Ten times larger than anything previously built in Europe, the Ariane 5's solid-booster motors each develop 6,367kN (1.44 million lb) of thrust, slightly more than half the thrust of the Shuttle solid boosters, and about the same as the Titan 4. The six test firings to date at Kourou have demonstrated "flawless" performance, says Jean-Pierre Ledey, general manager of Europropulsion, an equally owned subsidiary of France's SEP and Italy's BPD, which is responsible for development, production and technical co-ordination of the booster. A final, flight-standard, test is set for July.

The nozzles are produced by SEP at Bordeaux, with the motor casing subcontracted to Germany's MAN. Propellant and internal thermal insulation is the responsibility of BPD, which also supplies the propellant for the top section from its plant in Italy. Propellant casting for the centre and aft sections, is carried out by Regulus, a joint subsidiary of SEP and SNPE in French Guiana.

The booster design follows the general practice of other manufacturers. "We wanted to use well known, qualified technologies, to reduce risk, ensure reliability, and keep costs to a minimum," says Ledey. "It is a very simple, very conservative design, and the results of testing show that we were right to do that."

The 8mm-thick low-alloy carbon steel casing is manufactured in seven segments, and assembled at the MAN plant into three main segments, consisting of the lower and middle three sections, and a single top section. The "field joints" between the main segments are sealed with double "O" rings, and their design takes the Shuttle Challenger accident into account. "We have tested the field joints under exceptionally tough conditions," says Ledey, "and there have been no problems."

Testing at Kourou has, he adds, shown that "...we are within all of the figures set at the beginning of the programme". Critical to success was the need to demonstrate that the thrusts of the boosters, which at take-off produce 92% of the total thrust of the launcher, remain within ±3% of each other as they burn down. "We are well within that," says Ledey.

Again for simplicity, the nozzle is vectored by means of a flexible bearing, made of alternating slices of elastomer and metallic shims. It is vectorable (using two large actuators driven by pressurised helium acting on the hydraulic oil) to a maximum of 6¡ in any direction. To ensure a low rate of erosion, the 1m-diameter nozzle throat is made from carbon-carbon composite, its exit cone from light alloy, with phenolic carbon and silica insulation.

The propellant weighs 238t and consists of a mix of polybutadiene, ammonium perchlorate (the oxidiser), and aluminium (the fuel). After mixing, it is poured into the segments, around a mandrel, and cast under a vacuum. For the top section of the booster, the fuel is cast with a star-shaped internal cavity, to provide the thrust peak required in the first 20s of burning, while for the rest of its length the cavity is conical, tapering slightly towards the top. Burn time is 132s, with thrust reducing to 4,000kN at maximum dynamic pressure, and increasing gradually almost to the peak again, reached after 90s. The thrust then tails off again, to nil at 130s.

Ignition of the propellant is achieved with a small rocket motor, itself ignited by a pyrotechnic charge. The motor generates an extremely hot gas jet for half a second, which is directed on to the propellant through several nozzles. As with the Shuttle, the boosters are ignited only after the main engine has been started and checked.

The entire thrust of the boosters, each of which, is more powerful than an entire Ariane 44L (four liquid-boosters) launcher, is transmitted to the main stage through the forward mounting, which means that the cryogenic tanks can be made thinner, but introduces the problem of vibration near to the shock-sensitive payload section. A rubber/steel damping system has therefore been developed for the forward-engine mount. "We're confident that it will not give us any problems," says Leday.

The boosters are not reusable, but at least in early flights, will be recovered by parachute for inspection.

Source: Flight International