In astrodynamics, gravity drag (or gravity losses) is inefficiency encountered by a spacecraft thrusting while moving against a gravitational field. Orbital mechanics or astrodynamics is the application of Celestial mechanics to the practical problems concerning the motion of Rockets and other Spacecraft The term inefficiency has several meanings depending on the context in which its used Allocative inefficiency - Allocative efficiency theory A spacecraft is a Vehicle or machine designed for Spaceflight. Thrust is a reaction force described quantitatively by Newton 's Second and Third Laws. A gravitational field is a model used within Physics to explain how gravity exists in the universe

Percentage losses due to gravity drag depend on the relative sizes of the acceleration due to the thrust and due to gravity as well the direction the thrust is applied in. Percentage of gravity losses are lower when the thrust is mostly lateral, and when the acceleration is large compared to the gravity.

Introduction

Once a vehicle has left the ground and until it reaches a stable orbit, the acceleration due to gravity must be opposed by the vehicles engines, at the cost of some propellant. Gravity drag is the delta-v needed due to this.

As an extreme example, consider a launch from Earth. One could plan a rocket that climbs to 1000 feet above the ground and then hovers there for a minute before proceeding onward and eventually into orbit. Certainly, this rocket will burn more fuel than one which proceeds directly to orbit without a hover. The reason for this inefficiency is that the thrust is simply supporting the weight of the rocket (during the hover) and not increasing the rocket's speed. This illustrates the rule of thumb that a time-consuming launch is inefficient.

It might be supposed that given that gravity is around 9. 8 m/s^2, that a loss of delta-v of 9. 8 m/s would occur per second. However, this greatly overestimates the losses in many cases.

Calculation

If the gravitational acceleration vector is g and the thrust vector per unit mass (acceleration produced by the engine) is a, then the actual acceleration of the craft is a − g, while using delta-v at a time-rate of a; that is, the delta-v of the vehicle used is | a | / | a − g | times the actual increase in speed. In Astrodynamics, the term delta-v, literally "change in velocity" (see symbol delta) has a specific meaning it is a Scalar which takes In the case of a very large thrust during a very short time, a desired speed increase can be reached with little gravity drag, while for a only slightly more than g, the gravity drag is very large.

When applying delta-v against gravity to increase specific orbital energy, it is advantageous to spend delta-v at the highest speed possible, rather than spending some, being decelerated by gravity, then spending some more, or spending it at less than full capacity. In Astrodynamics, the term delta-v, literally "change in velocity" (see symbol delta) has a specific meaning it is a Scalar which takes In Astrodynamics the specific Orbital energy \epsilon\\! (or vis-viva energy) of an Orbiting body traveling through Space Gravity drag 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 spend the delta-v when the rocket is going fast; in the case of being decelerated by gravity this means as soon as possible.
• It is wasteful to lift fuel unnecessarily: use it right away, and then the rocket does not have to lift it.

These effects apply whenever climbing to an orbit with higher specific orbital energy, such as during launch to Low Earth orbit (LEO) or from LEO to an escape orbit. A Low Earth Orbit (LEO is generally defined as an Orbit within the locus extending from the Earth’s surface up to an altitude of 2000 km An escape orbit (also known as C 3 = 0 orbit is a high-energy Parabolic orbit around the central body

Vector considerations

A pure rocket vehicle maintaining vertical velocity/altitude can have an effective Lift to drag ratio comparable to that of airliners. In Aerodynamics, the lift-to-drag ratio, or L/D ratio ("ell-over-dee" in the US "ell-dee" in the UK is the amount of lift generated

Acceleration is a vector quantity, and the direction of the acceleration has a large impact on the overall efficiency. For instance, gravity drag would reduce a 3 g thrust directed upward to an acceleration of 2 g, for an efficiency of 67%. g-force (also G-force, g-load) is a measurement of an object's Acceleration expressed in g s However, the same 3 g thrust could be directed at such an angle that it had a 1 g upward component, completely cancelled by gravity drag, and a horizontal component of 2. 8 g. Achieving 2. 8 g acceleration with 3 g thrust gives an efficiency of over 94%.

As orbital speeds are approached, the efficiency climbs further as the vehicle needs less vertical acceleration to maintain altitude, as momentum (or equivalently centrifugal effects in the rotating frame of reference around the center of the Earth) cancel the gravitation of the Earth, and more of the thrust can be used to accelerate.

It's important to note that "efficiency", in this sense, is not the only objective of a launching spacecraft. Rather, the objective is achieve the position/velocity combination for the desired orbit. For instance, the way to maximize acceleration is to thrust straight downward, leading to "efficiencies" over 100% because gravity actually aids the rocket's acceleration; however, thrusting downward is clearly not a viable course of action for a rocket intending to reach orbit.

On a planet with an atmosphere, the objective is further complicated by the need to achieve the necessary altitude to escape the atmosphere, and to minimize the losses due to atmospheric drag during the launch itself. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " Altitude is the Elevation of a point or object from a known level or datum (plural data In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a These facts sometimes inspire ideas to launch orbital rockets from high flying airplanes, to minimize atmospheric drag, and in a nearly vertical direction, to minimize gravitational drag like in the above calculations.

See also

• Atmospheric drag
• Delta-v budget
• Oberth effect