A summary is presented of ablation performance data for carbon-carbon nosetip models obtained over a range of pressures from 10 to 168 atm. Two classes of tests are reviewed: (1) steady state, in which a constant environment is imposed on the model, and (2) ramp, in which the pressure is increased from 10 to 80 atmospheres to simulate re-entry pressure history. Comparison of arc test parameters with typical reentry vehicle parameters is included, to assess the adequacy of the test simulation. Based on this comparison, recommendations are made for facility developments which would yield improved simulation capability for reentry vehicle nosetip ablative performance

Nestler, D. E.

English

NASA Technical Reports Server

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N79-19040
Paper No. 32
ABLATIVE PERFORMANCE OF CARBON-CARBON NOSETIPS
IN SIMULATED RE-ENTW/ ENVIRONMENTS
D. E. Nestler, General Electric Company, Re-Entry and Environmental
Systems Division, Philadelphia, Pennsylvania
._BST]_.CT
A summary is presented of ablation performance data
for carbon-carbon nosetip models obtained over a
range of pressures from 10 to 168 aim. Two classes
t of tests are reviewed: (1) steady state, in which a
constant environment is imposed on the model, and
v (2) ramp, in which the pressure is increased from
10 to 80 atmospheres to simulate re-entry pressure
history.
l
Steady state recession rates which differ by a
factor of two for a given material over the pres-
sure range tested are shown to be compatible with
the JANAF thermochemical theory for graphite, pro-
vided that an augmentation of turbulent heating due
to surface roughness is introduced.
Ramp tests are used to deduce the pressure at which
boundary layer transition occurs, from photograrhic
observation of gouging due to turbulent heating.
Different camera views, as well as post-test evalu-
ation, show that boundary layer transition on 3-D
carbon-carbon is not uniform; transition occurs
earlier at 45 degrees between the orthogonal weave
axes •
Comparison of arc test parameters with typical re-
entry vehicle parameters is included, to assess the
adequacy of the test simulation. Based on :his
comparison, recomendations are made for facility
developments which would yield improved simulation
capability for re-entry vehicle nosetJp ablative
performance.
INTRODUCTION
Three-d/mensional weaves of carbon-carbon are prime contend-
ers for nosetlps of future re-entry vehicles. The 3-D carbon-
carbon material can provide ablative performance comparable to
that o, 8zap,,,_e. but with superior mechanical properties.
393
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1979010842-385
https://ntrs.nasa.gov/search.jsp?R=19790010869 2020-03-20T18:11:51+00:00Z
It has become current practice to test advanced carbon-
carbon noeetlp materlals in high pressure arcs to determine their
relative ablative performance prior to fllsht-testlng selected
candidate materlals. Two types of tests are commonly conducted:
(1) steady state, in which a constant enviror_ent is imposed on
the model, and (2) ramp, in which the pressure is steadily in-
creased by advancing the model towards the nozzle exit.
The objective of this paper is to present the results of
evaluations of selected arc test data of carbon-carbon nosetip
models, _ncluding an assessment of the adequacy of the test
simulation.
NOMENCLATURE
B dimensionless blowlns parameter; B _ -
Ch
Ch heat transfer coefficient for actual surface
Cho heat transfer coefficient for smooth surface
=. E Euler number; E = S due
Ue dS
HCL centerllne enthalpy ,_farc flow
Hs enthalpy of solid parulcles belns mechanically ."
rmoved from ablating surcace.
