Info

bag by the hot, high-pressure gas (see Ref. 12-15). One particular composition uses 65 to 75% NaN3, 10 to 28% Fe203, 5 to 16% NaN03 as an oxidizer, a burn rate modifier, and a small amount of Si02 for moisture absorption. The resultant solid nitride slag is caught in a filter.

The power P delivered by a gas generator can be expressed as

P = m(hx - h2) = [mTxRk/(k - 1)][1 - {p2/p,t~X)lk] (12-1)

where m is the mass flow rate, h\ and h2 the enthalpies per unit mass, respectively, at the gas generator chamber and exhaust pressure conditions, 7j is the flame temperature in the gas generator chamber, R the gas constant, p2/p\ is the reciprocal of the pressure ratio through which these gases are expanded, and k the specific heat ratio. Because the flame temperature is relatively low there is no appreciable dissociation, and frozen equilibrum calculations are usually adequate.

Smokeless or Low-Smoke Propellant

Certain types of DB propellant, DB modified with HMX, and AN composites can be nearly smokeless. There is no or very little particulate matter in the exhaust gas. These minimum-smoke propellants are not a special class with a peculiar formulation but a variety of one of the classes mentioned previously. Propellants containing Al, Zr, Fe203 (burn rate modifier), or other metallic species will form visible clouds of small solid metal or metal oxide particles in the exhaust.

For certain military applications a smokeless propellant is needed and the reasons are stated in Chapter 18 (Exhaust Plumes). It is very difficult to make a propellant which has a truly smokeless exhaust gas. We therefore distinguish between low-smoke also called minimum-smoke (almost smokeless), and reduced-smoke propellants, which have a faintly visible plume. A visible smoke trail comes from solid particles in the plume, such as aluminum oxide. With enough of these particles, the exhaust plume will scatter or absorb light and become visible as primary smoke. The particles can act as focal points for moisture condensation, which can occur in saturated air or under high humidity, low temperature conditions. Also, vaporized plume molecules, such as water or hydrochloric acid, can condense in cold air and form droplets and thus a cloud trail. These processes create a vapor trail or secondary smoke.

Several types of DB propellant, DB modified with HMX, nitramine (HMX or RDX) based composites, AN composites, or combinations of these, give very few or no solid particles in their exhaust gas. They do not contain aluminum or AP, generally have lower specific impulse than comparable propellants with AP, and have very little primary smoke, but can have secondary smoke in unfavorable weather. Several of these propellants have been used in tactical missiles.

Reduced-smoke propellants are usually composite propellants with low concentrations of aluminum (1 to 6%); they have a low percentage of aluminum oxide in the exhaust plume, are faintly visible as primary smoke, but can precipitate heavy secondary smoke in unfavorable weather. Their performance is substantially better than that of minimum-smoke propellants, as seen in Fig. 12-1.

Igniter Propellants

The process of propellant ignition is discussed in Section 13.2, and several types of igniter hardware are discussed in Section 14.3. Propellants for igniters, a specialized field of propellant technology, is described here briefly. The requirements for an igniter propellant will include the following:

Fast high heat release and high gas evolution per unit igniter propellant mass to allow rapid filling of grain cavity with hot gas and partial pres-surization of the chamber.

Stable initiation and operation over a wide range of pressures (subatmo-spheric to chamber pressure) and smooth burning at low pressure with no ignition overpressure surge.

Rapid initiation of igniter propellant burning and low ignition delays.

Low sensitivity of burn rate to ambient temperature changes and low burning rate pressure exponent.

Operation over the required ambient temperature range.

Safe and easy to manufacture, safe to ship and handle.

Good aging characteristics and long life.

Minimal moisture absorption or degradation with time.

Low cost of ingredients and fabrication.

Some igniters not only generate hot combustion gas, but also hot solid particles or hot liquid droplets, which radiate heat and impinge on the propellant surface, embed themselves into this surface, and assist in achieving propellant burning on the exposed grain surface.

There have been a large variety of different igniter propellants and their development has been largely empirical. Black powder, which was used in early motors, is no longer favored, because it is difficult to duplicate its properties. Extruded double-base propellants are used frequently, usually as a large number of small cylindrical pellets. In some cases rocket propellants that are used in the main grain are also used for the igniter grain; sometimes they are slightly modified. They are used in the form of a small rocket motor within a large motor that is to be ignited. A common igniter formulation uses 20 to 35% boron and 65 to 80% potassium nitrate with 1 to 5% binder. Binders typically include epoxy resins, graphite, nitrocellulose, vegetable oil, polyisobutylene, and other binders listed in Table 12-7. Another formulation uses magnesium

12.6. LINERS, INSULATORS, AND INHIBITORS 509

with a fluorocarbon (Teflon); it gives hot particles and hot gas (Refs. 12-16 and 12-17). Other igniter propellants are listed in Ref. 12-18.

