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Subject: Sol fly by rss

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Luke Stirling
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andrewdoull wrote:
Does anyone know why there's no fly by path over Sol?

In essence, I'd say that all trajectories on the entire board are dictated in a significant way by Solar gravity. As for a close Sol fly-by, well, those usually don't end too well.
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Kyrill Melai
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This question has been asked in the yahoo groups. Search for the oberth effect and you should find it. There is a very extensive answer by Phil there.The topic is titled: Solar flyby/slingshot?
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Phil Eklund
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andrewdoull wrote:
Does anyone know why there's no fly by path over Sol?


I agree (partially) with Luke. A gravity slingshot is useful in two ways: (1) by momentum transfer with the planet or star, and (2) by the Oberth effect (essentially a thrust advantage by expending fuel at the lowest possible gravitational potential energy).

The first way, momentum transfer, is useful for surmounting delta-v with respect to a third body. You gain kinetic energy and speed, but not with respect to (wrt) the body you are slingshotting. For instance, to escape the Earth's gravity field, it can be useful to slingshot Luna. But you can't use Earth's gravity to help escape from the Earth gravity. Rather than make a rule, I tried to represent this impossibility by the geometry of the map, especially those paths which cross but do not intersect close to planets.


A flyby around Earth neither gains nor loses energy with respect to Earth. But a pass "in front of" Earth as it travels around the Sol can sling you to Venus. Or a pass "behind" Earth can get you to the superior planets like Mars or Jupiter. You can visualize the momentum transfer slingshot by throwing a tennis ball at an on-coming vehicle. From the point of view of the driver, the ball arrives at a certain speed, bounces off the windshield and exits at the same speed. So no velocity gain, wrt to the driver. Yet wrt the thrower, the ball acquired an additional velocity due to the speed of the vehicle. (And the vehicle lost velocity slightly to conserve momentum).


By the same reasoning you can't really use Sol as a slingshot to attain a particular orbit within the solar system. You can use Sol only as slingshot to get to another solar system. In High Frontier Interstellar, I have special rules to do this (especially the Jupiter-Sol-Jupiter exit).

This reasoning is complicated by the second method, the Oberth effect. A close pass to Sol makes your propellent expenditure more effective, and thus it can be used to help attain orbits within the solar system, contradicting everything I said about momentum transfer. The Oberth is hard to visualize, but can be thought of as the efficiency gain by throwing something away at the bottom of a hill as opposed to dragging it up a hill then throwing it away. But the Oberth is not so good for most of the rockets in High Frontier, who can only discharge propellent at a very limited rate. So I left Sol slingshots off.
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Phil Eklund
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NoDDs wrote:
This question has been asked in the yahoo groups. Search for the oberth effect and you should find it. There is a very extensive answer by Phil there.The topic is titled: Solar flyby/slingshot?


Oops, I forgot I already answered this. I'm praying I didn't contradict myself.
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Jorik
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phileklund wrote:
NoDDs wrote:
This question has been asked in the yahoo groups. Search for the oberth effect and you should find it. There is a very extensive answer by Phil there.The topic is titled: Solar flyby/slingshot?


Oops, I forgot I already answered this. I'm praying I didn't contradict myself.

quickly go to yahoo, copy your answer and past it into your post, no-one will notice
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Dom Rougier
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There's a discussion of this in the back of the rulebook, and from memory the answer was a resounding "maybe".

I'm not sure it's a massively useful route to take in any case - a little like the Moon, making use of it can be more trouble than it's actually worth.
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