Grain configuration for solid propellant rocket engine

Pierce, R. M.

English

NASA Technical Reports Server

June 29, 1965 R. M. PIERCE 
PROPELLANT GRAIN FOR ROCKET MOTORS 
~ i l e d  larch 8, 1961 
N 
s 
~ 3 0 - 3 5 5 3 4 .  
(ACCESSION NUMBER) ' (TKUI 
INVENTOR. 
RALPH M. P/ERCE 
" 7 h  E/Z;r(dYL/ 
AGENT 
https://ntrs.nasa.gov/search.jsp?R=19700026218 2020-03-17T01:50:49+00:00Z
3,191,379 United States Patent Ogce ,,,, ,, , ,, 
3,191,379 
PRgBPEkLAIqT GRAIN FBBR.RO631HmT MOTOF 
Ralph M. Pierce, Wedlands, Cahf.; granted to NaJat~onal 
Assenantics and Space Adnninls&aaiebn sandier provisiarns 
of 42 U.8.C. 2457Ba) 
Piled Mar. 8,1961, Berr. No. "3,229 
3 Claims. (6311. 60-35.6) 
This invention relates to solid propellant rocket motors, 
and more varticularlv to a novel ccnfi~uration of the 
propel!ant Gain, which is loaded in the motor case. 
Solid propellant rocket motors, as presently developed, 
usually comprise a cylindrical shell containing a mono- 
lithic grain of propellant having a central bore which 
opens directly into the nozzle throat. Ignition of the 
propellant is initiated over the exposed surface of the 
central bore, and the gaseous combustion products pass 
out through the nozzle. Combustion proceeds normal to 
the burning surface, and during the burning period, cer- 
tain portions of the propellant burn out to the shell before 
other portions, exposing the shell to the extremely high 
temperatnre combustion process as slivers of propellar~t 
continue to burn. This results in severe heating of the 
case, which necessitates the use of heavy insulation to  
prevent case failure. 
The primary object of the present invention is to pro- 
vide a so!id propellant rocket motor which is so con- 
structed and arranged that the propellant grain burns 
through to the case over its entire inner surface at  the 
instant of complete burnout. Thus, during the burning 
period of the rocket motor, the propellant itself serves as 
thermal insulation for the case, shielding the latter against 
the high temperature combustion gases, and at the moment 
any of the case is first exposed to the hot gases, the com- 
bustion process ceases, due to exhaustion of the propel- 
lant. 
By eliminating exposure of the motor case wall to the 
combustion process, the need for thermal insulation to 
prevent case failure is likewise eliminated. An important 
advantage of the invention is that it permits a substantial 
reduction in dead weight, and provides a rocltet motor 
having an extremely high ratio of propellant weight to 
total engine weight, which is essential in a high perform- 
ance motor. 
Another object of the invention is to provide a solid 
propellant rockct motor in which the nozzle serves both 
as a part of the aft head closure, and also as a hollow 
supporting rod for the inner propellant tube. By thus 
eliminating a separate supporting rod for the inner propel- 
lant tube and a full aft head closure entirely separate from 
the nozzle, the present invention further eliminates dead 
weight and increases the ratio of propellant weight to 
total engine weight. 
Si l l  a further object of the present invention is the 
provision of a new and improved propellant grain con- 
figuration in which one portion of the burning area 
plogreszes during combustion, while another portion of 
the buining area retrogresses. The net progressivity of 
the total motor can thus be contro?led to prevent develcp- 
; ment o: excessive conlbustion chamber pressure during 
the burning period. 
I These objects are achieved in the present invention by 
using a propellant grain configuration consisting of a 
hollow outer shell of propellant which completely covers 
the inner surface of the n~otor  case, and an inner tube of 
propellant disposed concentrically within the said hol- 
low shell, the said inner tube of propellant being sup- 
ported at one end on an inwardly projecting nozde. 
Combustion gases generated by burning of the propellant 
on the facing areas of the outer shell and tube flow for- 
wardly and then are reversed in direction to flow rear- 
wardly through the hollow center of the tube and out 
Ifil.ough the nozzle. The thickness of the outer shell of 
propellant is such that the entire inner tube is consumed 
bcfore the o ~ ~ t e r  shell burns through to the case. Thus, 
at the moment the case is first exposed, combustion ceases 
due to exhaustion of the propellant. 
The foregoing and other objects and advantages of the 
present invention will become apparent to those skilled 
in the art upon consideration of the following detailed de- 
scri~tion of the preferred embodiment thereof, reference 
-- A 
being had ,to the accompanying drawing, wherein; 
FIGURE 1 is a longitudinal sectioned view through a 
