Author Topic: MCT Speculation and Discussion Thread 4  (Read 505251 times)

Online Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #220 on: 06/23/2015 01:41 PM »
No, not remotely.

First off, all sides of an object doing reentry need Thermal protection systems because hot air swirls around the back of a capsule shaped vehicle, so the habitat your placing on the top would need extensive TPS which then gets left on Mars.  Likewise their would need to be yet more on the now exposed top of the propulsion stage to allow it to land on Earth.  Lastly their is no way to send anyone or anything back to Earth which is required.

The crane necessary to remove this habitat would be monstrous, and it would need to be mobile both before and AFTER picking up the habitat for it to do anything other then put it on the ground right next to the propulsion section which needs to blast off again, a very bad place to be.  The crane would have a higher mass then what it is lifting and would be extremely dangerous.

I'm proposing a habitat that is INSIDE the lander and deployed by WHEELS down a ramp, I can't see anything being simpler then that, and am perplexed why anyone feels this needs improving.
I value your critique.
 
You mentioned needing TPS on all sides of the MCT; I am only familiar with TPS on one side, where it serves as a leading edge during aerobraking on Mars,i.e. Design Reference Architecture 5A. Is the all-around TPS now a requirement for all Mars landers? Did your design include this?

I have seen a suggested design for MCT that is an enlarged version of a Dragon V.2, which sports a 15 m shield on the bottom and heat-resistant metal or composite for the rest of the "capsule'". Is this approach now obsolete?

The system I suggested could have TPS on all surfaces, but that would probably be expensive. If so, it would be less desirable to leave a whole section on Mars permanently. The large crane I suggested could be replaced by a different, low-mass system. But the first issue is TPS.
« Last Edit: 06/23/2015 01:48 PM by Ionmars »
* Mars' orbit: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

Online Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #221 on: 06/23/2015 03:25 PM »
...
...
I'm proposing a habitat that is INSIDE the lander and deployed by WHEELS down a ramp, I can't see anything being simpler then that, and am perplexed why anyone feels this needs improving.
Your proposal is excellent and should be employed. I would add only this:
After you have unloaded your habitat, now go back and unload the whole top section of the MCT as another habitat.

Maximum cargo delivered in just one trip of the MCT.  Win-win for SpaceX!

###
[Edit: A bonus -- we will be returning to Earth the absolute minimum mass that is physically possible.]
« Last Edit: 06/23/2015 03:36 PM by Ionmars »
* Mars' orbit: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #222 on: 06/23/2015 04:22 PM »
...
...
I'm proposing a habitat that is INSIDE the lander and deployed by WHEELS down a ramp, I can't see anything being simpler then that, and am perplexed why anyone feels this needs improving.
Your proposal is excellent and should be employed. I would add only this:
After you have unloaded your habitat, now go back and unload the whole top section of the MCT as another habitat.

Maximum cargo delivered in just one trip of the MCT.  Win-win for SpaceX!

###
[Edit: A bonus -- we will be returning to Earth the absolute minimum mass that is physically possible.]

Except that the proposals are in headon conflict with two basic principles Elon Musk has stated over and over again.

One is mass fraction. Best possible mass fraction is required for full reusability. Having an outer shell capable of withstanding atmospheric reentry at interplanetary speeds and the resultant heating plus an inner habitat capable of holding pressure for crew is extremely mass inefficient.

The second is full reusability. I think it might be possible that some low value but heavy equipment not needed for a smaller return crew might be removed if necessary. Especially if they can be reused on Mars. But except early on as a special startup arrangement the complete MCT will go back to reach the cost goals.

I have suggested before, that the whole cabin or cargo compartment may be removable and reused as habitat space on Mars. But that as an initial method. Not in a later stage when large numbers of colonists are transfered, that means the 100 people per flight are actually transported. At that stage the colony needs to be able to provide habitats and work places for all the arriving colonists.

Offline cambrianera

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Re: MCT Speculation and Discussion Thread 4
« Reply #223 on: 06/23/2015 04:36 PM »
First off, all sides of an object doing reentry need Thermal protection systems because hot air swirls around the back of a capsule shaped vehicle, so the habitat your placing on the top would need extensive TPS which then gets left on Mars. 

