Author Topic: MCT Speculation and Discussion Thread 4  (Read 626333 times)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #740 on: 10/13/2015 09:36 PM »
Has anyone suggested the possibility of SpaceX's announcement including "a reusable, nuclear (fission, fusion, either or) upper stage for the BFR" yet?

As far as I know their are few advocates for Nuclear propulsion left on these forums and none have advanced it for use by SpaceX, it is hard to imagine that a company which finds Hydrogen 'not worth the effort' would go Nuclear.  I've read some old threads in which the Nuclear camp got schooled badly by a guy named Sorenson, even NASA has basically been downplaying the idea compared to decades past.

As for the technology itself, Nuclear Thermal rockets are not reusable, in fact they are difficult to even fire more then once because the core will want to melt-down from decay heat after propellant flow stops.  Nothing Fusion based is anywhere near to even getting off a chalk board right now.

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #741 on: 10/13/2015 09:51 PM »
I think a nuclear fusion engine using magnetic control of the plasma and having one end expel the plasma for propulsion with the other end feeding liquid hydrogen in and continuously making plasma for propulsion would work.  It would still require a lot of power just to use lasers to create the fusion at one end.

However if you are going to use some other type of power source to create the fusion for the engine, one still needs a lot of power, and you have to have heavy hydrogen to operate.  How much I don't know? 

So it seems SEP with very large arrays and a cluster of a lot of ion engines and a large tank of propellant could also make a very large in space, higher speed cargo or even human run to Mars. 

A small NEP reactor, sealed, use for several years, and throw away, by shooting it to the Sun.  It might be somewhat smaller and could make several Mars runs before depleting its fuel.  Technology for small sealed reactors should be improved first. 

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Re: MCT Speculation and Discussion Thread 4
« Reply #742 on: 10/13/2015 10:00 PM »
I think the biconic shape is the way to go.  I do think concentric fuel/oxygen tanks running the entire length of the MCT in the center would allow for more balance.  Say with liquid methane in the center tube and the lox in a donut tube around the methane.    Cargo, solar panels, landing legs, thrusters and thruster fuel, etc could be around this central tanking system.  The picture on post 733 is what I like except with concentric center fuel/lox tanking.  One can still dock on the nose, as tank dome could stop short of the nose.  Heavy cargo could be stored near the bottom for easier unloading.  Instead of the plug nozzle engine as shown, standard Raptors vacuum engines could be on the bottom.  The interstage could double as heat protection flaps when coming back through the atmosphere. 
« Last Edit: 10/13/2015 10:02 PM by spacenut »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #743 on: 10/13/2015 11:17 PM »
Has anyone suggested the possibility of SpaceX's announcement including "a reusable, nuclear (fission, fusion, either or) upper stage for the BFR" yet?

I think a nuclear fusion engine using magnetic control of the plasma and having one end expel the plasma for propulsion with the other end feeding liquid hydrogen in and continuously making plasma for propulsion would work.  It would still require a lot of power just to use lasers to create the fusion at one end.

However if you are going to use some other type of power source to create the fusion for the engine, one still needs a lot of power, and you have to have heavy hydrogen to operate.  How much I don't know? 

So it seems SEP with very large arrays and a cluster of a lot of ion engines and a large tank of propellant could also make a very large in space, higher speed cargo or even human run to Mars. 

A small NEP reactor, sealed, use for several years, and throw away, by shooting it to the Sun.  It might be somewhat smaller and could make several Mars runs before depleting its fuel.  Technology for small sealed reactors should be improved first.
Yeah...  these are tangents that really don't belong in this thread.  Fusion engineering is comparable in scope to the most expensive Mars programs proposed.  Fusion propulsion is way, way out there, quite possibly postdating practical fusion power;  Not that it shouldn't be explored, but this is a thing that Elon Musk wants to fly in a decade, not in half a century.

Here's what we *thought* when we started work on fusion, and what actually happened:



ITER won't start even experimental work on the easiest D-T reaction for another 12 years, and it weighs 23,000 tons and isn't even designed for the extraction of useful electrical power.  Newer IEC fusion approaches are optimistic, but tiny in the amount of effort being expended; see the "actual funding" line for the tokomak work that's ended up monopolizing most of our efforts.

