Dragonfly: Thermal Design Overview

ICES-2020-160 Dragonfly: Thermal Design Overview G. Allan Holtzman1, Robert F. Coker2, Carl J. Ercol3 and Douglas S. Adams4 Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723 and Loren C. Zumwalt5 Lockheed Martin Space Systems Company, 12257 S Wadsworth Blvd, Littleton, Colorado 80125 Dragonfly is a NASA New Frontiers mission that will send a rotorcraft lander to Titan, Saturn’s largest moon. Titan has a dense atmosphere and low gravity, making flight an ideal way to traverse its surface. Powered flight will be achieved by employing a battery charged by a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). The temperature of Titan at its surface is nearly constant but extremely cold at 94 K, so the lander’s thermal control system (TCS) will retain the excess heat from the MMRTG and distribute it throughout the lander body with a pumped fluid loop. During cruise to Titan, a cruise stage with a second pumped fluid loop will maintain spacecraft components and the MMRTG within allowable flight temperatures. The performance of the lander TCS as well as all components exposed to the Titan environment will be verified with testing on Earth in a Titan-relevant environment. This paper will provide an overview of the lander and cruise stage still-evolving thermal design details with a discussion of the planned thermal testing campaign. Nomenclature APL = Johns Hopkins Applied Physics Laboratory AU = Astronomical Unit CCHP = Constant Conductance Heat Pipe CFC-11 = ChloroFluoroCarbon fluid, also known as freon-11 EDL = Entry Descent and Landing EOM = End Of Mission HGA = High Gain Antenna I&T = Integration and Test IMU = Intertial Measurement Unit LIDAR = Laser Imaging, Detection, And Ranging LM = Lockheed Martin Space Systems Company MMRTG = Multi-Mission Radioisotope Thermoelectric Generator MSL = Mars Science Laboratory N2H4 = Hydrazine PSU = Power Switching Unit or Penn State University RCS = Reaction Control System RF = Radio Frequency TCM = Trajectory Correction Maneuver TCS = Thermal Control System TEL = Thermal Exterior Losses document TVAC = Thermal VACuum TWTA = Traveling Wave Tube Amplifier 1 Dragonfly Lead Thermal Engineer, Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723. 2 Dragonfly Instrument and Analysis Thermal Lead, Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723. 3 Dragonfly Lander Fluid Loop Thermal Lead, Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723. 4 Dragonfly Spacecraft System Engineer, The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723. 5 Dragonfly Cruise Thermal Lead, Lockheed Martin Space Systems Company, 12257 S Wadsworth Blvd, Littleton, Colorado 80125. Copyright © 2020 Johns Hopkins Applied Physics Laboratory and Lockheed Martin Space Systems Company I. Introduction On Titan, the molecules that have been raining down like manna from heaven for the last 4 billion years might still be there largely unaltered, deep-frozen, awaiting the chemists from Earth. — Carl Sagan, Pale Blue Dot Dragonfly will allow our Earth chemists to explore this organically rich, ocean world at multiple locations on Titan’s surface by flying through its thick atmosphere (surface pressure = 1.5 bar), aided by the relatively low Titan gravity (g = 1.35 m/s²). The Titan environment is extremely cold at 94 K (-179°C), and although the lander’s thermal control system must take these harsh conditions into account, they are known and constant, changing very little over diurnal or even seasonal cycles. A single MMRTG supplies not only the heat needed to survive the Titan and cruise environments, but also all the electrical power for the lander and spacecraft throughout the mission including spacecraft trajectory correction maneuvers and atmospheric flight on Titan, stored for use during these transient events using a large lithium-ion battery. Figure 1. Dragonfly rotorcraft lander, surface configuration with HGA deployed. In order for Dragonfly to soar majestically through the Titan atmosphere, it must first get to Titan. During cruise, and on the launch pad once the MMRTG is installed, a pumped fluid loop that runs parallel to the lander’s internal loop (intended only for surface operations) manages heat dissipation from the MMRTG, which is near 2000 W at the beginning of the mission. The pump assembly, which is similar to the lander pumps, runs at a constant power, while passive mixing valves on the cruise stage radiator segments adjust for thermal variations. The cruise stage fluid loop attaches to a heat exchanger port on the MMRTG to maintain the fin root temperature near its optimum power generation value. This heat dissipation will thermally control all cruise and entry vehicle components, such as the tanks, valves, and electronics, as well as the aeroshell that surrounds the lander. Electrical heater power is not required in normal operations and is minimized for transient events by this design approach. The radiator will have two zones to allow the system to adjust to changing environmental conditions as the spacecraft travels through the inner solar system and then outward to Titan and still maintain constant MMRTG and propellant tank temperatures. 2 International Conference on Environmental Systems Figure 2. Cruise stage thermal block diagram. Similar to the cruise stage thermal control system, the lander’s thermal control system harvests heat from the MMRTG and distributes it throughout the body of the lander using a pumped fluid loop, but is completely separate from the cruise stage fluid loop, which will have been jettisoned with the rest of the cruise stage on arrival to Titan. The fixed amount of heat provided by the MMRTG used protect the lander’s internal components from the cold Titan thermal environment will be preserved by carefully designing the level of thermal isolation between the lander interior and its environment. External components on the lander will be designed to have survival temperatures < 94 K, such that they will not require survival heaters during hibernation. The performance of the lander TCS as well as all components directly exposed to the Titan environment will be verified with testing on Earth in a Titan-relevant environment. Figure 3. Lander thermal block diagram. II. Lander Thermal Control System The heart of the thermal control system for the lander is the MMRTG. Heat generated by the MMRTG is distributed throughout the lander body using a pumped fluid loop to maintain a typical thermal environment for its internal components. An insulating layer of Rohacell™ foam protects the interior of the lander from the cryogenic 3 International Conference on Environmental Systems and convective Titan environment by fully enclosing the MMRTG, battery, electronics and most science instruments. Structural attachments, such as the mobility motor arms, landing legs, wires, camera and LIDAR windows, and other penetrations to the insulating foam layer, are designed to minimize the heat leak from the lander interior to the external environment. Some components, such as the mobility and sampling system motors, are outside the lander interior and are designed and tested to survive in the Titan environment, with preheaters (in some cases) to reach their minimum operational temperatures. The thermal insulation for the lander is a 5-cm-thick layer of Rohacell™ 31 HF foam, which is a closed-cell rigid foam based on polymethacrylimide chemistry with an approximate density of 31 kg/m3 and extremely low dielectric constants with particularly favorable RF transmission properties at high frequencies. The effective thermal conductivity through a 5-cm-thick layer of foam from 0°C (approximate lander interior) to −179°C (approximate Titan ambient) is ≤0.035 W/m/K, verified by testing at the Johns Hopkins Applied Physics Laboratory (APL) in a flight-relevant environment. The thermal conductivity of the foam is less than the datasheet value for this application due to the decrease in material thermal conductivity with decreasing temperature. The external surface temperature of the lander, including the outside of the MMRTG enclosure, is greatly attenuated by the foam insulation, resulting in lander skin temperatures only 10–15°C above ambient (worst-case hot condition assuming no wind). A thin skin of aluminized Kapton will be bonded to the outer surface of the foam for grounding during Titan surface operations and handling during lander integration and testing (I&T). The lander pumped fluid loop distributes the MMRTG heat, which is predicted to be 1670 W EOM, to components within the lander body. The loop consists of a 1-cm inner diameter metal tube that runs along the length of the lander, up to the lander top deck, and back, attaching to most boxes and the battery with flanges on the tube, typically at the base of each box to simplify I&T (particularly for the battery). The heritage working fluid for previous MMRTG missions is CFC-11 at 2 L/min, and a design trade is currently in progress for Dragonfly to determine the optimum fluid and fluid loop architecture, for flight and for testing, for the lander and cruise stage, that takes into account thermal performance, temperature limits, pump power, fluid availability, safety, government regulations, reliability, radiation susceptibility, etc. The lander fluid loop will use one of the heat exchanger ports of the MMRTG, as done on Mars Science Laboratory (MSL) [Paris et al., 2004; Bhandari et al., 2013]. This approach will maintain the MMRTG fin root temperature near its optimum level. Thermal trim devices, located one on each side of the lander near its aft end, will provide a Vernier-style adjustment of the lander internal temperature, allowing for precise control of the MMRTG fin root temperature on Titan. The thermal trim devices conceptually consist of an aluminum sheet metal, covered with a 5-cm-thick layer of Rohacell™ foam, which opens using a linear actuator to expose the outside of the lander interior wall directly to the Titan environment.

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