Jeff Findley wrote on Thu, 30 May 2019
07:33:03 -0400:

In article ,
says...

We know what is in the Martian atmosphere. We know how to extract fuel
from it.


Well, no, we don't, in any effective way, unless your plan is to use
up over half your cargo capacity hauling LH2 to Mars.

Yes, that's the initial plan, at least for SpaceX. They plan on sending
a couple of Starships, uncrewed, to Mars to work on propellant
production ahead of sending a crew. Those initial Starships would be
carrying all the LH2 they need to produce the return propellant needed
for later crewed Starships.


This plan doesn't work very well when you look at the details. It's
also not the SpaceX plan that I've seen.

https://www.inverse.com/article/5129...rting-a-colony

According to this, the missions look like this:

2022 (two cargo ships): "The ships would place power, mining and life
support infrastructure for future flights. They would also confirm
water resources and identify hazards."

2024 (two cargo ships plus two manned ships): "They will be tasked
with setting up a propellant production plant, combining Martian
water, ice, and carbon dioxide to create methane and liquid oxygen to
fuel the ships and come back home. The humans would be tasked with
collecting one tonne of ice every day to fuel the plant."

Note that use of Martian ice collected by humans to fuel the
propellant plant?

And you're going to need water for your people.

You don't have to send everything on one ship.

On the first mission, you send to uncrewed Starships to make propellant
for the return trip. Yes, something on the order of half their cargo is
LH2. The rest is processing equipment, power generation, power storage,
and etc. Pretty much everything on those first two ships exists just to
make propellant.


Over 60% of their cargo must be LH2 if you're going to make enough
methane to fuel two Starships for the return trip. And note that your
plan is not the SpaceX plan. Your plan presumably leaves four
Starships sitting on Mars while two of the six make the return trip to
Earth using the fuel that the first two ships made.

The SpaceX plan says you bring lots of other stuff beside 'fuel
production' on those first two ships, including vehicles, life
support, ****loads of solar panels, etc. The SpaceX plan says those
first two ships bring rovers to locate water. Fuel production doesn't
start until humans are on site with the start of the second mission.
'Stay time' of that mission will be on the order of 14-18 months. That
says that SpaceX expects some 420-544 tonnes of Martian ice will be
made into fuel during the manned stay. That nominal 500 tonnes of ice
gets you around 55-60 tonnes of LH2, or enough to refuel a single
Starship. So the SpaceX plan doesn't seem to work, either, unless you
leave five Starships behind.

Obviously, bringing your own LH2 doesn't work (it reduces your cargo
too much for you to do much other than very long 'flags and
footprints' sorts of missions) and requiring people to be on scene to
move Martian ice at a tonne a day doesn't work (you can't move enough
ice at that rate to get all the ships refueled). So, equally
obviously, you either need the manual workers to move a lot more ice
than that (unlikely) or else you need to somehow automate the ice
collection.


On the second mission, you send two crewed Starships. Those would have
the water necessary for the crewed missions.


How big are your crews? You're going to need a tonne and a half of
water for each person. Plus they, too, are going to have to carry
that 60+ tonnes of LH2. Plus food. Plus vehicles (which your plan
doesn't send on the first ships). Plus everything else needed to
establish some sort of base, if you're doing more than 'flags and
footprints'.

If you're going to do more than
'flags and footprints' (and you are, because you can't make fuel for
the return trip fast enough), you need two years or so of water per
person you send. That's around a tonne and a half of water per
person. That hundred tonnes of cargo per ship is going fast...

Again, water comes on the crewed Starships. Also, with some water
recycling, you don't need the full two years or so of water per person.
How much water you need depends on how efficient the recycling is. We
have a lot of experience with this on ISS, so it's "simply" a matter of
running the numbers and doing the engineering trades.


There's a trade off here between what the recycling equipment weighs
vs what the water weighs.

And if your plan is to use in situ water (and pretty much every plan
includes that as a requirement) you're going to have to build that
water extraction plant that you can't know how to build until you go
and analyze the hell out of everything (according to Jeff) but you
can't go do THAT until you can build a water plant.

That wouldn't come until later. Until a Mars base can bootstrap in-situ
water production, you're stuck sending more LH2 and processing equipment
on uncrewed Starships and sending more water with crewed Starships.

But yes, once that in-situ water production gets going, you can replace
the LH2 and some of the water on future Starships with other cargo
and/or more people.

Musk's ambition is to set up a Mars colony. As such, he's already
planning on sending a crap ton of Starships to Mars. His ambition is
the opposite of flags and footprints.


And Musk doesn't appear to be planning to haul hundreds of tonnes of
LH2 to Mars. Musk appears to be planning on relying on in situ water
from the get go. And that's not practical, either, unless they can
move a hell of a lot more than a tonne a day of water.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw