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The Search for Life Using Submersible Heated (SLUSH) drill is a hybrid, thermo-mechanical Europa probe under development through NASA SESAME project. SLUSH utilizes a mechanical drill to break the formation, and heat to partially melt the fragments to form slush. The resulting slush behaves like liquid despite being partially frozen, enabling a significant reduction of the power required for melting the full volume of ice and to clear the ice. As the probe moves down, the slush freezers behind it. SLUSH is 5 m long and 57 cm in diameter, it has a heated drill bit in front and antitorque system on the side. SLUSH uses the Kilopower reactor for both thermal and electrical needs. The fission reactor can be turned on/off and is self-moderating, significantly reducing the risk of overheating if poorly conductive layer is encountered. There are two architectures to facilitate communication between the probe and surface lander: tether and wireless. The tethered approach provides high bandwidth and the exact depth of the probe (from the reel encoder). If a fiberoptic element is used, the fiber can be used to measure the thermal gradient, stress, and pressure. If tether breaks, the broken microfilaments can be used as an antenna for so called “Tunable Tether” approach. Tethers are used in wire-guided missiles, torpedoes, and drones; as such, they are robust and can fit into small tubes. We are not pursuing RF communication under COLDTech since other groups are investigating it already. SLUSH has several spool bays on top that are left behind in the ice once the spool is depleted. This allows each tether section to be purpose-designed. For example, the top section, which may see 150 kPa shear stresses on a diurnal cycle, will be reinforced with Kevlar. Leaving the spools behind also shortens the probe length making penetration more efficient. It should be noted that refrozen channel left behind as the probe descents repairs pre-existing cracks and creates a protecting ice cylinder for a tether. During SESAME funded project, we investigated “Tunable Tether” approach for communication. We found that fiber optic and coax cables are not suitable options. However, our models demonstrate the effectiveness of a balanced pair (two impedance matched parallel conductors) that can recover communication when the tether breaks, even with a channel gap (>1 cm) and a rotational offset of 45 and 90 degrees. We also did not find a pronounced increase in loss with the balanced pair with misorientation as we did with the coax approach. Our wireline drills (AutoGopher and WATSON) used tether hence we have experience with this technology as well. Per COLDTech call, we propose technology to enable communication through many kilometers of ice thickness. Building up on SESAME results, under this COLDTech we would: 1. Perform material trades for the tether (e.g. Kevlar, Vectran, pure copper wire etc.), perform shear and tensile tests in warm and cryogenic ice. Downselect ideal material. 2. Fabricate tethers (inner core components and sheath), perform shear and tensile tests in warm and cryogenic ice while measuring data rates and channel losses. 3. Design and fabricate TRL 5 spooler and attach to a SLUSH prototype (manufactured under SESAME project) 4. Perform tests in warm and cryogenic ice. 5. Refine Tunable Tether models and perform tests in warm and cryogenic ice Kris Zacny (PI) will be responsible for the overall project. He will be assisted by Systems and Communication Lead (Bolek Mellerowicz), Engineering Lead (Joey Palmowski), Chief Engineer (Gale Paulsen), and Project Manager (Dean Bergman). Europa ice shell models will be provided by Sam Howell at JPL. Tunable Tether work will be done by Mike Tipton from Integrity Communications.
Developing Instrument or spacecraft technology to improve measurements for future planetary science missions
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