Fission Nuclear Thermal Rockets have seen plenty of speculation, starting with Von Braun if not earlier, but they're questionable from a performance perspective, a fairly immature technology (vacuum restart is much less plausible than for turbopumped chemical rockets) , the very worst thing you can get permission to fly through the air, and do not scale down to convenient test units.  NTR still looks interesting to NASA because it permits less complex missions with fewer launches, but the progress in SEP has been vast since the last time it looked like they were actually going to develop something, under Reagan's Star Wars projects.  The best use case seems to be a fairly small engine (but they only get so small) pushing a large load through interplanetary burns (where TWR isn't such an issue);  But that delves into the difficult problem of long-term storage of hydrogen, the most efficient propellant for NTRs.  I've seen a proposal to do it using water, or ammonia, but not sure how serious that is, and it further damages the numbers relative to SEP.

I think the biconic shape is the way to go.  I do think concentric fuel/oxygen tanks running the entire length of the MCT in the center would allow for more balance.  Say with liquid methane in the center tube and the lox in a donut tube around the methane.    Cargo, solar panels, landing legs, thrusters and thruster fuel, etc could be around this central tanking system.  The picture on post 733 is what I like except with concentric center fuel/lox tanking.  One can still dock on the nose, as tank dome could stop short of the nose.  Heavy cargo could be stored near the bottom for easier unloading.  Instead of the plug nozzle engine as shown, standard Raptors vacuum engines could be on the bottom.  The interstage could double as heat protection flaps when coming back through the atmosphere.

Remember: The methalox propellant has to be stored at about 100K.  The humans have to be stored at about 295K.  The black-body temperature of Solar radiation is around 255K.  Increasing the surface area where the propellant and the human habitat come together, and increasing the variety of angles that the sun can strike the propellant tank, increases the amount of thermal load you have to deal with.  You're describing a worst-case-scenario thermally.  It may well be a best-case-scenario for solar & galactic cosmic ray protection, but the steady flow of GCRs are only slightly affected by any practical amount of mass (they penetrate better, being higher energy), and the solar ones only occur in short-timespan events, solar storms.  This is why there's a degree of elegance to preserving a spot at ~100K inside the propellant tank, and only heating it up for human habitation (incurring lots of boil-off) during the solar storms that should happen for around 12 hours every fifth mission or so (IIRC?).
« Last Edit: 10/13/2015 11:37 PM by Burninate »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #744 on: 10/14/2015 01:41 AM »

I'll also add that in addition to the trip duration, there's the "land the whole thing" mentality.   If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.

If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.

But again, this is about MCT.

We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle.  And that such a vehicle trades very well against a single massive direct vehicle.
It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!
« Last Edit: 10/14/2015 01:43 AM by Robotbeat »
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Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #745 on: 10/14/2015 01:47 AM »
Has anyone suggested the possibility of SpaceX's announcement including "a reusable, nuclear (fission, fusion, either or) upper stage for the BFR" yet?

I think many of us have considered it, but it'd actually be rather disappointing since it'd stack additional (legal) impediments against the possibility of a SpaceX mars mission. It's also unnecessary when you have something as big as the BFR.

Seeing a gigantic nuclear interplanetary colony ship descending through Earth's atmosphere bound back from Mars would truly be an incredible experience to have in one's lifetime though.

Edit:
It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!

I agree that it's possible and would be great on the fuel extraction/colonial front if you can extend them again on the surface, but it's horribly terrifying. If at any point they don't retract/extend properly, you have a problem. If whatever protective cover you have over them whilst stowed doesn't lock properly, you have a problem. Whilst it would solve radiator and power issues, the concept is still pretty frightening.

Edit edit: As the ISS has proved to an extent, big, movable solar panels in space can be rather unreliable, although there's probably ways to engineer that issue into the floor.
« Last Edit: 10/14/2015 01:54 AM by The Amazing Catstronaut »
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Offline Rocket Surgeon

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Re: MCT Speculation and Discussion Thread 4
« Reply #746 on: 10/14/2015 03:02 AM »
To go back to balancing the Biconic for a bit, would it be possible to have a configuration similar to the Phoenix (see attached), except applied to a Biconic and with some sort of escape system in the nose (sorry, really like the idea of the nose section being a dedicated 'lifeboat'. I just see it as a way to solve so many problems) It would still have the central column, as a way to move down to the main pressurized area.
Not sure what this would do to the mass in total, but wouldn't it allow for a more evenly distributed mass during EDL?

I do like that image, it is close to what I have in mind for an MCT. But I think a lengthy tunnel through a cryogenic tank might have issues.

If you insist on have a nose-mounted LAS, another way around it might be to have just a giant central tank (split between oxidizer and fuel), and then have a lower cargo bay (just above engines), and an upper cargo bay in the nose. It would force some level of cargo balance, but that would allow you to put all the pressurized volume up top. And as a bonus the unpressurized/heavy cargo would be easily offloaded near the surface.

I do like this idea, it seems elegant. It helps solve the problem of mass distribution, allows for the nose mounted Lifeboat, and allows you easily access, once landed, to large payloads that don't need to be pressurized during the flight (earth moving equipment possibly?)

Just on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight, but it allows for something I've been worried about, Abort during transit.
If we assume these things are traveling in a fleet (say 10 MCT's for 1000 colonists to Mars?), if one suffers a mishap during flight, then the colonists can load up and punch out from the failing MCT and rendezvous with another nearby one. Granted, depending on the mishap, the 'other' MCT could just come to them anyway, but there may be issues necessitating a quick departure, for example during Mars approach and entry...I'm pretty sure you can't keep 100 people alive on an MCT for 2 years waiting for free-return to Earth if you miss.
The other MCT would have to put up with 200 people for a short time, while they can be distributed amongst the rest of the fleet, giving you 9 MCT's with 110 people each for the rest of the trip.
And while the Lifeboat would be expensive, it wouldn't be as bad as the MCT, give it the capability to cram in 100 people as tightly as possible and keep them alive for say, 10 hours, long enough for rescue. Like current lifeboats are used for.
It may be expensive both in terms of mass and money, but it is terrible engineering to hope that 'it won't fail'... Especially when 'failure' includes the loss of 100 lives. Most risk assessments I've seen would still have that as an 'extreme risk', no matter how small the likelihood.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #747 on: 10/14/2015 05:39 AM »
It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!

Yes, they have run into their share of problems. Solar arrays for an MCT lander would need to be a lot more mechanically reliable to allow multiple open and close events during a round trip. And you'd want arrays that could function in space as well on the Martian surface.

It's a tricky engineering problem to solve, even for a relatively small set of Arrays. And if you to also provide a SEP cruise boost, you need to make them even larger. Easier said than done, even for smaller arrays.

EDIT: Here's an old animation showing the complexity of the ISS array deployment:
« Last Edit: 10/14/2015 05:57 AM by Lars-J »

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #748 on: 10/14/2015 05:50 AM »
Just on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight,

Don't make the mistake of thinking that all an abort capsule needs to do is taking passengers away from an imminent explosion, to just survive the imminent danger. That works on Earth, because rescue is nearby, and the environment is survivable.

But that is NOT enough for Mars. An abort capsule is USELESS if it just dooms you to death a couple of hours later. To be a proper abort/rescue vehicle it basically needs to be a fully independent spacecraft/lander, built into another spacecraft/lander. By adding enough rescue/survival capability, you get into a dark spiral that eventually leads you to conclude that your escape capsule needs another escape capsule.

But there is another way. Build safety margins and redundancy into your MCT/lander and forego an abort system that is effectively useless.

but it allows for something I've been worried about, Abort during transit.
If we assume these things are traveling in a fleet (say 10 MCT's for 1000 colonists to Mars?), if one suffers a mishap during flight, then the colonists can load up and punch out from the failing MCT and rendezvous with another nearby one. Granted, depending on the mishap, the 'other' MCT could just come to them anyway, but there may be issues necessitating a quick departure, for example during Mars approach and entry...I'm pretty sure you can't keep 100 people alive on an MCT for 2 years waiting for free-return to Earth if you miss.
The other MCT would have to put up with 200 people for a short time, while they can be distributed amongst the rest of the fleet, giving you 9 MCT's with 110 people each for the rest of the trip.

Being able to abort/rescue during transit is a completely separate aspect from whether or not you have a launch abort capsule. Any reasonable MCT proposal would have a fleet of them going during a launch window, so that rescue would be available even without abort capsules.

And no, I don't think you need to have a rapid way to escape your MCT during transit. At some point you just have to accept that there will be riskier times during a trip. Those who cannot accept risks can stay behind.
« Last Edit: 10/14/2015 05:53 AM by Lars-J »

Offline Rocket Surgeon

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Re: MCT Speculation and Discussion Thread 4
« Reply #749 on: 10/14/2015 06:22 AM »
Just on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight,

Don't make the mistake of thinking that all an abort capsule needs to do is taking passengers away from an imminent explosion, to just survive the imminent danger. That works on Earth, because rescue is nearby, and the environment is survivable.

But that is NOT enough for Mars. An abort capsule is USELESS if it just dooms you to death a couple of hours later. To be a proper abort/rescue vehicle it basically needs to be a fully independent spacecraft/lander, built into another spacecraft/lander. By adding enough rescue/survival capability, you get into a dark spiral that eventually leads you to conclude that your escape capsule needs another escape capsule.

But surely if we're assuming a mostly developed Mars Colony ready to accept 1000 new immigrants, then it is reasonable to assume that they could mount a quick rescue of an abort capsule? Something like a dedicated MCT with enough fuel to do a sub orbital hop, on station and ready to go could make it to the capsules landing site in a few hours, and if the capsule was close enough, you could even go over land. That's why it would only need to sustain the colonists for a few hours, not days or weeks. That keeps the capsule more simplistic. Simple CO2 scrubbers, maybe a dedicated radiator if the base hull rejection rates are naturally too low and batteries for power. Add some pressure fed hypergolic rockets, or methalox ones that are made so the tanks can be 'charged' from the main tanks when they are likely to be needed (for those who don't like hypergols) and a parachute or two to help slow down. Again, I'm not saying it'll be 'cheap' either in terms of mass or money, I'm saying that it is worth it. It would have to cost more than $1.5 billion to save 100 lives for it to be considered 'Grossly improportional' and for SpaceX to have a reasonable argument that is was too expensive.

Being able to abort/rescue during transit is a completely separate aspect from whether or not you have a launch abort capsule. Any reasonable MCT proposal would have a fleet of them going during a launch window, so that rescue would be available even without abort capsules.

And no, I don't think you need to have a rapid way to escape your MCT during transit. At some point you just have to accept that there will be riskier times during a trip. Those who cannot accept risks can stay behind.

I agree, but if you could have a system that does double duty, why not? and as I stated in my original post, it is unlikely that you'll need to rapidly escape a MCT in flight, but what about during Mars approach? if the engines/OMS fail or if a flaw is found in the heat shield and you cannot enter Mars orbit, you've condemned a ship of 100 people to slowly starve to death in deep space. Unless there is a way to keep them alive for several years on a free-return trajectory. Another possible option there, though would be to have cycling 'rescue' stations that could rendezvous with any MCT that 'misses' Mars. Could pull double duty as a deep space research station, or ship....but that's getting dangerously 'Star Trek' :P
« Last Edit: 10/14/2015 06:23 AM by Rocket Surgeon »

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #750 on: 10/14/2015 06:52 AM »

I'll also add that in addition to the trip duration, there's the "land the whole thing" mentality.   If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.

If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.

But again, this is about MCT.

We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle.  And that such a vehicle trades very well against a single massive direct vehicle.
urm, that is quite a logic leap.

