The external tank (ET) intertank structure for the space shuttle, a 27.5 ft diameter 22.5 ft long externally stiffened mechanically fastened skin-stringer-frame structure, was a labor intensitive manual structure built on a modified Saturn tooling position. A new approach was developed based on half-section subassemblies. The heart of this manufacturing approach will be 33 ft high vertical automatic riveting system with a 28 ft rotary positioner coming on-line in mid 1985. The Automatic Riveting System incorporates many of the latest automatic riveting technologies. Key features include: vertical columns with two sets of independently operating CNC drill-riveting heads; capability of drill, insert and upset any one piece fastener up to 3/8 inch diameter including slugs without displacing the workpiece offset bucking ram with programmable rotation and deep retraction; vision system for automatic parts program re-synchronization and part edge margin control; and an automatic rivet selection/handling system

Lumley, D. F.

English

NASA Technical Reports Server

AUTOMATIC FASTENING LARGE STRUCTURES - A NEW APPROACH 
Donald F. Lumley 
Senior Staff Engineer 
Production Engineering 
Martin Marietta Aerospace 
New Orleans, LA. 
ABSTRACT 
The External Tank (ET) for the Space 
Shuttle is produced by Martin 
.Marietta Aerospace at the NASA - 
Michoud Assembly Facility in New 
Orleans, Louisiana. The assembly of 
the ET Intertank Structure, a 27.5 
ft. diameter 22.5 ft. long 
externally stiffened mechanically 
fastened skin-stringer-frame 
structure, was a labor intensive 
manual build on a modified Saturn 
tooling position. 
A new approach was developed based 
on half-section subassemblies. The 
heart of this manufacturing approach 
will be 33 ft. high vertical 
automatic riveting system with a 28 
ft. rotary positioner coming on-line 
in mid 1985. 
The Automatic Riveting System 
incorporates many of the latest 
automatic riveting technologies. 
Key features include: vertical 
columns with two sets of 
independently operating CNC 
drill-riveting heads; capability to 
drill, insert and upset any one 
piece fastener up to 3/8 inch 
diameter including slugs without 
displacing the workpiece; offset 
bucking ram with programmable 
rotation and deep retraction; vision 
system for automatic parts program 
re-synchronization and part edge 
margin control; automatic rivet 
selection/handling sys tem permits 
stack height sensor or parts program 
selection from 16 feeders; riveting 
cycle control by servo upset force 
or head height; automatic tool 
changer; automatic hole inspection; 
and printout capability for hole 
size, upset force, and head height. 
https://ntrs.nasa.gov/search.jsp?R=19860001767 2020-03-23T06:20:08+00:00Z
INTRODUCTION j ,, 
The Space Shuttle is America's 
economical and effective Space 
Transportation System (STS) 
developed by the National 
Aeronautics and Space Administration 
(NASA) to conduct space missions for 
projected national and international 
space program activities. The Space 
,Shuttle vehicle conaists of three 
major elements: a reusable manned 
Orbiter, an expendable External Tank 
(ET) containing the Orbiter 
propellant, and, two reusable Solid ' 
Rocket Boosters (SRBS). The 
External Tank, built by Martin 
Marietta Aerospace at the NASA 
Michoud Assembly Facility in New 
Orleans, Figure 1, is'the largest 
element of the Space Shuttle, It 
serves as the backbone structure for 
attachment of the Orbiter and SRBs 
and also contains and delivers 
propellants for the three Orbiter 
main engines. The External Tank 
accommodates the complex stresses 
created by its own weight and that 
of the Orbiter prior to launch, then 
the thrust generated by the Orbiter 
and the SRBs during launch. The 
overall Shuttle has a gross lift-off 
weight of 4.5 million pounds. Fuel 
can be supplied from the External 
Tank to the three main engines at 
the rate of 45,283 gallons of liquid 
hydrogen ( L H ~ )  per minute and 16,800 
gallons of liquid oxygen (~02) per 
minute . 
Figure I .  
Completed External Tank Being Transported From Michoud Assembly Facili tg.  
The External Tank is 153.8 feet long 
and 27.6 feet in diameter, It 
weighs approximately 69,000 pounds 
empty and when loaded with 
propellants at launch weighs 
approxiinately 1,660,000 pounds. 
Three primary structures make up the 
ET; a LO2 Tank, an Intertank, and a 
LH2 Tank. Both propellant tanks are 
constructed of aluminum alloy skins 
with support or stability frames as 
required, and their skins are butt 
fusion welded to provide reliable 
sealed joints. The Intertank ' ' 
aluminum structure utilLzes. 
mechanically fastened skin$ and 
stringers with stabilizing 'frames. 
