Note: Descriptions are shown in the official language in which they were submitted.
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The in~ention relates to rocket motors, especially
those employing double-base propellants, and to means or reducing
the smoke emission therefrom. The expression rocket motor as
used in this specirication re~ers to a casing vented a-t one end
and containin~ a combus-tible propellant charge.
It is an essential feature of line-of-sight optically
guided missiles that smoke emitted during flight must not obscure
the missile or the target. In the case of military weapons, smoke
is additionally undesirable since it assists an enemy in pin-
pointing the launch area. Most double base propellant systemsare, in themselves, relatively smoke free, but even rocket motors
employing such propellant system still produce unacceptable
levels of smoke emission, a significant contribution being that
from the inhibiting coating applied to the solid propellant to
produce constant mass rate burning, and also the thermal insulant
between the propellant charge and the motor casing.
The term "double-base prope].lant" as used herein
refers to propellant systems containing both nitro-cellulose and
a nitrated polyhydric alcohol, eg nitro-glycerine, tri-ethylene
~0 ~Jlycol dinitrate or di-ethylene glycol dinitrate. Composite
modified double-base propellants containing ammonium perchlora-te
produce e~haust containing hydrogen chloride which forms mist
in moist air and hence such composite modified systems are not
generally employed when a smoke-free system is required. How-
ever, the present invention normally will reduce the smoke ernis-
sion from the inhibiting coating and thermal insulation even in
these systems.
Further, the invention is not limited to double-base
systems, but may be employed to reduce the smoke emission from
rocket motors employing any propellant system, althoucJh rnaximum
beneflt will obviously be ob-tained when the propellant sys-tem
itself is smoke-free or "low smo~e".
Currently used lnhibitors and -thermal insulants, for
e~ample cellulose acetate and ethyl cellulose, both plasticised
~ith phthalate esters and "Hypalon" rubber~ (chlorosulphonated
polyethylenes) give unacceptably large amounts of smoke and often
show other undesirable properties such as high nit:roglycerine
absorption or low softening temperature. Silicone elastomers
~0 have many desirable properties for use as inhibitors and/or
thermal insulants such as low temperature flexibility, low nitro-
glycerine absorption and high thermal stability, but when unfilled
or filled with commercial fillers such as CaCo3 sio2 or Tio2 give
copious smoke emission. It is an object of the present invention
to provide compositions for use as inhibitors and/or thermal
insulants in rocket motors which on burning do not emit unaccept-
able amounts of smoke.
According to the present invention, a rocket motor has
a propellant charge situated within, and incompletely filling a
`~ motor casing, the space between the said propellant charge and
the motor casing being at least partially filled by an elastomeric
composition comprising a silicone elastomer having asbestos dis-
persed therein~
According to one embodiment of the invention, the
elastomeric composition is situated immediately adjacent the . .
yropellant charge to act as an inhibitiny coa-ting to ensure
constant mass-rate burning.
According to a further embodiment of the invention the
elastomeric composition is situated adjacent the motor casing
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and ~cts as a thermal insulant be-tween the propellant charge
and the said casing.
According to a preferred embodiment of the invention,
therefore, the elastomeric composition fills substantially the
entire space between the motor casing and the propellant charge
and acts as both a thermal insulant and an inhibiting coating.
In this preferred embodiment the thermal insulant and inhibi-ting
coating may be applied separately to the motor casing and pro-
pellant charge respectively before the rocket motor is assembled;
or a single continuous layer of silicone elastomer having asbestos
dispersed therein may be cast in situ between the propellant
charge and the motor casing and thereby form both the inhibi-ting
coatin~ and the thermal insulant.
The propellant charge may be composed of any propellant
system, but is preferably a double-base propellant sys~em contain-
ing both nitro-cellulose and at least one nitrated polyhydric
alcohol.
