A valve is a fluid component which allows or blocks the flow of fluid through a pipe. In liquid rocketry, pressure gradients of over 6,000psi may be placed over these valves. These valves must also be able to open and close with high reliability, as they can be single points of failure in a feed system. As such, the selection and proper implementation of valves is critical to the success of a liquid rocket.
Valves can be open or closed using several different control methods.
The simplest valves are manually controlled, meaning that a person must physically open or close the valve for its operation. In a rocket feed system, such valves are useful for allowing operators to open and close fill ports when fueling the rocket.
To allow a computer to very easily control a set of valves, a solenoid valve can be used. In a solenoid valve, a coil of wire is used to generate an electromagnetic field, which can force a valve open or closed.
Such valves can operate off of DC or AC voltage, depending on the design. Often, a 12-24V DC supply is sufficient to successfully operate larger solenoid valves. A GPIO pin connected to a MOSFET or other type of switch can allow a computer to enable or disable the solenoid, allowing direct control over the feed system.
Electromagnets often take large amounts of current to supply them, meaning these valves can drain significant energy from a rocket's power supply. Additionally, solenoid valves rated for high pressures and cryogenics are very expensive to obtain.
Pneumatic valves are open and closed by pressurized air. By pressurizing an inlet on the valve to a certain amount (often in the realm of 70-100psi), the valve will be forced open or closed. To control these valves, a pneumatic feed system must be implemented.
A pneumatic feed system is controlled with another set of valves, such as 3-way or 5-way solenoid valves. A computer controlling the solenoid valves can allow the pressurized air to open or close the pneumatic valve. These solenoid valves are small, cheap, and consume smaller amounts of energy than a solenoid valve which directly operates on the main feed system.
Pneumatic valves are often very cheap and robust, making them a good option for feed system control. However, there comes the added complexity of implementing a pneumatic feed system in addition to the main feed system.
Implementing this additional feed system is still relatively easy to do, since the pressure regimes are much lower, and normal air can be used in the pneumatics. Additionally, the range of pressures which will successfully operate the pneumatic valve are relatively large, meaning a precise pressure regulator for the pneumatic feed system is generally not necessary.
In a pressure-fed rocket, a small, low-volume 100psi pressure regulator can be attached to the pressurant tank and used as the source for the pneumatic feed system. Flexible plastic tubing can be used to route the pressurized air through the solenoid valve and into the pneumatic valve.
Relief valves automatically open when the pressure inside them reaches a predetermined amount. These relief valves are attached to sections of feed which could experience dangerous pressure spikes. They "relieve" excess pressure which reaches the inside of the feed system before the pressure can build too high.
These valves are also very audible when they go off, giving operators a clear indication that something is going wrong in the feed system.
Relief valves are an essential safety component in feed systems. However, they are not a saving grace for all dangerous scenarios. If the pressure in the system builds faster than the valve can relieve, the system still puts the surrounding area in imminent danger. Relief valves should always be used in conjunction with other safety features, such as rupture disks.
A rupture disk, or burst disk, is a one-time use disk designed to rupture when the pressure in a feed system reaches a predetermined amount. When the disk is destroyed, gases are allowed to escape into the environment, quicly relieving pressure before a dangerous situation develops.
Burst disks can provide a much higher mass flow rate than relief valves, and also serve as an important safety feature inside feed systems.
Valves have several different physical designs depending on their control method and intended use.
A ball valve is a housing which uses a rotating sphere to control the fluid flow through the valve. The sphere has a cylindrical hole drilled through it. When the hole in the sphere is aligned with the openings in the valve housing, the fluid is allowed to pass through the valve. If the ball is rotated 90 degrees, the hole is now perpendicular to the openings, and fluid can no longer get through the ball.
Ball valves are very simple, durable, and reliable. They are a common and excellent choice for manual control valves. With some modification using mechanical devices, they can be converted into pneumatically-controlled or electrically-controlled valves by attaching actuators or motors to the handle.