One of his early Apollo design questions was how much heat shielding to install on the lee side of the Apollo capsule to protect it when it reentered the earth's atmosphere upon returning from the moon. "Based on intuition, not calculations, I said you didn't need to put anything on it," Faget says. "But the people who were doing calculations were ultraconservative. They put about an inch of ablative material on the lee side. Sure enough, when the thing reentered, it still had its thin mylar dust sheet. So my intuition would have saved at least four or five pounds a square foot, carried all the way to the moon and back, absolutely useless.

Max Faget: Master Builder
http://www.astronautix.com/articles/maxilder.htm
Oh to be young again. . .

Online Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #224 on: 06/23/2015 04:57 PM »
...
...
...
I have suggested before, that the whole cabin or cargo compartment may be removable and reused as habitat space on Mars. But that as an initial method. Not in a later stage when large numbers of colonists are transfered, that means the 100 people per flight are actually transported. At that stage the colony needs to be able to provide habitats and work places for all the arriving colonists.
I agree. And I bow to your earlier posting (reference?)
« Last Edit: 06/23/2015 04:58 PM by Ionmars »
* Mars' orbit: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #225 on: 06/23/2015 05:40 PM »

I agree. And I bow to your earlier posting (reference?)

No need for that. I mentioned it only to show that I am not generally against it. It is quite long ago and I would be hard pressed to find it now. The idea was to remove the whole upper part and have another heatshield on the tank dome of the propulsion section for earth reentry.

It may be most efficient to remove the cargo hold of some cargo MCT and use them as pressurized habitat. The ECLSS would be mostly not suited for Mars surface operation as that would be based on biological, plant ECLSS. So better send that back to earth for reuse.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #226 on: 06/23/2015 06:52 PM »
No, not remotely.

First off, all sides of an object doing reentry need Thermal protection systems because hot air swirls around the back of a capsule shaped vehicle, so the habitat your placing on the top would need extensive TPS which then gets left on Mars.  Likewise their would need to be yet more on the now exposed top of the propulsion stage to allow it to land on Earth.  Lastly their is no way to send anyone or anything back to Earth which is required.

The crane necessary to remove this habitat would be monstrous, and it would need to be mobile both before and AFTER picking up the habitat for it to do anything other then put it on the ground right next to the propulsion section which needs to blast off again, a very bad place to be.  The crane would have a higher mass then what it is lifting and would be extremely dangerous.

I'm proposing a habitat that is INSIDE the lander and deployed by WHEELS down a ramp, I can't see anything being simpler then that, and am perplexed why anyone feels this needs improving.
I value your critique.
 
You mentioned needing TPS on all sides of the MCT; I am only familiar with TPS on one side, where it serves as a leading edge during aerobraking on Mars,i.e. Design Reference Architecture 5A. Is the all-around TPS now a requirement for all Mars landers? Did your design include this?

I have seen a suggested design for MCT that is an enlarged version of a Dragon V.2, which sports a 15 m shield on the bottom and heat-resistant metal or composite for the rest of the "capsule'". Is this approach now obsolete?

The system I suggested could have TPS on all surfaces, but that would probably be expensive. If so, it would be less desirable to leave a whole section on Mars permanently. The large crane I suggested could be replaced by a different, low-mass system. But the first issue is TPS.

No, the issue that killed the idea for me was the crane which is huge and dangerous, the TPS is just added inefficiency on top of that.

I think DRM 5 was for some kind of high-altitude airobraking shield far wider then the payload to create low ballistic coefficient, where as your proposing a Dragon/Soyuz shaped capsule in which the sides are quite exposed to hot air flow, and I doubt the DRM ideas were actually analyzed to account for back-side heating.  Every probe sent to Mars has had a back-shell to protect it.  This is not to say that it is impossible or even prohibitively heavy, you just don't get to ignore the entry heating on this habitat or the back side of the propulsion unit which is what it seemed to me your were doing.

The crane is what I find totally impractical and I do not see how it is replaced by a 'low mass system', if your hab is just sitting on top of a rocket stage their is no way to get it down without using either a crane or a second rocket or legs with wheels that extend all the way around and past the propulsion stage.  All of which look extremely impractical compared to simple ramp+wheels deployment.

Now to be clear cranes CAN be useful if they are the right TYPE, specifically gantry cranes in the room of the payload bay are an excellent means of loading and unloading containerized cargo.  They use the vehicle itself for support and for achieving a wide base and only need to have rails that telescope out a few meters to clear the edge of the vehicle and deposit containers into waiting ground vehicles.


