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Many small satellites have been launched piggyback fashion alongside the larger primary
payloads. For these reasons a number of dedicated small satellite launchers have emerged and many more are under development, serving the increasing demand for critical small satellite missions requiring specific orbits. Although small satellites have been launched alongside many larger payloads on many launchers, currently the ARIANE ASAP is the only repeatable launch mechanisms available for small satellites as a secondary payload. In addition the Scout launcher was developed as a dedicated small satellite launcher, although it is no longer made, and the Russian Federation Zenit and Cosmos launchers have also regularly been used, but employing ad-hoc payload accommodation. Recently other launchers have been and are being developed, such as the Orbital Sciences Pegasus and Taurus, and the Lockheed Martin Athena. Secondary payloads are typically launched inert (not powered), and are then activated on deployment using a separation switch. For safety reasons the downlink can often only be switched on when the satellites have drifted apart sufficiently, typically requiring several hours. Deployment from the launcher is frequently achieved by a mechanical spring, which is released by pyrotechnic bolt cutters, commonly referred to as 'pyros'. The spring imparts a tiny velocity on the payloads in the range 0-1m/s, and when multiple secondary payloads are deployed the velocities are chosen to ensure the payloads will drift and stay apart. On some launches the third stage will perform some orbit or attitude changes for this reason, between deployment of payloads. |
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On
most larger launchersy liquid fuel and solid rocket boosters are added to a launcher in
fixed increments until the required lift capability is exceeded. Dummy mass is then to
added to fine-tune the required orbit injection point . Small payloads can be substituted
making effective use of this mass margin, often providing a very cost effective route into
orbit,however the main drawback to being a hitchhiker on a launcher is that the orbital
injection point is determined by the primary payload. Small payloads have limited scope
for carrying fuel for orbital manoeuvres, and so piggyback launches could be restrictive
for some applications, for instance to deploy many small satellites in a constellation. In
addition, most small satellites prefer LEO orbits, and the majority of launches are into
GTO, further restricting the available launch opportunities.