Dragonfly: Thermal Design Overview

ICES-2020-160

Copyright © 2020 Johns Hopkins Applied Physics Laboratory and Lockheed Martin Space Systems Company

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.

International Conference on Environmental Systems

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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.


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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


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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. The pumped fluid loop will be routed to the lander interior wall at these locations to maximize

the heat rejection capability of the device while physically separating the lander interior from the Titan atmosphere.

Because one trim device is at the fluid input to the MMRTG and the other is at the MMRTG output (and at the input

to the battery), the devices can be offset from one another to bias the MMRTG temperature warmer or colder

relative to the battery, in addition to biasing the lander internal bulk temperature warmer or colder. The trim devices

will be designed to leak only a minimal amount of heat to the external environment when fully closed. The size and

heat rejection capability of the trim devices will be determined during the typical detailed design process going

forward, and will be verified with component and lander level thermal testing.

The battery assembly is attached to flanges on the fluid loop to keep the battery above 0°C during cold survival

conditions and provide cooling for scenarios during which the battery dissipates only a moderate amount of heat.

The high thermal capacitance of the battery keeps its bulk temperature below its upper operational temperature limit

of 35°C during transient high heat dissipation cases, which only occur during atmospheric flight on Titan. The

battery bulk temperature only rises by 10°C during 30 min of powered flight, during which many lander internal

component heat dissipations (including the battery discharge) are at a maximum. To control temperature gradients

across the battery cells during these events, aluminum fin extrusions are placed between the cell stacks, with small

tube-axial fans envisioned to reject the heat to the lander interior, using the atmosphere that Titan generously

provides. The fans would operate only on the surface and only during atmospheric flight, and can be commanded to

turn on before the flight begins to precool the battery if needed (keeping in mind that the battery cannot be charging

if its temperature drops below 0°C).


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Most internal electronic boxes in the lander are mounted to the internal structure and dissipate heat to the

structure as well as the internal atmosphere (at Titan atmospheric composition but lower density due to the higher

temperature). Some higher heat dissipating boxes, such as the power switching unit (PSU) and inertial measurement

units (IMUs), are attached in close proximity to the fluid loop to maintain their cold survival and hot operational

temperatures within limits. The mobility rotor drive electronics will also have aluminum fin extrusions and fans to

limit their temperature during flight operations. A few internal items, such as the X-Band traveling-wave tube

amplifiers (TWTAs), are mounted directly to the lander interior top deck and dissipate heat to the wall by

conduction and to the interior fluid by convection (with one leg of the cruise stage fluid loop going to the top deck,

which passes through the lander and MMRTG, to limit the maximum operational temperatures of the TWTs during

cruise, during which fans will not be used).

The internal portion of the high gain antenna (HGA) gimbal assembly is thermally connected to the fluid loop,

either by the loop going across the top deck or by using constant conductance heat pipes (CCHPs) to make the

thermal connection (on Titan, the orientation of the pipes are conveniently appropriate for reflux operation). The

portion of the gimbal assembly that extends above the lander top deck is insulated with a 5-cm dome of Rohacell™

foam. The insulated dome is designed to allow the HGA dish to articulate in azimuth and elevation, and the

attachment between the HGA gimbal assembly and the dish is thermally isolated to limit the heat leak across that

interface. Conduction through the HGA gimbal stack, along with natural convection, keeps both HGA gimbals well

above their cold survival temperature limit of −100°C. A small 10-W preheater is used for 15 min to raise the

gimbal actuator temperature to ≥-40°C before operation and can drop to 5 W to hold that temperature during HGA

operation.

The lander has eight body-mounted cameras, which require holes in the insulation for transparent windows. The

survival temperature for these cameras is −135°C, so the heat leak associated with each one will be minimized by

isolating the cameras from the lander interior with a 2.5-cm-thick covering of Rohacell™ foam. The conduction

from the cameras to the lander exterior surface will be tuned such that the resulting camera hibernation temperature

is just above their survival limit, thereby minimizing the resulting heat leak to the Titan ambient. Thermal analysis

indicates that a single window pane is optimal for this configuration, which has been confirmed by preliminary

component-level testing. Each camera will have a 35 W preheater to raise the camera temperature before operation.