wall enthalpy
K surface roughness
mm thermomechanical mass loss rate
_C thermochemical mass loss rate
Pr Prandtl number
: p local static p_essure
Ps stagnation pressure
_c convective heat flux
: _RR reradi_cive heac flux
_T turbulent convective hsat flux
Re* local sonic point unlC length Reynolds number;
Re* = De Ue/_e
S wetted surface length measured from stagnation point
steady scats recession rate
Te static cemperacure at edge of boundary layer
Tw wall temperature
I Tu turbulence level
Ue velocity at edge of boundary layer
cons half angle
£T compressibility correction in turbulent heat flux
relation
_e absolute viscosity ac edgs of boundary layer
_s density aC edge of boundary layermoBentum thickness of boundary layer
394
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1979010842-386
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STEADY STATE TESTS
The two facilities moat frequently u_e_ for _d 7 _tate
tests are the 50-meg_att arc cf the Alr Force _'- ,)_namics
Laboratory and the HIP facJllty of the M_c_ooneIl-_,>,_las Re-
search Laboratory. In :he 50-meg_watt arc, in_:i_ly spherical-
nose models sharpen to a blconlc shape (Figure IA) wlth a quasi-
steady state turbulent recession rate. In the HI_ facility, the
models are initially bicontc, and retain their t_rbulent shape
durin$ exposure (Figure 1B). Nominal test cor_tions for steady
state tests in these facilities are given below:
ENTHALFY MODELDIAM.
FACXLX_ P_SS_E (ATMl _L_S_ (_) (in.)
50 MWArc 80 11600 $000 1.27 0.50
HIP 124 6914 3000 0.762 0.30
HIP 168 8027 3800 0.762 0.30
The enthalpy values quoted are flow centerline values which
are inferred from calorimeter heat flux measurements as required
to match theory, and tacitly assume no hast transfer augmentatlon
due to free stream turbulence in the arc flow.
Centerllna recession history correspondlng to Figure I^ is
shown in Fiaure 2. Averase recesslon rates are determlned from
such plots for the quasl-steady or equilibrium ablated shape.
Recession rates for identical carbon-carbon models were cot-
elated empirlcally with test conditions for a range of pressure
from 80 to 168 atmospheres (Fisure 3). The corcelatlns parameter
employs exponents of 0.8 and 0.2 on sta_natlon pressure and model
dimmer-r, respectively, since these exponents appear in simple
turbulent boundary layer heat flux theory•
It is relatively straishtforward to show quali_atively by
means of the sraphie_ thermochemlcal response models that an In-
crease in the value of HCL or Pa from one steady state run to
another wlll _auaa an increase In steady state recession rate _.
The derivation of an explicit proportionality expression relatlnS
to HCL and Ps Is not stralshtforvard, however, 4us to the com-
plex variation of the mass loss rate parameter B with pressure
and wall temperatu:e (Figure 4).
In the 50 MW or HIP hlgh pressure arc anvlronmen[s, the heat
flux is 8o hlgh that the surface temperature is driven l'to the
sublimation realms. In a steady state test, Tw is deteruined by
the steady state eneraY balance:
0. " - - ;,", (1)
395
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1979010842-387
wFIGURE 1. TYPICAL SHAPE C_LANGE HISTORIES
2.09
135 ° l, _8
1,67
l. 46
1.25
1.0._
0. _.1
0. C_._
O, 42
0.2!
0 SEC
TIME INCI(EMENT 0. 10 ,_ECJ
B. HIP - 124 Arm.
, 300
J
s
.°
,i
. p_l, _ .v, _"
1979010842-388
_r 0.45 - [3 FILM DATA UREMENT7A LENGTH CHANGE MEAS
Z 0.40 -- K
0.35 --
O.30 --
LEAST
0,25 -- _*_
../ SQUARES FIT
/: _ 0.20 --0.15 --
rn_j ' -J s = 0.249 IN/SEC
0.I0 -- " =
0.05 [3
o.oo P i I I I I I I I I I
" 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200
' TIME, MILLISECONDS
FIGURE 2. Typical Recession Rate Determination (50 MW Test) **
0.4 -
50 MW - STM 80 ATM T_fl_-" y
' O HIP - STM 124, 168 ATM .]. /Jl 10.3 -
, O" HIP - RACE 80 ATM I /
= _ O_ HIP - REVMAT 10
_ 0.2 --
0°1
I I 1
5
1 " 2 x lO
'* FIGURE 3. CORRELATION OF STEADY
(arm)O'e (btu/ib)/(in)0'2
' STATE RECESSION RATE
• 397
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1979010842-389
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1979010842-390
Neglecting thermomechanJcal mass loss rate mM, Eq. (1) may be
written as:
qc " _R_+ _ (2) #
or: _ ""
Ch (HcL - Hw) = fiRR(Tw)+ _Hw (3)
in which:
Hw = f (B', p) (4)
as shown in Figure (5).
