MICRO FLUIDIC VALVE FOR SATELLITE PROPULSION SYSTEM by iaemedu

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									International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July - August (2013) © IAEME
                             AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)                                                         IJMET
Volume 4, Issue 4, July - August (2013), pp. 171-179
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       MICRO FLUIDIC VALVE FOR SATELLITE PROPULSION SYSTEM


            CH. Sreenivasa Rao1, BK.Venkataramu2, Dr.M.M.Nayak3, Dr.E.S.Prakash4
1
    Scientist, Liquid Propulsion Systems Centre, Indian Space Research Organisation, Bangalore, India
      2
        Associate Director, Liquid Propulsion Systems Centre, Indian Space Research Organisation,
                                              Bangalore, India
           3
             Visiting Professor, Centre for Nano Science and Engineering, IISc., Bangalore, India
         4
           Professor, Dept of Mechanical Engineering, UBDT College of Engg, Davanagere, India



ABSTRACT

        The advent of remote sensing satellites is gaining vital importance for national resource
surveys. The control systems required for such satellites call for stringent propulsion systems with
high precision actuators. These satellites use monopropellant propulsion systems which need to
deliver impulses as less as 30 mNs with an actuator (thruster) on time of 30ms for day to day attitude
corrections or even to extended period of operating up to a few thousand seconds firing of the
actuators for major orbital plane corrections which may have to be carried out few time during the
life span of the satellite. Further the propellant delivery to the actuators is by actuation of a solenoid
valve which plays a vital role on the overall capability of the propulsion system. Any minute leakage
though these valves could generate continuous thrust of a few mN which could affect the quality of
the imagery obtained by the on board remote sensing devices. Extreme leak tightness through the
valve is an important criterion for design.
        The design and development of a micro flow control valve was undertaken for a satellite
propulsion system. Primarily it is important to design a valve, having large electromagnetic force to
be produced through a relatively short stroke. It is characterized by having a magnetic circuit of
extremely shorter length and more cross sectional area. This may be achieved by better design of
electromagnetic circuit and also by reducing the mass of the moving parts. The configuration with
armature type of magnetic circuit is selected for the micro fluidic valve.
The valve is normally closed and electromechanically actuated with hard metal on soft seat
configuration. The focus was on adapting solenoid type design techniques to provide less than 10 ms
response time for monopropellant hydrazine thruster application. The valve has an extended
operating temperature of –15oC to +70oC with a total mass around 120g and nominal power draw of
9W @28V. Coil is designed to have operating capability between 28V to 42V DC. The valve has a

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6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July - August (2013) © IAEME

nominal outlet tube port of 2 mm to provide maximum flow rate of 16 cc/s of hydrazine, at an
operating pressure of 2.4 MPa with a pressure drop of 0.1 MPa. Low flow rate around 1.0 cc/s of
hydrazine flow is possible with a pressure drop of 0.005 MPa. Further micro flow rates can also be
achieved just by changing the outlet tube port dia from existing 2mm to the minimum required
diameter to meet the rated flow. The valve was demonstrated for 50,000 cycles. The developmental
valve was tested functionally and valve test results are satisfactory.
This paper describes the design methodology, construction of the valve, tests carried out and salient
test results.

Keywords: Hydrazine, Propulsion, Response, Seat, Spacecraft, Valve, Vibration

1. INTRODUCTION

        Currently the state of art in thruster system for small satellites is monopropellant propulsion
system. The specific impulse of the monopropellant propulsion systems are typically about 200sec
[1]. Propulsion system consists of different types of flow control components to maintain attitude and
orbit control of a spacecraft. The design and utilization of flow control components in spacecraft
necessitate a wide variety of operational and environmental considerations related to each intended
application [2]. Different types of components such as Fill Valves, Vent Valves, Single shot
operated valves, Unidirectional valves, Pressure regulators, Isolation valves and Flow control valves
are used in the propulsion system of spacecrafts. The flexure based micro fluidic valve falls under the
category of flow control valves. These valves are required to operate for more than 5 years and to
withstand rigorous high vacuum, radiation, temperature extremes with highly reactive propellants
[2]. These environments, in general, deteriorate materials and degrade functional performance. The
very low pressures of space can cause evaporation and sublimation of materials to the point where
the normal function is impaired or destroyed. Functional requirements for valves include zero
leakage, good cycle life, low mass, lowest pressure drop, minimum size and low power consumption.
The design and selection criteria are operating pressure, leakage requirements, pressure drop and
flow capacity, flow medium, type of actuation, actuation time, power, mass and size, operating life,
sensitivity to contamination and environmental conditions. This paper describes the design
methodology, construction, tests carried out and salient test results of micro fluidic valve.

