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Aviation History
1963
1963 - 2211.PDF
1012 START or SUPER- ORBITAL. RE-ENTRY Vi 36,0O0 TT/SEC v =2; 400,000 rr y^ -6' SKIP-OUT TRAJECTORY V*S" 29,OOOF"T/SEC PULL-OUT V«* 32,000 FT/SEC •M«= 115.000 n Y- o- START OF SUB-ORBITAL RE-ENTRY V dfc at, 000 rr/scc y ^= 2EO,000 FT FLIGHT Internationo 19 December 1% Typical super-orbital re-enti trajectory (altitude and rang plotted to same scale), f>oi "Supercircular re-entry fljgj path control requirements by T. C. Sanial, reproduce in the RAE paper by Crabtre and Peckbam Missiles and Spaceflight and where, without modulation of angle of attack, maximum aero dynamic loads would occur. For the second stage of re-entry the vehicle must generate sufficient negative lift (say, by rolling through 180°) to balance the centrifugal force and keep the vehicle within the atmosphere, even though its speed is greater than the orbital value. Of course, provided the speed at this point is less than escape speed, the vehicle could negotiate a "multiple pass" trajectory, skipping in and out of the atmosphere until it has decelerated to orbital speed. However, this is generally reckoned to be inadvisable for several reasons— for example, the danger of passing through the Van Allen radiation belts, the excessive time involved, and so on. This second stage then corresponds to the overshoot boundary previously described, and the vehicle gradually decelerates until orbital speed is reached. Since the centrifugal force gradually decreases as the speed approaches orbital speed, then the altitude for inverted flight gradually increases during this phase. In the example shown, about 1,000 miles is covered during these first two stages in about 2imin. Now the height between undershoot and overshoot boundaries is referred to as the entry corridor depth as already described, and this depth increases rapidly with increase of L/D. Guidance and control problems are of course eased if a deep corridor can be achieved. Even so, the flight path characteristics such as angle, attitude and rate of climb, change but slowly compared with horizontal rates. As a result, sensing the flight path conditions (for control or display in the cockpit) poses a severe problem, particu larly over the wide range of values that must be measured throughout the entry. The third stage, with speed gradually decreasing below orbital, involves lift directed away from the Earth again, but here the accelerations normal to the flight path are small, of the order of g/(L/D). The final phase, below about 100,000ft, involving the transition to low supersonic flight, includes also the final approach and landing. For a hypersonic (L/D)max of 2, a low-speed value of about 4 would probably be obtained, so that with a wing loading of say 351b/sq ft a landing speed of not more than 150kt is feasible, and this should not pose very difficult problems. The experience gained with the X-15 will bt especially relevant here. Heating Characteristics The lifting re-entry vehicle experiences lower convective heating rates than a compar able ballistic vehicle but, for longer times, the net result is a greater total heat input. This means that the heat-sink and/or ablative heat-shield solutions used with ballistic vehicles become unattrac tively heavy for a lifting vehicle, which must therefore lose a large part of the heat input by radiation. This is more efficient with a higher surface temperature. If heat shielding is by absorption (which is understood to include ablation, etc) minimum heat-shield weight is related to minimum heat load, and this tends to be achieved by making entry vehicles as small as is practical consistent with their payloads. If heat shielding is by radiation from the surface, relatively large vehicles are required for a given payload in order to limit the surface temperatures, and vehicles with a high lift capability appear applicable at satellite entry speeds. With increasing entry speeds, however, heating rates become especially severe and radiation cooling alone may be insufficient. Thus for higher speeds (of the order of escape speed and above it may be necessary to concede some ground on design for minimur convective heating if by so doing the radiative heating can b drastically reduced. For example, Allen has suggested the use of slender cone with a continuously ablating central rod issuing fron the apex like a propelling-pencil. To summarize:— (a) For re-entry at satellite speeds the high lift and high L/I vehicle with radiation shielding is attractive (i.e., aircraft-like) (b) For speeds greater than satellite speeds, some ground may hav to be conceded in that some absorption heat shielding may b necessary in addition to radiation cooling. In that case a lowe L/D capability must be accepted—as with a lifting blunt body, fo example. (c) For speeds greater than escape speed higher convective heatin; rates must be accepted in order to reduce the radiative heating Thus the vehicle will have a still lower lift capability, and will haw more absorption heat shielding than for (b). However, blunt nose may have to be avoided to some extent, and more slender design; adopted. In this regime there may be a case for a variable-geometn design, as also in case (b), whereby folding wings are deployed tc produce adequate L/D for low-speed manoeuvrability and landing Problem Areas There are many aerodynamic problem: connected with a lifting re-entry vehicle in addition to heal transfer, as well as structural and systems engineering problems. Some of these problems are:— Flow fields and pressure distributions It is important to have a detailed knowledge of the local flow conditions and the complete pressure distribution around the vehicle for loading calculations, heat transfer estimates, and stability and control calculations, to name but a few. Designing for maximum lift and drag is not too difficult in general terms, and the actual body shape is of little importance. But the flow pattern over delta wings is very complex over the whole incidence range from 0 to 90°. Heat transfer and thermal protection systems Problems connected with both convective and radiative heat transfer at super-satellite speeds are complicated by chemical changes in the air due to the high temperatures involved—dissociation, ionization, and chemical reactions between the different constituents of air at high tempera tures. Non-equilibrium phenomena can play a major role in deter mining the heat transfer under such conditions. Guidance, stability and control The guidance problems associated with even the relatively deeper lifting re-entry corridors have already been mentioned. However, the question of stability and control is perhaps as difficult as any for lifting re-entry. This becomes im mediately apparent when it is seen that the whole angle-of-attack range from 0 to 90° has to be considered. Studies have shown how completely the flow pattern changes over this wide incidence range, which will affect the operation of aerodynamic control surfaces, for example. T. G. Sanial has studied a number of problems connected with flight-path control requirements for re-entry at super-satellite speeds. For two particular shapes (blunted half-cones) Saniai found that a pitch rate of at least 5°/sec is required to avoid skipping out of the atmosphere on the overshoot boundary; and the maximum time-lag that can be tolerated in initiating the pull-up manoeuvre, for instance, is about 5 sec. . , In the aeroelastic field there are likely to be problems of classic^ and panel flutter, in addition to instabilities similar to transom buzz on extensible drag brakes and other aerodynamic conn • These problems are of course intensified by the severe t'16"11^ environment of the vehicle. Again, since manoeuvring in roll at rug angles of attack is likely to be called for in lifting re-entry, there potential problems in non-linear cross-coupling dynamics.
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