Introduction W HEN a thermalprotectionsystemdesigneris confrontedwith a leading-edge design for a hyersonic vehicle, the available options are either passively cooled, heat-pipe-cooled, or actively cooled systems. The upper use limit for passive leading edges may be determined by evaluating the material properties in light of the thermal and mechanical loads. If passive leading edges cannot survive the environmental conditions, heat-pipe-cooledor actively cooled leading edges may be required. Preliminary design studies at NASA Langley Research Center indicate that a refractorycomposite/refractory-metal heat-pipe-cooled leading edge can reduce the leading-edge mass by over 50% compared to an actively cooled leading edge, can completely eliminate the need for active cooling, and has the potential to provide fail-safe and redundant features.1 Though heat pipes are often a viable and lightweight option, the analysisrequiredto determinethe feasibilityfor a particular application can be extensive and can preclude their use. It is therefore beneŽ cial to have a simple set of closed-form equations that can be used to determine if the heat-pipe option is feasible.Having a simple analysis technique availablemay prevent the unnecessary incorporation of active cooling systems when heat pipes may provide a cheaper and lightweight alternative and may also eliminate the need for a complex, three-dimensional Ž nite element analysis (FEA) to answer the initial question of feasibility. The purpose of this Note is to present a set of simple, closedform design equations that can be used to determine a preliminary design of a heat-pipe-cooled leading edge. The design equations presented here are only for thermal design purposes and do not include any stress analysis. Temperatures obtained from the design equations are compared to three-dimensional for both a large and small leading-edgeradius.Thoughsome restrictionsapplyto the use of these equations,they appear to be a useful tool for the preliminary design of heat-pipe-cooled leading edges. Use of these equations will quickly answer questionssuch as, Is a heat-pipe-cooledleading edgeevenfeasible?What is theheatpipeoperatingtemperature?Are refractory metal or superalloy heat pipes required? Is a refractorycompositestructurerequired?What is the requiredheat-pipelength? If the preliminarydesignequationsindicatea feasibledesign,a more detailed analysis should follow.