A comparison of steady state rc:esslon rates measured at
different facilities and test conditions can be obtained by using
the steady state energy balance (Eq. 3) to determine the convec-
tive heating rate which must have existed to have produced the
measured mass loss rate. This convective heating rate can then
be compared with the theoretical value predicted for the given
test conditions. If the inferred experimental heating rate ex-
ceeds the theoretical value, the degree of augmented heatlag due
to surface roughness can be computed.
Since a quasl-steady shape change is achieved during the -
steady state tests, the axial recession is equal at all locations
on the model surface. Due to difficulties in defining the effec-
tive nose radius of the sharpened equilibrium shapes, it is more
convenient to work with turbulent heating to the conical region
of the model, rather than with laminar stagnation point heating.
The theoretical smooth surface turbulent heating transfer co-
efficient Cho for a cone is predicted by the Eckert reference
enthalpy relation:
- qT - .0348 (0eUe)0._( _ C*2Ch° HR - RW PrZf_ _- _I £T (5)
The mass loss rate on the co_ical region is determined by:
" 0 _ sin _ (6)
in'which S is the measured axla_==_=_lrecession rate, and 6 is the
cone half-angle. The re-radlatlon heat flux qRR is computed
from the measured surface temperature. Eq. (3) is now used to
solve for Ch by iteration, using Figure 5 to determine Hw and
Figure 4 to determine B'. Comparison of Ch and Cho yields the
augmented heating ratio Ch/Cho.
The analysis procedure described above was applied to the
124 and 168 atm HIP test conditions, and the 80 atm 50 MW arc
test condition, for ablation tests of 3-D carbon-carbon. Local
properties needed in evaluating Eq. (5) were determined by isen-
399
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1979010842-391
#_ .,
k
tropic expansion from centerline stagnation conditions to local
pressure appropriate to the quasl-steady cone angle for each
condition.* The test conditions selected for the 50 MW arc are
representative average values of many runs, while specific runs
were selected for HIP tests. The test conditions and calculated "_
values of Cho, Ch, and Ch/Cho are shown in the following table.
MEASURED CALCULATED
Ps HCL _ B Tw Ch ICh/Cho
Facility arm kJ/kg btu/ib cm/sec deg °K Cho
50 MW 80 11600 5000 .635 45 4000 3.15 4.40 1.4
HIP 124 6914 2980 .508 57 4167 4.56 6.40 1.4
HIP 168 8027 3460 .787 57 4167 5.86 8.80 1.5
It is seen that a consistent degree of augmented heating is infer-
red from the measured recession rates and wall temperatures for
the three different test conditions.
The values of Ch/Cho computed above should be regarded as
first approximations, due to the inherent uncertainties in test
conditions. The effects of relaxing some of the assumptions made
in the analysis are summarized below. _
Assumption Effect of Relaxing Assumption
• No thermomechanlcal loss Finite thermomechanlcal loss
, yields lower Ch/Cho
• "Standard" HCL Lower HCL yields higher Ch/Cho
e.g., HCL = 8120 kJ/kg (3500
btu/ib) yields Ch/Cho = 1.9
instead of 1.4 for 50 MW arc.
RAMP TESTS
The ramp test technique has become a standard test procedure
in the 50 MW arc, and has also been used at the HIP facility.
Typical determination of transition nressure is shown in Figure
6. By using three cameras, transition can be observed on 6 rays
for each model, as inferred from the onset of gouges in the pro-
file.
The results of simply averaging the recorded transition
pressure according to camera view are shown below for two test
series.
