Viking '75 Mars Lander Construction, Looking for Viking lander design/construction information Options

[Tom Dahl]

Greetings all! I am searching for detailed construction and design information about the NASA Viking '75 Mars project hardware, particularly for the lander, aeroshell, base cover, and bioshield. Can anyone recommend good sources? I am especially looking for engineering drawings and under-construction photographs.

To set the stage, here is an album of about 100 drawings and photos which I've collected so far.

I have already read the "usual" books, such as NASA RP-1027 "Viking '75 Spacecraft Design and Test", the press kits, the scientific papers produced about the mission, a number of industry papers covering various instruments and subsystems, the major Martin Marietta books, etc. I am hoping to find additional sources. Any ideas?

Also, does anyone know if there are aeroshell, base cover, or bioshield components lurking in a museum or in storage somewhere?

FYI, I have visited three of the best landers still on Earth: The Proof Test Capsule in the Smithsonian NASM, the Flight Capsule 3 (backup) in the Museum of Flight near Seattle, and the Science Test Lander in the Virginia Air and Space Center. I've taken nearly 1,000 photos of the three of them (most of which are publicly available in other Picasa Web albums of mine). I've taken a few measurements, but I would dearly love to find more authoritative drawings of more hardware (interior, exterior, everything). I have begun submitting some Freedom of Information Act requests to NASA/JPL which has started to bear some trivial but kind of fun fruit.
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Update as of March 2017: During the past few years I have been fortunate enough to collect a significant amount of information on the Viking lander hardware. My thanks to a number of organizations for providing me access to their resources:

• Viking Mars Missions Education and Preservation Project
• The Museum of Flight in Seattle
• NASA Langley Research Center
• California Science Center
• Virginia Air and Space Center
• Smithsonian Air and Space Museum

Here is a list of on-line albums with Viking information that I have gathered. Essentially all the photographs - about 3500 currently - were taken by me (with the notable exception of those in the "Misc diagrams" album, which come from historic sources). The photos have descriptions that explain the major subject of each image. To see the descriptions with the current Google Photos web album interface, click on an image and then click the small letter i in circle (i) icon in the upper right corner. This displays an information panel that includes the description. You can also zoom and pan to see the full resolution image detail. Many images in a number of the albums show the 1000 or so measurements I have been able to capture from actual Viking hardware, thanks to the kind cooperation of the above folks.

Flight Capsule 3 in Seattle Museum of Flight (756 photos)
Dimensioned diagrams of the FC3 lander

PTC Lander at Smithsonian NASM 2013 (466 photos)
PTC Lander at Smithsonian NASM 2016 (888 photos)

Lander at Virginia Air and Space Center (622 photos)
Dimensioned diagrams of the VASC’s lander

Lander at California Science Center (456 photos)
Dimensioned diagrams of the CSC's lander

Misc diagrams, unusual photos (over 350 images)
Body assembly blueprints

Collector Head Shroud Unit at NASA LaRC (99 photos)
Biology instrument at Cleveland MoNH (36 photos)
Meteorology Sensor Assembly (60 photos)
Meteorology Electronics Assembly (22 photos)
Tape Recorder (53 photos)
High Gain Antenna photos and measurements (96 images)
XRFS Instrument (42 images)

Viking lander contractor historic scale model (14 images)
My Viking project documents collection

The main focus of my efforts during the past few years has been to create an accurate and high-fidelity digital 3D model of the Viking lander. I've chosen to use the SketchUp software to build the model because a near-full-featured free version is available, allowing other people to use my model. The 3D model itself, as a work-in-progress, is available via DropBox. I update that model file periodically as major elements get added. I've created an album containing numerous renderings of digital model components, and I have a YouTube channel with some videos about the modeling project. I have also uploaded the lander core body and the Surface Sampler Collector Head to the SketchUp 3D Warehouse so that other people can easily access those components (the 3D Warehouse can be accessed from within SketchUp, or via web browser). The file on DropBox lister earlier contains those components and others.
-- Tom

[Tom Dahl]

I have recently completed modeling the Viking lander’s two Radioisotope Thermoelectric Generators (RTGs). Here are some overall views:

Here is an overall cut-away view of the RTG:

The RTGs are a Viking-specific variant of the SNAP-19 type which was used on earlier spacecraft (Nimbus III, Pioneer 10 and 11). SNAP is an acronym for System for Nuclear Auxiliary Power, which is a series of generators stretching back to the 1950s. Odd-numbered SNAP designs convert the heat of natural radioisotopic decay directly into electricity. Even-numbered designs contain a nuclear reactor in which a radioisotope undergoes accelerated nuclear fission; the resulting heat is converted into electricity in a variety of means.

The Viking SNAP-19 RTGs were fueled with Plutonium 238 in the form of Plutonium Dioxide Cermet (rather than the more hazardous pure Plutonium 238 metal), which is 83% PuO2 and 17% Molybdenum. The fuel was shaped into 18 discs or pucks, seen on the far left in the following exploded views:


The RTG has no moving parts and is completely passive with no means of control. It weighs about 34 pounds and is a bit over 15 inches high (including the dome on top). The span of the six cooling fins is 23 inches across opposite pairs. There are 20,600 curies of fuel (equivalent to a few pounds) sufficient to generate 682 thermal watts from the heat of natural radioactive decay at the time of fueling (which was some nine months prior to launch). At the beginning of the mission the RTG produced about 42 watts of electricity at 4.4 volts DC. The conversion is quite inefficient but the generator has extreme reliability and robustness, and the excess heat was critical to the mission (heating the lander’s interior electronic components).