so:id propellant rocket motor embodying the principles 
of the present invention; and 
15 FIGURE 2 is a transverse sectional view of the same, 
taken at 2-2 in FIGURE 1. 
Referring more particularly to the drawing, the rocket 
mottor of the present inveiltion comprises a generally elon- 
gated cyliildrical shell or case 5 having a rounded end 
20 ciosure 6 at one end thereof, and an open end 7 at the 
other extremity through which a nozzle projects. The 
nozzle 8 may be secured to the case 5 in any srritable 
manner, as by the threaded connection 9. The nozzle 8 
projects into the interior of the case 5 for a substantial 
25 distance, and only the outer end thereof projects rearward- 
ly beyond the end of the case. The nozzle 8 is provided 
with a restricted throat section 10, which is preferably 
enlarged in thickness at 11, and this throat section merges 
at its rear end into an outwardly flared, bell-shaped ex- 
30 pansion section 12, in which the combustion gases expand 
and accelerate to supersonic velocities. At its front end, 
the throat section 10 expands in diameter at 13 to form 
what is referred to  in the art as the "co~lvergent section." 
The propellant charge of the present invention com- 
35 prkes an outer shell 14 and a separate inner tube 15, each 
of which is a single propellant grain. The outer shell 14  
covers the entire inner surface of the case 5 and end 
closure 6 with a substantially uniform thickness of prcpel- 
lant, which is proportioned so as lo burn out to the case 
40 simultaneously over the entire inner surface thcreof. 
Where it is found that the outer shell 14 of propellant 
tends to burn a.t a non-uniform rate owing to non-uniform 
pressure and gas flow conditions within the colnbustion 
chamber, it may be necessary to increase the thiclmess 
45 of the shell 14 in certain areas in order to obtain sirnul- 
taneous burn-out at all points. 
The inner tube 15 of propellant is generally tubular in 
configuration, with a cylindrical bore 16 extending through 
50 the center thereof, and a slightly conical outer surface 17 
which cooperates with the facing inner surface 3.8 of the 
shell 1 4  to form a forwardly diverging annular passage 
2@. Tne front end of the tube 15 is rounded at 21 and 
spaced from the shell of propellant covering the end clo- 
55 sure 6 so as to provide an inwardly turned passageway 
connecting the front end of the passage 20 with the front 
end of the bore 16. At the extreme rear end of the pas- 
sage 28, the tube 15 is coated with a layer 22 of ignition 
inhibitor, and the facing area of the outer shell 14 is like- 
FO wise coated with a layer 23 of ignition inhibitor. 
Ignition of the propellant grains 14  and 15 is initiated 
by any suitable igniter means, and combustion takes place 
over the entire exposed surface of the propellant. Com- 
bustion gases generated by burning on the facing surfaces 
G5 17  and 18, flow forwardly through the annular channel 
288 and then reverse in direction to flow rearwnrdly 
through the central bore 16 and out through the nozzle 
throat 16) and expansion section 12. Burning proceeds 
normal to the exposed surface, and the burning surfaces 
E6 and 18 progress in surface area, v~hile the burning 
70 siarface 17  retrogresses in surface zrea. This retrogres- 
sivity of the tube burning surface 17  counteracts, to some 
L, --L .I 
3,191,379 ., = - l r *  
3 4 
extent, the progressivity of the shell burning surfaces 16 central bore so that the combustion gases generated at 
and 18, with the result that the net progressivity of the the burning surfaces of said propellant on opposite sides 
total motor remains within reasonable bounds, and de- of said passageway pass forwardly along said passage- 
velopment of excessive pressure in the combustion cham- way and then reverse in direction and pass rearwardly 
ber is prevented. 5 along said central bore to exhaust through said nozzle, 
One important relationship of the present invention is said tubular body of propellant being of a thickness such 
that the respective thicknesses of the tube 15 and shell that it is entkly consumed before said shell is consumed, 
14 must be such that the tube 15 is entirely consumed and the thickness of said shell being such that it burns out 
before the shell 14 burns out against the case 5. Since to said case simultaneously over the entire area thereof, 
the tube 15 burns on both its inner surface 16 and outer 10 so that combustion ceases due to exhaustion of said pro- 
surface 17, the total thickness of the tube 15 may be some- pellant at the same instant said case is exposed to com- 
what more than the thickness of the shell 14. Also it is bustion gases. 
contemplated that the tube 15 might be made of one type 2. A solid propellant rocket motor comprising a cylin- 
