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NASA NTRS2000

SEM Characterization of Extinguished Grains from Plasma-Ignited M30 Charges

M30 propellant grains that had been ignited in interrupted closed bomb experiments were characterize by scanning electron microscopy (SEM). Previous chemical analysis of extinguished grains had given no indications of plasma-propellant chemical interactions that could explain the increased burning rates that had been previously observed in full-pressure closed bomb experiments. (This does not mean that there is no unique chemistry occurring with plasma ignition. It may occur very early in the ignition event and then become obscured by the burning chemistry.) In this work, SEM was used to look at grain morphologies to determine if there were increases in the surface areas of the plasma-ignited grains which would contribute to the apparent increase in the burning rate. Charges were made using 30 propellant grains (approximately 32 grams) stacked in two tiers and in two concentric circles around a plastic straw. Each grain was notched so that, when the grains were expelled from the bomb during extinguishment, it could be determined in which tier and which circle each grain was originally packed. Charges were ignited in a closed bomb by either a nickel wire/Mylar-capillary plasma or black powder. The bomb contained a blowout disk that ruptured when the pressure reached 35 MPa, and the propellant was vented into a collection chamber packed with polyurethane foam. SEM analysis of the grains fired with a conventional black powder igniter showed no signs of unusual burning characteristics. The surfaces seemed to be evenly burned on the exteriors of the grains and in the perforations. Grains that had been subjected to plasma ignition, however, had pits, gouges, chasms, and cracks in the surfaces. The sides of the grains closest to the plasma had the greatest amount of damage, but even surfaces facing the outer wall of the bomb had small pits. The perforations contained gouges and abnormally burned regions (wormholes) that extended into the web. The SEM photos indicated that a grain from the top tier, which was farther away from the plasma ignition source, sustained more plasma-induced damage to the perforations and the web than did the grains on the bottom tier.

Kinkennon, A.↗

NASA NTRS2003

Localized Ignition And Subsequent Flame Spread Over Solid Fuels In Microgravity

Localized ignition is initiated by an external radiant source at the middle of a thin solid sheet under external slow flow, simulating fire initiation in a spacecraft with a slow ventilation flow. Ignition behavior, subsequent transition simultaneously to upstream and downstream flame spread, and flame growth behavior are studied theoretically and experimentally. There are two transition stages in this study; one is the first transition from the onset of the ignition to form an initial anchored flame close to the sample surface, near the ignited area. The second transition is the flame growth stage from the anchored flame to a steady fire spread state (i.e. no change in flame size or in heat release rate) or a quasi-steady state, if either exists. Observations of experimental spot ignition characteristics and of the second transition over a thermally thin paper were made to determine the effects of external flow velocity. Both transitions have been studied theoretically to determine the effects of the confinement by a relatively small test chamber, of the ignition configuration (ignition across the sample width vs spot ignition), and of the external flow velocity on the two transitions over a thermally thin paper. This study is currently extending to two new areas; one is to include a thermoplastic sample such poly(methymethacrylate), PMMA, and the other is to determine the effects of sample thickness on the transitions. The recent results of these new studies on the first transition are briefly reported.

Kashiwagi, T.↗

NASA NTRS2013

Material Ignition and Suppression Test (MIST) in Space Exploration Atmospheres, Summary of Research

