Author Topic: MCT Speculation and Discussion Thread 4  (Read 626327 times)

Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1220 on: 12/15/2015 03:11 AM »
The one design criteria most overlooked is minimum operating costs. This is probably the most significant item for the design and delta -V requirements that the BFR must meet. These operating costs are for sending an MCT to Mars meaning less tanker flights means much lower costs. An increase of the BFR's deliverable delta V by 750m/s increases the tanker prop load from 100mt to 200mt. This nearly halves the total cost for an MCT to Mars by almost 50% from 10 flights to only 5 flights. This is without any change to the design of the BFS itself. The BFS is fairly easily sized because the three design flight uses: as second stage, as EDS, and as Mars SSTO all are very close to the  same propellant loads requirements.

Could not agree more.
I was in a high-tech industry engineering products where low production cost was paramount.  We did the same cost driver analysis on every single aspect that Musk does.
I see the 1st stage as going low & slow under 3 Km/sec while the delta V is in the MCT stage two which makes it utilitarian for all Mars purposes.  Large propellant tanks make 'mods" for a tanker minimal. 
I also think those speculating that the BFR might be larger than minimal models may be onto something as they reduce # of flights to refuel the MCT in LEO or wherever.  It's a complex system analysis whether to make a minimum parts cost re-useable BFR vs making one a bit larger (a bit more expensive per unit) that reduces # of flights to fuel up an MCT in orbit for a Mars journey.
The range of LEO capability given by SpaceX way back for the MCT was 180-250mt. As we have discussed tremendous about what the BFS dry weight is possible 80mt is sort of a average or consensus value + 100mt of payload, making the 180mt to LEO the absolute minimum that the system must meet. But what if the performance was closer to the other end 250mt. That means that about 70mt of extra propellant is delivered per flight.

The whole reason for the larger BFR and less flights would be the same regardless of whether the BFR is reusable or expendable. If the minimum design BFR takes 10 flights to accomplish sending a single BFS to Mars but a bigger BFR with the exact same design BFS that takes only 5 flights although the BFR costs 50% more per flight still gives a reduction to per mission of 75% of the 10 flights configuration. There are other advantages to requiring half the flights and that is pad availability. In order to support sending 4 BFS's to Mars the 10 flights minimal BFR configuration would require 40 launches in the 780 day period (a launch every 19.5 days).  In order to support sending 4 BFS's to Mars the 5 flights large BFR configuration would require 20 launches in the 780 day period (a launch every 39 days). For the 10 cargo to 1 crew ratio of missions making the possibility of 20 cargo and 2 crew missions in a synod (780 day period) that would require
a) 10 flight config-> a launch every 3.5 days
b) 5 flight config-> a launch every 7 days

As mission counts increase the number of launches becomes a greater cost factor than any other consideration.

@oldAtlas_Eguy By your argument for fewer BFR flights to support each BFS going to Mars. It seems to make sense to go bigger with the BFR to to take one less flight per Mars bound BFS. Especially since even the smaller BFR being discuss will need specialized logistics infrastructure anyway. Maybe a bigger 15+ meter diameter BFR is cheaper with fewer flights needed for each Mars bound BFS.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1221 on: 12/15/2015 11:06 AM »
Could the first stage go up almost straight similar to the BO New Shepard, but maybe up to 150km peak altitude, eating all the gravity and air resistance losses and use the second stage for the task of building up orbital speed? On the way down with its large diameter it may not need a reentry burn or only a very small one. Reuse fuel would be mainly only the small amount of landing fuel.

Everyone hit you over horizontal velocity, but there may also be an issue with the vertical. Going to 150km (rather than staging below 100km with enough vertical velocity to carry the two stages above 100km), means that the first stage will have a 100km free-fall before it hits the atmosphere.

While the entry speed may be technically lower than a 3km/s horizontal entry, the rate of atmospheric density increase will be extremely sharp. That induces stresses on the stage in addition to mere re-entry heating.

