A sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not involve putting a vehicle into orbit. Manned and unmanned sub-orbital flights have been undertaken to test spacecraft and launch vehicles intended for later orbital flight, but some vehicles have been designed exclusively to reach space sub-orbitally: manned vehicles such as the X-15 and SpaceShipOne, and unmanned ones such as ICBMs and sounding rockets.
The sub-orbital spaceflight should not be confused with a partial orbital spaceflight: a low Earth orbit, with deorbiting after less than one full orbit, as in the Fractional Orbital Bombardment System.
During freefall the trajectory is part of an elliptic orbit as given by the orbital equation. The perigee distance is less than the radius of the Earth, hence the ellipse intersects the Earth. The major axis is vertical, the semi-major axis is more than one half of the radius of the Earth, and almost always less than the radius.
If the objective is just to reach space, sub-orbital flights are appealing because this is very much easier (it simply means going higher than the edge of space) than to achieve orbit (which requires a velocity of about 8 km/s (18,000 mph)). A dedicated sub-orbital spacecraft can therefore be built and operated much more cheaply than an orbital spacecraft. Less powerful sub-orbital craft may not reach speeds much higher than around 1.1 km/s to 1.3 km/s (2,500-3,000 mph).
However, for intercontinental ballistic space flights, like that of an ICBM, or a possible future commercial spacecraft, a typical speed is / might be 7 km/s.
For more information on the difference between sub-orbital and orbital spaceflights, refer to the article Difference between sub-orbital and orbital spaceflights.
While there are a great many possible sub-orbital flight profiles, it is expected that some will be more common than others.
Sub-orbital tourist flights will initially focus on attaining the altitude required to qualify as reaching space. The flight path will probably be either vertical or very steep, with the spacecraft landing back at its take-off site.
The spacecraft will probably shut off its engines well before reaching maximum altutude, and then coast up to its highest point. During a few minutes, from the point when the engines are shut off to the point where the craft begins to slow its descent for landing, the passengers will experience weightlessness.
In 2004, a number of companies worked on vehicles in this class as entrants to the Ansari X Prize competition. SpaceShipOne was officially declared by Rick Searfoss to have won the competition on October 4, 2004 after completing two flights within a two week period.
Another potentially large market is research payloads. Often researchers want to run experiments in microgravity or above the atmosphere. There have reportedly been several offers from researchers to launch experiments on SpaceShipOne, which have been turned down until the next version of the vehicle (http://news.bbc.co.uk/1/hi/sci/tech/3722596.stm).
Another possibly lucrative market for sub-orbital spacecraft is intercontinental flight. A sub-orbital flight could travel from Europe to North America in less than an hour. Due to the high cost, this is likely to be initially limited to high value cargo such as courier flights, or as the ultimate executive jet.
Reaching for orbit
Commercial spacecraft operators may use sub-orbital flights to allow a constant progression towards full orbital flight. The test craft will reach higher and higher velocities until they reach low earth orbit. There is considerable debate about the validity of this approach, however, as the scale of the two problems (sub-orbital and orbital flight) are very different.
History of manned sub-orbital spaceflight
- U.S. - X-15, 1959-1968; above 100km altitude from 1963
- U.S. - Mercury program, Mercury-Redstone 3, Mercury-Redstone 4, 1961, Alan Shepard & Virgil Grissom
- U.S.S.R. - Soyuz 18a, Vasili Lazarev & Oleg Makarov - launch emergency caused suborbital flight
- U.S. private - SpaceShipOne, June 2004 (first non-governmental space flight)
- U.S. private - SpaceShipOne, September 2004 (first Ansari X-Prize qualifying flight)
- U.S. private - SpaceShipOne, October 4, 2004 (Ansari X-Prize winning flight)
Future of manned sub-orbital spaceflight
Privately-held companies such as Blue Origin are taking an interest in sub-orbital spaceflight, due in part to ventures like the Ansari X Prize. NASA and others are experimenting with scramjet based hypersonic aircraft which may well be used with flight profiles that qualify as sub-orbital spaceflight.