International Journal of Fluid Machinery and Systems DOI: http://dx.doi.org/10.5293/IJFMS.2021.14.2.161 Vol. 14, No. 2, April-June 2021 ISSN (Online): 1882-9554 Original Paper Design and Mathematical Modelling of Pressurisation System for Inflatable Aerodynamic Decelerator Chandan Saxena1, Lala Surya Prakash2, Unnikrishnan S1, Nikesh S1, Dilip V1, Radhakrishnan M1 1 Control Systems and Umbilical Division, Liquid Propulsion Systems Centre, Indian Space Research Organisation, Valiamala, India, [email protected], [email protected], [email protected], [email protected] 2 Human Space Flight Group, Liquid Propulsion Systems Centre Indian Space Research Organisation, Valiamala, India, [email protected] Abstract The paper depicts design philosophy of a pneumatic pressurization system configured for Inflatable Aerodynamic Decelerator (IAD). The objective is to fine tune parameters in conjunction with mathematical model allowing a better understanding of process. System configuration has been discussed and mathematical model is formulated for estimating thermodynamic properties and mass flow of gas transferring from high pressure storage to IAD through orifice. Theoretical analysis and calculations were made and compared with the simulations carried out in LMS AMESim platform. Experiments were conducted in vacuum conditions to verify pressure change in high pressure storage and in IAD and this experimental data is used for validation of mathematical model. Keywords: Inflation, Pressurisation System, Inflatable Decelerator, Mathematical Modelling 1. Introduction Inflatable Aerodynamic Decelerators (IAD) are 3-dimensional internally pressurized, axi-symmetric inflatable structure used for atmospheric re-entry on earth or on surface of other planets as part of interplanetary missions. IAD provides large cross-sectional area to increase drag and aerodynamic stability at higher dynamic pressures and Mach numbers to slow down the re-entry vehicle in atmosphere [6][10]. Indian Space Research Organisation (ISRO) has initiated its programme to develop technology for reusability of vehicle and spent stages, using Inflatable Aerodynamic Decelerator (IAD). Towards this, sounding rocket RH300MkII has been identified for technology demonstration. Vehicle is designed to attain apogee of 70-80 km and IAD is proposed to be deployed during its return to earth. The inflation has to be carried out by on-board pressurization within a definite time. This technology has already been proved in inter-planetary missions like Viking, Pioneer, Galileo, Beagle 2, Huygens, Genesis, [1] LOFTID, IRVE. The paper presents configuration of pneumatic pressurisation system or Inflation System Module (ISM) configured for IAD. The design of overall pressurization system and selection of its components are discussed. In later part of the paper, mathematical model developed for the system is depicted, and compared with experiments conducted in vacuum conditions. Mathematical modelling involves pressurization of an inflatable decelerator from a rigid pressure vessel. This is a case of charging of finite deformable volume from a finite volume pressure source. The model is validated using AMESim platform’s pneumatic and control simulation modules, and results are compared from the experiments done in vacuum environment. Problems like gas charging of a cylinder from high pressure feed line, or venting of high-pressure gas cylinder into atmosphere are frequently encountered in most of the engineering applications. Such problems can be solved by many approaches like Steady Flow Energy Equation (SFEE approach) [7] as they deal in constant source pressure or constant sink pressure or very high ratio of volume of high- to low-pressure vessel. As ISM involves comparable volume on both sides, it results in significant variation of pressure simultaneously on both sides. Moreover, it involves the deformable volume which is getting pressurized, inflated and then deformed due to a rigid gas bottle source. Comprehensive solution needs to be developed for such problem. Received November 15 2020; accepted for publication March 27 2021: Review conducted by Prof. Hideaki Tamaki. (Paper number O20051J) Corresponding author: Chandan Saxena, [email protected], [email protected] 161 2. Design Inputs for Pressurisation System Pressurisation system has to store the high-pressure gas to inflate IAD as per the mission requirements, usually done after stage burnt- out. Preliminary specification of pressurization system is given in Table 1. Table 1 IAD and ISM specifications IAD Pressurisation System (ISM) S. No. Specification Value Unit S. No. Specification Value Unit 1. Number of tori 3 Nos. 1. Final pressure 1.4+0.1 bar (abs.) 2. Diameter of torus 210 mm 2. Operating temperature 250 K 3. Total volume 220 litre 3. Time for full inflation 15 s 4. Burst pressure 5.0 bar (g) 4. Maximum temperature 353 K 5. Max. aerodynamic pressure 20 kPa 5. Maximum mass 5 kg Objectives of IAD may vary with mission requirements, and are of paramount importance while designing pressurisation system. Objectives of ISM are listed below: • Achieve required pressure after attaining equilibrium between gas storage and IAD. • Completion of Inflation in a pre-defined time interval. • Store and maintain the pressurised gas under high-dynamic loads. • Vent out the gas to atmosphere after touchdown if required. 3. Configuration Pneumatic pressurisation system involves titanium gas bottle as high-pressure gas source, a flow regulating device (constant throat area type orifice), an isolation valve and inflatable volume. Additionally, another valve is mounted as can be seen in Figure 1, facilitating venting of circuit. The ISM operates with pressurized gaseous nitrogen (GN) 2 in a blow down mode. System schematic is shown in Figure 1. ISM leverages existing flight technologies and hardware to realize a cost- efficient system for demonstrating reusability technology. ISM is planned to be hosted as a secondary payload on a proved sounding vehicle RH300MkII. Figure 1: Inflation System Module (ISM) 4. Selection of System Elements 4.1 Selection of Gas Nitrogen is selected as pressuring gas as it is inert, easily available, compatible with most metals, non-hazardous, easy to handle, cheap and less leak-prone as compared to Helium. However, it is comparatively heavier than Helium and also possess higher inversion temperature. These differences are significant when amount of gas required in system is large and mass-margin of system is low. However, in present study, this difference is not significant. However, in case of larger IADs, Helium may be used on the basis of mass advantages. Gas generator systems may be proved to be even more efficient (around 20-50 % with LH or LO), but such a system is not considered 2 2 because of absence of any heritage [9]. 4.2 High Pressure Source Spherical gas bottle is selected for the storage of high-pressure gas source considering mass and envelope constraints [10]. Size of gas bottle is optimised based on the trade-off study between total mass (gas mass + vessel dead mass) of the bottle and its volume, and pressure requirement varying with volume. Total mass of the gas bottle is expressed as 𝑚 = 𝑚 +𝑚 (1) 𝑡 𝑔 𝑏 Mass of the gas stored is a function of pressure and temperature. 𝑚 = 𝑝𝑔.𝑉𝑔.𝑀 = 𝑚 (𝑝 ,𝑟) (2) 𝑔 𝑔 𝑔 𝑖 𝑅.𝑇𝑔 162 Initial pressure of the gas required can be expressed as equation 3[3]. 𝑝 = 𝑝 .(1+𝑉𝑓 + 𝑉𝑚) (3) 𝑔 𝑓 𝑉𝑔 Where, 𝑝 is the pressure required at which IAD is targeted to be pressurised. V is the volume of attachments to the IAD volume 𝑓 m downstream of isolation valve between gas bottle and IAD. Assuming volume of IAD and attachment volume to be constant, and considering spherical gas bottle initial pressure can be expressed in terms of radius (inner) of spherical gas bottle, i.e. 𝒑 = 𝒑 (𝒓 ) (4) 𝒈 𝒈 𝒊 Dependency of variable m is deduced from Eq. 2 and 4, as: g 𝑚 = 𝑚 (𝑟) (5) 𝑔 𝑔 𝑖 Now, Inert mass of the gas bottle is expressed assuming it to be a perfect sphere, 4 3 𝑚 = 𝜋((𝑟 +𝑡 ) −𝑟3)𝜌 𝑏 3 𝑖 𝑔 𝑖 𝑔 m = m (r ,t ) (6) 𝑏 𝑏 𝑖 𝑔 , for any particular material. The thickness of the gas bottle is derived based on strength criteria. For minimising the error, general equation of spheres can be derived using stress-strain equations. [5] 𝜎 𝑝𝑔.𝑟𝑖3{(𝑟𝑖+𝑡𝑔)3+2.𝑟3} = 𝐹𝑂𝑆 2.𝑟3.((𝑟𝑖+𝑡𝑔)3−𝑟𝑖3) Assuming appropriate factor of safety with permissible material stress σ, 𝑟 being the radius under consideration, the above equation is transformed to express shell thickness 𝑡 as: 𝑔 𝑡 = 𝑡 (𝑟) (7) 𝑔 𝑔 𝑖 From the above equations, m =m (r) (8) t t i Trade off study for optimisation of gas bottle specification can be done using equation 4 and equation 8. Trade-off is shown in Fig. 2 for present study. Variation of total mass and pressure required is plotted against volume of the gas bottle. The pressure requirement in eq. 3 is derived assuming certain conditions like ideal gas expansion and thermal equilibrium of gas. Based on experiments conducted in vacuum conditions, initial pressure of 81 bar is finalised corresponding to 94 mm internal radius. Hence, internal radius of 100 mm is selected being conservative selection. 4.3 Isolation Valve Isolation valve is used for isolating stored high-pressure gas from inflatable volume. It is critical in terms of safety and reliability for mission. Pyrotechnic Valve (Normally Closed type) is selected in place of solenoid/latch valve due to its uncompromising leak tightness, faster response, Fig. 2 Trade-Off study for gas bottle redundancies in initiators, flight heritage and reliability. 4.4 Flow Regulation Device One of the important functions of inflation system is to operate in a specific time. At the instant of operating pyro isolation valve, flow takes place from gas source to inflatable volume. Orifice have advantages over mass, cost, reliability and ease in operation [2]. Active pressurization system with mechanical/electronic pressure regulator and solenoid valve in series was ruled out considering its complexity and lower reliability. Thus, ISM requirements narrowed down to a pressure flow regulation by means of fine-tuned orifice. As shown in Fig. 5, larger orifice diameter calls for shorter time duration to achieve required IAD pressure. For RH300MkII, required inflation duration is 15 seconds, which corresponds to 3 mm diameter from LMS AMESim simulation in 1-dimension. However, based on the subsequent experiments, orifice of diameter 3.2 mm is finalized. 4.5 Charging and Passivation Charging of gas bottle needs to be done remotely from safety point of view. A fill valve with triple seal redundancy is selected for the aforementioned application. Once, the system is recovered, system is required to be vented. For this, a solenoid valve, or latch valve has to be included in gas circuit. 163 5. Mathematical Modelling A one-dimensional lumped parameter mathematical model of the IAD pressurization system is generated by simplifying the system to four interconnected control volumes i.e. Gas Bottle and three tori of IAD. All control volumes are assumed interconnected by an orifice. This simplified system is represented mathematically in terms of set of coupled governing equations in unsteady form. It is assumed that the gas is ideal and the process is adiabatic process and the coefficient of discharge is constant. Also, as the inflation is carried out at high altitude, external work done by atmosphere on IAD is neglected. 5.1 Governing Equations Gas Bottle: Gas bottle is a rigid spherical volume with only one exit. Conservation of mass and energy yields following governing equations for gas bottle. 𝑉 𝑑𝜌𝑔 = −ṁ (9a) 𝑔 𝑒,𝑔 𝑑𝑡 𝑑(𝑚𝑔𝑢𝑔) = −ṁ ℎ (9b) 𝑒,𝑔 𝑔 𝑑𝑡 IAD: It’s a non-elastic deformable volume and has three tori connected in series. Control volumes, representing first and second torus, are modelled with single inlet and outlet. Control volume associated with third torus has a single inlet. Initially, IAD is constrained using ties to provide structural rigidity during storage. Ties are ruptured automatically during inflation. For the model, volume of IAD is assumed constant when the ties are not ruptured initially and rupture happens simultaneously at same pressure. Each torus is modelled by applying conservation of mass and energy with volume variation. First Torus 𝑑(𝜌𝑡1𝑉𝑡1) = ṁ −ṁ (10a) 𝑒,𝑔 𝑒,𝑡1 𝑑𝑡 𝑑(𝑚𝑡1𝑢𝑡1) = ṁ ℎ −ṁ ℎ −𝑝 𝑑𝑉𝑡1 (10b) 𝑒,𝑔 𝑔 𝑒,𝑡1 𝑡1 𝑡1 𝑑𝑡 𝑑𝑡 0, 𝑡≤𝑡 𝑑𝑉𝑡1 ={ 𝑐 (10c) 𝑑𝑡 𝐾(𝑝𝑡1−𝑝𝑎𝑚𝑏),𝑡>𝑡𝑐 | 𝑉𝑡1 =𝑉𝑡1,𝑓 Second Torus 𝑑(𝜌𝑡2𝑉𝑡2) = ṁ −ṁ (11a) 𝑒,𝑡1 𝑒,𝑡2 𝑑𝑡 𝑑(𝑚𝑡2𝑢𝑡2) = ṁ ℎ −ṁ ℎ −𝑝 𝑑𝑉𝑡2 (11b) 𝑒,𝑡1 𝑡1 𝑒,𝑡2 𝑡2 𝑡2 𝑑𝑡 𝑑𝑡 0, 𝑡≤𝑡 𝑑𝑉𝑡2 ={ 𝑐 (11c) 𝑑𝑡 𝐾(𝑝𝑡2−𝑝𝑎𝑚𝑏),𝑡>𝑡𝑐 | 𝑉𝑡2 =𝑉𝑡2,𝑓 Third Torus 𝑑(𝜌𝑡3𝑉𝑡3) = ṁ (12a) 𝑒,𝑡2 𝑑𝑡 𝑑(𝑚𝑡3 𝑢𝑡3) = ṁ ℎ −𝑝 𝑑𝑉𝑡3 (12b) 𝑒,𝑡2 𝑡2 𝑡3 𝑑𝑡 𝑑𝑡 0, 𝑡≤𝑡 𝑑𝑉𝑡3 ={ 𝑐 (12c) 𝑑𝑡 𝐾(𝑝𝑡3−𝑝𝑎𝑚𝑏),𝑡>𝑡𝑐 | 𝑉𝑡3 =𝑉𝑡3,𝑓 The parameter 𝐾 in equations 10c, 11c and 12c is a constant proportionality factor and is conjectured to be dependent on geometry of torus. As all three tori have similar geometry, 𝐾 is assumed constant. Parameter 𝐾 and t was evaluated experimentally. c Orifice: All control volumes are interconnected by orifice and the mass flow rate from one control volume to other is controlled by these orifices. The following set of equations are used for computing the mass flow rate through each orifice [4]. 𝑝 ṁ = 𝐶 𝐴𝑓( 𝑑) 𝑜𝑟𝑓 𝑞 𝑝𝑢 𝛾 𝑝 = ( 2 )𝛾−1 𝑐𝑟 𝛾+1 If Pd >P , flow is not chocked and f(Pd) is given by: Pu cr Pu 164 2⁄γ (γ+1)⁄γ 𝑓(𝑝𝑑)= 𝑝𝑢 .√ 2𝛾 .√(𝑝𝑑) −(𝑝𝑑) (13) 𝑝𝑢 √𝑅𝑇𝑢 𝛾−1 𝑝𝑢 𝑝𝑢 If Pd <P , flow is not chocked and f is given by Pu cr 𝛾+1 𝑓(𝑝𝑑)= 𝑝 .√𝛾( 2 )𝛾−1 (14) 𝑝𝑢 √𝑅𝑇𝑢 𝛾+1 Ideal Gas: The macroscopic properties of the pressurization gas are interrelated using following equation of state: 𝑝 =𝜌𝑅𝑇 (15) Specific enthalpy and internal energy, for an ideal gas is expressed in terms of state variables: ℎ =𝑐 𝑇 ; 𝑢 =𝑐 𝑇 (16) 𝑝 𝑣 Above equations eq. 9 to 16 forms mathematical model of ISM with 11 independent variables viz., p , T , p , p , p , T , T , g g t1 t2 t3 t1 t2 T , V ,V and V . Set of equations are solved in coupled manner, with initial conditions, using odeint function in scipy module on t3 t1 t2 t3 Python platform. 5.2 Validation of Model Validation is carried out by comparing simulated data with LMS AMESim model and development test which was carried out to assess the performance ISM under vacuum. For this, pressure was monitored in the gas bottle and 3rd torus. Experimental and LMS AMESim data are utilized. Simulation was carried out using above developed model with initial condition same as that of test. Simulation is performed with system parameters (fixed) and initial conditions (corresponding to 11 independent variables) as tabulated in Table 2. Table 2 Values of system variables and Initial conditions for validation of mathematical model System Parameters Initial Conditions (t = 0 s) V =4.2×10−3 m3 p =81×105 Pa g g V =33.7×10−3 m3 T =303 K t1,f g V =76.8×10−3 m3 p =p =p =0.2×105 Pa t2,f t1 t2 t3 V =120×10−3 m3 T =T =T =303 K t3,f t1 t2 t3 t =70×10−3s (from experiment) V =V =V =5×10−3 m3 c t1 t2 t3 C =0.70 (Gas Bottle downstream) q - C =0.65 (inside IAD) q Time history of pressure inside gas bottle and third torus of IAD are obtained from model with initial conditions. Third torus being outermost torus, promises static pressure reading inside tori. The data is compared with the experimental data and AMESim model [8]. Fig. 3 shows IAD pressure rise as per mathematical model developed against experimental data and AMESim. Pressure spikes are observed in former two, because of residual trapped gas inside cotton tied IAD during packing and folding. Fig. 3 Pressure Variation in Gas Bottle (left) and IAD 3rd Torus (right) 165 Fig. 4 Absolute Pressure Deviation between Experimental and Simulated data in Gas Bottle (left) and IAD 3rd Torus (right) The deviation in the model is quantified using absolute difference between experimental data and simulation data and is shown in figure 4. A maximum deviation of 4 bar is noticed in Gas Bottle whereas a maximum deviation of 0.14 bar is noticed in IAD. The mean squared error (MSE) for gas bottle and IAD are 2.2 bar2 and 0.001 bar2 respectively. 5.3 Parametric Study As the mathematical model is validated in the previous section, a parametric study is carried out by varying gas bottle initial pressure and orifice diameter. The objective is to find out the sensitivity of pressurization time and the final IAD pressure on the selected parameters. The initial dynamics of rupturing of ties are neglected for this study as it does not have any impact on the objective. Fig. 5 Gas bottle and IAD pressure (in bar(abs.)) variation characterized by Orifice Diameter (mm) Fig. 6 Gas bottle and IAD pressure (in bar (abs.)) variation characterized by Initial Pressure (bar) 166 Figure 5 depicts the sensitivity of orifice diameter on gas bottle pressure decay and pressure build-up in IAD 3rd torus. Above graphs have been plotted with same gas bottle initial pressure of 80 bar (absolute). Pressure decay shows more sensitivity to ‘change in orifice diameter’ at lower values. Figure 6 shows the effect of changing initial gas bottle pressure at constant volume on gas bottle pressure decay and IAD 3rd torus pressure build-up. Comparison has been made at constant orifice diameter of 3.0 mm. Sensitivity study done in this section allow us to determine the permissible tolerance on initial pressure requirement and orifice diameter. For present study, IAD pressure requirement is 1.4 bar with 0.1 bar upper tolerance. For meeting inflation requirements under 15 seconds, permissible upper tolerance on orifice diameter is 0.1 mm (3.2+0.1 mm), and on initial gas bottle pressure 6.7 bar (81+6.7 bar). 5.4 Conclusion The configuration and selection of system elements for IAD pressurization system is discussed in-depth for RH300MkII sounding rocket. The system can easily be scaled up for different sizes of rocket. A prototype model was developed to validate the proposed configuration and was subjected to vacuum level test. The experiment showed the adequacy of proposed pressurization system to meet mission objective along with the mission constraints. A simplified theoretical model was developed for simulating IAD pressurization system. The variable volume inflatable was also accounted in the model to capture the initial dynamics of pressurization process. The model was validated using the data obtained from experiment. The simulated results shows good fit with the experimental data with MSE of 2.2 bar and 0.001 bar for Gas Bottle and IAD respectively. A parametric study is also carried out to show the sensitivity of gas bottle initial pressure and orifice size. As part of future study, it is proposed to carry out experiments at different ambient pressure levels to make the model more robust. Nomenclature 𝑝 : Instantaneous gas pressure ṁ : Exit mass flow rate from control volume 𝑒 𝑉 : Instantaneous Volume 𝑉 : Volume of components downstream of IAD 𝑚 𝑚 : Instantaneous mass 𝐾 : Proportionality factor 𝑅 : Gas constant 𝑡 : Critical time for inflation of torus 𝑐 𝑇 : Instantaneous temperature 𝑝 : Ambient pressure 𝑎𝑚𝑏 𝛾 : Heat capacity ratio ṁ : Mass flow rate through orifice 𝑜𝑟𝑓 𝜌 : Gas density 𝐶 : Orifice flow coefficient 𝑞 𝑡 : Time variable 𝐴 : Orifice flow area 𝑀 : Gas molecular mass 𝑝 : Downstream pressure 𝑑 𝜎 : Permissible stress in gas bottle material 𝑝 : Upstream pressure 𝑢 𝑟 : Radius of spherical gas bottle 𝑝 : Critical pressure 𝑐𝑟 𝑢 : Specific internal energy Subscripts ℎ : Specific enthalpy 𝑔 : Gas bottle conditions 𝑚 : Gas bottle dead mass 𝑡 : IAD first torus conditions 𝑏 1 𝑚 : Total gas bottle and gas mass 𝑡 IAD second torus conditions 𝑡 2 𝑐 : Constant pressure specific heat 𝑡 : IAD third torus conditions 𝑝 3 𝑐 : Constant volume specific heat 𝑓 : IAD assembly specifications 𝑣 References [1] Juan R. 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