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The average travel time to Mars has been quoted to be around nine months (~ 270 days). This assumes current propulsion methods and when Mars and Earth are near each other.

Musk has been quoted to say that his ITS space rocket system could make it to Mars in about 80 days - or less than a third of the time current techniques could get us there.

I'm curious to learn more about this statement. What is unique about the ITS that would make the travel time so much shorter? Is the propulsion method described in existence today?

I've reviewed a few other questions on stack but have come up empty so far:

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    $\begingroup$ I don't think anything explicitly prevents humanity from throwing a rocket hard enough at Mars to get there in 80 days with current rocketry. It's just that A) When we throw things at Mars, we want to throw as much useful payload as we can, and 2) There's nothing we're currently throwing at Mars that needs to get there faster than the next Hohmann transfer window lets it. $\endgroup$
    – notovny
    Jan 3 at 23:29
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    $\begingroup$ Does the name "Hohmann transfer" mean anything to you? I'm asking because your first paragraph makes it unclear what you understand the current (non-ITS) baseline for travel is (specifically "near eachother" is often misunderstood to imply straight line travel). $\endgroup$
    – Flater
    Jan 5 at 13:09
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    $\begingroup$ This is a great comment @Flater. Thanks. I recently learned about Hohmann transfers and now understand why that baseline is so much longer. Thanks. $\endgroup$
    – FontFamily
    Jan 5 at 22:50

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What is unique about the ITS that would make the travel time so much shorter? Is the propulsion method described in existence today?

The current manifestation of the ITS is Starship, and the performance estimates of such a vehicle have shrunk considerably since the public reveal of the ITS (it is very much so a different vehicle though so not an apples to apples comparison). The propulsion system on all manifestations of ITS/BFR/Starship is not exactly revolutionary. It uses chemical rocket engines the same way that the Apollo missions did 50+ years ago (albeit with different propellants and significantly better cost & mass efficiency). The in-space long term storage, transfer, and eventual use of cryogenic propellants is a key enabler of SpaceX's visions for Starship, and this is not a well explored technology (though in-space transfer of room-temp fluids is routine).

The uniqueness of a fully fueled Starship (when in orbit) lies in its maneuvering capabilities ($\Delta V$). Using the rocket equation and current Starship specs:

Here is a porkchop plot for the ~2028 Earth to Mars transfer window subject to $\Delta V<6.9$ $km/s$:

Earth to Mars porkchop

Which shows that 80 days is not quite possible. The impressive $\Delta V$ of Starship does wonders to expand the width of a transfer window, but the tyranny of the rocket equation dictates that it cannot significantly shorten the transfer period.

FWIW, here is the relationship of $\Delta V$ to $C3$ for this example (250 km parking orbit assumed). The region in grey is the typical C3 range for spacecraft sent to Mars:

C3 vs delta V

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    $\begingroup$ This is great and very helpful. I saw that Earth and Mars will be "close" to each other again in 2035. When I do a porkchop plot for that time period, I am getting a ToF of about 80 days for a Delta-V at 6.9 km/s. $\endgroup$
    – FontFamily
    Jan 4 at 2:20
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    $\begingroup$ @FontFamily if you are using this website then note that it is giving you $V_\infty$, not $\Delta V$ (it's mistakenly labeled). The difference is non-negligible. $\endgroup$ Jan 4 at 2:30
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    $\begingroup$ Looks like there's a very narrow window around Jan-Feb 2029 when it could be made in about three months (maybe a few days over 90), if they don't leave any margin, though. That's still pretty impressive! $\endgroup$
    – CBHacking
    Jan 4 at 9:43
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    $\begingroup$ How about human-populated round-trip to Mars, returning alive? $\endgroup$
    – Ricardo
    Jan 4 at 21:24
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    $\begingroup$ @Ricardo I suppose that for a round-trip, it might be feasible to bunker some fuel for the return trip ahead of time via a slower unmanned mission ..? $\endgroup$ Jan 5 at 16:45
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Probably the key term here is pork chop plot. The linked page includes one from 2005 for Mars where the lowest C3 departure was 15.5 and 400 or so days, but doubling that to 30 got a transfer in 125 days, by leaving Earth faster and burning off more energy on arrival.

For science payloads it generally makes sense to use the minimum fuel load and pack as much equipment as possible and wait the extra days.

For humans they need to eat and breath, so there is math to be done removing consumables and adding fuel. In the 2005 porkchop plot going from 15.5 C3 to 16 gets access to a new minimum on the lower part of the plot, dropping from 400 to 200 days, and pushing to 16.5 gets to 175 days. So for Humans the physically optimal transfer is probably never going to be the best choice, it just becomes a question of trade offs.

With SpaceX, having made Starship the all in one solution for Mars they have a heat shield that is (hopefully) robust enough to re-enter at Earth, and has enough performance to reach Earth orbit in it's own right (many Mars designs assume ion engines). So they have a design that is consumable/space constrained but has surplus performance for departure and capable of aerobrake/aerocapture at Mars, which makes high energy routing more possible than for a larger but lower thrust/less robust vehicle.

Possibly most critical Starship uses cryogenic Methane and Oxygen so every day in space loses some to evaporation. So Starship has further choices to burn fuel on departure for faster transit or have to carry more spare fuel to make up for more evaporation.

It is possible that the 90 day transit is being forced on them by evaporation rather than a true feature of the design. At the moment (January 2022 before first sub orbital Starship flight) not enough information is available for outsiders to apply useful math to the question.

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    $\begingroup$ This is helpful. It sounds like approaching 90 days for a transit is doable. Thank you. $\endgroup$
    – FontFamily
    Jan 4 at 0:46

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