Author Topic: MCT Speculation and Discussion Thread 4  (Read 614313 times)

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #980 on: 10/30/2015 03:52 AM »
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.

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.
1. Otherwise it has no abort capability, and it has no ability to handle off-nominal EDL that put it outside the expected landing area.
2. The heavy equipment necessary for doing the preparing a solid landing pad could not be delivered by a "Red Dragon". It just isn't possible. A tiny rover (MSL size or smaller) cannot do it, other than *maybe* push some small boulders out of the way, and that's not going to help much. The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover.

So while a prepared landing pad for MCT will be of great value to increase the life expectancy of the MCT hardware (engines and landing gear) down the road, you cannot demand it to be there. And any equipment that is able to build MCT pads will by necessity be large enough to need delivery by MCT.

But since (IMO) the first few landed MCTs will likely just one way pathfinder cargo delivery vehicles for ISRU equipment, it also makes sense to at the same time also deliver heavy equipment for soil movement and landing pad preparation. This will allow the next MCTs (that actually are planned to fly again) to land on better "pads".

This is of course challenging for the leg design. It will likely require them to be much shorter and sturdier than F9R legs. Maybe even fixed legs that are integral to the thrust structure with a small spring/dampener element. Imagine the DC-X (below), but slightly shorter legs.
« Last Edit: 10/30/2015 04:12 AM by Lars-J »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #981 on: 10/30/2015 04:12 AM »
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.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #982 on: 10/30/2015 04:15 AM »
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.

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.

EDIT: It could certainly end up being the case that no reuse/takeoff from Mars will be practical without having landed on a properly prepared surface, which also needs to be able to support the launch propellant mass. But this goes with my earlier assertions that a Mars outpost will need to be "seeded" with a few not-reused MCTs to deliver initial equipment.
« Last Edit: 10/30/2015 04:33 AM by Lars-J »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #983 on: 10/30/2015 04:29 AM »
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.

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.
It's not about strength, it's about center of mass and stability;  Excavation by those engines is going to fragment the landing surface erratically, and even a small slope will tip the thing over in a static case, to say nothing of the levering action of actually colliding with the surface.

{Unprepared soil, all thrust from rear engines, small legs}: Pick two if you're lucky, one if you're unlucky in the engineering trade study.  Of the couplets, Unprepared soil / small legs  (but higher engines) seems the hardest, unprepared soil / all thrust from rear engines (but massive wide legs) the middle, and all thrust from rear engines / small legs  (but prepared levelled hard pad) the easiest eventually, but impossible to start with.

IMO we should conservatively assume big legs / unprepared soil / mix of canard and rear engines, because landing at an unprepared site is how the project will start out regardless.  I think the legs have to be *more capable* than F9R's legs.
« Last Edit: 10/30/2015 04:39 AM by Burninate »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #984 on: 10/30/2015 06:52 AM »
Mostly unprepared underground yes. Maybe some boulders removed. However I watched that NASA workshop about selecting landing sites for manned missions to Mars. For each of the proposed sites it was mentioned that there are flat hard surfaces available to land on. The NASA landers would not be as big as MCT though.

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.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #985 on: 10/30/2015 09:37 AM »
There are a few flat areas of exposed rock on Mars, with limited covering of dust and few stones and boulders.

See e.g. 1057pdf in http://www.nasa.gov/sites/default/files/atoms/files/mars-c-abstracts_in_order_of_presentation10242015_0.pdf

There is probably no need for special preparation of the initial landing site.


Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #986 on: 10/30/2015 09:57 AM »
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.

Is the hoverslam an assumption?

They may plan earlier, lower thrusts (requiring good control and variability).

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #987 on: 10/30/2015 10:03 AM »
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.

However, F9R lands with minimal fuel and lots of engines, making it bottom heavy and therefore more stable. The MCT, at least the biconic version, must be more balanced through its length. Swings'n'roundabouts.

