NASA Technical Memorandum 101537

7

DESIGN, FABRICATION, AND TESTS

OF A MI_TALLIC SHELL TILE

THERMAL PROTECTION SYSTEM

FOR SPACE TRANSPORTATION

_- (NAsA-TH-IO15BT) DFSIGN, FABRICATION, AND

TESTS UF A METALLIC SHELL TILE THFR_AL

PROTECTION SYSTPM FOR SPACE TRANSPORTATION

(NASA) 27 p CSCL 22B

N_I-2422'k

Unclas

G3/18 0019_i

lan O. MacConochie

H. Neale Kelly

June 1989

Date for general release June 30,,, 1991

N/kSA

National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23665
I |||
DESIGN, FABRICATION, AND TESTS OF A METALLIC SHELL TILE THERMAL

PROTECTION SYSTEM FOR SPACE TRANSPORTATION

fan O. MacConochle and H. Neale Kelly

Langley Research Center

SUMMARY

A new design of a thermal protection tile has been investigated as a

posslble alternative to some of the current Space Shuttle Orbiter tiles or as

a system for future aerospace transports. The tiles differ from currently

used reusable surface insulation (RSI) in that the proposed design consists

of an outer load-carrying shell which is filled internally with low-density

flexible insulations. One such tile has been fabricated from O.O04-in.

superalloy for the top half of the tile and O.O04-in. titanium for the lower

half and is filled with high-temperature ceramic flexible fibrous

insulation. The tile has been tested in a radiant heat lamp facilitY up to

1600°F where it protected the underlying structure to 345°F. Each 2-3/8 in.

x 6 in. x 6 in. test tile weighs 0.33 Ib, with a unit weight of 1.328 Ib/ft 2

for a four-tile array. The tiles appear to be competitive from the

standpoint of insulating qualities and weight with current RSI. Further

testing, however, would be required to verify the design for all aspects Of

the real flight environment.

INTRODUCTION

The Shuttle Orbiter employs an RSI over approximately 70 percent of the

vehicle surface (ref. I). This insulation (applied in small blocks) has

proven to be one of the lightest, most Insulatlve materials which could have

been used for the current Space Shuttle. However, improvements in subsystems

are constantly being sought, and alternate solutions for the surface

insulations are being studied particularly with a view toward reducing
initial cost and improving durability (ref. 2). In addltlon to the above

goals for the current Shuttle, future aerospace transports may requlre a

somewhat different type of thermal protection system. These systems will

differ In entry environment, and therefore, the study of alternative systems

is warranted. One notable area In which future transports differ is that of

entry planform loadlng which, may be as little as 50 percent of the Shuttle

values. This characteristic of future transports makes It possible to use

lower heating rate trajectories (ref. 3) and, therefore, lower (than RSI)

temperature capabillty materials (ref. 4).

In this paper, an alternate thermal protection system (TPS) is described

In which a flexible insulation is encapsulated in a 6 in. x 6 in. semi-rlgid

shell (ref. 5). Radiant heat lamp test data for one such tile (having a

metallic shell) are reported.

SHELL TILE CONCEPT

On the current Space Shuttle Orbiter, the RSl serves both as an

insulator and structurally as a rigid outer surface to resist aerodynamic

loads. In the proposed design, insulation and load-carrying functions are

separated, that is, the outer shell carries loads, while the insulation

package serves only for insulation.

The shell tile (shown in the cros section in figure 1) consists of a

lower shell which is mechanically attached to the structural skin and an

upper shell which is attached to the lower shell. The upper shell is

stiffened with a dimpled sheet of material welded to the shell's inner

surface. The dimpled sheet also acts as a radiation barrier. The top edge

of the bottom shell Is reinforced with a small rib to add stiffness. The

bottom of the bottom shell is lef t open In the interest of weight saving with

a flange provided to accommodate four mechanical fasteners.

