inflatable structures: development of a recovery...

INFLATABLE STRUCTURES: DEVELOPMENT OF A RECOVERY SYSTEM

C. BARDET EADS Space Transportation, Bordeaux, FRANCE

([email protected])

Abstract : This paper gives an overview of the work that was performed by EADS Space Transportation (Company belonging to EADS Corporation -European Aeronautic Defense and Space Company-) in order to develop an inflatable Recovery System for a 50 Tons Mock Up that impacts on water at a maximal velocity of 17 m/s. This paper presents the major steps that have been achieved to design and justify this inflatable system of a new scale regarding size (40 m3), deployment duration (1.8s), and applied external loads (210 Tons). Materials are presented. Design methods, based on a dual approach combining analytical criteria and Finite Element models, used for this inflatable application are detailed. This system has been qualified in 2002 and is operational since 2003.

1- Introduction In the frame of the validation of its hydrodynamic codes EADS LV has chosen to develop a dedicated Mock Up for analyzing launch submarine phases. These tests are aimed at measuring some important features of such launch submarine trajectories like velocity Vs time and hydrodynamic pressure map along the profile of the Mock Up.

9th SAMTECH Users Conference 2005 1/13

In that frame, EADS Launch Vehicles Industrial Operations Directorate has been working for 5 years on the development of a Mock Up dedicated to these submarine testings. To recover this Mock Up after testing, a challenging Inflatable Recovery Sub System (IRSS) composed of two large balloons has been developed. This paper presents the major steps of this IRSS development phase, which has been successfully qualified in 2002. The plan of the paper is as follows: - Mock Up Mission description and its IRSS General Requirements - Description of the IRSS - Principles of the IRSS - IRSS Design and Justification - Milestones - Conclusion

2- Mock Up Mission Description & IRSS General Requirements The Mock Up is a 50T structure that shall be recoverable after performing an initial launch submarine trajectory. Figure 1 illustrates the sketch of the Mock Up mission: 1- Launched from a submarine platform which upper roof is –24 m below the water free surface (see bottom left), the Mock Up runs up to the water surface - at that time, the two balloons of the IRSS are folded inside their individual pressurized container-. During this phase, trajectory data are recorded. 2- At this point starts the recovery phase: after emergence of the Mock Up, the two doors of the containers are released and the order for balloon deployment is supplied by an internal countdown of the Mock Up. A ballistic trajectory is achieved – during which the two balloons are inflated up to a pressure larger than 1 absolute bar so as to obtain the balloons final volume of 20m3 - and followed by an impact on the water free surface. The recovery phase is finally ended in water and the balloons allow the Mock Up final stabilization at the free water surface (see middle right). The main features of the Mock Up recovery phase are presented hereafter: - Impact Velocity on water after emergence and ballistic phase : 3 m/s < Vi = Vertical Velocity < 17m/s (velocity of the central body rear face when it impacts water free surface) - Attitude (longi axis w.r.t local vertical axis) at impact : -60° < Theta < 60° - Ballistic Phase duration after emergence : 2s < T < 5s The main requirements for the Recovery System are: - Maximal Depth for collision avoidance with submarine platform : -24m w.r.t water free surface - Reliability : < 5. 10e-4 - Reusability : 10 Times



After performing a trade off among several solutions, it has been chosen to develop an Inflatable Recovery Sub System (IRSS) composed of two inflatable balloons of 20m3 each for fulfilling the above requirements.

3- Description the IRSS The IRSS is composed of: • 2 inflatable balloons :

Envelope : Nitride PVC, thickness : 1.4 mm, Allowable : 1100 daN / 5 cm Net : Polyester cable : Strength : 15 T / cable 20 aramid cables (link between steel Ring and Container) : Strength : 35 T / cable 1 steel Ring (link between Net & Balloon and aramid cables)

4 flexible lines for balloon pressurization (link between capacity and balloon): Length: 1m, Strength: 250 bar Balloon feature: Folded: 1 mP

3P, 300 Kg Unfolded (deployed): 20 mP

3P

• 2 pressurized capacities of 400 l / 100 bar : one per balloon, for their inflation Figure 2 illustrates the interfaces between the balloon and its pressurized container: see the situation obtained just after the deployment order (left) and the fully deployed situation before water impact (right). Figure 3 presents these interfaces before a deployment test. Figure 4 presents the folded balloon inside its container (left) and its rear side (right). Note that the folding procedure is a very important point due to the fact that the space inside the container does not present large margins. In addition the beginning of the deployment phase is a critical event as no damage neither on the envelope f the balloon nor on the flexible lines shall occur: initial kinematics shall not present any blocking point. Indeed, at that time, internal pressure developed by the 4 flexible lines is close to 100 Br and the volume of the balloon is almost zero (fully folded). Figure 9 presents a general view of the balloon at the end of the deployment phase. Note the regular spacing of the net around the envelope. The net permits to confine the envelope and thus it reduces the loads (internal overpressure & hydrodynamic load) applied onto the envelope when the balloon impacts water free surface: part of these loads are deviated inside the net.

