Space Shuttle External Tank
The external tank of the Space Shuttle ( English Space Shuttle External Tank , abbreviation in the following: ET) contained the cryogenic rocket propellants liquid hydrogen (fuel) and liquid oxygen (oxidizing agent). The external tank was built by Lockheed Martin in the Michoud Assembly Facility . The ET was also part of the Ares rocket family developed as part of the Constellations program and is the basis of the main stage of the future manned American space launch system .
Use during a mission
The orbiter itself carried only small amounts of maneuvering propellants and propellants for the fuel cells for on-board power supply. The three main engines SSME arranged at the stern of the orbiter were supplied from the external tank during take-off and ascent. After the main engines burned out, the tank was separated from the orbiter, re-entered the earth's atmosphere about half a orbit around the world, most of it burned up and the rest fell about 18,500 km from the launch site into the ocean - usually depending on the orbit inclination in the Indian or the Pacific Ocean, far away from known shipping routes. The external tank was therefore the only large component - in contrast to the reusable solid fuel boosters and the orbiter - that was lost on every mission and had to be procured anew every time.
The tank narrowly missed orbit; a speed of about 100 m / s was missing to stay in an orbit. The orbiter, now separated from the external tank, made up the lack of speed to reach a stable orbit by means of the orbital maneuvering system auxiliary engines with fuel carried on board; in a burning phase shortly after separation from the tank, the apogee of the orbit was raised when it was reached At the furthest point from the earth, the path was circularized by a second burning phase.
overview
The outer tank was the largest and (if filled) heaviest individual component of the space shuttle. It was 46.88 m long, had a 8.4 m diameter and consisted of three main components:
- Liquid oxygen tank in the upper area
- Unpressurized area where most of the electrical components were located
- Tank for the liquid hydrogen in the lower part - this is the largest part, but its content was lighter than that of the oxygen tank.
The external tank was detachably connected to the underside of the orbiter with a double strut called a bipod in the front area and at two points in the rear area. On the two rear struts there were also the pipe couplings for the main fuel lines to the three engines of the orbiter, for the compressed gas lines for maintaining the tank internal pressure and cables for control signals and power supply between the tank and the orbiter. Electrical signals and control commands between the orbiter and the two solid fuel rockets, which were arranged alongside the ET, were also transmitted through these lines.
Coloring
The tanks used on the first two shuttle launches - STS-1 and STS-2 - were painted white with titanium dioxide . The NASA had feared that the solar radiation would otherwise heat up the tank too. During the preparations for the maiden flight in April 1981, however, it became apparent that the rust-brown insulation layer was completely adequate protection. This is why the white paint has been dispensed with since STS-3 (the STS-2 tank was already painted when the decision was made). This saved one work step and 270 kg of mass.
Weight-optimized developments
Starting with the STS-6 mission , a lighter ET ( called Lightweight Tank (LWT) by NASA ) was introduced. This tank was used for most of the shuttle flights until the failed flight of STS-107 . In 1998, NASA began using the so-called Super Lightweight Tank (SLWT) for flights to the International Space Station (ISS) , which further reduced the weight through a different design. Even if new tanks could differ slightly, they had an approximate curb weight of 30 t (SLWT: 26.5 t). The last tank from the first series had an approximate curb weight of 35 t. Each decrease in tank weight increases the payload weight by approximately the same number. The weight reduction was achieved by removing portions of the struts (structural struts that are attached lengthwise to the tank). In addition, fewer stiffening rings were used and the structure of the hydrogen tank was changed. Some areas of the tank were made narrower and the weight of the rear suspension for the solid rocket rockets was reduced by using a stronger, yet lighter and cheaper titanium alloy .
The anti-geyser line was removed from the construction a little earlier , which again saved a few hundred kilos. The anti-geyser line mated to the liquid oxygen line and provided a liquid oxygen circuit to reduce the build-up of gaseous oxygen during pre-launch fill-up. After data on the fuel load had been evaluated during tests on the ground and the first shuttle missions, the anti-geyser line was no longer used on the STS-5 mission and subsequent missions. However, the overall length and diameter of the external tank has not changed.
Components
Liquid oxygen tank
The liquid oxygen tank was a self-supporting aluminum construction made of laser-welded, pre-formed, rolled metal sheets, which were reinforced inside by ring frames and longitudinal stringers. He worked in the pressure range from 240 to 250 kPa (absolute pressure). The tank was bolted to the intermediate tank section with its flange at the rear end. The tank included baffles to minimize sloshing and swirling . They made sure that the liquid moved as little as possible and made sure that only oxygen in the liquid phase gets into the supply line to the turbo pumps of the three SSME .
The tank passed into the supply line with a diameter of 43 cm, which conducted the liquid oxygen through the intermediate tank and through a pipe bend in the same, then along the outer wall of the external tank on the right-hand side to the connection point between the external tank and orbiter. The diameter of the line allowed the oxygen to flow at a mass flow of 1264 kg / s, while the SSMEs ran at 104% of their nominal thrust and had a maximum flow rate of 1.1099 m³ / s. The pointed-elliptical shape of the liquid oxygen tank reduced the air resistance and heat build-up due to air friction when flying through the lower layers of the atmosphere. The nose cone contained the ascent air data system in nine specimens and served as a lightning rod.
