Here are the grain numbers:
| Grain # | Casting Tube |
Propellant Length |
Propellant Weight | Density |
| 1 | 4.83"@ 25.11 gr. | 4.72" | 1.6887 | .0601 |
| 2 | 5.144"@ 26.75 gr. | 5.019" | 1.786 | .0612 |
| 3 | 5.25"@ 27.3 gr. | 5.07" | 1.8324 | .0621 |
| 4 | 5.47."@ 28.44 gr. | 5.27" | 1.8841 | .0614 |
| 5 | 5.36"@ 27.87 gr. | 5.22" | 1.8594 | .0612 |
| 6 | 5.43"@ 28.24 gr. | 5.17" | 1.8333 | .0610 |
| Totals | 31.49"@163.71 gr. | 30.469" | 10.8839 pounds | .06139 |
Number of Grains: 6
Propellant Weight: 10.8839 pounds
Density: .06139 pounds/cubic inch
Average Grain Length: 5.0782"
Grain Diameter: 3.0"
Core Diameter: 1.26"
Throat Diameter: .75"
Kn: 431, 477, 445
Work on the rocket is complete, I finished up a few little things such as motor retainers and tests of the electronics module. Now it was time to do a last stability check. The Barrowman Center of Pressure is 77.58" from the nose cone, so I fully assembled the rocket with loaded motor and found the center of gravity to be 67.5625" from the nose. That leaves me at about 10.01" of stability, or 2.15 margin or caliber's of stability.
Using a Cd of .61, Flight simulations indicate the apogee to be 11,479' at 24.8 seconds. With a Cd of .75 apogee sims out at 10,205' at 23.1 seconds. I'll set my back up timer at 25 seconds. Speed at motor burn out is not highly dependent on drag, so the maximum speed with a nominal burn should be about 1218 fps or about 830 mph (mach 1.1 ish).
This will be my first flight using the new ARB-3D recording altimeter from Accel Techtronics. It should be interesting looking at the accelerometer data from the flight, this will be the first flight where I've had the ability to look at flight speed, actual deployment time, etc. I really would have liked to have used my own altimeter as well, but I wanted to fly the Cosmo 2 on the same day, and I didn't want to try swapping the altimeter back and forth. Not to mention the fact that it may take some hours to locate the rockets after landing.
Back Up Timer: 27 seconds
Main Deploy: 500'
Prim 2 Charge: .36 grams
Apogee Charge: 4.95 grams
ARB3D Firmware: ARB3Dv120.bxb
Lift Off Weight: 32.2 pounds
To making loading the grains easier, I laid out all the grains on the EPDM rubber sheet. Then rolled the EPDM rubber around the grains and used aluminum foil tape to seal and close the liner. The EPDM likes to stick to the aluminum casing, making it hard to insert the 11+ pound package. So I use talcum powder on the outside of the EPDM liner and it slides right in.
Launch Day:
I'd been watching the weather with the keenest of interest for all of a week. As Launch day drew near the outlook wasn't good. Windy, with clouds moving in during the afternoon and possible showers overnight and into Sunday. So it looked as though it would have to be Saturday or not at all. When I woke up Saturday morning, my heart sank, it was gray and gloomy, with the wind whistling through the trees. I had told my ground crew (John and Jace) to come in about 10:30am, I really thought they would call in first because of the weather, and we'd cancel the launch. But they both showed up, despite gloomy skies we decided to head out to the launch site and see how it developed. As it turned out, the closer we got, the skies started clearing. By the time we arrived at the site, there were only a few thin high clouds, but the wind was still blowing 15 to 20 mph, and perhaps gusting even higher.
After calling in to activate the wavier, we went about setting up the launch rail and control circuit. I decided to launch the RFR as a sounding rocket, to get an idea of the wind direction, then decide on what to launch after the first flight. The RFR was successfully launched and recovered a little over 1/2 mile from the pad. The flight to 2,800' had drifted about as far as it went high. At that point I had pretty much decided not to launch the Prelude, and settle for another flight of the Comso 2 rocket. So I removed the largish drogue from the Cosmo 2, and installed the smallest drogue I had. The Cosmo 2 was prepped and on the pad, when the altimeter fired the PIRM2. Hmm, that happened on the first flight of the Cosmo 2 as well. For some reason I'm getting current to the main channel. I was going to reload the PIRM2, then changed my mind. Clouds were now building to the West, there was time for one more launch. So we prepped the launch tower for the Prelude.
