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Day 31 -
Thrust Measurement Rig and Thrust Data
Configuration of the thrust measurement rig
This dampner was too big due the induced
waves in the water.
A smaller dampner set up that worked well to
absorb the vibration.
Measuring out an exact dose of foaming
agent.
Test firing foam.
Foam tests are much more fun.
The aftermath of a test.
Date: 6th April
and 8th April
2007 9:30am -
12 noon
Location:
Backyard
Conditions: Overcast, partly sunny, with
occasional shower.
Rockets:
Name
Capacity
Notes
Polaron IV
8.8 L
This is the rocket body without
any recovery system or fins, used
purely for testing performance.
Team Members at
Event:
PK, GK, AK, John K and Paul K. Number
of Firings: 10
We have finished constructing the thrust
measurement rig, and after a couple of
modifications to it we managed to obtain
some good quality data for both water and
foam. For details of the rig see the section
below. This is a bit of a long write up so
you may want to grab a cup or skip ahead to
the sections you
are interested in.
Day's Events
When we initially set up the test rig we
ran a 4 tests to see how effective it was
and if our measurements were within range of
the scale. We first fired the rocket 2
times, once with water and once with foam.
These were uncalibrated tests and were meant
to give us an idea how well the set up was
working. One the most obvious issues was
that the scale needled bounced around so
much due to the scale's spring mechanism
that reliable readings could not be made
with any accuracy.
During online forum discussions it was
suggested by Clifford Heath to use a dampner
made from a disk on a stick sitting inside a
bucket of water. Anti-gravity Research had
used a similar setup for their thrust
measurement. We placed a small disk ~100mm
in diameter in a bucket and it certainly
reduced the wild oscillations but it was
still bouncing around too much. We then
switched to a much larger disk in more water
and that settled the needle down really
well on the subsequent 2 test firings.
On review of the data we found that
the large disk had induced a fairly large
wave in the bucket which then reflected off
the sides and gave a great big sine wave in
the data. We were getting close but we
needed to get rid of the wave effects inside
the bucket.
We then built a much smaller dampner from
a 1.25L PET bottle but this time the disk
had holes in it and fit fairly snugly in the
bottle - a lot like a coffe plunger. This allowed water to pass through
the holes and around the edges but no big
waves were created and that smoothed up the
data really well.
Data
The thrust curve data shown below was for
a rocket with the following parameters:
Parameter
Value
Notes
Capacity
8.8L
4 x 2L bottles joined
neck to base with a 8mm Robinson
coupling. The actual volume of each of
the 2L bottles is 2210mL
Water
2100ml
When bubble bath was
added then it was 2050ml Water and 50ml
Bubble bath for equivalent weight.
The rocket used our Jet Foaming
technique to create foam.
Pressure
100 psi
For all tests (air)
Nozzle
7 mm
straight through hole for all tests
Experiment setup and Explanatory Notes
We chose a restricted nozzle to help
stretch out the data so it could be more
accurately measured and visualised.
The thrust measurement rig was balanced
so that it read zero Newtons with the rocket
empty. This was done so that the data
obtained would reflect the thrust (force)
that contributes only to rocket
acceleration, as it already incorporates the
force needed to overcome the weight of the
water in the rocket. Hence the thrust measurement
does not represent the absolute thrust.
Ultimately we are only interested in how
much force is contributing to the
acceleration of the rocket upwards anyway.
If we had balanced the rig while the
rocket is loaded with water, by the time the
thrust ended and all the water was out, the
scale would read the weight of the water
since it would now be unbalanced by that
amount. This would have introduced a bias to
the data. Since we do not know at what rate
the water empties from the rocket, we later
cannot remove this bias from the thrust
data.
Three firings were made with water only,
and three firings were made with foam so
that reproducibility and accuracy of data
could be verified. All the parameters for
the 6 firings were identical except in the
foam tests, 50 ml of water was replaced by
50 ml of normal bubble bath solution. For
the foam tests the water was also
redistributed in the rocket so that Jet
Foaming could be used. See previous tests
for more details.
The total volume and weight of water was
the same for both tests.
Analysis
It should be first noted that these are
static tests and in real life an
accelerating rocket may behave differently,
however, these tests are a good
approximation to what really happens.
We obtained the following thrust values
by video taping the kitchen scale on the rig
at 60 frames per second and then
painstakingly (and I mean painstakingly)
stepping through frame by frame in the video
and recording the values in a spreadsheet.
Digital logging is definitely needed. Much
thanks goes to my wife for helping to enter
the data.
Water
NOTE: Click on
the graphs for more detail
Water only thrust
curves
From the curves you can clearly see that
the data is quite clean and test 1 and 3
were almost identical. Test 2 shows a
slightly lower thrust at first and remaining
lower, however, it is compensated by a
longer burn. Why this happened we are not
sure but perhaps something to do with the
turbulence around the seal in the nozzle.
The area under the curve represents the
total impulse generated by the rocket and
this was:
Test
Total Impulse
(N-s)
1
86.647
2
84.833
3
86.661
Average
86.047
Which is within ~3% of each other
giving a fairly accurate reading for
reproducing the data.
