Some questions for our Rocket Scientists regarding the coast phase?

[smartcooky]

After a rocket is launched with a payload to be taken to geostationary orbit, the second stage engine cuts off and the spacecraft coasts for a while in a parking orbit. Then the second stage starts up to push the spacecraft into a GTO before an apogee burn circularises the orbit at 36,000 km.

My questions are

1. Why is there a coast phase, i.e. Why does the second stage not just keep burning to put the spacecraft into a GTO?

2. Does it have anything to do with the fact that the rocket is launched from a latitude some distance from the equator.     

[Allan F]

It has to be in the right position for a GTO. Otherwise, it would end up in GSO out of position. Not much applause to a satellite designed to be above 10 deg west, when it hangs 80 deg east.

[QuietElite]

Rockets are normally injected into GTO while flying over the equator. The reason is so that either the ascending or descending node in relation to the equator is right at Apoapsis. You do that because satellites that are put into GTO normally want to reach a geostationary orbit so they have to do a inclination change at one of the nodes and this is most efficient at Apoapsis.

Placing your node at Apoapsis by doing your GTO burn over the equator has 2 advantages:

1. You have to do your inclination at Apoapsis which is the most efficient since at Apoapsis you have the lowest orbital velocity so pushing "sideways" to change the inclination has more impact if you are slow.

2. You can combine your prograde burn to raise your Orbit with your inclination change since they both happen at Apoapsis which also saves fuel. You can imagine this by trying to walk to the opposite corner on a square with sidelength 1. You can walk along the corners in two separate moves and have to travel 2 units. But if you walk on the diagonal line you only have to walk the distance SQRT 2 or about 1.4 units.

[smartcooky]

OK, so if I understand this correctly, the coast phase is there to "wait" until the spacecraft crosses the equator, at which point, the GTO insertion burn is made, and that point will be come the perigee of the GTO, and the apogee will be 180° around the far side of the earth.

Would it be correct to say that the coast phase would not be necessary if the launch site was directly on the equator?

[gwiz]

Yes, Ariane 5 launches from a near-equatorial site have a single burn of the upper stage.