This article was first published in print in the September 2020 issue of Horological Times, the publication of the American Watchmakers-Clockmakers Institute.
Humans have long been fascinated by mechanical automata. In Greek mythology, Hephaestus, the god of blacksmiths and master craftsman of Olympus, created a large humanoid automaton named Talos to guard Crete. The Greeks themselves produced the Antikythera Mechanism, the earliest known example of a mechanical computer. In the 15th century Leonardo da Vinci became famous for his various automatic devices. But it wasn’t until World War II that mechanical computing devices reached a zenith with the advent of mechanical fire control computers before the transistor was invented in the 1950’s. Accordingly, modern spacecraft feature a dizzying array of computers, controllers, sensors, and other electrical components. But even the hardiest of these components cannot survive in one of the most extreme environments in the solar system: the surface of Venus.
Long referred to as Earth’s sister planet due to its similar size and location in the solar system, Venus is a much more perilous world. The surface is an extraordinary 860°F, hot enough to melt lead, while the atmospheric pressure is 92 times that of Earth, high enough to crush a nuclear submarine. In the history of space exploration only a handful of Soviet spacecraft have ever landed safely on the surface of Venus. Each spacecraft featured a large pressure vessel that kept sensitive electronics insulated from the Venetian atmosphere for a couple of hours, just long enough to send data back to Earth. Current mission proposals would only survive for up to 24 hours despite using state-of-the-art technology. To enable long term exploration of this exotic world a return to age-old technologies may be in order.
The Automaton Rover for Extreme Environments
The Automaton Rover for Extreme Environments (AREE), Figure 1, is a concept to enable long term exploration of the surface of Venus. AREE replaces vulnerable electronics with more robust mechanical systems. For example, on Venus power can be generated from the dense wind using a turbine, energy stored in springs, and motion transmitted to wheels using gearboxes. Basic mechanical computing and sensing, similar to the mechanical fire control computers of the 1940s, would be used to direct the rover. High temperature electronics, although not eliminated, are used sparingly due to their low computing capabilities. In short, AREE enables the exploration of Venus by utilizing clever mechanism design where it makes sense, and high temperature electronics where the technology is mature enough.
To take AREE from concept to reality, basic functionality must first be proven on a small scale. For AREE that means demonstrating complex mechanisms operating at Venus conditions. There are precious few examples of this in history. Some Soviet landers carried sample collection devices, but the design details for those missions have been lost to history. More recently, HoneyBee Robotics has successfully built electric motors and gearboxes for a Venus drill. However, no one has ever attempted to build a device as complex as a fully integrated rover, and starting with a mechanical clock provides an ideal proving ground.
The Venus Clock
A clock requires use of basic mechanisms such as springs, gears, and bearings, and is therefore a useful precursor to more complex devices. Furthermore, some sort of clock or timing device would be required on an actual Venus rover to synchronize actions of the rover and regulate power. At this early stage though the goal is simple: demonstrate that a clock can run at 860°F, build confidence in the AREE concept, and gain experience in building high temperature mechanisms.
Everything must be built out of something, so material selections was an early priority for the Venus clock. Fortunately, many options exist. Special alloys of steel are in use in jet engines and power generation turbines that can withstand temperatures in excess of the surface of Venus. Other metals, like Tungsten, have a melting temperature of over 6,000°F, and there are heat resistant ceramics that survive temperatures even higher than that. Unfortunately, none of those are affordable with a limited research budget. More common metals like aluminum are too soft and would nearly melt on Venus, but titanium and stainless steel alloys are a compelling middle ground: they have high melting temperatures but are also readily available. For this reason, all structural components on the Venus clock are made from 300 series stainless steel.