Electricity is generated via the thermoelectric effect, by which a temperature differential across a thermoelectric couple produces electricity. In the SNAP-19 generator there are a total of 90 thermoelectric conversion couples, each consisting of a P-leg and an N-leg:

The P-leg materials are 15% a mix of Tellurium, Antimony, and Silver, and 85% a mix of Germanium and Tellurium, given the designation TAGS-85, plus a Tin-Tellurium segment on the hot side (on the right in the above image). The N-leg materials are a Lead-Tellurium mix designated 3M-TEGS- 2N(M).

The 90 couples are grouped into six thermoelectric conversion modules of 15 couples each. The following image shows two of the modules in exploded views:

The couples are connected by copper straps. Pairs of couples are connected in parallel, and the pairs are connected in series. The couples in adjacent modules are interconnected by #9 gauge wires, forming a partial hex pattern above and below the RTG core that is visible in the overall exploded views. The full hex wire below the core is a magnetic compensation loop. The hot shoe (inboard end) of each couple is up against the graphite heat shield surrounding the fuel capsule (actually separated from the heat shield via a thin mica sheet for electrical insulation). The temperature of the heat shield surface - forming the hot side of the couple - is about 950F (about 510C). To maximize the temperature differential across the thermoelectric couples, a set of “cold end” hardware provides a thermally conductive path from the copper interconnect straps (which are soldered to the outboard ends of the P- and N-legs of the couples) to the relatively cool exterior RTG housing. The temperature of the exterior housing at the root of each of the six large cooling fins - which are directly adjacent to the six aluminum heat sink bars - is typically about 330F (about 160C).

Starting from the moment of assembly, the RTG power output gradually decreased due to a number of factors:

• Reduction in the RTG core temperature due to decay of the Plutonium fuel, reducing the temperature differential across the thermoelectric couples and thus the efficiency of the thermoelectric conversion process. With a half-life of about 88 years, this was actually not a significant factor during the expected single-digit-years lifetime of the mission.
• Sublimation loss of material from the thermoelectric conversion components due to continuous exposure to high heat.
• Increase in Helium gas within the RTG (produced spontaneously by decay of the Plutonium fuel). Helium has a relatively high thermal conductivity, which lessened the temperature difference across the thermoelectric couples and reduced their efficiency.

The effect of the Helium buildup was partly counteracted by a novel solution. The prominent dome on top of the generator, seen in section view in the following image, is a sealed reservoir filled with a 95% Argon / 5% Helium mix at assembly.

The generator’s cylindrical body was filled with a 90% Helium / 10% Argon mix. The initial high Helium fraction within the generator body was deliberately chosen to reduce the beginning-of-life power output. The months-long series of final RTG tests and storage prior to launch in Earth’s warm dense sea-level atmosphere would otherwise have produced RTG temperatures higher than desired. During cruise to Mars (in vacuum) and on the Mars surface (in a very cold low-pressure atmosphere) a higher energy conversion rate - and thus higher temperature differential - was wanted. The interface between the RTG body and reservoir dome was sealed as described earlier, but with an intentionally leaky seal. A dummy electrical receptacle with a Viton O-ring seal was installed between the body and dome. The dummy receptacle is the threaded object at top center of the ribbed RTG upper cover in the above section image. The O-ring permitted an extremely slow but effective gas exchange between RTG body and reservoir, raising the fraction of Argon (which is less thermally conductive than Helium) in the RTG and improving power generation.

A great deal of effort was applied to mitigate risks of Plutonium fuel release that might occur due to launch accidents (explosions) or unintended spacecraft reentry into Earth’s atmosphere (if the booster rocket did not impart sufficient Earth-escape launch velocity). The following cut-away image shows the RTG’s “heat source” (as the encapsulated plutonium fuel assembly was termed) in close-up:


To mitigate the effect of physical shocks and impacts, the fuel stack (olive-drab pucks) was encapsulated in a four-layer metal capsule consisting of the following, from inner to outer:

1. Inner Liner made of a Molybdenum-46% Rhenium alloy, 0.009 inch thick. This is formed of two slip-fit overlapping shells, in direct contact with the Plutonium fuel. (In order to exactly fill the volume of the capsule, Molybdenum shims were placed as needed between fuel pucks. The fuel capsule I modeled, VF-3, has five such shims, three of which are visible.)
2. Liner made of a Tantalum-10% Tungsten alloy, 0.020 inch thick. This layer is welded closed and provides the main decontamination and health-physics encapsulation for the fuel.
3. Strength Member made of T-111, a Tungsten-8% Hafnium-2% Tantalum alloy, 0.090 inch thick. This layer provides the primary impact resistance necessary in case the overall RTG or a separated fuel capsule fell from a great height onto a hard surface.
4. Capsule Clad made of a Platinum-20% Rhodium alloy 0.018 inch think, providing an oxidation barrier during capsule handling. Handling difficulties were exacerbated by the fact that the capsule’s outer surface was thermally very hot - nearly 1400F (about 760C). (The core of the Plutonium was almost 1900F, 1040C.)

To mitigate the extreme heating that could occur if the RTG were to reenter the Earth’s atmosphere at nearly orbital speeds, the above capsule was first enclosed in three pyrolytic graphite sleeves, and then within a large hexagonal heat shield made of AXF-Ql fine grained isotropic Poco graphite, enclosed on both ends with graphite caps attached via stub Acme threads. The graphite is tolerant of extreme heating. In fact two earlier-version SNAP-19B RTGs were inadvertently subjected to moderate re-entry conditions when the NIMBUS B-1 spacecraft to which they were attached suffered a launch abort about one minute after liftoff in May 1968. The RTG housings were completely destroyed, but the heat sources were found and recovered from the Pacific ocean floor in October 1968 and examined in detail. The graphite heat shields were essentially intact.

The SNAP-19 Viking RTG is marvelously simple in principle but subtly designed for extreme reliability.