1. Introduction

Hypergolic propellants ignite solely upon contact between the fuel and oxidizer. This characteristic allows for reignition without the need for separate ignition devices, thereby reducing the complexity of the system. Additionally, due to their excellent storability and high specific impulse, hypergolic propellants are widely used as propellants for spacecraft. Representative combinations are monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) and unsymmetrical dimethylhydrazine (UDMH)/NTO. MMH/NTO is preferred in the United States, while UDMH/NTO is favored in Russia [1,2].

Due to the various advantages of hypergolic propellants, research on hypergolic propellants has been conducted over the past several decades. However, the understanding of the ignition mechanisms of hypergolic propellants remains insufficient. Therefore, both numerical and experimental studies must be conducted in parallel to elucidate the ignition phenomena of hypergolic propellants and to develop engines. For example, in the 2010s, Japan conducted both experimental and numerical studies on hypergolic propellants for the development of lunar lander engines [3,4].

Hypergolic propellants are highly toxic substances that require extreme caution during handling. MMH is an acutely toxic substance with an LD50 (oral, rat) of 32 mg/kg, which causes the death of 50% of test animals upon a single dose. Even minimal exposure can be fatal, and it is associated with carcinogenicity, reproductive toxicity, and environmental hazards to aquatic life [5,6,7,8]. NTO is also an acutely toxic substance, posing significant risks of vapor inhalation due to its high vapor pressure [8,9,10,11,12]. In order to avoid the toxicity issues of MMH/NTO, ongoing research seeks to develop low-toxicity hypergolic propellants with similar performance to existing hypergolic propellants [13,14,15,16,17,18,19,20]. Low-toxicity hypergolic propellants consist of combinations of oxidizers, such as hydrogen peroxide, nitric acid, and nitrous oxide, with reactive and catalytic fuels containing igniters, as well as ionic fuels. However, fuels with igniters have limitations regarding the storage stability of the igniters, while ionic fuels face challenges related to production efficiency and cost. To date, low-toxicity hypergolic propellants have only been assessed for their potential applicability, with few instances of actual implementation in propulsion systems. This lack of practical application results in limited reliability. Due to these technical limitations, replacing conventional hydrazine-based hypergolic propellants with low-toxicity alternatives remains challenging. Therefore, MMH/NTO is still used in spacecraft such as the U.S. Space Force’s reusable unmanned spacecraft, X-37B [21], China’s lunar lander, Chang’e, India’s lunar lander, Chandrayaan [22], and SpaceX’s manned and unmanned spacecraft, Dragon [23,24]. Research on MMH/NTO-based hypergolic propellants is essential for spacecraft development, and safe handling guidelines must be established beforehand.

As the goal of Korea’s national space development roadmap is to launch a lunar lander by 2032 [25], experimental research for the development of hypergolic propellant engines is necessary. However, the lack of safe handling guidelines has led to insufficient research on hypergolic propellants. This paper aims to establish a foundation that enables university-level researchers to conduct experimental research safely and efficiently by examining the trends in experimental studies of hypergolic propellants and establishing safe handling guidelines for hypergolic propellants.

2. Trends in Experimental Research

2.1 Drop test

The drop test, as shown in Fig. 1, is an experiment in which an oxidizer droplet is dropped into a droplet or pool of fuel, or vice versa, to measure the ignition delay time (IDT), a critical performance evaluation metric for hypergolic propellants. This experiment uses only a small amount of propellant and features a simple experimental setup, leading to numerous studies conducted in university laboratories. In particular, for hydrazine-based hypergolic propellants, the kinematic viscosity is below 20 cSt, resulting in similar values for ignition delay in the drop test and in engine tests [26]. Therefore, the drop test can be used to predict the ignition delay of actual liquid rocket systems.

In 1991, Daimon et al. [27] conducted a drop test by dropping fuel droplets into a pool of oxidizer at room temperature (18-20°C). The results are shown in Fig. 2. Hydrazine, MMH, and UDMH were used as fuels, while NTO and fuming nitric acid (FNA) were used as oxidizers. The experimental results confirmed that as the impact velocity increased, chemical reactions occurred more rapidly and violently.

Fig. 1.

Schematic of drop test apparatus.

Fig. 2.

