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Completed TRL 6 (started at 4, targeting 6)
There are increased interests on developing reusable rocket launch platform for fast turn-around launch deployment for commercial applications. Professor Amrutur Anilkumar at Vanderbilt University, working under AIAA's Reusable Launch Vehicle Technical Committee, has been conducting research on how to certify structural integrity of reusable rockets before, during and after deployment. The ideal case will be sensors integrated within the flight hardware that can validate structural health after each deployment, where there are many interests from the commercial section. This effort has been proposed via utilized a custom WDM (wavelength division multiplexing) fiber optics interrogator to show the feasibility of placing fiber bragg grating (FBG) sensors throughout the carbon fiber rocket to validate structural integrity before, during, and after launch events. During this effort NASA AFRC has designed and built a prototype WDM-based fiber optics interrogator that uses a tunable laser with no moving parts, with a comparable 40 nm wavelength bandwidth, and can be packaged inside the 6” diameter limit of the reusable rocket. This project’s objective is to develop a ruggedized WDM (wavelength division multiplexing) fiber optics interrogator for supporting the structural health monitoring of reusable launch vehicles. This innovation seeks to integrate fiber bragg grating (FBG) sensors into the carbon fiber rockets to validate structural integrity before, during, and after launch events. The system utilizes a continuously tuned light source and incorporates a novel time-sharing concept to enable multi-channel sensing while maintaining a compact size. The collaboration between AFRC and Vanderbilt University aims to create a flight-ready, low-SWaP (size, weight, and power) interrogator that can be used for health monitoring and recertification of rocket structures.
Current state-ot-the-art WDM interrogators available commercially based on tunable laser is the Hyperion si155 unit from Luna/Micron Optics, where they have an expanded 80nm wavelength range, but full spectrum measurement is only at 10Hz (10 samples per second). Another drawback is the unit dimension is 8.1” x 10.8” x 3.2”, and over 6lbs, which will not be able to fit inside any rocket. Other WDM systems is based on the broadband light source/spectrometer combination, which is not being investigated. This system design weighs ~3.5 lbs with dimensions of 6.5” x 3.5” x 3.5”, can sample at 50-500 Hz (potential to 5k Hz with newly acquired laser), about 12W of power, and most importantly it has no moving parts and has been designed to withstand the extreme environments associated with aeronautics and space flight.
The AFRC in-house developed WDM flight system will be able to support various flight opportunities, such as Vanderbilt’s reusable rocket launch platform, as well as others Aerospace’s company reusable rocket platforms. A low-weight, low profile FBG interrogation system ready for various space and aeronautics uses, with dimensions of 6.5” x 3.5” x 3.5”, which would fit inside Vanderbilt’s rocket payload bay. Typical WDM system are limited to 1x channel systems. The reason for this, in a WDM every channel needs its dedicated laser source, and a FBGA for processing. This causes multi-channel system to grow size, weight, and power requirements. Implementing a newly developed time sharing concept (not new in general, but new in fiber optic based interrogators), with nano-second precision and timing we are able to time share the laser and FPGA with multiple channels. The aim for this CIF is to target a 8 channel system, we can go up to a 16 channel system using this time sharing concept. As a result, the current system design, although ruggedized for flight, is smaller than anything currently on the market. The development of the ruggedized WDM fiber optics interrogator has the potential to revolutionize structural health monitoring for reusable launch vehicles. By integrating FBG sensors and implementing a time-sharing concept, the system achieves a compact design while supporting high-channel count measurements. This innovation could significantly improve the efficiency and safety of reusable rocket launches, allowing for real-time monitoring of structural integrity throughout all phases of flight. The technology's successful transition to flight testing, as planned, could open new possibilities for the aerospace industry, enabling more frequent and reliable rocket launches while enhancing industry-wide structural certification practices.
Current fiber optics interrogator based on WDM technology is limited by two form factors. First type WDM interrogator can use (relatively) inexpensive broadband light source but use both an optical beam-splitter (etalon) and a faster readout CCD sensor to filter out the different wavelengths, which is called a wavelength spectrometer. These spectrometers can be expensive, costings tens of thousands of dollars each, and the number of sensors that can be measurement concurrently is limited. Also the reliability of the optical beam-splitter during shock environment is questionable. Second type of WDM interrogator uses a continuously tuned light source, and the reflected light from the FBG is collected temporally via a photo-detector. In this case the tunable light source is expensive; however, the number of detectors available are scalable, which tens of uniquely center-wavelength FBG sensors can be concurrently measured. Current COTS interrogator based on tunable light source are relatively bulky for flight application. The FBG interrogator proposed under this CIF is based on continuously tuned light source. The proposal addresses several challenges in structural health monitoring for reusable launch vehicles. Current fiber optics interrogators based on WDM technology face limitations in terms of size, cost, and scalability. The existing options either employ expensive spectrometers with limited concurrent measurements or bulky tunable light sources. These limitations hinder the integration of comprehensive sensor arrays into rockets for accurate structural validation before, during, and after launch. This projects innovation aims to overcome these challenges by introducing a ruggedized WDM interrogator with a novel time-sharing concept, enabling a compact and scalable design capable of supporting multiple FBG sensors, thereby enhancing the reliability and efficiency of reusable rocket launches. With this nano-second precision time-sharing concept, the laser and FPGA can be shared for multiple sensing fibers vs other systems that are limited to a single sensor.
Most importantly, at the end of this CIF year 2 phase, we will have a flight ready, flight experienced, Low-SWaP fiber optic based interrogator that can help support health monitoring of reusable launch vehicles and give the industry a new way to recertify structures for next flight.
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