Quote from: oldAtlas_Eguy on 06/20/2015 09:07 pmQuote from: Robotbeat on 06/20/2015 08:45 pmWhy would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?I would guess more like 30-35 tons.Just the propellant tank portion, 5 Raptor engines and possibly landing legs too would weight ~40mt. Now add the reentry shield and the cargo bay structure. Of course the cargo variant will not have as high a dry weight as the crew variant but where is the tradeoff in crew payload size and crew vehicle dry weight increase. If you could get the cargo variant to have a dry weight as low as 60mt then reduce the payload size of the crew variant (crew + supplies) to only 60mt on a crew variant that dry weight 100mt things will work out better in that the overall system becomes smaller. You shrink the size and maybe some savings on the propellant tank dry weight due to smaller tanks.My only problem with the estimates is that the more detail we go the heavier the MCT gets.My visualization for the MCT version of the BFR upper stage is 4 raptors, but the hardware to cant them for Mars landing/take off. I think 60t works for the dry weight of a cargo only version, and I am not committed one way or the other yet as to whether the passenger ECLSS and quarters are just cargo 'modules' that fit on an otherwise standard MCT or a seperately designed and built MCT. What I do expect is that a passenger MCT is less loaded with payload than the cargo only one so that it has more ΔV partly for slighlty shorter transit time, partly for more safety margin.My visualization for a reusable Earth orbit tanker Upper Stage for the BFR is a slightly smaller volume all fuel vehicle that adds little to the launch weight of the BFR and has about 10% lower dry weight than the MCT cargo. Running some numbers this morning I am only seeing 120mt of propellant left over (after margins) to transfer to a depot. Note the idea is that there doesn't need to be active cooling on the tanker since it goes immediately to its offload rendezvous. I want this vehicle to be specially designed because operationally it flies the most. With 4.1 flights per MCT going to Mars, plus whatever flights get made to satisfy other BLEO business the depot, MCTs and other LEO and Earth based Mars infrastructure gets used for. So a small fleet of these makes sense. Cargo MCTs may well work for just launching cargo bound anywhere in LEO or beyond and I suspect that there will be far less volume of this than keeping the depot topped up so no need to specifically develop a LEO cargo MCT.
Quote from: Robotbeat on 06/20/2015 08:45 pmWhy would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?I would guess more like 30-35 tons.Just the propellant tank portion, 5 Raptor engines and possibly landing legs too would weight ~40mt. Now add the reentry shield and the cargo bay structure. Of course the cargo variant will not have as high a dry weight as the crew variant but where is the tradeoff in crew payload size and crew vehicle dry weight increase. If you could get the cargo variant to have a dry weight as low as 60mt then reduce the payload size of the crew variant (crew + supplies) to only 60mt on a crew variant that dry weight 100mt things will work out better in that the overall system becomes smaller. You shrink the size and maybe some savings on the propellant tank dry weight due to smaller tanks.My only problem with the estimates is that the more detail we go the heavier the MCT gets.
Why would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?I would guess more like 30-35 tons.
Quote from: nadreck on 06/21/2015 05:32 pmQuote from: oldAtlas_Eguy on 06/20/2015 09:07 pmQuote from: Robotbeat on 06/20/2015 08:45 pmWhy would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?I would guess more like 30-35 tons.Just the propellant tank portion, 5 Raptor engines and possibly landing legs too would weight ~40mt. Now add the reentry shield and the cargo bay structure. Of course the cargo variant will not have as high a dry weight as the crew variant but where is the tradeoff in crew payload size and crew vehicle dry weight increase. If you could get the cargo variant to have a dry weight as low as 60mt then reduce the payload size of the crew variant (crew + supplies) to only 60mt on a crew variant that dry weight 100mt things will work out better in that the overall system becomes smaller. You shrink the size and maybe some savings on the propellant tank dry weight due to smaller tanks.My only problem with the estimates is that the more detail we go the heavier the MCT gets.My visualization for the MCT version of the BFR upper stage is 4 raptors, but the hardware to cant them for Mars landing/take off. I think 60t works for the dry weight of a cargo only version, and I am not committed one way or the other yet as to whether the passenger ECLSS and quarters are just cargo 'modules' that fit on an otherwise standard MCT or a seperately designed and built MCT. What I do expect is that a passenger MCT is less loaded with payload than the cargo only one so that it has more ΔV partly for slighlty shorter transit time, partly for more safety margin.My visualization for a reusable Earth orbit tanker Upper Stage for the BFR is a slightly smaller volume all fuel vehicle that adds little to the launch weight of the BFR and has about 10% lower dry weight than the MCT cargo. Running some numbers this morning I am only seeing 120mt of propellant left over (after margins) to transfer to a depot. Note the idea is that there doesn't need to be active cooling on the tanker since it goes immediately to its offload rendezvous. I want this vehicle to be specially designed because operationally it flies the most. With 4.1 flights per MCT going to Mars, plus whatever flights get made to satisfy other BLEO business the depot, MCTs and other LEO and Earth based Mars infrastructure gets used for. So a small fleet of these makes sense. Cargo MCTs may well work for just launching cargo bound anywhere in LEO or beyond and I suspect that there will be far less volume of this than keeping the depot topped up so no need to specifically develop a LEO cargo MCT.Yes, the depot could be a specially equipped MCT for 0 boil-off since it would have large enough tanks to refuel 1+ MCT's for Earth departure. This would make it easy to orbit the depots since they are just another cargo specialized version of the MCT which are then manufactured in the 10's to 100's.
