A microgravity experiment to elucidate the cool flame dynamics in a multi-droplet system was conducted using the TEXUS-60 sounding rocket. The cool flame position was successfully observed through formaldehyde chemiluminescence. This allowed the study of cool flame propagation speed along n-decane droplet arrays, with droplet spacings of 8 mm (nine droplets) and 16 mm (five droplets). The spread speed was analyzed at an ambient temperature of 570 K and a pressure of 0.1 MPa, using air as the ambient gas. For both droplet configurations, the cool flame occurred from nearly the same position 4.7 s after the droplet was inserted and settled at the combustion position. The cool flame spread speeds decreased from around 250 mm/s to approximately 100 mm/s after the spontaneous ignition, and then increased. The maximum speed reached around 600 mm/s for the 16 mm droplet spacing case. Under these experimental conditions, the results suggest that in the first stage the cool flame spread follows the premixed propagation mode, while in the latter stage the acceleration may be driven by the sequential spontaneous ignition.

The flame spread of thin PMMA sheets has been studied both in opposed configuration under microgravity (9.3 s drop tower) and in downward configuration under normal gravity conditions. The position of the flame front has been determined with the help of an infrared camera. Propagation rates have been measured over a range of 30–200 mm/s forced opposed flow (only microgravity), 60–101.3 kPa ambient pressure, and oxygen concentrations of 21–35.4 vol. %, which corresponds to real environmental conditions on spacecraft. In addition to variations along the normoxic curve with constant oxygen partial pressure, the parameters oxygen and pressure were examined independently of each other in order to be able to determine their respective effects. The variation of the flow velocity was carried out at 60 kPa and 35.4 vol. % oxygen. The results for both microgravity and normal gravity are discussed separately on the basis of the respective atmospheric effect. Along the normoxic curve, a significant increase in the flame spread rate was found with increasing oxygen concentration and correspondingly decreasing pressure. This results in a significant increase in the risk of fire with regard to future exploration missions. The effect of the flow velocity cannot be neglected in the investigated velocity range and was linked to the influencing parameters in a correlation. Based on this parameterization, a linear function is given that reflects both the downward and the opposed results well. This function can be used to predict the flame spread in the given parameter space. Under normal gravity, the tested samples show a slightly increased propagation rate in most cases. This can essentially be attributed to the difference in velocity between the buoyant flow and the forced flow in µg.

The preliminary results are presented for the last flight of the Spacecraft Fire Experiment (Saffire VI) which was conducted on an orbiting Cygnus spacecraft. These experiments directly address the risks associated with our understanding of spacecraft fire behavior at practical length scales and geometries. The lack of this experimental data has forced spacecraft designers to base their designs and safety precautions on 1-g understanding of flame spread, flame self-extinguishment, fire detection, and suppression. However, low-gravity combustion research including the prior Saffire flights have demonstrated substantial differences in flame behavior in reduced gravity. The Saffire experiment was developed by an international team of investigators with the goal of addressing open issues in spacecraft fire safety. NASA's Spacecraft Fire Safety Demonstration Project was designed with the goal of conducting a series of large-scale experiments in spacecraft environments that represent practical spacecraft fires. The final flight examined concurrent spread over large samples (all 41 cm wide) including a thin sheet of flammable fuel (cotton/fiberglass 50 cm long); 2-sided spread over 1 cm thick polymethyl methacrylate (18 cm long) ; 1-sided spread over 0.5 cm thick (18 cm long); and Nomex fabric (7 cm long). These experiments were performed on two separate unmanned ISS re-supply spacecraft after they had delivered their cargo and had begun their return journeys to Earth (ultimately destructive reentry). Preliminary flame spread rates and flammability assessments are presented for the conditions studied with comparison to prior data. Temperature and carbon dioxide sensors were placed throughout the vehicle which will be compared to a vehicle transport model to develop the ability to predict the impact of a fire in a spacecraft.

