Scramjet engine design

[Jacki43]

Hello all,

I want to model a scramjet engine with c++ software, and I am a beginner with JSBSIm. I wanted to ask you is it possible in JSBSim to model a scramjet engine?
Can you please give me a brief guide on how can I do that?
Thanks in advance

[seanmcleod]

I suggest you take a look at the source code for the existing engine models in src\models\propulsion as a first step to get familiar with how engine models are integrated in JSBSim. Which should give you an idea of how to go about adding a scramjet engine model.

[Jacki43]

Thank you for your reply.
I think the engine model that exists in JSBSim which is similar to the scramjet engine, is FGTurboProp.cpp. Because both of them are airbreathing. Consequently, I think I should eliminate the compressor and turbine that are used in the turboprop model and add the nozzle and diffuser into that to implement a scramjet engine. Do you agree with that?
Is there any idea on how to consider diffuser and nozzle to implement scramjet?

Thanks in advance

[seanmcleod]

I was suggesting more that you look at the engine models to get an idea of the classes involved and the methods you need to implement. I wouldn't try and start will all of the FGTurboProp or FGTurbine code and try and remove the compressor modeling.

What is the model (set of equations) you have for a scramjet, what are the inputs required including configuration options to then run through your equations to calculate the amount of thrust given the inputs?

[bcoconni]

I think the engine model that exists in JSBSim which is similar to the scramjet engine, is FGTurboProp.cpp. Because both of them are airbreathing.

Please don't do that. I would recommend against using FGTurboProp.cpp or FGTurbine.cpp as a start for your scramjet model.

Consequently, I think I should eliminate the compressor and turbine that are used in the turboprop model and add the nozzle and diffuser into that to implement a scramjet engine. Do you agree with that?

You can't. There are no compressor/turbine/nozzle in FGTurboProp.cpp and FGTurbine.cpp. They are not engineering models. You will not find any physics/thermodynamics model in them.

As @seanmcleod said, your best course of action is to build a new model from the ground up using the set of equations you have.

Basically, you need to:

• Write a new class Scramjet (or any name that you see fit) that inherits from FGEngine
• Declare the new type of engine in FGPropulsion.cpp. For that, you need to add a new if condition with your model name scramjet and call the constructor of your class as in the code extract below.
  

  jsbsim/src/models/FGPropulsion.cpp

  Lines 417 to 425 in c68ef9f

  	} else if (engine_element->FindElement("electric_engine")) {
  	HaveElectricEngine = true;
  	if (!IsBound) bind();
  	Element *element = engine_element->FindElement("electric_engine");
  	Engines.push_back(make_shared<FGElectric>(FDMExec, element, numEngines, in));
  	} else {
  	cerr << engine_element->ReadFrom() << " Unknown engine type" << endl;
  	return false;
  	}

• For a start you can skip fuel management and use stub methods for anything that is linked to fuel and tanks.
• You should rather use the FGElectric class for an example as it contains the bare minimum for an FGEngine class derivation.
  • In FGElectric you will see that the method Calculate is the place where the engine parameters are computed with respect to the throttle command, atmospheric properties, etc.

[Jacki43]

Thank you so much @bcoconni, @seanmcleod.

I will consider your suggestion to design a scramjet engine. However, at first, I want to clarify something for myself. As you know, the JSBSim-master\engine\PW125B.xml is a turbojet. I wanted to know the trust equation of this turbojet has been defined in which place of the code?
You mentioned that:

They are not engineering models. You will not find any physics/thermodynamics model in them

So, how the trust force of PW125B has been defined in JSBSim?

I wanted to apply the trust equation of scramjet, but I do not know where can I define the equation and parameters (for instance, mass flow rate, pressure, velocity, and area for stream and exit conditions). I thought that if I find the source files of turbojet and try to change the trust equation of turbojet to the equation of scramjet, I will have an initial result for scramjet engine design.
Can you please let me know if there is an idea about that @bcoconni?

Thanks in advance

[seanmcleod]

As @bcoconni mentioned use FGElectric as an example/template for your FGScramjet class. Also take a look at the FGTurbine class to see how it makes use of tables/functions in case you need that sort of functionality.

