Method for testing rocket nozzles at high tensile stress level

Estes, E. G.

English

NASA Technical Reports Server

IS 
ROCKET NOZZLE TEST METHOD 
Filed April 38, 1968 
FIG. I . n  
0 
THERMAL STRESS (OVERTEST) 
NET STRESS (OVERTEST) 
' NET STRESS(FLIGHT1 
PRESS. STRESS (OVERTEST) 
--....I-- - - _ - -  - PRESS. STRESS (FLIGHT) 
https://ntrs.nasa.gov/search.jsp?R=19710006168 2020-03-17T02:39:29+00:00Z
acted by larger compressive stresses due to the increased 
chamber pressure. In fact, the net tensile stress is gen- 
erally lower at higher mass flow rates. 
Other methods have been proposed for testing sub- 
merged nozzle throats at amplified tensile stress levels. 
One method is to apply heat shocks to a sample of the 
same type of material as that used in a nozzle throat to 
induce large thermal stresses. However, it is questionable 
whether these tests are a good indication of the perform- 
ance of the materials in an actual nozzle. Another method 
involves the use of propellants which burn at an ex- 
tremely high temperature to increase the tensile stress 
level without a corresponding increase in compressive 
stresses from higher chamber pressures. While this 
flow rates, 15 method provides a better indication of nozzle reliability 
than the simple testing Of a piece of nozzle material, the 
special PrOPellalltS required are often costly to procure. 
In addition, the different chemical composition of the 
'IJecia1 propellants can affect the performance Of the 
3,534,597 
'' 
3 Claims 
t. CB. GOPsln 15/00 
U.6. a. 73-146 
A method of testing a rocket nozzle, of the type hav- 
ing a throat submerged in a motor &amber, at high ten- 
sile stress levels. The method comprises briefly operat- 
ing the rocket motor at higher than usual 
until inner and outer portions of the throat walls reach 
a maximum temperature difference. At this time, the mass 
flow rate is suddenly lowered. men the flow rate 
is cut, the pressure drops rapidly while the temperature 
differential decreases only slowly. For a period after the 2o and give 
mass flow rate is cut, the thermally induced stresses, OBJECTS AND SUMMARY OF THE INVENTION 
which are tensile, are only slightly counteracted by the One object of the present invention is to provide a 
method for inducing large thermal stresses in a rocket pressure induced stresses, which are compressive, SO that 
25 nozzle without correspondingly large pressure induced a high net tensile stress level is obtained. 
This invention relates to methods for testing rockets. 
40 
Description of the prior art 
The throat area of many high performance rocket 
nozzles are subjected to high temperatures, such as 5,000" 
F., and high pressures such as 700 p.s.i. In order to with- 
stand such conditions, the throat areas of high perform- 
ance nozzles are often constructed of special graphite 45 
materials. Reliability testing of the nozzles, and particu- 
larly of the throat areas, generally requires the inducing 
of stresses at a level above those to be encountered in 
normal use. 
In some rocket motor designs, the nozzles are posi- 50 
tioned entirely behind the chamber wherein fuel is burned. 
In the case of such motor designs, higher stress levels can 
be realized by simply burning fuel at a higher rate. The 
resultant higher mass flow rate causes a larger pressure 
within the throat area and a larger temperature grandient 55 
between the inner and outer portions of the throat. Both 
the higher pressure and higher temperature gradient in- 
duce higher tensile stresses in the throat material. There- 
fore, this over-testing method provides a good indica- 
tion as to the reliability of the nozzle under the lower GO 
tensile stresses encountered in normal use. 
In many rocket motor designs, the throat of the nozzle 
is located within the motor chamber wherein fuel is 
burned. As a result, the outside of the throat is subjected 
to large compressive forces from the burning gases which 65 
have not yet exited from the motor chamber. These 
pressures induce large compressive stresses in the nozzle 
throat walls. Such nozzles cannot be subiected to higher 
stresses. 
Another object is to provide a method for more ac- 
curately testing a rocket motor wherein the nozzle throat 
is submerged within the motor chamber. 
