Mars Mission Study 2013

Update 4-8-2024:  Should any readers want to learn how to do what I do (estimating performance of launch rockets or other space vehicles),   be aware that I have created a series of short courses in how to go about these analyses,  complete with effective tools for actually carrying it out.  These course materials are available for free from a drop box that can be accessed from the Mars Society’s “New Mars” forums,  located at http://newmars.com/forums/,  in the “Acheron labs” section,  “interplanetary transportation” topic,  and conversation thread titled “orbital mechanics class traditional”.  You may have scroll down past all the “sticky notes”. 

The first posting in that thread has a list of the classes available,  and these go far beyond just the two-body elementary orbital mechanics of ellipses.  There are the empirical corrections for losses to be covered,  approaches to use for estimating entry descent and landing on bodies with atmospheres,  and spreadsheet-based tools for estimating the performance of rocket engines and rocket vehicles.  The same thread has links to all the materials in the drop box. 

The New Mars forums would also welcome your participation.  Send an email to newmarsmember@gmail.com to find out how to join up.

A lot of the same information from those short courses is available scattered among the postings here.  There is a sort of “technical catalog” article that I try to main current.  It is titled “Lists of Some Articles by Topic Area”,  posted 21 October 2021.  There are categories for ramjet and closely-related,  aerothermodynamics and heat transfer,  rocket ballistics and rocket vehicle performance articles (of specific interest here),  asteroid defense articles,  space suits and atmospheres articles,  radiation hazard articles,  pulsejet articles,  articles about ethanol and ethanol blends in vehicles,  automotive care articles,  articles related to cactus eradication,  and articles related to towed decoys.  All of these are things that I really did. 

To access quickly any article on this site,  use the blog archive tool on the left.  All you need is the posting date and the title.  Click on the year,  then click on the month,  then click on the title if need be (such as if multiple articles were posted that month).  Visit the catalog article and just jot down those you want to go see.

Within any article,  you can see the figures enlarged,  by the expedient of just clicking on a figure.  You can scroll through all the figures at greatest resolution in an article that way,  although the figure numbers and titles are lacking.  There is an “X-out” top right that takes you right back to the article itself. 

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Update 8-17-18:  I revisited this very study in 2016,  with electric propulsion to send the unmanned assets ahead.  That got a huge reduction in launched mass.  The basic manned vehicle notions got refined in 2016,  and I used essentially the same two-way one-stage landers.  That updated version is on this site as "Mars Mission Outline 2016" dated 5-28-16.  Use the navigation tool on the left.  Click on 2016,  then May.  It is the only thing posted during May that year.
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original article:
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This is the culmination of about 3 years' effort on my part toward roughing-out practical manned Mars mission designs.  I started with nuclear rocket solutions that drew many objections.  This study is what it looks like when you combine all-chemical propulsion for the transit vehicles and the landers,  with historically-valid concepts regarding the purpose of exploration,  and with the ethical consideration of designing-in "a way out" for crew self-rescue at every step possible.  And,  this is what it looks like when you pay attention to what we already know is crucial for successful long-distance travel in space (another ethical consideration).  Updates 12-20-13 in red.  Update Dec 31 2013 at end in blue.

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In November and early December,  I roughed-out a series of chemically-powered manned Mars mission scenarios somewhat similar to my nuclear-powered Mars Society 2011 paper (ref. 1),  and my associated subsequent second thoughts (ref. 2).   This was based around the same ideas of multiple landings in the one trip,  and a reusable one-stage chemically-powered Mars lander (similar to the design in ref. 3).  To that mix,  I added the notions of starting some sort of base at one of the sites visited,  a visit to Phobos,  and storing all the reusable and salvage assets in stable locations (not even one propellant tank is discarded,  every single thing is salvaged for re-use!).  All of this is actually a feasible thing to do.  Done right,  the price tag is order-of-magnitude closer to $100 billion not $1 trillion.