I set one camera pretty close to the launch tower, John had one and Jace had the last camera. I had the RDF receiver and was at the launch controller.
All clear. 5,4,3,2,1 and ignition!
The Prelude came to life instantly...
Video Camera 1: Divx File stationary
camera close to the pad. Good sound.
Video Camera 2: Jace's Video: Divx File, caught the entire boost on tape. (New file added 10.30.2005 as the first one seemed to have problems.)
Video Camera 3: John's Video: Divx File, camera has touchy zoom, only the first part of flight caught but good sound.
Launch Test 112 audio combined: Divx File, Jace's video with audio combined from two other cameras.
If you don't have Divx, you really need to get it. Click Here to go the the Divx web site.
Here's the moment of ignition. Notice the flame, it's from the magnesium shavings in the starter mix I painted on the grains to aid ignition.
Here's my first erythritol based propellant rocket coming off the pad.
Now the Prelude has left the tower.
There goes the Prelude on her maiden flight.
It was picture perfect, straight off the pad into a flawless boost. The sound of the burning motor getting lower as the speed and distance of the rocket stretched the sound waves. The Prelude was quickly out of sight, we all stood around and stared into the sky, but nothing was to be seen. I swept the antenna back and forth, beep, beep, beep. I started counting seconds, it should have deployed by now, but still nothing could be seen. Time crept by, I kept sweeping the sky with the antenna. Beep, beep, beep. It must be under the drogue by now... Then I started narrowing in on the location of the rocket with the antenna. I quickly discovered I could get a very good fix on the location with a quick sweep of the antenna.
The rocket, as expected, had flown into the wind, and now was drifting directly overhead. I continued homing in on the rocket, without ever seeing the rocket I could tell where it was in the sky. Enough time had passed now, I knew the drogue had successfully deployed. I tracked the rocket to the North, then the receiver went silent. We quickly jumped back into my van for the hunt. Once back on the road, at the top of a hill I did a sweep with the receiver, no signal from the rocket. So I called the FAA to close the wavier, then moved on down the road. We drove out about 1.25 miles from the launch site, and stopped on another hill.
I turned on the receiver, beep, beep, beep. A huge sigh of relief, we were close, I had the receiver set to medium range. So I set out on foot, sweep the antenna back and forth, walk towards the strongest signal. Child's play really, it was that easy. Within minutes I came upon this scene.
Those cows are really interested in something there. Hey, I think I see something red.
I got over a fence, and sure enough, there was the Prelude. As in the Cosmo 2 flight, a little close to the water for my liking.
Here's the Prelude as found.
The rocket had a perfect dual deployment and had landed undamaged. I listened to the altimeter beeping out the altitude. Had I made 10,000'? Let me count those beeps one more time, yep, 10,402'. Success!
Here's the crew back after recovering the Prelude. From the left, John H., Jace M. and of course me (Scott) I really was much happier than I look in this picture.
To sum up this launch in one word, it would have to be flawless. Everything worked exactly as planned. The new Accel Techtronics flight computer did fantastic, the transmitter and receiver were great and simple to use. The KN/ER propellant worked great, the motor had no problems, the rocket survived the flight undamaged, the rocket flew straight and true, the PIRM2 worked great again... Another great day of flying rockets.
Here is the Accel Techtronics file from the flight.
The Accelo-Graph software is free to download from Accel Techtronics software page.
Flight Stats Summary:
Max Altitude: 10,402'
Max Acceleration: 17.59 g's
Max Velocity: 1,022 fps, 833 mph, mach 1.103
Time to Apogee: 24 seconds
Descent Rate Under Drogue: 57 fps
Liftoff to Landing: 206.88 seconds
Apogee Charge to Deployment: 1.43 seconds
Here's a capture of the Accelograph software from the flight. I didn't have the Prelude parameters such as liftoff weight, propellant weight and burn time programed in before the flight. So the motor performance can not be calculated, nor the velocity in the rocket performance area.
