The idea of using a gravity turn maneuver to land a vehicle was originally developed for the Lunar Surveyor landings, although Surveyor made a direct approach to the surface without first going into lunar orbit. the propellant tanks filled during the rocket burn. A computer program called Lander that simulated gravity turn landings applied this concept by simulating a gravity turn launch with a negative mass flow rate, i.e. This was essentially a launch in reverse except that a landing spacecraft is lightest at the surface while a spacecraft being launched is heaviest at the surface. ![]() The Apollo Lunar Module used a slightly modified gravity turn to land from lunar orbit. ![]() Then later during the flight the rocket coasts between stage firings, allowing it to level off above the atmosphere, so when the engine fires again, at zero angle of attack, the thrust accelerates the ship horizontally, inserting it into orbit.īecause heat shields and parachutes cannot be used to land on an airless body such as the Moon, a powered descent with a gravity turn is a good alternative. This reduces both aerodynamic drag as well as aerodynamic stress during launch. Because gravity turns the flight path during free flight, the rocket can use a smaller initial pitchover angle, giving it higher vertical velocity, and taking it out of the atmosphere more quickly. The technique is also useful when launching from a planet with a thick atmosphere, such as the Earth. This is particularly useful in very high thrust rockets, where if the engines were fired continuously, the rocket would run out of fuel before leveling off and reaching a stable orbit above the atmosphere. Some time must be allowed for stage separation and engine ignition between each successive stage, but some rocket designs call for extra free-flight time between stages. If the rocket is a multi-stage system where stages fire sequentially, the rocket's ascent burn may not be continuous. The rocket is producing thrust though, and rather than leveling off and then descending again, by the time the rocket levels off, it has gained sufficient altitude and velocity to place it in a stable orbit. If the rocket were not producing thrust, the flight path would be a simple ellipse like a thrown ball (it is a common mistake to think it is a parabola: this is only true if it is assumed that the Earth is flat, and gravity always points in the same direction, which is a good approximation for short distances), leveling off and then falling back to the ground. Because gravity acts straight down, the new velocity vector is closer to being level with the horizon gravity has "turned" the trajectory downward.Īfter the pitchover, the rocket's flight path is no longer completely vertical, so gravity acts to turn the flight path back towards the ground. As before, the launch vehicle's new velocity is the vector sum of its old velocity, the acceleration from thrust, and the acceleration of gravity. ![]() When used in this context, it is similar to a gravitational slingshot the difference is that a gravitational slingshot often increases or decreases spacecraft velocity and changes direction, while the gravity turn only changes direction.Ī diagram showing the velocity vectors for times t during the downrange acceleration phase. The term gravity turn can also refer to the use of a planet's gravity to change a spacecraft's direction in situations other than entering or leaving the orbit. This minimizes transverse aerodynamic stress on the launch vehicle, allowing for a lighter launch vehicle. Second, and more importantly, during the initial ascent phase the vehicle can maintain low or even zero angle of attack. First, the thrust is not used to change the spacecraft's direction, so more of it is used to accelerate the vehicle into orbit. ![]() It offers two main advantages over a trajectory controlled solely through the vehicle's own thrust. It is a trajectory optimization that uses gravity to steer the vehicle onto its desired trajectory. A gravity turn or zero-lift turn is a maneuver used in launching a spacecraft into, or descending from, an orbit around a celestial body such as a planet or a moon.
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