IR videography has proven useful for tracking the position of the pyrolysis front under a flame propagating upward in 1g. Since PMMA burns with low soot formation in 1g conditions, the radiation density of the gases above the sample is very low and the flame is hardly visible. As a result, the surface can be easily followed and the temperature at the pyrolysis front can be determined. The phenomenological definition of the pyrolysis front remains difficult. In experiments on concurrent propagation along cylindrical PMMA samples under µg conditions during the Sounding Rocket flight TEXUS 57, it was shown that the drastically increased soot production compared to terrestrial conditions leads to a significantly increased radiation fraction by the gas phase above the sample. Here, pyrolysis front temperatures were determined to exceed 450 °C and include a significant radiative fraction of soot formed between the flame and the sample. This also sheds light on the significantly elevated local temperatures under µg conditions, which can lead to self-sustaining burning of some materials in microgravity that are inflammable or extinguishing on ground– this despite the overall reduced mass burn rate observed in microgravity. The paper compares pyrolysis front velocities in upward configuration (1g) along cylindrical PMMA samples with diameters of 5, 20, and 25 mm with concurrent propagation (µg) of the same samples. Both the 1g and µg experiments were performed in hypoxic exploration atmosphere (70 kPa, 26.5% O2). While the 1g experiments were subjected to buoyant convection only with a typical but averaged flow velocity of 20 to 30 cm/s, a forced inflow between 20 and 30 cm/s was set for the µg experiments. Soot formation was also analyzed in the microgravity tests.

The combustion of single liquid oxygen droplets in gaseous hydrogen is investigated experimentally under microgravity conditions to shed light on spray combustion processes in rocket engines. Using a drop tower apparatus, experiments are performed varying the ambient pressure between 0.1-5.7 MPa, which corresponds to a reduced pressure of oxygen p(r) between 0.02-1.12. The combustion is investigated using high-speed shadowgraph imaging to track the droplet shape and OH-chemiluminescence to identify the flame zone. At low pressures (p(r) < 0 . 15 ), the droplet shape is found to change significantly during combustion likely due to the formation of a water ice layer around the droplet. Small jets of oxygen appear to break out of this ice layer, leading to an observed increase in linear and angular momentum of the droplet. At higher pressures, the visible effect of ice formation near the droplet surface decreases. The combustion process at different pressures in the subcritical and the supercritical regime is compared and discussed. The pressure has a limited influence on the flame standoff ratio, whereas it influences the burning rate constant substantially. Specifically, the experimental data suggest a maximum of the burning rate constant near the critical pressure, which is consistent with several experiments on hydrocarbon droplet combustion. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

To investigate the basic combustion processes in a cryogenic rocket engine, the combustion of individual liquid oxygen droplets in a hydrogen atmosphere under microgravity conditions is experimentally and numerically investigated. For this purpose, an experimental setup is used in the ZARM (Center of Applied Space Technology and Microgravity) drop tower. In a combustion chamber cooled with liquid nitrogen, a single oxygen droplet is produced on a quartz suspender and ignited with a laser-induced plasma spark. The subsequent combustion is investigated with various diagnostics. Shadowgraph images are used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame is determined by OH-chemiluminescence. The initial droplet width varies between 0.66-0.93 mm and combustion is investigated at pressures between 1 and 45 bar. The effects of ambient pressure on combustion are discussed by means of the burning rate and the droplet-to-flame ratio. As the pressure increases, the burning rate increases, but the droplet-to-flame diameter ratio decreases slightly. The condensation or freezing of water vapor outside the reaction zone and near the droplet surface is investigated for different pressures. As the pressure increases, the distance between the condensation/freezing zone and the flame decreases. In addition, a numerical model for hydrogen-oxygen combustion is presented. Simulation results predict the emergence of three, distinct flame regions during the ignition transient; two premixed flames and a central, diffusion flame. For the quasi-steady diffusion flame that results, the numerical results point to condensation/freezing outside the reaction zone, qualitatively confirming the experimental results.

