Rocket fuel contains fuel and oxidizer and, unlike jet fuel, does not need an external component: air or water. Rocket fuels, according to their state of aggregation, are divided into liquid, solid and hybrid. Liquid fuels are divided into cryogenic (with the boiling point of the components below zero degrees Celsius) and high-boiling (the rest). Solid fuels consist of a chemical compound, a solid solution, or a plasticized mixture of components. Hybrid fuels consist of components in different aggregate states, and are currently in the research stage.
Historically, the first rocket fuel was black powder, a mixture of saltpeter (oxidizer), charcoal (fuel) and sulfur (binder), which was first used in Chinese rockets in the 2nd century AD. Ammunition with a solid propellant rocket engine (solid propellant rocket engine) was used in military affairs as an incendiary and signaling means.
After the invention of smokeless powder at the end of the 19th century, a single-component ballistite fuel was developed on its basis, consisting of a solid solution of nitrocellulose (fuel) in nitroglycerin (an oxidizing agent). Ballistite fuel has a multiple of higher energy compared to black powder, has a high mechanical strength, is well formed, retains chemical stability for a long time during storage, and has a low cost price. These qualities predetermined the widespread use of ballistic fuel in the most massive ammunition equipped with solid propellants - rockets and grenades.
The development in the first half of the twentieth century of such scientific disciplines as gas dynamics, physics of combustion and the chemistry of high-energy compounds made it possible to expand the composition of rocket fuels through the use of liquid components. The first combat missile with a liquid propellant rocket engine (LPRE) "V-2" used a cryogenic oxidizer - liquid oxygen and a high-boiling fuel - ethyl alcohol.
After World War II, rocket weapons received a priority in development over other types of weapons due to their ability to deliver nuclear charges to a target at any distance - from several kilometers (rocket systems) to intercontinental range (ballistic missiles). In addition, rocket weapons have significantly supplanted artillery weapons in aviation, air defense, ground forces and the navy due to the lack of recoil force when launching ammunition with rocket engines.
Simultaneously with ballistic and liquid rocket fuel, multicomponent mixed solid propellants developed as the most suitable for military use due to their wide temperature range of operation, elimination of the danger of component spills, lower cost of solid-propellant rocket engines due to the absence of pipelines, valves and pumps with higher thrust per unit weight.
The main characteristics of rocket fuels
In addition to the state of aggregation of its components, rocket fuels are characterized by the following indicators:
- specific impulse of thrust;
- thermal stability;
- chemical stability;
- biological toxicity;
- density;
- smokiness.
The specific thrust impulse of rocket fuels depends on the pressure and temperature in the combustion chamber of the engine, as well as the molecular composition of the combustion products. In addition, the specific impulse depends on the expansion ratio of the engine nozzle, but this is more related to the external environment of rocket technology (air atmosphere or outer space).
Increased pressure is provided through the use of structural materials with high strength (steel alloys for rocket engines and organoplastics for solid propellants). In this aspect, liquid-propellant rocket engines are ahead of solid propellants due to the compactness of their propulsion unit in comparison with the body of a solid-fuel engine, which is one large combustion chamber.
The high temperature of the combustion products is achieved by adding metal aluminum or a chemical compound - aluminum hydride to the solid fuel. Liquid fuels can use such additives only if they are thickened with special additives. Thermal protection of liquid-propellant rocket engines is provided by cooling with fuel, thermal protection of solid propellants - by firmly fastening the fuel block to the walls of the engine and the use of burn-out inserts made of carbon-carbon composite in the critical section of the nozzle.
The molecular composition of the combustion / decomposition products of the fuel affects the flow rate and their state of aggregation at the nozzle exit. The lower the weight of the molecules, the higher the flow rate: the most preferred combustion products are water molecules, followed by nitrogen, carbon dioxide, chlorine oxides and other halogens; least preferred is alumina, which condenses to a solid in the engine nozzle, thereby reducing the volume of expanding gases. In addition, the aluminum oxide fraction forces the use of conical nozzles due to the abrasive wear of the most efficient parabolic Laval nozzles.
For military rocket fuels, their thermal stability is of particular importance due to the wide temperature range of rocket technology operation. Therefore, cryogenic liquid fuels (oxygen + kerosene and oxygen + hydrogen) were used only at the initial stage of the development of intercontinental ballistic missiles (R-7 and Titan), as well as for launch vehicles of reusable space vehicles (Space Shuttle and Energia) intended for launching satellites and space weapons into low-earth orbit.
