FGRocket Class Reference

Detailed Description

Models a generic rocket engine.

The rocket engine is modeled given the following parameters:

• Specific Impulse (in sec)

Additionally, the following control inputs, operating characteristics, and location are required, as with all other engine types:

• Throttle setting (in percent, from 0 to 1.0)
• Maximum allowable throttle setting
• Minimum working throttle setting
• Sea level fuel flow at maximum thrust
• Sea level oxidizer flow at maximum thrust
• X, Y, Z location in structural coordinate frame
• Pitch and Yaw

The nozzle exit pressure (p2) is returned via a call to FGNozzle::GetPowerRequired(). This exit pressure is used to get the at-altitude thrust level.

One can model the thrust of a solid rocket by providing a normalized thrust table as a function of time. For instance, the space shuttle solid rocket booster normalized thrust value looks roughly like this:

    
 <thrust_table name="propulsion/thrust_time" type="internal">
   <tableData>
      0.0   0.00
      0.2   0.91
      8.0   0.97
     16.0   0.99
     20.0   1.00
     21.0   1.00
     24.0   0.95
     32.0   0.85
     40.0   0.78
     48.0   0.72
     50.0   0.71
     52.0   0.71
     56.0   0.73
     64.0   0.78
     72.0   0.82
     80.0   0.81
     88.0   0.73
     96.0   0.69
    104.0   0.59
    112.0   0.46
    120.0   0.09
    132.0   0.00
   </tableData>
 </thrust_table>

T
he left column is time, the right column is normalized thrust. Inside the FGRocket class code, the maximum thrust is calculated, and multiplied by this table. The Rocket class also tracks burn time. All that needs to be done is for the rocket engine to be throttle up to 1. At that time, the solid rocket fuel begins burning and thrust is provided.

@author Jon S. Berndt
@see FGNozzle,
FGThruster,
FGForce,
FGEngine,

Definition at line 125 of file FGRocket.h.

#include <FGRocket.h>

Inheritance diagram for FGRocket:

Collaboration diagram for FGRocket:

Public Member Functions
	FGRocket (FGFDMExec *exec, Element *el, int engine_number, struct FGEngine::Inputs &input)
	Constructor.
	~FGRocket (void)
	Destructor.
double	CalcFuelNeed (void)
	The fuel need is calculated based on power levels and flow rate for that power level.
double	CalcOxidizerNeed (void)
	The oxidizer need is calculated based on power levels and flow rate for that power level.
void	Calculate (void)
	Determines the thrust.
std::string	GetEngineLabels (const std::string &delimiter)
std::string	GetEngineValues (const std::string &delimiter)
bool	GetFlameout (void)
	Gets the flame-out status.
double	GetIsp (void) const
double	GetMixtureRatio (void) const
double	GetOxiFlowRate (void) const
double	GetThrustVariation (void) const
	Returns the thrust variation, if any.
double	GetTotalImpulse (void) const
	Gets the total impulse of the rocket.
double	GetTotalIspVariation (void) const
	Returns the Total Isp variation, if any.
double	GetVacTotalImpulse (void) const
	Gets the total impulse of the rocket.
void	SetIsp (double isp)
void	SetMixtureRatio (double mix)
void	SetThrustVariation (double var)
	Sets the thrust variation for a solid rocket engine.
void	SetTotalIspVariation (double var)
	Sets the variation in total motor energy.
Public Member Functions inherited from FGEngine
	FGEngine (int engine_number, struct Inputs &input)
virtual const FGColumnVector3 &	GetBodyForces (void)
virtual bool	GetCranking (void) const
virtual double	getFuelFlow_gph () const
virtual double	getFuelFlow_pph () const
virtual double	GetFuelFlowRate (void) const
virtual double	GetFuelFlowRateGPH (void) const
virtual double	GetFuelUsedLbs (void) const
virtual const FGColumnVector3 &	GetMoments (void)
virtual const std::string &	GetName (void) const
size_t	GetNumSourceTanks () const
virtual double	GetPowerAvailable (void)
virtual bool	GetRunning (void) const
unsigned int	GetSourceTank (unsigned int i) const
virtual bool	GetStarter (void) const
virtual bool	GetStarved (void) const
virtual double	GetThrottleMax (void) const
virtual double	GetThrottleMin (void) const
virtual double	GetThrust (void) const
FGThruster *	GetThruster (void) const
EngineType	GetType (void) const
virtual int	InitRunning (void)
void	LoadThruster (FGFDMExec *exec, Element *el)
void	LoadThrusterInputs ()
virtual void	ResetToIC (void)
	Resets the Engine parameters to the initial conditions.
virtual void	SetFuelDensity (double d)
virtual void	SetFuelFreeze (bool f)
virtual void	SetName (const std::string &name)
virtual void	SetRunning (bool bb)
virtual void	SetStarter (bool s)
virtual void	SetStarved (bool tt)
virtual void	SetStarved (void)
Public Member Functions inherited from FGModelFunctions
std::string	GetFunctionStrings (const std::string &delimeter) const
	Gets the strings for the current set of functions.
std::string	GetFunctionValues (const std::string &delimeter) const
	Gets the function values.
std::shared_ptr< FGFunction >	GetPreFunction (const std::string &name)
	Get one of the "pre" function.
bool	Load (Element *el, FGFDMExec *fdmex, std::string prefix="")
void	PostLoad (Element *el, FGFDMExec *fdmex, std::string prefix="")
void	PreLoad (Element *el, FGFDMExec *fdmex, std::string prefix="")
void	RunPostFunctions (void)
void	RunPreFunctions (void)
Public Member Functions inherited from FGJSBBase
	FGJSBBase ()
	Constructor for FGJSBBase.
virtual	~FGJSBBase ()
	Destructor for FGJSBBase.
void	disableHighLighting (void)
	Disables highlighting in the console output.

