Engine Model Components

An engine model component models the thrust and specific impulse (I_sp) of a rocket engine.

Constant Thrust and I_sp

The Constant Thrust and I_sp engine model uses constant, specified values for thrust and I_sp. The parameters of this model are described in the following table.

Constant Thrust and I_sp Engine Model Parameters

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)
Isp	Specific impulse: Enter a value in the scenario's specific impulse unit (seconds).
Thrust	Thrusting force: Enter a value in the selected force unit (e.g., Newtons).

Constant Acceleration and I_sp

The Constant Acceleration and I_sp engine model employs user-supplied constant values for acceleration and I_sp. At each step in time, the model adjusts the thrust to hold acceleration and I_sp constant given a changing mass. The parameters of this model are described in the following table.

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)
Acceleration	Constant acceleration
Isp	Constant specific impulse

Custom Engine

The Custom Engine model defines an engine on the basis of Custom Functions. The parameters of this model are described in the following table.

Custom Function Engine Parameters

Parameter	Description
EvalFunction	Custom function to call at every thrust evaluation
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)
PostFunction	Custom function to call after all propagation ends
PreFunction	Custom function to call before any propagation begins
SegStartFunction	Custom function to call at the beginning of each segment
UpdateFunction	Custom function to call at the beginning of each integration step

Ion Engine

The Ion Engine model calculates three parameters: Thrust, V_e (Exhaust Velocity) and(mass flow rate).

V_e = I_spg

where I_sp is specific impulse (in seconds), and g is the gravitational acceleration on the surface of the Earth.

where MassFlowRate is described below. The value of should 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.

Bothand Thrust are affected by the Throttle value. The Input Power is clipped based on the minimum and maximum power attributes.

The parameters of this model are described in the following table.

Ion Engine Parameters

Attribute	Description
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). This is the value used to convert Isp to exhaust velocity Ve: Ve = Isp(g)
Input Power Source	This is the object that computes the power input to the engine.
Minimum Required Power	This is the minimum power required for the engine to produce thrust.
Maximum Input Power	This is the maximum power that the engine can use to produce thrust.
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.
Reference Epoch	This is the date and time used as a reference epoch for degradation.
Percent Throttle	This is the percentage of available thrust to use (100 is full, 0 is off).

Equations

The Equations pane helps you verify that the coefficients of the engine are accurate. Enter an Input Power value and Epoch and then click Calculate. You can review the results of the equations to see if any of the values are incorrect. You can edit and retest the coefficients as often as you want. The values that you enter in this pane are only used to compute the equations that it comprises and are not applied to the actual model.

If you enter an Input Power that is less than the Minimum Required Power for the engine, then all the equation values will be zero. If you enter an Input Power that is greater than the Maximum Input Power for the engine, the utility will calculate all the equations based on the Maximum Input Power.

Engine definition

I_sp Coefficients

Coefficient	Description
IspModel.C0	Constant coefficient.
IspModel.C1	Linear coefficient.
IspModel.C2	Quadratic coefficient.
IspModel.C3	Cubic coefficient.

Mass Flow Rate Coefficients

In the Function of field, select Power, Isp, or PowerandIsp as the independent variable of the equation.

Coefficient	Description
FlowRateModel.C0	Constant coefficient.
FlowRateModel.C1	Linear coefficient.
FlowRateModel.C2	Quadratic coefficient.
FlowRateModel.C3	Cubic coefficient.

Mass Flow Efficiency Coefficients

In the Function of field, select Power or Isp as the independent variable of the equation.

Coefficient	Description
MassFlowEfficiencyModel.C0	Constant coefficient.
MassFlowEfficiencyModel.C1	Linear coefficient.
MassFlowEfficiencyModel.C2	Quadratic coefficient.
MassFlowEfficiencyModel.C3	Cubic coefficient.

Power Efficiency Coefficients

This formula is available 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
PowerEfficiencyModel.C0	Constant coefficient.
PowerEfficiencyModel.C1	Linear coefficient.
PowerEfficiencyModel.C2	Quadratic coefficient.
PowerEfficiencyModel.C3	Cubic coefficient.

Utility for Testing Coefficients

The Utility for Testing Coefficients pane is a condensed version of the Equations pane on the Engine Parameters tab. The only value that you must supply is the Input Power.

Plugin Engine

The Plugin engine model uses a COM component plugin to define the model attributes. The parameters of this model are described in the following table. For more information about using COM components with STK, see the COM Based Engine Plugins topic in the STK Programming Help.

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	Comprises one or more fields. Each field displays an attribute of the selected COM component.
PluginIdentifier	The ProgID of the COM component that you are using for this model.

If you want to change a C Sharp plugin, you may have to exit STK and then launch the application again after you have changed it.

Polynomial Thrust and I_sp

The Polynomial Thrust and I_sp model uses calculated values for thrust and I_sp that are based on coefficients that you supply.

Engine definition

In the Engine Definition pane, enter a value for the Gravitational Acceleration Constant, at sea level on the Earth. This is the value used to convert I_sp to exhaust velocity V_e:

V_e = I_sp(g)

Click Edit Thrust Coefficients ... to enter the coefficients of the thrust equation that you want to use for this engine model. Thrust coefficients must be entered in terms of Newtons and Pascals, regardless of the selected unit settings. Click Edit ISP Coefficients ... to enter the coefficients of the specific impulse equation that you want to use. I_sp coefficients must be entered in terms of seconds and Pascals.

Utility for Testing Coefficients

The Utility for Testing Coefficients pane helps you verify that the coefficients of the engine are accurate. Enter values for Pressure and Temperature and then click Calculate. You can review the results of the equations to see if any of the values are incorrect. You can edit and retest the coefficients as often as you want. The values that you enter in this pane are only used to compute the equations that it comprises and are not applied to the actual model.

Throttle Table engine

A Throttle Table engine model uses a tabular representation of engine data. The data include values such as thrust, specific impulse, and mass flow rate as functions of the thruster input power. The power magnitudes at which propulsion data are tabulated are called Throttle Levels.

Attribute	Description
Throttle Table File	Browse to a .throttletable file. A Throttle Table file contains tabulated engine data.
Engine Operation Mode	Choose how Thrust, Isp and Mass Flow Rate will be interpolated against Engine Input Power. Continuous with regression polynomial fit- Fit a least-square polynomial of a specified degree through the data Continuous with piecewise linear fit- Interpolate engine performance data with a piecewise linear interpolant Discrete power levels- Perform zero-order hold interpolation of thrust, mass flow rate, and Isp against power.
Regression polynomial degree	This is the degree of the least-square fitted polynomial to the tabular engine data for the default operation mode.
Gravitational Acceleration Constant	This is the 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).
Input Power Source	This is the component that computes the power input to the engine.
Percent Degradation Per Year	This is the degradation factor is (1 - x)n, where n is the time since epoch in years, and x is the percent degradation per year.
Reference Epoch	This is the date and time used as a reference epoch for degradation.