1968-02-01

Short Downrange Entry 680370

Most previous studies of entry maneuverability of high L/D vehicles from low altitude earth orbit have been concerned with defining the maximum crossrange performance without regard to the downrange distance. However, for certain “quick return” missions, entry within a limited downrange distance may also be desirable. Results of a study to define the effects of various entry techniques in minimizing the distance to the heel of the footprint, within reasonable heating limitations, are presented.
Studies were made for a spacecraft having a maximum hypersonic L/D of 2.2. A computer program was utilized that simulated entry in four phases:
  1. 1.
    An entry pullout phase,
  2. 2.
    A modulated bank angle phase, to suppress skip,
  3. 3.
    A constant bank angle phase, held until a desired heading was achieved.
  4. 4.
    A terminal glide phase at maximum L/D.
Any combination of bank angle and angle of attack could be selected for each phase, except Phase 2, where bank angle was modulated as necessary to prevent skip.
Maneuver parameters were investigated for both exo-atmospheric and atmospheric entry stages. The exo-atmospheric parameters included orbit altitude and retrograde velocity increment, thrust-to-weight ratio, and thrust angle. Parameters in the atmospheric stage included initial entry angle, angle of attack during Phases 1, 2, and 3; bank angle during Phases 1, 3, and 4, and the heading angle change to initiate Phase 4. The effect of each of these variables was investigated parametrically to establish its influence on downrange-crossrange capability.
The entry techniques used to shorten the downrange distance generally decreased crossrange performance also. The downrange decrease attainable for a given crossrange requirement is therefore dependent on the excess crossrange capability of the spacecraft.

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