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E.R.D.E. 3/M/63 CONFIDENTIAL

MINISTRY OF AVIATION

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EXPLOSIVES RESEARCH & DEVELOPMENT ESTABLISHMENT

TECHNICAL MEMORANDUM No. 3/M/63

An Investigation of Materials for Choke Discs and Expansion Nozzles for 2-inch Is*

Experimental Rocket Motors ^D

R.S. Smith G.W. Stocks

m 90

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E.R.D.E. 3/H/63 CONFIDENTIAL WAC/151/010

MINISTRY OF AVIATION

EXPLOSIVES RESEARCH AND DEVELOPMENT ESTABLISHMENT

/ •0 - TECHNICAL MEHOHUTOUK NO. 5/l'i/63

An Investigation of Materials for Choke Discs and Expansion Nozzles for 2-inch Experimental Rocket Motors CL

^ R.S. Smith and G.!7. Stocks

Approved: .{*.

W.G. JILLIAMS S.P.1

Approved for 1 j, U Circulation: '' ,-

G.H.sY YOUNG P.S.(D)

CAth March, 196^)

17ALTHAH ABBEY jifS.oiljJi.

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CONTENTS

1. Summary

2. Introduction

3. Experimental

3.1 Specimens 3.2 Measurement of Erosion 3.3 Procedure

k. Results

5. Conclusions

5.1 Choke Disc Materials 5.2 Expansion Nozzle Materials

6. Reference

Tables 1 - 8

Figures 1-6

Page No.

1

1

3

3 3 3

if

5

3 3

5

6-16

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Reference: WAC/151/010

1. SUMMARY

Measurements of the amount of choke erosion in a two-inch solid propellant motor containing approximately 3A lb. of aluminised polyurethane propellant have been made, using as choke materials a wide range of commercial grades of graphite and a limited number of processed grades of graphite supplied by the Metallurgy and Physics Department, R.A.E. Farnborough. The erosion of metal chokes made from tungsten, molybdenum, and tantalum was also measured. Under the conditions of these tests it was found that of the graphites investigated, Nobrac grade M3/HF had the highest resistance to erosion. Tungsten was the only metal tested that was superior to the best grades of graphite.

The throat erosion of expansion nozzles using the same type of motor was also investigated. In this case the study was restricted to nozzles made from four materials, viz. one commercial grade of graphite, a silicon carbide/graphite mixture, a copper-base alloy, and molybdenum. For firings with a propellant containing 5 per cent aluminium the copper-base alloy, Hidurel 6, proved superior to the other materials tested. All the materials tested eroded substantially when fired with propellants containing 20 per cent aluminium.

2. INTRODUCTION

Measurements of rates of burning and specific impulse of experimental propellant formulations are carried out at E.R.D.E. in a two-inch rocket motor (K-round) containing approximately }i lb. of propellant. When this motor is used for measurement of rates of burning, it is fitted with a carbon disc or choke, drilled to the appropriate size for the desired combustion pressure; for specific impulse measurement, a graphite expansion nozzle, having a nominal expansion ratio of 7;1, is fitted.

The excessive erosion of these carbon chokes and nozzles that occurs with motors containing high-energy polyurethane-propellant formulations causes a marked decrease in chamber pressure with time, and a loss in thrust efficiency. The assessment of propellant ballistics must consequently become more difficult, and less accurate. Gains in performance of a few units in specific impulse are now regarded as significant, consequently, methods of measuring the ballistic properties must be improved.

The primary purpose of this study is to find a choke and a nozzle material that will maintain reasonably constant operating conditions for the brief duration of a K-round firing, normally between 0.5 and 2 seconds. The material must also be reasonably inexpensive and easy to machine, in view of the great variety of sizes needed.

The choice of nozzle and choke materials for this motor had hitherto been confined to three grades of carbon, Nobrac Ltd. h'3/50, Morgan Crucible Ltd. SY.9106, and Powell Duffryn Ltd. B.2., all of which were unimpregnated, electro-graphitised, pressed grades with quoted apparent densities of 1.75, 1.66 and 1.60 g/cm3 respectively. These grades were satisfactory with

/plastic

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plastic propellant of low and medium energy; under slightly more severe conditions the Powell Duffryn grade was marginally superior to the other two. With the introduction of high-energy polyurethane propellants these materials proved less suitable and increases in choke orifice area of up to JO per cent and occasionally as high as 60 per cent during one firing have been observed.

