Author Topic: MCT Speculation and Discussion Thread 2  (Read 281895 times)

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 2
« Reply #340 on: 01/23/2015 04:18 pm »
Mars EDL is pretty hard, so you'd try to get down as much useful cargo as you can. If a transfer stage can remain in orbit, why not keep it there? It can be refuelled in LEO, and used again for the next trip.

I don't understand how you use a transfer stage to boost you out of LEO and most of the way (delta V wise) to Mars, and yet you "keep it there" (the transfer stage in LEO).  If you already need the engines in the Raptor for some of the delta V to Mars (and Mars decent and landing), it seems you pay only a very small mass penalty to make the tanks big enough make the complete delta V burn.  Especially compared to the extra mass of an additional transfer stage with its engines and especially if that stage needs to reserve fuel to negate its extra delta v and return back to LEO.  Or maybe I'm just not understanding your concept?

In any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. 

I think I agree with you here.  I wouldn't be surprised that when Musk announces the rough design for MCT that it will be scaled down to, say, 50 passengers, and that in reality it will never come close to having that many.  I think the first several MCT missions and synods will see it with a 'modest' crew of about 6 to start, building up to at most 15-20.  These folks will be busy exploring (esp. water sites and extraction methods), and setting up infrastructure (habs, power, prep'd landing site, greenhouses, etc.).

Then after a decade and a half, we'll see some type of Aldrin Cycler for transferring 100 (or more!) folks per trip.  This vehicle will have much more volume per person that the MCT could ever offer, have artificial gravity via spinning modules, have a high degree of closed life support, offer good solar flare protection, etc.  The MCT's mission thereafter will transition very nicely to ferrying people and cargo from Earth's surface up to the cycler, and then at Mars from Mars orbit down to the surface (i.e. MCT's won't make the Earth-Mars 'crossing' any longer).

Offline symbios

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Re: MCT Speculation and Discussion Thread 2
« Reply #341 on: 01/23/2015 04:35 pm »
SpaceX has said themselves that it will take many trips before they start to send 100 persons per trip.

Since they can only make one trip every 2 years. Say that it will take 5 trips or 10 years before they start sending any serious amount of people. By that time we are into 2035 to 2040...

The MCT will have gone through many iterations between 2020 and 2035. It will not be the same MCT as we are thinking about today...  And remember this is SpaceX time...

I do not think that this MCT will ever see 100 people on it.
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Offline Owlon

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Re: MCT Speculation and Discussion Thread 2
« Reply #342 on: 01/23/2015 04:39 pm »
In any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. 

I think I agree with you here.  I wouldn't be surprised that when Musk announces the rough design for MCT that it will be scaled down to, say, 50 passengers, and that in reality it will never come close to having that many.  I think the first several MCT missions and synods will see it with a 'modest' crew of about 6 to start, building up to at most 15-20.  These folks will be busy exploring (esp. water sites and extraction methods), and setting up infrastructure (habs, power, prep'd landing site, greenhouses, etc.).

Then after a decade and a half, we'll see some type of Aldrin Cycler for transferring 100 (or more!) folks per trip.  This vehicle will have much more volume per person that the MCT could ever offer, have artificial gravity via spinning modules, have a high degree of closed life support, offer good solar flare protection, etc.  The MCT's mission thereafter will transition very nicely to ferrying people and cargo from Earth's surface up to the cycler, and then at Mars from Mars orbit down to the surface (i.e. MCT's won't make the Earth-Mars 'crossing' any longer).

It really isn't that hard to fit 100 people in a 15m capsule, even if a large portion of it is propellant tanks. You could easily have 500+ m3 of pressurized space. If everyone gets a little enclosed 2-3 cubic meter bunk/private area, at any given time you can have 1/3 of the crew sleeping, 1/3 taking private time in their bunk, and 1/3 out in 200+ m3 of public space exercising, eating, socializing, and whatnot. You only need an open area that can comfortably accommodate about 30-40 people.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 2
« Reply #343 on: 01/23/2015 05:18 pm »
Basically I believe that MCT in the long run will be its own launch abort system. Capable of separating from the first stage and soft land. But probably not capable of speeding away from a fireball in case the first stage has a catastrophic failure without prior warning.

I believe that after a sufficiently large number of flights the launcher will prove that this kind of catastrophic failure is rare enough that it needs not be considered in operation. But in discussion it seems I am quite alone with that position.