(Travelling 100km through the first ten kilometres of atmosphere, then 100km through the second 10km... vs travelling 30km through the first 30km.... See what I mean. 200km of deceleration before you reach 30km altitude, vs just 30km deceleration and you're already deep in the atmosphere at the 20km mark.)

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1222 on: 12/15/2015 11:55 AM »
While the entry speed may be technically lower than a 3km/s horizontal entry, the rate of atmospheric density increase will be extremely sharp. That induces stresses on the stage in addition to mere re-entry heating.

(Travelling 100km through the first ten kilometres of atmosphere, then 100km through the second 10km... vs travelling 30km through the first 30km.... See what I mean. 200km of deceleration before you reach 30km altitude, vs just 30km deceleration and you're already deep in the atmosphere at the 20km mark.)

The vertical component is still the same, just a horizontal component added. I don't see at the moment how this could be less harsh. I am not 100% sure though.

Offline OxCartMark

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Re: MCT Speculation and Discussion Thread 4
« Reply #1223 on: 12/27/2015 01:52 AM »
Just re-read parts of the GQ article.  Here is a bit that is just below the previous quote that I think belongs in this thread -


"Musk has previously said that he would publicly present some specifics of his Mars-colonization plans later this year, though he tells me that it may now be early next year. "Before we announce it, I want to make sure that we're not gonna make really big changes to it," he says. "Um, yeah. I think it's gonna seem pretty crazy, no matter what."

Just because it's so far beyond what people would imagine?

He laughs. "It's really big." And laughs again. "It's really big. There's not been any architecture like this described that I'm aware of."


That's from December 2015.

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1224 on: 12/27/2015 10:01 AM »
Just re-read parts of the GQ article.  Here is a bit that is just below the previous quote that I think belongs in this thread -


"Musk has previously said that he would publicly present some specifics of his Mars-colonization plans later this year, though he tells me that it may now be early next year. "Before we announce it, I want to make sure that we're not gonna make really big changes to it," he says. "Um, yeah. I think it's gonna seem pretty crazy, no matter what."

Just because it's so far beyond what people would imagine?

He laughs. "It's really big." And laughs again. "It's really big. There's not been any architecture like this described that I'm aware of."


That's from December 2015.

It's that quote that I keep thinking of when people start saying what they think the architecture will be, big capsules etc. I don't think anyone has yet described an architecture that would fit with Musk's statement.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1225 on: 12/27/2015 06:55 PM »
Is he saying it is bigger even then Nova?  Bigger then the UR-700M?   What is his limit for 'described'?
« Last Edit: 12/27/2015 06:58 PM by Impaler »

Offline TheTraveller

Re: MCT Speculation and Discussion Thread 4
« Reply #1226 on: 12/30/2015 04:25 AM »
SpX Mars suit worn by Elon Musk?
BFS in the background?

https://www.instagram.com/elonmusk/?hl=en

Is this a Martian rescue fleet arriving after a Earth comet slam?
« Last Edit: 12/30/2015 04:41 AM by TheTraveller »
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Offline Coastal Ron

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Re: MCT Speculation and Discussion Thread 4
« Reply #1227 on: 12/30/2015 05:01 AM »
SpX Mars suit worn by Elon Musk?
BFS in the background?

https://www.instagram.com/elonmusk/?hl=en

Is this a Martian rescue fleet arriving after a Earth comet slam?

Nice catch!  That must being showing Musk on Earth though with his helmet off, unless he figures he'll be able to terraform Mars before he gets there.

Also, the vehicle in the background has aerodynamic features, so it's not just ballistic like the Falcon 9 1st stage.

Of course all this assumes that the drawing reveals some real details, but it could be pure fantasy...
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline TheTraveller

Re: MCT Speculation and Discussion Thread 4
« Reply #1228 on: 12/30/2015 05:10 AM »
SpX Mars suit worn by Elon Musk?
BFS in the background?

https://www.instagram.com/elonmusk/?hl=en

Is this a Martian rescue fleet arriving after a Earth comet slam?

