Quote from: philw1776 on 12/30/2015 10:26 pmI 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 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.
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.
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.
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.
Umbrella, do you even read other posts, or is this a post-only thread for you?
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.
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.
Quote from: philw1776 on 12/31/2015 01:00 pmMy 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.
Quote from: Impaler on 12/31/2015 02:57 amI 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...
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?
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%.
Mass ratios are roughly equivalent to today's. Probably among the most pessimistic/conservative cited here.