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The study deals with the investigation of energetic ionic liquids capable of serving as both monopropellants and bipropellants on a single mission. Various blends of the monopropellant hydroxyethylhydrazinium nitrate (HEHN), which was found to be non‐hypergolic with red fuming nitric acid (RFNA) and white fuming nitric acid (WFNA) under ambient conditions, were formulated with known hypergols to impart hypergolicity and to reduce the viscosity and surface tension of pure HEHN. Considering the miscibility and the ignition delays of the chosen hypergols, unsymmetrical dimethylhydrazine (UDMH) was chosen as the candidate for combustion characterization through measurement of ignition delays. UDMH‐HEHN blends containing a minimum of 30 % and 40 % UDMH by weight were found to be hypergolic with RFNA and WFNA, respectively. Meticulous experiments were conducted to measure the ignition delays in a drop test setup, equipped sequentially with a high‐speed camera, which was primarily utilized to observe the physico‐chemical events governing ignition, as well as an optoelectronic diagnostic setup, which was utilized to segregate the physical and chemical ignition delays. Standard hypergolic propellants were chosen to compare the veracity of the data from the optoelectronic diagnostics setup with those obtained using a high‐speed camera. The hypergolic blend containing 60 % UDMH was found to be the best candidate, owing to its low ignition delay of 5.8 ms with RFNA, 80 % lower vapor pressure compared to UDMH, and 30 g‐s/cm3 higher predicted vacuum density specific impulse than UDMH with IRFNA.

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The accurate measurement of temperature in sooty flames remains a challenging task. In this study, the procedure for calibration and use of a consumer-grade digital single-lens reflex (DSLR) camera for the measurement of temperature in sooty flames using color-ratio pyrometry (CRP) is elucidated. Owing to the necessity of acquiring RAW images for CRP, investigations conducted thus far have been limited to stable flames. In this work, the potential of a CANON EOS 550D DSLR camera for measuring temperature during a transient process such as a droplet combustion event at frame rates up to 50 fps is demonstrated. The spectral response curves of the camera-lens system were obtained with the help of a tunable laser source and a laser power meter, which were subsequently used to generate lookup tables for blackbody as well as soot radiation. Soot radiation was assumed to vary with wavelength (λ) as λ−α, where α is the soot dispersion exponent. The blackbody lookup table was validated against blackbody calibration data from 1023 to 1773 K. Experiments were conducted on a candle flame as well as a McKenna flat flame burner with ethylene–air mixtures of equivalence ratios 2.1 and 2.3. For estimating temperatures using CRP, soot particles were classified as nascent and mature soot with soot dispersion exponent (α) values of 4 and 1.38, respectively. The CRP results were found to yield a decent match with thermocouple measurements as well as data reported in the literature.

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