Valves are used to control the flow of fluids. Several kinds of valves can be found in mechanical supply catalogs:

	Ball valves
	Gate valves
	Globe valves
	Needle valves
	Butterfly valves
	Check valves
	Solenoid valves
	Relief valves
	Pressure regulating valves
	Control valves
	Float valves

Of these valve types, ball valves, needle valves, check valves, relief valves and pressure regulating valves are the most useful as amateur rocket components.

Check and relief valves are self-controlling. Check valves allow flow in only one direction. Relief valves open at a specified pressure to prevent over-pressurization. And regulating valves, once adjusted for a desired pressure, also are self-controlled. Their function is to provide a constant exit pressure. Needle valves are useful for fast proportional control, as is needed in thrust vector control of multiple engines.

Ball valves are the most commonly used kind of valve for controlling propellant flow and feed pressure to the propellant tanks. Ball valves are controlled by a valve stem. It is connected by means of a shaft coupler to an actuator such as a gear motor. A ball valve has entrance and exit ports, usually threaded for standard NPT pipe connections. Between the ports is a channel containing a sphere (the "ball") with a hole through its center and suspended by seating material that holds the ball in place and seals the volume outside the ball from fluid flow around it. The diameter of the ball channel is the specified diameter of the ball valve. Typical sizes for amateur rockets is from one-half to one inch.

Valves used to control the flow of cryogenic fluids, such as liquid oxygen, require special consideration. First, the fluid within the channel of a closed ball can expand when warmed and cause valve failure. To avoid ball over-pressurization, a drainage path is provided. A small vent channel is drilled into the ball at a right angle to the main channel. It is positioned on the exit side of the valve so that when the valve is in the off position, the channel fluid will drain to the exit, thus vacating the ball.

Second, the opposite problem is that of freezing the ball in place in the valve so that it cannot be moved when actuated through the stem. The Percheron rocket and an Amroc hybrid rocket both failed because the valve heater wires, instead of being connected to a heating circuit, were disregarded. Residual water in the oxidizer tanks (left over from cleaning the tanks, plus condensed moisture) collected at the bottom of the tanks and drained into the valve plumbing. Because the closed valve is the lowest point, the accumulated moisture froze around the valve seating, immobilizing the valve. The lesson is: either make sure no moisture is present in cryogenic valves or else heat the valve to keep the water in a liquid state.

Low-cost commercial ball valves ordinarily do not have a valve heater built into them. Heating tape can be used instead, wrapped around the valve and powered from a ground electrical source while on the pad or test stand.

Brush and synchronous permanent-magnet motors have greatest torque at stall (zero speed). This matches well with the (zero-speed) stiction of valves. (Stiction is static friction.) Once the ball begins moving, the friction decreases markedly, as does the required torque to turn it.

The size of motor required depends on the required torque and the actuation speed. The product of torque and rotational speed determines the motor power, which relates to its size:

For on/off control, the ball is rotated 90 degrees. For cryogenic valves, rotation must be bidirectional, to keep the drain channel on the exit-port side of the valve when off.

A typical motor will produce much less torque at stall and run much faster than desired. To trade off speed for more torque, a gearbox is used. A combination motor and gearbox is called a gearmotor. For the example, the gear ratio is 560:1. Gearbox output speed is 1/560 motor speed and output torque is 560 times motor torque. The example above used gearmotor output speed and torque values.

For non-cryogenic valves, a unidirectional controller can be used which rotates the ball 90 degrees each actuation. The schematic diagram of the control electronics is shown on the opposing page. This design uses two optical light-path sensors (D1/U1, D2/U2) to sense the valve stem position, by sensing the gearbox output shaft coupled to it. The sensors are placed 90 degrees (mechanical) apart. Each sensor has an infrared light-emitting diode (LED: D1 and D2) and a phototransistor with signal-processing circuitry (U1 and U2), mounted in the two arms of the U-shaped structure of the sensors, across from each other. The LED shines across the gap and activates the phototransistor unless something blocks the light path.

