potato launcher

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KATHMANDU UNIVERSITY

SCHOOL OF ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

Design and Fabrication of Potato Launcher

Submitted By: Submitted To:

Prabin Dhital(42060) Assoc. Prof. Dr. Bivek Baral

Sudeep Parajuli(42068) Department of Mechanical

Sujeet Regmi(42074) Engineering

Arun Satyal(42075)

Saugat Timilsina(42087)

July 2012


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Table of ContentsAbstract ......................................................................................................................................................... 2

Acknowledgement ........................................................................................................................................ 3

Background ................................................................................................................................................... 4

Introduction ................................................................................................................................................... 4

Combustion Launcher ............................................................................................................................... 4

Design of potato launcher: ............................................................................................................................ 5

Design of combustion chamber: ............................................................................................................... 5

Range calculation and optimization .......................................................................................................... 7

Construction .............................................................................................................................................. 8

Safety Measures ........................................................................................................................................ 8

Problem Encountered ................................................................................................................................ 8

Conclusion .................................................................................................................................................. 10

References ................................................................................................................................................... 11

Appendix-I .................................................................................................................................................. 12

AppendixII ............................................................................................................................................... 14


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Abstract

For the fulfillment of the course Design of Thermal System a mini project Design and

Fabrication of Potato Launcher was undertaken. We undertook the mini project for the

following purpose; to understand the design procedure of a product from scrap, to gain

knowledge in thermal system design and to study modeling and simulation procedures and

techniques involved during designing of any product.

Potato launcher was fabricated after modeling and simulation. During modeling and simulation

the range of the potato was calculated to be 86m at projection angle of 360, so we fabricated a

stand with inclination of 360 and launched the projectile, but the range achieved was 110m in

average.

Safety measures were adopted thoroughly as the combustion of the air fuel mixture inside the

combustion chamber results in very high temperature and pressure products.


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Background

For the fulfillment of the course Design of Thermal System a mini project Design and

Fabrication of Potato Launcher was undertaken. Potato is one of the highly consumed and

desired food all around the world, launching of potato through a launcher can be as tasteful as

delicious potato dish. We undertook the mini project for the following purpose:

a. To understand the design procedure of a product from scrap.b. To gain knowledge in thermal system design.c. To study modeling and simulation procedures and techniques involved during designing

of any product.

Introduction

A potato launcher is pipe-based cannon which uses combustion energy of a gaseous fuel

(Butane) to launch projectiles at high speeds. It is widely known as spud gun or spudzooka

among the potato launcher enthusiast. There are various kinds of potato launchers based on the

firing mechanism viz. Combustion Launcher, Pneumatic Launcher, Dry-ice Launcher and

Hybrid Launcher [1]. For our mini project we designed and fabricated a combustion launcher.

Combustion Launcher

A combustion potato launcher uses expansion of a burnt gas in a fixed volume chamber to propel

the potato through the barrel and launch it.

In order to fire, the operator loads a projectile into the barrel, adds fuel to the combustion

chamber and triggers the ignition source. The fuel then ignites, creating hot expanding gases, and

forcing the projectile out of the barrel. Distances vary greatly depending on many factors,including the type of fuel used, the efficiency of the fuel/air ratio, the combustion chamber/barrel

ratio, and the flight characteristics of the projectile. Common distances vary from 100 to 200

meters [1].

In a combustion potato launcher there are basically four elements:

A fuel system A combustion chamber An ignition source

A barrel


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Design of potato launcher:

Design of combustion chamber:

Consideration:

Combustion chamber volume 1 liter. Initial pressure at start of combustion 1 atmospheric pressure (101 kPa). Fuel used Butane (C4H10). Expansion after combustion of the air/fuel mixture is isentropic.We used butane (C4H10) as a combustion fuel. The combustion equation for stoichiometric

condition is:

C4H10 + 6.5 (O2 + 3.76 N2) 4 CO2 + 5 H2O + 24.44 N2

The temperature of the burned gas when the fuel is burned at stoichiometric amount was

calculated considering law of conservation of energy (First Law of Thermodynamics) as:

hreactant = h( C4H10) + 6.5 h(O2) + 24.44 h(N2) , at 250 C.

hproduct = 4 h(CO2) + 5 h(H2O) + 24.44 h(N2)

hreactant = hproduct

Where, h(Compound) denotes the enthalpy of corresponding compound.

These equations were solved using Engineering Equation Solver to get the final temperature after

combustion.

The temperature correlates to the rise in pressure as combined gas law,

Here, combustion chamber volume is constant, V1 = V2,which reduces the equation to

Here, P, V and T represent the pressure, volume and temperature at state 1 and state 2.