A trial was carried out to determine whether commercial grades of graphite, other than those in current use, might be more satisfactory as choke and nozzle materials. A number of processed grades (l) supplied by the Metallurgical Department, 3.A.E., Farnborough were also investigated. The opportunity was also taken to check briefly the suitability of the high- melting-point metals, tungsten, molybdenum, and tantalum.

Most ceramic materials, despite their high melting points, possess the inherent weakness of having low thermal shock resistance, which with a few exceptions renders them unsuitable as constructional materials for nozzles or chokes. In combination with other materials, however, mostly better thermal conductors, satisfactory results have been achieved. Only one of these materials, a sintered silicon carbide/graphite mixture developed by Hard Metal Tools Ltd., was included in these tests.

Because of the comparatively short burning time, it appeared likely that a metal with a high thermal diffusivity, not necessarily a high melting point, might also render satisfactory service. Transient-heat conduction calculations based on conditions similar to those found with these expansion nozzles have shown the superiority of copper compared with some other metals and alloys. For this reason one high-copper-content alloy, Hidurel 6 (Langley Alloys Ltd.) was examined in these tests in the form of an expansion nozzle.

The majority of th'ese screening tests were confined to various brands of graphite, however, because graphite had in the past proved a useful choke and nozzle material for routine ballistic assessment of propellants; it is readily available at a relatively low cost, and is easy to machine in the many different choke sizes required for ballistic assessment.

In certain circumstances (e.g. where it is necessary for a large number of firings requiring only a small range of choke or nozzle sizes) the use of less amenable, more expensive materials, such as tungsten or molybdenum might be justified, provided that their erosion resistance was superior and the assessment of the propellant performance therefore more accurate than it would have been with graphite nozzles. Possible cases in point would be:

(a) Control rounds for specific impulse,

(b) The firing of Phoenix propellant formulations where only two sizes of nozzle are used,

(c) Firings as a means of production control, possibly applied to the polyurethane propellants.

Most of the tests were carried out with materials machined in the form of choke discs because they were less expensive to produce than expansion nozzles. Hidurel, molybdenum and silicon carbide/graphite were tested in the form of nozzles, however, because these materials were under consideration

/for

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for applications (a) and (b) above, for which nozzles only were required.

3. ^XPERIiiENTAL

3.1 Specimens

The choke materials sampled were machined in the form of discs, 1.385 inches diameter and 0.25 inch thick, with a hole 0.302 inch diameter drilled centrally in each disc, with the exception of a few specimens of molybdenum, tantalum and tungsten which had an outside diameter of 0.750 inch (Table 7, p.15)« Each disc was shrunk into a mild steel housing when used as a motor choke (Fig. 1).

A limited number of specimens were machined in the form of expansion nozzles with a throat diameter of 0.302 inch and a nominal expansion ratio of 7:1« The molybdenum and Hidurel nozzles used for this test were machined from solid bars (Fig. 2) whilst the graphite and silicon carbide/graphite nozzles were inserts shrunk into mild steel cases (Fig, 3)«

3.2 Measurement of Erosion

A magnified silhouette of the choke orifice or nozzle throat was pro- jected on to half-plate bromide paper, before and after a firing (Fig. k). The areas of the photographed images were measured with a planimeter and the relative increase in area was used as an index of erosion of the material.

;o Procedure

Two aluminised polyurethane propellants U.2l6/D.6/1.0 and 11.2^1/0.6/1.0, the compositions of which are given in the Table below, were employed in this investigation.

FI?0PELLANT COMPOSITIONS

Designation Ingredient % by Weight Flame Temperature at 1000 p.s.i., theoretical, K

U. 216/D.S/1.0 Ammonium Perchlorate 72.50 3,200°

Daltorol 1^5

Triethyleneglycol- dinitrate (TEGN)

6.75

Tolylene 2,if-di- isocyanate (TDI)

1.30

Aluminium 5.00

U. 2 WC 6/1.0 Ammonium Perchlorate 60.00 3,550°

Daltorol 13.40

TEGN 5.00

TDI 1.60

Aluminium 20.00 ..,, i

/The

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The choke specimens were tested in the thin-walled motor (Fig. 5) and the nozzle specimens in the thick-walled motor (Fig. 6).