The silicone elas-tomer may be any of -the we]l known
silicon based elastomer systems having the general formula
~0 ( n (4-n)/2)m
wl~erein n is 0, 1, 2 or 3; m is greater than 2; and R represents
an alkyl, aryl, fluoroalkyl, alkenyl, alkaryl or aralkyl group
or a mixture of such groups with each other and/or with other
groups such as hydrogen or chlorine atoms, alkoxy, aryloxy or
alkylamino groups. The silicone elastomer may be formed from
any of the well-known precursor systems such as acetoxy-terminated,
acetic acid eliminating, one pot systems or hydride-terminated,
systems. However for ease of application to the propellant
charge and for safety, it is preferable to use a room-temperature
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curing two part silicone rubber formed by condensing an organo-
polysiloxane with an alkyl silicate ester according to an equation
of the type
! R R aR 1
~0-- si ---o ~ -- si , o~ -- y,o----- --~ o -l;Y -- xo ' s;i--- ------ X
R In R OR(or R~ ,R i n
Ca-talys-t
W
jR 1 fR
~o Lsi-~ o ~ . _ Si o --fi ---~ ----t---l i o l x
R n R OR (orR) R ~n /
Eqn 1.
~herein X represents a hydrogen atom and Y an alkyl group. Alter-
natively an organopolysiloxane ester wherein X represents an alkyl
~roup may be reacted with a hydroxy terminated polysiloxane.
The catalyst used may be, for example, a monocarboxylic
2~ acid salt of a metal such as tin or lead.
The term "asbestos" as used herein reEers to those in-
combus~ible mineral silicates commonly known as asbestos which may
vary in colour and may consist of simple silicates such as magne-
sium silicate (chrysotile) or complex silicates such as magnesium
iron silicates (amosite) or sodium iron silicates (crocidolite).
The asbestos may be in any physical form, for example
as long fibres, mos-tly having a length of about 50 to 500 ~m and
a breadth of about 5 ~lm, or as short fibres ("Elour") mostly having
a lenqth of about S to 50 ~m and also a bread-th oE about 5 -to 50 ~m.
~referably, however, the asbestos is in the form oE long fibres
havin~ an aspect ratio (length : breadth) of at least 10 : 1 since
these give a ~reater reduction in smoke emission than shor-t fibres
and also avoid puffs of smoke observed when sho~t fibres are usedO
Long fibres also give increased mechanical strength to the sili-
cone elastomer composition. The asbestos should advan~ageously
comprise at least 10~, preferably at least 20%, by weight of the
composition. The upper limi~ of asbes-tos conten-t is set by -the
~ifficulty of blending high loadings of asbestos, especially long
fibres, with the silicone elastomer to form a wor~able mixture.
~ith simple blending appara-tus the limit of workability is reached
at about 25% w/w of long fibres or ~0% w/w of flour, bu-t with
more efficient blending systems higher loadings may be reached.
The asbestos should be ad~ed to the mixture of the precursors of
the elastomer before said precursors are cured. Thus the asbestos
may, for example, be mixed with the organopolysiloxane and the
alkyl silicate and catalyst added. The mixture is then applied
to the area where thermal insulation is required or coated on to
the surface of the propellant charge, preferably after painting
_~ the said surface with a silane coupling agent composition, for
example that comprising ~-glycidoxy propyl trimethoxy silane
and an aromatic amine curing agent in a weight ratio of about 1:1
(as described by Stenson and Manners, 5th Int Congress of Surface
Activity, 1968, pp ).
Examples illustrating the smoke emission from various
inhibitors for double-base propellants will now be described with
reference to the àccompanying drawings which illustrate the con-
figurations of rocket motors used. All propellants used in -the
examples were nitrocellulose/nitroglycerine double-base propellant
containing various ballistic modifiers.
