1 /*****************************************************************************
4 Author: Christian Mayer
8 -------- Copyright (C) 1999 Christian Mayer (fgfs@christianmayer.de) --------
10 This program is free software; you can redistribute it and/or modify it under
11 the terms of the GNU General Public License as published by the Free Software
12 Foundation; either version 2 of the License, or (at your option) any later
15 This program is distributed in the hope that it will be useful, but WITHOUT
16 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
17 FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
20 You should have received a copy of the GNU General Public License
21 along with this program; if not, write to the Free Software
22 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
24 Further information about the GNU General Public License can also be found on
25 the world wide web at http://www.gnu.org.
27 FUNCTIONAL DESCRIPTION
28 ------------------------------------------------------------------------------
29 A hot air balloon simulator
32 ------------------------------------------------------------------------------
33 01.09.1999 Christian Mayer Created
34 03.10.1999 Christian Mayer cleaned the code by moveing WeatherDatabase
35 calls inside the update()
36 *****************************************************************************/
38 /****************************************************************************/
40 /****************************************************************************/
49 #include <simgear/constants.h>
51 #include <Aircraft/aircraft.hxx>
53 #include "BalloonSim.h"
55 /****************************************************************************/
56 /********************************** CODE ************************************/
57 /****************************************************************************/
59 /****************************************************************************/
62 /* Set the balloon model to some values that seem reasonable as I haven't */
63 /* got much original values */
65 /****************************************************************************/
69 ground_level = 3400.0;
71 gravity_vector = SGVec3f(0.0, 0.0, -9.81);
72 velocity = SGVec3f(0.0, 0.0, 0.0);
73 position = SGVec3f(0.0, 0.0, 0.0);
74 hpr = SGVec3f(0.0, 0.0, 0.0);
76 /************************************************************************/
77 /* My balloon has a radius of 8.8 metres as that gives a envelope */
78 /* volume of about 2854 m^3 which is about 77000 ft^3, a very common */
79 /* size for hot air balloons */
80 /************************************************************************/
82 balloon_envelope_area = 4.0 * (8.8 * 8.8) * SGD_PI;
83 balloon_envelope_volume = (4.0/3.0) * (8.8 * 8.8 * 8.8) * SGD_PI;
85 wind_facing_area_of_balloon = SGD_PI * (8.8 * 8.8);
86 wind_facing_area_of_basket = 2.0; //guessed: 2 m^2
88 cw_envelope=0.45; //a sphere in this case
91 weight_of_total_fuel = 40.0; //big guess
92 weight_of_envelope = 200.0; //big guess
93 weight_of_basket = 40.0; //big guess
94 weight_of_cargo = 750.0; //big guess
97 max_flow_of_fuel_per_second=10.0*1.0/3600.0; //assuming 10% of one hour of total burn time
98 current_burner_strength = 0.0; //the throttle
100 lambda = 0.15; //for plasic
101 l_of_the_envelope = 1.0/1000.0; //the thickness of the envelope (in m): 1mm
103 T = 273.16 + 130.6; //Temperature in the envelope => still at ground level
106 void balloon::update()
108 /************************************************************************/
109 /* I'm simplifying the balloon by reducing the simulation to two */
111 /* the center of the basket (CB) and the center of the envelope (CE) */
116 /* cg (=center of gravity) */
120 /* On each center are forces acting: gravity and wind resitance. CE */
121 /* additionally got the lift (=> I need to calculate the weight of the */
122 /* air inside, too) */
124 /* The weight of the air in the envelope is dependant of the tempera- */
125 /* ture. This temperature is decreasing over the time that is dependant */
126 /* of the insulation of the envelope material (lambda), the gas used */
127 /* (air) and the wind speed. For a plane surface it's for air: */
129 /* alpha = 4.8 + 3.4*v with v < 5.0 m/s */
131 /* The value k that takes all of that into account is defined as: */
133 /* 1 / k = 1 / alpha1 + 1 / alpha2 + l / lambda */
135 /* with 'l' beeing the 'length' i.e. thickness of the insulator, alpha1 */
136 /* the air inside and alpha2 the air outside of the envelope. So our k */
139 /* k = 1 / [1/4.8 + 1/(4.8+3.4v) + l/lambda] */
141 /* The energy lost by this process is defined as: */
143 /* dQ = k * A * t * dT */
145 /* with Q being the energy, k that value defined above, A the total */