My visualization for the MCT version of the BFR upper stage is 4 raptors, but the hardware to cant them for Mars landing/take off. I think 60t works for the dry weight of a cargo only version, and I am not committed one way or the other yet as to whether the passenger ECLSS and quarters are just cargo 'modules' that fit on an otherwise standard MCT or a seperately designed and built MCT. What I do expect is that a passenger MCT is less loaded with payload than the cargo only one so that it has more ΔV partly for slighlty shorter transit time, partly for more safety margin.
Quote from: nadreck on 06/21/2015 05:32 pmMy visualization for the MCT version of the BFR upper stage is 4 raptors, but the hardware to cant them for Mars landing/take off. I think 60t works for the dry weight of a cargo only version, and I am not committed one way or the other yet as to whether the passenger ECLSS and quarters are just cargo 'modules' that fit on an otherwise standard MCT or a seperately designed and built MCT. What I do expect is that a passenger MCT is less loaded with payload than the cargo only one so that it has more ΔV partly for slighlty shorter transit time, partly for more safety margin.I don't believe integrated habitat and direct Earth-reutrn are compatible.
My favorite, Quote from: MuskI mean, if you do a densified liquid methalox rocket with on-orbit refueling, so like you load the spacecraft into orbit and then you send a whole bunch of refueling missions to fill up the tanks and you have the Mars colonial fleet - essentially - that gets built up during the time between Earth-Mars synchronizations, which occur every 26 months, then the fleet all departs at the optimal transfer point. Elon Musk at MIThttp://shitelonsays.com/transcript/elon-musk-at-mits-aeroastro-centennial-part-2-of-6-2014-10-24
I mean, if you do a densified liquid methalox rocket with on-orbit refueling, so like you load the spacecraft into orbit and then you send a whole bunch of refueling missions to fill up the tanks and you have the Mars colonial fleet - essentially - that gets built up during the time between Earth-Mars synchronizations, which occur every 26 months, then the fleet all departs at the optimal transfer point.
Quote from: Impaler on 06/21/2015 06:06 pmQuote from: nadreck on 06/21/2015 05:32 pmMy visualization for the MCT version of the BFR upper stage is 4 raptors, but the hardware to cant them for Mars landing/take off. I think 60t works for the dry weight of a cargo only version, and I am not committed one way or the other yet as to whether the passenger ECLSS and quarters are just cargo 'modules' that fit on an otherwise standard MCT or a seperately designed and built MCT. What I do expect is that a passenger MCT is less loaded with payload than the cargo only one so that it has more ΔV partly for slighlty shorter transit time, partly for more safety margin.I don't believe integrated habitat and direct Earth-reutrn are compatible.An integrated hab for 6-10 people could probably mass less than 25 tonnes, so it seems possible for initial missions. Later missions with more would need a larger hab and that could not be integrated. So I believe you have made an important point, integrated habs have no long term future on the MCT, so will probably not be designed in the first place.
The landing gear mass is almost certainly driven by the force of impact with the surface NOT the static weight of the vehicle, in other words objects still have inertia irregardless of gravity. And even if static weight weight were the concern you would need to size the legs based on the gross take off weight which we all agree will be greater then landing weight.F9 first stage has 8% of dry mass in the leg system, and this is designed for flat artificial surfaces and is not carry precious human cargo. The LEM had around 3% of touch down mass in legs, but that was a soft-touchdown with a deeply throttling engine, not the SpaceX 'hover-slam'.I think Gross take off Weight will be ~450 mT total, not these monstrous 1000 ton figures. And their would not be any kind of integral habitat in a 'crew' version. Their will just be a single version with an unpressurized cargo bay into which a habitat module would be placed.Max Q is aerodynamic pressure peak, in the Martian atmosphere it is an almost irrelevant force compared to the force experienced during launch from Earth, it is not the same as max g-forces which is what would be relevant for not crushing the vehicle.
You are correct about mass vs. weight, but I would think the Martian landing will be at lower speed due to the lower gravity (wider tolerance for v=0 altitude=0 point). Good point about Mars GLOW though I think we would both agree that that should not be the peak force on the legs (Earth landing will).
Good comparison on the lunar lander vs F9 S1 legs. The F9 legs had the additional constraints of having to be deployable, aerodynamic when folded, support a higher COV vehicle, and as you mentioned a higher speed impact (through the nominal should be close to 0). Probably all the same constraints the MCT legs will have.
The reason I think they will all be pressurized is that I think having an aero shell and a separate pressure vessel is a waste of mass. The exception to this would be if you are leaving the habitat behind on mars. With a top or side TPS I think a pressurized volume is all but required. Are you assuming a capsule design with the TPS on the bottom?
