Figure 1. The components of Ariane-5 launcher
The Ariane-5 launcher is now in the qualification phase of its development programme. That phase mainly involves the testing of the various components of the launcher. Testing is well underway. Upon the successful completion of all tests, the launcher will be ground qualified and ready for two demonstration flights in 1995 and 1996 that will mark the end of the Ariane-5 development and the start of the operational life of the launcher.
The development programme for the Ariane-5 launcher began in 1988 and since then, the configuration of the launcher has changed greatly in response to the evolution of system requirements. The main configuration para-meters are currently frozen. The critical design review has already taken place and the qualification phase is now underway.
The launcher qualification can be divided into two parts. The first part consists of the test and analysis activities that will permit ground qualification. The second part includes the first two launches, which will mark the end of the development of the launcher and the start of the operational phase.
The ground qualification, in turn, is made up of qualification programmes at the different launcher-element levels. The status of the individual programmes varies, some are nearing completion while others are less advanced. All however are progressing in keeping with the general planning which foresees a first demonstration launch at the end of 1995.
The status of the main testing activities for the various launcher elements is summarised here. The main elements of the launcher are shown in Figure 1.
Figure 1. The components of Ariane-5 launcher
Two large and very powerful solid boosters will provide the main propulsion during the first two minutes of Ariane-5's flight. The structural qualification of the booster involves the testing of its casing, joints, lower skirt and forward cone. Firing stage tests are also part of the qualification.
Being made of steel and carrying 236.5 tonnes of solid propellant, the booster's casing is mainly dimensioned by the pressure achieved during the combustion of the propellant. Therefore the most relevant tests of the casing are two hydraulic pressure tests performed until the casing ruptures. In both tests, the safety margin obtained has been shown to be about 1.4 times the maximum operating pressure.
The design of the booster's joints has been of particular concern since the beginning of development because of the catastrophic problems that the Space Shuttle 'Challenger' encountered. A testing programme using flat-plate connections was undertaken to optimise the design by changing the initial gaps, missing pin, etc. Pressure tests were then performed with two full-scale cylinders and closed domes using different load cycles under nominal and downgraded conditions (Fig. 2).
Figure 2. Preparing for the testing of the booster joints
Note: For background information on Ariane-5 and the testing campaign, see ESA Bulletins 68 (November 1991) and 69 (February 1992).
Finally, the casing rupture tests mentioned above have demonstrated the safe behaviour of the joints.
Lower skirt and forward cone
The booster's lower skirt supports the launcher while the launcher is waiting to be fired. The stiffness and strength tests of the lower skirt have been completed, and the safety margins obtained were found to be suitable (Fig. 3).
Work on the forward cone is not as advanced. This structure transmits thrust to the launcher main body and provides for aerodynamic booster shaping. Combustion pressure oscillation detected during the booster firing tests required the late introduction of a softening device to attenuate the transmission of the low-frequency thrust oscillation to the main launcher body. The definition of this device is now being finalised, and the configuration of the forward cone can then be frozen.
Booster firings
Firing tests began using a reduced-scale configuration of the booster, composed of a forward segment, a central segment and an aft segment. The purpose was to begin testing the booster's internal ballistics. The results showed the presence of combustion pressure oscillation and, as previously mentioned, a softening device has had to be introduced in the booster to cryogenic stage interface.
The full-scale tests are performed with the booster in a vertical position in a test-stand, with the nozzle firing downward. The tests can be divided into two parts:
The aim of the B1 test was to verify the behaviour of the nozzle and the segmented propellant configuration. The test was successful: the proper behaviour of the internal ballistics, the thermal protection and the nozzle actuation was demonstrated. It also confirmed the level of thrust oscillation of about 4%.
After demonstrating the satisfactory behaviour of the casing and joints, and after the success of B1, the next firing test, the M1 firing, was authorised. That test was performed using flight-type hardware but as in B1, the forward cone was replaced by a structure allowing the thrust reaction along the booster axis. The firing was well performed and showed a thrust oscillation of about 3%, lower than in B1.
During the subsequent M3 test (Fig. 4) and the M4 test, the booster was supported laterally, as it will be in flight. Another M test will still be performed before the two Q firings that mark the end of the qualification phase.