The Material Ignition and Suppression Test (MIST) project has had the objective of evaluating the ease of ignition and the fire suppression of materials used in spacecraft under environmental condition expected in a spacecraft. For this purpose, an experimental and theoretical research program is being conducted on the effect of space exploration atmospheres (SEA) on the piloted ignition of representative combustible materials, and on their fire suppression characteristics. The experimental apparatus and test methodology is derived from the Forced Ignition and Flame Spread Test (FIST), a well-developed bench scale test designed to extract material properties relevant to prediction of material flammability. In the FIST test, materials are exposed to an external radiant flux and the ignition delay and critical mass flux at ignition are determined as a function of the type of material and environmental conditions. In the original MIST design, a small-scale cylindrical flow duct with fuel samples attached to its inside wall was heated by a cylindrical heater located at the central axis of the cylinder. However, as the project evolved it was decided by NASA that it would be better to produce an experimental design that could accommodate other experiments with different experimental concepts. Based on those instructions and input from the requirements of other researchers that may share the hardware in an ISS/CIR experiment, a cylindrical design based on placing the sample at the center of an optically transparent tube with heaters equally spaced along the exterior of the cylinder was developed. Piloted ignition is attained by a hot wire igniter downstream of the fuel sample. Environment variables that can be studied via this experimental apparatus include: external radiant flux, oxidizer oxygen concentration, flow velocity, ambient pressure, and gravity level (if flown in the ISS/CIR). This constitutes the current experimental design, which maintains fairly good consistency with Dr Tien's and Dr Olson's project approaches. A further goal of the project has been to develop a combined solid/gas phase numerical model based on the MIST test methodology to predict the flammability behavior of practical materials in spacecraft.

flammability↗

NASA NTRSDate not supplied

Spatial Structure of Precipitation and Lightning in the Vicinity of Wildfire Ignitions in the United States

The horizontal storm structure surrounding 92512 lightning ignited wildfires is examined in the mid to eastern sections of the United States from 2003-2015 using Vaisala’s National Lightning Detection Network (NLDN), NCEP’s Stage IV gauge corrected radar precipitation mosaic, and the US Forest Service’s Fire Occurrence Database. Though lightning flash density peaks strongly around fire ignitions on the instantaneous 1 km scale, on the hourly 10 km scale both the lightning and precipitation peaks are typically offset from fire ignitions. As expected, lightning density is higher and precipitation lower around ignition points compared to non-ignition points. The average spatial distribution of total lightning flashes around fire ignitions is symmetrical, while that of precipitation and positive flashes is not. The results of regressing positive and negative currents against ignition efficiency are ambiguous. These results can aid in modeling storms that are likely to produce the "dry lightning" associated with wildfire ignitions.

Brian Vant-Hull↗

NASA NTRS1970

Fundamental ignition study for material fire safety improvement, part 1

The investigation of preignition, ignition, and combustion characteristics of Delrin (acetate terminated polyformaldehyde) and Teflon (polytetrafluoroethylene) resins in air and oxygen are presented. The determination of ignition limits and their dependence on temperature and the oxidizing media, as well as the analyses of the volatiles produced, were studied. Tests were conducted in argon, an inert medium in which only purely pyrolytic reactions can take place, using the stagnation burner arrangement designed and constructed for this purpose. A theoretical treatment of the ignition and combination phenomena was devised. In the case of Delrin the ignition and ignition delays are apparently independent of the gas (air, oxygen) temperatures. The results indicate that hydrogen is the ignition triggering agent. Teflon ignition limits were established in oxygen only.

Paciorek, K. L.↗

NASA NTRS1972

Initiation system for low thrust motor igniter

A test program was carried out to demonstrate an igniter motor initiation system utilizing the bimetallic material Pyrofuze for a solid propellant rocket with controlled low rate of thrust buildup. The program consisted of a series of vacuum ignition tests using a slab burning window motor that simulated the principal initial ballistic parameters of the full scale igniter motor. A Pyrofuze/pyrotechnic igniter system was demonstrated that uses a relatively low electrical current level for initiation and that eliminates the necessity of a pyrotechnic squib, with its accompanying accidental firing hazards and the typical basket of pyrotechnic pellets. The Pyrofuze ignition system does require an initial constraining of the igniter motor nozzle flow, and at the low initiating electrical current level the ignition delay time of this system was found to be quite sensitive to factors affecting local heat generation or loss rates.

