We’re assuming the first two ships don’t set up autonomous mining, so we’re not even sending an ISRU plant with them.  That means Constellation and Finity’s End have no ability to go home when the first return window opens, 550 days after landing.  Instead, after Heart of Gold and Nostromo arrive, they generate enough propellant that Constellation and Finity’s End can go home in the second return window, after 1340 days on Mars (3 years, 8 months), 1520 mission days total.  A 180 day return yields a total mission elapsed time of 1700 days (4 years, 8 months).  Essentially, the first two ships set out on a 5 year mission to explore a strange new world…

Heart of Gold and Nostromo arrive 26 months after Constellation landed.  Heart of Gold will also stay on Mars for 1340 days, launching back to Earth on its mission day 1520; landing back on Earth after being gone for 1700 days total.

During the third Earth-Mars launch opportunity (4 years, 4 months since initial launch; 3 years 10 months since initial landing), another ship heads for Mars—and now, we need a third MCCS.  This ship launches while Constellation is enroute back to Earth (60 days into the return flight).  Heart of Gold will have been on Mars 610 days when this ship launches.  And, we also need another name.

We’ve done whimsical, we’ve given nods to the sci-fi that changed a generation, and we’re still reserving Mayflower for a particular ship.  Where to next?  Well, what came before the Mayflower?  The Jamestown settlement was founded in 1607 and became the first permanent English settlement in the new world.  The colonists came in three ships:  Susan Constant, Discovery, and Godspeed.  We don’t really want to use a proper name, and Discovery was already a space shuttle, so how about Godspeed?  Regardless of religiosity, it conveys the right tone.  Perhaps more important, it continues our tradition, to date, of using names that should remind us how dangerous this work can be, and how we’d like to avoid mistakes of the past.  (And, of course, they’re just placeholders anyway.  So, MCCS-3 Godspeed.)  Let’s assume each launch window includes a MCCS and a freighter, so looking at the Jamestown resupply missions, we discover that the first resupply included a ship called Phoenix.  Perfect.  (The other ship was John and Francis, somewhat less meaningful today.)

Now, with a 180 day trajectory each way, each ship will have 670 days on Earth for maintenance before relaunching in the next Earth-Mars window.  (They get back to Earth just after the current Earth-Mars window closes.)

We had a base population of 12 with Heart of Gold; we’ll go up with Godspeed, and drop back when Heart of Gold leaves.   What does Godspeed bring to Mars on its first trip?  More of everything, including more people.  We’re four years past the initial launches to Mars, so increasing the life support capacity on the third crew-capable ship to Mars also seems reasonable.  Let’s assume Godspeed brings 16 people, so the population will go up to 28 when it arrives.  Godspeed will stay 3.7 years, and Heart of Gold goes home halfway through that stay.  

Heart of Gold takes crew back to Earth when it leaves.  Constellation could also take crew, if necessary.  We’d prefer Constellation doesn’t take crew, just because it would be nice to get a couple ships back without extra drama, and we need the workforce available on Mars.  But, think “cancer” and the opportunity suddenly looks good.

How many crew will return on Heart of Gold is an open question.  All the first crews need to be open to the idea of staying on Mars for a long time—it’s not possible to be 100% certain how long this is, but Heart of Gold’s crew is committed to at least an 18 month stay, and is planned for nearly 4 years.  No one, no matter how confident, can be sure of their attitude towards Mars after four years, when no one has ever done it before.  Perhaps everyone will want to stay.  Or, perhaps four years on Mars will have become the equivalent of a military “remote” tour—interesting, tolerable, but in these early days, not someplace to spend the rest of one’s life.  In particular, crew selection for this ship and others to follow will have to consider what happens to the crew as they age, whether all crew will be couples, and when/whether children will enter into the equation.

This is important for planning out the base size—colonizing Mars is our goal, but at this point, Mars is the remotest of remote assignments.  The US military doesn’t ask volunteers to sign up and spend the rest of their lives overseas, even in the best locations.  Even State Department Foreign Service Officers rotate back to the States. At some point, most of the crew will really be colonists.  But, at least for the first few missions, a four year maximum “tour” for most personnel is more reasonable.  Among other factors, we want the best crew we can get, not the best crew that’s willing to go to Mars forever.  (I’d go to Mars in a minute.  My wife, not so much.  I’d rather not have to choose, and I doubt I’m unique in that respect.  From that perspective, the National Geographic Mars mini-series is probably a bit more science fiction dream than astronaut reality.)  

Mission design needs to consider reverse flow for another reason—it drives decisions about how much cross-training to do with individual crew, particularly since it will be impossible to predict precisely which individuals want or need to return home.  This cross-training decision also has to consider a worse situation, accidents.  Fatal accidents remove certain skillsets from the remaining crew, which is a problem.  They’re also highly likely to drive remaining crew to reevaluate their life decisions.  Reverse flow is definitely non-trivial.  

There is some good news here, however.  Individual crewmembers can return to Earth after only 550 days on Mars.  This could be useful in the event of sickness, but this can also be part of our mission architecture after Heart of Gold’s initial mission.  Part of the crew can return after only 18 months on the next ship departing.  Part can stay for four years, returning on their own ship.  An 18 month tour would be ideal for visiting scientists, who could manage a 2.5 year expedition much more practically than 5.  The longer term crew would build out the base infrastructure, with some eventually becoming colonists.  This would be somewhat similar to McMurdo or Amundsen-Scott South Pole Station, where some crew and scientists return many times, and others accomplish their objectives in one visit.

With a mix of tour lengths and personal missions, we also get a better social mix—science gets done by scientists, excited to return home with their discoveries.  The base is built out by Mars settlers, with an extra point of pride during the time between science visits, when only a select few “winter over” at the base.  And, each long term crew is joined at their halfway point by a new group of people, who inject a fresh perspective into the tour, again increasing the manpower available for exploration, science, and base assembly.

