Author Topic: MCT Speculation and Discussion Thread 4  (Read 626178 times)

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #1860 on: 03/29/2016 11:42 AM »
Nice rendering, though I agree with others that the airlock tunnel is structurally impractical. Just have an ordinary airlock above the tanks and a ladder down the side, with a flip-out hook for hoisting down stuff.

Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #1861 on: 03/29/2016 02:12 PM »
Guys, what happened to "Keep It Simple Stupid"?

The design concepts I've seen presented of late are more and more complex, with many added potential points of failure on items that, while convenient, also become mission critical failure points.

The base outer design of the MCT will most likely be something like a jumbo sized Dragon 2 capsule, (assuming the Dragon 2 works as planned) combined with a separate living quarters module and strongback structure to which propulsion, fuel and cargo modules would be attached.  Sending a single MCT at a time to Mars, with 100 colonists each, would take many decades to achieve the population intended.  Likely, when the actual colonial effort starts, they will effectively be building a "Liner" style system, carrying hundreds of people for each trip.

Obviously, the first expeditions will likely be single craft, small crews as more proof of concept and scouting type missions, but later flights will be for the establishment of the initial "beach head" colony.

Adding things like internally stored cargo, skyscraper tall landers, and elevators, amongst many other concepts, simply adds to the overall complexity of the mission profile, as well as adding unneeded dangers to an already very dangerous operation.

Something similar to the old, post Apollo capsule based lander design, (except as a SSTO Mars design) seems the most simple and least failure prone type of craft.  (The addition of inflatable Hypersonic decelerator systems would be a help too).
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Offline jsgirald

Re: MCT Speculation and Discussion Thread 4
« Reply #1862 on: 03/29/2016 04:32 PM »
The base outer design of the MCT will most likely be something like a jumbo sized Dragon 2 capsule, (assuming the Dragon 2 works as planned) combined with a separate living quarters module and strongback structure to which propulsion, fuel and cargo modules would be attached.  Sending a single MCT at a time to Mars, with 100 colonists each, would take many decades to achieve the population intended.  Likely, when the actual colonial effort starts, they will effectively be building a "Liner" style system, carrying hundreds of people for each trip.

It is quite reasonable, the problem here is Musk saying that they intend to 'land the whole thing'.
Also, the 100 colonists per ship looks more a like long term goal (for future versions of the system) rather than a requirement. A bit like the 80000 persons/year.

"For every expert, there is an equal and opposite expert".

Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #1863 on: 03/29/2016 04:45 PM »
The base outer design of the MCT will most likely be something like a jumbo sized Dragon 2 capsule, (assuming the Dragon 2 works as planned) combined with a separate living quarters module and strongback structure to which propulsion, fuel and cargo modules would be attached.  Sending a single MCT at a time to Mars, with 100 colonists each, would take many decades to achieve the population intended.  Likely, when the actual colonial effort starts, they will effectively be building a "Liner" style system, carrying hundreds of people for each trip.

It is quite reasonable, the problem here is Musk saying that they intend to 'land the whole thing'.
Also, the 100 colonists per ship looks more a like long term goal (for future versions of the system) rather than a requirement. A bit like the 80000 persons/year.

He's also said that the design is currently fluid.  It's likely to change before they start cutting metal for it.
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Offline jsgirald

Re: MCT Speculation and Discussion Thread 4
« Reply #1864 on: 03/29/2016 05:13 PM »
He's also said that the design is currently fluid.  It's likely to change before they start cutting metal for it.

Functionally we may define three modules: lander/payload delivery, propulsion and living quarters/ECLSS. If you separate them you'll need to duplicate, e.g. in space propulsion vs landing. It may be simpler in the end to build a jack-of-all-trades ship, it won't be optimal but it will be simple to operate as a system and require a lot less mass.
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Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #1865 on: 03/29/2016 11:03 PM »

...

Edit: since it runs by the engines, wouldn't it make sense to allow engine compartment access from the tunnel?

{laughs} I really like the idea, but suspect that the amount of maintainable systems you could access would be limited, and number of situations where you have the vehicle safe in space but with an engine failure would be even more limited.

The only scenario that's realistic is on the Martian surface, where they landed safely but either something glitched or they just want to do a check-out and service before lift-off for Earth-return. In which case, you'd be better off designing for access to the entire engine system from outside (as you would on Earth for a first stage after a launch-abort).

Humbly, I disagree with this assertion. It seems you are looking for scenarios under the streetlight of status quo, while the vastness of a space mass-transport system, comprised of reusable components contains plenty of them, if a bit concealed by the contrast :).

