Saturn System Mission Opportunities Using a Titan Aerogravity Assist for Orbital Capture Robert M. Booher(1) and J.E. Lyne(2) (1)
Undergraduate Student, Department of Mechanical, Aerospace, and Biomedical Engineering, 414 Dougherty Engineering Building University of Tennessee Knoxville, TN 37996-2210 USA Email:
[email protected]
(2)
Associate Professor, Department of Mechanical, Aerospace, and Biomedical Engineering, 414 Dougherty Engineering Building University of Tennessee Knoxville, TN 37996-2210 USA Email:
[email protected]
In previous papers authored by our group, the use of an aerogravity assist (AGA) maneuver at Titan for orbital capture about Saturn was evaluated for a Cassini-class vehicle.1 These studies confirmed that the proposed maneuver is a viable alternative to the use of a traditional propulsive orbital insertion maneuver. A vector diagram of the maneuver is shown in Figure 1. In the current paper we build on this concept and discuss mission opportunities for future return voyages to the Saturn system. Specifically, four candidate mission plans were developed based on overall performance including total ΔV, flight duration, launch year, and launch energy. (Initial trajectories were taken from the European Space Agency Advanced Concepts Team website2 and then optimized through SAIC Trajectory Optimizer3.) The candidate trajectories, displayed in Table 1, range from an October 2018 departure date to a departure in December of 2024. Though likely too near in the future for an actual Saturn return, the launch window in 2018 provided the best results of any tested trajectory and is included for comparison. In the table, the “Trajectory type” column indicates the planetary encounter sequence, including gravity assisted flybys.
Initial Saturnocentric velocity of probe Final probe velocity WRT Saturn
Inbound V
Turn angle
Titan orbital velocity (VT) = 5.58 Fig. 1 Vector diagram of AGA maneuver
Outbound V
Once promising mission opportunities had been identified, the four candidate trajectories were then evaluated for their arrival conditions at Titan. The Saturn arrival declination and V∞ were used to find the probe’s velocity relative to Titan (VE,Titan) at an altitude of 1000 km, corresponding to the Titan
atmospheric interface. Our coordinate system was defined such that the probe arrives from the negative y direction as Titan orbits posigrade about Saturn at the origin; the Titan-spacecraft intercept position in the Saturn-centered reference frame is designated by the angle Q. The coordinate system for the Saturn system arrival geometry is illustrated in Figure 2, and a graph of intercept position versus
atmospheric entry speed for each candidate mission opportunity is shown in Figure 3. LEO departure date(yyyy.mm.dd)
Trajectory type
Total V (km/s)
Duration (years)
C3 (km2/s2)
Saturn arrival V∞
2018.10.13 (DB)
EVEES
4.130
8.22
16.092
8.482
-6.910
2020.03.14 (DB)
EVVEES
5.179
10.99
11.550
7.825
12.260
2021.11.05 (DB)
EVEES
4.797
8.78
10.937
6.858
6.116
2024.12.04 (DB)
EVVES
4.805
10.44
19.398
6.027
13.379
(km/s)
Saturn Arrival Dec (deg)
Table 1. Candidate Trajectories for Saturn system mission
The various missions opportunities were then examined to determine the desirable range of intercept positions that would provide Titan atmospheric entry velocities of 6 to 10 km/s. This velocity range was chosen to avoid the excessive aerothermal environment that would be associated with higher entry speeds. The approach trajectories for each candidate mission opportunity was then compared to Titan ephemeris data, and the Saturn system arrival dates were adjusted to accurately target the optimal Titan intercept positions. Using these final trajectories, the AGA maneuvers were computed using the Program to Optimize Simulated Trajectories (POST).5 The final paper will present a description of the candidate mission opportunities, including details of the interplanetary trajectories and data on the Titan AGA maneuvers.
Fig. 2 Schematic representation of probe intercept with Titan.
Fig. 3 Intercept Position (ϴ) vs. Titan Atmospheric Entry Velocity
References [1] Philip Ramsey and James Evans Lyne, “An Investigation of Titan Aerogravity Assist for Capture into Orbit About Saturn,” Journal of Spacecraft and Rockets. Vol. 43, No. 1, pp. 231-233, Feb. 2011. [2] Mission Analysis Advanced Concepts Team, European Space Agency. Nov. 2010. . [3] SAIC Trajectory Optimizer, Science Application International Corporation. [4] Philip Ramsey and James Evans Lyne, “Enceladus Mission Architecture Using Titan Aerogravity Assist for Orbital Capture About Saturn,” Journal of Spacecraft and Rockets. Vol. 45, No. 3, pp. 635-638, Feb. 2011. [5] Brauer, G. L., Cornick, D. E., Olson, D. W., Peterson, F. M., and Stevenson, R., “Program to Optimize Simulated Trajectories (POST),’ NASA CR NAS1-18147, Sept. 1989.
