Sub-orbital spaceflight

A sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not involve putting a vehicle into orbit. Sub-orbital flights have been undertaken to test spacecraft and launch vehicles intended for later orbital flight, but some vehicles, such as the X-15 and SpaceShipOne have been designed exclusively to reach space sub-orbitally.

Table of contents

1 Overview

2 Flight profiles

2.1 Tourist flights
2.2 Intercontinental flights
2.3 Reaching for orbit

3 History of manned sub-orbital spaceflight

4 Future of manned sub-orbital spaceflight

Overview

Sub-orbital flights are appealing because it is very much easier to reach space (which simply means going higher than the edge of space) than to achieve orbit (which requires a velocity of about 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 2,500-3,000 mph. 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 altutude, and then coast up to its highest point. 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.

As of 2004 there are a number of companies working on vehicles in this class as entrants to the ANSARI X PRIZE competition.

Intercontinental flights

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)

Future of manned sub-orbital spaceflight

Private industry is starting to take an interest in sub-orbital spaceflight, due in part to ventures like the ANSARI X PRIZE (widely expected to be won in 2004).

In addition, NASA and others are experimenting with scramjet based hypersonic aircraft which may well be used with flight profiles that qualify as sub-orbital spaceflight.