George
George was built to develop my experience with model rocketry after not having built or flown a rocket in over a decade.
Description
George is an Estes Patriot kit (model 2056) designed to fly on inexpensive B and C motors. The Estes kit itself is based on the MIM-104 “Patriot” PAC-2 GEM surface-to-air missile designed and built by Raytheon. This particular kit was chosen mainly because it looked cool, but also because it seemed easy to build and was based on a well-tested, known-stable prototype rocket.
The kit was purchased from Amazon for $14.54, tax included. There were no significant additional costs associated with assembly, aside from the expected purchase of adhesives and tools.
The name “George” is the result of a hasty decision made on launch day when I discovered that I would have to choose a name for each of the two rockets I would be launching. I was rushed and couldn’t think of anything better on the spot, so I went with George and Frank.
Details
All measurements are from the finished rocket.
- Height: 54 cm
- Body Diameter: 4.1 cm
- 4 fins
- 1 stage
- Width with Fins: 11.2 cm
- Paint: 1 coat white primer
- 1/8" launch lug
- Hexagonal parachute, 36.8 cm on the long axis, 22 cm on the short (side length 18.6 cm), with a 5.7 cm diameter hole in the middle. Total surface area 797 cm^2.
Construction
George was built during August of 2022. Construction was straightforward, and no significant modifications were made to the kit.
The fins were first sanded smooth on their large, lateral surface, then the leading and trailing edges were rounded to a semicircular profile. This profile was chosen solely because it seemed more aerodynamic; I did not spend a lot of time researching the shape.
The fins were then positioned on the body tube according to the fin template provided by the kit. Small pencil marks were made on either side of the cutout, then extended out to the length of the tube by using a doorframe as a straightedge.
All parts were test fit before being glued in place.
The fins were attached to the body tube using superglue (cyanoacrylate) along the entire proximal edge of the fin. An Estes Fin Alignment Guide (part 2231) was used to try to maintain alignment, but there was quite a bit of wobble using this method. Instead, I printed Justblair’s parametric fin jig with 0.5 mm added to all measured widths and diameters to account for the quality of my admittedly low-end 3D printer. No filets were made between the fins and body.
The engine mount assembly had significant play between the centering rings and body tube, so, following in Apollo 13’s tradition of tearing off parts of the flight plan, 5 mm wide strips were cut off the assembly instructions and glued with wood glue around the circumference of the centering rings until the fit was snug. The motor mount was assembled and attached to the body tube with cyanoacrylate.
Flashing on the nose cone was removed first with a hobby knife, then the nose cone was sanded with 400, 800, and 1200 grit sandpaper until it was smooth. The nose cone fit snugly inside of the body tube without any modification.
The parachute, shock cord, and shock cord mount used were all stock.
The finished rocket was placed on a 3D printed stand for painting. Time constraints allowed only one coat of white primer to be applied, which was sanded smooth after it had been given time to dry. The yellow color of the upper (forward) body tube did show through somewhat, but the coloration is not noticeable at a distance.
Simulation
A model of the rocket was built in OpenRocket after construction of the actual rocket had been completed. In retrospect, waiting until all parts were glued together to weigh everything was not the right way to do this, because this led to mass estimations being used for several components. It seems I was a little too eager to get everything assembled and in the air.
A model of the craft, shown above, was built in OpenRocket. The mass component discussed below is visible towards the aft of the rocket.
After weighing, a 12.4 g discrepancy was noted between the measured rocket (heavier) and the simulated craft in OpenRocket (lighter). Initially, a 12.4 g mass component was simply added to the simulation and was given the same length and diameter as the rocket itself. This, however, incorrectly assumed a uniform distribution of excess mass, which was not the case. The excess mass is hypothesized to have come from adhesives (and the paper strips added to the engine centering rings), which are concentrated around the motor mount and fins. Paint is another source of excess mass, but is spread more or less evenly across the craft. The mass component was sized and moved until mass measurements and the position of the center of gravity predicted by the model agreed with those measured from the completed rocket. The final mass component was 10 cm tall and placed with its aft end in line with the aft end of the craft. This placement is consistent with adhesive applied to the motor mount and fins, confirming the earlier hypothesis.
