Water Rocket Extra Credit Program "rockthrust"

This extra credit exercise requires that a participating et181 Rocket team collaberates with a Junior Lab team in order to compare the rocket thrust vs time with experiment. This quarter a number of junior lab teams have expressed ibterest in developing an experiment to measure and record the thrust force of the water rocket vs time. Recall from the rocket analysis that as long as the water is being expelled through the nozzle, the thrust force is given by the following equation:


where:
P is the pressure of the compressed air
Pa is the atmospheric pressure
AN is the area of the nozzle

Since the pressure P varies with time as the air expands to expel the water the thrust force varies in unison. In this extra credit exercise we wish to plot the thrust force vs time, and compare the results with those measured by the Junior Lab team.

The Computer Program "rockthrust"

The program rockthrust is a modification of the last exercise rockplot in that we wish to plot the thrust force vs time, and save the thrust force array together with the time, velocity and height arrays on a data file rocket.out to allow off-line plotting using MATLAB. We first get the rocket data from a data file in order to construct a rocket , and subsequently invoke the method findvelht to evaluate the thrust. As before we show a typical example of a datafile as follows:
urieli.data
43.0  591.0  0.4  0.3  4.0  0.3  8.0
################################################
The small urieli root beer water rocket -10/8/05
The above data items are (in sequence):
ms [gm] - solid mass (rocket + payload)
vtot [cc] - total volume (water + air)
dnoz [cm] - diameter of nozzle
fill - fill ratio (water/total volume)
pgage [atm] - charge gage pressure [atmospheres]
cdrag - Coefficient of drag
dbottle [cm] - outside diameter of bottle
################################################

The method findvelht is modified such that the local variable thrust becomes an argument passed by reference, as shown in the schematic below. The prototype of the modified method findvelht can be seen in the header file rockthrust.h


A typical execution is shown below together with the output data file rocket.out. As in program rockplot, the input data defining the rocket is read from the input data file. All data items that are input by the user are shown in italics.

enter the filename containing the rocket data: urieli.data
 get rocket data from the "urieli.data" file
data read:
43, 591, 0.4, 0.3
4, 0.3, 8
################################################
The small urieli root beer water rocket -10/16/06
The above data items are (in sequence):
ms [gm] - solid mass (rocket + payload)
vtot [cc] - total volume (water + air)
dnoz [cm] - diameter of nozzle
fill - fill ratio (water/total volume)
pgage [atm] - charge gage pressure [atmospheres]
cdrag - Coefficient of drag
dbottle [cm] - outside diameter of bottle
################################################
Water rocket initialized as follows:
Solid mass[gm]: 43, total volume[liters]: 0.591
absolute charge pressure[kPa]: 500
initial air charge volume[liters]: 0.4137
Nozzle free flow area [sq.cm]: 0.125664
Bottle cross sect. area [sq.cm]: 50.2655
Density - water: 1000kg/m^3, outside air: 1.2kg/m^3
Ratio of specific heat capacities for air (k): 1.4
Drag coefficient: 0.3
gravity acceleration: 9.807m/s^2
enter the max time to test (sec)
1
value entered is 1(s)
enter the number of points to evaluate
21
number of points is 21
plot width is 61 characters
rocket thrust (N) vs time(s):
           0    I                                                           *
        0.05    I                                                       *    
         0.1    I                                                   *        
        0.15    I                                                *           
         0.2    I                                             *              
        0.25    I                                          *                 
         0.3    I                                        *                   
        0.35    I                                      *                     
         0.4    I                                    *                       
        0.45    I                                  *                         
         0.5    I                                *                           
        0.55    I                               *                            
         0.6    *                                                            
        0.65    *                                                            
         0.7    *                                                            
        0.75    *                                                            
         0.8    *                                                            
        0.85    *                                                            
         0.9    *                                                            
        0.95    *                                                            
           1    *                                                            
minimum value: 0, maximum value: 10.0531
rocket upward velocity(m/s) vs time(s):
           0    *                                                            
        0.05    I   *                                                        
         0.1    I      *                                                     
        0.15    I          *                                                 
         0.2    I              *                                             
        0.25    I                  *                                         
         0.3    I                       *                                    
        0.35    I                           *                                
         0.4    I                                *                           
        0.45    I                                      *                     
         0.5    I                                            *               
        0.55    I                                                   *        
         0.6    I                                                           *
        0.65    I                                                        *   
         0.7    I                                                      *     
        0.75    I                                                   *        
         0.8    I                                                 *          
        0.85    I                                               *            
         0.9    I                                            *               
        0.95    I                                          *                 
           1    I                                         *                  
minimum value: 0, maximum value: 29.5986
enter the filename: rocket.out
21 rows of data written to "rocket.out" file

The plot of thrust vs time should be compared to the measured data obtained by the Junior Lab team using your rocket. In your report you should describe the experiment and discuss the results obtained. The rocket.out file is shown below - notice the fourth column that includes the thrust force in Newtons.

rocket.out
     0.000                0.000           0.000          10.053
     0.050                1.801           0.045           9.348
     0.100                3.632           0.181           8.729
     0.150                5.512           0.409           8.181
     0.200                7.461           0.733           7.692
     0.250                9.502           1.156           7.253
     0.300               11.661           1.685           6.857
     0.350               13.973           2.325           6.497
     0.400               16.478           3.085           6.169
     0.450               19.234           3.977           5.868
     0.500               22.320           5.014           5.592
     0.550               25.854           6.216           5.336
     0.600               29.599           7.609           0.000
     0.650               28.261           9.056           0.000
     0.700               26.936          10.433           0.000
     0.750               25.775          11.759           0.000
     0.800               24.597          13.014           0.000
     0.850               23.488          14.213           0.000
     0.900               22.442          15.360           0.000
     0.950               21.436          16.453           0.000
     1.000               20.513          17.516           0.000

You can now use the data file in a similar manner to the previous exercise rockfile in which you used the FTP program to fetch the file and save it on the local computer disk. You should read the file into the MATLAB application and plot the thrust vs elapsed time, as shown below.

Note the following: Even though your team will share the results of the rocket experiment data, the program, MATLAB plot, and report should be your individual work, done without colloboration or help from any source. The MATLAB plot should contain a box with your name printed (not handwritten) on the plot, including the basic data used to determine the plot as shown. Your program (named rockthrust.cpp) should be saved in your home directory together with the nielsen.data data file by 10:00am of the due date shown on the calendar, however the report can be submitted at any time before the final exam date.

Telnet: "condor"

Wow - teaming up with the Junior Lab to compare our calculated thrust to actual measurement - this is REAL engineering!