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Comment by ssorallen | Hacker News Reader

Elon Musk has said from the beginning of SpaceX that trips to Mars would have to return as well. It's not because of the humans but because the cost of producing the space ships will be so large that it's not feasible to shoot them to Mars once and leave them there; they have to be reusable.

> The principle behind all of its vehicles is total re-usability; every single system must be able to be serviced and pressed back into operation. This is to cut down on the already astronomical cost of space flight (pun intended), making it more affordable for private companies to conduct missions into the heavens. The company’s main goal is to eliminate the equipment cost for space travel, leaving fuel as the only financial burden.

http://www.extremetech.com/extreme/122923-spacex-ceo-claims-...

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> It's not because of the humans but because the cost of producing the space ships will be so large that it's not feasible to shoot them to Mars once and leave them there; they have to be reusable.

The problem with the return trip is Earth reentry. Coming in at interplanetary speeds (falling to the sun from a "height" of Mars' orbit to Earth's), it is impossible for humans to reenter the Earth's atmosphere safely. It's barely possible from the moon.

This means that there has to be about as much deceleration before reentry than there was there was acceleration on the way out. So now you have to carry double the fuel, and the fuel to carry that, etc. This vastly complicates the problem with a manned mission to mars.

Lunar speed reentry is quite feasible, and has been practiced. Apollo did it fine, and Orion did it a few weeks ago. State of the art ablative shields, like the PICA-X used on Stardust and Dragon, are quite capable of handling entry at Mars transfer speed. (Lunar entry profile is about 11km/s, Mars 13-14km/s--though it can go as high as 20km/s deformation on design.)

The real problem with the return trip is getting off Mars. Specifically, putting enough propellant on the surface to get you back up. Sure, it's easier than Earth, but it's still a planet. If NASA were serious about Mars, they would be sending prototype ISRU plants every cycle, because having rocket fuel waiting for you on the surface is the real enabling technology for Mars exploration. Or Lunar or Venus for that matter.

It's not just getting fuel to Mars. It's building and operating launch infrastructure there. Unmanned LEO missions from KSC get delayed and fail. On Mars there won't be spare parts or separate ground crews, and the spacecraft will have to be readied having been through launch and touchdown cycles rather than a slow gentle roll from a VAB to launch pad.

The speed of light means no real time support from Houston. If ten people go out, then ten people have to get an interplanetary space program established on Mars. It ain't gonna be a matter of pushing the easy button.

This is one reason why it's often thought that a long-term station in Earth orbit (possibly at one of the Lagrange points) is a pre-requisite for a Mars mission. So, you could decelerate to the station instead of to re-entry. But, it is possible if one sent enough supplies ahead.

One way of handling things if there's a long-term station is to build the ship to Mars in space (or at least refuel at the station). If you send the return fuel ahead (or find a way to make it while at Mars) you aren't required to carry a large amount of fuel.

You still have to carry twice the fuel. What about a refuel station orbiting mars? Replenish the mars fuel station with fuel before the manned mission departs, they can just use that fuel stored at Mars when they return from the manned mission.

Deploying a station orbiting Mars would be pretty expensive.

Why re-enter?

That doesn't mean you have to return from the ground. Have the large reusable craft swing by Mars without slowing down, dropping off a small daughter ship/lander, and continuing back to Earth.

The ship itself returning would be relatively trivial as long as it didn't need to degravitate from Mars.

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