Qualitative measurements of the effective heats of ablation of several materials in supersonic air jets at stagnation te...

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Title:
Qualitative measurements of the effective heats of ablation of several materials in supersonic air jets at stagnation temperatures up to 11,000° F
Series Title:
NACA RM
Physical Description:
25 p. : ill. ; 28 cm.
Language:
English
Creator:
Rashis, Bernard
Witte, William G
Hopko, Russell N
Langley Aeronautical Laboratory
United States -- National Advisory Committee for Aeronautics
Publisher:
NACA
Place of Publication:
Washington, D.C
Publication Date:

Subjects

Subjects / Keywords:
Ablation (Aerothermodynamics)   ( lcsh )
Supersonic planes   ( lcsh )
Aerodynamics -- Research   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
Abstract: The ablation rates and effective heats of ablation were determined for a number of materials in supersonic ceramic-heated and electric-arc-powered air jets at stagnation temperatures ranging from 2,000° F to 11,000° F. The results indicated that the effective heats of ablation were from 7 to 40 times greater than the heat-absorption capacity of copper and increased with increasing aerodynamic heat flux. In addition, the results indicated that for the materials having phenolic-resin binders, the effective heats of ablation decreased with increasing resin content.
Bibliography:
Includes bibliographic references (p. 12).
Statement of Responsibility:
by Bernard Rashis, William G. Witte, and Russell N. Hopko.
General Note:
"Report date May 5, 1958."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003810791
oclc - 135481797
sobekcm - AA00006136_00001
System ID:
AA00006136:00001

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NACA RM L58E22

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS


RESEARCH MEMORANDUM


QUALITATIVE MEASUREMENTS OF THE EFFECTIVE HEATS OF

ABLATION OF SEVERAL MATERIALS IN SUPERSONIC

AIR JETS AT STAGNATION TEMPERATURES

UP TO 11,0000 F *

By Bernard Rashis, William G. Witte, and Russell N. Hopko


SUMMARY


The effective heats of ablation of a number of materials were
derived from tests in supersonic air jets at stagnation temperatures
ranging from 2,0000 F to 11,0000 F. The materials included the plastics
Teflon, nylon, Lucite, and polystyrene; the inorganic salts ammonium
chloride and sodium carbonate, several phenolic resins of varied resin
content and type of reinforcement; and a melamine-fiber glass laminate.

The results indicate that the effective heats of ablation range
from 7 to 40 times greater than the heat-absorption capabilities of cop-
per not undergoing ablation at the conditions tested. The effective
heats of ablation for Teflon, nylon, and Rocketon, which were tested in
both a ceramic-heated and an electric-arc-jet, increased with increasing
aerodynamic heat flux. For several glass-reinforced phenolic-resin
models, the resin content of which varied from 27 percent to 65 percent,
the effective heats of ablation, for the same aerodynamic heat fluxes,
decreased with increased resin content.

The inorganic salts, ammonium chloride and sodium carbonate, from
which models were constructed by cold-pressing of crystals, did not
compare well with the other materials with regard to strength; however,
these models did show the highest values of effective heat of ablation
at comparable aerodynamic heat fluxes in comparison with the other
materials tested.


*Title, Unclassified.







2 NACA RM L58E22


INTRODUCTION


In order to reduce dispersion error due to winds and density
variations for long-range ballistic missiles, consideration is being
given to configurations with high weight-drag ratios. This require-
ment greatly intensifies the heating problem, the heat inputs to the
nose being aLmost 20 times greater than for subsonic impact. These
heating rates are of such magnitude that the heat-flux inputs which
the nose-cone material must absorb are almost 40 times greater than
those which conventional materials such as copper can withstand, as
indicated by the analysis of reference 1, which takes into account the
temperature gradient through the material.

Previous investigations of this problem by the Langley Pilotless
Aircraft Research Division have involved the study of transpiration
cooling (ref. 2), endothermic decomposition (ref. 5), upstream ejection
of water (ref. 4), upstream ejection of solids (ref. 5), and radiation,
latent heat of fusion, pyrclysis, and ablation (ref. 6). All these
schemes may be considered as a means of increasing the effective heat
capacity of the nose shape. The purpose of this paper is to present
some qualitative measurements of the effective heat-absorption
capacities of a number of materials undergoing ablation. The tests
were conducted in a ceramic-heated jet of the Langley Pilotless Air-
craft Research Division and an electric-arc-powered air jet of the
Langley Structures Research Division at a nominal Mach number of 2.0.


