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This is all theoretical and would never be done in real life because it accomplishes too many stated space exploration objectives, tests too many technologies required for interplanetary travel, and isn't complex enough to satisfy the complexity cravings that NASA has.

We build three SEP tugs and four landers.  The fourth lander is a contingency spare kept at L1 along with a smaller SEP tug for station keeping (the same design that we'll use to return satellites to ISS for repair/refurbishment/refueling/repurposing).

We're going to design the tugs to be capable of transferring payloads to Mars from the outset rather than wasting time and money screwing around with a bunch of intermediate designs requiring subsequent testing.  Every functional part will be designed such that it is on-orbit replaceable without tools, if possible.  As new tech comes out of our labs, the tugs are upgraded.  Think of it as propulsion legos for astronauts to play with.

The SEP tugs enable LEO/L1/LEO transfers of heavy payloads between refuelings, but you have to be willing to wait a few months for the transfers.  This is not a major problem because we can only maintain a relatively low launch cadence for crewed launches.  The heavy landers only use chemical propulsion to get to the lunar surface from L1 and then back to L1.  This maximizes useable propellant mass for our tugs and landers, as additional propulsion hardware and propellant would be required to supply the depot in any orbit other than LEO.  After the initial support infrastructure is in place at ISS, subsequent launches provide consumables and replacement parts only.

If we're willing to launch the landers and SEP tugs with minimal propellant mass and use subsequent launches for fueling, it's possible to make this work with F9H only.  When F9 and F9H become reusable, we can purchase ten or more F9H flights for the price of a SLS flight.  F9/F9H are the American equivalents of Soyuz and Proton.  If the upper stages are also reusable, the crew can launch to ISS aboard a F9, instead of the more expensive F9H.

I was thinking methalox for the landers because it makes the stack shorter since the tankage is smaller and it doesn't have the storage issues associated with LH2.  The propellants are moderately cryogenic, but cooling requirements are within the capabilities of existing active cooling systems that NASA's contractors have already expended considerable effort to develop.  The attitude control thrusters should also use methalox.

The general idea is to test the propulsion systems and landers through years of actual operations in space in the actual configuration required for a Mars mission, less components unique to atmospheric operations like inflatable heat shields.

Dragon is mass efficient enough that a F9H and RL-10 powered upper stage (or something similar to it that doesn't cost so much) can transfer it between LEO and L1, but more importantly, back to LEO without refueling.  Active cooling is required, but we're supposed to be testing that tech because NASA says it's required for mid term space exploration objectives.

If we're doing space exploration on the cheap, we might as well have a concept of operations that makes that possible.

I think we can look at a typical resupply mission and easily determine that a significant mass fraction of the cargo is devoted to propellant, breathing gases, and water.  It highlights the requirement for any long duration mission requiring a high degree of self sufficiency to use electric thrusters for station keeping and have reliable closed loop ECLSS.

For ISS, or any lunar or Mars missions for that matter, to remain within the far reaches of the realm of affordability, much better propulsion and life support systems need to be tested aboard ISS.  All we could establish by tabulating cargo masses would be resupply rate for the consumables.  Consumption rates for oxygen were recorded on Expedition 12, although that was a number of years ago and we'd need more data to baseline average oxygen consumption.  One would think that NASA would have this information.

Rob,

By overhaul do you mean complete redesign?  That's what it would take to fly one.  There's but a handful of people alive who have any experience with the design.  If NASA doesn't permit the contractors to devise some type of impossibly massive craft or issue design requirements that are impossible to meet, it would be simpler to start from scratch and devise a lander with the features you proposed.

SpaceNut,

There is no "fixing" Orion.  It's too damn heavy for any launch system we'll have in the near future to send it and a payload that can do something other than carry astronauts into LEO.  Ares I and Ares V had to be operated in conjunction with each other to do anything useful.  Someone at NASA must have a manual that delineates how to make otherwise simple tasks impossibly complex and expensive.  It's either that or entertainment for engineers.  Simpletons, like myself, just scratch their heads and wonder what they're smoking.  Whatever it is, it must be good.

Are you asking what available upper stage/engine combination could throw 37t to TLI or if there's an upper stage that only weighs ~7t that could throw 37t to TLI?  If it's the latter, then there is no upper stage that only weighs ~7t that can throw 37t to TLI.

SLS doesn't have the performance required to throw Orion and a lander to TLI because it wasn't properly designed to begin with.

There's no way to "fix" SLS, short of redesigning the damn thing to be what it was intended to be in the first place.  Composite tanks, lighter SRM casings, and using a more energetic propellant for the SRM's may get 115t to LEO or so, but that's where the design tops out at.  The 150t to LEO was a requirement that came from multiple studies of what type of throw a heavy lift rocket needed for it to be of utility for a lunar or Mars program.  The plan to use 5 or even 6 RS-25's on a 10M core stage wasn't something random that an engineer pulled out of his butt.

Obviously that's not what NASA designed, so launching a capsule and an usable lander to TLI is out of the question.