A cap mounted on a swivel arm on the service building covered the vent opening at the tip of the ET and thus the oxygen tank during the countdown phase and was only swiveled away about two minutes before the start. This cap sucked off escaping oxygen vapor, which could have led to ice deposits at the top of the tank and thus posed a danger to the spacecraft during launch.
The liquid oxygen tank contained a separate, pyrotechnically operated vent valve at its front end. When the orbiter was separated from the external tank shortly before reaching orbit, this valve was opened in order to tip the tank away from the orbiter by the recoil of the escaping residual gas and thus support the separation maneuver and enable better control of the re-entry aerodynamics of the external tank.
The volume of the liquid oxygen tank was 554 m³. It had a diameter of 8.41 m, was 15 m high and weighed 5.4 t when empty.
Intermediate tank section
The intermediate tank section ( “Intertank” ) was the cylindrical connection made of steel and aluminum in a half-shell construction with ring-shaped connecting flanges at each end, which connected the two individual tank containers. At the same time, the SRB solid fuel boosters were introduced to thrust via their front attachment in this structural part. These connection fittings were arranged on opposite sides of the outer wall of the section. The thrust was distributed to the rest of the structure via a central ring bulkhead, and the connections were directly connected to a steel strut running across the section to absorb the radial compressive forces of the SRB. The planking consisted of stringer-reinforced light metal panels.
The intermediate tank contained the instruments and avionics of the external tank and a connection unit. Before take-off, an arm on the launch frame was docked to it, which flushed the space with inert nitrogen gas and extracted hydrogen gases in order to prevent oxyhydrogen explosions . The intermediate tank section was connected to the outside atmosphere during the flight.
The intermediate tank section had a length of 6.9 m, a diameter of 8.4 m and weighed 5.5 t.
Liquid hydrogen tank
The liquid hydrogen tank was an aluminum component made up of four laser-welded, barrel-shaped segments, five inner ring frames and two arched tank bottoms at the front and rear .
The working pressure was between 220 and 230 kPa. The liquid hydrogen tank also contained a system to prevent vortex formation and a suction fitting that conducted the liquid hydrogen through a 43 cm wide supply line to the line coupling on the left external connecting strut to the orbiter. The mass flow of LH 2 was 211 kg / s during a performance of the SSMEs of 104% or a maximum flow rate of 2,988 m³ / s. At the front end of the tank was the front ET orbiter connecting strut, at the rear end the two rear ET orbiter suspensions and the rear SRB-ET stabilizing supports. The liquid hydrogen tank had a diameter of 8.4 m, a length of 29.46 m, a volume of 1515.5 m³ and an empty weight of 13 t.
Heat protection
The ET heat protection consisted of heat-dissipating materials and a sprayed-on foam insulation . The system also used phenolic thermal insulators to prevent air condensation on metallic parts of the hydrogen tank. The heat insulators should also reduce the heating of the liquid hydrogen. The heat protection weighs 2.2 t.
However, the heat protection was problematic and turned out to be a fatal weak point of the shuttle missions. Until 1997, foam insulation was made with freon , a chemical known for its damaging effect on the ozone layer . Although NASA exempted from a law calling for a reduction in the use of freon and the amount of freon used on the tank was small, the composition of the foam was subsequently changed. However, the new foam fell off much more easily on take-off and increased the number of hits on the shuttle's heat tiles by ten times. In addition, ice often formed on the outside of the tank after it was filled with the cryogenic liquids, which poses a threat to the shuttle during the flight.
During the launch of STS-107 , a piece of foam insulation came off and hit the leading edge of the wing of the Space Shuttle Columbia at very high speed. The impact destroyed several reinforced carbon heat tiles on the front of the wing, so that when it re-entered the earth's atmosphere, super-hot plasma was able to penetrate the inside of the wing. This led to the destruction of the wing structure and consequently the breaking of the Columbia and the death of the crew members. The problem of the pieces of foam falling off couldn't be completely resolved at first. Cameras attached to the shuttle recorded a piece of foam flying away from the ET on the STS-114 . However, this part did not hit the space shuttle.
As a result, NASA suspended all further shuttle launches until the problem was understood and resolved. The so-called PAL thresholds (Protuberance Air Loads) have been identified as a possible cause of the problems with the insulation. These thresholds cover the fuel lines to the orbiter running on the outside of the tank with foam to protect them against air turbulence. Tests in the wind tunnel have shown that foam also detaches from the IFR ice / frost ramps. For the next flight ( STS-121 ) in July 2006, the PAL thresholds were dispensed with; the IFRs were left unchanged.