I went through a couple of scenarios based on capture into a high-apogee elliptical orbit,  which reduces arrival velocity requirements a bit,  and makes a visit to Phobos relatively easy.  However,  a one-stage reusable chemical lander of significant payload fraction is far too limited in velocity capability for anything but a low circular orbit to be practical.  This orbit needs to be inclined at the value of the farthest latitude of interest,  so as to eliminate the plane changes the lander cannot afford. 

These scenarios,  and those in the references,  all begin by addressing what is really to be accomplished by sending people to Mars.  That is a far harder task than going to the moon was,  by far.  Figure 1 explains why that is true.  But,  this is now within our technological grasp,  so we should go.  Figure 2 relates the drive to do this to the entire history of humanity.  It is the manifestation of an urge that is simply built into us.  Exploring and colonizing new places is what we have always done.

Figure 1 – Why Sending People to Mars and Back is So Hard

Figure 2 – The Drive to Go to Mars Is Part of a Very Old Urge

Going to and settling new places is colonization,  pure and simple.  It is no different with space travel.  There are new places out there to explore,  and maybe settle.  Thinking long-term,  colonization is what it is all about,  even though the initial trips of course have a far more limited scope (they always have).

The most successful approach used half a millennium ago settling the New World is the 3-step process shown in Figure 3.  Those are given names here as (1) exploration,  (2) adaptation,  and (3) colonization.  While shown as separate blocks in the figure,  these steps inherently overlap a little (they always have). 

Exploration,  properly done,  answers two very deceptively-simple questions,  shown in the figure.  I mean them exactly as they are phrased,  word-for-word!  Until you have answered those,  you cannot learn how to live in the new place,  much less establish settlements,  or a full-blown colony,  because you still don’t know what is there that you can use.  How simple is that?  Yet,  so very challenging!

The concept of “ground truth” gets into this with the technologies we have today.  Remote sensing can tell you that there seems to be water in this place on Mars,  and not that one.  But,  you do not know for sure the sensing result is correct,  you do not have any information regarding how much is really there,  or that its quality is something you can use.  Even today,  we must visit the site with a drill rig and look deep underground in multiple places to quantify whatever resources are there.  So far,  we have no robots who can do that job on Mars.  That’s why people must go:  to complete the exploration process. 

The second step is adaptation,  which is basically learning how to use local resources to live there.  That’s your first base or settlement,  maybe more than one,  maybe not.  There’s two broad categories here:  dependent and independent living.  You start out dependent on supplies shipped from home,  augmented as best you can by what you can produce locally.  On Mars,  food,  water,  and air are all very big problems.  So is propellant production.  So is all sorts of infrastructure. 

Later,  as you learn to do more with the local resources,  you gradually shift into independence from critical supplies shipped from home.  That does not happen very fast.  It’s not something that can be planned thoroughly ahead of time,  precisely because you are adapting to an environment new to you. 

But,  once you are independent of basic survival supplies shipped from home,  that’s when you can begin shifting from being a base doing adaptation,  into being a real settlement or colony making your own living in the new place.  It happens very,  very gradually.  It always has.  You don’t just “land one!”

Figure 4 shows the best historical approach for who pays for what phases in this 3-step process.   The most successful colonies came from doing exactly those 3 steps,  funded in this way.  However,  great thought must be given to what sort of trade economy will be built as the colony matures.  It has to go beyond simple extraction of local resources that folks back home might find valuable.  Historically,  those colonies who never got beyond the resource extraction goal are now mostly still Third World countries.  Those for which a real two-way trade economy got established early are now prosperous nations.  This was done more by chance back then,  we should not make that mistake today. 

Figure 3 – The Historical 3-Step Process That Was Most Successful Colonizing the New World

Figure 4 – The Historically Most-Successful Way That Colonization Is Funded

All that being said,  the realities of the politics-of-money today suggests that there will be one and only one manned expedition sent to Mars at government expense.  It does not matter to that conclusion whether one government does this alone,  or several governments band together to do this.  It does not matter if a few visionary private entities participate,  it is government that funds the lion’s share of that first trip,  just as it was long ago.  Initial exploration is what governments have always funded,  it is unreasonable to expect that going to Mars will be any different.  It is also unreasonable in today’s world to think they fund two or more successive trips. 

