is then 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: where φ is the flight path angle measured from the local horizontal (perpendicular to r.) This allows the calculation of φ at any point in the orbit, knowing radius and velocity: Note that flight path angle is a constant 0 degrees (90 degrees from local vertical) for a circular orbit. True anomaly as a function of time It can be shown that the angular momentum equation given above also relates the rate of change in true anomaly to r, v, and φ, thus the true anomaly can be found as a function of time since periapsis passage by integration: Conversely, the time required to reach a given anomaly is: Orbital maneuvers Once in orbit, a spacecraft may fire rocket engines to make in-plane changes to a different altitude or type of orbit, or to change its orbital plane. These maneuvers require changes in the craft's velocity, and the classical rocket equation is used to calculate the propellant requirements for a given delta-v. A delta-v budget will add up all the propellant requirements, or determine the total delta-v available from a given amount of propellant, for the mission. Most on-orbit maneuvers can be modeled as impulsive, that is as a near-instantaneous change in velocity, with minimal loss of accuracy. In-plane changes Orbit circularization An elliptical orbit is most easily converted to a circular orbit at the periapsis or apoapsis by applying a single engine burn with a delta v equal to the difference between the desired orbit's circular velocity and the current orbit's periapsis or apoapsis velocity: To circularize at periapsis, a retrograde burn is made: To circularize at apoapsis, a posigrade burn is made: Altitude change by Hohmann transfer A Hohmann transfer orbit is the simplest maneuver which can be used to move a spacecraft from one altitude to another. Two burns are required: the first to send the craft into the elliptical transfer orbit, and a second to circularize the target orbit. To raise a circular orbit at , the first posigrade burn raises velocity to the transfer orbit's periapsis velocity: The second posigrade burn, made at apoapsis, raises velocity to the target orbit's velocity: A maneuver to lower the