The cryogenic main stage stands 30 metres high and carries 156.2 tonnes of liquid oxygen and hydrogen. It is ignited at lift-off and provides the only thrust after booster separation. Its main elements are the oxygen and hydrogen tanks, the upper skirt, the thrust frame and the Vulcain engine.
Figure 3. Test set-up for the static test of the booster's lower skirt
Figure 4. M3 booster firing test in Kourou in June 1994, with flight-type hardware and the booster supported in its flight position
The development of the Vulcain engine is in itself a complete programme and thus it is not described here. The tests, however, are ongoing and hours of firings on several engines in two test benches are accumulating (Fig. 5).
Figure 5. The Vulcain engine during a firing test
Main tanks
The two main tanks, the oxygen and hydrogen tanks, are made of 2219 welded aluminium sheets with a common bulkhead and spherical domes. As with the testing of the booster casing, the most relevant test of the tanks relates to their pressure performance. The test, which has already been successfully performed, has validated the tanks' dimensioning, and has provided the information required for a future mass-saving analysis.
Upper skirt and thrust frame
The upper skirt provides the link between the cryogenic stage and the upper composite. The qualification tests of the skirt have been completed (Fig. 6). The rigidity and safety margins were found to be well in accordance with predictions. As the structural margins obtained were higher than necessary, a mass-saving analysis has been performed.
The thrust frame supports the Vulcain engine. Its configuration has evolved several times because of its complexity and the great number of interfacing elements, but its design is now frozen (Fig. 7). A vibration test was performed to confirm the acceleration levels at the different equipment locations. Rigidity and strength tests are being prepared and an acoustic test is also planned.
Stage test
The stage tests will start with the 'Battleship' (BS) version at the Ariane-5 launch facilities in Kourou, French Guiana. This version consists of two industrial cryogenic tanks, a lower part of a flight representative stage and the main functional electrical equipment. The main purpose of this campaign is to test the stage functions that will be used for the M and Q tests, to validate the electrical interfaces as well as the ground equipment and operating procedures. The complete stage qualification includes BS, M and Q tests
Figure 6. The cryogenic stage's upper skirt during a static test
Figure 7. The cryogenic stage's thrust frame (yellow) with the Vulcain engine installed (silver cone)
Combined tests
Since the upper composite is not a unit, testing of each structure can be difficult without proper representation of the adjacent structures. Therefore, some tests are performed with a combination of two or more structures. This is the case for the vibration and shock tests of the and the shock test of the Speltra with the fairing pyrotechnic system.
However, the most important test in terms of structures contribution will be the shock and acoustic test using the vehicle equipment bay, the L9 stage, the Speltra and the upper skirt of the cryogenic stage.
Fairing
The fairing structure is made in two halves: it will open to allow the payload to be released into orbit. It also protects the payload and provides for the launcher aerodynamic shaping. Two qualification models of the whole structure have been manufactured. One of them has been used for two in-vacuo separation tests (Fig. 9), and is now being used for the acoustic tests with a third separation test envisaged afterward. The second model has been used for the static tests which have been successfully completed.
Figure 9.The two halves of the fairing during a separation test
Speltra
The Speltra is used for multiple satellite launches. It is located above the vehicle equipment bay. It holds one of the payloads while it supports the fairing and another payload. The stiffness and strength tests have already been completed (Fig. 10). A shock test with a horizontal separation system of the fairing has also been performed, and finally the separation tests will finalise the qualification phase.
Figure 10. Preparing for the Speltra static tests
Vehicle equipment bay
The vehicle equipment bay is a structure containing the launcher's main electrical boxes. A major testing activity concerning this structure has been the shock programme performed to properly define the dampers of the equipment platform (see 'Shock Induced at Separation' below).
Preparations for the static tests were already being made when the system tests confirmed the presence of radial deformations induced by eccentric transmission of the booster thrust. That implied that a change in the design of the lower part of the structure was required. In addition, the strength tests will be performed with an upper skirt model instead of a simple adjacent structure, to properly introduce the booster loads.