I am sure you can appreciate maximum access when changing on orbit a couple of Raptors after an unfortunate MMOD event on one of a couple of hundreds of craft you total fleet comprises. Btw, my suggestion was made with engine room being unpressurised in mind, not part of the interior, but rather the exterior of the ship for that matter.
« Last Edit: 03/29/2016 11:12 PM by mfck »

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #1866 on: 03/29/2016 11:11 PM »


Guys, what happened to "Keep It Simple Stupid"?

...

Not to disagree with the rest of your post, I'd like to suggest that by the time this thing gets built, Musk's and his engineers' concept of Simple will be interesting to behold.

Also, completely aside, I'd note that that phrase, addresses stupid, which people building MCT are not


Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1867 on: 03/30/2016 04:44 AM »
Adding things like internally stored cargo

Where else would you put the cargo for a vehicle capable of EDL on a planet with an atmosphere?

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1868 on: 03/30/2016 04:56 AM »
I am sure you can appreciate maximum access when changing on orbit a couple of Raptors after an unfortunate MMOD event on one of a couple of hundreds of craft you total fleet comprises.

You aren't going to be transporting spare Raptors inside the passenger ship, nor moving them through a crew-access tunnel. (Nor even the major parts of the engine.) They'd be stored in one of the cargo ships. So the repair crew still need to exit the ship.

Having a crew access tunnel that opens inside the engine area offers a trivial advantage over an EVA-airlock anywhere else on the ship.

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #1869 on: 03/30/2016 06:14 AM »
I am sure you can appreciate maximum access when changing on orbit a couple of Raptors after an unfortunate MMOD event on one of a couple of hundreds of craft you total fleet comprises.

You aren't going to be transporting spare Raptors inside the passenger ship, nor moving them through a crew-access tunnel. (Nor even the major parts of the engine.) They'd be stored in one of the cargo ships. So the repair crew still need to exit the ship.

Having a crew access tunnel that opens inside the engine area offers a trivial advantage over an EVA-airlock anywhere else on the ship.

Not in the design discussed, though
« Last Edit: 03/30/2016 06:15 AM by mfck »

Offline kaoru

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Re: MCT Speculation and Discussion Thread 4
« Reply #1870 on: 03/30/2016 04:55 PM »
After reading the past few posts, there seems some disjoint between the opinions put forward and reality of the requirements and the technology available.  First, scaling up the Dragon 2 spacecraft to be an upper stage, lander, and long term spacecraft is impractical.  However, scaling up an traditional upper stage isn't and it's because of the requirements.  Any design will be a trade off but there are some absolutes imposed by the requirements and the available technology.  I identify the requirements as these:

Starting with propellants & propulsion, CH4/LOX is required since ISRU/refuelling has to be employed.  Propulsion has to work efficiently in thin atmosphere to vacuum and have highest ISP (for CH4/LOX).  The modes are ascent, injection, entry, descent, and landing.  Ascent and injection requires high thrust and specific impulse which means no cosine/expansion thrust loss and using a lot of propellant.  Using a lot of propellant means "independent" refuelling on surface and in orbit with all the plumbing/pumps, etc. on board the spacecraft.  The remaining modes entry, descent, and landing requires throttling/control for supersonic retro-propulsion (EDL) which means adaptable nozzles and canting of the engines (with cosine loss to lower thrust).

As well, RCS/ullage propulsion has to be based on CH4/O2 (gaseous) in bladder/piston tanks, as hydrazine/nitrogen/helium can't be refueled (aka CO2 can be used as the pressurizing gas).  Also, momentum wheels will have to be employed for stability to lower propellant expenditures.  Of course, main propulsion (aka the US Raptors) will require cryogenic CH4/LOX storable for long term which means cryogenic plant (aka IVF) with pumps, compressors, etc.  Also, for a closed loop ECLSS and for ISRU, a Sabatier reactor also has to be included.  All of this will take a lot of energy and being pure solar is impractical both on orbit and on surface.  The energy budget will have to be balanced.  This most likely means a hybrid solution between solar/batteries and an ICE running the pumps/compressors and generator.  All of these technologies will be used on orbit and on surface and are absolutely critical/high maintenance.  This maintenance requirement assumes access to the technology timely/effectively and in any mode, in other words, in a pressurized environment.