What that means is that the first expedition must accomplish fully the first step (exploration),  and put the assets in place for a first base (start the second step) at the best possible site.  That is what is shown in Figure 5.  Once that first base is in place,  even if it is not initially manned permanently after the expedition leaves,  it acts to draw the visionary private entities who “smell” an opportunity.  This is not fundamentally different today than 500 years ago.  “If you build it,  they will come” is actually true.

We already have technologies we can take to Mars that could potentially make water,  air,  and perhaps even rocket propellants on Mars,  given the right site resources and conditions.  (Food production is a much tougher problem,  that’s part of what the adaptation base is eventually for.)  We might even be fairly sure of our technologies when we go that first time.  The problem isn’t the technologies so much as it is getting the “right” site.  There,  just like here,  every site will be different and none will be exactly “average”.  To expect otherwise is either naïve,  or unreasonable,  or both. 

Thus it is imperative that we visit more than one site in that first exploratory trip.  We have to pick the “best” one of those,  meaning the one where the local resources best match up with the technologies we brought with us.  We’ll have more than one potential landing site identified before we go.  The real objective for the people on the expedition is to find out,  with actual ground truth,  which candidate is actually the best site for the base.  You do that by trying out your adaptation technologies at all of the sites.  That is why Figure 5 says what it says. 

If you don’t visit more than one site,  your chances are much lower of building an adaptation-development base that is successful enough to draw the private entities into a majority funding position for subsequent expeditions.  If you don’t build such a base on your first trip,  then your first expedition essentially devolves into a “flags-and-footprints” mission.  Given the difficulty and expense of going all the way to Mars with people at this time,  that would be entirely pointless!

That imperative to pick a “best” site and start a base is why I do not think there is much value to the many minimalist mission plans I have seen proposed that just make one landing,  direct or otherwise,  even though their price tags are closer to $10 billion than $100 billion.  Our remote sensing is just not good enough to bet lives and potential settlement-attractiveness on it.  So,  this old saying is actually quite true about sending people to Mars:  “Go whole hawg or none”.

Figure 5 – Goals for the First Mission

Roughing-Out A Mission:  Start With the Lander

Given the imperative to make multiple landings,  it is the landing craft and its propellant supplies that will “drive” the mission design.  This is because that will be one of the larger masses that must be “dead-headed” to Mars,  the other being the propellant supply for returning the crew to Earth.  Betting their lives on return propellants manufactured on Mars is unwise and unsafe,  because you don’t really know your equipment will work well enough at the site you pick,  until you actually try it!  The answer could just as easily be “no it won’t work adequately” as “yes it will”.  The point is,  you don’t know for sure. 

That is why in Ref 1 and 2,  and this study,  I send enough propellants from Earth to accomplish the entire mission safely.  Anything made locally just augments that supply,  making more sites potentially explorable,  and making the base left behind more attractive to those who follow.   You do that in the second half of the stay,  when you are surface-based,  by using your landers in suborbital flights,  with the propellants you have made while you are there.  

In refs. 1 and 2,  I was looking at single-stage nuclear-powered reusable landers.  But,  unlike the transit vehicles,  in a lander there is little radiation shielding available from the structure and the distances.  So,  in ref. 3 I looked at a family of chemically-powered landers,  to be used from a low Mars orbit at 200 km.  All had the same 3 ton payload.  The liquid oxygen (LOX) – liquid hydrogen (LH2) produced the smallest lander mass by far.  The others,  including otherwise-attractive LOX-liquid methane (LCH4),  were all 60 tons or higher in ignition mass.  Yet all were feasible as single-stage reusable vehicles,  to be refueled on-orbit from orbiting supplies,  and flown again.  That payload fraction is a bit low at 5%.