The fact that SpaceX were looking into SEP could mean they were looking at an all-in-one vehicle with both SEP and chemical propulsion.

The fact is that we don't know what kind of SEP approach they were considering (though it is probably safe to assume "all of them") and that unless we get a definitive statement on MCT architecture from SpaceX, all we have to go on is fragments.

So unless Elon or Gwynne or someone else from SpaceX says otherwise, MCT will remain a "land the entire thing" architecture, no matter how disagreeable you find the prospect of it.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #751 on: 10/14/2015 08:18 AM »
The fact that SpaceX were looking into SEP could mean they were looking at an all-in-one vehicle with both SEP and chemical propulsion.

As was already mentioned in the thread, that would require the large solar arrays not only somehow be retracted into the MCT itself but also be deployed again after re-launch from Mars, and thus be so reliably shielded from re-entry (and issues on the surface like dust) that you can guarantee they will redeploy for the return trip to Earth. There's simply no advantage to justify that complexity.

MCT will remain a "land the entire thing" architecture, no matter how disagreeable you find the prospect of it.

The original land-the-entire-thing comment was made in the context of the typical multi-stage launch/landing architectures. It doesn't mean that Musk has ruled out having a SEP stage that remains in orbit.

And given that he's happy to do EOR refuelling, to avoid the restrictions of monolithic launches that are typical of Mars architectures, I can't see why anyone would think he'd arbitrarily rule out such an option just to satisfy one interpretation of a throw away quote.

We might find out in a month or so. Until then, these options are on the table... "No matter how disagreeable you find the prospect."

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #752 on: 10/14/2015 01:15 PM »
My idea of the fuel tanks would be tall cylinders concentric.  Inside about 5m diameter would be methane.  Next outer ring would be lox at say 7m.  Then insulation.  Then human/cargo area.  Then outer walls and insulation.  All concentric circles so that you maintain balance.   On the ground our large liquid natural gas tanks were double hulled with a vacuum pulled in between.  That could be done also.  Doesn't have to be super thick, just enough to hold a vacuum.  Since liquid methane and lox are only about 20 degrees C apart, one inside the other vertically would eliminate any balance problems landing and taking off vertically.  Humans could be at the top.  Cargo can be around the sides, since most cargo shouldn't have a freezing problem.  I'll try to sketch it and put it in a reply. 

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Re: MCT Speculation and Discussion Thread 4
« Reply #753 on: 10/14/2015 01:27 PM »
My question is, why not send the crew up in traditional capsules to dock and load into the MCT.  If the MCT has to refuel in LEO.  The refueling station could be where the crew transfers in.   No need to put launch escape system on MCT.  It will only be landing on Mars and on Earth.  One take off on Mars with its lower gravity shouldn't be a problem.  If the MCT explodes on take off on Mars, you are not getting home anyway.  Even if you did have, you would be stranded on Mars unless there was room in another MCT to get back. 

Also, the first wave of MCTs are going to build and establish the colony living quarters, power production equipment, fuel production equipment, greenhouse equipment, among other things.  This will take some time so why not minimal crew of say 12, leaving 10, returning two to earth on each trip.  The ones who stay will be busy getting everything up and running, building, exploring, etc.  This will take some time.  They could stay for say 2 years with replacement crews eventually coming and going with more staying each time and staying longer.  This would be like military duty except longer.  Volunteers can stay longer or return.  At least a hospital would need to be built for a more permanent colony.  There might be childbirth at some point.  How would these children fare if they return to earth with heavier gravity? 

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #754 on: 10/14/2015 01:39 PM »
My question is, why not send the crew up in traditional capsules to dock and load into the MCT. 

Because that'd be way expensive cumulatively than launching the MCT itself, and cost is a factor.
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #755 on: 10/14/2015 01:56 PM »
How would these children fare if they return to earth with heavier gravity?

We can only speculate. However they are genetically still humans. I am quite sure they can adapt though it might be hard to start with. It is only 2.6 times what they are used to. We do see people move around with 2.6 time the weight they are supposed to have and we are not genetically designed for that.