Nice catch!  That must being showing Musk on Earth though with his helmet off, unless he figures he'll be able to terraform Mars before he gets there.

Also, the vehicle in the background has aerodynamic features, so it's not just ballistic like the Falcon 9 1st stage.

Of course all this assumes that the drawing reveals some real details, but it could be pure fantasy...

Elon is known to like dropping hints, bread crumbs, like he is doing with Chris on L2.

Time will tell if it is fantasy or not.

Elon's suit is bigger than Watney's Martian suit.

Is clear to see the bent traffic control stop light, left of his foot on the ladder. Also shattered buildings to his left & right. So not Mars.

Enhanced BFS image attached.
« Last Edit: 12/30/2015 06:09 AM by TheTraveller »
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Offline cscott

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Re: MCT Speculation and Discussion Thread 4
« Reply #1229 on: 12/30/2015 08:04 AM »
Doesn't look like the SpaceX suit design to me.  Doesn't particularly look like Elon, either, for that matter.

The only thing that is familiar is the Dragon-like canted thruster arrangement.
« Last Edit: 12/30/2015 08:05 AM by cscott »

Offline TheTraveller

Re: MCT Speculation and Discussion Thread 4
« Reply #1230 on: 12/30/2015 08:41 AM »
Doesn't look like the SpaceX suit design to me.  Doesn't particularly look like Elon, either, for that matter.

The only thing that is familiar is the Dragon-like canted thruster arrangement.

Looks like a surface / EVA suit and not a Crew Dragon flight suit.
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Offline Stardhingy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1231 on: 12/30/2015 05:37 PM »

It's that quote that I keep thinking of when people start saying what they think the architecture will be, big capsules etc. I don't think anyone has yet described an architecture that would fit with Musk's statement.

I'm just going to suggest an outrageous one. The MCT is an Earth SSTO vehicle sized near the weight limit of LC39A that can deliver 236mTons to LEO. With a BFR under it it reaches orbit high on fuel. Fuel capacity is significantly higher than required for the trip to Mars, so one BFR flight can fuel up more than one MCT. Early flights can de-risk ISRU by leaving an MCT in Mars orbit with fuel for TEI and landing with enough fuel to get back to orbit.

I think that's probably big enough.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1232 on: 12/30/2015 07:59 PM »

It's that quote that I keep thinking of when people start saying what they think the architecture will be, big capsules etc. I don't think anyone has yet described an architecture that would fit with Musk's statement.

I'm just going to suggest an outrageous one. The MCT is an Earth SSTO vehicle sized near the weight limit of LC39A that can deliver 236mTons to LEO. With a BFR under it it reaches orbit high on fuel. Fuel capacity is significantly higher than required for the trip to Mars, so one BFR flight can fuel up more than one MCT. Early flights can de-risk ISRU by leaving an MCT in Mars orbit with fuel for TEI and landing with enough fuel to get back to orbit.

I think that's probably big enough.
1) Um... why?
2) Deliver 236 tons payload?
2A) Assuming 'yes', and you mean 236 tons of bricks to orbit on top of the rocket:


The structural mass fraction of the F9 first stage is currently estimated here at 6.1%.  The structural mass fraction of the Shuttle External Tank (SLWT edition) is estimated here at 3.5%.  Let's split the difference and call the estimated rocket structural mass fraction 5%.
  EDIT: Actually, the Shuttle tank's structural mass fraction should be higher (worse) than the Falcon tank according to first principles;  This is an apples to oranges comparison because the Shuttle External Tank had no engines.  Tank mass fraction is supposed to scale inversely with propellant density;  The density of methalox is lower than the density of RP-1-LOX.  Perhaps I should rerun this with 7% or 8%.

