This TechTalk highlights 10 key considerations for engineers when selecting pneumatic orifices for flow dividers. Precision and reliability are key, especially when the flow dividers are performing critical functions in space.
Ion propulsion systems offer more efficient use of fuel and electrical power as compared to other propulsion technologies. Therefore, ion thrusters are the preferred option for a wide range of space missions such as satellite station-keeping, orbit insertion, and deep space exploration. Spacecraft powered by ion thrusters can achieve incredibly high speeds—reaching up to 200,000 miles per hour. However, the trade-off is that ion thrusters generate low thrust when compared to alternatives such as chemical propulsion. To achieve the same overall change in momentum, they must operate reliably for longer durations, in some cases for many thousands of hours.
Gas feed systems are critical to ion thruster performance
The gas feed system is one of the critical systems in ion propulsion. It controls the flow of a noble gas, such as xenon or krypton, to the inlet of the anode and cathode ports of the ion thruster. The flow through the anode is used to provide thrust; flow through the cathode is used to balance the charge emitted by the ion beam and to initialize and sustain the plasma discharge. It is critical to ensure the precise ratio of flow between the anode and cathode.
The cathode is one of the most complex aspects of thruster design because it is fragile and difficult to operate. It is also critical to the stability of the thruster during start-up and to the efficiency of the system. Any excess flow through the cathode beyond that which is used to balance the charge is simply lost.
It is no longer available to provide additional thrust, thereby reducing the efficiency and life of the system. One method of creating this precise flow ratio is employing precision calibrated flow orifices to meter the propellant into the thruster.
Generally, a single orifice metering element is the simplest and most accurate way to meter flow. However, the flow requirements of ion thrusters are typically extremely low and require very small orifice sizes; these are highly susceptible to contamination and less practical to manufacture using conventional production methods. As a result, alternate methods of restriction should be explored to produce the desired flow performance.
Key considerations for selecting orifices for metering propellant
Propellant metering orifices represent critical components within a propellant feed system and play an important role in the efficiency of an ion thruster. When selecting a metering orifice or device, an engineer must consider several design parameters, including the following:
Metered Flow Rate – The critical feature of a precision orifice is that it provides the optimal flow rate of propellant to the appropriate location. The properties of the gas, the entry, and exit geometries, and the length and diameter of the orifice all impact the flow rate. The propellant and target flow rate should be specified.
Flow Rate Tolerance – In a mass production environment there will inevitably be some variation from orifice to orifice. A critical system requirement is an understanding of the way production tolerances of individual orifices influence total tolerance. The tolerance stack-up of multiple orifices in parallel can be described by the following equation:
Reliable, multi-orifice restrictors utilizing Visco technology
Lee Visco Jets and JEVAs offer an alternate solution: integrating a complex fluid passage with numerous orifices in series to create a multi-orifice restrictor in a miniature package. The combined effect of taking pressure drops across a series of orifices offers very high restrictions with a large minimum passage size. For example, a Lee Visco Jet can replace a single 0.0004″ orifice with an equivalent restriction that includes a minimum passage of 0.005″; this allows for a more reliable device that is much less susceptible to contamination. Even with the larger passage size, the Lee Visco Jet and JEVA are in a miniature, lightweight package: it is less than an inch long with integral safety screens on both sides of the metering element for additional protection.
The Lee Company 100% flow tests each multi-orifice restrictor because accurate metering of the propellant is required for the anode and cathode feed lines. Acceptance testing may include flow testing using nitrogen at specific pressure drops to simplify the correlation to alternative noble gases. This will ensure the most reliable and accurate performance.
The Lee Visco Jet can be customized with flow rate accuracies as low as +/-3%. With the cathode flow typically representing about 10% of total mass flow, this variation accounts for only about +/-0.3% of the total mass flow rate.
Lee multi-orifice restrictors are constructed entirely from stainless steel to handle the extreme pressures and temperatures common in space systems. They are available in a variety of installation configurations to allow for design flexibility.
The Lee Company developed a field-proven locking end that allows the component to be installed directly into a manifold. Utilizing this self-retaining feature eliminates the need for elastomeric seals and securely locks the component into place without any bypass leakage. Line mount configurations are also an option and may include special tube ends that can be welded directly in-line.
The Lee Company has an on-site cleanroom facility capable of cleaning parts to meet specific customer cleanliness levels such as NAS 1638 or IEST-STD-CC1246.
In addition to pedigree on satellite and spacecraft thrusters, Lee multi-orifice restrictors are operating in a wide range of space applications: launch vehicles, crew capsules, landers, on the International Space Station, and on spacesuits. These precision orifices:
Regardless of the application, Lee multi-orifice restrictors offer a reliable long-term solution where precision metering of liquids or gases is required.
The Lee Company has a long history of supplying devices for use in the space industry. We have been at the forefront of fluid control technology since 1948, supplying millions of innovative products worldwide from our state-of-the-art manufacturing facilities in Connecticut, USA. The Lee Company transforms complex problems into deliverable solutions; we do this through ongoing research, design, development, and commitment to quality and innovation. Our in-depth application knowledge enables us to collaborate with customers and provide personal technical support through a wide network of experienced sales engineers who are ready to address any challenge.
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