Looking at the NASA example you linked to:

At a minimum you would need to read in configuration parameters for the area of the exit nozzle Ae and area of the inlet nozzle etc. Here is an example showing how FGTurbine reads in the bypass ratio configuration option.

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 435 to 436 in 33d429e

	if (el->FindElement("bypassratio"))
	BypassRatio = el->FindElementValueAsNumber("bypassratio");

Then in your FGScramjet::Calculate method you would make use of the configuration values plus make use of the input commands like the throttle command and atmospheric properties to calculate the thrust as per the equation shown in the NASA example.

The throttle command and atmospheric inputs are available via:

jsbsim/src/models/propulsion/FGEngine.h

Lines 117 to 142 in 33d429e

	struct Inputs {
	double Pressure;
	double PressureRatio;
	double Temperature;
	double Density;
	double DensityRatio;
	double Soundspeed;
	double TotalPressure;
	double TAT_c;
	double Vt;
	double Vc;
	double qbar;
	double alpha;
	double beta;
	double H_agl;
	FGColumnVector3 AeroUVW;
	FGColumnVector3 AeroPQR;
	FGColumnVector3 PQRi;
	std::vector <double> ThrottleCmd;
	std::vector <double> MixtureCmd;
	std::vector <double> ThrottlePos;
	std::vector <double> MixturePos;
	std::vector <double> PropAdvance;
	std::vector <bool> PropFeather;
	double TotalDeltaT;
	};

Then in FGScramjet::Calculate you'll have:

thrust = NASA equation;
Thruster->Calculate(thrust);

[Jacki43]

Thank you so much for your detailed response.

Then in your FGScramjet::Calculate method you would make use of the configuration values plus make use of the input commands like the throttle command and atmospheric properties to calculate the thrust as per the equation shown in the NASA example.

As you mentioned, I applied the codes that were related to the bypass ratio configuration to the FGSceamjet. However, when I want to use the atmospheric properties in the thrust equation, I have the error: the properties are undefined. Is there any suggestion on how to read these properties? Should be added specific library to the FGScramjet?

Thanks in advance

[bcoconni]

However, when I want to use the atmospheric properties in the thrust equation, I have the error: the properties are undefined. Is there any suggestion on how to read these properties?

Could you please show us the lines of code that cause the compilation errors ?
Could you also show the error messages that you get ?

[Jacki43]

Thank you for your reply. I only applied some changes in the FGElectric file that you mentioned to implement FGScramjet.
The line of code that I got error is Thruster->Calculate(vt-vc); . Please note that vt-vc is a sample thrust formula to test that whether the atmospheric properties can be read.
`
void FGScramjet::Calculate(void)
{
RunPreFunctions();

if (Thruster->GetType() == FGThruster::ttPropeller) {
((FGPropeller*)Thruster)->SetAdvance(in.PropAdvance[EngineNumber]);
((FGPropeller*)Thruster)->SetFeather(in.PropFeather[EngineNumber]);
}

RPM = Thruster->GetRPM() * Thruster->GetGearRatio();

LoadThrusterInputs();
Thruster->Calculate(vt-vc);

RunPostFunctions();
`

Also, the error is:

[seanmcleod]

Vt and Vc are not member variables of FGEngine, take a look at the code snippet I shared earlier, they're part of a struct. If you take a look at the constructor for FGElectric or any of the other engine models you'll see a reference is passed to this struct.

FGElectric::FGElectric(FGFDMExec* exec, Element *el, int engine_number, struct FGEngine::Inputs& input)
  : FGEngine(engine_number, input)
{

Which is then passed to the constructor of FGEngine which is the base class. If you take a look at the FGEngine class you'll see it has the following member variable.

  struct Inputs& in;

You'll see this member variable in actually being used in the propeller code you copied.

Why have you copied the propeller related code into your scramjet model?

[seanmcleod]

@Jacki43 I had mentioned in my first response -

I suggest you take a look at the source code for the existing engine models in src\models\propulsion as a first step to get familiar with how engine models are integrated in JSBSim.

The idea being to understand how engine models are structured, what base classes to inherit from, where and how they get their configuration data, where and how they get their required input parameters from etc.