| Motor | Mass (g) | Cp (cm from nose) | Cg (cm from nose) | Stability (cal) |
|---|---|---|---|---|
| none | 68.5 | 40.2 * | 30.5 | 2.37 ** |
| Estes B6-4 | 86.7 * | 40.2 * | 34.9 * | 1.30 ** |
| Estes C6-5 | 92.5 * | 40.2 * | 35.7 * | 1.10 ** |
* Calculated by OpenRocket
** Derived from (Cp - Cg) / Body Diameter
After the model had been matched closely to the actual rocket, it became possible to make flight predictions:
| Motor | Apogee (m) | Velocity at Chute Deploy (m/s) | Optimum Deploy Delay (sec) | Max Velocity (m/s) | Max Accel (m/s2) | Time to Apogee (sec) | Flight Time (sec) | Landing Velocity (m/s2) |
|---|---|---|---|---|---|---|---|---|
| B6-4 | 81.4 | 5.93 | 3.44 | 40.6 | 130 | 4.23 | 21.9 | 4.72 |
| C6-5 | 197 | 4.22 | 4.64 | 67.8 | 140 | 6.41 | 48.1 | 4.56 |
All values calculated by OpenRocket
Flights
| Flight | Datetime | Site | Pad | Variant | Motor | Attempts | Outcome |
|---|---|---|---|---|---|---|---|
| 1 | 2022-08-13 11:52 | Lucerne | A2 | Estes B6-4 | 1 | Success | |
| 2 | 2023-04-08 13:55 | Lucerne | B1 | Estes C6-5 | 2 | Success |
Flight 1
Launch attempt 1 lifted off from pad A2 at 2022-08-13 11:52 on an Estes B6-4 and flew without incident. The craft arced downwind during flight. The ejection charge fired properly and the parachute was deployed without any snags or tangles. Descent was smooth and controlled. Craft landed approximately 100 ft (30 m) generally WNW from the launchpad. No damage to the rocket was found upon inspection. Paint was intact, the motor was successfully retained and all wadding was cleanly ejected. There were no burn marks or singeing to the parachute or cord.
No video was taken of the launch due to the rushed timeframe of launch day.
Flight 2
Attempt 1 did not result in engine ignition. The Launch Control Officer reported that the igniter had continuity, but it either shorted or was manufactured improperly. Igniter was retained and investigated. The most likely cause of failure to the igniter was physical damage to the pyrotechnic material of the igniter sustained when the igniter plug was pushed into the motor. It appears as if the plug pushed the two wires just far enough apart that they were unable to get hot enough to cause the pyrotechnic material to ignite. This is a known failure mode of this type of igniter, and is a large part of why the Estes motor kits include six igniters for three motors.
A new igniter was inserted into the motor for attempt 2. Flight 2 lifted off from pad B1 at 2023-04-08 13:55 on an Estes C6-5 and flew without incident. Ejection charge fired near apogee. Descent was controlled and appeared to be smooth from the ground. The motor was retained and all wadding was cleanly ejected. Parachute and elastic cord were intact and not singed. The rocket was dragged across the ground for a short distance after landing.
Upon inspection, a dent approximately 4 cm wide (perpendicular to long axis) and 1.5 cm tall indentation was found to the forward end of the main body tube. At its greatest extent, the tube had been deflected approximately 0.5 cm towards the midline. No damage was noted to the nose cone, indicating that the dent could not have been sustained prior to the firing of the ejection charge.
There was some chipping to the layer of primer on the outside of the body tube. No significant structural damage was found and the body tube was easily hand-manipulated back into a state where immediate reflight would be possible.
Video
Video was recorded on a Sony Xperia 5 IIImy phone (2160 x 3840 @ 23.97 fps progressive scan) and brought into Sony Vegas 16 for frame-by-frame analysis.
All times are in seconds relative to liftoff. Owing to the 23.97 fps rate of the video, all events (except liftoff) include a +/- 41.7 ms uncertainty.
| T+ (sec) | Description |
|---|---|
| 0.000 | Liftoff - the last frame in which the rocket shows no motion relative to the pad. |
| 0.042 | Rocket is in motion and a smoke plume is clearly visible. |
| 0.125 | The rocket has cleared the launch rail. |
| 0.135 | Sound from the rocket reaches the camera. Using 340 m/s (761 mph) for the speed of sound, this translates to a distance of 46 m (~151 ft) between the rocket and camera, which seems reasonable. Exact positions of the camera and launch pad were not recorded and cannot be reconstructed. |
| 1.920 | Motor burnout. This is defined as the first frame in which the smoke output from the motor noticeably decreases, as seen below. |
| 5.675 | The ejection charge begins to fire, as evidenced by a puff of smoke appearing at the rocket and trailing out the back. |
| 6.301 | Smoke trail from ejection charge stops. |
| 6.802 | Main ejection charge fires. A large puff of smoke is seen near the rocket. |
| 7.011 | Wadding can be seen falling free from the rocket. |
| 7.512 | Chute can be seen. |
| 42.649 | Contact with ground. |
Analysis
| Simulated (sec) * | Actual (sec) | |Difference| (sec) | |
|---|---|---|---|
| Ignition | 0.000 | 0.000 | 0.000 |
| Burnout | 1.874 | 1.920 | 0.046 |
| Chute Deploy | 6.875 | 6.802 | 0.927 |
| Deploy Delay | 5.001 | 4.882 | 0.119 |
| Landing | 42.066 | 42.649 | 0.583 |
* All simulated data is from OpenRocket
Total flight duration was 42.649 seconds. Powered ascent was 1.920 seconds long, ballistic travel (ascent and descent) was 5.58 seconds long. Controlled or Slowed descent was 35.137 seconds long, fully 82.4% of the rocket’s flight.
