Mars EDL is pretty hard, so you'd try to get down as much useful cargo as you can. If a transfer stage can remain in orbit, why not keep it there? It can be refuelled in LEO, and used again for the next trip.
In any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle.
Quote from: DLR on 01/23/2015 02:21 pmIn any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. I think I agree with you here. I wouldn't be surprised that when Musk announces the rough design for MCT that it will be scaled down to, say, 50 passengers, and that in reality it will never come close to having that many. I think the first several MCT missions and synods will see it with a 'modest' crew of about 6 to start, building up to at most 15-20. These folks will be busy exploring (esp. water sites and extraction methods), and setting up infrastructure (habs, power, prep'd landing site, greenhouses, etc.).Then after a decade and a half, we'll see some type of Aldrin Cycler for transferring 100 (or more!) folks per trip. This vehicle will have much more volume per person that the MCT could ever offer, have artificial gravity via spinning modules, have a high degree of closed life support, offer good solar flare protection, etc. The MCT's mission thereafter will transition very nicely to ferrying people and cargo from Earth's surface up to the cycler, and then at Mars from Mars orbit down to the surface (i.e. MCT's won't make the Earth-Mars 'crossing' any longer).
Basically I believe that MCT in the long run will be its own launch abort system. Capable of separating from the first stage and soft land. But probably not capable of speeding away from a fireball in case the first stage has a catastrophic failure without prior warning. I believe that after a sufficiently large number of flights the launcher will prove that this kind of catastrophic failure is rare enough that it needs not be considered in operation. But in discussion it seems I am quite alone with that position.
I'm not up to date on all of Elon's comments regarding the MCT, but here's my take.I think the MCT will basically be a large, scaled-up version of Dragon 2, with large fuel tanks and Methalox engines, perhaps with a 15m diameter heat shield (I don't think Musk will go for anything other than the capsule shape, since that's what he has experience with).
SpaceX has said themselves that it will take many trips before they start to send 100 persons per trip.Since they can only make one trip every 2 years.
Quote from: symbios on 01/23/2015 04:35 pmSpaceX has said themselves that it will take many trips before they start to send 100 persons per trip.Since they can only make one trip every 2 years.Not if it has the thrust to widen the available window, no, they won't have this issue. Even one Vac raptor would provide them with this thrust, and we can safely assume that it'll have the fuel for it.Besides, what's to stop them from sending out a flotilla at once when they get into the swing of it? They'll certainly have an enormous revenue stream by then if they don't screw up, and the cost of the BFR+MCT is meant to be low; comparable to modern expendables or less. Each MCT and BFR is meant to be good for multiple flights, so assuming referb/re-fuel is good, you're going to end up with a stockpile of them over time. If they build one MTC+BFR every year, and they'd certainly do faster rates, you'd end up with two by the next window. As the first returns, you get three, then so on. Then couple in increased production rates.SpaceX has used the phrase "Mars Colonial Fleet": Certainly, they will require one if they want to get to the "sending 100 people at a time" to Mars phase within our lifetimes.I believe they'll make it work.
Manabu - I'm one of the people who believes at least some sea level optimized raptors on MCT are a good idea. I think they're needed for landing the MCT on Earth, useful for abort scenarios, useful for reducing gravity losses at launch from Earth (fueled from the booster), and they don't have gigantic flimsy nozzles. Yes even with a 20+ second loss in isp. I suspect that the 2nd stage (tanker stage) will have vacuum optimized nozzle. Described better and with diagrams and other slightly more speculative features (such as argon thrusters for interplanetary phase, Zubrin's 1000 s isp silane ramjets as Mars descent/ascent workhorses, tripwater transpiration and argon flash regenerative inconel silane/exhaled CO2 microcombustor heatshield (paints itself in ablative silica and carbon while firing) etc.) on L2 threads.
I don't understand how you use a transfer stage to boost you out of LEO and most of the way (delta V wise) to Mars, and yet you "keep it there" (the transfer stage in LEO). If you already need the engines in the Raptor for some of the delta V to Mars (and Mars decent and landing), it seems you pay only a very small mass penalty to make the tanks big enough make the complete delta V burn. Especially compared to the extra mass of an additional transfer stage with its engines and especially if that stage needs to reserve fuel to negate its extra delta v and return back to LEO. Or maybe I'm just not understanding your concept?
Quote from: DLR on 01/23/2015 01:09 pmI'm not up to date on all of Elon's comments regarding the MCT, but here's my take.I think the MCT will basically be a large, scaled-up version of Dragon 2, with large fuel tanks and Methalox engines, perhaps with a 15m diameter heat shield (I don't think Musk will go for anything other than the capsule shape, since that's what he has experience with). I dunno, they are accumulating quite a bit experience with cylindrical stages as well. (not quite the same reentry, but a violent one nonetheless)
Quote from: GORDAP on 01/23/2015 04:18 pmI don't understand how you use a transfer stage to boost you out of LEO and most of the way (delta V wise) to Mars, and yet you "keep it there" (the transfer stage in LEO). If you already need the engines in the Raptor for some of the delta V to Mars (and Mars decent and landing), it seems you pay only a very small mass penalty to make the tanks big enough make the complete delta V burn. Especially compared to the extra mass of an additional transfer stage with its engines and especially if that stage needs to reserve fuel to negate its extra delta v and return back to LEO. Or maybe I'm just not understanding your concept?Just to explain how such would work and not that I'm of the mind that its what SpaceX will do but... Your "transfer-stage" boosts the stack out of orbit into a TMI trajectory and then boosts-BACK some to loop back to LEO for refueling and to be used on the next out-going stack. Since the majority of the "mass" in the system is nothing but propellant and you supposedly HAVE that in LEO there is no real "mass-penalty" associated with the system. This is ONLY workable with fuel depots or tankers since none of the "Mars-Direct-ish" missions can economically afford this.
