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SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

TREND S AN D F U T U R E D E V E L O P M E N T S IN A E R O S P A C E M A T E R I A L S

Le o E. Gatzek and Jame s L. H. P e ck A e r o s p a c e Corporation

Spacecraft Design and P e r f o r m a n ce Departmen t E l Segundo, California

A B S T R A C T

This d i s c u s s i on treats the current state of the m a t e r i a ls art and the developmental trends indicated for the period to

1970. S t r e s s es and skin temperatures of manne d v e h i c l es can b e expected to i n c r e a se as v e h i c le operations evolve kinet-

ically f r om M e r c u ry orbital m i s s i o ns through Dyna Soar b o o s t- glide r e - e n t ry and eventual lunar r e - e n t r y. Man y d e s i r a b le an d promising m a t e r i e ls for load-bearing and nonstructural use under the natural and induced environments of a e r o s p a ce operations s i m p ly cannot be produced yet in the d e s i r ed quan›

tities or shapes. B e c a u se of a lack of oxidation-resistant alloys and suitable coatings for the r e f r a c t o ry m e t a l s, their u se has been r e s t r i c t ed to nonstructural applications. The availability of s t r u c t u r a l - g r a de uniform alloy sheet is s t i ll not optimu m for molybdenu m and columbinum a l l o y s. Tungsten an d tantalum alloy sheet of usable s i z es is not generally a v a i l› able. The behavior under conditions of s t r e s s, t e m p e r a t u r e, an d oxidizing atmosphere is not too w e ll understood and r e›

quires considerable study. Behavior with coatings under these conditions has not been investigated sufficiently to formulate design concepts. Graphite is not currently available in the n e c e s s a ry s i z es or uniformity for immediate application as a structural m a t e r i a l; the need is shown for r e f r a c t o ry carbide development . C e r a m i cs do not presently p o s s e ss the r e q u i› site properties for immediate u s e.

A general evaluation of promising new m a t e r i a ls includes superalloys such as Nicrotung, Rene 4 1, Udime t 700 and Inco 7 1 7 C . P r o j e c t ed improvement s for r e f r a c t o ry m e t a ls are e x›

amined , together with brief d i s c u s s i o ns of c o m p o s i t e s, p l a s› t i c s, f o a m s, metal f a b r i c s, filament-wound s t r u c t u r e s, and the u se of m e t a ls as f u e l s. Coatings, e l a s t o m e r s, and adhe- sives a re d i s c u s s e d. Attention is a l so directed towards the cryogenic behavior of both m e t a ls and nonmetals.

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Introduction

Futur e a e r o s p a ce operations -- through the sensible atmospher e into vacuum space and return -- w i ll deman d both the innovation of new m a t e r i a ls and the refinement, adaptation, or combination of man y currently available m a t e›

r i a l s. The nature and scope of the a e r o s p a ce m i s s i on w i ll determine the performance and equipmen t specifications. The equipmen t requisites w i l l, in turn, dictate the m a t e r i al r e›

quirements. The environmental p a r a m e t e rs to be encountered -- and survived -- w i ll v a ry with different types of m i s s i o n s, each of which w i ll i m p o se upon the vehicle a certain range of kinetic conditions.

Th e natural environments and the induced environments impose d by m i s s i on performance b e c o m e interactive f r om the momen t fueling of the vehicle stages c o m m e n c e s . The d e g r ee to which vehicle operating loads, temperature gradients and hea t flux, c o r r o s i on and oxidation, radiation effects, acoustic f a c t o r s, and other considerations interact will v a ry in rate an d amoun t with the type of operation. F i g u re 1 shows, for example , how projected average h o t - s i de skin temperatures of manne d vehicles can be expected to i n c r e a se as operations evolve kinetically f r om Dyna Soar b o o s t - g l i de to orbital r e - e n t ry and through lunar r e - e n t r y. Tactical v e h i c l es m a y experience even higher deceleration m a x i m a and heat flux.

This d i s c u s s i on treats the current state of the art for m e t a ls and nonmetals and the development trends indicated for the period to 1970. It is now obvious that meaningful m a › terials data mus t be available before commencemen t of the engineering of spacecraft. M o r e o v e r, production technology for new m a t e r i a ls generally lags f r om two to four y e a rs b e›

hind the technological advances mad e during development.

B e c a u s e of this, lead t i m es for m a t e r i a ls procuremen t mus t b e extended. Man y d e s i r a b le and promising m a t e r i a ls proved in r e s e a r ch and development activities s i m p ly cannot be p r o›

duce d yet in the d e s i r ed quantities or shapes.

A n example can be seen in the c a se of the solar c e l ls used in the Explorer VI and Pioneer V paddlewheel s a t e l l i t e s. (1) Boron-diffused silica c e l l s, coated with a 3 - m il layer of filter g l a ss w e re ordered f r om two s o u r c e s. Sample batches worked wel l in the S TL lab. But procuremen t of a sufficient numbe r of these fragile c e l ls that would meet quality control specs posed a serious problem. Both satellites w e re launched with only enough c e l ls on hand, in each c a s e, for a single, spare solar paddle. The two launches w e re seven month s apart. As another example, the a u s - f o r m ed steels developed in r e s p o n se to the demand s for ultra-strength m a t e r i a ls (2) a re still in the

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Figure 1. Temperature Trends for Manne d Re-entry Vehicles.

LOZ AVERAG E HO T SID E SKI N TEMR , F 8000

6000 4000 2000

1000 800 600

400 I960 1965 1970

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O R B I T AL R E - E N T RY

L U N AR

R E - E N T RY

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developmenta l stage; pilot work has not yet been translated into production r e q u i r e m e n t s.

Direction of Effort

In an effort to examine current trends and future d e v e l o p›

m e n t s , it is instructive to contemplate briefly s o me of the past an d present developmental trends. A l m o st without exception, surveys (3) have indicated that the m o st pressing needs a re 1) m o re complete understanding of the space environment in whic h the m a t e r i a ls mus t survive, and 2) comprehension of the nature of a specification before extensive m a t e r i a ls d e v e l› opmen t is undertaken. There have been instances of m i s u se of m a t e r i a ls by d e s i g n e r s, and c a s es in which p r o g r a ms w e re conducted at a high level of effort only to be abandoned even›

tually because the f i r st statement of the p r o b l em proved to be erroneous. (4)

Th e design of r e - e n t ry vehicles in the 1 9 5 0 ’s is a c a se in point. Typical ways of expressing the c h a r a c t e r i s t i cs required for nose cone m a t e r i a ls w e re founded on the assumption of equilibrium conditions. By this postulation it could be shown ho w temperatures at, or exceeding, the melting point of any know n m a t e r i al w e re to be expected. Exhaustive m a t e r i a ls

studies w e re initiated, but the p r o b l em was not r e s o l v ed by this approach.

