Transient Simulation Capabilities for the ORION-ESM Propulsion System Development Corinna von Graeve§ , Francesco Di Matteo∗ , Nicola Ierardo‡ §
ESA-ESTEC MPCV Programme Department,∗ ESA-ESTEC, Aerothermodynamics and Propulsion Analysis ‡ ESA-ESRIN LAU-DVL (full department description ∗§ Keplerlaan 1, 2201AZ Noordwijk, Netherlands,‡ Via Galileo Galilei, 00044 Frascati RM, Italy Abstract The NASA ORION Multi-Purpose Crew Vehicle (MPCV) is a pressurized, crewed spacecraft, designated to transport up to four crew members from Earth’s surface to a destination beyond Low Earth Orbit. It provides for all services necessary to support the crew while on-board and ensures a safe return to Earth at the end of the mission. Based on an agreement between NASA and ESA, the ORION will thereby be supported by a European Service Module (ESM), which is based on ATV heritage and provides the following main resources/functions: • • • • •
Propulsion / Thrust Power supply Thermal control Consumables for the Crew Module Payload volume
Due to the main objectives of the ORION spacecraft, the ESM shall be designed for very low mass, low ressource demands and especially high crew safety. For an according design of the ESM Propulsion Subsystem (PSS) and for the consideration of its insintric complexities, a prediction of the functional behaviour in nominal and contingency operations becomes necessary. The present work is succeeding the development of a transient functional model of the complete ORION propulsion subsystem and the simulation of different mission scenarios in nominal and off-nominal conditions as presented during the Space Propulsion Conference in 2014. Major design changes such as the replacement of the mechanical pressure regulator by an electrical one led to the necessity to update the previous performance model. Updated functional models are presented, allowing for the investigation of the pressure regulation scheme’s impact on the system and engine performances. Furthermore, transient phenomena occuring in the system during its operation may threaten the compliance with the system’s hardware limits and are assessed in this work. The models are built using the European Space Propulsion System Simulation (ESPSS) on the software platform EcosimPro, an object oriented tool, capable of modelling various kinds of dynamic systems. The proposed model features also original component models developed by the authors in order to improve the accuracy of simulations and analyses.
Nomenclature ATV AUX BIV EM EPR ESM ESPSS HEM ICPS LOI MMH MON MPCV OMS-E
PSS PCA QIV RCS TLI
Automated Transfer Vehicle Auxiliary thrusters Branch Isolation Valve Exploration Mission Electrical Pressure Regulator European Service Module European Space Propulsion System Simulation Homogenious Equilibrium Model Interim Cryogenic Propulsion System Lunar Orbit Insertion Monomethylhydrazine Mixed Oxides of Nitrogen Multi-Purpose Crew Vehicle Orbital Manoeuvering System Engine
∗ Corresponding
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Propulsion Subsystem Pressure Control Assembly Quad Isolation Valve Reaction Control System Trans-Earth Injection
Introduction
The NASA ORION Multi-Purpose Crew Vehicle (MPCV) will serve as a manned spacecraft for human exploration beyond Low Earth Orbit (LEO). The European Space Agency (ESA) is responsible for the procurement of the Service Module (ESM), delivering mainly in-orbit propulsion, power, thermal control and consumables for the crew. The pressure-fed, bi-propellant propulsion subsystem (PSS) embedded in the ESM provides for thrust and at-
author:
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titude control using MMH and MON as storable propellants. The Orbital Manoeuvring System Engine (OMSE), taken from NASA’s Space Shuttle Program, delivers the thrust for translational manoeuvres while 8 Auxiliary thrusters (AUX)are used for trajectory correction manoeuvres. Attitude control is provided by the 24 thrusters of the Reaction Control System (RCS). Special features of the PSS are the separated pressurization systems in order to prevent any dangerous propellant mixing, resulting in a high-pressure helium tank and a Pressure Control Assembly (PCA) for each propellant side. Moreover, two serial propellant tanks are mounted on each propellant side. The flow schematic of the PSS can be seen in Figure 1. Preliminary performance analyses have been carried out previously as described in [3]. Due to design changes, mainly the replacement of the former mechanical pressure regulator by an electrical one, the previous functional models have to be updated and the impact of the electrical pressure regulation scheme has to be investigated. The progress of the PSS design in comparison to [3], leads to the necessity to also thoroughly investige major transient phenomena that occur during PSS operation, such as waterhammer effects in the Pressure Control Assembly (PCA), priming peaks in the propellant feed network as well as cross-talk effects due to pulsed engine operation. According models are presented and the capabilities of the used software tool are demonstrated in the present paper.
