Model Rocket Engine Force
Experiments from Team Labs

Experiment Profile

Connections:		Geometry, Trigonometry, Physics, Chemistry, Meteorology, Rocket Design
Skills:		Scientific Inquiry, Graphing, Analyzing, Measuring, Inferring
Math Concepts:		Difference, Sum of Difference, Absolute Value
Duration:		1 Class Period
Team Size:		2-3 students per group (with teacher/adult supervision), or whole class demonstration
Content Standards:		Science Standard A (grades 9-12) Science Standard B (grades 9-12) Science Standard E (grades 9-12) Math Standard 1, 4, 5, 9, 12, 13 (grades 9-12)

Summary

In this experiment, students will gain a better understanding of force. An Estes Rocket Company model rocket engine will be tested using a Force Probe and computer-based data collection software. This experiment will measure the thrust produced by a model rocket engine. This measurement will include the engine’s impulse force, total force and its ejection charge in Newtons. The results can be compared to the literature that is sent with the Estes Model Rocket engine/s. Because the manufacturer uses strict engineering tolerances and standards in its design, it will be interesting to see how close the results of this experiment are--using the 16 bit resolution of the Think Station and extreme sensitivity and accuracy of Team Lab’s Force Probe-- when compared to the Thrust Profile supplied by the manufacturer with its engines.

Materials

Host Computer

ThinkStation SP16 Interface Kit

ThinkStation Interface

Power Supply

Communications Cable

Excelerator 2000 Software

Force Probe with API (Automatic Probe Identification)

Force Probe Table Adapter (from the accessory kit)

Table

Two Ring Stands or Table Stands with clamp attachments

Several small clamps

Rocket and rocket engine

Metal or PVC pipe, slightly larger in diameter than the rocket body. Cut the pipe to about half the length of the rocket.

Cork or rubber stopper to fit diameter of rocket

Background

During World War II, one of the many reactionary endeavors in the United States to the war was the creation of the Reaction Research Association in 1943. It was established as a vehicle to promote scientific discovery in the area of rocketry and propellants. This encouraged thousands of model rocket experiments with self-produced propellants that were ultimately fatal to the budding young scientists, teachers and students seeking to contribute to the war effort. With awareness and interest in rocketry beginning to peak after the war during the 1950s, the first model rocket was developed and patented in 1957 as an educational aid for the science curriculum.

With the advent and development of the safe-use model rocket engine under the Model Rocket Engine Standards of the Federation d’Aeronautics Internationale (FAI), over 300 million accident-free launches have occurred worldwide. This is mainly due to the development and manufacture of a cartridge-type pre-assembled engine that was designed to strict military standards for the use of black powder explosives. Remote electrical ignition is used to launch the rocket. This launch process has also helped to establish its extraordinary safety history.

Designing and launching model rockets has become an international hobby. Several U.S. Space Shuttle pilots have noted that their interest in space exploration started in model rocketry. There has also been Massachusetts Institute of Technology (MIT) doctoral students that have solved professional rocketry problems with model rocketry. Today, the mass-produced model rocket engines are pre-loaded with a nontoxic propellant (not recommended for ingestion!), with non-metallic casings of paper and clay, rendering it expendable and biodegradable (reloading is strictly forbidden). All in all, this is one of the safest and creative hobbies that has many students and adults alike thinking and dreaming of their next design and launch. These dreams will forge the next generation of improved design and engine efficiency in model rocketry and beyond.

