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 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 gravitational potential energy needs to be overcome: for the Kármán line this is approximately 1 MJ/kg. W=mgh, m=1 kg, g=9.82 m/s2, h=105m. W=1*9.82*105≈106J/kg=1MJ/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.
What we have in the past fifty years usually meant, though, by spaceflight has included remaining in space for a period of time, rather than going up and immediately falling back to earth. This entails orbit, which is, at altitudes prevalently entered by such means, not a matter of altitude, but rather of velocity, which is not to say that we need not discuss air friction and relevant altitudes in relation to that and orbit. At much, much, higher altitudes than many orbital ones maintained by satellites, altitude begins to become a larger and larger factor and speed a lesser one. At lower altitudes, due to the high speed required to remain in orbit, air friction is a very important consideration affecting satellites, much more than in the popular image of space. At even lower altitudes, balloons, with no forward velocity, can serve many of the roles satellites play.
Many cargoes, particularly humans have a limiting g-force 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.
Launchers vary with respect to their reliability for achieving the mission.
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 optimization is the process of designing a trajectory that minimizes or maximizes some measure of performance 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 problem. This was problem was first studied by Robert H. Goddard and is also known as the Goddard problem.
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 (lift) histories. These histories can be controlled by a number of means including such techniques as using an angle of attack 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.
Sub-orbital space flight is any space launch that reaches space without doing a full orbit around the planet, and requires a maximum speed of around 1 km/s just to reach space, and up to 7 km/s for longer distance such as an intercontinental space flight. An example of a sub-orbital flight would be a ballistic missile, or future tourist flight such as Virgin Galactic, or an intercontinental transport flight like SpaceLiner. Any space launch without an orbit-optimization correction to achieve a stable orbit will result in a suborbital space flight, unless there is sufficient thrust to leave orbit completely. (See Space gun#Getting to orbit)
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 the Earth's surface corresponds to a sideways speed of about 7.8 km/s (17,400 mph), an energy of about 30MJ/kg. This is several times the energy per kg of practical rocket propellant mixes.
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 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.
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 vehicle's speed. If the kinetic energy exceeds the potential energy then escape occurs. At the Earth's surface this occurs at a speed of 11.2 km/s (25,000 mph), but in practice a much higher speed would be needed due to airdrag.
Rocket launch is the only current way to reach space. In some cases an airbreathing (jet engine) first stage has been used as well.
Non-rocket space launch is a launch into space where some or all of the needed speed and altitude are provided by something other than expendable rockets. A number of alternatives to expendable rockets have been proposed. In some systems such as Skyhooks, rocket sled launch, and air launch, a rocket is used to reach orbit, but it is only part of the system.