Strand, L. D.↗

NASA NTRS1976

Ignition Transients of Large Segmented Solid Rocket Boosters

A model is described which provides a means for analyzing the complexities of ignition transients and pressure peaks of large, high performance, segmented solid rocket boosters. The method accounts for: (1) temporal and spatial development of the flow field set up by the head end igniter discharge, (2) ignition and flame spreading coupled to chamber flow, (3) the steep velocity, pressure, and temperature gradients that occur during the early phases of ignition, and (4) the interactions that produce ignition spikes (i.e., compression of chamber gases during pressurization, erosive burning, and mass added effect of igniter discharge). The technique differs from earlier models in that the flow interactions between the slots and main chamber are accounted for, and the original computer program for monolithic motors is improved. The procedures were used to predict the ignition transients of the current design for the space shuttle booster.

Caveny, L. H.↗

NASA NTRS1978

Space Shuttle SRM Ignition System

This paper presents the Space Shuttle SRM Ignition System, which consists of a large solid propellant main igniter, a small solid propellant initiating igniter and an electromechanical safety and arming device containing two NASA Standard Initiators and a B-KNO3 pyrotechnic booster charge. In development motors, the igniter also has a valve through which CO2 is injected for post-firing quench of the SRM. The igniter has redundant, testable seals at all pressurized joints and three major reusable components; the case, the adapter, and the S&A device. Two development problem areas are discussed. One problem area was transverse mode combustion instability in the main igniter with maximum amplitude of 340 psi peak-to-peak at a frequency of 1500 Hz, which was reduced by a propellant grain configuration change and a change from a 2% aluminum content propellant to a formulation containing 10% aluminum. The other problem area was an excessively rapid rise of thrust in the SRM, which was reduced by reducing the igniter mass flow rate. This mass flow rate reduction was accomplished by removing portions of the grain starpoints in the head end.

Bolieau, C. W.↗

NASA NTRS1983

Ignition of confined gaseous mixtures by hot surfaces and hot wires

Ignition times and spatial and temporal variations of temperature and concentration in gaseous mixtures confined between two infinite parallel walls or two infinite cylinders have been obtained by numerical integration of the appropriate conservation equations written in Lagrangian coordinates. Ignition times and ignition energies are presented for the case of an isothermal wall in terms of the initial mixture pressure and equivalence ratio for both one and two-step chemical reaction mechanisms. The numerical results indicate that there is a critical mixture pressure for which the ignition time is minimum. The values of this critical pressure are larger (smaller) than 1 atm for the one- (two-) step reaction mechanism. The critical pressure for the ignition time is not equal to the critical pressure for the ignition energy. The ignition time and energy decrease with the equivalence ratio within a certain range and then remain constant.

Ramos, J. I.↗

NASA NTRS1987

Ignition overpressure study from solid rocket motor firings

The objective was to investigate, through experimental means, the basic mechanisms influencing ignition overpressure and to determine ways to suppress ignition overpressure. Ignition overpressure was studied using solid rocket motors with geometry scaled at 1 percent of the Shuttle's Solid Rocket Boosters. Both water injection and aerosol foam were examined as a mean of reducing ignition overpressure. The results of the water injection tests indicate that a relatively small amount of water is sufficient to provide significant suppression. Of the flow rates tested, the lower water injection flow rates provided the best reduction of the ignition overpressure wave. Also, the test results show there is an optimum water flow rate range that provides the best suppression, and as this range is exceeded the effectiveness of water to reduce ignition overpressure is decreased. Aerosol foam provided very little reduction of ignition overpressure, but only small volumes of foam were used and further testing is necessary to determine its total effectiveness as a means of suppression.

Counter, Douglas D.↗

NASA NTRS1986

Ignition of metals and alloys by high-velocity particles

The ignition of metals and alloys by impacting high-velocity particles in gaseous oxygen was investigated. A convergent/divergent nozzle was used to accelerate the flowing oxygen, which in turn accelerated the particles to velocities greater than 305 m/s (1000 ft/s). The test sample (target) was placed at the end of the chamber in the flow path. Aluminum 6061, type 316 stainless steel, type 304 stainless steel, and Inconel 718 were ignited with 1600-micron aluminum 2017 particles at elevated temperatures and pressures whereas Monel 400 could not be ignited. The ignition susceptibility of metals and alloys appeared to increase as the inlet pressure, sample temperature, and particle size were increased. Type 304 stainless steel particles required more extreme conditions for ignition of materials than similar sized aluminum particles. The results indicated that ignition and subsequent burning of the particles were required for ignition of the target materials with the possible exception of aluminum 6061.

