in space exploration and interplanetary missions is controlled entry and re-entry into planetary and Earth atmospheres, which requires the dissipation of considerable kinetic energy as the spacecrafk decelerates and penetrates the atmosphere. As such, effective heat load management of stagnation points and acreage heating remain a technological challenge and pose significant risk, especially for human missions.

An active flow control concept using counterflowing jets to significantly modify the external flowfields and strongly weaken or disperse the shock-waves of supersonic and hypersonic vehicles to reduce the aerothermal loads and wave drag was investigated. Experiments were conducted in a trisonic blow-down wind-tunnel, complemented by pre-test computational fluid dynamics (CFD) analysis of a 2.6% scale model of Apollo capsule, with and without counterflowing jets, in Mach 3.48 and 4.0 freestreams, to assess the potential aerothermal and aerodynamic benefits of this concept. The model was instrumented with heat flux gauges, thermocouples and pressure taps, and employed five counterflowing jet nozzles (three sonic and other two supersonic with design Mach numbers of 2.44 and 2.94) and nozzle exit diameters ranging from 0.25 to 0.5 inch. Schlieren data show that at low jet flow rates of 0.05 and 0.1 lb,/sec, the interactions result in a long penetration mode (LPM) jet, while the short penetration mode (SPM) jet is observed at flow rates greater than 0. Ilb,/sec., consistent with the pre-test CFD predictions. For the LPM, the jet appears to be nearly hlly-expanded, resulting in a very unsteady and oscillatory flow structure in which the bow shock becomes highly dispersed such that it is no longer discernable. Higher speed camera Schlieren data reveal the shock to be dispersed into striations of compression waves, which suddenly coalesce to a weaker bow shock with a larger standoff distance as the flow rate reached a critical value. The pronounced shock dispersion could significantly impact the aerodynamic performance (L/D) and heat flux reduction of spacecrafk in atmospheric entry and re-entry, and could also attenuate the entropy layer in hypersonic blunt body flows. For heat transfer, the results show significant reduction in heat flux, even giving negative heat flux for some of the SPM interactions, indicating that the flow wetting the model is cooling, instead of heating the model, which could significantly impact the requirements and design of thermal protection system. These findings strongly suggest that the application of counterflowing jets as active flow control could have strong impact on supersonic and hypersonic vehicle design and performance. IntroductionOne of the technical challenges in space exploration and interplanetary missions is controlled entry and re-entry into planetary and Earth atmospheres, which requires the dissipation of considerable kinetic energy as the spacecrafk decelerates and penetrates the atmosphere. As such, effective heat load management of stagnation points and acreage heating remain a technological challenge and pose significant risk, especially for human missions.

Tremendous amounts of heat and drag loads occur during entry and re-entry into planetary and earth atmospheres have posed grave challenges on maintaining structure integrity of space exploration vehicles. Though various thermal protection systems (TPS) have been employed to manage the heat loads, both localized and transient spikes at stagnation points, the use of TPS can substantially increase the weight of the vehicle. Hence, various concepts, such as aeroassisted orbital transfers and aerobraking, have been designed to mitigate the high heating rates such that the TPS weight can be minimized. Among those concepts, the use of a flow augmented thermal management system for entry and re-entry environments has shown some promises in reducing heat and drag loads. This concept relies on jet penetration from supersonic and hypersonic counterflow jets, that could significantly weaken and disperse the shock-wave system of the spacecraft flowfield to reduce wave drag and aerothermal loads. Greatly reducing wave drag and aerothermal loads significantly enhances aerothermal performance, allowing thinner or much lighter TPS to be used, which translates into spacecraft weight and cost savings. Other benefits include better aerodynamic efficiency and improved down-range and cross-range maneuverability. There are two jet penetration modes involved in a supersonic/hypersonic flow interacting with counterflow jets: short penetration mode (SPM) and long penetration mode (LPM) interactions. Previous studies have shown that the LPM jet significantly increases the shock stand-off distance, thus reducing the strength of bow shock, which leads to a reduction in wave drag. The LPM jet acts as, in essence, a "pencil" of fluid with high dynamic pressure, penetrating into the incoming freestream, to attenuate the shock system. Though the function of the LPM jet has been demonstrated in the previous study, further experimental and computational analyses through trade studies are required to determine the optimum operating conditions of the LPM jet.The objective of this study is to provide a practical design approach to the development of flow control technologies as spacecraft subsystem(s) for better thermal management, aerodynamic efficiency, control authority and range, and improvements in payload mass fraction of the spacecraft in entry and re-entry atmospheres using counterflowing LPM jets to significantly modify/reshape the external flow environment and weaken the shock. To achieve this goal, we have conducted numerous time-accurate and steady-state computational fluid dynamics (CFD) simulations of the supersonic flow around an Apollo-type capsule with and without the counterflow jet using a Reynold-averaged Navier-Stokes (RA S) flow solver--UNIC code. Axisymmetric RANS simulations were first conducted to investigate the grid convergence for this type of flowfield. Parametric studies of different freestream Mach numbers (3.48 and 4.96),

A study was undertaken to capture the best practices for the development of reliable and robust spacecraft structures for NASA's next generation cargo and crewed launch vehicles. In this study, the NASA heritage programs such as Mercury, Gemini, Apollo, and the Space Shuttle program were examined. A series of lessons learned during the NASA and DoD heritage programs are captured. The processes that "make the right structural system" are examined along with the processes to "make the structural system right". The impact of technology advancements in materials and analysis and testing methods on reliability and robustness of spacecraft structures is studied. The best practices and lessons learned are extracted from these studies. Since the first human space flight, the best practices for reliable and robust spacecraft structures appear to be well established, understood, and articulated by each generation of designers and engineers. However, these best practices apparently have not always been followed. When the best practices are ignored or short cuts are taken, risks accumulate, and reliability suffers. Thus program managers need to be vigilant of circumstances and situations that tend to violate best practices. Adherence to the best practices may help develop spacecraft systems with high reliability and robustness against certain anomalies and unforeseen events.

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