Sub-orbital spaceflight
From Wikipedia, the free encyclopedia
A sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not leave the vicinity of Earth, and where the spacecraft does not enter orbit. For example, any object that reaches 100 km above sea level, and then falls back to Earth, is considered a sub-orbital spaceflight. Manned and unmanned sub-orbital flights have been undertaken to test spacecraft and launch vehicles intended for later orbital spaceflight, while other 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 low Earth orbit, or a spaceflight that attains orbit but deorbits after less than one full orbit, known as a partial orbital spaceflight (for example, the Fractional Orbital Bombardment System).
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[edit] Definition
A spaceflight is defined as travel through outer space, which is usually defined as anything higher than 100 km above sea level to coincide with the Karman line. 100 km was chosen because that is roughly the point where a vehicle would have to fly faster than orbital speed (which is a function of altitude) in order to derive sufficient aerodynamic lift from the atmosphere to support itself. So any vehicle that travels higher than 100 km yet slower than orbital speed is said to be in Sub-orbital spaceflight.
If the trajectory of a sub-orbital spaceflight is part of an elliptic orbit, then the perigee distance is less than the radius of the Earth, so the ellipse intersects the Earth, and hence the spacecraft won't complete an orbit.
[edit] Speed requirements
Sub-orbital spaceflights are possible with maximum speeds of around 1 km/s, compared to spaceflights in low Earth orbit which need speeds around 8 km/s. However, some sub-orbital spaceflights, such as Intercontinental ballistic missiles, or possibly future commercial spaceflights, speeds of over 7 km/s may be achieved. This difference is caused by the need for the spacecraft to travel a considerable distance horizontally, as opposed to simply flying straight up and coming straight back down.
So if one's goal is simply to "reach space", for example the teams who were competing for the Ansari X Prize, then there is no need for horizontal motion. Consequently, the astrodynamical measure, delta-v, for a sub-orbital flight can be as low as 1.4 km/s, compared to around 10 km/s required for orbital spaceflights.
It should be noted that any spaceflight that returns to the surface, including sub-orbital ones, will undergo atmospheric reentry. But because the speeds of sub-orbital spaceflights may be considerably lower than orbital spaceflights, the aerodynamic heating caused will be much less.
[edit] Flight profiles
While there are a great many possible sub-orbital flight profiles, it is expected that some will be more common than others.
[edit] 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.
[edit] 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[1].
[edit] 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.
[edit] 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.
[edit] 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
[edit] 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.
