Thrust in Space - The Nuances of Thruster Valve Design
Applications for thruster valves are growing daily. More and more companies are designing rockets, satellites, and reusable space vehicles that require thruster valves to position, maintain, or change vehicle orbit in space. In particular, launch systems designed to place small, micro, and nano satellites into orbit are proliferating. This new generation of satellites is designed to provide everything from communications, imaging, GPS, weather forecasting, climate change monitoring, and even monitoring of farmland irrigation and fertilization needs. The demand for space-based images especially drives the need for more imaging satellites.
The thruster valve plays a crucial role in the guidance and positioning of space vehicles, rockets and satellites. The thruster valve must reliably respond to commands from the vehicle guidance system to fire when, and for as long as needed, to move the spacecraft to a different position. When needed, thrusters do not normally fire for long periods of time. Rather, they are fired in short bursts to control the vehicle position.
Since there is no friction in space, a short burst of a thruster results in the creation of motion in the appropriate direction. Once in motion, the vehicle will continue to travel until other thrusters are fired, creating a braking action to slow or stop the vehicle from moving. For example, a satellite in geosynchronous orbit may need its orbit tweaked occasionally to keep it in the right place relative to the surface of the earth. On the other hand, surveillance satellites need to drop to a lower orbit to take images, then raise to a higher orbit to keep them out of harm’s way. For a thruster to do its job reliably, the valves that are part of it must be extremely reliable, and accurately respond when needed.
“The design of a thruster valve is no small feat,” states Rich Kelly, Senior Project Engineer. “Many application parameters must be carefully considered before the design process begins.” Typical information required includes fluid pressure, maximum flow, pressure drop, and end connections. Voltage and current limits are also important, since power for the solenoid valves is provided by solar panels connected to storage batteries.
The media is also critical. Thrusters use several liquids to create hot gases to propel the vehicle, including hydrazine, hydrogen peroxide, and nitrogen tetroxide. Smaller satellites use cold inert gases, typically nitrogen. When the application calls for hydrogen peroxide, special precautions are taken, as hydrogen peroxide can cause corrosion of the solenoid valve components.
For reusable space vehicles, eliminating corrosion and contamination is imperative in order to provide a long operating life without the need for servicing the valve. Valcor’s extensive experience with designing solenoid valves for corrosive propellants enables engineers to draw on a wide array of metals and trim materials for thruster valve designs. Valcor’s experience with these materials shortens the development time for the valve, as material qualification is kept to a minimum. Size and weight must be minimized, as there is very little room available for the valves among the multitude of other components in a spacecraft. Thruster valves can be as large as a three-pound coffee can or as small as a thimble.
Another factor for consideration is the amount of vibration to which the valve is subjected. The valve must remain closed during the extreme vibration created during the vehicle launch. If the valve leaks, precious media is lost. Valcor’s valves contain several proprietary features that enable them to survive and excel in space environments. Some of these proprietary features greatly improve the performance of the valves, specifically with regard for cycle life, high pressure, and high temperature operating conditions.
Cycle Life: Valcor’s solenoid thruster valves are mostly sliding fit, plunger types, but with a special design feature that suspends the magnetic plunger within the coil bobbin. By virtually eliminating contact between the plunger (which reciprocates to open and close the valve) and the coil bobbin, these valves are capable of extremely high cycle life. Millions of cycles are demonstrated by valves in service, which is typical for this design.
High Pressure: Valcor valves have unique seat/poppet designs that control the stress on the seal materials, providing very low leakage in high-pressure service. This is particularly notable for bi-propellant propulsion systems that require materials compatible with hydrazine and nitrogen tetroxide. These compatible materials (polymers) have low structural material properties, so designing leak-free, high-pressure-capable valves has been a challenge for the aerospace valve industry. Valcor’s engineers developed a creative design that reduces the stress on these materials, thereby allowing the valves to achieve exceptionally long service life with bi-propellant gases under high pressure.
High Temperature: Valcor has a large variety of valves designed for high-temperature gas and solid propellant applications. By managing how the hot gas travels throughout the valve, the impact of high temperatures on the materials of construction can be controlled. Controlling the impact of high temperature on the valve components enables longer operating service life for the valve.
Spacecraft and satellites, and the launch vehicles that put them into space are known to be very expensive. Over the last 20 years, a number of privately-funded companies are attempting to dramatically reduce the cost of placing spacecraft in orbit, as well as the launch vehicle itself. The focus on reusable launch vehicles emphasizes the importance of valves that consistently work reliably from mission to mission.
To that end, Valcor has been aggressively working with spacecraft companies to eliminate non-value-added operations and focus on those processes that add value. Valcor also works closely with the customer’s engineering design team to find ways to eliminate potential risk by ensuring a robust system design. In many cases, Valcor takes on the task of building and testing these systems, providing a turnkey assembly that the customer installs in their vehicle.
Recently, Valcor partnered with a well-respected veteran of the space industry to create a thruster valve for a new reusable space vehicle. The customer had already selected a valve supplier and incorporated that supplier’s thruster valves into their design. However, the customer had some reservations about their selection and turned to Valcor Engineering for an alternative. There were substantial design constraints, as the project was well underway at this point. The size and configuration of Valcor’s design had to fit the existing design of the system. Further, the customer needed to go into production in an expedited time, so there was precious little time to design, prototype, and produce the new thruster valve. Valcor undertook the project and is fast-tracking the design and development of a thruster valve that meets the tight schedule requirements.
With many projects, the initial focus is on the design and manufacture of the solenoid valve, but by closely partnering with the customer, Valcor often expands the scope of the project by taking on the design of the sub-system or entire system in which the valve is used. The result of this design effort is a fully assembled and tested system that saves the customer time, effort, and cost in the manufacture of their vehicle.
This article was written by Paul Meyers, Aerospace Sales & Marketing Director and Louis J. Arcuri, Marketing, Valcor Engineering Corporation (Springfield, NJ). For more information, visit here .