A'_-mass balance calculation showed that the boundary layer on the
cone surface is fed by a small central core (.061 cm radius for
HiP at 168 atl,, .i12 cm for 50 MW arc at 80 atm), such that
center_Ine stagnation conditions are a reaspnable approximation.
400
, _, 6 ,,_
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1979010842-392
t' P = 1000 ATM
: P I00 ATM
P 10 ATM
: 8000 - P 1.0ATM '0 1 ATM "_
_ P = 0.01 ATM
, _ 6000 -
4000-
: OR:GaNALPAGE IS
OF POOR QUALITY
] 2000 -
:, 0 i I
: 0 1.0 2.0
/ B', f_w/Ch
FIGURE 5, HW vs. B _ for JANAF (Aerotherm) Thermochemist
'_ _ SECONDS _. -"_'__ _,i FLARED
_ _ Ol NOZZLE
"i I
ER-6.18 •
: WD-6.9011
I 100
p_
_ , :i _ _
7" .J i,
,: i" " " 60 O4D
,, -_" 40 _ I_
_ .."*_ _
0
• 2.5 2.0 1.5 1.0 0.5 0
DISTANCE FROM NOZZLE EXIT, INCHES
¢
:, FIGURE 6. TYPICAL DETEF&IINATION OF TRANSITION PRESSURE
#
L
f
I, 401
_t
:" 7
tNorth & South Cameras Top Camera
Test (45 ° Between Weave Axes) (Along Weave Axes)
Series # Readings Avg. PTR (atm_ # Readings Avg. PT_ (atm) '
A 38 71.6 20 77.7
E 24 73.5 10 77.4
A clear bias is seen to exist for transition to occur earlier
(lower FTR) along the 45 ° direction between weave axes.
A study of transitio,_ gouges occurence from post-test exam-
inatlon of a group of models also showed a consistent bias for
transition to occur more frequently on 45° rays (Figure 7), in
agreement with previous results of Heinonen and Jumper (Ref. i).
This non-axisynnnetric character of transition is due to the
three-dimensional weave geometry of carbon-carbon, and is typi-
C' fled by the post-test view of the model shown in Figure 8.
: The numerical values of transition pressure given above are
; typical of good quality processing for a particular 3-D carbon-
carbon. Figure 9 shows the decrease in transition pressure which
occurs as the open porosity of the test specimen is i_creased.
It appears that the transition pressure can be controlled by vary-
ing the open porosity of the material.
COMMENTS ON ADEQUACY OF TEST SIMULATION
In any ground test designed to simulate flight conditions,
i the question arises: is the simulation adequate? In the present
case, the 50 MW and HIP arcs provide a high pressure, high en-
", thalpy environment in Which comparative performance of material
candidates is obtained; hence, exact simulation would appear to
be of secondary importance. However, the lack of one-to-one
similitude zn certain parameters which may be signifJcant In
ablation performance suggests the need for caution in inter-
preting even comparative arc test results. In the following
:, paragraphs, comments are given concerning the adequacy of test
simulation for steady state and ramp tests, followed by recom-
mendations for facility developments which would provide improved
simulation capability.
Steady State SlmulatJon
Recession rates obtained in steady state tests reflect the
combined effects of thermochemlcal and thermomechanical material
removal. The simulation of ablative performance of purely
thermochemical nature would appear to be adequate for comparative
assessment; however, a subtle difference in arc and flight flow
-. field character may result in larger differences in flight per-
formance. Specifically, the different degrees of nose sharpening
, associated with different surface roughness has little effect on
turbulent forecone heatln_ in a low supersonic environment such
402
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1979010842-394
_!T. .- ,.
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1979010842-395
as that of the arcs, but has a large effect in hypersonic re-
entry due to the bow shock curvature-streamllne swallowing
phenomenon illustrated in Figure I0.