Concurrently, a c r i t i c al appraisal of the thermal environ›

men t during r e - e n t ry led to the conclusion that transient heating was the design consideration. (4) Calculations of the magnitud e of the heat pulse w e re verified by experimentation, (5) and a mean s of simulating the heat pulse was developed.

With these test methods, it b e c a m e p o s s i b le to s c r e en man y promising m a t e r i a ls and to utilize existing m a t e r i a ls for the

solution of the nose cone r e - e n t ry problem. (4)

A decade later, a somewha t s i m i l ar p r o b l em is posed by the need for negotiating the a e r o s p a ce environment. W e do no t yet understand the properties required of m a t e r i a ls well enought o guide development p r o g r a m s. (4) A ll aspects of the space environment that could effect the behavior and p e r f o r m›

anc e of m a t e r i a ls require clarification for the creation of evaluation techniques.

Considerable difference of opinion exists at this time as to the state of our m a t e r i a ls efforts. While s o me highly r e›

garded m a t e r i a ls people feel that our p r o g r a ms a re p r o g r e s s› ing fairly well and in the proper direction, others a re of divergent opinion:

" T h e re is wasteful work on exotic refractory m e t a ls and overlapping r e s e a r ch activities in certain of the newer metal

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applications. . . There a re waves of fashionable r e s e a r ch which , unfortunately, lead to insufficient r e s e a r ch on older m a t e r i a l s .¹¹ (6)

O n the other hand, there is considerable evidence that the d i s c o v e r y, development, or adaptation of new m a t e r i a ls could conceivably lead to a breakthrough that would advance m a rk - edly the state of the art in a e r o s p a ce m a t e r i a l s. The e m e r›

genc e of germanium helped advance the field of e l e c t r o n i c s. In certain specialized f i e l d s, titanium is providing the founda›

tion for a whole new light m e t al technology comparable to that provided by aluminum . Z i r c o n i um has been transformed f r om a laboratory curiosity into a vital substance for atomic power technology. The entire nuclear p r o g r am was founded on two m e t a l s: uranium, a r e l a t i v e ly r a re and unknow n curiosity in the 1 9 4 0^!s; and plutonium, an entirely m a n - m a d e c h e m i c al element. (7)

S i m i l a r l y, the design of a t a i l o r - m a de m a t e r i al for s o me specific application holds both challenge and reward for the m a t e r i a ls engineer. The revolutionary t r a n s i s t or depended upo n the development of an appropriate semiconductor. Cubic boron nitride and the artificial diamond a re the direct out›

growth s of m a t e r i a ls s c i e n c e. (2)

It i s, then, obvious that activity is justified in both d i r e c› tions. Adaptation a n d / or combination of existent m a t e r i a ls mus t be pursued v i g o r o u s l y. And the innovation of new special application m a t e r i a ls mus t be encouraged. The level of effort in either direction will be determined l a r g e ly by m i l i t a ry and civilian a e r o s p a ce m i s s i on r e q u i r e m e n t s. It will be d e t e r› mine d a l so by the development of evaluation techniques which a re based on a broader fundamental understanding of m a t e›

r i a ls and their behavior in c o m p l ex a e r o s p a ce structures and components .

A t the current state of the a r t, three general approaches a re being taken in the design of spacecraft to insure structural integrity. M a t e r i a ls people a re seeking, f i r s t, to u se r e f r a c› tory m e t a ls in a r e as of e x t r e m e ly high t e m p e r a t u r e, despite the relatively difficult fabrication p r o b l e m s. Certain d e s i r› able property i m p r o v e m e n t s a re indicated in figure 2 for the m o s t readily available m a t e r i a l s. The second approach seeks to devise cooling techniques through which t e m p e r a t u r es can b e maintained within the capabilities of superalloys or other alloys of lower melting point. (8) The third approach is to establish the vehicle g e o m e t r y, maneuvering p r o c e d u r e, and operating r e g i m e - - s u ch as the Dyna Soar r e - e n t ry c o r r i d o r -- whic h p e r m i ts temperature reduction to the d e g r ee where superalloys can be employed.

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Refractory Metals

Th e application of refractory metals in both h i g h - M a c h- numbe r aircraft and r e - e n t ry v e h i c l es has been confined largely to d i s c u s s i on of temperatures between Z 0 0 0 F and 4 0 0 0 F, the c o r r i d or which is created by 5 - d e g r ee and 5 0-

degree r e - e n t ry angles of attack, r e s p e c t i v e l y. Below 2 0 0 0 F, the lighter weight superalloys a re suitable; the heavier r e f r a c› tory m e t a ls can remain useful up to 4 0 0 0 F to 5 0 0 0 F.

F o r the stagnation area on a nose cone, r e - e n t ry t e m p e r›

atures of 4 0 0 0 F - 5 0 0 0F will require tungsten, tantalum, c o m p o s i t e s, and reinforced p l a s t i c s. H e r e, heat r e s i s t a n ce or heat dissipation is the p r i m a ry requirement, with strength of only secondary consideration. Leading edges and other v e ry hot a r e as of the structures will develop temperatures whic h can be accommodate d by molybdenu m or columbium in the p r i m a ry structure. (8)

A t present, only tungsten, tantalum, molybdenum , and c o l u m b i u m - - a n d their alloys -- remain for potential applica› bility for structures designed to operate above 2 0 0 0 F. On a

strength-to-weight b a s i s, the lighter weights of columbium an d molybdenu m provide superiority to tungsten and tantalum in the 2 0 0 0 F to 2 5 0 0 F r e g i m e. Above this range, m o l y b›

denu m and tantalum a re c o m p a r a b l e, with tungsten superior in a ll r e s p e c ts up to its useful limit of a l m o st 4 0 0 0 F. (8)

Lac k of high temperature oxidation r e s i s t a n ce is a c r i t i› cally limiting factor in the use of r e f r a c t o ry m e t a l s. C o l u m › bium and tantalum a re superior to tungsten in this c h a r a c t e r› i s t i c. They a re far better than molybdenum , which oxidizes so rapidly at 1 7 0 0 to 1 8 0 0 F that its dense, bluish smok e literally obscures shop a r e as when the metal is being h o t- worked . However , the oxidation rate of all four m a t e r i a ls is too high to p e r m it their use without a coating in r e - e n t ry v e›

h i c l e s. Inasmuch as r e - e n t ry conditions include the probability that temperatures created by the f i r st bounce will be in the neighborhood of 4 0 0 0 F, and since the limit of presently d e v e l› oped coatings is approximately 3 0 0 0 F, great strides in coating improvement s a re n e c e s s a ry if the full potential of r e f r a c t o ry metals is to be r e a l i z e d. (9) High priority e x p e r i› ment s a re underwa y with molybdenu m disilicide coatings and other p r o c e s s es in connection with the Dyna Soar p r o g r a m.

Anothe r significant design criterion that mus t be e s t a b› lished for the individual vehicle and m i s s i on is the ultimate tension allowable, based on the temperatures encountered.

