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Aviation History
1963
1963 - 1202.PDF
28 FLIGHT International, 4 July 1963 Missiles and Spaceflight edges; sufficient leading-edge suction is attained to provide an adequate L/D for landing without the need for high-lift leading- edge devices, which would be extremely difficult to engineer. A fin or fins will generally be needed for stability and control within the atmosphere. Since a fin in the conventional position on the upper rear fuselage would be shielded in a partial vacuum at high speed and high incidence, a more satisfactory arrangement is a pair of fins at the tips. Further, since fin leading-edge sweep with respect to the local airflow largely depends on the incidence of the vehicle, aerodynamic heating considerations will place the fins on top of the wing tips rather than below. Ground clearance during landing will probably dictate this also, since the fuselage will generally be positioned above the wing; this latter arrangement minimizes the wetted area subjected to the high-pressure region below the aircraft, and hence minimizes both the skin-friction drag and the heat input to the aircraft. There will, however, be some loss of performance at supersonic speeds, because favourable interference could have been achieved with a configuration having both the fuselage and the fins below the wing. The above considerations fix the general configuration of a high lift re-entry vehicle as envisaged at present (cf. Dyna-Soar). The re-entry manoeuvre which such a vehicle can be expected to under take can now be described, and this will be done for the general case of re-entry from outer space at speeds greater than that of a satellite. At one time it was thought that a series of braking ellipses, in which the vehicle makes temporary grazing contact with the atmosphere, would be a satisfactory way of reducing the speed of a vehicle to a more acceptable value for re-entry. However, it is now known that belts of intense radiation (the Van Allen belts) exist close to the Earth, and repeated passage through these would be extremely hazardous for the crew of a space vehicle. Accord ingly, as far as return to Earth is concerned, emphasis is now being placed on "single-pass" entry manooevres, which are far more critical because of the high speed involved; however, the time taken for entry is much less and much easier to predict. It is apparent that the point of re-entry cannot be varied to any great extent without the expenditure of a great deal of fuel; thus unless the flight has been timed extremely accurately, the re-entry point may be a long way from the desired landing site. This factor indicates the need for achieving very-long-range glides within the atmosphere, and to this end a four-phase technique has been post ulated, as follows2-3:— (1) An initial penetration of the atmosphere, followed by a pull-up to the fringe again. The penetration is taken to sufficient depth and is of sufficient duration such that, when the vehicle is again established at the fringe of the atmosphere, the excess velocity above satellite speed has been destroyed. The vehicle is pulled over into a sensibly horizontal flight path, and is then effectively in a low orbit just on the edge of the atmosphere. (2) An orbital glide to take the vehicle to within striking distance of the pre-selected landing site. The vehicle attitude (incidence) is so chosen that the drag slows the vehicle up in the required distance. (3) A second and final penetration of the atmosphere, during which the vehicle loses the remainder of its velocity and approaches the landing site in a long subsatellite glide. (4) The landing manoeuvre proper. For return to Earth, Ref. 2 gives the order of magnitude of the first three phases, in terms of distance and duration, as follows: entry, 1£ to lOmin (usually less than three), 450 to 2,300 n.m.; transatellite glide, three to 150min, 900 to 35,000 n.m.; subsatellite glide, approx 40min, 7,000 n.m. Considering the re-entry manoeuvre as a whole, it is clear that the re-entry vehicle must have the following characteristics:— (a) The ability to withstand the high aerodynamic heating and accelerations attained in phases (1) and (3). (b) High lift, to control the penetration and pull-up of phase (1). (c) High L/D in phase (2), to achieve long range if necessary. (d) Adequate lift (or L/D for a glider) at low speeds to facilitate landing (phase 4). (e) Good controllability at all speeds and incidences. In addition there is a requirement for the vehicle to be capable of high drag during the initial penetration of the atmosphere. This enables the vehicle to decelerate fairly rapidly in low-density air, thus minimizing the kinetic energy to be absorbed lower down where the heating rate and acceleration are higher. The initial penetration of the atmosphere, which becomes effective at an altitude of about 400,000ft, is carried out with the flight path at a shallow angle to the local horizontal. For a successful, safe return, this flight-path angle must lie within what is known as the re-entry corridor (diagrams on page 29), the limits of which are referred to as overshoot and undershoot. Overshoot represents the case of too shallow an angle; this means insufficient penetration at super-satellite speed, and the vehicle, unable to hold itself within the atmosphere, skips out into space again. Undershoot represents the case of too steep an angle; the vehicle penetrates too deeply into the atmosphere and exceeds one or other of the two design limits (aerodynamic heating or acceleration). The "width" of the re-entry corridor represents the range of re-entry angles which can be used, and depends on the vehicle design limits, the type of re-sntry manoeuvre employed and the speed at re-entry. Generally the corridor is quite narrow—only a few degrees at satellite speed—and decreases rapidly in extent as re-entry speed is increased. For ballistic return to Earth with a lOg acceleration limit, the corridor becomes vanishingly small at an entry speed of 46,C00ft/sec. It should perhaps be pointed out that deceleration and heating are both dependent on vehicle velocity with respect to the atmo sphere. Therefore re-entry becomes less critical if it is carried out in the direction of the Earth's rotation3. 400 300 p -200- •IOO - SUBSATELLITE GLIDE ^-^^*^ TRANSATELLITE GLIDE /L\. SHALLOW >- TV^ ENTRY *-^"^ ^ / \ STEEP / V ENTRY ' I I VELOCITY SATELLITE ENTRY Typical lifting entry manoeuvres for atmospheric entry from interplane tary speed. This diagram is taken from Ref. 2, which is referred to near the foot of Col I on this page Although the altitude at which maximum deceleration occurs varies considerably with vehicle drag, for a given entry angle the actual value of the maximum deceleration varies only slightly; for the ballistic case it lies between about 7.4g and 9.4g over a very wide range of drag levels*. The fact that it varies at all is attributable to the change in atmospheric temperature with altitude. As Chapman points out,6 the temperature : altitude relation is strongly dependent on latitude, season, solar activity and time of day. Design of drag level to give minimum deceleration is therefore of little use. For a given drag level, however, it is helpful to have the ratio of pressure drag to skin-friction drag fairly high, as this reduces kinetic heating. One can infer from this that external drag devices (airbrakes, balloons, parachutes, rotors) might be used to advantage, since these give predominantly pressure drag. Etkin has shown6 that, if a large-drag auxiliary body is towed behind a lifting vehicle during a 7g entry from orbit, the total heat load can be reduced by up to 75 per cent; a corresponding saving in deceler ation could be achieved if the trajectory were programmed to reduce acceleration rather than heating. Clearly the extra weight and complexity of the auxiliary body must be balanced against any simplification and weight-saving in structure and cooling system. This proposal by Etkin represents a simple use of variable drag; the auxiliary body is jettisoned as soon as the vehicle achieves an equilibrium glide trajectory and requires high L/D. More sophis ticated variable drag devices have been suggested, including seg mented flared-skin and umbrella configurations. These are intended to be retractable and fully controllable as regards exposed drag area, and can be applied to both lifting and non-lifting vehicles.
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