The induction times of pairs of droplets of n-decane and n-tetradecane have been investigated experimentally in microgravity. The results were compared to the induction times of single droplets. N-decane is a well matching model fuel for synthetic kerosene while n-tetradecane is well matching the ignition behavior of synthetic diesel fuel. All droplets had a diameter of 0.8 mm. The experiments were carried out at pressures of 0.3 and 0.5 MPa and in the temperature range of 650 to 850 K. The spacing of the droplet pairs was 1, 2, 4 and 6 mm (center distance). The LIF on formaldehyde was applied. Formaldehyde is the stable product of the cool flame reaction and is formed parallel to the OH-radical whose concentration in the cool flame is too low for detection. As formaldehyde is immediately decomposed at hot ignition, the whole ignition process is displayed by applying this technique. The results show that for both fuels the induction times of a pair of 1 mm distance are slightly longer than for a single droplet. Beyond 1 mm the induction times drop below that of a single droplet. The 6 mm spacing showed the shortest induction times. With widening gap between the droplets, the instant of hot ignition moves from near the droplets interspace to in between the droplets and moves back to an arbitrary outside location at 6 mm gap. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

To investigate the fundamentals of liquid oxygen droplet combustion in hydrogen under microgravity conditions, a drop tower apparatus has been developed. In a cryogenic combustion chamber cooled with liquid nitrogen, single oxygen droplets with a diameter of 1 mm are suspended at the tip of a thin suspender. Ignition is accomplished after microgravity conditions are reached by a near-infrared laser, which generates a plasma spark positioned in the immediate vicinity of the oxygen droplet. The subsequent combustion is investigated with various optical diagnostics. Shadowgraph imaging is used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame are determined by OH chemiluminescence imaging. The oxygen/hydrogen combustion at two different pressure levels is reported. At a pressure of 0.1 MPa, irregular structures appear to form on the droplet surface during the combustion process, which lasts 137 ms before the droplet is completely consumed. The formation of these irregular structures is consistent with the condensation and freezing of water vapor in the cold-temperature region near the droplet surface. At a higher pressure of 4 MPa (reduced pressure pr,O2 =0.79), the water ice formation is significantly reduced due to the shorter burning time of 66 ms and the closer proximity of the flame to the droplet surface. The measured burning constant is k0.1 = 5.5 mm2/s at 0.1 MPa and k4 = 7.1 mm2/s at 4 MPa, respectively. This increase with increasing pressure is consistent with the smaller flame standoff distance.

Spacecraft fire safety is an important consideration when designing future exploration missions. Concurrent flow flame spread experiments were carried out aboard the Cygnus spacecraft while in orbit to address current knowledge gaps related to solid fuel combustion in microgravity. The experiments used PMMA (polymethylmethacrylate) samples that were 50 mm wide and 290 mm long with two variations-one sample was a 10 mm thick flat slab, while the other was a 10 mm thick flat sample that had a 4 mm thick grooved center section. The thickness variation had a major impact on the flame shape, and the grooved sample developed a deep inverted-V shaped notch as the thin center section burned through. This notch enhanced heat transfer from the flame to the solid through an effectively wider flame base, which is the part of the flame with the highest temperature. Unlike in normal gravity (and also likely in partial gravity), where buoyant flow promotes acceleratory upward flame growth, the microgravity flames reached a steady size (limiting length) for a fixed forced convective flow in agreement with theory (i.e. there is a zero net heat flux at the flame tip). The limiting length implies that the spread rate of the flame will be controlled by the regression rate (burnout rate) of the material because the flames remains anchored to the upstream end of the fuel samples. This is a significant finding for spacecraft fire safety, and makes the probability of flashover in a spacecraft unlikely as the flame size will be small for low convective ventilation flow environments typical in spacecraft. On the other hand, long-burning flames in small vehicles will generate significant quantities of fuel vapor that may reach the lean flammability limit and cause a backdraft. The rapid extinction of the flame when the flow was turned off also supports the existing fire mitigation strategy on the ISS to deactivate the ventilation system in the event of fire alarm. These findings can be applied to improve the safety of future space exploration missions.

