Quote from: dante2308 on 05/30/2016 12:35 amI don't know how much it has been stated, but the real issue I'm seeing is with second stage reuse being tied to Mars transportation. I'm not quite understanding the business case for reusing an interplanetary stage that makes 4 trips per decade (assuming Mars launch windows) unless it also doubles as a well-used transportation system to Earth orbit. However Elon is adamant that the Falcon line will take care of the satellite market for the foreseeable future and has indicated that the second state reuse plans aren't compatible with the Falcon rocket architecture. I attached a porkchop plot showing the delta-v space for Mars as the sum of hyperbolic excess velocities. Sorry, I didn't spend any time making it pretty. The point the plot makes is simply that the Mars windows are not really voluntary.Realize that the interplanetary stage will be reused multiple times per each mission cycle:1) launch2) refuel and fire again from Earth orbit toward Mars3) landing on Mars4) refuel and ascent from Mars(possible refueling in Mars orbit, perhaps not required)5) landing on EarthThe stage has to be intact at each stage, not shedding parts that it'll need in the rest of the mission. So you ALREADY need a reusable stage (really a spacecraft, but I'll stick with your terminology) just so the architecture works, so you might as well use it again. And getting 10-15 mission cycles out of each manufactured stage makes a non-insignificant reduction in cost! ...even if it is over 2-3 decades....additionally, SpaceX will require the stage to do refueler duty and cargo duty. They need the stage to be capable of a lot more than 10-15 reuses in those configurations.And yeah, the potential cost reduction of 4-5 mission cycles per decade is still important. They need the cost as low as possible in order to achieve the sub-$500k/person ticket price, and at the full swing of 80,000 people to Mars per year, they'll need that reuse just to keep up.
I don't know how much it has been stated, but the real issue I'm seeing is with second stage reuse being tied to Mars transportation. I'm not quite understanding the business case for reusing an interplanetary stage that makes 4 trips per decade (assuming Mars launch windows) unless it also doubles as a well-used transportation system to Earth orbit. However Elon is adamant that the Falcon line will take care of the satellite market for the foreseeable future and has indicated that the second state reuse plans aren't compatible with the Falcon rocket architecture. I attached a porkchop plot showing the delta-v space for Mars as the sum of hyperbolic excess velocities. Sorry, I didn't spend any time making it pretty. The point the plot makes is simply that the Mars windows are not really voluntary.
The reusable second stage will be shuttling enormous amounts of propellent and cargo in LEO, roughly 5000 tonnes per 100 people sent to Mars. That's not feasible with disposable upper stages.
Quote from: envy887 on 05/30/2016 01:49 amThe reusable second stage will be shuttling enormous amounts of propellent and cargo in LEO, roughly 5000 tonnes per 100 people sent to Mars. That's not feasible with disposable upper stages.5,000 tons? About a quarter of the LEO mass will make it to Mars. That's quite the haul.
Quote from: dante2308 on 05/30/2016 01:57 amQuote from: envy887 on 05/30/2016 01:49 amThe reusable second stage will be shuttling enormous amounts of propellent and cargo in LEO, roughly 5000 tonnes per 100 people sent to Mars. That's not feasible with disposable upper stages.5,000 tons? About a quarter of the LEO mass will make it to Mars. That's quite the haul.Musk estimated 10 cargo flights per passenger flight, all hauling 100t of payload and probably about as much dry mass (engines, structure, heatshield, etc.). That's thousands of tons in LEO, once you include propellent.
I don't see how a bigger expendable launcher is any cheaper. Colonization isn't feasible unless the launcher is cheaply and quickly reusable, including the second stage. That's true no matter what the scale.
The vast majority of launches will not be interplanetary though. To send a 11 ships to Mars (one with 100 passengers, ten with 100t cargo each) will require not 11 but about 50 launches of 100t each to LEO (to orbit the 5000t I mentioned before).A cheaply reusable super heavy lift upper stage is absolutely indispensable to making colonization work.
It would seem having so much expensive infrastructure sitting around for 2 years at a time with nothing to do would doom such a project to economic failure. Trying to move 100 – 800 people every 2 years with an eventual goal of 800,000 – 1 million spread over 2 to 3 decades would also appear to be a recipe for failure. It would make more sense for the 1st few cycles to carry 10 – 20 people at a time with their associated equipment to establish the initial colony and then expanded the system into something much more efficient.As has probably been discussed before, using a Mars cycler with transit stations at both Earth and Mars would make much more sense. The cargo would go by SEP which could be launched at any time and spiral out until they can leave during the appropriate window. When it gets closer to the appropriate window the people can launch and stay at a station at ELL2 and then leave to catch the Mars cycler. At the Mars end they could depart and wait at the Mars station to catch the landing craft down.The beauty of using a Mars cycler is that almost all of the living space, radiation shielding, power and food/water production is only launched once. It doesn’t slow down at the Mars end nor at the earth end. By using the infrastructure to build more infrastructure you are more akin to building a road than just a simple boat like the Mayflower.On each cycle SEP tugs could bring more modules and supplies to the Mars cycler. These cycles would be on both the inbound and the outbound trip thus doubling the number of opportunities. The people would only be going on the outbound side. On each outbound the people could add the new modules and thus increase the number of people this cycler could handle on each trip. This means the system could be expanded and thus make use of all of the infrastructure over the entire departure Mars cycle instead of just being clustered around a small 2 to 3 week window.As most of the people would be going outbound at 1st there would only be in need in the beginning for an outbound cycler. As commerce increases eventually a Mars – Earth inbound cycler could be constructed. In the beginning the most logical cycler to use would be the Aldrin cycler. At the moment there are 18 other cyclic orbits known. Each of these with different time periods per cycle and earth to Mars transit times. But as the commerce between Earth and Mars increases it might make sense to add at least some of these other cyclic periods to the overall system. This would increase the number of transit times available to Mars from approximately once every 2 years to an occasional of multiple times every 2 years.What SpaceX’s long-term plans are only SpaceX knows. But it would seem that the economics would force the maximum use of the infrastructure if a long-term project is to have any chance at all of succeeding.
