NASA's participation in the multi-nation ExoMars 2018 Rover mission includes a critical astrobiology Mass Spectrometer Instrument on the Rover called the Mars Organic Molecule Analyzer (MOMA). The Aperture Valve is a critical electromechanical valve used by the Mass Spectrometer to facilitate the transfer of ions from Martian soil to the Mass Spectrometer for analysis. The MOMA Aperture Valve development program will be discussed in terms of the Initial valve design and subsequent improvements that resulted from prototype testing. The Initial Aperture Valve concept seemed promising, based on calculations and perceived merits. However, performance results of this design were disappointing, due to delamination of TiN and DLC coatings applied to the Titanium base metals, causing debris from the coatings to seize the valve. While peer reviews and design trade studies are important forums to vet a concept design, results from testing should not be underestimated. Despite the lack of development progress to meet requirements, valuable information from weakness discovered in the Initial Valve design was used to develop a second, more robust Aperture valve. Based on a check-ball design, the ETU /flight valve design resulted in significantly less surface area to create the seal. Moreover, PVD coatings were eliminated in favor of hardened, nonmagnetic corrosion resistant alloys. Test results were impressive, with the valve achieving five orders of magnitude better sealing leak rate over end of life requirements. Cycle life was equally impressive, achieving 280,000 cycles without failure

Canham, John S.

Engler, Charles D.

NASA Technical Reports Server

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Aperture Valve for the Mars Organic Molecule Analyzer (MOMA) 
 
Charles Engler*, John Canham^  
 
Abstract 
NASA's participation in the multi-nation ExoMars 2018 Rover mission includes a critical 
astrobiology Mass Spectrometer Instrument on the Rover called the Mars Organic Molecule 
Analyzer (MOMA). The Aperture Valve is a critical electromechanical valve used by the Mass 
Spectrometer to facilitate the transfer of ions from Martian soil to the Mass Spectrometer for 
analysis. The MOMA Aperture Valve development program will be discussed in terms of the 
Initial valve design and subsequent improvements that resulted from prototype testing. The 
Initial Aperture Valve concept seemed promising, based on calculations and perceived merits. 
However, performance results of this design were disappointing, due to delamination of TiN 
and DLC coatings applied to the Titanium base metals, causing debris from the coatings to seize 
the valve. While peer reviews and design trade studies are important forums to vet a concept 
design, results from testing should not be underestimated. 
Despite the lack of development progress to meet requirements, valuable information from 
weakness discovered in the Initial Valve design was used to develop a second, more robust 
Aperture valve. Based on a check-ball design, the ETU /flight valve design resulted in 
significantly less surface area to create the seal. Moreover, PVD coatings were eliminated in 
favor of hardened, nonmagnetic corrosion resistant alloys. Test results were impressive, with 
the valve achieving five orders of magnitude better sealing leak rate over end of life 
requirements. Cycle life was equally impressive, achieving 280,000 cycles without failure. 
 
 
 
 
 
 
*NASA Goddard Space Flight Center, Greenbelt, MD 20771 
^ATK Space Systems, Beltsville, MD 20705 
https://ntrs.nasa.gov/search.jsp?R=20140013463 2019-08-31T17:04:39+00:00Z
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Introduction 
By studying organic molecules (the chemical building blocks of life as we know it), MOMA is 
designed to help answer questions about whether life existed on Mars, as well as its potential 
origin, evolution, and distribution on the Red Planet. The MOMA Instrument is principally a 
mass spectrometer used to detect the masses and relative 
concentrations of atoms and molecules in a substance. MOMA 
is designed to analyze the types and amounts of chemicals 
that make up organic and inorganic compounds found in rock 
and soil samples on Mars. The instrument will address a top 
science goal seeking the signs of past or present life on Mars. 
Since the surface of Mars is bathed in ultraviolet and cosmic 
radiation, complex organics in the uppermost surface layers 
could be degraded. The 2018 Rover will be equipped with a 
drill to obtain samples from as deep as 2 meters in the hope 
that organics at that depth would still be viable. Core samples 
of Martian soil will be ablated by a UV laser, producing ions 
which pass thru the Aperture Valve and are subsequently 
trapped and analyzed by the Mass Spectrometer to determine the molecular composition of 
the ion, which then determines the molecular composition of the sample.  
 
 The MOMA Mass Spectrometer illustrated in figure 1. 
Principle components are the Mass Spectrometer 
Housing, 266nm UV Laser Head and Aperture Valve. The 
Martian sample is moved to the Ultra-clean zone directly 
below the Aperture Valve. A laser mounted onto the 
Mass Spectrometer Housing creates a focused beam of 
energy which oblates the soil and ionizes soil particles. 
The valve opens, allowing ions to flow into the Mass 
Spectrometer, then quickly closes. Ions are trapped by a 
Linear Ion Trap, scanned for molecular composition, 
then ejected.  
 