-2-
A thermal Isolation pad (TIP) ls placed on the vehtcle surface. The

lower half of the shell tlle ls then fastened to the vehicle and filled wlth

Insulation, and the upper shell ls attached wlth lts Insulatlon package

already In place. In the basellne design, a self-locking mechanism ls used

to hold the upper and lower shells together (fig. 2). This locklng system

conslsts of pairs of thln wedges welded to the top and bottom shells at four

locations at the midpoints of the sldes of the shells. Stops are attached to

the lower shell near each corner of the tile to lnsure proper locktng action

and correct tile helghto To remove the tiles, four paddle shaped tools are

lnserted between the tiles and rotated 90 ° to disengage the four wedges

(fig. 2).

As an alternate to the self-locking wedges, upper and lower shells can

be fastened (by rIvetlng). In fastenlng the upper and lower shells together

prlor to attachment to the vehicle, access to the Inslde of the tile is no

longer posslble; therefore, the t11e must elther be bonded or 11mlted in use

to structure that Is accesslble from the Inslde such that fasteners could be

applled from the Interloro

In addltlon to the mechanlcal deslgn options described, many optlons are

avallable In materials and Internal Insulations. For example, candidates for

the outer shell Include pyrollzed composltes, refractory metals, superalloys,

stalnless steels, and titanlum a11oys. In the two tiles shown In flgure 3,

the left-hand tlle (upper she11) Is fabrlcated from the refractory metal

columblum whlle the rlght-hand t11e Is fabrlcated from the superalloy Rene

41. For the lower (unexposed) shells, stalnless steels, titanlum a11oys, or

more moderate temperature capab111ty organlc composltes are 11kely

candidates. Stalnless steel and titanlumwere used for the left and rlght

bottom shells, respectively, In flgure 3.

-3-
For internal insulation, a wlde varlety of flexlble refractory fibrous

insulations or radiation shields can be used in multiple layers; each layer

is selected for an optimum based on weight, insulating qualities, and

temperature capabilities. Whereas rlgidized ceramic fibrous materials used

in making RSl typically have densities in the range of 9 to 22 Ib/ft 3,

suitable flexible insulations for a shell tile concept have densities In the

range of I to 6 Ib/ft 3. Since insulation qualities and insulation weight are

both relevant to system efficiency, the density-conductlvity (pK) products

are often compared (ref. 6). These values are shown for one rigidized

(LIgOO) and two flexible insulations at sea level pressures (fig. 4). For

example, the pK product for LIgO0 Is over 2-i/2 times that of the material

used in the shell tile design. (Compare RSl and microquartz in fig. 4 at

1600°F.) Since insulation in the shell tile can be stratified, even lower

density and lower temperature capability insulations can be used in the lower

stratified layers of the shell tile insulation such as the I Ib/ft 3 material

shown in figure 4. This material has a temperature capability of up to

approximately 700°F.

Deterrents to extremely high efficiency for the current system are the

weight and conductivity of the load carrying sidewalls. For example, 0.007-

in-thick Rene 41 has a thermal conductivity of 14.6 Btu/(hr-ft-°F) compared

with 0.704 Btu/(hr-ft-°F) for the O.015-in. coating on the RSI. A carbon

composite (also a likely candidate for the upper shell) has a conductivity of

2.5 Btu/(hr-ft-°F) (parallel to the graphite fibers). This value is still

over 3 times greater than that for an RSl coating. What renders the shell

tile competitive with RSl, however, are the extremely low pK products for the

insulation materials which range from one third to one seventh of the values

for the rigidlzed ceramic fiber blocks (i.e., RSI).

-4-
An additlonal deterrent to heat flow down the sides of the shell is the

top-to-bottom shell Juncture. In this regard, commercially available

flexible high-temperature tapes can be placed in the Juncture. Also, the use

of lower conductivity materials such as titanium for the lower shell

minimizes heat flow. Titanium has a conductivity of B Btu/(hr-ft-°F), or

about 25 percent less than Rene 41 at the mean design operating temperature.

In lower temperature (leeslde) areas of the vehicle, titanlum could also be

used for upper shells and even lower temperature capability materials for

lower shells.

Because the shell tile can be mechanlcally fastened, the permissible

interface temperature is greatly increased, and therefore, a "warm or hot

structure" approach to design could be utilized. For exampe, if the surface

to which the tile is attached for the optimal design is titanium, temperature

at the interface could be allowed to reach 750°F without detriment to the

attachment system.