12.6. LINERS, INSULATORS, AND INHIBITORS

These three layers at the interface of a grain were defined in Section 11.3. Their materials do not contain any oxidizing ingredients; they will ablate, cook, char, vaporize, or distintegrate in the presence of hot gases. Many will burn if the hot combustion gas contains even a small amount of oxidizing species, but they will not usually burn by themselves. The liner, internal insulator, or inhibitor must be chemically compatible with the propellant and each other to avoid migration (described below) or changes in material composition; they must have good adhesive strength, so that they stay bonded to the propellant, or to each other. The temperature at which they suffer damage or experience a large surface regression should be high. They should all have a low specific gravity, thus reducing inert mass. Typical materials are neoprene (specific gravity 1.23), butyl rubber (0.93), a synthetic rubber called ethylenepropylene diene or EPDM (0.86), or the binder used in the propellant, such as polybutadiene (0.9 to 1.0); these values are low compared with a propellant specific gravity of 1.6 to 1.8. For low-smoke propellant these three rubber-like materials should give off some gas, but few, if any, solid particles (see Ref. 12-19).

In addition to the desired characteristics listed in the previous paragraph, the liner should be a soft stretchable rubber-type thin material (typically 0.02 to 0.04 in. thick with 200 to 450% elongation) to allow relative movement along the bond line between the grain and the case. This differential expansion occurs because the thermal coefficient of expansion of the grain is typically an order of magnitude higher than that of the case. A liner will also seal fiber-wound cases (particularly thin cases), which are often porous, so that high-pressure hot gas cannot escape. A typical liner for a tactical guided missile has been made from polypropylene glycol (about 57%), a titanium oxide filler (about 20%), a di-isocyanate crosslinker (about 20%), and minor ingredients such as an antioxidant. The motor case had to be preheated to about 82°C prior to application. Ethylenepropylene diene monomer (EPDM) is linked into ethylenepropylene diene terpolymer to form a synthetic rubber which is often used as polymer for liners; it adheres and elongates nicely.

In some motors today the internal insulator not only provides for the thermal protection of the case from the hot combustion gases, but also often serves the function of the liner for good bonding between propellant and insulator or insulator and case. Most motors still have a separate liner and an insulating layer. The thermal internal insulator should fulfill these additional requirements:

1. It must be erosion resistant, particularly in the insulation of the motor aft end or blast tube. This is achieved in part by using tough elastomeric materials, such as neoprene or butyl rubber, that are chemically resistant to the hot gas and the impact of particulates. This surface integrity is also achieved by forming a porous black carbon layer on its heated surface called a porous char layer, which remains after some of the interstial materials have been decomposed and vaporized.

2. It must provide good thermal resistance and low thermal conductivity to limit heat transfer to the case and thus keep the case below its maximum allowable temperature, which is usually between 160 and 350°C for the plastic in composite material cases and about 550 and 950°C for most steel cases. This is accomplished by filling the insulator with silicon oxide, graphite, Kevlar, or ceramic particles. Asbestos is an excellent filler material, but is no longer used because of its health hazard.

3. It should allow a large-deformation or strain to accommodate grain deflections upon pressurization or temperature cycling, and transfer loads between the grain and the case.

4. The surface regression should be minimal so as to retain much of its original geometric surface contour and allow a thin insulator.

A simple relationship for the thickness d at any location in the motor depends on the exposure time te, the erosion rate re (obtained from erosion tests at the likely gas velocity and temperature), and the safety factor / which can range from 1.2 to 2.0:

Some designers use the simple rule that the insulation depth is twice the charred depth.

The thickness of the insulation is not usually uniform; it can vary by a factor of up to 20. It is thicker at locations such as the aft done, where it is exposed for longer intervals and at higher scrubbing velocities than the insulator layers protected by bonded propellant. Before making a material selection, it is necessary to evaluate the flow field and the thermal environment (combustion temperature, gas composition, pressure, exposure duration, internal ballistics) in order to carry out a thermal analysis (erosion prediction and estimated thickness of insulator). An analysis of loads and the deflections under loads at different locations of the motor are needed to estimate shear and compression stresses. If it involves high stresses or a relief flap, a structural analysis is also needed. Various computer programs, such as the one mentioned in Refs. 12-20 and 12-21, are used for these analyses.

An inhibitor is usually made of the same kinds of materials as internal insulators. They are applied (bonded, molded, glued, or sprayed) to grain surfaces that should not burn. In a segmented motor, for example (see Fig. 14-2), where burning is allowed only on the internal port area, the faces of the cylindrical grain sections are inhibited.

Was this article helpful?

0 0
Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment