UT in Phased Array applications for Control of Structures and Piping of stages in the European Space Launchers Elena Tosti NDT specialist and NDT Plant Engineering Responsible AVIO S.p.A.; Colleferro, Italy [email protected] Abstract. The non-destructive controls applied to components of the Stages manufactured by AVIO for the small European Launcher VEGA, include UT when possible as a http://www.ndt.net/?id=22998 consequence of the low cost and absence of environmental hazards. UT in phased array are in development to check the structure of solid state propellant rockets, which is obtained by using the wrapped carbon filament technology. The composite resulting after polymerization of the structure made in filament winding, is characterized by a degree of compactness which generates high attenuation of the ultrasound signal. This requires a generator of high power pulse when high frequency probes are used as consequence of the resolution required to the NDT control of the structure. Recently two major improvements have allowed good results in UT application to this kind of structures. The improvement More info about this article: comes from both the type of technology that produces more compact structures as well as from the UT last generation systems. The ultrasound techniques are a practical and cost effective non-destructive inspection for this kind of structures and are prone to the automatization as recommended in the industrialization of manufacturing cycles. On the other hand, for the control of piping in the liquid propellant stage also, is under development the application of UT in phased array. The UT method avoids complications associated with safety, which are necessary when radiographic inspection is made during assembling and that make the times unacceptably long. The paper presents the results obtained through UT Systems in Phased Array, developed as prototypes for non-destructive control of the cylindrical part of the insulated structure of Solid Propellant Rockets and on piping of liquid propellant stage. Keywords: Solid Propellant Motor, Non Destructive Control, UT techniques. 1. Introduction In the year 2020 the Qualification Flight of the new European Launcher Ariane 6 is expected. In this Program, AVIO is involved as solid propellant boosters maker. Namely, as manufacturer of the large size solid propellant rocket P120 (3,4 m in diameter and 11,7 m in length). The solid propellant rocket P120 is also used as first stage of the small Italian launcher VEGA-C. Ariane 6 and VEGA Programs will require a production of P120 units up to 36 units/year. On the other hand, VEGA launcher uses other two solid propellant stages in dimensions reduced with respect to P120, but entirely tested through Non Destructive Inspection at AVIO premises. The large dimensions of the new P120 solid propellant rocket, as well as the evaluation of the amount of hours consumed in NDI during Ariane 6 and VEGA production, has needed investments in new NDI plants and also an effort in investigate complementary NDI methods applicable to the components of these launchers. As for inspection of critical parts of the solid propellant rocket structure when not yet loaded and after loading, the Radiographic Inspection is irreplaceable and it has involved upgrades of existing plants when possible, otherwise the construction of larger plants. Among the possible NDI methods, the UT is considered as a potential complementary method to the RT method and applicable to the not critical parts of the rocket structure when not yet loaded. This has given impetus to feasibility studies aimed above all, to verify that ultrasound inspection could be industrialized through automatic machines. These studies have been circumscribed to zones of the structure which allow a more immediate implementation and not requiring particularly complex UT plants not currently seen as necessary. Other than the structures of solid propellant rockets, also piping system of the liquid propellant upper stage of the VEGA launcher, has been investigated to evaluate the feasibility of UT as complementary NDI when applied to welds during assembly activities. In this paper results of test activities finalized to propose UT as NDI complementary to Radiographic Inspection for control of parts in the solid propellant rockets of large size as well as of liquid propellant stage and used for space applications, are presented. The complementarity is intended to be a way to unburden particularly busy facilities and / or to have a reliable back-up method in case of unforeseen circumstances. 