For added thermal protection, very thin foils (O.O0i-in. thick) can be

added in alternate dimpled and flat Sheets between the outer stiffened shell

and the top shell insulation package. Because of their thin gauge and the

absence of contact welds between sheets, they make an efficient deterrent to

heat flow to the shell tile Interior. (Note: The effectiveness of

multi-layers of unbonded foil cannot be compared directly with a multi-wall

construction, described in references 7 and 8, inasmuch as the latter are

Joined at the contact points and are fabricated from much thicker materials,

namely, 8 to 14 thousandths versus I to 3 thousandths for the foil).

Currently used RSI tiles expand negliglble amounts during peak heating,

but gaps must be left between the mechanically weak tiles to prevent contact

during flexure of the substructure to which they are attached. By the end of

-5-
the peak heating period, the alumlnum structure has heated up, and the gaps

between the tile have increased.

With the shell tlle design, the Inltlal gaps can be sized so that the

gaps essentially close durlng peak heatlng. Because the tiles are metallic,

some contact Interference ls permissible and should cause no damage to the

system. After peak heatlng and durlng atmospheric fllght, because of their

low thermal mass, the tiles quickly cool to match local flight conditions.

In the contracted condition, the tile gaps are again rendered adequate to

allow for deflections of the underlying structure such as those associated

with subsonlc maneuvers and higher dynamic pressures.

Tile slzes can be varied; the higher the predicted heatlng rate for a

given location, the thicker the tile and the smaller the width dimensions

needed to mlnlmlze the gap slze changes between ambient temperature and peak

heating "at entry. In general, however, tile sizes should be kept small,

since thewhole design ls predicated on small physlcal size to minimize

weight, thermal distortions, and the Initial ttle gap width.

MANUFACTURING

All mechanlcal joints on the shell tile design were made using a

capacitor discharge resistance welding device. In general, the tiles are

easy to fabricate. Rene 41, however, is somewhat harder to form, and the

dimpled stiffener sheets used In the top of the upper shells had to be formed

in a two-step process wlth a 75-percent deformation followed by a 100 percent

fu11-depth forming of the dimples. This was done to minimize cracking of the

Rene 41.

Although different toollng would be used for mass production, the design

and development costs are expected to be relatively low because of the

simpllcity of shell tile design. Further, there are no left- or right-hand

-6-
tiles; therefore, tooling requirements are reduced. Fabrlcatlon of

triangular or rectangular shell tiles should present no special problems.

Shell tiles, havlng compound curvature, would be costly to manufacture, and

speclal toollng would be required to form Individual tiles. However, most of

the tile dlmensfons are small relative to the large radii of curvature

anticipated for the future vehicles, and therefore, flat tiles should suffice

In most locations.

TEST TILE

The tlle used In the radiant heat lamp tests was fabricated from 0.004-

ln. gauge Rene 41 for the upper shell and 6-A1-4V titanium for the lower (or

inner) shell. As stated under the section on manufacturing, a O.O04-tn.

dimpled Rene 41 sheet was resistance welded to the inside surface of the

upper shell In order to stiffen the surface. In thts case, the lower shell

was also stiffened by bending the titanium sidewall material 90 ° to form

frames at the top and bottom of the lower shell (fig. 1). The upper and

lower caps of the test tile were fastened with four 3/32-1n-diameter blind

rivets located in the overlap zone 3/8 ln. from each of the four corners of

the tile. A glass tape 3/32-1n. thick by 1/2-tn. wide was captured in the

upper to lower shell overlap zone.

The tile was fastened from the back side of the simulated aluminum

structure wlth 1/8-1n. nut plates located inside and mounted on the lower

flange of the lower shell. A 0.17-1n. Nomex thermal insulation pad was

placed beneath the shell tile assembly to reduce the heat flow down the

sidewalls of the assembled tile lnto the structure beneath. The pad is of

the same material as the strain isolation pad (SIP) used with RSI but Is not

required for strain isolation, since the floating nut-plate-fasteners (and

clearance holes in the bottom shell) accommodate differential thermal

-7-
expansion or mechanical strains between the shell tile and the structure.