Figure 1: MPCV-ESM Propulsion Subsystem
pneumatic complex systems with heat transfer and control are easily evaluated with the 1-D fluid flow library. The ORION ESM model is built using the European Space The pipe component, for instance, is not a lumped paramPropulsion System Simulation (ESPSS) [2] on the software eter but a fully 1-D component. It uses an area-varying platform EcosimPro [1], an object oriented tool capable of non-uniform 1-D mesh, it computes heat exchanges with modelling various kinds of dynamic systems. The pro- the wall and the environment as well as pressure drop and posed model features also original component models de- mass flow rate based on these governing equations: veloped by the authors in order to improve the accuracy of ∂u ∂f (u) simulations and analyses. The results are compared with + = S(u) ∂t ∂x the test data provided during the subsystem qualification test campaign in order to provide a reliable tool to fol- where low the complete subsystem design phase and predict the performance for future flights. ρ ρv nc nc The ESPSS library is composed by a set of different ρx ρvx u = A libraries [2]: fluid properties, 1-D fluid flow, tanks, com ρv ; f (u) = A ρv 2 + P ; bustion chambers and turbomachinery. ρE ρvH The Fluid Properties library features detailed real flu −ρAkwall (∂P/∂t) ids properties based on the NIST database [4]. Two-phase, −ρxnc Akwall (∂P/∂t) two-fluid mixtures of a real fluid in any thermodynamic S(u) = −0.5(dξ/dx)ρ v|v|A + ρgA + P (dA/dx) state with a non-condensable (ideal) gas are included. The Homogeneous Equilibrium Model (HEM) is used to calq˙w (dAwet /dx) + ρgvA culate the properties (quality, void fraction, etc...) of a real fluid in two phase conditions, with or without a non- where xnc is the non-condensable gas mass fraction and condensable gas. kwall is the wall compressibility, A is the cross section The 1-D Fluid Flow library allows transient simula- area, ρ the density, v the velocity, E the internal energy, tion of two-fluid, two-phase flow systems. Hydraulic or P the pressure and H the enthalpy.
1.1
Simulation tool
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The Tanks library enables the transient simulation of rocket engine and spacecraft tanks. Tanks components can be connected with 1-D fluid flow components to simulate a complete rocket engine cycle. The Combustion Chambers library enables the computation of rocket engines and thrust chamber elements. Combustion gas mixture properties (transport properties and heat capacity) are calculated from adequate coefficients based on each chemical species that is present in the combustion product. Minimisation of Gibbs free energy is applied to find equilibrium molar fractions at a mixture of reactants.
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Update of MPCV-ESM PSS functional performance model
A preliminary performance analysis of the MPCV-ESM PSS has been previously presented in [3], featuring a Pressure Control Assembly (PCA) with a mechanical pressure regulator system to ensure the required propellant tank pressure for the engine operation during all mission scenarios. After detailed trade-off studies, the mechanical pressure regulator has been replaced by an electrical one, leading to the necessity of re-evaluating the system in terms of pressurant gas mass budget, consistency with pressure and temperature requirements in the system as well as the influence on the engine performances. In principle, the Electrical Pressure Regulator (EPR) is composed of two serial simultaneously operated solenoid regulation valves (HPSV), a Pressure Regulation Unit (PRU) and a flow controlling orifice per propellant side. The PRU receives the actual tank pressures from pressure transducers, compares the received value with given pressure thresholds and sends an according position command to the valves. This allows for controlled pressurant gas flow when the propellant tank pressure exceeds the lower pressure threshold or, on the contrary, stopping of the gas flow in case of exceedance of the upper pressure threshold.
