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ROCKET PERFORMANCE LAB

Objectives
At the end of this laboratory, you should be able to:
•	Determine the Impulse of a rocket engine
•	Predict the performance of a model rocket under zero-drag conditions
•	Explain the effects that drag & wind will have on a rocket’s flight path and performance
•	Predict the center of mass and center of pressure locations of a model rocket design
•	Assess the stability of a model rocket design
•	Explain and follow range safety procedures
•	Successfully launch a model rocket

Problem Statement
In this lab, you will work on a small team (3-4 students) to design, analyze, and then flight test a model rocket. The design goal is to build a reliable rocket that will maximize downrange flight distance when launched at an elevation angle that you choose. This requires ensuring that the rocket is stable in flight and will have as little drag as possible.

We will first launch a calibration rocket (made by the Instructor) at a 45° elevation angle to factor in the wind effects on a given launch day. Then your team will launch your rocket, and we will assess the distance your rocket flies downrange compared to the calibration rocket. The goal is to be the team with the largest percent increase in downrange distance relative to the calibration rocket.

Your team will receive a set of rocket materials from which you will construct your rocket. You are not allowed to use any additional materials (other than adhesives) or to make any test flights before the official data-collection flight.

Your team will hand in a single report documenting your work. Your grade will depend 50% on the whole team grade, and 50% on your own contribution. We expect each team member to take on an equal load in terms of time and effort. Note that just flying the rocket will not count for much effort.

1. Performance prediction
We will begin by predicting the performance of a model rocket while ignoring the effects of drag, wind, or the mass change as fuel is burned. Assume that the rocket thrust is maintained at a constant elevation angle until burnout, after which the rocket flies ballistically.

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Assume the following parameters are known: Average rocket motor thrust: T
Rocket motor burn time:	tb
Total rocket mass:	m (assumed constant)
Launch elevation angle:	q (measured from the horizontal)

(a)	Find an equation or set of equations to compute the maximum altitude attained by the rocket. Hint: Do separate computations of just the horizontal components of thrust, acceleration, velocity, and position, and another set of computations for just the vertical components of thrust, etc.

(b)	Find an equation or set of equations to compute the total downrange distance flown by the rocket. Same hint as for (a).

(c)	The parameters for the calibration rocket are given below. Compute the estimated downrange distance the calibration rocket will fly using your solution from part (b).

Average rocket motor thrust: T = 3.03 N
Rocket motor burn time:	tb = 0.36 s
Total rocket mass:	m = 0.027 kg
Launch elevation angle:	q = 45° (measured from the horizontal)

2 Experimental Determination of Thrust Curves 
Thrust curves will be determined using a Pasco force meter connected to a computer. The equipment should be setup outside to minimize the smoke dispersal through the building. Calibrate the force meter with known masses. Have your instructor check your setup before firing the rocket engines. 
The Rocket Engine Test Bracket is designed for use with the Force Sensor and a computer interface to determine the magnitude of impulse of Estes model rocket engines (series A – D). 
Setup
Set up the equipment
1. Remove the hook from the end of the Force Sensor. Use a small screwdriver to connect the Rocket Engine Test Bracket to the Force Sensor in place of the hook (Figure 1).
Figure 1
Attaching the Rocket Engine Test Bracket to the Force Sensor

2. Mount the Force Sensor on the support rod in one of two ways: horizontally as shown in the diagram.

3.	Insert the model rocket engine into the test stand with the engine’s exhaust port pointing away from the Force Sensor.
Set up Logger Pro
1.	Set up the Force Sensor in Logger Pro..
2.	Adjust the sampling rate for the Force Sensor to 500 Hz.
Prepare for Ignition
1.	Move the equipment to the launch site.
2.	Insert the igniter into the rocket engine and spread the wires apart, leaving the paper separator in place (Figure 3).

Figure 3
Inserting the igniter into the model rocket engine

Note: Carefully follow the instructions that accompany the rocket engine and igniter.

3. Clip the alligator clips of the cables from the launch controller to the igniter wires.
Note: Do not allow the igniter wires or
alligator clips to touch each other. Be certain that the safety key is not in the launch controller. Insure that the cables are well away from the exhaust area  and the computer interface is as far away from the exhaust area as possible.

4.  Check the exhaust area to ensure it is clear of any combustible material, people, or animals.

Data Collection
1.	Insert the safety key into the launch controller.
2.	Begin recording data and move to 15 feet (5 meters) away from the Rocket Engine Test Bracket.
3.	Ignite the rocket.
4.	After the burn, stop recording data.