2. Regions of straightforward UT implementation for NDI applied to the structure in composite of the rockets. The structure of a solid propellant rocket is composed of an internal thermal protection made in rubber on which is wrapped carbon fibre pre-impregnated by resin. The polymerization of these materials makes them thermally and structurally resistant to the temperatures and pressures generated by the combustion of the propellant during the flight. The shape of the structure is comparable to a cylinder with two domes at the ends (Fig. 1). This cylindrical part is a percentage up to 70% of the whole structure and with exception of the two “skirts” at the two ends, by means of which the rocket structure is assembled as a stage of the launcher, no particular criticality is associated to this region. In consideration of this “not criticality” of the cylinder and of the large area to be inspected, it’s tempting proceed with an immediate and easy NDI method alternative to the X-Ray. To an NDI method is required the detection and sizing of defects like delamination in composite and rubber as well as de-bonding between rubber and composite. UT is an NDI method well responsive to this requirement Dome Skirt (composite) (composite) Cylindrical Body (composite) Thermal Protection (rubber) Figure 1 – Typical structure of a solid propellant rocket before loading. 2 The challenge in this NDI method, when applied to these structures obtained by using the wrapped carbon filament technology, is to obtain an amplitude of the signal significantly higher with respect to the background signal generated by the structure itself, whose homogeneity is lower than that of other composites used, for example, for aeronautical structures. This challenge is won with relative simplicity using a local immersion UT technique that allows an excellent coupling and then high frequency probes. But other two conditions are important and allowed to obtain results never obtained in the past when the feasibility of a such application was tested. Precisely, both the construction technique of these composite structures had an evolution to the point of generating greater body homogeneity, as well as the pulse-generators available today are such as to be able to have high energy signals that can still emerge with sufficient amplitudes from large thicknesses traversed, that for structures of large rockets, can even reach 40-50 mm. 3. Development Phases for the design of an UT automatic machine applied to the control of the cylindrical region of a rocket composite structure. In order to design an automatic machine for UT control of a rocket composite structure, some tests have been performed by using a prototype probe-holder designed to allow a continuous wetting of the composite thus ensuring a good coupling probe-surface. This UT technique by “local immersion in water”, it is well used at AVIO since three decades for UT control applied to Ariane 5 booster on steel structures where un-bonding with insulating rubber wrapped in the internal surface of the structure is checked. The innovations recently implemented for application to structures of Ariane 6 and VEGA rockets, is the use of a Phased-Array probe and the control applied to composite which is the material replacing the steel typical for Ariane 5 boosters. To test the suitability of the innovation on structures of interest without face the complication of the largest dimension, the smaller rocket composite structure produced for VEGA, has been used (Fig. 2). This structure, with a diameter of 2 m and a length of 3.5 m, reproduces the typical configuration of each structure of a solid propellant rocket designed for VEGA and Ariane 6. This “small scale” test allowed to verify the adequacy of the coupling during the combined motions: rotation of the structure and longitudinal motion of the probe-holder, for a complete helical scan of the object. Hydraulic Power Unit UT data treatment Rocket Structure Rotating Support Probe-Holder in Control Console Longitudinal motion Figure 2 – Prototype of an UT in PA control machine for use on a small rocket structure. 3 Only after the success of this test, the effect of the maximum thickness implied in the rockets structure of largest size has been evaluated. A meaningful assessment needs a simulation of typical defects in the structure of largest thickness. For this application a P120 mock-up has been manufactured with artificial defects inside (Fig. 3). The mock-up was 3,4 m in diameter and a total thickness like those of the P120 critical area in the cylindrical region, where two composite shells of 17,3 mm thickness are coupled each other by 2 mm of rubber. [A] Composite wet region Mock-up Rotating Support Probe-Holder [B] Pipes and water collection circuit Pneumatic circuit Figure 3 – Prototype (A) and detail (B) of an UT in PA control machine for use on the largest rocket structure. Results of tests using the configurations in Fig. 2 and Fig. 3 are described in details in the following paragraphs. 3.1. Results from functional tests of a prototype UT machine in PA for automatic control of a small rocket structure With reference to the Fig. 2, in the figure below (Fig. 4) is detailed the composition of the structure in the stratification of the tested sections.