Talk:Gravity turn

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2008[edit]

This is a fantastic article. I have been doing trajectory optimzation for years and this is the best discussion of gravity turns I have ever seen.Skimaniac (talk) 03:36, 11 March 2008 (UTC)

Yes, it's quite good. Question: should the term "gravity turn" be used in connection with what are ordinarily called "gravity assist" trajectories? Or should it be limited to launches and landings? While it's technically true that all use gravity to turn the trajectory of a spacecraft, these are sufficiently different applications in practice that it might confuse people to use the same term.

Launches and landings are distinguished from gravity assist trajectories in that they contact the planet at some point, are continuously or nearly continuously powered, and if the planet has an atmosphere, involve highly significant aerodynamic considerations. Gravity assist trajectories are usually (but not always) unpowered and rarely if ever intersect the atmosphere of a planet, much less its surface, so aerodynamic considerations like angle of attack don't apply.

I'd recommend that this article acknowledge that gravity assists are technically gravity turns, but limit the discussion to the use of gravity turns in launches and landings. The separate articles on gravity assist should be consulted for the use of gravity to create hyperbolic trajectories that do not contact the planet.

Oh, another thing. I don't think you can say that gravity assists change the speed of a spacecraft while gravity turns do not. It all depends on your own frame of reference; if the planet is moving in your reference frame, then a gravity assist always changes the speed of the spacecraft as measured in that reference system as the spacecraft exchanges kinetic energy with the planet. But if the planet is stationary (assuming infinite planet mass), then the spacecraft speed is always the same before and after the assist and only the direction changes. You can't really say that *this* trajectory is inherently a gravity assist while *that* one is not, because it's subjective to the observer's reference frame. Karn (talk) 02:57, 26 February 2009 (UTC)

Did Surveyor also use a gravity turn during landing? Although it made a direct approach to the lunar surface without first entering orbit as Apollo did, it still followed a (hyperbolic) orbit so the gravity turn principle should still apply. Because it used a large solid fuel retrorocket, it might not have been able to follow a nearly ideal gravity turn as the Apollo LM did. Comments? Karn (talk) 23:39, 5 March 2009 (UTC)

The maths[edit]

What's n? what's tau? Greglocock (talk) 01:29, 26 September 2009 (UTC)

n=noatical miles, tau=terrified ant underestimaiters

Can we compare with other launch trajectories?[edit]

I have been given to believe the Skylab launch followed another sort of trajectory called a direct insertion. It, too, leaned over, but at a much higher altitude, so that it was in its final orbital trajectory on completion of its initial burn. It presumably had excess delta-v capability.

If true, would "direct insertion" rate another article? Or would it serve as a contrast to the way launches are usually done?