Knowing pressure after combustion, for the design of combustion chamber the geometry can be

calculated using design consideration for thin walled pressure vessel.

Stress in a thin-walled pressure vessel,


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Where hoop stress, or stress in the circumferential direction is, is stress in the

longitudinal direction, p is internal gauge pressure, ris the inner radius of the sphere, and tis

thickness of the cylinder wall.

PVC (Poly-Vinyl-Chloride) pipe was used for the construction of the pressure vessel. The

pressure developed in the combustion chamber after combustion of the fuel is about 707.4 kPa.

For given geometry of PVC pipe of standard 75 mm diameter and 2 mm thickness, the principle

stress developed given by:

Where, is the principle stress developed due to hoop stress and longitudinal stress12.70 MN/m2.

The yield modulus of the PVC pipe is 51.71 MN/m2 [2], which is in accordance with the

principle stress calculated for the design of the vessel.

Thus, combustion chamber of 226 mm with internal diameter 75 mm forming volume of 1 liter is

used.

After the design of the combustion chamber the barrel length and diameter was arbitrarily taken

as 1 m and 37 mm respectively.

Calculation of the velocity of potato:

The velocity attained by the potato is due to the pressure difference experienced by the potato

between the pressure side in the combustion chamber and the pressure at the open atmosphericair.

Where, f is the force experienced by the potato, d is the internal diameter of the barrel, P 2 is the

pressure developed in combustion chamber after combustion and ffr is the frictional force

experienced by the potato which varies according to the fit of the potato but in average it was

found to be 18 N.

Where, a is the acceleration of the potato and m is the mass taken as 50g.


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Where, v is the velocity attained by the potato and s is the length of the barrel is 1m. The velocity

thus was calculated to be 44 m/s.

Range calculation and optimization

With the velocity of 44m/s the range was calculated using Eulers Projectile method for airresistance. The range and velocity for projectile in Eulers method is given by:

Where, r0 = [0 0], is initial position of the projectile.

v0 = [vcos vsin ], is initial velocity in x-direction and y-direction, is angle ofprojection.

a = [ax ay], is acceleration in x-direction and y-direction.

, Where, cd is coefficient of drag, is density of air, A is area ofprojection of in the direction the projectile motion, v is thevelocity of projectile and m is the mass of the projectile.

ay = -g , where g is acceleration due to gravity.

The range optimization was done varying the angle of projection.

Figure: Optimization of range with respect to angle of projection, .It was seen that the maximum range is 86m at 36

0angle of projection.


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Construction

A working prototype of potato launcher was fabricated in Mechanical Workshop. The materials

used in the fabrication are:

S.N. Parts Dimension Use

1. PVC pipe 75mm X 226 mm Combustion chamber2. PVC Plug 75mm Seal combustion chamber3. Reducer 75mm to 37mm Connect combustion chamber and barrel4. PVC pipe 37mm X 1000 mm Barrel5. Gas Lighter - Ignition

Method of assembly:

A PVC pipe of75mm was cut into 226 mm length to acquire 1 liter combustion chambervolume. One side of the pipe was sealed with PVC plug of diameter 75mm. Reducer reducedthe combustion chamber diameter of75mm to barrel diameter 37mm. Barrel was attached tothe reducers other end. All the piping system was glued together using PVC cement. A hole of

18 mm was drilled in combustion chamber for fuel inlet and to place gas lighter into combustionchamber for ignition.

Safety Measures

i. The pressure development inside the combustion chamber is very high with respect toatmospheric pressure, so all joints needed to be sealed perfectly thus we used PVC

cement which formed a tight fit joints resistible to pressure fluctuation inside the

combustion chamber.

ii. To avoid the leak from the gas lighter position in the combustion chamber we drilledhole whose diameter was equal to the diameter of the lighter (i.e. 18 mm).

iii. The potato was launched in a open empty field to avoid accident, as high speedprojectile can cause fetal injuries if it hits anybody.

Problem Encountered

i. For safety purpose we tried different electrical mechanisms for ignition to ignite theair/fuel mixture from a safe distance.

a. A spark plug ignition was used whose ignition energy was not sufficient to ignitethe air/fuel mixture inside the combustion chamber.

b. A broken filament bulb was used which did not ignite the air/fuel mixture insidethe combustion chamber.

c. A broken filament bulb was used with match stick powder to ignite the air/fuelmixture, but during the combustion of the match stick powder used the air inside

the chamber leaving insufficient air inside the combustion chamber for air/fuel

mixture within combustible limit.


