Author Topic: MCT Speculation and Discussion Thread 4  (Read 503229 times)

Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1240 on: 12/30/2015 11:08 PM »
To get a lot smaller than 34,000 MT requires more smaller vehicles, rendezvous, etc. per flight.  It also likely increases total dV by increasing the number of different intermediate parking/rendezvous  orbits.  That likely increases total production and operation cost well over one giant stack vehicle for each 100 MT cargo (or 100 persons) mission.  In terms of affordable building in mass, WW II Liberty ships, produced cheaply in mass (2,700 built in only a few years), had deadweight over 10,000 MT.  An MCT stack 34,000 MT fueled would be less than 5,000 MT structural construction.  It seems like as hard as it is one giant stack is probably the best hope for $500k/person.
« Last Edit: 12/31/2015 01:31 AM by Umbrella »

Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1241 on: 12/31/2015 12:15 AM »
On the economics of $500k/person,  suppose $500k is in real (not inflated) dollars and each MCT stack makes 30 round trips, one every 26 months, over 65 years.  That may seem like a long operational life, but we are still flying B-52s and KC-135s constructed over 50 years ago. So, in real (but undiscounted) dollars total ticket revenue is $1.5 billion per stack.  Suppose that financing (real interest rate) and operational costs over 65 years eat up 2/3 of this, leaving $500 million for capital construction.  (Note, 2/3 operational finance might seem low for 65 years but there would possibly be extra revenue to partially offset costs by using the stack for other missions between Mars flights.) 

Could Spacex mass produce 333 MCT (BFS+BFR) stacks at $500 million a piece to transport 1 million colonists over 65 years?  That seems possible since they can make Falcon 9 for $60 million and there will be economies of scale.   

What is missing from the calculation is supply cargo, which Musk has estimated at 10 times the number of passenger missions (seems reasonable even with ISRU).  In fact there need to be a lot of early cargo flights just to build the refueling depot.

In summary the economics of $500k/person seems maybe feasible with only passengers, but I cannot get to $500k/(person + cargo allocation).

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1242 on: 12/31/2015 02:57 AM »
I think these wildly huge BFR/MCT estimates are way, way off.  The thing has to be affordable to build in quantity and has to be able to launch without evacuating the surrounding populace and rebuilding puny steel & concrete launch pads.

Yeah, we keep pingponging between "But the math and some educated guesses say it has to be at least this big, even optimistically" and "But Musk said it would be $500k/ticket.  Hundreds of thousands of passengers.  To do that it has to be a quarter that mass and twice the speed!  Build it out of unobtainium!".

I think there may be some middle ground in having one vehicle with multiple *configurations* for different purposes.

The primary bottomline variable is delta-V capability.  I showed above that a 9.2km/s stage for MCT is extremely problematic, perhaps impossible depending on structural mass fraction.  There are several steps (starting with ISRU and working on up) where refueling can drop the dV capability needed of the vehicle by splitting the longest leg of the mission, very substantially.

It's not that an 88-Raptor MCT is definitely impossible, it's just impractical & unnecessary;  There are easier ways, lower-hanging fruit.  It doesn't make *sense* to avoid propellant depots, to avoid ISRU, to avoid LEO cargo loading.

I think the latest round of speculation is quite over the top, people seem to have forgotten that the LAST round of speculation was already have been the largest launch vehicle ever built.  Musk's comments about really big are for a lay audience who's basis of comparison is the Saturn 5, the Shuttle and possibly SLS, they are not directed at this thread and should not be construed to mean the earlier speculations were too small.

I see a GLOW of around 4,600 mt, 31 Raptor engines on the booster and a MCT/BFS of only 75 mt dry.  A vastly more achievable size.

This vehicle dose initial missions but in order to scale up to huge colonization efforts you have to augment it with SEP transit vehicles and habitats which raise the efficiency by orders of magnitude by letting the BFS serve as a rapidly cycling landing craft at Mars.  Over 1 synod a MCT/BFS staying at Mars could make more then 100 surface-orbit-and-back-again runs meaning you can actually get more like 3000 uses of the vehicle over it's lifetime rather then a mere 30.

People need to stop pretending that the initial vehicle is the one and only thing that will ever be used and SpaceX would just manufacture zillions of copies of a vehicle and fly massive armadas of them at terribly bad amortization rates while never inventing another vehicle ever again.

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #1243 on: 12/31/2015 05:59 AM »
I see a GLOW of around 4,600 mt, 31 Raptor engines on the booster and a MCT/BFS of only 75 mt dry.  A vastly more achievable size.

Which assumes 75mt dry mass for a reusable 100mt Mars lander is realistic...
« Last Edit: 12/31/2015 05:59 AM by Oli »

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1244 on: 12/31/2015 06:11 AM »
To get a lot smaller than 34,000 MT requires more smaller vehicles, rendezvous, etc. per flight.

Yes, but smaller vehicles will cost less. And a higher flight rate will lower costs. And geez, if we are still afraid of *rendezvous* (mentioned like a word that shall not be spoken in some circles), then we might as well give up on the whole space thing.

It also likely increases total dV by increasing the number of different intermediate parking/rendezvous  orbits.

Propellant is dirt CHEAP.

And there will be lots of propellant tanker missions for each "MCT" no matter how you slice it, the question is just how many. And if the BFR can't be reused and fly economically with a high flight rate, the whole Mars thing is not going to happen anyway.

In terms of affordable building in mass, WW II Liberty ships, produced cheaply in mass (2,700 built in only a few years), had deadweight over 10,000 MT.  An MCT stack 34,000 MT fueled would be less than 5,000 MT structural construction.  It seems like as hard as it is one giant stack is probably the best hope for $500k/person.

Your point would be well taken if we needed to build *thousands* of BFR/MCTs. Going oversized will A) cost more in development, B) cripple mass production benefits (raising costs), and C) reduce your flight rate (further increasing cost)
« Last Edit: 12/31/2015 06:14 AM by Lars-J »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1245 on: 12/31/2015 01:00 PM »
My smallest BFR/MCT is 12.5m in diameter, has 25 engines and 4630mT GLOW. Delivers 180mT to LEO at a mass fraction of 3.9% or 26:1 wet to dry mass ratio.  Goes low & slow.  Propellant reserve for abort to Earth landing. Built for rugged quick turn RTLS and re-launch.  The 1st stage is the easier vehicle for SpaceX to design.  Meets the claimed requirement of 100mT cargo to LEO.

My larger preferred BFR/MCT is 15m diameter, has 28 engines and 5100mT GLOW. Delivers 185mT to LEO at a mass fraction of 3.6% or 28:1 wet to dry mass ratio.  Goes low & slow.  Built for rugged quick turn RTLS and re-launch. I think Musk goes 15m diameter to allow for future engine growth and provide margin when a 85mT dry weight MCT turns out to be too optimistic with complications from things like TPS and complex extra Mars landing engines located "higher" along the structure.

The 80-85mT dry weight upper stage is "near SSTO" with 7.7Km/sec capability fully fueled.  Allows for "fast" transits to Mars.  Less refueling for slower cargo flights to Mars.

When you calculate the mass of propellant for a 15m vehicle you realize that the "BFR" is short and stout, the opposite of the F9 family.
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1246 on: 12/31/2015 04:02 PM »
At the time Saturn V was developed it was 5 times heavier than the next largest rockets, Titan III and Proton.  How, 45 years later, can building a rocket that is 10-20 times larger (only about twice as big as was confidently proposed in the 1960s and 1970s with Nexus, Sea Dragon, etc.) than Saturn V be such an incredibly hard challenge?  The Germans built 900 foot long airships 100 years ago.  We built Liberty ships as described above in WW II.  We have been building aircraft carriers at 100,000 MT dry mass since the 1960s.  We have been building super tankers 500 meters long and 300,000 MT dry since the 1970s. 

Empty mass of Saturn V was about 10% of GLOW.  Building for a 34,000 MT to 100,000 MT GLOW is building a 3,400 MT to 10,000 MT vehicle.  That is 1/5 to 1/2 the size of an Ohio class nuclear submarine.

The construction and operation of an elaborate network of smaller Saturn V or Nova class rockets, refueling tankers, inter orbital transfer vehicles, intermediate point space habitats, etc.  has got to be a lot more difficult and expensive than one big dumb rocket and one refueling depot on Mars for surface to surface and back transit.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1247 on: 12/31/2015 04:42 PM »
Where do you launch this behemoth?  Not at Cape Canaveral or the new Texas site.  Too close to population.  Noise too.  And what magic materials to you construct your launch pad with?  Think thrust force & joules of heat energy.  TOTALLY impractical. 