Additional Inherited Members
Public Types inherited from FGEngine
enum	EngineType {   etUnknown , etRocket , etPiston , etTurbine ,   etTurboprop , etElectric }
Public Types inherited from FGJSBBase
enum	{ eL = 1 , eM , eN }
	Moments L, M, N. More...
enum	{ eP = 1 , eQ , eR }
	Rates P, Q, R. More...
enum	{ eU = 1 , eV , eW }
	Velocities U, V, W. More...
enum	{ eX = 1 , eY , eZ }
	Positions X, Y, Z. More...
enum	{ ePhi = 1 , eTht , ePsi }
	Euler angles Phi, Theta, Psi. More...
enum	{ eDrag = 1 , eSide , eLift }
	Stability axis forces, Drag, Side force, Lift. More...
enum	{ eRoll = 1 , ePitch , eYaw }
	Local frame orientation Roll, Pitch, Yaw. More...
enum	{ eNorth = 1 , eEast , eDown }
	Local frame position North, East, Down. More...
enum	{ eLat = 1 , eLong , eRad }
	Locations Radius, Latitude, Longitude. More...
enum	{   inNone = 0 , inDegrees , inRadians , inMeters ,   inFeet }
	Conversion specifiers. More...
Static Public Member Functions inherited from FGJSBBase
static const std::string &	GetVersion (void)
	Returns the version number of JSBSim.
static constexpr double	KelvinToFahrenheit (double kelvin)
	Converts from degrees Kelvin to degrees Fahrenheit.
static constexpr double	CelsiusToRankine (double celsius)
	Converts from degrees Celsius to degrees Rankine.
static constexpr double	RankineToCelsius (double rankine)
	Converts from degrees Rankine to degrees Celsius.
static constexpr double	KelvinToRankine (double kelvin)
	Converts from degrees Kelvin to degrees Rankine.
static constexpr double	RankineToKelvin (double rankine)
	Converts from degrees Rankine to degrees Kelvin.
static constexpr double	FahrenheitToCelsius (double fahrenheit)
	Converts from degrees Fahrenheit to degrees Celsius.
static constexpr double	CelsiusToFahrenheit (double celsius)
	Converts from degrees Celsius to degrees Fahrenheit.
static constexpr double	CelsiusToKelvin (double celsius)
	Converts from degrees Celsius to degrees Kelvin.
static constexpr double	KelvinToCelsius (double kelvin)
	Converts from degrees Kelvin to degrees Celsius.
static constexpr double	FeetToMeters (double measure)
	Converts from feet to meters.
static bool	EqualToRoundoff (double a, double b)
	Finite precision comparison.
static bool	EqualToRoundoff (float a, float b)
	Finite precision comparison.
static bool	EqualToRoundoff (float a, double b)
	Finite precision comparison.
static bool	EqualToRoundoff (double a, float b)
	Finite precision comparison.
static constexpr double	Constrain (double min, double value, double max)
	Constrain a value between a minimum and a maximum value.
static constexpr double	sign (double num)
Public Attributes inherited from FGEngine
struct Inputs &	in
Static Public Attributes inherited from FGJSBBase
static char	highint [5] = {27, '[', '1', 'm', '\0' }
	highlights text
static char	halfint [5] = {27, '[', '2', 'm', '\0' }
	low intensity text
static char	normint [6] = {27, '[', '2', '2', 'm', '\0' }
	normal intensity text
static char	reset [5] = {27, '[', '0', 'm', '\0' }
	resets text properties
static char	underon [5] = {27, '[', '4', 'm', '\0' }
	underlines text
static char	underoff [6] = {27, '[', '2', '4', 'm', '\0' }
	underline off
static char	fgblue [6] = {27, '[', '3', '4', 'm', '\0' }
	blue text
static char	fgcyan [6] = {27, '[', '3', '6', 'm', '\0' }
	cyan text
static char	fgred [6] = {27, '[', '3', '1', 'm', '\0' }
	red text
static char	fggreen [6] = {27, '[', '3', '2', 'm', '\0' }
	green text
static char	fgdef [6] = {27, '[', '3', '9', 'm', '\0' }
	default text
static short	debug_lvl = 1
Protected Member Functions inherited from FGEngine
void	Debug (int from)
bool	Load (FGFDMExec *exec, Element *el)
Protected Member Functions inherited from FGModelFunctions
virtual bool	InitModel (void)
Static Protected Member Functions inherited from FGJSBBase
static std::string	CreateIndexedPropertyName (const std::string &Property, int index)
Protected Attributes inherited from FGEngine
bool	Cranking
const int	EngineNumber
double	FuelDensity
double	FuelExpended
double	FuelFlow_gph
double	FuelFlow_pph
double	FuelFlowRate
bool	FuelFreeze
double	FuelUsedLbs
double	MaxThrottle
double	MinThrottle
std::string	Name
double	PctPower
bool	Running
double	SLFuelFlowMax
std::vector< int >	SourceTanks
bool	Starter
bool	Starved
FGThruster *	Thruster
EngineType	Type
Protected Attributes inherited from FGModelFunctions
FGPropertyReader	LocalProperties
std::vector< std::shared_ptr< FGFunction > >	PostFunctions
std::vector< std::shared_ptr< FGFunction > >	PreFunctions
Static Protected Attributes inherited from FGJSBBase
static constexpr double	radtodeg = 180. / M_PI
static constexpr double	degtorad = M_PI / 180.
static constexpr double	hptoftlbssec = 550.0
static constexpr double	psftoinhg = 0.014138
static constexpr double	psftopa = 47.88
static constexpr double	fttom = 0.3048
static constexpr double	ktstofps = 1852./(3600*fttom)
static constexpr double	fpstokts = 1.0 / ktstofps
static constexpr double	inchtoft = 1.0/12.0
static constexpr double	m3toft3 = 1.0/(fttom*fttom*fttom)
static constexpr double	in3tom3 = inchtoft*inchtoft*inchtoft/m3toft3
static constexpr double	inhgtopa = 3386.38
static constexpr double	slugtolb = 32.174049
	Note that definition of lbtoslug by the inverse of slugtolb and not to a different constant you can also get from some tables will make lbtoslug*slugtolb == 1 up to the magnitude of roundoff.
static constexpr double	lbtoslug = 1.0/slugtolb
static constexpr double	kgtolb = 2.20462
static constexpr double	kgtoslug = 0.06852168
static const std::string	needed_cfg_version = "2.0"
static const std::string	JSBSim_version = JSBSIM_VERSION " " __DATE__ " " __TIME__

