Viking '75 Mars Lander Fuel Tanks

The next phase of work is under way for my long-term project to model the Viking '75 Mars lander: the fuel tanks and associated supports and fuel-system plumbing. I recently completed the fuel tanks and their supporting struts.

Published reference material is scant for the lander’s fuel system, but I was fortunate to be able to study in detail lander test units that are exhibited at the California Science Center, the Virginia Air and Space Center, and the Smithsonian National Air and Space Museum. It has been a challenge to reconcile differences between these test units, to decide what is representative of the Flight units that were launched to Mars in 1975. A few press photos of the actual landers during assembly (bought on eBay in recent years) have been helpful.

Here are a few more views (including just the bare-bones lander body for context). All the components are solid.

The fuel tanks (23.5 inches inside diameter) held sterilized hydrazine propellant. This powered the lander’s three Terminal Descent Engines which provided the force to slow the spacecraft during the final 45 seconds or so before touchdown on Mars. The hydrazine was also used to power four small roll-control thrusters that oriented the (immobile) lander such that it would face in the desired direction at landing. The TDEs and roll thrusters will be a future modeling effort.

Next to be modeled will be all the fuel and pressurant plumbing lines (tubes) and valves. I’ve captured many dozens of measurements from the test unit seen below, but it’s still going to be a challenge to make it as accurate as I can. The plumbing for the two tanks are somewhat mirror images of each other, which helps a bit.

Earlier posts on this project are linked below:


When you think it can’t get better it does ! Epic :+1:

Excellent work. The level of detail is amazing.

Fantastic work Tom! I’m a huge fan of this project.

I’m curious: what would the total part count be for the lander — and do you have a total of how many parts you’ve modelled to-date?

To date, the model consists of just over 1000 unique components. Hard to estimate what the total will be, assuming that I’m able to “finish” it; probably another thousand or two. Note that this is to represent essentially all of the externally-visible piece-parts of the lander hardware, plus selected items that are not visible (such as the two Radioisotope Thermoelectric Generators, hidden under large wind covers).

Regarding the actual Viking lander hardware, there are a lot of internal parts. Much of that consists of electronics assemblies (which mostly consist of discrete electronics components - resistors, capacitors, etc. plus some small 8-pin integrated circuits) and wickedly-complex wiring harnesses. But there are a few super-complex mechanical systems inside, such as the Biology instrument (which has many thousands of internal parts itself) and the Gas Chromatograph Mass Spectrometer. I’m not planning on modeling any of that (lack of energy, time, and reference material).

If I have any modeling energy when the “external” lander is done, the next aspect of Viking that I would model (some future decade) is the two-layer capsule in which the lander was sealed prior to landing on Mars - the inner layer of aeroshell (heat shield) and base cover (top-half), plus the outer layer of bioshield base and cover. Those components have complex mechanical systems, probably on the order of a thousand unique piece-parts. (That doesn’t include what was inside another mass spectrometer and other science instruments mounted on the aeroshell to take readings during the few minutes of descent.)

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I marvel at your persistence and patience — as well as your technical knowledge of the lander, and evident modelling skill.

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.


I like your work, the level of details and the technical description. Amazing.

Your dedication and modeling skills are superb. Trimble should be paying for images of your Mars Lander for publicity campaigns. Perhaps calling it
“ You thought SketchUp is a toy”

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I have added a model of the ITT Cannon bayonet connectors (with Glenair backshell strain-relief) that were used on the Viking lander’s pyrotechnic devices. The photograph of a partially-exploded connector is of a similar unit that I was able to purchase recently; that unit’s coupling nut (large-diameter shell with splined end) has a shoulder or step, unlike the type used on the lander. I also didn’t want to fully disassemble it, fearing permanent damage. I especially like the “wave spring” that is within the coupling nut, used to provide pressure between the coupling nut and the connector barrel or body, to keep a mounted connector in place. All the components are solid in SketchUp terms.

Here is a view of two such connectors attached to the normally-closed fuel valve that is mounted at the fuel inlet of one of the lander’s three Terminal Descent Engines (TDEs); the rocket engine itself has not yet been modeled. The red object is a protective tape wrap on all the lander’s external wiring, included here for completeness (though it sadly hides some of the knurling on the connector’s backshell!).

