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Lab #4 – Rocket Engine Impulse. Rev 9/21 PHY 101 University Physics I
Glendale Community College
Name
Experiment 4 Rocket Engine Impulse
Objective
Determine thrust, impulse and specific impulse of a rocket engine. Understand the connection
etween impulse and a force versus time curve.
Equipment
Force sensor, labquest mini interface, rocket engine and igniter, mass balance, ignition system,
lab stand, engine holder and computer.
Procedure
1. Obtain a rocket engine and mass it before it is burned.
Pre-burn Mass = grams
2. Insert an igniter into the rocket exhaust nozzle and hold the igniter in place with a plug.
3. Set up the force sensor including attached engine holder on a lab stand and clamp the engine
in the holder.
4. Connect force sensor to a labquest mini, connect mini to computer and launch Loggerpro.
Ensure a force versus time graph is displayed.
5. Take an ignition system, the computer and all peripheral devices outside.
6. Set up the rocket engine on the stand away from the building, cars, flammable objects,
pedestrians, ra
its, snakes, quail, road runners and security.
7. Weigh down the stand with a large rock.
8. Making sure the program will measure force, set the recording time to at least 30 seconds, if it
isn’t already set.
9. Hit collect, then shortly after data collection starts fire the engine.
10. Let the motor burn out completely, including firing the ejection charge before stopping the
program, or just let the program time out. If the motor doesn’t ignite, call for your instructor.
11. Record the maximum force and get the area under the curve. Only include the time interval
when it was firing, but observe and note what happens when the ejection charge goes off (a burst
of flame will shoot out the forward end).
Maximum force = N
burn duration (look for delta t) = XXXXXXXXXXs
Impulse = Area under curve (integral) = N s
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Lab #4 – Rocket Engine Impulse. Rev 9/21 PHY 101 University Physics I
Glendale Community College
12. Check your results with your instructor. If the results do not look good, obtain another
engine and igniter and repeat the process.
13. If the data look good,
ing everything back inside and mass the engine again.
Post burn Mass = grams
Analysis and Questions
1. Calculate the mass of propellant burned by subtracting the post-burn mass from the pre-burn
mass.
Mass of propellant = grams
2. Impulse = area under the force versus time graph and is measured in Newton – seconds =
Ns. The impulse is labeled as “integral” in the image. Write the impulse here. Ignore –
Impulse = Ns
Impulse is also the change in momentum of a mass, where momentum = mv. m = mass (kg)
and v = velocity m/s. Using the impulse you just determined, calculate the final velocity of a
ocket starting from rest when its average mass is 38.0 grams. What is the necessary unit of
mass?
3. The force time curve does not display a constant force. The force quickly peaks then drops to
an approximately constant level then burns out. Why does the force shoot up so high initially?
Think about a toy rocket on a launch pad with that engine in it. What would you observe when it
launches? Watch a you-tube video. What does the rocket have to do quickly? What purpose is
served by the fins?
4. What is the purpose of the thrust leveling off during the bulk of the time the engine fires?
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Lab #4 – Rocket Engine Impulse. Rev 9/21 PHY 101 University Physics I
Glendale Community College
5. Convert the impulse to pound seconds using the impulse from above and the conversion that
4.44 N = 1.00 lb.
6. Convert the mass of propellant burned to pounds using the conversion 454 grams = 1.00 lb.
7. Calculate the rocket engine’s specific impulse. Specific impulse = total impulse in units of
pound second divided by pounds of propellant burned, thus the unit is just seconds. A rocket
motor with high specific impulse has a value of 500 s. How do these engines stack up in
comparison?
8. Sketch the force versus time curve below. Indicate maximum thrust, constant thrust and
engine burn out.
Force (N)
XXXXXXXXXXTime (s)
Physics 115 Spring 2020
Addendum to Rocket Engine Impulse Experiment
Also view the video of the engine burning.
Preburn mass of engine Engine with igniter beside holde
Complete apparatus
XXXXXXXXXXRod stand
XXXXXXXXXXForce senso
XXXXXXXXXXLabquest mini computer interface
XXXXXXXXXXRocket engine in holder and attached to force senso
XXXXXXXXXXIgnition system
The rocket engine is massed before and after it is burned to determine the amount of propellant
urned. Propellant mass must be determined to calculate specific impulse.
The complete setup is fairly simple. The engine holder is connected to the force sensor which is
connected to the labquest mini which is connected to a computer. The computer takes force
versus time data. See the next few figures for data you need to answer the questions and perform
calculations.
Area under the curve
You may ignore the minus
sign.
Force, that is, engine thrust
t = the duration of the
highlighted portion of the
graph.
t is the amount of time
the engine actually fires.
Looking at the very tip of the force
curve (top if it were inverted. Note the
location of the cursor. Again, ignore
negative values.
Coordinate of the tip
(time (s), force (N))
All force values are negative because of direction, just like every other vector problem we’ve
considered. If the engine could have been turned around somehow, the forces would have been
positive. Think about what the force vs. time curve would look like if it were rotated 180
o
about
the x (time) axis.
When you draw the red curve on page 2 in your lab report, you may flip it upright.
The impulse generated by the engine = area under the curve. This is not a geometrical area, its
unit = N s, which is a unit of impulse.
The experiment procedure pages contain all the data you need to perform calculations and
answer questions.