A rocket is defined as a technologically advanced reaction motor that can carry its oxidant to propel a launch vehicle. A reaction motor is used as a propulsion device to generate a forward push by expelling the rocket structure in a backward direction. A missile is an unmanned rocket vehicle. It is a combination of the guidance system and an explosive material. A missile is guided by a ground-based communication center.
The function of the propulsion system is to produce thrust, which is the force that moves a rocket through air and space. Different propulsion systems generate thrust in different ways, but always through some application of Newton’s third law of motion. In any propulsion system, a working fluid is accelerated and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas.
A large fraction of the rocket engines in use today are chemical rockets; that is, they obtain the energy needed to generate thrust by chemical reactions to create a hot gas that is expanded to produce thrust. In Liquid fuel propulsion, the propellant is comprised of two composites: fuel and oxidizer. They are stored separately in tanks in the liquid phase and are pumped into the nozzle combustion chamber where burning occurs. The engine can stop the combustion and the thrust by turning off the propellant flow. Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks.
Chemical rockets are mainly used when high thrust to weight performance is required such as powering space launch vehicles, guided missiles, and spacecraft. A Hybrid Rocket Engine (HRE) is one of three categories of chemical rockets – the other two being Solid Rocket Motor (SRM) and Liquid Rocket Engine (LRE).
Liquid propellant rockets are subdivided into two sub-categories: Monopropellant and Bi-Propellant. Monopropellant Rocket Engines generate thrust through a process of chemical decomposition, usually with a catalyst; whereas, Bi-Propellant Rocket Engines generate combustion by mixing a liquid oxidizer and a liquid fuel under high pressure within a combustion chamber.
Liquid Monopropellant rockets, though significantly lower in specific impulse compared to Liquid Bi-Propellant rockets, are mechanically much less complex and thus considered more reliable. They can be throttled, stopped, and restarted on-command. Those commonly used for satellite, upper-stage, and spacecraft propulsion have used hypergolic propellants like Unsymmetrical Dimethyl Hydrazine (UDMH) that are toxic, environmentally unfriendly, and carcinogenic.
A hypergolic propellant combination used in a rocket engine is one whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. The main advantages of hypergolic propellants are that they can be stored as liquids at room temperature and that engines which are powered by them are easy to ignite reliably and repeatedly. These fluids are used in many different rocket and aircraft systems for propulsion and hydraulic power including orbiting satellites, manned spacecraft, military aircraft, and deep space probes.
Because hypergolic rockets do not need an ignition system, they can fire any number of times by simply opening and closing the propellant valves until the propellants are exhausted and are therefore uniquely suited for spacecraft maneuvering and well suited, though not uniquely so, as upper stages of such space launchers as the Delta II and Ariane 5, which must perform more than one burn. Restartable non-hypergolic rocket engines nevertheless exist, notably the cryogenic (oxygen/hydrogen) RL-10 on the Centaur and the J-2 on the Saturn V. The RP-1/LOX Merlin on the Falcon 9 can also be restarted.
The most common hypergolic fuels, include hydrazine (N 2 H4 ) and its derivatives including; monomethylhydrazine (MMH), unsymmetrical dimethylhydrazine (UDMH), and Aerozine 50 (A-50), which is an equal mixture of NA and UDMH. The oxidizer used with these fuels is usually nitrogen tetroxide (N 2O4 ), also known as dinitrogen tetroxide or NTO, and various blends of N 2O4 with nitric oxide (NO), are all liquid at ordinary temperatures and pressures. They are therefore sometimes called storable liquid propellants. They are suitable for use in spacecraft missions lasting many years. The cryogenic of liquid hydrogen and liquid oxygen limits their practical use to space launch vehicles where they need to be stored only briefly
Although commonly used, hypergolic propellants are difficult to handle due to their extreme toxicity and/or corrosiveness. However, in recent years satellite thrusters using ‘green’ propellants such as AF-M315E, a hypergolic hydroxyl ammonium nitrate fuel/oxidizer blend, Nitrous Oxide, and Hydrogen Peroxide have been developed and tested. Liquid Monopropellant rockets range in Specific Impulse from 76 sec. vac. for a typical cold gas generator to a high of 220 sec. vac. for a hot gas thruster using Hydrazine propellant. Some hypergolic engines have demonstrated specific impulse as high as 322 sec. vac.
Bi-propellant pump-fed rocket engines range in specific impulse depending upon the propellant combination from a low of 303 sec. vac. for rockets using Liquid Oxygen and RP-1 to a high of 455 sec. vac. for those using Liquid Oxygen and Liquid Hydrogen.