Quote from: envy887 on 05/30/2016 02:26 amThe vast majority of launches will not be interplanetary though. To send a 11 ships to Mars (one with 100 passengers, ten with 100t cargo each) will require not 11 but about 50 launches of 100t each to LEO (to orbit the 5000t I mentioned before).A cheaply reusable super heavy lift upper stage is absolutely indispensable to making colonization work.I'm having a hard time wrapping my head around a ship that is equally useful as a LEO hauler and a manned Mars expedition ship. I would assume two separate versions under normal circumstances.
Probably three: manned, cargo, and propellent tanker, differing mostly in how payload is stored. They would use the same launcher and engines, and probably be built on the same production line. All would need about the same payload and Delta v performance, and all would need to be capable of Mars and/or Earth EDL. I don't see how building one version to be reusable while the others are expendable saves any cost or time. They all need landing capability (and return, for Mars ships) anyway.Quote from: dante2308 on 05/30/2016 02:33 amQuote from: envy887 on 05/30/2016 02:26 amThe vast majority of launches will not be interplanetary though. To send a 11 ships to Mars (one with 100 passengers, ten with 100t cargo each) will require not 11 but about 50 launches of 100t each to LEO (to orbit the 5000t I mentioned before).A cheaply reusable super heavy lift upper stage is absolutely indispensable to making colonization work.I'm having a hard time wrapping my head around a ship that is equally useful as a LEO hauler and a manned Mars expedition ship. I would assume two separate versions under normal circumstances.
I never suggested anything should be expendable.
I'm just wondering how feasible it is to lower the cost of an interplanetary stage by aiming to reuse it 4 times/decade. It sort of seems like the goal you have after you've been building something similar for decades...
I'm having a hard time wrapping my head around a ship that is equally useful as a LEO hauler and a manned Mars expedition ship. I would assume two separate versions under normal circumstances.
Quote from: dante2308 on 05/30/2016 02:33 amI'm having a hard time wrapping my head around a ship that is equally useful as a LEO hauler and a manned Mars expedition ship. I would assume two separate versions under normal circumstances.I see it working out this way: A MCT will have an engine section and a cargo or passenger section. The engine section will have tanks big enough to work as a tanker as well. With tanks that big it can do fast transfer even in unfavorable launch windows. It just needs another fueling flight.So a new MCT will start its life as a tanker. No passenger or cargo section added, saving weight. Only a cap with heatshield on top. It will do 5 or whatever flights as a tanker until it is proven reliable. Then a cargo or passenger section gets added and it will do ~10 or whatever flights to Mars. At the end of its life the cargo section gets removed and it works as a tanker again. This way you can get at least 40 launches out of the expensive part, the engine section.
Gucky, I like the plan. However, I would put less Mars flights on the menue. If they keep flying the same engine section ~10 times to Mars, they will need to support the same hardware, software, all the replacement parts, etc for about 30 years. Thats a pretty long time. It is more likely that technology advances fast enough to make a replacement of an MCT as a Mars shuttle every ~3 trips more likely. That way, they can phase out old technology every 10 years or something which is much more reasonable.It doesnt really hurt either because the engine section can do just more tanker runs, or even cargo to LEO or GTO missions. In fact, a MCT for GTO missions would be quite cost efficient since the entire rocket gets re-used naturally. Dont know what the payload limits would be though.
It doesnt really hurt either because the engine section can do just more tanker runs, or even cargo to LEO or GTO missions. In fact, a MCT for GTO missions would be quite cost efficient since the entire rocket gets re-used naturally. Dont know what the payload limits would be though.
<snip>Here is my second worry beyond the frequency of interplanetary stage reuse. How do you iterate towards the correct design? We all watched SpaceX redesign the Falcon 9 over the years before they got the stages to land and we can expect several more iterations before they achieve "rapid" reuse. That's just the nature of bleeding-edge engineering. How many chances do they get to nail down MCT's architecture in a way that really optimizes against any contingency in a sustainable way? How many flights before you even put people on that voyage? Rationally, the process to reusable interplanetary spacecraft should take at least half a century if we were in a rush. However, rather than just be skeptical, might I suggest that perhaps a more sustainable path to this kind of thing might actually be the moon after all? So many technologies can be qualified getting there and it's only a few days away. Tickets will sell just as well or better too.