A spring powers the Venus clock much as one would power AREE. Springs store energy through elastic deformation, which means the spring material can return to its original shape and is not permanently bent. Many metals act as springs at room temperature. However, at higher temperatures temporary elastic deformation becomes permanent, energy is dissipated, and the metal is no longer a very good spring! Put another way, metals have a tendency to take a ‘set’ at high temperatures. One of the big surprises in this project was just how easily that happened. Common high temperature metals like stainless steel and titanium showed permanent deformation even when bent to only 10% of the yield strength (the point at which the metal is permanently deformed) and heated to Venus temperatures. More specialized alloys were investigated, and a nickel-chromium-iron alloy called Inconel 718 showed adequate performance at temperature. Inconel was selected as the spring material, but necessitated the procurement of an expensive custom spring.
The final power spring had an outside diameter of 3.25,” an arbor diameter of 0.5”, provided an average torque of 2.5 in-lbs, and had 27 turns. These numbers were selected to match the performance of other steel springs in testing, and also maximize the life of the clock in the oven as there is no way to wind it while in a test chamber. The spring is fixed on the outer end and attached to a rotating shaft on the inner end.
Like most mechanisms, clocks feature a number of rotating shafts. In the Venus clock, all shafts are oriented vertically. The weight of each shaft is simply supported on a point at the end of the shaft. This method was influenced by jeweled bearings which mitigate friction through reduction of contact area. The shafts are stabilized with bushings made from Graphalloy, a self-lubricating graphite-metal alloy designed for use in furnaces.
Mechanical clocks today are typically made of small, precise components. The Venus clock is not a typical clock. The device must operate over a wide span of temperatures and be fabricated from custom components with a tight budget in mind. These concerns are addressed by using larger components with relatively loose tolerances. That way, small differences in thermal expansion are less likely to cause the mechanism to bind. By loosening tolerances, cheaper fabrication methods can be used as well. Most Venus clock components were cut from sheet metal with a waterjet. Only the escapement and pallet were CNC machined. The clock is relatively large with dimensions of 7” x 7” x 4” due to the manufacturing methods used.
The basic layout of the clock is shown in Figure 2. The Inconel power spring sits on top and drives a gear train. The escapement is a simple pin-pallet design, known to be robust to manufacturing errors if not particularly precise. Historically, pin-pallets are relatively inexpensive and were a staple in low end watches until quartz technology filled the niche. Finally, the balance wheel meters the escapement through the use of another Inconel spring. This balance spring is a simple a strip of metal that engages a pin on the outside of the wheel.
Testing and Results
In the spirit of the design-build-test-repeat methodology, multiple iterations of the clock were constructed before coming to a final design. Early versions were made from laser cut acrylic plastic and 3D printed components to verify the design. After graduating to metal, testing occurred in a small oven that reached Venus temperature but not pressure. Initial results were promising but revealed some issues. Most notably, there was significantly more friction in the gear train after exposure to high temperatures than before, but the clock returned to its original performance after disassembled and put back together. Although the specific cause of this issue hasn’t yet been verified, a possible explanation is that the high temperature evaporated off any oils or other lubricating contaminates that could have reduced friction. There also could have been some other chemical or molecular change on the surface of the stainless steel. The problem was addressed by decreasing the gear ratio between the spring and escapement, thereby delivering more power to overcome friction but reducing the spring life.
After all the iterations and troubleshooting the Venus clock did indeed operate at 860°F, fulfilling the initial design goal. Just as importantly, it provided a base of experience with which to go after a more ambitious project: an actual rover capable of roving in a Venusian environment.
Applying lessons learned from the Venus clock, designs were made for a simple benchtop scale rover that could move forward, sense an obstacle with a bumper, and reverse direction, all at the surface temperature of Venus. AREE will not have cameras or sensors typical of mobile robots; these electronics would not survive the extreme environment. Instead, AREE senses obstacles by simply bumping them with a mechanical sensor, causing the rover to reverse and find a new path forward. The benchtop rover mimics this process.
The clock and rover share the same power spring and are both made of 300 series Stainless Steel, but the similarities largely end there. The power springs on the rover are mounted horizontally, releasing power into a planetary gearbox and then into the wheels. The planetary gearbox incorporates a reversing mechanism that triggers when the bumper is contacted. Also, instead of using bushings, the rover incorporates full ceramic ball bearings.
