Small space debris are a high risk for the walls of Composite Overwrapped Pressure Vessels (COPV), by making small holes and causing the fuel leak. Commonly the self-healing materials are used to keep the mechanical structure strength, here the hermeticity of the repaired portion is a stringent requirement, to prevent any potential fuel leak from the cryogenic tank in vacuum. The efficiency is compared for protective walls composed of a combination of various layers, using strong materials (Kevlar, Nextel) and self-healing commercial materials developed as bulletproof, e.g. the Ethylene-co-Meth Acrylic Acid (EMAA) and Reverlink TM .The small debris impact dynamic was detected and monitored with Fiber Bragg Gratings (FBG) sensors at very fast acquisition frequencies, up to 0.5 GHz (2 ns), measuring the variation of the total reflected signal by the FBG. The acquisition system is based on commercially available products. To measure the total wavelength spectrum, the fastest available spectrometer can go up to 2 MHz acquisition (Micron-Optics), which is insufficient to detect the hypervelocity impact. The impact pressure evolution of the FBG, placed in the middle layer, was compared with commonly used strain gauges placed a few layers further or on the back of the last layer. The measured impact time delay and relative intensity were compatible between the two sensing methods.Some samples were characterized in details using the X-ray Computed Tomography at ESTEC, they permitted us to confirm the results by observing the details of the healing and follow the impact trajectory visually.

The best plan to curtail measurement errors is to modulate the input signal before it enters a sensor. As far as linear directional sensors are concerned, VMM (the vector modulation method) can make the earliest modulation and appropriate demodulation to curtail measurement errors and then improve the precision of measurement. According to the modulation–demodulation principle and the linear system theory, especially the frequency-remaining character and the superposition theorem of linear systems, a precise response to the input signal will be resolved by means of demodulation. VMM curtails the main elements of errors, including quantization error and bias of a measurement system, along with bias, drift, noise and disturbance of sensors. Experiments measuring ERR (the Earth's rate of rotation) are presented, and a design for an experiment measuring gravitational acceleration with VMM is presented. The preliminary experiments have given positive support. VMM, which can remarkably get rid of errors, is much clearer in theory and easier to implement in engineering practice.

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