Sub-orbital spaceflight

A sub-orbital space flight 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.

Usually a rocket is used, but experimentally a sub-orbital spaceflight has also been achieved with a space gun.^[1]

Altitude requirement

Isaac Newton's Cannonball. Paths A and B depict a sub-orbital trajectory.

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.^[2] The US military and NASA award astronaut wings to those flying above 50 miles^[3] (80.47 km), although the US State Department appears to not support a distinct boundary between atmospheric flight and space flight.^[4]

Orbit

During freefall the trajectory is part of an elliptic orbit as given by the orbit equation. The perigee distance is less than the radius of the Earth R including atmosphere, 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 a is more than R/2. The specific orbital energy is given by:

where is the standard gravitational parameter.

Almost always a < R, corresponding to a lower than the minimum for a full orbit, which is

Thus the net extra specific energy needed compared to just raising the spacecraft into space is between 0 and .

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 than 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.

Flight duration

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.

Flight profiles

Profile for the first manned American suborbital flight, 1961. Launch rocket (not shown) lifts the spacecraft for the first 2:22 minuts. Dashed line: zero gravity.

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

The X-15 (1958-68) would lift itself to an altitude of app. 100 km and then lose height and glide down.

Ballistic missiles

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 (97 km).^[5] Then in the 1950s the USA and USSR concurrently developed much longer range Intercontinental Ballistic Missiles (ICBM)s all of which were based on the V-2 Rocket and the work of the scientists at Peenemunde. There are now many countries who possess ICBMs and even more with shorter range IRBMs (Intermediate Range Ballistic Missiles).

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 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. The Scaled Composites 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. It has since been completed with eight seats (one pilot, one co-pilot and six passengers) and has taken part in captive-carry tests and with the first mother-ship WhiteKnightTwo, or VMS Eve. It has also completed solitary glides, with the movable tail sections in both fixed and "feathered" configurations. The hybrid rocket motor has been fired multiple times in ground-based test stands, and was fired in a powered flight for the second time on 5 September 2013.^[6] Four additional SpaceShipTwos have been ordered and will operate from the new Spaceport America. Commercial flights carrying passengers are expected in 2014, and will be guided by a "safety-driven schedule".

Scientific experiments

A major use of suborbital vehicles today are as scientific sounding rockets. Scientific suborbital 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 .

Suborbital transportation

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.

However, 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, but the difference is not large.^[7]

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.

The SpaceLiner is a hypersonic suborbital spaceplane concept that could transport 50 passengers from Australia to Europe in 90 minutes or 100 passengers from Europe to California in 60 minutes.^[8] The main challenge lies in increasing the reliability of the different components, particularly the engines, in order to make their use for passenger transportation on a daily basis possible.

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.^[9]

Notable unmanned sub-orbital spaceflights

• The first sub-orbital space flight was in early 1944, when a V-2 test rocket launched from Peenemünde in Germany reached 189 kilometres altitude.Peenemünde',^{[10][citation needed]}
• 8 September 1944, the world's first successful ballistic missile (V-2, launched by Germany) hits its target for the first time, Chiswick in London, England. Three civilians were killed and seventeen injured, a massive crater was left. By September 1944, the V-2s routinely achieved Mach-4 during terminal descent.^{[citation needed]}
• Bumper 5, a two stage rocket launched from the White Sands Proving Grounds. On 24 February 1949 the upper stage reached an altitude of 248 miles (399 km) and a speed of 7,553 feet per second (2300 meters per second approx.)^[11] which is nearly Mach-7.
• USSR — Energia, 1986, Polyus payload failed to reach orbit; this was the most massive object launched into suborbital spaceflight to date

Manned sub-orbital spaceflights

Above at least 100 km in altitude.

	Date (GMT)	Mission	Crew	Country	Remarks
1	1961-05-05	Mercury-Redstone 3	Alan Shepard	United States	First manned sub-orbital spaceflight, first American in space
2	1961-07-21	Mercury-Redstone 4	Virgil Grissom	United States
3	1963-07-19	X-15 Flight 90	Joseph A. Walker	United States	First winged craft in space
4	1963-08-22	X-15 Flight 91	Joseph A. Walker	United States	First person and spacecraft to make two flights into space
5	1975-04-05	Soyuz 18a	Vasili Lazarev Oleg Makarov	Soviet Union	Failed orbital launch. Aborted after malfunction during stage separation
6	2004-06-21	SpaceShipOne flight 15P	Mike Melvill	United States	First commercial spaceflight
7	2004-09-29	SpaceShipOne flight 16P	Mike Melvill	United States	First of two flights to win Ansari X-Prize
8	2004-10-04	SpaceShipOne flight 17P	Brian Binnie	United States	Second X-Prize flight, clinching award

Future of manned sub-orbital spaceflight

Private companies such as Virgin Galactic, XCOR, Armadillo Aerospace, Blue Origin and Masten Space Systems 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. Non-profit entities like ARCASPACE and Copenhagen Suborbitals also attempt rocket-based launches.

See also

• Point-to-point sub-orbital spaceflight
• Orbital spaceflight
• Spaceflight
• Spaceport
• List of rocket launch sites
• Office of Commercial Space Transportation
• Canadian Arrow
• Supersonic Transport

References

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