Your proposal is excellent and should be employed. I would add only this:
After you have unloaded your habitat, now go back and unload the whole top section of the MCT as another habitat.

Maximum cargo delivered in just one trip of the MCT.  Win-win for SpaceX!

###
[Edit: A bonus -- we will be returning to Earth the absolute minimum mass that is physically possible.]

Huuu??  The top section of the lander I've been proposing is propellent tanks, why on Earth would their be a second habitat of any kind on the vehicle.  If I'm deploying by ramp and wheels I would put EVERYTHING on thouse wheels to maximize it's efficiency and operational simplicity.  Anything that is returned to Earth will be loaded back in as cargo, NOTHING should be integrated into the vehicle which is not physically necessary for launch and landing.



Except that the proposals are in headon conflict with two basic principles Elon Musk has stated over and over again.

One is mass fraction. Best possible mass fraction is required for full reusability. Having an outer shell capable of withstanding atmospheric reentry at interplanetary speeds and the resultant heating plus an inner habitat capable of holding pressure for crew is extremely mass inefficient.

The second is full reusability. I think it might be possible that some low value but heavy equipment not needed for a smaller return crew might be removed if necessary. Especially if they can be reused on Mars. But except early on as a special startup arrangement the complete MCT will go back to reach the cost goals.

I have suggested before, that the whole cabin or cargo compartment may be removable and reused as habitat space on Mars. But that as an initial method. Not in a later stage when large numbers of colonists are transfered, that means the 100 people per flight are actually transported. At that stage the colony needs to be able to provide habitats and work places for all the arriving colonists.

If your saying that the Thermal protection can be integrated with the pressure hull then that's a non-starter for the simple reason that it provides a direct heat path into the interior and will COOK people.  The pressure hull has to have a stand-off gap between it and the TPS, likewise for propellent tanks.

Thus a removable habitat inside of an unpressurized bay is hardly any less efficient then an integrated one, the only extra mass is a little sheet metal wall for the payload bay and the system to secure the payload during launch and landing.  The benefits are getting BIG habitats deployed on the surface for early missions and reducing the mass at launch from Mars surface to a minimum.

What is inefficient is doing high speed entry from interplanetary speeds, I'm advocating for much much lower speeds which will tax the vehicle design far less, which should more then make up for mass costs of a removable hab.  And it will also allow a single type of lander to do both crew and cargo flights, a key factor in making it cheaper to develop, manufacture and operate.
« Last Edit: 06/23/2015 06:57 PM by Impaler »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #227 on: 06/23/2015 07:21 PM »
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.

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.

The Saturn SII was about 45mt dry, including give J2 engines.  I think a similar expendable stage built today would be somewhat lighter.  So 40mt is probably a good conservative number. 30-35mt is more aggressive, but plausible.  Of course, it depends on what such a stage would be volume-wise compared to the SII.  Less efficient engines (but only about 40s less), but more dense fuel.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #228 on: 06/23/2015 07:36 PM »

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'.

Only the F9 booster hoverslams.  We don't know that they'd do that for a crewed vehicle.  Which is why although there's certain advantages to landing on Raptors if they can manage it, I think we can't rule out dedicated landing thrusters with soft landing ability.  Dv2 will land pretty softly as it seems, and that will have people.


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.


That's what I've been wondering too, although that comes back to my favored concept of a common basic MCT platform that would be configured for various roles like tanker, instead of a dedicated 2nd stage.
But essentially a basic cylindrical aeroshell with a blunted nose as the outer mold line.  All of these platforms would look like that from the outside. Inside there's tanks and a space for installing things.  Tankers would have nothing there.  Depots would have perhaps active cryogenic refrigerators there to reduce boiloff during it's long stay in LEO.  Later people movers would have a large hab module with accommodations for 100 people there.  Cargo movers would have accommodations for stowing and locking in 100mt of cargo containers/rovers/etc.
Whatever you put in this modular volume would total 100mt.  100mt of cargo, 100mt large pressurized hab plus 100 people plus provisions for them, etc,

But the initial MCT's would have that space with more of a combination of small exploration crew accommodations and cargo area for equipment and supplies.  With probably a Sabatier reactor and rolls of solar film cells?  Whatever's needed for support a crew of 7-14 and do surface exploration and refueling, without any prepositioned support like later missions would likely have.
So maybe a 25mt hab+7 crew+provisions, and 75mt worth of cargo and supplies.  Or something.
« Last Edit: 06/23/2015 07:47 PM by Lobo »

