Aerocapture design study for a Titan polar orbiter

Abstract

In 2014 a team at NASA Goddard Space Flight Center (GSFC) studied the feasibility of using active aerocapture to reduce the chemical ΔV requirements for inserting a small scientific satellite into Titan polar orbit. The scientific goals of the mission would be multi-spectral imaging and active radar mapping of Titan's surface and subsurface. The study objectives were to: (i) identify and select from launch window opportunities and refine the trajectory to Titan; (ii) study the aerocapture flight path and refine the entry corridor; (iii) design a carrier spacecraft and systems architecture; (iv) develop a scientific and engineering plan for the orbital portion of the mission. Study results include: (i) a launch in October 2021 on an Atlas V vehicle, using gravity assists from Earth and Venus to arrive at Titan in January 2031; (ii) initial aerocapture via an 8-km wide entry corridor to reach an initial 350×6000 km orbit, followed by aerobraking to reach a 350×1500 km orbit, and a periapse raise maneuver to reach a final 1500 km circular orbit; (iii) a three-part spacecraft system consisting of a cruise stage, radiator module, and orbiter inside a heat shield; (iv) a 22-month mission including station keeping to prevent orbital decay due to Saturn perturbations, with 240 Gb of compressed data returned. High-level issues identified include: (i) downlink capability-realistic downlink rates preclude the desired multispectral, global coverage of Titan's surface; (ii) power-demise of the NASA ASRG (Advanced Stirling Radioisotope Generator) program, and limited availability at present of MMRTGs (Multi-Mission Radioisotope Generators) needed for competed outer planet missions; (iii) thermal-external radiators must be carried to remove 4 kW of waste heat from MMRTGs inside the aeroshell, requiring heat pipes that pass through the aeroshell lid, compromising shielding ability; (iv) optical navigation to reach the entry corridor; (v) the NASA requirement of continuous critical event coverage for the orbiter, especially during the peak heating of the aerocapture when the radio link will be broken. In conclusion, although Titan aerocapture allows for considerable savings in propellant mass, this comes at a cost of increased mission complexity. Further architecture study and refinement is required to reduce high-level mission risks and to elucidate the optimum architecture.

The Composition and Chemistry of Titan’s Atmosphere

“…However, Dragonfly will only investigate the low latitudes, including dunes and the crater Selk. In the future, an orbiter, as envisaged by several published studies, − would provide the important benefit of a truly global picture, including potentially complete global mapping of the atmosphere and surface at uniform resolution, with time-domain information to search for changes occurring. Other elements, such as a balloon, airplane, and/or floating lake probe, could provide valuable in situ information about other environs. − …”

Analysis of Transfer Trajectories Utilizing Sequential Saturn-Titan Aerocaptures

“…This proposal aims to utilized aerocapture to enable the trajectory of a polar orbiter around Titan. Titan is a prime candidate for aerocapture and aerobraking maneuvers because it possesses a dense but also expansive atmosphere due to its low surface gravity and cold temperatures [12]. Aerocapture, which will be discussed in more detail in Section 2.4, is the capture of a hyperbolic trajectory into an elliptical orbit via passing through the atmosphere.…”

“…This process becomes more effective with atmospheres like Titan's with large scale heights. Titan's scale height in the upper atmosphere is roughly 60 km which is 5 times that of Earth or Mars [12]. The primary concern for this missions trajectory is arriving at the Saturn system with minimal energy and preserving geometry to allow for the direct Titan aerocapture.…”

“…This is a common gravity assist chain used to reach the system. This trajectory never enters a Saturn orbit and instead aims to reach Titan at the Saturn frame periapsis [12]. The final chosen trajectory has a 12 year time of flight and arrives at Saturn with a v ∞ of 5.28 km/s.…”

“…Following the methods discussed in Section 3.3, impact parameter break-point variation from changing v ∞ was examined for a range of v ∞ between 4 and 10 km/s. This range includes commonly used v ∞ from other Titan focused missions [4,12,15]. The impact parameter variation was performed with ballistic coefficients of 1, 60, and 180 kg/m 2 (chosen based on other Titan mission proposals [12,15]) in order to determine whether ballistic coefficient had an appreciable change in the results or could be held constant moving forward into other analysis.…”