IEEE TRANSACTIONS ON EDUCATION, VOL. 38, NO. 4, NOVEMBER 1995 32 1

An Interdisciplinary Senior Design Course Utilizing Electronically Guided Model Rockets

A. Scottedward Hodel, Member, IEEE, and Thomas A. Baginski, Member, IEEE

Abstract-Electrical engineering graduates are faced with a wide variety of technological and managerial decisions. The ability of the engineer to interact with the entire spectrum of individuals involved in marketing a product is a critical component of the overall success of a project. A senior level design course is presented that integrates the numerous phases of project development in order to prepare the student for the industrial environment.

I. INTRODUCTION HE evolution of a product from concept to production can T be an extremely involved process that proceeds through

various phases such as design, simulation, prototype construc- tion, testing and evaluation. The complexity of the process generally requires the engineer to interact with numerous technical disciplines. Patent protectiodinfringement, product licensing, cost analysis, marketing and documentation are all critical parameters in determining the ultimate success of a de- sign. Additionally, the ability to effectively communicate with technical as well as nontechnical individuals in both a written and oral format is imperative for successful implementation of a project.

An electrical engineering senior design course was struc- tured that addressed these topics. The course was offered as a two quarter sequence which is mandatory for graduation. Successful completion of the first quarter is necessary for students to proceed in the sequence. The project required students to design and flight test a guided model rocket that had to meet a variety of specifications. The class was divided into 5 groups of 5 to 6 students per group. Each group had to submit a formal proposal to a panel of faculty that acted in a managerial role. The class was structured in order to lead students through the key steps involved in the industrial development of a product.

The remainder of this paper is organized as follows. A sum- mary of course administration and student project guidelines and constraints is provided in Section 11. Following this, a description of student project development with an example of one student team’s results is presented in Section 111. Conclusions and future course possibilities are presented in Section IV.

11. PROJECT GUIDELINES AND CONSTRAINTS

The course project was selected in order to provide students with an opportunity to apply their academic experience in a

Manuscript received December 4, 1992; revised June 19, 1995. The authors are with the Department of Electrical Engineering, Auburn

University, AL 36849-5201 USA. IEEE Log Number 9413969.

single systems-engineering project requiring a broad base of engineering background. Students were required to design a guided model rocket that would locate and tum toward the sun when launched from up to 30 degrees from off vertical. This project required students to design sensing, guidance, actuation and power circuitry while meeting requirements set by the Federal Aviation Administration, the National Association of Rocketry and performance guidelines set by the instructors. These requirements included limits on the total mass of the rocket and its propellant, the materials used in the rocket struc- ture and the maximum attainable altitude of the rocket. The use of commercially available rocket engines was mandatory as well as rocket parts such as body tube, control surfaces and nose cone. Safety was a vital concem during all phases of the project. The project was divided into two phases. During Phase 1 (the first academic quarter), students were to design and bench test their guidance electronics and various subsystems. During Phase 2 (the second academic quarter) students were to install their electrical systems on board a model rocket and to launch and recover their rocket a total of 3 times.

It was desired to give students as much responsibility and flexibility as possible while providing necessary resources for successful completion of the project. Students were organized into design teams of 5 to 6 members, individual members of which were required to have completed at least one subject sequence relevant to the project. Further, each team was required to have at least one member who had completed course sequences in electronics, control systems, and digital systems, respectively. In this way, teams were composed of “experts” with diverse backgrounds, and could operate autonomously with a minimum of instructor input. When questions or difficulties arose outside the abilities of the group as a whole, the instructors (also of differing areas of expertise) were available to advise student groups. Since the design project had no formal textbook, budgetary constraints were met by requiring students to pool the associated funds (approx. $50 each) for a quarterly budget of $25&$300 per team. This amount proved sufficient to meet all project material needs.

