Zero fuel consumption/constant mass

[sthelia]

Hi!
I'm working on testing some flight mechanic properties and would like to test some settings where I keep the mass of the aircraft constant, ie no fuel is being consumed.
I've been looking through the source code to find how the fuel is being used to see if I could set a property to 0 (I tried the tsfc property for my turbine engine, that did not work. I read the discussion on TSFC #381 and am thinking that that might be the reason it doesn't work to set tsfc to zero in the engine file.

I found one line that seems to be where this is calculated:

https://github.com/airware-legacy/jsbsim/blob/c70bd1a61efeb0514984e3fe3e5bfb5ca9236d9e/src/models/FGPropulsion.cpp#L259-L260

but then:

jsbsim/src/models/propulsion/FGEngine.cpp

Lines 95 to 101 in 375f5be

	double FGEngine::CalcFuelNeed(void)
	{
	FuelFlowRate = SLFuelFlowMax*PctPower;
	FuelExpended = FuelFlowRate*in.TotalDeltaT;
	if (!Starved) FuelUsedLbs += FuelExpended;
	return FuelExpended;
	}

So that requires me to understand where SLFuelFlowMax comes from, which I only find Rocket engine references to. My engine is currently a turbine engine, but I would like to be able to set the fuel consumption to zero for other kinds of engines as well.

[seanmcleod]

The FGEngine::CalcFuelNeed() method you found is the base class implementation, however most of the derived classes like FGTurbine override the CalcFueldNeed() method.

So for example for the FGTurbine case doesn't make use of SLFuelFlowMax:

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 380 to 386 in c68ef9f

	double FGTurbine::CalcFuelNeed(void)
	{
	FuelFlowRate = FuelFlow_pph / 3600.0; // Calculates flow in lbs/sec from lbs/hr
	FuelExpended = FuelFlowRate * in.TotalDeltaT; // Calculates fuel expended in this time step
	if (!Starved) FuelUsedLbs += FuelExpended;
	return FuelExpended;
	}

And in the FGPiston case:

jsbsim/src/models/propulsion/FGPiston.cpp

Lines 531 to 536 in bcd3f98

	double FGPiston::CalcFuelNeed(void)
	{
	FuelExpended = FuelFlowRate * in.TotalDeltaT;
	if (!Starved) FuelUsedLbs += FuelExpended;
	return FuelExpended;
	}

So I'm not sure there is a trivial way to ensure that all engine types don't consume fuel.

For the turbine case it looks like if you set TFSC to 0 you'll end up with the idle fuel flow rate.

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 222 to 224 in c68ef9f

	correctedTSFC = TSFC->GetValue();
	FuelFlow_pph = Seek(&FuelFlow_pph, thrust * correctedTSFC, 1000.0, 10000.0);
	if (FuelFlow_pph < IdleFF) FuelFlow_pph = IdleFF;

Where IdleFF is calcuated by:

jsbsim/src/models/propulsion/FGTurbine.cpp

Line 524 in c68ef9f

IdleFF = pow(MilThrust, 0.2) * 107.0; // just an estimate

[sthelia]

Ah, I see!
In that case, does it currently require me to modify the source code to set the flow rate to zero?

Maybe it will be easier to add fuel in the same rate as it is being consumed, if that's possible.

[seanmcleod]

Yep, hacking the source code is one option.

The other option, sort of in the vein of refueling at the same rate the fuel is being burnt, and which wouldn't require modifying the source code is to add an option in the script file to continuously set/reset the fuel tank(s) contents to the original contents specified at startup via the contents-lbs property of the tank(s).

jsbsim/src/models/propulsion/FGTank.cpp

Lines 452 to 458 in f4ab491

	void FGTank::bind(FGPropertyManager* PropertyManager)
	{
	string property_name, base_property_name;
	base_property_name = CreateIndexedPropertyName("propulsion/tank", TankNumber);
	property_name = base_property_name + "/contents-lbs";
	PropertyManager->Tie( property_name.c_str(), (FGTank*)this, &FGTank::GetContents,
	&FGTank::SetContents );

[bcoconni]

In addition to @seanmcleod's suggestions, you can consider :

• Using an electric engine which, by definition, does not burn fuel.
• Replacing the engine by a thrust force with <external_reactions> (below is an example of how to use <external_reactions> to model a parachute)

jsbsim/aircraft/ball/ball.xml

Lines 100 to 132 in 375f5be

	<external_reactions>
	<!-- "Declare" the reefing term -->
	<property>fcs/parachute_reef_pos_norm</property>
	<force name="parachute" frame="WIND">
	<function>
	<product>
	<property>aero/qbar-psf</property>
	<property>fcs/parachute_reef_pos_norm</property>
	<value> 1.0 </value> <!-- Full drag coefficient -->
	<value> 20.0 </value> <!-- Full parachute area -->
	</product>
	</function>
	<!-- The location below is in structural frame (x positive
	aft), so this location describes a point 1 foot aft
	of the origin. In this case, the origin is the center. -->
	<location unit="FT">
	<x>1</x>
	<y>0</y>
	<z>0</z>
	</location>
	<!-- The direction describes a unit vector. In this case, since
	the selected frame is the WIND frame, the "-1" x component
	describes a direction exactly opposite of the direction
	into the wind vector. That is, the direction specified below
	is the direction that the drag force acts in. -->
	<direction>
	<x>-1</x>
	<y>0</y>
	<z>0</z>
	</direction>
	</force>
	</external_reactions>

[seanmcleod]

@sthelia another option I came across recently is the SetFuelFreeze option, see - #462