Man y mechanical properties of the r e f r a c t o ry m e t a ls a re p r e›

sented in t e r ms of the strain-hardened (cold-worked) condition. However , these strength levels ma y never be utilized, as the

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uz

Figure 2. Projected Improvement s of Refractory Metals.

TEMPERATURE F

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e x t r e m e heat of the f i r st r e - e n t ry m a n e u v e r - - e v en though it b e of short d u r a t i o n - - is sufficient to r e c r y s t a l l i ze the m i c r o- structure and reduce the allowable strength by as muc h as 50 percent. A molybdenu m 0 . 5 - p e r c e nt titanium alloy, for e x›

a m p l e , can reach an ultimate tensile strength level of 115, 000 p s i, with 50 percent reduction by rolling. However , after a short exposure at 2 5 8 0 F the ultimate strength in tension is l e ss than 2 5, 000 p s i.

Thus , the magnitude of the high temperatures during the f i r st r e - e n t ry bounce, for example, can completely r e c r y s t a l› l i ze the thin gages in even a few seconds. Representative m a t e r i al thicknesses a re 0.008 inch and 0.010 inch for the majority of applications.

Superalloys

It is in the temperature range of 1 5 0 0 to about 2 0 0 0 F that the n i c k e l - b a se or c h r o m e - b a se superalloys have been widely adopted b e c a u se of their ability to withstand high

s t r e s s es for sustained periods of t i me at these t e m p e r a t u r e s, an d because superalloys a re available in a wide v a r i e ty of the required f o r m s. This, together with the experience a c c u m u › lated with these a l l o y s, indicates that superalloys w i ll b e c o m e increasingly important in a e r o s p a ce technology during the next few y e a r s. The r e f r a c t o ry m e t a l s, as indicated p r e v i o u s l y, a re still not in the l a r g e - s c a le production stage.

A significant expansion of the operating limits of s u p e r- alloys has taken place with the development of m o re than a dozen new n i c k e l - b a se and c o b a l t - b a se m a t e r i a l s. Certain of these alloys a re fabricable into sheet, while others a re s u i t› able only for p r e c i s i on casting.

Th e new n i c k e l - b a se superalloys have muc h in c o m m o n insofar as composition is concerned. A ll consist b a s i c a l ly of a n i c k e l - c h r o m i um solid solution, to which have been added various amount s of tungsten (8%), molybdenu m (4 to 10%), and columbiu m (2%). The latter a re used to strengthen the m a t r ix solid solution and to participate in carbide formation. Except for Inconel " 7 13 C ," all the new n i c k e l - b a se superalloys a l so contain appreciable amount s of cobalt, which i n c r e a s es the c r e ep strength of precipitation-hardenable, n i c k e l - b a se a l l o y s; on e (Unitemp 1753) contains 9.5 percent iron. Precipitation hardening, which contributes to the h i g h - t e m p e r a t u re strength, is brought about by the p r e s e n ce of aluminum (1.5 to 7.5%) and titanium (0.75 to 4%). Boron and zirconium a re present in s e v e r al of the newer superalloys and appear to have a b e n e f i› cial effect on the h i g h - t e m p e r a t u re strength and ductility. In order for the alloys to be workable however, limitations have been placed upon the percentages of titanium, aluminum , zirconium, and boron present. (10)

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Figure 3. Operating Temperature for 100-Hour Rupture Time and 15, 000-psi Stress Level for Various Superalloys.

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Th e new c o b a l t - b a se superalloys contain approximately 1 9- to 2 5 - p e r c e nt chromium ; this, together with the carbon content, yields a high-strength b a se m a t e r i al hardened by a carbide-precipitation m e c h a n i s m . Nickel is usually included in the a l l o y s. At least one of the h i g h - m e l t i n g, c a r b i d e- forming elements such as tungsten, molybdenum , tantalum, an d columbium is present in various amounts. Boron addi›

tions to the S - 8 16 alloys have yielded improved h i g h - t e m p e r a›

ture mechanical p r o p e r t i e s, while titanium is an important addition in the wrough t alloys J - 1 5 70 and J - 1 6 5 0. (10)

F i g u re 3 illustrates how operating temperatures have been i n c r e a s e d, based on a 100-hour rupture t i me at 15, 0 0 0 - p si s t r e s s. It should be noted that the properties of the casting alloys a re for the a s - c a st condition, the condition in which they a re n o r m a l ly used. S i m i l a r l y, the workable alloys a re delineated in the h e a t - t r e a t ed condition. These r e p r e s e n t a› tive properties a re for illustrative purposes only.

H i g h - N i c k el Steels

Th e new 2 0- to 2 5 - p e r c e nt nickel iron alloys with s t e e l› like c r y s t al structures p o s s e ss great potential for solid p r o- pellant casings and p r e s s u re v e s s el construction. These s t e e ls currently exhibit uniaxial yield strength approximating 2 8 0, 000 psi; they a l so indicate a m e a s u r a b le toughness s u p e›

rior to that demonstrated by m o st of the presently employed high-strength s t e e l.

High-Strength Titanium A l l o ys

Curren t and projected analyses of the alpha, alpha-beta, an d beta of the titanium alloys indicate considerable p r o m i se for solid propellant c a s i n g s. Effort is being extended on the improvemen t of s o l i d - s t a te structures concurrent with c o n›

stituent modification for the purpose of refining m i c r o s t r u c- t u r e s, increasing the working strength, and improving weld ability. Heat treatments a re a l so being studied which p r o m i se to improve mechanical properties that a re not attainable at the present state of the a r t.

F r o m the 190, 0 0 0 - p si tensile strength available f r om the present 6 - a l u m i n u m / 4 - v a n a d i um titanium a l l o y, it m a y be e x›

pected that 2 5 0, 0 0 0 - p si ultimate tensile strength can be d e v e l› oped f r om an a l l - b e ta titanium alloy in 1965, approximately 2 9 0, 000 psi by 1968, and probably 315, 000 psi by 1970.

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High-Strength Aluminum A l l o ys

Recen t investigations by Alcoa have resulted in the developmen t of C u - M g - Zn alloy which exhibits s t r e n g t h - t o- density ratios of the order of 1 million inches. ( T h i s, of c o u r s e, is based on an ultimate tensile strength level of a p›

proximately 100, 000 psi and a density of 0.10 pounds per cubic inch.) The alloy at present offers limited weldability. Further development effort w i ll provide high weld-joint effi› ciencies and perhaps a 125, 0 0 0 - p si ultimate strength.

