This web page documents the brief life and death of a high power sport scale model. I built a model of the first flight test round of the Army Tactical Missile System (ATACMS) flown at White Sands Missile Range on 26 April 1988. It took only about a month to build and paint the model. As DARS club members know, I brought the model in a very early stage of construction to the July club meeting. Two weeks later it was ready for test flights at the club launch. And two weeks after that it was painted and ready for LDRS in Argonia Kansas (1999).
The model is not very "scale." The nose cone is a standard LOC PNC5.38L which is about a 4-1 ogive. Except that the real thing is a 3.07 Von Karman ogive. I had purchased a LOC I-ROC kit some time ago and never got around to building it. Then I started noticing that it might make a decent ATACMS... I also had to lengthen the body a bit in order to fit the L330 in it. And I made the fins just a bit oversize.
I flew the completed (but unpainted) model twice using first a I284 and then a J275. It flew great both times. Too great actually. I had badly mis-estimated the CD of the model and it went much higher than I had counted on. Since I weighed the model before flight and had an Adept ALTS2 on board, I was able to back out the actual CD. The J275 resulted in a flight to almost 3800 feet which was bit higher than the 3000 feet I had predicted. Using WRASP I was able to determine the actual CD as 0.26. Much better than I would have thought. I have just finished doing a rough plan using RockSim and it also calculates a CD of 0.26. I wish I had it before (I picked up a copy from Tim during NARAM).
The details of both flights were:
|Motor||Takeoff weight||CG location (from tail)||Altitude|
|I284||8 lbs. 0 oz.||18 inches||2468 feet|
|J275||8 lbs. 9 oz.||17 5/8 inches||3774 feet|
With the new CD of 0.26 WRASP was predicting an altitude of over 10,000 feet on the L330. This was going to require a tracking transmitter. I had PIC programmer problems and was unable to build a transmitter in time. Pete Kerckhoff loaned me his for the flight. Be sure to check out Pete's web page.
For some bizarre reason, I didn't take any pictures of the painted model! On the day of the flight, Dave Schaefer came out to the pad with me (thanks Dave!). And I handed him my camera to take a pre-launch photo. Except I forgot I had just loaded a fresh roll of film and left the camera in manual mode. I figured this out when I looked at the developed film. This one frame is very badly under exposed. I must have noticed it when I was getting ready for launch and reset it to automatic exposure control without thinking about it.
But not to worry, there was a healthy DARS contingent at LDRS and they gave me couple of pictures. On the left is the ATACMS ready and waiting. On the right is a picture of Dave Schaefer and I about to head out to the pads.
Anyway, here are the launch pictures. The first shows it just off the launch rail heading straight up. The second shows it making a sharp left (south) turn.
Fortunately, the rocket went parallel to the flight and crowd line. But it was headed almost horizontal to the south. I watched anxiously for the altimeter to kick out the chute but never saw it. Since the Ellis Mountain motors do not have any sort of delay or smoke charge, the rocket vanished as soon as the motor burned out. Then it was time to go looking for the remains.
I was certain that it had gotten at least as far as the highway that was one mile south of the launch area. So I looked in the disked wheat stubble in that quarter section (take a square mile (one section) and cut it in quarters) on the south side of the road. I did not even think about looking in the quarter section just east of there. It had a milo crop in it and I would never find it in there. After fortifying myself with plenty of water, I trudged out to the south edge of the quarter and then back. On the way back I took every opportunity to stop under a shade tree. Did I mention it was hot? No luck. I decided I would continue the search in the morning.
Overnight a thunderstorm rolled through the area. Several awnings had their support poles bent into interesting designs and others just blew away. I decided to watch the rockets fly for awhile and let the roads and fields dry out a bit before I ventured out to continue my search.
Sunday I wanted to look in the disked wheat stubble field that ran 1 1/2 to 2 miles from the launch site. So I drove to the south edge (love those section line roads) picked a likely place to park and started out. I saw my rocket just as soon as I turned to look into the field. It was only 200 yards (give or take) from the road. I grabbed my camera to record the gory details and headed towards it. Here is a shot looking back towards the range head. It is just barely visible in the center and is almost 2 miles away.
The nose is on the right and the motor section is just visible as the spot of orange to the left. Here is a closer shot of the remains.
I looked for evidence that the rocket had hit the ground and bounced to this point but could find none. The fin stuck in the ground next to the nose tells me that the nose had not separated before impact. I found a hole burnt into the shock cord protector. This indicates that the at least one of the ejection charges went off. But it might have gone off on impact. Both the altimeter and timer were destroyed.
Here are a couple of closer shots of the booster. Hey I can salvage the rail button!
The only parts that I can reuse are the rail guides and the recovery system (tubular nylon shock cords, Pratt parachute and shock cord protectors, and Rocketman parachute). The rocket is a total loss along with the motor case and electronics. An expensive lesson.
