The Butterfly And The Bullet

Paradoxically, compared to flights that would follow, the first mission of the Space Transportation System - dubbed 'STS-1' - was relatively straightforward. Its objective was to fly Columbia into space, test her systems and bring her home two days later to a desert landing in California. Yet with so many unknowns and a history of technical problems, it was also the most complicated mission ever attempted. Not only would Columbia herself be tested, but so would the untried boosters and the giant External Tank (ET) that would feed the Shuttle's engines with over 1.9 million litres of liquid propellants.

''Like bolting a butterfly onto a bullet'' was how veteran astronaut Story Musgrave, who flew Columbia in late 1996, described the unusual appearance of the combined Shuttle, tank and boosters. It is an appropriate description. The 46.6-m-long ET, reminiscent of an enormous aluminium zeppelin standing on end, is indeed bullet-like, but is actually far more than 'just' a container. Its upper quarter houses liquid oxygen and its lower three-quarters carry liquid hydrogen.

Separating the two sections of the ET is an unpressurised 'intertank', which contains instrumentation and umbilical interfaces to the launch pad's purging and hazardous-gas-detection systems. Above the intertank, the liquid oxygen tank holds up to 542,640 litres of oxidiser and, beneath it, the liquid hydrogen tank holds around 1.4 million litres of fuel. Both are then fed through two 43-cm-wide fuel lines into disconnect valves in the Shuttle's aft compartment, and from thence into the main engines' combustion chambers.

''[The main engine is] very high-performance,'' said Henry Pohl, ''[with a] very high chamber pressure for that day and time [and] very lightweight for the thrust that they were producing. I would say that we came out with that program in the only time in [US history] when it would have been successful. If we had waited another two years before starting development on [the] Shuttle, we probably would not have been able to do it, [because] the people that designed the main engine were the same that designed [previous rocket] engines. That group of people had designed and built seven different engines before they started the Shuttle development. A lot of [them] retired [and so] if we'd waited another two or three years, those people would all been gone and we would have had to learn all over again on the engine development.''

Built by Rocketdyne - formerly part of the Shuttle's prime contractor, Rockwell International, but now owned by Boeing - the engines burn for about eight minutes of ascent and are shut down a few seconds before the ET is jettisoned, right on the edge of space. Each engine measures 4.2 m long, weighs 3,400 kg and is 'throttleable' at 1% incremental steps from 65% to 104% rated thrust. This ability, which is controlled by the Shuttle's onboard General Purpose Computers (GPCs), helps to reduce stresses on the vehicle during periods of maximum aerodynamic turbulence.

Despite the immense thrust generated by each engine and the colossal amount of propellant needed to run them for such a short length of time, they in fact provide only 20% of the power needed to get the Shuttle into space. The remainder comes from the two 45.4-m-long SRBs, which are the only solid-fuelled rockets ever used in conjunction with a manned spacecraft. Loaded with a powdery aluminium fuel and an oxidiser of ammonium perchlorate, the boosters, built by Morton-Thiokol in Utah, are mounted like a pair of Roman candles on either side of the ET.

This unusual combination, referred to as 'the stack', is not, in fact, totally reusable and came about following a series of financial and technical compromises dating to the early 1970s. The Shuttle in its present form is designed to fly a hundred times before major modifications become necessary and the SRBs about a quarter of that figure. The ET, on the other hand, is discarded about eight-and-a-half minutes after launch to burn up in the atmosphere over the Indian Ocean. It was considered more costly to modify the tank for reusability than to simply build a new one for each mission.

''The Shuttle is an asymmetric vehicle,'' said former NASA flight director Neil Hutchinson. ''It doesn't look like it ought to launch right, because it's not a pencil! Some of us, in the early days, wondered how that was going to work. In fact, it's [still] a very tricky vehicle to launch. It has to be pointed carefully in the right direction at certain times or you'll tear the wings off or tear it off the External [Tank]. It's not a casual launch process.''

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