Early Fire Detection Technology in Manned Spacecraft

Abstract

During the manned space missions, the crucial concerns of the spacecraft and the crew's safety require long-term reliable, real-time, and accurate smoke monitoring of the cabin atmosphere to provide onset warning of fire. However, the absence of the thermal buoyancy effects in the microgravity environment makes the smoke particle movement more random and thereby brings more difficulties to the smoke detection. The prior spacecraft smoke detectors include the UV-sensing fire detectors, the Brunswick Defense particle-ionization smoke detector, and the Allied Signal light-scattering smoke detector. The need of more reliable and accurate fire detection technology is strengthened by a number of reported incidents of false alarms and no alarms in space missions. In order to increase the reliability of detection and reduce the response time, a novel early fire detection system for space missions, integrating the laser scattering smoke detection technology with the near-infrared laser photoacoustic (PA) trace gas measurement technology, is presented in this article. Since PA sensor measures the acoustic pressure wave that is directly related to the absorbed energy by gas molecules and detected by second harmonic technology, it is immune to the background signal and easily achieves ppm level of detection limits. The concentration of carbon monoxide is very low at normal atmosphere, but dramatically increases in nearly all types of fire events, so it was selected as the target gas for the fire signature. The PA part consists of two distribute feedback lasers: the gas cell with particle filters, and the ultrasensitive acoustic measurement unit with mechanical vibration isolation frame. The smoke detector part consists of the laser diode as light source, the photodiode to sense forward scattered light (30 °), and the labyrinth obscuration cell. The sample gas of both parts is sucked from the cabin air through a micropump, which can keep the system response time less than 2 s, even in the microgravity environment. The experimental results of the prototype device show that the detection limit of carbon monoxide is 0.3 ppm and attenuation sensitivity of the smoke is 0.05%/m with satisfying the detection requirements for space missions. The device is designed to reliably operate 5 years on orbit. With a lightweight (1.5 kg) and low power consumption (2 W), the proposed system, combining the PA gas detection technology with the laser scattering smoke detection technique, is suitable for reliable early fire detection in spacecraft.