Difference between sub-orbital and orbital spaceflights
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There sometimes appears to be confusion among the general public about the difference between sub-orbital and orbital spaceflights. This article is an attempt to clarify this issue. It also elaborates on the technical implications of the differences between orbital and sub-orbital spaceflights.
A spaceflight is a flight into or through space. The craft which undertakes a spaceflight is called a spacecraft.
The general public often thinks of orbital spaceflights as spaceflights and of sub-orbital spaceflights as "something less than actual spaceflights". This is not accurate; both orbital and sub-orbital spaceflights are true spaceflights.
The term orbit can be used in two ways: it can mean a trajectory in general, or it can mean a closed trajectory. The terms sub-orbital and orbital spaceflights refer to the latter: an orbital spaceflight is one which completes an orbit fully around the central body.
For a flight from Earth to be a spaceflight, the spacecraft has to ascend from Earth and at the very least go past the edge of space. The edge of space is, for the purpose of space flight, often accepted to lie at a height of 100 km (62 miles) above mean sea level. Any flight that goes higher than that is by definition a spaceflight. Where the Earth's atmosphere ends space begins but the atmosphere fades out gradually so the precise boundary is difficult to ascertain - hence the need for an arbitrary altitude for the edge of space.
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Angular velocity
An orbital spaceflight is achieved when the spacecraft travels around the Earth in space at sufficient lateral velocity (or equivalently, enough angular velocity) for the centrifugal force to cancel out the pull of Earth's gravity. Lateral velocity is the speed of something around an object and it is this which is the critical factor. Although the angular velocity required is a function of the height of the orbit, orbital spaceflight is possible at any altitude beyond the edge of space.
A body which does not have sufficient angular velocity cannot orbit the Earth. The actual speed of a sub-orbital spacecraft could exceed that of an orbital one and the height that a sub-orbital spacecraft attains may even exceed that of an orbital one, but the critical difference between the two - the achieving of an orbit - depends crucially on the angular velocity. Travelling straight up will never result in an orbit, doing so faster than escape velocity will have the obvious effect and orbit is still not attained.
Difference in the real world
That said, typical sub-orbital craft need go only just past the accepted edge of space at 100 km (62.5 miles) for the flight to be a spaceflight. At this arbitrary boundary there is still too much atmosphere present for a long term stable low earth orbit (LEO). In order to be stable for more than just a few weeks or months the satellite or spacecraft is placed in orbit at an altitude where drag from the atmosphere truly is negligible. A stable LEO is usually at least 350 km up.
But again, the difference in height should not be overemphasized: Whether the altitude is 100 km or 350 km the distance from the centre of the Earth is only different by less than four percent.
The difference between the lowest speeds required for orbital and sub-orbital space flights is substantial: a spacecraft must reach about 29,000 km/h (18,000 mph) to attain orbit. This compares to the relatively modest 4,000-4,800 km/h (2,500-3,000 mph) typically attained for sub-orbital crafts.
The important difference in energy requirements between a sub-orbital spaceflight such as that required for the X Prize and for an orbital spaceflight is that no lateral or angular velocity is required for the sub-orbital flight. The energy required to get to 100 km or even 350 km altitude is dwarfed by the energy required for the necessary lateral velocity of orbital space flight.
In terms of energy: accelerating a spacecraft to orbital speed requires about 31 times as much net energy as just lifting it to a height of 100 km (together 32 times), see computation.
But this is the energy which must be imparted to the orbiting mass: For a rocket the fuel and oxygen (and their tanks) must be accelerated as well and so the energy requirement is actually much more than the factor of 32 identified. (See the rocket equation article for a more detailed treatment).
In terms of the semi-major axes <math>a<math> of the elliptic orbits: the total specific orbital energy is Template:Il where <math>\mu\,<math> is the standard gravitational parameter. Being at rest at the surface of the Earth corresponds to <math>a=R/2<math> (with <math>R<math> the radius of the Earth). Reaching a height of 100 km means an increase of <math>a<math> of 50 km, while a LEO requires an increase of <math>a<math> of more than 3000 km. See also low-energy trajectories.
A vertical sub-orbital flight with the same energy as a LEO would reach a height of ca. 7000 km above the surface.
Atmospheric reentry a much bigger challenge with orbital flights
Because of that speed difference, atmospheric reentry is much more difficult for orbital flights than it is for sub-orbital flights. Note however, that such considerations only apply to orbital flights where the vehicle needs to return to Earth intact. If the vehicle is, say, a satellite that is ultimately expendable, then there naturally is no need to worry about reentry.
Returning craft though (including all potentially manned craft), have to find a way of slowing down as much as possible while still in higher atmospheric layers and avoid plunging downwards too quickly. To date (as of 2004), the problem of deceleration from orbital speeds has mainly been solved through aerobraking, ie. using the atmospheric drag itself to slow down. On an orbital space flight initial deceleration is provided by the retrofiring of the craft's rocket engines. Aerobraking in turn has so far mainly been achieved through orienting the returning space craft to fly at a high drag attitude coupled with ultra strong heat shields on the space craft, to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at supersonic speeds. The thermal energy is dissipated mainly as infrared radiation. Sub-orbital space flights, being at a much lower speed, do not generate anywhere near as much heat upon re-entry.
This has allowed maverick aircraft designer Burt Rutan recently (July 2004) to demonstrate an alternative or complementary approach to heat shield dependant reentry with the suborbital SpaceShipOne. It may be possible that the concepts utilized in SpaceShipOne's design can be applied to orbital space craft design and result in a markedly reduced need for a massive heat shield. Currently however, the need for an ultra high-performance and ultra reliable heat shield is a major difference between crafts designed for orbital flights (as opposed to sub-orbital ones), demonstrated in the [Mercury program] wherein the orbital flights used a typical ablative heat shield while the sub-orbital flights relied simply on a large metal heat-sink.
Summary
- Sub-orbital spaceflights flights are spaceflights just as orbital flights are.
- Both go beyond the atmosphere and past the edge of space.
- A sub-orbital flight can reach a higher height than an orbital one.
- The most important requirement for an orbital flight over a sub-orbital one is speed.
- The shock wave produced by high speed atmospheric reentry generates lots of heat from which the spacecraft must be protected.