STS External
Tank
External Tank
The Space Transportation System’s External Tank is
one of the four major components that NASA
contracted for development in 1972
 Lockheed-Martin won the ET contract and moved its fabrication
plant to the NASA facilities in Machoud, Louisiana
 The Machoud plant’s location on the Gulf of Mexico allowed
shipping the completed External Tanks to the Kennedy Space
Center by barge
 The ET is the largest component on the STS, and could not be
shipped by rail or by cargo aircraft
 The Machoud plant will be turned over to Boeing for conversion
into the upper-stage Ares I fabrication facility as the STS
project comes to an end
External Tank Primary Components
 Oxygen tank
 Intertank
 Hydrogen tank
External Tank Primary Components
 Oxygen tank
 Intertank
 Hydrogen tank
External Tank
ET SLWT specs (super lightweight tank =
SLWT)
 Length - 46.9 m (153.8 ft)
 Diameter - 8.4 m (27.6 ft)
 Empty weight - 26,559 kg (58,500 lb)
 Gross liftoff weight - 762,136 kg (1.680 million lb)
 3% empty/gross weight ratio
External Tank
To minimize weight, the External Tank is constructed
as a pressurized, thin-walled, dual tank assembly
 A rigid, light-weight cylindrical shell connects both
liquid oxygen (top) and liquid hydrogen (bottom)
tanks
 The Orbiter's top attach point and the SRB's top
attach points are both located on the rigid intertank
section
 SRB Attach points are connected on the intertank through a
crossbrace
 The intertank is subjected to some of the highest loads on
the STS during the ascent phase
External Tank
Flight dynamics of the ET required the high-density
liquid oxygen be placed above the low-density liquid
hydrogen tank
 After separation of the SRBs, the thrust line of the
main engines runs through the center of mass of the
Orbiter and the ET, which would be much lower if the
oxygen tank was below the fuel tank
 The result would be a large rotation moment and a much
larger thrust vector correction
 The arrangement also Improves rotational stability of
the ET-Orbiter if one or two of the SSMEs were to fail
or shut down prematurely
External Tank
Other ET components include
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Fuel and oxidizer transfer lines to the Orbiter
Fuel and oxidizer pressurization lines to the tanks
Tank vents
Service ports and lines
Tank depletion sensors
Thermal insulation
Structural attachment points for the Orbiter and
SRBs
 Electronic power and control subsystems
External Tank
The first External Tanks, designated Standard
Tanks (STs), were constructed with an
empty weight of 35,045 kg (77,100 lb) and a
capacity of:
 LH2 - 395,582 gallons
 LOX - 146,181 gallons
The first two External Tanks were painted
white
 STS-1
 STS-2
Four more STs were launched unpainted which
had a weight savings of approximately 2,727
kg (6,000 lb)
External Tank
Lightweight Tanks (LWT) were introduced on the STS-6
mission
 Included lighter framed and thinner metal sections
 Same aluminum alloys as the standard weight tank
 Primarily Al 2219
 Lightweight ET tanks were flown on the majority of the
STS missions
 The ill-fated Columbia STS-107 mission which was the
last to use this tank
 As of 2007, a total of 86 lightweight ET tanks were flown
 Each had a dry weight of 29,894 kg (65,767 lb)
External Tank
Super Lightweight Tanks (SLWT) were
introduced on the STS-91 mission in
1989
 SLWT have been used on STS missions
since 1989 with the exception of STS-99
and STS-107
 Design structure of the super
lightweight tank is the same as the
lightweight tank
 Includes lighter aluminum-lithium alloys
for some of the elements
 Super Lightweight ET is the lightest but
most expensive tank
 Dry weight of 26,509 kg (58,319 lb)
ET Plumbing
Propellant feed lines for the SSME transfer the
fuel from the bottom of both tanks to the
Orbiter umbilical lines at the bottom OrbiterET attach points
 Propellant feed is a simple gravity feed since the
tanks and fuel are exposed to a minimum of 1-g from
launch to main engine cutoff (MECO)
 Tank pressurization is still necessary because the
turbopumps continually pull propellants from the
tanks to feed the SSMEs
ET Plumbing
 Without positive pressure on the liquid
propellants, the tanks would be pulled to a
vacuum by the SSME turbopumps resulting
in gas in the propellant lines which could
damage or destroy the SSMEs
 Hence, a small part of the fuel and oxidizer preinjection gas is isolated, regulated, and then sent
to the propellant tanks for positive pressurization
 A second line to the liquid hydrogen (LH2) tank is
used to return excess liquid hydrogen to the LH2
tank since not all of the LH2 is consumed in the
fuel feed loop to the SSMEs
ET Plumbing
ET LOX Tank
 The LOX tank is an aluminum-lithium alloy
monocoque structure (the skin serves part of the
structure)
 Capacity of 619,090 kg (1,362,000 lb) liquid oxygen at liftoff
 Equivalent to 542,639 l (143,350 gal)
 This is only one-third the volume of the hydrogen
tank, but a propellant weight six times that of the
liquid hydrogen
 Tank pressurization is generated from oxygen gas
warmed by the SSME's turbopump preburner
 The LOX feed line extends on the outside of the tank
from the LOX tank bottom to the Orbiter umbilical
disconnect at the aft attach point
ET Feed
 Feed line into the SSME
manifold abuts the lower
Orbiter support, with the
hydrogen feed line is
located at the opposite aft
attach point
 Feed-through from the ET
tank line to the Orbiter is
accomplished with a 430
mm (17") inner diameter
quick-disconnect on the
Orbiter's umbilical door
ET – Socket Joint
 Ball and socket joint
(lower left)
 17” quick
disconnect feed line
(lower right)
ET Feed
ET LOX Tank
 During pad operations,
tank venting is made
via a top vent valve that
extends through the
metal tip of the ET
 The metal tip also
provides an
aerodynamic transition
for the tank top, and
serves as a conductor
for electric field
dissipation to reduce
lightning strike hazards
during ascent
ET LOX Tank
 Tumble valve installed on the upper tank shelf of
earlier ETs in order to spin the tank end-over-end
was thought to offer a more reliable reentry
trajectory
 Later removed because of the potential hazard of a
rotating ET striking the Orbiter after separation
ET LOX Tank
 The ET LOX
tank includes
internal
baffles to
reduce
sloshing, fluid
rotation, and
vortex motion
ET LOX Tank
High-density LOX and an elongated tank that is not
perfectly rigid can and does generate low-frequency
oscillations during launch acceleration
 These oscillations induce pulsations at the low-pressure
oxidizer turbopump input called pogo oscillations which
can damage the SSME turbo machinery
 The pogo effect that first appeared on the Atlas ICBM
 Reduced on the STS with the addition of fluid expansion
chambers (accumulators) in the LOX feed line
ET LOX Tank
LOX/LO2 tank specs
 Composition:
Aluminum-lithium alloy
 Length:
16.6 m (54.6 ft)
 Outer diameter: 8.41 m (27.6')
 Volume:
560.7 m3 (19,786 ft3)
 LOX temp:
-183oC (-297oF)
 Weight (empty): 5,455 kg (12,000 lb)
 Flow rate (max): 1,267 kg/s (2,787 lb/s, 17,597 gal/min)
 Tank pressurization: 20-22 psi
ET Intertank
ET intertank is an
unpressurized,
rigid, cylindrical
structure used to
connect the
oxygen tank to
the hydrogen tank
 Aluminum-lithium
alloy semi
monocoque skinstringer structure
 Approx. 12,000 lb
in weight
ET Intertank
 Used to attach the upper Orbiter and the upper SRBs
to the ET
 Intertank also houses the electronics and
instrumentation for the ET
 Transfers power and signals from the Orbiter to the
attached SRBs
 The design utility of the intertank allows separate
bulkheads for the fuel and oxidizer tanks
 Makes a simpler structure with fewer operational constraints
 Unpressurized cylindrical piece has attach points and
reinforced thrust panels to transfer launch and flight
loads between the SRBs and the Orbiter
ET Intertank
 Tank fill and hydrogen vent access, ports and lines
run from an umbilical plate on the intertank to the
two propellant tanks
 GH2 vent and LO2 and LH2 fill lines on launch pad
are connected to the umbilical panel
 Quick disconnects for release as the umbilical arm
is retracted at liftoff
ET Intertank