The two sensors are mounted on the face of the gearbox (see photograph) with hot glue, and a disk of transparent acrylic plastic sheeting rotates through the light paths. By placing opaque material, such as tape, on the disk, two slits can be formed that are used to detect the on and off valve positions. The disk is mounted to the gearbox shaft by hot-gluing it to a collar, which has a set-screw for tightening it to the shaft. The collar can be aligned so that the slits activate the sensors in the on and off valve positions.

The opto-mechanical scheme is shown below. Two slits at 180 degrees (in-line) allow 90 degree rotation. As the disk rotates clockwise, the slit goes into the OPEN sensor, stopping rotation. When the disk moves again (CW again), the slit goes into the CLOSED sensor, stopping motion again. The other slit is now positioned at 9 o'clock, ready to move under the OPEN sensor on the next move. The controller is commanded by being powered on, and each time power-on occurs, it moves 90 degrees, alternating between on and off positions.

In the electrical circuit diagram, resistors R1 and R2 power the LEDs, causing them to emit infrared light. U1 and U2, when activated by this light, cause their outputs (at pin 2 of the VTL10G3s) to be at a logic high level (near +12V(D)). The outputs are not internally pulled to the high state; R3 and R4 are "pull-up" resistors that cause the outputs to go to a logic high level when the circuitry driving pin 2 does not pull it low.

This controller has the simple interface of being commanded to change on/off state by being powered on. Consequently, power-on must be detected. The power-on reset circuit is based around comparator U5:C. When power is applied, the decoupled power supply, +12V(D), rises in voltage, causing U5:C, pin 8 to rise, through resistive voltage divider R6, R7. Until +12V(D) exceeds 6.2 V, Zener diode Z1 is off and pin 9 (noninverting input to comparator) is at 0 V. With the – input higher than the + input, the output of U5:C is low. U4:C inverts this low level and applies it to an RC differentiator, C6, R17. When +12V(D) is high enough to run the valve controller (> 6.2 V), the U5:C output goes high, causing a positive voltage spike on U4:D, pins 12 and 13. This spike is buffered by U4:D and applied to U3:A, pin 1. U3:A forms an RS FF with U3:B, and is toggled by alternating high-level inputs. The power-on circuit drives U3:A, pin 1 high momentarily; it is otherwise low, allowing pin 6 (the S input) to be toggled.

Initially, the disk is randomly positioned and neither slit will necessarily be under a sensor. Then both OPEN and CLOSED sensor outputs are low. U3:D and U3:C form an OR gate. (U3:C is an inverter.)

Logic gates are the elemental electronic components of digital and computer circuits. Digital circuits have two valid voltage levels (high/true/1 and low/false/0). The inputs and outputs (I/O) of digital components can therefore be referred to as 1s and 0s instead of voltage values, where voltages ranges for the two levels are specified.

The simplest logic component is an inverter, shown below. The "bubble" at the output indicates inversion.

When its input is 0, its output is 1, and vice-versa. To specify the function of such logic devices, a truth table is used, showing all combinations of inputs and their outputs. For an inverter – the logic NOT function – the truth table is:

A basic logic gate is the AND gate, shown below:

The truth table for an AND gate is:

In other words, the output of an AND gate is high (1) only when all (both) inputs are high. The output is low when any (either) of the inputs is low.

An OR gate has a high output if any (either) input is high. The OR gate symbol is shown below.

The OR gate truth table is:

An OR gate has a low output only when all inputs are low.

When an inverter follows a gate it becomes a NOT AND (NAND) or NOT OR (NOR) gate. The symbols for these gates is the same as AND and OR gates but with an inverter "bubble" at their outputs, as shown below.

Commercial gates are available as ICs. Usually, several gates are available in one IC package. The 4001 contains 4 NOR gates, designated as A - D.

When both OPEN and CLOSED are low at power-on, the S input (U3:A, pin 1) is momentarily pulsed high, but the R input (pin 6 of U3:B) remains low. When the S input returns low, the low R input (pin 6) allows the output (pin 4) to go high, turning on the motor. The randomly oriented disk will rotate until the slit is under one of the sensors and the valve is in either the closed or open state. Then either of the sensor signals, OPEN or CLOSED, will go high, causing U3:B, pin 4 to go low, turning off the motor. The valve is now in a defined state, indicated by the OPEN and CLOSED indicator outputs (connector pins 4 and 5).