SYMBOLS


A surface area of nose shape, sq ft

Cp,g specific heat of gas layer, Btu/lb-OF

D diameter of nose shape, ft

h heat-transfer coefficient, Btu/(sq ft)(sec)(OF)

heff effective heat of ablation, Btu/lb

He boundary-layer-recovery enthalpy, Btu/lb

Ht stagnation specific enthalpy, Btu/lb


gas-layer specific enthalpy, Btu/lb






NACA RM L58E2?


i average heat flux to nose shape, Btu/(sq ft)(sec)

qt stagnation heat flux, Btu/(sq ft)(sec)

Tt stagnation temperature, OF

Tw surface temperature, OF

w average nose-shape ablation rate, lb/sec

w- average unit-area nose-shape ablation rate, lb/(sq ft)(sec)



GENERAL CONSIDERATIONS


In figure 1, there are shown schematic drawings of bodies under-
going two types of ablation: In figure l(a) the body first melts
forming a relatively thin layer of molten material or a liquid film
which is then vaporized, the vaporized material forming a relatively
thick gas layer; this process is hereinafter referred to as ablation by
melting and vaporization. In figure l(b) the body is directly vapor-
ized, the vaporized material also forming a relatively thick gas layer;
this process is hereinafter referred to as ablation by sublimation. All
the materials investigated underwent ablation in one of these two ways.
References 7 and 8 give detailed mathematical explanations of the
mechanism of ablation by melting ani vaporization. A brief discussion
of the ablation mechanism as described in these references is given in
the following paragraphs without resort to mathematical analysis. A
brief discussion is also given of ablation by sublimation.

The following schematic diagram shows the heat flux for both types
of ablation:







NACA RM L58E22


qmas s
ejection


Gas layer


heat of
fusion


heat of
vaporization

sensible

chemical
reaction


qmaess
ejection


heat of
sublimation

Sensible


chemical
reaction


Melting and vaporization Sublimation


For ablation by melting
equation is


qaero chemical
reaction


heat of
vaporization


and vaporization, the general heat-balance


- ({ass
\ ejection


+ liquid +. heat of +
film fusion


+ sensible + radiation) = 0


For ablation by sublimation, the general heat-balance equation is


_aero q aero






NACA RM L58E22


qaero I chemical (Qmass + heat of + sensible +
reaction \ ejection sublimation


radiation = 0


The term qaero is the aerodynamic heating due to the movement of
the airstream over the body and the temperature difference between the
airstream and the nose-shape surface. The term chemical is the
reaction
heating due to the reaction of the body material with the hot airstream.
The term liquid is the heat which is absorbed by the liquid film.
film
The liquid film is moved downstream by the drag exerted on it by the
boundary layer; thus, heat is picked up at the stagnation point and moved
rearward. A result of the detailed analysis of reference 8 is that the
liquid film is capable of blocking 50 percent of the heat flux into the
liquid film from reaching the nonflowing solid interior for the specific
case analyzed. The terms qheat of and heat of are the heats
fusion vaporization
absorbed in melting or vaporizing the material undergoing ablation. The
term sensible is the heat which is absorbed by the ablated material in
being raised from its initial temperature to the melting temperature in
ablation by melting and vaporization and to the sublimation temperature
in ablation by sublimation. The term radiation is the heat emitted
from the body by radiation.

