In recent years, NASA has become interested in densified fuels such as solid hydrogen. A change from liquid to solid-state fuel storage would result in an approximately 15% smaller onboard fuel tank, and thus a lower gross vehicle lift off weight. A lower lift off weight would allow for heavier payloads, more crewmembers, or longer space flight missions. The ability to store and use solid-state fuels would also lend to the possibility of more powerful atomic based propellants, such as boron or carbon, in the future. However, currently used techniques for liquid based mass gauging, required for quantifying the remaining mass in onboard fuel tanks, are not applicable to solid mass gauging. A new mass gauging technique is required to implement the use of solid-state fuel. It is required that this new mass gauging technique be capable of continuous measurement despite variations in fuel distribution, changes in gravitational forces, and other effects associated with mass in motion experienced during space flight. Furthermore, this technique and its related equipment must be minimally invasive to the fuel system, both mechanically and thermally. Advanced Technologies Group (ATG), has recently developed an optical mass gauging system with promising results in ground based tests on liquid hydrogen. The optical mass gauging system developed by ATG is coupled to a fuel tank via fiber-optic cables and utilizes the unique absorption spectra of molecular hydrogen, a tunable laser light source, a pseudo-integration optical sphere, and a spectrometer to gauge mass. A nearly monochromatic light, including an absorption wavelength for molecular hydrogen at a given intensity, is reflected uniformly within the pseudo integration sphere containing hydrogen. The intensity of the absorption wavelength is attenuated by hydrogen mass absorption, and the remainder is uniformly reflected about the internal surface of the pseudo-integration sphere. A ratiometric calculation is then used to approximate the attenuation due to mass, and ultimately the mass present, based on intensity measurements taken for an absorption wavelength and a non-absorption wavelength from the spheres internal surface. This system is minimally invasive and can be used to gauge quantities of solid mass by adjusting the emitted spectra to overlap the primary absorption wavelength of solid hydrogen at approximately 797.4 [nm]. In the present work, a solid hydrogen particle generator was designed and fabricated to test the response of the solid hydrogen optical mass gauging system (SHOMGS) prototype developed by ATG. The solid hydrogen particle generator consists of several components. Pre-cooled hydrogen gas (~80 K) was introduced from a cold trap into an encapsulated temperature controlled reservoir that was partially submerged in a bath of liquid helium at 4.2 K. This reservoir utilized the latent heat of the liquid helium bath as well as the heat capacity of the helium vapor to condense the hydrogen gas into liquid at approximately 19 K. Following condensation of a desired quantity of liquid hydrogen, the ullage in the reservoir was pressurized with helium gas to create a favorable pressure gradient for injection. A valve at the base of the reservoir was then opened to inject a fine spray of liquid hydrogen through an injection nozzle into a SHOMGS equipped pseudo-integration sphere containing a bath of liquid helium at approximately 4.2 K. The liquid helium bath of the sphere is used to solidify the droplets of liquid hydrogen into solid particles. A coaxial capacitor liquid level sensor was used in the liquid hydrogen reservoir to quantify the amount of mass injected from the particle generation system during each injection. Seven experiments were conducted. In each experiment, 10 to 20 mass injections were made to determine the response of the SHOMGS and the reproducibility of the results from the particle generation system. Raw data was recorded of the liquid hydrogen conditions before and after each injection, as well as associated changes in capacitance. These values were then used to calculate the injected mass. In addition, raw data was recorded from the SHOMGS regarding changes in reflected light intensity corresponding to each injection. Ratiometric analysis was performed on the light intensity data and this response was plotted against the quantities of mass injected to correlate the SHOMGS response. Following this battery of tests, several conclusions were determined. The solid hydrogen particle generator is capable of repeatable results and can provide known quantities of solid hydrogen with a calculated mass error of 10-20% dependant largely on the amount injected. The SHOMGS developed by ATG exhibits responses correlated to changes in mass injected. Following further development, this prototype could be modified for use on future space flight platforms.
