Launch Speeds and Earth Rotation


The first concept we must clarify before answering this question is that orbital speed is orbital speed, no matter how you get there. For example, the Space Shuttle typically travels between 17,000 and 18,000 mph (27,355 to 28,960 km/h) when in orbit. No matter which direction the vehicle was headed in its orbit, that speed would be the same for a given circular orbit at a given altitude.

The only difference in speeds that comes into play is related to the direction in which a rocket is launched. In a previous question on our site, we explained how the rotation of the Earth assists in launching into orbit.

Part of that explanation discussed the "speed bonus" that a rocket gets by launching closer to the equator. A Space Shuttle launched from Cape Canaveral gains 915 mph (1,470 km/h) because the Earth rotates at that velocity at that location. The Shuttle launches to the east because this is the direction in which the Earth rotates. As a result, the Shuttle does not need to accelerate from 0 to 17,000 mph, but only from 915 to 17,000 mph. In other words, the Shuttle only has to accelerate by 16,085 mph (25,880 km/h) to reach its orbital speed because that extra 915 mph is provided by the Earth itself.

The situation you are asking about is the reverse--a launch towards the west. In this case, the Shuttle would experience a speed penalty of 915 mph (1,470 km/h). It would now have to accelerate to 17,915 mph (28,825 km/h) to reach orbit because it has to overcome the initial velocity imparted on it by the rotation of the Earth.

Finally, it should be noted that there are additional reasons for launching the Shuttle to the east in addition to the speed bonus. The primary reason for launching to the east from Cape Canaveral is because of range safety constraints. A Shuttle will not launch due west from the Cape since it would be launching over populated land. The same rule applies for launches off the West coast. If it is necessary to launch a rocket to the west or into polar orbit, it must be done from the West coast, again because of range safety.
- answer by Joe Yoon, 16 March 2003

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