Regarding max Q and max Gs. I agree with you in the general sense, but I think we may be talking past each other. Different phases of flight have different masses, and a larger mass at a given G load requires more structure. There are also different "sources" of force acting on the MCT (inner stage, engine thrust structure, TPS, nose). The highest G phase will be the peak pressure on most structural members, but not all. At max Q (earth assent) the MCT structure has to support the aerodynamic pressure plus the payload mass (times Gs) plus the full propellant mass (times Gs) (yes I'm assuming combined S2, you might not be). My point was that certain structural members will see a higher pressure at max Q than at higher G phases of the flight (MECO, SECO, mars assent, and mars or earth entry)Edit: spelling
A 5-person crew's luxurious tourist hab is a 25-person crew's adequate expedition hab is a 100-person crew's short-term transfer hab. Design once, and use it for multiple campaigns.
Agree that static weight is not an issue, it is speed of impact with the surface. But I disagree that gravity will be the main determinant of that, on both Mars and Earth the vehicle will be under propulsive decent and gravity is simply a part of that 'dance' the Raptor engine is TOO MUCH thrust to hover on at either Earth or Mars (230 mT hover on Earth, 605 mT on Mars), no one belives MCT would have that much mass at touchdown. I favor a set of vernier engines specifically to make a softer touchdown (and avoid cratering the surface)
I have never heard of an space vessel in which the outer aero shell IS the pressure vessel, I would speculate that it presents for too much of a thermal pathway into the vessel and would literally COOK the passengers, note that reentry capsules get quite warm inside during re-entry and this is with considerable insulation between the TPS and pressure vessel. So I do not believe what your describing is possible.Leaving habitats on surface is exactly what I proposed. The overall shape I'm going with (originally Lobo's configuration) is that of a biconic with TPS on the top/sides, engines and legs on the bottom and an unpressurized cargo-bay door on the side.
Direct-return vehicles are essentially a dead-end configuration because they have a low maximum flight rate, so why go down that path.
Sometimes I use a first approximation and say the cargo is as much as transport cost. That would average 500000 $ for one t. Averaged between simple tools and Intel CPUs.
Quote from: Impaler on 06/21/2015 10:02 pmDirect-return vehicles are essentially a dead-end configuration because they have a low maximum flight rate, so why go down that path.But SEP stages and transit habs also have a low flight rate. So replacing a direct-return MCT with SEP, transit hab and a smaller MCT lander does not seem to be a win.The main advantage of a direct-return MCT is that it can be maintained, refurbished and upgraded on Earth. Any architecture which involves space-only or Mars-only stages has to explain how they will be maintained in space or on Mars. This is not an easy problem to solve, especially if we assume new and upgraded versions of SEP, transit hab and MCT lander are produced every few years. Earth has so many advantages, presence of jigs and tooling, unlimited supplies of water and other working fluids, a full local supply chain, local presence of the design engineers, clean rooms, large hangers under pressure, etc. I think that the overheads of maintaining equipment off-earth removes any advantage your architecture might have in terms of lower initial mass in LEO, at least until we reach colony sizes of 100,000.
Impaler and CyclerPilot, this idea should be compatible with the COOPS you are developing:Consider the passenger version of MCT, the trans-Hab and the surface-Hab to be all the same unit. By this I mean that the passenger section and the propulsion section of MCT would be separate and functionally independent units, i.e. propulsion avionics entirely in the propulsion unit and ECLSS components entirely within the passenger unit.. The passenger unit would sit on top of the propulsion unit, which sports a wide heat shield on its underbelly for EDL. The bottom rim of the passenger unit would be joined to the top rim of the propulsion unit only by a ring of bolts through both rims.Passengers would ride in this vehicle from Earth and land on Mars' surface. A mobile robotic arm that was previously deployed onto Mars' surface would remove the bolts holding the units together. Then a pre-deployed crane would raise the passenger unit and place it on the ground in a desirable location. The passengers have now landed on Mars along with a permanent habitat unit complete with ECLSS. The propulsion unit, now rather lightweight, could then be launched back to Earth and reused. Note that the heat shield on the underbelly also returns.This system could also pre-deploy habitats on Mars prior to the first human landing.As more colonists arrive and build ISRU-based habitats, these original habs would continue to be employed as backup in case of emergency or simply additional housing to give colonists more living space. Also note that not all habs would be permanently located on Mars; some would be launched back to Earth with persons wishing to return.A cargo version of MCT could also perform double-duty. Once landed, the cargo unit would likewise be removed and set on the ground. Unloading cargo would proceed from ground level to ground level. After unloading, the cargo hatch door(s) would be closed and permanently welded shut, both the interior pressure vessel and the exterior shell. Now we have a sizable tank for storing propellants or other liquids produced on Mars.Do you think this is feasible?
Quote from: Impaler on 06/21/2015 10:02 pmDirect-return vehicles are essentially a dead-end configuration because they have a low maximum flight rate, so why go down that path.But SEP stages and transit habs also have a low flight rate. So replacing a direct-return MCT with SEP, transit hab and a smaller MCT lander does not seem to be a win.Any architecture which involves space-only or Mars-only stages has to explain how they will be maintained in space or on Mars.