DOI: 10.1002/prep.201800381 Ignition Delays of Mixtures of the Non-hypergolic Energetic Ionic Liquid Hydroxyethylhydrazinium Nitrate Blended with Unsymmetrical Dimethylhydrazine Umakant Swami,* [a] Krishnamachary Senapathi, [b] Krishna Mohan Srinivasulu, [b] Jayaraman Desingu, [b] and Arindrajit Chowdhury [a] Abstract: The study deals with the investigation of en- ergetic ionic liquids capable of serving as both monop- ropellants and bipropellants on a single mission. Various blends of the monopropellant hydroxyethylhydrazinium ni- trate (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 con- taining a minimum of 30% and 40% UDMH by weight were found to be hypergolic with RFNA and WFNA, re- spectively. 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 chos- en to compare the veracity of the data from the optoelec- tronic 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/cm 3 higher pre- dicted vacuum density specific impulse than UDMH with IRFNA. Keywords: Energetic ionic liquids · hypergolic propellants · drop test setup · ignition delay 1 Introduction Hypergolic bipropellants are fuel-oxidizer pairs that ignite spontaneously when brought in contact with each other, rendering other ignition systems redundant and thus sim- plifying engine design and operation [1]. Engines deploying hypergolic propellants are widely used in various stages of spacecraft and missile launch vehicles as well as for orbital manoeuvring and attitude control in space stations and sat- ellites. The commonly used hypergolic propellants are hy- drazine and its derivatives as fuels, with dinitrogen tetr- oxide as an oxidizer. Hydrazine based fuels, such as monomethylhydrazine (MMH) and unsymmetrical dime- thylhydrazine (UDMH) are toxic, carcinogenic, highly vola- tile, and sensitive to adiabatic compression [2,3]. Ignition in hypergolic systems depends primarily upon exothermic liq- uid-vapor chemical reactions at low temperatures. The fac- tors influencing ignition are vapor pressure, viscosity, sur- face tension, and density of fluids involved, as well as condensed and gas phase reaction rates [4,5]. Ignition delay is defined as the time elapsed between the first contact be- tween the fuel-oxidizer pair and subsequent ignition. As a general rule of thumb, the ignition delay should be less than ten milliseconds to avoid excessive accumulation of propellants in the engine, leading to avoid dangerous pres- sure spikes, known as ‘hard starts’ [6]. The propellant synthesis community continuously at- tempts to find an alternative fuel with lesser toxicity, but with comparable ignition delays and performance as the hydrazine family. In the past two decades, ionic liquids (ILs) have been one of the most scintillating entrants to the field of energetic compounds [7,8]. In addition to energetic ionic liquids (EILs) being tailored as monopropellants and ex- plosives [9], hypergolic EILs based on dicyanamide anions were proposed as well [10]. As compared to the widely used hydrazine-based fuels, hypergolic ILs as alternative fuels are expected to exhibit various attractive properties, such as extremely low vapor pressure, high bulk density, [a] U. Swami, A. Chowdhury Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Fax: + 91-22-2572-6875 *e-mail: uswami@iitb.ac.in [b] K. Senapathi, K. M. Srinivasulu, J. Desingu Defence Research Development Laboratory, DRDO, Hyderabad, 500058, India Full Paper These are not the final page numbers!¼ �� Propellants Explos. Pyrotech. 2019, 44, 1–9 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1