Introduction
NASA Glenn Research Center (NGRC), our sponsor, has asked us to use design, build, optimize,
verify, and document the thrust of a pulsejet; to do so, we used a steel shroud, or ejector, placed
behind the pulsejet. The results from our experiment can be used for the study of the more
expensive and complex Pulse Detonation Engine (PDE) Propulsion. We have completed our
study of the pulsejet, and found that the thrust does, in fact, increase with the use of an ejector.
The purpose of this report is to provide a complete documentation of our study.
Details and History of Project
Currently, NGRC is researching the PDE system. The PDE system works by mixing the fuel
with air. Once the detonation is initiated, it moves through the fuel/air mix. This results in high
pressure gas filling the detonation chamber. The detonation wave exits the engine and air is
drawn in by the reduction in pressure restarting the cycle. There are many benefits of using the
PDE. They include a reduced cost compared to regular turbojets, a high efficiency at higher
speeds, a greater reliability than other systems, and a potential to reduce hazardous emissions.
The pulsejet is used to model the PDE because the operation of the pulsejet is very similar to the
pulse detonation of the PDE. Because it has a more simple design and is less expensive, the
pulsejet is easier to test. The pulsejet system works by injecting fuel into the combustion
chamber with air from the mouth of the engine. Increased pressure in the combustion chamber
closes the valve at the mouth of the engine and forces the products out the rear end producing
thrust. After the products leave and the pressure in the chamber returns to its normal state, the
engine repeats the process by opening the valve and taking in more fuel. The exhaust from the
pulsejet then enters the ejector with additional outside air. This results in increased mass flow,
which causes an increase in the thrust. The pulsejet and ejector system are shown in figure 1.
Figure 1: Pulsejet and Ejector System Setup
In the past, groups have performed tests using various ejectors, but have not produced good
results. The past experiments have also not been well documented. There has been a problem
with the fuel system of the pulsejet. At speeds of approximately eighty miles per hour, the fuel
line would kink and no fuel could reach the pulsejet. Another group has fixed the fuel system
problem this semester, and we were able to run the pulsejet at high speeds with no problems.
In our study of the pulsejet, we constructed three (3) steel ejectors of various lengths. We then
tested each ejector with the pulsejet in the five-by-seven foot wind tunnel, and also obtained the
thrust of the pulsejet alone. Finally, we analyzed the data and determined whether the use of an
ejector does, in fact, increase the thrust of a pulsejet.
Method
The thrust of a pulsejet can be increased with the use of an ejector of the right design. The
purpose of the ejector is to increase the mass flow rate which then increases the thrust. The
exhaust gases from the pulsejet enter the ejector with additional outside air. This causes an
increase in the mass flow. The thrust from the ejector can be maximized from the position and
the design of the ejector. We were only able to examine the design of the ejector, but not the
position behind the pulsejet.
We used a design for the pulsejet that has been used by previous groups. We made three (3)
conical ejectors of various lengths: six, eight, and ten inches. We used an inlet produced by a
previous group and constructed the three conical shrouds. For the testing, we attached the inlet
to each of the ejectors with magnets. Each of the ejectors was made of steel, as was the inlet.
We chose this material because it is inexpensive and is able to withstand high temperatures
without melting.
To construct each ejector, we started with a sheet of twenty-two (22) gauge sheet metal. Using
the pattern shown in figure 2, we cut the metal, and then rolled the sheet into a cone. The ends
were then welded together to keep the cone shape. Using the remaining sheet metal, we
constructed stands for each of the three ejectors to be attached to the pulsejet test stand.
Figure 2: Sample plans for 8" ejector
After each of the ejectors was constructed, we tested the ejector and the pulsejet in the five-byseven foot wind tunnel for three days. We used the six-component balance in the wind tunnel.
This balance measures the drag, lift, and side forces as well as the pitch, roll, and yaw moments.
The balance uses six transducers to measure these forces and moments; the signals from the
transducers are sent to a data acquisition system that translates the voltage of the transducers into
forces and moments.
We spent most of the first day of testing calibrating the balance in the wind tunnel to determine
any error in the balance. To do this, we applied a known force to the pulsejet setup in each
direction separately. For example, we first applied a known drag force to the pulsejet and
measured the voltage output by the transducers. We then used the same procedure for each of
the other forces. We found that all of the forces and moments were linearly related to the voltage
output from the transducers.
