On Mar/11/2021 at 14:11, JF Mezei wrote :
On 2021-03-11 08:17, Jeff Findley wrote:

Due to orbital mechanics, every couple of years there is a "window" when
you can send spacecraft to Mars using a minimum amount of delta-V.

And this is the problem. Not much point in sending fuel ahead if by the
time you realise the fuel vehicle crashed, the crew are already on their
way to Mars. And using different windows means a very long time between
sending supplies and sending crew.

No one is planning a mission the way you describe it here. You don't
send fuel from Earth to Mars. You send some kind of apparatus that will
make fuel on Mars using in situ resources (some mission architecture
plan on sending hydrogen from Earth, but that is a small mass fraction
of the oxidiser and fuel). You do that at least one launch window before
sending humans. You don't send humans before your fuel factory on Mars
has succeeded.

Good question. I'm sure the mission planners at NASA know this answer.
But, free return trajectories do exist for an earth-Mars-earth trip.
Cite:

https://en.wikipedia.org/wiki/Free-return_trajectory

But that brings up ECLSS and food. If you have to slingshot and be
autonomous for another year to get back to Earth, that means that when
you leave earth, you leave with a hell of a lot of supplies and water.
And it also means you need some pretty fancy ECLSS to reclaim as much as
you can for water and oxygen. I have to wonder if SpaceX would take ISS
system designs or make their own from scratch. In later case, they are
a long way from having a Mars crewed mission.

A mission profile with a free return (nearly free return) trajectory
follows a path similar to a mission with a short stay on Mars. It
wouldn't take a year. More like 7 months. But it is more energy
expensive than a mission following a Hohmann transfer trajectory.

Almost worth considering putting people in induced coma for 6 months
with intravenous feeds to keep them alive followed by strong exercise
regimen to bring them back to shape.

That would be a whole new ballgame.

Another consideration: you might need 2 starships to return from Mars.
You send one from Earth to Mars solely with supplies needed for the
return journey. Lands on Mars. Once the crewed vehicle comes in, they
need to fuel both ships which take off and meet in space so the
food/water can be transfered to the crewed vehicle.


Coming from Earth, would landing and then taking off back to Earth end
up requiring less food than entering Mars orbit and when the crewed
vehicle takes off from Mars, it forst goes to orbit to meet with supply
ship and then heads to Earth? (thsi woudld also allow refueling
assuming both the tanker and the supply ship are in same orbit).

A Starship should be able to take-off from Mars and go to Earth without
in orbit resupply or refuel.


Alain Fournier