NASA team designing sub to explore Titan’s seas
An engineering team at NASA’s Glenn Research Center (GRC) in Cleveland, Ohio, is designing one of the most unusual vehicles in NASA’s history – a submarine.
No, NASA has not taken over for the Navy. This is not just any submarine. This submarine is designed to explore an environment never-before-seen by humans – the liquid hydrocarbon lakes and seas of Saturn’s largest moon, Titan.
“The early explorers explored the seas on ships here on Earth,” said Steve Oleson, leader of the NASA Glenn team designing the sub. “But I think they missed a lot of stuff. So, for Titan’s seas, I said why not a submarine?” Oleson proposed the concept of the Titan Sub to NASA’s Innovative Advanced Concepts (NIAC) program and was awarded funding to develop the design.
The current mission outline has the sub arriving at Titan sometime in the 2040s when the northern region of Titan goes into its summer phase. The sub would splashdown aboard a USAF X-37 spaceplane, which would then sink and release the sub into the sea. The approximately 1-tonne (2,200 lb) sub is an autonomous submersible vehicle, powered by two Stirling radioisotope generators that utilize the heat produced from the natural radioactive decay of plutonium-238 and convert it into electricity. The sub is designed to carry out a 90-day, 1,200-mile (2,000-kilometer) voyage through Titan’s largest northern polar methane/ethane sea, called Kraken Mare.
Instruments aboard the sub would allow scientists to measure the trace organic components of the sea, which may exhibit prebiotic chemical evolution. A benthic sampler will collect and analyze sediment from the seabed. Examining the sediment, along with side-scan sonar readings of the seafloor’s morphology, may reveal evidence of historical cycles of filling and drying of Titan’s seas.
The Cassini spacecraft has made multiple flybys of Titan since it arrived at Saturn in 2004, sending back imagery and data of Kraken Mare and the moon’s other vast northern polar hydrocarbon seas. Cassini’s radar has measured the depths of these bodies, some at only a few meters, while others, such as Kraken Mare, measured more than 660 feet (200 meters) deep, the maximum depth the Cassini radar can penetrate.
However, creating a sub that can survive, travel, and successfully operate in the cold temperatures and strange conditions of a cryogenic sea was the challenge of the sub’s designers. Meeting those kinds of engineering challenges are the specialty of the COMPASS team at NASA Glenn.
COMPASS stands for Collaborative Modeling for Parametric Assessment of Space Systems. It is a multidisciplinary team of engineers who use a face-to-face concurrent design process to develop spacecraft designs through integrated vehicle systems analyses. Specialists in propulsion, power, thermal environments, command and data handling, communications, guidance and navigation, all come together in an intensive developmental process. The COMPASS team is similar to the Jet Propulsion Laboratory’s (JPL) Team X and Goddard Space Flight Center’s (GSFC) Mission Design Lab. However, there is one significant difference.
“We mainly do design reference missions for new technologies,” said COMPASS team leader, Steve Oleson, “whereas Team X and Mission Design Lab at Goddard, they’re doing actual proposals for Discovery missions. They’re very new technology averse. Here, it’s all about the technology. If you have a new power system or a new propulsion system, how can that revolutionize how things work? Well, if you don’t run it through a design reference mission, not only do you not know what it’s good for, you don’t really know what other requirements go along with utilizing that technology. A new power system may sound really good in the laboratory, but when you actually put in on the moon or somewhere else, maybe we didn’t think the thermal problems would be that bad. Or if we need to bounce-land it, we didn’t know the g-forces would be that much. So it goes back and forth.”
This back and forth, face-to-face, integrated analysis process, is essential to develop successful designs. Oleson insists it is all about having the right people. For the Titan sub design, as with other projects, Oleson was quick to bring in the perspectives and specialization of people from other centers.
“I called Michael Paul at Penn State,” Oleson said. “He had worked on a Venus lander with us. And I knew they did torpedoes and all kinds of stuff there.” Paul is an engineer with Penn State’s Applied Research Lab.
“I had some experience in the past working on Titan mission concepts,” Paul explained, “and the Applied Research Lab at Penn State is very forward in the field of submarine design, atomic vehicle design, propulsion, control, all those things that need to go into a submarine, which is not typical of space labs.”
Paul made another important recommendation.
“In my previous Titan work, I had worked closely with Ralph Lorenz,” Paul said, “and I was able to convince Ralph to join us as well.” Lorenz is a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory (APL). “Ralph is one of the leading Titan scientists on the planet,” Paul added.
“Michael (Paul) said I have the perfect scientist for you,” said Oleson, who took Paul’s advice and recruited Lorenz to be the primary scientific investigator on the Titan sub design. “So the three of us in just a few minutes kind of got it put all together.”
Paul and Lorenz worked well with Oleson and the COMPASS team at NASA Glenn Research Center.
“I had a lot to say about how to balance the different resources that were available,” Paul said. “Volume, mass, power, and to maintain a balance between those things while still being focused on the science goals that Ralph as the chief scientist for the project was saying we need to accomplish. And this (the Compass team) is a really awesome environment to do some of this thinking in […] so there is a lot of really good intellectual interchange in these concurrent design environments.”
