# Gravity turn/ja

この方法は周回軌道から着陸する場合にも使える。水平方向の速度を無くしてから垂直にゆっくり降下するより、斜め方向に噴射して水平垂直両方向を同時に相殺したほうが効率が良い。

## メカニズム

ピッチオーバーによって重力ターンを始める高度は、いくつかの要素で決定される。大気のある天体では、大気圧と空気抵抗が効率的な重力ターンを決定する。

## タイミング

カービンから高度70kmに到達するまでの軌道

1. 飛行経路上のあらゆる障害物
2. 大気の密度、空気抵抗、最大推力
3. 天体の重力と機体の推力重量比(TWR)の兼ね合い

## 着陸

The presence of the atmosphere gives a great aid as the drag is always pointing to the opposite direction on the velocity (if not effected by aerodynamic lift or any extra powered assist) - means prograde required for the gravity turn. The drag forces a kind of gravity turn on the vehicle - and the effect can be augmented using chutes. The parachutes can decrease the descending speed (= terminal velocity) for safe landing without any power assist in a properly dense atmosphere.

The situation of same on celestial bodies without atmosphere, just the maneuver has to be accomplished without the aid of always well-orientated "free engine" called drag. The pitchover maneuver must be started immediately after leaving the surface (minimal vertical climbing), and has to accelerate with full power for the most efficient ascending on bodies without atmosphere. It means (as playing it backward) the full-power deceleration (retrogade burn ()) has to be started as late as possible to be finished turning to vertical just above the surface. The approximating the "ideal" trajectory means increasing efficiency - and danger too. Rationally, some clearance is needed, but the descending speed has to be maximized reasonably after the craft turned to vertical orientation.