Looked at another way,  the high specific impulse (Isp) afforded by LOX-LH2 simply produces higher payload fraction at otherwise equal conditions,  by far.  These November-December 2013 studies were aimed at a larger fixed payload mass of 11 to 12 tons.  This time,  the only feasible configuration was a 79 ton LOX-LH2 lander with a rather attractive 14% payload fraction.  Even LOX-LCH4 never exceeded about 5% payload fraction,  or got under about 200 tons ignition mass.  Numbers like that very quickly push you into building unaffordable “Battlestar Galacticas” just to get there and back.  It's bad enough just reusing every single asset,  including propellant tanks.  

LOX-LH2 also makes the best sense for transit propulsion,  again due to its substantially-higher Isp.  If you use the same propellant for both transit and lander,  you only have to worry about one kind of propellant tankage to contain it,  and you could use similar,  if not exactly the same,  rocket engines for all vehicles.  Those are things that simplify design and reduce weight. 

That kind of thinking leads immediately to a modular vehicle design with a common propellant module that is easily launched,  plus the use of multiple assembled vehicles,  so that assembly and transits-to-Mars can be spread-out over time before the people go.  If you use the landers themselves as their own transit propulsion,  that saves even more inert weight.    This was the basis of the nuclear LH2 vehicle fleets in ref. 2,  and it is the basis of the LOX-LH2 vehicle fleet in this study. 

Accordingly,  the lander design requirements used in this study are given in Figure 6.  Note the slightly-higher orbit altitude of 300 km,  guaranteeing good stability for the year this fleet will spend at Mars.  That orbit will be raised at departure to 500 km for even longer-term storage stability of the reusable and salvage assets left at Mars.  My terminal descent requirement is (I hope) a conservative guess. 

The basic lander design criteria and assumptions are given in Figure 7.  20% total inert fraction is barely credible for a vehicle that flies several-to-many times.  Experience with a variety of vehicles says that robustness is heavy.  The 11 ton dead-head payload includes a crew of 3 plus a month’s supplies,  and about 9 tons of exploration and experimentation equipment.  This would include a rover with a big drill rig on it,  capable of reaching around a kilometer down.  The surface habitation is in the lander itself. 

Figure 8 shows the lander weight fractions I got with LOX-LH2 and LOX-LCH4.  There is no question that LOX-LH2 is the better choice.  A 200+ ton lander with LOX-LCH4 just drives you into building “Battlestar Galacticas”.  79 tons with LOX-LH2 is bad enough:  it will require assembly from smaller components in low Earth orbit (LEO),  but that can actually be done,  as partially proven by the construction of the ISS. 

Lander layout and weight statement is shown in Figure 9.  Note that the stance is about as wide as the lander is tall,  so it will have good stability,  even on rough terrain.  The dimensions are consistent with the masses and densities of the propellants,  and the masses and dimensions of the other cargo items and crew living space requirements.  The decks above the main cargo floor remain pressurized for the crew to live in,  on the surface.  The cargo deck is around the engine compartment,  and between the 3 (or 4?) retractable-landing leg bays.  The crew cabin is actually a minimal abort capsule to get the crew to the surface without killing them;  it has 6 seats,  even though I planned on a usual crew of 3. 

This lander has an engine compartment that is sealed gas-tight to the rest of the vehicle.  Its only openings are the ports the four engines fire through;  and those have no covers,  they remain wide open during hypersonic entry into Mars’s atmosphere.  Because the compartment is sealed,  there is no through-flow of oncoming superheated gas,  quite unlike the shuttle Columbia’s damaged-wing situation that killed it and its crew.  Nothing insulates as well as a gas column. 