For both cases, the predominate term is mass which is the
ejection
heat that has been blocked by the gas layer. A simple physical picture
of how this blocking is achieved is obtained by considering the hot air-
stream as having a specific enthalpy Ht and the'ejected gas as having a
specific enthalpy Hg which is much less than Ht. Assuming that all
of the ejected gas mixes with the hot airstream, a gas layer is formed
over the body which is at a specific enthalpy level He. The value of
He is greater than Hg but is less than Ht. The effect of mass
ejection thus reduces the specific enthalpy level of the boundary layer.
In addition, the ejected gas has absorbed the quantity of heat (He Hg).
Since the heat input to the body depends on the enthalpy or temperature
difference between the boundary layer and the body surface, the ejected
gas has been of twofold action in blocking heat that would have gone into
the body material. This twofold action accounts for the fact that the
shielding or blocking effect increases nonlinearly with the aerodynamic







NACA RM L58E22


heat input. As a consequence, the term q-mass tends to predomi-
ejection
nate at high values of aerodynamic heat flux. This explanation has
considered only first-order effects. In actuality, the mass-ejection
mechanism is very complex.

It should be noted that the ablation mechanism acts in the direc-
tion that tends to keep the surface temperature relatively constant over
wide ranges of qaero- It is thus deduced that in ablation by melting
and vaporization an increase in qaero will show up essentially as a
decrease in the thickness of the liquid film more so than as an increase
in the surface temperature. In ablation by sublimation an increase in
qaero will show up as an increase in the enthalpy difference across the
gas layer much more so than as an increase in the surface temperature.

It should not be inferred that since ablation by melting and vapori-
zation has an additional heat-absorbing term it is a more effective
method than ablation by sublimation. In ablation by melting and vapori-
zation the term qmass is evaluated for only the amount of the
ejection
ablated material which is vaporized, which may be a small fraction of the
melted material. In ablation by sublimation all of the ablated material
is ejected into the boundary layer, and qmass may be considerably
ejection
larger than for ablation by melting and vaporization.


TEST FACILITY AND TEST MODELS


The ceramic-heated jet (laboratory model) and the electric-arc-
powered air jet were utilized for the present tests. Reference 9 gives
some of the details of construction and operation of these two facili-
ties. Several preliminary tests, conducted in the ceramic-heated jet,
were made of models having configurations B and D shown in figure 2.
The basic configurations A and C (fig. 2) were adapted for the remainder
of the tests conducted in the ceramic-heated jet. Configuration E
(fig. 2) was tested in the electric-arc-powered air jet. Only the
materials Teflon, nylon, and Rocketon were used for this configuration.

The Teflon, Lucite, nylon, polystyrene, Rocketon, and Planeton
models were machined to size from commerically obtained materials. The
ammonium chloride ani sodium carbonate models were machined to size from
rrds formed by cold-pressing commerically obtained crystals. One-half-
inch-diameter phenolic-resin models were machined from one-half-inch-
diameter rods supplied by the Cincinnati Tecting Laboratory; these rods






NACA RM L58E22


were all of high-temperature phenolic resin no. 37-9-X, but varied in
percent of resin content and reinforcement material. The preliminary
test models of configuration D were machined from commercially obtained
reinforced phenolic and melamine resin rods. The Haveg Rocketon and
Planeton materials are similar in composition to the phenolic-resin
materials.

For all the tests conducted in the ceramic-heated jet, the jet
exit Mach number was approximately 2.0 and the exit pressure was main-
tained at approximately sea-level pressure. The models were mounted on
a side-injection-type sting and were inserted into the stream only after
the required flow conditions were established. A timer which was
synchronized with the sting was visually recorded along with the model
for all the tests by high-speed motion-picture cameras. For the ceramic-
heated jet, the stagnation temperatures were determined from calibrated
pyrometer readings of the top surface of the heated ceramic bed. For
the electric-arc-powered air jet the stagnation temperature (11,0000 F)
was determined from a calculation based on the measured jet static tem-
perature which was observed with a spectroscope.

The surface temperatures of the models were not measured during the
tests. The values used in the calculations were obtained from static
test measurements of the melting temperatures for all the materials
which exhibited a liquid film. In later tests, the surface temperatures
were measured by an optical pyrometer while the models were undergoing
ablation. The values obtained by this method were in agreement with the
values previously assumed from the static tests. The sublimation and
melting temperatures given in reference 10 were used for the ammonium
chloride and sodium carbonate, respectively. The Teflon does not have
a definite sublimation temperature; however, a survey of the available
literature on Teflon indicated that a surface temperature of 1,0000 F
would be reasonable for the present tests. In table I there are
listed the densities, thermal conductivities, and the specific heats of
most of the materials tested. A list of all the materials tested is
given in table II.


