# What does a flight path angle of zero degrees mean at launch?

Total beginner here so please be gentle. I'm looking at the Apollo 11 flight path angle plot from Apollo Saturn V Postflight Trajectory AS 506 which shows a zero degree angle at launch.

Isn't path angle measured from the horizontal? In fact, this plot levels back out at zero after orbital insertion. What does this zero degree at launch mean--shouldn't it be 90 degrees for a vertical launch?

In this plot from Apollo Saturn V Postflight Trajectory AS 506

the confusion arises because this is the inertial flight path angle (in Apollo documents "space-fixed" == "inertial")

So sitting on the pad the vehicle in the "space-fixed" coordinates has a horizontal velocity of ~ half a km/s (due to the rotation of the Earth) and a flight path angle of 0.

Check out Appendix A of the document for definitions of the variables.

It's well explained here

The plot starts at a point in which the inertial flight path angle is zero and the velocity is non zero. That is because space has been chosen as the frame of reference, that implies that Earth's rotation is taken into account. So the launch vehicle has already a horizontal space fixed velocity of about 410 meters per second due East when it is sitting on the launch pad in Florida. When the launch vehicle is ascending the vertical component of the velocity is increasing and therefore the inertial flight path angle is increasing. But at the same time the launch vehicle is starting to pitch over to gradually maneuver into an orbit in which it is aligned along the local horizon (flight path angle is zero.). So the flight path angle increases at lift-off, will reach a maximum at which the vertical component of the velocity will reach a maximum and will then taper off to zero when an orbit has been obtained.

• This is absolutely the plot I'm looking at. I scoured and missed this definition in the appendix. Thanks for the quick answer! Oct 29, 2022 at 14:48
• @MarkStevenson I see that you have accepted this answer. It's also good practice to upvote the answer; click on the arrowhead that points upward. (Clicking on the downward pointing arrowhead is a down-vote, which I assume is not what you want given that you accepted the answer.) Oct 29, 2022 at 14:53
• I suppose "space-fixed" means some kind of coordinate system where the center of earth is fixed, otherwise you'd not just need the rotation, but also the movement of the earth. Oct 30, 2022 at 13:14
• I see that Appendix A does indeed describe "flight path angle" as the angle between the vehicle velocity vector and the local horizontal. There is no name given to the angle between the vehicle body and its velocity vector - I see that isn't what this document is about but is that usually referred to as the angle of attack (or what else)? Oct 31, 2022 at 9:47
• @Puffin I'm away from my sources and good internet for a while. There is an Apollo coordinate systems document that I have found, presumably by googling, that may have the answer to that. It might be on ntrs which google sometimes misses. Oct 31, 2022 at 14:10