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http://en.wikipedia.org/wiki/Earth_to_orbit

Earth to Orbit (ETO) describes the process or means of placing a payload into an orbit around the Earth. Traditionally this has used rockets, however ETO can also include much more exotic forms of transportation such as railguns and hypersonic cannons and Space elevators.

Methods

Earth to orbit transportation can be roughly broken up into four groups, these are:

1. Rocket transportation
2. Ballistic methods
3. Orbital transportation
4. Relay transportation

Rocket Based Transportation

Titan II Rocket Launch

A series of suborbital rockets are used in order to accelerate a payload to orbital velocities. This is the only method of placing a payload into an earth orbit which has been successfully used. It involves constructing a series of rockets which are usually stacked on one another, imparting their change in momentum to the payload before dropping away.

Although a series of suborbital rockets are usually used. Other schemes involving SSTO or Single Stage To Orbit vehicles have been proposed. However they all suffer from a low Mass fraction, reducing their effectiveness for transporting payloads into orbit.

Ballistic methods

A stationary platform at least partly in Earth's atmosphere that delivers the required momentum by using the Earth to react against. Examples of this may be a rail gun, coil guns, a hypervelocity gun or a Lofstrom loop. Because the payload is accelerated to velocities greater than required for orbital velocity by the time the payload has left the device, either very high accelerations are required, or very large structures (thousands of kilometres long) are needed.

A Lofstrom loop would accelerate its payloads above the atmosphere, but the others are typically subject to extreme deceleration due to air drag (500m/s^2). Lofstrom loops also have low accelerations but gun designs typically use much higher accelerations that are unsuitable for humans.

Orbital Transportation

This method uses a satellite, already in earth orbit to impart part of the required momentum to accelerate the payload to orbital velocities. The most accepted candidates used various configurations of a Tether. The most promising is the Skyhook elevator which uses momentum exchange to boost the payload from a suborbital trajectory to an orbital trajectory . As momentum is transferred from the momentum exchange tether to the payload it loses orbital energy, decreasing in altitude.

The loss in altitude of the tether must be recovered, otherwise the tether itself may reenter the Earth's Atmosphere. Although this can be done in many ways. One way is to use a conductive tether and pass electricity through it, forming an Electrodynamic tether to generate a force to accelerate the tether after a loss of momentum.

The end of the skyhook tether dips in the atmosphere. However because it has zero relative velocity to the Earth's surface. It is not effected by atmospheric heating that would otherwise destroy an object entering earth's atmosphere without a Heat shield.

Relay Transportation

This method uses a combination of the above methods over to cover the various parts of the journey into orbit.

See also

• Orbital spaceflight
• Spacecraft propulsion
• Space elevator
• Space fountain
• Tether propulsion
• Skyhook

External links

NASA documents

• Highly Resuable Space Transportation: Approaches for Reducing ETO Launch Costs to $100-$200 per Pound of Payload

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Published in July 2009.

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