The Aerodynamics of Grid Fins
The University of Alabama
Tuscaloosa, AL
Rocket Girls
May 6, 2013
NASA University Student Launch Initiative
Post-Launch Assessment Review

1.0 Project Summary
The Rocket Girls chose to study the basic aerodynamics of grid fins. The team used strain gauges on the attachment points of the fins to the bracket system to measure the bending strain during flight. The rocket reached an apogee of 4,971 feet during the competition launch and was recovered successfully. The team reached over 3,000 persons through outreach and stayed within a $5,000 budget throughout the year.
2.0 Dottie Overview
Dottie was 8 feet tall, 4 inches in diameter, and made using LTM 45 carbon fiber. The rocket was built in six sections but came apart into three during flight. At launch, Dottie weighed approximately 16 pounds containing the payload and recovery electronics, 24inch and 84-inch parachutes, and flew four custom 3D-printed grid fins and brackets design by the team. The Rocket Girls studied the aerodynamics of grid fins by placing strain gauges on the fins that were flown perpendicular to the rocket body in place of traditional planar fins.
2.1 Vehicle Flight Summary
Dottie flew on a CTI L730 motor at the competition launch and reached an apogee of
4,971 feet. The flight was stable with the center of gravity (C
G
) nearly two body lengths forward of the center of pressure (C
P
). Shown below is an altitude versus time graph from the avionics PerfectFlite Stratologger altimeter that shows the deployment at apogee of the drogue parachute and of the main parachute at 900 feet. The drift distance was within the required distance and there was no damage to the vehicle, fins, or electronics at landing.

Figure 1. Altitude versus Time Graph
3.0 Payload Flight Summary
The payload electronics worked as intended during launch. The four strain gauges ran through the booster section from the fins and attached to a breadboard with four capacitors. As the capacitors ran their cycle, they received data from a specific strain gauge and then sent it to the Raspberry Pi Datalogger powered by an external battery.
The data from the Raspberry Pi was stored on an SD card and then read using a program in Python. Due to the stability and integrity of the bracket design for the grid fins, there was minimal strain data on the fins during launch to accurately understand the aerodynamic forces during flight.
3.1 Scientific value
Grid fins have been used by the Russians on their missiles since the 1970’s as a control surface or abort system. Not until recently has the United States begun using grid fins on the MOAB and as a potential control system on the Orion. Although these fins have been available for 40 years, knowledge about their effects on the aerodynamics and flight of the vehicle is limited. The data collected in testing, as well as sub-scale and fullscale flights allowed the Rocket Girls to determine if grid fins have the potential to be as successful in their stability and flight control planar fins.

3.2 Visual data observed
The visual data was minimal due to the strength in design of the bracket system. The fins were securely mounted to the booster section of the rocket to prevent shearing of the fins during launch; therefore the strain during flight was too sporadic and negligible to properly acquire any useful data.
4.0 Overall Experience
4.1 Lessons learned
Over the course of the year, as the team developed different grid designs that needed to be tested, The Rocket Girls learned it was imperative to have scheduled additional flights in case of weather or unsuccessful launches, since this was a major factor in trying to keep on schedule. This project also taught the team that their goals were capable of changing as they further researched grid fins and how to adapt to changing circumstances on launch day.
4.2 Summary of overall experience
Originally, the team attempted to design grid fins with a lower coefficient of drag (C
D
) than that of traditional fins. The goal eventually transformed into finding the aspect ratio that could be best modeled in RockSim for the full-scale vehicle as well as studying the aerodynamic forces on the fins during launch. Unfortunately, the payload received minimal data from the strain gauges. The Rocket Girls also learned a great deal about data acquisition systems and electronics as there were no electrical engineering students on the team.
5.0 Educational Engagement Summary
This year, the UA Rocket Girls far exceeded the set educational engagement outreach goal of 2,000 people. The team was able to reach a total of 3,419 people with over 500 of the total being 6 th through 9 th grade students. By using lesson plans, PowerPoint presentations, and incorporating lessons suggested by the teacher the Rocket Girls were able to inspire and educate students, teachers, and parents about science, math, engineering, and rockets. Table 1 shows a list of all educational engagement events during the 2012-2013 USLI competition year.

Table 1. Engagement Events Attended
Event
E-Day
UA Engineering Alumni
Tailgate
Tuscaloosa Story Lab
Into to Aerospace
Engineering
Echols Middle School
Selma High School
Tuscaloosa County High
School
Event Location
Tuscaloosa, AL
Tuscaloosa, AL
Tuscaloosa, AL
Tuscaloosa, AL
Tuscaloosa, AL
Selma, AL
Tuscaloosa, AL
Number of Persons
Reached
750
248
51
84
191
141
35
McWane Center
6.0 Budget Summary
The Rocket Girls spent an estimated $3,500 of their $5,000 given by the Alabama
Space Grant Consortium. They later received $600 from USLI which will be saved for a future project. Dottie’s estimated price on the pad was $2,056 even though the team’s expense for the vehicle and electronics was only $550. The chart for the price on the pad is shown below. The Rocket Girls spent an additional $2,950 throughout the project on the sub-scale rocket, NAR 1 certifications for new team members, and educational engagement activities. The remaining $1,500 will be spent before May 31, 2013 on power tools, altimeters, and team member reimbursements of small purchases.
Table 2. Price on the Pad
Item Amount
1
Birmingham, AL
Total
Team's
Expense
Structure
Donated
Estimated
Price Each
220.8
1919
3419
Total Price
220.8 Carbon Fiber
Airframe
Fiberglass
Couplers
1 Donated 11.5 11.5
Centering Rings
Fiberglass Bulk plates
Nosecone
Fiberglass Sleds
2
2
1
3
Donated
28
32
42
60
14
32
14
120
28
32
42

Grid Fins
Outer Brackets
Inner Brackets
PerfectFlite
Altimeters
Duracell
Batteries
GPS
Raspberry Pi
Strain Gauges
U-bolts
Fin Pins
C-Clips
Shear Pins
Rivets
Nuts
#6 Screws
Threaded
Rods
Drogue
Parachute
Main
Parachute
Main
Parachute
Protector
Drogue
Parachute
Protector
Shock Chords
Motor
Motor Case
Rear Closure
Total
1
1
1
3
4
1
1
4
12
40
8
6
4
4
4
8
4
4
4
1
Payload Components
Free to Print
Free to Print
Free to Print
Electronics
Reused
4.25
5.5
3.75
79.95
48
Reused
35
Donated
Hardware
10
2.8
3.6
0.4
1.56
20
3.2
12
Recovery System
Reused
12
400
35
49
45
2.5
0.7
0.9
0.05
0.13
0.5
0.4
2
Reused
10
180
10
Reused 7
45
180
10
7
17
22
15
239.85
48
400
35
196
10
2.8
3.6
0.4
1.56
20
3.2
12
1
1
1
2
Other
161
100
39
$548.56
22 44
165.95
106.95
39.95
165.95
106.95
39.95
$2,079.56