Coatings

In c o m m o n with other metallic e l e m e n t s, W , Ta, Cb, and M o display a considerable affinity for oxygen at elevated t e m›

p e r a t u r e s. Thus, the limiting factor in the c a se of m o st r e f r a c t o r i es is the availability of coatings which will protect the m a t e r i al f r om oxidation at high temperatures and f r om c o r r o s i on at r o om t e m p e r a t u r e. Developmen t of such coatings will enable us to take advantage of the full useful strength ranges of r e f r a c t o ry m e t a l s. Of the various techniques by whic h structural m e t a ls can be c o a t e d - - n a m e l y, aluminum hot-dipping, m e t al spraying, e l e c t r o - p l a t i n g, e l e c t r o l e ss plating by c h e m i c al reduction, and vapor p l a t i n g - - it appears that coatings f o r m ed by vapor deposition might be particularly d e s i r a b le because the temperature at which the coating is applied m a y be actually higher than that which the structure will reach in u s e.

F o r relatively narrow limits of radiation environment and internal heat generation l o a d s, it has been p o s s i b le to control the a v e r a ge equilibrium temperature within a closed vehicle (wher e thermal conduction and convection is negligible) by providing a surface of proper " c o l o r ," or e m i s s i on and r e f l e c› tion c h a r a c t e r i s t i c s. Variations can be mad e to a limited extent in this color by the use of movable panels or vehicle orientation relative to radiation s o u r c e s. (11)

T h e r m o c h r o m i c pigments, whose c o l o rs change with t e m›

perature, a re known ; a properly compounde d mixture of such substances might automatically provide a control s y s t e m.

Alternatively, application of mechanical, e l e c t r i c a l, c h e m i c a l, or t h e r m al signals to suitably sensitive colored m a t e r i a ls could be mad e f r om within the v e h i c l e.

Th e theory of the equilibrium temperature of a body in space as a function of radiation flux is w e ll understood, and data a re now being accumulated f r om s a t e l l i t e s, space p r o b e s, an d a s t r o n o m i c al observations as to radiation flux patterns in the a e r o s p a ce r e g i m e.

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Developmen t is required now of temperature control coatings with variable reflective and e m i s s i ve c h a r a c t e r i s t i cs for control of temperatures in orbiting v e h i c l es containing variable heat generating d e v i c e s. Such control should be maintained within the limits of 5 9 to 8 6 F through automatic or independently controlled variation of the reflective and e m i s s i ve c h a r a c t e r i s t i cs of the coating m a t e r i a l. (11)

Meta l F a b r i cs

Th e use of metal woven fabrics has interesting p o s s i b i l i› ties for a e r o s p a ce applications. W i re m e sh shows p r o m i se for spaceship antennas, s o l ar c o l l e c t o r s, r e - e n t ry s t r u c t u r e s, drag b r a k e s, and certain inflatable, or expandable, s t r u c t u r e s.

F o r these representative applications, a m a x i m u m t e m›

perature of 1 5 0 0 F s e e ms adequate at this t i m e. One manu › facturer selected Rene 41 as m o st promising, as considerable w i r e - d r a w i ng experience was available and fabrication m e t h›

od s w e re not radically difficult. It appears that m e t a ls such as Udime t 700 ma y replace Rene 41 in s o me applications as m o r e fabrication experience is obtained with s u p e r a l l o y s. (12)

Ren e 41 w i re of 0 . 0 0 1 6 - i n ch diameter has been woven into 200 200 count cloth (warp f i ll count per inch) in plain, twill, and basket w e a v e s. (This represents the finest w e a v e, s i m i l ar to p e r c a le sheet or d r e ss shirt m a t e r i a l .) R e p r e s e n›

tative r e - e n t ry type tests have been conducted on these m a t e›

rials with suitable surface coatings. The tear strength of this wove n cloth is greater than that of stainless steel shim stock of three times the c r o s s - s e c t i o n al a r e a. The weave weighed approximately 13.60 ounces per square yard; this included a coating weight of 8.0 ounces per square yard applied to one side. The coating of a m e t al cloth ma y provide protection f r om oxidation. The coating m a t e r i al mus t have good adhesion to the basic m a t e r i al under static and dynamic conditions and good flexibility to facilitate packaging when required. Media such as metal powders, c e r a m ic m a t e r i a l s, powdered a l u m i›

nu m and antimony, carbon black, and iron oxide can be added to the base e l a s t o m e rs for improvemen t of h i g h - t e m p e r a t u re p r o p e r t i e s. (12)

Refractory metals ma y a l so find application in this area whe n properly coated. Recent experiments indicate that tungsten w i re filaments or m e sh can be protected against oxidation in stagnant or moving air at temperatures up to 3 0 0 0 F for periods as long as 20 minutes. This was achieved b y applying a multilayer coating of chromium , silicon, and

rhodium . It has been suggested that further improvemen t can b e mad e if complete diffusion alloying, bonding, and melting of the surface coat is achieved over the entire length of the specimen . (13)

216

(13)

LlZ

OXIDE SUBLIMATION OR ENERGY CONTENT BOILING TEMP

IOOO BTU/LB OXIDE

^e

F MOLECULAR WEIGHT OF COOLED PRODUCT

HYDROGEN 6.8 212 18

LITHIUM 8.5 4220 30

BERYLLIUM 10.3 7280 25

BORON 7.8 4060 70

CARBON 3.8 -110 44

MAGNESIUM 6.4 5570 40

ALUMINUM 7.2 6380 102

SILICON 6.3 3590 60

F i g u re 4. M e t a ls As F u e l s.

(14)

Meta l Fuels

Th e use of m e t a ls as fuel constituents has r e c e i v ed considerable attention in recent y e a r s. Figure 4 shows the m o r e important m e t a ls which m a y be used as fuels. Of p a r›

ticular interest are the lighter m e t a ls including lithium, b e r y l l i u m, boron, magnesium , and aluminum . Although none ha s the heating value of hydrogen, s o me compound s of boron an d hydrogen have been developed as fuels that have better handling properties than either hydrogen or the elemental m e t a l. A m a j or difficulty with the boron-hydride c l a ss of fuels is the high fusion and vaporization temperatures of the oxide, resulting in liquid condensation in the working t e m p e r›

ature range. This condensation can have a deleterious effect in the thrust produced by these fuels when burned in rocket m o t o r s . (5)

Th e use of aluminum as a componen t of solid rocket fuels is now making possible the production of fuels with appreciably improve d specific i m p u l s e s. However , high m o l e c u l ar weight such as exhibited by aluminum oxide is not a desirable c h a r›

acteristic of a working propellant. Thus, the lighter m e t a ls lithium, b e r y l l i u m, and boron, and the hydrides of lithium an d boron m u s t be considered as solid-propellant constituents. (5)

F o a m e d M a t e r i a ls

F o a m e d plastics have demonstrated their high potential for a e r o s p a ce applications. These p o l y m e rs m a y be c a r r i ed to the deployment location in compact, powdered f o rm and expande d to man y times the original volume without any i n›

c r e a se in weight. In addition to the considerable rigidizing qualities, the foams offer p r o m i se as a m i c r o m e t e o r i te atten- uant; they can s e r ve as ablating heat shields and m a y prove useful as h i g h - e n e r gy impact absorbents or vehicle b u m p e r s . On e application features double-wall construction, with foame d m a t e r i a ls sandwiched between; the inner wall s e r v es as the load-bearing m e m b e r . The f o a m - i n - p l a ce s y s t e ms are favored, additionally, because of their ability to be foamed within, or outside, the vehicle in space after launch. P o l y- urethane r e s i ns capable of being " h e a t - t r i g g e r e d" to foam after a period of suspended animation at low temperatures are receiving major attention. Polyvinyls, p o l y e s t e r s, e p o x i e s, s i l i c o n e s, and phenolics have been designated for evaluation.