4-Principles of the IRSS Two of the major IRSS requirements expressed above concern the Mock Up velocity impact on water free surface (< 17m/s) and the collision avoidance with the platform roof (> -24m w.r.t water free surface). In order to fulfill them, an inflatable solution relying on the use of two Balloons was chosen among other concepts, because, this solution presents the following advantages:


- Extended buoyancy during the submarine trajectory of the recovery phase: + 40 m3 (2 balloon volumes) - Extended hydrodynamic drag : +18 m2 (2 balloon equivalent surfaces) These aspects, as it is demonstrated in the next paragraph, give the IRSS an outstanding robustness with regards to the former requirements. Two major constraints are imposed by the chosen inflatable concept: - the balloons shall be able to sustain the loads induced by the water impact (see below: hydrodynamic pressure profile applied along balloon lower hemisphere is equivalent to 105 Tons as an external load combined to an internal overpressure of 2.15 absolute bar produced by balloon volume reduction) - to have deployed balloons at water impact – ie Pabsolute = (1+ ε) bar – in order to prevent the balloon from any collapse during this critical phase.

5- IRSS Design and Justification The Design and Justification of the IRSS has been split into 3 steps: 1- Step 1 was dedicated to the choice of the deployed balloon volume and its final internal pressure in the air. These choices were driven by the depth requirement (not to exceed -24m w.r.t water free surface) and had to consider the limited space that was available in the balloon container. This led also to the 100 bar nominal pressure of the 400 l capacities. This step was tackled by a SAMCEF Finite Element model called “trajectory model” (see below). This model was correlated to two “scale 1” release tests (ELEVE 1 & 2 tests) with a 50T Dummy on which 2 balloons were installed, using the nominal container interface. 2- Step 2 was aimed at the analysis and validation of the Balloon deployment phase with the pressurized capacities. Some analytical analyses were performed but this step mainly relied on “scale 1” tests at IRSS level only (Balloon deployment tests) 3- Step 3 was dedicated to the design and justification of the IRSS in order that it sustains loads when Mock Up impacts water surface. This was realized by combining some analytical approaches and the use of a SAMCEF Finite Element model called “IRSS model” in order to set up “design rules”. These rules were confirmed by the mean of two additional “scale 1” release tests (ELEVE 3 & 4 tests) with the same 50T Dummy on which 2 balloons with final design were installed. These tests were aimed at the qualification of the IRSS. We now come back to a detailed presentation of each step. UStep 1: In order to find out the “optimal” volume/ final pressure of the balloon, a FE model was developed to determine the Mock Up trajectory during the ballistic, impact and hydrodynamic phases following the water emergence event. This model was called the “trajectory model”. As the water impact non-stationary phenomenon are complex to


model, while the optimization loop requested several iterations, some model simplifications were proposed. It was chosen to represent the transient phases following the emergence event by macroscopic hydrodynamic laws traditionally valid for stationary states like added mass effect and hydrodynamic drag coefficient. In addition, the link between the balloons and the central body was represented by a simple linear spring. The central body was assumed to be a rigid body whereas the balloon volume was driven by static equilibrium between the balloon inflating law (see step 2) and the water hydrostatic pressure. The following equations present all the terms that have been modeled.

Motion Equations: For the central body: MUMUwaterlinkMUwterMUMUXMUwaterMUMUMU mlKgVolVCSgmm γρργ /