Currently, the military uses exclusively high-boiling liquid fuel based on nitrogen tetroxide (AT, oxidizer) and asymmetric dimethylhydrazine (UDMH, fuel). The thermal stability of this fuel pair is determined by the boiling point of AT (+ 21 ° C), which limits the use of this fuel by missiles under thermostated conditions in ICBM and SLBM missile silos. Due to the aggressiveness of the components, the technology of their production and operation of missile tanks was / is owned by only one country in the world - the USSR / RF (ICBMs "Voevoda" and "Sarmat", SLBMs "Sineva" and "Liner"). As an exception, AT + NDMG is used as a fuel for the Kh-22 "Tempest" aircraft cruise missiles, but due to problems with ground operation, the Kh-22 and their next generation Kh-32 are planned to be replaced with jet-powered Zircon cruise missiles using kerosene as fuel.
The thermal stability of solid fuels is mainly determined by the corresponding properties of the solvent and polymer binder. In the composition of ballistic fuels, the solvent is nitroglycerin, which in a solid solution with nitrocellulose has a temperature range of operation from minus to plus 50 ° C. In mixed fuels, various synthetic rubbers with the same operating temperature range are used as a polymer binder. However, the thermal stability of the main components of solid fuels (ammonium dinitramide + 97 ° C, aluminum hydride + 105 ° C, nitrocellulose + 160 ° C, ammonium perchlorate and HMX + 200 ° C) significantly exceeds the similar property of known binders, and therefore it is relevant search for their new compositions.
The most chemically stable fuel pair is AT + UDMG, since a unique domestic technology of ampulized storage in aluminum tanks under a slight excess nitrogen pressure for an almost unlimited time has been developed for it. All solid fuels chemically degrade over time due to the spontaneous decomposition of polymers and their technological solvents, after which oligomers enter into chemical reactions with other, more stable fuel components. Therefore, solid propellant rocket checkers need regular replacement.
The biologically toxic component of rocket fuels is UDMH, which affects the central nervous system, mucous membranes of the eyes and the human digestive tract, and provokes cancer. In this regard, work with UDMH is carried out in insulating chemical protection suits with the use of self-contained breathing apparatus.
The value of the fuel density directly affects the mass of the LPRE fuel tanks and the solid propellant rocket body: the higher the density, the less the parasitic mass of the rocket. The lowest density of the hydrogen + oxygen fuel pair is 0.34 g / cu. cm, a pair of kerosene + oxygen has a density of 1.09 g / cu. cm, AT + NDMG - 1, 19 g / cu. cm, nitrocellulose + nitroglycerin - 1.62 g / cu. cm, aluminum / aluminum hydride + perchlorate / ammonium dinitramide - 1.7 g / cc, HMX + ammonium perchlorate - 1.9 g / cc. At the same time, it should be borne in mind that the solid propellant rocket engine of axial combustion, the density of the fuel charge is approximately two times less than the density of the fuel due to the star-shaped section of the combustion channel, used to maintain a constant pressure in the combustion chamber, regardless of the degree of fuel burnout. The same applies to ballistic fuels, which are formed as a set of belts or sticks to shorten the burning time and acceleration distance of rockets and rockets. In contrast to them, the density of the fuel charge in the solid propellant rocket engine of end combustion based on HMX coincides with the maximum density indicated for it.
The last of the main characteristics of rocket fuels is the smoke of combustion products, visually unmasking the flight of rockets and rockets. This feature is inherent in solid fuels containing aluminum, the oxides of which are condensed to a solid state during expansion in the rocket engine nozzle. Therefore, these fuels are used in solid propellants of ballistic missiles, the active section of the trajectory of which is outside the enemy's line of sight. Aircraft missiles are fueled with HMX and ammonium perchlorate fuel, rockets, grenades and anti-tank missiles - with ballistic fuel.
Energy of rocket fuels
To compare the energy capabilities of various types of rocket fuel, it is necessary to set comparable combustion conditions for them in the form of pressure in the combustion chamber and the expansion ratio of the rocket engine nozzle - for example, 150 atmospheres and 300-fold expansion. Then, for fuel pairs / triplets, the specific impulse will be:
oxygen + hydrogen - 4.4 km / s;
oxygen + kerosene - 3.4 km / s;
AT + NDMG - 3.3 km / s;
ammonium dinitramide + hydrogen hydride + HMX - 3.2 km / s;
ammonium perchlorate + aluminum + HMX - 3.1 km / s;
ammonium perchlorate + HMX - 2.9 km / s;
nitrocellulose + nitroglycerin - 2.5 km / s.
