MCT will necessarily have to rely on some preplaced ground infrastructure. It will be incredibly foolish to hobble the fundamental design just because of the very first mission. If it is a problem, SpaceX can use Dragon to pre-land a rover or a crane, or some one-shot modifications made to MCT to allow it to land on an unprepared surface.
I'm not sure you get to have short, stubby legs and rear engine hoverslam landings at the same time on unprepared soil. Excavation by the plume is a problem.
Quote from: Burninate on 10/30/2015 04:12 amI'm not sure you get to have short, stubby legs and rear engine hoverslam landings at the same time on unprepared soil. Excavation by the plume is a problem.Nobody said it would be easy. There will have to be a engineering trades, but I certainly believe a MCT gear must be very strong - and to save mass, short. Remember that it also needs to be capable of supporting a full propellant load before takeoff.The legs will need to be more like Dragons legs than F9R.
I believe the landing legs won't need to be like the Falcon 9 first stage legs. F9 is long and slender. MCT will be short and stubby in comparison.
But, in general, when I say that the IBMCT will mass less than an MCT + S2, and thus will have more gross LEO capability, that's simple logic. You aren't duplicating your hardware and systems, like you are with a S2R + MCT. You have one TPS, not two. [...]
So if you assume the same booster for either [...]IF you were doing a 3 stage to LEO, that might be different. But you are talking TSTO either way [...]
No, I *strongly* disagree with this. MCT will need to be capable of landing on unprepared Martian ground. There is no question about it, if you think about it.
I do apologize if I allude to anything I can't expand on. I'm trying to stay away from doing that because it's poor form.
And remember DC-X demonstrated the Swan Dive maneuver, a transition from slightly angled down nose-first entry to vertical landing. A small aerosurface or two might help if you wanted to go at an even greater angle, but I see nothing that suggests the transition is impossible.
BTW, I guess that conceptually, I think of MCT as a less-ambitious version of DC-Y/DeltaClipper/DC-I. Less ambitious because: It wouldn't need to be SSTO (6-8km/s is all that's needed, which makes a HUGE difference vs 9.5km/s... basically it means you can afford TWICE the dry mass including payload!), it uses methane (which in spite of the Isp hit probably would make SSTO easier due to the FAR higher bulk density) instead of hydrogen, and it'd basically always operate in vacuum except for final landing at Earth, thus making an aerospike nozzle unnecessary.
I believe the landing legs won't need to be like the Falcon 9 first stage legs. F9 is long and slender. MCT will be short and stubby in comparison. I believe also that the legs will not need to support MCT fully fuelled for launch. They could add supports for that purpose before tanking takes place.
Quote from: Lobo on 10/29/2015 11:24 pmBut, in general, when I say that the IBMCT will mass less than an MCT + S2, and thus will have more gross LEO capability, that's simple logic. You aren't duplicating your hardware and systems, like you are with a S2R + MCT. You have one TPS, not two. [...] C'mon, you know you can't linearly sum non-linear systems like that. The systems on two vehicles will be individually lighter (and designed for a lighter vehicle) than the systems on a larger integrated vehicle. For example, the TPS on the second stage will not need to cope with Mars direct-return velocity. Ie, 7km/s instead of 11, or just 40% of the energy and at a lower g-load. Same with all the flow-on effects of requiring larger systems, higher mass, then sturdier structures to deal with the higher mass, increasing the mass further...
Quote from: Lobo on 10/29/2015 11:24 pmSo if you assume the same booster for either [...]IF you were doing a 3 stage to LEO, that might be different. But you are talking TSTO either way [...]The MCT has tanks and engines. Why wouldn't you take advantage of that and use it as a third stage to increase payload?However, for me the clincher is that the if we assume SpaceX take the same incremental approach they did with the development of F9R and Dragon, then the development path goes through the lower requirements of a reusable second stage. Incrementally developing that stage will provide them with key insights in developing the MCT. For example, in theory, a larger stage will be easier to re-enter due to its lower density; but in practice, structural strength has been an issue. Which one dominates in the MCT design? A less demanding second stage should help them learn as they go. If you can more easily solve the structural issues, you go big - Integrated stage MCT. If structural issues dominate, you split the vehicles - MCT with separate second stage. Developing directly to MCT will invariably involve making decisions early that cause problems later, since you don't know in advance which systems are going to work better than expected and which are going to be harder, more expensive, or higher maintenance.[Example, SpaceX has apparently found for FH that increasing the performance of the Merlin engines is easier than cross-feed. And for BFR, that more engines on a single core is easier to manage than more cores; which goes against previous industry assumptions.]The issue isn't "what is the optimum Mars vehicle that I, and my chums on L2, can design", instead it's "what is the likely lowest-cost development path for SpaceX for the whole system". That path goes through a second stage. Where it leads after that... depends on how that second stage performs.
The issue isn't "what is the optimum Mars vehicle that I, and my chums on L2, can design", instead it's "what is the likely lowest-cost development path for SpaceX for the whole system". That path goes through a second stage. Where it leads after that... depends on how that second stage performs.
My take is if the second stage can come back through the thicker earth's atmosphere, and land in the heavier gravity than Mars, then it should be easy to modify for the MCT, thus serving two purposes. Bottom half could be the same. Top half would either be empty for a payload, or the modular MCT cargo, human habitation, and solar panels for power along with the metholox production equipment.
...The issue isn't "what is the optimum Mars vehicle that I, and my chums on L2, can design", instead it's "what is the likely lowest-cost development path for SpaceX for the whole system". That path goes through a second stage. Where it leads after that... depends on how that second stage performs.
In addition to your two points I would add that with the ability to land on unprepared surfaces the MCT would be usable for sub-orbital hops of perhaps several thousand miles and thus serve as a means of rapid transit between base locations and the hinterlands, effectively the same role that helicopters and bush-planes would serve on Earth and we see heavy use of such vehicle in wilderness/pioneering areas.
Quote from: Paul451 on 10/30/2015 12:51 pmThe issue isn't "what is the optimum Mars vehicle that I, and my chums on L2, can design", instead it's "what is the likely lowest-cost development path for SpaceX for the whole system". That path goes through a second stage. Where it leads after that... depends on how that second stage performs.This is exactly the way that SpaceX operates - they have learned the Soviet incremental approach to goals, but with vertically integrated manufacturing.
For the same dry mass and tank size
Quote from: Robotbeat on 10/31/2015 01:35 amFor the same dry mass and tank sizeThat's the major flaw in the work your siting, at equal volume the difference in propellant density means a huge difference in gross take off weight and gross vehicle weight would follow that relationship not remain static. And costs scale with dry vehicle weight not volume so it is simply not a fair comparison....