ChemicalThruster

Use of ChemicalThruster Resource in Conjunction With Maneuvers

A ChemicalThruster resource is used only in association with finite maneuvers. To implement a finite maneuver, you must first create both a ChemicalTank and a FiniteBurn resource. You must also associate a ChemicalTank with the ChemicalThruster resource and you must associate a ChemicalThruster with the FiniteBurn resource. The finite maneuver is implemented using the BeginFiniteBurn/EndFiniteBurn commands. See the BeginFiniteBurn/EndFiniteBurn command documentation for worked examples on how the ChemicalThruster resource is used in conjunction with finite maneuvers.

Thrust and ISP Calculation

Unscaled thrust, F_T, and Isp, as a function of Pressure, in kPa, and Temperature, in degrees Celsius, are calculated using the following polynomials.

The thrust, T, output in Newtons, is scaled by the Duty Cycle and Thrust Scale Factor. The thrust acceleration direction vector (the direction of the actual acceleration not the thruster nozzle) is given by ThrustDirection1-3 and is applied in the input Coordinate System. The Isp is output in seconds.

The mass flow rate and the thrust equations are shown below where F_T and Isp are defined above, f_d is the duty cycle, f_s is the thrust scale factor, R_iT is the rotation matrix from the thrust coordinate system to the inertial system, and T_d is the unitized thrust direction.

Local Coordinate Systems

Here, a Local coordinate system is defined as one that we configure "locally" using the ChemicalThruster resource interface as opposed to defining a coordinate system using the Coordinate Systems folder in the Resources Tree.

To configure a local coordinate system, you must specify the coordinate system of the input thrust acceleration direction vector, ThrustDirection1-3. If you choose a local coordinate system, the four choices available, as given by the Axes sub-field, are VNB, LVLH, MJ2000Eq, and SpacecraftBody. VNB or Velocity-Normal-Binormal is a non-inertial coordinate system based upon the motion of the spacecraft with respect to the Origin sub-field. For example, if the Origin is chosen as Earth, then the X-axis of this coordinate system is the along the velocity of the spacecraft with respect to the Earth, the Y-axis is along the instantaneous orbit normal (with respect to the Earth) of the spacecraft, and the Z-axis completes the right-handed set.

Similarly, Local Vertical Local Horizontal or LVLH is also a non-inertial coordinate system based upon the motion of the spacecraft with respect to the Origin sub-field. Again, if we choose Earth as the origin, then the X-axis of this coordinate system is the position of the spacecraft with respect to the Earth, the Z-axis is the instantaneous orbit normal (with respect to the Earth) of the spacecraft, and the Y-axis completes the right-handed set.

MJ2000Eq is the J2000-based Earth-centered Earth mean equator inertial coordinate system. Note that the Origin sub-field is not needed to define this coordinate system.

SpacecraftBody is the attitude system of the spacecraft. Since the thrust is applied in this system, GMAT uses the attitude of the spacecraft, a spacecraft attribute, to determine the inertial thrust direction. Note that the Origin sub-field is not needed to define this coordinate system.

Caution When Setting the ChemicalTank Temperature and Reference Temperature

Note that both the thrust and ISP polynomials have terms that involve the ratio, (Temperature / Reference Temperature). For GMAT, this temperature ratio is calculated in Celsius units, and thus, there is a discontinuity when the Reference Temperature is equal to zero. For this reason, GMAT requires that the absolute value of the input Reference Temperature is greater than 0.01.

Note also that the form of the Thrust and ISP polynomial has some behavior, when the Reference Temperature is near 0 degrees Centigrade, that you need to be aware of. Because of the previously mentioned discontinuity, the polynomials do not vary smoothly when the Reference Temperature is near zero. For example, consider the two Reference Temperatures, -0.011 and + 0.011 degrees Centigrade. These two temperatures are close to each other in value and one might expect that they have roughly similar thrust and ISP values. This may not be the case, depending upon your choice of thrust/ISP coefficients, since the temperature ratios associated with the two Reference Temperatures have the same magnitude but different signs. You may choose to set the input Reference Temperature equal to the input Temperature, thus eliminating any dependence of thrust and ISP with temperature when using the currently implemented ChemicalTank model based upon Boyle’s Law where the fuel Temperature does not change as fuel is depleted.

Create a default ChemicalTank and a ChemicalThruster that allows for fuel depletion, assign the ChemicalThruster the default ChemicalTank, and attach both the ChemicalThruster and ChemicalTank to a Spacecraft.

%  Create the ChemicalTank Resource
Create ChemicalTank FuelTank1
FuelTank1.AllowNegativeFuelMass = false
FuelTank1.FuelMass = 756
FuelTank1.Pressure = 1500
FuelTank1.Temperature = 20
FuelTank1.RefTemperature = 20
FuelTank1.Volume = 0.75
FuelTank1.FuelDensity = 1260
FuelTank1.PressureModel = PressureRegulated

%  Create a ChemicalThruster, that allows fuel depletion, and assign it a ChemicalTank
Create ChemicalThruster Thruster1
Thruster1.CoordinateSystem = Local
Thruster1.Origin = Earth
Thruster1.Axes = VNB
Thruster1.ThrustDirection1 = 1
Thruster1.ThrustDirection2 = 0
Thruster1.ThrustDirection3 = 0
Thruster1.DutyCycle = 1
Thruster1.ThrustScaleFactor = 1
Thruster1.DecrementMass = true
Thruster1.Tank = {FuelTank1}
Thruster1.GravitationalAccel = 9.810000000000001
Thruster1.C1 = 10
Thruster1.C2 = 0
Thruster1.C3 = 0
Thruster1.C4 = 0
Thruster1.C5 = 0
Thruster1.C6 = 0
Thruster1.C7 = 0
Thruster1.C8 = 0
Thruster1.C9 = 0
Thruster1.C10 = 0
Thruster1.C11 = 0
Thruster1.C12 = 0
Thruster1.C13 = 0
Thruster1.C14 = 0
Thruster1.C15 = 0
Thruster1.C16 = 0
Thruster1.K1 = 300
Thruster1.K2 = 0
Thruster1.K3 = 0
Thruster1.K4 = 0
Thruster1.K5 = 0
Thruster1.K6 = 0
Thruster1.K7 = 0
Thruster1.K8 = 0
Thruster1.K9 = 0
Thruster1.K10 = 0
Thruster1.K11 = 0
Thruster1.K12 = 0
Thruster1.K13 = 0
Thruster1.K14 = 0
Thruster1.K15 = 0
Thruster1.K16 = 0

%  Add the ChemicalThruster and the ChemicalTank to a Spacecraft
Create Spacecraft DefaultSC
DefaultSC.Tanks = {FuelTank1}
DefaultSC.Thrusters = {Thruster1}

BeginMissionSequence

Create two ChemicalTanks (called aTank1 and aTank2) and a ChemicalThruster, attach both the ChemicalThruster and ChemicalTanks to a Spacecraft, and configure the thruster to draw four times as much fuel from aTank1 than aTank2.

%  Create the ChemicalTank Resource
Create Spacecraft aSat
aSat.Tanks = {aTank1,aTank2}
aSat.Thrusters = {aThruster}

% Create two tanks
Create ChemicalTank aTank1 aTank2

%  Configure thruster to draw four times as much fuel 
%  from aTank1 than aTank2
Create ChemicalThruster aThruster
aThruster.Tank = {aTank1,aTank2}
aThruster.MixRatio = [4 1]

BeginMissionSequence