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Ascent Trajectories and the Gravity Turn
by Staff Writers
Bethesda, MD (SPX) Oct 04, 2016
 File image.

Almost all space launch vehicles liftoff from the ground in the vertical direction and continue to orbit along an ascent trajectory that is usually optimized for the conditions in order to maximize performance while maintaining conservative safety margins. The actual shape of the path to orbit is influenced by a number of factors, including winds and the desired payload injection parameters.

However, the ideal trajectory profile is based on reaching orbital speed, altitude and orientation as the upper stage completes its injection burn. In most cases, the trajectory is designed to avoid any aerodynamic side load, i.e., the angle of attack is kept at zero.

Ignoring wind factors, this is achieved through the use of a "gravity" turn or "zero-lift" turn. This is a trajectory optimization technique that uses the transverse component of gravity (that is perpendicular to the launch vehicle's longitudinal axis) to turn the velocity vector as it ascents toward orbit. Control is achieved by carefully changing the pitch orientation of the vehicle during its powered ascent.

The gravity turn offers the advantage of a natural ascent profile without wasting any of the vehicle's propellant. Furthermore, by keeping the angle of attack near zero, transverse aerodynamic stresses are kept to a minimum, allowing a lighter launch vehicle.

At liftoff, the rocket begins its vertical ascent, gaining both speed and altitude. Initially, gravity acts directly against the thrust of the rocket, limiting its vertical acceleration and acting as "gravity drag."

As soon as the vehicle clears any service towers and performs any required roll maneuvers, a "pitchover maneuver" is executed in order to steer the rocket's longitudinal axis toward the downrange direction and to establish the ultimate orbit plane. This maneuver is accomplished by gimbaling the rocket engines slightly to direct some of the thrust to one side, creating a net torque on the vehicle.

Once this is completed, a small part of the gravitational force is directed perpendicular to the longitudinal axis. This is the beginning of the gravity turn. From this point until orbit injection, the transverse gravity component continues to grow and causes the vehicle's velocity vector to rotate toward the horizon as it ascends.

The exact initial pitchover angle depends on the specific launch vehicle and is orbital destination. As soon as the pitchover maneuver is completed, the rocket engines are returned to their non-gimbaled orientation. Note that this small steering maneuver is the only one needed during an ideal ascent in which thrust must be used for the purpose of steering. In reality, wind forces do cause minor gimbal-induced corrections during ascent.

Since all space launch vehicles consist of at least two stages, each stage is fired sequentially, resulting in slight discontinuities in thrust. Therefore, ascent sequences are designed to deal with lower-stage shutdown, separation and upper-stage startup.

An actual ascent sequence involves many events and steps. For all those who are involved, concerned and interested in launch vehicle performance, let Launchspace bring you up to date on this topic. Check out our three-day presentation.

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