For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called "Saffire," studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20-25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

As part of the Saffire project, solid materials were burned aboard orbiting spacecraft in two sets of experiments. The materials, mounted within a large air flow duct, were substantially larger than fuel samples in all previous microgravity tests. Large-than-typical samples could be accommodated because the tests were remotely conducted in unmanned ISS supply vehicles just days before their controlled re-entry and burn-up in the atmosphere. In the first experiment, a large cotton-fiberglass fabric measuring 40.6 × 94 cm was burned in two separate tests (concurrent and opposed). In the second experiment, nine samples measuring 5 × 30 cm in area were burned in succession. Of these nine, two were sheets of cotton-fiberglass fabric, identical to the material burned in the first experiment, and were burned in the concurrent-flow configuration. Two digital video cameras were used to record flame behavior and spread rate. Other diagnostics included radiometers, thermocouples, oxygen, and carbon dioxide sensors. Results demonstrate the unique features of purely forced flow in microgravity on flame spread, the dependence of flame behavior on the scale of the experiment, and the importance of full-scale testing for spacecraft fire safety.

Since 2012, a series of Spacecraft Fire Experiments (Saffire) have been under development by the Spacecraft Fire Safety Demonstration (SFS Demo) project, funded by NASA's Advanced Exploration Systems Division. The overall objective of this project is to reduce the uncertainty and risk associated with the design of spacecraft fire safety systems for NASA's exploration missions. The approach to achieving this goal has been to define, develop, and conduct experiments that address gaps in spacecraft fire safety knowledge and capabilities identified by NASA's Fire Safety System Maturation Team. The Spacecraft Fire Experiments (Saffire-I, -II, and -III) are material flammability tests at length scales that are realistic for a spacecraft fire in low-gravity. The specific objectives of these three experiments are to (1) determine how rapidly a large scale fire grows in low-gravity and (2) investigate the low-g flammability limits compared to those obtained in NASA's normal gravity material flammability screening test. The experiments will be conducted in Orbital ATK's Cygnus vehicle after it has unberthed from the International Space Station. The tests will be fully automated with the data downlinked at the conclusion of the test before the Cygnus vehicle reenters the atmosphere. This paper discusses the status of the Saffire-I, II, and III experiments followed by a review of the fire safety technology gaps that are driving the development of objectives for the next series of experiments, Saffire-IV, V, and VI.

The interaction between two neighboring n-decane droplets during the autoignition process in air was experimentally investigated under microgravity conditions in the drop tower Bremen. The initial droplet diameter was 0.8 mm in all experiments. Single droplets and droplet pairs with a center distance of 1, 2, 4 and 6 mm were investigated. The air temperature was varied between 650 and 850 K with an increment of 25 K. The examined pressure conditions were 0.3 and 0.5 MPa. This results in a total number of 90 drop experiments. The ignition process was observed applying the laser induced fluorescence (LIF) on formaldehyde with high temporal and spatial resolution. Both, the cool flame process and the hot flame ignition could be measured regarding induction times and the temporal and spatial evolution of the formaldehyde formation.

The concept for a next-generation drop tower system is presented that is motivated by the scientific demand for much higher experiment repetition. This demand resulted in repetition rates of over 100 experiments per day which exceed the current capabilities of operating drop towers by far. High experiment repetition rates can for instance be realized through the novel application of a guided electro-magnetic linear drive system in a fully automatic drop tower operation. Such a new kind of drop tower system combines beneficial technologies of different free fall systems like freely falling drop capsules, capsule-in-capsule systems, and the vertical parabola method as already utilized in ZARM’s worldwide unique catapult system. This proposed next-generation drop tower system named GraviTower Bremen does not only enable experiments with an outstanding microgravity quality (10−6 g, where g is the Earth’s gravitational acceleration) and a duration of 6 s but also novel experiments under partial gravity conditions (0.1 g to 0.4 g) matching those of Moon or Mars with durations of up to 8.5 s. Due to its linear drive system the GraviTower allows the same very low initial acceleration and following deceleration loads onto the experiment. These can be selected according to the experiment’s needs with only 1.5 g or 4 g. The engine power of the linear drive system allows also large payload dimensions and masses. The features and capabilities of the proposed GraviTower Bremen combine all advantages of current drop towers and represent the next technological step forward in ground-based research under space conditions.