DATA REDUCTION


The ablation rates of the various materials were determined by two
methods. One method involved the weighing, on an analytical balance,
of the models before and after testing. The other method involved
measuring the velocity at which the stagnation point receded from
enlargements of high-speed motion-picture film. The best results with
regard to sharpness and clarity of detail were obtained when the
16-millimeter motion-picture film was magnified 25 times. An arc was






NACA RM L58E22


then fitted to the nose contour and the radius of this arc was used in
the calculation of the volume and the heat-transfer coefficient. Both
methods checked except in the case of Teflon, which has a relatively
large expansion when heated. In all cases, the motion pictures indi-
cated that after a certain time had elapsed, the velocity at which the
stagnation point receded was constant with time.

The average aerodynamic heat input into the nose shape was com-
puted for the air-jet tests from


q = 0.5q = 0.5 (Tt Tw) (1)



where the values of (hf/) were taken from the curve of figure 3(a),
which was taken from reference 11. The constant 0.5 is the ratio of
the average heat flux for the entire nose to that at the stagnation
point, assuming laminar flow over the nose shape. Laminar heating was
assumed since the Reynolds numbers of the tests based on model diameters
were less than 60,000. The Mach number, static-pressure, velocity, and
static-temperature ranges in the ceramic-heated jet are also given in
figure 3.

For tests in the electric-arc-powered air jet the heat flux at the
stagnation point was determined from a calorimeter model to be approxi-
mately 3,140 Btu/(sq ft)(sec). The average heat flux was assumed to be
one-half this value or 1,570 Btu/(sq ft)(sec).

The effective heat of ablation was computed from



heff = (2)


where A is the surface area of the nose shape. The weight-loss values
were corrected for extraneous losses from the model sides and the values
of D and A were averaged to account for the slight changes in dimen-
sions which occurred during the tests.


RESULTS AND DISCUSSION


In figure 4 there are shown the average ablation rates per unit
area as a function of the average aerodynamic heat flux for Teflon,
iylon, and Rocketon. It is clearly indicated from figure 4 that the







NACA RM L58E22


ablation rates determined from the tests in the arc jet are considerably
lower than those that would be indicated by a linear extrapolation of the
values obtained in the ceramic-heated jet. Since the diameters of the
models tested in the two jets, as well as the test conditions, were
different, it cannot be assumed that I alone is the correlating param-
eter; however, the marked dependence of the ablation rates upon q cer-
tainly establishes it as a parameter of primary importance.

In figure 5 there is shown the variation of heff with q for the
data shown in figure 4. It should be noted that the values of heff
obtained from tests in the arc jet are approximately two to three times
greater than the values of heff corresponding to the highest value of
q for the tests in the ceramic-heated jet. Although the Teflon indi-
cates a rather high value of heff for the lowest heating rate, it is
felt that the results at low heating rates are influenced somewhat by
conduction into the solid and hence do not reflect values of effective-
ness for steady ablation. This conduction effect was investigated
experimentally for the Teflon by testing a series of models at approxi-
mately constant heating rate (f = 68 Btu/ft2-sec) for successively longer
periods of test time. The results shown in figure 6 indicate that the
ablation rates rise sharply and then level off. Only the steady value
was utilized for the results presented herein. The resulting error due
to this finite time requirement to obtain steady ablation is about
10 percent; application of a correction for this error to the heff
value of 5,100 Btu/lb at q = 76 Btu/(sq ft)(sec) results in the cor-
rected value of heff = 2,850 Btu/lb.

Although the results shown in figure 5 have a small conduction
error, it should be noted that the materials tested may very well be
affected by conduction under prolonged low heating rates. For compari-
son purposes the calculated useful heat capacity of a copper heat sink
(ref. 1) is shown.

In figure 7 there is shown the variation of the effective heat of
ablation with resin content for two values of q for the 1/2-inch-
diameter glass-reinforced phenolic-resin models. The results indicate
that the value of heff decreases with increasing phenolic-resin con-
tent. The heat liberated from the reaction of the resin with the hot
stream increases with the increasing resin content. At the higher heat
condition the percentage effect of the burning decreases.