Constructor & Destructor Documentation

FGRocket()

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

Constructor.

Parameters

exec	pointer to JSBSim parent object, the FDM Executive.
el	a pointer to the XML Element instance representing the engine.
engine_number	engine number

Definition at line 54 of file FGRocket.cpp.

  : FGEngine(engine_number, input), isp_function(nullptr), FDMExec(exec)
{
  Load(exec, el);
 
  Type = etRocket;
  Element* thrust_table_element = nullptr;
  ThrustTable = nullptr;
  BurnTime = 0.0;
  previousFuelNeedPerTank = 0.0;
  previousOxiNeedPerTank = 0.0;
  PropellantFlowRate = 0.0;
  TotalPropellantExpended = 0.0;
  FuelFlowRate = FuelExpended = 0.0;
  OxidizerFlowRate = OxidizerExpended = 0.0;
  SLOxiFlowMax = SLFuelFlowMax = PropFlowMax = 0.0;
  MxR = 0.0;
  BuildupTime = 0.0;
  It = ItVac = 0.0;
  ThrustVariation = 0.0;
  TotalIspVariation = 0.0;
  VacThrust = 0.0;
  Flameout = false;
 
  // Defaults
   MinThrottle = 0.0;
   MaxThrottle = 1.0;
 
  std::stringstream strEngineNumber;
  strEngineNumber << EngineNumber;
 
  auto PropertyManager = exec->GetPropertyManager();
  bindmodel(PropertyManager.get()); // Bind model properties first, since they might be needed in functions.
 
  Element* isp_el = el->FindElement("isp");
 
  // Specific impulse may be specified as a constant value or as a function -
  // perhaps as a function of mixture ratio.
  if (isp_el) {
    Element* isp_func_el = isp_el->FindElement("function");
    if (isp_func_el) {
      isp_function = new FGFunction(exec, isp_func_el, strEngineNumber.str());
    } else {
      Isp = el->FindElementValueAsNumber("isp");
    }
  } else {
    throw("Specific Impulse <isp> must be specified for a rocket engine");
  }
  
  if (el->FindElement("builduptime"))
    BuildupTime = el->FindElementValueAsNumber("builduptime");
  if (el->FindElement("maxthrottle"))
    MaxThrottle = el->FindElementValueAsNumber("maxthrottle");
  if (el->FindElement("minthrottle"))
    MinThrottle = el->FindElementValueAsNumber("minthrottle");
 
  if (el->FindElement("slfuelflowmax")) {
    SLFuelFlowMax = el->FindElementValueAsNumberConvertTo("slfuelflowmax", "LBS/SEC");
    if (el->FindElement("sloxiflowmax")) {
    SLOxiFlowMax = el->FindElementValueAsNumberConvertTo("sloxiflowmax", "LBS/SEC");
    }
    PropFlowMax = SLOxiFlowMax + SLFuelFlowMax;
    MxR = SLOxiFlowMax/SLFuelFlowMax;
  } else if (el->FindElement("propflowmax")) {
    PropFlowMax = el->FindElementValueAsNumberConvertTo("propflowmax", "LBS/SEC");
    // Mixture ratio may be specified here, but it can also be specified as a
    // function or via property
    if (el->FindElement("mixtureratio")) {
      MxR = el->FindElementValueAsNumber("mixtureratio");
    }
  }
 
  if (isp_function) Isp = isp_function->GetValue(); // cause Isp function to be executed if present.
  // If there is a thrust table element, this is a solid propellant engine.
  thrust_table_element = el->FindElement("thrust_table");
  if (thrust_table_element) {
    ThrustTable = new FGTable(PropertyManager, thrust_table_element);
    Element* variation_element = el->FindElement("variation");
    if (variation_element) {
      if (variation_element->FindElement("thrust")) {
        ThrustVariation = variation_element->FindElementValueAsNumber("thrust");
      }
      if (variation_element->FindElement("total_isp")) {
        TotalIspVariation = variation_element->FindElementValueAsNumber("total_isp");
      }
    }
  }
 
 
  Debug(0);
}

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~FGRocket()

~FGRocket

(

void

)

Destructor.

Definition at line 148 of file FGRocket.cpp.

{
  delete ThrustTable;
  Debug(1);
}

Member Function Documentation

CalcFuelNeed()

double CalcFuelNeed ( void  )

virtual

The fuel need is calculated based on power levels and flow rate for that power level.

It is also turned from a rate into an actual amount (pounds) by multiplying it by the delta T and the rate.

