The Space Shuttle STS 70 mission in June will be a routine Tracking and Data Relay Satellite deployment and experiments flight, with a five-person crew under the command of US Air Force Col. Terrence Henricks. Its first eight minutes of flight, however, will make history.
One of the Discovery's main engines (SSMEs), No 2036, will be operating with a high-pressure, liquid-oxygen turbo-pump, developed by Pratt & Whitney. This replaces a turbo-pump produced by SSME main contractor Rocketdyne. Called the Block 1 SSME upgrade, the new engine includes other improvements. The P&W turbo-pump is designed to give the engine a 55-mission, 7.5h design life before overhaul.
This is the first phase in a $1 billion NASA programme to develop more reliable and long-lasting Shuttle engines. Block 2 SSMEs will begin flying in September 1997 with P&W liquid-hydrogen turbo-pumps too.
While the Block 1 and 2 SSME upgrade programme was originally planned with safety, cost and reliability in mind, the improvements have recently taken on another, equally critical, raison d'etre. The USA and Russia are now committed to a series of joint Shuttle/ Mir missions, as a prelude to the operation of the international Alpha space station.
Unlike the original US-led Freedom space station, which was to have operated in a 28.5° inclination orbit, the Alpha, with its large Russian will fly at a higher inclination, 51.6°. The best advertised Shuttle performance to 28.5° is 24,950kg. This compares with 1980's originally advertised 29,500kg, now decreased by shuttle weight increases and to accommodate better understood lift off load levels.
For 57° orbits, the Shuttle's best payload performance is reduced to 18,600kg, basically because it does not benefit from its usual advantage of following the path of the Earth's rotation. In giving way to the need for a high-inclination, NASA has placed the Space Shuttle programme under enormous pressure.
HIGH VALUE
To achieve meaningful transportation of space station hardware to 51.6¡ during an advertised 20-plus flights to assemble the Alpha, the Space Shuttle will depend on improved SSMEs which can operate for longer periods and at a higher rated thrusts, with better levels of performance and reliability, without jeopardising safety.
NASA's problem is that, despite the bullish official hype the new Block 1 engine has passed certification tests and proved to be more efficient than the current SSMEs, but it has not fully demonstrated its new performance. This is measured as specific impulse, the ratio of the thrust produced to the rate at which matter is expelled by the engine. In addition, the Advanced Solid Rocket Motor programme, which would have assisted the Shuttle in taking heavier payloads to orbit, was cancelled in 1993.
If the new engine continues to fall short on performance, the Shuttle will not be able to carry maximum payloads to 51.6°. It will be 600kg short on payload performance, possibly making missions carrying the heaviest components, together with other equipment on one flight impossible, increasing the number of missions required to construct and maintain the station. This is another example of the tenuous state of the US/Russian partnership, which was forged because neither country could build a station on its own, (Flight International, 1-7 March).
The all-Rocketdyne SSMEs, designed with 1970s technology, have flown 67 launches since April 1981, including the Challenger/51L flight in 1986, which ended in disintegration. Two hundred SSMEs performed nominally on the 67 launches, amassing 96,480s of operational firing. One engine, on the STS 51F mission in August 1985, was shut down at T+4min55s, causing an abort to orbit.
While this is a good record, the original SSMEs have required far more maintenance and component-replacement after each mission than originally planned. Each SSME develops 1,668kN thrust at lift-off, consuming liquid-oxygen and liquid-hydrogen at a mix ratio of 6:1. The engines can be throttled over a thrust range of 65% to 109%. Thrust, is developed by a staged combustion process that involves pre-burning. The propellants are combusted in dual pre-burners to produce high-pressure hot gas to drive the high-pressure turbo-pumps, to deliver oxidiser and fuel to the engine and the pre-burner combustion chambers. Low-pressure turbo-pumps are used to provide the necessary pressure on the high-pressure turbo-pumps to prevent cavitation.
HIGH-PRESSURE
The high-pressure liquid-oxygen (oxidiser) turbo-pump operates at a nominal speed of 29,057rpm, and the high-pressure liquid hydrogen fuel turbo-pump at 35,000rpm. After each flight these pumps have needed to be removed for routine inspection, maintenance, overhaul and component replacement. In the early years of Space Shuttle operations the pumps, and particularly the turbine bearings and blades, caused concern at NASA because they did not demonstrate the predicted reusability.