So, let’s assume Heart of Gold’s entire crew goes home after 4 years, and it also takes 6 of Godspeed’s crew.  This leaves 10 people on Mars from Godspeed’s crew.  Since we had 28 people on Mars temporarily, we’d like to build out base capacity to around 56—building out ahead of need, but also maximizing life support redundancy.  

Constellation, returning in the fourth window on its second trip to Mars, now brings 32 more at the 6 year point, temporarily increasing the population to 42.  A goal of Constellation/Finity’s End cargo will be to expand base life support capability to around 80.  Godspeed will go home with its remaining 10 crew, plus half (16) of Constellation’s, dropping the actual base population back to 16.

For the science/settler concept to work this pattern needs to hold…a fairly large arriving contingent at each launch window, but with most of those people departing at the next return window.  A smaller cadre remains for a longer time, and a still smaller group elects to remain indefinitely.  Table 1 shows a notional mission flow, and Table 2 shows an associated population curve.

At the fifth Earth-to-Mars launch opportunity (now 8 years since initial landing), the cycle continues.  Heart of Gold comes back with more supplies on its second journey.  We’ll also assume Heart of Gold has been upgraded to carry 32 people.  This doesn’t necessarily mean cabins and “room” on board for 64, but does imply we should be able lose half the life support without endangering the crew.  Establishing the reliability of life support components is absolutely critical, both for immediate safety and to determine what the future design margins need to be.  Presumably, Heart of Gold will be accompanied by a new freighter, since Nostromo stayed on Mars.  

We now have 48 people on Mars until Constellation goes home—half of Constellation’s first crew and Heart of Gold’s second.  At this point, we’re seriously faced with the reverse flow issue.  If the entire crew of each ship returns after 4 years, then the base population never expands.  At some point, we have to assume a small percentage doesn’t view this as a “remote,” but sees Mars as their new home.  When Constellation goes home, we’d like to plan on it only taking 10 of its remaining 16 crew.  That drops us to 22 people on Mars, long term, but also starts the population buildup. (22 = 6x Constellation + 16x Heart of Gold.)  Importantly, it’s not necessary that the 6 crewmembers remaining on Mars commit to live there forever.  Really, we’re just looking for 6 crew who want to stay another two years.

Let’s follow the population buildup over the next several missions.  Assuming a single MCCS travels at each launch opportunity accompanied by an unmanned freighter, we also assume that the crewed ships on their subsequent trips bring either 32 or 36 people.  If the returning ships all take 26 back to Earth, then the base size increases steadily.  First launch, Constellation, no crew.  Second launch, Heart of Gold:  12.  Third launch, Godspeed: 16 (28 on Mars until Heart of Gold takes its entire crew and 6 of the Godspeed crew back).  Ten on Mars long term, until Constellation returns.  Fourth launch, Constellation:  32 arrive, for a temporary population of 42.  Godspeed returns to Earth with the remaining 10 of its crew and 16 of Constellation’s--total of 26 returning.  Fifth launch, Heart of Gold’s second journey:  32 arriving.  Constellation takes the remaining 16 of its crew and 10 from Heart of Gold.  Twenty-two Heart of Gold crew remain on Mars, as longer-term stays begin.  Sixth launch, Godspeed’s second journey:  36.  As the third MCCS built, Godspeed will benefit from life support lessons on the first two, so it should be able to support a slightly higher capacity.  The population increases to 58 until Heart of Gold returns home, taking 26—16 Heart of Gold, leaving 6 on Mars for 6 years, and 10 from Godspeed, leaving 26 of Godspeed’s crew on Mars.  The population temporarily drops back to 32.  Seventh launch, Constellation’s third journey, bringing 32.   This results in a Mars population of 64, short term.  Note this signals an approaching decision point; somewhere between the seventh and ninth launch, a single MCCS-class ship no longer has enough life support to evacuate the entire base.  We will need to have sufficient confidence in the on-Mars infrastructure to let this happen.  Alternatively, at some point, we will probably have more ships available, so perhaps this point actually determines when we need another set of MCCS or full passenger BFR’s.

The next return, Godspeed, takes 26 again, dropping us back to 38 short term.  Eighth launch is Heart of Gold’s third trip, again carrying 32.  This puts us at 70 people on Mars.  Next return, Constellation, takes 26 again, reducing the long term population to 44.  Ninth launch, Godspeed’s third journey, again 36 arriving.  These are MCCS vessels, so cargo remains the imperative versus personnel.  (Recall the stated plan for BFR is 40 cabins, double occupancy.)  Population increases to 80 until Heart of Gold takes 26 home, dropping to 54 long-term.  Tenth launch, Constellation’s fourth journey, again bringing 32, upping the population to 86.  Next return, Heart of Gold with 26; long-term population 60.  Eleventh launch, Heart of Gold’s fourth trip, again with 32.  Total population 92 until Connie returns home with 26, dropping us again to 66.  Twelfth launch window, Godspeed’s fourth trip, 36.  Base population:  108.  This reaches our base population goal of 100.  Although it will drop back to 76 temporarily when Heart of Gold returns to Earth with 26, all the long-term life support requirements have to assume the higher, temporary population numbers.  Twelve launch windows, each spaced 26 months apart, equate to reaching the goal 24 years after the first crew lands, 26 years into our missions.

Twelve missions, each with a MCCS and a freighter taking a reasonably low-energy trajectory to Mars in order to maximize available cargo.  Somewhere between 250 and 300 tons of cargo per mission window should be available.  That’s 3000 to 3600 tons of cargo, more than any NASA planner or sci-fi author ever dreamed.

Next, let’s take a look at how we’ll decide what to bring.