Hyperenvironment

During the past decades progress in aeronautics and astronautics has been remarkable because people have learned to master the difficult feat of hypervelocity flight. A variety of manned and unmanned aircraft have been developed for faster transportation from one point on earth to another. Similarly, aerospace vehicles have been constructed for further exploration of the vast depths of space and the neighboring planets in the solar system. Plastics have found numerous uses in specialty areas such as hypersonic atmospheric flight and chemical propulsion exhaust systems. The particular plastic employed in these applications is based on the inherent properties of the plastics or the ability to combine it with another component material to obtain a balance of properties uncommon to either component.

Plastics have been developed for uses in very high temperature environments. It has been demonstrated that plastic materials are suitable for thermally protecting structures during intense rocket and missile propulsion heating. This discovery became one of the greatest achievements of modern times, because it essentially initially eliminated the thermal barrier to hypersonic atmospheric flight as well as many of the internal heating problems associated with chemical propulsion systems.

Modern supersonic aircraft experience appreciable heating. This incident flux is accommodated by the use of an insulated metallic structure, which provides a near balance between the incident thermal pulse and the heat dissipated by surface radiation. The result is that only a small amount of heat has to be absorbed by mechanisms other than radiation. With speeds increasing (8,000 fps) heating increases to a point where some added form of thermal protecdon is necessary to prevent thermostructural failure. Hypervelocity vehicles transcending through a planetary atmosphere also encounter gas-dynamic headng. The magnitude of heating is very large, however, and the headng period is much shorter.

This latter type of thermal problem is frequendy referred to as the reentry headng problem, and it posed one of the most difficult engineering problems of the twentieth century. A vehicle entering the earth's atmosphere at 25,000 fps has a kinetic energy equivalent to 12,500 Btu/lb of vehicle mass. Assuming the vehicle weighs a ton, it possesses a thermal energy equivalent to 25,000,000 Btu. This magnitude of energy greatly exceeds that required to completely vaporize the entire vehicle. Fortunately, only a very small fraction of the kinetic energy converted to heat reaches the body while the remainder is dissipated in the gas surrounding the vehicle. Materials performance during hypersonic atmospheric flight depends upon certain environmental parameters. These thermal, mechanical, and chemical variables differ gready in magnitude and with body position.

In general, they are concerned with temperatures from about 2,000 to over 20,000F (1,100 to 11,000°C), gas enthalpies up to 40,000 Btu/lb, convective/radiative heating from 10 to over 10,000 Btu/ft2/see. The stagnation pressures is less than 1 to over 100 atm., surface shear stresses up to about 900 psf, heating times from a few to several thousand seconds, and gaseous vapor compositions involving molecular, dissociated, and ionized species. To operate in these extreme conditions ablative materials can be used.

During ablation its surface material is physically removed. The injected vapors alter the chemical composition, transport properties, and temperature profile of the boundary layer, thus reducing the heat transfer to the material surface. At high ablation rates, the heat transfer to the surface may be only 15% of the thermal flux to a non-ablating surface. Up to tens of thousands of Btu's of heat can be absorbed, dissipated and blocked per pound of ablative material through the sensible heat capacity, chemical reactions, phase changes, surface radiation and boundary layer cooling of the ablator.

The heating rate or environmental temperature does not limit ablative systems, but rather by the total heat load. In spite of this limitation, however, the versatility of ablation has permitted it to be used on various hypervelocity atmospheric vehicles. No single, universally acceptable ablative material has been developed. Nevertheless, the interdisciplinary efforts of materials scientists and engineers have resulted in obtaining a wide variety of ablative compositions and constructions. These thermally protective materials have been arbitrarily categorized by their matrix composition, and typical materials are given in Table 6.2.