of propellant, and the shell 14 another type of different drical casing having one end closed and the other end - 
burning rate, in which case the thickness dimensions 15 open, a grain of solid propellant bonded over the entire . 
would vary according to the burning rate of the propellant inner surface of saidacasing, a tubular configuration of 
involved. propellant grain inserted within the open end of. said i 
In FIGURE 1, the reference numerals 16', 17' and 18' casing and spaced from the grain on said casing, said 
designate the burning surfaces at a point slightly more than tubular configuration being characterized in that the pro- 
halfway through the burning period. It will be noted 20 pellant is combustible on all surfaces of the tube, a nozzle, 
that the shell 14 has burned back to slightly less than half means for attaching said nozzle to the open enld of said 
its original thickness, whereas the tube 15 has burned casing, said nozzle projecting into said casing and into 
.down to a very thin wall thickness, tapering down to said tubular configuration, means including a portion of 
a knife edge at its front end. At this point, the tube 15 said nozzle for closing the space between said casing and 
is about to start burning back rapidly from its front, or 25 said tubular configuration adjacent the open end of said 
left hand, end, and in a very brief period of time will casing, whereby upon ignition of said propellant com- 
have burned away completely, leaving only the shell 14 bustion progresses on the casing and tubular configura- 
and a thin collar of propellant encircling the inner end of tion simultaneously and the combustion products pass 
the nozzle. through the space toward the closed end of said casing 
While I have shown and described in considerable de- 30 and out through the tubular portion and nozzle. 
tail what I believe to be the preferred form of my inven- 3. A solid propellant rocket motor comprising a cylin- 
tion, it will be understood by those skilled in the art that drical casing having a rounded end closure at one end 
various changes may be made in the shape and arrange- and an opening at the other end, a tubular body of solid 
ment of the several parts without departing from the propellant grain having a cylindrical bore and positioned 
broad scope of my invention, as defined h the appended 35 within said casing concentrically with and spaced there- 
claims. from and defining an annular space, a grain of solid pro- 
I claim: pellant bonded to the entire inner surface of the casing, 
1. A solid propellant rocket motor comprising a gen- means closing the annular space between the open end of 
erally cylindrical case having a rounded end closure at the casing and said tubular body, an outwardly flared 
the front end thereof and a nozzle at the rear end thereof, 40 nozzle attached to the open end of the casing and extend- 
said nozzle including a restricted throat section and an ing into the cylindrical bore, whereby after ignition the 
outwardly flared expansion section, said case being joined combustion products of said grain pass forward through 
to said nozzle adjacent the expanded rear end of said the annular space toward the closed end of the casing and 
flared expansion section so that the nozzle forms a sub- then out through the cylindrical bore, combining with 
stantial portion of the rear end closure of said case, said 45 the combustion products of the tubular body and con- i 
nozzle projecting into the interior of said case, a shell of tinuing out through said nozzle. 
solid propellant bonded to the interior of said case over 4 
the entire inner surface thereof, said shell being of sub- References Cited by the Examiner 
stantially uniform thickness and having an inner surface UNITED STATES PATENTS [ 
which is ipoited form a bdg surface, a tubular 50 1,879,579 9/32 stoUa et alal 6(&35&6 ' ;E "' propeuant the projecting 2,440,305 4/48 Skinner - 6(&35:6 portion Of said nozzle and bonded said tubular 2,623,465 12/52 Jasse ,-----------,---, 60-35.6 body having a cylindrical central bore which opens into 2,814,179 11/57 Edelman et al. 660--35.6 j 
said throat section of said nozzle, and said tubular body 2,865,456 12/58 Dennis 6---,-----____6(3..47 having a slightly conical outer surface which coo~a-ates 55 2,914,484 3/61 Cooley 6,e33.6 with the facing surface of said shell to form a reaxyardly 
diverging annular passageway, said annular passageway 
connecting at its rear end in sealing engagement to the SAMUEL LEVINE, Primory Examiner. 
case connecting at its front end to the front end of said SAMUEL FEINBERG, Examiner, 
, , 
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Propellant grain for rocket motors Patent

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