Optical investigation of hypergolic ignition and combustion in paraffin-based hybrid rockets [electronic resource]
Keywords: Aeronautics and Astronautics.

The operation of paraffin-based hybrid rockets has, at times, been limited by fuel grain structural integrity and unreliable ignition. This study investigates (1) the ability of two blackening agents to increase the opacity of paraffin over wavelengths associated with radiative heat transfer, thereby improving the structural integrity and regression rate of the fuel; (2) optical pyrometry as a tool for estimating C* efficiency in a gaseous oxygen/paraffin hybrid rocket; and (3) the use of green hypergolic propellants for in-space hybrid rocket propulsion. Optical characterization of paraffin wax and two blackening agents was conducted from the ultraviolet (UV) to the mid-infrared (MIR) in order to better understand radiative heat transfer within the combustion chamber of a paraffin-based hybrid rocket and inform future work related to laser ignition of paraffin. Absorption spectra are reported for (1) neat paraffin, (2) paraffin doped with up to 1\% blackener dye by mass, and (3) paraffin doped with up to 0.68% carbon black by mass. Blackener dye exhibited an absorption spectrum which makes it ill-suited as a radiation-enhancing additive in paraffin-based hybrid rockets. Carbon black, on the other hand, demonstrated the ability to improve absorbance of paraffin over wavelengths associated with thermal radiation, while proving difficult to disperse uniformly in paraffin. Analysis of emission spectra acquired in the plume, fore end, nozzle exit and post-combustion chamber during 33 hot fire tests of a gaseous oxygen/paraffin-based hybrid rocket demonstrated the accuracy of optical pyrometry for flame temperature estimation for the purpose of inobtrusively measuring C* efficiency. Absolute irradiance measurements were compared with computationally-derived values and numerous lessons were learned regarding experimental methods to maximize the fidelity of spectral acquisition from within the combustion chamber for a paraffin-based hybrid rocket. A droplet ignition chamber was designed, fabricated and utilized to investigate the viability of analytical reagent-grade nitric acid (69.3% mass concentration) and lithium aluminum hydride (LiAlH4, LAH)-doped paraffin wax as green hypergolic propellants for in-space propulsion. One hundred and four droplet ignition experiments were conducted while varying pressure (0.069 to 2.1 MPa), oxidizer type and temperature, proportion of LAH and paraffin within the fuel, and atmospheric conditions (pure nitrogen and air). Analysis of high-speed video obtained for each ignition event consistently yielded ignition delay times of 5-12 ms for nitric acid/LAH at pressures exceeding atmospheric -- values which are on par with state-of-the-art hypergolic propellants. Ignition delays were found to inversely relate to ignition chamber pressure. An LAH loading of 50% by mass in paraffin was the lowest concentration at which hypergolic ignition was reliably achieved in a pure nitrogen environment at standard temperature and pressure. Emission spectra acquired during hypergolic ignition informed a qualitative analysis of the self-ignition phenomenon for high- and low-ignition-delay events.