What is meant by "a high-energy geostationary orbit"? (SpaceX Arabsat 6A)

Question

The Teslarati article SpaceX’s April 7th Falcon Heavy launch a step toward new commercial markets says:

While there is some inherent uncertainty surrounding the (once again) fairly new rocket, SpaceX has now officially filed a plan with the Cape Canaveral range authorities that would see Falcon Heavy nominally conduct a critical static fire test as soon as March 31st, followed one week later by a launch target of no earlier than (NET) 6:36 pm EDT (22:36 UTC), April 7th. Set to place the ~6000 kg (13,200 lb) Arabsat 6A communications satellite in a high-energy geostationary orbit, a successful mission that ultimately proves Falcon Heavy’s commercial utility could also raise global launch market interest in the rocket, including potential anchor customers like NASA.

1. What (if anything) is meant by "a high-energy geostationary orbit"?

2. It SpaceX bringing the satellite all the way to a cis-GEO orbit, or is this a launch only to GTO?

Answer 1

The other answerer is on the right track. I believe the description of a "high energy geostationary orbit" is a mistake on the part of the author: it should be called a "high energy geostationary transfer orbit" (hereafter GTO).

First, let me detail how a traditional GTO is used from an inclined launch site.

1. The launcher will throw the satellite into a transfer orbit with an apogee of geosynchronous altitude (35,786 km).

2. At apogee, the satellite will perform a burn which simultaneously reduces the inclination to 0° and raises the perigee to geosynchronous altitude.

3. After this combined burn, the satellite is in geostationary orbit.

The reason why the inclination change and perigee raise maneuvers are combined is simple trigonometry--burning diagonally requires less total energy to achieve the same final velocity (not just speed, the direction matters a lot!) than burning one way and then burning perpendicular to that direction. Additionally, this is done at geosynchronous altitude because the satellite is traveling slower there than it was at low earth altitude. Thus, changing direction does not require as much change in velocity (delta-V).

Falcon Heavy launched Arabsat 6a into a transfer orbit with an apogee of 90,000 km, well above the geosynchronous altitude of 35,786 km. At that altitude, it was traveling very, very slowly, so the combined inclination change and perigee raise burn required even less dV from Arabsat than if it had happened at GEO. But Arabsat would not yet have been in geostationary orbit. It needed to perform one more circularization burn at perigee to slow itself back down and lower its apogee from 90 Mm to geosynchronous altitude.

This kind of an "overshooting" transfer into a higher orbit is not a Hohmann transfer orbit, it is a bi-elliptic transfer. Despite requiring three burns (1: [over]raise apogee, 2: raise perigee, 3: lower apogee) rather than just the two of the Hohmann (1: raise apogee, 2: raise perigee), bi-elliptic transfers can require less dV in some cases. Raising from LEO to GEO is not one of these cases. The total dV required is greater than a traditional GTO.

However, the lower-energy burns required of Arabsat to enter GEO from its transfer orbit expended much less dV than the single burn to enter GEO from a traditional GTO.

The difference in energy was made up by Falcon Heavy, which placed Arabsat in the high energy geostationary transfer orbit which threw it out to 90 Mm instead of 35.7ish Mm.

Answer 2

The transfer orbit for Arabsat 6a went to an apogee of 90,000km. At that height it's travelling very slowly, so adding delta-V needs less energy. The sat needs both to remove the launch inclination (i.e. 28.5 -> 0) and at the same time raise the perigee to geostationary altitude. Then, when the sat reaches perigee the engine is fired again to circularise the orbit. This needs less energy from the sat's own motor but requires more from the launcher.