 Intertank specs
 Length:
6.86 m (22.5')
 Diameter:
8.40 m (27.6')
 Weight:
5,455 kg (12,000 lb)
ET Hydrogen Tank
LH2 tank is the largest of the ET tanks
 Fusion-welded lightweight aluminum and aluminumlithium alloy used fpr semi monocoque structure
 Interior contains anti-vortex and anti-slosh baffles to
keep gas from entering the liquid feed to the engines
 29,000 lb tank has nearly 3 times the volume of the
LO2 tank
ET Hydrogen Tank
 To prevent complete fuel depletion and serious
damage to the SSMEs, four depletion sensors are
placed near the bottom of the LH2
 Sensors can initiate the command a shutdown on all
thee SSMEs if main engine cutoff was not reached
before either the fuel or the oxidizer was exhausted
 For safety and SSME protection, 1,100 lb extra fuel is
included in the LH2 tank to assure a rich mixture at
MECO
ET Hydrogen Tank
ET Hydrogen Tank
ET Hydrogen Tank
LH2 tank specs
 Composition:
Aluminum-lithium alloy
 Length:
46.9 m (153.8')
 Outer diameter:
8.41 m (27.6')
 Volume:
1,498 m3 (52,882 ft3)
 LH2 temp:
-253oC (-423oF)
 Weight (empty):
5,455 kg (12,000 lb)
 Flow rate (max):
1,267 kg/s (2,787 lb/s, 17,597 gal/min)
 Tank pressurization: 32-34 psi
Early assembly of ET with the hydrogen tank on the
left and the oxygen tank on the right
ET Thermal Protection
Thermal protection (TP) is needed on the ET to
protect the loaded cryogenic fluids from
heating on the launch pad and during ascent
 TP coatings consists of seven types of ablation and insulation
foam layers
 Light-weight ablation materials are molded, hand formed or
sprayed on several areas on the outer surfaces of the ET to
reduce heat flow during high-speed atmospheric ascent
 Outer ablation layers reduce heating from aerodynamic drag
during ascent
 Other foams such as polyurethane are injected, poured,
sprayed, or molded for specific applications that require
durability and efficient thermal insulation properities
ET Thermal Protection
 Compliance with Federal environmental regulations
required that several of the chlorofluorocarbons be
replaced with hydrochlorofluorocarbons beginning
with STS-79 and in increasing amounts since
 Phenolic thermal isolators are placed between the
extremely low-temperature LH2 tank, the tank
attachments, and tank supports in order to reduce
heat inflow as well as condensation and ice
formation on the outer ET shell
 Total weight of the ET thermal protection materials is
2,188 kg (4,823 lb)
ET Thermal Protection
ET Thermal Protection
NASA has had difficulty preventing fragments of foam
from detaching during flight for the entire history of
the program
 STS-1, 1981 - Crew reports white material streaming past
windows during Orbiter-External Tank flight segment. Crew
estimated size of debris from 1/4-inch to fist-sized. Postlanding report describes probable foam loss of unknown
location, and 300 Orbiter thermal tiles needing outright
replacement due to various causes.
 STS-4, 1984 - Protuberance Air Load (PAL) ramp lost
Additional 40 Orbiter tiles require replacement
 STS-5, 1982 - Continued high rate of tile loss
ET Thermal Protection
 STS-7, 1983 - 50x30 cm bipod ramp loss
photographed, dozens of spot losses
 STS-27, 1988 - One large loss of uncertain origin,
causing one total tile loss and hundreds of small
losses
 STS-32, 1990 - Bipod ramp loss photographed; five
spot losses up to 70 cm in diameter, plus tile
damages
ET Thermal Protection
 STS-50, 1992 - Bipod ramp loss. 20x10x1 cm
tile damage
 STS-52, 1992 - Portion of bipod ramp,
jackpad lost. 290 total tile marks, 16 were
greater than an inch
 STS-62, 1994 - Portion of bipod ramp lost
ET Thermal Protection
 STS-107, 2003 – Foam insulation detached from one of the
tank's bipod ramps and struck the leading edge of Space
Shuttle Columbia's wing at several hundred miles per hour. The
impact is believed to have damaged several reinforced carboncarbon thermal tiles on the leading edge of the wing, which
allowed super-heated gas to enter the wing superstructure
several days later during re-entry. This resulted in the
destruction of Columbia and the loss of its crew.