In 1986, three years after NASA began a Phase 2 SSME improvement programme, the agency appointed P&W - with a $186 million contract - to develop new high-pressure turbo-pumps. These were intended to last ten missions without requiring inspection, overhaul and maintenance after each flight, and increasing safety margins and reliability. As a result, the new turbo-pumps incorporate the latest technology derived from P&W's extensive experience in gas-turbine engine development, including single-crystal turbine blades and advanced ceramic bearing technology.
BLOCK INTRODUCTION
The Block 1 SSME upgrade will be introduced on one of the three SSMEs on STS 70. Three current engines will then power the STS 69/Endeavour mission in August and the STS 73/Columbia will fly with all three new Block 1 SSMEs in September, for the second US Microgravity Laboratory mission, if engineers are satisfied the Block 1 is as efficient as it has been advertised to be. Otherwise, more ground testing may be required before its re-introduction. The plan is that after STS 73, current and Block 1 engines will continue to be flown alternately.
The Block 1 engine features the P&W Alternate High Pressure Oxidiser Turbo-pump (AHPOT), designed as a line-replaceable unit, which successfully completed testing on 15 March following a programme of firings, totaling 20,000s, at NASA's Stennis Space Centre, equivalent to 40 Shuttle missions. "Throughout certification we surpassed all expectations," said John Price, AHPOT programme manager at P&W. "Now we are ready to support our first mission." Behind the smiles, lies concern about efficiency, although this has not been blamed on the AHPOT but on a new hot-gas manifold from Rocketdyne.
The Block 1 test programme was completed in nine months and included eight long-duration, simulated, abort missions of about 760s each at up to 109% rated power level. A typical SSME ascent lasts 520s at 104% except at the throttle-down for Alpha missions may require higher rated thrust, to 109%, for longer periods.
In October 1994, the first of a pair of AHPOT-equipped engines, No 2036, reached the flight-certification programme's mid-point, by logging 10,000s of test time. The second engine, 2037, completed the programme with an additional 10,000s. It was these engines, although proving to be more efficient than the current SSMEs, which first revealed that the Block 1 did not meet the programme performance goals. The AHPOTs have also logged more than 86,000s of development and certification test time.
ELIMINATING WELDS
For the AHPOT, the turbo-pump housing is produced through a casting process, eliminating all but six of the 300 welds on the current pump. The new pump uses new ball bearings with advanced cooling and ceramic elements, using silicon nitride, a type of ceramic, which offers several advantages over steel bearings.
The material is 30% harder than steel and has an ultra-smooth finish, which generates less friction during pump operation. The new bearing will help eliminate concerns over excessive wear to the pump-end ball bearing. The AHPOT runs at 23,700rpm, with a maximum discharge pressure of 545bar and absorbing 19,285 shaft kW.
Also flying with the new turbo-pump is the new Rocketdyne two-duct hot-gas manifold, or power head. This routes propellants to the combustion chamber, to significantly improve fluid flows within the engine system by decreasing pressure and reducing maintenance, and enhancing overall performance. This replaces an original gas manifold with three smaller hot-gas transfer system ducts in the current design, with two enlarged ducts to improve engine performance. This will increase hot-gas-flow uniformity, decrease turbulence levels and eliminate the need for injector baffles.
NASA has revealed that during the Block 1 tests at Stennis, operating the new power head required more coolant for the combustion chamber, resulting in the lower engine performance. Rocketdyne will have its work cut out to improve this before NASA is out of the fire.
The Block 1 engine features a single-coil heat exchanger, replacing a multi-part unit. The new heat exchanger pressurises the liquid oxygen in the large external tank by bringing oxygen through the hot-gas system in the power head. The original was made with seven critical welds. A new process has made a single coil possible with no welds and increased tube wall thickness, improving durability.
The Block 2 SSME upgrade, to be introduced in 1997, includes the P&W High Pressure Fuel Turbo-pump (HPFT) and what is considered to be a major safety improvement, the Large-Throat Main Combustion Chamber, with a 10% increase in throat diameter.
COOLANT CHANNELS
It will also feature an increase in the number of coolant channels, a decrease in wall thickness to reduce chamber pressures and lower temperatures in the combustion wall. This reduces stress on all engine components so the engine can be operated at high power for longer.
After the Block 1 and 2 programmes went $260 million over budget in December 1991, NASA halted the HPFT programme until it was resumed in May 1994.
For all its improvements, the AHPOT, with thicker walls for increased tolerance, is heavier than the current SSME turbo-pump, contributing to the engine's 150kg weight increase, thereby adding to the criticality of the Shuttle's weight margin.
Source: Flight International
