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

ET SLWT specs (super lightweight tank = SLWT) 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

Other ET components include External Tank Other ET components include 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)

Lightweight Tanks (LWT) were introduced on the STS-6 mission 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 Orbiter-ET 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 pre-injection 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

Ball and socket joint (lower left) ET Feed ET – Socket Joint Ball and socket joint (lower left) 17” quick disconnect feed line (lower right)

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

LOX/LO2 tank specs ET LOX Tank 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

Aluminum-lithium alloy semi monocoque skin-stringer structure 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 skin-stringer structure Approx. 12,000 lb in weight

Used to attach the upper Orbiter and the upper SRBs to the ET 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

Intertank specs Length: 6.86 m (22.5') Diameter: 8.40 m (27.6') ET Intertank Intertank specs Length: 6.86 m (22.5') Diameter: 8.40 m (27.6') Weight: 5,455 kg (12,000 lb)

LH2 tank is the largest of the ET tanks ET Hydrogen Tank LH2 tank is the largest of the ET tanks Fusion-welded lightweight aluminum and aluminum-lithium 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

LH2 tank specs ET Hydrogen Tank 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. Post-landing 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

STS-50, 1992 - Bipod ramp loss. 20x10x1 cm tile damage 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 carbon-carbon 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

Beginning with STS-79, this system was no longer used 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