For the test, the entire upper and lower shells were ftlled with 3.5-1b/ft 3-

density micro-quartz (because of availability). As mentioned earlier, lower

density, lower temperature capability flextble Insulations could be used for

the lower layers. The upper Insulations package used was 3/4-in. thick; the

lower, 1-1/2-tn. thick. Test tile component weights are given In table I and

centerltne properties, In table II. Various assumptions for boundary layer

heat transfer coefficient can be made from adiabatic to h - 30 x 10.4

Btu/(sec2-ft2-°F).

TEST APPARATUS AND PROCEDURE

For the tests described herein, a shell tile was mounted In a 6 In. x

6 In. cutout in a test frame which in turn was attached to a 1/4-in. aluminum

plate to simulate structure (fig. 5). The test frame (mounted on four

standoffs) was filled with the same Insulation as the test tile. Four

adjustable sheet metal angles were provided In the frame openlng to simulate

adjacent tiles and provide for adjustable tile gaps. The assembled tile and

frame measured 20 in. x 30 In.

Six fixtures with eight infrared lamps in each were used. The lamps

were rated at appPoximately 2500 watts each at voltages of 460 to 500 volts.

Power to the lamps was regulated by a sillcon-controlled rectifier. The

lamps were provided with forced air for coollng and high-emlsslvity

reflectors. A 12-channel printer recorder was used to record the tile

temperatures every 30 seconds. Thermocouples of the chromel-alumel type were

utlllzed. The top surface thermocouples were protected with 304 stainless

sheathlng. Thermocouple locations are described In table Ill and are shown

on figures 6 and 7 as an Inset. The principal objective In the selection of

thermocouple locations was to measure the temperature drops across major

-8-
components down the tile centerllne and down the tile sldewallso

To simulate an entry, the controller was programmed to ralse the outer

surface of the tile to the designated temperature 11nearly wlth time In 500

sac from room temperature. Peak temperature on the surface was held for

three different tlme Intervals, namely 500, 800, and 1100 sec to determine

the effects of extended entry times on structure temperatures. Table IV

shows the programmer Inputs for the 500-sec hold ttme° At the end of the

peak heating perlod, the panel was programmed to cool to room temperature In

approximately 500 sec, but no acttve coollng systems were provlded so that

cooling perlods were frequently longer, particularly for the high-peakheating periods and temperatures.

TEST RESULTS

Sample test results obtatned on the recorder are shown tn table V and

figures 6 and 7. In figure 6, a 500-sec-duratlon peak heatlng period at

900°F was used. The aluminum structure (thermocouple 8) reached a

temperature of lOT°F, or 37°F above ambient temperature. The second prtntout

Is shown for the same peak entry temperature, but the heating perlod was held

for 1100 sec to simulate a higher heat load trajectory (I.e., longer entry

time). Maximum alumlnum structure temperature was ll7°F for this case, or

only IO°F htgher than the 500-sac peak heattng duration Just cited. This

suggests that the Incremental Increases In structural temperature resulting

from extended entry tlmes is extremely small.

In flgure 8, temperature of the alumlnum structure ls plotted versus

tlle surface temperature. At 1600°F, structure temperatures reached 345°F

for a 500-sec peak heating period, 5°F below the 350°F deslgn limit used for

the current Shuttle aluminum structure. (Also see table V data at 1600°F.)

-9-
i

For entry trajectories tailored for the longer peak heating perlods

(for example the 800 and 1100 sec shown), the amount of tile insulation would

have to be increased if the 350°F structural temperature limit Is to be

malntalned for a 1600°F surface temperature.

The weight of four test tiles in an array is 1.55 Ib for a I ft2 area of

coverage. When the tlle deslgn Is optimized and the Insulation packages

stratified with lower density Insulatlon In the lower layers, the welght of a

four-tile array is estimated to be 1.328 lb. This weight has been plotted in

flgure g from reference g, In which several other metallic shell deslgns and

Shuttle RSl are shown.