My design speculates the above requirements and provides a solution to each as simply as possible and with the least amount of dry mass (that can be speculated).  Also, the "tunnel" provides a lot of advantages.  First, the tanks are a torus which are structurally stronger than a straight tank.  It also provides a more direct path for plumbing/wiring which means less mass (the Saturn V had the same idea for the fuel lines passing thru the tanks, five pipes in total).  The bottom airlock and tunnel provides the means to off-load cargo without a heavy crane or such.  My idea is to use an electro-magnetic lift system (like on the US Ford class carrier) installed in the tunnel to a bottom airlock area.  Then a simple pulley/cable system lowers to the ground.  This bottom pressurized area also allows access to the afore mentioned system like IVF/Sabatier reactor, ICE generator, batteries, etc. for maintenance on orbit or on surface.  Also, the systems can be connected/plumbed externally for Mars ISRU operations.

I'm still devoted to finishing my model.  I must be since I bought the Pro version of SketchUp to the tune of $700.  My justification to my wife was that my daughter (who's doing computer science/game development) can use it for game modelling.  She seemed to buy that.

My .02 worth or more,
Kaoru

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1871 on: 03/30/2016 05:27 PM »
  This most likely means a hybrid solution between solar/batteries and an ICE running the pumps/compressors and generator.  All of these technologies will be used on orbit and on surface and are absolutely critical/high maintenance.  This maintenance requirement assumes access to the technology timely/effectively and in any mode, in other words, in a pressurized environment.

I'm still devoted to finishing my model.  I must be since I bought the Pro version of SketchUp to the tune of $700.  My justification to my wife was that my daughter (who's doing computer science/game development) can use it for game modelling.  She seemed to buy that.

My .02 worth or more,
Kaoru


I'm still on the free Sketchup ;-)  Am I missing anything except for the converters?

Did you check the fuel and oxygen requirements for the ICE (Internal Combustion Engine)?  for the 100 kW power level I chose for my own design, I calculated about 2 tonnes of solar arrays and converters.
When I checked the power to mass ratio of CH4+3O2=CO2+2H2O in an ICE, I found I would only get 7.1 Mj/kg, or 7100 MJ per tonne.  This works out at about 1 tonne per day of combustion required for 40 kW of power + 60 kW of heat, so it was really bad choice. The oxygen is very heavy.

You might choose to pressurize your RCS ullage gas tanks with methane and oxygen and avoid pressurants altogether?

looking forward to seeing the whole model!




« Last Edit: 03/30/2016 05:36 PM by lamontagne »

Offline kaoru

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Re: MCT Speculation and Discussion Thread 4
« Reply #1872 on: 03/30/2016 06:28 PM »
I'm still on the free Sketchup ;-)  Am I missing anything except for the converters?

Did you check the fuel and oxygen requirements for the ICE (Internal Combustion Engine)?  for the 100 kW power level I chose for my own design, I calculated about 2 tonnes of solar arrays and converters.
When I checked the power to mass ratio of CH4+3O2=CO2+2H2O in an ICE, I found I would only get 7.1 Mj/kg, or 7100 MJ per tonne.  This works out at about 1 tonne per day of combustion required for 40 kW of power + 60 kW of heat, so it was really bad choice. The oxygen is very heavy.

You might choose to pressurize your RCS ullage gas tanks with methane and oxygen and avoid pressurants altogether?

looking forward to seeing the whole model!
I was on the SketchUp Pro trial which expired and then I used SketchUp Make for a while to see the differences.  While on the Pro trial, I implemented dynamic components for the engines, airlocks, and hatches; they actually move when clicked.  Also, I used the solid tools (intersect/union, etc.) for implementing a lot of the components.  When I went to the free version, those tools were unavailable. I could still model but it was more time consuming/tedious and I couldn't modify the dynamic components I done.  I researched around for various modelling software and SketchUp is the only one that was applicable with a reasonable price (perpetual license; no subscription required).  All other software was subscription based and way to expensive anyway (aka Autodesk wanted $700 per year).

To keep on point, the energy requirements (for a Sabatier reactor, compressors, pumps) are substantial.  I have some good ideas for light but large/efficient solar panels but that's stretching things.  An ICE will be needed and it'll burn CH4/O2.  Your right that the RCS/ullage can be autogenously pressurized but the waste CO2 would also work and be available (as well as the ECLSS CO2) to the Sabatier reactor to make CH4 and O2 from CO2 and H2O.  The challenge is the energy required, which is obvious.  Solar panels and radiators can only be so big, batteries are only a buffer, and ICE generator requires fuel.

