The situation described above can, however, be prevented through the appropriate use of wings. In the case of a standard airplane, the wings are pitched upward so that, as a result of the motion of flight, the lift occurs that is supposed to carry the airplane (Figure 42). In our case, the wings are now adjusted in the opposite direction, that is, pitched downward (Figure 43). As a result, a pressure directed downward towards the Earth occurs, exactly offsetting the centrifugal force excess by properly selecting the angle of incidence and in this fashion forcing the vehicle to remain in the circular flight path (Figure 44).
Figure 42. The fundamental operating characteristics of wings during standard heavier-than-air flight: The "lift" caused by air drag is directed upward and, therefore, carries the airplane.
Key: 1. Lift; 2. Air drag; 3. Direction of flight; 4. Weight of the vehicle; 5. Wings; 6. Surface of the Earth.
For performing this maneuver, the altitude was intentionally selected 75 km above the Earth's surface, because at that altitude the density of air is so thin that the space ship despite its high velocity experiences almost the same air drag as a normal airplane in its customary altitude.
Figure 43. The operating characteristics of wings during the "forced circular motion" of a landing space ship. Here, air drag produces a "negative lift" directed towards the Earth (downward), offsetting the excessive centrifugal force.
Key: 1. Centrifugal force excess; 2. Air drag; 3. Direction of flight; 4. Negative lift; 5. Wings; 6. Surface of the Earth.
During this "forced circular motion," the travel velocity is continually being decreased due to air drag and, therefore, the centrifugal force excess is being removed more and more. Accordingly, the necessity of assistance from the wings is also lessened until they finally become completely unnecessary as soon as the travel velocity drops to 7,850 meters per second and, therefore, even the centrifugal force excess has ceased to exist. The space ship then circles suspended in a circular orbit around the Earth ("free circular motion," Figure 44).
Since the travel velocity continues to decrease as a result of air drag, the centrifugal force also decreases gradually and accordingly the force of gravity asserts itself more and more. Therefore, the wings must soon become active again and, in particular, acting exactly like the typical airplane (Figure 42): opposing the force of gravity, that is, carrying the weight of the space craft ("gliding flight," Figure 44).
Figure 44. Landing in a "forced circular motion." (The atmosphere and the landing spiral are drawn in the figure for the purpose of a better overview higher compared to the Earth than in reality. If it was true to scale, it would have to appear according to the ratios of Figure 8.)
Key: 1. Return (descent) trajectory; 2. Travel velocity of 11,000 m/sec; 3. Free circular motion; 4. The part of the atmosphere 100 km high useable for landing; 5. Free Orbit, in which the space ship would again move away from the Earth if the wings fail to function; 6. Forced circular motion; 7. Landing; 8. An altitude of 75 km above the Earth's surface; 9. Start of braking; 10. Boundary of the atmosphere; 11. Gliding flight; 12. Earth; 13. Rotation of the Earth.
Finally, the centrifugal force for all practical purposes becomes zero with further decreasing velocity and with an increasing approach to the Earth: from now on, the vehicle is only carried by the wings until it finally descends in gliding flight. In this manner, it would be possible to extend the distance through the atmosphere to such an extent that even the entire Earth would be orbited several times. During orbiting, however, the velocity of the vehicle could definitely be braked from 11,000 meters per second down to zero partially through the effect of the vehicle's own air drag and its wings and by using trailing parachutes, without having to worry about "overheating." The duration of this landing maneuver would extend over several hours.