On entry,  this vehicle comes out of hypersonics (local Mach 3) about 10 to 15 km altitude,  which is way too low for a chute to deploy,  much less do any deceleration “good” on an object this big and heavy.  So,  instead,  the engines simply fire up in supersonic retro-propulsion,  for a direct rocket-braking landing.  This process is actually what sizes the four lander engines,  which are slightly canted at about 10 degrees,  to provide plume stability during supersonic retro-propulsion.  Engine specifications and design data are given in Figure 10.  I did not include the detailed results of my entry analysis here,  although the crew feels not more than about 1.5 gees the whole way. 

Figure 6 – Velocity Requirements for the Lander with Low Orbit Basing

Figure 7 – Basic Design Criteria and Data for the Lander

Figure 8 – Verifying Propellant Choice

Figure 9 – Lander Layout and Weight Statement

Figure 10 – Lander Engine Specifications and Design Data

This same lander pushes its own landing propellant supply to Mars one-way.  The basic idea is to divide the approximately-one year stay at Mars into two phases:  explore a few sites based from orbit the first few months,  and then establish a base at the best one of those sites,  where everybody stays the remainder of the time at Mars.  During that second phase,  the crew leaves the return vehicle and return propellant supply in orbit,  until departure.  This process is depicted in Figure 11. 

This two-phase process also designs-in maximum crew safety.  During the first (orbit-based) phase,  3 go to the surface in a lander,  while the other 3 do science from orbit,  with a “reserve” lander available for rescue.  If you send 3 not 2 landers,  then you always have a rescue “bird” available,  even if one should fail.  That way,  you need not abort the mission if one lander fails. 

During the second phase,  you bring all 3 landers and all 6 crew to the base at the best site.  If you are able to make propellant locally,  it can be used to support suborbital missions to yet other sites,  using those same landers.  But,  you still have a rescue “bird” and a backup,  if something goes wrong.  One of those 3 landers is all it takes to get the entire crew back to orbit,  although the initial concept calls for leaving as many landers as possible in orbit,  when the crew departs to come home.   

The philosophy here is designing-in a “way out” at every phase of the mission.  Nothing else is ethical. 

Figure 11 – The Two-Phase Process to Both Explore and Establish an Initial Base on Mars

Roughing-Out A Mission:  Decide Upon Some Modules

I ran enough numbers for space per person and supplies per day in ref. 4 to rough-out a generic habitat module (see fig. 12) and a generic stored supplies module (see fig. 13).  Two of each are enough for a crew of 6 for nearly 3 years.  So,  I just used those design numbers here.  The habitat modules at just under 25 tons each (as loaded) could be launched one-at-a-time by the heavy-lift variant of Atlas-V,  or two-at-once by a Falcon-Heavy.  The storage modules are just under 50 tons each,  requiring a Falcon-Heavy to launch them one-at-a-time.  Both are 5 m in diameter.   

My prior studies were hydrogen-propelled nuclear thermal designs.  Those modules are not quite what is needed here with LOX-LH2 chemical propulsion,  but all the same ideas and features apply.  I roughed out spherical insulated cryo-tanks for LOX and for LH2 that would fit end-to-end within a shell 5 m in outside diameter.  This shell would cover some truss structure,  and have an outer layer that is really multiple layers of foam-and-foil meteoroid shielding.  See fig. 14.

There is plenty of volume in the ends of this propellant module for fold-out docking gear,  accommodating both end-to-end and side-by-side docking,  in a variety of stack configurations.  Solar cells along the outer surface would power a small cryo-cooler in each module.  I simply guessed that 5% inerts would cover all of this,  since these tanks get launched once,  and stay in space thereafter,  even if and when they ever get reused. 

Figure 12 – Habitat Module (2 Required for Crew of 6)

Figure 13 – Supplies Storage Module (2 Required for Crew of 6)

Figure 14 – Common Propellant Module for LOX-LH2 Vehicles

My original idea was to push everything to Mars with the landers.  This did not make as much sense for the manned vehicle,  because that would bring one lander back to Earth instead of leaving it at Mars where it could be used again.  As the vehicles sized out,  the landers pushing dead-head propellant were larger and heavier than the two-way manned vehicle.  The better solution was an engine module specifically for the manned vehicle,  with two engines for redundancy.  The fleet concept is shown in Figure 15. 