Ignition or explosion delay time of various hypergolic propellants [27].

The researchers at Purdue University conducted drop tests using MMH and oxidizers from the NTO or nitric acid series. Due to NTO’s low boiling point, they replaced it with nitric acid oxidizers for experiments until the early 2010s [28]. Kubal et al. [29] reported that when red fuming nitric acid (RFNA) droplets were dropped into a pool of MMH in 2010, the ignition delay varied from 2.5 ms to 10.5 ms, even though all other conditions were identical, due to changes in the droplet’s impact point. Forness et al. [30,31] revealed through subsequent research that the reaction patterns between MMH and nitric acid oxidizers depend on the type of droplet impact. The types of impact can be classified into three categories: explosion, bounce, and splash. They argued that when the Weber number (We) exceeds 300, splash impact occurs, while when We is below 300, a smaller impact angle φ and impact parameter B, as shown in Fig. 3, increase the probability of explosion impact. Since only explosion impact can be predicted by existing reaction mechanisms, they highlighted the need for a new reaction mechanism that can also explain bounce and splash impacts. In 2012, Dambach et al. [32] measured the flame temperature of RFNA and MMH in a nitrogen environment using a thin-filament pyrometer. Subsequently, Heister et al. [33] validated the reaction mechanisms by comparing the measured flame temperatures with numerical analysis results. In 2017, Black et al. [34,35] conducted a drop test of MMH/NTO. Due to NTO’s high vapor pressure and low boiling point, they could not use the syringe and pipette method for propellant charging, as was done with nitric acid oxidizers. Instead, they supplied gaseous NTO to a condenser, cooled it in the condenser to liquefy it, and then opened a valve to drop NTO droplets into the MMH pool. The experimental results indicated that when the oxidizer was dropped from a height of 2.5 inches, MMH/NTO exhibited an ignition delay of 1.45 ± 0.60 ms. In 2023, Beaver et al. [36] studied the impact of NO content in MON (mixed oxides of nitrogen), an oxidizer used in actual rocket engines, on ignition delay. The study found that the ignition delay of MON-25 (25% nitric oxide in nitrogen tetroxide) was significantly greater than that of NTO, MON-3, and MON-10, and that the ignition delay became increasingly sensitive to initial pressure conditions as the NO content increased.

Fig. 3.

Impact parameter B and impact angle φ [29].

As shown in Fig. 4, hypergolic propellants ignite through stages of condensed-phase reaction, mixed-phase reaction, and gas-phase reaction [37]. Since the 0-D ignition delay calculated using reaction mechanisms considers only the reactions in the gas phase, the time for condensed-phase reactions must be excluded from the ignition delay to validate the reaction mechanisms.

Fig. 4.

Three stages of the hypergolic propellants ignition process [37].

In 1997, researchers at The University of Alabama in Huntsville [38,39,40,41,42,43,44,45] categorized IDT into liquid reaction time (LRT) and chemical delay time (CDT) and designed an experimental setup, as shown in Fig. 5, to measure these parameters. Using this setup, as illustrated in Fig. 6, the first phototransistor records the moment of droplet contact and gas generation, while the second phototransistor records the ignition moment, allowing the simultaneous measurement of IDT, LRT, and CDT in a single experiment. They conducted experiments by dropping RFNA droplets from a height of 1.5 cm and measured ignition and chemical delays while varying temperature and O/F ratio. The experimental results showed that CDT decreased as the temperature increased. They also reported that ignition failed or ignition delay significantly increased when the O/F ratio was extremely high or low.

Fig. 5.

Schematic of chemical delay time measurement device [38,40].

Fig. 6.

Measurement of chemical delay and ignition delay with phototransistor [40].

In 2012, researchers at Pennsylvania State University [46,47] conducted drop tests using nitric acid-based oxidizers and MMH. They measured ignition delay using a microphone, photodiode, and high-speed camera and recorded temperature using a thermocouple. Based on the measured temperatures, they divided the ignition process of liquid hypergolic propellants into three stages: the stage where the propellant’s temperature rises to its boiling point through a liquid-phase reaction, the stage where the gas temperature increases due to initial gas-phase reactions, and the stage where the temperature reaches the flame temperature after ignition. Additionally, they reported that when MMH droplets were dropped into a pool of RFNA, the reaction between NO₂ gas released from the oxidizer and the fuel, prior to droplet contact with the RFNA pool, resulted in inconsistent ignition delay measurements.