[Jacki43]

Thank you so much for your response. I got the point.
I should use the Vt, Vc in the formula as follows:
Thruster->Calculate(in.Vt-in.Vc);
I tested it and I do not have any error right now.

Why have you copied the propeller related code into your scramjet model?

I just wanted to try to apply the thrust equation, so I did not check all of the things. As you mentioned, I should go through the sample engine codes and make sure I am designing the scramjet engine in the right way.

It will be great if this issue stays open so that I can ask my questions and get your opinions that are very useful for me.
Thanks in advance

[bcoconni]

I should use the Vt, Vc in the formula as follows:
Thruster->Calculate(in.Vt-in.Vc);

Not quite so. The Thruster instance must be supplied with a thrust not a velocity.
In addition, I am pretty sure the difference between the true airspeed in.Vt and the calibrated airspeed in.Vc is irrelevant to the computation of a ramjet thrust.

[Jacki43]

Thank you so much for your help.

I am trying to implement the scramjet NASA thrust equation as I mentioned. However, all of the variables that I need, do not exist in jsbsim/src/models/propulsion/FGEngine.h. I need the pressure, mass flow rate, and velocity of both stream and exit conditions.
Do you think for the exit condition (nozzle), should I use the variables that exist in jsbsim/src/models/propulsion/FGNozzle.cpp?
I think the velocity of stream condition is Vt. What about the velocity of exit condition, and what is its variable?

Thanks in advance

[bcoconni]

I need the pressure, mass flow rate, and velocity of both stream and exit conditions.

You will have to compute them. The mass flow rate being constrained by the nozzle exit area which I guess is always chocked for a ramjet.

[seanmcleod]

Yes at some point in most cases you won't have every single variable available but will need to calculate and derive the variables you need from the existing variables.

So for example how would you derive the incoming mass flow rate?

[Jacki43]

Thank you for your reply.
I rewrote the thrust equation in such a way that I use the existing variables. However, before applying I wanted to ask you please let me know what are thePressure and Density. Are they related to the exit condition?
I found a description for Vt :

velocities/vt-fps is the total airspeed (in feet per second) : it is the
norm of the aircraft velocity with respect to the air measured in the body
frame.

Consequently, we have Ve= Vt+V0. Is it true?
Thanks in advance

[bcoconni]

However, before applying I wanted to ask you please let me know what are thePressure and Density. Are they related to the exit condition?

Pressure and Density are the air pressure and density just upstream of the aircraft. From the scramjet point of view, Pressure is the static pressure. You'll need to convert it to stagnation pressure to compute the conditions at the entry of the scramjet. And you will have to do the same for the temperature.

Obviously these conditions colud be significantly different at the exit. In the case of an ideal ramjet:

• The pressure will be the same at the exit than it was at the entry/inlet
• The temperature will be higher
• The density will be lower (as a result of the ideal gas relationship P=rho*R*T)

Consequently, we have Ve= Vt+V0. Is it true?

True airspeed Vt and Equivalent airspeed Ve are not linked by a linear relationship as can be seen in Wikipedia article on true airspeed

[seanmcleod]

let me know what are the Pressure and Density. Are they related to the exit condition?

Did you search through the code to see where and how they're set?

jsbsim/src/FGFDMExec.cpp

Lines 501 to 525 in 193d28c

	case ePropulsion:
	Propulsion->in.Pressure = Atmosphere->GetPressure();
	Propulsion->in.PressureRatio = Atmosphere->GetPressureRatio();
	Propulsion->in.Temperature = Atmosphere->GetTemperature();
	Propulsion->in.DensityRatio = Atmosphere->GetDensityRatio();
	Propulsion->in.Density = Atmosphere->GetDensity();
	Propulsion->in.Soundspeed = Atmosphere->GetSoundSpeed();
	Propulsion->in.TotalPressure = Auxiliary->GetTotalPressure();
	Propulsion->in.Vc = Auxiliary->GetVcalibratedKTS();
	Propulsion->in.Vt = Auxiliary->GetVt();
	Propulsion->in.qbar = Auxiliary->Getqbar();
	Propulsion->in.TAT_c = Auxiliary->GetTAT_C();
	Propulsion->in.AeroUVW = Auxiliary->GetAeroUVW();
	Propulsion->in.AeroPQR = Auxiliary->GetAeroPQR();
	Propulsion->in.alpha = Auxiliary->Getalpha();
	Propulsion->in.beta = Auxiliary->Getbeta();
	Propulsion->in.TotalDeltaT = dT * Propulsion->GetRate();
	Propulsion->in.ThrottlePos = FCS->GetThrottlePos();
	Propulsion->in.MixturePos = FCS->GetMixturePos();
	Propulsion->in.ThrottleCmd = FCS->GetThrottleCmd();
	Propulsion->in.MixtureCmd = FCS->GetMixtureCmd();
	Propulsion->in.PropAdvance = FCS->GetPropAdvance();
	Propulsion->in.PropFeather = FCS->GetPropFeather();
	Propulsion->in.H_agl = Propagate->GetDistanceAGL();
	Propulsion->in.PQRi = Propagate->GetPQRi();

