Enabling and Enhancing Science Exploration Across the Solar System: Aerocapture Technology for SmallSat to Flagship Missions

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Non-Equilibrium in the Mars Entry Shock Layer Characterized via Laser Absorption Spectroscopy

Predicting and managing heat transfer during planetary entry is a critical engineering challenge for current and future space exploration missions. This work aims to improve the understanding of thermodynamic non-equilibrium during Mars entry via experimental studies to capture chemical kinetics and the rates of energy transfer between translation, rotation, and vibration for the dominant molecular species (CO2 and CO). The key elements of this research can be split into two distinct phases: (1) develop and demonstrate non-equilibrium high speed sensing of CO and CO2 on a high enthalpy shock tube at UCLA. (2) Deploy the sensor at NASA Ames on a representative Mars entry flow. A mid-infrared laser absorption strategy for simultaneous measurement of translational, rotational, and vibrational temperatures of carbon monoxide (CO) at high speeds was developed for application to high temperature non-equilibrium environments relevant to Mars atmospheric entry. The sensing strategy is shown to resolve each targeted transition with temporal and spectral resolution sufficient for quantitative multi-temperature measurements over a wide range of temperatures and pressures (2100 - 5500 K, 0.03 - 1.02 atm), including behind incident shock waves traveling up to 3.3 km/s. A similar strategy is employed on CO2 transitions from the ν3(00^00) and ν3(01^10) states. Vibrational relaxation times were resolved at temperatures relevant to Mars backshell heating (2,000 - 3,000 K) in various CO2 - Ar mixtures and found to be in good agreement with the Simpson rate model. The final effort of this project deployed a multi-species sensor on the Electric Arc Shock Tube facility at NASA Ames to study a recreated shock layer similar to that experienced on the Mars2020 mission. Temperature and number densities of CO2 and CO were extracted from the data and compared to various chemistry models and a simultaneous emission measurement. At shock velocities below 3.1 km/s, the agreement between the measurements and the Johnston mechanism is typically within 5\% for temperature and within 10\% for number density. At shock velocities above 3.1 km/s, the CO2 measurement becomes sensitive to a thin boundary layer and corrections of this effect are presented. On test cases with enough energy to dissociate CO2, a quantum cascade laser scanned the P(2, 20), P(0, 31), and P(3, 14) transitions of the CO fundamental band at 4.98~�m. CO formation rate is measured to be close to the Johnston kinetic mechanism at low velocities, and then trending towards the Cruden kinetic mechanism at high velocities. In summary, this work has advanced laser absorption techniques to include high speed (MHz) multi-temperature measurements of CO2 and CO on non-equilibrium flows relevant to Mars planetary entry

High-speed interband cascade laser absorption sensor for multiple temperatures in CO2 rovibrational non-equilibrium

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High-speed mid-infrared laser absorption spectroscopy of CO2 for shock-induced thermal non-equilibrium studies of planetary entry

Abstract A high-speed laser absorption technique is employed to resolve spectral transitions of CO $$_2$$ 2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO $$_2$$ 2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO $$_2$$ 2 asymmetric stretch fundamental bands ( $$\Delta$$ Δ v $$_3$$ 3 = 1) near 4.19 \upmu \hbox {m} μ m . The sensor probes a wide range of rotational energies as well as two vibrational states (00 $$^0$$ 0 0 and 01 $$^1$$ 1 0). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO $$_2$$ 2 non-equilibrium with a single vibrational temperature

RF-waveform optimization for MHz-rate DFB laser absorption spectroscopy in dynamic combustion environments

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Multi-line Boltzmann regression for near-electronvolt temperature and CO sensing via MHz-rate infrared laser absorption spectroscopy

International audienceA mid-infrared laser absorption technique is developed for sensing of temperature and carbon monoxide (CO) number density from 2000 K to above 9000 K. To resolve multiple rovibrational lines, a distributedfeedback quantum cascade laser (DFB-QCL) is modulated across 80% of its current range using a trapezoidal waveform via a bias-tee circuit. The laser attains a spectral scan depth of 1 cm −1 , at a scan frequency of 1 MHz, which allows for simultaneous measurements of four isolated CO transitions near 2011 cm −1 (4.97 µm) with lower-state energies spanning 3,000 to 42,000 cm −1. The number density and temperature are calculated using a Boltzmann regression of the four population fractions. This method leverages the information contained in each transition and yields a lower uncertainty than using a single line pair. The sensor is validated in shock tube experiments over a wide range of temperatures and pressures (2300-8100 K, 0.3-3 atm). Measurements behind reflected shock waves are compared to a kinetic model of CO dissociation up to 9310 K and are shown to recover equilibrium conditions. The high temperature range of the sensor is able to resolve rapid species and temperature evolution at near electronvolt conditions making it suitable for investigations of high-speed flows, plasma applications, and high-pressure detonations

Extended tuning of distributed-feedback lasers in a bias-tee circuit via waveform optimization for MHz-rate absorption spectroscopy

International audienceVariations in injection-current waveform are examined using diplexed RF-modulation with continuouswave distributed-feedback (CW-DFB) lasers, with the aim to maximize the spectral tuning range and signal-tonoise ratio for MHz-rate laser absorption spectroscopy. Utilizing a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold during the modulation period. The effect of waveform duty cycle and leadingedge ramp rate are further examined. A spectral scan depth on the order of 1 cm −1 at a scan frequency of 1 MHz is achieved with a representative CW-DFB quantum cascade laser near 5 µm. Distortion of high-frequency optical signals due to detector bandwidth is also examined, and limitations are noted for applications with narrow spectral features and low-bandwidth detectors. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth and target spectra. A representative optimization is presented for measurements of sub-atmospheric carbon monoxide spectra with a 200-MHz detection system. The methods are then demonstrated to resolve transient gas properties (pressure and temperature) via laser absorption spectroscopy at MHz rates in a detonation tube and shock tube facility. An appendix detailing a first-order model of high-speed distributed feedback laser tuning dynamics is also included to support the experimental observations of this work

Mars2020 entry shock layer thermochemical kinetics examined by MHz-rate laser absorption spectroscopy

International audienceA mid-infrared laser absorption diagnostic was deployed to examine the evolution of thermophysical properties across a simulated Mars2020 shock layer in the Electric Arc Shock Tube (EAST) facility at NASA Ames. Rapid laser tuning techniques using bias-tee circuitry enabled quantitative temperature and number density measurements of [Formula: see text] and CO with microsecond resolution over a shock velocity range of 1.30–3.75 km/s. Two interband cascade lasers were utilized at 4.17 and 4.19 μm to resolve rovibrational [Formula: see text] lines spanning across [Formula: see text] to [Formula: see text] in the asymmetric stretch fundamental bands. In test cases with enough energy to dissociate [Formula: see text], a quantum cascade laser scanned multiple transitions of the CO fundamental bands near [Formula: see text]. The results are compared to the Data Parallel Line Relaxation (DPLR) code and Lagrange shock tube analysis (LASTA) simulations of the shock layer. A numerical simulation of the compressible boundary layer is used to account for measurement sensitivities to this flow feature in the EAST facility. Temperature and species transients are compared to multiple chemical kinetic models. The laser absorption data presented in this work can be used to refine the models used to simulate the aerothermal environment encountered during Mars entry