Before the commencement of this investigation it had been found, during the course of routine ballistic assessment firings, that propellant U.21S eroded graphite chokes more severely than any other propellent developed at E.R.D.E. up to that time, and it was therefore chosen for these tests. ;.fter the preliminary screening of a wide range of commercial grades of graphite had been completed, a new propellant, U.2^f 1, which had been developed was found considerably more erosive than U.21o. The investigation was continued, therefore, using propellant U.2^1. The results of the tests with both propellants are contained in this Technical Memorandum.

From the initial screening of graphite chokes with propellant U.21o three samples of each of the two best grades, together with some additional grades that became available after the preliminary trial had been completed, were fired with propellant U.2^1. A limited number of chokes made from the high-melting-point metals were also fired with this propellant. The final assessment of commercial grades of graphite was based on a further series of firings with ten samples of each of six grades, selected on the basis of results obtained in the two preliminary tests. V.'here differences in erosion of the best grades were marginal, consideration was given to their relative cost in making an assessment.

The nozzle materials './ere tested with U.21S and U.2^1 propellant, except in the case of molybdenum from which only one nozzle was made; as it cracked during the firing with U.216 propellant, no further consideration was given to this material. For this investigation Nobrac graphite II3 was used mainly as a control for the tests with the other materials which were, in general, tested as they became available.

Each choke disc or nozzle was used for one firing only throughout these tests, except for the Eidurel 6 nozzle which was used three times, as indicated in Table 8 (p.16).

k. RESULTS

The results of the tests with the processed grades of graphite supplied by R.A.E. are contained in Table 1 (p. 6). It was apparent that both the furfuryl alcohol and gas-impregnation processes reduced erosion. Erosion of a sample of EY.9 gas-impregnated graphite resulting in a carbon gain of 8.55 per cent was approximately half that of the untreated sample. It is difficult to explain why the second sample, with a carbon gain of 0.95 per cent, showed little improvement over the untreated sample, but further trials are necessary before any definite conclusions can be reached.

The detailed results of the tests with choke disc materials are contained in Tables k, 55 6 and 7> (pp. 9 - 15 ) and those with nozzle materials in Table 8. Summaries of the results of the tests with the best choke and nozzle materials are contained in Tables 2 and 35 respectively (pp. 7 and 8).

From Table 2 it can be seen that Nobrac graphite, grade INJ/HF, showed least erosion under the conditions of the tests, although in one of the pre- liminary tests (Table 5) it eroded slightly more than the Le Garbone grade

/F.5890

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P.5S9O. This discrepancy can possibly be accounted for by batch to batch variability of a particular grade of graphite, a contingency for which there •./as no provision in any of the tests. It is also evident, from Table 2, that of the refractory metals investigated as choke disc materials, only tungsten was superior to the best grades of graphite.

The summary of results contained in Table 3 shows that the silicon carbide/graphite nozzles are superior to those made from graphite grade M3, although when used with propellent formulation U.2^-1 the superiority is not as marked as might be expected. Hidurel 6 proved to be very satisfactory when fired with propellant U.216, but not with propellant U.2^1. As a result of this investigation, nozzles made from Hidurel 6 ere in current use at E.R.D.E. with some of the less erosive, high-energy propellants.

5. CONCLUSIONS

5.1 Choke Disc liaterials

Under the conditions of these tests it was found that of the graphites investigated, Nobrac grade M3/HF had the highest resistance to erosion. Of the metals tested tungsten was the only one that was superior to the best grades of graphite.

5.2 Expansion Nozzle liaterials

For the motors filled with propellant U.216 uncooled nozzles made from Hidurel 6 proved superior to the other materials tested. All materials eroded substantially when exposed to the combustion products of propellant U.2^1.

6. REFERENCE

1. R.L. Bickerdike, A.R.G. Brown, G. Hughes, W. Johnson and VJ. ',/att, Nuclear Fower, 1959, ±, 86.-

/TABLE 1

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?ABLE 2

Nummary of Erosion Trials with Graphite and Refractory Metal Chokes

Orifice diameter 0.302 inch. Choke thickness 0.250 inch.