Why are we going to baseline this?  A Mars mission without closed loop ECLSS requires slightly insane mass, given current lift capabilities.  CL-ECLSS is, more or less, a hard requirement for any realistic Mars mission.

For a crew of four, assuming a 600 day surface stay (or shorter stay and contingency consumables, however you wish to think about it) and 360 day round trip transit time, that's well north of 140t to LEO, never mind the mass required for TMI.  We've never landed something with the mass that the surface habitat would require, in consumables alone, for a stay of that length.  Why concern ourselves with how much it would weigh?

Once development has been completed, let's fly VPCAR and TSAC aboard ISS and see how well that works.

CL-ECLSS and ISRU make the mission doable from a mass and cost perspective.

No, there's no way to support another STS mission.  The launch and recovery infrastructure has been destroyed or decommissioned.

If I had my way, all the orbiters would be taken from the museums, the avionics would be upgraded and structural enhancements made to the landing gear, and flight capability restored.  From there, the vehicles would simply be stored for future use.  Instead of paying lip service to having a contingency flight capability, we'd actually have hardware available for that purpose should the need arise.

There's no point in actually flying an orbiter if other less expensive vehicles are available for the same purpose.  STS should be used for high value cargo recovery only.

There was nothing "worn out" about STS except for all the excuses and failures to provide a more economical and capable replacement or complementary capabilities like SLS.

As I've stated in another thread, funds could have been devoted to upgrading the avionics and ECLSS for STS and automated methods for repairing or replacing the STS TPS could have been developed.  Congress could have mandated that the components for STS and SLS be manufactured and assembled in one location.  Those simple cost saving measures never materialized because there was no economic incentive on the part of the contractors to do so and there's no political support because everyone wants a piece of the pie.  There's simply no good reason why a partially reusable spacecraft had to cost so much.  It was an entirely contrived aspect of the program.  If Congress wanted to dictate something to NASA that made sense, that made all the sense in the world from an economic standpoint.

There's also no reason why ISS should have cost $100B.  Something akin to SLS should have been in development with STS, using common hardware.  The 5 segment boosters and ATK's proposed Dark Knight boosters contribute very little to the actual performance of SLS, but cost billions to develop.  If even the slightest amount of practicality was applied to the SLS development program, a 5 engined core stage would have been developed and a more energetic propellant used in the same basic SRM design utilized by STS.

STS had one capability that capsule systems lack- the ability to return with sizable and heavy cargo.  If SLS was not designed to return the expensive RS-25's for refurbishment and reuse, then a STS flight should have accompanied every SLS flight to retrieve the hardware.  At $72M a pop, they're too expensive to dump in the ocean after less than ten minutes of use.

If SLS was available to construct ISS with, it would not cost $100B.  ISS has been durable enough to remain on orbit for more than a decade.  The MDM's on ISS have only been replaced once and upgraded once, that I know of.  The Apollo, Saturn V, and Skylab programs incorporated analog and mechanical systems for control purposes.  In short, analog and mechanical methods of systems control are heavy and less capable than digital systems.

We've yet to test a high reliability, fault-tolerant closed loop ECLSS subsystem or artificial gravity generation aboard ISS.  I'm starting to think that that's never going to happen, so why not bring it down when Russia is ready to take back their station modules to save money and start development work on Skylab II?  If it were up to me, I'd bring it down today.

Succinctly, if we want to reconfigure ISS to incorporate technologies required for space exploration, like artificial gravity, active radiation shielding, closed loop ECLSS, ROSA and SEP for station keeping, SEP tugs to haul space junk back to ISS, and adding a manufacturing and repair facility to repurpose the space junk, then ISS and commercial space flight programs have a reason to exist.  If not, we're just throwing money at ISS maintenance and supply.

Getting rid of the old Russian modules will force us to come up with our own solutions to replace the few capabilities lost when they leave and is a step in the right direction.

After decades of operations conducted at multiple space stations, is there still a lot of critical science work that must be performed at a space station in LEO in order to advance human space flight activities required for a Mars mission?

We could use ISS for closed loop ECLSS experiments, but that's about it.  ISS will never be used as a staging point for future manned exploration missions because there's no mission benefits, as far as I am aware, to going to the station first.  What would it matter if all partners involved wanted to bring the station down in 2024?

Is ISS merely a destination for astronauts/cosmonauts to go to to maintain the appearance of doing something necessary or relevant for future manned space exploration missions or is the platform critical path for NASA's stated manned space exploration objectives?

In order to gain experience required for future long duration deep space missions, sooner or later we're going to have to actually build and test deep space habitats, radiation mitigation technologies, and advanced propulsion systems.  We're not going to do any of that aboard ISS.

With or without international cooperation, ISS is an enormous funding suck and isn't advancing our manned space exploration capabilities.  ISS could have been many things, but thus far it has been an orbital construction project and microgravity exposure experiment and little else.  If the Russians are serious about further manned space exploration, they likely see ISS in the same light.  There's a political component to it, but from a practical perspective maintaining ISS won't advance any manned space exploration objectives.

