How much speed do I need to put something to orbit on Titan? I know it is a simple question but look among those maps that are there but they are not clear to me.
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asked Feb 29, 2020 at 17:15
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2 Answers
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6
From Ulysse Carion's "subway map", the delta-V required is approximately:
- 9400 m/s to get from Earth's surface to low Earth orbit;
- 3210 m/s + 4500 m/s to intercept Saturn;
- 3060 m/s to intercept Titan;
- 660 m/s to get into orbit around Titan (at 1000km altitude to stay out of Titan's extremely deep atmospheric envelope).
Thus, from Earth, you need a total of around 20,830 m/s.
Note that all these values are approximate. The exact values will depend on the exact trajectories taken on the exact dates, and even with the design of the rocket (especially for the Earth ascent phase) The subway map can tell you if a mission design is generally feasible, but that's all.
The subway map also doesn't show any opportunities for cleverness. NASA's trajectory browser shows there's a launch opportunity in 2030 that takes a gravity assist from Jupiter in order to do the LEO-to-Saturn portion of the mission using 6410 m/s instead of 7710 m/s, for a total of 19,530 m/s. The delta-v needed at the Jupiter flyby is very small.
answered Feb 29, 2020 at 17:40
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3$\begingroup$ Neat. LEO really is "halfway to anywhere". $\endgroup$ Feb 29, 2020 at 18:22
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3$\begingroup$ 1000 km altitude over Titan is not likely to be a typo. Titan's atmosphere is 200 to 800 km deep and 1000 km altitude seems a sensible place to orbit. $\endgroup$– PereFeb 29, 2020 at 19:14
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1$\begingroup$ Oh wow, so it is. Thanks, I've corrected my answer (and learned something cool). $\endgroup$ Feb 29, 2020 at 19:25
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$\begingroup$ I love this answer and the subway map, though I'm having trouble finding Central Park and Haight Ashbury. Did you use generic imgur instead of bringing it into SE's imgur for a reason? $\endgroup$– uhohFeb 29, 2020 at 23:56
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2$\begingroup$ 1000 km is probably not high enough for a sustained orbit. When I was working Cassini operations we had a Titan flyby pass where we got to 950 km, and the drag was so high we almost lost attitude control. When I worked on the "Titan Saturn System Mission" study at JPL we used 1300 km for the lowest circular orbit we'd consider, finally converging on a 1500 km orbit. For aerobraking we went as low as 600 km, but we could put up with the atmospheric densities at that altitude because Titan orbits are much slower than most other places with atmospheres. $\endgroup$ Mar 3, 2020 at 19:42
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Delta-v map is good enough for direct transition trajectories.
If you are interested how the real space missions to Titan (or at least the real designs) work, the picture will be some different.
First difference - when we leave the Earth's orbit, we don't need to burn rockets so much. We can borrow most of remaining delta-v by gravity assists. Earth and Venus are the most suitable velocity donors here, mainly because their otbital configurations allow launch almost every year. Jupiter can be used for final gravity assist too, but opportunities are rare and happen about a couple of years in two decades timeframe. When arriving to Saturn - part of insertion delta-v can also be borrowed, this time from Saturn's satellites, mainly Titan. Cassini spacecraft did exactly that.
Second point, specific for Titan - it's the only moon with atmosphere. Because of this Titan is only moon where soft landing requires less effective delta-v than orbit insertion. Trere is currently one project in implementation phase - the Dragonfly rotorcraft, planned to be launched in 2026. Here is .pdf with trajectory design for Dragonfly.
For orbiting around Titan - the atmosphere also allows aerobraking for lowering the apoapsis, some economy of effective delta-v too. In 2010 there was a study for Titan-Saturn System Mission (TSSM). Some details about the trajectory and delta-v for TSSM can be found in this paper. You can see the spacecraft combines gravity assists with solar electric propulsion (SEP) by ion thrusters. Total delta-v is about 2370 m/s for chemical propulsion (table 11 at page 14) and 3330 m/s for electric propulsion (table 12 at page 15). Also the aerobraking at Titan atmosphere shown at pages 11-12.