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~XAMPLE 1
Preliminary screening was carried ou-t using the arrange-
ment shown in Figure ] wherein an uninhibited double-base pro-
pellant 1 is burned at approximately 8 MN/m2 in a motor casing 2of diameter 51 mm and the exhaust directed through a tube 3 of
~iameter 51 mm having a lining 4 of a~out 5 mm thickness of the
inhibitor being tested. The smoke emission after 1 second was
estimated on an arbitrary scale 0 to 10. The results are shown
in table I.
TABLE I
Particle %(w/w) of
Inhibitor Filler % w/w of size ,um Smoke Emission inhibitor
Filler of Filler eroded
K96601'2 None o _ 8.7 22.4
" SlC 50 40 7.6 26.3
" MgO 50 40 8.0 21.3
" MgO 35 40 9.2 21.0
K9660 ' 2 3 50 40 7.6 20.5
" Alkon3 25 7.0 21.1
" Asbestos flour255 to 50 3.6 15.0
" " " 35 " 2.2 4.7
" " " 45 " 3.2 3.4
Asbestos flour 35 " 2.7 3.5
Boric acid 10
Cellulose5
Acetate ~ 4 23
Ethyl - - - 3 21
Cellulose
Hypalon BaSO~ 60 - 4 7
CL 4851
O = zero smoke 10 = complete obscuration
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~OTES
1 K9660 is a polydimethylsiloxane/a1kyl silicate ester
~UM stock produc~ accor~ to equation 1 and Eorms
-the basls of the filled material sold by Midland Sili-
cones Ltd. under the sales no MS 9161.
2 Supplied by Midland Silicones Ltd.
3 ~lkon is a polyoxymethylene copolymer, now sold by ICI
under the Registered Trade Mark "Kemetal".
~ Lenyth and breadth of particles each in -the range 5 to
50 ,um.
Plasticlsed with phthalate esters.
6 Hypalon CL d851, is a chlorosulphonated polyethylene
cross-linked with lead oxide, PbO, supplied by the
Northern Rubber Co.
E~A~IPLE 2
Tests on fibrous asbestos filled K9660 silicone elastomer were
carried out in the flameless motor configuration shown in Figure
2 wherein a motor casing 11 contains only a short charge 12 of
propellant, but the inhibitor coating 13 on the propellant extends
over the whole length of the motor casing. This configuration
gives a higher inhibitor to propellant ratio than would a full
ch~rge system and involves passing exhaust over a considerable
length of inhibitor coating even at the commencement of burning~
Hel~ce smoke emission would be expected to be worse wi-th -this
system than with a full charge system,
Tests were carried out using a cool propellant (chamber
temp 2290K) and smoke emission was assessed on an arbitrary
scale 0 to 10 after 5, 10, 15 and 20 seconds burning time under
flameless conditions, The results are shown in Table II,
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TABLE II
Smoke emission
Inhibitor E'iller %w/w of Filler ( sec from iyni-tion)
K 9660Asbestos fibrel 10 7.8 8.2 8.2 8.6
" " 20 2.8 2.8 2.1 3.0
"Asbestos flour 10 7 8 8 9
Cellulose acetate
(phthalate ester
plasticised) - - 8.7 8.9 8.2 7.6
1 Length 50 to 500 ~m, breadth about 5 ~m.
2 Length 5 to 50 ~m, breadth 5 to 50 ,um.
EX2~1PLE 3
Example 2 was repeated for asbestos flour and asbestos
fibre filled silicone K9660 using a hotter propellant (chamber
temperature 2620K) under flameless conditions. The results are
shown in table III.
TABLE III
Length Preadth Smoke Emission
Inhibitor Filler %w/w of Filler range range (sec from
(jum) (jum) ignition)
5 10 15 20
K 9660 Asbestos flour 20 5-505-50 2.2 5.1 3.0 9.9
" " " 25 5-50 5-501.7 8.0 2.7 9.4
" " " 30 5-50 5-501.7 2.4 4.4 8.8
" Asbestos fibre 25 50-500 about 51.5 1.5 2.6 5.6
,,
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