Students were evaluated on the basis of 1) quarterly oral presentations, 2) quarterly written reports 3) quarterly hard- ware demonstration, and 4) personal effort. Criteria 1)-3) were assigned on a team-by-team basis, while criteria 4) was applied to individual teammates. Oral presentations were made to both faculty members teaching the design course as well to other faculty within the department. Written reports were to follow an outline of presentation provided by the instructors; other than general topic headings, students were instructed to document their project sufficiently so that another group

0018-9359/95$04.00 0 1995 IEEE

322 IEEE TRANSACTIONS ON EDUCATION, VOL. 38, NO. 4. NOVEMBER 1995

of students could duplicate their work using only their final project as a guide.

Student teams were also required to submit quarterly project proposals with weekly milestones indicated. The end of the quarter project reports were to indicate when these milestones were achieved. The first quarter hardware demonstrations were performed on an ad hoc basis; as students completed their hardware, an instructor would examine circuit operation in the lab and give credit for completed work. The second quarter flight tests were scheduled on two separate weekends. Students were required to complete their required flight- recovery tests on these dates (or upon alternate dates selected in case of inclement weather). Personal effort was measured by weekly progress reports and by quarterly peer reviews. Weekly reports were to detail work performed including any significant developments and/or difficulties in the project.

Student progress was also monitored with weekly meetings. All students met with the instructors during a scheduled meeting during which were discussed general questions and issues relating to project development in general and the senior design project in specific. Faculty illustrated team organization and management, patent application and protection, project development and scheduling, etc., as an aid to students devel- oping an approach to overall project development. Students also shared their experiences in class on such topics as rec- ommended rocket construction techniques, parts acquisition, and circuit testing.

Response of the students to the senior design project was overwhelming in terms of both effort and quality of work- manship. It was normal for the groups to spend many late nights in the lab and machine shop. The extreme enthusiasm was attributed to the students being able to build and test a complete system in addition to computer simulations. The designs constructed clearly showed students the difference between circuit simulations and “real world problems such as parasitic coupling and proper mounting techniques.

111. STUDENT PROJECT RESULTS

The design project class was divided into 5 student teams (nicknamed Boing, Martin Matta Hari, McDonalds Douglas, Southrup, and TRUU), with a total of 28 students. All stu- dent teams met project requirements in both quarters. While each team was given the same design requirements, each team’s rocket design differed significantly from the others. For example, Boing emphasized reduction of mass in their rocket design, while Martin Matta Hari emphasized structural integrity of design. (The Boing team rocket weighed 20 oz without its F25-4 motor, while the Martin Matta Hari rocket weighed 40 oz.) Similarly, the McDonalds Douglas team used an air-brake style of guidance surface (see Fig. 1) while the remaining teams used a swiveled canard slightly forward of the rocket center of gravity. Teams used a variety of sensors, electronics, and guidance configurations, so that at the conclusion of the course not one rocket bore physical or structural resemblance to another.

The remainder of this section outlines student efforts in the design, modeling, and testing of their guided model rockets.

Rocket design description is presented in terms of aerodynam- ics, sensing, guidance surfaces and actuators, control electron- ics, power, and recovery system. Details of the McDonalds Douglas team rocket design are provided as an example of each rocket subsystem. As shown in Fig. 1, this team’s rocket was constructed in four modular sections. These in- clude nose cone/sensor section, the control surface/servomotor section, the electronics/power supply section, and the stabi- lizer fin/engine/recovery section. The rocket illustrated is 46.5 inches tall and weighs 22.9 ounces. It utilizes an elliptical delta type fin design with a 45 degree leading edge sweep angle, and was launched with an E30-4 reloadable Aerotech model rocket engine. Following the design description the simulation software and results are presented.’

A. Aerodynamics

The senior design course was conducted in an electrical engineering department, and thus students had little or no insight into the aerodynamics of standard model rockets, much less guided model rockets. Nevertheless, students quickly began to acquire information from model rocketry publications [ 11, [3], [4], periodical literature [2] and aerospace engineering professors and students. Students soon learned that a detailed analysis of the aerodynamics of their rockets would require wind tunnel testing and supercomputer analysis of turbulent air flow about their guidance surfaces, which was clearly beyond the intent of the course. However, standard model rocketry magazines provided a number of rules of thumb that were applied in model rocket designs:

1) For conservative design stability, the aspect ratio (the ratio of rocket length to rocket diameter) should be M 10.