2. DETAILS

2.1     Working Mechanism
        Over the past decade, there has been wide variety of actuation mechanisms and methods
employed for construction of micro valves including electro-static, electro-magnetic and piezo-
electric actuation [3]. Electrical signals from spacecraft guidance system actuate the ON/OFF valve
which meters gas or propellant flow and thereby controls rocket thrust to generate the control forces
required by the spacecraft control system. ON/OFF valves are basically shut off valves that feed gas
or propellant into the thruster at constant flow rates. These are remotely operated type, capable of
actuating either continuously or in pulse mode on electrical command. These are to have longer
operational life in terms of years and are to operate millions of times under extreme environmental
conditions under steady state or pulse mode with extreme leak tightness (< 1X10-5 Standard cc/sec of
gaseous helium). These valves are normally closed type and flow requirements are decided by the
rating of thruster used in the control system. The solenoid valve mainly consists of a bobbin over
which the excitation coil is wound. An armature with an elastomer seat is kept pressed against a hard
seat in the normally closed position. When the coil is energized, the armature moves against the

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spring force, thus opening the valve. As the coil is de-energized, the valve closes due to the
compression in the spring. A non-magnetic spacer is introduced between two sides of flanges in the
bobbin to improve the response of the valve. The cross section of idealized solenoid valve is shown
in Fig.1




                              Fig 1 schematic of idealized solenoid valve

        The selection process of an optimum propulsion system and components for specific
applications require mission analysis. Major evaluation criteria are mass, size, cost, reliability and
performance [4]. The valve has an actuating mechanism, seating mechanism and fluid filtering
mechanism, all in one unit within a small envelope. The armature movement is guided by flexures,
positioned on both sides of armature. The armature is spring loaded to control loading such that the
stresses remain within elastic range. In the closed position, the spring-loaded armature with elastomer
seat acts against a raised controlled width land on the hard seat. The differential pressure acting on
the seat effective area generates additional closing forces. When the solenoid coil is energized, the
coil builds up magnetic field at the center and as a result armature assembly is attracted towards the
stationary stop against closing forces due to spring compression and fluid pressure thus opening the
valve outlet port. On de-energizing the coil, the magnetic field disappears, the armature assembly
moves back and closes the valve port due to spring force and fluid pressure force. The Schematic
drawing of micro fluidic valve is shown in “Fig.2” and 3D model in Unigraphix is shown in “Fig.3”.




                                 Fig.2 schematic of micro fluidic valve


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                                Fig 3: 3D model of micro fluidic valve


2.2     Salient Design Features
  Salient design features of Micro fluidic valve include:
      • Flexure guided armature
      • Armature type with helical compression spring
      • Propellant compatible materials
      • All electron beam welded construction
      • No dynamic seals
      • Electrical termination by pig tail leads
      • Cylindrical type filter element at the inlet of the flow path
      • Hydrazine, Gaseous helium, GN2 fluid media
      • The volume of the elastomer button is kept very small to ensure negligible swelling when
          exposed to the propellant

2.3     Materials and Construction
        Simplified construction reduces the valve manufacturing cost and no dynamic seals are used
[5]. The materials chosen for the Micro fluidic valve shall be compatible with the Hydrazine
propellant medium. A highly permeable material like SS 430 is used for parts, which come in the
magnetic flux path. Parts in the propellant wetted cavities are made of non -magnetic material like SS
304L and PTFE elastomer as seat button. Flexures are made of SS 302. In order to ensure positive
force on the armature and to achieve leak rates less than 1X10-5 Scc/s of gaseous helium at an
operating pressure of 2.4MPa, helical spring is used, which is made up of SS 302. Further to protect
the elastomeric seal button from the weld heat and due to the complexities in the weld joints in
intricate cavities, electron beam welding is adopted.

2.4    Valve Orifice sizing
       The orifice is sized, based on the maximum required flow of 15 g/s of hydrazine at the
pressure drop of 0.01Mpa, by using the standard formulae

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July - August (2013) © IAEME

          Q = Cd A √ (2g∆P /ρ)

           Where A = area of orifice = (Π/4) d2
                Cd = Coefficient of discharge
                 d = diameter of orifice
                 g = acceleration due to gravity
                ∆P = Pressure drop
                ρ = Density
                Q = flow rate
After determining the orifice diameter, final orifice diameter is fixed by considering factor of safety.

2.5   Valve Stroke
The minimum stroke (lmin) required for the valve is calculated based on the formulae

           (Π/4) d2 = Π d lmin
           Where d        = diameter of orifice
                   l min = minimum stroke
Final stroke is fixed by considering allowances like allowance for teflon squeeze, allowance for
teflon seat swelling (while in contact with the propellants) and allowance for thermal expansion of
teflon seat at the extreme temperatures.

2.6 Photograph
The photographic view of the assembled Micro fludic valve is shown in Fig.4




                            Fig.4 photographic view of micro fluidic valve

3. ACCEPTANCE TESTING OF VALVE

       Acceptance tests were carried out on one development hardware of micro fluidic valve to
characterize the valve performance.

  The Functional tests include
     • Internal Leak test
     • External Leak test
     • Insulation Resistance
     • Pull in Voltage
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      • Drop out Voltage
      • ON Response
      • OFF Response
      • Flow Calibration
        Internal leak test indicates the leak rate through the valve seat and External leak test reveals
the leak through the weld joints. Actuation voltage is the minimum voltage at which the valve fully
opens. ON / OFF Response indicates the time in milli seconds to open or close the valve. Flow test is
for verification of the required flow at the given pressure drop. The test levels followed are as per
Qualification levels during valve testing.

The sequence of the testing followed is
     • Functional tests
     • Proof pressure test (1.5 times MEOP)
     • Functional tests
     • Vibration tests (sine & random)
     • Functional tests
     • Thermo vacuum test
     • Functional tests
     • Cycle test for Qualification of valve
     • Functional tests

4. PERFORMANCE EVALUATION

        The Resistance of the coil used is 84 ohms for the selected gauge of 35.5 AWG with 1000
number of turns. Valve was tested for its functionality & found satisfactory. The Internal leak rates
are in the order of 1X10-6 Scc/Sec of gaseous helium at 2.4MPa. External leak rates are in the order
of 1X10-8 Scc/Sec of gaseous helium at 2.4MPa. The micro valve ON/OFF responses @ 28VDC is in
the rage of 3 – 7ms. Valve responses are less than the specified value of 10ms at various voltages
between 28 to 42VDC with an operating pressure of 2.4MPa. Typical on response plot @ 28VDC
actuation with opening response of 6.8ms is shown in Fig.5.




                                  Fig. 5 Typical valve on response plot

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       Variation of flow through the valve for the given pressure drop is shown in “Fig.6” and
Variation of valve actuation voltage for different inlet pressures is shown in “Fig.7”.




                            Fig.6 Variation of flow with pressure drop




                  Fig.7 Variation of actuation voltage with valve inlet pressure




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  Variation of valve current required to actuate the valve for different inlet pressures is shown in
  “Fig.8”.




              Fig.8 Variation of current required for actuation with valve inlet pressure



4. CONCLUSION

        The development of a micro fluidic valve for satellites propulsion system was carried out.
The development hardware underwent functional testing to qualification levels. Internal leak rates of
the valve are 1X10-6 Scc/s of gaseous helium and are stable for varying pressures. The micro valve
was demonstrated for a maximum flow rate of 16cc/sec with a pressure drop of 0.1MPa, however
with 1 cc/sec is obtained with 0.005 MPa pressure drop. The micro valve actuation voltages are in
the range of 5-22VDC, which are well within the specified value of 25VDC. The micro valve
ON/OFF responses @ 28VDC is in the rage of 3 – 7ms. In totality, valve responses are less than the
specified value of 10ms at various voltages between 28 to 42VDC with an operating pressure of
2.4MPa. The valve was demonstrated for flow, leak, actuation voltage and responses to meet the
specific requirement to use in small satellite propulsion systems. The development valve functional
test results are satisfactory.




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5. REFERENCES

Journal Papers
[1] K. Ramamurthi and G.Madhavan Nair “Evolution and Growth of Spacecraft Propulsion
    Systems”, Journal of Spacecraft Technology, Vol. 8, No.1, Jan 1998
[2] B.K.Venkataramu, Dr.K.Anantharam, “Normally Closed Series Redundant Solenoid Valve for
    Satellite Control System”, IE(I) Journal, Vol 74, September 1993
[3] P. Selvaganapathy, E.T.Carlen, C.H.Mastrangelo, “Electrothermally actuated in line micro fluidic
    valve”, Science Direct, Sensors and Actuators A 104(2003)275-282
Hand Books
[4] Chemical Micro thruster options, Page 6, NASA Contractor Report 198531, Oct 1996
International Conference
[5] R.H.Reinicke and S.A.Harris, “Bi latch Isolation Valve for Spacecraft Propulsion systems”, 31st
    AIAA/ASME/SAC/ASEE joint propulsion conference and Exhibition, AIAA 95-3098 July 10-12
Website
[6] EADS, Astrium website on propellant flow control valves
    http://www.astrium.eads.net/en/ , dated 10/06/2012




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