Thermomechanlcal contribution to recession rate at a given " ,_
test condition is properly simulated only if the full scale nose-
tip is tested. The test models in the arcs are smaller than -'
flight nosetlps, particularly for the HIP facility where pressure
is more representative of re-entry. Local dynamic pressure along
the biconlc turbulent ablated shape will be lower in arc tests
than in re-entry, for common stagnation pressure, due to flow
field effects (supersonic vs. hypersonic freestream Mach number).
Thus, pressure gradient forces (dp/ds) will be larger in arc tests
than in re-entry; while dynamic pressure and shear forces will
be smaller. Hence, differences in recession rate between two
materials may be obtained in arc tests which will not occur in
flight.
_ Transition Simulation
Boundary layer transition on a nosetip may be effected by :
the following variables:
' K/% relative roughness o_
Tw/T e wall-to-stream temperature ratio "_
B" dimenslonless 51owlng parameter
dimensionless pressure gradJent parameter
E (Euler number)
Tu turbulence level
Comparison of arc and flight environments is given below.
Typlcal Re-entry 50 MW Arc (80 atm)
Ps (atm) 80 - 150 80
HCL (kJ/kg) 9280 - 18560 11600
(btullb) 4000 - 8000 5000
(K/9)K-.0025 cm 3 to 8 3.0
(i roll)
Tw/Te 0.6 to 1.3 0.7
Re*/ft x 106 5 to 20 8.0
B" 0.2 to 0.5 0.4
E Arc higher than re-entry due to radial
gradients in approach flow
Tu Arc higher than clear-alr re-entry
It is seen that reasonably good simulation of several para-
meters which affect transition is achieved by the arc test con-
ditions, except for E and Tu. Also, an indirect influence of
pressure and enthalpy on roughness height of the ablating sur-
face may have significance, since the arc values are on the low
404
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1979010842-396
|i
Re-entry Arc'
FIGU'REi0. ARC SIMULATION DEFICIENCY OF OBLIQUE SHOCK
STREAMLINE SWALLOWING
405
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1979010842-397
end of the _e-entry values._'
Difference• in arc and re-eutry environment can result in
different comparative transition performanc; between two mater-
ials. For example, if free•tream turbulence controls transition
in arc tests (as it may for smoother material•), little differ-
ence in transition pressure may be observed; yet during clear
air re-entry, larger differences in transition may result because
surface roughness controls transition.
Facility Recommendation•
It i• obvious that the ideal facility should provide high
enthalpy, uniform low turbulence approach flow, hypersonic Math
number, and a large enough nozzle to test full scale no•etip•.
Since such a facility is not likely in the foreseeable future,
more practical specific recommendations are given below for
improved arc test simulation of nosetlp ablative performance.
• Reduce the freestream turbulence level by use of a •till-
ing chamber, and observe the affect on transition pres-
sure.
• Provide a moderate increase in pressu_e-enthalpy°nozzle
diameter (model size) similitude, such that variation
in test model diameter could be accomplished at a given
test condition.
• Provide a thorough calibration of the arc, including
measurement of radial and axial gradients of flow pro-
perties. This information would permit application of
nose shape change computer codes to extrapolate dif-
ferences in ablation performance in arcs to re-entry
conditions.
CONCLUDING REMARKS
An evaluation of typical high pressure arc test results for
3-D carbon-carbon nosetip models has demonstrated that steady
state recession rates over a wide range of test pressures show
a consistent degree of augmented heating due to surface roughness.
Th_ non-axisymmetrlc nature of boundary layer transition h__ been
quantified. The test simulation has been assessed to be reason-
ably adequate, with the exception of thermomechanical contrlbu-
tion to ablation, and arc freestream turbulence effects on trans-
ition.
REFERENCES
I. Heinonen, E.W., and Jumper, G.Y., Jr., "A Method for the Eval-
uatlon of Damage on the Surface of Carbon-Carbon Ablation Samples",
Proc. of Eighth Conf. on Space Simulation, NASA SP-379, Nov., 1975.
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Ablative performance of carbon-carbon nosetips in simulated re-entry environments

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