2) The center of pressure of the rocket (the center of mass of the rocket silhouette) should be aft of the rocket center of gravity (CG).

3) Flight stability should be tested by “spin testing;” that is, by attaching a string about the rocket CG and rotating the rocket overhead. If the rocket is open-loop stable, then the nose should turn into the wind during a spin test.

With these guidelines in mind, most teams purchased standard model rocket kits and then modified the bodies and fins to accommodate the additional hardware and larger engine mounts required for flight.

The McDonalds Douglas team rocket was built using an eighteen inch Estes BT-70 body tube, an Aerotech motor mount, custom-made basswood engine rings, and 3/32 inch aircraft-grade plywood. The only modification to the original unit was the addition of two square inches of rear stabilizer fin surface area to ensure stability with the longer body length required for the guidance electronics.

An additional fin assembly was built to accommodate the larger 29 mm (E, F, and G class) motors. The four components can be attached and each joint, except the fin-electronics junction, (rocket Sections 3 and 4, Fig. l), can be sealed with transparent tape. The rocket is finished with automotive grade

’ Digitized videotape images of a guided model rocket flight may be viewed at WWW address http://www.eng.aubum.eduTscotte/rocket.html.

HODEL AND BAGINSKI: AN INTERDISCIPLINARY SENIOR DESIGN COURSE UTILIZING ELECTRONICALLY GUIDED MODEL ROCKETS 323

C o n c e p t u a l DIagram (not to sca le )

Control Surfaces

Electronics

Fig. 1. McDonalds Douglas team rocket schematic.

lacquer spray paint. The fin-electronics junction is friction fit to allow for separation at point of parachute deployment.

E. Sensing

In order to determine the bearing from the current rocket heading to the sun, the rockets had to carry on-board sensors to drive the control electronics. For this purpose, all groups used four photosensors mounted in “east-west’’ and “north- south’ pairs; the output of each pair in turn was transmitted to a comparator in the control circuitry. The various groups used photovoltaics, phototransistors, or photoresistors, based on their own preferences and experimentation.

The McDonalds Douglas group used four cadmium sulfide photocells that were mounted orthogonally with epoxy on the inside of the PNC-50 KA nose cone. A six conductor shielded cable was connected to the sensors and routed through the nose cone and terminated with a 9-pin DIN connector which fits in the base of the TA-6070 balsa adapter. The advantages of the photocells were low cost, adequate response time and lack of susceptibility to transients and saturation.

C. Guidance

The guidance system of the rocket refers to the control surfaces and accompanying actuators (electric motors) used to

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drive the control surfaces. Four groups opted to use rotating guidance fins, while the McDonalds Douglas group used “air- brakes” that were thrust into the oncoming wind in order to increase drag on one side of the rocket. Some groups used a three-position (lefdrighdcenter) control scheme similar to the Sidewinder missile, while other groups used the full range of motion (proportional control) of their mechanical guidance systems in order to avoid “chattering” in their guidance hardware.

The selection of guidance surfaces had to be made with aerodynamic constraints in mind. If the guidance fins (usually mounted forward of the center of gravity) were too large, then the center of pressure would move too far forward on the rocket and the conservative margin of stability would be lost. On the other hand, if the fins were too small for the weight of the rocket then the guidance system was rendered ineffective. It was also necessary to determine the amount of torque required to be applied to the rotating fins in order to ensure that the driving servomotors would be able to guide the rocket without stripping the relatively fragile internal gears.

The McDonalds Douglas team used proportional control in their guidance system. Their test flights were noted for rapid response time of the control circuitry while having no observable overshoot in the rocket path.