Pressure and level measurement
There were eight sensors ( Engine Cutoff Sensors, ECO ) to detect the running out of fuel and oxidizer shortly before reaching the final altitude and to switch off the Space Shuttle Main Engines (SSME) in an orderly manner; the engine cut-off ( MECO - Main Engine Cut Off ) had to take place with excess fuel; A shutdown in the event of a lack of fuel and therefore an oxidizer-rich mixture could have caused burns and severe corrosion of the engine components. The SSME turbopumps were also only allowed to suck in media in the liquid phase, as otherwise they would have over-revved without load and could have exploded. There were four sensors each for fuel and four oxidizer. The sensors for the end of the hydrogen were at the bottom of the fuel tank, those for the oxidizer in the fuel line leading from the tank. While the main engines (SSME) were in operation, the orbiter's computers continuously calculated the current mass of the vehicle as a result of fuel consumption. Normally the engines were switched off at a given target speed. However, if two of the fuel or oxidizer sensors had detected dryness, the engines would have been shut down beforehand.
The level sensors for the liquid oxygen were arranged in such a way that a maximum operating time of the main drive was made possible with the lowest possible, non-usable residual quantities in the tanks, without the oxygen pumps running dry. In addition, the liquid hydrogen tank was refueled with a reserve of 320 kg beyond the mixing ratio of 6: 1 (oxidizer to fuel) required for combustion.
Four pressure sensors each at the top of the liquid oxygen and liquid hydrogen tanks monitored the gas pressure in the respective tank.
Technology of the external tank
The external hardware, ET Orbiter connectors, utility connections, electrical and safety systems weigh together 4.1 t (9100 pounds). Each fuel tank has a vent and pressure relief valve at the front end. This double-function valve can be opened from the ground as a ventilation in the pre-launch phase and during the flight when the overpressure in the tank for liquid hydrogen reaches 360 kPa or in the tank for liquid oxygen 270 kPa.
Each of the two rear connection points of the ET is connected to a corresponding unit on the orbiter, which is also used for alignment between the two units. A physically strong connection is ensured by bolts. If the GPC initiates the separation of the external tank, the bolts are blown by means of pyrotechnic units.
The ET has five fuel valves that allow connection to the orbiter's fuel system, two for the liquid oxygen tank, three for the liquid hydrogen tank. One of the oxygen valves is installed for liquid and one for gaseous oxygen. The supply line of the hydrogen tank has two valves for liquid and one for gaseous hydrogen. Another fuel line for hydrogen is only used to recirculate liquid hydrogen during the relaxation phase shortly before take-off.
Two electronic connections provide power to the ET from the orbiter and route data from the ET and the two SRBs to the orbiter.
Use after the space shuttle
Constellation program
As the ET as part of after the Columbia disaster in 2003 by should the solid rocket boosters President Bush proclaimed Constellation program in the next generation of transport aircraft which Ares V (formerly Cargo Launch Vehicle (Calvados)) for cargo and the Ares I (previously Crew Launch Vehicle (CLV)) for the manned spacecraft Orion (formerly Crew Exploration Vehicle (CEV)).
In contrast to the Space Shuttle, the Ares V was originally supposed to have five SSME engines attached directly to the ET, but the number and model of the engines varied during the ongoing planning process, with the oxygen tank being located on the Saturn V rocket should be at the bottom. The solid rocket, made up of five segments, should be attached to the side as before. The upper stage ( Earth Departure Stage ) for leaving Earth orbit and the payload (for moon flights this should have been the Altair lunar module ) should have been installed above the tank .
The second stage of the Ares I, which was supposed to run on liquid fuel, should also consist of a scaled-down shuttle tank on which the Orion spaceship would have been mounted. The stage itself should be mounted on an adapted solid matter booster from the space shuttle program. This second stage was originally intended to receive a single SSME, but possible problems with an SSME that only started during flight prompted NASA to reduce the size of Orion and switch to an adapted version of the J-2 engine . However, due to the negative report by the Augustine Commission after only one suborbital test flight of the Ares I in 2009, in which no components of the ET were involved, the program was discontinued by President Barack Obama , who succeeded George W. Bush, in spring 2010.
Space Launch System
In 2010, however, the United States Congress decided to incorporate parts of the concepts of the Constellation program into future manned missions beyond low Earth orbit, so the main stage of the new manned space launch system (SLS) is to be derived from the ET and have the same diameter have two solid fuel boosters mounted on the side of the space shuttle for the time being and carry the Orion-MPCV spacecraft taken over from Constellation over an upper stage, which is dimensioned depending on the payload and mission profile . Between the upper level and the Orion or instead of the Orion, there is space for the appropriate payload, e.g. B. as for the LEM during the moon landings of the Apollo program . The main stage is equipped with three, four or five RS-25D / E (derived from SSME) engines, depending on the mission profile. The basic design of the SLS is based on the concepts of the Ares IV and Ares V from the Constellation program.
Web links
- Lockheed Martin: External Tank (English)
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- ↑ NASA: The External Tank (English)
- ↑ NASA: ECO Sensor PowerPoint Slide (PowerPoint presentation, English; 485 kB)
- ↑ Space Shuttle ECO Sonsors: an in-depth view ( memento of the original dated December 12, 2007 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (English)
- ↑ STS-114 Engine Cut-off Sensor Anomaly Technical Consultation Report , NASA, November 3, 2005 , PDF file, in English, accessed December 19, 2015