Solid fuel based on ammonium dinitramide is a domestic development of the late 1980s, was used as a fuel for the second and third stages of the RT-23 UTTKh and R-39 rockets and has not yet been surpassed in energy characteristics by the best samples of foreign fuel based on ammonium perchlorate. used in the Minuteman-3 and Trident-2 missiles. Ammonium dinitramide is an explosive that detonates even from light radiation, so its production is carried out in rooms illuminated by low-power red lamps. Technological difficulties did not allow to master the process of manufacturing rocket fuel on its basis anywhere in the world, except in the USSR. Another thing is that the Soviet technology was routinely implemented only at the Pavlograd chemical plant, located in the Dnepropetrovsk region of the Ukrainian SSR, and was lost in the 1990s after the plant was converted to produce household chemicals. However, judging by the tactical and technical characteristics of promising weapons of the RS-26 "Rubezh" type, the technology was restored in Russia in the 2010s.
An example of a highly effective composition is the composition of solid rocket fuel from Russian patent No. 2241693, owned by the Federal State Unitary Enterprise Perm Plant named after CM. Kirov :
oxidizing agent - ammonium dinitramide, 58%;
fuel - aluminum hydride, 27%;
plasticizer - nitroisobutyltrinitrate glycerin, 11, 25%;
binder - polybutadiene nitrile rubber, 2, 25%;
hardener - sulfur, 1.49%;
combustion stabilizer - ultrafine aluminum, 0.01%;
additives - carbon black, lecithin, etc.
Prospects for the development of rocket fuels
The main directions for the development of liquid rocket fuels are (in the order of priority of implementation):
- the use of supercooled oxygen in order to increase the density of the oxidizer;
- transition to a fuel vapor oxygen + methane, the combustible component of which has 15% higher energy and 6 times better heat capacity than kerosene, taking into account the fact that aluminum tanks are hardened at the temperature of liquid methane;
- adding ozone to the oxygen composition at the level of 24% in order to increase the boiling point and energy of the oxidizer (a large proportion of ozone is explosive);
- the use of thixotropic (thickened) fuel, the components of which contain suspensions of pentaborane, pentafluoride, metals or their hydrides.
Supercooled oxygen is already being used in the Falcon 9 launch vehicle; oxygen + methane-fueled rocket engines are being developed in Russia and the United States.
The main direction in the development of solid rocket fuels is the transition to active binders containing oxygen in their molecules, which improves the oxidation balance of solid propellants as a whole. A modern domestic sample of such a binder is the polymer composition "Nika-M", which includes cyclic groups of dinitrile dioxide and polyetherurethane butylenediol, developed by the State Research Institute "Kristall" (Dzerzhinsk).
Another promising direction is the expansion of the range of used nitramine explosives, which have a higher oxygen balance in comparison with HMX (minus 22%). First of all, these are hexanitrohexaazaisowurtzitane (Cl-20, oxygen balance minus 10%) and octanitrocubane (zero oxygen balance), the prospects of which depend on reducing the cost of their production - currently Cl-20 is an order of magnitude more expensive than HMX, octonitrocubane is an order of magnitude more expensive than Cl -twenty.
In addition to improving the known types of components, research is also being carried out in the direction of creating polymer compounds, the molecules of which consist exclusively of nitrogen atoms connected by single bonds. As a result of the decomposition of a polymer compound under the action of heating, nitrogen forms simple molecules of two atoms connected by a triple bond. The energy released in this case is twice the energy of nitramine explosives. For the first time, nitrogen compounds with a diamond-like crystal lattice were obtained by Russian and German scientists in 2009 during experiments on a joint pilot plant under the action of a pressure of 1 million atmospheres and a temperature of 1725 ° C. Currently, work is underway to achieve the metastable state of nitrogen polymers at ordinary pressure and temperature.
Higher nitrogen oxides are promising oxygen-containing chemical compounds. The well-known nitric oxide V (a planar molecule of which consists of two nitrogen atoms and five oxygen atoms) is of no practical value as a component of solid fuel due to its low melting point (32 ° C). Investigations in this direction are carried out by searching for a method for the synthesis of nitric oxide VI (tetra-nitrogen hexaoxide), the framework molecule of which has the shape of a tetrahedron, at the vertices of which there are four nitrogen atoms bonded to six oxygen atoms located on the edges of the tetrahedron. The complete closure of interatomic bonds in the molecule of nitric oxide VI makes it possible to predict an increased thermal stability for it, similar to that of urotropin. The oxygen balance of nitric oxide VI (plus 63%) makes it possible to significantly increase the specific gravity of such high-energy components as metals, metal hydrides, nitramines and hydrocarbon polymers in the solid rocket fuel.
