Effect of Water and Humidity on Hypergolic Propellant Ignition Delay
Figure 2. Freezing point curve of HNO
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- III. Experimental Design A. Propellant Aging Tests
- B. Hypertester
| Figure 2. Freezing point curve of HNO 3 + H 2 O system 18 Wang and Thynell suggest that the liquid-phase reactivity in nitric acid reactions can be affected by the equilibrium composition of the nitric acid-water solution. They completely neutralized MMH to monomethylhydrazinium nitrate using 70% HNO 3 , but only partially neutralized MMH and partially oxidized it to form several other compounds using 90% HNO 3 7 III. Experimental Design A. Propellant Aging Tests To understand the rate of hygroscopic absorption of TEAB, vials with 5 ml of TEAB were placed in a humidity- controlled oven. All of the vials were rinsed with isopropyl alcohol and dried with nitrogen. Once the TEAB was transferred to the vial, the vial was capped until it reached the testing oven. Increasing the oven temperature accelerates the aging rate of the samples twofold for every C increase in temperature, so maximizing the oven temperature minimizes the time needed to age propellants As temperature increases, the relative humidity that corresponds to the same vapor density as 100% RH at C decreases. Since the magnesium chloride hexahydrate salt solution used can only control the RH as low as 35%, the oven temperature was set to C, which maximized aging rate and simulated 100% RH at CA water tank with a solenoid valve dispensed water automatically into the salt solution whenever the RH dropped below the target value. This system is described in further detail by Harman. 20 TEAB samples received from two different sources were tested in the humidity oven. In the first batch, seven vials were filled with TEAB from Sigma-Aldrich and placed uncapped in the humidity oven. Every two days the vials were pulled from the oven, capped, and stored for an intended test duration of two weeks. The TEAB in the vial pulled after six days showed signs of solidifying and the four remaining vials were pulled out of the oven on the seventh day. The second batch of testing used the same humidity oven with the same settings, but used one vial of TEAB synthesized by Ramachandran and Kulkarni. 21 After seven days in the same oven, the sample was removed. As well as aging TEAB in a humid environment, another test article was used to conduct aging in a dry environment. An 18 in x 11 in x 13.5 in acrylic and aluminum box with 10 feedthrough ports was used to maintain a dry air or dry nitrogen environment. A manual regulator allowed air or nitrogen to purge the box and a needle valve Downloaded by PURDUE UNIVERSITY on July 21, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.2015-3867 American Institute of Aeronautics and Astronautics 6 was slightly opened to let it escape. A hotplate was set to C and the vial was placed on top of the hotplate. All of the feedthroughs were plugged to prevent ambient air from entering the box. Five milliliters of TEAB from Sigma-Aldrich was used for both of these tests. The sample in dry air was kept in the box for 7 days and the sample in nitrogen is kept in the box for 14 days. B. Hypertester The Hypertester is a modular platform used to conduct hypergolic drop-on-pool tests. The original system was based on earlier work at China Lake. Gradually, the system has been improved to include more instrumentation. Prior to this study, the Hypertester included an impact sensor to record the moment of droplet impact and a photodiode to measure when ignition occurs. For this test campaign, a piezoelectric diaphragm is used as the impact sensor and a Thorlabs DET10A photodiode is used to record light released from the reaction. In addition to these standard measurement devices, the Hypertester is outfitted with a microphone, a high speed video camera, and a spectrometer/streak camera combination. The microphone is a PCB Piezotronics B prepolarized free-field microphone. Video is recorded with a Vision Research Phantom v camera from 5000 fps to 20 000 fps depending on the test. A grid of white-light LEDs is used to illuminate the falling drops. The spectrometer is a Princeton Instruments PI-ACTON SP spectrometer using a grating of 600 g/mm at a 300 nm blaze angle. The spectrometer divides the light signal vertically by wavelength and the light is passed to a Sydor Instruments Ross 2000 streak camera. The streak camera sweeps the incoming light signal horizontally across a 1360 by 1024 pixel CCD sensor. The resulting D image maps light intensity by time and wavelength. At the grating used, the vertical direction resolves 0.093 nm per pixel. Tests were conducted with a 30 ms nominal sweep time producing a resolution of 25 s per pixel. A more detailed description of this setup is described by Dennis To provide the light signal to the spectrometer, a Thorlabs UM Solarization-Resistant Multimode fiber optic cable runs from the test article to the spectrometer inlet. A collimating lens is used to reduce the capture area of the fiber to a 3 mm diameter beam with the bottom edge of the beam contacting the top of the crucible on the opposite side of the lens. The oxidizer pool is contained in a polytetrafluoroethylene (PTFE) crucible with interior diameter of 0.44 in and a depth of 0.13 in. The crucible sits on top of the impact sensor and is located in the same spot by four sliding plastic guides. Al syringe unloads drops of fuel 5 in above the crucible. Just below the syringe, an optical cage lines up a laser diode on one end with another DET10A photodiode on the other side. As the drop passes through the laser beam, the signal to the photodiode drops. This system is referred to as the laser interrupter. A delay generator reads the output from the laser interrupter to time signals going out to the streak camera, the video camera, and the two high frequency data acquisition systems used to record the other instrumentation outputs. These trigger signals allow all of the data to be synchronized in time. A low frequency data acquisition system records the data from a T-type thermocouple and an Omega HX71 humidity meter. The Hypertester is contained within the same test article box used foraging TEAB in dry air and nitrogen environments. A PTFE rod goes through the box to actuate the syringe. The humidity meter sits inside the feedthrough below the PTFE rod. The top feedthrough on the right side connects to a household humidifier. The two ports below it are plugged with rubber stoppers and three cables are routed through the stoppers for the microphone and photodiode outputs. On the left side of the box, dry shop air is connected to one feedthrough. Two feedthroughs pass the wires for the laser, the impact sensor, the optical fiber, and the thermocouple. The remaining ports are plugged. The test article is shown in Figure 3. Downloaded by PURDUE UNIVERSITY on July 21, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.2015-3867 |