Most cameras, once preheated, will maintain acceptable temperatures due to their internal operational heat

dissipation, and the others will need a small amount of preheat to remain applied during operation.

Two panorama cameras are mounted to the structure of the high gain antenna, with the window of each camera

resting on the external surface of the lander’s top deck when the antenna is stowed. The side and back of the

cameras’ packaging will be insulated with a 5-cm-thick layer of Rohacell™ foam, with a thermally-isolated

structural attachment to the antenna. Thermal isolation through the lander’s top deck foam insulation at these

locations will be decreased by just enough to allow the right amount of heat to leak into the cameras to keep them

above their minimum survival temperature of −135°C during lander hibernation. The cameras will require a small

amount of heat when deployed to maintain their minimum operational temperatures when in use, and their minimum

survival temperatures if not in use but still deployed.

The lander thermal control design exposes a limited number of components directly to the Titan environment,

including the mobility motors, sampler drills and blowers, and landing gear. Special design and operational

considerations have been made for these components. Thermal modeling shows external components, such as the

landing struts, reach equilibrium with the Titan environment before initial landing, which will be verified with

component and lander level thermal testing.


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Figure 4. Lander external (left, cold survival) and internal (right, cold survival and hot operation)

temperatures on Titan.

The lander’s mobility motors are designed to survive in the Titan environment and require no survival heater

power. The motor bearings will be preheated with ~90 W each for 5 min before operation to a temperature of

−65°C. The heaters are located near the center of the motor, directly adjacent to the bearings. The internal structure

of the motor is largely titanium, so the windings will be colder, but this is desirable to limit the internal resistance of

the motor. During flight, the motor can dissipate up to 300W of heat but the temperature remains very cold, just

above the motor minimum operational temperature limit, depending on the exact motor operational profile. Unlike

motors of this type used in terrestrial applications, the Dragonfly mobility motors have no cooling fins. The cold

survival, preheating, and hot operational performance of the motor have been verified with preliminary thermal

testing in a Titan-relevant environment at APL and Penn State University (PSU), and the design of the mobility

motor will be evolved during Phase B and verified with testing.

III. Cruise Stage Thermal Control System

The Dragonfly spacecraft thermal design builds on concepts proposed by Lockheed Martin (LM) and APL in the

NASA New Frontiers Step 1 & 2 mission design architectures. The cruise stage fluid loop relies on MMRTG waste

heat to regulate cruise stage, lander avionics, and aeroshell while maintaining MMRTG temperature throughout

cruise period. The cruise stage design employs curved radiators and multi-layer insulation closeouts to provide

efficient entry system solar shading over the mission extremes of 0.615–9.07 AU. Figure 5 depicts the integrated

Dragonfly spacecraft thermal model which incorporates an APL Step 1 lander model that has been modified for

cruise simulations to assess hardware margins and set initial thermal conditions for the EDL assembly and lander.

No electrical power except fluid loop pump power is permitted or required by the cruise stage thermal subsystem

except for the reaction control system (RCS)/trajectory correction maneuver (TCM) propulsion and telecom

hardware elements to support TCM and telecom transients.


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Figure 5. Integrated thermal model is being used to validate configuration and CONOPS.