(MCT would be more stable in a belly landing, if you can tolerate the extra mass of dedicated landing engines. Plus tilted (unprepared) ground would put less strain on the legs.)

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #988 on: 10/30/2015 12:51 PM »
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. [...]

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...

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 [...]

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.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #989 on: 10/31/2015 12:33 AM »

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.

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.


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. 

Understandable but still highly annoying, if you can't show L2 figures here can you take my figures and post them into the L2 discussion and report back the substance of the comparison and reaction to them without actually getting into specifics.  If you need a concise write up I can do that to make the job easier.




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.

If this is in reference to a 2nd stage doing nose first re-entry it is misplaced.  My concern is not that the vehicle can not turn in the lower atmosphere once under thrust, that is clearly achievable even if we would prefer to avoid it.  My concern is that the vehicle will tumble during the peak dynamic pressure of entry when it is without thrust and control surfaces are at minimum effectiveness.  A cylindrical body with rounded ends is not stable unless the heavy end is forward, and on a rocket stage the engines are the heavy end so bottom first entry is the preferred method, this is already the method used for the first stage and it experiences a far more mild entry then a 2nd stage would.  DC-X is a conical vehicle and somewhat stable in tip forward configuration when the center of mass is in the rear.  For the MCT itself which I see as being a bi-conic it would have considerable flaps at the bottom/rear of the vehicle to bring the center of drag back closer to the center of mass, also I'm looking at a vehicle much less slender like then the DC-X.



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.

In general shape yes but you greatly exaggerate the effect propellant density on the vehicles performance.  Propellant density helps in three main ways, thrust density, drag reduction and reduced gravity loss.  All these factors make dense propellants great at launch (even to the point that horrible solid boosters look good), but by the time you get the the altitude of staging (which should be about the same for either the integrated MCT or the separate 2nd stage) these factors are no longer significant and high ISP clearly out-performs.

And as I have said repeatedly, getting 8 km/s DeltaV from Metho-Lox rockets requires the same dry mass fraction as 9.5 km/s requires from a Hydro-Lox vehicle 12% for all vehicle structure and payload.  So this is not significantly easier.

I am favorable to use of Ethane or methyl-acetylene as these would improve bulk density and require less hydrogen/water collection on mars, so long as it dose not foul the engine of course.  Hopefully the Raptor will be able to run on both these propellants and LNG.


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.

My solution would be to place around a dozen small vernier engines up on the side about 3/4ths of the way up the vehicle canted outwards where they would not impinge on the ground or the landing gear.  These have just enough thrust to hover the vehicle at landing so the primary propulsion system 'slams' at a height of a 100 m shuts down (after successful start of the verniers is verified) and allows these vernier engines to do the final touchdown at lower speed.  On Earth the Raptor may not be able to fire due to instability and the Verniers may have to fire over a longer period though the lower terminal velocity should help.

These engines also act as orbital maneuvering thrusters and assist in flipping the vehicle from it's nose first entry into landing orientation, the larger number and angle give good leverage and throttle control during touchdown.  Lastly they may be able to fire during assent and reduce or eliminate the need to gimbal the primary Raptor engines which could save some mass and eliminate a point of failure in the hydraulic system.  They do not offer a solution to Raptor plumes impinging on the ground at launch, this will need to be mitigated by site alteration prior to launch, likely via some kind of protective material being placed on the ground.




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.

I don't think this is likely, the landing mass will be close to 200 mT and take off mass around 400 mT (for my mars orbital rendezvous mission design).  This is a modest enough difference that holding static weight of twice the touchdown should not be a problem as it would easily fall with the necessary structural margin of a rougher landing.

The leg types I see are telescoping pneumatic Shock absorber arrayed around the vehicles outer edges and sloping at an angle matching the outer mold line of the vehicle.  This spreads the legs (6 of them btw) as widely as possible and allows for up to 1 leg to fail.  Being along the outer surface the legs can be very long and extend nearly half the length of the vehicle which will keep the large engine bells from hitting the surface.  The tops of the legs structures will connect to some kind of structural disk/floor in the mid vehicle (where their is either cargo or a structure to support a propellant tank/s in the nose) so the lower portion of the vehicle is clear of intrusions.  In my design the cargo is in the lowest portion of the vehicle making it as bottom heavy as possible during landing.