Paul was also able to bring one of his Penn State colleagues, Justin Walsh, along on the project. “Justin is a submarine designer,” Paul said, “so his contributions were specific to the buoyancy control and the thrust from the propellers and attitude control and so on.”
“We don’t usually design boats,” Oleson said, “let alone a submarine. So we got the guys from Penn State who designed unmanned underwater vehicles. However, that’s water they are in. But what’s this stuff we’re going to be in? Liquid methane? Liquid ethane? Very different.”
“This is a completely foreign sea to us,” Paul concurred. “This is as alien as you can imagine. This is such a different pressure, temperature, and composition range from anything that we’ve ever floated before; that some basic assumptions that you would make when designing an autonomous underwater vehicle like this go right out the window.”
The strange environment of Titan’s methane/ethane sea threw one of its biggest curves at the team as they tried to design a ballast system for the sub.
“On Earth, you have the atmosphere and ballast tanks where you pump things in and pump things out,” Oleson said. “We were going to do that on Titan because Titan has two atmospheres of nitrogen, and that sounds great. However, the temperatures there are so low, when you get down to [a] pressure of about 100 meters or so, the nitrogen collapses back into a liquid.” With the nitrogen collapsed into a liquid, the sub would be unable to use it to pump the sea’s methane fluid out of the ballast tanks to rise to the surface. “So, our solution is to bring neon tanks and have a piston system,” Oleson said, “and you basically pressurize with neon and push out the ballast methane/ethane so that you can float. Neon won’t collapse at the temperatures we’re talking about.”
This realization of the properties of nitrogen in Titan’s environment led to another discussion relating to its effect on the operation of the sub. Titan’s methane/ethane seas have been there for millions of years, which means significant amounts of the moon’s nitrogen atmosphere has been dissolved into that methane/ethane liquid.
“What the heck is going to happen to all that dissolved nitrogen?” Paul wondered. “When we turn a propeller in this, there could be so much dissolved nitrogen in the methane, the sub could be pushing through so much bubbles, it’s going to be like driving through a glass of Alka-Seltzer – where all the bubbles are being formed and the nitrogen is coming out of solution, and we don’t have anything for the propellers to push against.”
The Titan sub project completed its NIAC-funded first design last year. It was approved for further NIAC funding this past spring and is now moving into Phase-2 design work, allowing the COMPASS team to further explore solutions to the dissolved nitrogen issue, and develop the sub design in greater detail.
In Phase 1, the team designed the sub as a stand-alone vehicle, not communicating with any other spacecraft or orbiter. Atop the sub, running the length of the vehicle, is an upright panel that appears to be a solar array.
“That is a phased array antenna that will talk directly back to Earth,” Oleson explained. “In the 2040s, it will be summertime on Titan, and the Sun will always be in the sky. And within 6 degrees of the Sun will be this little planet called Earth. So all you need is a sun sensor to point there, and you point your antenna and you talk back to Earth. We will do about 8 hours of submerged science, and then 16 hours of cruising on the surface, where we can do surface observations and talk back to Earth.”
With the newly approved NIAC Phase-2 funding, the COMPASS team will now explore additional possibilities for the sub’s design and mission.
“In Phase 2, we’ll also do a COMPASS design on an orbiter-based submarine,” Oleson said. “How does this design change if you base it on an orbiter? Some things get a lot easier. The delivery is different. We could even entertain not using radioisotope power. There is a lot of fuel where we’re landing. So if I could take enough oxidizer along, I could burn it in a fuel cell and get heat off of that. Problem is, I need a ballast system, and then I’d have all this oxidizer to deal with.”
The COMPASS team will continue to work these problems – a prospect Michael Paul looks forward to with excitement.
“To do this first order sort of work, if I can call it that, where we’re saying, fundamentally – What does this environment look like? – is for me as an engineer a real sense of exploration and discovery,” Paul said. “I’ve always had that in other work where we’ve gotten our spacecraft to fly and to see new things that we can’t see here on Earth. That’s always been wonderful for me. But to be able to do that, years in advance of a mission, to essentially discover something new about the way that world works, is really exciting.”
The work that Oleson, Paul, Lorenz, and the COMPASS team are doing on the Titan Sub design may lead to an exciting mission of discovery on Titan in the 2040s. Their design work may also be paving the way for other subsurface explorations of alien seas and lakes in the future in other parts of the Solar System.
Video Courtesy of NASA Glenn Research Center
Michael Cole is a life-long space flight enthusiast and author of some 36 educational books on space flight and astronomy for Enslow Publishers. He lives in Findlay, Ohio, not far from Neil Armstrong’s birthplace of Wapakoneta. His interest in space, and his background in journalism and public relations suit him for his focus on research and development activities at NASA Glenn Research Center, and its Plum Brook Station testing facility, both in northeastern Ohio. Cole reached out to SpaceFlight Insider and asked to join SFI as the first member of the organization’s “Team Glenn.”