So they're the free stream values, the 0 subscript values in the NASA equation.

Also, think about it, how would the generic base class FGEngine know anything about your specific engine to figure out how to calculate an exit value? What if you're modeling an electric engine?

[Jacki43]

Thank you for your quick response.

Also, think about it, how would the generic base class FGEngine know anything about your specific engine to figure out how to calculate an exit value? What if you're modeling an electric engine?

I did not find jsbsim/src/FGFDMExec.cpp, so I was confused.
Thank you so much.

[bcoconni]

I did not find jsbsim/src/FGFDMExec.cpp, so I was confused.

May be you could search for src/FGFDMExec.cpp?

[Jacki43]

Thank you for your reply @bcoconni. I searched and found it. Just one question is that what is the property of "direct" trust value that I could put in notify section and get its value during the flight?
When I use <property>propulsion/engine/thrust-lbs</property>, its value is 0.

Also, the value of the properties <property>atmosphere/sigma</property> and <property>atmosphere/rho-slugs_ft3</property> that I used them in the thrust equation, are 0. I do not have any errors in my code. However, when I change the engine type from the rocket engine to the electric engine, the value of these properties becomes 0. Do you have any idea that this problem is related to which part of the code?

[bcoconni]

Just one question is that what is the property of "direct" trust value that I could put in notify section and get its value during the flight?
When I use <property>propulsion/engine/thrust-lbs</property>, its value is 0.

It is most likely because you did not pass the thrust value (in pound-force) to Thruster in your C++ class:

  Thruster->Calculate(thrust);

Also, the value of the properties <property>atmosphere/sigma</property> and <property>atmosphere/rho-slugs_ft3</property> that I used them in the thrust equation, are 0.

• Air density in slug/ft^3 is available from the variable in.Density in your C++ class.
• sigma a.k.a. density ratio is available from in.DensityRatio.

[Jacki43]

Thanks for your reply.

It is most likely because you did not pass the thrust value (in pound-force) to Thruster in your C++ class.

I had defined the thruster equation in Thruster in my code as follows. Do you think is there anything wrong with this part of the cod?

[bcoconni]

I see a number of errors in your code:

• in.PressureRatio is the ratio of the ambient pressure to the sea level pressure.
  

  jsbsim/src/models/FGAtmosphere.h

  Lines 152 to 153 in d528f8b

  	/// Returns the ratio of at-altitude pressure over the sea level value.
  	virtual double GetPressureRatio(void) const { return Pressure/SLpressure; }

• in.DensityRatio is the ratio of the ambient density to the sea level density.
  

  jsbsim/src/models/FGAtmosphere.h

  Lines 175 to 176 in d528f8b

  	/// Returns the ratio of at-altitude density over the sea level value.
  	virtual double GetDensityRatio(void) const { return Density/SLdensity; }

• I guess that pe, re, ve, Ae and mdote are respectively the pressure, density, velocity, area and mass flow at the ramjet exit ? If that is indeed the case then your equations are incorrect. Since you are modelling a perfect ramjet, it is assumed that there are no pressure losses and that there are no compressors/turbines therefore the stagnation pressure at exit equals the stagnation pressure at inlet. In addition, as explained in the MIT site that I mentioned above, the inlet and exit Mach numbers are also the same. Hence the inlet and exit static pressure are equal as well i.e. pe == po at any time.

