Rockets propellants

Rockets use the principle of jet propulsion. This is the application of Newton s famous Third Law of Motion that action and reaction are equal and opposite . If 

Rocket propellants are similar to gun propellants as they burn smoothly and without detonation. Rocket propellants are required to burn at a 

Stabilizer Carbamite (diphenyl diethyl), urea methyl centralite (diphenyl dimethyl urea), chalk, and diphenylamine Increase shelf life of propellant 

Plasticizer Dibutyl phthalate, carbamite, and methyl centralite Gelation of nitrocellulose 

Rocket propellants are similar to the propellant charge powders discussed above, because they combust in a controlled manner and do not detonate. However, propellant charge powders bum considerably quicker than rocket propellants, which results in a significantly higher pressure for gun propellants in comparison with rocket propellants. Typical pressures in the combustion chambers of rockets are 70 bar, which can be compared with up to 4000 bar for large artillery and NAVY cannons. 

As is the case for propellant charges, the specific impulse for rocket propellants is also one of the most important performance parameters. 

The specific impulse (/ p) is the change in the impulse (impulse = mass x velocity or force X time) per mass unit of the propellant. It is an important performance parameter of rocket motors and shows the effective velocity of the combustion 

The force F is the time dependent thrust F (r), or the average thrust F b is the combustion time (in s) and m is the mass of propellant (in kg). Therefore the specific impulse has the units N s kg or m s Sometimes, predominantly in English speaking areas, the specific impulse is given based on the gravitation of the Earth g g = 9.81 m s ), which is on the mass of the propellant, and therefore has the unit seconds (s)  

Whereby y is the ratio of the specific heat capacities of the gas mixture, R is the gas constant, Tc is the temperature (K) in the combustion chamber and M is the average molecular weight (kg mol ) of the formed combustion gases  

Rocket propellants are very similar to gun propellants in that they are designed to burn uniformly and smoothly without detonation. Gun propellants, however, bum more rapidly due to the higher operating pressures in the gun barrel. Rocket propellants are required to burn at a chamber pressure of 7 MPa, compared with 400 MPa for gun propellant. Rocket propellants must also bum for a longer time to provide a sustained impulse. 

The specific impulse 4 is used to compare the performances of rocket propellants and is dependent on the thrust and flow rate of the gases through the nozzle as shown in Equation 8.4. 

The value for 1 is dependent on the velocity and pressure of the gaseous products at the nozzle exit. An expression for Is is given in Equation 8.5, 

There are two main types of solid rocket propellant these are composite and double-base propellants. 

Composite rocket propellants are two-phase mixtures comprising a crystalline oxidizer in a polymeric fuel/binder matrix. The oxidizer 

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Dimeihylamine, C2H7N, (CH3)2NH. Colourless, inflammable liquid with an ammoniacal odour, mp -96" C, b.p. 7°C. Occurs naturally in herring brine. Prepared in the laboratory by treating nitrosodimetbyl-aniline with a hot solution of sodium hydroxide. Dimethylamine is largely used in the manufacture of other chemicals. These include the solvents dimethylacetamide and dimethyl-formamide, the rocket propellant unsym-metrical dimethylhydrazine, surface-active agents, herbicides, fungicides and rubber accelerators. 

Elemental fluorine has been studied as a rocket propellant as it has an exceptionally high specific impulse value. 

Rocket Propellants. SoHd rocket propellants are mostly based on chemically cross-linked polymeric elastomers to provide the mechanical properties required in launchings and the environmental conditions experienced in storage, shipment, and handling (see Elastomers, synthetic). Double-and triple-based nitrocellulose propellants are also employed as rocket propellants. 

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

Table 1. Typical Components of Composite Rocket Propellants...

Rocket propellants must not contain sizable cracks, pores, or cavities. They are inspected using x-rays and ultrasonics, and firings are conducted in strand burners, intermpted burners, and in reduced or full-scale rocket motors (see also Nondestructive evaluation) (16—20). 

