Gravity turn
A gravity turn is a technique used to reach orbit from the surface of a celestial body while using minimal fuel. In order to escape the surface, a craft must rise faster than gravity pulls it down. In order to maintain a stable orbit, the craft must gain enough sideways momentum at a high enough altitude to avoid colliding with any surface features or getting slowed down by the atmosphere, if any. A gravity turn combines these two steps into one maneuver, saving fuel in the process.
As a craft starts with ascending vertically, it slowly turns until by the end of the turn it points sideways. For an analogy, imagine getting to orbit without a gravity turn: it would be a straight up flight, then a 90 degree turn once high enough to orbit. Think of a gravity turn as "cutting the corner", so to speak. It's a shorter path, so it saves more fuel. A reversed gravity turn can be performed to land on a surface from orbit as well.
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Mechanics
Gravity turns work by using gravity as an acceleration assist. If a launched craft followed a purely vertical flight path, then it would fight gravity throughout its entire burn, putting all of the acceleration work on its engines which greatly decreases the net Delta-V. Using a gravity turn, gravity pulls the velocity vector of the craft down and the craft tilts to follow it. Because of this, the craft not only saves fuel that would otherwise be lost from pointing in a non-vector direction, it also saves fuel by not fighting gravity through the entire burn. By the point the velocity vector has no vertical speed, no fuel is wasted fighting gravity.
Gravity turns also are timed on bodies with atmospheres to minimize drag generated during the turn. If a craft turns too early in its flight, it will waste fuel to drag trying to push a longer distance through the atmosphere. If a craft turns too late, it will waste fuel fighting gravity when less fuel would be used resisting drag.
Timing
The timing and rate of tilt of a gravity turn are based on mainly on two things: the gravitational pull of the body it is on and the density of its atmosphere, though the thrust-to-weight ratio (TWR) of the craft also comes into play.
The least efficient gravity turn, regardless of the body the craft starts on, is to launch the craft and maintain a vertical heading, and once the craft is either outside of the atmosphere or above surrounding obstructions, turn to face horizontally, and burn to circularize the orbit. In this gravity turn, all fuel in the initial burn is spent resisting gravity. On bodies with no atmosphere, this is very close to the proper turn, however. With no atmosphere, a craft need not worry about any drag generated, and thus should turn to face near horizontal as early on in the launch as possible given its TWR. Doing this minimizes the fuel spent resisting gravity, while putting the maximum force into achieving an orbit.
On planets with an atmosphere, however, timing is crucial to the success and efficiency of a gravity-turn. On Kerbin, it is most efficient to begin the gravity turn at 10 kilometers, with the ship and vector marker facing at 30 to 40 degrees above the artificial horizon. Because gravity turns are not always perfect, the craft may need to pause its burn and coast to apoapsis, and do a circularization burn at apoapsis, though the most efficient gravity turn will have a continuous burn through circularization.
TWR
The TWR of a craft can greatly influence the gravity turn of a craft, especially with more advanced forms of staging, notably asparagus staging. On some crafts, especially those with advanced staging, the TWR of a ship can drop as staging progresses, eventually leading to a point of a TWR too low to properly circularize and orbit before re-entering the atmosphere or colliding with the ground of the body launched from. If the craft has sufficient delta-V for the launch, gravity turns can be prolonged, with a longer vertical burn and slower rate of tilt. Though this will put the payload of the launch into a higher orbit, sometimes significantly for crafts with final stages with exceptionally low TWRs, it will often put the craft into orbit without a need to alter it.
See also
- Gravity turn on Wikipedia