This is a graph after exporting the file from Accelograph into Excel. This is only the liftoff to apogee. We see some crazy readings from the barometric sensor just after peak thrust. I would guess this was the sonic shock wave when the rocket exceeded the speed of sound.
Here is another graph from Excel. This shows barometric altitude only. Notice the dip just after apogee, no doubt that's ejection gases adding some pressure to the electronics bay.
After looking closely at the data, it appears the ARB3D doesn't start recording until 2 g's plus have been detected. Which leaves a little data behind. The accelerometer recorded a maximum of 17 g's, which also seems high to me. Burn time as indicated by initial positive g's was 4.2 seconds, add another portion of a second for thrust to develop and the unit to start recording and that's about right for burn time, although I would have expected a slightly longer burn, perhaps another .75 second or so.
We also have to keep in mind what an accelerometer chip of this type is capable of, they do great for detecting apogee, and give us some nice basic data. But they aren't going to give absolute data, it's more ballpark type of data.
Looking at the apogee data, it almost looks to me like the timer fired the drogue charge. The unit is still recording negative g's, about -1.56, then the next sample goes to -20 g's and the barometric pressure increases. To me, that indicates the apogee charge went off. Then, a third of a second later the accelerometer records +4.5 g's, indicating drogue deployment. Not knowing how the program was written to detect apogee, I'm not really sure though. Regardless of what triggered the drogue, it appears to have fired 126' below apogee at T + 27.23 seconds. Apogee of 10,402' occurred between T+ 23.2 to 24.8 seconds.
Here is another chart I did in Excel. I painstakingly calculated the velocity increase for each sample, then added up all the velocities to come up with rocket speed in feet per second. The bottom line numbered from 1 to 131 is a sample, each sample was taken .0317972 seconds apart. So this graph represents the recorded time from start to about 4.8 seconds. I should note again, the unit does not record the first moments of liftoff, it started recording at about 3.4 g's. So the velocity data I have here will be less than actual speed.
What is interesting here is the altitude line (bright blue). When looked at in this fine of detail, it's a very hectic curve. Some of that will be noise in the signal, some may be due to turbulence around the rocket. Not that it means a lot, I just thought it was interesting.
I still can't come up with an exact velocity for the rocket. Not including the initial moments of the flight, the velocity works out to 1058 fps, assuming the initial 3.4 g's for 1 second not included, I end up with a maximum velocity of 1167.48 fps, or 795.98 mph.
Ok, so I decided there was another way to calculate the rockets maximum speed. Using the altitude versus time, I can calculate the average velocity quite easily. The problem is, the barometric sensor would would suffer from the effects of mach transition. So I'd have to get an average speed after motor burn out. So I used the the values from 5.21 seconds to 7.21 seconds into the flight, during that time the rocket averaged 1222.2 feet per second or 833.3 mph. That's actually going to be a little low, as maximum speed would have occurred right at motor burn out. So my initial software prediction of 860 mph is probably just about dead on.
One last thing I looked at in the data was the main deployment. During descent under the drogue the accelerometer reading was about -.25 g's. At 493' the accelerometer recorded +4.36 g's, indicating the main chute had deployed and opened. At its landing site, the rocket was 276.5' higher than its launch point. To be honest, I hadn't really thought about that. We did launch from a low spot, and I'm not surprised the elevation was 276' higher, as we did drive up a couple of good inclines to get to the landing site. But that only left a margin of 224' for the main to deploy. I suppose I should really have the main deploy at 700' from this launch site.
The ARB3D seems to have performed well. I suppose I'd have to admit the graphing software is a little rough around the edges compared to some units on the market. But the price and features make it very attractive.
The above picture was taken after the motor burn. The forward closure end is to the right, and was virtually intact. The nozzle end melted to the casing a little, and I had to tear it away to get it out. But all in all it did a great job of protecting the aluminum motor case.
Here is a satellite image* showing the launch site and landing sites of the rockets launched.
* Iowa Geographic Map Server - Iowa State University Geographic Information Systems Support & Research Facility