I repeatedly discuss entrepreneurial Human Space Expeditions. Obviously, a proven Earth return and Re-entry system will be required for these efforts. I have made the point that entrepreneurial rocket motor developments are not far from what is needed for these flights (if expedition mass and cost are minimized as they are for extreme terrestrial adventures). Adequate control systems have been flown. Communications and navigation is no longer a difficult problem. Similarly we have demonstrated that SCUBA + Mountaineering hardware can be adapted and upgraded for human Life Support in space. A variety of problems NASA publicizes are simply not significant compared to the risks serious explorers are regularly facing. The regulatory, launch license issue, has also been addressed, and will not be a barrier. The return and re-entry systems remain the biggest obstacle, but are not out of reach if an appropriate approach is taken. An “appropriate” approach provides what is necessary – reasonably available and affordable – and does not waste time dreaming about concepts which are not ready to use. Pioneers on the Oregon and California Trails did not wait for “Reusable” transportation. Neither wagons nor their animal propulsion systems returned to make regular, multiple trips! Leave dreams of reusable spacecraft to another generation and keep in mind the ENORMOUS cost of completing a railroad to the Pacific so that Reusable Transportation to those territories was available. Focus instead on an older idea: when transportation is expensive, pack light and go anyway! I will continue to discuss the achievable costs and opportunities which will open “Entrepreneurial Spaceflight” using lightweight versions of conventional rocket propulsion. But for the moment I want to focus on “Conventional” re-entry. That involves, of course, the ablative heat shield used in the Mercury, Gemini and Apollo. This technology has virtually never failed. Of course it is possible for design or manufacturing of a future unit to be so poor that fatal failure results. And there lies a significant problem and entrepreneurial opportunity. I will review details of re-entry design later, and provide insights to help others understand the available literature. But as an overview: Hypersonic re-entry with a traditional (blunt) heat shield brings the relative gas flow to a standstill at some central point, the “Stagnation Point”. This produces a “Detached Shock” some distance ahead of the heat shield as the supersonic gas flow is forced to accommodate the obstruction by slowing down or stopping. Such shock waves, and the accompanying deceleration of the relative gas flow, always increases the gas density, pressure and temperature in that region. Behind the Detached Shock, the hot, compressed gas follows the laws of classic, subsonic aerodynamics! This includes the formation of a boundary layer, separating the solid surface from the relatively uniform bulk gas in the flow field around the heat shield. At some distance from the Stagnation Point, the gas flowing over the heat shield usually accelerates again to supersonic flow, with modestly decreased density, pressure and temperature. But experimental data proves that the greatest thermal load on the heat shield occurs at or near the Stagnation Point, so for our purposes this may be the only point we need to understand. In these discussions, it makes no difference if the gas is rapidly moving past a fixed object, or if an object is moving rapidly through a gas initially at rest: the local physics is identical. In either case a large force is exerted on the solid object, and a correspondingly large amount of energy (that force times the flow velocity) is deposited in the gas flow behind as heat. The heated gas flows “gradually” inward when it is past the body, but a moderate taper can keep the after body separated from this hot gas. A much smaller amount of heat is conducted through the boundary layer to the walls of the re-entry vehicle! (Often less than 1% of the total heat generated by re-entry.) Typically, the boundary layer will average half the flowfield velocity and temperature, and only the heat transferred out of that small portion of the gas flow will reach the heat shield. (In other words: the Boundary Layer itself is the most important Heat Shield!) The boundary Layer does not have a sharp boundary on the outside, nor a uniform velocity or temperature inside. Rather, it has a velocity gradient running from zero at the re-entry body wall to a speed approaching that of the adjacent gas flow at its outside. Its velocity profile tends to retain its shape as its thickness changes. The aerodynamic boundary layer grows on the surface of a sharp edged plate, parallel to the gas flow for minimum drag, with the square root of the distance from its leading edge (where the gas flow first encounters the surface). This is required by the nature of classical viscosity and basic momentum equations. (The surface drag force acting on the body is balanced by the momentum loss of decelerated gas as the boundary layer grows into the slipstream.) The same equations require that the boundary layer Have Uniform Thickness over the spherical leading portion of a body obstructing the gas flow (and producing a stopped flow, Stagnation Point). (In this case, the flow stream lines are squeezed closer to the body as the gas flow accelerates away from the Stagnation Point. Some of these stream lines cross into the fixed thickness boundary layer and that gas slows down. This produces the momentum loss required to balance the surface drag, as in the flat plane case.) Both flow situations are characterized by defining Reynold's Number (Re) = (Rho*V*L/Mu), where Mu is the standard viscosity, Rho is the gas density, V is the gas flow velocity and L is a characteristic dimension. Re is a dimensionless parameter when quantities in appropriated units are inserted in the calculation. The boundary layer has a typical thickness of L/Sqrt(Re), although a multiple of this is used to identify the distance at which the local flow practically equals its velocity outside the boundary layer. In many situations, including practical re-entry vehicles, Re = Reynold's Number runs from 10,000 to 1,000,000 or more, so the boundary layer often is < 1% of the scale parameter (L). (A similar situation exists inside pipes and rocket motors, where L is the inside diameter. The rocket nozzle is the smallest diameter section, and has an increased velocity and Reynold's Number, so the boundary layer is thinnest there. This layer also limits the heat transfer to the throat material, but its thinned dimension allows the greatest heat transfer in that throat region.) Over a spherical “nose”, L is the radius of curvature of that spherical surface. Thus making that radius larger increases the boundary layer thickness and decreases the heat flow through that layer! The limit of “large radius” is flat, but that face is not stable in subsonic or hypersonic gas flow. The curved surface produces stable re-entry aerodynamics if the center of gravity of the capsule is more or less enclosed within that curved surface. Aerodynamic stability, like the boundary layer properties discussed, may be dominated by subsonic gas flow in the region behind the hypersonic shock and can be modeled by something as simple as a dropped Styrofoam bowel or plate. Note that while the critical portion of re-entry involves a modest range of velocities (entry speed to about ½ that speed), a very large range of gas densities is involved. Thus Reynolds Number (Re) has a large range during the process. As the gas density increases, Re increases linearly (normal viscosity Mu does not change much with gas pressure and density, but DOES change with gas temperature.) As the density increases (deeper in the atmosphere) Re increases exponentially, and the boundary layer (proportional to L/Sqrt(Re)) becomes thinner, allowing greater heat transfer. Eventually, the large form drag of the blunt capsule reduces the velocity enough so that the reduced stagnation gas temperature drops the heat transfer below its peak value. Eventually, the reduced velocity also reduces Re, allowing the boundary layer to grow somewhat. If the heat shield radius (L) is increased, Re increases proportionally, but L/Sqrt(Re) still increases producing a thicker boundary layer with less heat transfer. Note that a low mass reentry vehicle of conventional (Mercury Capsule) size, will decelerate higher in the atmosphere, where the air density is lower. This will produce a Lower Reynolds number when the heat transfer is greatest, a thicker boundary layer and less heat transfer. However, the Heat Transfer is a Larger Portion of the total kinetic energy of reentry, so the heating problem is somewhat worse. If reentry mass is decreased by a factor of 6.25, the heat transferred is reduced to 40% of that for the heavier unit, but this still represents 2.5 times the relative proportion. So heat shield mass will become a modestly larger percentage of the reentry mass. To Be Continued...