Here is a view of two connectors attached to the pyrotechnic pin-puller device at the bottom of a landing leg. The pin-puller was electrically fired shortly before landing to release the stowed or folded landing legs, allowing them to extend for landing on Mars.


Impressive work.

Hi Tom.

As a mechanical engineer involved in a lot of projects for piping design I can really appreciate the work involved in your models. It is not only amazing, it is astonishing. I like very much the exploded views of the piping components.

I read your resumé in the forum and saw that you are also interested in the space program. I was also interested in all form of space exploration, having followed all the missions of the Mercury, Gemini and Apollo programs. I also watched all the Space Shuttle missions that I could and I also watched with great interest the two Crew Dragon launches and rendez-vous with the ISS.

I like it when someone pushes SketchUp to the limit and beyond.

Keep up the good work going.


I have added the various fuel lines that serve fuel tank 1 on the Viking '75 Mars lander work-in-progress model:

The photograph is of a test lander. The details of the fuel-system plumbing on that test unit differ slightly from the actual Flight landers that went to Mars. I’ve modeled the Flight-type fuel system based on the limited reference material I have that actually shows the Flight lander plumbing.

The larger-diameter (3/4 inch) lines supply fuel from fuel tank 1 (the large sphere) to the lander’s three Terminal Descent Engines (TDEs, not shown). The smaller-diameter (1/4 inch) lines are for filling and draining the tank and to allow bleed-in through the system during pre-launch testing and preparations. The fuel lines and unions are hollow, the coupling nuts and other fittings are threaded, and everything is Solid, naturally.


The fuel lines and associated brackets and fittings that serve fuel tank 2 on the Viking '75 Mars lander model have been added. The general layout of lines and valves for tank 2 is similar (mirrored) to that of tank 1, but there are many exceptions. As is the case for fuel tank 1 posted above, the reference test lander unit (photographed in the Virginia Air and Space Science Center, which is also the official visitor center for the NASA Langley Research Center in Hampton VA, original home of the Viking Program Office) differs in various details from the two landers launched to Mars in 1975.


The Viking '75 Mars lander’s fuel system model is complete. Here are overall views of the work-in-progress lander (which is still missing many large external components) with the fuel tanks and all the lines and valves in place, along with the four roll-control thrusters that are mounted on the fuel tanks (two thrusters per tank). Front view:

Top view:

The next image is just the fuel system, without any other parts of the lander. There are about 300 unique Solid components in the fuel system: valves, lines, fittings, and roll-control thruster assemblies. The fuel lines (3/4 inch OD and 1/4 inch OD) were manufactured in sections, which were sometimes joined by welding and other times via couplings. The model represents each distinct section before welding or coupling.

The four small fuel lines in a vertical cluster right of center are for filling the fuel tanks with hydrazine (N2H4) propellant, which was done after the final two-day heat sterilization of the entire sealed lander capsule. The two right-most lines of the cluster are for the lander’s two fuel tanks. The two left-most lines of the cluster are to fill two additional fuel tanks within the aeroshell capsule that protected the lander (not in the model). The aeroshell had 12 small rocket thrusters that were used to brake the capsule out of Mars orbit. The pair of aeroshell tank fill lines are routed horizontally through a couple of turns, then go nearly vertically down to dead-end near the bottom of lander fuel tank 1 on the right. At the dead-end point, a pyrotechnic tubing cutter on the aeroshell severed the lines before the aeroshell was jettisoned, about a minute before landing on Mars.

Here are comparisons between the SketchUp model and the test lander unit at the Virginia Air and Space Science Center. This test unit differs in various ways from the actual Flight landers on Mars (and is missing some parts), but it was my primary reference (having taken many many dozens of photographs and measurements of it in 2014). Photographs of the Flight landers during build-up that show these details are extremely limited. Most historic photographs were taken after insulating blankets were installed that completely hide the tanks and nearby plumbing.

