By:
Laura Romrell,
[Return to Research Page  Background Info  Purpose  Review of Literature  Hypothesis  Operational Definitions  Procedures  Data  Conclusion  Bibliography  Related Links]
Background Information: (Back to top)
A water rocket is exactly what it sounds like, a rocket that uses water as its
counter mass to propel it through the air. The most commonly used form is a
modified refreshment container, other wise known as a soda bottle. Water is
propelled out of the bottom of the bottle by pressurized air encapsulated above
the water in the bottle. The release of this air and water creates a counter
thrust effect. The same basic principles between normal rockets and bottle
rockets are the same (3). There are three main forces which control the
direction of the rocket: thrust (flap), drag (Ffr),
and weight (w=mg). The thrust is provided by the release of the reaction mass,
which in this case is the water in the water rocket. This thrust occurs because
of the difference in atmospheric pressure artificially created inside the water
rocket when compared to the “natural surroundings” (4). The drag force exist
because of the friction caused by air molecules along the side of the rocket
(3).variance in the shape of the rocket will affect the amount of drag the
rocket experiences, thus, because this is not the factor we wish to test, we
will need to make sure this remain as constant as possible. Lastly weight is
simple the mass of the rocket multiplied by gravity. In order for your rocket
to launch, the thrust force must be stronger than its weight. The physics
principal behind a bottle rocket works is
Purpose: (Back to top)
The purpose of this experiment is to find the relationship (independently and
collectively) between the amount of water in the water rocket, the amount of
pressure and the horizontal displacement.
Review of Literature: (Back to top)
When constructing our rockets there are 5 critical factors we must remain aware
of. These are: reliability, rigidness, precision, weight, and drag. (2) Reliability
refers to the consistency with which the rocket flies. This is important,
because if there are huge discrepancies between the distances when all factors
are left constant, our uncertainty value will be too high to be able to judge
the impact of out independent variable. The second one, rigidness, Singleton
(2) stresses is one of the most overlooked. If the rocket or any part of the
fins is able to flex, there is no way to accurately calculate how it will
perform in the air, and thus it is not reliable. Precision refers to the actual
construction process. Again, Singleton stresses that even only one millimeter
adjustment can affect whether your rocket is even able to take off. Weight will
be one of out independent variables so we do not have to such great lengths to
make sure every thing is equal, since it will be adjusted. Lastly drag will
need to be constant on all our rockets and launches in order to obtain
analyzable data (2).
Hypothesis: (Back
to top)
Our hypothesis is three fold.
We believe that graphically (with mass as the independent variable and distance
as the dependant) our results will resemble that of a bell curve with the
extremely heavy and the extremely light rockets traveling the shortest distances.
This is because the lighter masses will have less propellant and the heavier
masses will be too heavy to be lifted and launched efficiently.
We believe that the graph with pressure as the independent variable and
distance as the dependent variable will show a positive exponential
relationship, because the more pressure applied, the greater the force pushing
the rocket forward. Lastly, we predict that the optimum combination will be
either 3 or 4 cups at 60psi, for the reasons written above.
Operational Definitions:
(Back to top)
Mass will be defined as the numbers of cups of liquid put into a two
liter bottle rocket. Our data set consists of 1 cup to 6 cups.
Pressure will be determined by the amount read on a pressure gauge. Our
data set consists of 30psi to 60psi.
Distance will be measured in yards by using the football field.
Place the launcher at the 0 yard line of a football field facing into the
field, prop it up at an angle with something heavy i.e. a bucket etc. take a
picture with a camera at ground level to analyze later to find the launch
angle. Fill rocket with the set amount of water. Place rocket over PVC pipe and
attach metal clip to secure it in place. Now, with someone watching the
pressure gage, begin pumping the bicycle pump until the desired pressure is
reached. Once this occurs, stand back and pull the rope attached to the metal
clip to release the rocket. Now walk onto the field, find the rocket and record
the amount of yards it traveled. Repeat this process with three trials for each
for data point. (It is recommended to keep one of the variables consistent
while manipulating the other, ex 2 cups at 30psi, 2 cups at 40psi ... then
switch to 3 cups at 30 psi, 3 cups at 40 psi... etc.)
Raw Data:
(Trials indicated by commas; numbers
in yards)

30 psi 
40 psi 
50 psi 
60 psi 
1 cup 
23, 29, 26 
34, 31, 30 
33, 31, 34 
44, 44, 41 
2 cups 
34, 27, 29 
30, 32, 31 
39, 36, 33 
36, 41, 45 
3 cups 
43, 35, 38 
36, 40, 35 
42, 39, 43 
42, 38, 45 
4 cups 
37, 37, 39 
35, 39, 41 
39, 43, 42 
29, 37, 33 
5 cups 
30, 27, 28 
37, 44, 37 
41, 38, 43 
30, 31, 34 
6 cups 
22, 20, 18 
26, 26, 20 
31, 33, 24 
29, 34, 31 
Data File (text tab delimited)
Averages (Back to top)
(in yards)