Returns

Total fuel requirement for this engine in pounds.

Reimplemented from FGEngine.

Definition at line 222 of file FGRocket.cpp.

{
  if (ThrustTable != 0L) {          // Thrust table given - infers solid fuel
    FuelFlowRate = VacThrust/Isp;   // This calculates wdot (weight flow rate in lbs/sec)
    FuelFlowRate /= (1 + TotalIspVariation);
  } else {
    SLFuelFlowMax = PropFlowMax / (1 + MxR);
    FuelFlowRate = SLFuelFlowMax * PctPower;
  }
 
  FuelExpended = FuelFlowRate * in.TotalDeltaT; // For this time step ...
  return FuelExpended;
}

CalcOxidizerNeed()

double CalcOxidizerNeed ( void  )

virtual

The oxidizer need is calculated based on power levels and flow rate for that power level.

It is also turned from a rate into an actual amount (pounds) by multiplying it by the delta T and the rate.

Returns

Total oxidizer requirement for this engine in pounds.

Reimplemented from FGEngine.

Definition at line 238 of file FGRocket.cpp.

{
  SLOxiFlowMax = PropFlowMax * MxR / (1 + MxR);
  OxidizerFlowRate = SLOxiFlowMax * PctPower;
  OxidizerExpended = OxidizerFlowRate * in.TotalDeltaT;
  return OxidizerExpended;
}

Calculate()

void Calculate ( void  )

virtual

Determines the thrust.

Implements FGEngine.

Definition at line 156 of file FGRocket.cpp.

{
  if (FDMExec->IntegrationSuspended()) return;
 
  RunPreFunctions();
 
  PropellantFlowRate = (FuelExpended + OxidizerExpended)/in.TotalDeltaT;
  TotalPropellantExpended += FuelExpended + OxidizerExpended;
  // If Isp has been specified as a function, override the value of Isp to that,
  // otherwise assume a constant value is given.
  if (isp_function) Isp = isp_function->GetValue();
 
  // If there is a thrust table, it is a function of propellant burned. The
  // engine is started when the throttle is advanced to 1.0. After that, it
  // burns without regard to throttle setting.
 
  if (ThrustTable != 0L) { // Thrust table given -> Solid fuel used
 
    if ((in.ThrottlePos[EngineNumber] == 1 || BurnTime > 0.0 ) && !Starved) {
 
      VacThrust = ThrustTable->GetValue(TotalPropellantExpended)
                * (ThrustVariation + 1)
                * (TotalIspVariation + 1);
      if (BurnTime <= BuildupTime && BuildupTime > 0.0) {
        VacThrust *= sin((BurnTime/BuildupTime)*M_PI/2.0);
        // VacThrust *= (1-cos((BurnTime/BuildupTime)*M_PI))/2.0; // 1 - cos approach
      }
      BurnTime += in.TotalDeltaT; // Increment burn time
    } else {
      VacThrust = 0.0;
    }
 
  } else { // liquid fueled rocket assumed
 
    if (in.ThrottlePos[EngineNumber] < MinThrottle || Starved) { // Combustion not supported
 
      PctPower = 0.0; // desired thrust
      Flameout = true;
      VacThrust = 0.0;
 
    } else { // Calculate thrust
 
      // PctPower = Throttle / MaxThrottle; // Min and MaxThrottle range from 0.0 to 1.0, normally.
      
      PctPower = in.ThrottlePos[EngineNumber];
      Flameout = false;
      VacThrust = Isp * PropellantFlowRate;
 
    }
 
  } // End thrust calculations
 
  LoadThrusterInputs();
  It += Thruster->Calculate(VacThrust) * in.TotalDeltaT;
  ItVac += VacThrust * in.TotalDeltaT;
 
  RunPostFunctions();
}

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GetEngineLabels()

string GetEngineLabels ( const std::string &  delimiter )

virtual

Implements FGEngine.

Definition at line 248 of file FGRocket.cpp.