Relative to their mass, traditional hypergolic propellants are less energetic than such cryogenic propellant combinations as liquid hydrogen / liquid oxygen or liquid methane / liquid oxygen. A launch vehicle that uses hypergolic propellant must therefore carry a greater mass of fuel than one that uses these cryogenic fuels. Cryogenic propellants like Liquid Oxygen and Liquid Hydrogen, though inexpensive, add even more complexity and potential sources of failure, many of which can cause the vehicle to go out of control, catch fire, or explode. Because their propellants cannot be stored long-term on-vehicle and require considerable preparations for use, they are rarely used today for military applications.
Hypergolic fluids are toxic liquids that react spontaneously and violently when they contact each other. N 2O4 itself is non-flammable, non-explosive, and does not exothermically decompose; however, when added to fire it will increase the intensity of combustion and burning rate by providing an additional oxygen source to the air.
Several unintentional hypergolic fluid-related spills, fires, and explosions from the Apollo Program, the Space Shuttle Program, the Titan Program, and a few others have occurred over the past several decades. Spill sites include the following government facilities: Kennedy Space Center (KSC), Johnson Space Center (JSC), White Sands Test Facility (WSTF), Vandenberg Air Force Base (VAFB), Cape Canaveral Air Force Station (CCAFS), Edwards Air Force Base (EAFB), Little Rock AFB, and McConnell AFB.
High-performance hypergolic propellants for space rockets based on the materials genome
The chemical energy released by a propellent (rocket fuel) forms the power source of rockets and spacecraft, and can determine the altitude range and service life of a spacecraft. Examples include the Atlas-Centaur rocket based on liquid dihydrogen and oxygen fuel, bound for Mars and Venus, as well as the Long March 3B rocket containing UDMH (unsymmetric dimethyl hydrazine)/dinitrogen tetroxide to the moon. However, these high-performance rocket propellants or fuels are limited by high toxicity and decomposition, alongside their stable existence only at extremely low temperatures.
A new generation of rocket propellants for deep space exploration such as ionic liquid propellants with long endurance and high stability, are attracting significant attention. However, ionic liquid propellants are strongly restricted by their inadequate hypergolic (spontaneous ignition) reactivity between the fuel and the oxidant, where this defect can cause local burnout and accidental explosions during rocket launch. The need for an efficient and systematic method to design high-performance hypergolic additives therefore exists. The materials genome method can reduce the period of investigation required to develop such new materials.
A recent strategy to discover new materials presents a method based on “materials genomes,” which relies on big data analysis of the structures and properties of target materials to discover new materials. Researchers aim to construct artificial intelligence programs and screening to analyze a large number of possible structures in a short timeframe using the method. Yuan et al. applied the materials genome method in this work to predict the most probable hypergolic additive.
In a new report, Wen-Li Yuan and a research team in Chemistry at the Sichuan University in China and the Idaho University in the U.S. have proposed a visual model to demonstrate features of propellants to estimate their performance and applications. The materials genome and visualization model of the propellants greatly improved the efficiency and quality of developing performance propellants with applications to discover new and advanced functional molecules in the field of energetic materials.
To establish a hypergolic materials genome database, the team identified key structures of hypergolic compounds and explored their structure-activity relationships. A hypergolic reaction is an exothermic redox reaction (i.e., combustion) where components can spontaneously ignite on contact in a rocket combustor. Such compounds are typically made of gas-generating elements such as carbon and nitrogen. Much like the relationship between the gene and its base pair, the diverse hydrogen (H), carbon (C), nitrogen (N) and other elements constitute a series of hypergolic functional groups and frameworks to generate hypergolic compounds as suitable rocket propellants. The materials had to have an ignition delay time, a high enthalpy of combustion and a high specific impulse to determine the total energy payload capacity of rockets. Propellent additives should also be stable and compatible. Based on these requirements, Yuan et al. provided a direct method to identify key structures of hypergolic additives from the elemental composition of their functional structures.
Nitrogen-rich energetic propellants can boost energy beyond traditional fuels to improve the specific impulse of rocket fuels. Using existing literature, the researchers found the relationship of more than 1000 propellants and their mixtures to understand the connection between their elemental composition and thermal decomposition properties. Propellants containing 30 to 50 percent nitrogen content had the highest thermal stability with decomposition temperatures in excess of 200 degrees Celsius. The researchers deduced an appropriate nitrogen content to meet the specific requirements and thermal stability for high-performance propellants. The carbon element content also generated substantial amounts of combustion heat and gaseous carbon dioxide necessary for spacecraft propulsion to provide sufficient chemical energy to overcome gravity. Based on the enthalpy of combustion between carbon and nitrogen, the enthalpy of propellant combustion was positively related to the carbon content. To design the propellants, the team combined the limits of nitrogen elements in propellants with a highest allowable carbon content to achieve the best performance for specific impulse and combustion enthalpy.