Translucent B e r y l l i um Oxide

Translucent b e r y l l i um oxide is one of the m o r e promising a e r o s p a ce m a t e r i a l s. Original interest in this c e r a m ic was

2 1 8

(15)

created by its application to nuclear r e a c t o r s. M o r e properly know n as polycrystalline dense b e r y l l i um oxide (BeO), this m a t e r i al has properties which mak e it desirable for use as a neutron moderator or reflector in high temperature reactor c o r e s. B eO is second only to graphite as the b e st heat c o n›

ductor of a ll - m e t a l s• The m a t e r i al is being considered for numerou s a e r o s p a ce applications in which c e r a m ic s t r u c› tural m a t e r i al would be subjected to high temperatures o c c u r›

ring simultaneously with high heat fluxes. The r e f r a c t o ry properties of dense B eO and its r e s i s t a n ce to c o r r o s i on in a gaseous environment mak e the m a t e r i al attractive for use as n o z z l es and other component s in rocket m o t o r s.

Th e transparency of B eO to m i c r o w a v e t r a n s m i s s i o n, combine d with its strength at high t e m p e r a t u r es and its r e s i s t› anc e to t h e r m al shock, mak e it particularly suitable for r a- dom e applications. B eO appears to be somewha t superior to alumin a in this application b e c a u se it has three to four t i m es the r e s i s t a n ce to t h e r m al shock while having equivalent strength and e l e c t r i c al p r o p e r t i e s.

Th e r e s i s t a n ce of B eO to c o r r o s i on by liquid m e t a ls s u g›

g e s ts potential u s es in various m e t a l l u r g i c al p r o c e s s i ng o p e r›

ations such as the vacuum melting of r e f r a c t o ry m e t a l s.

W h i s k e r s

Fine s i n g l e - c r y s t al f i l a m e n t s, c o m m o n l y r e f e r r ed to as

" w h i s k e r s ", are synthesized from various inorganic r e f r a c t o r› ies and can attain high elastic strengths approaching 4 m i l l i on psi, with correspondingly high modulu s of elasticity l e v e l s.

Th e use of whiskers in reinforcement of certain organic m a t e r i a ls can produce c o m p o s i t es with mechanical strength levels five t i m es greater than the strength of f i b e r g l a ss r e i n› forced p l a s t i c s. The p r o b l e ms a s s o c i a t ed with the m a s s p r o›

duction of w h i s k e rs a re c o m p l e x, as is the subsequent f a b r i› cation into composite components. It is estimated that c o m › m e r c i a l production of w h i s k e rs suitable for reinforcements in both organic or metallic m a t r i c es m a y not be achieved until the late s i x t i e s. The potential, however, is so great that i n c r e a s ed effort m u s t be expended to devise methods of synthesizing d e f e c t - f r ee long w h i s k e r s. It should be e m p h a › s i z ed that only those c r y s t a l l i t es which exhibit the highest strengths and elastic moduli are useful; no i n c r e a se in strength occurs in the composite unless the modulu s of e l a s› ticity of the fiber reinforcement exceeds that of the m a t r i x.

Som e p r o g r e ss has been mad e in the development of t e c h›

niques for m a s s producing AI2O3 w h i s k e r s. F i b e rs of other substances such as B 4 C , SiC, ZrC^, and MgO a re also under consideration.

2 1 9

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Composit e M a t e r i a ls

A n interesting trend is seen in the development of

synthesized composite m a t e r i a ls based on structural p l a s t i c s. Thes e m a t e r i a ls exhibit lightness, a re thermally insulative, an d have low erosion rates - - c h a r a c t e r i s t i cs which, for e x›

a m p l e , suit them admirably to rocket nozzle applications. Th e m a t e r i a ls a re b a s i c a l ly structural plastic substrates c o v›

ered, in turn, with porous insulators and e r o s i o n - r e s i s t a nt coatings.

Composit e s y s t e ms which have been found to be optimum consisted of an outer refractory surface l a y e r, then a v e ry thin metallic film; next cam e a porous insulating inorganic layer, and finally a structural plastic b a se m a t e r i a l. The properties of individual component s of the protective surfaces hav e been studied in subsonic and supersonic h i g h - t e m p e r a t u re gas s t r e a m s. With the aid of these experimental r e s u l t s, composite surfaces w e re formulated and subsequently exposed in the s a me high temperature environments. Results showed a significant lag in heat penetration into the substrate s t r u c› tural plastic during hyperthermal exposure. Tungsten-faced c e r a m i cs exhibited d e s i r a b le performance c h a r a c t e r i s t i cs in h i g h - t e m p e r a t u re reducing a t m o s p h e r e s; n i c k e l - c o a t ed z i r-

conia surfaces w e re optimum in h i g h - t e m p e r a t u re oxidizing environments. The principal p r o b l e ms encountered with the protective composite s y s t e ms w e re developing suitable m e t h›

od s of fabrication, and preventing separation of laminates during exposure to temperatures up to 5 4 0 0 F. (15)

Ablative P l a s t ic Chars

R e s e a r ch on ablative plastic chars has uncovered new i n›

formation on their physical structure. New data on the c h e m › ical m e c h a n i sm of the ablative p r o c e ss has a l so been r e v e a l e d. On e important ablative m e c h a n i sm is that s o me p l a s t i cs d e›

compos e into g a s es which undergo further decomposition into carbon and lower m o l e c u l a r - w e i g ht g a s e s. This carbon is deposited in the surface region of the char, reinforcing it and makin g it m o r e resistant to erosion. Results of this r e s e a r ch a re being used to synthesize new improved ablative p l a s t i c s.

(16)

Technique s have been developed for studying the m i c r o- structure of ablative plastic c h a r s. One method consists of impregnating the porous char with plastic so that its structure is permanently rigidized in its a s f o r m ed state. The i m p r e g›

nated char is surface-ground and polished, then magnified photograph s of the surface are made . F r om the photographs,

220

(17)

ιζζ DEGREE S

e

F(XIOO )

44

^ι

40 -

36 - 32 - 28 - 24 - 20 -

16 <Z>

12 - 8

^{2 2 2}

4 0 ’ I960

BALLISTIC

MISSILE APPLICATION S MANNED SPACE CRAFT SUPERSONIC J ET

AIRCRAFT ^ ^ - ^ ^

1970 YEARS

1965

F i g u re 5. E l a s t o m er Applications.