30 psi 
40 psi 
50 psi 
60 psi 
1 cup 
26 
31.333333 
33 
37 
2 cups 
30 
31 
36 
43 
3 cups 
39 
37 
41 
42 
4 cups 
38 
38 
41 
33 
5 cups 
28 
39.666666 
41 
32 
6 cups 
20 
24 
29 
31.33333333 
Data File (text tab delimited)
Calculated uncertainty values (Back to top)
(in yards)

30 psi 
40 psi 
50 psi 
60 psi 
1 cup 
+/3 
+/2 
+/1.5 
+/3 
2 cups 
+/3.5 
+/1 
+/3 
+/1.5 
3 cups 
+/3.5 
+/2.5 
+/2 
+/3.5 
4 cups 
+/1 
+/3 
+/2 
+/4 
5 cups 
+/1.5 
+/3.5 
+/2.5 
+/2 
6 cups 
+/2 
+/3 
+/3.5 
+/2.5 
Graphs (Back to top)
Conclusion: (Back to top)
Our first hypothesis (that the relationship between the amount of water and the
distance traveled by the rocket would resemble a bell curve) was correct. When
all of the trials were averaged together (Average distance by Water Level) a
bellshape curve is clearly visible, and just as we predicted, 3 and 4 yielded
the farthest rockets. This was most likely because of the same reasons we
mentioned in our hypothesis: the lower masses don't have as much thrust power
as the middle ones, but the heavy masses are weighted down by their extra mass
more than the additional thrust power compensates for it. In addition to being
seen independently, this bell curve is also seen across varying psi levels
(Effect of Water Amount on Distance by psi level). The best example of this, is
with the "30 psi" values, however, for each level the effect is
observed. The fact that this effect is seen in both the independent and
collective graphs suggests greater certainty of this outcome.
Our second hypothesis, however, was not as clear cut. While the distance
increased as the psi level increased when 1, 2, 3 or 6 cups were used (Effect
of Pressure on Distance by Water Amount), the data for both 4 and 5 cups
heavily drops at 60 psi. These two discrepancies were enough to cause no effect
to be observed when psi levels were analyzed independently (Average Distance by
psi Level). We attribute this mostly to error rather than any scientific
phenomenon. For example, half way through conducting the 5 cup series, we ran
out of water, and had to go get more. Because it was cold outside, we decided
to put warm water in so our hands wouldn't get as cold, forgetting that a change
in temperature would probably have an adverse effect on the graph. In addition,
often our rockets didn't fly completely straight, which caused them to land
before they had reached their full potential distance. Our calculated
uncertainty for each data point was also very high, almost making the
discrepancies we did observe between values obsolete. These factors, in
addition to others, may have been what caused 60 psi to not show as high of
results.
Our third hypothesis would suggest that the farthest distance should have been
reached at either 3 or 4 cups, and 60 psi. While the absolute highest value
obtained (45) was at 3 cups and 60 psi, the highest average was actually found
at 2 cups and 60 psi (43 versus 42 for 3 cups and 60 psi). Although this value
doesn't completely match our hypothesis, it does fit in with the general idea
that a high psi level and a moderate amount of water is the optimal combination
in order to achieve the greatest distance.
Were we to do this experiment again, we would want to use video analysis to
allow us to also calculate the vertical displacement. This would allow us to
also analyze whether higher pressures cause the rocket to go higher in addition
to farther.
Bibliography: (Back to top)
(1) Slife, Erika. "Imaginations soar in
elementary school's water rocket seminar."
SunSentinel [
(2) Singleton IV, Leo C. Bottle Rocket Handbook. 2001. 7 Nov. 2007
<http://www.pulsejets.com/phpbb2/files/_rocketry bottle_rocket_handbook_952.pdf>.
(3) "How Water Rockets Work." Water Rocket Porta1.26 Jan. 2007. 7
Nov. 2007
<http://wrockets.tribdesign.com/index.php?project=NICK&page=howtheywork>.
(4) Braeunig, Robert A. Rocket
Propulsion.] Jan. 2007. NASA. 7 Nov. 2007
<http://www.bracunig.us/space/propuls.htm>.
(5) Glazier, Colin and Spindel, Aaron. Bottle Rockets. May. 2001
< http://tuhsphysics.ttsd.k12.or.us/Researcti/lbOI/SpinGlaz/index.htm>