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. [...]

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...

Well said

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 [...]

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.

I've considered using MCT tanks for 3rd stage benefits, it might be worth 2-3 km/s if full at launch.  But in manned launches I believe it would make MCT too massive for rapid abort.  Some propellant would be needed for that abort, around 20-40 mT I estimate and this is allowed for in sizing the whole launch vehicle.  In a cargo launch it is an option and provides some margin in case of a premature 2nd stage shutdown, or it may be of assistance in raising a ~200 km orbit up to ~400 km where rendezvous with SEP would need to take place due to drag on such vehicles being dangerous below this level.

The rest of your post I strongly agree with, SpaceX needs a spiral development path not a single miracle vehicle that springs from our foreheads like Athena.  I've been thinking about what comes before and after the primary lander (aka MCT), the stuff before it should help retire risk and make money, and the things after it should act as force multipliers to get more cargo and people to mars at lower cost then could be done with just a single vehicle type.
« Last Edit: 10/31/2015 03:01 AM by Impaler »

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #990 on: 10/31/2015 01:04 AM »
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. 

Offline Bob Shaw

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Re: MCT Speculation and Discussion Thread 4
« Reply #991 on: 10/31/2015 01:21 AM »

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.

This is exactly the way that SpaceX operates - they have learned the Soviet incremental approach to goals, but with vertically integrated manufacturing.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #992 on: 10/31/2015 01:24 AM »
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.
Right, we are in violent agreement. :)
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #993 on: 10/31/2015 01:35 AM »
Impaler:

My analysis comes from Dunn's work comparing SSTO propellants. I am not exaggerating the improvement in mass ratio you get for using methane/LOx over hydrolox. For the same dry mass and tank size, you can get 2.25 times the propellant using methane (plus you don't need thick insulation), thus more than doubling your mass ratio. Sure, you get 15-20% higher Isp with hydrolox, but the 2.25x mass ratio of methane/LOx can overcome that advantage, even with the logarithm. See here: http://web.archive.org/web/20130826050557/http://dunnspace.com/alternate_ssto_propellants.htm

Another advantage over hydrogen is that to produce a certain mass of methane/LOx takes much less water and power than does producing the same mass hydrolox propellant.

And instead of ethane or propyne (shorter name for methylacetylene), propylene would be a better propellant because it can be fairly easily made on Mars and gets nearly the same performance as propyne. Propylene and ethylene (often coproduced when synthesizing from Methane) also can be easily polymerized to make structures on Mars. Of course, with these higher hydrocarbons you lose some of the advantage methane has in being highly resistant to coking.


Additionally, 8km/s is a lot easier to achieve in vacuum (higher Isp) when you don't need an T/W ratio greater than 1 (means you also don't need to throttle down as much). But 8km/s is at the VERY high end that I think MCT would be capable of. More likely 6-7.5km/s (6km/s is /just/ enough to do a direct return to Earth from Mars surface on a slow trajectory using an equatorial launch site).



...and for the record, building an SSTO RLV isn't impossible, just difficult. Especially at sea level.
« Last Edit: 10/31/2015 02:10 AM by Robotbeat »
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Re: MCT Speculation and Discussion Thread 4
« Reply #994 on: 10/31/2015 02:02 AM »
...
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.

Put some cargo doors on that second stage, and we're in violent agreement.
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Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #995 on: 10/31/2015 03:18 AM »
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.

No an expensive massive spacecraft like MCT would not be used in such a way. Regardless of whether or not it can land on an unprepared surface, landing pads will be built after the first one lands if it can't be done by equipment delivered by Dragons.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #996 on: 10/31/2015 03:23 AM »

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.

