Landing in Braking Ellipses


In the method just described, transitioning from the descent orbit into the free circular orbit and the required velocity reduction from 11,000 to 7,850 meters per second occurred during the course of the "forced circular motion." According to another Hohmann recommendation, this can also be achieved by performing so-called "braking ellipses" (Figure 45). In this landing procedure, the wings are not used initially, but braking is performed as vigorously as the previously explained danger of excessive heating will permit by means of a trailing parachute as soon as the vehicle enters into sufficiently dense layers of air.

Figure 45. Landing in "braking ellipses." (The atmosphere and landing orbit are drawn here higher than in reality, exactly similar to Figure 44. Reference Figure 8.)

Key: 1. First braking ellipse; 2. Second braking ellipse; 3. Third braking ellipse; 4. Fourth braking ellipse; 5. Glided flight; 6. Earth; 7. Rotation of the Earth; 8. Braking distance of the ellipses; 9. Landing; 10. Return (descent) orbit.

However, the travel velocity, as a result, cannot be decreased to such an extent as would be necessary in order to transition the space ship into free circular motion. An excess of velocity, therefore, still remains and consequently also a centrifugal force that pushes the vehicle outward so that it again exits the atmosphere and moves away from the Earth in a free orbit of an elliptical form (first braking ellipse). The vehicle, however, will not move away to that distance from which it originally started the return flight because its kinetic energy has already decreased during the braking (Figure 45). Due to the effect of gravity, the vehicle will re-return to Earth after some time, again travel through the atmosphere with a part of its velocity again being absorbed by parachute braking; it will move away from the Earth once again, this time, however, in a smaller elliptical orbit (second braking ellipse), then return again, and so on.

Therefore, narrower and narrower so-called "braking ellipses" will be passed through one after the other corresponding to the progressive velocity decrease, until finally the velocity has dropped to 7,850 meters per second and as a result the free circular motion has been reached. The further course of the landing then occurs with the help of wings in gliding flight, just as in the previously described method. The entire duration of the landing from the initial entry into the atmosphere to the arrival on the Earth's surface is now around 23 hours; it is several times longer than with the method previously described. Therefore, the wings provided anyway for the Hohmann landing will be used to their full extent even at the start and consequently the landing will be performed better in a forced circular motion.


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