In accordance with the present invention, a method 
is provided for testing rocket nozzles to produce large 
thermally induced stresses without correspondingly large 
pressure induced stresses. The method is especially adapt- 
able to the testing of rocket motors wherein the nozzle 
25 throat is submerged within the motor chamber. In the 
case of such nozzles, the method of the invention pro- 
duces large net tensile stresses in the throat section to test 
it for reliability. 
The method of this invention comprises the initial op- 
eration of the rocket motor at a high mass flow rate. 
This results in rapid heating of the inner surface of the 
nozzle throat, so that a large temperature differential 
exists across the thickness of the throat walls, resulting 
in large tensile stresses. Of course, the high mass flow 
rate also results in a high pressure with correspondingly 
high pressure induced stresses. In the case of a submerged 
throat, the high pressures produce compressive stresses 
that counteract the thermally induced stresses. However, 
after a few seconds, when the thermal gradient in the 
nozzle walls has almost reached a maximum level, the 
mass flow rate is cut to a low level. 
The pressure of the gaseous fluid on the nozzle throat 
drops rapidly as the mass flow rate is rapidly lowered. 
However, the thermal gradient decreases a t  a substan- 
tially lower rate. During the time when the pressure is 
a t  a low level but the thermal gradient is still very large, 
the tensile stresses in the nozzle throat are at a maximum 
IeveI. If a high mass flow rate is used, the tensile stresses 
existing immediately after the mass flow rate is cut are 
substantially higher than the levels which can be achieved 
during continuous operation at any constant mass flow 
rate. The ability of the nozzle to withstand these high 
tensile stresses is a good indication that the nozzle will 
reliably withstand the lower net tensile stresses encoun- 
tered under normal use conditions. 
The invention will best be understood from the fol- 
lowing description when read in conjunction with the 
accompanying drawings. 
QRIGIN OF INVENTION 
The invention described herein was made in the Per- 
farmance of work under a NASA contract and is sub- 30 
ject to the provisions of Section 305 of the National Aero- 
nautics and Space Act of 1958, Public Law 85-568 (72 
Stat. 435; 42. U.S.C. 2457). 
BACKGROUND OF THE INVENTION 
Field of the invention 
tensile stress levels merely by operating the motors at 
higher mass flow rates. Although the higher mass flow 70 
rate causes larger temperature gradients and therefore 
larger tensile stresses, these higher stresses are counter- 
DESCRIPT1oN OF THE DRAWINGS 
FIG. 1 is a sectional perspective view of the rear por- 
tion of a rocket motor, showing a typical rocket nozzle 
rapid heating of the inner surface 50. 
Counteracting the thermally induced tensile stresses in 
the throat insert 16 are compressive stresses caused by 
the high pressure of the burning gases. The burning fuel 
produces a large pressure on the outer surface 18 of the 
nozzle. A substantially lower pressure exists at the 
inner surface 20 because of the high velocity of the gases 
flowing therethrough. The pressure at  the narrowest part 
of the throat insert 16 is only approximately 60% of 
the pressure at the outer surface 18. The compressive 
stresses resulting from the pressure of gases lowers the 
nozzle stresses to a moderate tensile stress level. 
Typical operating conditions of high performance rocket 
motors result in temperatures at the inner surface 20 
on the order of 5,000" F. and gas pressures on the 
outer surface 18 of approximately 700 p.s.i. Higher per- 
formance rocket motors often use graphite material for 
the throat insert 16 to withstand these conditions. Such 
materials have relatively low tensile strengths on the 
order of 2,000 to 3,000 p.s.i., and failure is most likely 
to occur from high tensile stresses. The graphite ma- 
terials are generally stronger a t  higher temperatures, so 
it is actually the cooler portions of the nozzle, that is, 
the portions near the outer surface 18, that are weakest 
in operation. The highest tensile stresses occur in two 
planes perpendicular to the center line of the nozzle, 
these being the planes indicated by the lines 22 and 
24. A location 26 in the plane indicated by line 24, and 
between the inner and outer surfaces of the nozzle, is 
where the nozzle is most likely to  fail under the high 
tensile stresses on the order of 3,000 p.s.i. produced dur- 
ing operation. 
