This thesis describes the design, fabrication and performance evaluation of the novel micro igniter which was designed to improve ignition performance and ignition stability of micro solid propellant thruster. A micro solid propellant thruster is used in nano-satellites for its attitude and orbit control. It has been developed in an array-type to compensate the lack of reusability of solid propellant. In the array-type, the performance-uniformity of unit thrusters and high ignition-success rate are important issue. Although many research teams were successfully developed micro solid propellant thruster array, studies on the uniformity and the high ignition-success rate have not yet been reported. To carry out these studies, the study on ignition stability of a micro igniter in the micro solid propellant thruster is essential because the igniter can have a great effect on overall performance of the thruster. This is why the heater-contact igniter was developed in this study.
The structural stability of the micro igniter is also important because it should be operated in harsh space-environments. It is noteworthy that if the structural stability of the membrane in the micro igniter is low, the micro solid propellant thruster cannot be operated normally because the propellant cannot be protected from outside. In this work, a glass wafer was used to fabricate thick glass-membrane. The decrease of ignition ability was reported in previous research on the micro igniter which had a thick glass-membrane. The heater-contact igniter in this work was also designed and developed in order to compensate this deficiency.
The heater-contact igniter had two main characteristics: one is the position of a heater and the other is the fabrication method of the membrane. The heater was placed under the membrane in order to directly contact with propellant. So improvement of ignition characteristics and ignition stability could be achieved because the heat generated by heater could be transferred to propellant directly. The membrane in this igniter was fabricated using the polishing process to make its thickness uniform. This uniform thickness could minimize the ignition instability caused by the membrane.
The prototype thruster was designed for performance evaluation of the heater-contact igniter. 7 unit thrusters were arranged in different distances between them to find the allowable minimum distance in the thruster array. The fabrication process of this thruster including the heater-contact igniter were successfully developed and carried out in this work.
The firing test of the prototype thruster was carried out for performance evaluation of the heater-contact igniter. First, ignition characteristics were evaluated. The ignition delay and ignition energy exponentially decreased as input voltage increased. The measured minimum ignition delay and ignition energy were 11.04 ms and 17.7 mJ, respectively. Second, ignition stability was evaluated. All unit thrusters were successfully ignited, so ignition-success rate reached 100%. The ignition characteristics of all unit thrusters were measured, and the calculated standard deviations of ignition delay and ignition energy were 1.35 ms and 3.11 mJ, respectively. Finally, the propagation characteristic of combustion was evaluated. When thruster number 6 was ignited, thruster number 7 was simultaneously ignited. The distance between thruster number 6 and 7 was 0.25 mm, so the measured allowable minimum distance in an array was 0.5 mm.
마이크로 고체 추진제 추력기의 점화성능 및 점화안정성 향상을 위하여 히터 접촉식 점화기로 명명된 새로운 방식의 마이크로 점화기를 개발하고 이의 성능평가를 수행하였다. 점화기의 재료로는 구조적 안정성 확보를 위하여 유리 웨이퍼를 사용하였으며, 이전의 유리 웨이퍼 기반 점화기와는 달리 점화코일이 박막 아래로 향하도록 함으로써 추진제와 직접 접촉하여 생성된 열이 추진제로 직접 전달 될 수 있도록 설계하였다.
점화실험을 통한 점화기의 성능 평가를 수행하기 위하여 히터 접촉식 점화기와 노즐층, 챔버층, 바닥층이 포함된 추력기 시제품을 설계하고 이의 제작공정을 수행하였다. 또한 배열체의 집적도 향상 연구를 위하여 추력기 시제품 설계 시 각 단위 추력기들이 서로 다른 거리로 배열되도록 설계하였다. 바닥층은 유리 웨이퍼의 다이싱(dicing) 공정을 통하여 제작되었으며, 노즐층과 챔버층은 감광유리의 노광 및 식각 공정을 통하여 제작되었다.
히터 접촉식 점화기는 먼저 한 웨이퍼에 니켈의 Lift-off 공정을 통하여 점화코일을 형성한 후 챔버층에 접착함으로써 제작되었다. 히터 접촉식 점화기의 제작공정 시 기존의 UV접합법을 이용할 경우 추진제인 lead styphnate가 두 층 사이의 틈으로 새어버리는 현상을 방지하기 위해 새로운 UV접합법의 개발을 통하여 히터 접촉식 점화기 및 추력기 시제품의 제작공정을 완료하였다.
성능 평가 결과 측정된 최소 점화 지연 시간 및 점화 에너지는 각각 11.04 ms, 17.7 mJ 이였으며, 이 값은 앞서 개발된 유리 기반 점화기보다 점화 지연 시간은 60%, 점화 에너지는 9.3% 증가된 값을 나타내었다. 점화기의 성능 안정성 평가 결과 점화성공률은 100%을 나타내었으며, 측정된 각 단위 추력기들의 점화 지연 시간 및 점화 에너지의 표준편차는 1.35 ms, 3.11 mJ 를 나타내었다. 0.5 mm 미만의 거리에서 연소열의 전파 현상이 발생하여 허용가능한 단위 추력기 사이 최소 거리는 0.5 mm로 결정되었다.