In table II, there is given a summary of all the test conditions.
Except for Teflon, nylon, and Rocketon, for which arc-jet tests were
conducted, all the tests were conducted in the ceramic-heated jet at
values of q ranging from 70 to approximately 250 Btu/(sq ft)(sec). It






NACA RM L58E22


is of interest, however, to note that the inorganic salts indicated
relatively high values of heff as compared with the other materials.
Also, the models having configuration D (fig. 2), which were machined
from commercial materials, indicate an increase in heff with finer
mesh fiber glass reinforcement. Since the finer mesh model probably
had a smaller percent of resin content, the results are probably
affected in the same manner as the 1/2-inch-diameter resin models. The
values of heff as a function of q for the materials for which tests
were conducted only in the ceramic-heated jet are shown in figure 8.


Motion-Picture Observations

Examination of the color motion pictures and the models tested in
the ceramic-heated jet indicated that all the Teflon models acted in
essentially the same manner. The Teflon surface was slick; there were
no visible signs of vapor, flaming or melting, just a slow disappearance
of the Teflon. The nylon, lucite, and polystyrene at approximately
2,2000 F had a liquid film which appeared to extend for some distance
rearward of the hemisphere-cylinder .j.ncture. At approximately 5,8000 F
this liquid film appeared to vaporize downstream. The Haveg Rocketon and
the resin models all acted somewhat similarly. At approximately 2,9000 F
and 5,8000 F the noses of the models glowed brightly and liquid appeared
to be coming from the nose and gradually vaporizing as it moved downstream
along the sides of the models. The two inorganic salts, ammonium chloride
and sodium carbonate, acted in the same manner as the Teflon. There were
no visible signs of flaming or melting or glowing of the surfaces, just a
slow disappearance of the material for about 5 to 6 seconds for the
ammonium chloride and for about ) to 4 seconds for the sodium carbonate.
The models began to break up after these times. All the other materials
exhibited good strength characteristics; however, very minute fractures
sometimes developed into large cracks during the test, particularly for
the Haveg Rocketon models. Figure 9 shows some typical models after
testing.

The color motion pictures of the arc-jet tests indicated no change
in the manner in which ablation occurred on the Teflon as compared with
the models tested in the ceramic-heated jet. The nylon model did not
zhow any clear-cut liquid film, but the nose surface was slick in
appearance and there may have been some film. The Rocketon model
appeared to vaporize.

The motion pictures indicated that during the tests in the ceramic-
heated jet noticeable ablation did not start instantly for the Teflon and
nylon models. There was a lag time of approximately 0.7 second for the






NACA RM L58E22


Teflon at a value of q of 76 Btu/ft2-sec. This lag time decreased to
about 0.3 second at a value of q of approximately 200 Btu/ft2-sec.

The nylon lag times ranged from approximately 0.2 second at a value
of q of 110 Btu/ft2-sec to slightly less than 0.1 second at a value of
Z of 250 Btu/ft2-sec. These lag times are due possibly to a finite time
requirement, in practical cases, for the surface to reach a temperature
at which melting or sublimation will occur. Motion pictures of tests in
the arc jet show no lag times for the materials tested.


SUMMARY OF RESULTS


The ablation rates and the effective heats of ablation were eval-
uated for a number of materials in supersonic ceramic-heated and electric-
arc-powered air jets for stagnation temperatures ranging from 2,0000 F to
11,0000 F. The average heat fluxes ranged from 20 to 1,570 Btu/ft2-sec.
The following results were obtained:

1. All the materials tested indicated effective heats of ablation
ranging from 7 to 40 times greater than the useful heat absorption of a
copper heat sink not undergoing ablation.

2. For Teflon, nylon, and Rocketon, which were tested over the com-
plete range of heating rates, the results indicated that the effective
heats of ablation would increase with increasing heating rates.

3. For several glass-reinforced phenolic-resin models, of which the
resin content varied from 27 percent to 65 percent, the effective heats
of ablation, for the same aerodynamic heat flux, decreased with increased
resin content.