(18)

information on c e ll s t r u c t u r e - - i n c l u d i ng c e ll diameter and wal l t h i c k n e s s - - a re being obtained. These data a re then c o r›

related with ablation performance and m a t e r i a ls constructions. Investigation of ablative plastics and composites in s i m u›

lated propulsion combustion gas environments has provided insight in promising m a t e r i a ls constructions and revealed new concepts of t h e r m al protection. P l a s t i cs containing vitreous s i l i ca fibers oriented n o r m a l to the gas flow have outstanding r e s i s t a n ce to erosion. Phenolic r e s i ns with high carbon c o n›

tent and high c r o s s - l i n k i ng appear m o st p r o m i s i n g. The c o n›

cept of using highly gasifying plastics u p s t r e am of the throat section in nozzle s p e c i m e ns has been found in exploratory wor k to have considerable p r o m i s e.

Ne w concepts in l o w - e r o s i o n - r a te plastic c o m p o s i t es for r e - e n t ry environments have been investigated. One p r o m i s›

ing m a t e r i a l, containing carbon fiber reinforcement had e x›

t r e m e ly s m a ll dimensional l o ss and good surface and shape retention after exposure in a 15, 0 0 0 F air p l a s ma s t r e a m. (15)

E l a s t o m e rs and Compliant M a t e r i a ls

E l a s t o m e rs a re c l a s s i f i ed generally by c h a r a c t e r i s t i cs of elasticity as contrasted with their d e g r ee of b r i t t l e n e s s. Highe r temperature and shorter exposure trends continue in

e l a s t o m er applications. P r e s e nt organic e l a s t o m e rs a re u s e›

ful for approximately 100 hours at 5 0 0 F and for s e v e r al hour s at 6 0 0 F. Inorganic composite s e al m a t e r i a ls show p r o m i s e for use at 1000 d e g r e e s, but lack the high d e g r ee of r e s i l i e n ce and e a se of deflection d e s i r ed in an ideal s e al m a t e r i a l. (See F i g u re 5.)

Hig h temperature adhesion studies of the better heat resistant e l a s t o m e rs (namely r e s i n - c u r ed butyl, f l u o r o e l a s t o- m e r s , silicone rubbers) with the new wholly aromatic p o l y- amid e fiber (HT) have been v e ry encouraging. P r e l i m i n a ry evaluations have shown this e l a s t o m e r - f i b er combination to be superior with regard to high temperature p e r f o r m a n ce ( t e m›

perature range f r om 3 5 0 F to 5 5 0 F) to previously available e l a s t o m e r - n y l o n, g l a ss or w i re cord combinations. Such i m›

proved f i b e r - r e i n f o r c ed e l a s t o m er s y s t e ms would be e x t r e m e›

ly useful in applications such as flexible connectors, fuel containers, t i r e s, etc. Improved f l u o r o - e l a s t o m er and s i l i› con e rubber compound s have been developed which a re r e s i s t› an t to temperatures of 6 0 0 to 1 0 0 0 F for s h o r t - t i me exposures of five minutes or l e s s. (17) These e l a s t r o m e r ic m a t e r i a ls ma y prove especially useful under r e - e n t ry r e g i m e s.

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P l a s t i cs Developmen t

Efforts a re being directed toward synthesis of improved lightweight, t h e r m a l ly r e s i s t a nt insulating m a t e r i a ls for p r o›

pulsion s y s t em applications and u se on manne d hypersonic v e h i c l e s. P r e s e nt effort is directed towards conversion of p o l y m e r s - - w i th and without r e a c t i ve f i l t e r s - - to r e f r a c t o ry s p e c i e s. New r e s i ns with higher carbonization efficiency, and inorganic p o l y m e rs with m o l e c u l ar backbones of elements other than carbon a l so appear p r o m i s i n g. S e m i - r i g id ablative plastics with improved r e s i s t a n ce to rupture and c r a ck f o r m a›

tion appear to have potential for propulsion environments.

(See F i g u r es 6 and 7.)

Organi c and Inorganic Adhesives

Th e increasing needs for h i g h - s t r e n g t h, t e m p e r a t u r e- resistant structural adhesives require i n c r e a s ed activity in the field of inorganic m a t e r i a ls as a s o u r ce for componen t m a t e r i a l s. Principal current investigations a re centered on c e r a m i c, c e r m e t, inorganic p o l y m e r s, and m e t a l l o - o r g a n ic type b a se m a t e r i a ls and combinations thereof. A temperature of 2 0 0 0 F is considered as the target. F or l e ss s e v e re r e›

quirement s in the range of approximately 7 0 0 F, investigations a re under way on two promising organic p o l y m er s y s t e m s -- polyisocyanurates and p o l y m e r ic c h e l a t e s.

R e s e a r ch p r o g r a m s on the h e a t - s t a b i l i z i ng effects of v a r›

ious mettalic oxides as additives to adhesive formulations has resulted in an adhesive which displays target tensile shear strength (1000 psi) in s h o r t - t i me ( 1 0 - m i n u t e) exposures to 6 0 0 F and 750 p si after 200 hours at 6 0 0 F.

A t w o - p a r t, l o w - p r e s s u r e, a m b i e n t - t e m p e r a t u r e - c u r i ng adhesive s y s t em based on an epoxylated g l y c e r i n e - m o d i f i e d, a m i n e - c u r ed novolac epoxy was developed to meet p r i me t a r› get strength and pot life r e q u i r e m e n t s. One c o n t r a c t o r ’s c o n›

cept of using metallic chromates as f i l l er m a t e r i al was an entirely new approach to the p r o b l em of overcoming a h e r e t o› fore general susceptibility to salt spray exposures of eposy r e s i n - b a s ed adhesive m e t a l - t o - m e t al bonds. A d h e s i v es of this type are of particular need in the field repair of flight vehicle structures where the thermosetting adhesives a re not at hand.

Inorganic nonpolymeric ( c e r a m ic type) adhesives have been developed which in m e t a l - t o - m e t al bonds on stainless s t e el adherents display mea n tensile strength values of nearly 2000 psi at r o om temperature and c l o se to 1000 p si at 8 0 0 F.

Sandwic h panels with c o re facings of stainless s t e el a re show›

ing substantial flexure and edgewise c o m p r e s s i on strengths.

22J

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REINFORCED LAMINATE S ^A^HESIVES* "

^{F 0 A M}^S Figure 6. Plastic Materials for 0. 1-Hr Service.

f[ZZ TEMPERATUR E F

2500 2000 1500 1000 500 0

1965-70

1960-65

PRESENT

(21)

Bond s have been prepared with Iconel X as the adherent m e t a l, that display r o o m - t e m p e r a t u re tensile strengths in the order of 1800 p s i, with no appreciable weakening at temperatures to 1 1 0 0 F. W o r k is continuing along various approaches to d e›

velop adhesives which can be adapted to bonding still h i g h e r- t e m p e r a t u r e - r e s i s t a nt m e t a ls such as r e f r a c t o r i es to provide structurally sound bonded c o m p o s i t e s.

