Phobos and Deimos on the return to Earth

[Russel]

There was another option floating around in my head that is worth mentioning.

It involves first having a modest habitat in Mars orbit which happens to have SEP capability. The principle here is that you want to spend as much time as possible in a comfortable and relatively safe environment.

Initially your MAV delivers you to low Mars orbit where you join the habitat. It spirals out to Phobos L1 and loiters there while you use modest means to visit the surface. After you've explored Phobos you spiral out to do the same at Deimos. Finally you use SEP to achieve something closer to a capture orbit. You rendezvous with the Earth return vehicle and it uses chemical thrust to return to Earth.

Reason I'm not that fussed about this is that given sensible parameters its still a couple of months worth of extra mucking around in Mars orbit. Which means more supplies. So its doubtful if it really wins over just using the MAV as a "taxi" for the moons. But worth considering.

[Hop_David]

Given an approximately 1000 km tether going up from Phobos and an ~3000 tether going down from Deimos, there exists a ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos and Deimos.

It'd be possible to travel between the two moons using almost zero reaction mass.

[Russel]

Where do you end up if you miss catching the Deimos tether

[Hop_David]

Where do you end up if you miss catching the Deimos tether

In some ways the problem is like landing a passenger jet on an runway. For a soft landing you need the vehicle and runway at the same place and time with close to the same velocity vectors.

But it differs in some ways. The approach to a Deimos or Phobos tether would be more gradual than a jet’s approach to a runway. There would be time to adjust the trajectory if the approach is too fast or too slow.

Also the acceleration is different. On earth’s surface, acceleration is 9.8 m/s^2. At the top of 937 km Phobos tether the acceleration is .14 m/s^2. At the bottom of a Deimos tether the acceleration is about .03 m/s^2.

The milder the acceleration, the more forgiving the catch. It’d be like doing an earth catch in slow motion.

At the Phobos tether, the catch would be about 70 times as slow as an earth catch. At the Deimos tether it’d be about 290 times as slow as an earth catch.

[Russel]

Don't you also have phasing issues? How often are both Phobos and Deimos in the right points in their orbits to do this?

[Hop_David]

Don't you also have phasing issues? How often are both Phobos and Deimos in the right points in their orbits to do this?

Launch windows between Phobos and Deimos tethers would open each 10.25 hours or so. Trip time between moon tethers is about 9 hours.

[redliox]

Doing a review breakdown on the most fuel-efficient routes from Mars...

So for Phobos, the end result is a delta-v of 7 km/s
1: Launch from Mars to Phobos Transfer Orbit: 4.3 km/s
2: Periapsis Raise: 0.6 km/s
3: Phobos Ops: 0.16 km/s
4: Direct Trans Earth Injection: 1.9 km/s

And for Deimos, including Oberth Effect, the end results is a delta-v of 7.25 km/s
1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s
2: Periapsis Raise: 0.7 km/s
3: Deimos Ops: 0.16 km/s
4: Periapsis Brake:  0.64 km/s
5: Trans Earth Injection: 1.05 km/s

(Hop_David are these figures accurate including for Oberth?)

There could be several different options for a Mars-Deimos/Phobos-Earth route:
1) Single vehicle (Mars Direct [tweaked to include moon visit])
2) Two vehicles: MAV+Orbital ERV (Mars Semi-Direct route with ERV doing moon visit])
3) Two vehicles: Primary Vehicle+Moon Vehicle
4) Three vehicles: Options 2 or 3 +booster stage

I believe the easiest (or perhaps most efficient being correct wording) would be options 1 and 3.  The fuel would come chiefly from Mars and have the option of returning directly to Earth if the moon visit is ruled out.  In option 3 the Moon Vehicle could specialize in rendezvous plus remove the ~160 m/s budget from the Primary's fuel.  Option 4 is probably the best for safety since you would have the TEI waiting w/o Martian ISRU; the main downside would depend on what kind of booster you use; current solids or hypergolics (and their available engines) wouldn't have enough kick to meet much more than ~500 m/s even with a large stage and the boil-off from liquid hydrogen is concerning (methalox I'll give a 'maybe' to).