Laminar-Turbulent Transition on Reentry Capsules and Planetary Probes

Introduction N ASA interest in reentry vehicles has recently taken a dramatic shift away from lifting bodies (X-33, X-38) and winged vehicles (shuttle, X-34) towards capsules. This follows the loss of the Columbia and the Presidential announcement of a program focused on the moon and Mars [1]. The Presidential Commission report mentions “entry, descent, and landing: precision targeting and landing on ‘high-g’ and ‘low-g’ planetary bodies” as one of 17 critical areas identified for the initial focus ([1], p. 28). NASA is developing a crew exploration vehicle (CEV) to fulfill these missions, following the planned retirement of the shuttle circa 2010. It has been determined that this CEV is to be an Apollo-like capsule. Although moderate levels of lift can much improve the safety and flexibility of entry trajectories ([2], Chap. 6), the Apollo geometry is proven in the very high aeroheating of a moon-return entry {11 km=s ([3], p. 19)}, and easier to integrate with a launchescape rocket. The sameCEV reentry geometry is to be used for both the low-energy entries from low earth orbit (LEO) and the very different high-energy entries from hyperbolic lunar and interplanetary missions. The planetary program also continues to fly blunt bodies as entry probes; it seems efficient to consider these similar shapes in parallel with the capsules. Transition is one aerothermodynamic problem that is not yet well understood. Will the uncertainties in estimating transition [4] impact the design of these vehicles? Is transition important to the development of new capsules a la Mercury, Gemini, Apollo, or Soyuz? It is generally thought that transition is more important for slender vehicles than for blunt vehicles, although few studies have been carried out to support this idea [5]. Because there is much uncertainty about the properties of any such capsules (geometry, ballistic coefficient, mission needs, and so on), this question cannot be answered definitively. Nevertheless, the following reviews the public-domain literature from blunt-body and capsule programs, with a view towards identifying the available data for transition under these conditions, the physics of the transition process, and the conditionswhere laminar-turbulent transition is important. Particular attention is paid to manned capsules, where funding levels are likely to support more detailed analyses. Although the Russian Soyuz vehicle may be an important forerunner to the CEV [6], detailed technical information was not available to the author, so it is not discussed. This survey, updated from [7], is certainly incomplete, and the author would appreciate any additional information that the reader might be able to provide. Laminar-turbulent transition in hypersonic boundary layers is important for prediction and control of heat transfer, skin friction, and other boundary layer properties. Vehicles that spend extended periods at hypersonic speeds may be critically affected by the uncertainties in transition prediction, depending on their trajectories, geometries, and surface properties. However, the mechanisms leading to transition are still poorly understood, even in low-noise environments. Many transition experiments have been carried out in conventional ground-testing facilities over the past 50 years [4]. However, these experiments are contaminated by the high levels of noise that radiate from the turbulent boundary layers normally present on the wind tunnel walls [8]. These noise levels, typically 0.5–1% of the mean, are an order of magnitude larger than those observed in flight [9,10]. These high noise levels can cause transition to occur an order ofmagnitude earlier than inflight [8,10]. In addition, themechanisms of transition operational in small-disturbance environments can be changed or bypassed altogether in high-noise environments; these changes in themechanisms change the parametric trends in transition [9]. Although transition can become completely dominated by roughness [11] or perhaps ablation effects, these effects are usually only one of several factors whose effect must be understood to reliably predict flight [9,12]. Because no single ground-test facility can simultaneously duplicate the velocity, scale, freestream noise, freestream chemistry, and surface temperature of reentry flight, partial simulations in the available facilities must be combined to develop computational models that can then be extrapolated toflight. Mechanism-based methods must be developed to provide reliable predictions. Such mechanism-based prediction methods are now becoming feasible for complex three-dimensional flows at hypervelocities, due to ever-increasing computational capabilities. For example, Johnson et al. recently provided stability-based transition analyses for a planetary probe, although their preliminary results are only for a simplified axisymmetric case [13]. Because the best mechanism-based methods will still require many assumptions, development and validation will require measurements of the