The thrust level of one lander engine was sufficient,  so the engines on the manned vehicle engine module would be very similar to the lander engine,  just half thrust.  I took an educated guess for engine weights and structure for the module using 50:1.  Those numbers are given in Figure 16.

The velocity requirements for the trip to and from Mars were figured for worst-case planetary alignment (Mars at aphelion and Earth at perihelion).  See Figure 17.  An orbit-raising requirement was also computed,  as the landers and empty tanks left there need an orbit stable over several years,  not just one.  I included this orbit-raising maneuver in the propellants computed for the unmanned vehicles,  and in the manned vehicle return configuration.  The plane change at arrival was assumed to be direct entry to 40 degrees inclined from the interplanetary transfer ellipse,  and the same for return.

Note that both the departure and arrival (and any orbit change) requirements are included in the total velocity requirement.  No empty propellant tanks are staged off after each burn,  every single asset is left where it can be recovered and re-used.  

Figure 15 – Concepts for Vehicle Fleet to be Assembled in Low Earth Orbit

Figure 16 – Engine Module for the Manned  Vehicle

Figure 17 – Velocity Requirements for a Low Orbit-Based Mission at Worst-Case Planetary Alignment

In Figure 18 are the requirements for a Phobos excursion from the low inclined orbit.  This includes a plane change maneuver at apoapsis to reach Phobos,  and the same to return.  As it turns out,  the velocity requirement for this trip is only slightly less than the design velocity requirement for any one landing on Mars.  Thus it takes the same 3-propellant-module quantity of propellant to fuel up for a round trip to Phobos,  as it does to land on the surface of Mars.  The dead-head supply for the landers is thus 6 landings on Mars and one trip to Phobos:  7 trips,  for 21 propellant modules. 

The rest of the dead-head propellant supply is that required for manned vehicle return (to Earth orbit for reuse!!).  For safety purposes,  it is assumed that rendezvous and docking in Mars orbit will be reliable,  as it is here.  Apollo made that same assumption at the moon.  On the return trip, it is assumed that the supplies are 2/3 depleted at start of the return voyage,  and wastes have been left behind.  This lightens up the two storage modules by quite a bit.  That configuration needs about 17 propellant modules to make the return voyage,  as shown in Figure 19.  The heavier configuration for the outbound voyage at full supply weights requires more modules (some 28).  This is also shown in Figure 19.  Both configuration spin end-over-end for 1 full gee artificial gravity as shown.

This total dead head propellant supply is split up among the 3 landers,  and propellant modules added until they can meet the velocity requirements to reach orbit about Mars.  Each lander thus has 54 modules to push and draw from,  as shown in Figure 20. 

Figure 18 – The Phobos Excursion

Figure 19 – Manned Vehicle Configured for the Return,  and For Outbound to Mars

Figure 20 – Dead-Head Propellants and Landers as One-Way Unmanned Vehicles

The launch manifest for assembling this fleet in low Earth orbit is given in Figure 21.  This listing includes everything that departs for Mars,  plus 9 more propellant modules that fuel up the landers for departure.  These 9 modules remain in Earth orbit.  With one important exception,  everything to be launched fits atop existing commercial launchers (Atlas-V,  Delta-IV,  and Falcon-9),  or one that will fly in 2014 (Falcon-Heavy).  These vehicles are assembled by simple docking in orbit,  the same as was the International Space Station.  There is no new technology to be developed here. 