You are not alone, I have been arguing the same for quite some time. If we are ever going to transport a large number of people to orbit, we are going to have to let go of the most extreme LAS requirements. (we don't equip airliners with parachutes for all) The MCT itself could provide abort capability throughout most of the launch.
« Last Edit: 01/23/2015 05:19 pm by Lars-J »

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 2
« Reply #344 on: 01/23/2015 06:01 pm »
I'm not up to date on all of Elon's comments regarding the MCT, but here's my take.

I think the MCT will basically be a large, scaled-up version of Dragon 2, with large fuel tanks and Methalox engines, perhaps with a 15m diameter heat shield (I don't think Musk will go for anything other than the capsule shape, since that's what he has experience with).

I dunno, they are accumulating quite a bit experience with cylindrical stages as well. (not quite the same reentry, but a violent one nonetheless)

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 2
« Reply #345 on: 01/23/2015 06:31 pm »
SpaceX has said themselves that it will take many trips before they start to send 100 persons per trip.

Since they can only make one trip every 2 years.

Not if it has the thrust to widen the available window, no, they won't have this issue. Even one Vac raptor would provide them with this thrust, and we can safely assume that it'll have the fuel for it.

Besides, what's to stop them from sending out a flotilla at once when they get into the swing of it? They'll certainly have an enormous revenue stream by then if they don't screw up, and the cost of the BFR+MCT is meant to be low; comparable to modern expendables or less. Each MCT and BFR is meant to be good for multiple flights, so assuming referb/re-fuel is good, you're going to end up with a stockpile of them over time. If they build one MTC+BFR every year, and they'd certainly do faster rates, you'd end up with two by the next window. As the first returns, you get three, then so on. Then couple in increased production rates.

SpaceX has used the phrase "Mars Colonial Fleet": Certainly, they will require one if they want to get to the "sending 100 people at a time" to Mars phase within our lifetimes.

I believe they'll make it work.

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Offline symbios

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Re: MCT Speculation and Discussion Thread 2
« Reply #346 on: 01/23/2015 06:51 pm »
SpaceX has said themselves that it will take many trips before they start to send 100 persons per trip.

Since they can only make one trip every 2 years.

Not if it has the thrust to widen the available window, no, they won't have this issue. Even one Vac raptor would provide them with this thrust, and we can safely assume that it'll have the fuel for it.

Besides, what's to stop them from sending out a flotilla at once when they get into the swing of it? They'll certainly have an enormous revenue stream by then if they don't screw up, and the cost of the BFR+MCT is meant to be low; comparable to modern expendables or less. Each MCT and BFR is meant to be good for multiple flights, so assuming referb/re-fuel is good, you're going to end up with a stockpile of them over time. If they build one MTC+BFR every year, and they'd certainly do faster rates, you'd end up with two by the next window. As the first returns, you get three, then so on. Then couple in increased production rates.

SpaceX has used the phrase "Mars Colonial Fleet": Certainly, they will require one if they want to get to the "sending 100 people at a time" to Mars phase within our lifetimes.

I believe they'll make it work.

I to believe they will make it work. But you need to get the infrastructure in place to receive the people, you need to test all the equipment etc. etc. They will not send 100 people on the first or the second flight.

I believe that SpaceX will make a return flight on every window. But that window only opens every 2 year.

So I stand by my statement that it will probably be 2035 (SpaceX time) before any "fleet" with people on board will be sent.
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Offline ArbitraryConstant

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Re: MCT Speculation and Discussion Thread 2
« Reply #347 on: 01/23/2015 06:58 pm »
Manabu - I'm one of the people who believes at least some sea level optimized raptors on MCT are a good idea.  I think they're needed for landing the MCT on Earth, useful for abort scenarios, useful for reducing gravity losses at launch from Earth (fueled from the booster), and they don't have gigantic flimsy nozzles.  Yes even with a 20+ second loss in isp.  I suspect that the 2nd stage (tanker stage) will have vacuum optimized nozzle. 

Described better and with diagrams and other slightly more speculative features (such as argon thrusters for interplanetary phase, Zubrin's 1000 s isp silane ramjets as Mars descent/ascent workhorses, tripwater transpiration and argon flash regenerative inconel silane/exhaled CO2 microcombustor heatshield (paints itself in ablative silica and carbon while firing) etc.) on L2 threads.
Having seen this, I think essentially every one of these concepts by themselves introduces a >99% likelihood of failure to an already ambitious concept. Together failure would be guaranteed.