Procedure

Collecting Data with the Force Probe

1. Select an area to set up your experimental apparatus, either outside or in a well-ventilated warehouse or garage area free of any combustible materials.
View from side and slightly above	2. Attach a ring stand with a clamp attachment to a table. Attach the pipe to the ring stand with a small clamp, so that the pipe is parallel to the ground. (Here, we have placed a board on the table to provide the proper thickness for the clamps to attach securely. This is not necessary if your table is thick enough.)
	3. Attach another ring stand to the table with a clamp attachment. Prepare the Force Probe by snapping the Table Adapter (flat surface) onto the end of the probe. Be sure to push it on hard enough that it snaps over the o-ring of the tip. A slight amount of lubrication (such as lip balm or petroleum jelly) can be used to make this easier. Attach the Force Probe to the ring stand so that it aligns with one end of the pipe. Leave about 10 cm of space between the tip of the probe and the end of the pipe. View of clamp setup, from side and slightly above
View from side and slightly above
4. Connect the Force Probe (API) to the ThinkStation interface by inserting the connector into any analog jack (outlined in black).
Closeup of rocket with cork inserted, aligned with Force Probe	5. Remove the rocket nosecone. Slide the rocket into the pipe, and place a fitted cork or rubber stopper into the front of the rocket. Be sure that the corked end of the rocket is oriented to the Force Probe in a horizontal position.
Closeup of engine inserted	6. Insert the engine to be tested in the tail end of the rocket. Remove or bend away the clip that holds the engine in place to the rocket body. This will ensure that when the ejection charge occurs--which would push out the parachute through the nosecone--it will instead eject out of the end of the rocket, once again applying force to the system. This will allow the engine to expend itself, and to prevent combustion and burn from occurring within the rocket inside the pipe; the engine will simply blowout the end of the rocket. Therefore, a safety zone of 10 to 20 meters will need to be cleared around and beyond the end of the pipe depending on the size of the engine being tested.
View of setup, from different angles
Closeup of engine with leads live	7. Insert the igniter, ensuring that the ends are not crossed so that it does not short out, and that the igniter’s wires are touching the propellant grains. Insert the igniter plug and bend the wires out and back in preparation for attachment to the micro-clips. 8. Put on your safety glasses (and insist that all others helping within the test site area do so). Attach positive and negative (red and black, respectively) alligator micro-clips to the engine close to the tape strip that is on the igniter, ensuring they are not touching.
Show me	9. Prepare your electrical ignition source (a fresh 12 volt battery source will do if you do not have an official model rocket launching system). DO NOT attach your positive and negative lead wires to your electrical source at any time. You will simply touch the ends of the lead wires to the battery’s connectors at the proper moment (step 14).
Show me	10. On your computer, launch Excelerator 2000 and click on the Connect&GOTM icon. Excelerator will automatically identify the Force Probe and create a graph of force vs. time.
11. Set your sample rate to 100 Hz (samples per second). Set your time to 20 seconds. 12. Zero your Force Probe.
	13. Start data collection by clicking the green GO button on the left side of the Excelerator toolbar.
14. Wait approximately 2 seconds, and then touch your alligator micro-clips to the corresponding positive and negative leads of your electrical source.
15. Observe the experiment over time and make sure you are clear of the end of the pipe where the engine will eject its byproducts of combustion and the engine itself with its ejection charge after the delay that is indicated on the rocket--this occurs after the initial thrust and burn.
16. Retrieve the expelled engine and save your results.

Analysis of the Data

A. This is the Estes Rocket company’s Thrust Profile for a B6-4 Engine. You will note the similarities in the Excelerator Fast Graphs to follow. Note: we tested a B4-2 engine. The code on the engine refers to: B = Total Impulse (total power in N-s) produced by the engine 4 = Average Thrust or average push in N (4.45N = 1 lb.) 2 = Delay; the time delay in seconds between thrusting and the ejection charge.
View screen	B. Our Initial Data Profile for the B4-2 engine. This is comparable to the manufacturer’s literature that is included with a B4-2 engine.
View screen	C. Maximum Thrust achieved: at 6.9 s = 10.04 N This is comparable to the manufacturer’s literature noting a B4 engine to produce about 13 N of thrust.
View screen	D. Total Impulse in N-s (which is found by analyzing the area under the curve of the total impulse, i.e. when the engine is producing thrust for propulsion of the rocket) = 4.37 This is comparable to the manufacturer’s literature noting a B4 engine to have a Total Impulse of 5 N-s.
View screen	E. Average Thrust in N (indicated in Statistic Table) = 4.01 N This is comparable to the manufacturer’s literature noting a B4 engine to have an average thrust of 4.15 N.
View screen	F. Delay Period (no measurable thrust, the delay designed into the engine to await deployment of the parachute) = 1.93 s This is comparable to the manufacturer’s literature noting a B4 engine to have a delay period of 2 seconds.

Conclusions

The Estes Rocket that we tested using the Force Probe allowed us to confirm the manufacturer’s claims within a ± range of about 10%, which is allowed for and indicated on the engine’s specifications. Storage and shelf life of the engine may also affect the results. The sensitivity and accuracy of the Force Probe/Thinkstation/Exclerator 2000 apparatus made it clear when each of the successive stages of the rocket’s design came into play--initial thrust maximum, burn-out phase, delay period and ejection charge.

The 10.04 N of the initial impulse corresponded to approximately 2.25 pounds of thrust. It is interesting to compare this to the thrust produced by the Space Shuttle’s Main Engine, which is featured on the front of Team Lab’s Physics curriculum book, which has been calculated to be 1,739,107 N at sea level. One could include this experiment as a precursor to the design and modeling of a model rocket for testing, for it would help to indicate what size and type of engine you may want to design, depending on the goals of your rocket.

Extensions

• Try differing engine sizes and note their indicated impulse thrust and profile to the manufacturer’s Thrust Profiles.
• Calculate approximate distance acquired per Newton from your Force Probe data and actual test launches of differing model rockets.
• Do aerodynamic design considerations come into play?
• What other sources of thrust could be used to launch model rockets?
• Try testing them against the Force Probe and compare their thrust rates and efficiency.

For further explanation of the mathematics and design considerations involved with creating and launching model rocketry, as well as links to other helpful sites, visit http://www.execpc.com/~culp/rockets/rckt_eqn.html#Calc.

About the author... Marc Mueller is the Secondary Curriculum Specialist at Team Labs. His background includes packaging and mechanical engineering, secondary science, technology and vocational instruction. His real-world experience is mirrored in his curricula as he has been focused on engineering, creating applied technology laboratories, and the creation of pre-engineering, computer technology and vocational coursework and activities throughout his career.

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