R e s e a r ch efforts on adhesives will continue to be p r i m a r›

ily directed toward attaining high temperature durability in m e t a l - t o - m e t al bonds. B a s ed on the m o re promising d e v e l o p›

ment s to date in the field of inorganic nonpolymeric, c e r a m ic type, a d h e s i v e s, efforts w i ll be continued in this a r e a. H o w › e v e r, work w i ll continue a l so in exploiting the fullest capabil› ities of organic p o l y m e r ic and m e t a l l o - o r g a n ic s y s t e ms as potential adhesives components.

P r o p e r preparation of adherent metal surfaces prior to adhesive application has been shown to be a n e c e s s a ry and integral factor in the ultimate strength and durability of a bonde d joint. This has been shown to be specific for both a given adhesive type and a given m e t al or alloy. Investigation of such factors a re being mad e in direct connection with the individual adhesive r e s e a r ch p r o g r a m s.

Particular emphasis in the investigations of c e r a m i c - t y pe adhesives w i ll be placed on obtaining bonds with r e s i s t a n ce to temperatures muc h higher than those required to mature or cure the bonds. This effort w i ll include investigations of a i r- setting adhesives and a mean s of developing m o i s t u re a b s o r p›

tion. Further r e s e a r ch w i ll be conducted toward developing formulations and p r o c e s s i ng techniques which w i ll impart ductility and toughness - - as opposed to b r i t t l e n e s s- - in the bonds .

Filamen t Windings

Recen t studies in the field of m a t e r i a ls have shown that large weight savings can be mad e through extensive use of filament windings as the b a s ic structural m a t e r i al for a e r o›

space s t r u c t u r e s, particularly p r e s s u r i z ed components.

Som e of the structural advantages of filament winding a re a high strength-to-weight ratio, c o r r o s i on r e s i s t a n c e, and lack of creep at working s t r e s s. The p r e s s u r e - v e s s el type s t r u c› ture can be woun d in such a manne r as to provide a ratio of allowable r a d i a l - t o - a x i al s t r e s s. This allows efficient design for radial loads due to burst p r e s s u r es and longitudinal loads resulting f r om both burst p r e s s u re and axial loads. This is p o s s i b le b e c a u se the filaments have uniaxial strength that can b e oriented in any required geometric pattern. Engineering metals are essentially isotropic in behavior, and thus do not

225

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923

TEMPRATUR E F 1500 1000

500 0 REINFORCE D LAMINATE S STRUCTURA L FOAMS

ADHESIVES

Figure 7. Plastic Materials for 1000-Hr Service,

1965-70

1960-65

PRESENT

(23)

exhibit m a x i m u m efficiency in any particular orientation; for example , the longitudinal direction of cylindrical p r e s s u re v e s s e l s.

Man y g l a s s - r e i n f o r c ed p l a s t i cs structures a re stronger on a unit-weight b a s is than m e t a l s. But putting the reinforced composite together in such a way that the p r i m a ry load-bearing m e m b e r s - - n a m e l y the g l a ss f i l a m e n t s - - a re used to m a x i m u m efficiency in the final structure is the a im of the filament windin g technique.

Th e fact that the individual filaments a re p r i m a r i ly loaded only unidirectionally in tension make s it p o s s i b le to design structures of m a x i m u m efficiency. F or example, in c y l i n d r i› cal p r e s s u re v e s s e l s, the ratio of hoop s t r e ss to longitudinal s t r e ss acting in the wall is 2 : 1.

In m e t al p r e s s u re v e s s el design, the benefit of the sphere over the cylinder is that the s a me s t r e ss is present in both d i r e c t i o n s. In designing cylinders in m e t a l, although the

longitudinal s t r e ss is only one-half the hoop s t r e s s, the wall thickness mus t be designed to withstand the hoop s t r e s s.

Consequently, since m e t a ls are e s s e n t i a l ly isotropic in strength, the product of operating p r e s s u re and v o l u me c a p a c›

ity (PV) per unit weight of m a t e r i al is l e ss in the cylinder than in the sphere for a given peak s t r e s s. (18)

In filament-wound c y l i n d e r s, however, the reinforcement is so oriented and proportioned that the hoop strength of the cylinder w a ll is actually twice the longitudinal strength.

Mos t of the work p e r f o r m ed to date on the l a r g e r, i r r e g› ularly shaped, woun d structures has been through d e v e l o p›

menta l trial and e r r o r. One of the l a r g e st p r i m a ry structures yet produced by filament winding was a 950-pound propellant tank 8 feet in diameter and 17 feet long.

A n e x t r e m e ly interesting m a t e r i a l, f r om the m a t e r i al engineer¹ s standpoint, is a f i l a m e n t - o r i e n t ed prepreg m a t e r i a l. Th e purpose of the m a t e r i al is to combine the preorientation of filament inherent in filament winding with the shape f l e x i b i l› ity inherent in molding flat p r e p r eg reinforced p l a s t i c s.

This m a t e r i al is mad e by winding impregnated g l a ss roving on a cylindrical m a n d r e l in a predetermined helix. Th e cylindrical structure thus produced is then slit axially, flattened and molded, or the r e s in can be ´ - s t a g e d. The m a › t e r i al can be molded by bag or matched m e t al techniques.

A t present, conventional E - g l a ss in the f o rm of roving is b y far the m o st c o m m o n l y used reinforcement in filament winding . But improvement s in both type and f o rm of r e i n f o r c e› men t p r o m i se future property i m p r o v e m e n t , as w e ll as i n›

c r e a s ed v e r s a t i l i ty in the p r o c e s s. Developmen t interest is no w centered on higher modulu s g l a ss reinforcement.

2 2 7

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9ZZ SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

Figure 8. Comparison of Steel V e r s us G l a ss Reinforcement for Filament Winding.

FILAMENT FILAMENT

LAMINATE

HOOP LONGITUDINAL VALUES

GLASS STEEL GLASS STEEL GLASS STEEL GLASS STEEL GLASS STEEL

ULT T EN S T R,

IOOO PSI ^{2 0 0} ^{4 5 0} 140 315 93 210 4 7 105 70 158

T EN MOD OF ELAST,

I 0⁶ PSI ¹⁰ ²⁹ ^{7.0 20.3}^4.67 13.5 2.35 6.6 3.5 10.2

T O T AL ELONGATION,

% ^{’ 2.0} ^1.6 2.0 1.6 2.0 1.6 2.0 1.6 2.0

DENSITY,

LB/CU IN. ^0.92 ^0.283 0.077 0.209 0.077 0.209 0.077 0.209 0.077 0.209

S T R E N G T H - W E I G HT

RATIO, I 0⁶ IN. ^2.17 ^1.59 1.82 1.51 1.21 1.0 0.60 0.50 0.91 0.75

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F o r higher temperature applications, high s i l i ca and quartz fibers could be useful. Unfortunately, high s i l i ca fibers such as R e f r a s il have only about one-fourth the strength of ¯ g l a s s. Quartz f i b e r s, although somewha t s t r o n g e r, a re e x t r e m e ly expensive; namely,about $60 per pound. F or long- t e r m , h i g h - t e m p e r a t u re applications, ¯ g l a ss is satisfactory for u se with any of the r e s in s y s t e ms developed to date. (Of p o s s i b le future interest also a re yarns mad e of c e r a m ic f i b e r s .)