The External Tank primary structure a 
is shown in Figure 2. 
J 
~ i ~ u r e  2. 
External Tank Primary Structure. 
Figure 3. 
Intertank Structure 
I .  
DISCUSSION 
The Intertank, Figure 3, is the ET 
structural connection that joins 
with both the LO2 and LH2 tanks to 
provide structural continuity 
between these assemblies. Its 
primary functions are to receive and 
distribute all thrust loads from the 
SRBs and transfe loads between 
propellant tanks. The Intertank 
also functions as a protective 
compartment for housing 
instrumentation, range safety 
components, and other subsystems. 
The Intertank cylindrical structure 
consists of two integrally machined 
thrust panels and six mechanically 
fastened stringer stiffened panels. 
It is 27.6 feet in diameter and 22.5 
feet long. The two thrust panels 
distribute the concentrated axial. 
SRB thrust loads to the LO2 and LH2 
tanks and adjacent skin panels. The 
thrust panels are selectively 
machined with tapered skin 
thicknesses and external integral 
ribs. The six stringer stiffened 
panels are similar to each other 
except for penetrations, system 
installation provisi,ons, and an 
access door. Each panel is 10.8 
feet wide and 22.5 feet long and 
includes a forward and aft chord for 
attachment to the LO2 and LH2 tanks. 
The skidstringer panels are each 
made of two aluminum skins 
mechanically spliced longitudinally 
by internal and external butt 
straps. Skin doublers provide 
necessary reinfbrcement for areas 
where the skin is penetrated and 
localized reinforcements to 
distribute thrust loads. There are 
18 aluminum hat section strirgers 
mechanically fastened to etch 
skin/stringer panel. 
The six stringer stiffened panels 
and two thrust panels are 
mechanically spliced using 
longitudinal butt splices to form 
the Intertank skin shell. 
One main frame is employed to 
transmit the transverse SRB thrust 
loads and the intermediate ring 
frames stabilize the cylindrical 
shell. The main frame is 
constructed of machined outer and 
inner chords joined to webs to form 
an I-beam 20 inches deep. The four 
intermediate ring frames are 
constructed similar to main frame 
and are 12 inches deep. 
The SRB beam assembly running 
through the middle of the Intertank 
is a rectangular box beam. It is 
42.95 inches deep at the center, 
tapers to 26 inches at the ends, and 
is 15 inches wide. Two SRB thrust 
fittings,.machined aluminum 
forgings, are attached at either end 
of the beam and provide for SRB 
attachment. 
The Intertank structural assembly is 
performed at the NASA Michoud 
Assembly Facility (MAF) in New 
Orleans, Louisiana. Major 
components including thrust panels, 
stringer panels, frame quadrant 
sections, and the SRB beam are 
subcontracted. 
The Development and early production 
Intertanks were built on modified 
Saturn S-1C tooling remaining from 
the previous ~aturn/Apollo program 
at Michoud. A new Intertank 
manufacturing approach was developed 
utilizing a half section 
subassembly. New tooling was 
designed and built and implemented 
in a manual mode in the fourth 
quarter of 1983. The heart of the 
half section manufacturing approach 
will be a new.and unique Automatic 
Riveting System for large 
structures. The new riveting system 
is being built by GEMCOR and is 
currently in preliminary checkout at 
their facility. It will go into 
operation at Michoud in the third 
quarter 1985. 
The Intertank half section, Figure 
4, consists of three skin/stringer 
panels, 180 degree sections of the 
five frames, butt splices, and 
miscellaneous items. The components 
are joined using blind fasteners 
(under hat section stringers), 1/4 
inch diameter A286 Hi-Sets, and 
Hi-Loc fasteners. 
The first position in the half 
section family of tooling is the 
panel and frame tack fixture. In 
this fixture, the four 180 degree 
intermediate frame segments, 180 
degree main frame segment, the three 
45 degree skin/stringer panels are 
positioned and located for tack 
fastening. Blind fasteners located 
under the hat section stringers are 
the primary tack fasteners. 
The third position is for finish, 
inspection, repair and bracket and 
subsystem support structure 
installation. In the current manual 
mode, fasteners that will be 
installed by the Automatic Riveting 
System are installed in this 
fixture or the tacking fixture. 
Figure 4 .  
I n t e r tank  Half Sect ion  On Transportation Dolly. 
These two tooling positions as well 
as the foundation foot-print for the 
Automatic Riveting System are shown 
in Figure 5, 
The second position in the family 
will be the Automatic Riveting 
System which will install the 
fasteners to secure the skin 
panels t o  the frames and 
complete the butt splices. 