 
 
Mass  
Spectrometer 
Housing 
Aperture  
Valve 
Figure 1- MOMA Mass Spectrometer 
with Aperture Valve 
Ion Inlet 
3 
 
Initial Aperture Valve Design 
 
Over 125 requirements for the valve were levied, which were then ranked according to 
importance and distilled down to the 25 top “driving’ requirements. The initial Aperture Valve 
design is illustrated in figure 2. Sealing the inlet was accomplished by means of a Sliding Pin, 
which operated to open and close off the Ion Tube. A solenoid was connected to the sliding pin 
connected to a pull-type solenoid, which when energized, pulled the Pin to the open position, 
allowing ions to travel from a Martian sample below the valve, up the inlet cone and thru the 
hole at the base of the cone. After power to the solenoid was removed, the solenoid rapidly 
closed from stored energy in a recoil spring. Close tolerance between the sliding Pin and Valve 
Body produced a pressure leak rate consistent with the requirement of 10E-3 cc/sec He for 
beginning of life and 10E-2 cc/sec He, 
end of life, which was defined as 
125,000 actuation cycles. 
To meet operational and performance 
requirements, materials for Aperture 
Valve components were comprised of 
Titanium 6AL-4V. Titanium is well 
known material of choice for 
aerospace design with benefits 
including: high strength-to-weight 
ratio, corrosion resistance and non-magnetic. Coatings applied to the Titanium were added for 
surface hardness, lower coefficient of friction and prevented galling between the Sliding Pin 
and Valve body. The choice of coatings on these parts and application of the coatings to the 
Titanium base metal would prove problematic. Figure 3 illustrates two versions of the Initial 
Requirement 
Pressure leak rate 10E-3 cc/sec He 
Operational cycle life 125,000 cycles 
Operational temperature -20°C to 50°C 
Valve open/close time <50ms 
Mass 90grams 
Failure Mode Fail closed 
Power 5 watts 
Material limitations Non-magnetic 
Figure 2- Aperture Valve early design 
4 
 
Valve design. Both feature the same mechanical sliding pin type concept. Clearance between 
the outside diameter of the Sliding Pin and bore hole within the Valve body were tightly 
controlled to .0002” clearance. The valve body was coated with Titanium Nitride (TiN) by means 
of physical vapor deposition or PVD, a process of sublimating pure titanium with nitrogen in a 
high energy, vacuum oven.  
The result is an extremely 
hard, thin, gold colored film, 
commonly found on tool bits. 
The sliding pin was coated 
with Diamond-Like-Carbon 
(DLC), another PVD coating 
having a desirable 
combination of a low 
coefficient of friction and high 
micro hardness. In general, 
PVD coatings have a micro-
hardness greater than 80Rc, well above 
the hardness of tool steel. Both coatings 
were applied at 2.5 micron (.0001in.) thickness. 
 Problems with TiN and DLC Coatings 
Traditionally, the largest inherent 
problem with ceramic based coatings 
such as TiN and DLC has been the 
issue of adhesion:  typically the 
higher the diamond percentage in a 
DLC film, the harder the DLC film but 
the higher the compressive stress 
within that film.  In extreme cases, 
such stresses can create unstable 
interfaces (i.e adhesion and/or 
cohesion issues) and the film may 
become prone to delamination.  
 
 
 
Figure 3- variations of coated valves 
Figure 4- Coating delamination on valve parts 
5 
 
Figure 4 illustrates the results of Aperture Valve testing after less than 100 actuations. Figure 4 
(left) illustrates the bore of the valve body coated with TiN. Flaking of gold colored material 
inside the bore and complete film separation from the base substrate is evident. From our 
experience, the PVD process used to apply TiN onto internal cavities and bore holes does not 
produce acceptable adhesion of the TiN film. It is not clear whether additional factors 
associated with substrate preparation within the bore were contributing factors. Figure 4 (right) 
illustrates excessive wear of DLC coating on the sliding Pin. This was probably the result of 
delaminated TiN flakes abrading the DLC surface. TiN on the cone and smaller shaft feature on 
the Sliding Pin were requested by the science team. While monolithic coatings are most 
common, multi-layer PVD coatings are applied with acceptable results, depending on number 
of layers, layer thickness, hardness of the coatings, etc. In this case, TiN was applied over the 
DLC coating with poor results, as flaking was evidenced here as well.  
In the laboratory, the assembled valve initially operated well, meeting open and close times of 
<50ms per the requirement. Leak rate on the valve was also initially within requirement of 1E-3 
cc/sec He. After less than 100 open-close cycles however, there was a notable sticking of the 
sliding pin; the solenoid could quickly open the valve, however the recoil spring could not 
return the pin quickly due to particulates generated from TiN fragments. Finally, the valve pin 
ceased to open or close, 
overwhelmed by friction 
between sliding surfaces. 
Moreover, scratches on 
the sliding surfaces 
prevented the valve from 
being refurbished 
resulting in a failure. The 
valve test chamber is 
illustrated in figure 5. 
Valve operates at 5-7 torr 
(Mars ambient pressure) 
in a CO2 environment. Upper 
Chamber (not shown) operates at 
1E-6 torr. Physical dimensions of the Initial Valve are 98mm x 20mm x 20mm (LxWxH) 
 
