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Aviation History
1963
1963 - 1237.PDF
'••• .• :. ••• • FLIGHT International, 11 July 1963 Missiles and Spaceflight have been devised are generally suitable for one or other of the two tvpes of heating, but not for both. The simplest cooling technique is radiation cooling, on which the X-20 relies very largely during its long, shallow glide. With this method the temperature of the aircraft skin is allowed to rise to an equilibrium value. At this temperature, which may be 2,000°C, the rate at which the heat is radiated out from the skin is equal to ENTRY ANGLE (DEG) Curves showing the approximate variation of re-entry heating with entry angle. These curves are taken from "The Effect of Trajectory and Aerodynamic Characteristics on Entry Heating of Lifting Spacecraft," by Lowe, Greene and Gervais, read before the IAS last year the rate it is absorbed. A large radiating surface is required; hence the vehicle will have a low wing loading. Maximum heat radiation will be achieved with a matt black surface, although during the period of highest heating much of this surface will be white hot. A thermally shielded structure is employed, which to a large extent separates the load-bearing and heat-control functions9. The amount of insulation needed depends on the exchange rate between insulation weight and structure weight to give the necessary strength during re-entry. As the amount of insulation is reduced, the structure thickness (and therefore weight) will have to be in creased to allow for the degraded material properties at the increased temperature; parametric studies will indicate the combination of insulation and structure which gives minimum weight. Note that, as the structure temperature goes up, more advanced materials (titanium, Inconel X) will have to be used. The yield-strength/ weight ratio of the materials declines rapidly as the temperature rises, dropping by a half between 450 and 900°C. This emphazises the need for an effective heat-shield and insulation. Research has proved materials such as the alloys of molybdenum, colum- bium, tantalum and tungsten, which have the necessary strength at high temperatures; unfortunately these same materials have a very low resistance to oxidation at these temperatures. Much work is therefore being carried out in attempts to find suitable coatings which will prevent oxidation. Such a coating must withstand the severe conditions of temperature, acoustic vibration and flexing. It is again unfortunate that a small, compact (and therefore rigid) airframe which would minimize flexing is inherently unsuitable for radiation cooling, because of the low radiating surface area and the shorter trajectory which it would use. If the heat is not radiated away from the vehicle, it must be absorbed either internally or externally. The criterion for cooling >s primarily the total heat load, since this must be within the capa city of the absorption (heat-sink) system; this type of cooling is thus suitable for small, compact vehicles which have a high wing loading and which use relatively short, steep re-entry trajectories. j^nce the structure will generally be completely inadequate as a heat sink, internal absorption implies a complex system of pipes located close to the aircraft skin. A pumping arrangement is used circulate a cooling fluid (possibly fuel, possibly a liquefying rnetal) to some radiators situated in cooler parts of the vehicle. disadvantage is that the system may not be able to cope with a emporary excess heating rate, even though the quantity of coolant carried may be adequate for absorbing the total heat load. 65 This disadvantage is not shared by the simplest of the two ex ternal-absorption techniques which have been devised—ablation cooling. With this method the parts of the vehicle subjected to the greatest heating are coated with an expendable layer of a material which during re-entry absorbs heat by melting, burning or sub liming. The thickness of material required is a direct function of the total heat load expected; it is important that no great distor tion of the shape of aerodynamic surfaces should occur during ablation. The absorption process is self-regulating in that, if an extra-high heating rate is encountered, ablation merely speeds up to compensate. Ablative cooling has already been extensively used for re-entry to Earth, for satellites and for ballistic missiles. The second external-absorption technique is known as transpira tion cooling. Simple in principle, it consists of a porous skin cover ing the vehicle such that, when re-entry commences, a suitable fluid can be forced under pressure through the pores in the surface. By this means a great deal of the heat can be absorbed by the fluid (especially if a liquid is used, because of the latent heat of evapora tion). More important than this, however, the fluid/air mixture forms a layer of relatively cool gas next to the vehicle's skin, and this is a very effective form of insulation. The rate at which the fluid is pumped through the pores is regulated according to the heating rate. Manufacturing and engineering difficulties, particu larly the problem of clogging of the skin pores, have so far prevented the use of transpiration cooling; it does, however, hold promise for the future. An ingenious use of transpiration has been suggested by Bog- donoff and Chapman5. The basis of the method is to maintain an insulating layer of separated flow over as much of the surface as possible. The separated flow is established at the nose and wing leading edges by a spike and sharp-edged plates (i.e., two-dimen sional spikes) respectively; it is maintained by transpiration from the downstream side of each of a series of ridges which are set at intervals over the vehicle's surface and which are transverse to the direction of flow. More likely to be used on hypersonic-cruise aircraft, the method might find an application with re-entry vehicles since it promises a very low heating rate to the vehicle as well as a low skin-friction drag. One further cooling technique deserves mention; this involves magnetohydrodynamics (MHD) and makes use of the fact that, at the very high temperatures attained during re-entry, the air surrounding the vehicle is ionized. Since an ionized gas is readily influenced by a magnetic field, in principle electromagnetic equip ment could be carried on board and used to make the bow shock stand farther away from the surface. The reduced shear stresses and gas displacement so obtained would give a corresponding reduction in heat transfer to the surface. But the science of MHD is still very much in its infancy, and the weight of electrical equip ment which would be needed according to the present state of the art is prohibitive. These disadvantages, plus the not insignificant heating effect of the coils themselves, put MHD out of the running as a present-day cooling technique. A possible breakthrough may Another artist's impression of the Boeing/USAF X-20 Dyna-Soar (a land ing scene was depicted last week). Apart from the ASSET programme (illustration overleaf), this is the only vehicle outside the Soviet Union at present scheduled to give information on controllable re-entry. The pilot will maintain a high angle of attack during re-entry, to minimize heating over the cockpit (protected at this stage by a refractory shield) and equipment bay
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