Previous RLV concepts like VentureStar failed taking smaller risks with better funding and more time, while SpaceX has done extremely well with the approach of basically throwing engineering margin at the problem and structuring their efforts to do this economically.

Offline RanulfC

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Re: MCT Speculation and Discussion Thread 2
« Reply #348 on: 01/23/2015 06:59 pm »
I don't understand how you use a transfer stage to boost you out of LEO and most of the way (delta V wise) to Mars, and yet you "keep it there" (the transfer stage in LEO).  If you already need the engines in the Raptor for some of the delta V to Mars (and Mars decent and landing), it seems you pay only a very small mass penalty to make the tanks big enough make the complete delta V burn.  Especially compared to the extra mass of an additional transfer stage with its engines and especially if that stage needs to reserve fuel to negate its extra delta v and return back to LEO.  Or maybe I'm just not understanding your concept?

Just to explain how such would work and not that I'm of the mind that its what SpaceX will do but... :)

Your "transfer-stage" boosts the stack out of orbit into a TMI trajectory and then boosts-BACK some to loop back to LEO for refueling and to be used on the next out-going stack. Since the majority of the "mass" in the system is nothing but propellant and you supposedly HAVE that in LEO there is no real "mass-penalty" associated with the system. This is ONLY workable with fuel depots or tankers since none of the "Mars-Direct-ish" missions can economically afford this.

I'm not up to date on all of Elon's comments regarding the MCT, but here's my take.

I think the MCT will basically be a large, scaled-up version of Dragon 2, with large fuel tanks and Methalox engines, perhaps with a 15m diameter heat shield (I don't think Musk will go for anything other than the capsule shape, since that's what he has experience with).

I dunno, they are accumulating quite a bit experience with cylindrical stages as well. (not quite the same reentry, but a violent one nonetheless)

Mars EDL with a simple "capsule" shape gets harder the more massive (and larger) you go. There's a serious "hard-limit" where you have to rely on more and more propulsive braking the bigger/heavier you are. Most of the current "assumed" MCT designs seem to be coming up on that limit and the more "area" you can offer during aerobraking the better your profile gets. So the idea of using more of the "side" of the vehicle to offer more area for using the atmosphere to break in the better hence the "side" profile ideas. I should also mention that landing "horizontally" eases a large number of issues with getting people and equipment out of the lander as well. Musk has stated that they are going through and using the Falcon-9/FH to 'learn' about thing they need to know to design the MCT and I don't think we can say at this point that they are wedded to any single ideas and seem to be willing to experiment and learn.

Randy
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Offline GORDAP

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Re: MCT Speculation and Discussion Thread 2
« Reply #349 on: 01/23/2015 08:00 pm »
I don't understand how you use a transfer stage to boost you out of LEO and most of the way (delta V wise) to Mars, and yet you "keep it there" (the transfer stage in LEO).  If you already need the engines in the Raptor for some of the delta V to Mars (and Mars decent and landing), it seems you pay only a very small mass penalty to make the tanks big enough make the complete delta V burn.  Especially compared to the extra mass of an additional transfer stage with its engines and especially if that stage needs to reserve fuel to negate its extra delta v and return back to LEO.  Or maybe I'm just not understanding your concept?

Just to explain how such would work and not that I'm of the mind that its what SpaceX will do but... :)

Your "transfer-stage" boosts the stack out of orbit into a TMI trajectory and then boosts-BACK some to loop back to LEO for refueling and to be used on the next out-going stack. Since the majority of the "mass" in the system is nothing but propellant and you supposedly HAVE that in LEO there is no real "mass-penalty" associated with the system. This is ONLY workable with fuel depots or tankers since none of the "Mars-Direct-ish" missions can economically afford this.

Randy, thanks for the explanation.  This is pretty much what I assumed people meant with the 'transfer stage', but I'm still struggling to make sense of it. 

You already have tanks and engines on the MCT, so why not simply size the tanks for the MCT to make the complete LEO to Mars Surface burn, rather than add another complete stage to do half the job, especially considering that that stage would then have to reserve fuel to boost itself back to LEO once it's done it's job.  I can't see how the numbers could possibly favor the latter approach.  On one side of the ledger, we have the mass needed to make MCT tanks go from 'V' to '2V'.  On the other side of the ledger we have the mass for the transfer stage to hold 'V' (which is already more than the mass needed to stretch the MCT tanks), plus the mass of the engines of the transfer stage, plus the boost back fuel for the stage, plus the coupling/decoupling hardware on both stages...  How can this possibly make sense?  We're considering LEO refueling (or fuel depots) in both cases, so that doesn't help the case at all.