On the next power-on, the R input (U3:B, pin 6) will be high because a slit is aligned under one of the sensors. This allows the RS FF S input (pin 1) to be pulsed high by the power-on reset pulse. Then pin 4 goes high and the motor turns on. When a slit moves under a sensor, pin 6 goes high, and with pin 1 high, the RS FF changes state and the motor is turned off.

The motor driver first stage is the U5:A comparator. When +12V(D) has come to its steady-state value of about 12 V, the R6, R7 divider outputs about 3 V to U5 pins 8, 6, 10 and 4. This voltage is between low and high logic-level voltages. When U5:A, pin 4 is high, it is at a higher voltage than pin 5 and the output, pin 2, goes low. This pulls the base of Q1 low, through R9 and Q1 conducts, turning on Q2 and the motor. R18 and R10 turn off Q2 and Q1. R9 limits the base current of Q1 and Q2, for the base of Q1 is but two diode drops down from the supply and the U5:A output is near ground. R9 sets the base current of Q1.

Inverters and gates are combinational logic devices because they merely combine inputs according to their logic function and output the result. Time is not really involved. Sequential logic devices have memory or states. (The idea of a logic state is essentially the same as that of a thermodynamic state in chapter 4.)

A flip-flop is a basic memory device that can store one bit of information. The simplest "flop" can be made of two inverting gates. NOR and NAND flops are shown below. For the NOR-implemented flop, assume that the Q output is 0 and /Q = 1. (The "/" is "NOT" so that /Q is the logic inverse or complement of Q.) The flop is in the reset state. Assume also that the S (set) and R (reset) inputs are at 0.

The S input logic level now changes from 0 to 1. This causes the upper NOR gate output to change to 0, so that /Q = 0. The /Q input to the lower NOR gate allows its output to change so that Q = 1. The flop has changed state, from Q = 0, to Q = 1. The Q input to the upper gate forces its output to remain 0 so that the S input no longer matters; it can be 0 or 1 and the flop state remains. The flop "remembers" the state it changed to regardless of the subsequent S input. Consequently, only a high pulse on S causes the flop to be set (Q = 1). A high pulse on R will now cause the flop to change state (be reset) so that Q = 0.

For the NAND-gate RS flop, works similarly, but with low input pulses. By alternately applying low pulses to its S and R inputs, the flop alternates between states.

Upon turn-off, current is flowing in the motor, which appears as an inductor in series with winding resistance. Inductors cause their currents to continue to flow – somewhere. Diode D3 provides a current path for motor current when Q1 and Q2 turn off. Without D3, the voltage across the motor would rise to maintain the current, causing possible voltage breakdown in its driver transistors. C5 filters out current and voltage spikes during current switchover to D3, which switches on and off somewhat slowly. (It is not a "fast recovery" diode.)

When the motor has stopped running (and is in either the on or off state), then pin 3 of U3:A will be high. If the valve is in the closed position, the CLOSED sensor output is also high and U4:A, pin 1 is high. When both conditions are true, the U4:A NAND gate output is low, causing the U5:D comparator output to be low, turning on Q3 and causing the CLOSED output indicator to go to about the voltage on the +COM input. Similarly, OPEN goes high when U4:B, pin 4 goes low. When the OPEN and CLOSED outputs are not asserted (near +COM volts), the R11 and R16 pull-down resistors keep the outputs near ground.

The extra connector input, +COM at pin 3, allows versatile interfacing to computers or other equipment. Most computers operate with +5 V logic, and +COM of 5 V allows the OPEN and CLOSED outputs to be TTL-level compatible, though the valve controller is run off an unregulated +12 V supply.

The shaft collars are available from McMaster-Carr Supply (Chicago tel. (708) 833-0300; NJ tel. (908) 329-3200). Penn one-piece steel collars are about $0.45 for the 5/16 inch bore (5/8 inch outside dia., 11/32 inch width) part number 6432K13. Troy sintered metal shaft collars (also 5/16 inch bore) are about $0.98 (part # 6166K22).