The only option that's viable for future manned space exploration is to design, built, and test a new class of space exploration habitat capable of long duration missions.  This could be viewed as a deep space transit habitat or space station.  I guess it all depends on what type of propulsion module is connected to it.  Russia is moving on to space exploration and science objectives that are of interest to them and I think we, and our international partners, should do the same.  If our space exploration objectives coincide, then cooperation is mutually beneficial.

I think getting rid of the ISS maintenance task is beneficial to all involved.  If US and/or foreign companies find benefit in maintaining the space station, that's fine.  If the Russians want their modules back, give them their modules back.  The future of manned space exploration does not depend on Russian ISS modules.

GW,

So, if the multiple O-ring design was an issue with the STS SRM's, have the SLS SRM's switched to using a single O-ring design?  If not, then there's no change between STS and SLS, so that's not a valid argument for why STS was "unsafe" (bearing in mind that there's no such thing as "safety" that exists anywhere except between someone's ears).

Regarding avionics upgrades, would it be easier to develop new avionics for an existing vehicle for which there was lots of experience from actual operations or a completely new vehicle with unproven flight characteristics?

I agree that a giant rocket isn't necessary, but it does make some aspects of operations simpler.  It made no sense whatsoever to build an expendable launch vehicle using the most expensive reusable hardware ever devised.  NASA should have simply declined that Congressional mandate.  If Congress wanted to shut down the manned space program, it could have.  However, if everyone at NASA stood firm on that position, Congress would also have suffered the backlash from their corporate owners and public opinion.  It's just a jobs program at this point.  We can sugar coat it any way we like, but it is what it is.

At this juncture, we're going to get a giant rocket whether it's needed or not.  Given the length of time SLS has already been in development and the length of time it will be in development before its first flight, I don't think there's any way that it could have been more expensive to simply re-create Saturn V.  If we'd done that, we'd have a 130t class rocket as a starting point.

The type of GCNR I had in mind was a coaxial flow open cycle vessel using Americium fissile material and tungsten seeded hydrogen propellant.  The reactor would probably have a toroidal core shape to inhibit fuel loss and maximize the propellant's exposure to the thermal flux produced by the core.  This complicates flow somewhat but also solves lots of other problems.  Ideally, initial models would not require external radiators and thus would be limited to an Isp between 2500s and 3000s.  I want to determine how small a reactor could be created to replicate the thrust level of the RL-10 and retain desirable operating characteristics (relatively small quantity of fissile material required for operation, low fissile material loss, modest operating pressure, lightweight moderator, no external radiator required for heat rejection, minimal seeding material required for nozzle cooling).

The tungsten-seeded hydrogen propellant can protect the walls of the containment vessel and nozzle for thermal flux required to achieve an Isp of up to 7000s.  With a system that achieves an Isp of up to 3000s, no radiator is necessary.  For Isp ranges between 3000s and 6500s, a radiator is required.  One of the odd but serendipitous things determined through actual experimentation was that the seed material was only required until the hydrogen was heated to 15,000K, at which point it rapidly became opaque (absorbed radiated photons from the UF6 plasma) at the operating pressures involved.  Increasing the operating pressure increased the opacity of the propellant.

Actual experimentation with Americium demonstrated a dramatic reduction in fissile material required to sustain fission.  Using UF4 and UF6 for fissile material requires much larger core radii or a more reflective (heavier) moderator.  It was determined that there was an optimum moderator thickness of about .46 meters (if using UF6, not Americium).  The moderator accounts for a significant portion of the system's mass (unless a higher Isp is desired, at which point radiator mass is a significant portion of the system's mass).

Everything I've read says that this is entirely doable with current materials tech and simply requires funding.  If I'm wrong, then I am, but all the scientists who actually did experiments directed towards development of this technology had results that were favorable for continuing development.  Then the politicians killed funding.

I advocate for development of NTR tech because it strikes a balance between power and efficiency.  If we complete the argument Impaler made in his last post, either more power generation mass or more reaction mass are required to "go faster".  The Boeing SEP tug is already at the limit of what a fully evolved SLS booster could lift.  If more funding was directed to SEP technology development by canceling costly and redundant capsule systems (Orion and CST-100 immediately come to mind), then perhaps by the time SLS is ready to fly we could have a SEP tug ready to test as well.  Impaler shot that idea down.

We can spend money on Cadillac class capsule systems that don't carry the supplies necessary to make a trip to Mars or we can spend money on the propulsion and environmental control tech required to simply get us there and then concern ourselves with how fancy our ISS servicing spacecraft are.

I don't want to wait 20 years for tech to "improve", I want to know what we can do between today and the time SLS is man rated.  Why waste time and money playing with space rocks or developing 3 different vehicles to service ISS when we can send a SEP tug and dummy payload to Mars?

To their credit, Boeing is actively looking for ways to reduce the cost and weight of the ET.  Robotic FSW and spun domes are part of the initial plan and composite tanks and thrust structures are further down the road.  Testing on sub scale test articles was completed last year.  If testing on full scale test articles goes well, SLS would come really close to the 130t target with advanced solids and the 4 RS-25 core stage.

I'm crossing my fingers and praying to the rocket gods.