- #1

BitWiz

Gold Member

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Examples of KE often use a car to demonstrate that doubling the velocity quadruples the stopping distance. I get that, but though velocity is a goal in space travel, I would think acceleration is a more important energy management element in frictionless, gravity-free space.

Then there's the difference in how cars and rockets work:

A Chevy and a rocket are at a stop light. The light turns green . . .

The Chevy's engine cylinders fire at equidistant points along the road; fuel usage is proportional to distance.

The rocket is not connected to the road; fuel usage is proportional to time.

If it's fair to plug proportions into the acceleration equation (d / t^2), then I get Fuel is proportional to t^2 for the Chevy, and sqrt(d) for the rocket, i.e:

If we double the acceleration time, the Chevy needs four times the fuel, the rocket just twice the fuel.

If we quadruple the acceleration distance, the Chevy needs four times the fuel, the rocket just twice the fuel.

So my question is, is it fair to use Earth-observer-based KE to determine the fuel requirements for a rocket?

Thanks!

Chris