An important test of rocket motor nozzle reliability is 
a test wherein the throat insert 16 is subjected to  tensile 
stresses of a higher level than are encountered in normal 
flight use. This is because the failure of the throat insert 
under tensile stress is one of the most likely causes 
of failure. It might be expected that the tensile stresses 
can be increased by merely operating the rocket motor 
at a higher mass flow rate. However, a higher mass flow 
rate does not result in higher tensile stresses at the 
throat insert for submerged nozzles. 
As the mass flow rate increases, and pore gas flows 
through the nozzle, the convective heat transfer to the 
throat (which is the primary means of heating) increases 
3) 534) 597 
3 
which can be tested by the method of the present inven- 
tion; and 
FIG. 2 is a graph showing stresses produced at an 
inner area of the nozzle of FIG. 1. 
by the 0.8 power of the pressure. Because of the ther- 
mal response of the throat material, this heating increases 
the tensile stress in the throat insert. However, the pres- 
sure loading increases by the 1.0 power of the pressure 
within the rocket case, producing compressive stresses 
that offset the increased tensile stress. The pressure in- 
duced stresses increase at a higher rate (the 1.0 power) 
FIG. 1 illustrates a rocket motor for which the test than the rate of increase of tensile stresses (the 0.8 
method of the present invention is particularly useful. power), so that the net tensile stresses actually decrease 
The motor comprises a motor case 10 which contains a for higher mass flow rates. 
solid propellant 12. A nozzle assembly 14 is located at In accordance with the invention, large tensile stresses 
the aft end of the rocket, and has a throat insert 16 dis- are produced in the throat insert 16 by initially operat- 
posed within the motor case. The location of the throat ing the rocket motor at a high mass flow rate, generally 
insert 16 results in its outer surface 18 receiving the gas .one which exceeds the rate used in normal flight. This 
pressure produced by the burning propellants before they 16 results in rapid heating of the inner surface of the 
pass through the inner surface 2cP of the throat section. nozzle throat, producing large temperature differentials 
These pressures on the outer throat surface decrease the and correspondingly large thermally induced stresses. Of 
tensile stresses on the throat insert, thereby increasing course, the compressive stresses produced by the high 
its reliability. However, this has heretofore also resulted pressure counteract the thermal stresses, and generally 
in increased difficulty in reliability testing. 20 produce a net stress which is somewhat below the level 
The stresses induced in the nozzle insert 16 come encountered in flight operations. 
from several sources. When the rocket motor i s  started, After several seconds of operation, when the thermal 
all portions of the nozzle insert 16 are at ambient tem- differential and corresponding thermally induced stresses 
perature. The rush of the hot gases through the throat reach nearly a maximum level, the mass flow rate is 
rapidly heats the inner surface 20 of the throat insert. 25 suddenly cut from the high range shown prior to time I1 
The outer surface 18 is also heated, though at a sub- to the low range which is reached a short time there- 
stantially lower rate. The heating of the insert 16 produces after. The mass flow rate generally is not reduced 
tensile stresses due to expansion. Also, tensile stresses to  Era, because the resultant net tensile stresses may 
of a high order of magnitude are produced by reason far exceed the capability of the throat material, thus 
of the thermal gradient resultinn from the extremely 30 causing nozzle failure, and preventing achievement of 
the test obiectives. If a maximum tensile stress level is 
DESCRIPTION OF THE PREFERRED 6 
BMBODIMENTS 
desired, however, the mass flow rate may be cut to 
zero. The cut in mass flow rate results in a rapid drop 
in pressure, but not in temperature differential so that 
35 the net tensile stress becomes very large for a brief 
period. 
FIG. 2 is a graph showing the stress levels produced 
in a typical nozzle po&ion during an over-test, when 
the mass flow rate is initially higher than the rate to  be 
40 used during a typical flight. The location 26 in the throat 
insert represents one such critical area which is among 
those most likely to fail by tensile stresses. When the mo- 
tor is started, the gas pressure rises to a high level in a 
very short period. As a result, the pressure induced stress 
45 level indicated by the graph line 30 reaches an appreciable 
level during a short period of time. The pressure induced 
stresses produced in the critical area of the throat are 
initially compressive, and therefore are shown at a level 
below the zero stress level. As the gases move through the 
60 throat, the inner surface 20 is rapidly heated, resulting 
in thermally induced stresses shown by the graph line 32, 
these stresses being tensile. The net stress, shown by the 
graph line 34 is equal to the difference between the pres- 
sure induced and thermally induced stresses. 
