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Aviation History
1963
1963 - 2210.PDF
FLIGHT International, 19 December 1963 1011 RE-ENTRY PROBLEMS THIS article, outlining some of the many problems involved in the safe recovery and landing of a vehicle from a satellite orbit or deep- space mission, is a shortened version of the paper "Re-entry and Landing" presented by L. F. Crabtree and D. H. Peckham of the Royal Aircraft Establishment, Farnborough, at the symposium on aerospace vehicles held by the British Interplanetary Society in London on November 13. THERE are many problems involved in the safe recovery and landing of a vehicle from a satellite orbit or space mission. Since a large part of the deceleration is provided by the atmosphere, a re-entry vehicle must be designed to withstand enormous heat loads as well as to control the actual deceleration loads. There are two principal methods of achieving recovery from satellite orbits:— (a) a pure ballistic re-entry as typified by the Mercury capsule, (b) a glide re-entry in which aerodynamic lift provides manoeuvr ability, and in which the peak heating rates are less than in (a), but the total heat load is greater due to the longer time involved in the re-entry manoeuvre. A typical value of maximum lift/drag ratio for a winged vehicle of this latter class is 2 at hypersonic speeds. (This would yield a value of max L/D of about 4 at low speeds.) Trimmable ballistic :onfigurations are visualized for Gemini and Apollo with a maxi mum L/D ratio of about 0.5, so in this case only minor trajectory Dorrections will be possible. Again, a lifting body of somewhat boat-like shape with a maximum L/D»ratio of about unity has been suggested. With such a vehicle the heating and deceleration prob lems are greatly eased, but of course the actual landing manoeuvre would not be so easy as with the winged vehicle. Up to the present only ballistic re-entry with final recovery (or landing") by parachute has been employed, for the main reason that the weight is thereby kept to a minimum. This has been unavoidable since available booster rockets were limited in thrust. For the future, however, with the ever-increasing scale of orbital *nd space operations, greater flexibility in the choice of the final landing area will be necessary and this means controlled lifting 'e-entry. For military operations this feature will be imperative if he orbital vehicle needs to be capable of initiating the re-entry Manoeuvre at short notice. Let us first look at the general characteristics of the two main Masses of orbital re-entry vehicle. The non-lifting vehicle has the 'irtue of simplicity, is lighter than a comparable lifting version and, mlike the winged vehicle, has little unfavourable aerodynamic feet on the stability of its rocket launcher. It is usually a bluff body vhose front face when re-entering the atmosphere forms a heat shield behind which the vulnerable parts of the vehicle are protected. puring its passage through the atmosphere the only aerodynamic orces on the vehicle are drag loads so that, once the initial retard- »tion from orbit is made, the trajectory is fixed and virtually no manoeuvres are possible. The peak deceleration reached during 'e-entry is quite high—8g or 9g even for a few degrees angle of sntry, and with manned satellites it is necessary to preserve correct orientation of the occupants to the flight path in order that they can withstand these decelerations. Because of the lack of manoeuvring ability, the final recovery of ion-lifting vehicles needs a fairly complex organization, since significant landing dispersion can arise from the day-to-day 'ariations in atmospheric characteristics, and from operational/ ''loting errors. The recovery of the Mercury capsules affords an Sample of the scale of the organization required. In spite of such Reparations, some damage is likely to result from the landing mpact. The limitations of the ballistic trajectory can be mitigated to some stent by offset of the centre of gravity for control of re-entry, and he use of deployable flexible sail wings, such as the Rogallo wing, a ^t allows recovery on land through horizontal touch-down on 'kids. These features are incorporated in the Gemini capsule. Controlled lifting re-entry is as yet unproven but the technique is being investigated in the USA with the X-20 (Dyna-Soar) and Asset projects. A winged vehicle has the great advantage of manoeuvr ability, enabling the trajectory to be controlled and near-conven tional aircraft landings to be made. Because of their lifting capa bility these vehicles have a much wider flight envelope than non- lifting vehicles; the trajectory can be chosen so as to maintain a desirable balance between the heating conditions to which the vehicle is subjected and the decelerations which the occupants have to withstand. Small amounts of lift significantly reduce the deceler ations, for example a lift/drag ratio of 0.5 reduces the peak deceler ation to about 2g. In general, therefore, the question of crew orientation with regard to deceleration tolerance can be ignored. In addition, the heating rates on the vehicle can be reduced to values below those on a comparable non-lifting vehicle. The time involved in the lifting re-entry manoeuvre is much greater, of the order of half an hour for a lift/drag ratio of 1, and is roughly proportional to L/D. As a result, quite large distances are travelled during re-entry and, with the manoeuvre capabilities, this opens up the possibility of a wide choice of landing sites in the large areas available. For example, a vehicle with an L/D ratio of 2 could reach latitudes up to 55° from an equatorial orbit and could, therefore, use landing sites in the UK, South Africa or Australasia. Using conventional aircraft techniques, a lifting vehicle could have approach speeds of perhaps 150kt, which is not excessive by present standards, so that it could possibly use standard airfields. Provided adequate measures are taken to ensure that the heating conditions can be withstood without structural damage, the vehicle could be re-used. So far we have been discussing only re-entry from satellite orbits. For re-entry from space missions where the speeds are much higher, then all the problems are magnified. For instance, at speeds above escape speed, heating of the vehicle by radiation from the hot gas cap becomes significant, and affects the whole design philosophy. Guidance problems also become more acute in that the "corridor" depth for safe re-entry becomes smaller. Lifting: Re-entry Up to now recovery from orbit has been along a ballistic trajectory. For future systems, however, the characteristics of ballistic re-entry appear altogether too restrictive, and some degree of manoeuvrability will be needed during re-entry to give control over range, deceleration and heating rates. This means the ability to generate lift. In this way a relatively deep corridor (compared with the ballistic case) is obtained within which a safe re entry can be accomplished. The boundaries of this re-entry corridor have been termed "undershoot" and "overshoot," and correspond to steeper and shallower initial flight path angles respectively. The overshoot boundary corresponds to the smallest angle at which re-entry can be made without the vehicle passing through the atmosphere and returning to space. For a ballistic vehicle this limiting angle must be steep enough to decelerate the vehicle to orbital speed before the opposite boundary of the atmosphere is reached, but a lifting vehicle can use negative lift to hold itself in the atmosphere at greater than orbital speeds while decelerating; it can therefore re-enter at a shallower angle. The undershoot boundary corresponds to the steepest re-entry angle for which the deceleration or heating are kept within prescribed limits. A typical lifting re-entry from greater than orbital speed is illustrated in the diagram (overleaf). The first stage involves lifting away from the Earth at or near the CLmax of the vehicle, keeping the acceleration normal to the flight path within a pre scribed limit (say 8-1 Og). This then is the undershoot boundary as just described, where the use of positive lift is to avoid penetrating to too low an altitude at too high speed. Modulation of the lift during this phase can be used to obtain the steepest possible re-entry angle. Thus initially the vehicle would be at the angle of attack for maximum lift and high drag, and the angle of attack would be reduced as the vehicle penetrated deeper into the atmosphere so as to maintain accelerations and lower surface heating within acceptable limits. Thus the maximum pull-up and deceleration forces are generated at the highest possible altitude and the heating at the bottom of the pull-up is minimized. The end of this initial phase of re-entry occurs when the flight path angle becomes zero
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