High-strength m e t al filament ( 0 . 0 04 to 0 . 0 0 5 - i n. diameter w i r e) reinforcement provides substantial i n c r e a s es in strength an d modulus, as shown by the strength data and design values show n in F i g u re 8. However , strength-weight ratios a re still lower than those obtainable with g l a s s.

On e of the m o st recent developments in metal w i re for reinforcement is a 5 7 5, 0 0 0 - p si high carbon s t e el w i re in d i a m e t e rs of approximately 0 . 0 04 inch. Although this strength level i m p r o v es strength-weight r a t i o s, ratios a re still not so high as those of g l a s s - r e i n f o r c ed s t r u c t u r e s. (18)

Metals at Cryogenic T e m p e r a t u r es

B e c a u s e of the behavior of certain m a t e r i a ls at low t e m›

p e r a t u r e s, the selection of m a t e r i a ls for u se in forthcoming liquid hydrogen propellant m i s s i le s y s t e ms presents a c h a l› lenge to the engineer. F i r st generation Atlas and Titan I m i s›

s i l es a re built of structural and nonstructural m a t e r i a ls for s e r v i ce at temperatures as low as - 3 0 0 F. The second g e n›

eration liquid propellant m i s s i l es w i ll utilize m a t e r i a ls at temperatures as low as - 4 2 3 F. Although the temperature variation between LH ^ and lox is not too great, the difference in m a t e r i a ls properties can be significant.

F r o m the dearth of data available, it appears that certain structural m a t e r i a ls now in use should be satisfactory at L H 2 t e m p e r a t u r e s. Others that can be used at lox temperatures appea r l e ss satisfactory for s e r v i ce at the lower LH £ t e m p e r a›

ture range. (19)

Th e effect of cryogenic temperatures on m a t e r i a ls p r o p e r›

ties v a r i es with the alloy content, m i c r o s t r u c t u r e, and the condition of the alloy. Behavior of an alloy at cryogenic t e m›

peratures is a l so considerably dependent on design configura› tion and g e o m e t r y. The designer is faced frequently with d e c r e a s ed toughness in a m e t al which b e c o m e s increasingly n o t c h - s e n s i t i ve at lower t e m p e r a t u r e s. In general, the strength of m e t a ls i n c r e a s es as the temperature d e c r e a s e s. Bu t along with this i n c r e a se in strength is a corresponding d e c r e a se in ductility. The strength properties attained at low temperatures a re usually d e s i r a b le with, however, an unwel›

c o m e tendency toward brittle behavior.

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Figure 9. Results of Tensile T e s ts on Notched and Unnotched Specimens of 7 0 7 9 - T6 Aluminum A l l oy Billet.

TEMP,

e

F Etu»

KSI Ftyi KSI

E,

I 0

⁶

PSI T E N S I LE RATIO

ROOM 72.3 60.5 10.0 101.0 1.40

-110 75.2 64.8 10.1 103.0 1.37

-321 83.2 74.4 11.3 105.0 1.26

- 4 23 93.2 82.0 10.6 99.6 1.07

230

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Th e principal type of b r i t t l e n e ss encountered at low temperatures is attributed to d u c t i l e - t o - b r i t t le transition b e›

havior. T h i s, fortunately, is unique to certain m e t al s y s t e m s.

Th e transition temperature m a y be defined as that range below whic h the m e t al f r a c t u r es with little or no plastic deformation. Anothe r cause of b r i t t l e n e s s, p r i m a r i ly in ferrous b a se a l l o y s, is attributed to m e t a l l u r g i c al phase transformation. F i g u r es 9 through 12 show that certain alloys show particularly good behavior to t e m p e r a t u r es as low as - 4 2 3 F.

Certain design applications a re dependent on m a t e r i a ls whic h do not exhibit the d u c t i l e - t o - b r i t t le transition. These

include aluminum and austenitic stainless s t e el a l l o y s. The cold-worked aluminum a l l o y s, 5 0 5 2 - H 38 and 5 4 5 6 - H 3 8, and the precipitation-hardenable 2 0 2 4 - T6 alloy have been used with considerable s u c c e ss at t e m p e r a t u r es as low as - 4 2 3 F. AIS I 3 0 1 - XH s t a i n l e ss s t e el has exhibited s i m i l ar advantages.

(19)

Th e alpha titaniums also show exceedingly good p r o m i se for l o w - t e m p e r a t u re d e s i g n s, with considerable experience available on T i - 5 A l - 2 . 5 S n. An alpha-beta type titanium alloy ha s been fabricated for L H ^ containment by one manufacturer.

Whe n consideration is given the behavior of nonmetals at cryogenic t e m p e r a t u r e s, it is noted that, compared to m e t a l s, the nonmetals demonstrate poor l o w - t e m p e r a t u re ductility. (See F i g u re 13.)

M e a s u r e m e n t Techniques

In seeking to develop m a t e r i a ls and evaluate them for u se over periods up to three or four y e a rs in a e r o s p a ce environ›

m e n t s , activity w i ll include determination and study of n o r m a l physical p r o p e r t i e s, along with optical and e l e c t r i c al p r o p e r›

ties as applicable. T e s ts mus t be devised f r om which long- life properties can be predicted f r om s h o r t - t i me t e s t s. Key experiments on test v e h i c l es to c o r r e l a te test r e s u l ts in s i m›

ulated environments with actual a e r o s p a ce results a re a l so unde r way.

Considerable study is required to determine a n d / or d e v e l› op the n e c e s s a ry instrumentation b e st suited to m e a s u re and t e l e m e t er changes in c h a r a c t e r i s t i cs of m a t e r i a ls during l o n g - t i me flight in a e r o s p a ce environments. This w i ll provide a mean s for a limited numbe r of key experiments which can b e used for validating r e s u l ts f r om simulation t e s t s.

M o s t effects on m a t e r i a ls have been predicted f r om s i m u›

lated environmental t e s ts on the ground, or f r om m e a s u r e›

ment s on the environment and prediction of probable effects on m a t e r i a l s. One available method that can be adapted is based on the change in e l e c t r i c al r e s i s t a n ce with the change

2 5 1

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2J2

e