Assuming 5% structural mass fraction on the rocket:

The burn mass ratio for a liftoff burn of 9.2km/s at an average Isp of 370s, will be about 12.6:1 wet to dry.

Plug that into here with a 9200m/s LEO, and you need 7143mt launch mass to reach orbit with 236mt of payload atop an empty rocket with calculated mass 329mt.  Add about 25% for liftoff thrust margin, and you need 87.5MN liftoff thrust. Raptor thrust is ~= 2250kN;  That makes for 39 Raptors.

LC-39A is said to be designed to 12.5Mlbf == 55.6MN.

2B) Assuming 'no', and you meant 236t total mass to orbit:

Plugging the same parameters in (with a little iteration to work the linked calculator backwards) but with an m1 (total mass to orbit) of 236t, you get 98.6mt payload mass atop a 137.4mt empty rocket to orbit, using a launch mass of 2984mt.  With the same coefficient, that's 36.6MN launch thrust, which is within the scope of LC-39A.   That makes for 17 Raptors... but below 100mt to orbit.

EDIT: Updated calcs two posts down
« Last Edit: 12/30/2015 08:43 PM by Burninate »

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #1233 on: 12/30/2015 08:09 PM »
SpX Mars suit worn by Elon Musk?
BFS in the background?

https://www.instagram.com/elonmusk/?hl=en

Is this a Martian rescue fleet arriving after a Earth comet slam?

Well that's interesting. Hard to believe that's just some fantasy vehicle.

Looks like a biconic to me, although its hard to tell.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1234 on: 12/30/2015 08:41 PM »

It's that quote that I keep thinking of when people start saying what they think the architecture will be, big capsules etc. I don't think anyone has yet described an architecture that would fit with Musk's statement.

I'm just going to suggest an outrageous one. The MCT is an Earth SSTO vehicle sized near the weight limit of LC39A that can deliver 236mTons to LEO. With a BFR under it it reaches orbit high on fuel. Fuel capacity is significantly higher than required for the trip to Mars, so one BFR flight can fuel up more than one MCT. Early flights can de-risk ISRU by leaving an MCT in Mars orbit with fuel for TEI and landing with enough fuel to get back to orbit.

I think that's probably big enough.
1) Um... why?
2) Deliver 236 tons payload?
2A) Assuming 'yes', and you mean 236 tons of bricks to orbit on top of the rocket:


The structural mass fraction of the F9 first stage is currently estimated here at 6.1%.  The structural mass fraction of the Shuttle External Tank (SLWT edition) is estimated here at 3.5%.  Let's split the difference and call the estimated rocket structural mass fraction 5%.
  EDIT: Actually, the Shuttle tank's structural mass fraction should be higher (worse) than the Falcon tank according to first principles;  This is an apples to oranges comparison because the Shuttle External Tank had no engines.  Tank mass fraction is supposed to scale inversely with propellant density;  The density of methalox is lower than the density of RP-1-LOX.  Perhaps I should rerun this with 7% or 8%.

Assuming 5% structural mass fraction on the rocket:

The burn mass ratio for a liftoff burn of 9.2km/s at an average Isp of 370s, will be about 12.6:1 wet to dry.

Plug that into here with a 9200m/s LEO, and you need 7143mt launch mass to reach orbit with 236mt of payload atop an empty rocket with calculated mass 329mt.  Add about 25% for liftoff thrust margin, and you need 87.5MN liftoff thrust. Raptor thrust is ~= 2250kN;  That makes for 39 Raptors.

LC-39A is said to be designed to 12.5Mlbf == 55.6MN.

2B) Assuming 'no', and you meant 236t total mass to orbit:

Plugging the same parameters in (with a little iteration to work the linked calculator backwards) but with an m1 (total mass to orbit) of 236t, you get 98.6mt payload mass atop a 137.4mt empty rocket to orbit, using a launch mass of 2984mt.  With the same coefficient, that's 36.6MN launch thrust, which is within the scope of LC-39A.   That makes for 17 Raptors... but below 100mt to orbit.