A commercially available valve controller, the Innovatia VC1, has an on/off command input line and on power-on, goes to a closed state by default. The VC1 is shown below.

Bidirectional valve operation is needed for proper use of cryogenic ball valves. These are valves with a small drain hole drilled into the ball at right angles to the main channel. When the valve is closed, cryogenic fluid that otherwise would be trapped in the valve can drain to the exit side through the drain hole. Without a drain hole, the fluid heats at ambient temperature, pressures increases and the valve seating bursts. To keep the drain hole on the exit side, the ball must be turned back from the opened to the closed position. A drain hole on the pressurized inlet side of the valve will not drain the ball channel.

A 12 V regulated supply is derived from the unregulated input power, +12V(U). If the power polarity is reversed, the power-bridge MOSFETs (Q12-Q16) conduct in the reverse direction and blow the system fuse (not shown). Also  included is a transient voltage suppressor (TVS) that is placed across the +12V input to ground. It clamps the supply if it exceeds the TVS voltage rating, and blows the fuse.

This design uses a full bridge to drive the gearmotor that couples to the valve stem, with four switches. When the voltage applied to the motor is reversed, the motor turns the other way. When diagonally opposed switches are on, the M+ motor terminal is +; when the other pair are on instead, M– is +.

If the MOSFET gates are not driven hard enough (that is, with a high enough voltage to turn the MOSFETs on hard), then the MOSFETs operate in their linear region with channel (drain-to-source) resistance that is too high and excess power is dissipated in them. The power MOSFETs are operated as switches – either off (very high R_DS) or fully on (very low R_DS). In both cases, power dissipation in R_DS is low.

Another fault protection mechanism, common to power circuits, is undervoltage lockout. When +12V(U) is less than 9 V,  the logic IC is held in a state of reset, which turns off the bridge transistors. As +12V(U) exceeds 11.5 V, reset is negated. What this accomplishes is to keep the VC1 in a reset state until the input voltage is high enough so that the MOSFET gates can be driven hard enough.

Yet another fault protection is the current-limit circuit that protects the MOSFETs from excessive current. While a system-level current limiter might do this, it will usually be too slow to act to protect the fast MOSFETs, and a faster current-limiter is needed. A small-resistance sense resistor  is the sensor of the current-limit threshold.  This provides a low level into the logic IC that causes it to turn off the MOSFET drive.

The main function of the VC1 is implemented in the logic IC and by Hall-effect device (HED) sensors. The logic IC is a programmable logic device (PLD), with 8 flop outputs and 8 logic inputs that are combined in the programmable section of the PLD. The flops are clocked at pin 1 by the oscillator that provides high-side gate drive to the all-N-channel power bridge.

The HEDs are positioned 90 degrees apart on the VC1 circuit-board, and are actuated by button magnets mounted on a collar on the output shaft of the gearbox. The HEDs are unipolar; they actuate (low output) only when one magnetic polarity is applied, and when it is removed, they negate (high output). The two HEDs are actuated by opposite-polarity magnetic fields, and the magnets are reversed from each other, so that only one magnet will actuate its corresponding HED. This allows absolute position sensing, and avoids what to the PLD are multiple opened or closed positions.

The open/close (O/C) command is input to overvoltage or reverse voltage protection circuitry. If O/C is open (no connecting signal), a default close command occurs. If the O/C connection breaks, the valves close.

Finally, the VC1 is but a subsystem component in the propulsion feed system, which is controlled by the flight computer. To increase reliability and controllability, the flight computer is sent signals indicating the opened or closed states of the valve, based on the HED shaft position sensors. The PLD outputs these signals through interface terminals, which can be connected to light-bulbs or LED indicators (for manual checks), or be used instead as logic inputs to the flight computer, according to a  jumper setting. The jumper is left in to power the OPENED and CLOSED outputs from the system supply (+12V(U)), and removed so that the COMMON terminal (J3) can be connected to the computer 5 V supply, making the two outputs logic-compatible with the computer. Current-limiting resistors protect these outputs from overcurrenting in case the outputs are shorted.