High-Speed Diagnostic Tools Applied to the Study of New Hypergolic Hybrid Rocket Fuels

Le secteur spatial connait depuis quelques années déjà un essor fulgurant, notamment avec l’apparition d’un large écosystème de compagnies privées qui révolutionnent l’accès à l’espace. Le secteur spatial est encore aujourd’hui dominé par les moteurs à ergols liquides, qui font usage d’un carburant et d’un oxydant sous forme liquide pour générer de la poussée. Bien que forts performants, ces moteurs comportent souvent de hauts risques opérationnels inhérents aux ergols liquides. Ces substances sont souvent à des températures cryogéniques, très volatiles, et leur nature fluide engendre un risque élevé de déflagration ou de détonation en cas de fuite. Certains carburants liquides de haute performance, dits hypergoliques, sont extrêmement toxiques et dangereux pour l’environnement. Un carburant est dit hypergolique lorsqu’il a la capacité de s’enflammer spontanément en présence d’un oxydant à pression et température ambiante. Les dangers associés à l’utilisation de moteurs fusées liquides demeurent un facteur limitant quant à la réduction des coûts et des impacts environnementaux. Deux fronts de recherche scientifique et industrielle pourraient ensemble fournir une partie de la solution à ces problèmes. En premier lieu, les moteurs fusées dits hybrides pourraient servir d’alternative plus sécuritaire aux moteurs liquides. Un moteur fusée est dit hybride lorsqu’il fait usage d’un ergol sous forme solide et d’un ergol sous forme liquide. Les moteurs hybrides sont considérés plus sécuritaires que les moteurs liquides, puisque la différence de phase entre les deux ergols empêche leur mélange en cas de fuite. Les risques d’explosion dûs au mélange d’une grande quantité d’ergols en sont grandement diminués. En second lieu, des substances alternatives aux carburants hypergoliques toxiques traditionnels sont étudiées. Encore une fois, les moteurs hybrides peuvent être une source de solutions, puisque plusieurs substances solides à température ambiante ont démontrées de bonnes propriétés d’hypergolicité. Ce projet de recherche se penchera donc sur la combinaison de ces deux fronts de recherche : le développement de carburants pour moteurs fusées hybrides et hypergoliques. Plus particulièrement, le projet à pour but d’étudier le comportement à allumage de carburants enrichis de borazane, une substance solide, à faible toxicité, et hypergolique avec plusieurs oxydants communs. Ce borazane est mélangé à des carburants hybrides habituellement non- hypergoliques pour leur conférer cette propriété. Des carburants tels que de la paraffine enrichie de borazane connaissent déjà un fort intérêt dans la littérature, mais il y a encore plusieurs artères de recherche à explorer.

Abstract

The space sector has seen a resurgence in recent years, in part thanks to the explosion of new private enterprises which have revolutionized access to space. The rocket engine industry of today is dominated by liquid propellant rocket engines, which use fuels and oxidizers in liquid form to produce thrust. Although boasting high performance, these rocket engines often have high operational risks, owing to the liquid propellants they use. Liquid propellants are often cooled to cryogenic temperatures to become liquid at atmospheric pressure. Their volatile nature and fluidity makes the risk of deflagration or detonation relatively high in the event of a leak and subsequent mixing. Additionally, many high performance \textit{hypergolic} liquid fuels are extremely toxic and dangerous for the environment. A fuel and oxidizer combination is called \textit{hypergolic} if they have the ability to spontaneously ignite upon contact at room temperature and pressure. Despite the increased accessibility to space flight, the high risks associated with the use of liquid rocket engines remain a limiting factor with regards to price reduction and environmental impacts. Two avenues of research could together offer a solution. First, \textit{hybrid} rocket engines could serve as a safer alternative to liquid rocket engines. A rocket engine is considered \textit{hybrid} when it makes use of a propellant in solid form and another in liquid or gaseous form. Most commonly, the fuel is in solid form, placed inside the combustion chamber, where it reacts and burns with the injected liquid oxidizer. Hybrid rocket engines are considered safer than than liquid rocket engines, as the difference in physical phase between the fuel and oxidizer prevents mixing of large quantities. Second, new alternatives to traditional hypergolic fuels, which are highly toxic, are being developed. Many promising new hypergolic fuels are solids at room temperature, making them hybrid rocket fuels. This research project focuses on these avenues of research : the development of hypergolic hybrid rocket fuels. Specifically, the project aims to study the ignition behavior of hybrid fuels doped with ammonia-borane, a solid substance with low toxicity and hypergolic with many common oxidizers. The ammonia-borane is mixed with non-hypergolic hybrid fuels to make them hypergolic. Fuels such as ammonia-borane-doped paraffin wax are already being studied in the scientific literature, yet much remains unknown, such as the effect of physical properties on the ignition behavior, the exact chemical kinetics that lead to ignition, and the effect reliability of hypergolic additives in multiple stop and start applications. To better understand the ignition mechanism of hybrid fuels loaded with ammonia-borane, high speed imaging techniques are adapted to oxidizer drop tests. High speed visible spectrum images are used to determine the time delay between initial contact of the propellants and ignition. A Schlieren imaging system is developed to observe the release of gases at the fuel surface prior to ignition. A high speed infrared camera is used to collect data on the propellant temperature profile in the short time span between contact and ignition. These three imaging techniques are used simultaneously to study the effect of additive particle granulometry on the hypergolic ignition of ammonia-borane-doped paraffin wax. The results show that smaller ammonia-borane particles lead to longer ignition delays than large particles, at equivalent mass loading in the fuel. The reduction in ignition delay between small and large particles is found to be between 3 and 5. A hypothesis is presented, which suggests that smaller additive particles lose a larger portion of the heat they generate to the surrounding medium compared to larger ones, which increases the time required to ignite. The hypothesis suggests that heat loss is one of the main factors that increase the ignition delay of ammonia-borane in hybrid fuel matrices. With the insights obtained in the ammonia-borane granulometry study, new sorbitol-based fuel mixtures are developed. If the hypothesis that heat loss is an important factor in ignition delay times, then the properties of sorbitol should make it advantageous over paraffin for reducing ignition delay. Ignition delay tests performed on sorbitol-based fuel samples had conclusive results. Ammonia-borane-doped sorbitol fuel samples ignited faster and more strongly than paraffin based samples at equivalent ammonia-borane mass fraction. Sorbitol based samples loaded with 10\% ammonia-borane by weight ignited in 8.20 ms, while paraffin based samples of identical ammonia-borane mass loading ignited in 60.25 ms on average. The results of this research project show that the use of a large range of imaging techniques and diagnostic tools in the study of hypergolic propellants is crucial to better understand the underlying physical and chemical mechanisms behind the ignition process. This improved understanding can lead to the creation of new fuel mixtures that may be better suited to the needs of the space propulsion industry. Access to space could become more affordable, but also safer for people, communities, and the environment.