Orbit insertion
Orbit insertion is the task for entering a desired stable orbit after launch or arrival in the sphere of influence of another celestial body.
Orbit insertion after launch
After launch, the important task is to avoid all obstacles on the way to the desired orbit:
- Atmosphere
- Terrain
Direct ascent
The simplest way to to fly at a point on the new orbit and accelerate to the orbit velocity at this point in one powered phase. This is a preferred strategy for rocket stages with engines that can't be restarted, but causes very high gravity losses.
Two-Burn strategy
For reducing gravity losses, one way is to aim for inserting into a nearly stable orbit just above the obstacles (for example 130 km for earth or 15 km on the moon), which has its apogee at a point of the target orbit. A second burn matches the velocity vector to the orbit velocity of the target orbit.
Example: Shuttle Ascent
The ascent trajectory of the Space Shuttle is full of examples how safety constraints influence the maneuvers. The initial ascent trajectory is shaped to send both ET and Shuttle on a trajectory after MECO, that allows the ET to crash over a small region in the Indian Ocean, while giving the Shuttle still the opportunity to return to KSC in a Abort Once Around maneuver, should the first burn of the OMS engines (OMS-1) fail. The OMS-1 raises the apogee from the initial low apogee to the target orbit. At the new apogee, about 45 minutes later, the OMS-2 inserts the Shuttle Orbiter into its desired trajectory. Should the OMS-2 fail, the shuttle could still be maneuvered to a safe reentry before its orbit would have decayed to a uncontrolled reentry over the course of the next few days.