4. The inorganic salts, ammonium chloride and sodium carbonate from
which models were constructed by cold-pressing of crystals did not compare
well with the other materials with regard to strength; however, these
models did show the highest values of effective heats of ablation at com-
parable aerodynamic heat fluxes, in comparison with the other materials
tested.


Langley Aeronautical Laboratory,
National Advisory Committee for Aeronautics,
Langley Field, Va., May 5, 1958.





NACA RM L58E22


REFERENCES


1. Stalder, Jackson R.: The Useful Heat Capacity of Several Materials
for Ballistic Nose-Cone Construction. NACA TN 4141, 1957.

2. Rashis, Bernard: Exploratory Investigation of Transpiration Cooling
of a 400 Double Wedge Using Nitrogen and Helium as Coolants at
Stagnation Teuperatures of 1,2950 to 2,9100 F. NACA RM L57F11,
1957.

3. Modisette, Jerry L.: Preliminary Investigation of Lithium Hydride as
a High-Temperature Internal Coolant. NACA RM L57F12a, 1957.

4. Rashis, Bernard: Preliminary Indications of the Cooling Achieved by
Ejecting Water Upstream From the Stagnation Point of Hemispherical,
800 Conical, and Flat-Faced Nose Shapes at a Stagnation Temperature
of 4,0000 F. NACA RM L57103, 1957.

5. Kinard, William H.: Feasibility of Nose-Cone Cooling by the Upstream
Ejection of Solid Coolants at the Stagnation Point. NACA RM L57K22,
1958.

6. Casey, Francis W., Jr., and Hopko, Russell N.: Preliminary Investi-
gation of Graphite, Silicon Carbide, and Several Polymer-Glass-
Cloth Laminates in a Mach Number 2 Air Jet at Stagnation Tempera-
tures of 5,0000 F and 4,0000 F. NACA RM L57K15, 1957.

7. Lees, Lester: Similarity Parameters for Surface Melting of a Blunt-
Nosed Body in a High Velocity Gas Stream. Rep. No. M-TM-184
(Contract No. AF 18(600)-1190), The Ramo-Wooldridge Corp., Guided
Missile Res. Div., July 29, 1957.

8. Sutton, George W.: The Hydrodynamics and Heat Conduction of a
Melting Surface. Jour. Aero. Sci., vol. 25, no. 1, Jan. 1958,
pp. 29-52, 36.

9. Purser, Paul E., and Bond, Aleck C.: NACA Hypersonic Rocket and
High-Temperature Jet Facilities. Rep. 140, AGARD, North Atlantic
Treaty Organization (Paris), July 1957.

10. Hodgman, Charles D., ed.: Handbook of Chemistry and Physics.
Thirty-sixth ed., Chemical Rubber Publishing Co., 1954-1955.

11. Fields, E. M., Hopko, Russell N., Swain, Robert L., and Trout,
Otto F., Sr.: Behavior of Some Materials and Shapes in Supersonic
Free Jets at Stagnation Temperatures up to 4,2100 F, and Descrip-
tions of the Jets. NACA RM L57K26, 1958.






NACA RM L58E22


TABLE I.- MATERIAL PROPERTIES


Material Density, Thermal conductivity, Specific heat,
Ib/cu ft Btu/ft-sec-F Btu/lb-F
(a)


Teflon

Glass
impregnated
with 91-LD
phenolic resin

41-percent
phenolic resin

65-percent
phenolic resin

44-percent
phenolic resin

27-percent
phenolic resin

37-percent
phenolic resin

Phenolic nylon
(57-percent
phenolic)

Ammonium chloride

Sodium carbonate

Nylon

Polystyrene

Lucite


150

115




87.4


100.1


114.0


152.7


95

74.3

69.9

66.1

75.6


35.5 x 10-6

41.7 x 10-6




2 x 10-6


52 x 10-6


52 x 10-6


52 x 10-0


52 x 10-"


52 x 10-


55.2 x 10-4

25.2 x 10-4

48.4 x 10-4


0.5

.25


aAll values of specific heat
that of Teflon, which is based on
perature to approximately 6000 F.


are for room temperature except
the average value from room tem-








NACA RM L58E22


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