In astrodynamics and rocketry, gravity loss is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field. In other words, it is the cost of having to hold the rocket up in a gravity field. Gravity losses depend on the time over which thrust is applied as well the direction the thrust is applied in. Gravity losses as a proportion of delta-v are minimised if maximum thrust is applied for a short time, or if thrust is applied in a direction perpendicular to the local gravitational field. During the launch and ascent phase, however, thrust must be applied over a long period with a major component of thrust in the opposite direction to gravity, so gravity losses become significant. For example, to reach a speed of 7.8 km/s in low Earth orbit requires a delta-v of between 9 and 10 km/s. The additional 1.5 to 2 km/s delta-v is due to gravity losses, steering losses and atmospheric drag. Example Consider the simplified case of a vehicle with constant mass accelerating vertically with a constant thrust per unit mass a in a gravitational field of strength g. The actual acceleration of the craft is a-g and it is using delta-v at a rate of a per unit time. Over a time t the change in speed of the spacecraft is (a-g)t, whereas the delta-v expended is at. The gravity loss is the difference between these figures, which is gt. As a proportion of delta-v, the gravity loss is g/a. A very large thrust over a very short time will achieve a desired speed increase with little gravity loss. On the other hand, if a is only slightly greater than g, the gravity loss is a large proportion of delta-v. Gravity loss can be described as the extra delta-v needed because of not being able to spend all the needed delta-v instantaneously. This effect can be explained in two equivalent ways: The specific energy gained per unit delta-v is equal to the speed, so efficiency is maximized when the delta-v is spent when the craft already has a high speed, due to the Oberth effect. Efficiency drops drastically with increasing time spent thrusting against gravity. Therefore, it is advisable to minimize the burn time. These effects apply whenever climbing to an orbit with higher specific orbital energy, such as during launch to