Oh, and I just realized, the MCT fuel tanks already have to be the larger size.  This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v.  So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 2
« Reply #350 on: 01/23/2015 08:18 pm »
Oh, and I just realized, the MCT fuel tanks already have to be the larger size.  This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v.  So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?

Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.

People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.

Offline RotoSequence

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Re: MCT Speculation and Discussion Thread 2
« Reply #351 on: 01/23/2015 08:36 pm »
Oh, and I just realized, the MCT fuel tanks already have to be the larger size.  This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v.  So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?

Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.

People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.

This might be a dumb question, but I should probably ask - are the Delta-V requirements to go from Earth Orbit to the surface of Mars any different than the requirements to go from the surface of Mars to Earth Orbit?

Offline Jcc

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Re: MCT Speculation and Discussion Thread 2
« Reply #352 on: 01/23/2015 10:06 pm »
Oh, and I just realized, the MCT fuel tanks already have to be the larger size.  This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v.  So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?

Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.

People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.

This might be a dumb question, but I should probably ask - are the Delta-V requirements to go from Earth Orbit to the surface of Mars any different than the requirements to go from the surface of Mars to Earth Orbit?

Yes. You can use aero capture to slow down, but need to push uphill from Mars surface.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 2
« Reply #353 on: 01/24/2015 12:39 am »
Oh, and I just realized, the MCT fuel tanks already have to be the larger size.  This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v.  So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?

Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.

People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.

This might be a dumb question, but I should probably ask - are the Delta-V requirements to go from Earth Orbit to the surface of Mars any different than the requirements to go from the surface of Mars to Earth Orbit?

Yes. You can use aero capture to slow down, but need to push uphill from Mars surface.

Yes, but you can also do aero capture into LEO, so it is pretty much a wash.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 2
« Reply #354 on: 01/24/2015 01:14 am »
In any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. The ISS alone masses a good 400 tonnes, and any credible transfer habitat designs I have seen are at least in the thirty to sixty tonnes range, for crews of six!

What are you counting into the habitat mass? A BA 330 is supposed to be 20 tonnes and it is 330 m^3, which is really huge, especially in zero g when you can use volume much more efficiently (in 3-D).

Because of that, I think artificial gravity is a bad idea for going to Mars. People have spent much more time than that in zero-g, and on Mars the gravity is less anyway.

Quote
I hope it's a more credible and detailed design than Zubrin's ERV from The Case for Mars, which was hopelessy mass-constrained and would never have worked in reality.

Why wouldn't it have worked?

Offline colbourne

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Re: MCT Speculation and Discussion Thread 2
« Reply #355 on: 01/24/2015 01:55 pm »
Has Musk released any plans of using any vehicle other than MCT for landing the first person on Mars.
Has he any plans for a one way mission (at least until their is capability to return people from Mars) ?

Offline docmordrid

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Re: MCT Speculation and Discussion Thread 2
« Reply #356 on: 01/24/2015 02:01 pm »
Has Musk released any plans of using any vehicle other than MCT for landing the first person on Mars.
Has he any plans for a one way mission (at least until their is capability to return people from Mars) ?

No and no. Unmanned is another matter, as in if a Red or Ice Dragon mission flies.
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Offline MichaelBlackbourn

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Re: MCT Speculation and Discussion Thread 2
« Reply #357 on: 01/24/2015 10:03 pm »
I love that you have the weapon storage so carefully marked in that MCT layout... Will the M4 Carbine work in the martian atmos? Or is it for maintaining lawful colonists in route.

Edit: This was in response to a now deleted post with a MCT design with 100 carbines listed on it.
« Last Edit: 01/24/2015 10:45 pm by MichaelBlackbourn »

Offline Clyde

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Re: MCT Speculation and Discussion Thread 2
« Reply #358 on: 01/24/2015 10:53 pm »
I did not find any estimates of the per-launch-cost of MCT in the early stages (=before regular trips to mars). I will not try to estimate that cost, but I think there is a cost threshold perhaps around $500m per BFR/MCT launch below which MCT could make a profit if used for paying passenger tours to LEO.

