Space launch
Space launch is the earliest part of a flight
Flight is the process by which an object moves either through an atmosphere or beyond it by generating lift or propulsive thrust, or aerostatically using buoyancy, or by simple ballistic movement....

 that reaches space. Space launch involves liftoff
Liftoff (disambiguation)
- Culture :* Lift Off , an Australian educational television series* "Liftoff" , an episode of The West Wing* "Lift Off", an electronic music instrumental for Ztar composed by Les Fradkin...

, when a rocket or other space launch vehicle leaves the ground at the start of a flight. Liftoff is of two main types: rocket launch
Rocket launch
A rocket launch is the takeoff phase of the flight of a rocket. Launches for orbital spaceflights, or launches into interplanetary space, are usually from a fixed location on the ground, but may also be from a floating platform such as the San Marco platform, or the Sea Launch launch...

, the current conventional method, non-rocket spacelaunch
Non-rocket spacelaunch
Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...

 where other forms of propulsion are employed, including airbreathing jet engines or other kinds.

Definition of space

Space has no physical edge to it as the atmospheric pressure gradually reduces with altitude; instead, the edge of space is defined by convention, often the Kármán line
Karman line
The Kármán line lies at an altitude of above the Earth's sea level, and is commonly used to define the boundary between the Earth's atmosphere and outer space...

 of 100 km. Other definitions have been created as well, in the US for example space has been defined as 50 miles.


Therefore, by definition for spaceflight to occur, sufficient altitude is necessary. This implies a minimum specific
Specific may refer to:* Specificity* Specific, a cure or therapy for a specific illnessLaw:* Specific deterrence, focussed on an individual* Specific finding, intermediate verdict used by a jury in determining the final verdict...

 gravitational potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...

 needs to be overcome: for the Kármán line this is approximately 1 MJ/kg.

In practice, a higher energy than this is needed to be expended due to losses such as airdrag, propulsive efficiency, cycle efficiency of engines that are employed and gravity drag
Gravity drag
In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field...



Many cargoes, particularly humans have a limiting g-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 that they can survive. For humans this is about 3-6 g. Some launchers such as gun launchers would give accelerations in the hundred or thousands of g and thus are completely unsuitable.


Safety is the probability of causing injury or loss of life. Unreliable launchers are not necessarily unsafe, whereas reliable launchers are usually, but not invariably safe.

Apart from catastrophic failure of the launch vehicle itself other safety hazards include depressurisation, and the Van Allen radiation belts which preclude orbits which spend long periods within them.

Trajectory optimisation

Trajectory optimization
Trajectory optimization
Trajectory optimization is the process of designing a trajectory that minimizes or maximizes some measure of performance within prescribed constraint boundaries...

is the process of designing a trajectory
A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...

 that minimizes or maximizes some measure of performance
Figure of merit
A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application...

 within prescribed constraint boundaries. While not exactly the same, the goal of solving a trajectory optimization problem is essentially the same as solving an optimal control
Optimal control
Optimal control theory, an extension of the calculus of variations, is a mathematical optimization method for deriving control policies. The method is largely due to the work of Lev Pontryagin and his collaborators in the Soviet Union and Richard Bellman in the United States.-General method:Optimal...


The selection of flight profiles that yield the greatest performance plays a substantial role in the preliminary design of flight vehicles, since the use of ad-hoc profile or control policies to evaluate competing configurations may inappropriately penalize the performance of one configuration over another. Thus, to guarantee the selection of the best vehicle design, it is important to optimize the profile and control policy for each configuration early in the design process.

Consider this example. For tactical missiles, the flight profiles are determined by the thrust and load factor
Load factor
Load factor may refer to:* Load factor , the ratio of the lift of an aircraft to its weight* Load factor , the ratio of the number of records to the number of addresses within a data structure...

 (lift) histories. These histories can be controlled by a number of means including such techniques as using an angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

 command history or an altitude/downrange schedule that the missile must follow. Each combination of missile design factors, desired missile performance, and system constraints results in a new set of optimal control parameters.

Orbital launch

In addition, if orbit is required, then a much greater amount of energy must be generated, in order to give the craft some sideways speed. The speed that must be achieved depends on the altitude of the orbit - less speed is needed at high altitude. However, after allowing for the extra potential energy of being at higher altitudes, overall more energy is used reaching higher orbits than lower ones.

The speed needed to maintain an orbit, near to the Earth's surface corresponds to a sideways speed of about 7.8 km/s, an energy of about 30MJ/kg. This is several times the energy per kg of practical rocket propellant
Rocket propellant
Rocket propellant is mass that is stored in some form of propellant tank, prior to being used as the propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuel propellant is often burned with an oxidizer propellant to produce large volumes of very hot...


Gaining the kinetic energy is awkward as the airdrag tends to slow the spacecraft, so rocket-powered spacecraft generally fly a compromise trajectory that leaves the thickest part of the atmosphere very early on, and then fly on for example, a Hohmann transfer orbit
Hohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....

to reach the particular orbit that is required. This minimises the airdrag as well as minimising the time that the vehicle spends holding itself up. Airdrag is a significant issue with essentially all proposed and current launch systems, although usually less so than the difficulty of obtaining enough kinetic energy to simply reach orbit at all.

Escape velocity

If the Earth's gravity is to be overcome entirely then sufficient energy must be obtained by a spacecraft to exceed the depth of the gravity potential energy well. Once this has occurred, provided the energy is not lost in any non conservative way, then the vehicle will leave the influence of the Earth. The depth of the potential well depends on the vehicle's position, and the energy depends on the vehicles speed. The kinetic energy exceeds the potential energy then escape occurs. At the Earths surface this occurs at a speed of 11.2 km/s, but in practice a much higher speed would be needed due to airdrag.

Rocket launch

Rocket launch is the only current way to reach space. In some cases an airbreathing (jet engine) first stage has been used as well.
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