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #756 on: 10/14/2015 02:52 PM »
My question is, why not send the crew up in traditional capsules to dock and load into the MCT. 

Because that'd be way expensive cumulatively than launching the MCT itself, and cost is a factor.

Depends on the capsule size. Two or three capsules, 30-50 people each. Small compared with the MCT itself. Fully reusable, cost is approx. the cost of the fuel. So as long as the cost of the fuel for 2 or three flights is less than the cost to launch the whole MCT, it won't be more expensive.

Given the size requirements of the MCT, it's going to weigh more than 2-3 times that of a 30-50 person capsule. So capsules would probably be cheaper.

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Re: MCT Speculation and Discussion Thread 4
« Reply #757 on: 10/14/2015 03:48 PM »
I was thinking in terms of the beginning of colonization, 100, 50 even 30 people will not be going initially.  6 in two capsules to load would only require 2 launches of Falcon 9.  MCT then would not have to have LAS.  Later after the MCT has proven itself taking cargo to Mars with no crashes, then passenger flights could begin.  They have already said, it would take 10 cargo flights per one passenger flight to set up a colony.  So... why not take along 12 crew per MCT flight and say 90 tons of cargo.  Smaller requirements for human habitation in transit, thus more room for cargo.  Even one crew of 6 per MCT would only require one docking other than refueling. One BFR refueling tanker, one MCT launch, and one Falcon 9 launch with a crew of 6 per MCT trip to Mars. 

Four people could stay on Mars till the next synod, while two return with the MCT.  10 MCT landings per synod to begin with, would have 40 colonists.  Next Synod, with production, fuel depots, more MCTs could come to Mars, and reusable boosters, reusable MCT's, and more in production, say 80 people next synod, then 120.  As production of BFR's and MCT's increase, so would colonization.  No real need to send 100 at a time until a large living, production, greenhouses, etc have been established on Mars.  Also. Martian manufacturing of living, life support, and various other manufacturing production would have to be in full swing for construction of cities for mass migrations of 100 people at a time.  I think it will start with a trickle, then grow exponentially over time.  Over 50 years after the first landing, there might be 50-100,000 colonists, especially if some other companies or governments, build reusable MCT's to match SpaceX.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #758 on: 10/14/2015 05:04 PM »
But surely if we're assuming a mostly developed Mars Colony ready to accept 1000 new immigrants, then it is reasonable to assume that they could mount a quick rescue of an abort capsule? Something like a dedicated MCT with enough fuel to do a sub orbital hop, on station and ready to go could make it to the capsules landing site in a few hours, and if the capsule was close enough, you could even go over land. That's why it would only need to sustain the colonists for a few hours, not days or weeks. That keeps the capsule more simplistic. Simple CO2 scrubbers, maybe a dedicated radiator if the base hull rejection rates are naturally too low and batteries for power. Add some pressure fed hypergolic rockets, or methalox ones that are made so the tanks can be 'charged' from the main tanks when they are likely to be needed (for those who don't like hypergols) and a parachute or two to help slow down. Again, I'm not saying it'll be 'cheap' either in terms of mass or money, I'm saying that it is worth it. It would have to cost more than $1.5 billion to save 100 lives for it to be considered 'Grossly improportional' and for SpaceX to have a reasonable argument that is was too expensive.

It is 'grossly improportional' if it also cripples your architecture. And I still don't think you are understanding how capable your abort capsule would have to be to avoid black zones on Mars. And parachutes won't help much for something this size. (they don't scale up well, and Mars atmosphere is thin to begin with) Consider this... Delta-V to reach low Mars orbit is over 3 km/s. So if you have a problem when you are *almost* in orbit, you now have ~2(?) km/s of propulsive capability that this capsule needs to have in order to land softly - and you will land far over the horizon, on the other side of the planet. Any capsule capable of 2+ km/s that is large enough to hold 100(!) people is going to be massive. MASSIVE. Your simple capsule just won't cut it. 

