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Aviation History
1963
1963 - 1203.PDF
FLIGHT International, 4 July 1963 29 -io° 'g' BOUNDARY SKIP BOUNDARY ^RE-ENTRY CORRIDOR _ 400,000 ft If \\ / \ I I / Planar view of entry paths, with the avail able re-entry corridor shaded. The vertical scale is exaggerated in relation to the radius of the Earth When a vehicle first comes into contact with the very thin upper atmosphere, its deceleration will be extremely small unless the drag is made to be very high; as noted previously, it is desirable (within the design acceleration limits) to lose as much speed as possible as quickly as possible in this low-density region in order to minimize aerodynamic heating. Thus, if a variable drag device is employed, it will be held in the fully open position at the start of re-entry and will be gradually closed as the vehicle descends into the denser lower atmosphere. For ballistic re-entry, a peak deceleration of 4 to 5g is possible with some reduction in heat transfer; this result is necessarily poor compared with that for lifting re-entry, because high drag in the ballistic case automatically produces a steeper trajectory. Nevertheless the benefits are useful and complement a further advantage which such a device has when fitted to a ballistic re-entry vehicle—trajectory control. An Avco brake7, which has been designed in detail, is claimed to provide a ± 150-mile variation in landing site just after the start of re-entry. The weight penalty for such a device is not excessive; in fact Avco have shown that for re-entry from a low orbit (about 100 miles) the drag obtained by merely opening out the brake is sufficient to cause the commencement of re-entry. Thus the retro- rocket system is redundant, and there is a net weight saving. In most cases, however, a retrorocket system is a necessary item of equipment; Marshall has shown analytically8 that, in such cases, sparing use of the available thrust can be advantageous during the initial stage of re-entry to supplement the very low aerodynamic drag. Significant reductions in kinetic heating and peak deceler ation are obtained at relatively low cost in fuel weight, provided that high aerodynamic drag is achieved as soon as possible. The benefits obtained are increased if the rocket unit is vari-directional, and can be used to provide initial flight-path control independently of deceleration. For winged re-entry vehicles capable of fairly high lift, the need for external drag devices is greatly reduced owing to the fact that the wings themselves can form a useful high-drag surface. This implies that the initial penetration be made at very high incidence, thus obtaining a high pressure-drag from the wings. It has in fact been suggested that re-entry might be made at 90° incidence, i.e., with the planform at right angles to the flight path. Feasibility studies have shown, however, that (a) the drag at 90° for a typical re-entry configuration is not significantly higher than that attained at the maximum-lift incidence (about 5501); (b) whereas the normal aerodynamic controls are adequate for use at 55°, a completely new reaction control system would have to be developed and installed to provide control at 90°; and (c) entry at 90° does not give the vehicle a vertical lift component necessary for maintaining the desired flight path and preventing overshoot and undershoot. For these reasons, then, lifting re-entry will generally be started by an initial penetration at an incidence of around 55°. It is, how ever, evident that with the vehicle in the upright position the magni tude of the lift component may become an embarrassment, since it tends to reduce penetration of the atmosphere. The problem be comes obvious when supersatellite speeds are considered, for here negative (downward) lift is necessary to hold the vehicle within the atmosphere. A possible solution here is to commence re-entry in the manner considered above, then gradually reduce incidence as required until the incidence is negative and the necessary downward Typical entry corridors plotted in the form of entry angle against velocity for a height of 400,000ft. This diagram is taken from Ref. 4; the accompanying text is Col 1 on the opposite page. The ballistic (LID=0) corridor vanishes at 46,000ftjsec entry lift is attained. The disadvantages of this "altitude modulation" method are two: first, a thermally insulated structure is required on the top surface as well as on the lower surface (the pilot's cockpit would be particularly vulnerable if stepped as on the X-20); second, as the incidence is reduced down through zero, the drag level falls off, thus defeating the object of re-entering at high incidence. The suggested alternative procedure for achieving the required positive or negative vertical lift component, without sacrificing drag level, is the use of banked flight. With this method the pilot main tains the high angle of incidence during the first part of re-entry but undertakes a rolling manoeuvre; by altering the angle of bank in this way, the vertical lift component is adjusted to maintain the planned flight path. For re-entry at supersatellite speeds most of this phase will be carried out at bank angles greater than 90°, i.e., the vehicle will be inverted. As the vehicle penetrates deeper into the atmosphere, a stage is reached when the resultant force on the vehicle is of such magnitude that the limiting acceleration is reached. To prevent this limit from being exceeded, the altitude modulation which was postulated above for lift is now brought into use; the incidence is progressively re duced from the nominal 55°, so that the lift and drag are reduced and the resultant acceleration kept within the design limit. The reduction in lift will generally cause an unwanted change in the vertical component of that lift, and therefore, the bank angle must be altered to compensate. This procedure uses banked flight and altitude modulation to achieve what is effectively a wider entry corridor than could be achieved by simpler techniques. With this in mind it should be emphasized that, if re-entry is made under conditions well within the corridor, then the entry manoeuvre should be modified such that it borders on the aerodynamic heating limit rather than on the acceleration limit. There is little point in subjecting the crew to, say, lOg in order to keep aerodynamic heating unnecessarily low, when a re-entry trajectory with a maximum acceleration of 6g could be employed without exceeding the design heating limit. To be concluded REFERENCES 1. Yoler, Y. A.: Dyna Soar: A Review of the Technology. Aerospace Engineering, Vol 20, No 8, August 1961. 2. Smith, R. H. and Menard, J. A.: Supercircular Entry and Recovery with Manoeuvrable Manned Vehicles. IAS Paper 61-114-1808, 1961. 3. Lowe, R. E., Greene, R. B. and Gervais, R. L.: The Effect of Trajectory and Aerodynamic Characteristics on Entry Heating of Lifting Spacecraft: Proc. of Aerospace Forum II Session, IAS 30th Annual Meeting, January 22-24, 1962. 4. Hildebrand, R. B.: Manned Return from Space. Second Inter national Congress of the International Council of the Aero nautical Sciences, Zurich. September 12-16, 1960. 5. Bensen, O. O. and Strughold, H. (editors): Physics and Medicine of the Atmosphere and Space; Wiley, 1960. 6. Etkin, B.: On a Relatively Cool Transition from a Satellite Orbit to an Equilibrium Glide: Proc. of Aerospace Forum II Session, IAS 30th Annual Meeting. January 22-24, 1962. 7. Aviation Week, Vol 70, No 25, June 22, 1959. 8. Marshall, F. J.: Optimum Re-entry into the Earth's Atmosphere by Use of a Variable Control Force: WADC TR 59-515, October 1959.
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