MISSION CONCEPT: that may be the mission to KRAKEN MARE SEA in 2038!
Possible MISSION TIMELINE
The choice of a 2047 landing date on Titan’s Saturn moon it’s not only to ensures continuous lighting conditions for surface imaging, but also allows for direct communications with Earth.
Why? From the Kraken Mare, Earth is never more than 6° from the Sun. As such, it was decided to not use an orbiter (like Cassini) for the mission and so, it we possible to double the isotope power system of the submarine.
HOW COMMUNICATE TO EARTH?
It will be possible to communicate Directly to Earth (DTE System) when the sub is on the surface. Despite the power available, the DTE antenna would need to be large to covering about 1.2 billion km to Earth.
Because the distance from Earth to Titan vary from 8 to 11 AU (where 1 AU = 149,661,688 km), a round trip communications between those two can take 67 to 92 minutes.
The Submarine’s concept design features a ‘sail’ or ‘dorsal fin’ above the hull with a 4 X 0.5-m fixed X-Band Phased Array Antenna. This antenna is partitioned into 3.5 X 0.5-m for data transmission and 0.5 X 0.5-m for receive.
By its shape, the antenna can greatly increase the sub’s drag when submerged and, the power not anymore needed for communications (~250 W), can be used for the propulsion system.
Below, we see the sail structure mounted vertically to the top of the hull. Antennas are patched on both sides of that structure and a deployable mast is mounted just in the front. Placed on the top of this mast, a half-meter mast contains the science (Surface) Imager and a Meteorology Sensor and, just in the middle-way below, an X-Band Omni Antenna on each side.
Titan Submarine External Components
For the communications system, 330 W has been allocated on a 24 V DC S/C bus and, for down-link communication, 34 m DPN Earth station antenna of 34 meter will be used.
During moving or stationary surface operations, the sub communicates with Earth through the phased array antenna for high data-rate communications. Surface transits must be planned accordingly so that one side of the array can maintain Earth lock. If communications are lost, the sub will go to “All Stop” and use Sun sensor data to position the sub to re-acquire Earth via one of the X-band Omni antennas.
The two-high gain omni-directional antennas, one on each side of the mast will be used for emergency, at very low bit rate communications or, simply, for a ranging “I’m alive” signal.
Thus, the X-Band communications DTE system can give ~800 bps during 16 hr DSN passes each day on surface in using 250 W DC provided by Phased Array Dorsal Antenna. Also, the Dual X-band Omni Antennas can provide communication by an Autonomous Command and Data Handling (C&DH) for 16 hr/d on surface and 8 hr/d when submerged for exploration.
The Command and Data Handling (C&DH) system provide computer control and data storage for the submarine’s equipment and the aero-vehicle carrying it to Titan. This system has a single fault environment tolerant main processor rated for 50 krad total dose radiation. Enclosure inside the pressure shell of the Sub in a temperature of 280 K, it will be able to storage 25 Megabit/day of science data.
Methane is the primary ingredient in many of Saturn’s moon’s lakes and seas. Scientists have designed a submarine that can withstand the volatile chemicals that will be found in these methane lakes.
Credit: Science Channel
To provide propulsion and maneuvering below the surface, the sub will use four ~100 W motors attached on booms. This power provides up to 1.6 m/s when submerged and 0.9 m/s on surface speeds. These thrusters were chosen for redundancy to accommodate a motor failure, to eliminate the need for actuator/fins, to allow at sub to maneuvering at low speeds above and below the surface. For security purpose, the back provides easy access to the rear of the hull when the time will come to load the Stirling Radioisotope Generators (SRG) into the sub.
Location of navigation components
A Sun sensing camera is used to obtain the direction to the Sun. Using ephemeris stored in the onboard computer, the direction to the Earth is then calculated, and the vehicle is oriented such the phased array antenna is well pointed for direct communications. After, the vehicle obtains updates to its position and heading during this phase. A reasonable estimate on the expected accuracy for a position fix while on the surface is about 1 km.
Titan Submarine Internal Components
At the launch, the power needed will be 78 W and will go to 842 W during submarine activation and checkout. In fact, the average power requirements during multiday surface or subsurface cruise will be about 800 W and the bus voltage in the Sub will be 28 V (± 6). The Titan nominal minimum mission duration is assumed to be three months, until 3 years. Because of the long duration cruise (~ 10 yr) at the maximum power energy, the storage is not used for power peaking.
Because of the liquid ethane and methane sea temperature, heat rejection should be similar or superior to 4 K deep space operation for which most space power systems are designed.
For the Power System, the Stirling Radioisotope Generators (SRGs) are privileged because it can provide high power density, long life and low power degradation, proven flight heritage, 2X500W, 200 kg and, 3800W waste heat to be removed, where excess waste heat is available to maintain warm ambient temperatures for internal components.
For NASA, one of the prime requirement is to maintain the submarine interior at ~ 290 K to 310 K (17 °C to 37°C) while operating within the seas. To reach that, the approach will be based on a thermal balance between the heat generated by the SRGs heat used to provide power, and the heat loss surroundings the submarine exterior. The waste heat is then distributed from the insulation and the RTGs system throughout the interior to obtain the correct temperature difference with the exterior.
A pump loop coolant system to move the heat from the isotope system and distribute it within the interior. And, a cold plates and heat pipes to move the heat from the electronic sources to the interior.
The selected power system uses a pair of eight General Purpose Heat Source (GPHS)-SRGs. This type was chosen as the baseline power system, proving 900 W of DC power, for the 840 W uses in 13 yrs.
Consisting of tow Stirling convertors operating in a dual opposed configuration, the Submarine’s GPHS-SRGs system will minimize vibration. These convertors use a MarM-247 heater head with hot cycle temperatures of 760 °C and, we estimate the rejecter temperature at 120 °C at beginning of mission (BOM). Solid insulation surrounds the GPHSs drive the heat into the convertors.
Each SRG is about 1 m long and 36 cm in diameter with a mass of 65 kg. Heat is removed via a pumped loop system attached to the radiator housing. Overall heat in to DC power conversion efficiency at beginning of life (BOL) is 25 percent.
During the launch and transit, a heat removal system similar to that used on the Mars Curiosity rover is assume with a pumped loop system removing heat to a radiator located on the lifting body/cruise stage. Heat load for the Titan submarine is about 4,000 W.
SRG heat removal
The two SRGs are connected to the S/C bus and provide 26 (±) V. (See below the electrical architecture) Each SRG has its own shunt if no power is being drawn from the unit, just as in the SRG. An S/C shunt balances S/C power requirements and power generated from the SRGs. Additionally, a 6,000 μF bus capacitance via a 1 kg lithium ion battery is required to help control voltage fluctuations. This battery is bookkept under the PMAD for the sub.
The insulation covering the interior of the submarine – Specs: Insulation (aerogel foam)/3.0 cm thick - 300 W/m2 heat loss thru outer skin
Mechanical: Pressure vessel capable of withstanding an external pressure of 10 bar; titanium (Ti) skin and ring stiffeners; internal truss to carry equipment through launch; composite hydrodynamic fairing; dorsal sail to hold phased array antenna and surface science
Inside of the hull skin and its six ring supports are covered with a 3-cm thick layer of foam insulation to maintain the inside temperatures and minimize the heat that escapes it.
How we know it is that thickness we need? Because, inside, we have two large structural rings, one placed near the front and, the other near the rear. These structures cut the insulation and provide a heat sink between the inside and outside of the hull. This heat sink was accounted to determining the thickness of the insulation need to meet the thermal requirements.
Primary, these two large rings provide support for the internal structural “cage” and give the interfaces to all of the other internal components.
The structural “cage” located within the hull provides the mounting interface for all of the subsystem components contained within the pressurized hull structure. That thing is desired because the extra sensitives science electronics have to be far away from the SRGs to avoid any interference from it.
As well all the science electronics were located in the front of the hull and mounted in the front inside the “cage” structure, the UI Sensor Box is still placed to a panel. This location provides to the imager to look out a 4 in2 window located at the front of the hull.
The IMUs and electronics for the GN&C system are located directly behind the science stuff inside the cage along with the flight controller of the C&DH system. And, finally, just behind is the Power Management and Distribution (PMAD) electronics. At the end, we have the two SRGs.
Apart that distribution of materials inside the Sub, the biggest problem for operation activities is when it submerges in the sea.
Terrestrial submarines use various techniques from diving planes and thrust to ballast tanks filled and then ‘blown’ using compressed atmospheric gases to venture beneath the waves then returns to the surface.
One of the first steps in designing the buoyancy system for the submarine was to select a buoyancy gas to provide the lift needed to surface and remain on the surface for as much time as desired.
So, how we do that in Titan’s conditions? If we use a compressed gas ballast system, like the Titan’s primarily nitrogen atmosphere gas, it will be infeasible. Why? Because, ethane, and especially methane, can quickly absorb the nitrogen and, at –180 °C, it collapses to a liquid below 4 bar, limiting the depths at ~200 m. We need minimum 1,000 m!
That the reason why, in phase I, the interest is using cylindrical ballast tanks in Titanium with either free floating pistons or bladders pressurized by neon (Ne). The gas will be brought from Earth and reclaimed after each dive by a compressor during the 16 hr of surface operations.
Neon was selected since it is inert and will not react with the liquid methane. Also at the ambient temperature of 94 K it will remain gaseous even at high pressures.
The liquid volume displaced per ballast tank is 0.229 m3 and it can isothermally be compress. The ballast measure is 4-meter-long by a diameter of 0.27 m. When the Sub is on surface for its 16 hours’ communication, it required only a pump power less than 20W.
To be sure the Ne gas buoyancy approach work, it will be necessary to conserve it between each ascent and descent. So, it is requiring to be captured after ascent and re-pressurized prior to the next ascent.
To accommodate these requirements, a system was devised that utilized a Ne gas pressure tank, control valves, and a piston all housed within the main buoyancy tanks.
Buoyancy tank layout
The buoyancy tank is a large outer cylindrical tank, symmetric about this center axis and separated into front and rear halves. At the center of the are the neon pressure tanks, where on either side is a piston that can travel the length of the cylindrical portion of the ballast tank up to the spherical ends. Also, there are stops placed near the neon tanks and the cylindrical ends to limit the range of the piston motion.
In this conception, there are four control valves that regulate the flow of liquid methane and atmospheric nitrogen into and out of the buoyancy tanks. There are also control valves and a compressor that regulate the flow of the compressed neon into and out the tanks.
So, beginning at the surface, the pistons are fully extended up against the stops at the spherical portions of the tank. The main buoyancy tank body is filled with neon gas at a pressure of approximately 150 psi, which is the approximate hydrostatic pressure at the lowest desired depth.
On the buoyancy tank, the lower control valves, exposed to the liquid methane are closed and the upper control valves exposed to the atmosphere are open.
Green circle on top: Valves Open / Red circle at the bottom: Valves Closed
In this arrangement, the submarine is buoyant and will float on the surface of the methane sea. The surface operations will require approximately 8 to 10 hr, where during this time the submarine is preparing for its next descent.
To do so, the Ne gas is pumping back into the Ne pressure tank, going from 150 psi when the buoyancy tank is filled to 1,000 psi in the pressure tank.
Buoyancy configuration at the surface
In this arrangement, the submarine will begin to descend. To control the descent, the top control valves can be closed, trapping the nitrogen gas within the tanks and slowing or stopping the descent. The ability to control or stop the descent will depend on how much variation there is in the density of the liquid methane with depth.
Once all of the nitrogen gas has left the buoyancy tank, the submarine will descend to its maximum depth. Which one limit? This will be determined based on the weight and its volume, say how the density of the methane varies with depth. If there are no variance, the upper and lower valves can be left open or closed.
Neutrally Buoyant Operation
To begin the ascent, the upper control valves are opened. The valves from the Ne pressure tank are opened allowing the Ne gas to flow into the buoyancy tank and begin to move the piston toward the spherical end of the tank. As the piston moves, the liquid methane is pushed out of the buoyancy tank through the upper control valve. At this stage the lower control valves can be either closed or opened. As the submarine nears the surface they will also be opened so that the liquid methane can flow out the bottom as the upper valves break the surface and are exposed to the atmosphere. This will prevent the spherical ends from staying filled with liquid methane when the submarine is on the surface.
Buoyancy configuration at the surface
The final Sub design shown up has a mass of approximately 1,386 kg, long of 6 m with a 0.62 m diameter pressure vessel.
Because the closed Ne ballast tanks are external that will allow for submerging and hovering a depth possibility of 1,000 m at a pressures resistance up to 10 bar.
Hydrodynamic & Propulsion
A streamlined torpedo-like pressure hull is fitted with external cylindrical ballast tanks mounted high up on each side and covered with a free-flooded hydrodynamic fairing. Four fixed fins are mounted near the tail in an X configuration to provide hydrodynamic stability. Additional floatation and static ballast are distributed to provide hydrostatic stability. Small electric thrusters attached to the tip of each stabilizer fin provide propulsion, the thrust levels of each may be controlled independently to provide directional control.
Fluid properties at the surface were assumed to be those of liquid ethane at 94 K and 1.5 bar. The liquid density is assumed to be 660 kg/m3 and the kinematic viscosity of 147.6 lb/ft-hr resulting in a Reynolds number that is 1/6th that of an identical submarine at the same speed in terrestrial saltwater.
The surface temperature of Kraken Mare is estimated to be 94 K, with a liquid density that is 2/3rds that of saltwater and gravitational acceleration 1/7th that of Earth.The thrust produced on Titan would be at least 1/3rd less due to the lower fluid density.
Navigation using Inertial Measurement Unit (IMU), Sun direction, Earth tracking, liquid velocity Doppler, sonar scanning.
KRAKEN MARE EXPLORATION – MISSION SCENARIO
Once afloat, the submarine deploys its mast with the X-band Omni antennas, its MET and its surface imaging system. With its Sun sensor data, it obtains a lock on Earth and communicate its position and health status via one of the Omni antennas. A low-rate beacon is then activated. Contact with Earth last for approximately 16 hr.
During its first communications, the sub play back entry-decent-splashdown (EDS) data and system-wide housekeeping data. In the same time, it returns its first measurements of wind and current sea speed, air and sea temps from the MET and show its first images from Kraken Mare.
With these data received, Earth perform a health check to assess hull integrity and system-wide thermal balance.
The rest will come soon with others concepts missions and technologies, as balloons, space planes, rovers, etc.
CHECK FOR UPDATE!
Phase II Study Plans
The following is summary of conceptual design activities that would be conducted under a Phase II NIAC study
• Examine Science Payload in more detail (e.g., immersion-tolerant meteorology package; seabed sampling options, etc.)
• Evaluate subsurface communication DTE, and via Relay orbiter, noting RF transparency of some Titan liquids
• Evaluate impact on CONOPS and downlink data benefit of a relay orbiter, identify most useful complementary science from orbiter (e.g., could a space borne radar detect the sub or its wake?)
• Create a conceptual design of the aero-vehicle/Titan entry system and create a MEL for that system
○ Develop the details of SRG installation accommodations, procedures and GSE required
○ Assess thermal protection system capability to withstand Titan entry heating
○ Assess sub mounting in aero-vehicle
○ Assess systems for separation of aero-vehicle from sub after Titan splashdown
○ Assess detailed LV interfaces including LV adapter
• Perform trajectory design to determine LV performance requirement, transit time to Titan, and any necessary planetary gravity assist maneuvers required to assure sub arrival at Titan summer
• Assess other Titan entry systems based on entry speed and heating
○ Inflatable heat shield/decelerator
• Assess alternative methodologies for Titan descent and splashdown
○ Circular chutes and guided parasail landing system
• Assess alternate sub deployment schemes from entry system
○ Sub separation after splashdown
○ Sub separation before splashdown (separate splashdowns of sub and entry system)
• Assess required targeting to assure landing on Kraken Mare
• Analyze communications systems requirements for the trans-Saturn cruise phase, and the Titan EDL phases of the Titan Submarine mission
MISSION TIMELINE / WWW.NASA.GOV
NASA Innovative Advanced Concepts - NIAC
Launch 2038 Trans-Titan/ Insertion
Interplanetary Cruise ~7 Years
Titan Atmospheric / Entry & Descent ~2.5 hours Kraken Mare Splashdown
Sub Activation & Checkout ~4 days
First Transit ~7 days
Map and Explore Kraken-1 ~90 days
Map and Explore Kraken-2 ~90 Days
Return to Ligeia/Mare and enter if possible ~120 Days
End Of Primary Mission
Cruise Day: • 8 hrs Submerged Science/Transit • 16 hrs Surfaced shore imaging/meteorology/data return