L9 stage
The structure of the L9 stage has undergone major modifications throughout the development process. Presently, the configuration is frozen and the static tests are under preparation.
A development model of this structure was used during the shock tests of the full-scale vehicle equipment bay. A model of the complete stage is shown in Figure11.
Successful firing tests with the Aestus engine have been performed. Stage firing tests followed, with the last one being undertaken on 5 October 1994 and lasting for 1075 seconds.
Figure 11. The L9 stage being installed in its container
The system mechanical activities include static and modal tests and acoustic and shock programme testing. Their results allow the verification of the system mechanical inputs and the validation of the dynamic mathematical models that will be used in the remaining system studies.
Static and modal testing
The tests have been divided into three campaigns corresponding to the different main launcher assemblies: the cryogenic stage, the upper composite and the booster.
Three kinds of tests have been performed with the cryogenic stage: stiffness tests, overflux tests (non-homogeneity of stress distribution), and modal testing. In general, the stiffness and overflux obtained in testing are in agreement with predictions.
The upper composite tests have been performed with the vehicle equipment bay, the upper stage, the Speltra, spacecraft mock-ups and an adjacent structure at the lower interface. The same three types of tests as for the cryogenic stage have been performed: stiffness, overflux and modal tests. The stiffness found in testing is globally higher than foreseen, as are the radial deformations. The frequency is as expected.The test campaign relating to the booster has been performed using the third full-scale test model (M2) with a reinforced upper structure and a representative thrust frame. Lateral modal tests have been performed in a clamped con-figuration. The results correlate well with the predicted characteristics of the first modes.
Acoustics
Maintaining the acoustic environment inside the spacecraft compartments, fairing and Speltra at the time of launch is a problem that had to be addressed from the beginning of launcher development. Experience with previous launcher programmes showed that this question required an early effort to characterise the external sound field and the acoustic behaviour of the upper structures.
The external field has been defined using test data obtained with reduced-scale models. Water injection at different locations on the launcher pad was tested at the same time as a method of attenuating the high levels of sound expected.
Fairing acoustic tests are being performed. The test article is one of the two full-scale structures used in the fairing qualification, equipped with foam panels including special acoustic absorbers (resonance cavities). The use of helium for noise attenuation is also being evaluated.
Another test campaign with the upper structures is planned. The test configuration includes the vehicle equipment bay, the upper stage and the Speltra from the upper part of the launcher, and the upper skirt of the cryogenic stage. The aim of being installed in its container this campaign is to evaluate the vibro-acoustic performance of the structures and the supported equipment.
Shock induced at separation
The shock produced at the time of the pyrotechnic separations has also been taken into account very early in the development, especially for the vehicle equipment bay. Shock induced at separation appears at the upper and lower interfaces between the boosters and the main launcher body, at the vehicle equipment bay where the upper composite separates, in the lower part of the Speltra and at the fairing horizontal and vertical separation planes.
Testing of flat articles was initiated as part of the shock evaluation programme for the vehicle equipment bay. It was very soon realised that the accelerations induced at the equipment level were very high and that a damper was required. Such a system was implemented at the interface between the platform support and the cone and was validated through testing.The fairing-separation shock was measured in an early test using a cylinder with a height of 2 metres and a diameter of 5.4 metres. The levels transmitted from the fairing's horizontal separation system through the Speltra cone to the payload adaptor have been measured on the qualification structure of the Speltra with the fairing system.
Shock testing is also to be performed with the upper structures and cryogenic skirt, the configuration already mentioned in relation to the acoustic tests. Shock will be induced at the booster to cryogenic skirt level, vehicle equipment bay separation system and Speltra horizontal pyrotechnic chord device. Operational deployment In order to validate the mechanical integration of the launcher (at the Guiana Space Centre), the following test campaigns are scheduled:
The first MDO1 test campaign took place in September 1993 (Fig. 12).
With the configuration of the Ariane-5 launcher now frozen, the testing activities required in the ground qualification phase are proceeding without any major problems. The flight elements for the first two models are now being manufactured to be ready for the flight part of the launcher qualification in 1995.
Figure 12. MDO1 campaign: the cryogenic stage and booster mock-ups in the integration building
ESA Bulletin Nr. 80.