Benz, F. J.↗

NASA NTRS1988

Modeling of the transient ignition of a nonmetal/oxygen system by heterogeneous reaction - Effects of oxygen pressure

A theoretical model of the transient ignition of a nonmetallic polymer in a pure oxygen environment was developed and used to assess the safety of an oxygen system under elevated pressure. Results demonstrated that, for a general oxygen system, the ignition temperautre is sensitive to the external heating rate as well as such system parameters as the minimum ignition heat flux and the corresponding minimum ignition temperature. As the oxygen pressure increases, the minimum ignition temperature decreases, until, at a sufficiently high oxygen pressure, spontaneous ignition occurs. Numerical results generated by the model were correlated with the reported ignition data for polyethylene, Teflon, and nylon.

Yuen, Walter W.↗

NASA NTRS1989

Ignitability test method and apparatus

An apparatus for testing ignitability of an initiator includes a body with a central cavity, initiator holder for holding the initiator over the central cavity of the body, an ignition material holder disposed in the central cavity of the body and a cavity facing the initiator holder which receives a measured quantity of ignition material to be ignited by the initiator and a chamber in communication with the cavity of the ignition material holder and the central cavity of the body. A measuring system for analyzing pressure characteristics is generated by ignition material by the initiator. The measuring system includes at least one transducer coupled to an oscillograph for recording pressure traces generated by ignition.

Bement, Laurence J.↗

NASA NTRS1993

Ignition and subsequent flame spread over a thin cellulosic material

Both ignition and flame spread on solid fuels are processes that not only are of considerable scientific interest but that also have important fire safety applications. Both types of processes, ignition and flame spread, are complicated by strong coupling between chemical reactions and transport processes, not only in the gas phase but also in the condensed phase. In most previous studies, ignition and flame spread were studied separately with the result that there has been little understanding of the transition from ignition to flame spread. In fire safety applications this transition is crucial to determine whether a fire will be limited to a localized, temporary burn or will transition into a growth mode with a potential to become a large fire. In order to understand this transition, the transient mechanisms of ignition and subsequent flame spread must be studied. However, there have been no definitive experimental or modeling studies, because of the complexity of the flow motion generated by buoyancy near the heated sample surface. One must solve the full Navier-Stokes equations over an extended region to represent accurately the highly unstable buoyant plume and entrainment of surrounding gas from far away. In order to avoid the complicated nature of the starting plume problem under normal gravity, previous detailed radiative ignition models were assumed to be one-dimensional or were applied at a stagnation point. Thus, these models cannot be extended to include the transition to flame spread. The mismatch between experimental and calculated geometries means that theories cannot be compared directly with experimental results in normal gravity. To overcome the above difficulty, theoretical results obtained without buoyancy can be directly compared with experimental data measured in a microgravity environment. Thus, the objective of this study is to develop a theoretical model for ignition and the transition to flame spread and to make predictions using the thermal and chemical characteristics of a cellulosic material which are measured in normal gravity.

Nakabe, Kazuyoshi↗

NASA NTRS1993

A study of ignition phenomena of bulk metals by radiant heating

Early research on combustion of metals was motivated by the knowledge of the large heat release and corresponding high temperatures associated with metal-oxygen reactions. The advent of space flight brought about an increased interest in the ignition and combustion of metallic particles as additives in solid rocket propellants. More recently, attention has been given to the flammability properties of bulk, structural metals due to the number of accidental explosions of metal components in high-pressure oxygen systems. The following work represents a preliminary study that is part of a broader research effort aimed at providing further insight into the phenomena of bulk metal combustion by looking at the effects of gravity on the ignition behavior of metals. The scope of this preliminary experimental study includes the use of a non-coherent, continuous radiation ignition source, the measurement of temperature profiles of a variety of metals and a qualitative observation of the ignition phenomena at normal gravity. The specific objectives of the investigation include: (1) a feasibility study of the use of a continuous radiation source for metal ignition; (2) testing and characterization of the ignition behavior of a variety of metals; and (3) building a preliminary experimental database on ignition of metals under normal gravity conditions.