I used Ref.[1] for an estimate of the demand elasticity. A $10m ticket price could capture a market of ~500 passengers/year, hence 10 MCT @50 passengers / year.  For a  $20m ticket price one gets about 200 passengers / year or 4MCT / year. Note that with a $30m ticket price the number of passengers more or less drop to less than 1 per year which is where we are at the moment. (I used figure fig.5 to scale down the 600 passengers @$5m/seat of table 7).

Paper [1] assumes that the tourists would stay two weeks in some orbital facility, hence one needs to include the cost of some orbital Bigelow-like facility.

Could passenger tours to LEO + visit to an orbital Bigelow-like facility be offered at ticket prices of $10m-$20m and make a profit on it?

If the cost (BFR/MCT+orbital facility maintenance) can be maintained lower than $500m per batch of 50 passengers it looks like it would be possible to make a profit and that there could be enough passengers for between 4 and 10 MCT LEO trips per year.



[1] http://www.spaceportassociates.com/pdf/orbital_tourism.pdf


Offline BobHk

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Re: MCT Speculation and Discussion Thread 2
« Reply #359 on: 01/24/2015 10:57 pm »
In any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. The ISS alone masses a good 400 tonnes, and any credible transfer habitat designs I have seen are at least in the thirty to sixty tonnes range, for crews of six!

What are you counting into the habitat mass? A BA 330 is supposed to be 20 tonnes and it is 330 m^3, which is really huge, especially in zero g when you can use volume much more efficiently (in 3-D).

Because of that, I think artificial gravity is a bad idea for going to Mars. People have spent much more time than that in zero-g, and on Mars the gravity is less anyway.

Quote
I hope it's a more credible and detailed design than Zubrin's ERV from The Case for Mars, which was hopelessy mass-constrained and would never have worked in reality.

Why wouldn't it have worked?

I took a look at the proposal and an article about the history of the SEI (http://www.wired.com/2013/04/mars-direct-1990/).  It struck me that you can now, with SpaceX's accurate landing capability, refit a Red Dragon as a reactor (replace the return to earth stage on the landed Red Dragon with a massive RTG), land it, THEN land the ERV nearby and have its 'rover' simply attach the two.  This would eliminate the need to have the reactor on the ERV as well as allow you to send Red Dragon Reactors at will (daisy chained reactors wherever you need them) as needed without combining the ERV and reactor.

Amerecium-241 is plentiful (over 200 tons laying about on Earth now) and has a 400 year half life.  Putting it to use on Mars and getting it off this planet seems an optimal solution to two issues.

perhaps this:

Quote
"Zubrin and Baker opted for a conjunction-class Mars mission; that is, one in which the crew would travel to Mars on a six-month minimum-energy trajectory, remain at Mars for about 500 days, and return to Earth on a six-month minimum-energy trajectory. In practice, travel and stay times would vary from one conjunction-class mission opportunity to the next because Mars has a somewhat elliptical orbit about the Sun. Cautious NASA planners have generally opted for opposition-class Mars missions, which would see the crew stay at Mars for perhaps a month. "

...is why some consider it a possible fail...thats a long time to spend on Mars with 1990s tech.  But that type of mission, conjunction-class, saves energy/fuel versus a opposition-class mission.

Quote
"One of the mission’s objectives would be to seek water ice."

Which would mean a north polar landing if you want your life to be easy and not require water condensation from atmosphere, mining and processing of water ice from soil, et cetera.

The alternative version using an NTR (Nuclear Thermal Rocket) to boost "...both a habitat lander and a propellant factory/ERV toward Mars..." brings up its own host of issues least of which is radiation protection for the crew from the rocket.  I'd wager we don't need NTRs now so its a non issue.

Quote
"Several difficulties became obvious to NASA planners as they sought to integrate Mars Direct with their FLO-derived Mars plans. For example, the 7.1-metric-ton ERV cabin was too small to support all the needs of four people during a six-month weightless journey from Mars to Earth. Zubrin did not reveal to his Penn State audience the ERV’s planned habitable volume, though he did compare it to the Space Shuttle’s roughly 70-cubic-meter two-deck crew cabin. Controls, pilot and commander seats, equipment, and storage compartments occupied a large portion of the Shuttle crew cabin volume; Zubrin did not indicate whether the same would apply to the ERV cabin. In addition, the lone pressurized rover could not be used to its full potential; if it ventured beyond the range of an astronaut on foot and became stuck or broke down, then its occupants would become stranded without hope of rescue."

Tech seems to have solved a number of these issues: expandable habitat modules, smaller and lighter weight control systems, lots of improvement in 25 years.






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