Launch Segment

Use the Launch segment to model a simple spacecraft launch from Earth or another central body. The method is like that employed by the Simple Ascent propagator for Launch Vehicle and Missile objects. The trajectory produced is a simple curve rising vertically from the launch pad that turns over smoothly to insert the launch vehicle into orbit with the specified flight path angle at the insertion point using the specified velocity. If a trajectory with the specified flight path angle cannot be computed, a zero flight path angle trajectory is used with a discontinuity in the last point to give the desired final velocity.

Typically, a launch vehicle manufacturer supplies a mission analyst with an insertion vector in the ECF frame as well as the time of flight. Using Astrogator's Launch segment, you can then model the launch data received from the manufacturer and vary the launch time to meet other mission requirements.

To define a Launch segment, you must define parameters for the launch trajectory, the launch location, the burnout point, and (optionally) the burnout velocity. Specific launch location and burnout point parameters are framed in separate sections of the window, and you can access the burnout velocity parameters from the burnout section.

The following table describes the general trajectory parameters of a Launch segment:

Field Description
Central Body The default central body of any Launch segment is the Earth. To select a different central body for the launch, click to open the Select Central Body window. Click the desired central body to highlight it and then click OK.
Step Size Specify the time interval between calculated ephemeris output points.
Use State From Previous Segment Select to define the launch location parameters by using the state of the spacecraft at the end of the previous segment. The launch location parameters and the Pre-Launch Time field become unavailable.
Pre-Launch Time

Specify the amount of time before the spacecraft's launch that it appears in the scenario. The vehicle will remain at the launch position until beginning of the launch epoch.
Ascent Type Specify the order of the spline used to generate the motion along the ellipse:
  • Ellipse: Cubic Motion - the motion generates on the basis of given positions and velocities.
  • Ellipse: Quartic Motion - the motion generates on the basis of given positions and velocities and initial acceleration.
Initial Acceleration If you select Quartic Motion as the Ascent Type, specify the initial acceleration.

Launch Parameters

After defining the general parameters of the launch, you must define the location and start time of the launch. The following table describes the launch location parameters:

Field Description
Launch Coordinate Type Select between:
  • Geocentric (Planetocentric) - the location is measured from the center of mass of the Earth or other central body.
  • Geodetic (Planetodetic) - the location is measured along the normal to the surface of an ellipsoid defined with reference to the Earth (or other central body).
Epoch Specify the date and time of the launch.
Set Mission Elapsed Time Epoch to Launch Epoch Select to set the Mission Elapsed Time epoch to the Launch epoch when running the MCS.
Latitude Specify the latitude of the launch location.
Longitude Specify the longitude of the launch location.
Radius/Altitude Specify the radius (planetocentric) or altitude (planetodetic) of the launch location.

Import Facility Location

You can load a facility from the STK facility database to define the launch location by clicking Import Facility Location... to open the Facility Database window. STK will not add the facility to the current scenario. Instead, it imports the facility information to populate the launch location parameter fields.

Select Facility

You can select a facility in the current scenario to define the launch location by clicking Select Facility... to open the Select Facility window; click the desired facility to highlight it and then click OK. This button is only displayed if there is at least one facility in the scenario.

Burnout Parameters

After defining the general and launch location parameters, you must define the location of the burnout point and the burnout velocity.

The option Specify burnout state in enables selection between parameter sets that include various spherical coordinate systems and the central- body-fixed Cartesian system.

Select the central-body-fixed Cartesian system provides the option to specify the time-of-flight and to fully specify the six position and velocity components of the burnout state in the standard STK central-body-fixed Cartesian system for the current central body.

Selecting "A spherical coordinate system" allows for specification of time-of-flight, as well as, position and velocity in some combination of the following parameters. This depends on the the sub-selections of the burnout positions and velocity types.

Burnout

The burnout location parameters are described in the following table:

Field Description
Burnout Point Definition Select between:
  • Geocentric (Planetocentric) - the burnout point is measured from the center of mass of the Earth or other central body.
  • Geodetic (Planetodetic) - the burnout point is measured along the normal to the surface of an ellipsoid defined with reference to the Earth (or other central body).
  • Launch Az / Alt - the burnout point is defined in reference to distance downrange along an azimuth, measured from the surface of the Earth or other central body.
  • Launch Az / Radius - the burnout point is defined in reference to distance downrange along an azimuth, measured from the center of mass of the Earth or other central body.
Time of Flight Specify the time of flight (the time from launch to burnout).
Latitude

Specify the latitude of the spacecraft at burnout.

(Geodetic or Geocentric)
Longitude

Specify the longitude of the spacecraft at burnout.

(Geodetic or Geocentric)
Azimuth

Specify the azimuth of the launch trajectory. This is the angle defining the launch direction, measured from North towards East.

(Launch Az / Alt or Launch Az / Radius)
Downrange Dist

Specify the downrange distance of the spacecraft from launch at burnout. This is the distance along a great arc from the launch location to the burnout sub-satellite point.

(Launch Az / Alt or Launch Az / Radius)
Radius/Altitude Specify the radius (Geocentric or Launch Az / Radius) or altitude (Geodetic or Launch Az / Alt) of the spacecraft at burnout.

Burnout Velocity

The following table describes the parameters used to define burnout velocity:

Field Description
Burnout Options Select between:
  • Use Fixed Velocity - the inclination of the final state of the launch segment is determined by the arc between the launch and insertion positions, and the horizontal flight path angle is set to zero.
  • Use Inertial Velocity - the final state of the launch segment is solely and completely determined by the burnout position and velocity.
Fixed Velocity If you selected the Use Fixed Velocity option, specify the velocity magnitude in the fixed frame.
Inertial Velocity If you selected the Use Inertial Velocity option, specify the velocity magnitude in the inertial frame.
Inertial Velocity Azimuth If you selected the Use Inertial Velocity option, specify the inertial velocity azimuth. Inertial velocity azimuth is the angle from the projection of north in the local horizontal plane to the inertial velocity vector, right handed.
Inertial Horizontal Flight Path Angle If you selected the Use Inertial Velocity option, specify the inertial horizontal flight path angle. Inertial horizontal flight path angle is the angle from the local horizontal to the inertial velocity vector, positive towards radius. It is also 90 degrees minus vertical flight path angle.

Spacecraft Parameters and Fuel Tank

Use the Spacecraft Parameters and Fuel Tank tabs to manually define the Spacecraft Configuration.

User Variables

Use the User Variables tab to set initial values for user variables defined for the mission.