Explanation of
different parts of the thrust curve for
water only
The initial spike is caused by the
release of the rocket and can be ignored, or
averaged out.
There is a clear almost flat line for the
thrust in the water phase, and then there is
a clear transition to air-pulse which
remains constant for a short while before
tapering off to zero. The shape of this
curve was identical in all three tests.
Foam
Foam thrust curves
This is where it gets a little more
interesting. All three foam thrust curves show an almost
identical shape although the thrust curve is
a lot more rough than for water only. This
is most likely caused by the foam density
varying throughout the firing. If the foam
density was homogeneous then this curve
would be a lot smoother.
Foam thrust curve
explained
Again the curves show the initial launch
spike, but also show an unusual effect where
the thrust varies markedly for about 500 ms
approximately 400 ms after the start. This
isn't an anomaly of a single firing as all
three firings showed this. This is most
likely related to the Jet Foaming technique
or the blow-through effect and we will
explore this region further in future tests.
At the moment we are referring to it as the
Foam Thrust Anomaly (FTA) until we can
identify the cause.
It also doesn't appear to be a resonance in
the measurement system as the data does not
show this effect in any of the other tests
or any other portions of the thrust curves.
The area under the curve represents the
total impulse generated by the rocket and
this was:
Test
Total Impulse
(N-s)
4
74.689
5
72.911
6
73.933
Average
73.844
Which is within ~3.5% of each other
giving a fairly accurate reading for
reproducing the data.
The curve when averaged is a fairly
slowly decaying curve with no transition to
air-pulse.
The maximum thrust generated
during the flight is about 50% of the
maximum for water only. The average thrust
was 9.98N for foam compared to
20.4N for water only.
Now when we compare the total impulse
generated by water only and foam it is
evident that foam results in lower total
thrust. Foam produced only 86% of the
total impulse of water.
Thrust curves combined
for foam and water
However, the foam generated thrust for
almost twice as long as the water only
rocket! In one test the thrust lasted for
8.5 seconds!
Total impulse isn't the the whole story
when calculating the apogee of a rocket.
When you take drag into account and the fact
that it is proportional to the square of the
velocity consider this: Is a faster rocket
burning for shorter amount of time going to
fly higher than a slower rocket burning for
a longer time?
We pose this as a challenge to the reader
to use the data provided to calculate the
rocket apogee. You can consider the dry mass
of the rocket to be 650 grams.
(If the video does not play, try the latest
Flash player from Macromedia)
Thrust Measurement Rig
The thrust measurement rig consists of a
balance with the rocket mounted on one side
with a kitchen scale on the other. The
kitchen scale measures 0 - 500grams.
Attached to the lever arm is a dampner that
prevents the spring in the scale from
oscillating as the rocket induces vibration
into the rig.
The scale cost about $6 and will be
eventually replaced with an electronic load
cell and data acquisition system to make
measurements faster.
The scale dial is video taped at 60
frames per second which gives us a stable
time reference for the measurement samples.
These are then transferred to a spreadsheet
by replaying the video frame by frame.
The balance can swing all the way around
on the pivot allowing us to permanently
mount the rocket on it, while giving us the
ability to easily fill it with water.
The nozzle is 85mm from the pivot point,
while the scale is at 1415mm from the pivot
point.
Refer to the photos at left and the video
for further details on the operation of the
rig.
Lessons Learned
Foam produces a lower total impulse
compared to water in this test.
When a water only rocket fired, there
was a brief period during the air pulse
where constant thrust was produced for
about 300ms. This was unexpected as you
would expect there to be constant decay in
the pressure. This may be due to the
bottle's elastic property. The bottle
contracts as the air exits keeping the
thrust constant until they stop
contracting and you get the expected decay
in thrust.
There is no clear transition to air
pulse with a foam rocket and the thrust is
a lot rougher most likely due to
non-homogeneous foam density.
The foam thrust lasted about twice as
long as the water only thrust.
What's Next
We will perform more tests with different
sized nozzles, various pressures, different
amounts of foaming agent, and different
foaming agents.
The big test will be the construction of
a number of CD nozzles to see if the total
impulse can be lifted above that of a water
only rocket. We are hoping that with the
lower drag and an improved nozzle efficiency
we will be able to improve on water only
water rockets.
Update: The
CD nozzle results are now available here:
Day 32
Acknowledgements
Thanks also to Jim Fackert for suggesting
the camera pointed at the scale to take
readings. We should also thank Gordon M,
Richard W, David L, Trevor, Brian and many
others for useful suggestions on how to set
up the thrust measurement rig.
Test Record
Test
Rocket
Pressure (PSI)
Notes
0a
Polaron IV
80
Uncalibrated water only test with 7
mm nozzle. No dampner and the data
was unusable
0b
Polaron IV
80
Uncalibrated foam test with 7 mm
nozzle. No dampner and the data was
unusable
0c
Polaron IV
80
Uncalibrated water only test with 7
mm nozzle. Large dampner. Data was
unusable due to wave reflections in
the bucket.
0d
Polaron IV
80
Uncalibrated foam test with 7 mm
nozzle. Large dampner. Data was
unusable due to wave reflections in
the bucket.