Propellant formulation U.2^1 (20 per cent Al)

Material Source and

Grade Mo. of Firings

Time of Burning,

sec

; ean Values

Mean Combustion Chamber Pressure, p.s.i.

Combustion Chamber Pressure

Drop During Firing, p.s.i.

Increase in

Orifice

<y /o

Graphite Nobrac M3 10 1.18 1460 313 22

n Nobrac M3/28/HF

8 1.21 1339 327 26

>i Nobrac 113/28 9 1.18 1418 tf12 22

il Nobrac EF/28 10 1.17 1^62 317 21

il Le Carbone F.5890

10 1.15 15^0 267 21

;i Powell Duffryn E2.B

3 1.01 153^ 150 22

Graphite Nobrac M3 10 1.17 l¥f2 279 2k ii Nobrac M3/KF 9 1.15 1522 199 15

Graphite (tantalum carbide coated)

Le Carbone F.589O

3 0.97 1600 365 2k

: Molybdenum (sintered)

Electro- Alloys

5 1.05 1^65 225 30

Molybdenum (cast and forged)

A.R.D.E. (W/HLW)

2 0.99 1^57 293 35

Molybdenum (cast)

A.R.D.E. CJ/HLQ)

3 0. 9k H*5 502 50

Tantalum Mure:; 1 1.C3 1^23 67^+ 105 ,

Tungsten (sintered)

Murex 3 0.91 1727 -160

89 3k •

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/TABLE 3

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TABLE 4

Initial Screening of Graphite Chokes

Fropellant formulation U.216 (5 per cent Al)

Manufacturer Grade

Increase in Orifice Area Mean Combustion

Chamber Pressure, p.s.i.

Time of Burning,

sec

% Mean #

Le Carbone it ,i

il .!

P.5890 ;i

.t

9 3 7

6 1333 1654 1660

1.34 1.16 1.17

National Carbon ii n

II ;i

Graphite ATJ II 11

11 ii

12 9 6

9 1332 1566 1605

1.35 1.14 1.19

Nobrac n

M3 11

it

10 7 14

10 1338 1^33 1560

1.35 1.22 1.14

Morgan Crucible ;t II

II :i

EYX.49 11

it

2k 13 15

17 1128 1447 1419

1.43 1.22 1.22

Powell Duffryn ;t II

II il

B.2 :i

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13 18 33

23 1445 1393 1315

1.20 1.24 1.23

Nobrac M3 (50) II 11

ii ;i

27 17 28

2k 1375 1403 1290

1.23 1.24 1.25

Acheson • i

c.s. il

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2k 2k

1244 * *

1-39

Morgan Crucible ;i ii

;i II

EY.9106 II

i i

23 30 36

30 1405 1432 1240

1.22 1.15 1.29

Powell Duffryn „ i,

B.1 it

29

27 j 28 28 j

I

1362 1382 1324

1.23 1.20 1.25

Powell Duffryn ;i it

,i ,i

B.2/R ,1

> I

32 ! 29 30 28 !

1362 1371 1339

1.24 1.23 1.25

* Morgan Crucible i

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3Y.9 II

11

32 33 30 26 ,

1057 113^ 1199

1.44 1.30 1.31

1

*Disc fractured.

/TABLE 4 (contd.)

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TABLE h (contd.)

Manufacturer Grade

Increase in Orifice Area Mean Combustion

Chamber Pressure, p.s.i.

Time of Burning,

sec

% Mean %

Powell Duffryn n it

II ,1

C3 i

28 30

33

30 1175 15^0 1V?6

1.V3 1.11 1.12

Morgan Crucible 11 ii

.; ;i

CYX.105 (EY.115)

II

37 36 3k

36 1210

1157 1^20

1.28

1.31 1.17

Morgan Crucible ;; it

ii \\

CYX.114 ti

it

h3

39

k2 1175 1104

12^5

1.30 1.29

1.27

Morgan Crucible ;i PI

II ii 1

CYX.106 .i

ii

53 80

71

68 1103 956 10V?

1.3<f 1.34 1.31 I

/TABLE 5

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CONFIDENTIAL

TixBLE 6

Final Erosion Trial with Graphite Chokes

Propellant formulation U.241 (20 per cent Al)

Increase in Orifice Area

Mean Combustion Time of

Manufacturer Grade Chamber Pressure,

p.s.i.

