Constant Thrust and Isp | Constant Acceleration and Isp | | Custom Engine | Ion Engine | Plugin Engine | Polynomial Thrust and Isp
The Component Browser provides several ways to model the thrust and specific impulse (Isp) of a rocket engine (any of these approaches can be used to create new engines for use in your space mission scenario):
Constant Thrust and Isp
This model employs user-supplied, constant values for thrust and Isp. A Component Edit window contains the following editable fields:
| Field | Description | Default Values |
|---|---|---|
| g | Gravitational acceleration constant at sea level on the Earth. Enter a value in the selected distance unit per the selected time unit squared (e.g. km/sec2). This is the value used to convert Isp to exhaust velocity Ve:
Ve = Isp(g) |
0.00980665 km/sec2 |
| Isp | The specific impulse for this engine. Enter a value in the scenario specific impulse unit (seconds). | 300 s |
| Thrust | The thrust for this engine. Enter a value in the selected force unit, e.g., in Newtons (N). | 500 N |
| User Comment | Enter information that will help you later to recall the purpose of this engine model. | none |
Constant Acceleration and Isp
This model employs user-supplied, constant values for acceleration and Isp. At each step in time, the model adjusts the thrust to hold acceleration and Isp constant given a changing mass. The fields described in the following table are available for edit for this component.
| Field | Description | Default Values |
|---|---|---|
| g | Gravitational acceleration constant at sea level on the Earth. Enter a value in the selected distance unit per the selected time unit squared (e.g. km/sec2). | 0.00980665 km/sec2 |
| Acceleration | Constant acceleration provided by the engine. | 0.000500000 km/sec2 |
| Isp | Constant specific impulse of the engine. | 300.0 sec |
| User Comment | Enter information that will help you later to recall the purpose of this engine model. | none |
Custom Engine
Select the Custom Engine option to define an engine on the basis of Custom Functions.
Custom Function Parameters
| Parameter | Description | Default Values |
|---|---|---|
| EvalFunction | Custom function to call at every thrust evaluation. | none |
| g | Earth surface gravity acceleration for Isp conversions. | 0.00980665 km/sec2 |
| PostFunction | Custom function to call after all propagation ends. | none |
| PreFunction | Custom function to call before any propagation begins. | none |
| SegStartFunction | Custom function to call at the beginning of each segment. | none |
| UpdateFunction | Custom function to call at the beginning of each integration step. | none |
Ion Engine
The Ion Engine model calculates three parameters: Thrust, Ve (Exhaust Velocity) and
(mass flow rate).
Ve = Ispg
where Isp is specific impulse (in seconds), and g is the gravitational acceleration on the surface of the Earth.
where MassFlowRate is described below. Note thatshould be negative; this represents the loss of mass when the engine is on. The MassFlowRate coefficients should return a positive number, and STK enforces the correct sign.
where D is the degradation factor (PercentageDegradationPerYear) measured from the reference epoch.
Note that bothand Thrust are affected by the throttle setting. The input power is clipped based on the minimum and maximum power attributes.
Engine Parameters
| Attribute | Description | Default Values |
|---|---|---|
| Gravitational Acceleration Constant | Gravitational acceleration constant at sea level on the Earth. Enter a value in the selected distance unit per the selected time unit squared (e.g. km/sec2). | 0.00980665 km/sec2 |
| Input Power Source | Object that computes the power input to the engine. | ProcessedPower |
| Minimum Required Power | Minimum power required for the engine to produce thrust. | 27 dBW |
| Maximum Input Power | Maximum power that can be used by the engine to produce thrust. | 33.6 dBW |
| Percent Degradation Per Year | The degradation factor is (1 - x)n, where n is the time since epoch in years, and x is the percent degradation per year. | 0 |
| Reference Epoch | Date and time used as a reference epoch for degradation. | Scenario/Analysis Start time |
| Percent Throttle | Percentage of available thrust to use (100 is full on, 0 is off). | 100 |
Equations
This feature helps you to make certain that your coefficients were entered accurately. Enter an Input Power value and define the Epoch. Click Calculate and observe the values displayed in the equations below to ascertain whether any error has occurred. If so, you can edit and retest the coefficients as often as necessary to obtain the correct results.
If the specified Input Power is less than the Minimum Required Power for the engine, then the utility will display all values as zero. If the specified Input Power is greater than the Maximum Input Power for the engine, then the utility will calculate all values based on the Maximum Input Power.
