Friday, September 5, 2014

Unrestricted Motor

I wrote this a long time ago and just noticed it sitting in the post section as a draft.  This launch was ten years ago in the spring of 2004 and apparently I was thinking about boosted darts back then as well.  I posted some pictures in 2009...I don't recall why this account wasn't posted at that time.  I also noticed that I mention the atlimeter was reporting the peak altitude but apparently I didn't record that info as its not mentioned. 

Launch of L-class unrestricted motor

Unrestricted (burning on all surfaces) propellant grain

I designed this motor with several ideas in my mind. First was to have a motor capable of lifting heavier and larger payloads than I’ve done in the past. Secondly, I wanted a larger motor that didn’t require casting multiple grains, and finally I wanted a motor that could serve as a booster for a future boosted dart project. The motor design uses a single unrestricted burning grain, producing a relatively high thrust short duration burn profile. The motor serves as the casting mold reducing the amount of hardware needed for the casting process. A well-lubricated, thin paper liner is inserted into the motor before the propellant is cast. After the grain has cured, the liner is removed and discarded. This ensures that the grain has a slightly loose fit in the motor. KNO3/Sorbitol is the propellant, which, with its lower melting temperature, eases casting on cool, windy nights at the MTA. Finally, the motor is constructed entirely of steel which eliminates the need for any insulation.
The nozzle and bulkhead are fabricated from 1018 cold roll steel. The nozzle has a 45 degree convergence angle, and a 12 degree divergence angle, and has an area expansion ratio of 4. Both are retained in the casing by eight 8-32, ¼ inch low head cap screws and have a single Nitrile O-ring. Both the nozzle and bulkhead retaining screws are tightened against the nozzle or bulkhead flange. As such, the casing holes are sized to the head diameter of the fasteners. Load transfer is thus accomplished through bearing of the fastener head against the casing wall. The casing is made of mild steel. It has a 3” OD and a .065” wall thickness. The entire motor weighs a total of 4.19 lbs (1900 grams) which breaks down as follows; nozzle 1.04 lbs (472 grams), bulkhead 0.39lbs(176grams), casing 2.73 lbs. (1239 grams), retaining screws and O-rings .03 lbs (0.15 grams).
I had been planning on static testing this motor at the launch in March. Then, later it would be used as the booster on a two-stage airframe that Peter had been constructing. However, several days before the launch, we decided that we could fly the two-stage as a single stage with no motor in the upper portion of the rocket. Without the added weight of the motor in the upper part of the rocket we knew that it would be somewhat over stable. With the relatively low expected altitude, if it veered into the wind we wouldn’t be looking at too long of a walk.
We got to the MTA late Friday night and cast the grain before turning in for the night. In the morning, I had to remove the grain from the motor case so that I could peel the liner off of the outside of the grain. It didn’t take me long to realize that I had forgotten to grease the inside of the motor case the night before. It took about a half an hour of hammering with a rubber mallet (note second photo above) before we finally got the grain out. Once out of the case, the liner peeled away from the grain with no problem. The grain was still not completely cured as I could indent it with my fingernail. The end that we had hammered looked like it had been hammered on as it appeared slightly deformed but had no visible cracks in it. I think that if it had been fully cured it would have likely fractured from the hammering.

Once we got the motor ready, we turned our attention to the airframe and recovery system. This launch was as much a test of the airframe material as of the motor. The airframe was made of 4inch ABS pipe with ABS sheet used for the fins. The upper fins were attached only with ABS cement. The lower fins used the ABS cement and were additionally reinforced with a layer of epoxy and fiberglass. The entire airframe went together quickly and for minimal cost.
The total weight of the rocket as it went to the tower was just less than thirty-one pounds. After a few last minute details, we were ready to launch. The motor came up to pressure quickly, and the rocket roared out of the tower. After an initial turn into the wind as it cleared the tower, the rocket flew very straight considering its rough construction. It continued to look great up thru apogee. Shortly reaching apogee, the main charge ignited. The lower section, with the motor in it, came loose and began a ballistic decent. It was at that moment we realized that we had forgotten to install a set of screws meant to hold the two stages together when the airframe was flown in the single stage configuration. The upper portion of the rocket descended under the drogue chute until the secondary charge ignited at 1000 feet and blew that section apart. The upper part then dropped a thousand feet, and the nose cone landed gently under the main chute. Fortunately we recovered everything. Amazingly, all but one fin were still securely attached to the airframe, and the motor was in good condition. The casing did have a small deformation near the bulkhead end, which may have been attributable to the fact that we had to hammer the grain out of the casing, or may have been from the impact. Fortunately a new casing is easy to make. The altimeter was reporting the max altitude, even after falling from 1000 feet. The altimeters pressure sensor was ruptured by the excessively large secondary charge. The manufacturer (Perfectflite) replaced it and tested out the unit for a minimum charge.
Over all we were very pleased with the launch. All of the problems we had should be easily solved with a prelaunch checklist and a little less guesswork about the proper amount of BP for ejection charges. The ABS material turned out to be very versatile, easy to work with and exhibited enough strength for our purpose. The ABS sheet probably doesn’t have enough rigidity for a high-speed rocket fins, but worked fine at slower speeds. We reached a speed of around 380 mph with this launch. Considering the cost, we will probably use it again in the future.

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