Based on this report, page 5 figure 4, 6.8% is a reasonable structural mass fraction for a typical expendable RP-1/LOX core stage;  In achieving 6.1% SpaceX invented an engine with record-setting TWR.  Raptor will end up being a more complex, heavier FFSC engine in the interest of high specific impulse.

This report contains a table on expendable hydrogen stages.  The largest ones have structural mass fractions of 6.25% (not a first stage though) to 10.1%.

This leads me to believe 7% is a decent optimistic estimate for structural mass fraction of an expendable methalox stage with no thermal management / insulation & Raptor engines.  The Big Question is how much propellant depot capability adds to that... but since that's hard to quantify, let's leave that aside for a while and work from a best case scenario where it adds nothing.

Going back to the calculator and subbing in 7%, I end up with:
2A)236mt payload to orbit
empty rocket mass of 1041mt
16100mt launch mass
197MN;  88 Raptors

2B)236mt mass to orbit
empty rocket mass of 192.4mt;  Payload of 43.6mt
2984mt launch mass (unchanged)
36.6MN; 17 Raptors (unchanged)

If you get pessimistic and raise structural mass fraction to 10% to account for thermal insulation etc, launch becomes impossible in either vehicle, anything below that 12.6:1 ratio we worked out earlier (~= 7.9%) hits an asymptote and can't reach orbit.

I think we're way too close to the wire here.
« Last Edit: 12/30/2015 09:11 PM by Burninate »

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1235 on: 12/30/2015 08:42 PM »
Elon's Instagram acct image looks like it comes from anime or manga. It has that style. I can't find the series, but throwing it into Google image search only returns other manga images, suggesting the colour palette is the same.

As for the vehicle, rotate it to the right, and it's shape seems pretty aircraft/shuttle-like. Which is common in anime.

Offline Stardhingy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1236 on: 12/30/2015 09:17 PM »
>Why?

For 80,000 people per year at 100 per vehicle you would need to average two flights per day. Really though you need to do it over about 3-months or you leave passengers in Earth orbit for months before departure. So we get to 860 flights per day.

Then you need to fuel up to get to a high orbit, and fuel up again. So if its 5 flights to fill up in LEO and use use half the fuel to get to the staging orbit, then its probably another 10 to get fuel to the staging orbit. You could save on the high orbit fill ups by using SEP tugs, but a SEP tug can't make many trips.

It works, for dozens of ships, but it doesn't scale to thousands well. An SSTO would simplify ground operations, and you could start going to Mars without the BFR, since the MCT would be capable of taking fuel up.

Also... I just wanted to get a guess out there that we can be pretty sure is not an underestimate of what he means by BFS.

Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1237 on: 12/30/2015 09:50 PM »
I think size estimates have generally been too low.  Start with Earth surface to Mars surface and back to Earth surface, only refueling on Mars surface, everything reusable.  BFS when it lands on Mars, dry of fuel, is going to be 300 MT or more.  Even an empty 747 is over 200 MT.  BFS will contain 100 MT cargo (100 passengers + life support, etc. will probably weigh the same) when it lands and have empty tanks+engines+heat shield+landing legs for return. 

Falcon Heavy is claimed (on Spacex website) to be able to send 13 MT to Mars with liftoff weight of 1463 MT, for liftoff to payload ratio of about 113 - not even clear if that includes reusability.  (That is a lot better than the ratio for MSL, which was about 167 using an Atlas V.) So, it seems like BFS+BFR at Earth liftoff will be at least 34,000 MT, but more likely a lot more.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1238 on: 12/30/2015 10:26 PM »
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.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1239 on: 12/30/2015 10:40 PM »
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.
« Last Edit: 12/30/2015 10:52 PM by Burninate »

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