324 IEEE TRANSACTIONS ON EDUCATION, VOL. 38, NO. 4, NOVEMBER 1995

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All groups used standard Futaba radio-controlled aircraft electric motors, retailing for approximately $40 each. These motors require a pulse-length encoded signal at 52 Hz in order to set the motor position. In order to make use of these motors, the rocket control circuitry required the use of timer chips and some extra logic so that the desired signal could be sent to the motor inputs. In spite of the additional complexity of the circuitry involved, this hardware configuration proved to be superior in terms of weight and power requirements, to that of standard voltage-level driven motors that were available to the students. The reduced power requirements in turn allowed students to remove extra dc batteries from their rockets, and thus provided even more weight reduction.

D. Control Circuitry

The rocket control circuitry for each team served as an interface between the sensor voltage output and the 52 Hz pulse-length encoded inputs of the Futaba servomotors. In all cases, the outputs of sensor pairs were run through a comparator in order to determine in which direction the sunlight was brightest. Once this decision was made, the guid- ance configuration (three-position or proportional guidance) determined the remainder of their control circuitry architecture.

Teams that used three-position guidance configurations hard-wired three waveform generators into their circuitry and used digital logic to select one of the three waveforms for input to the servomotor. The McDonalds Douglas team used proportional guidance, and for this purpose compared the output of each sensor-comparator to a sawtooth wave in order to construct a 52 Hz pulse train whose pulse-width is a representation of the difference of light detected by the opposing sensors. A schematic diagram of their circuit is shown in Fig. 2. In theory, the response of the circuit should be independent of the light level because the gain is a ratio of the photoresistors. Two pairs were used on orthogonal axes so that control of each control surface or control surface pair could be done independently.

The McDonalds Douglas team also incorporated electronic safeguards into their control circuitry in order to ensure that servomotors were not run to the limit of their motion. This is accomplished by the circuit in Fig. 2 by op-amps U2A, U4A and by the AND gate U5A for the N/S servomotor and by corresponding elements U2C, U4B and U5B for the E/W servomotor. The potentiometers P4 and P5 (P3 and P6) set the upper and lower limits for the N/S (W) servomotor. As long as the dc level at the output of U1A lies between the reference set at P4 and P5, the outputs of U2A and U4A remain high

HODEL AND BAGINSKI: AN INTERDISCIPLINARY SENIOR DESIGN COURSE UTILIZING ELECTRONICALLY GUIDED MODEL ROCKETS 325

and the position control signal is allowed to pass to the N/S servomotor. Similarly, as long as the dc level at the output of U1B lies between the references set at P3 and P6, the outputs of U2C and U4B remain high and the position control signal is allowed to pass to the E N servomotor. Alternatively, if the dc levels at the output of U1A is out of range, a logic low will exist at one of the inputs of U5A and the position control signal will be restricted from reaching the N/S servomotor. In this situation, the servomotor retains its current position and is not overdriven and damaged.

The input to the servomotor required conditioning by an amplitude control circuit that was added to maintain the position control signal amplitude at 1.4 volts dc. Fig. 2 shows a 3.6 volt zener diode that maintains a constant 3.6 volts across a voltage divider that holds the input to the servomotor at the desired voltage.

E. Power Requirements

Other than the rocket motor itself, the dominant source of weight in the student team’s initial model rocket designs were the batteries to power the on-board electronics. The batteries used ranged from standard 9 V transistor batteries to special purpose high-output batteries used in radio-controlled aircraft. Final rocket designs typically used one set of batteries to power the guidance servomotors, and a second set of batteries to provide power to the control electronics. This circuit isolation was used in order to avoid power transients that occurred when rockets in the three-position guidance configuration moved from one motor position to the other.

The McDonalds Douglas team used a +4.5 and - 1.5 volt supply made up of four AAA batteries to power the electronic circuitry and a 4.8 volt 500 mAh Nickel Cadmium battery pack to power the two servo motors. (The - 1.5 volt rail was chosen to bias the input offset voltage of the op-amps.) The 4.8 volt battery used to power the motor was manufactured by the same company that made the servomotors, and was chosen because it has a current rating sufficient for powering both servomotors. A small double-pole, double-throw switch was mounted on the rocket body to allow switching ordoff of the f4.5 volt and -1.5 volt electronic dc power supplies.

