Editing Spacecraft flight dynamics (section)

===Flight path angle===
The ''specific angular momentum'' of any conic orbit, ''h'', is constant, and is equal to the product of radius and velocity at periapsis. At any other point in the orbit, it is equal to:{{sfnp| Bate| Mueller| White| 1971| p=18}}
<math display="block">h = r v\cos\varphi,</math>
where ''φ'' is the flight path angle measured from the local horizontal (perpendicular to&nbsp;''r''.) This allows the calculation of ''φ'' at any point in the orbit, knowing radius and velocity:
<math display="block">\varphi = \arccos\left(\frac{r_p v_p}{r v}\right)</math>

Note that flight path angle is a constant 0 degrees (90 degrees from local vertical) for a circular orbit.