Oh, and I just realized, the MCT fuel tanks already have to be the larger size. This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v. So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?
Quote from: GORDAP on 01/23/2015 08:00 pmOh, and I just realized, the MCT fuel tanks already have to be the larger size. This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v. So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.
Quote from: Lars-J on 01/23/2015 08:18 pmQuote from: GORDAP on 01/23/2015 08:00 pmOh, and I just realized, the MCT fuel tanks already have to be the larger size. This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v. So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.This might be a dumb question, but I should probably ask - are the Delta-V requirements to go from Earth Orbit to the surface of Mars any different than the requirements to go from the surface of Mars to Earth Orbit?
Quote from: RotoSequence on 01/23/2015 08:36 pmQuote from: Lars-J on 01/23/2015 08:18 pmQuote from: GORDAP on 01/23/2015 08:00 pmOh, and I just realized, the MCT fuel tanks already have to be the larger size. This is because they have to be sized for the Mars surface to Earth surface burn, which I believe is on the order of 5100 m/s, which is larger (right?) than the LEO to Mars delta v. So since you already have tanks large enough to do the job, why not put the fuel in them in LEO rather than a transfer stage?Bingo. *IF* you want to go from Mars surface to LEO (or Earth surface) in one stage - and that's what Musk has been saying, a transfer stage makes no sense.People have a tendency to want to inject their own favorite Mars architecture elements into the MCT (such as cyclers, transfer stages, two-stage MCT) but that does not match what we have heard from anyone at SpaceX.This might be a dumb question, but I should probably ask - are the Delta-V requirements to go from Earth Orbit to the surface of Mars any different than the requirements to go from the surface of Mars to Earth Orbit?Yes. You can use aero capture to slow down, but need to push uphill from Mars surface.
In any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. The ISS alone masses a good 400 tonnes, and any credible transfer habitat designs I have seen are at least in the thirty to sixty tonnes range, for crews of six!
I hope it's a more credible and detailed design than Zubrin's ERV from The Case for Mars, which was hopelessy mass-constrained and would never have worked in reality.
Has Musk released any plans of using any vehicle other than MCT for landing the first person on Mars.Has he any plans for a one way mission (at least until their is capability to return people from Mars) ?
Quote from: DLR on 01/23/2015 02:21 pmIn any case, I have a hard time seeing how Musk intends to shuttle a hundred colonists to Mars without some dedicated in-space transfer vehicle. The ISS alone masses a good 400 tonnes, and any credible transfer habitat designs I have seen are at least in the thirty to sixty tonnes range, for crews of six! What are you counting into the habitat mass? A BA 330 is supposed to be 20 tonnes and it is 330 m^3, which is really huge, especially in zero g when you can use volume much more efficiently (in 3-D).Because of that, I think artificial gravity is a bad idea for going to Mars. People have spent much more time than that in zero-g, and on Mars the gravity is less anyway.QuoteI hope it's a more credible and detailed design than Zubrin's ERV from The Case for Mars, which was hopelessy mass-constrained and would never have worked in reality. Why wouldn't it have worked?
"Zubrin and Baker opted for a conjunction-class Mars mission; that is, one in which the crew would travel to Mars on a six-month minimum-energy trajectory, remain at Mars for about 500 days, and return to Earth on a six-month minimum-energy trajectory. In practice, travel and stay times would vary from one conjunction-class mission opportunity to the next because Mars has a somewhat elliptical orbit about the Sun. Cautious NASA planners have generally opted for opposition-class Mars missions, which would see the crew stay at Mars for perhaps a month. "
"One of the mission’s objectives would be to seek water ice."
"Several difficulties became obvious to NASA planners as they sought to integrate Mars Direct with their FLO-derived Mars plans. For example, the 7.1-metric-ton ERV cabin was too small to support all the needs of four people during a six-month weightless journey from Mars to Earth. Zubrin did not reveal to his Penn State audience the ERV’s planned habitable volume, though he did compare it to the Space Shuttle’s roughly 70-cubic-meter two-deck crew cabin. Controls, pilot and commander seats, equipment, and storage compartments occupied a large portion of the Shuttle crew cabin volume; Zubrin did not indicate whether the same would apply to the ERV cabin. In addition, the lone pressurized rover could not be used to its full potential; if it ventured beyond the range of an astronaut on foot and became stuck or broke down, then its occupants would become stranded without hope of rescue."