Branch, Melvin C.↗

NASA NTRS1993

Ignition and combustion of lunar propellants

The ignition and combustion of Al, Mg, and Al/Mg alloy particles in 99 percent O2/1 percent N2 mixtures is investigated at high temperatures and pressures for rocket engine applications. The 20 micron particles contain 0, 5, 10, 20, 40, 60, 80, and 100 weight percent Mg alloyed with Al, and are ignited in oxygen using the reflected shock in a shock tube near the endwall. Using this technique, the ignition delay and combustion times of the particles are measured at temperatures up to 3250 K as a function of Mg content for oxygen pressures of 8.5, 17, and 34 atm. An ignition model is developed which employs a simple lumped capacitance energy equation and temperature and pressure dependent particle and gas properties. Good agreement is achieved between the measured and predicted trends in the ignition delay times. For the particles investigated, the contribution of heterogeneous reaction to the heating of the particle is found to be significant at lower temperatures, but may be neglected as gas temperatures above 3000 K. As little as 10 percent Mg reduces the ignition delay time substantially at all pressures tested. The particle ignition delay times decrease with increasing Mg content, and this reduction becomes less pronounced as oxidizer temperature and pressure are increased.

Burton, Rodney L.↗

NASA NTRS1956

High-temperature oxidation and ignition of metals

A study of the high-temperature oxidation of several aircraft construction materials was undertaken to assess the possibility of ignition under high-temperature flight conditions.Tests have been made both in open and closed jets, and, in addition, the burning of metals has been observed under static conditions in a pressurized vessel containing either air, oxygen, or nitrogen. When heated in an atmosphere of oxygen or when heated and plunged into a supersonic airstream, titanium, iron, carbon steel, and common alloys such as 4130 were found to have spontaneous-ignition temperatures in the solid phase (below melting) and they melted rapidly while burning. Inconel, copper, 18-8 stainless steel, Monel, and aluminum could not be made to ignite spontaneously at temperatures up to melting with the equipment available. Magnesium ignited spontaneously in either type of test at temperatures just above the melting temperature.A theory for the spontaneous ignition of metals, based on the first law of thermodynamics, is presented. Good correlation was obtained between calculated spontaneous-ignition temperatures and values measured in supersonic jet tests. There appears at the present time to be no need for concern regarding the spontaneous ignition of Inconel, the stainless steels, copper, aluminum, or magnesium for ordinary supersonic airplane or missile applications where the material temperature is kept within ordinary structural limits or at least below melting. For hypersonic applications where the material is to be melted away to absorb the heat of convection, the results of the present tests do not apply sufficiently to allow a conclusion.

Hill, Paul R↗

NASA NTRS1995

Discharge ignition behavior of the Space Station plasma contactor

Ignition testing of hollow cathode assemblies being developed for the Space Station plasma contactor system has been initiated to validate reliable multiple restart capability. An ignition approach was implemented that was derived from an earlier arcjet program that successfully demonstrated over 11,600 ignitions. For this, a test profile was developed to allow accelerated cyclic testing at expected operating conditions. To date, one hollow cathode assembly has been used to demonstrate multiple ignitions. A prototype hollow cathode assembly has achieved 3,615 successful ignitions at a nominal anode voltage of 18.0 V. During the ignition testing several parameters were investigated, of which the heater power and pre-heat time were the only parameters found to significantly impact ignition rate.

Sarver-Verhey, Timothy R.↗