A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution.
For example, the path of an object launched from Earth that reaches 100 km (62 mi) above sea level, and then falls back to Earth, is considered a sub-orbital spaceflight. Some sub-orbital flights have been undertaken to test spacecraft and launch vehicles later intended for orbital spaceflight. Other vehicles are specifically designed only for sub-orbital flight; examples include manned vehicles such as the X-15 and SpaceShipOne, and unmanned ones such as ICBMs and sounding rockets.
Sub-orbital spaceflights are distinct from flights that attain orbit but use retro-rockets to deorbit after less than one full orbital period. Thus the flights of the Fractional Orbital Bombardment System would not be considered sub-orbital; instead these are simply considered flights to low Earth orbit.
By one definition a sub-orbital spaceflight reaches an altitude higher than 100 km above sea level. This altitude, known as the Kármán line, was chosen by the Fédération Aéronautique Internationale because it is roughly the point where a vehicle flying fast enough to support itself with aerodynamic lift from the Earth's atmosphere would be flying faster than orbital speed. The US military and NASA award astronaut wings to those flying above 50 miles although the US State Department appears to not support a distinct boundary between atmospheric flight and space flight.
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, and hence the spacecraft will fail to complete an orbit. 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.
Speed, range, altitude
To minimize the required delta-v (an astrodynamical measure which strongly determines the required fuel), the high-altitude part of the flight is made with the rockets off (this is technically called free-fall even for the upward part of the trajectory). The maximum speed in a flight is attained at the lowest altitude of this free-fall trajectory, both at the start and at the end of it.
If one's goal is simply to "reach space", for example in competing for the Ansari X Prize, horizontal motion is not needed. In this case the lowest required delta-v is about 1.4 km/s, for a sub-orbital flight with a maximum speed of about 1 km/s. Moving slower, with less free-fall, would require more delta-v.
Compare this with orbital spaceflights: a low Earth orbit (LEO), with an altitude of about 300 km), needs a speed around 7.7 km/s, requiring a delta-v of about 9.2 km/s.
For sub-orbital spaceflights covering a horizontal distance the maximum speed and required delta-v are in between those of a vertical flight and a LEO. The maximum speed at the lower ends of the trajectory are now composed of a horizontal and a vertical component. The higher the horizontal distance covered, the more are both speeds, and the more is the maximum altitude. For the V-2 rocket, just reaching space but with a range of about 330 km, the maximum speed was 1.6 km/s. Scaled Composites SpaceShipTwo which is under development will have a similar free-fall orbit but the announced maximum speed is 1.1 km/s (perhaps because of engine shut-off at a higher altitude).
For larger ranges, due to the elliptic orbit the maximum altitude can even be considerably more than for a LEO. On an intercontinental flight, such as that of an intercontinental ballistic missile or possible future commercial spaceflight, the maximum speed is about 7 km/s, and the maximum altitude about 1200 km. Note that an intercontinental flight at an altitude of 300 km would require a larger delta-v, that of a LEO. It should be noted that any spaceflight that returns to the surface, including sub-orbital ones, will undergo atmospheric reentry. The speed at the start of that is basically the maximum speed of the flight. The aerodynamic heating caused will vary accordingly: it is much less for a flight with a maximum speed of only 1 km/s than for one with a maximum speed of 7 or 8 km/s.
In a vertical flight of not too high altitudes, the time of the free-fall is both for the upward and for the downward part the maximum speed divided by the acceleration of gravity, so with a maximum speed of 1 km/s together 3 minutes and 20 seconds. The duration of the flight phases before and after the free-fall can vary.
For an intercontinental flight the boost phase takes 3 to 5 minutes, the free-fall (midcourse phase) about 25 minutes. For an ICBM the atmospheric reentry phase takes about 2 minutes; this will be longer for any soft landing, such as for a possible future commercial flight.
Suborbital flights can last many hours. Pioneer 1 was NASA's first space probe, intended to reach the Moon. A partial failure caused it to instead follow a suborbital trajectory, reentering the Earth's atmosphere 43 hours after launch.
While there are a great many possible sub-orbital flight profiles, it is expected that some will be more common than others.
The first suborbital vehicles which reached space were ballistic missiles. The very first ballistic missile to reach space was the German V-2 on October 3, 1942 which reached an altitude of 60 miles. That in fact was the first man-made object of any kind to reach space. Then in the 1950s the USA and USSR concurrently developed much longer range Intercontinental Ballistic Missiles (ICBM)s. There are now many countries who possess ICBMs and even more with shorter range IRBMs (Intermediate Range Ballistic Missiles).
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 atmosphere begins to slow down the downward acceleration, 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.
A major use of suborbital vehicles today are as scientific sounding rockets. Scientific suborbtial flights began in the 1920s when Robert H. Goddard launched the first liquid fueled rockets, however they did not reach space altitude. Modern sounding rocket flights began in the late 1940s using vehicles derived from German V-2 ballistic missiles. Today there are dozens of different sounding rockets on the market, from a variety of suppliers in various countries. Typically, researchers wish to conduct 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 .
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.
The size of rocket, relative to the payload, necessary to achieve this, is similar to an ICBM. ICBMs have delta-v's somewhat less than orbital; and therefore would be somewhat cheaper than the costs for reaching orbit.
Thus due to the high cost, this is likely to be initially limited to high value, very high urgency cargo such as courier flights, or as the ultimate business jet; or possibly as an extreme sport, or for military fast-response.
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 Reaction Engines Skylon do exist, so it might not be a totally unreasonable approach.
Tether launch assist
There have been proposals to use tethers (commonly referred to as skyhooks) to put suborbital payloads into orbit. For example, an orbiting space station could extend a tether, and a suborbital vehicle rendezvous with the end of the tether and dock to it. If practical, this would be considerably less expensive than launching payloads directly into orbit on rockets on a per flight basis..
Notable unmanned sub-orbital spaceflight
Manned sub-orbital spaceflights
Future of manned sub-orbital spaceflight
Private 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.
Published - July 2009
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