Tuesday, February 24, 2015

Electron Cont'd (8)

Let's talk launch trajectory design.
The basic steps included in many orbital launch profiles are the following:
  1. Vertical Launch: Pitch angle (angle from horizontal)=90 degrees
  2. Pitchover Maneuver: Rocket is tilted slightly and so is no longer vertical, initiating the gravity turn
  3. Gravity Turn: Rocket pitch angle is increased so that the rocket flies a curved path and the angle of attack is nearly zero, minimizing aerodynamic stresses at the expense of not gaining altitude as rapidly
  4. Max Q Throttling: Engines are throttled down while passing through maximum dynamic pressure ("Max Q") to reduce aerodynamic stresses; usually happens at around 70 seconds into flight just prior to hitting the sound barrier
  5. Engine Cut-Off: Often abbreviated to MECO when referring to main engine cut-off, engines are shut down once fuel is used up and thrust falls below a certain level, or deliberately to coast
  6. Staging: Stages that have spent fuel are jettisoned to reduce mass
  7. Payload shroud (fairing) Jettison: Shroud protecting the payload at the top of the rocket is jettisoned early to reduce mass once altitude is high enough and air is thin enough
  8. Payload/Upper stage Deployment: Payload is deployed by mechanical means (e.g., springs) or given a boost to a different orbit by an upper stage kick motor
Here is a great long-exposure photograph of the OCO-2 launch by Rick Baldridge illustrating the entire launch flight path including the characteristic curvature of the gravity turn. (Visit the web page to learn more)

Some rockets may also perform a Roll program during launch to roll the rocket to a certain orientation about its central axis. For example, the Space Shuttle rolled over to make sure its wings faced up during the gravity turn and reduce aerodynamic loads. Here is a gif illustrating it from NASA TV coverage of STS-129:

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