Info

Source: Courtesy of Air Force Phillips Laboratory, Edwards, California.

Source: Courtesy of Air Force Phillips Laboratory, Edwards, California.

A typical composition diagram for a composite propellant is shown in Fig. 12-4. It shows how the specific impulse varies with changes in the composition of the three principal ingredients: the solid AP, solid Al, and viscoelastic polymer binder.

For double-base (DB) propellant the theoretical variations of Is and Tx are shown in Figs. 12-1 and 12-5 as a function of the nitroglycerine (NG) or plasticizer percentage. The theoretical maximum specific impulse occurs at about 80% NG. In practice, nitroglycerine, which is a liquid, is seldom found in concentrations over 60%, because the physical properties are poor if NG is high. There need to be other major solid or soluble ingredients to make a usable DB propellant.

For CMDB propellant the addition of either AP or a reactive nitramine such as RDX allows a higher Is than ordinary DB (where AP or RDX percent is zero), as shown in Fig. 12-6. Both AP and RDX greatly increase the flame temperature and make heat transfer more critical. The maximum values of Is occur at about 50% AP and at 100% RDX (which is an impractical propellant that cannot be manufactured and will not have reasonable physical properties). At high concentrations of AP or RDX the exhaust gases contain considerable H20 and 02 (as shown in Fig. 12-7); these enhance the erosion rate of carbon-containing insulators or nozzle materials. The toxic HC1 is present in concentrations between 10 and 20%, but for practical propellants it seldom exceeds 14%.

Nitramines such as RDX or HMX contain relatively few oxidizing radicals, and the binder surrounding the nitramine crystals cannot be fully oxidized. The binder is decomposed at the combustion temperature, forms gases rich in hydrogen and carbon monoxide (which reduces the molecular weight), and

Oxidizer concentration, %

Oxidizer concentration, % Oxidizer concentration, %

FIGURE 12-3. Variation of combustion temperature, average molecular mass of the combustion gases, and theoretical specific impulse (at frozen equilibrium) as a function of oxidizer concentration for HTPB-based composite propellants. Data are for a chamber pressure of 68 atm and nozzle exit pressure of 1.0 atm. (Reproduced from Ref. 12-2 with permission of the AIAA.)

FIGURE 12-4. Composition diagram of calculated specific impulse for an ammonium perchlorate-aluminum-polyurethane (PU is a polyester binder) at standard conditions (1000 psi and expansion to 14.7 psi). The maximum value of specific impulse occurs at about 11% PU, 72% AP, and 17% Al. (Reproduced from Ref. 12-3 with permission of the American Chemical Society.)

FIGURE 12-4. Composition diagram of calculated specific impulse for an ammonium perchlorate-aluminum-polyurethane (PU is a polyester binder) at standard conditions (1000 psi and expansion to 14.7 psi). The maximum value of specific impulse occurs at about 11% PU, 72% AP, and 17% Al. (Reproduced from Ref. 12-3 with permission of the American Chemical Society.)

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