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #229 on: 06/23/2015 08:07 PM »
If you have to have 10 flights of cargo with 1 passenger flight of 100 with say 4 crew, why not have a crew of 4 with 10 passengers on each flight.  Have the rest as cargo.  That way every MCT would be identical, and so it only carries about 80-90 tons of cargo.  However, the cargo could be loaded in modules, that could be unloaded and when emptied can be used for habitat.  No need for separate cargo and human flights.  The 10 colonists could stay on Mars to work, build habitats, landing pads, solar power stations, and maintain ISRU equipment.  10 MCT's would get you 100 colonists. 

Cargo modules could be made from Falcon 9 cores with openings on each end.  Once emptied of solar panels, a rover or so, the inside can have modular rooms/furniture installed for living quarters.  They could be laid down and covered with Martian soil for radiation protection, except on the openings to the outside.  Or, Bigelow inflatable habitats could be used until more permanent cover can be made. 

Having 14 in an oversized capsule similar to Dragon would allow and extra layer of safety during launches on earth and Mars. 

On another note, several MCT's with minimum crew only would have to land first to set up fueling for return, that way the colonists would have a way to escape Mars in case of some unforeseen calamity. 

« Last Edit: 06/23/2015 08:12 PM by spacenut »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #230 on: 06/23/2015 08:25 PM »
So, as a thought for the layout of MCT itself, I'm still favoring this basic cylinder, with a blunted nose cap or nose cone on it.  Probably with ablative TPS on the nose and along the ventral half of it.  But we need a way to deploy fairly large solar arrays for the cruise each way, which can then be retracted.  We've kicked this around a few times, but here's a perhaps simple and plausible method.

So to this cylindrical MCT, picture a raised blister along it's whole dorsal side on the cylinder body, and the modular compartment on the bottom, between the tanks and engines (although it could be at the top of putting it at the bottom was too problematic with design).
That raised blister is a long series of mini cargo bay doors housing retracted solar arrays, similar to how the ISS arrays looked when they were stowed.  Once the blister doors are open, the arrays, which are fixed in position, extend directly out to the sides.  Not nearly as far as the ISS's, probably just far enough to block the sun hitting the MCT hull.  So once MCT does the TMI burn, it flips to put it's "back" to the sun and then these arrays both shade MCT do reduce solar energy absorption, and they'll always be facing the sun for maximum electricity production.

So, you have a combo solar array and sun shield (acting like the one they installed on Skylab to get the temperatures down).  And they don't have much in the way of moving parts.  little doors that fold down, and rectangular arrays that simply extend out stretching out folded solar panels, and then pull back in at the end of the cruise and the blister doors are closed.

I think this would be far more simple method than deploying a large array out of a cargo bay door like X-37B does, and reduces the heat loads on MCT that must be compensated for with radiators.  They would also help reduce boiloff in transit as the tanks are shaded. 

This system would be independent of what that MCT platform was configured as.  It'd be an integral part of the platform, as no matter what it's doing, it'll need power.  A depot would need power (and benefit greatly from shading).  Even just a simple tanker would probably need power, or a lot of batteries.

I suppose even -more- clever than this would be just to coat the whole dorsal side of the MCT cylinder with solar panels, like Dv2's trunk or HTV.  But could such a system be made that could withstand EDL?  And would there be the same sunlight blocking advantages as there would be to separate physical arrays crating actual shade?  The temps the arrays might get to really wouldn't be transferred to MCT, where they would with integrated solar panels on MCT's surface through conduction.  And a deployable separate arrays would be flat for maximum exposure instead if curved around half of a cylinder. 

But, there'd be a nice advantage to such a static/passive system if possible.  It's why JAXA went with it on HTV and why SpaceX went to it on Dv2.



As an interesting note, if the designers are clever, such an array could possibly be deployed after landing on Mars...although they wouldn't be much use if you were close to the equator unless they could rotate up some too.  Probably not enough power to fuel up MCT, but should be enough to provide system power.  Which would be useful until larger surface arrays are deployed. 

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #231 on: 06/23/2015 08:28 PM »

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.

Bingo.

Plus it'd be a depot that could periodically return to Earth for repair/refurbishment and then be relaunched for another tour of duty.

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #232 on: 06/23/2015 08:41 PM »
So it would take one MCT depot to refuel one MCT going to Mars.  The Depot if emptied each time it goes up could come back to be refueled and checked out then.  No need to worry about boil off.  Launch the Fuel depot, launch the MCT to dock, refuel and head to Mars.  Fuel depot returns, refuels, and relaunches with the next MCT. 

I too like the idea of a cylinder MCT. It could be stretched for a fuel depot without much expense. 

Offline rklaehn

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Re: MCT Speculation and Discussion Thread 4
« Reply #233 on: 06/23/2015 08:58 PM »
So it would take one MCT depot to refuel one MCT going to Mars.  The Depot if emptied each time it goes up could come back to be refueled and checked out then.  No need to worry about boil off.  Launch the Fuel depot, launch the MCT to dock, refuel and head to Mars.  Fuel depot returns, refuels, and relaunches with the next MCT. 

I too like the idea of a cylinder MCT. It could be stretched for a fuel depot without much expense.

I think a depot needs to be larger than a MCT. The economics are much better if you can fill the depot over the two years of a mars synodic cycle where there is no good launch window to mars, and the MCT fleet is somewhere in transit.

If you would refuel the MCT from another tanker MCT, you would have to do all your launches in the few weeks per synod or so that have good launch opportunities.
Try the ISS 3D visualization at http://www.heavens-above.com/ISS_3D.aspx

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #234 on: 06/23/2015 11:01 PM »


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.


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.

Yup.   See the Bigelow BA-330.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #235 on: 06/23/2015 11:27 PM »
So it would take one MCT depot to refuel one MCT going to Mars.  The Depot if emptied each time it goes up could come back to be refueled and checked out then.  No need to worry about boil off.  Launch the Fuel depot, launch the MCT to dock, refuel and head to Mars.  Fuel depot returns, refuels, and relaunches with the next MCT. 

Bingo.

I too like the idea of a cylinder MCT. It could be stretched for a fuel depot without much expense.

Yup, nice and simple geometry.  Makes the potential of TPS panels/tiles all one uniform size and shape possible around most of MCT, aside from the nose. 

It could be stretched, but that's a different spacecraft with different EDL profile, etc. 
But I don't think a Mars bound MCT will need full fuel in LEO, if we assumed direct return, as direct return will be the greatest dV requirement I suspect.  So a full depot wouldn't need the ability to transfer every drop of it's propellant into Mars-MCT necessarily.  Perhaps it'd transfer over enough propellant for TMI and Mars EDL, and have enough left so it can do it's own Earth EDL. 

And assuming it stays in orbit for 1-1.5 years receiving regular tanker offloads, and then pumps up one Mars-MCT per synoid, it could then come home, spend a few months getting refurbished, and then get relaunched and start taking tanker deliveries for the next 1-1.5 years in preparation for another Mars-MCT departure the next synoid.

I see the depot as much larger than that. I would prefer to see passenger carrying MCT's launch in pairs as close to simultaneously as possible. Also because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth. I also see the depot with a hab as transit station, and a place where PicaX can be recoated on MCT's along with engine swaps (engines taken off BFR tanker stages)

My response to Spacenut applies here too. Why a larger Depot?  If you are launching two MCT in a synoid, then place two MCT-depots in LEO a year or so ahead of time and then launch enough tankers to fill them both up ahead of the MCT-Mars launch.  Where they will dock with their respective depots, tank up, and burn for Mars together.

As for active cooling, see my earlier post about MCT having dorsal solar/shade arrays.  In place of "cargo" it could have the refrigerators and run off those solar arrays.

Now, all of that said, if you wanted to make a much larger depot that never came back down, that might be an application for the flexible tanks Impaler has been talking about?
I wonder if they could be set up with some sort of air bladder that could be inflated to push the propellant to the Mars MCT, and then pumped down again.  Like an air diaphram pump? Of course the MCT's deliverying the propellant would be in rigid tanks and would need some method of pumping propellant out of them, so that might not necessary accomplish much.  However, of the inflatable tanks were cheap, maybe such a depot could be cheap and thus expendable?
« Last Edit: 06/23/2015 11:43 PM by Lobo »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #236 on: 06/24/2015 02:48 AM »
One of the advantages of a SEP transit vehicle is that all this depot stuff largely goes away.  The vehicle itself would hold all the propellents needed, it would not be cryogenic and the 'fleet' would be stationed at high Earth orbit so the small amounts of chemical propellents in the landing craft are much easier to keep refrigerated.  Filling this fleet of vehicles is just a matter of putting propellents in LEO and having members of the fleet shuttle it up to high orbit.

Each synod the fleet departs for Mars and a few months later the empty return vessels arrive as they depart for Earth at approximately the same time.  Assuming 3 month transfer this means 23 months of loiter at Earth which means the fleet available 88% of the time for receiving propellents and non-perishable cargo as well.

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Re: MCT Speculation and Discussion Thread 4
« Reply #237 on: 06/24/2015 02:54 AM »
If you have to have 10 flights of cargo with 1 passenger flight of 100 with say 4 crew, why not have a crew of 4 with 10 passengers on each flight.  Have the rest as cargo.  That way every MCT would be identical, and so it only carries about 80-90 tons of cargo.  However, the cargo could be loaded in modules, that could be unloaded and when emptied can be used for habitat.  No need for separate cargo and human flights.  The 10 colonists could stay on Mars to work, build habitats, landing pads, solar power stations, and maintain ISRU equipment.  10 MCT's would get you 100 colonists. 
Lots of reasons this is less efficient.

The opportunity cost of those 36 seats that could have been paying customers.  The substantial salaries of those 36 extra crew.

The added safety margins, colonist accommodations, and extra propellant for a faster transit that were not necessary on 90% of flights but now are.

This also makes earth orbit operations harder because now you either have to launch everything near the TMI window instead of having the freedom to launch cargo MCTs months or years ahead of time.

Edit: spelling
« Last Edit: 06/24/2015 04:04 AM by CyclerPilot »

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #238 on: 06/24/2015 03:07 AM »
So it would take one MCT depot to refuel one MCT going to Mars.  The Depot if emptied each time it goes up could come back to be refueled and checked out then.  No need to worry about boil off.  Launch the Fuel depot, launch the MCT to dock, refuel and head to Mars.  Fuel depot returns, refuels, and relaunches with the next MCT. 

I too like the idea of a cylinder MCT. It could be stretched for a fuel depot without much expense.
Not clear from your post that you need multiple refuel flights.  A single launch does not have enough propellant for a MCT TMI.  Even the most optimistic mission designs require at least 3 refuel flights.  I have seen some estimates as high as 8.

Having the depot semi-permanently in orbit with good insulation / shading and active cooling allows the operational freedom to launch propellant all throughout the synod.  As opposed to a high flight rate sprint followed by 20 months of inactivity.

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #239 on: 06/24/2015 04:01 AM »


If your saying that the Thermal protection can be integrated with the pressure hull then that's a non-starter for the simple reason that it provides a direct heat path into the interior and will COOK people.  The pressure hull has to have a stand-off gap between it and the TPS, likewise for propellent tanks.

Thus a removable habitat inside of an unpressurized bay is hardly any less efficient then an integrated one, the only extra mass is a little sheet metal wall for the payload bay and the system to secure the payload during launch and landing.  The benefits are getting BIG habitats deployed on the surface for early missions and reducing the mass at launch from Mars surface to a minimum.

What is inefficient is doing high speed entry from interplanetary speeds, I'm advocating for much much lower speeds which will tax the vehicle design far less, which should more then make up for mass costs of a removable hab.  And it will also allow a single type of lander to do both crew and cargo flights, a key factor in making it cheaper to develop, manufacture and operate.
If there is an issue with thermal flux to the structure / pressure vessel, a layer of insulation could be added between them over most of the area.  If it gets as hot as you say, your design will probably need this insulation layer too because your load bearing layer will lose strength at elevated temperatures.

In your design isn't the "little sheet metal wall for the payload bay" a critical and heavy load bearing member?  It has to support the load of the propellant tanks, propellant, TPS, and aerodynamic drag during launches.  If it is a combined S2 on earth launch (full prop tanks at max Q and MECO) I would say the added mass requirement is a non-starter.  If your MCT launches as an empty S3 then the load is much less but still significant.  Hard to say if Earth or Mars launch would be a higher peak load without doing the math.

For nose first reentry that wall has to support the load of the engines and cargo during max G.  You said you were using a low speed entry, so probably not a major constraint.  Are you accomplishing this propulsively with an oberth burn at Mars?  SEP deceleration?  Slower transits?  Aerobraking?  EM drive?

I think your design would be much better and lighter if you made your crew pressure vessel (and cargo containers) load bearing. They could still be modular and removable.

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