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It's only not economical with our current mostly disposable rockets and lack of experience with ISRU and long term management and transfer of cryogenic fuels in

It's only not economical with our current mostly disposable rockets and lack of experience with ISRU and long term management and transfer of cryogenic fuels in space. Luckily we're working towards gaining experience on all those fronts. While generating sufficient Methane is probably too big of an ask without involving extensive infrastructure, in methalox rockets (which many next gen rockets are), the larger mass is the LOX (iirc ~75% of propellant mass on Starship is LOX), which is a lot easier to devise automated methods for ISRU. Thus the problem becomes a lot more solvable. You send ahead a system to collect and store LOX, then you bring enough Methane for the return trip, and you also spend the 2 years till the return window collecting LOX. Although, if you're sending stuff ahead of time and have the expected economics of Starship (ie down from hundreds of millions per ton to the Martian surface to millions per ton), you really can afford to just send ahead enough propellants for at least a round trip.

Fred34 3y ago
I haven't kept track much on how feasible the Starship rocket is. Maybe it's not that hard to make fuel on mars. Maybe humanity is making good progress figuring out how to transfer fuel in space. You sound much more educated than me on the advancements being made. I still think it would way more feasible to just to find 5-6 guys in the 25-30 year range to send on a one-way expedition. Instead of spending all of their time maintaining a factory making fuel these guys could be spending time exploring the planet and doing experiments. In return humanity would memorialize them and try to send them stuff to stay alive. After a few years we would try to send another crew to join the first crew to make more progress on the planet and build from there.
- b112 3y ago
  "Prisoners of conscience" perhaps? Some people are accused and convicted of a crime, know they are at fault, and even want to be sentenced for their crime. EG, they knew they were negligent, made a mistake they should not have, etc. Such types could potentially feel almost relief with such exile, if their personalities aligned with that of an explorer. And if they truly desire censure, punishment, "doing their time" on Mars, instead of rotting in a jail cell, may seem very good to them. Of course, lots of profiling would need to be done -- but, that is already done for such missions.
  - Fred34 3y ago
    I don’t think it would be an issue finding people for a one-way mars mission. Guys already sign themselves to fight in wars that they don’t expect to return from. Being memorialized and known is enough (especially for the case for men) for some people to go on one-way missions. Being part of something greater than themselves is enough for some people.
credit_guy 3y ago
> iirc ~75% of propellant mass on Starship is LOX I remember something similar. The stoichiometric mix is 80% oxygen, but the actual mix is fuel rich, because CH4 is a light molecule, and the end result is a higher specific impulse. Still, one could imagine that you could run the rocket in oxygen rich mode; you give up some specific impulse, but you might need to carry less CH4 with you for the return trip. All that said, I just run the numbers, and I had a huge surprise: if I didn't make any mistakes, the Starship has enough fuel for the return trip, without any need for fancy refueling in Mars orbit. According to the wikipedia delta-v map [1], the delta-v needed to go from GST orbit to the Mars transfer orbit is only 1.16 km/s (yes, not a mistake). Then from there to low Mars orbit it's a further 2.1 km/s, but aerobraking is possible. Also out of the 1.16 km/s from Mars transfer to GST, 0.77 km/s can be done with aerobraking on the return trip. But let's be conservative and ignore all the assistance from aerobraking (although the Starship was designed specifically with that in mind). We end up with a total of 3.26 km/s for one way, and 6.52 km/s for both ways. The exhaust velocity of the Starship is 3.56 km/s [2], which leads to a ratio of delta-v to exhaust velocity of 1.83, the exponential of which is 6.24. So you can have a ratio of initial mass to final mass of 6.24. The ratio of the Starship gross mass to dry mass is 1300/100 = 13, so more than twice the minimum. Of course, the delta-v map assumes the most economical routes, and for human travel you might need to splurge a bit, to get up there faster. Still, 13 vs 6.24 seems like a good margin to me. [1] https://upload.wikimedia.org/wikipedia/commons/9/93/Solar_system_delta_v_map.svg https://upload.wikimedia.org/wikipedia/commons/9/93/Solar_sy... [2] https://en.wikipedia.org/wiki/SpaceX_Starship_(spacecraft) https://en.wikipedia.org/wiki/SpaceX_Starship_(spacecraft)
- ben_w 3y ago
  I suspect that this capability, and the earth orbit refuelling going with it, was designed specifically so that it would be able to return in this way. The first few trips almost certainly won't have the opportunity to do ISRU, and until there's a lot of redundancy it would be betting the farm with each manned mission if they didn't have an unassisted return capability.

Fred34 3y ago

I haven't kept track much on how feasible the Starship rocket is. Maybe it's not that hard to make fuel on mars. Maybe humanity is making good progress figuring out how to transfer fuel in space. You sound much more educated than me on the advancements being made. I still think it would way more feasible to just to find 5-6 guys in the 25-30 year range to send on a one-way expedition. Instead of spending all of their time maintaining a factory making fuel these guys could be spending time exploring the planet and doing experiments. In return humanity would memorialize them and try to send them stuff to stay alive. After a few years we would try to send another crew to join the first crew to make more progress on the planet and build from there.

b112 3y ago
"Prisoners of conscience" perhaps? Some people are accused and convicted of a crime, know they are at fault, and even want to be sentenced for their crime. EG, they knew they were negligent, made a mistake they should not have, etc. Such types could potentially feel almost relief with such exile, if their personalities aligned with that of an explorer. And if they truly desire censure, punishment, "doing their time" on Mars, instead of rotting in a jail cell, may seem very good to them. Of course, lots of profiling would need to be done -- but, that is already done for such missions.
- Fred34 3y ago
  I don’t think it would be an issue finding people for a one-way mars mission. Guys already sign themselves to fight in wars that they don’t expect to return from. Being memorialized and known is enough (especially for the case for men) for some people to go on one-way missions. Being part of something greater than themselves is enough for some people.

credit_guy 3y ago

> iirc ~75% of propellant mass on Starship is LOX I remember something similar. The stoichiometric mix is 80% oxygen, but the actual mix is fuel rich, because CH4 is a light molecule, and the end result is a higher specific impulse. Still, one could imagine that you could run the rocket in oxygen rich mode; you give up some specific impulse, but you might need to carry less CH4 with you for the return trip. All that said, I just run the numbers, and I had a huge surprise: if I didn't make any mistakes, the Starship has enough fuel for the return trip, without any need for fancy refueling in Mars orbit. According to the wikipedia delta-v map [1], the delta-v needed to go from GST orbit to the Mars transfer orbit is only 1.16 km/s (yes, not a mistake). Then from there to low Mars orbit it's a further 2.1 km/s, but aerobraking is possible. Also out of the 1.16 km/s from Mars transfer to GST, 0.77 km/s can be done with aerobraking on the return trip. But let's be conservative and ignore all the assistance from aerobraking (although the Starship was designed specifically with that in mind). We end up with a total of 3.26 km/s for one way, and 6.52 km/s for both ways. The exhaust velocity of the Starship is 3.56 km/s [2], which leads to a ratio of delta-v to exhaust velocity of 1.83, the exponential of which is 6.24. So you can have a ratio of initial mass to final mass of 6.24. The ratio of the Starship gross mass to dry mass is 1300/100 = 13, so more than twice the minimum. Of course, the delta-v map assumes the most economical routes, and for human travel you might need to splurge a bit, to get up there faster. Still, 13 vs 6.24 seems like a good margin to me. [1] https://upload.wikimedia.org/wikipedia/commons/9/93/Solar_system_delta_v_map.svg https://upload.wikimedia.org/wikipedia/commons/9/93/Solar_sy... [2] https://en.wikipedia.org/wiki/SpaceX_Starship_(spacecraft) https://en.wikipedia.org/wiki/SpaceX_Starship_(spacecraft)

ben_w 3y ago
I suspect that this capability, and the earth orbit refuelling going with it, was designed specifically so that it would be able to return in this way. The first few trips almost certainly won't have the opportunity to do ISRU, and until there's a lot of redundancy it would be betting the farm with each manned mission if they didn't have an unassisted return capability.