- Normally Closed Isolation demonstrated to internal leakage
rates less than 1x10-5 sccs GHe at pressures of 2.5 bar to 207
bar (36 to 3,000 psia) and temperatures from -54 to 93°C
(–65 to 200°F)
- Precise, modulating closed loop pressure/flow regulation (pressure,
flow rate or anode current feedback) from 0 to 80°C (32
to 176°F)
- Electronically adjustable set-point demonstrated to mass flow
turn-downs greater than 25 to 1 and pressure reductions from
207 bar to 35 millibar (3,000 to 0.5 psia)
The device can also operate as an open/closed
2-way valve.
This multi-function capability can significantly
reduce the number of xenon feed system components required
(reducing both integration costs and overall system weight)
while increasing system capability and reliability.
Technology
In designing the Multi-Function Valve, Marotta applied
and developed two advanced technologies (a magnetostrictive actuator
and a tight shut-off metal seat) to satisfy the unique and demanding
needs of Hall or Ion thrusters. This technology can also be utilized
for other cold or inert gas systems.
Hall Effect Thrusters (HETs) and Ion Thrusters operate at very
high specific impulse (over 1500 and over 3000 seconds, respectively),
and at very low thrust (often within 10 to 200 millinewtons),
so they only require a tiny xenon gas flow rate. This miniscule
flow rate (at times could be considered a closed valve leak rate!)
requires a tiny valve stroke measured in microns, or even parts
of a micron.
Xenon feed systems for Ion Thrusters and HETs must be “mass spectrometer”
gas leak tight for many (often more than 10) years, to avoid depletion
of xenon gas from the supply tank(s). Also, the xenon supplied
on these types of Thrusters must be of very high purity, so all-metal
construction, including the valve closure, is desirable. Mass
spectrometer tight shutoff, using hard seating materials, requires
a very high force valve actuator such as the magnetostrictive
material utilized in the MFV.
Some Thrusters need precise, electronically variable, xenon flow
modulation for power and thrust level management, while the Neutralizers
need independent flow adjustment of the xenon supply to optimize
thruster performance through the spacecraft mission. All of these
very unique valve actuator requirements – tiny poppet stroke,
high closure force, and precise flow rate modulation – are provided
by Marotta’s advanced magnetostrictive actuator, operating an
advanced, all-metal hard poppet/seat closure and throttling element.
MFV for Xenon Gas Supply
Spacecraft using Electrostatic “Hall” or “Ion” Electric
Propulsion Thrusters require propellant feed systems which provide
leak-tight isolation and precision delivery of propellant from
high-pressure, blow-down supply tanks to the Thruster. Internal
leakage for propellant isolation is typically near, or less than,
1x 10-4 sccs GHe at supply tank pressures as high as 150 bar (2175
psia). The total flow rate delivered to a present-day Thruster
often varies depending on the Thruster size and type. Flow rates
have varied from fractions of milligrams per second to tens of
milligrams per second.
Marotta’s MFV provides both propellant isolation
and precise mass flow delivery to a variety of Thruster sizes
using the same all metal wetted construction hardware. The MFV
has the variable set-point functionality of a “Bang-Bang” system
with the smooth precise flow characteristics of a mechanical regulator.
In addition the MFV has the ability to vary the desired flow almost
instantaneously as compared to much longer durations typically
seen from thermal devices.
MFV Description
As with most precision small flow, variable inlet
pressure regulation devices, MFV operation relies on sensor feedback
for xenon regulation rather than open loop control; this compares
to the sensing orifice in a mechanical regulator or the transducer
feedback of an electronically pulsed regulator. The MFV uses an
electronic feedback signal and commanded set-point as shown in
Figure 1. Sensors may vary depending on the desired control parameter.
Tested sensors include a pressure transducer for pressure control,
a mass flow sensor for mass flow control, and a current sensor
for Hall Effect Thruster anode discharge current control.
MFV construction compares to what the fluid controls
industry would call a normally closed, pull-type plunger solenoid
valve. The plunger (or armature) is simply replaced by magnetostrictive
material. As the solenoid coil creates a magnetic field, the material
grows in length, eventually contracting the poppet that seals
against the valve seat. The poppet lifts away from the seat as
magnetostrictive growth continues from increased magnetic field.
By varying the magnetic field the stroke is varied permitting
modulating flow control. When the solenoid’s magnetic field is
removed, the material returns to its original normally closed
state. The magnetostrictive material’s inherent high force from
strain permits larger sealing loads which subsequently permitted
metal to metal sealing to internal leakage values less than 1x10-5 sccs
GHe (refer to AIAA paper
99-2561, Multifunction Valve Extended Development Testing;
652K PDF file).
Design Demonstration & Heritage
MFV
Heritage of the proposed MFV comes directly from
work performed under two Ballistic Missile Defense Organization
(BMDO; now the Missile Defense Agency, MDA) Small Business Innovative
Research (SBIR) contracts for a Magnetostrictively Actuated, Multi-Function
Xenon Gas Valves, or as we call it, the “MFV”. All MFV efforts
to date have benefited from SBIR guidance and funding, particularly
NASA GRC who served as the Technical Monitor for the effort.
The Phase 1&2 SBIR MFV Program was completed
in 1999. Proof of concept MFV hardware provided precision, single
stage, closed-loop pressure control at design inlet pressures
from 2000 psia blowing down to 100 psia. Accomplishments using
SBIR design hardware include:
- End of cycle test (>100,000 closed to open to closed cycles)
internal leakage rate less than 1x10-5 sccs GHe.
- <1x10-5 sccs GHe during thermal testing from
-54 to 93°C.
- Pressure transducer feedback control of a T-160 Hall Effect
Thruster at NASA-GRC as well as with Astrium using a RITA Thruster
at Giessen University.
- Anode current feedback control of a SPT-100 Hall Effect Thruster
at NASA-GRC.
- High to low pressure Xenon operation (including 3000 psia)
at ambient temperature (with no heaters required).
- Operational performance testing from 0 to 80°C, including
pressure feedback testing from 10 sccm to 250 sccm at 138 bar
BOL & 6.9 bar EOL pressure at the same closed loop control
gain setting (though use in variable gain control systems is
an option).
- Random Vibration testing to 23.6 Grms while pressurized &
internal leakage monitored at 2000 psig.
- Successful testing with a RITA Thruster at Giessen University.
- Space Power Inc. (since acquired by Pratt & Whitney Space
Propulsion) T-140 HET test using pressure and anode feedback
at TRW (now Northrop Grumman Space Technology)
Most recently the MFV was qualified for use on the
Astrium GOCE Satellite. The MFV provides precise flow control
to the Ion thruster throughout a 0.02 to 0.63 gram per second
xenon flow range. The qualification accomplishments achieved in
this program, adding to the SBIR accomplishments, include:
- End of cycle test (80,000 closed to open to closed cycles,
including portion at 0 & 80°C) internal leakage less
than 1x10-5 sccs GHe.
- 8 x 0 to 60°C thermal vacuum cycles with functional testing
at extremes on first and last cycles, and internal leakage tests
at -30°C.
- Shock testing.
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