The majority of the fasteners 
used in this operation are 1/4 
inch A286 Hi-Sets, 
SYSTEM DESCRIPTION 
The Automatic Riveting System 
utilizes a new and unique system of 
driving the drill/rivet heads under 
computer numerical control on 
vertical inner and outer columns 
with the work piece rigidly 
supported on a 360 degree rotary 
positioner. This Vertical Drivmatic 
is capable of installing any one 
piece fastener, including slugs, up 
to 3/8 inch diameter without 
displacing the workpiece. The inner 
Figure 5. 
Footprint For In-bertank Automatic Riveting System 
Shorn Between First  and Third Tooling Positions. 
to outer head load bias to the 
workpiece is limited to a maximum of 
50 pounds under the heaviest 
riveting.conditions. 
The system is capable of riveting 
structures with both internal and 
external stiffening. The rivet 
installation rate exceeds twelve 
fasteners per minute in the Dual 
head mode and seven per minute in 
the single head mode. The key 
features of the system are described 
in the following paragraphs. 
Vertical ~rill/~ivet Columns - 
Rotary Positioner 
permits 28.5 feet of vertical travel 
or 20.7 feet of working range with 
either set of carriages parked. 
Each pair of carriages are 
controlled by separate CNC systems. 
The inner column carriages (slaves) 
provide synchronous movement to the. 
outer carriages and are positioned 
accordingly. See Figure 6. 
The outer Z-axis carriage houses the 
Y-axis carriage which, in turn, 
houses the transfer head. The 
transfer head carries the drill 
spindle, hole inspection system, and 
bucking ram and is actuated by the 
transfer cylinder. The outer Z-axis 
The columns are 33 feet high and carriage also mounts the automatic 
measure eight feet by ten feet at tool changer, automatic injector 
the base. Two sets of opposing changer, vision system and TV 
carriages, Z axis, are positioned by camera. Six inches of in and out 
electro servomotor rack and pinion head travel, Y axis, is provided for 
drives with dual hydraulic motor clearing external stiffeners and 
counterbalances, This arrangement accommodating work plane variations. 
4 
The inner column carriage contains carriage to permit independent edge 
the upset ram which has 24 inches of margin control, 
programmable retraction and 
programmable rotation of 350 degrees The CMC rotary positioner, C axis, 
about the rivet centerline. is a 28 foot diameter ring shaped 
rotary table which positions the 
Plus or minus one-half inch X - axis part holding fixture and work piece 
translation is provided in each for the drill rivet operations, 
Figure 6. 
GEMCOR Automatic Riveting System During Prelim-;nary Checkout, 
Control System 
The Control system includes the 
operator's console; two 
Allen-Bradley 7320 Computerized 
Numerical Controls (cMC) ; three 
Allen-Bradley Programmable Logic 
Controllers (PLC 2/30); and the 
General Electric Vision System, The 
controls are interfaced to permit 
independent and simultaneous 
operation of both sets of heads, 
The control system block diagram is 
shown in Figure 7. 
Primary control of the Vertical 
Drivmatic is accomplished through 
utilization of two CNC controllers, 
One controls the Upper Head, while 
the other controls Lower Head 
movement. With the dual CNC system 
I 
one will be considered the master, 
which controls the 'C' Axis 
programming and is responsible for 
locking out any head motion that 
could cause workpiece damage. 
Either CNC can have the capability 
of controlling the C Axis, but only 
one at a time will do so. 
Head functions will be controlled by 
two PLC's in communication with the 
CNC, In addition a third PLC will 
be used to control both ARS 
Systems. The Tack Resynchronization 
Visual System provided for each head 
will interact with its associated 
Cl\TC, 
The Operatorss console houses all 
operator controls in a single main 
control panel, The two 
I 
Figure 7 ,  
Control Systems Bloek Diagram, 
Figure 8. 
@erator9s Console Provides Independent Control of Dual Drill/Riueting Systems. 
Allen-Bradley CNC main control upset force or head height can be 
panels with the CRT display and selected from the console or the 
alpha-numeric keyboard are located parts program. 
in the operator's console along with 
all controls and indicators 
necessary for the operation of the Rigid Workpiece Location And Clamp 
system and riveting process. The 
Operator" Console, shown in Figure 
8, also houses 'the color TV 
monitors and vision system displays 
for each set of heads, 
The system is capable of controlling 
the riveting process by either servo 
. 
upset force or head height control. 