• The exit temperature Te is a result of the temperature rise that occurred in the combustion chamber. I see no such computation of the temperature in your model.

• The exit density re is then obtained from the ideal gas law: re=pe/(R*Te)

• The computation of the exit velocity ve is using the Bernoulli's principle for incompressible fluids which is incorrect. Gases are generally assumed to be incompressible for Mach numbers lower than 0.3 but a ramjet is a propulsion device for supersonic/hypersonic vehicles so the hypothesis of incompressibility falls.

• The computation of the exit area Ae breaks the conversation of mass. The form that you are using could be valid for incompressible fluids (i.e. when re == ro) but, as explained above, this assumption is not met. So you should have re*Ae*ve == ro*Ao*vo at any time.

Have you checked that the value of Thrust is not zero ? If you are running your simulation at sea level then in.DensityRatio and in.PressureRatio are equal to 1.0 which should explain why Thrust is equal to 0.0.

[bcoconni]

This issue (#392) has been moved to Discussions.

Support requests and questions are now resolved in Discussions while Issues are restricted to bugs reports and feature requests.

This change is only motivated by project organization and will not affect the support provided. We'll gladly continue to answer your questions 😄.

[seanmcleod]

I was also going to mention that a number of the parameter calculations didn't seem to make sense, but my main question is what sort of debugging have you tried?

In particular if you set a breakpoint on line 120 where the thrust is calculated what value do you see calculated? If it is 0 then that answers your question below. If it is 0 then go back and check the parameters you use to calculate thrust, do they make sense, are there calculation bugs?

When I use propulsion/engine/thrust-lbs, its value is 0.

If your thrust calculation isn't 0 then step into Thruster->Calculate(thrust). What do you see happening in that method in the debugger?

What thruster do you have connected in your model? I'm a bit suspicious that you've added your code to FGElectric and so I'm wondering what sort of thruster you have connected. Why haven't you implemented your own FGRamJet class?

[Jacki43]

Thank you so much for your detailed answer @bcoconni. My assumption was wrong and you are right. I applied some new equations with the help of this website and this one, so the value of thrust has not been zero. However, when I apply this scramjet model to X15 (instead of the XLR99 that it is already used), the value of thrust in the "climb" event is: propulsion/engine/thrust-lbs = -265.837163. It is very low and negative. I wanted to make sure that whether this result is related to the equations that I applied or any other part of the code. Consequently, please let me describe here the changes that I applied to have a scramjet engine @seanmcleod .

I changed the engine and thruster type as follows:

    <propulsion>
        <engine file="scramjet">
            <location unit="IN">
                <x> 600 </x>
                <y> 0 </y>
                <z> 0 </z>
            </location>
            <orient unit="DEG">
                <roll> 0.0 </roll>
                <pitch> 0 </pitch>
                <yaw> 0 </yaw>
            </orient>
            <feed>0</feed>
            <feed>1</feed>
            <thruster file="direct">
                <location unit="IN">
                    <x> 600 </x>
                    <y> 0 </y>
                    <z> 0 </z>
                </location>
                <orient unit="DEG">
                    <roll> 0.0 </roll>
                    <pitch> 0.0 </pitch>
                    <yaw> 0.0 </yaw>
                </orient>
            </thruster>
        </engine>
        <tank type="FUEL">    <!-- Tank number 0 for Main engine -->
            <location unit="IN">
                <x> 282.3 </x>
                <y> 0 </y>
                <z> 0 </z>
            </location>
            <capacity unit="LBS"> 9470 </capacity>
            <contents unit="LBS"> 0 </contents>
        </tank>
        <tank type="FUEL">    <!-- Tank number 1 for Main engine -->
            <location unit="IN">
                <x> 408.3 </x>
                <y> 0 </y>
                <z> 0 </z>
            </location>
            <capacity unit="LBS"> 8236 </capacity>
            <contents unit="LBS"> 6 </contents>
        </tank>
        <tank type="FUEL">    <!-- Tank number 2 for RCS -->
            <location unit="IN">
                <x> 140 </x>
                <y> 0 </y>
                <z> 0 </z>
            </location>
            <capacity unit="LBS"> 12 </capacity>
            <contents unit="LBS"> 0 </contents>
        </tank>
    </propulsion>

The content of scramjet.xml engine file is:

      <electric_engine name="scramjet">
         <augmented>         0  </augmented>
         <injected>          0  </injected>
       </electric_engine>

Finally, the equations that I applied for thrust are:

[bcoconni]

Have you checked your units ?