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ii. A non-return valve was used to introduce gas into the combustion chamber, but thepressure of the butane gas canister was very high, so we could not introduce the

calculated amount of fuel through the valve.

iii. Potato needed to be tight fit in the barrel for proper transfer of pressure energy formthe combustion chamber to the potato, so at the beginning we had problem with

launching the potato to an optimum range. So, we started using potato of larger size

which would mesh in the barrel and barrel would act as a template for the potato

inside the barrel.


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Conclusion

A potato launcher was fabricated after modeling and simulation. During modeling and

simulation the range of the potato was calculated to be 86m at projection angle of 360, so we

fabricated a stand with inclination of 360 and launched the projectile, but the range achieved was

110m in average. The inconsistency in the simulated value and actual performance is because of

the assumptions that were made during the simulation. The difference between the simulated

value and actual performance can be minimized by approximating the actual scenario.

Safety measures must be adopted thoroughly as the combustion of the air fuel mixture inside the

combustion chamber results in very high temperature and pressure products which can cause

accident of high degree burn to the operator.


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References

[1] Wikipedia- A Free Encyclopedia. http://en.wikipedia.org/wiki/Spud_gun#\Combustion_launchers, July 7, 2012.

[2] PVC-Polyvinyl Chloride, Plastic Distributers, Distributors to the World.7600 Anagram Drive-Eden Prairie, MN 55344.


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Appendix-I

EES program to simulate combustion process and calculate the velocity of the potato

"C3H8 +x(O2+3,76N2)----> 3CO2+4H2O+3.76xN2+(x-5)O2"

"Subprogram to calculate the temperature of the burned mixture"

"******"SUBPROGRAM UBT(x : T)

HR=enthalpy(C3H8, T=25)+x*enthalpy(O2, T=25)+3.76*x*enthalpy(N2, T=25)

HP=3*enthalpy(CO2, T=T)+3.76*x*enthalpy(N2, T=T)+(x-5)*enthalpy(O2,

T=T)+4*enthalpy(H2O,T=T)

HP=HR

End

"******"

"Function to calculate the pressure knowing the temperature of the gas"

"******"

Function PRESS(T)

P2:=(1/1000)*(273+T)/298*1000PRESS:=P2

End

"******"

"Function for iterating for pressure with x"

"******"Function Check(A)

x=5

12:CALL UBT(x:T1)

P=Press(T1)

x=x+.1

p_ref=6895*8*.003/.05If (P>p_ref) Then GoTo 12 "PRESSURE CHECKING FOR MAXIMUM

ALLOWABLE PRESSURE"Check:=x

End"******"

"******Velocity on the potato


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*******"

Function velocity(P)F:=(P-101)*1000*(pi*.05^2)/4 -18 "Frictional Force of 18N"

Acceleration:=f/.05

vel:=(2*acceleration*2)^(1/2)

velocity:=velEnd

"******"

X= Check(1)equ=5/X

CALL UBT(X:T2)

pre=press(T2)

"Velocity=velocity(pre)"


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AppendixII

% MATLAB Program to compute the trajectory of a potato using the Euler method.

clc;

clear all;

%* Set initial position and velocity of the potato

%y1 = input('Enter initial height (meters): ');

y1=0;

speed = input('Enter initial speed (m/s): ');

% Iterating for the optimum value of theta for maximum horizontal range

theta =20;

fprintf('Angle \t \t Time of Flight(s) \t \t Range(m) \n')

while (theta


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%* Loop until potato hits ground or max steps completed

maxstep = 1000; % Maximum number of steps

for istep=1:maxstep

%* Record position for plotting

xplot(istep) = r(1); % Record trajectory for plot

yplot(istep) = r(2);

t = (istep-1)*tau; % Current time

xNoAir(istep) = r1(1) + v1(1)*t;

yNoAir(istep) = r1(2) + v1(2)*t - 0.5*grav*t^2;

% Acceleration of potato

accel = air_const*norm(v)*v; % Air resistance

accel(2) = accel(2)-grav; % Gravity

%* Calculate the new position and velocity using Euler method

r = r + tau*v; % Euler step

v = v + tau*accel;

%* If potato reaches ground (y


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%* Print maximum range and time of flight

fprintf('%f \t \t %f \t \t \t %f \n',theta,istep*tau,r(1));

% Mark the location of the ground by a straight line

xground = [0 max(xNoAir)]; yground = [0 0];

for j=1:1000

if (yplot(j)>=0)

xplot1(j)=xplot(j);

yplot1(j)=yplot(j);

else

break;

end

end

% Plot the trajectory and parabolic, no-air curve

plot(xplot1,yplot1,'--',xNoAir,yNoAir,'-',xground,yground,'-');

hold on;

theta =theta+2;

end

legend('Euler method','Theory (No air) ');

xlabel('Range (m)'); ylabel('Height (m)');

title('Projectile motion');


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Appendix-III

potato launcher

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