SUMMARY REMARKS

The concept appears to be weight competitive with other types of thermal

protection systems. Advantages of the shell tile concept over other types

are greater durability and flexibility than RSI in the substitution of

materials, and greater ease of servicing and replacement. Even after the

tiles are installed, the upper half of the tile could be removed and

substitutions of other materials for the cap made that are nearly optimal for

the local heating rate. Similarly, insulation types and thicknesses could be

made that are more nearly optimal for the heat load. These changes could be

made after the vehicle is in service since the changes in insulation would

not result in any changes in the outer moldllne.

-10-
REFERENCES

1. Cooper P. and Holloway, P. F.: The Shuttle Tile Story. Journal of

Aeronautics and Astronautics, pp. 24-36, January 1981.

2. Hays, D.: An Assessment of Alternate Thermal Protection System for the

Space Shuttle Orbiter. NASA CR-1657go, February 1982.

. Wurster, Kathryn E. and Eldred, Charles H.: Technology and Operational

Considerations for Low-Heat-Rate Trajectories. AIAA Paper No. 79-0890,

presented at the Conference on Advanced Technology for Future Space

Systems, Hampton, Virginia, May8-11, 1979.

. Wurster, Kathryn E.: Mass Reduction for Advanced Winged Entry Vehicles

Through Integrated Thermostructural-TraJectory Design. AIAA Paper No.

80-0363, presented at the 18th Aerospace Sciences Meeting, Pasdena,

California, January 14-16, 1980.

5. MaConochie, I. 0.; Lawson, A, G.; and Kelly, H. Neale: Shell Tile

Thermal Protection, United States Patent No. 4,456,2085.

6 Varlsco, A.; Bell, P.; and Wolter, W.: Design and Fabrication of

Metallic Thermal Protection Systems for Aerospace Vehicles. NASA

CR-145313, February 1978.

. Blair, W.; Meaney, J. E., Jr.; and Rosenthal, H. A.: Design and

Fabrication of Titanium Multi-wall Thermal Protection System (TPS) Test

Panels. NASA CR-159241, February 1980.

. B1alr, W.; Mesney, J. E.; and Rosenthal, H. A.: Fabrication of Titanium

Multi-Wall Thermal Protection System (TPS) Test Panel Arrays. NASA

CR-159383, December 1980.

9. Kelly, H. N. and Webb, G. L.: Assessment of Alternate Thermal Protection Systems for the Space Shuttle Orbiter. NASA TM-84491, May 1982.

-11-
Table I

Test Tile Component Weights

Basic shell tile assembly Wt, Ib

Upper shell (0.004-in. Ren_) ................................. 0.1272

Lower shell (0.004-in. Ti) ...................................... 0.0463

Top insulation (microquartz) ................................. 0.0525

Bottom insulation package (microquartz) .............. 0.1113*

Glass joint insulation ............................................. 0.0020

0.3393

Accessories

Thermalinsulation pad(0.16-in-thick Nomex) ...... 0.0215

Fastenem .............................................................. 0.0269

0.0484

Total weight per test tile ............................................. 0.3877

4-tile array (3.54-1b/ft 3 insulation density) ............. 1.551"

4-tile array (stratified insulation densities) ............. 1.328

*Notes: Insulation with a density of 3.5 Ib/ft3 was used here

in the test tile, but materials with a density as low

as 1 Ib/ft 3 are commercially available. These lower

density insulations could be used in locations away

from the outer surfaces of the tile.

-12-
Table il

Shell Centerline Properties

Temperature,

OF

Specific Heat, Cp,

Btu/(Ib-°F)

Thermal Conductivity, K,

Btu/(Ib-sec-°F)

Layer 1 - Ren_ (tile outer shell )

[t = 3 x 104 ft; p = 515 Ib/ft3; _ -- 0.90]*

540

1040

1540

0.11

0.12

0.13

1.7 x 10-3

2.9 x 10-3

4.0 x 10-3

Layers 2 and 3 - microquartz insulation (in top and bottom shells)

[t = 0.0625 ft (layer 2) and 0.i25 ft (layer 3); p = 3.5 Ib/ft 3]

