I have completed modeling the two types of large pyrotechnic fuel valves used on the Viking '75 Mars lander.
- One type is known as “normally open” (NO), which is manufactured and assembled to allow fuel to flow through the valve. When triggered, the NO valve will permanently close, preventing any further fuel flow.
- The other type is “normally closed” (NC), which is manufactured and assembled to prevent fuel from flowing through the valve. When triggered, the NC valve will permanently open, allowing fuel to flow.
Pyrotechnic valves were chosen for Viking (and most other spacecraft) because of their high reliability, low weight, and small size. They are one-shot devices which cannot be reset.
Seen in these renderings and photograph of a test lander, the valve bodies are approximately T shaped. In this case (adjacent to the lander’s fuel tank 1), the inverted-T valve on the left is the normally-closed type; the upright-T valve on the right is the normally-open type. Fuel flows from right to left in these images: out the bottom of the large spherical tank, then through the valve pair and on toward the lander’s three Terminal Descent rocket engines (used to soft-land the spacecraft on Mars).
During launch from Earth, cruise to Mars, and the first few minutes of entry and descent into the Mars atmosphere, both valves are in their initial as-manufactured configuration. The NO valve on the right allows pressure-fed fuel to emerge from the tank, until the fuel reaches the NC valve on the left. The NC valve ensures that the lander’s three rocket engines cannot prematurely ignite, even in the event of a closed-but-leaky throttle valve on an engine. (An additional NC valve is located right in front of each rocket engine, as a redundant factor to prevent early firing.)
When it is time to fire the three terminal descent rocket engines (when the lander descends to about 4000 feet above the surface of Mars, with less than a minute until landing), the NC valve on the left is triggered to permanently open. (The three NC valves in front of each engine are simultaneously triggered to open.) Fuel can then flow into the throttle valves of each engine, which open as directed by the lander’s computer and sensors to decelerate the spacecraft and perform a soft landing.
The moment that any of the lander’s three footpads touch the surface of Mars and trip an electrical switch, the NO valve (on the right) is triggered to permanently close. This ensures that additional fuel cannot reach the engines (even in the event of a leaky throttle valve), guaranteeing engine shutdown.
The lander has a second fuel tank with a similar pair of NO/NC valves. Here is an exploded view of a valve pair:
Visible just right-of-center is a wedge-shaped object that is located within the body of the normally-open (NO) valve. When triggered, that wedge is driven downward (as oriented here) puncturing a thin wall section of the fuel line and sealing off both sides of the cut.
Just left of center is a flattened cylinder with a large hole in it, located within the body of the normally-closed (NC) valve. When triggered, that ram is driven upward (as oriented here) to shear off discs that close the inlet and outlet sides of the fuel line. With the discs rammed out of the way, the hole (“port”) in the ram aligns with the inlet and outlet fuel lines to enable fuel to flow. Here is a close-up view with labels:
Each of these valves is actuated by a pair of redundant Viking Pressure Cartridges (VPCs), which are in turn triggered by a Viking Standard Initiator (VSI):
Seen above horizontally are a VSI on the far left, and an exploded (forgive the pun) VPC at center-right. The VSI has a three-pin bayonet plug for an electrical cable on the left end, and threads on the right for mounting into the pyrotechnic device. For some devices, the VSI produces sufficient gas pressure to actuate the device (such as “pin-pullers”). For these valves, however, much more force is needed - provided by the Viking Pressure Cartridge. An initiator threads into the top of a VPC body (at center). Within the VPC is a two-stage explosive charge represented by the tan and dark gray cylinders.
An electrical impulse triggers a small explosive charge in the VSI. The resulting hot high-pressure gas enters the top of the VPC and bursts a thin disc that protects the VPC charges. The VPC charges then ignite, producing a large blast of high-pressure gas that ruptures the bottom sealing discs and flows into the body of the valve. Hollow passages within the valve body (not seen here, but included in the SketchUp model) direct that gas to flow to the head of a piston on the valve’s wedge or ram. A pair of red O-rings surrounding each piston ensure that the gas presses on the piston head to force it into the valve body, actuating the valve.
Next to be modeled with be some of the electrical wiring, and a maze of small-diameter fuel lines for system fill, bleed, and supply to small Roll Control thrusters.
