The Ariane 5 launcher promises to provide a much-needed boost to the European space industry.
Julian Moxon/PARIS
When the Ariane 5 launcher finally roars away from the Kourou launch pad in French Guiana in early 1996, European launch capability will receive a badly needed shot in the arm if Europe is to compete against the increasingly tough competition from around the world.
At a stroke, the cryogenically fuelled Ariane 5 will increase the payload capacity for launches to geostationary orbit by more than 40% over that of the current most-powerful version of the Ariane 4, carrying a maximum payload of 6.9t to geostationary transfer orbit (GTO). It should also bring an increase in reliability to 98.5% (against 90% for Ariane 4) which is sorely needed by the hard-hit satellite insurance market.
Ariane 5 programme management follows the same lines as that for previous Arianes. Technical direction is carried out by French Space Agency CNES, which places the industrial contracts on behalf of the European Space Agency (ESA). Aerospatiale is the industrial architect of the programme. The development cost was set originally at Fr35 billion ($7.2 billion), with a 20% "contingency allowance". The seven-month programme slippage to date will ensure that most of the contingency is spent, however.
The current Ariane series began with the Ariane 1 (first launched in December 1979), and continues today with the Ariane 4 range, with its maximum payload capability of 4.6t to GTO. The decision to design a follow on was made in the early 1980s, says Guy Laslandes, Ariane 5 programme manager for CNES. "There was a demand for a manned capability, which meant very high reliability."
The requirement for a manned capability was dropped with the termination of the Hermes space plane programme in 1993 - although Laslandes says that he is "certain" that it will return with the eventual need to supply crew members to the international space station. Meanwhile, the greater reliability, which was needed for manned flights, will benefit commercial prospects at a time when competition has become exceptionally tough.
Around 30 different launcher configurations were studied, with the clear aim of finding the design yielding the lowest operational costs. There was a further payload requirement that the launcher had to be able to accommodate the new generation of Shuttle-compatible satellites measuring 4.57m in diameter.
The resulting mix of solid-fuel boosters and a cryogenic main stage is similar to that of the Shuttle, the Titan and other cryogenic launchers. "There's a simple reason," says Laslandes. "It is because this is the most economical configuration in terms of manufacturing and provides the most reliable solution, with the best performance."
Stage dimensioning was finalised in mid-1992 and the launcher critical-design review was held in early 1994, confirming the engineering concept, launch operations and compatibility of the various elements of the programme.
All Ariane 5s will use the same basic module, called the main-stage propulsion system, or lower composite, consisting of the two solid boosters and the main cryogenic stage. To this is added the upper composite, consisting of the vehicle-equipment bay (VEB), the storable-propellant stage, the Speltra carrier structure for double launches, the fairing, which can be long (17m) or short (12.7m) depending on the payload, and the payload adaptors. (Ariane launchers remain unique in being able to deliver two payloads to independent orbits.)
The VEB carries almost all of the launcher's dual-redundant avionics, including the on-board computer, along with the attitude-control system, the latter fuelled with 70kg of hydrazine. The storable-propellant stage carries 9,700kg of propellant for the geostationary mission - enough for 1,100s of third-stage burn.
LIFT-OFF
At lift-off, the solid boosters each produce 5,774kN (7,298,000lb) of thrust - the Vulcain 1,120kN (see boxes). The boosters separate at an altitude of 55-70km (35-43 miles), the nose fairing being jettisoned at 110km. Separation of the upper composite occurs at 140km, some 10min after launch, the cryogenic stage continuing in ballistic flight, rotating about its longitudinal axis until it falls into a pre-designated "safe" area in the Pacific.
The Aerospatiale-built main cryogenic stage measures 5.5m in diameter, and is 30.5m high. The single main engine is attached to a thrust frame and rear skirt, the latter housing the propellant and fluid system for various operations of the stage, such as pre-cooling of engine components, and pressurisation.
Thrust is transmitted to the main structure by the conical thrust-frame, which also contains the nozzle-actuation jacks for vectoring the engine up to 6¡ in any direction. As with the Shuttle, the entire weight of the launcher is supported on the boosters, which are free standing on the ground. Three supports attach the boosters to the lower part of the liquid-hydrogen tank. A single mounting joins the top to the forward skirt, transmitting the entire thrust of the boosters (to a maximum of 6,367kN) to the launcher.
The tanks are constructed from aluminium alloy by Cryospace (an Aerospatiale/Air Liquide consortium) and are separated from each other by a common bulkhead. Capacity of the liquid-hydrogen tank is 26t, while its liquid-oxygen counterpart holds 132t.
Within the forward skirt, and carried on an annular rack within it, is housed most of the functional electrical equipment required for flight, telemetry and safety operations. The skirt, in turn, carries the storable-propellant stage, containing six pressurised tanks: two each containing the fuel for the Aestus rocket engine (mono-methyl hydrazine and nitrogen tetroxide), the remaining two containing helium to pressurise the fuel. Integrated with the stage is the vehicle-equipment bay, carrying the two sets of three small hydrazine thrusters to provide attitude control during the launch.
The top of the launcher consists of the carbonfibre nose fairing, either long or short, and the Speltra satellite support structure, which, if two satellites are being launched, carries the upper craft while housing the lower.
Many of the problems with the Ariane 4 system have occurred in the third, cryogenic, HM7 stage and, here, Arianespace has taken a new step with the Ariane 5 in using an extremely simple propergol-fuelled design which self-ignites and needs no turbo-pumps, and hence has virtually no moving parts. The Aestus engine operates at a lower, 27.5kN, thrust level, however, so that the third stage takes 19min instead of the HM7's 12min to reach GTO from release.
SATELLITE DELIVERY
The satellite delivery system is similar to those of previous Arianes. After ignition, the Aestus engine pushes the upper composite to GTO arriving, some 29min after launch. In the dual-payload case, the upper composite is spun-up before the release of the first satellite. Rotation is then stopped while the stage is oriented for the next release, the payload adaptor jettisoned, and the composite spun-up again for release of the second satellite.
Laslandes says that the precision of the system is such that "...we expect to achieve better than 100m accuracy" for placing satellites in the right position before they continue under their own steam to full geostationary orbit. This, and the fact that the third stage reaches an altitude of 600km, against the Ariane 4's 200km, means that less satellite fuel is needed for positioning, increasing on-station time.
Dual-satellite launches, which were an option on the Ariane 4, will become the norm with the Ariane 5, helping Arianespace to keep its launch prices competitive. They mean, however, that the marketing organisation will have to find customers willing to launch their satellites at the same time, which, in the increasingly tough environment, may prove to be a disadvantage.
FIRST LAUNCHES
The first two launches are paid for by ESA, with the first commercial launch planned for 1 October 1996. Arianespace then takes over the programme, and is close to concluding negotiations for 14 launchers. The transition phase from the Ariane 4 begins in the year to October 1997, during which two Ariane 5s will be launched, rising to three in the following year, then four, and, in the 1999-2000 period, five. Launch frequency is expected to remain at this level.
Source: Flight International