A popularly used plastic ablative heat protective material is plastic-base composites that include a TS plastic organic matrix. Their response to the heat abrasion occurs in a variety of ways. There are the depolymerization-vaporization (polytetrafluoroethylene), pyrolysis-vaporization (phenolic, epoxy) and decomposition melting vaporization (nylon fiber reinforced plastic). The principal advantages of plastic-base ablators are their high heat shielding capability and low thermal conductivity, however they are limited in accommodating very high heat loads due to the high erosion rates. Most TS plastics and highly crosslinked plastics (especially those with aromatic ring structures) form a hard surface residue of porous carbon. The amount of char formed depends upon various factors:

1. Carbon-to-hydrogen ratio present in the original plastic structure.

2. Degree of crosslinking and tendency to further crosslink during ablative heating.

3. Presence of foreign elements like the halogens, asymmetry and aromaticity of the base plastic structure.

4. Degree of vapor pyrolysis of the ablative hydrocarbon species percolating through the char layer.

5. Type of elemental bonding.

The behavior of the char during flight is pertinent to its success as an ablative material. Once the carbonaceous layer forms, the primary region of pyrolysis gradually shifts from the surface to a substrate zone beneath the char layer. The newly formed char structure is attached to the virgin substrate material and remains thereon for at least a short period of time. Meanwhile, its refractory nature serves to protect the temperature-sensitive substrate from the environment.

Gaseous products formed in the substrate pass through the porous char plastic layer, undergo partial vapor phase cracking, and deposit pyrolytic carbon (or graphite) onto the walls of the pores. As the organic plastic or its residual char are removed by the ablative aspects of the hyperenvironment, the reinforcing fibers or particle fillers are left exposed and unsupported. Being vitreous in composition, they undergo melting. The resultant molten material covers the surface as liquid

Plastics and other high temperature performance materials (Courtesy of Plastics FALLO).

Ablative Plastics

Elastomer

Ceramic

Metal

Polytetrafluoroethylene

Silicone rubber filled

Porous oxide

Porous refractory

with microspheres

(silica) matrix

(tungsten infiltrated

and reinforced with

infiltrated with

with a low melting

a plastic honeycomb

phenolic resin

point metal (silver)

Epoxy-polyamide resin

Polybutadiene-acrylo-

Porous filament

Hot-pressed

with a powdered oxide

nitrile elastomer

wound composite

refractory metal

filler

modified phenolic resin

of oxide fibers and an

containing an

with a subliming powder

inorganic adhesive, impregnated with an organic resin

oxide filler

Phenolic resin with

Hot pressed oxide,

an organic (nylon),

carbide, or nitride

inorganic (silica), or

in a metal

refractory (carbon)

honeycomb

reinforcement

Precharred epoxy

impregnated with a

noncharring resin

Major property Of interest

Type of plastics

Propulsion system application

Ablative

Phenol-formaldehyde

Charring resin for rocket nozzle

Chemical resistance

Fluorosilicone

Seals, gaskets, hose linings for liquid fuels

Cryogenic

Polyurethane

Insulative foam for cryogenic tankage

Adhesion

Epoxy

Bonding reinforcements on external surface of combustion chamber

Dielectric

Silicone

Wire and cable electrical insulation

Elastomeric

Polybutadiene-acrylonitrile

Soli propellant binder

Power transmission

Diesters

Hydraulic fluid

Specific strength

epoxy-novolac

Resin matrix for filament wound motor case

Thermally nonconductive

Polyamide

Resin modifier for plastic thrust chamber

Absorptivity : emissivity ratio

Alkyd silicone

Thermal control coating

Gelling agent

Polyvinyl chloride

Thixotrophic liquid propellant

droplets, irregular globules, and/or a thin film. Continued addition of heat to the surface causes the melt to be vaporized. A fraction of the melt may be splattered by internal pressure forces, or sloughed away when acted upon by external pressure and shear forces of the dynamic environment.

Thermoplastic and elastomeric plastics tend to thermally degrade into simple monomeric units with the formation of considerable liquid and a lesser amount of gaseous species. Littic or no solid desired residue generally remain on the ablating surface. Elastomeric-base materials represent a second major class of ablators. They thermally decompose by such processes as depolymerization, pyrolysis, and vaporization. Most of the interest to date has been focused on the silicone plastics because of their low thermal conductivity, high thermal efficiency at low to moderate heat fluxes, low temperature properties, elongation of several hundred percent at failure, oxidative resistance, low density, and compatibility with other structural materials. They are generally limited by the amount of structural quality of char formed during ablation, that restricts their use in hyperthermal environments of relatively low mechanical forces.

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