In 2017, researchers at Purdue University used a high-speed camera to measure the time from droplet contact to the onset of gas generation in a drop test, defining it as the liquid phase induction time. The liquid phase induction time for MMH/NTO, as measured with the high-speed camera, was 9 µs at 300 K [35].

2.2 Impinging jet test

The impinging jet test is commonly used to simulate impinging injectors widely applied in liquid rockets, allowing for the observation of spray and combustion characteristics when fuel and oxidizer jets collide. The setup for the impinging jet test is generally configured as shown in Fig. 7, and it can also be built in a simpler form using syringes and linear actuators.

When the fuel and oxidizer jets injected through the injector collide, the propellant evaporates due to chemical reactions, forming a gas film that hinders the mixing of liquid propellants in the impingement region. Additionally, since hypergolic propellants undergo rapid chemical reactions within milliseconds, there is insufficient time for proper mixing, which creates challenges in injector design [48,49]. Therefore, developing hypergolic propellant engines requires experimental investigation of the mixing, atomization, vaporization, and combustion characteristics of the propellants injected through the injector.

Fig. 7.

Schematic of impinging jet test apparatus.

Since the late 1950s, NASA researchers have conducted impinging jet tests using hydrazine and NTO. In 1959, Elverum and Staudhammer [50] reported that if the mixing of fuel and oxidizer is not achieved quickly, combustion efficiency significantly decreases. In 1965, Johnson [51] observed that the spray pattern of hypergolic propellants differs completely from that of non-reactive fluids due to rapid chemical reactions occurring at the interface between the oxidizer and fuel jets. In 1967, Evans et al. [52] reported that the gas film formed by chemical reactions at the interface hinders the mixing of liquid propellants, resulting in a phenomenon called reactive stream separation, where the oxidizer and fuel become separated. They also revealed that the design of the injector determines whether reactive stream separation occurs. In 1968, Clayton [53] reported a phenomenon called popping, in which repeated explosions occur due to heat transfer from the flame or exothermic reactions of the propellants, followed by a pressure rise from the explosion and subsequent flow blockage caused by the pressure increase. Lawver and Breen [54] argued that ignition occurs at the interface, resulting in reactive stream separation, when the ignition delay (τ_ign) is shorter than the residual time for mixing at the jet impingement region (τ_res), i.e., when τ_ign/ τ_res < 1, as shown in Fig. 8. They also reported, as illustrated in Fig. 9, that high propellant temperatures promote stream separation, while lower temperatures and larger jet diameters increase the likelihood of popping. In 1968, Mills et al. [55] reported that popping occurs when ignition takes place in a well-mixed liquid sheet. In 1970, Houseman [56] identified a phenomenon called penetration, where, contrary to separation, an excess of fuel appears on the oxidizer orifice side, and an excess of oxidizer appears on the fuel orifice side. They observed that penetration occurred when the orifice diameter was 0.508 mm, separation occurred at a diameter of 1.854 mm, and when the diameter was 0.736 mm, the flow transitioned from penetration to mixing and eventually to separation as the chamber pressure increased. From this, they confirmed that the mixing conditions lie between penetration and separation conditions.

Fig. 8.

Mixing and separation conditions for hypergolic stream [54].

Fig. 9.

Criteria for mixing, popping, and reactive stream separation [54].

In the 2010s, researchers at Purdue University conducted impinging jet tests with MMH/RFNA and MMH/NTO. Dennis et al. [37,57,58] found that as the mixing time of MMH/RFNA jets increased, ignition became more stable; however, when the mixing time exceeded 10-15% of the ignition delay, reactive stream separation occurred [57,58]. In 2017, Black [34] conducted impinging jet tests with MMH/NTO and confirmed that the ignition delay was similar to that observed in drop tests. Additionally, they explored the feasibility of using gelled propellants to reduce risks during propellant handling but reported that their performance was significantly lower compared to that of pure propellants [29 ,59].

