Sub-orbital spaceflight

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.

Overview

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). 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.

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.

Flight profiles

While there are a great many possible sub-orbital flight profiles, it is expected that some will be more common than others.

Tourist flights

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 altitude, 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.

In 2005, Sir Richard Branson of the Virgin Group announced the creation of Virgin Galactic and his plans for a 9 seat capacity SpaceShipTwo named VSS Enterprise.

Scientific experiments

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^*.

Intercontinental flights

Another possibly lucrative market for sub-orbital spacecraft is intercontinental flight. Research, such as that done for the X-20 Dyna-Soar project suggests that a semi-ballistic 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 business 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. Still, winged, single stage to orbit designs like Skylon do exist, so it might not be a totally unreasonable approach.

History of manned sub-orbital spaceflight

U.S. — X-15, 1963, Joseph A. Walker — two flights above 100km altitude
U.S. — MR-3 & MR-4, 1961, Alan Shepard & Virgil Grissom
U.S.S.R. — Soyuz 18a, 1975, Vasili Lazarev & Oleg Makarov — launch emergency caused suborbital flight
U.S. (private) — SpaceShipOne, 2004, Mike Melvill & Brian Binnie — Ansari X-Prize winner

Future of manned sub-orbital spaceflight

Privately-held companies such as Rocketplane Limited and 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.

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