2,2

1−∆−−−=

For the Balloons: BBwaterlinkBwaterBBXBwaterBBB mlKgVolVCSgmm γρργ /2

,21

−∆+−−=

Kinematics relation: 0)( Lxxl MUB −−=∆ This “trajectory model” is a 1 dimensional vertical model, but is sufficient as the maximal loads and depth are reached for vertical impact. Added masses terms are obtained from literature [LAMB] (however, they do not correspond to predominant terms) and hydrodynamic drag coefficients were obtained by a fitting of the model with two “scale 1” release tests (ELEVE 1 & 2) on water. This fitting was only needed for the central body and balloons decelerations at water impact time and permits to determine the equivalent drag coefficients for both. This simple model was found to be very efficient, indeed: - thanks to the smooth and soft profile of the balloon, there is no shock at balloon water impact : there is a constant adaptation of the balloon envelop to the hydrodynamic pressure map surrounding it making the proposed hydrodynamic representation of the model quite accurate - water impact duration of the Mock Up is short with regards to the total duration of the hydrodynamic phase, making negligible the transient effects that are not represented in this model This trajectory model was developed under the Mecano Module of SAMCEF Software. Macroscopic laws were introduced through SAMCEF Mecano User’s Functions (these functions are fully programmable by the User). It is composed of 3 external Degrees of Freedom and 18 internal Degrees of Freedom. Figure 5 presents a typical output of the model. We have plotted here the time dependence of displacement / velocity / acceleration of the rear face of the Mock Up and also the interface loads between Balloons and Mock Up. This calculation was performed for a Mock Up rear face release from an altitude of 14m above the water free surface. Figure 6 presents an overlay of the central body measured acceleration during ELEVE n° 2 “scale 1” release test and the same parameter supplied by the model. Note the good correlation:


- The first highly dynamic transient phase corresponds to the rear face central body shock at water impact. This is not represented into the model, however note how model acceleration fits well with average test acceleration. During this phase, the acceleration is mainly driven by the central body hydrodynamic drag coefficient. For the central body, we found a value of Cx=0.5 - The second acceleration increase corresponds to the balloon impact on the water. Note the good correlation and also the quasi static measured acceleration (there is no high frequency contribution) due to the smooth and soft profile of the balloons. During this phase, the acceleration is mainly driven by the IRSS hydrodynamic drag. For the balloon, we found an equivalent drag coefficient value of Cx=0.8 - The final part is driven by the buoyancy effects of the balloons (Archimède law) To summarize this first step, one can say that: - Once fitted, there is an excellent correlation between the model and “Scale 1” development tests regarding displacement / velocity / acceleration of the Mock Up during the recovery phase. Transient phenomenon not captured by the model (shocks at water impact) have negligible impact on Mock Up kinematics - The model led to the final choice of the parameters: for the Balloon a volume of 20 m3 was chosen, its final absolute internal pressure in the air is 1.6 Bar. This led also to the 100 bar nominal pressure of the 400 l capacities. By scattering the impact velocities and the various parameters of the model (especially hydrodynamic drag coefficients, balloon volume and inflating law parameters) we obtained by analysis a maximal theoretical Depth of - 22 m, an IRSS design load of 105 Tons per balloon and a design absolute internal pressure for the balloon of 2.15 Bar. UStep 2: The Balloon deployment phase was approached by deployment tests aimed at validating the dynamic pressure inside the flexible lines and the balloon deployment phase duration. 10 deployment tests were performed, all successful. Following features were obtained: - The balloon deployment duration is 1.8s with a scatter of 0.2s at 2σ. This time corresponds to the delay between pressurized capacity valve opening and volume stabilization of the balloon -ie Pabsolute = (1+ε) bar-. As we explained before, this last condition is mandatory to have a successful recovery, preventing from any collapse of the balloon at water impact. - In the air, the asymptotic pressure obtained in the balloon is 1.6 absolute Bar. This pressure is obtained after 5s since valve opening Figure 7 presents the pressurized capacity connected to the container (rear view of the test facility). Figure 8 presents a dummy of the container door that is ejected when the balloon is deployed (top view of the facility): the mass of the door is close to 80Kg. Figure 9 presents the deployed balloon after 5s, time at which internal pressure stabilizes at 1.6 absolute Bar.


Note that the tests were performed vertically, which is a conservative configuration: indeed, the balloon deployment happens during the ballistic phase that follows the emergence of the Mock Up. UStep 3: Balloon Design and sizing rely on three approaches: some analytical methods used to define the IRSS structural architecture, FE analysis in order to consolidate the analytical approach and “scale 1” tests in order to qualify the IRSS design. The impact duration (delay between balloon initial contact with water and its full immersion) is around 0.3s. This quite long time permits to consider stationary pressure (pressure is the same every where in the balloon and is only depending on time) inside the Balloon as the sound wave travel is one order of magnitude higher than the largest dimension of the balloon. Therefore, balloon sizing can be approached by quasi-static methods. An analytical approach was developed in order to define the dimensioning criteria of the balloon. Thanks to the “trajectory model” presented above, the balloon external loads were identified. We found out that for an impact velocity of 17m/s the hydrodynamic resultant load Fh was 80 Tons per balloon. Due to the reaction Rap between the balloon and the central body, we determine the reaction load Rs at the interface between aramid

cables and balloon insert. hFRsαcos

1= where α represents the equilibrium position

reached by the aramid cables when balloon impacts water. This value was obtained by geometrical construction (α = 40°) and Rs = 105 Tons. This gives the maximal overpressure inside the balloon at water impact:

2R

RsPi

π=∆ where

R is the balloon equivalent radius when assumed as a sphere) and iP∆ = 1.15 Bar Figure 10 presents the loads applied on the balloon at water impact. In order to define the mechanical architecture of the balloon (material choices, skin thickness, reinforced areas) we chose the following dimensioning mechanical flux criteria: - for balloon upper hemisphere :

2i

dimPR∆

=φ which is a bi-axial criterion driven by the

existing overpressure between the balloon and atmospheric pressure

- For balloon lower hemisphere: )(2

)(

2)(

)(xr

xFPxrx hydroi

dim πφ +

∆= where x represents the

longitudinal altitude of a balloon circular section for flux calculation, r(x) is the radius of the section, Fhydro is the contribution of the hydrodynamic pressure up to altitude x , the reference altitude being at the bottom point of the balloon. This is a mono axial criterion obtained by writing the global equilibrium of the balloon zone located between the reference point and the section at altitude x.


These criteria are calculated along x for each section of the balloon and are compared to the material allowable considering the local design of the balloon. The minimal safety factor is

)()(

minx

xksdim

allowablex φ

φ= = 1.4. This value corresponds to

theoretical reliability of 10P

eP-4 per balloon.

A SAMCEF Mecano FE model of the balloon (called the “IRSS model”) has been set up in order to consolidate some major hypotheses of the former analytical approach. Three points were addressed: - the internal pressure evaluation inside the balloon : this pressure is one major driver of the dimensioning flux presented above and the analytical approach makes some major hypotheses that need to be confirmed - the flux evaluation inside the balloon membrane at various altitudes x : this evaluation permits a comparison with the two dimensioning criteria presented above - to obtain a representation of the balloon deformation, especially in the vicinity of the flexible lines The model is based on the assembly of membrane and cable elements representing the initial geometry of the balloon. The external loading is performed by an hydrodynamic pressure map (integral of which is Fh) applied on the membrane elements of the balloon lower hemisphere and a contact area is managed along the central body. SAMCEF Mecano robust algorithm (Newton Raphson implicit Method applied on a Newmark (HHT) time integration of the motion equations), dedicated to this difficult large deformations / large displacements problem, provides after several time steps with the final equilibrium position of the balloon. In order to determine the pressure inside the balloon SAMTECH company has developed a User’s function to evaluate at each time step the internal volume of the membrane elements. This function was finally coupled to internal pressure by an isotherm law PV = cste. Figure 11 presents the equilibrium position obtained by the “IRSS model”. The results of the analytical and SAMCEF Mecano FEM approaches were as follows, assuming a cosines hydrodynamic profile for the balloon lower hemisphere: - FEM overpressure is 20% below analytical one - Dimensioning criteria are +/-15% close to the FEM ones - Complex static position of the balloon with an �angle close to the 40° estimated angle This dual approach (analytical/ numerical) and its good correlation gave us a great confidence in the balloon design.

This third step was concluded by two “Scale 1” release qualification tests (ELEVE 3 & 4). Their goals were: - To demonstrate the compliance of the IRSS with the “depth” requirement and correlate the “trajectory model” - To qualify the IRSS strength and validate the design and justification performed by the dual approach described above.

These Tests were successful. The ELEVE n°3 qualification test was performed at an impact velocity of 17 m/s. Following results, in accordance with the models, were obtained: - Depth : -18 m w.r.t water surface - FH = 75 Tons / Balloon - Balloon internal over pressure: 1 bar Figure 12 presents the test configuration of the ELEVE n°3 test and the final IRSS stabilization.

6- Milestones

- The IRSS qualification has been pronounced in June 2002. - The IRSS is operational since 2003.

7- Conclusion This Recovery System is a highly targeted challenge, and has given the following:

- A threshold has been achieved : a new IRSS scale has been industrialized (fast deployment, large balloons, important loads) and qualified - Regarding Design and Justification, this project has proved that analytical approaches remain a pragmatic method for mechanical architecture choices and structural dimensioning: blocking points are to be avoid by conception, simple load path have to be identified, design must evidence robustness… - Several SAMCEF Mecano FE models have been implemented to have a deeper insight of the loading and of the mechanical behavior of the Balloons. These models were very useful all along the development phase to improve the comprehension of complex phenomena and to identify the most important among them. EADS LV has now a complete approach / understanding of this kind of concept: - System Vision for hydrodynamic phase representation Industrial Vision for Conception, Design, Justification, Manufacturing & Testing EADS LV believes that this IRSS concept can also find its application among other heavy structures that shall be recoverable after water impact (boosters, vehicles…).


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inflatable structures: development of a recovery...

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