(666b) Designing an Extraterrestrial Submarine for Titan. I. Nitrogen Solubility Modeling in the Titan Hydrocarbon Seas

An extraterrestrial submarine is currently being designed to explore the depths of the hydrocarbon rich seas on the Saturn moon Titan under the NASA Innovative Advanced Concepts phase 2 study. Cassini flyby data compiled over the past decade indicates that Titanâ??s northern pole sustains stable seas of variable concentrations of ethane, methane, and nitrogen at a surface temperature of 93K and 152 kPa pressure. Meanwhile the atmosphere near the seas is primarily gaseous nitrogen, with small amounts of gaseous methane. The average density of the Titan seas is half that of Earth while Titan gravity is 1/6 that of Earth. Other than Earth, Titan is the only other known body in the solar system that sustains stable, accessible seas. The driving force for exploration of Titan is both geological and astrobiological in nature, to determine if hydrocarbon based life is possible on Titan and to improve our understanding of the history and evolution of hydrocarbons in the solar system.

A one year exploration mission is baselined to study the seas of Titan. Multiple vehicle concepts were studied, but a submarine proved to be the most robust option. To meet science exploration objectives, the submarine must operate autonomously, study atmosphere/sea exchange, interact with the seabed at pressures up to 10 atm, traverse large distances, hover at the surface and at any depth within the seas, and be capable of tolerating wide variations in Titan properties. Results of the Phase 1 study are available in the literature [1], along with detailed information on the trade studies and preliminary design of the power, thermal, and ballast control subsystems for the Saturn Titan submarine [2]. In short, a 200kg, 1000W Stirling Radioisotope Generator system was baselined for power, yielding 3800 W waste heat into the seas. Aerogel insulation, a pump coolant loop system, and cold plates were baselined for thermal control to maintain internal science and communication electronics at ambient temperature. A high pressure gaseous neon bottle and pressure-fed system with mechanical separation was used to control the amount of liquid inside the ballast tanks to control the vertical ascent and descent of the sub. Mechanical separation was required to avoid solubility issues of neon dissolving in the ethane/methane seas and to avoid impurities when recovering the pressurant.

Critical to the design and operation of the submarine is knowledge of the amount of dissolved nitrogen gas within the cryogenic liquid ethane and methane seas. Solubility is defined as a property of a solute to dissolve in a particular solvent. Solubility is a special case of vapor/liquid equilibrium (VLE) in a subcritical mixture. Estimates taken during the initial trade studies showed that GN₂ had a relatively high solubility of 4% in pure liquid ethane at the surface; solubility of air in water is negligible (<0.1% at 1.03 MPa) during terrestrial submarine operation. The solubility of GN₂ in liquid methane is expected to be even higher. It is to be determined if heat leak rates from the power source into the liquid are high enough to cause effervescence. However, dissolved nitrogen gas that comes out of solution will cause several problems for the submarine, in particular, there are three concerns:

1. In a quiescent case, GN₂ bubbles could interfere with sensitive science measurements while the submarine is hovering.
2. In a moving case, GN₂ bubbles that form along the craft could coalesce at the aft end of the submarine and cause cavitation in the propellers.
3. GN₂ that come out of solution in the liquid inside the ballast tanks could hinder pressure control and thus affect vertical control of the submarine. Accurate solubility models are therefore needed to mitigate these concerns.

This presentation presents brief details on the Phase 1 design of the submarine, along with thermal design concerns to be addressed in the Phase 2 design. Then, a review of the literature is presented to gather relevant VLE data for binary nitrogen/methane and nitrogen/ethane mixtures. It is shown that there is a large volume of VLE data for nitrogen/methane and nitrogen/ethane mixtures at higher pressures and temperatures typical of those incurred during transport and refinement of liquefied natural gas here on Earth. However, VLE data for nitrogen/methane/ethane mixtures are poorly represented in the literature, especially near Titan sea conditions near the freezing point. Analysis of the data does in fact confirm that nitrogen solubility is higher in liquid methane, and that solubility increases with both increasing pressure and decreasing temperature. For pure liquid methane, at 152 kPa and 93K, the solubility of GN₂ in liquid methane is about 17.5%.

This work was funded through the NASA Innovative Advanced Concepts (NIAC) Phase 2 Titan Submarine Project.

References

[1] Oleson, S., Lorenz, R., and Paul, M. â??Phase I Final Report: Titan Submarineâ? NASA-TM-2015-218831 July, 2015.

[2] Hartwig, J.W., Colozza, A., Lorenz, R.D., Oleson, S., Paul, M., and Walsh, J. â??Exploring the Depths of Kraken Mare â?? Power, Thermal Analysis, and Ballast Control for the Saturn Titan Submarineâ? Cryogenics 74, 31 â?? 46. 2016.