UT in Phased Array Applications for Control of Structures and Piping of Stages in the European Space Launchers

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.

1 - Composite 2 - Composite, rubber, composite, rubber 3 - Composite, rubber (two thicknesses) 4 - Composite, rubber (thinnest part)

Figure 4 – Composition of the stratifications at the tested section of the small rocket.

4 [2]

[1] [3]

[4] [5]

Figure 5 – From left to bottom of each picture: [A], [S] and [C] scans in the five sections tested in the small rocket structure.

In Fig. 5 are well visible the interfaces expected in each section (ref. Fig. 4), being the signals are of amplitude significantly higher with respect to the background noise. This high amplitude is due to a composite quality obtained after improved manufacturing procedure. In all sections, the scan of the overall cylindrical section is represented in a [C] scan. The [A] scan and [S] scan are visualized on the selected section of the [C] scan.

3.2. Results from functional tests of a prototype UT machine in PA for automatic control of a mock-up of a structure of a large rocket.

With reference to the Fig. 3, in the figure below (Fig. 6) is detailed the layout of the artificial defects inserted during stratification of the mock-up.

(1) h d (2) (1) Composite (2) Rubber (3) (3) Composite

Figure 6 – Layout of the artificial defects in the stratification of the of the mock-up representing the critical part in the cylindrical region of the largest size rocket.

The produced mock-up it is a representative piece of the large scale rocket. A scaled mock-up, mainly for UT investigation is not representative and then, the P120 mock-up has been scaled only in length. This mock-up has the same diameter of the P120 rocket structure and the same thicknesses in the skirt-cylinder overlapping region which is the most critical part in the P120 cylinder. The manufacturing procedure of the reduced length in the mock-up however, is necessarily different from that of the true structure because in particular the automatic machine for stratification compaction can’t operate due to limited space on the mock-up. This limitation can generate a density of the composite resulting after polymerization, not properly representative of the true structure. An un-perfectly compacted composite generates an high attenuation of the UT signal, but this situation is

5 conservative because the results in the real composite structure are expected be of greater amplitude. A serious limit, instead, concerns the detectability of small artificial defects because they can be confused with unwanted defects and consequent to the manual manufacturing process. The presented results point out limitation of such type. In the Fig. 7 below, an evaluation of the density uniformity of the mock-up in its section, is presented. This evaluation is made by radiographic inspection through the analysis of the “grey level vs position” along the cross section of the two concentric shells separated by 2 mm rubber. In considering the law of attenuation when an X-Ray beam penetrates a material characterized by a linear attenuation factor µ (I = I0●ln- µ x) and in consideration of the material traversed by the X-Ray beam in the cylindrical section as a function of the tangential distance from the cylinder axis, it is found a significant the difference of the experimental behaviour from the theoretical one. While in the theory, only the variation due to the increase of the material thickness affect the X-ray beam attenuation, experimentally two separate behaviours are visible for the two shells and this is due to two different densities characterizing the two regions: the external shell of lower density and the internal shell of greater density.

Grey Level Line Profile direction

Figure 7 – Analysis of mock-up material internal inhomogeneities.

The lowest density in the external shell implies presence of internal material inhomogeneities visible in the UT scans (Figs. 8, 9, 10 and 11) particularly when resolution is increased to detect smaller artificial defects.

Defect at depth 8,5 mm

Defect dimensions 25x24 mm

Figure 8 – Example of artificial defect detected in the external shell.

6 Defect at depth 39,4 mm

Defect dimensions 15x12 mm

Figure 9 – Example of artificial defect detected in the inner shell.

When defect of smallest dimensions (6 mm) are investigated, many “natural defects” became visible and they blend with artificial ones of variable size and also comparable to those of artificial defects (Fig. 10).

Natural defects (> 6 mm) Natural defect (5 mm)

Figure 10 – Example of natural defects detected in the external shell.

4. Other application of UT in PA for control of welded pipes during assembling of the VEGA fourth stage

At their final assembly of piping partially welded, on the liquid propellant stage performed in a clean room, radiographic inspection of welding shall be performed after evacuation of the area with consequent stop of all activities carried out in parallel in the same environment. This stop is incompatible with production times, especially in view of the production rates connected with the evolution of the VEGA and Ariane 6 programs. In view of this, a feasibility study was started to replace the radiographic inspection of these welds with a UT inspection. In Fig. 11 is a sketch of the system designed for control of pipes of small size (diameter from 6 mm to 9 mm and wall thickness from 0,4 to 0,7 mm).

Several samples of typical titan pipes welded have been used and radiographic inspection performed in order to select samples with acceptable weld and samples with defect on the weld. Results from tests using this system are shown in Fig. 12 where the sample #3 is a weld well done, while in the sample #2 the weld is affected by an unacceptable excess of material in the weld. On each side of the radiography is shown the results obtained through the UT inspection of the sample. The weld well done (in the right

7 side) shows an echogram without anomalous echoes that, on the contrary, are present in the left echogram and that are due to the geometric edges generating signal reflection.

Figure 11 – System UT-PA for piping control.

[sample 2] [sample 3]

Figure 12 – Example of welded pipes control through UT-PA system.

5. Conclusions

Inspections using UT methods are always been used at AVIO with techniques using multiple probes [2, 3] as well as Phased Array [1]. Tests presented in this paper show that UT as NDI method can be proposed as complementary to the already qualified RT inspection techniques and replace it for inspection of not critical regions in structure of solid propellant rockets and for inspection of welds in pipes of upper liquid propellant stage.

Acknowledgements

The author wishes to acknowledge to acknowledge the suppliers Mr. Bushati A. and Mr. Lippolis D.. for their cooperation in the project.

References

[1] Tosti E., « NDT in the Production of Solid Propellant Motors for Space Applications », 3AF Space Propulsion, Cologne, Germany, 2014. [2] Tosti E., « Controlli NDT nell’ambito della produzione di Motori Spaziali a Propellente Solido Una Panoramica delle Attività svolte da AVIO SPAZIO S.p.A. alla sua seconda generazione di impianti NDT », 15° AIPnD, Trieste, Italy, 2013. [3] Tosti E., « Advanced NDT Diagnostics applied to the Control of Space Motors. The experience in AVIO Space Division », 18th WCNDT, Durban, South Africa, 2011.