The exception to all-assembly-by-docking is the lander design:  this has a base diameter of 12 meters and so is far too wide to fit any of these launchers,  even though the dry weight for the entire lander is feasible for Falcon-Heavy at just under 27 tons.  The landers will require “real” assembly from smaller components on-orbit,  something not so very practical with today’s typical spacesuits.  Conceptually,  I divide a lander into a less-than-5-m diameter core,  a load of decking and framing plus landing legs,  and a load of shell plating and heat shield panels plus the cargo-handling gear.  These would be nominal 9 ton loads,  launchable by 3 Falcon-9’s to build one lander.    But,  it has to be put together in zero-gee and vacuum,  and that’s what we have never done before.  It’s more new skills than it is new technology. 

The total tonnage in the manifest is 3725 metric tons,  most of which is propellant modules that simply dock together,  and their plumbing gets coupled up.  Atlas-V and Delta-IV are costing in the vicinity of $5000/kilogram when flying at or near full load.  Falcon-9 is significantly less expensive,  and Falcon-heavy is projected as a lot less expensive.  That produces the launch costs given in Figure 22.  A wild guess puts launch costs at 20% of program costs,  for something a lot closer to $100 billion than $1 trillion.  It is about half the numbers that I got for the high-orbit basing scenarios. 

Figure 21 – Launch Manifest for the Fleet

Figure 22 --  Conclusions and Costs – Base in Low Orbit!

Concluding Remarks

What I have presented is a manned Mars mission plan that (1) makes sense in terms of what exploration really is and does,  (2) makes sense in terms of the politics-of-money,  (3) requires no major new technology developments,  (4) reuses every single asset,  including all empty propellant tanks,  and (5) builds in safety and a self-rescue “way out” for the crew to the maximum extent possible every step of the way.  That last is important because the hard lessons of the Apollo 1 fire and the two lost shuttles is that “nothing is as expensive as a dead crew”.

Items (1) and (2) are very important.  Sending people to Mars and back is far too difficult and expensive to waste it on a “flag-and-footprints” stunt.  It is very likely that only one mission will ever be sent on largely government funding,  and even that is not a sure thing.  That one mission had better lay the groundwork work for the largely commercially-funded permanent base efforts later,  efforts that might eventually result in a permanent settlement and colony. 

Item (3) is a real “killer” for government-funded efforts.  Programs emphasizing major new technology developments simply do not provide flying vehicles.  Examples:  X-30,  X-33.  If you really want to go to Mars now,  you do it with what you have now!  Period!  This does not rule minor items like a new spacesuit,  but it does rule out major items like waiting for new propulsion.  Using what you have is how we went from nothing to the moon in about 10 years.  That’s what is required to reach Mars in only about 10 years. 

Item (4) is crucially important in a longer time frame sense,  because return missions to Mars (no matter who funds them) can use the assets already there,  and because vehicle assets recovered into Earth orbit can be re-purposed and re-used for other missions.  Why launch new hardware if you don't have to?  Just launch new propellants and supplies,  and make the trip,  whatever it is.  

The modular vehicles outlined here could easily by reconfigured to accomplish the other space missions beyond Earth-moon space that we might be interested in.  These would include missions to near-Earth asteroids for purposes of learning how to defend against asteroid impact.  It might include trips to the main asteroid belt beyond Mars.  It could include trips to Venus orbit,  and even to the surface of Mercury.  

If you put the water and wastewater tankage around the flight control station in one of the habitat modules,  and make it big enough for the whole crew for about a day,  you have solved the radiation protection problem.  By spinning end-over-end at the very modest rates indicated above,  you have solved the microgravity disease problem  with spin artificial gravity.  That also simplifies a variety of long-term life support issues as well. 

The needs that I forsee are twofold:  (1) we need a supple,  lightweight,  nonrestrictive spacesuit,  and (2) we need the skills and experience at real construction in Earth orbit to support building the landers “from scratch”.  Possession of those two items feeds back into the Mars mission and anything else we really want to do in space,  in so many ways that it is not feasible to list them here.  And,  these two items go together:  it is not possible to do nuts and bolts construction work,  or plumbing,  or wiring,  with the clumsy suits that we now have.  That is the real need that must be addressed to go to Mars. 