2.1
Figure 2: EcosimPro/ESPSS schematic for nominal performance analysis
tanks. Only pressure loss effects due to the depletion of the upper tank are neglected in this way. Moreover, due to the current state of the project at the end of the preliminary definition phase, several assumptions have to be taken for the spacecraft environment, equipment characteristics etc. The main model assumptions are summarized as follows:
Model description
A functional model for the MPCV-ESM PSS is set up to assess the system performance with the implemented EPR, as presented in Figure 2. In order to provide reliable model results in a reasonable computation time, the model is reduced to the main relevant components. The complete gas side is modelled in detail, while various simplifications are made for the liquid part of the PSS. Among others, the complex feed lines network is replaced by a simple pressure resistance, representing the accumulated pressure losses, and the assessment of the EPR influence on engine performance is limited to the OMSE. The serial propellant tanks are simplified into a single tank component, representing the same propellant mass, gas/liquid interface area and ullage volume as the serial
• • • • •
Isothermal propellant tanks Adiabatic helium tanks Perfect liquid properties for MMH Real fluid properties for MON Helium as real gas in the pressurization part and perfect gas in propellant tanks • Propellant feed line network simplified to single resistance to deliver nominal engine inlet conditions at the OMS-E inlet • AUX/RCS firings simulated as mass flow boundaries • Serial propellant tank simplified by single tank
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EM-2 Mission Scenario
thermodynamic properties of both propellants are computed by the model. Caused by its high vapour pressure and therefore volatile character, the oxidizer MON shows a strong self-pressurization capability during coasting phases, while the MMH tank reaches a lower equilibrium pressure. Therefore, the consideration of all these effects allow for monitoring of the the compliance with the pressure and temperature limits throughout the entire mission, for nominal and failure conditions. As a result of the electrical pressure regulation scheme, the according fluctuation of propellant tank pressures is directly transfered to the mixture ratio at the engine and thruster inlets. This effect and its influence on engine thrust and ISP is shown exemplarily for the LOI boost in Figure 6. With the aid of this model, the specific impact of the pressure regulation on the engine performance can be assessed throughout the entire mission. The compliance with the engine inlet boxes at every point in time can be verified and optimizations for pressure set-point and regulation bandwidth can be carried out. With the proposed model, a detailed analysis of the pressurization function can be carried out and relevant design criteria for the EPR layout and regulation settings can be given, e.g. optimized regulation bandwidth and pressure regulation set-point, flow controlling orifice diameter to meet beginning and end of life flow requirements, etc. The model is well suited to analyse the nominal and failure case EPR operation for all kind of mission scenarios.
To evaluate the PSS performance with an electrical pressure regulator, the simulation of the Exploration Mission 2 (EM-2) is selected. This mission is intended as the first manned mission of the ORION spacecraft with ESM.
Figure 3: EM-2 mission scenario In the EM-2 mission scenario as presented in Figure 3, the ORION spacecraft is launched on NASA’s Space Launch System and undergoes a perigee raise manoeuvre by the Iterim Cryogenic Propulsion System (ICPS) which will bring it on an orbit towards the moon. After a seperation boost and multiple days of transfer, the ESM propulsion system performs a Lunar Orbit Insertion (LOI) boost that propells the spacecraft in an orbit around the moon. The Trans-Earth Injection (TEI) is then entering the ORION in a transfer orbit back to Earth. In addition, the ESM propulsion system is performing attitude control and trajectory control manoeuvres.
2.3
He MMH pressure He MON pressure He MMH temperature He MON temperature
Model results
Pressure
Simulation results are presented for the conditions in the helium tanks, the propellant tank ullages for the entire mission and the OMS-E performance during LOI. As it can be seen from Figure 4, the model is capable of monitoring the pressure and temperature behaviour in the pressurant tanks during the mission. The model reproduces the temperature and pressure drop due to quasi adiabatic expansion of the gas during phases of active regulation. During coasting phases and non-regulated boost phases, a reheating of the gas by heat transfer with the tanks walls and subsequently a re-pressurization of the helium tank takes place. With the help of this model, detailed budget assessments can be carried out for nominal but also for failure case scenarios throughout the system development. The more complex heat and mass transfer mechanisms in the pressurized propellant tanks are also represented by the proposed model. As shown in Figure 5, pressure and temperature behaviour in the propellant tank ullages due to incoming gas flow, heat transfer with the tank walls, heat and mass transfer between the liquid propellant, propellant vapours and helium as well as due to the specific
Temperature
2.2
Time
Figure 4: Conditions in helium tanks for EM-2
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MMH pressure MMH temperature
MR
Pressure
Temperature
Mass flow rate
MMH MO
Time
Time
(a) MMH tank
(a) MR shift
MON pressure MON temperature
ISP
Thrust
Pressure
Temperature
Thrust ISP
Time
Time
(b) MON tank
(b) Thrust and Isp
Figure 5: Pressure and temperature behavior in propellant tank ullages for EM-2
Figure 6: MR shift and effect on thrust and ISP for EM-2 LOI
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Waterhammer analyses
and
priming MMH MON
Pressure
Transient waterhammer effects occur due to sudden change in the flow velocity of a fluid in a tubing system when compressibility effects play a major role. Upon hitting dead ends or closed valves, the former kinetic energy of a fluid is transformed into pressure energy, consequently leading to a pressure surge in the network. The pressure peaks travels through the network with the speed of sound and are reflected at other components or dead ends in the network. In addition to the sudden pressure surge, also low pressures can be induced locally due to the propagation of expansion waves and lead to cavitation issues. The height of the generated waterhammer pressure increase is described by the Joukowsky equation as
0
2
4
∆P = ρc∆U
8
10
12
(a) EPR pressure surges for ten switching cycles
with ρ as the density, c as the speed of sound and u as the fluid velocity. This work exemplarily presents analyses for the occurance of a gaseous waterhammer due to the regulation mechanism of the electrical pressure regulator (EPR) and the liquid priming of the propellant lines prior to engine start.
MMH MON
EPR waterhammer
Pressure
3.1
6
Time (s)
Due to the bang-bang mechnanism of the electrical pressure regulator, a gaseous waterhammer is induced at the upper solenoid valve inlet each time it is commanded closed. It has therefore to be assessed if the resulting pressure surge remains below the valve limits during all operations. This analysis is carried out on the model presented in Section 2.1. In order to verify the consistency with the pressure limits, the relevant pressures are monitored at beginning of life (BOL), leading to the highest waterhammer peaks due to highest mass flow rates.
1.3
1.35
1.4
1.45
1.5
1.55
1.6
Time (s) (b) EPR pressure surges for one switching cycle
Model results The simulation results for the EPR waterhammer analysis are presented in Figure 7, showing a period of ten switching cycles and a zoom on one single cycle. As it can be seen, the pressure drop upon valve opening as well as the pressure peak upon closing is calculated by the model, depending on the current PCA inlet pressure and helium mass flow rate. It can be assessed if the highest pressure peaks stay within the limits of the affected hardware. Moreover, an estimation of the waterhammer frequency can be given by the help of the proposed model.
Figure 7: EPR pressure surges for MMH and MON
3.2
Liquid priming
One of the most challenging phenomena to be investigated during a spacecraft propulsion system development is the priming process. Special attention has to be paid to the liquid priming of the propellant feed lines when the propellant isolation valves are commanded open and the liquid column enters the evacuated downstream network. The resulting waterhammer at closed thruster valves may exceed the sustainability limits of the hardware or even lead to a risk of detonation. The proposed model is capable to simulate these very complex phenomena occurring when two phase flows are involved. It is able to monitor ther-
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modynamic conditions of the fluid/gas mixture at every location in the tubing system and to simulate the priming shock wave propagation over time. In order to develop a suitable priming strategy for the ORION ESM propulsion system, parametric analyses can be performed with the help of the presented model. Based on the ORION ESM tubing layout, different isolation valve response times and opening characteristics can be included and their effect on the priming shock can be evaluated. Moreover, the possibilitiy of using a buffer gas in the downstream line to limit the priming pressure peak can be assessed in detail. Due to the subsequent priming of the individual branches and worst case pressure peak assumptions, the branch priming processess are carried out independently. To demonstrate the capabilites of the model, the pressure wave propagation for exemplary priming processes of the AUX branch and one RCS string are presented in the following. The results are limited to the oxidizer side, as the fluid properties of MON lead to more critical priming pressure peaks.
minimum absolute number of nodes that delivers conservative results. Therefore it can be assured that the highest pressure peaks are captured by the model. The simulation start is set to the opening command of the BIV, assuming a constant helium pressure in the downstream network.
Priming strategy To meet the safety requirements, the propellant tanks and the thrusters are separated by three barriers on ground: 2 serial isolation valves and 1 thruster inlet valve. The current priming strategy forsees an opening of the downstream isolation valves (QIV1,2,3/BIV2) and the induction of a helium gas buffer in the lines down- Figure 8: EcosimPro/ESPSS schematic for AUX branch stream the branch isolation valve (BIV) prior to priming. priming analysis This leads to a decrease of the liquid priming peak that occurs upon the consequent opening of the BIV. Model description The model for the evaluation of AUX and RCS branch priming strategies can be seen in Figure 8 and Figure 9. The relevant model assumptions for the priming analysis models are: • • • • •
Pipes assumed adiabatic to environment Real fluid properties for MON Dearated propellant Perfect gas properties for helium Homogenious Equilibrium Model (HEM) for two phase flow
Special focus has to be set on the discretization of the EcosimPro pipe components when simulating prim- Figure 9: EcosimPro/ESPSS schematic for RCS branch ing shock pressure waves. The discretization has to follow priming analysis the rule of inertia, stating that I=
dL = const A
Model results Following the above described priming strategy, the pressure and temperature at the closed AUX with dL = L , L as the tube length, n the number of nodes and RCS thruster valves inlets are monitored. The results n and A as the tube cross section. The total number of nodes can be seen in Figure 10 and 11. in this relative distribution is depending on the simulation Upon opening of the isolation valves, the pressure at case. It has been derived based on a trade-off study for the the thruster inlets rises up to the saturation pressure of
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MON_A1 MON_A2 MON_A3 MON_A4 MON_A5 MON_A6 MON_A7 MON_A8
Pressure
Pressure
MON_PT1 MON_PT2 MON_PT3 MON_PT4 MON_PT5 MON_PT6 MON_PT7 MON_PT8 MON_PT9 MON_PT10 MON_PT11 MON_PT12
0
0.5
1
1.5
2
0
Time (s)
0.5
1
1.5
2
Time (s)
Figure 10: Pressure at AUX thruster valve inlets during priming
Figure 11: Pressure at RCS thruster valve inlets during priming
the fluid. The quasi isentropic compression of the noncondensable gas in the pipes by the travelling liquid front induces a first pressure peak and a following pressure plateau. Subsequently, the liquid front hits the closed thruster valves and induces the actual waterhammer pressure peak. Due to the use of the HEM approach, multiphase phenomena at the liquid front are not taken into account by the model. E.g., liquid and vapour are assumed as a perfext mixture in a node, the fluid density is computed as a function of the local vapour mass fractions and the complex bubble effects are neglected. The induced pressure waves are reflected at the thruster inlets and travel back and forth through the piping network. The pressure peak frequency is therefore dependent on the pipe length, explaining the differences in pressure peak occurance and frequency not only in the AUX and RCS network, but also among the different thrusters. The amplitude of the pressure peaks is damped according to the friction factor of the pipe surface. However, despite the simplifications due to computation capabilities and the HEM approach, the model is capable of capturing priming peaks and pressure wave travel through the fluid network and an estimation of occuring pressure peaks for different priming strategies can be made.