 
 
Figure 5- Aperture Valve test chamber 
6 
 
 
ETU /Flight Aperture Valve design 
The second generation Aperture Valve for ETU and Flight was designed to specifically avoid the 
issues associated with PVD coatings and the Sliding Pin sealing concept requiring .0002 in. 
clearance to the Valve body bore to create the seal. While the previous design was based on 
creating hard, durable sliding surfaces 
between the Sliding Pin and Valve Body to 
minimize friction and particulate wear, the 
Initial Aperture Valve never achieved more 
than 1200 cycles before failing due to 
particulates. Obviously, the Initial Valve design 
could not meet the open-close requirement of 
125,000 cycles. Therefore, another valve 
design was developed which specifically 
avoided the design weaknesses described. Figure 6 illustrates the improved valve design. 
Common features to the Initial Valve include a pull-type solenoid, sliding carrier (not pin), valve 
body and recoil spring. Figure 6a illustrates the principle components of the new valve design. 
The innovation in the design may be found 
in the method of sealing the valve open and 
closed, similar to a check-ball design, 
whereby a ball fits into a spherical seat to 
create the seal. In this design, the ball is 
placed into a socket forcing the ball onto the 
seat by a small spring. A sliding Carrier 
containing the ball assembly translates to 
the right when pulled by the solenoid. Translation 
of the Carrier forces the ball out of the valve seat 
and into the Carrier. A hard stop prevents further 
motion of the Carrier, allowing a hole in the 
carrier to provide an open path for ions to travel 
thru the valve and up the inlet tube. When power to the solenoid is removed, the recoil spring 
sends the Carrier back, returning the ball to the valve seat to complete the seal. 
Results of ETU /Flight Aperture Valve Testing 
The ETU /Flight Aperture Valve design exceeded expectations for sealing and performance. The 
valve seat design was unique for this application and routinely achieved sealing leak rates as 
high as <E-9 cc/sec He.  In fact, 5E-9 cc/sec. in Helium was measured even after 125,000 cycles. 
Figure 6- ETU /Flight Aperture Valve 
 
 
Figure 6a- ETU /Flight Valve Section 
Recoil Spring 
Solenoid 
Sealing Ball 
Ion Inlet 
Valve Body 
7 
 
Closure times also exceeded requirements; open and closure times of <50ms (each way) were 
observed with an typical time of 26ms. The ETU /Flight Aperture Valve design proved superior 
to the original valve in meeting or exceeding design requirements.  Tests were conducted on 
valve endurance beyond requirements, finding a small reduction in sealing leak rate after 
250,000 cycles. Additional testing was subsequently performed on the same valve to assess 
endurance when contaminates were introduced into the test chamber environment. Mars 
regolith, an earth-based concoction of materials to simulate a typical Martian bedrock soil was 
used for the test. The test valve was run again for an additional 30,000 cycles in the 
contamination environment without failure. Leak rate was measured at 2E-8cc/sec He, 
approximately 5 orders of magnitude higher than the end of life requirement (1E-3 cc/sec He 
EOL spec vs. 2E-8 cc/sec He on test valve). 
 
Conclusion 
The MOMA Aperture Valve development program was briefly discussed in terms of design 
improvements resulting from prototype test results. The Initial Valve design was based on over 
125 requirements which were distilled down to the top 25 “driving” design requirements. The 
Initial Aperture Valve concept seemed promising, based on calculations and perceived merits. 
However, performance results of this design were disappointing, due to delamination of TiN 
and DLC coatings applied to the Titanium base metals, causing debris from the coatings to seize 
the valve.  
Despite the loss, valuable information from weakness discovered in the Initial Valve design was 
used to develop a second, more robust Aperture valve. Based on a check-ball design, the ETU 
/flight valve design resulted in significantly less surface area to create the seal. Moreover, PVD 
coatings were eliminated in favor of hardened, nonmagnetic corrosion resistant alloys. Test 
results were impressive, with the valve achieving five orders of magnitude better sealing leak 
rate over end of life requirements. Cycle life was equally impressive, achieving 280,000 cycles 
without failure. Due to schedule constraints, the ETU/ flight valve was never tested to actual 
end of life, however, it is estimated that the valve could achieve 500,000 cycles, due to the 
relatively few particulates discovered during post-test inspection of valve components.  
 
Acknowledgements 
The authors would like to thank Ken Blumenstock for engineering the solenoid design and 
Willie Barber for his manufacturing expertise and support on this project.  


Aperture Valve for the Mars Organic Molecule Analyzer (MOMA)

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