Launch Test 120

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The grain casting is complete for the A2MD flight, so I've done some new calculations using the propellant weight and geometry numbers.

Grain #	Grain Weight	Tube Length/weight	Propellant Weight	Propellant Length	Density
1	876.2	5.31/27.6	848.6	5.17	0.0624
2	853.3	5.12/26.6	826.7	5.0	0.0628
3	875.3	5.34/27.7	847.6	5.15	0.0625
4	969.8	6.02/31.3	938.5	5.81	0.0614
5	867.7	5.28/27.5	840.2	5.15	0.0620
6	875.3	5.34/27.8	847.5	5.19	0.0620
7	834.5	5.03/26.2	808.3	4.88	0.0629
8	759.3	4.53/23.5	735.8	4.4	0.0636
9	847.0	5.29/27.5	819.5	5.01	0.0622
10	860.1	5.28/27.5	832.6	5.14	0.0616
Total	8618.5gr. 19.0005lbs.	52.54" 273.2 gr.	8,345.3 gr. 18.3982 lbs.	50.9"	0.06233 lb./cu. in.

Here are the numbers on the motor:

Casing: Diameter 3.5", Wall .125"

Nozzle: Throat Diameter: 1.0", 60 degree convergent, 30 degree divergent, 2.22" exit, 4.93:1 expansion

Kn: 406 initial, 449 max, 419 ending

Propellant Weight: 18.3982 pounds

Casing Internal Length: 53.305"

Total Segment Length: 52.54"

Expected Total Impulse: 2,355 pound seconds, 10,474.9 Newton seconds, low "N" class

Lower body tube, complete, no propellant: 14.0 pounds

Upper body tube, complete less transmitter: 8.8 pounds

Propellant and liner tubes: 19.0005 pounds

Lift Off Weight: 41.4 pounds

Here's a screen shot from Fpred showing the expected thrust profile.

And a screen shot from Winroc.

I made a new 30" drogue, it should be pretty bullet proof as I used 9/16" tubular nylon for shroud lines, and sewed them to the apex of the chute. Assuming a weight of 23.4 pounds, descent rate under the drogue should be about 73.27 fps, under the 80" main chute descent rate should be about 26.19 fps.

Flight Electronics:

Accel Techtronics ARB3D:

Apogee Detection: Accelerometer, no mach inhibit required.

Apogee Charge: 3.5 grams

Main Deployment: Barometric, set to 800'.

Pirm 2 Charge: .4 grams

Output: LED

Transonic PK-6

Apogee Detection: Barometric, 15 second mach delay.

Main Deployment: Barometric, set to 800'.

Output: Piezo

One last check of stability once the rocket was assembled. I measured the center of gravity at 74.25", with a center of pressure between 87.65" to 97.27". That leaves me with 13.4" to 23.02" of margin, or 3.62 to 6.22 calibers. Liftoff weight is 41.4 pounds.

Launch Day:

As the forecast had predicted, it was a beautiful day, clear skies, light wind and a little cool with temps in the mid 50's. The ground was still a bit damp, which limited us to launching from slightly higher ground to keep out of the wet areas. All in all, no complaints in the weather department. Bill went with me in my old van, John and Kathy followed in her SUV, the four wheel drive proved handy getting equipment in to the actual spot we located the launch controller, while my "helpless in the mud" van stayed near the launch pad. While I had three rockets prepped and ready to fly, the A2MD was the only one I really wanted to fly, so I decided to launch it first, then see if time and conditions allowed for more flights.

We located the trailer with the launch rail on a bit of a hill, and really had to block up under the jacks to get the trailer level. I know it doesn't look level in the videos, but it was, the camera is the tilted one. I really didn't take too long to set up, the others strung out the over 500' of cable to the launch controller, while I finished setting up the rocket on the rail. I powered up the altimeters, made one last check of everything and headed to the launch controller. Bill took a video camera up the side of small hill to get a different camera angle. I was on the large video camera near the controller, John had another camera and Kathy had the honor of pushing the launch button after my count down.

One last check, all clear on the ground and in the sky...

Kathy prepares to light the A2MD motor.

5,4,3,2,1 and ignition!

Click Here for a quick low resolution video from my camera, great sound, nice close up of the launch but I lost the rocket on the way up.

Click Here for another short low res video from Bill's camera, pretty good sound and the entire boost was captured.

Click Here for another low res video from John's camera, wide shot has the entire boost, may show the actual speed the best.

Click Here for the camera sitting 50' from the pad at launch.

Click Here for a slightly better quality video with all four camera shots in one. (8.55 MB)

Here's a capture from my video just after ignition.

Again from my camera as the A2MD leaves the rail.

This is from Bill's perspective from the hill top.

Another capture from Bill's camera as the A2MD is well into it's flight.

Here's a capture from the JVC video of Johns.

The A2MD came to life with a wonderful roar, the 400+ pounds of thrust quickly vaulted the rocket off the pad into a beautiful, straight flight. I watched cautiously as the motor burned out, the first dangerous phase of the flight had been cleared (the motor burn). As the sound faded, there was nothing to see or hear, the rocket would coast another 25 seconds to apogee. So I grabbed my radio direction locator receiver, set on close range I scanned the sky, no signal, so I went to medium range on the receiver, still no signal. Now I was starting to worrying, even over three miles up I should be getting a signal. So I switched to long range and swept the sky again. This time I picked up a faint signal, then zeroed in on the location.

Shortly after picking up the signal we all heard a rather dismal sound, a little like a jet flying overhead.