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Thruster cross-talk
When operating thrusters in pulse mode like the Reaction Control Thrusters (RCS) of the MPCV-ESM, a coupling between the different parts of the fluid circuit due to pressure wave propagation and interference may occur. This consequently induces undesired effects anywhere in the network, called cross-talking effects. Special attention has to be given to these possible effects as they may be a major threat to the system. For this reason, a model is proposed to preliminarily evaluate this pressure wave interaction and its consequences. Approach In a first step, a waterhammer analysis is carried out for each thruster inlet valve individually while all other thruster valves remain closed. According to the pressure peak that is induced by each valve closure at all other valve inlets, the highest interaction between individual thrusters can be estimated. Afterwards, two of the thrusters with the highest estimated interaction probability are set to pulse mode in phase or with a random offset of 0.01 s. The pressure values at both valve inlets are then monitored in order to investigate in how far they are influenced by the pulsing of the respective other. Model set-up The EcosimPro schematic is shown in 12. It is based on the RCS priming model presented in 3.2. The dead ends are replaced by thruster simulators represented by orifice-outlet boundary combinations. The orifice cross section and pressure loss coefficients are tuned in such a way that the mass flow rate through the orifice equals the nominal mass flow rate per thruster. With this
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approach, the backpressure of the thrusters during startup is included in the analysis.
MON_PT1 MON_PT2 MON_PT3 MON_PT5 MON_PT6 MON_PT7 MON_PT8
Pressure
Figure 12: EcosimPro/ESPSS schematic for cross-talk analysis
Model results The investigation of single thruster shutdown induced pressure surges at all other inlet valves in the network lead to the conclusion, that the interaction between a neighbouring thruster pair is the strongest due to the short connection line. However, as it can be seen in Figure 13, closing thrusters No. 1 and No. 7 induce extensive pressure peaks at the respective other at a later point in time. Therefore, these thrusters are chosen for the simulateneous pulse mode assessment. Figure 14 displays the pressure at one pulsing thruster inlet when having the other thruster closed (dotted blue line), pulsed in phase (solid red line) and pulsed with a 0.01 s offset (solid green line). These figures show an increase of the highest waterhammer peak in Figure 14(a), the amplification of the pressure waves amplitude over time and the alteration of the wave frequency due to the interaction of the induced pressure waves in comparison with the single shut case. The proposed model is therefore capable to estimate possible pressure wave interactions between pulsing thrusters and show the importance of a careful consideration of these cross-talking effects in the tubing network.
0
0.002
0.004
0.006
0.008
0.01
Time (s) (a) Thruster No. 1
Pressure
MON_PT1 MON_PT3 MON_PT5 MON_PT6 MON_PT7 MON_PT8 MON_PT11
0
0.002
0.004
0.006
0.008
0.01
Time (s) (b) Thruster No. 7
Figure 13: Example for induced pressures at all other thruster inlet valves due to single valve waterhammer
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effect on the engine performance. Transient events, that play a major role in a propulsion system development, have been preliminarily examined. Pressure surges in the PCA due to the electrical pressure regulation scheme as well as pressure peaks due to priming of the propellant feed lines network have been assessed and the capabilities of EcosimPro/ESPSS have been pointed out. Moreover, the computation of pressure wave interaction due to pulse mode operation of the Reaction Control System has been presented and the probability of interferences has been demonstrated. In the next project phase, the proposed models can be updated with frozen design data and validated with the help of hydraulic model tests. They can then be used for detailed flight predictions and optmimization purposes throughout the all future project phases.
Pressure
In phase Offset -0.01s Closed
Closing of FCV#7
1
1.02
1.04
Time (s)
References
(a) at thruster No. 1
[1] Empresarios Agrupados. Ecosimpro: Continuous and discrete modelling simulation software. www.ecosimpro.com, 2007.
In phase Offset +0.01s Closed
[2] J. Steelant M. De Rosa and J. Moral. ESPSS: European Space Propulsion System Simulation. In Space Propulsion Conference, 2008. Pressure
[3] F. Di Matteo and N. Ierardo. MPCV Propulsion System Functional Model. In Space Propulsion Conference, 2014. [4] NIST. Standard reference database 69: Nist chemistry webbook. www.webbook.nist.gov, 2005.
1
1.02
1.04
Time (s) (b) at thruster No. 7
Figure 14: Pressure wave interference between thruster No. 1 and No. 7 for pulse mode in phase and with an offset
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Conclusion
Following the replacement of a mechanical pressure regulator by an electrical one, the previous performance model presented in [3] has been updated. The EM-2 mission has been simulated to investigate the influence of the regulation scheme of the the EPR on the system performance. Besides the consistency with pressure and temperature requirements in the gas and propellant tanks, special focus has been set on the MR shift at the OMS-E inlet and the
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