After calibrating the balance in the wind tunnel, we first tested the pulsejet without any ejector
on the stand to produce baseline data. We used the data produced from this test to compare to
the data from the tests with the ejectors on the stand. To examine the thrust of the pulsejet, we
only needed to study the change in the drag component of the balance. We first took samples
with the pulsejet not running using the data acquisition system; then, we started the pulsejet and
took multiple samples of the thrust at thirty feet per second (30 ft/sec) and sixty feet per second
(60 ft/sec). We chose these values for the speed of the wind tunnel because the fuel system was
still unreliable at high speeds. Also, because the fuel tank can only hold a limited amount of
fuel, we were only able to run the pulsejet for a maximum of ten minutes. It takes the wind
tunnel longer to get up to higher speeds which would reduce the amount of time to take samples
of the thrust.
Once we obtained our baseline data from the pulsejet alone, we tested each of the three ejectors
the last two days of testing. We placed each ejector approximately one and a half inches behind
the pulsejet. We then used the same procedure that we used for the pulsejet alone. We first took
samples with the pulsejet not running. Then, we ran the pulsejet and acquired samples at the two
velocities. To reduce error, we not only obtained samples with increasing velocity; once we
reached sixty feet per second, we decreased the velocity and took samples as the velocity of the
wind tunnel went down.
Problems Encountered
As with any project, we encountered a few problems while testing the different ejectors with the
pulsejet. Our main problem was caused by the exhaust of the pulsejet. The fumes from the
pulsejet can be very dangerous if exposed to them for extended amounts of time. During our
entire time testing in the wind tunnel, the air was constantly being tested by OSHA
(Occupational Safety and Health Administration). After running the pulsejet for an average of
five minutes, we were required to wait forty-five minutes before entering the wind tunnel again
to change the setup. At one point, we ran the pulsejet for ten minutes, and we had to wait over
two hours before the levels of dangerous gases dropped to the acceptable level of five (5) parts
per million. Because we were required to wait an extended amount of time before entering the
wind tunnel, the amount of tests we could perform was limited. If we were able to start the
pulsejet, but for some reason it shut off in the middle of testing, we had to wait forty-five
minutes until we could try the test again.
Another problem we encountered is that the inlet was not secure on the ejectors when we started
testing. We first tested the six inch ejector in the wind tunnel with the inlet only pushed on to the
front of the ejector. When we were starting the pulsejet for the test, the suction produced by the
pulsejet pulled the inlet off of the ejector. To fix this problem, we used two very strong magnets.
The magnets were placed behind the inlet to attach onto each ejector, so it wouldn't affect the
flow of air over the ejector. After the one incident, the inlet was secure for the rest of the tests.
The pulsejet is very difficult to start in cold weather. During our testing, the average temperature
inside the wind tunnel was about fifty degree Fahrenheit. Even at this temperature, the pulsejet
needed to be heated with a heating gun before starting it. If the pulsejet was left sitting too long
before starting, it would not start because it would be too cold. The pulsejet was also very
temperamental when starting. This caused us to not obtain as many samples as we had originally
planned.
Yet another problem we encountered was that the pulsejet would cease functioning during a run
for no apparent reason. After disassembling the pulsejet we would discover that the reed used in
the combustion chamber had broken and pieces of the reed were gone. You can see the missing
sections of the reed in FIGURE BLAH, in addition we noticed that the ends of some flanges
would chip after the pulsejet had run for an extended time. The chipping may have resulted in
changing the performance of the pulsejet.
Figure BLAH: The Two Reeds After Running in the Pulsejet and Closeup of Reed Two (right)
The last major problem we found was the variance in the wind tunnel velocity. With each test,
we obtained data at different speeds because the wind tunnel velocity is difficult to control.
Because of the variations in the speed, it was more difficult to analyze the data produced by our
tests. In some tests, we were able to obtain thrust samples at thirty and sixty feet per second.
However, in other tests, the speed varied as much as ten feet per second.
Results
Based on the data from our wind tunnel testing, the overall resulting thrust increased
significantly compared to the thrust produced only by the pulsejet itself. As we expected, the
ejectors increased the overall thrust and worked well with the non-steady thrust engine.
Referring to Figure # generated from the raw wind tunnel data, we could see a general trend of
increasing thrust with the ejectors. We obtained a thrust of as much as 43% better than the
pulsejet with no ejector.