• The gas constant R is 287.1 in J/K/kg (I'm surprised to read 2871 which seems 10 times as much ?)
• in.Pressure is given in psf (pound-force per square feet)
• in.Temperature is given in Rankine
• in.Vt in feet per second
• cp in SI units is gama*R/(gama-1) which is approximately 1004 in J/K/kg

So ro is computed from quantities expressed in a mixture of SI units and imperial/US/UK units and is most likely incorrect. In any case the air density is already available from the variable in.Density which is expressed in slug/ft^3. And since the variable mdot in your program is obtained from ro then the mass rate is most likely incorrect as well.

I would rather use a constant value for te because this is how a combustion chamber basically works: at full thrust you burn as much fuel as you can to produce the maximum heat that the materials can withstand. This is especially true for a scramjet where you are not limited by the maximum rotational speed that the rotors can withstand.

[bcoconni]

According to the site you mentioned NPR should be higher than 1.0 (total pressure is always higher than static pressure) and can easily be computed (assuming there are no pressure losses in the scramjet): NPR = in.TotalPressure/in.Pressure.

Also I would leave efficiencies apart for a start (eta = 1.0).

[bcoconni]

I guess you replaced 1-(1/NPR)^(gamma-1)/gamma by 0.03 because you got a negative number using NPR=0.9 ?

[seanmcleod]

I wanted to make sure that whether this result is related to the equations that I applied

So when you set a breakpoint on Thrust = mdot * (ve - vo); what do you see? If thrust is negative then either mdot is negative or ve - vo is negative.

[Jacki43]

Hello all,
I faced two issues.

-First, when I use the XLR99 engine, the value of Mach is 4 (in Apogee) while in the scramjet engine, Mach is less than 1. The scramjet should produce more thrust than the rocket engine, so the velocity should be more than that! I do not know is from which part of my code. When I multiply thrust by 10, the velocity reaches Mach 4.

-Furthermore, the propulsion/tank/contents-lbs do not decrease during the flight and are constant. Are there specific properties for this task that should be defined in the engine script file?

Is there any idea on these issues?
The following codes show the comparison of x15 in two different runs (one with XLR99 engine and another one with scramjet engine).

Apogee event ( run with XLR99 engine):

Apogee (Event 5) executed at time: 187.430000
velocities/mach = 4.141563
velocities/h-dot-fps = -0.198912
position/h-agl-ft = 274216.626387
attitude/pitch-rad = -0.013265
fcs/throttle-pos-norm = 0.000000
propulsion/engine/thrust-lbs = 0.000000
propulsion/tank[0]/contents-lbs = 0.000000
propulsion/tank[1]/contents-lbs = 59.514250
propulsion/tank[2]/contents-lbs = 12.000000

Apogee event ( run with scramjet engine):

Apogee (Event 5) executed at time: 71.220000
velocities/mach = 0.725899
velocities/h-dot-fps = -0.060488
position/h-agl-ft = 48040.043519
attitude/pitch-rad = 0.278655
fcs/throttle-pos-norm = 1.000000
propulsion/engine/thrust-lbs = 7550.042961
propulsion/tank[0]/contents-lbs = 9400.000000
propulsion/tank[1]/contents-lbs = 8236.000000
propulsion/tank[2]/contents-lbs = 12.000000

Thanks in advance.

[seanmcleod]

Well there are massive differences in this single data point (apogee) right?

The XLR99 combination took 187s to reach apogee, and it reached 274,261ft in altitude, versus only 71s and an altitude of only 48,040ft for your scramjet combination.

So given these massive differences I wouldn't be concerned about the difference in Mach at apogee.

Are you logging the data at even just 1Hz and comparing the thrust produced for the two combinations to compare? My guess (given the very limited data you've provided) is that your scramjet is producing a lot less thrust all the time compared to the XLR99.