Note that I did not specify the atmospheres to be used inside the vehicle modules or in the landers.  That would inherently go with the spacesuit design,  so that decompression for nitrogen-blowoff is unneeded for going outside.  Yet there are extreme fire dangers and health risks associated with pure-oxygen breathing. 

We have already done these selections for the shuttle and all the space station programs going back decades.  We simply need to do it again with the new suit that we have to have.  I suggest this should be a mechanical counter-pressure (MCP) suit,  but done in a new way as “vacuum-protective underwear”.  But,  that’s another topic.   Some fundamental compression requirements for it are given in ref. 5. 

Update 12-31-2013:  

This study is,  in many ways,  an upper bound on a practical and reasonably safe mission design that produces a great deal of results almost no matter what actually happens.  So,  to reduce tonnage launched and thereby mission cost,  what can you give up on?

To stay very productive no matter what,  you cannot give up on basing from orbit and visiting multiple sites early in the stay at Mars,  and your landers must refuel and fly multiple times.  Nor can you give up on establishing that adaptation base on that first mission,  given politics-of-money in our time.  

To stay safe,  you cannot give up on sending the exploration and return propellant supplies from Earth,  and you cannot give up on sending 3,  not 2 or 1,  landers.  

To stay healthy,  you cannot give up on supplying radiation protection and pretty near 1 full gee of artificial gravity.  A modular baton-shaped ship design that spins end-over-end is definitely the most effective way to do artificial gravity.  

What you can give up on is saving every single scrap of hardware for reuse or salvage.  I roughed-out my designs for the manned and unmanned vehicles by making both departure and arrival burns from a set of propellant tanks initially full,  without staging off any empties before making the arrival burn.  One could stage off those empties into deep space (actually the transfer orbit,  so there are long term collision risks) after the departure burn.  I have not yet investigated this,  but I'd guess the savings would be closer to 10% than factor-of-two.  

What you do not want to give up on,  unless forced to "at gunpoint",  is recovering the manned vehicle in Earth orbit at mission's end.  Jettisoning this vehicle into deep space (actually the transfer orbit,  so there are future collision risks) allows you to eliminate the arrival burn in favor of a free return.   This will save some propellant tonnage,  but it loses you future use of something very expensive to launch:  the assembled crew habitation and supply storage,  and its transit engines,  at the very least.  

This would be "penny-wise but pound-foolish" mismanagement,  because those very same items can return to Mars,  visit near-Earth asteroids,  or even go to Venus or Mercury.  With some hotter propulsion substituted,  those same components could even take men to the main asteroid belt.  

Why build them too fragile and throw them away,  again and again and again?  That's stupidity incarnate.  

Build them tough and launch them once,  and fly them to many places over many years.  

References

1. G. W. Johnson,  “Going to Mars (or Anywhere Else Nearby)”,  paper presented at the 14^th International Mars Society Convention,  Dallas,  Texas,  August,  2011.  A version is posted at http://exrocketman.blogspot.com,  dated 7-25-11,  same title.  (Nuclear-powered designs.)

2.   .2.  G. W. Johnson,  “Mars Mission Second Thoughts Illustrated”,  posted at http://exrocketman.blogspot.com,  dated 9-6-11.  (Nuclear-powered designs.)

3.    3.   G. W. Johnson,  “Reusable Chemical Mars Landing Boats Are Feasible”,  posted at http://exrocketman.blogspot.com,  dated 8-31-13.  (Chemical 1-stage reusable landers.)

4.    4.  G. W. Johnson,  “Rough-Out Mars Mission with Artificial Gravity”,  posted at http://exrocketman.blogspot.com,  dated 7-19-12.  (Nuclear transit,  undefined landers.)

5.       G. W. Johnson,  “Fundamental Design Criteria for Alternative Space Suit Approaches”,  posted at http://exrocketman.blogspot.com,  dated 1-21-11.  (Applicable to MCP suits)