5.8
Thrust (lb)
5.3
10 inch
4.8
8 inch
6 inch
4.3
No Ejector
3.8
3.3
20
30
40
50
60
Tunnel Veocity (ft/sec)
Figure Something:
Due to the frequent turning on and off of the wind tunnel, the raw data fluctuates greatly, as
shown in figure ### above. The fluctuations are the greatest with the six inch ejector. The
vibrations in the ejector due to a weak upright support and unsteady thrust also caused the
variations in the data from pulsejet.
We were not able analyze how the resulting thrust generated is a function of the ejector length
and wind speed. Thus, we compiled the raw data and produced figures showing the resulting
thrust with the ejector length and wind tunnel speed inputs.
Thrust vs Ejector length
6
Pulsejet system thrust
5
4
30 ft/sec
3
60 ft/sec
2
1
0
0
2
4
6
8
10
12
Ejector Length
Figure #@:
Figure ### shows an overall increase in thrust from the pulsejet with no ejector to the pulsejet
with the eight (8) inch ejector. It decreases from the eight inch ejector to the ten inch ejector.
As shown in figure ####, the thrust increases from less than four (4) pounds to as high as five (5)
pounds with the ejectors. The optimal ejector length for generating maximum thrust is
somewhere between eight and ten inches. This applies for both speeds of thirty (30) feet per
second and sixty (60) feet per second.
Thrust vs Windspeed
6
Thrust (lbs)
5
4
6 inch ejector
8 inch ejector
3
10 inch ejector
No Ejector
2
1
0
20
30
40
50
Wind Speed (ft/sec)
Figure*&:
60
70
Figure ### shows the eight inch ejector had a slightly higher thrust than the ten inch ejector at
low speeds up to forty-four (44) feet per second. At higher speeds, the ten inch ejector had the
best performance. We observed a trend of decreasing thrust as the tunnel velocity increased.
As shown in figure ###, the thrust produced with the ejectors decreased over time while the
thrust produced without the ejectors stayed constant. This shows that the ejectors became more
inefficient as the wind tunnel speed increased. Eventually, if the wind tunnel speed was
increased greatly, the ejectors may not have increased the thrust at all.
Overall, the ejectors were shown to be very effective in generating extra thrust from the pulsejet
at low velocities. The optimal length of the ejector is between eight and ten inches. The ejectors
were reliable in increasing the thrust of the pulsejet despite the unsteady thrust of the pulsejet and
the change in the wind tunnel velocity. Reducing the vibrations in the ejector by making the
upright support stronger would make the resulting thrust more stable.
Timeline
Figure (*%):
Costs
Overall, the pulsejet project was not very expensive as shown in figure #####. However, we did
spend more than we had originally planned. When we started, we had planned to spend
$ 6,524.75. Because of the extended time we spent in the wind tunnel, our cost increased about
$ 1,000.
The only cost to the group included the four (4) sheets of 22-gauge sheet metal, a digital video
tape to document our tests, the transparencies, and the poster for the final presentation. We used
the steel inlet used by previous groups, so we did not have to pay for one ourselves. We also
used the fuel system and fuel used by the pulsejet fuel delivery system group.
Unit Cost
Quantity
$130.00/hr
$50.00
9.6
1
Ejector
Steel Plate
Steel Inlet
$ 4.00
$15.00
4
1
$ 16.00
$ 15.00
Supplies
Fuel Cost
$25.00/gal
1
$ 25.00
Labor
Machinist
Technicians
Engineering
$65.00/hr
$65.00/hr
$50.00/hr
2
15
100
$ 130.00
$ 975.00
$ 5000.00
$30.00
$0.75
$30.00
2
25
1
$ 60.00
$ 18.75
$ 30.00
Facilities
Wind Tunnel
Test Stand
Documentation
Final Report
Transparencies
Poster
Total Cost
$ 1248.00
$ 50.00
$ 7,567.75
Total
Figure ####: Cost Diagram of the Pulsejet Project
Further Research
There are many things that can be done for further study of the pulsejet. The most relevant to
our study would be to build more ejectors of lengths between those we tested and get even more
resolution in the data. Another area for further study would be to design and build a mechanism
to change the separation between the ejector and the pulsejet. Building a mechanism to remotely
change the separation is necessary due to the limitations on entering the wind tunnel. In
addition, a new data acquisition system could be used to find time dependant values of the
pulsejet. Finally, to simplify the testing, a different test stand should be designed and built that is
specifically for thrust measurements.