In 2007, researchers at National Cheng Kung University in Taiwan [60] conducted impinging jet tests with MMH/NTO. They gradually increased the mixture ratio of MMH to NTO from 1 to the stoichiometric ratio of 2.5 at a impingement angle of 60° and measured the temperature 20 mm below the impingement point. The results showed that the temperature increased up to a mixture ratio of 1.4 but decreased beyond 1.6 due to a reduction in mixing efficiency. Based on these experiments, they successfully developed a rocket engine injector in 2012 [61].

In 2017, researchers at JAXA [3] conducted impinging jet tests with MMH/MON-3. They published images of the flow and flame when the oxidizer and fuel jets collided at angles of 20° and 60°. At the 20° impingement angle, ignition occurred before the liquid sheet broke into droplets, resulting in reactive stream separation and the observation of combustion instability, with flames oscillating longitudinally. At the 60° impingement angle, ignition occurred after the liquid sheet had fully broken up, preventing stream separation. Additionally, orange flames were observed near the oxidizer orifice, and blue-white flames appeared near the fuel orifice, with the flame distribution closely resembling the droplet distribution.

2.3 Observation of flame structure

Fig. 10 shows the experimental setup used to observe the flame structure of fuel droplets. The fuel is mixed with fumed silica or hydroxypropyl cellulose (HPC) to form a gel, which is suspended in the combustion chamber using either stainless steel or glass. When gaseous NTO is injected, the flame structure of the fuel droplets can be observed. When liquid fuel reacts with a gaseous oxidizer, a dual flame structure is observed, as shown in Fig. 11. The inner flame results from the thermal decomposition of the fuel droplet, while the outer flame is a diffusion flame formed by the oxidizer and either the fuel or its decomposition products [62]. Both the research team at Purdue University [63,64] and a Chinese research team [65,66] captured the dual flame structure around the fuel droplet during the early stages of combustion. As combustion progressed, they observed small-scale explosions caused by bubbles forming within the fuel droplet.

Fig. 10.

Schematic of droplet combustion experiment apparatus.

Fig. 11.

Droplet combustion model [62].

Hayashi et al. [67] conducted counterflow flame experiments with MMH/NTO by reacting MMH vapor, evaporated from an MMH pool at the bottom, with gaseous NTO supplied from the top, as shown in Fig. 12. The experimental results revealed three flame layers. Through comparison with numerical analysis results, the lower orange flame was attributed to the decomposition of MMH and the oxidation of NH₃, the middle blue-white flame was associated with excited OH* radicals, and the upper orange flame resulted from the recombination of NO₂.

Fig. 12.

Schematic of 1-D counterflow flame experiment apparatus.

2.4 Non-ignition evaluation

Non-ignition evaluation is conducted under conditions in which ignition does not occur, such as by maintaining the reactor temperature below the flash point or diluting the propellant. The purpose of this evaluation is to verify the initial chemical reactions of hypergolic propellants by analyzing the reaction products using mass spectrometry or infrared spectroscopy. In 1967, Mayer et al. [68] investigated the reaction between liquid MMH and NO₂ vapor at -11°C. The results revealed that the reaction products consisted of methylhydrazinium nitrate (MMH･HNO₃), nitromethane (H₃CNO₂), monomethyl-lnitrosamine (CH₃NHNO), nitramide (H₂NNO₂), H₂O, CO, CO₂, NH₂, NH₃, NO, NO₂, and CH₂. In 1972, Saad et al. [69] analyzed the reaction products of an NTO/CCl₄ solution reacting with MMH at -20°C. The analysis revealed that the main products included methylhydrazinium nitrate, monomethylnitrosamine, N-methyl formamide (CH₃NHCHO), methylamine (CH₃NH₂), dimethylamine ((CH₃)₂NH), methanol (CH₃OH), dimethylnitrosamine ((CH₃)₂NNO), H₂O, N₂, and various carbon and nitrogen oxides. Based on these results, they proposed a reaction mechanism for the liquid-phase reaction between MMH and NTO. In 2012, Wang and Thynell [46,47] reacted 0.5 µL of nitric acid with MMH at temperatures of 20°C, 50°C, 100°C, 150°C, 200°C, and 250°C and analyzed the reaction products using infrared spectroscopy. Based on the experimental results, they proposed a reaction mechanism for the liquid-phase reaction between MMH and nitric acid and the initial gas-phase reactions occurring prior to ignition.

