Angle of attack for rockets?

[newzilla7]

The definition of a perfect gravity turn according to Wikipedia (always reliable) is an ascent path such that at any given point (after the pitchover maneuver), your angle of attack is zero. Here's my issue: I want to test using this definition to optimize my gravity turns by keeping my angle of attack at zero, but I have no idea how to calculate the angle of attack for my rockets. Most internet information, such as pages on Wikipedia, focus on angle of attack for aircraft. What is the definition of angle of attack for a rocket? How do I find a rocket's angle of attack using tools in KSP?

[Plusck]

Basically, make it heavy at the top, put enough fins on the back and leave SAS off, and it should maintain a near-zero angle of attack for as long as there is enough of an atmosphere to sustain its "flight".

 

edit: As for tools: if you are perfectly centred on surface "prograde" in the atmosphere, your angle of attack is zero, by definition. After about 18-22km, rockets tend to lose prograde just using fins in KSP, so you'll need to use prograde hold (even though it always lags a little behind prograde). To maintain that zero angle of attack even above 36km, you'll need to manually click on the navball to keep it in surface mode after it switches to orbital.

Edited September 12, 2016 by Plusck

[LN400]

The AoA for a rocket is simply the angle between the longitudinal axis (the center marker on your nav ball) and the direction of flight (the prograde marker).

 

Basically, what it means is: Keep the nose of the rocket centered on the prograde marker.

 

EDIT

Have a look at page 2

http://www.philsrockets.org.uk/forces.pdf

Edited September 12, 2016 by LN400

[FyunchClick]

On ‎12‎-‎9‎-‎2016 at 10:06 PM, Plusck said:

To maintain that zero angle of attack even above 36km, you'll need to manually click on the navball to keep it in surface mode after it switches to orbital.

That brings up an interesting point. I suspect you'll want to follow surface prograde while in atmosphere because you'd be incurring a bit more drag by burning off that prograde not to mention it's your missiles natural tendency to point that way (if you disregard gravity anyways). However, I wonder if it's not actually the orbital speed prograde that's the more efficient path to follow? If so, you may not want to force it to follow surface prograde

[Plusck]

2 hours ago, FyunchClick said:

That brings up an interesting point. I suspect you'll want to follow surface prograde while in atmosphere because you'd be incurring a bit more drag by burning off that prograde not to mention it's your missiles natural tendency to point that way (if you disregard gravity anyways). However, I wonder if it's not actually the orbital speed prograde that's the more efficient path to follow? If so, you may not want to force it to follow surface prograde

Yes, I agree absolutely.

To maximise your orbital energy, you want to maximise the energy you are throwing into your exhaust (meaning that the exhaust must be aimed directly opposite your direction of travel), and you want that exhaust to be as horizontal as possible (so that you're not losing energy to the planet's gravity well).

So yes, the most efficient path must be the orbital speed prograde, since it's the higher of the two possible travel vectors. However, once you get up to orbital speeds (which should be the case around about 36 km)  there should be very little difference between orbital and surface (atmospheric) prograde. That difference (if any) is a very good indicator as to the perfection of your gravity turn.

Still, there is still significant drag all the way to about 50-55 km, so I'd guess that maintaining "surface" prograde until then should be the goal, and if that causes you to raise Ap too high then you've failed the OP's basic test. :shrug: