Airframe (CalVistor)
Fin design discussion:
We designed our fins based on the need of our rocket. Initially, we had fin dimensions that were larger. However, as our rocket design developed we realized that this design was not meeting our apogee and stability needs. Each change would need our fins to be altered which was constant. After our rocket design was set, we began our fin simulations with respect to our motor simulations. We wanted to make sure our fins and motor complimented each other. We did different fin-size simulations for each motor we felt could be used to meet the needs of our rocket. After many fin/motor simulations and utilizing the optimization tool in Open Rocket, we landed on our current fin dimensions.
Fin Semi-span (inches) *
Distance between root cord to tip cord (inches) - Numeric data only
7.4 in
Fin attachment *
Description of the method used for securing the fins to the airframe.
Sand areas of connection before applying epoxy. Clean off any fiberglass dust with IPA solution. Spread epoxy(JBweld) all around the bottom of the fin tab and side of fiberglass, so that it is completely coated.
Use the fin jigs to place the fins into the fin slots on the booster tube, so that the tabs are touching the motor tube and the aft side of the tab is touching the middle centering ring.
Create smooth epoxy fin fillets around each fin. After thoroughly mixing, wear gloves and use your finger to apply a generous amount of epoxy at the seams between the fins and the booster tube.
Once all three fins are cured and attached, align the aft centering ring with the rest of the rings, but do not epoxy it in. Keep multiple zip ties wrapped around the ring so that once it is perfectly aligned, it can be pulled out.
Epoxy the aft centering ring into the exact position established in the previous step. If it falls out of alignment, use one of the extra aft centering rings and try again, until it is successful. Allow the epoxy to cure for 24 hours.
This step will also take place after fin glassing and after the aft rail button is attached.
Final details
Fin glassing
General Procedure:
Roughen surface with 80 grit sandpaper.
Clean surface with 50% IPA, 50% water.
Cut 3 types of cuts of cloth:
First cut of cloth goes over the fillets, goes less than a third of the way up the fin, need 6 of these.
Second cut goes up ~⅔ of fin, and out to the midpoint between the fins on the airframe, need 6 of these.
Third cut goes from tip of one fin to tip of the adjacent fin, need 3 of these.
Mix epoxy in ratio given on instructions. Need a 3:1 ratio between epoxy and cloth.
Application:
Wet surface with epoxy.
Put a cloth layer on top of epoxy; it will soak the epoxy into itself.
Squeegee the excess epoxy and bubbles out of the layup to make it smooth.
Once all three layers are on, apply peel-ply over the layup.
Cure vertically.
Vacuum bag the entire fin area to ensure a smooth cured surface. Apply breather cloth to the peel-ply before bagging.
Once cured, sand down composite surface.
Fin Flutter Analysis *
Please describe the method and results used to determine the not to exceed flutter velocity or the divergence velocity.
We used the flutter boundary location to determine the rocket’s flutter velocity and ensure that our rocket’s velocity never exceeds this flutter velocity. Assuming our maximum altitude, which is the apogee of 10,000 feet, we were able to derive the estimated air temperature and estimated speed of sound. Finally, we were able to apply our fin geometry to calculate the fin flutter velocity, which is 1682.16 ft/s (1146.9 mph). We confirmed this value using the software FinSim, which combines our fin geometry with more specific rocket data to calculate this same value. Since our rocket’s maximum velocity is predicted to be 972 ft/s, we will not exceed flutter velocity.
Vehicle Weight (pounds) *
All vehicle, electronics, and recovery (pounds) - Numeric data only: NOT including motor case, propellant, or payload weight
48.897 lb
Empty motor case/structure (pounds) *
0.503 lb
Propellent weight (pounds) *
11.7 lb
Payload weight (pounds) *
Must be at least 8.8 lbs per IREC Rules (pounds) - Numeric data only
9.5 lb
Liftoff weight (pounds) *
Vehicle weight + propellant weight + motor case/structure + payload weight (pounds) - Numeric data only
70.6 lb
Center of Pressure (inches) *
The location of the center of pressure measured in inches from the tip of the nose cone. Numeric data only.
91.426 in
Center of Gravity (inches) *
The location of the center of gravity measured in inches from the tip of the nose cone. Numeric data only.
79.264 in
Static Margin *
The distance between the CG and the CP at lift-off is divided by the airframe diameter, measured in calibers. 1.5 is the highly recommended minimum
1.97 cal
Couplers & Airframe joints *
Discuss each airframe joint, including couplers, shoulders, and attachment method(s).
Ref. DTEG 8.5 - Airframe joints that implement “coupling tubes” should be designed such that the coupling tube extends no less than one body tube caliber on either side of the joint – measured from the separation plane. The nose cone is epoxied onto the payload tube. Airbrake coupler attached to airbrakes tube with epoxy. Booster tube coupler is attached to the airbrake tube with #6-32 black-oxide alloy steel screws and to the booster tube with epoxy. Av Bay coupler attached to the main parachute tube with epoxy.
#4-40 nylon screws placed between payload tube and Av Bay coupler.
#4-40 nylon screws placed between Av bay tube and main parachute tube.
Rocket construction narrative/ additional information *
Discussing the construction of your rocket including airframe, couplers, interstage couplers, nose cone, fins, fin attachment, composite materials, and identify commercial or SRAD components.
Booster tube assembly:
Recovery Tube Assembly Procedure:
Avionics Bay Assembly
Payload/Nosecone Assembly
GROUND TEST: The following full ground testing procedure was conducted on February 16th, in preparation for a test launch of the vehicle(with a ballast stack no payload) on February 18th.
Section 7: Propulsion System
Propulsion Type *
Solid
COTS - Commercial Off The Shelve
Propulsion Manufacturer *
Aerotech
COTS Motor - Manufacturers Designation
Aerotech M1939 W-P
Motor Letter Classification *
M
Average Thrust (N) *
1,939.0 N - Avg thrust given by thrustcurve.org/motors/AeroTech/M1939W/
Maximum Thrust (N)
2,429.7 N - Max thrust given by thrustcurve.org/motors/AeroTech/M1939W/
Total Impulse of all Motors (Ns) *
10,481.5 Ns - Total Impulse given by thrustcurve.org/motors/AeroTech/M1939W/
10,369 Ns - Total impulse given by RASP simulator file from thrustcurve.org/motors/AeroTech/M1939W/ ** explanation given in propulsion narrative and additional comments.
Motor Burn Time (s) *
Numeric data only (Seconds)
6.52 s - Motor burn time given by Open Rocket
6.2 s - Motor burn time given by thrustcurve.org/motors/AeroTech/M1939W/
Propulsion Narrative
Due to judges comments from our previous reports, we redid our propulsion simulations through OpenRocket. We used the RASP format simulator file from Thrustcurve.org. However, we wanted to preface a discrepancy between this file and the given information on the Thrustcurve site. For our motor, Aerotech M1939-W the written total impulse is given as 10481.5 Ns. However, the total impulse in the RASP file is 10369 Ns. All our simulations were done with this file, therefore using the impulse value of 10369 Ns.
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