I am still not positive why it made the turn. It was probably because it was too stable. When I first brought it up to the safety check-in, the Safety Check-in Officer was correctly very skeptical. In spite of assurances that I had weighted the model to match the CG location of the J275 flight, he would not sign off on my flight card. As I recall John Baumfaulk was called over and he said that he would like to see 1 caliber of stability. I replied that adding that much nose weight would make the model too heavy for the L330. It is VERY hard to move the CG of this short a model. Especially with an L330 in it.
At this point I should have just packed it in. Except that this was the only flight I had planned for LDRS. I had not brought any of my 54mm motor cases or the 3" to 54mm adapter I would need to use say a J415. Not to mention I hadn't arranged for the purchase of a J415 (pesky BATF :-) At this point I went stupid. I acquired a lead weight (I later (stupid again!) weighed it at 3 lbs.) and put as far forward in the nose as I could. This moved the CG an incredible 2 inches to give me 4.5" of static margin instead of the 2.5" I had started with. The combination of extra stability margin and slow liftoff speed combined to produce a horizontal flight.
I cannot fault the caution of the safety check officers at LDRS. They had a tough job to do. I should have been prepared to fly using a smaller motor to demonstrate the models stability for them.
I will build another ATACMS at this scale. But I think I will just put a 54mm motor mount in it this time. The J275 was great.
I finally broke down and purchased the videos produced by Point 39 Productions and Rocketman of LDRS. Lots of neat stuff. And the Rocketman video even included my ATACMS "flight." Watching the tape I could see that my rocket began to turn into the wind shortly after liftoff. At one point it was almost horizontal. Then the fins pushed it back until it was vertical. This would have been great except that the oscillations continued. With the result of a nearly horizontal flight.
This showed me a few things. Number one was that the model was extremely stable. The problem was that the fins just don't provide a lot of restoring force until they have a pretty good angle of attack on them. Speaking of angle of attack, viewing the tape showed me that model reached about 45 degrees angle of attack and did not go unstable as a result.
So how do I prevent a recurrence? I am sure that I will stick to smaller, higher thrust motors for this design from now on. The high thrust will get the model up to a speed where the fins are effective quicker. The other is to be careful about how much wind I fly it in. Kind of tough here in Texas, but calm days do happen. Occasionally even during a launch!
Here is a picture of the real thing. Thanks to Bob Wilson for scanning the photo he got from White Sands Missile Range.
The ATACMS returns to LDRS!
I built a new ATACMS based on LOC 5.5" parts except that this time I only put a 54mm motor mount in it. I finished painting it just in time to take it back to Kansas for LDRS 22 in Argonia.
I had wanted to fly it on a K550 but only a K695R was available. Good enough.
The boost was fast and extremely straight. Not even the slightest hint of a wiggle. The flight was marred by an early deployment which can be seen in the recorded AltAcc data.
Although the inertial velocity is up over 1,200 ft/sec and there are two noticeable anomalies in the pressure data, I don't think it quite broke through Mach 1 although it was certainly in the transonic region.
Because of the early deployment the parachute ended up tangled in its shroud lines. This was not entirely bad as the rocket landed within sight of the launch area. The impact opened a small crack in the body to tail cone joint but I don't think this is serious. I will put a little bit of CA in it and it should be ready to go again.
I think that the early deployment was due to the slip fit on the joint. I hadn't put shear pins at this joint because the coupler is paper and so is the body tube. But now I think I will fill at least part of the coupler area with foam and then insert some brass tube in the coupler and the body tube to support shear pins.
Alas, the picture I took only captured a bit of the red flame.
I seem to have accumulated enough flight data now that a flight log seems worthwhile. So here it is:
|1||LDRS 2003||AltAcc Data||Aerotech K695R-L||The first flight of my new ATACMS was marred by early deployment. Velocity (see graph) at deployment was just under 300 ft/sec.||6307'|
|2||May 2005||AltAcc Data||Aerotech J460T-L||Great flight. Photo by Bill Gee.||4405'|
|3||June 2005||AltAcc Data||Aerotech J275W-L||This flight included my first attempt at using the Defy Gravity Tether which didn't work as well as expected. Photo by Bill Gee.||3925'|
|4||August 2005||AltAcc Data||Aerotech K550W-L||Another attempt at using the tether and another failiure. neither of which were the fault of the Tether.||7540'|
|5||April 2008||AltAcc Data||Aerotech J275W-L||A very short motor delay and failed Tether deployment.||3500'|
|6||6 September 2009||AltAcc Data||Aerotech K695R||Perfect flight.||7,100'|
|7||22 May 2010||AltAcc Data||Aerotech J275W||Tried the Tether again. This time the main came out at apogee.||3,800'|
And a few pictures of the version I bashed out of an Estes Bullpup kit. The nosecone is much too long of course.