146 /* area of the envelope, t the time (in hours) and dT the temperature */
147 /* difference between the inside and the outside. */
148 /* To get the temperature of the air in the inside I need the formula: */
150 /* dQ = cAir * m * dT */
152 /* with cAir being the specific heat capacity(?) of air (= 1.00 kcal / */
153 /* kg * degree), m the mass of the air and dT the temperature change. */
154 /* As the envelope is open I'm assuming that the same air pressure is */
155 /* inside and outside of it (practical there should be a slightly */
156 /* higher air pressure in the inside or the envelope would collapse). */
157 /* So it's easy to calculate the density of the air inside: */
159 /* rho = p / R * T */
161 /* with p being the pressure, R the gas constant(?) which is for air */
162 /* 287.14 N * m / kg * K and T the absolute temperature. */
164 /* The value returned by this function is the displacement of the CB */
165 /************************************************************************/
167 /************************************************************************/
168 /* NOTE: This is the simplified version: I'm assuming that the whole */
169 /* balloon consists only of the envelope.I will improove the simulation */
170 /* later, but currently was my main concern to get it going... */
171 /************************************************************************/
173 // I realy don't think there is a solution for this without WeatherCM
174 // but this is a hack, and it's working -- EMH
178 // gain of energy by heating:
179 if (fuel_left > 0.0) //but only with some fuel left ;-)
181 float fuel_burning = current_burner_strength * max_flow_of_fuel_per_second * dt * weight_of_total_fuel; //in kg
183 //convert to cubemetres (I'm wrongly assuming 'normal' conditions; but that's correct for my special case)
184 float cube_metres_burned = fuel_burning / 2.2; //2.2 is the density for propane
186 fuel_left -= fuel_burning / weight_of_total_fuel;
188 // get energy through burning.
189 Q += 22250.0 * cube_metres_burned; //22250 for propan, 29500 would be butane and if you dare: 2580 would be hydrogen...
192 // calculate the new temperature in the inside:
193 T += Q / (1.00 * mAir);
195 //calculate the masses of the envelope and the basket
196 float mEnvelope = mAir + weight_of_envelope;
197 float mBasket = weight_of_total_fuel*fuel_left + weight_of_basket + weight_of_cargo;
199 float mTotal = mEnvelope + mBasket;
201 //calulate the forces
202 SGVec3f fTotal, fFriction, fLift;
204 fTotal = mTotal*gravity_vector;
206 // fTotal += fLift; //FIXME: uninitialized fLift
207 // fTotal += fFriction; //FIXME: uninitialized fFriction
209 //claculate acceleration: a = F / m
210 SGVec3f aTotal, vTotal, dTotal;
212 aTotal = (1.0 / mTotal)*fTotal;
214 //integrate the displacement: d = 0.5 * a * dt**2 + v * dt + d
215 vTotal = dt*velocity;
216 dTotal = (0.5*dt*dt)*aTotal; dTotal += vTotal;
218 //integrate the velocity to 'velocity': v = a * dt + v
219 vTotal = dt*aTotal; velocity += vTotal;
221 /************************************************************************/
222 /* VERY WRONG STUFF: it's just here to get some results to start with */
223 /************************************************************************/
225 // care for the ground
226 if (position[2] < (ground_level+0.001) )
227 position[2] = ground_level;
232 //cout << "BallonSim: T: " << (T-273.16) << " alt: " << position[2] << " ground: " << ground_level << " throttle: " << current_burner_strength << "\n";
235 void balloon::set_burner_strength(const float bs)
237 if ((bs>=0.0) && (bs<=1.0))
238 current_burner_strength = bs;
241 void balloon::getVelocity(SGVec3f& v) const
246 void balloon::setVelocity(const SGVec3f& v)
251 void balloon::getPosition(SGVec3f& v) const
256 void balloon::setPosition(const SGVec3f& v)
261 void balloon::getHPR(SGVec3f& angles) const //the balloon isn't allways exactly vertical
266 void balloon::setHPR(const SGVec3f& angles) //the balloon isn't allways exactly vertical
271 void balloon::setGroundLevel(const float altitude)
273 ground_level = altitude;
276 float balloon::getTemperature(void) const
281 float balloon::getFuelLeft(void) const
292 sgVec3 pos={0.0, 0.0, 0.0};
297 bool hysteresis = false; // moving up
300 for (int i=0; i<100; i++)
302 bal.update(0.1); acc_dt += 0.1;
303 bal.getPosition(pos);
311 if ((alt < 2990) && (hysteresis == true))
316 if ((bal.getTemperature()-273.16)>250.0)
317 burner = false; //emergency
327 //printf("Position: (%f/%f/%f), dP: (%f/%f/%f), burner: ", pos[0], pos[1], pos[2], dp[0], dp[1], dp[2]);
328 printf("%f \t%f \t%f \t%f\n", acc_dt/60.0, bal.getTemperature()-273.16, pos[2], bal.getFuelLeft());
332 bal.set_burner_strength(1.0);
337 bal.set_burner_strength(0.0);