# Talk:Gravity turn

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## 2008

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

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?

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?

## Doesn't compute?

I don't get the Moon landing part. A mortar shell has no thrust after firing but the nose will tip towards the ground after firing because the center of mass is in front of the surface areas where the most drag is generated. So it's a combination of the Earth's gravity, the projectile's inertia and atmospheric drag that turns the projectile downwards: Gravity pulls the projectile downwards, the drag on the lower rear of the projectile increases and generates torque around the center of mass that tilts the projectile's nose down. Isn't torque necessary to turn a body in an inertial frame of reference and hence a force that is not aligned with the center of mass? A rocket launched from Earth can behave similarly to a mortar shell. But when landing on the Moon, there is no atmosphere. The thrust of a rocket is usually aligned with the center of mass, except for the use of gimbal, as has been mentioned. The force of gravity is, by definition, aligned with the center of mass, except for gravity gradients, which in this case would probably be minute and tend to pull the spacecraft upright, which makes sense for landings but not for gravity turn takeoff. It seems to me that the article conflates two different issues concerning landing on a celestial body: attitude control and decreasing horizontal and vertical velocity. If the body has no atmosphere, it's obvious that the "gravity turn" is an elegant and efficient solution for the latter problem (all that remains is timing the thrust properly and making final corrections close to the ground), but I don't see how it would actually turn the spacecraft, that would still have to be done using engine gimbal, RCS or other means. Aragorn2 (talk)