 STS-114, 2004 - Cameras mounted on the tank recorded a piece
of foam separated from one of its PAL ramps, which are
designed to prevent unsteady air flow underneath the tank’s
cable trays and pressurization lines during ascent. The PAL
ramps consist of manually sprayed layers of foam, and are
more likely become a source of debris. That piece of foam did
not impact the Orbiter.
ET Thermal Protection
 STS-115, STS-116, and STS-121 experienced
“acceptable" levels of foam loss
 STS-118, 2007 - A piece of foam and/or ice approx. 10
cm in diameter separated from a feedline attachment
bracket on the tank, ricocheted off one of the aft
struts and struck the underside of the wing,
damaging two tiles. The damage was not considered
dangerous.
ET Prepared for
Assembly
Completed and
Mounted ET
ET Range Safety System
 Earlier External Tanks incorporated a pyrotechnic range safety
system to split the tanks and disperse the propellants if
necessary
 ET Range safety device included a receiver/decoder, an
independent battery power source, antennas, and ordnance
 Beginning with STS-79, this system was no longer used
 Range safety assembly was completely removed for STS-88 and
has not been present on any tank since
 Range safety destruct is placed in both SRBs to split the casing
and halt forward thrust in case of errant trajectory that could
jeopardize life or property
ET Transportation
Fabricated External Tanks are placed
on a barge at the Michoud, Louisiana
plant and towed to the Kennedy
Space Center by tug
ET Transportation
After delivery at KSC,
the ET is placed on a
carrier and towed into
the Vehicle Assembly
Building for
inspection and
storage
ET Flight Profile
 Approximately 8 1/2
minutes after launch, the
Orbiter is separated from
the ET with a delay of 10
seconds after MECO
 ET separation procedure
is initiated by a pyro
explosion of the three
attachment nuts, each of
which hold the ET strut to
the Orbiter
 Three bolts held by the
frangible nuts retract into
the Orbiter by spring
loading
ET Flight Profile
 At separation, the ET and Orbiter are traveling at the
same speed and direction but are physically
disconnected, including the umbilical lines
 RCS jets on the Orbiter then fire in the -Z direction to
pull the Orbiter away from the ET gradually
 Hydraulic actuators then close the two umbilical
doors tightly to seal the thermal tile-covered doors
to the Orbiter's bottom
ET Flight Profile
 After ET - Orbiter separation, the OMS engines are
fired to boost the Orbiter to its planned orbit
 Without the OMS burn, the Orbiter would soon
descend into the atmosphere just as the ET does
 The tank continues to arc upward after separation
and reaches an altitude of approximately 111 km (69
sm, 60 nm) post-separation
 Downrange trajectory brings it down approximately
1,296 km (700 nm) where any remains after the fiery
reentry splash down in a predetermined spot in the
Pacific or Indian Oceans, depending on launch orbit
inclination
ET Flight Profile
ET Flight Profile
ET – The Future
NASA Ares I and Ares V vehicles will employ similar
tanks to the ET LH2-LOX structure
 Ares I is the crew launcher consisting of a 5-segment SRB, with
an upper liquid fuel booster
 Same ET LH2 & LOX propellants
 Approximately 1/5 the volume of the STS ET
 Uses only spray-on insulation since crew vehicle rides on top of the
booster
 No insulation shedding hazard
 Ares V uses two 5-segment SRB boosters with a second-stage
that has five RS-68 engines
 RS-68 are also used on the Delta IV
 Second-stage uses the same ET structure concept with separate
LH2 and LOX tanks separated with an intertank structure
Launchers – Past, Present, and Future
The End