3. Safe Handling Guidelines of Hypergolic Propellants

3.1 Laboratory setup

To conduct hypergolic propellant experiments, an appropriate laboratory environment must be established. The laboratory should be divided into an experimental space, where the propellant is handled and experiments are conducted, and a control room, where the experimental apparatus is remotely controlled and personal protective equipment is stored. For experiments involving propellant quantities exceeding the threshold limit, such as impinging jet tests, the experimental apparatus must be operated remotely from the control room to protect personnel from potential hazards. The research team at Pennsylvania State University recommends setting the threshold limit based on values calculated from the ceiling exposure limits provided by the Occupational Safety and Health Administration (OSHA), as shown in Table 1, and the volume of the experimental space [42,43,44].

A fume hood and an air conditioning unit must be installed in the experimental space. To prevent hazardous reactants and harmful substances generated during experiments from being released without proper purification, a negative pressure of at least -2.5 Pa should be maintained [30,34,70]. Since NTO vaporizes as the temperature rises, making it difficult to conduct drop tests, the temperature of the experimental space must be maintained below the boiling point of NTO (21°C). Daimon et al. [27] conducted experiments while maintaining the temperature between 18°C and 20°C for optimal experimental conditions.

The laboratory must be equipped with fire suppression systems to prepare for potential fires, and an emergency shower and warning lights indicating that propellants are being handled must be installed outside the laboratory. Additionally, to prevent accidents due to power outages, all equipment must be connected to an uninterruptible power supply (UPS).

3.2 Use of proper materials

Hypergolic propellants are highly reactive and corrosive substances, making it essential to consider material compatibility when constructing laboratories and experimental apparatus. The use of incompatible materials may lead to corrosion, resulting in propellant leakage, fuel decomposition, or, in the worst case, fire and explosion [71,72,73]. NASA researchers tested the compatibility between propellants and materials and provided a list of suitable materials, as shown in Table 2[73,74,75]. All research institutions conducting experimental studies on hypergolic propellants have prioritized the use of materials listed in Table 2 when constructing experimental apparatus, with 304 stainless steel and Teflon being particularly common. However, materials such as butyl rubber, neoprene, and polyethylene have also been used in components where prolonged exposure is unlikely [74,75]. If it is unavoidable to use materials other than those listed in Table 2, compatibility testing with a small amount of propellant must be conducted under a fume hood [34].

3.3 Use of proper personal protective equipment

When handling hypergolic propellants, researchers must wear proper personal protective equipment. Both MMH and NTO are highly toxic substances, with vapor inhalation, ingestion, and skin or eye contact all posing severe risks to human health. NTO evaporates at a rate five times faster than that of water at room temperature [71], and no purification filter is available for it [76]. MMH is both a reproductive toxin and a carcinogen, and even exposure below permissible levels can have adverse effects on health if repeated. Accordingly, as shown in Table 1, strict exposure limits for MMH and NTO are applied by the Ministry of Employment and Labor of Korea, the American Conference of Governmental Industrial Hygienists (ACGIH), and the National Institute for Occupational Safety and Health (NIOSH). Therefore, when handling propellants, researchers must wear a full-body protective suit of Type 3 or higher, along with chemical-resistant gloves to prevent skin exposure, and must use a full-face air-supplied mask that provides respiratory air under positive pressure to protect the eyes and face. Additionally, regardless of whether propellants are being handled, a portable gas detector with an alarm must be carried when entering the laboratory. After completing the experiment, protective suits and gloves must be cleaned using an emergency shower located outside the laboratory before removal.

3.4 Propellant Storage Methods

Fuel and oxidizer must be stored in separate storage cabinets. Institutions handling hypergolic propellants, such as Purdue University, the University of Alabama in Huntsville, and NASA, commonly store fuel and oxidizer in separate locations to prevent fire and explosion due to contact between them [34,42,77].

MMH is a flammable liquid, and exposure to an ignition source at temperatures above its flash point of -8°C can lead to fire or explosion [78,79]. Therefore, MMH must be stored in sealed reagent bottles made of suitable materials, such as heat-resistant glass, at temperatures below its flash point (-8°C) but above its freezing point (-52°C).