A detailed cruise stage fluid loop concept has been implemented in the Dragonfly system thermal model to

validate the zero-heater power cruise thermal control approach and efficient distribution of MMRTG waste to

protect cruise stage and lander hardware elements while maintaining the MMRTG within its required operating

temperature range from 0.615 to 9.07 AU. See Figure 6. The cruise stage pump assembly is currently located in an

open bay below the MMRTG to minimize fluid line losses and pressure drops. Two parallel fluid control zones are

split off from the pump assembly to regulate the fuel tank (blue) and open (green-magenta line) bays. Each of these

fluid control zones contains four parallel sub-circuits to provide uniform control temperatures for the hardware

elements within each zone. This is especially important as maintaining uniform temperatures across the four fuel

tanks promotes equal N2H4 propellant usage from each tank and principle axis alignment over the course of the

mission. Independent, passive mechanical fluid bypass switches are deployed on each radiator segment to

autonomously regulate fluid temperatures as solar loading, spacecraft loads, and MMRTG waste heat output

changes over the mission. The aeroshell radiator indicated in Figure 6 is used to warm aeroshell interior and

backshell interface above historical flight allowable limits. The 4-96% fluid bypass valve control range is employed

with a refrigerant pumping at 2L/min throughout the cruise period. To further trim fluid temperatures, the open bay

zone is disabled beyond 3.5AU; this may or may not be necessary in the final implementation. The defined fluid line

length in the two-zone series-parallel fluid network is just under 150 ft when both zones are enabled. Development

and optimization of this fluid network and associated hardware implementation is ongoing.


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Figure 6. Cruise stage cooling loop concept implemented in Dragonfly system thermal model.

Entry analysis mimics the changing flight configuration and environments illustrated in Figure 7. Event timing

and environmental inputs follow the approach successfully used by LM in support of the Mars Phoenix and InSight

missions with the substitution of a powered descent by the Dragonfly rotocraft lander in lieu of the propulsive

braking using the N2H4 descent engine burns on these Mars heritage missions. Aeroshell bondline temperature

profiles are provided by the LM and NASA Ames aerothermal team for the entry, descent, and landing (EDL)

trajectory and aeroshell thermal protection system (TPS) design baseline. These are combined with the aeroshell re-

pressurization and atmospheric convection effects to estimate aeroshell and lander temperatures from entry interface

through backshell separation. The Dragonfly thermal model organization by spacecraft element as shown in Figure 8

permits the seamless transition between physical configurations across the EDL phase boundaries.


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Figure 7. Entry and Descent simulation facilitated by Dragonfly system thermal model.

Figure 8. Integrated thermal model elements combined to simulate system configurations from launch to

powered descent.


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IV. Thermal Analysis and Test Campaign

System-level thermal models for the Dragonfly spacecraft and lander will be used to verify the thermal control

designs. Thermal analysis cases for the spacecraft thermal model include launch, low-AU cruise, high AU-cruise,

transient trajectory correction maneuvers, and Venus flybys. Thermal analysis cases for the lander include the Titan

surface hibernation survival (cold case) and transient surface operations (hot cases, which utilize the thermal

capacitance of the components within the lander as a heat sink). Preliminary thermal models for the spacecraft and

lander show positive thermal margins for all components.

A Thermal Exterior Losses (TEL) budget has been created for Dragonfly and is used to manage the heat leak

through all interfaces that penetrate the foam insulation, such as structural mounts, antennas, science instruments,

and camera windows. The lander on Titan will be immersed in a cryogenic, convective environment and special care

will be needed to account for all the ways heat can leak out of the system, and thermal engineers will need to be

there every time a hole, wire, window, or other leak is created in the protective foam layer, to assess the impact of

the design details. The amount of heat the lander has to stay alive is a fixed value, slowly decaying over time to such

a point (well after the primary mission) that the lander will reach a state of permanent non-operation (freeze to

death). During the early stage of the project, contingency is applied to the budgeted heat leaks, with additional

uncertainty factors applied to heat leaks that are determined by analysis (in some cases up to 80%), which will be

reduced as the design matures and heat leak values are informed by testing. The TEL manages this “death by a

thousand cuts” threat that Dragonfly faces, to ensure that the lander thermal control design will be robust enough to

survive the Titan environment.

The lander TCS can be tested before flight with high confidence, because the thermal aspects of the Titan

environment can be conservatively replicated on Earth in a full-scale lander thermal demonstration testbed that will

include all relevant interfaces. Natural convective effects within the lander interior are similar on Earth as compared

to Titan, with the changes to density and gravity largely canceling each other. Cold survival for the lander is

determined by external forced convection at the maximum Titan wind speed, which can be replicated with fans in a

high-pressure thermal chamber (currently being installed at APL) at the appropriate speed. The Titan thermal

environment is constant, changing very little during the course of a diurnal cycle or even seasonally, within ±2K

[Cottini et al., 2012; Jennings et al., 2016], and the heat output from the MMRTG is a known, predictable quantity

as a function of time. Final adjustment of the lander TCS will be made using planned changes in the effective foam

thickness to tune the lander internal temperatures to the desired level, which will be biased slightly warm, allowing

the trim devices to fine-tune the temperatures once on Titan. The trim devices could be fully closed late in the

mission to conserve the diminishing heat output of the MMRTG, offering the possibility of a mission extension.

MMRTG simulator thermal balance tests will be conducted at APL in summer 2020, and are extremely

important tests for Dragonfly. Both the cruise and lander thermal control designs hinge on the amount of heat

harvested from the MMRTG by a fluid loop, and so thermal balance tests will be used to verify the thermal model

predictions. An MMRTG simulator will be placed in a Rohacell™ foam enclosure, and run in a TVAC chamber

with a flight-equivalent fluid and flow rate, at a cold wall temperature similar to that of the cruise internal aeroshell

environment. Temperatures will be monitored for the entire system, including the inlet and outlet fluid temperatures,

and the resulting heat balance will be verified. The MMRTG simulator will then be placed in a Titan thermal

chamber and run at a range of pressures, up to 1.5 atm, to understand the MMRTG heat balance for the lander. The

amount of heat extracted from the MMRTG for both the cruise stage and the lander will then be verified, a critical

feature to put heat leaks per the TEL in context.

A lander full-scale engineering model will be constructed in Phase C that will host all critical heat leak

interfaces, such as the motor arms, insulation panels, HGA gimbal assembly, etc., so that each of these heat leaks

can be measured in context of a full-scale lander geometry and adjustments can be made as necessary to ensure a

robust thermal control design. Final testing with the flight lander will be conducted in a Titan-relevant environment,

with planned final adjustments being made to the insulating foam to achieve a precise thermal balance for the lander.

The Titan approach, entry and descent transient conditions with regards to the removal of the cruise radiators may be

examined in a Titan chamber by running a transient thermal case that bounds the expected timeline, to ensure that

the lander is robust to this scenario.


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System level thermal balance testing of the integrated spacecraft (cruise stage, entry system, and the Dragonfly

lander) will be completed in a representative cruise vacuum environment using an MMRTG simulator to

demonstrate the flight interfaces between the major system elements and generate the thermal performance data

required to correlate the system level model which will be used to perform many of the system thermal performance

requirements for spacecraft, cruise stage, and entry system. Test requirements, critical data elements, and thermal

performance limits are still under development.

V. Conclusion

Titan has been waiting a very long time for our chemists to arrive, and it’s high time we got them there. The

Titan environment is extreme, but is a known quantity (Titan is not like a box of chocolates – you know what you’re

going to get), and this, coupled with the deterministic nature of the MMRTG heat output and the testable, adjustable

nature of the Dragonfly thermal control design, gives high confidence that the mission will be successful. The solid

design concept will of course undergo the typical detailed design process going forward, and along the way will

avoid “death by a thousand cuts” by use of the TEL, which will account for and track each heat leak. Atmospheric

flight on another world is a very cool thing…

Dragonfly, the coolest mission ever flown!

Acknowledgments

The Dragonfly mission is supported by the NASA New Frontiers Program under contract to The Johns Hopkins

University Applied Physics Laboratory. The authors acknowledge NASA and the Dragonfly Project Team for

support in the preparation and presentation of this paper.


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Dragonfly: Thermal Design Overview

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