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SpaceX's Interplanetary Transport System has a well-defined plan for getting to Mars:

  1. Launch the ITS spaceship into LEO
  2. Launch five ITS tankers to fill the spaceship's tanks.
  3. Launch the spaceship on a trans-Mars trajectory.
  4. Enter Mars's atmosphere, shedding most of the velocity aerodynamically, then landing usings its engines.

What will the plan for returning from Mars to Earth look like? If the spaceship is fully fueled from ISRU methane/oxygen, will it have the delta-v to get from the Martian surface back to Earth without refueling, or will it require refueling in Mars orbit?

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  • $\begingroup$ I realize this is somewhat speculative - I'm mainly looking for calculations about having the delta-V to return without refueling. $\endgroup$
    – DylanSp
    Commented Mar 6, 2017 at 18:38
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    $\begingroup$ A couple of quotes, this one is on the return "Using methane fuel harvested from the Martian regolith, Musk says, the spaceship would be able to lift free from Mars’ weak gravity and return to Earth." then this one address your point 4 "Upon arrival, heat shields on the ship’s belly will create mild friction with Mars’ scant atmosphere to help you brake. But the real stopping power will come from supersonic retropropulsion". I suspect the supersonic retropropulsion is .... different that "landing using its engines" $\endgroup$
    – Enigma Maitreya
    Commented Mar 6, 2017 at 22:19
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    $\begingroup$ @EnigmaMaitreya: Actually, "supersonic retropropulsion" is exactly "landing using its engines" except, at some point before touchdown, the rocket will transition from supersonic to subsonic, at which point it's just ordinary retropropulsion. I mean, the rocket will also land using its legs and whatnot, but Mars' atmosphere is much too thin for something with the mass of the ITS spaceship to aerobrake to subsonic speeds; the engines (acting as retrorockets) will be needed for that. $\endgroup$
    – CBHacking
    Commented Mar 6, 2017 at 22:28
  • $\begingroup$ @CBHacking - Indeed, my point was the majority of shed velocity will be done via the supersonic retro-propulsion not aerodynamic breaking. I should look but I am fairly certain that Red Dragon now schedule for 2020(?) will be the first technological demonstrator lander on Mars using this form of landing. $\endgroup$
    – Enigma Maitreya
    Commented Mar 6, 2017 at 22:54

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It takes 3.8 km/s DV to orbit Mars from the surface, 2.5km/s DV to reach an intercept with Earth, and most likely around 1 km/s to vertically land on Earth as proposed by SpaceX. Altogether the ITS ship would need around 7.3 km/s from the Martian surface to the Earth's surface.

The fully fueled ITS ship weighs 2400 tons, and it lands on Mars weighting around 300 tons. Since the exhaust velocity of a CH4/O2 fueled rocket is around 3538 m/s for the vacuum raptor engine.

DV=3538Ln(2400/300)=7.357km/s SpaceX plans to do this, but it'll be real tight However I didn't take into account gravity losses so my numbers are probably off.

Elon never talked about propellant tankers orbiting mars, and since the ITS ship has enough DV to come all the way back to Earth from the Martian surface, it will most likely be the way they do it.

SpaceX's plan is to generate methane on Mars using the Sabatier reaction using materials on Mars, once it generates the fuel it needs it will take off from Mars and in theory directly return to Earth.

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  • $\begingroup$ All of these numbers depend highly on the launch date, expected arrival date and the type of transfer. Could you specify these in your answers? Just eye-balling this, it seems like the best case scenario to only require 2.5 km/s to go from a Mars orbit to an interplanetary trajectory. $\endgroup$
    – ChrisR
    Commented Mar 6, 2017 at 21:58
  • $\begingroup$ Red Dragon will probably have some impact on the "how this is done" en.wikipedia.org/wiki/Red_Dragon_(spacecraft) $\endgroup$
    – Enigma Maitreya
    Commented Mar 6, 2017 at 22:32
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    $\begingroup$ If the spaceship is coming back without (much) cargo, you can knock some of the empty mass off. It probably won't be hauling 100 people and their equipment, life support, and consumables on (most) return journeys. Also, in theory, you could have the ITS spaceship refuel in Mars' orbit. It'd consume / waste a lot of locally-produced fuel, but the ITS tankers can (barely) SSTO from Earth; they'll have plenty of capacity to lift off from Mars, transfer a bit of fuel (just enough to have a safe margin) to an earth-bound ITS spaceship, and land on Mars again. $\endgroup$
    – CBHacking
    Commented Mar 6, 2017 at 22:35
  • $\begingroup$ @EnigmaMaitreya What do you figure Red Dragon has to do with this? The ITS doesn't use Dragons and the Dragon 2 doesn't have enough delta-V capacity to reach Mars orbit from the surface anyhow; its SuperDracos are only for emergency escape or retropropulsive landings (and not both; it doesn't have enough fuel to land on them if there was an emergency escape burn). The Red Dragon mission(s) will teach us about retropropulsive landings on Mars, but not about taking off again. $\endgroup$
    – CBHacking
    Commented Mar 6, 2017 at 22:38
  • $\begingroup$ @CBHacking did you read the technologies being tested? Did you miss the return Collected Samples? Or are you assuming it is permanently off the table and the Production of Fuel is .... well what do you make of that? $\endgroup$
    – Enigma Maitreya
    Commented Mar 7, 2017 at 0:12

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