A spacecraft is not like an ocean sailing vehicle.
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Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1248 on: 12/31/2015 04:52 PM »
Here are some designs done by Boeing in the 1970s for launch and landing facilities at KSC:

http://www.pmview.com/spaceodysseytwo/spacelvs/sld043.htm

http://www.pmview.com/spaceodysseytwo/spacelvs/sld044.htm

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1249 on: 12/31/2015 05:01 PM »
Umbrella, do you even read other posts, or is this a post-only thread for you?
« Last Edit: 12/31/2015 05:01 PM by Lars-J »

Offline Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #1250 on: 12/31/2015 07:12 PM »
Umbrella, do you even read other posts, or is this a post-only thread for you?
(I think they were responding to philw1776 asking about where and how this "behemoth" would be launched)

Offline Stardhingy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1251 on: 12/31/2015 09:50 PM »
My smallest BFR/MCT is 12.5m in diameter, has 25 engines and 4630mT GLOW. Delivers 180mT to LEO at a mass fraction of 3.9% or 26:1 wet to dry mass ratio.  Goes low & slow.  Propellant reserve for abort to Earth landing. Built for rugged quick turn RTLS and re-launch.  The 1st stage is the easier vehicle for SpaceX to design.  Meets the claimed requirement of 100mT cargo to LEO.

My larger preferred BFR/MCT is 15m diameter, has 28 engines and 5100mT GLOW. Delivers 185mT to LEO at a mass fraction of 3.6% or 28:1 wet to dry mass ratio.  Goes low & slow.  Built for rugged quick turn RTLS and re-launch. I think Musk goes 15m diameter to allow for future engine growth and provide margin when a 85mT dry weight MCT turns out to be too optimistic with complications from things like TPS and complex extra Mars landing engines located "higher" along the structure.

The 80-85mT dry weight upper stage is "near SSTO" with 7.7Km/sec capability fully fueled.  Allows for "fast" transits to Mars.  Less refueling for slower cargo flights to Mars.

When you calculate the mass of propellant for a 15m vehicle you realize that the "BFR" is short and stout, the opposite of the F9 family.

I think that's a little optimistic on mass ratios, but I do expect that a 15m diameter vehicle is going to have a very low ballistic coeefficient, with good lift it can spend a lot of time in the upper atmosphere for a low g and thermal load entry, giving a it a very good mass fraftion for an entry vehicle. But I also expect a design that doesn't depend on a never before achieved mass ratio.

On the order of a .9 mass fraction MCT and a .92 mass fraction booster, you get 236mT to orbit with a 5800 GLOW, and a 107mTon dry MCT. The MCT should have enough dV to do a reentry and landing development program before committing to sizing the BFR. Maybe you could get it orbit with Falcon Heavy boosters.  If you can't make the mass fraction on the MCT you can cut the payload, or tweak the BFR design. Around that size the whole thing is fairly insensitive to weight on the MCT, you could add 50tons and push the booster to .94 and still get 200mTons to orbit.

I can see maybe overcoming the problems with a super massive launch vehicle on Earth but the ISRU requirements get out of control. Can you really make several thousand mTons of propellent on Mars for each MCT? What's the footprint for a solar array to get that done in 2 years?


Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1252 on: 12/31/2015 10:37 PM »
12m is about the largest diameter standard river barges are.  Thus allowing manufacturing of fuel tankage almost anywhere with the vast Mississippi/Great Lakes systems as well as coastal areas.  The eastern inter-coastal waterway is from Brownsville, Texas to Maine.  The Mississippi river system reaches to Pittsburg, Mineapolis, probably Lincoln, Nebraska, and ties-in with the Tennessee-Tombigbee waterway going as far inland as Knoxville, Tennessee.  Canals connect the Mississippi through Chicago to the Great Lakes.  Anything larger than 12m will probably have to be made on the coast.  A three 8m core heavy version could be make almost anywhere with an 8m core single version replacing Falcon heavy.  Three core heavy for MCT. 

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1253 on: 12/31/2015 11:08 PM »

Anything larger than 12m will probably have to be made on the coast.  A three 8m core heavy version could be make almost anywhere with an 8m core single version replacing Falcon heavy.  Three core heavy for MCT.

I don't think they are planning a heavy version of the "BFR". Too much complexity for little gain, plus 3x infrastructure needed at pad. Such a massive launcher needs a lot of flights to have any chance of being economical, so you fly more often instead and assemble + refuel in orbit instead.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1254 on: 12/31/2015 11:09 PM »
Shotwell & Musk have said that the BFR will be made on site so river barges are irrelevant.
I'd bet big $ that BFR is single not tri-core.
« Last Edit: 12/31/2015 11:13 PM by philw1776 »
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1255 on: 12/31/2015 11:11 PM »
My smallest BFR/MCT is 12.5m in diameter, has 25 engines and 4630mT GLOW. Delivers 180mT to LEO at a mass fraction of 3.9% or 26:1 wet to dry mass ratio.  Goes low & slow.  Propellant reserve for abort to Earth landing. Built for rugged quick turn RTLS and re-launch.  The 1st stage is the easier vehicle for SpaceX to design.  Meets the claimed requirement of 100mT cargo to LEO.

My larger preferred BFR/MCT is 15m diameter, has 28 engines and 5100mT GLOW. Delivers 185mT to LEO at a mass fraction of 3.6% or 28:1 wet to dry mass ratio.  Goes low & slow.  Built for rugged quick turn RTLS and re-launch. I think Musk goes 15m diameter to allow for future engine growth and provide margin when a 85mT dry weight MCT turns out to be too optimistic with complications from things like TPS and complex extra Mars landing engines located "higher" along the structure.

The 80-85mT dry weight upper stage is "near SSTO" with 7.7Km/sec capability fully fueled.  Allows for "fast" transits to Mars.  Less refueling for slower cargo flights to Mars.

When you calculate the mass of propellant for a 15m vehicle you realize that the "BFR" is short and stout, the opposite of the F9 family.

I think that's a little optimistic on mass ratios, but I do expect that a 15m diameter vehicle is going to have a very low ballistic coeefficient, with good lift it can spend a lot of time in the upper atmosphere for a low g and thermal load entry, giving a it a very good mass fraftion for an entry vehicle. But I also expect a design that doesn't depend on a never before achieved mass ratio.



Mass ratios are roughly equivalent to today's.  Probably among the most pessimistic/conservative cited here.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1256 on: 12/31/2015 11:19 PM »
I see a GLOW of around 4,600 mt, 31 Raptor engines on the booster and a MCT/BFS of only 75 mt dry.  A vastly more achievable size.

Which assumes 75mt dry mass for a reusable 100mt Mars lander is realistic...

It's more realistic then most other proposals which presume the lander has >1000 mt of propellant tanks at the same mass.  I targeting just 300 mt for tank capacity resulting in a much smaller vehicle and a dry mass fraction of 20%.


I can see maybe overcoming the problems with a super massive launch vehicle on Earth but the ISRU requirements get out of control. Can you really make several thousand mTons of propellant on Mars for each MCT? What's the footprint for a solar array to get that done in 2 years?


I've estimated ~40 acres when using thin-film solar with 10% efficiency, occupying 50% of the ground surface to produce the power necessary to both collect water from the atmosphere and create Methane-Lox propellants of 300 mt over 1 synod.  The equipment including power should mass around 50 mt.

For comparison philw1776 anticipates between 1,170 and 1,404 mt propellant capacity in the MCT, presumably full at mars for the fastest possible Earth return.  A factor of between 4x and 5x more then my own target and would require between 200-250 mt of equipment for the same refueling time if the same technique is used.  Direct ice-mining might substantially improve the efficiency but the scale of mining must be very high because you require 45% of your final propellant mass in water for stoichiometric combustion of Methane-Lox.  This means a total mass of 526 to 631 mt, a bit under 1 mt per day.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1257 on: 12/31/2015 11:28 PM »
What would be wicked pissah would be a well conceived poll where we could select our BFR/MCT parameters before Elon makes his BFR/MCT announcement* and see who is at least in the ballpark.


* OK, I'm optimistically assuming we'll still be alive at that time.
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Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1258 on: 12/31/2015 11:38 PM »
It's more realistic then most other proposals which presume the lander has >1000 mt of propellant tanks at the same mass.  I targeting just 300 mt for tank capacity resulting in a much smaller vehicle and a dry mass fraction of 20%.

Does that include your self-deployable, disposable, wire landing/launch pad?

Offline Stardhingy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1259 on: 12/31/2015 11:49 PM »
Mass ratios are roughly equivalent to today's.  Probably among the most pessimistic/conservative cited here.

Is that accurate? I thought the highest ratios were around .94, or 16:1.

Edit: Oh, you mean total mass fraction to orbit, not per vehicle.
« Last Edit: 12/31/2015 11:54 PM by Stardhingy »

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