Parking Orbit - Disadvantages of Parking Orbits

Disadvantages of Parking Orbits

The most notable disadvantage is that a rocket needs to coast for a while, then restart while under zero g conditions. Furthermore the length of two of the burns (the initial injection burn, and the final burn) typically depend on where in the launch window the launch occurs. To do this without wasting fuel, a rocket stage that can fire, then stop, then start again is needed. This implies a liquid fuel engine since solid fuel rockets cannot be stopped or restarted - once ignited they burn to completion. But even in a liquid-fueled engine, this multiple restart capability is non-trivial for a number of reasons:

• During the coast, the propellants will drift away from the bottom of the tank and the pump inlets. This must be dealt with in some way. Common methods are tanks with diaphragms, or ullage rockets for settling the propellant back to the bottom of the tank.
• Longer life batteries and other consumables are needed.
• Some engines use special chemicals for ignition; multiple sets are needed for restarts
• Better insulation is needed, particularly on cryogenic tanks, to prevent too much propellant boiloff during coast.
• A better inertial guidance system is needed, to keep track of the state during the coast.
• A reaction control system is needed, to orient the stage properly for the final burn, and perhaps to establish a suitable thermal orientation during coast.

The Centaur and Agena families of upper stages were designed for such restarts and have often been used in this manner. The last Agena flew in 1987 but Centaur is still in production. The Briz-M stage often performs the same role for Russian rockets.