Throat Insert

Six groups of materials have been evaluated as nozzle throat inserts:

(1) Reinforced plastics

(2) Polycrystalline graphite

(3) Pyrolytic graphite and pyrolytic graphite codeposited with silicon carbide

(4) Refractory metals

(5) Carbon/carbon composites

The first four materials are in common usage, and the carbon/carbon composites (also called fibrous graphites or prepyrolyzed composites) are in advanced development. The ceramics, however, have poor thermal-shock characteristics; currently they are not considered by nozzle designers to be suitable for solid rocket application. Only the first five therefore are treated in the following discussion.

Reinforced plastics. — Reinforced-plastic throats are treated in the general discussion of thermal liners (sec. 2.2.2.2). For clarity, the material at the throat that forms the gas boundary will be referred to herein as the throat insert even if it is made of reinforced plastic.

Polycrystalline graphite. — The polycrystalline graphites (also called bulk graphites or monolithic graphites) are relatively inexpensive materials formed by either compression molding or extrusion. The fine-grain grades are used in many nozzle designs in limited areas because of their relatively low cost, high erosion resistance, and the unique characteristic (shared with pyrolytic graphite and carbon/carbon) of becoming significantly stronger as temperature increases (up to about 4500° F).

However, the relatively low strength of polycrystalline graphite requires that relatively thick sections be used or that the sections be well supported structurally. These restrictions limit the use of polycrystalline graphite in flight-type nozzles to throats, throat approaches, throat extensions, and blast tubes. However, polycrystalline graphite often is used in all sections of small test nozzles. Furthermore, nozzles for propellant or grain-design test < motors and some small operational motors often consist of nothing more than a nozzle shape machined from a cylinder of graphite held in a flanged steel shell (figs. 11 and 28).

Segmented Grains Rocket
Figure 28. - Nozzle machined from polycrystalline-graphite cylinder.

Failures of polycrystalline graphites in nozzles usually have occurred in the early part of the firing when the graphite surface first becomes hot and the backside is still cool. Graphite is relatively brittle, and the thermally induced stress frequently cracks the material, particularly if the graphite is not uniformly supported along its length. Furthermore, graphite often does not crack cleanly. The cracks tend to propagate spirally through the material, resulting in severe fracturing that usually leads to ejection. When the graphite has cracked and has not been ejected, segmenting the graphite at the crack location often has cured the problem in subsequent tests, as illustrated in figure 29. Figure 29(a) shows the cracked graphite after a test run. Segmenting the aft section of graphite before the next test corrected the problem (fig. 29(b)). Reference 32 discusses a similar example.

Sections of graphite therefore often are segmented axially into rings; ring cross sections varying from square to a 2:1 rectangle (axial length:radial thickness) are typical. Axial

(a) Nozzle with circumferential crack in aft graphite section

Solid Rocket Nozzle

(b) Redesigned nozzle with crack eliminated by segmenting the aft section into rings

Figure 29. - Prevention of thermal cracking of graphite by segmenting the graphite section into rings.

segmentation reduces stress levels and allows better escape of gases pyrolyzed from charring backup insulators. Failure of one nozzle was attributed to collapse from the external pressure built up by the gas released from the insulator (ref. 33). Segmentation of the graphite and provision of gas bleedoff paths eliminated the problem.

Analysis of poly crystalline graphite to predict cracking is imperfect because of (1) the lack of accurate high-temperature properties, (2) the wide variation in the material from piece to piece and within pieces, and (3) the lack of well-established failure criteria. Such analysis is often impractical because of the usually prohibitive expense of exacting plastic analysis. With increased size, the variability within a single piece increases, strength decreases, and NDT becomes more difficult, so that confidence in the survivability of large-diameter inserts is much less than the confidence with small-diameter inserts (refs. 34 and 35). Among npzzle designers, confidence in the successful use of polycrystalline graphite as a,, throat insert drops sharply if the inside diameter exceeds 12 in.

A further reason for limiting the use of graphite is its relatively high thermal diffusivity. Except in firings of very short duration, the graphite outer surface is at a high temperature throughout most of the firing. Most designs require a substantial thickness of insulation behind the graphite to drop the temperature to an acceptable level at the interface with the support structure. The relatively high coefficient of thermal expansion requires special design consideration. Provisions for the thermal growth of graphite in the axial direction relative to adjacent materials must be made. Gaps filled with an elastomeric material or other material that breaks down at low operating temperature are provided to allow for the graphite thermal growth in the axial direction. Inadequate allowance or no allowance for thermal growth has been a cause of nozzle failure.

Designers usually provide also for growth in the radial direction, since such growth (or restraint of it) can significantly affect stresses in the insert. Two methods are in use (fig. 30). One technique is to provide a cylindrical annulus behind the cylindrical graphite rings; the annulus may or may not be filled with a material such as that used in the axial gaps. The other method is to shape the back of the graphite rings as a ramp with the greater diameter at the forward end and support the rings with a matching ramp of insulation. The axial gap is placed at the forward end of the graphite-ring pack. The graphite-ring pack then thermally grows forward on the ramp into the axial gap. An additional advantage of this technique is that pressure forces on the rings push them against the ramp, thereby providing a pressure seal behind the insert.

One facility reports that the thermal-shock sensitivity of graphite is reduced by coating the inner surface with zirconium oxide (ref. 36); however, coating is not a general practice in industry.

References 37 through 40 discuss the effects of processing on graphite properties. References 41 through 43 discuss erosion characteristics of graphite.

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