Engine Definition
Isp Coefficients
| Coefficient | Description | Default Values |
|---|---|---|
| IspModel.C0 | Constant coefficient. | 3800 |
| IspModel.C1 | Linear coefficient. | 0 |
| IspModel.C2 | Quadratic coefficient. | 0 |
| IspModel.C3 | Cubic coefficient. | 0 |
In the Function of field, select Power, Isp, or PowerandIsp as the independent variable of the equation.
| Coefficient | Description | Default Values |
|---|---|---|
| FlowRateModel.C0 | Constant coefficient. | 4.43e-008 |
| FlowRateModel.C1 | Linear coefficient. | 0 |
| FlowRateModel.C2 | Quadratic coefficient. | 0 |
| FlowRateModel.C3 | Cubic coefficient. | 0 |
Mass Flow Efficiency Coefficients
In the Function of field, select Power or Isp as the independent variable of the equation.
| Coefficient | Description | Default Values |
|---|---|---|
| MassFlowEfficiencyModel.C0 | Constant coefficient. | 1 |
| MassFlowEfficiencyModel.C1 | Linear coefficient. | 0 |
| MassFlowEfficiencyModel.C2 | Quadratic coefficient. | 0 |
| MassFlowEfficiencyModel.C3 | Cubic coefficient. | 0 |
Power Efficiency Coefficients
This formula only becomes active if you have selected PowerandIsp as the independent variable of the Mass Flow Rate equation.
In the Function of field, select Power or Isp as the independent variable of the equation.
| Coefficient | Description | Default Values |
|---|---|---|
| PowerEfficiencyModel.C0 | Constant coefficient. | 1 |
| PowerEfficiencyModel.C1 | Linear coefficient. | 0 |
| PowerEfficiencyModel.C2 | Quadratic coefficient. | 0 |
| PowerEfficiencyModel.C3 | Cubic coefficient. | 0 |
Utility for Testing Coefficients
This feature works exactly as the Equation utility on the Engine Parameters tab, but without providing the option to specify the Epoch. The results are displayed below, though only the output is displayed - not the entire equation.
Plugin Engine
A Plugin engine model (see Integration help system) utilizes a COM component plugin to define the model attributes.
| Field | Description |
|---|---|
| g | Gravitational acceleration constant at sea level on the Earth. Enter a value in the selected distance unit per the selected time unit squared (e.g. km/sec2). This is the value used to convert Isp to exhaust velocity Ve:
Ve = Isp(g) |
| PluginConfig | Typically multiple fields, each one displaying an attribute of the selected COM component. |
| PluginIdentifier | The ProgID of the COM component you are using for this model. |
| User Comment | Enter information that will help you later to recall the purpose of this engine model. |
If you wish to make changes to a C Sharp plugin, you may have to exit STK to make the changes and then launch the application again.
Polynomial Thrust and Isp
This model uses calculated values for thrust and Isp, based upon user-supplied coefficients for thrust and Isp equations. A means is provided for testing your coefficients.
Engine Definition
The Engine Definition frame of the Thrust Engine Model window provides the following options:
| Option | Description | Default Values |
|---|---|---|
| Gravitational Acceleration Constant | Gravitational acceleration constant at sea level on the Earth. Enter a value in the selected distance unit per the selected time unit squared (e.g. km/sec2). | 0.00980665000 km/sec2 |
| Edit Thrust Coefficients... | Click this button and enter the appropriate value for each coefficient, depending upon the thrust equation you are using. |
C0: 490 T/Tr: 293 K |
| Edit ISP Coefficients... | Click this button and enter the appropriate value for each coefficient, depending upon the Isp equation you are using.
Thrust coefficients must be entered in terms of Newtons and Pascals, regardless of the selected unit settings. Similarly, Isp coefficients must be entered in terms of seconds and Pascals. |
C0: 300 T/Tr: 293 K |
Utility for Testing Coefficients
This feature helps you to make certain that your thrust and Isp coefficients were entered accurately. In the boxes provided, enter a Pressure value in the selected pressure unit and a Temperature value in the scenario temperature unit. Click Calculate and observe the values displayed at the bottom of the window to ascertain whether any error has occurred. If so, you can edit and retest the thrust and/or Isp coefficients as often as necessary to obtain the correct results.
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