Gun Propellents. Although the stresses on individual gun propellant grains are less severe because of the small size, these propellants must withstand much higher weapon pressures and accelerations. Formulation options are usually more limited for gun propellants than for rocket propellants because the products of combustion must not foul or corrode a gun, should have a low flame temperature, and should exhibit minimum flash and smoke characteristics. Gun propellants are examined microscopically for porosity, are tested for mechanical characteristics, and fired in closed bombs to determine the burning characteristics. 

Experimental Determination of the Burning Rate. Experimental determinations of the burning rate are made with the closed tomb for gun propellants and the strand burner for rocket propellants. The closed bomb is essentially a heavy-wahed cylinder capable of withstanding pressures to 689 MPa (100,000 psi). It is equipped with a piezoelectric pressure gauge and the associated apparatus requited to measure the total chamber pressure, which is directly related to the force of the propellant. It also measures the rate of pressure rise as a function of pressure which can then be related to the linear burning rate of the propellant via its geometry. Other devices, such as the Dynagun and the Hi—Low bomb, have also been developed for the measurement of gun propellant performance. 

Fig. 3. Effect of grain shape on pressure—time traces of rocket propellants. Cross section of grains are shown.

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

Solventless Extrusion Process. The solvendess process for making double-base propellants has been used ia the United States primarily for the manufacture of rocket propellant grains having web thickness from ca 1.35 to 15 cm and for thin-sheet mortar (M8) propellant. The process offers such advantages as minimal dimensional changes after extmsion, the elimination of the drying process, and better long-term baUistic uniformity because there is no loss of volatile solvent. The composition and properties of typical double-base solvent extmded rocket and mortar propellant are Hsted ia Table... 

R. A. McKay,M Study of Selected Parameters in S olid Propellant Processing,]et Propulsion Lab, Pasadena, Calif., Aug. 1986 J. L. Brown and co-workers. Manufacturing Technologyfor SolidPropellantIngredients/Preparation Reclamation, Morton Thiokol, Inc., Brigham City, Utah, Apt. 1985 W. P. Sampson, Eow Cost Continuous Processing of Solid Rocket Propellant, Al-TR-90-008, Astronautics Laboiatoiy/TSTR, Edwards AEB, Oct. 1990. 

The Annual Proceedings of the Joiat Army-Navy-Air Force (JANNAF) Propulsion Meetings, the reports of the special committees, and the periodic hterature surveys pubHshed by the Chemical Propulsion Information Agency including the aimual Chemical Propulsion Abstracts are iuvaluable sources of information on all aspects of Hquid and soHd gun and rocket propellants. They maybe classified. 

Monographs on rockets and rocket propellants by the National Aeronautics and Space Administration (NASA), Lewis Research Center, Cleveland. These iaclude the foUowiag Solid Propellant Selection and Characteri tion, Report SP-8064,1971 Solid Rocket Motor Peformance, Report SP-8039,1971 Solid Rocket Motor Igniters, Report SP-8051,1971 Solid Rocket Motor Metal Cases, Report SP-8025, 1970, and Captive Eire Testing of Solid Rocket Motors, Report SP-8041,1971. 

F. A. WiUiams and co-workers. FundamentalMspects of Solid Rocket Propellants, Agardograph No. 116, Technical Press, London, 1969. 

D. Altman and co-workers, Eiquid Rocket Propellants, Princeton University Press, N.J., 1960. 

A.luminum Hydride. Aluminum hydride is a relatively unstable polymeric covalent hydride that received considerable attention in the mid-1960s because of its potential as a high energy additive to soHd rocket propellants. The projected uses, including aluminum plating, never materialized, and in spite of intense research and development, commercial manufacture has not been undertaken. The synthetic methods developed were cosdy, eg. 

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

Oxidizing Properties. Nitric acid is a powerful oxidizing agent (electron acceptor) that reacts violentiy with many organic materials (eg, turpentine, charcoal, and charred sawdust) (19,20). The concentrated acid may react explosively with ethanol (qv). Such oxidizing properties have had military appHcation nitric acid is used with certain organics, eg, furfuryl alcohol and aniline, as rocket propellant (see Explosives AND PROPELLANTS). 

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