Data Analysis
1.	In Logger Pro, click the Autoscale button to resize the graph.
2.	Select the area under the curve and record the integrated value (impulse of the rocket engine).
3.	Use the smart cursor to determine the time to the ejection charge.
4.	Compare your recorded values with the expected values for the type of you are using.
Verify the thrust curve by comparing three engines of a given type. Plot your three data curves. 
1.	How much variation is there between the curves from the three trials? How much of this variation do you think is due to the instruments used to measure the thrust curves and how much is due to intrinsic variations in the rocket engines? 
2.	How well do your thrust curves compare to the manufacturer’s curves? Are the differences between them systematic or random? 
Come up with an experimental thrust curve that combines your trials and an estimate of the uncertainty for it. 

2.	Concept Development
As a group, brainstorm as many rocket design concepts as you can (varying body length, fin layout, etc.), and then downselect to at least three different likely design concepts. Sketch each of these three designs – these need not be engineering drawings, but just conceptual sketches. Also briefly describe the advantages and disadvantages of each of the three design concepts.

3.	Detailed Design Carefully measure and weigh the rocket components in your rocket kit. Then, generate a detailed design for your rocket. This should include:

Engineering drawings (side and top views, dimensioned)
Computation of the center of mass position (CM), measured from the nose of the rocket Computation of the center of pressure position (CP) using the Barrowman method (detailed on the Rocket Stability handout, linked to the syllabus)
Safety Margin: (CM – CP) / D, where D is the body diameter.
Desired launch elevation angle
Computation of the maximum altitude and downrange distance your rocket will fly, and the expected % increase or decrease relative to the calibration rocket
Forces vs. time profile and total impulse

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Your rocket must have a Safety Margin of at least 1, so make and document revisions to your design as necessary.

4.	Construction
Build the rocket to the specifications in your engineering drawings. After it is built, doublecheck the center of mass position by balancing the rocket on your finger or a small dowel and comparing against your estimate from part 3. If the CM is not where you predicted it would be, recompute the Safety Margin using its new position, and make and document any necessary design modifications to ensure the Safety Margin is at least 1.

5.	Launch
Flights will take place on October 11th. You must bring your documentation for the center of mass and center of pressure positions to the field at flight time in order to be allowed to fly. You must also follow all range safety procedures. During the flight, the Instructor will measure the actual downrange distance your rocket flies and compare it to the calibration rocket distance. As in the real world, you only get one chance when you launch a rocket, so re-launches will not be allowed.

6.	Final Report Make a single, final team report that includes the following elements:

•	Team member names and each member’s specific contribution to the project
•	Performance analysis from parts 1(a) – 1(c)
•	Design concepts, sketches, and a Product Design Matrix.
•	Detailed design, including all elements listed in part 3
•	Any notes on modifications made to the design during or after construction
•	A brief flight report, documenting downrange distance flown and comparing this to the
calibration rocket and to your predictions. Discuss what contributed to any discrepancies.
Also describe and discuss the overall performance of your rocket, whether it met (or
didn’t meet) your expectations, failed, etc.

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Rocket Construction Tips

As a team, pick up your rocket kit from the Instructor in the ab. Carefully measure and weigh each component so that you can design and analyze your rocket before actually building it. You may want to shorten the body tube and will make your own fins from balsa sheet. You can predict the mass of the fins or body by first determining each part’s mass/length or mass/area and then multiplying by the designed length or area. You only get one chance to build the rocket from the supplied materials, so be sure to carefully plan out your design before construction.

Fins will be strongest when the wood grain runs parallel to the leading edge of the fin. Lightly sanding the rocket body will make the fin attachments stronger when adhesive is applied.

The rocket is launched on a launcher that consists of a steel rod inclined at your chosen launch elevation angle. A short (1-2” long) plastic launch guide tube (a piece of a drinking straw) must be attached to the side of your rocket. Attach the tube starting approximately 2” from the bottom of the rocket, and ensure that the tube is parallel with the rocket body. Inclining the guide tube or fins will cause your rocket to spin about its long axis – this will decrease the energy that is applied toward the rocket’s forward kinetic energy, reducing its flight distance.

The rocket motors have a diameter less than that of the body tube. The rocket motor should first be mounted inside a smaller short tube that exactly fits the motor (provided in your kit), and then design a mount using balsa strips to hold the motor + tube inside the body tube. Design this mount so that the motor assembly fits tightly in the body without any adhesive, which will let you remove the used motor after your launch so the rocket can be reused if you’d like. Because the motor is not glued in place, you will also need to make and glue some form of stop inside the main rocket body to prevent the rocket motor from moving or pushing up inside the body during launch.