Igniter Characterization

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Rocket Igniter Characterization

A two-stage rocket project that we are working on drove the desire to select and characterize a workable method of air-starting the second stage of our rocket. We noticed that many rockets smoked for a prolonged period of time on the pad before the motor sufficiently lit to make the rocket airborne. These experiments focus on creating a motor igniter that would instantly ignite a motor in a safe and predictable manner.

Background:

Ammonium Perchlorate Composite Propellant (APCP) does not burn robustly at low pressures. Literature states that pressure greater than 180 psi is needed to sustain a stable APCP burn. The ideal motor igniter will produce sufficient gas to pressurize the motor's core above this level and sufficient heat to ignite the propellant. Many igniters sold commercially for amateur high power rocketry are based upon Thermite. Thermite generates a great amount of heat in the form of molten metal slag, but no gas. Although these igniters work, they rely on the propellant in the rocket to to create the gas needed to pressurize the motor chamber. It seemed to us that the inefficient combustion condition (very high temperature, but low pressure) was resulting in the ignition delay of the motors. We experimented using an igniter made from an e-match and several pyrodex pellets. The pyrodex pellets produce much gas and significant heat, though not nearly the as much heat as Thermite. The experiment resulted in a near instantaneous ignition and lift-off of the rocket. The success made us concerned that the Pyrodex was over-pressurizing the motor chamber. These experiments were constructed to measure the chamber pressure created by Pyrodex igniters and allow us to select the optimum amount of Pyrodex to ignite specific motors.

Goal:

Characterize Pyrodex igniters and determine the optimal amount of Pyrodex for M1419, M750, and N2000 commercial high power rocket motors. Using an instrumented chamber with the same volume as the core of selected motors and the correct nozzle throat orifice opening as that motor, burn the igniter and measure the pressure produced by it.

Equipment:

We adapted our propellant Strand Burner for the igniter experiments. The Strand Burner consists of a piece of 3000 psi rated steel pipe with pressure end-cap on one end and an electronics end-cap on the other. The pressure end-cap contains a pressure transducer (0-100 bar) and a safety burst diaphragm (1600 psi). The electronics end-cap was replace with a nozzle throat orifice cap for these experiments. The Nozzle Throat Cap consists of a Throat Plate bolted to its Nozzle Cap.

Igniter Test Chamber Click for Component Description

Igniter Test Chamber

Igniter Test Setup

The Throat Plate is a 1/4" aluminum disk with a circular hole at its center. This hole's diameter is cut to match the nozzle throat diameter of the selected motor. The Throat Plate is attached to the Nozzle Cap, which holds it in place and seals the end of the Strand Burner chamber, preventing gas from escaping except through the hole in the Throat Plate.

Nozzle Cap with igniter inserted

3D PDF (requires Acrobat Reader v8 or greater)

Drawing

The Strand Burner chamber's internal dimensions are internal diameter = 2.5", internal length = 12.19", yielding a volume of 59.83 cu-in. This volume needed to be reduced to match the core areas of the motors of interest.

Initial experiments focused on the M1419 motor. We reduced the volume of the Strand Burner chamber to 17.89 cu-in by inserting two cylindrical wood plugs. Each plug has an an OD of 2.47" and a .75" hole drilled in its center. One plug is 8.43" long the other is 1.4" long. When these plugs are inserted in the chamber and pushed to the end farthest from the Nozzle Throat Cap. The remaining 2.36" of the chamber wall surface was protected by a thin layer of aluminum foil.

The igniters were prepared by threading the e-match wires through the center hole in the Pyrodex pellets. 50 caliber, 50 grain pellets were used. The head of the e-match was bent over and taped to the side of the string of pellets, about at mid-point of the length. The pellets were taped together (using painter's masking tape and taped to the end of a 1/8" x 24" wooden dowel. The dowel was inserted through the Nozzle Throat Cap opening into the prepared chamber so that the top pellet was about 3" from the opposite end. A standard launch control box was used to ignite the e-match.

The Strand Burner's pressure transducer was attached to a PC via an Analog to Digital (A/D) converter. The launch trigger signal was also fed into another port of the A/D converter. The A/D converter was set to sample at 1000 samples per second.

Procedure:

1. The Pyrodex igniter was placed into the Strand burn chamber through the Nozzle Throat Cap and held in place by tape applied outside the chamber.
2. Electrical connections for recording and ignition were made.
3. The PC started recording A/D activity. The data was streamed to a disk file.
4. The e-match was fired.
5. The captured data was converted to a series on a spreadsheet chart.
6. The above sequenced was used for igniters containing 1, 3, 2, and then 4 Pyrodex pellets.
7. The chamber was not disassembled and cleaned until after all four igniters were tested.

Results:

 Igniter being tested. No rocket fuel is being burned, all this gas and flame is from the igniter.

The below chart shows the Pressure vs. Time characteristics of each igniter.

After the 1, 2, and 3 pellet burns, the wood dowel was removed intact. The 4-pellet burn burnt off the end of the dowel. We believe that some unburned Pyrodex may have been blown out of the chamber.

Upon disassembly, we noticed that the wood plugs were slid up against the Nozzle Throat Cap and the foil that was placed there was crumpled into a disk.

The chart data was determined to need more analysis and perhaps some adjustment to our equipment and procedure before we continued with testing the pressurization of the other motors on our list.

Preliminary Analysis:

The raw data from the pressure transducer indicated that room pressure was about 24 psi. We need to check the calibration of the transducer voltage output to psi conversion factors. The absolute psi values in the chart may not be accurate. However, the relative values are probably ok.

As we added more Pyrodex pellets, the area under the pressure curve should have increased. It did, except for the 4-pellet burn. This result combined with the burnt through dowel (mentioned above) makes us believe that some unburned Pyrodex was expelled.

As we added more pellets and chamber pressure increased, pressure oscillations started to occur and then grew in strength. We suspect that the shape of the volume in the chamber may have led to these oscillations. In an M1419 motor, the core is a cylindrical space of 1.125" diameter. In our chamber, the core consists of a 9.8" cylinder of .75" diameter adjacent to a 2.4" cylinder of 2.5" diameter. Although the total volume of the spaces are equivalent, we believe that the step in diameter may be responsible for these oscillations when it is subjected to very high speed airflow. We are considering redoing this experiment using an unstepped chamber (ID=1.125" length=18").

Analysis is still ongoing.