The planetary gearbox is configured such that when the rover moves forward, power is transmitted from the sun gear (the input) to the ring (the output) through stationary planets gears. This is a typical configuration for a planetary gearbox and causes the output to spin in the opposite of the input. When the bumper is depressed, all three components are effectively locked together and forced to rotate as one. The output now spins in the same direction as the input, and the drive direction of the rover is reversed. See the diagrams in Figure 3. After one revolution of the gearbox the system resets itself and the rover moves forward again. The completed gearbox assembly is shown in Figure 4.
The rover, Figure 6, is about two feet long and only 4 inches wide. The form factor was dictated by the Venus test chamber which is 8 feet long and 12 inches in diameter, allow the rover to roll along its considerable length.
During the test, the rover was held in place with a pin jamming the spokes on the rear wheel. When the test temperature obtained, the pin was pulled using a wire protruding out of the chamber. The rover then successfully moved down the chamber, contacted the end wall, and reversed direction. Figure 6 shows an image of the rover in the Venus chamber during a test that was performed only at Venus temperature. Note that the rover is in motion as evidenced by motion blur in the gearbox.
Traditional spacecraft often take decades to go from the drawing board to landing on another world and AREE is no exception. This development of a high temperature clock and rover has been immensely successful and sets the stage for more ambitious projects which, with a lot of hard work and some luck, may lead to the surface of Venus. At its core, AREE is a paradigm shifting idea. The concept forces individuals to reconsider what a spacecraft must look like and opens the door to more innovative uses of mechanical systems across the solar system. In this light, it is exciting to consider Leonardo da Vinci’s designs for helicopters, tanks, and even a small autonomous cart, and how his ideas continue to influence innovation today.
Jonathan Sauder was the PI on this project and, frankly, none of this would have happened without him. We would like to thank the AREE interns Tonya Beauty, Sean Dunphy, and Andrei Shahinian who worked tirelessly in the lab to bring these ideas to reality, as well as Nicholas Manousos and Kiran Shekar of the HSNY for providing input to the mechanical design. Countless others at NASA/JPL contributed ideas and motivation to this project, for which we are grateful. This work was funded by the NASA Innovative and Advanced Concepts (NIAC) program which funds early-stage transformative concepts for space exploration.
Notes and Further Reading
1. There is plenty of accessible information on the internet about the Antikythera Mechanism, for more of an academic treatment see: Freeth, T., Bitsakis, Y, et al. “Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism,” Nature, vol 444, Nov. 2006, pp. 587–591.
2. For a full discussion on various mechanisms that can be used as computing devices, see “Computing Mechanisms and Linkages” by Anthony Svoboda. An entertaining US Navy training film is also available on youtube under “Mechanical Computer — Basic Mechanisms in Fire Control Computers”: https://www.youtube.com/watch?v=s1i-dnAH9Y4&t=2180s.
3. For a more complete discussion on AREE, see the Phase 1 NIAC Report which is freely available online: Sauder, J. Hilgemann, E. et al. “Automaton Rover for Extreme Environments”
4. Kris Zacny’s paper “Development of a Venus Drill” covers both the history of the Soviet drilling efforts as well and HoneyBee’s recent efforts. A more easily accessed history can be found on Don Mitchel’s excellently researched website: http://mentallandscape.com/V_Venera11.htm.
Evan Hilgemann is a mechanical engineer at NASA’s Jet Propulsion Laboratory and also writes Explore & Observe, an email newsletter on modern-day exploration of earth and space.
Jonathan Sauder is a mechatronics engineer at NASA’s Jet Propulsion Laboratory and was the primary-Investigator on the Automaton Rover for Extreme Environments project.
This work was done as a private venture and not in the authors’ capacity as an employee of the Jet Propulsion Laboratory, California Institute of Technology. Any views and opinions expressed herein do not necessarily state or reflect those of NASA, JPL, or the California Institute of Technology.