F. Recovery

Guidance electronics may be damaged by the force of the hot gas of the parachute ejection charge. Furthermore, the electronics may baffle the ejection charge sufficiently to inhibit deployment of the rocket parachute. In order to avoid these difficulties, a number of innovative rocket recovery schemes were designed. The Boing team recovery system used the motor ejection blast to shoot the motor and its mounting out the back of the rocket; the motor mounting would in turn pull twin parachutes mounted outside the rocket body from their housing, and (if all went well) the rocket was recovered safely. The remaining teams moved the parachute assembly just forward of the motor and its mount, and placed a blast shield between the parachute and the guidance/control electronics. Hence, these rockets would separate the aft stabilizer fins from the rocket body, rather than at the nose cone as in a

standard model rocket kit. The McDonalds Douglas rocket used a recovery system of this type; safety clips were used to ensure that the shock cord would not separate from the rocket body in recovery.

G. Simulation on Digital Computer

In order to provide sufficient design verification, students were required to construct mathematical models of the rockets during flight. Students discussed aerodynamics with faculty in the aerospace engineering department as well as consulting a variety of texts on the subject matter. It was concluded that any exact analysis would require extensive wind tunnel testing and super computer simulation that was beyond the scope of effort of the course. The following is a description of a typical model. The emphasis was on the students developing a rational methodology and realizing the limitations of the computational analysis.

An empirical approach was taken to provide a data base from which to extract the rocket’s aerodynamic properties. Observations from test flights of prototype rockets were used by student teams to compute figures of merit that were used to parameterize a dynamic model of rocket flight in one dimension (the vertical axis) and in two dimensions (the vertical axis and one horizontal axis). Based on instructor suggestions, vertical acceleration dynamics were typically modeled as

mc, = -c1wz(t)2 + F ( t ) - mg

where vz(t) is the vertical velocity of the rocket, c1 is a coefficient of drag, F ( t ) is the rocket motor thrust profile (obtained from the manufacturer), and m and g are the rocket mass and the gravitational acceleration constant, respectively. Simulations of this type were useful in predicting rocket performance for planned variations in rocket guidance surfaces and motor thrust profile.

The McDonalds Douglas team used a Fortran Runge-Kutta integration routine to calculate the acceleration, velocity, and altitude for the case of a strictly vertical launch of a model rocket based on measured rocket parameters. A sample plot ob- tained from this simulation is shown in Fig. 3 ; this simulation corresponds to a D12 engine with a 24 oz rocket.

Students also attempted to develop a two dimensional simulation in order to estimate the maximum horizontal travel of the rockets during flight. Approximate equations of rocket dynamics were derived by students and instructors in a class meeting. Rocket dynamics were modeled approximately as

c2sin(@) 14 cos(+) w,= - c o s ( 8 ) - g - - ~ ~ ~ 2 c o s ( $ ) - - , T C1

m m m c2 sin(@)

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m m m

where c1, . . . , c4 are physical rocket parameters, U,, v, are

326 IEEE TRANSACTIONS ON EDUCATION, VOL. 38, NO. 4, NOVEMBER 1995

Rocket Flight Simulation D12 Engine

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z

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the respective horizontal and vertical velocity components, ‘U = d m is the velocity magnitude, J is the rocket’s rotational moment of inertia, and the angular variables 8, # and w 4 4 are defined as shown in Fig. 4. While simulation results did not exactly match observed rocket behavior, they served as guidelines for launch-test safety procedures. Due to the complexity of modeling rockets with dynamically varying guidance surface locations, it was not reasonable to expect high fidelity results of these simulations.