The mode of operation as well as the 
The Automatic Riveting System 
locates the work plane of the 
rigidly mounted Intertank half 
section and complete@ the riveting 
process without displacing the work 
piece*, The following explains the 
GEMCOR developed method of applying 
rivet upset forces while putting 
less than fifty pounds of bias force 
on the workpiece. The Y-Axis 
carriage moves in under system 
pressure. When the pressure foot, 
which is at low pressure, touches 
the work surface and collapes, the 
pressure foot encoder senses 
movement and stops the Y-Axis 
carriage. The Y-Axis then retracts 
until the encoder indicates home 
position. The pressure foot is now 
fully extended and can be 
pressurized with system pressure. 
The inner ram now advances until the 
clamp makes contact with the work 
surface. The clamp then switches 
from low to high'pressure and the 
work piece is fully clamped. The 
Drivmatic Drill and riveting process 
is then initiated. 
The vision system has the capability 
to detect the edge of a part and 
shift rivet location for a single 
hole or group of holes using X-axis 
translation so that a predetermined 
minimum margin is maintained. This 
mode of operation will be used to 
locate the edge of hat sections when 
riveting Intertank panels to 
frames. In the hole 
resynchronization mode, the vision 
system has the capability of 
resynching the parts program to a 
tooling hole. The resynchronization 
is accomplished by means of X-axis 
translation and/or Z-axis move. The 
system also has the capability of 
resynching on a second tooling hole, 
computing the offsets for 
intermediate fasteners, and shifting 
the fastener locations accordingly. 
Automatic Tool Changer 
An Automatic Tool Changer on each 
outer carriage changes the drill and 
bucking anvil by means of the CNC 
parts program or operator console 
input. The tool changer has the 
capability of accomplishing the tool 
change without moving the 
workpiece. The GEMCOR developed 
Automatic Tool Changer has 18 tool 
positions comprising six'bucking 
anvil positions and 12 drill 
positions. An Automatic Injector 
Changer accommodates up to five 
injectors. 
The Automatic Tool Changer can 
complete a drill change in less than 
30 seconds and a bucking anvil and 
injector change in 30 seconds. The 
Automatic Tool Changer is shown in 
Figure 9. 
Automatic Rivet Selection System 
An independent Automatic Rivet 
Selection System ( ARS) is provided 
for each set of riveting heads. 
Each ARS has a floor mounted feed 
station containing 16 vibratory bowl 
feeders for unscrambling and 
aligning the rivets. The rivet 
selection is controlled by the CNC 
using a miscellaneous (M) function, 
an automatic stock thickness 
measurement system, or through the 
operator console. Selected rivets 
are automatically dispensed one at a 
time, blown through the injector 
tube to the injector and fed into 
the drilled holes as part of the 
automatic riveting cycle. A backup 
or reserve feed capability is 
included to permit manual feed from 
three individual drop tubes for each 
riveting head. The ARS, which can 
be expanded to include additional 
feeders, is shown in Figure 10. 
Automatic Ins~ection 
A programmable probe type hole 
inspection system is incorporated in 
the outer head which performs a plug 
gage type inspection. A precision 
conical probe is inserted into the 
drilled hole and its displacement 
measured with a Linear Variable 
Displacement Transducer (LVDT) . 
Figure 9.  
Automatic Tool Changer Holds 1 2  Dri l ls  and 6 Bucking Anvils. 
From the linear displacement data, 
the ho,le diameter is computed and 
compared to the established 
tolerances prior to rivet 
insertion. The system is interfaced 
with the CNC system to provide the 
capability of interrupting the 
drill/rivet cycle and displaying an 
operator message on the CRT when a 
drill change becomes necessary to 
maintain hole tolerance. 
A rivet process data system provides 
real time hard copy printout of 
sequence number, upset force, hole 
size, and rivet head height. The 
frequency of printout is 
programmable to provide data as 
desired from every rivet cycle down 
to an exception only (out of 
tolerance) basis. 
The system is capable of programming 
the hole size, upset force, and head 
height tolerances and the frequency 
of hole size inspection, The basic 
programming of the frequency of hole 
inspection is not accessible from 
the operator" console; however, the 
operator has the capability to 
selectively add inspection of any 
holes without changing the basic 
program. 
SUMMARY 
This Automatic Riveting System, 
being procured from GEMCOR by Martin 
Marietta Aerospace for NASA, will 
incorporate a new and unique 
automatic riveting system approach. 
It will bring to the Michoud 
Assembly Facility and External Tank 
Program the latest automatic 
riveting technology permitting 
further ET cost reductions and 
capability for low cost production 
of large structures for future space 
programs. 
Figure 10. 
Automatic Riveting Selection System 
Selects & Feeds Fasteners From 1 6  
Vibratory Bowl Feeders. 


Automatic Fastening Large Structures: a New Approach

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