For the more sadistic of you (I guess that includes me because I put it here), here is a slightly enhanced sound file of the rocket coming in. I removed two segments where Bill spoke because it about blew out my speakers, and ran it through an equalizer. You'll still need to turn your PC sound up, but it's fairly easy to hear. 3.6 MB for 21 seconds of sound. lt120sound.wav

Something was wrong, the rocket was coming in too fast. Then deadly silence, I think I said something like, "!?X% no". Then I radioed Bill, he came back saying he had seen the rocket come in, with the drogue trailing behind the rocket. "Hmm, that was odd", I thought to myself. I didn't take long to find the rocket, it was sticking up out of the ground like a fence post about a third of mile from the launch pad.

John (left) and I (right) discuss what may have gone wrong.

Here's my rather expensive fence post! The rocket body is 8' long, that means there's about 4' in the ground...

Original Theory:I didn't take me long to see the problem, it was more bad luck than anything. The rocket had deployed the drogue just fine, then flew right through the shroud lines of the drogue. The tangled drogue, nose cone and deployment piston had been banging on the fin can the whole way down, as was evident by paint chips on the fin can.

Update April 10, 2006: How could I have missed this! I decided to go over the accelerometer data again, and found something very interesting. See the little dip at about 15,500' on the screen capture from the image below... It corresponds exactly to a nearly -9 g reading from the accelerometer. Look at the data below, it shows the g force and altitude in the suspect area.

After thinking about this some more. I'm not sure which unit fired the apogee charge for certain, but the drogue was definitely deployed too soon. Looking at the ARB3D date, it seems to have been the P6K. The ARB3D changes its sample rate from 30 samples per second to 10 samples per second after the apogee charge fires. The data indicates the change in sample rate occurred well after real apogee.

We leaned on the rocket a few times, but is wasn't going anywhere without a shovel. I leaned over the motor section, wait, did I hear beeping. Sure enough, from deep below the ground the Transonic PK6 was beeping out the max altitude... Start from the beginning, there's a one, now how many was that, eight, and lastly a four. Now add two zeros behind, 18,400'. Oh my, the A2MD had flown to just about where I had predicted!

But wait, it gets even better. I turned on my RDF receiver, and low and behold the transmitter, which was attached to the base of the nose cone was still working too. The little wire antenna was missing, but no big deal there.

After digging the rocket out, (we went to town and bought a shovel, as it's against my principles to bring a shovel to a launch), the body tube was packed with dirt but appeared undamaged. I still didn't know how the Accel Techtronics ARB3D had faired, but things were looking better. At the moment, I had only lost a wood nose cone and the drogue.

Once we got home, I set to cleaning out the body tube and extracting the electronics module. After a fair amount of work, the electronics module was removed. Incredibly, there wasn't any damage to anything in the electronics bay, all the batteries were in place, no cracked boards, hard to believe. So I fired up my PC to see if I could download the data from the ARB3D. After only a quick assessment of the data, here's the results from the ARB3D.

Max Altitude: 17,102'

Max Acceleration: 14.64 g's

Max Velocity: 1,577.37 fps, 1,073.7 mph, 1.4 mach

Descent Rate: 268 fps (way too fast!)

Time to Apogee: 29.2 seconds

Keep in mind, this unit has a strange feature that doesn't allow it to record the first moments of flight. So some data is lost. Actual max velocity would be slightly greater, as would time to apogee.

Here's a screen capture from Accelograph. I didn't program in the rocket parameters, so not all the information is available.

Click Here for the Accelograph file from the flight.

There is a significant discrepancy between the altitudes of the two altimeters. I guess if you average it, it comes out to 17,751' which is right about where it should have been according to simulations. I'll look at the data more closely in the coming days, but I don't expect to find anything more of real significance.

I'm not sure how to sum up the flight, I'm certainly not disappointed. It would have been nice if the recovery would have gone as well as the rest of the flight. Based on the accelerometer data, it looks like I'll have to use accelerometer based apogee, or accelerometer and timer back up only.

It was another great flight using KNER propellant, and the rocket flew great! Best of all, the rocket will fly again. All it needs is a new nose cone.

Here is the electronics module from the A2MD, you can see the remains of the apogee charge holder (latex glove fingers), the PIRM2 also fired normally as the pin had released the key. Notice how the heat and exhaust discolor the nozzle. A few minutes on the lathe with some emery cloth and the nozzle will look like new again.

Here's the A2MD back up on the wall after a good wash down. A new nose cone and she's good as new.

I exported the ARB3D data into Excel, and made this graph of the thrust/time curve from the flight. If you average out the peaks and valleys from the accelerometer data, this is a very nice flat thrust profile. Again, you can really see in this graph the data that is lost at the start of the recording.

This graph shows the barometric altitude vs. time. It looks like the rocket went into mach turbulence at about 3 seconds at about 2,500'. Motor burn out occurred around 4,000'.

Closing Thoughts:

The surface mount fin can is amazingly strong, when we tried to extract the rocket from the ground, we literally twisted with all our force on the fins, they didn't bend, crack, creak or budge even a tiny bit. I now have a very high degree of confidence on building future rockets using the same technique.

The KNER propellant continues to perform exactly as expected. Frankly, I'm still confused why I haven't heard of anyone else working with erythritol. Let's just say this, you don't know what your missing out on...

I'm not sure yet what my next project will be. I'll make a new nose cone this week for the A2MD, and I may fly the A2MD again this year. But, I tend to prefer to move on to bigger projects, we'll see. I'm afraid I'm reaching the upper limit for launches in my part of the country. I'm sure I can get a 25,000' wavier, and I suppose my next big project will target 20,000' to 22,500'. But with Spring planting just around the corner, it's unlikely I'll ask for another waiver until this Fall. With my radio direction finding equipment, I may do a small KNER rocket that can fly without a wavier. I'll just shoot it into a corn field, then use the transmitter to go get it.

I'll close the Launch Test 120 page, and wish you all happy flying!