[bcoconni]

First, when I use the XLR99 engine, the value of Mach is 4 (in Apogee) while in the scramjet engine, Mach is less than 1. The scramjet should produce more thrust than the rocket engine, so the velocity should be more than that! I do not know is from which part of my code. When I multiply thrust by 10, the velocity reaches Mach 4.

Ramjets are notoriously inefficient at low Mach numbers / subsonic velocities. You can plot your ramjet thrust against Mach to convince yourself: you should see a bell curve with a peak in the vicinity of Mach 2 or 3 and thrust decreasing when flying above or below that velocity like below.

This is the reason why vehicles equipped with ramjets are first accelerated with a rocket-like motor before the ramjet is ignited.

The other reason might be that your ramjet peak thrust is too low. In that case, you have 2 options to increase the thrust:

1. Increase the combustion temperature to a higher value (increase the value of tmax - according to the code you published earlier). This will increase the exhaust velocity and therefore increase the thrust.
2. Increase the mass flow by increasing the engine section Ao. Same as above: this will result in the increase of mdot and therefore increase the thrust as well.

Furthermore, the propulsion/tank/contents-lbs do not decrease during the flight and are constant. Are there specific properties for this task that should be defined in the engine script file?

For that you need to implement the fuel consumption in the method CalcFuelNeed which must return the amount of fuel (in pounds) that has been burnt in the last time step in.TotalDeltaT

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 380 to 386 in 518f2a7

	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;
	}

[Jacki43]

Increase the mass flow by increasing the engine section Ao. Same as above: this will result in the increase of mdot and therefore increase the thrust as well.

Thank you so much for your detailed answer. I had this idea in my mind, but I was not sure whether it is reasonable. As you mentioned, I increased the Ao and I got the result that I wanted.

For that you need to implement the fuel consumption in the method CalcFuelNeed which must return the amount of fuel (in pounds) that has been burnt in the last time step in.TotalDeltaT

As you mentioned, I considered the following code in my FGElectric.cpp file. However, I do not have fuel consumption yet. Is there any idea regarding this issue?

[bcoconni]

Basically fuel is used to heat up the air in the combustion chamber so you need a bit of thermodynamics to evaluate how much fuel is used.

1. The amount of energy stored in the air is the enthalpy which is basically H=mdot*Cp*T where T is the gas total temperature, Cp the specific heat at constant pressure and mdot the air flow.
2. The combustion chamber increases the enthalpy by burning a fuel quantity mdot_fuel (in kg/s) which has a Lower Heating Value Q_fuel (in J/kg).
3. So the air enthalpy H increases by mdot_fuel*Q_fuel when being heated up in the combustion chamber.
4. Using your variable names, the specific enthalpy before the combustion chamber is Cp*to and after the combustion chamber is Cp*te. So we have mdot_fuel*Q_fuel=mdot*(Cp*te-Cp*to) and finally mdot_fuel=Cp*(te-to)*mdot/Q_fuel.

You have already evaluated Cp to be 1004 (J/kg/K) and Q_fuel is typically 43e6 (J/kg) for kerosene (search "Lower Heating Value" on internet for other fuels such as methane, hydrogen, etc.).

Note that CalcFuelNeed is expected the return a mass of fuel in lbs so you need to integrate the fuel flow and convert the result to pounds if needed:

double FGElectric::CalcFuelNeed(void)
{
    const double Q_fuel = 43e6; // Kerosene (J/kg)
    FuelFlowRate = mdot*Cp*(te-to)/Q_fuel; // kg/s

    FuelFlowRate *= kgtolb ; // Convert from kg/s to lb/s
    FuelExpended = FuelFlowRate * in.TotalDeltaT;
    if (!Starved) FuelUsedLbs += FuelExpended;
    return FuelExpended;
}

Note that this is a very simplified evaluation of what actually is going on in a combustion chamber but I think this should meet your needs for a start.

[seanmcleod]

When the breakpoint is hit, the line hasn't been executed yet. The debugger is stopping the program before it executes the line, so the value you're seeing for the local variable Thrust is some uninitialized value.

Execute the line, in Visual Studio hit F-10 which will execute the line and then stop on the next line. But as you can see, ve < vo.