540

104O

1540

0.23

0.24

0.25

1.25 x 10-5

2.22 x 10-5

3.47 x 10-5

40

540

Layer 4 - Nomex felt (thermal isolation pad)

[t - 0.0133 ft ; p = 8 Ib/ft 3]

0.24

0.45

0.63 x 10-5

1.32 x 10-5

Layer 5 - Aluminum structure

[t -- 0.02 ft ; p = 173 Ib/ft 3]

70-350 0.22 0.03

*t = thickness, in.; p = density Ib/ft 3",e - emissivity

-13-
Table III

Thermocouple Locations

Thermocouple

no.

1

2

3

4

5

6

7

8

Tile surface centerline

Between dimpled stiffened sheet and top insulation pack

Between upper and lower insulation packages on tile centerline

On outside of top shell at upper-to-lower tile juncture

Inside surface of lower shell at upper-to-lower tile juncture

On bottom of lower tile sidewall above thermal isolation pad (TIP)

Underneath insulation pack on top of TIP at centerline of tile

On aluminum structure at centerline of tile

-14-
Table IV

Test Programmer Inputs for Time vs Temperature Tables

[500-sec hold time at maximum temperture]

Time, Temperature, °F, for Tma x ofsec 700°F 1320°F 1500°F 1600°F

0

100

200

300

400

5OO

60O

700

8OO

9O0

1000

1100

1200

1300

1400

1500

60

185

320

450

570

700

60

310

570

810

1050

1320

60

250

440

630

810

1500

q

570

450

320

185

60

1050

810

570

310

6O

,m,,

810

63O

440

250

60

60

370

680

990

1290

1600

i

i

i

1290

990

680

370

60

-15-
Table V

Maximum Thermocouple Temperature Readings

Peak

Temp,

oF

5O0

700

900

1000

1300

16OO

Lapsed _meat Peak

Heating Rate,

sec

Maximum Temperature, °F, at Thermocouple No.

1 2 3 4 5 6 7 8

500 490 462 295 333 327 160 202 80

800 490 465 333 360 355 185 121 90

1100 495 477 360 370 365 190 130 90

500 683 650 450 465 462 213 140 93

800 690 657 482 475 467 230 162 102

1100 720 690 536 502 495 .257 203 115

500 890 850 600 550 540 245 200 115

800 875 845 641 573 562 272 227 117

1100 885 850 675 596 585 285 245 120

500 940 925 745 705 320 263 190

500 1150 1180 860 820 363 340 225

800 1290 1340 960 925 465 332

1100 1300 960 925 490 _ 366

500 1600 1590 1133 1070 510 500 345

-16-
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-18-
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-25 -
NASA

State ,_slf_o_

1. Report No.

NASA TM- I01537

Report Documentation Page

2. Government Accession' No.

4. Title and Subtit_

Design, Fabrication, and Tests of a Metallic Shell

Tile Thermal Protection System for Space Transportation

7. Author(s)

Ian O. MacConochie

H. Neale Kelly

9. Performing Organization Name and Address

NASA Langley Research Center

Hampton, VA 23665"5225

12. Spon_ring Agency Name and Address

National Aeronautics and Space

Washington, DC 20546-0001

Administration

3. Recipient's Catalog No.

5. Re_ Date

June 1989

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

506-49-1 I-01

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

'l';t.'Sponsoring ._gency Code

15. Supplementary Notes

16. Abstract

A thermal protection title for Earth-to-0rbit transports is described. The

tiles consist of a rigid external shell filled with a flexible insulation. The

tiles tend tO be thicker than the current Shuttle rigidized silica tiles for the

same entry heat load but are projected to be more durable and lighter. The tiles

have been thermally tested for several similated entry trajectories.

17. Key Words(Suggested byAu_or(s))

Thermal Protection

Launch Reentry Vehicles

,19. Security Classif. (of this report)

Unclassified

i

NASA FORM 1526 OCT 86

18. Distribution Statement

20. Security Classif. (of this page)

Unclassified

T,

Subject Category 18

i21. No. of pages

26

22. Price