Although the safest method for storing NTO is to pressurize it with high-pressure gas, it must be stored as a liquid for experimental purposes even if a pressurized supply system is not in place. In this case, NTO should be stored in a pressure-resistant container under refrigerated conditions, close to its freezing point (-11°C), to account for the vapor pressure increase when the oxidizer is removed for use. Researchers at the University of Alabama in Huntsville stored RFNA, containing NTO, at temperatures below 0°C to maintain the composition of the oxidizer [42].

3.5 Propellant dispensing and filling

When handling MMH, operations must be performed in an environment of dry nitrogen with an oxygen concentration below 0.1%, typically achieved using a glove box. An oxygen concentration of 0.1% corresponds to 25% of the oxygen concentration at the upper flammability limit of MMH, which prevents ignition due to static electricity during propellant handling and also protects against oxidation. At the university laboratory level, operations are typically performed using syringes [34,42]. The filled syringe can be connected to the experimental apparatus and pressurized using a linear actuator or syringe pump to supply MMH.

Since NTO turns into a gas when pressure decreases, it cannot be dispensed or filled using micro syringes or micro pipettes that rely on negative pressure. It is common to pressurize an NTO tank with nitrogen for supply. If a pressurized supply system is not available and a syringe must be used, the syringe needle should be sealed and the piston removed before filling. This process should be conducted using a funnel in a glove box or fume hood to prevent vapor from entering the experimental space [80]. When transferring NTO into a syringe, the syringe should be cooled to minimize vaporization [81].

3.6 Post-experimental handling

The post-experiment process is divided into the treatment of combustion gases and the cleaning of experimental apparatus. The main products of the reaction between MMH and NTO are CO₂, H₂O, and N₂, which are harmless to humans, with only trace amounts of toxic substances, including residual propellants, present [80]. Although the combustion gases are harmless, researchers at the University of Alabama in Huntsville discharge them by passing them sequentially through a wet scrubber and an organic scrubber [38,39]. NASA also recommends using scrubbers to remove harmful substances [71].

After completing the experiment, the experimental apparatus should first be washed under a fume hood using a mildly basic solution that is harmless to humans, followed by rinsing with water and drying. MMH dissolves in water, but NTO reacts with water to produce nitric acid, nitrous acid, and nitrogen oxides. Therefore, if the residual oxidizer is not neutralized before washing with water, the apparatus may corrode. Researchers at the University of Alabama in Huntsville reported that all experimental apparatus that came into contact with the propellant, including used needles and syringes, are initially washed with a 2-4% dilute sodium hydroxide solution [38,39]. NASA researchers mentioned that mixing a small amount of MMH with a large quantity of water and discharging it into the sewage system is the simplest disposal method [75]. Wastewater from washing the apparatus can be discharged into the sewage system, as MMH solutions below 1% are not classified as hazardous substances [5].

4. Conclusions

This paper analyzes experimental research trends on hypergolic propellants and establishes safe handling guidelines based on previous studies conducted by research institutes.

Hypergolic propellants are essential elements in spacecraft development, and various combustion phenomena have been studied through diverse experiments conducted by several international research institutes. Major experiments include drop tests, impinging jet tests, flame structure observation experiments, and non-ignition evaluations, each of which can be utilized differently depending on the research objectives. For evaluating propellant performance, drop tests are suitable, while validating the reaction mechanism of gas-phase reactions requires both drop tests and counterflow flame experiments. To analyze the flame structure of fuel droplets, droplet combustion experiments must be conducted, and non-ignition evaluations are necessary to verify the reaction mechanism of liquid-phase reactions.

To achieve objectives such as the lunar lander launch and reusable spacecraft development, research on hypergolic propellants must also be conducted in Korea. However, due to the high toxicity of hypergolic propellants, research on safe handling guidelines must be conducted before experiments are performed. This paper presents safe handling guidelines to enable researchers to conduct experiments safely, covering the establishment of an experimental environment, the selection of appropriate materials, the use of personal protective equipment, methods for propellant storage and filling, and post-experiment procedures.

It is hoped that researchers intending to conduct future experimental studies on hypergolic propellants will use this paper as a foundational reference to design experiments suited to their respective objectives and conduct them safely. Through this, it is expected that research on hypergolic propellants will be further promoted in Korea.