{
  std::ostringstream buf;
 
  buf << Name << " Total Impulse (engine " << EngineNumber << " in lbf)" << delimiter
      << Name << " Total Vacuum Impulse (engine " << EngineNumber << " in lbf)" << delimiter
      << Name << " Roll Moment (engine " << EngineNumber << " in ft-lbf)" << delimiter
      << Name << " Pitch Moment (engine " << EngineNumber << " in ft-lbf)" << delimiter
      << Name << " Yaw Moment (engine " << EngineNumber << " in ft-lbf)" << delimiter
      << Name << " X Force (engine " << EngineNumber << " in lbf)" << delimiter
      << Name << " Y Force (engine " << EngineNumber << " in lbf)" << delimiter
      << Name << " Z Force (engine " << EngineNumber << " in lbf)" << delimiter
      << Thruster->GetThrusterLabels(EngineNumber, delimiter);
 
  return buf.str();
}

GetEngineValues()

string GetEngineValues ( const std::string &  delimiter )

virtual

Implements FGEngine.

Definition at line 267 of file FGRocket.cpp.

{
  std::ostringstream buf;
 
  buf << It << delimiter 
      << ItVac << delimiter 
      << GetMoments().Dump(delimiter) << delimiter
      << Thruster->GetBodyForces().Dump(delimiter) << delimiter
      << Thruster->GetThrusterValues(EngineNumber, delimiter);
 
  return buf.str();
}

GetFlameout()

bool GetFlameout ( void  )

inline

Gets the flame-out status.

The engine will "flame out" if the throttle is set below the minimum sustainable-thrust setting.

Returns

true if engine has flamed out.

Definition at line 164 of file FGRocket.h.

{return Flameout;}

GetIsp()

double GetIsp ( void  ) const

inline

Definition at line 170 of file FGRocket.h.

{return Isp;}

GetMixtureRatio()

double GetMixtureRatio ( void  ) const

inline

Definition at line 168 of file FGRocket.h.

{return MxR;}

GetOxiFlowRate()

double GetOxiFlowRate ( void  ) const

inline

Definition at line 166 of file FGRocket.h.

{return OxidizerFlowRate;}

GetThrustVariation()

double GetThrustVariation ( void  ) const

inline

Returns the thrust variation, if any.

Definition at line 200 of file FGRocket.h.

{return ThrustVariation;}

GetTotalImpulse()

double GetTotalImpulse ( void  ) const

inline

Gets the total impulse of the rocket.

Returns

The cumulative actual total impulse of the rocket up to this time.

Definition at line 154 of file FGRocket.h.

{return It;}

GetTotalIspVariation()

double GetTotalIspVariation ( void  ) const

inline

Returns the Total Isp variation, if any.

Definition at line 203 of file FGRocket.h.

{return TotalIspVariation;}

GetVacTotalImpulse()

double GetVacTotalImpulse ( void  ) const

inline

Gets the total impulse of the rocket.

Returns

The cumulative vacuum total impulse of the rocket up to this time.

Definition at line 158 of file FGRocket.h.

{return ItVac;}

SetIsp()

void SetIsp ( double  isp )

inline

Definition at line 174 of file FGRocket.h.

{Isp = isp;}

SetMixtureRatio()

void SetMixtureRatio ( double  mix )

inline

Definition at line 172 of file FGRocket.h.

{MxR = mix;}

SetThrustVariation()

void SetThrustVariation ( double  var )

inline

Sets the thrust variation for a solid rocket engine.

Solid propellant rocket motor thrust characteristics are typically defined at 70 degrees F temperature. At any other temperature, performance will be different. Warmer propellant grain will burn quicker and at higher thrust. Total motor impulse is not changed for change in thrust.

Parameters

var	the variation in percent. That is, a 2 percent variation would be specified as 0.02. A positive 2% variation in thrust would increase the thrust by 2%, and shorten the burn time.

Definition at line 188 of file FGRocket.h.

{ThrustVariation = var;}

SetTotalIspVariation()

void SetTotalIspVariation ( double  var )

inline

Sets the variation in total motor energy.

The total energy present in a solid rocket motor can be modified (such as might happen with manufacturing variations) by setting the total Isp variation.

Parameters

var	the variation in percent. That is, a 2 percent variation would be specified as 0.02. This variation will affect the total thrust, but not the burn time.

Definition at line 197 of file FGRocket.h.

{TotalIspVariation = var;}

---

The documentation for this class was generated from the following files:

• FGRocket.h
• FGRocket.cpp