H. Flight Tests

During Phase 1 of the design project students were not required to test-launch their rockets with electronics on board. Based on preliminary flight tests performed during Phase 1, it was decided in Phase 2 to special order E, F, and G class engines with appropriate ejection delays, and to refit existing rockets for these new engines. Based upon one-dimensional simulations, the most commonly selected motor was an Aerotech F25-4 engine, with a three second bum and roughly even thrust throughout. Phase 2 flight tests demonstrated that the simulations provided sufficient accuracy for the selection of engine thrust and delay times. Phase 2 also required three successful launches and recoveries. Based upon experience from the first quarter designs, student teams adopted a modular design to allow for rapid access to and, if necessary, replacement of broken or defective subsystems.

In the development of their rockets, students attempted to avoid any roll-rate difficulties by mounting their fins as precisely as possible. Preliminary design parameters required student rockets to have a low coefficient of drag related to the stabilizing fin area, as well as a minimal roll rate about the longitudinal axis. This required testing of various fin designs and control surfaces. The McDonalds Douglas team utilized a test rocket which was 25.25 inches long and weighing 2.53 ounces (71.7 grams), exclusive of the engine. An elliptical delta-type fin design was utilized with 7.7 square inches of area per fin. The test flights without control surfaces proved

Fig. 4. Definition of yaw variables.

exemplary. Negligible roll (<0.2 rev/s) was observed. Further tests were made with fixed-location control surfaces using all available rocket engine types. Acceptable results were obtained with all rocket engines. An estimated 15 degree average angle of attack was achieved using this control surface design.

Iv . CONCLUSION AND FUTURE PROJECTS

A senior level design course was developed to integrate the various aspects of design implementation in an industrial environment. The course consisted of designing and flight testing an electronically guided model rocket. An analysis of the rockets anticipated flight path was performed using a FORTRAN program which took into account such factors as stability, center of gravity, center of pressure, and engine thrust. The rocket was flight tested for three successful con- trolled flights and recoveries. Test flight results proved the design of the rocket to be very stable and respond quickly in flight.

This course is regarded as an ongoing effort and future projects are planned. Possible projects include hybrid imple- mentation of guidance circuitry or the use of a digital control system. There are numerous other possibilities, limited only by student imagination and the relatively small operating budget of the course. The universal reaction of students, faculty, and outside reviewers of this course has been exceptionally positive, both in terms of its use as an academic training exercise and as an enjoyable practical electrical engineering project.

REFERENCES

M. A. Banks, ed., Advanced Model Rocketry. Milwaukee, WI: Kalm- bach, 1985. D. Keeports, “Numerical calculation of model rocket trajectories,” Physics Teacher, 1990. D. Malewicki, “Centuri Technical Information Report 100: Model Rocket Altitude Performance,” Centuri Engineering Co., 1970. D. R. Pratt, Basics of Model Rockfry. Milwaukee, WI: Kalmbach, 1981.

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A. Scottedward Hodel (S’87-M’89) was born April 6,1962 in Grand Rapids, MI. He attended the University of Illinois at Urbana-Champaign where he received the B.S. degree in computer engineering (1984) and the M.S. degree (1986) and PkD. degree (1989), both in electrical engineering.

He has worked in such diverse areas as human factors engineering, theoretical computer science, numerical linear algebra, and control. He is an assistant professor in the Auburn University Department of Electrical Engineering, where his current research interests include computational meth- ods for reduced order control, computer aided control system design, and biomedical applications of control.

Dr. Hodel is a member of SIAM.

Thomas A. Baginski (M’87) was born in Erie, PA, on December 17, 1958. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Pennsylvania State University, University Park, in 1980, 1982, and 1984, respectively.

He is currently an Associate Professor of Electrical Engineering at Auburn University, AL, where he has resided since the completion of his doctorate. His current research interests include novel solid-state EMI-insensitive elec- troexplosive devices and the hazards of electromagnetic coupling of radiation to ordnance (HERO). Lk. Baginski is a member of the Material Research Society and the

Electrochemical Society.