Sometimes abort capability is impractical, and you mitigate risks other ways. You make your vehicle capable of abort, by having engine out capabilities and redundant systems. But you cannot remove all risks. Airliners still sometimes fall out of the sky, yet we don't have escape pods or parachutes.

But I'm tired of arguing these points over and over. For some, launch abort seems to be an essential that they just cannot comprehend going without.
« Last Edit: 10/14/2015 05:55 PM by Lars-J »

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #759 on: 10/14/2015 05:09 PM »
The fact that SpaceX were looking into SEP could mean they were looking at an all-in-one vehicle with both SEP and chemical propulsion.

As was already mentioned in the thread, that would require the large solar arrays not only somehow be retracted into the MCT itself but also be deployed again after re-launch from Mars, and thus be so reliably shielded from re-entry (and issues on the surface like dust) that you can guarantee they will redeploy for the return trip to Earth. There's simply no advantage to justify that complexity.
I was making an example of how the statement that they were "looking into SEP" could be understood in a different manner, not an actual plan.

MCT will remain a "land the entire thing" architecture, no matter how disagreeable you find the prospect of it.

The original land-the-entire-thing comment was made in the context of the typical multi-stage launch/landing architectures. It doesn't mean that Musk has ruled out having a SEP stage that remains in orbit.

And given that he's happy to do EOR refuelling, to avoid the restrictions of monolithic launches that are typical of Mars architectures, I can't see why anyone would think he'd arbitrarily rule out such an option just to satisfy one interpretation of a throw away quote.

We might find out in a month or so. Until then, these options are on the table... "No matter how disagreeable you find the prospect."
He wasn't comparing traditional mars architectures to MCT, he was saying that he was not intending on starting off with a cycler or using Dragons adapted for landing. When asked what vehicle he would use, he replied that "you should just land the entire thing."

Quote
Musk’s architecture for this human Mars exploration effort does not employ cyclers, reusable spacecraft that would travel back and forth constantly between the Red Planet and Earth — at least not at first.

"Probably not a Mars cycler; the thing with the cyclers is, you need a lot of them," Musk told SPACE.com. "You have to have propellant to keep things aligned as [Mars and Earth’s] orbits aren’t [always] in the same plane. In the beginning you won’t have cyclers."

Musk also ruled out SpaceX's Dragon capsule, which the company is developing to ferry astronauts to and from low-Earth orbit, as the spacecraft that would land colonists on the Red Planet. When asked by SPACE.com what vehicle would be used, he said, "I think you just land the entire thing."

Asked if the "entire thing" is the huge new reusable rocket — which is rumored to bear the acronymic name MCT, short for Mass Cargo Transport or Mars Colony Transport — Musk said, "Maybe."
source

I do not know how to interpret "land the entire thing" other than what they have in mind for MCT is an approach with no dedicated in-space hardware.

Also worthy of note, during his reddit AMA, he twice called the MCT a "spaceship and booster" system  and said they'd both be using raptor engine (and also, but not in the reddit AMA "We’re looking at our Mars transporter being around 15 million pounds of thrust" source)

Now, in there I managed to find a couple of sources that said SpaceX was looking into SEP, most notably Gwynne Shotwell's quote from the
Quote
around 4:25 "So we're looking at solar-electric propulsion. I think we're gonna umm, look at some other interesting IN-space propulsion technologies..."

and at AIAA 2011, Elon Musk said they were looking into using SEP as an "accelerator" for the trip to Mars.
(, he starts talking about it around 24:00)

But nowhere did I find any indication that they are stepping away from a monolithic MCT and looking into having dedicated in-space elements, which means the original statement by Elon that their design goal for the MCT is "land the whole thing" remains a valid criterion in ascertaining whether or not a marsbound architecture can be considered an MCT architecture or not.

Now, you may argue that this is me reading too deeply into quotes and that any design posted in this thread is, as it were, cut from whole cloth. While that is all true (Especially the former, as I love reading too deeply into things), the purpose of this thread is to design a vehicle and accompanying architecture based entirely on criteria extracted from the fragments of information we are able to scrounge up.

I like Impaler's proposed architecture. However, I disagree on it being a viable MCT.

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