The two MR-50F roll-control thrusters (made my Rocket Research Company, now part of Aerojet Rocketdyne) mounted on each of the lander’s two fuel tanks were mentioned earlier. The MR-50F SketchUp model is available by itself on the 3D Warehouse. Here is a view of the thruster pair on fuel tank 2:

The purpose of the roll-control thrusters was to control rotation or roll of the lander about its vertical axis during the final minute before landing on Mars. A pre-planned heading was chosen so that the final azimuth at landing would face in the desired direction for optimal mid-day solar illumination of the Mars surface for camera imaging during the landed mission. (This worked as planned for both Viking 1 and 2 when they landed in July and September 1976, respectively.)

The thruster’s gold cylinders are thin heat shields, to limit the heat radiated to the rest of the spacecraft from the thrust chamber within the shields. The black cylinder on the inboard end of each thruster are fuel valves. The light gray thruster nozzle is just barely visible protruding from the outboard end of the heat shield, pointing outward and slightly down (to cope with being off the lander’s vertical center of gravity).

Here is an exploded view of a mounted thruster (which includes a one-piece machined aluminum mounting fitting at center, behind the thruster body):

An exploded view of a thruster itself:

And a cut-away view of a thruster:

Hydrazine propellant was pressure-fed into the end of a dual solenoid valve (into a cavity in the upper-left corner of the above image). The dark striped ovoid forming the right side of the cavity is a fuel filter.

Two identical solenoid-actuated valves are in series. Two valves were included for redundancy. The biggest worry was a failed valve that stuck open, letting the thruster run continuously and forcing an opposed thruster to also fire continuously to counteract the torque. In addition to flight control complications, this would deplete fuel at a rate faster than anticipated. It was extremely unlikely that both of the dual valves would fail open. A failed-closed scenario was less of a concern, because the equivalent thruster on the other fuel tank could do the job (and fail-closed of both equivalent thrusters was also very unlikely).

The two solenoid electromagnet coils are the gold objects with lines representing the coiled wire. When electrically actuated they pulled an armature disc slightly toward the coil (to the left in the above image). The displaced armature compressed a small tapered coil spring at the center of each valve which opened a poppet mechanism (tiny white disc) to allow fuel to flow through the center of the valve, toward the right.

The pressure-fed fuel would then travel through the tiny straight channel connecting the valves to the thrust- or reaction-chamber just right-of-center, at the core of the thruster (within the gold heat shield). The feed channel is split into three tiny branches. The three branches lead to two stacked “Rigimesh” discs which further disperse the fuel before it reaches the upper catalyst bed - the thick light gray disc with a pebble texture. This volume of the thrust chamber was tightly packed with Shell 405 catalyst, which is granules of aluminum plated with iridium - the active catalyst ingredient. When hydrazine comes in contact with iridium, it immediately decomposes into nitrogen gas and hydrogen gas (producing a lot of heat in the process). This is how a monopropellant hydrazine engine works. there is no combustion, no need for a separate oxidizer (thus, “mono” or single-propellant).

Two additional larger Rigimesh screens separate the upper catalyst bed from the larger lower catalyst bed. The lower bed is packed with a smaller granule size (more surface area) to promote a more complete reaction of the remaining hydrazine that passed through the upper catalyst bed without reacting. Two final Rigimesh screens at the bottom of the lower catalyst bed, just before the narrow throat of the reaction chamber and expanding exhaust nozzle, retain the catalyst granules within the thruster. The catalyst is not consumed by the reaction; so long as hydrazine is fed into the chamber, the engine will continue producing thrust.

The hot nitrogen and hydrogen exhaust gasses finally exit the thruster via the expanding nozzle, producing 8 pounds of thrust (in the Viking configuration). This type of engine cannot be throttled, it is either off or on at full power. It can be pulsed in very short operations (milliseconds) for fine flight control.

Recently I was delighted to acquire an actual MR-50F thruster reaction-chamber-nozzle component. I spent a lot of time measuring the unit. The following image shows how I measured a stack of successive diameters of the nozzle exterior, using acrylic shims to control the vertical offset of the measurement plane. To measure the nozzle’s interior diameter I made a resin casting of it, and then used the same technique on the casting. I also made a casting of the chamber throat interior to determine its angle of convergence (which turns out to differ from a published figure) and the curves connecting the convergent and divergent (expanding) portions of the nozzle.

The overall work-in-progress Viking '75 Mars lander SketchUp model remains available on DropBox via this link (warning, the file is ~260MB).