Burning, sec

% Mean %

Standard Deviation

Nobrac M3/28/HF 30.1 29.5

28.6

27.1 18.8 28.6 22.0 23.0

26.0 3.9 1288 1312

*

1319 *

1225 1490 1310 1438 1332

1.24 1.22

1.21

1.23 1.17 1.21 1.15 1.21

Nobra.c {13/28 20.8

19.3 20.7 34.2 27.7 22.2 14.6 18.4

22.1 5.^ 1387 *

1465 1449 1349 1410 1445 1402 1446

1.20

1.18 1.19 1.20 1.18 1.16 1.18 1.17

21.2 1410 1.20

Nobrac EF/28 24.2 20.8 18.0 18.3 20.0 22.7 24.8 20.2 17.0 21.5

20.3 2.5 1419 1498 1470 1463 1490 1466 1444 1465 1498 1410

1.20 1.13 1.18 1.18 1.16 1.17 1.16 1.16 1.16 1.19-

Nobrac I'13 19.3 23.0 8.7 1404 1.19 (Standard)

•

21.6 43.1 15.^ 21.6 20.4 29.7 33.9 19.5 13.0

1446 1297 1498 1425 1472 1475 1401 1475 1530

1.16 1.24 1.16 1.16 1.16 1.15 1.17 1.16 1.14

/TABLE 6 (contd.)

- 13 -

CONFIDENTIAL

CONFIDENTIAL

TABLE 6 (contd.)

Increase in Orifice Area. Mean

Combustion Time of Manufacturer Grade Chamber

Fressure, p.s.i.

Burning, sec

% Mean %

Standard Deviation

Nobrac M3/HF 18.9 13.3 18.0 9.4 12.8 20.0 10.1 18.3 15.8

15.2 3.5 1519 1510 1497 1571 1531 1480 1525 1540 1525

1.14 1.16 1.16 1.14 1.14 1.16 1.14 1.14 1.15

6 Nobrac M3 20.2 21.9 6.3 1501 1.16 (•Standard) 30.8

28.6 17.4 25.2 10.9 20.0 13.0 23.7 29.1

1334 1380 1499 1411 1531 1508 1595 1377 1409

1.21 1.20 1.18 1.19 1.15 1.18 1.14 1.19 1.17

6 Le Carbone P.5890 23.7 18.3 19.1 30.9 25.5 19.5 18.3 16.3 13.2 21.8

20.3 4.7 1490 1549 1570 1530 1540 1582 1526 1540 1600 1451

1.16 1.18 1.12 1.16 1.14 1213 1.16 1.15 1.14 1.18

*Disc fractured

Fired with Batch B propellant

Fired with Batch A propellant

AABLE 7

- 14 -

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CONFIDENTIAL

E.R.D.E. 5/M/C3

CHOKE DISC

ILD STEEL CHOICE HOLDER

CHOKE DISC IN CHOKE HOLDER FIG.f

EXPANSION NOZZLE (MOLYBDENUM OR HIPUREL6) FIG.2

MILD STEEL CASE

GRAPHITE INCH

EXPANSION NOZZLE (GRAPHITE INSERT) FIG-3

CONFIDENTIAL

CONFIDENTIAL

E-R-P.E.3/M/63

A.

B.

CHOKE ORIFICE SHADOWGRAPHS BEFORE AND AFTER FIRING.

CONFIDENTIAL FIG-4.

E.R.D.E. 3/M/63

CONFIDENTIAL

COPPER/ASBESTOS WASHER

/ /

/

I • 1 O ' 2

INCH.

/

/

CHOKE DISC

CONE AND CYLINDER CHARGE PROPELLANT WEIGHT, ^- lb.

SR37IC CARTON IGNITER

TAPPED OUTLET FOR

PRESSURE TRANSDUCER

THIN-WALLED K-ROUND MOTOR, ASSEMBLED FOR RATE-OF-BURNING MEASUREMENT FIRINGS.

FIG.5.

CONFIDENTIAL

CONFIDENTIAL

E.R.D.E. 3/M/C3

GRAPHITE

PRQPELLANT

2 INCH K-ROUND ROCKET MOTOR •\ • ( ' .<

I -'•-"'I1-1- I' FIG-6.

INCH

CONFIDENTIAL