This is exactly the way that SpaceX operates - they have learned the Soviet incremental approach to goals, but with vertically integrated manufacturing.

Soviet's were/are vertically integrated too, I think the key difference is SpaceX has brought much higher quality control then they ever had in their heyday (and it has now got to crap).  Also SpaceX brought in the standard American light tanks that the Soviets never had or needed because their engines were/are so superior.  By combining the two SpaceX can actually contemplate reusable rocket that still have a payload.

But yes basically the SpaceX philosophy is WWKD "What Would Korolev Do", where as the rest of the industry takes after Wernher von Braun who in typical German fashion goes for maximum performance at any price.
« Last Edit: 10/31/2015 03:24 AM by Impaler »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #997 on: 10/31/2015 05:04 AM »
For the same dry mass and tank size

That'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.

If your backing off to a hohmann transfer speed direct from mars that would indeed be ~6 km/s and comes in at a dry mass fraction of 20%, considerably more reasonable and well below the mass fraction of SSTO vehicles of any propellant, if you were to include an efficient aerocapture system like magneto-plasma I could even see this vehicle being possible all be it optimistic.  But the transit time is now no good for passengers, only cargo.

But this dose give me an idea, if you were to take my concept which targets LMO 4 km/s when carrying the Musk specified 25 mT cargo and instead carry no cargo it should be right at that 6 km/s DeltaV.  As I've always assumed the 25 mT return is for passenger and no geological samples or martian wine exports it means that very few MCT will actually have this return cargo burden thus allowing them to do direct return to Earth all be it bone-dry and needing refueling in LEO to be able to land.



I see a nominal mission working something like this.  MCT with cargo is placed in LEO, 2 SEP tugs are placed in LEO, both are loaded with Xenon themselves and one is loaded with chemical propellants as a tanker, the other carries the MCT.  Both tugs to to EML1, propellant transferred to MCT, crew is launched on cis-lunar Dragon and sent to rendezvous and transfer to MCT.  MCT dose Earth swing-by burn to a 150 day transfer trajectory.

Tugs return to LEO and are refilled this time with both of them carrying chemical propellant loads, both spiral out and do fully electric powered transfer to mars arriving long after the MCT but just prior to Earth-Mars alignment.  MCT having produced/loaded propellant at mars launches to LMO 4 km/s, take on propellants from both tugs to reach full propellant load again and performs burn of another 4 km/s for another 150 day transfer to Earth.  SEP tugs return to Earth under electric power in time for next conjunction.

Cargo flights start in a similar manor but at EML1 each propellant carrying tug can service 2 cargo carrying MCTs.  At mars the MCTs do direct return empty of cargo so no SEP tugs need to transit to mars. 

For both manned and cargo flights the cadence is once every other synod for initial missions, later as infrastructure allows a more rapid cadence may be possible using an opposition return trajectory for the unmanned cargo MCT's if a propellant depot can be employed at mars and some MCT's tasked with running propellant too it.  In many years (looks like 40% of the time) it would still not be possible to get the vehicle back in time for the next launch window as opposition return is always slow.


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Re: MCT Speculation and Discussion Thread 4
« Reply #998 on: 10/31/2015 12:51 PM »
For the same dry mass and tank size

That'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....
Dry vehicle weight, to first order, scales with volume (look at the pressure vessel equation), so it is a fair comparison. Did you read my link?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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Re: MCT Speculation and Discussion Thread 4
« Reply #999 on: 10/31/2015 01:21 PM »
Quick thought that I haven't seen anyone mention.

When ISS visiting vehicles are opened up, the crew wear masks to avoid floating debris - and this is after the loading crews make efforts to avoid contamination.

For a crew that are living out of MCT on the surface, or even just making occasional visits, ISTM they will trek in a lot more debris than the CRS vehicles suffer.

How would this be mitigated? Crew wear masks and the ventilation turned to max to filter the air as quickly as possible?

Cheers, Martin

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