The thermally induced stresses, indicated by line 32, 
rise to a higher absolute magnitude than the pressure in- 
duced stresses, so that the net stress shown by line 34 
becomes tensile after a short period of operation. The 
dashed line 36 indicates the net stress level under norma1 
60 use conditions when the pressure is at the level shown by 
line 38. It can be seen that the net stress shown by line 
34 during the first period of the overtest is lower than 
the net stress of line 36 for a normal mass flow rate. The 
initially lower net stress shown at 34 is due to the fact 
65 that the increase in mass flow rate over the normal mass 
flow rate results in a greater increase in pressure induced 
(compressive) stress than the thermally induced (tensile) 
stress. 
The thermally induced stresses are dependent primarily 
70 upon the temperature gradient within the throat section. 
After a brief period of operation, the thermal gradient, 
and therefore the thermal stresses, reach a maximum and 
begin to decline. FIG. 2 shows the stress variation in the 
operation of a medium size nozzle having a minimum 
75 throat diameter of approximately 2 inches and a wall 
65 
3,534,597 
thickness at the throat of approximately 1 inch, wherein throat portion whose outer surface is subjected to the 
the maximum thermal stress was reached after approxi- high pressure gases which power the motor comprising: 
mately 3 seconds of operation. At a time tl which is just operating said motor at a mass flow rate within a first 
prior to the time of maximum thermal stress, the mass flow range; and 
rate is suddenly cut to a low level. The pressure induced rapidly reducing said mass flow rate to a second range 
stresses 30 quickly drop to a low level. However, the ther- that is substantially above zero, beginning at a time 
mally induced stresses 32 do no change rapidly. As a re- near the time of maximum thermally induced stresses 
sult, the net stress 34 increases so that a large tensile stress at predetermined portions of said nozzle throat por- 
is produced in the throat insert. For a certain time period, tion. 
the net stress 34 during the overtest exceeds the maximum 3. A method for testing apparatus which is subjected 
net stress 36 produced during normal flight operation. to hot fluids under high pressure, and which has thermal- 
If the nozzle insert does not fail during this critical period, ly induced stresses which reach a maximum level a pre- 
there is a strong indication that it will perform reliably determined time after subjection to said hot fluids under 
in normal flight use. said high pressure, said thermally induced stresses there- 
Although particular embodiments of the invention have 15 after declining from said level, to  establish high thermal 
been described and illustrated herein, it is recognized that stresses without correspondingly, high pressure induced 
modifications and variations may readily occur to those stresses, comprising: 
skilled in the art, and, consequently, it is intended that the initially subjecting said apparatus to said hot fluid while 
claims be interpreted to cover such modifications and said fluid is at high pressure, until a time when the 
equivalents. thermal stresses established in said apparatus are at a 
What is claimed is: high level; and 
1. A method for testing apparatus which is subjected decreasing the pressure of said fluid at said apparatus, 
to hot fluids under pressure, to establish high thermal at a rate substantially greater than the rate of change 
stresses without correspondingly high pressure induced of thermal stresses, beginning at a time when said 
stresses comprising: 25 thermally induced stresses are near said maximum 
initially subjecting said apparatus to said hot fluid while level. 
said fluid is a t  a high pressure which is greater than 
the maximum pressure of fluid to which said appara- 
tus is subjected under normal use conditions; and 
ct a rate substantially greater than the rate of change 
of thermal stresses, beginning at a time when the 
thermal stresses are a t  a high level, and continuing 
the decrease of said pressure only to a level which is 
substantially above zero. 
2. A method for testing a rocket motor having a nozzle 
20 
References Cited 
UNITED STATES PATENTS 
3,265,314 8/1966 Helms et al. _ _ _ _ _  239-128 X decreasing the pressure of said fluid at  said apparatus, 30 3*112i614 12/1963 Ellenberg et --- 239-591 
JERRY w. MYRACLE, Primary Examiner 
U.S. C1. X.R. 
35 73-15.6, 49.7 


Rocket nozzle test method Patent

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