The structural composition was another key feature of high-performance propellants to determine their stability, ignition behavior and biological toxicity. Ionic liquids composed of cations and anions have unique advantages of miscibility, volatility, hypotoxicity and thermal stability to greatly reduce the risk of exposing the operator to aerosols and deflagration. Using a screening method, Yuan et al. provided basic guidance to rapidly design and identify target compounds and considered other important indicators, including hypergolic reactivity and density, to select the best performance structure.
The team then conducted quantum analysis by investigating molecular orbital (MO) theory of anions as a criterion to determine hypergolicity and tested 15 anions, of which (1-methylhydrazinyl)tetrazolate (MHT) ionic liquids met all requirements of hypergolic additives. The genome database and screening process was therefore complete. Yuan et al. then studied the structure and physicochemical properties of MHT ionic liquids, including the density, thermal stability and detonation properties. Incidentally, the 1-butyl-3-methylimidazolium cation (Bmim+)-based MHT fuel had the highest thermal decomposition temperature, beyond 200 degrees Celsius, which was safe under extreme conditions in space. The team also tested two additional (Bmim+)-based propellants including Bmim-based dicyanamide (BmimDCA) and Bmim 5-aminotetrazole (BmimAT) ionic liquids.
Since toxicity was a serious problem in propellants, the team tested the toxicities of the ionic liquids using a Vibrio fischeri bacterium that can determine the environmental acceptability and toxicology parameter of materials. The combined BmimMHT/BmimDCA ionic liquids were advantageous as green propellants compared to traditional fuels. The DCA ionic liquid was more unique relative to toxicity, stability and volatility. Based on the guidance of the material genome method of propellants, Yuan et al. combined the DCA ionic liquid with BmimMHT, to make up for the insufficient hypergolic behavior of DCA.
In this way, Wen-Li Yuan and colleagues designed a previously unrealized family of high-performance propellant by using the propellant materials genome method. The MHT ionic liquid successfully solved the ignition behavior of the DCA ionic liquids. The design strategy summarized the structure-activity relationship of propellants combined with stability, hypergolicity and toxicity in a first-in-study materials genome method integrated in the field of propellants. The genome approach will guide and promote the molecular design and application of new materials to develop new high-performance propellants.
Sierra Space Advances its Storable Liquid Propulsion System
Sierra Space, the new commercial space subsidiary of global aerospace and national security leader Sierra Nevada Corporation (SNC), successfully completed testing of its hypergolic, or storable, liquid rocket propulsion system for orbit transfer, maneuvering and guidance control. Sierra Space’s patented VORTEX® engine cooling technology enables a compact and highly reliable propulsion system that can be stored for long periods of time on the ground and in space.
“We are excited to see Sierra Space’s unique rocket engine technology continue to evolve and push the boundaries of innovation,” said Tom Crabb, vice president of Sierra Space’s Propulsion & Environmental Systems group. “This success validates our intent to expand a family of engines using hypergolic propellants in a vortex chamber. New capabilities in next-generation engines could include throttling and multi-thrust capability in a single thruster. We seek reliable and affordable solutions for our customers.” Sierra Space’s VORTEX engine design promotes efficient, stable combustion while maintaining cool combustion chamber walls, enabling a more compact engine chamber while sustaining high-performance, and allowing rapid adaptation to multiple propellants.
The use of hypergolic propellants makes the propulsion system low-risk, extremely reliable and storable. Hypergolic propellants automatically ignite upon mixing, removing the mechanical complexity of the traditional ignition system. The new propulsion system can be scaled to suit a wide range of applications including upper stage boosters, missile systems, in-space propulsion, guidance, reaction control, extraterrestrial ascent and descent.
“The test campaign confirmed that these design improvements increased performance and we’re really proud of that,” said Dr. Marty Chiaverini, director of Propulsion Systems at Sierra Space. “It’s a highly scalable design that will allow us to perform quick redesigns for multiple thrusts, while still offering stable combustion.”
Sierra Space performed the hypergolic testing over a broad range of pressures and thrust levels, from 1,500 to 6,000 pounds of thrust. Other additional engines above 6,000 lbf thrust and thrusters less than 100 lbf are viable next options.
References and Resources also include: