Yuki SAKAMOTO

The Institute of Space and Astronautical Science (ISAS) of the Japan Aerospace Exploration Agency (JAXA) conducts a deep space exploration mission named Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science (DESTINY⁺). The mission requires a high-performance, compact solid kick stage with a high mass ratio and high system safety. The kick stage employes a newly developed laser ignition system to meet these requirements. We designed a laser unit for upper stages (LUUS), two types of laser-initiated pyrotechnic devices for solid motor ignition and a separation device actuator (the LID and LCTG) for the kick stage system. Optical fiber paths connecting the LUUS to LID/LCTG enables a continuity check by optical frequency domain reflectometry (OFDR). We successfully conducted continuity checks with OFDR and ran laser ignition tests to validate the design in simulating assembly- and launchsite operations.

Cryogenic fluids such as liquid hydrogen and liquid oxygen used as the rocket propellants easily evaporate and form the gas-liquid two-phase flow. The control of the two-phase flow is difficult due to the large fluctuation of the density. For high-precision control, it is necessary simultaneously to understand flow regimes which represent a gas-liquid distribution pattern. Therefore, in this paper, we developed a classifier that uses Bidirectional LSTM networks, which is part of a family of deep learning methods, with a measured value of a void fraction meter as input and gas-liquid flow rate conditions as output, in order to realize a flow regime classifier in the future. The classifier succeeded in classifying with more than 80% accuracy. In addition, in order to verify what features of the input data the classifier captures, a test data-set of which frequency was artificially changed was classified. As a result, it was confirmed that the classifier would use the frequency component of the input data as one of the basis for classification.

<p>Quality is an important parameter on the gas-liquid two phase flow for organizing its heat transfer, pressure loss properties and flow regime. However, a method to measure quality has not been established yet. A new quality measurement method has been developed by using the mixers and capacitive void fraction sensor. The method is based on the experimental results that the slip ratio of the two phase flow homogenized by the mixers correlates closely with quality. A slip ratio model is created using the mass flux and void fraction after the mixer. Helical type and cross type mixers are arranged in series to homogenize the two phase flow in wide range of the flow regime. Air and silicon oil are used as the working fluids in this experiment. Several types of the flow condition are made by changing the mass flow rates in the horizontal, vertical flow passages. As a result, quality can be measured within ±50% of the error at 92% cases of the whole experiments. Quality meter shows less or comparable errors compared with the Smith's formula which is generally used. In addition, quality meter can be used both for the developed and undeveloped flows.</p>

<p>Many space vehicles are powered by liquid hydrogen and liquid oxygen. Such fuel are cryogenic fluids, so they are easy to boil and become gas-liquid two phase flow. The LE-5B-3 engine has the capability of the idle mode firing same as the LE-5B-2 engine. Assessment of flow condition at the inlet of fuel turbo pump is important to operate the engine, because the fuel may flow in saturated condition under the idle mode in principle. In a two-phase flow state, void fraction is one of the most important parameters to assess the flow. Although many types of void fraction sensors were proposed, the capacitive technique has advantages to mount on the engine from the viewpoint of size, weight, toughness. In this study, plural circular electrodes capacitive void fraction sensor is developed for LE-5B-3 engines' ground firing test. The sensor was designed based on electric field analysis, and the specification was assessed prior to the ground test. The sensor was used in qualification test, and it was succeeded in achieving stable measurement and it helped to understand the fluid state during the engine operation. The sensor design technique, the assessment results and the ground test results are discussed in this paper.</p>

<p>Reducing the amount of propellant for re-cooling is an important issue for the rocket propulsion system using cryogenic fuel. Immediately after the start of the engine, the liquid fuel boils and becomes two-phase flow. In the state of two-phase flow, the void fraction, which is the gas-liquid ratio, is one of the important value for flow control. For above problem, we are developing void fraction measurement system for the cryogenic fluid. These devices were attached to the S310-43 sounding rocket for the purpose of "measuring two-phase flow behavior and heat transfer characteristics during coasting flight." These devices withstood the vibration shock test of 40G and succeeded to measure the void fraction of liquid/gas nitrogen two phase flow under vacuumed and microgravity circumstance. This report explains development and experiment results of the void fraction sensor and a capacitance amplifier. </p>

The hypersonic air-breathing engine, which is currently under development by Japan Aerospace Exploration Agency (JAXA), uses liquid hydrogen as the fuel. In order to accurately control the fuel flow rate during the start-up, it is essential to measure the heat transfer and pressure drop of the two phase flow. These two characteristics depend on void fraction, flow velocity and flow regime; thus, measurement methods for these values are required to be established. In this study, the void fraction measurement method by the high-speed image analyses has been developed. Two images taken from top and side directions by high-speed cameras of 1000 fps are used for “two-direction semi-automatic analysis”. We also develop “One-direction full automatic analysis” which uses only the side view as a full-automatic and high sampling rate method with little accuracy deterioration. The preliminary verification test using vertical pipe and acrylic ball shows favorable results within 2.2% error against the theoretical value. A cryogenic experiment using two-phase nitrogen flow was also conducted. Sampling rate of “One-direction full automatic analysis” can be up to 1000 Hz. The difference between the results of two methods was as minor as 5% when the void fraction was below 30%.

The Japan Aerospace Exploration Agency launched the S-310-43 sounding rocket from the Uchinoura Space Center on Aug.04, 2014 for the purpose of investigating such behavior as boiling and flow of cryogenic liquid rocket propellant in an environment simulating coasting flight on orbit by using the sounding rocket's sub-orbital ballistic flight. In the low-gravity state, the cryogenic fluid (liquid nitrogen) was introduced into the test sections of similar shapes to the flow channels in the cryogenic propulsion systems. The boiling of liquid nitrogen inside the test-sections and the transition of flow regimes from gas/liquid two-phase flow to liquid mono-phase flow were visualized. The temperatures, pressures and void fractions of each channels were measured as well. Development of the experimental equipment for S-310-43 sounding rocket is described in this paper.