Motor Performance
The motor burn time as measured from video is 1.920 seconds (+/- 41.7 ms), which is remarkably consistent with the 1.9 s measured burn time listed on thrustcurve.org, and 46 ms off from OpenRocket’s simulated burn time of 1.874 seconds.
There was only a 0.119 second difference between the expected 5 second deploy charge delay and the measured 4.882 second delay. The NAR lists an average measured delay of 4.28 s for this motor, though that report was last updated in August of 1996, and it is likely that Estes has improved on their delay time since then.
Apogee and Deployment
The OpenRocket simulation estimates an apogee of 197 m (~646 ft), and based on how well other aspects of the simulation match the actual flight, this estimate seems reasonable.
After the chute deployed, the craft appeared on video to be flipping end-over-end. This motion was not observed from the ground, though this is likely due to the difficulty involved in making out details at that distance.
Assuming an apogee of 197 m and taking the measured descent time of 35.847 sec, the average descent rate was 5.5 m/s. This differs significantly from the projected touchdown speed of 4.56 m/s. It is possible that the chute had not fully opened while the rocket was flipping end-over-end, and that there were two distinct modes of descent; one where the rocket is in near-freefall due to a partially opened parachute, and one where the rocket is descending under a fully-opened chute.
An alternate explanation is that the aerodynamics of descent under a parachute with a spill hole is different enough from a descent under a continuous parachute to be noticeable even under such a relatively short descent.
Landing and Recovery
The rocket first contacted the ground on two fins with the forward end of the body tube oriented towards the camera. The craft was then pulled downwind, away from the camera, with the two fins in contact with the ground acting as a fulcrum. From here, the rocket rotated until the body tube made contact with the ground, and the whole craft was dragged a short distance by the wind acting on the parachute.
It is interesting to note that although it seems likely that the dent on the forward end of the body tube would have come from this rotation into the ground about the two fins, the side of the body tube that made contact with the ground was the side directly opposite the dent. In fact, the dent was found on the side of the rocket facing up once the craft had come to a rest. This leaves the most likely cause of the dent to be contact between the body tube and nose cone during the end-over-end tumbling that the rocket experienced after parachute deployment. This hypothesis is further bolstered by the fact that no dirt or dust was found on or near the dent. The cause of this tumbling is not fully understood and warrants further investigation.
After landing, wind dragged the craft across the ground by the parachute. Video quality was not high enough to estimate how far the rocket was dragged, but the distance was not far (in the ballpark of 1 to 2 times the length of the body tube) and no significant damage was done to the craft.
Lessons Learned
Design and Simulation
- The simulation proved to be quite accurate and helpful, once it had been tuned. In the future, weighing components as they are assembled ought to lead to a much more accurate model, and therefore a much more accurate simulation. Mass inconsistencies can be handled as they arise, and would not need to be guessed at.
Construction
- The Estes fin jig is unreliable, and there is significant potential for fin cant. This is a problem with the design and material of the jig itself, and seems intractable: the clips which hold the fins to the jig are made of plastic, and clamp the fin to the jig. This forces the fin slightly to one side. If the clamping force were to be increased, would dig into the fin. If the clamping force were decreased, the fins would wobble even more and would not be true to the alignment lines along the body. Estes struck the best balance they could between too high and too low clamping force, and the result is adequate for low power rocketry, but I would not be willing to try such a jig with a medium- or high-power rocket.
- Using a doorjamb to draw alignment lines is terribly inaccurate. In the future, it would be best to use a metal angle extrusion to draw these lines.
Flight
- Estes igniters seem to be highly unreliable, and there is likely a good reason there are six igniters in a three-pack of motors. There are more reliable igniters on the market, but for much higher prices. For now, it would be best to simply expect a ≥50% failure rate. It may also be worthwhile to investigate making my own igniters.
- In order to track the distance traveled downrange by the rocket, the locations of both the launch pad and landing site need to be marked with GPS. This should be relatively trivial; it’s just a matter of remembering to do it.
- Similarly, a ruler or tape measure should be brought along: you never know what you may need to measure.
- Checklists for tools, supplies, and pre-launch checks need to be developed, tested, and used. It may be beneficial to have these checklists on paper.
- It is going to quickly become possible to lose track of rockets as they ascend. I have seen colored powder ejected with parachutes as a means of easily seeing both where and when a chute is ejected. This would be very helpful, and deserves research.
- Similarly, colored and/or reflective streamers added to the parachute or shock cord would aid in tracking the craft during descent.
- Tracking marks, such as alternating bands of black and white (or some other pair of highly contrasting colors) should be painted on the rocket to aid in investigating orientation and rotation.
- It is difficult to both record a launch and to watch it at the same time, and the desire to simply stand back and watch is always paramount. Instead of doing half a good job at either one, it would be good to plan research launches, during which all aspects of the flight are tracked and recorded, and recreational launches, where only minimal notes and no video need to be taken.