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What about Mr. Oberth?

20 Oct 2013, 18:43 UTC
What about Mr. Oberth?
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On a space forum I was singing the virtues of EML1 and 2, the earth-moon Lagrange regions closest to the moon. "What about Mr. Oberth?" asked a fellow who calls himself Rune.This is a common complaint from Zubrin fans who prefer to depart for Mars from Low Earth Orbit (LEO). Zubrinistas point out there's a greater Oberth benefit doing a burn deep in a gravity well.What is the Oberth benefit? Why is there a bigger Oberth benefit deep in a gravity well?The Oberth benefit gives a lot of extra kinetic energy for a small change in speed.Kinetic energy is equal to 1/2 * mass * velocity2. A way to visualize the product of three factors (mass * velocity * velocity) is as a rectangular solid:To get 1/2 mv2, just cut the square diagonally from corner to corner as shown above.What happens if you're already going fast and speed up a litte more? Say you increase your speed v by vb, velocity from a rocket burn. Here's a picture:Take 1/2 of m (v + vb)v2, and you get 1/2 mv2 as well as 1/2 mvb2, the kinetic energy you'd expect from adding these two speeds. On top of that, you also ...

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