Orbit

Use an Orbit type of modeling object to set all parameters that define an orbit. You can define spacecraft attitude, calculation positions, as well as sun and celestial body characteristics. You can model full orbits, orbit segments, and orbital maneuvers.

To model orbital maneuvers use parent and child orbits. You must use one Orbital Heating simulation object for each Orbit modeling section you define. Parent and child orbits are displayed independently in the Orbit Visualizer from each Orbital Heating simulation dialog box.

You must always use an Orbit modeling object in conjunction with an Orbital Heating simulation object to model the effect of thermal radiation on objects orbiting celestial bodies.

See Orbital Heating for more information.

Selecting a planet

Several celestial bodies such as the Earth, the Moon and some planets have predefined values for orbit definition.

You can select celestial bodies with predefined orbits, use or modify the available data, or define a generic entity for which you define all sun and planet characteristics.

Selecting an orbit type

When you select an Orbit Type certain orbit values are preset but you must specify additional orbit characteristics. Depending on the Planet you select different orbit types are available:

• For Earth, the available orbit types are : Beta Angle, Geostationary, Classical, Sun-synchronous, Shuttle, Molniya, and Sun Planet Vectors.

• For other predefined celestial bodies only Beta Angle, Geosynchronous, Classical, and Sun Planet Vectors orbit types are available.

Defining orbit parameters

The options on the Orbit Parameters tab let you define the orbit parameters of the object in orbit, the sun, and the selected celestial body.

• The Beta Angle orbit type does not specify a unique orbit plane. It defines orbits with the same exposure to the sun for a given beta angle and altitude.

• The options on the Orbit Parameters tab are automatically set for the unique Geostationary orbit type (Earth) and the Geosynchronous (all other predefined planets) orbit type.

• The Classical orbit type is the most flexible and intricate to define. For Earth, three special classical orbits are already partially defined: Sun-synchronous, Shuttle, and Molniya. See Understanding Classical, Sun Synchronous, Shuttle, and Molniya orbit type parameters for more information.

• For the Sun and Planet Vectors orbit type, the Spacecraft Attitude and Calculation Positions tabs are inactive. These geometric constraints are rolled up in values you have entered in the fields you select on the Orbit Parameters tab.

Alternatively, you can manually specify constant or time-varying sun and planet vectors. This bypasses much of the geometric calculation required before the solver can determine solar and planet view factors. For more information see Sun Planet Vectors orbit type parameters.

Defining sun planet characteristics

The options on the Sun Planet Characteristics tab let you define the planet size, period, gravity, and albedo fluxes for the planet. The options also define the time of year and solar flux. The software provides default values for selected predefined celestial bodies. You can specify the solar flux as a constant value or as a field that varies with time.

To calculate albedo and planet view factors, orbited celestial objects can be modeled implicitly or explicitly.

In the implicit representation, the thermal solver models the celestial object as a point. In the explicit representation, the planet is modeled as a hemisphere. Internally, the solver meshes the hemisphere using 2D elements. The explicit model:

• Allows ray tracing on specular and/or transparent surfaces.

• Traces the reflection and/or transmission of the radiative heat from the celestial object.

• Lets you increase the accuracy of the results and the processing time by increasing the values of either the Radial Mesh Density or the Circumferential Mesh Density boxes.

• Assigns a different albedo value that corresponds to the local latitude to each element of the mesh of the planet when you define a table field where the albedo is a function of the latitude (angle).

The implicit model:

• Provides faster solver times.

Example:

Consider a cube orbiting a planet with one face always pointing to nadir.

At low orbits, the side faces of the cube will see the planet; hence the explicit model will provide a more accurate solution.

If the planet is instead modeled with an implicit representation, the solution will be faster. However, the side faces will not be illuminated by the planet inducing error.

At high orbits the side faces of the cube will not see the planet; hence the implicit model can be used.

If the planet is instead modeled with an explicit representation, the processing time will be longer and the implicit or explicit solutions will be very similar.

The implicit model is used when the values of either the Radial Mesh Density or the Circumferential Mesh Density boxes are set to 1. For any other values the explicit model is used.

Defining spacecraft attitude

To define spacecraft orientation, you must specify two non-parallel vectors on the model, and associate them with two spatial direction options that you select from the Aim at list and the Align with list. The aim at (1) and align with (2) vectors originate from the spacecraft.

During the orbital simulation, the solver adjusts spacecraft orientation at each calculation position.

The options on the Spacecraft Attitude tab let you define:

• The orientation of the spacecraft with respect to orbital, solar, or star vectors.

• Rotation movements.A rotating spacecraft rotates slowly, with no more than a few revolutions per orbit. At each calculation position, the solver recalculates the spacecraft orientation based on the rotation since the previous calculation point. The software takes into account this slow rotation of the spacecraft when calculating view factors.

Specifying calculation positions

The options on the Calculation Positions tab let you specify the following:

• Whether to model a full orbit or a partial orbit.

• The angular interval between calculation points.

The solver calculates black body view factors, solar, earth and albedo view factors and resulting heat loads at each calculation position on the orbit. All calculation positions are referenced from the start angle you select, and the spacecraft position at the beginning of the transient analysis.

Regardless of any intermediate calculations you specify, results are always calculated at both the initial position of the spacecraft, and last defined position of the spacecraft. These two calculation positions are respectively 0° and 360° from the start angle for a full orbit, or the start angle and the end angle for a partial orbit.

To accurately capture thermal gradients, four additional calculation positions are always defined when you solve an orbit that has an eclipse.

Two are located at the start of the eclipse region and two at its end to capture the rapid heat flux variation that occurs in these regions.

Where do I find it?

Application	Pre/Post
Command Finder	Modeling Objects
Menu	Insert → Modeling Objects
Location in dialog box	Type list → Orbital Heating → Create

How do I

Set up an orbital heating simulation

View and animate satellite in orbit

Display satellite results in the orbit

Create an orbit modeling object

Learn more

Inputs to expressions

Look up more details

Orbit Visualizer user interface

Results Display Setup dialog box

Using the Orbit Visualizer

Understanding Beta Angle orbit type parameters

Understanding Classical, Sun Synchronous, Shuttle, and Molniya orbit type parameters

Sun Planet Vectors orbit type parameters

Auto-generated expressions

Quick links

Simcenter 3D Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal boundary conditions

Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

Browse Simcenter 3D help by product area

Orbit, Simcenter 3D 2021.1 Series

© 2020 Siemens

window.mainLanguage="en_US"

window.delivId=""

window.projectId=""

MathJax.Hub.Config({ TeX: { extensions: ["autoload-all.js"] }, tex2jax: { displayMath: [ ] }, "SVG": { scale: 125 } });