飞机加油问题

已知

1. 每个飞机只有一个油箱
2. 飞机之间可以相互加油(注意是相互，没有加油机)
3. 一箱油可供一架飞机绕地球飞半圈
4. 所有飞机从同一机场起飞，而且必须都安全返回机场，不允许中途降落，中间没有飞机场

问题：

为使至少一架飞机绕地球一圈回到起飞时的飞机场，至少需要出动几架飞机?

5

the 1st one come back at the point the angle is 45, the 2second come back at the point where the angle is 90. one plane keep flying. the third one flies to this one in the opposite direction to the location where the angle is 90. the fourth feuls the plane at the point where the angle is 45