I haven't run the numbers to size the employed technologies because there are too many unknown factors.  However for speculation, I just know that it will be an interesting balance.  I akin this to the systems on a boat, where you have to budget your energy/fuel over time between refueling.  My design will most likely run at a net energy loss for an individual module but with other modules can be made self-sufficient.

Kaoru.
« Last Edit: 03/30/2016 07:10 PM by kaoru »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1873 on: 03/30/2016 08:24 PM »
An ICE is a really bad idea. Batteries work great. Solar panels also work great. 100 days is much too long to be using a ICE. An ICE throws away 70-80% of your energy. At least a fuel cell only throws away half. And you already have to throw away about half your energy to produce the methane and oxygen in the first place. So you're left with a round-trip efficiency of between 10 and 25%, ie you're left with only one tenth to one fourth of the energy you started with. Bad bad bad. Solar panels, on the other hand, are producing energy and are also improving all the time.
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Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #1874 on: 03/30/2016 09:45 PM »
An ICE is a really bad idea. Batteries work great. Solar panels also work great. 100 days is much too long to be using a ICE. An ICE throws away 70-80% of your energy. At least a fuel cell only throws away half. And you already have to throw away about half your energy to produce the methane and oxygen in the first place. So you're left with a round-trip efficiency of between 10 and 25%, ie you're left with only one tenth to one fourth of the energy you started with. Bad bad bad. Solar panels, on the other hand, are producing energy and are also improving all the time.

I take it this applies to ACES IVF system as well? AIUI, its powered by a Roush Racing built ICE.
« Last Edit: 03/30/2016 09:47 PM by docmordrid »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1875 on: 03/30/2016 10:18 PM »
An ICE is a really bad idea. Batteries work great. Solar panels also work great. 100 days is much too long to be using a ICE. An ICE throws away 70-80% of your energy. At least a fuel cell only throws away half. And you already have to throw away about half your energy to produce the methane and oxygen in the first place. So you're left with a round-trip efficiency of between 10 and 25%, ie you're left with only one tenth to one fourth of the energy you started with. Bad bad bad. Solar panels, on the other hand, are producing energy and are also improving all the time.

I take it this applies to ACES IVF system as well? AIUI, its powered by a Roush Racing built ICE.
No, because the IVF is lasting for hours, maybe a few days. It'd need like a 50th the fuel of a 100 day trip that we're talking about.
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Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #1876 on: 03/30/2016 10:32 PM »
That's not what Boeing says.They say weeks, and mission descriptions out as asteroids sound longer than that.

http://spaceflightnow.com/2015/04/16/ula-gets-futuristic/

Quote
“We can take our first launch, big fuel tanks, supplies, food, water, if it’s a manned mission. And the next mission will bring up the spacecraft or the astronauts in their capsule. With this advanced upper stage, which can fly around for weeks, it’s up there waiting, we can put these pieces together, and outside the deep part of Earth’s gravity well with that much impulse and propellant, we can do anything.

“We can go out and tap the resources that are in space. We can asteroid mine, we can build the infrastructure required for a real and permanent human presence. Fuel depots, water depots, commercial human habitats. This is truly a game-changer and I couldn’t be more excited about what this will do for the future of space, all enabled by that advanced, high-performance, ultralong-duration upper stage,” Bruno said.
« Last Edit: 03/30/2016 10:34 PM by docmordrid »
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Offline jg

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Re: MCT Speculation and Discussion Thread 4
« Reply #1877 on: 03/30/2016 11:04 PM »
That's not what Boeing says.They say weeks, and mission descriptions out as asteroids sound longer than that.

http://spaceflightnow.com/2015/04/16/ula-gets-futuristic/

Quote
“We can take our first launch, big fuel tanks, supplies, food, water, if it’s a manned mission. And the next mission will bring up the spacecraft or the astronauts in their capsule. With this advanced upper stage, which can fly around for weeks, it’s up there waiting, we can put these pieces together, and outside the deep part of Earth’s gravity well with that much impulse and propellant, we can do anything.

“We can go out and tap the resources that are in space. We can asteroid mine, we can build the infrastructure required for a real and permanent human presence. Fuel depots, water depots, commercial human habitats. This is truly a game-changer and I couldn’t be more excited about what this will do for the future of space, all enabled by that advanced, high-performance, ultralong-duration upper stage,” Bruno said.

Note that the ICE has very high dynamic range (output power).  It can idle at very low power levels.  And the fuel used is not wasted: venting the water vapor overboard gives you just enough thrust to keep the propellants settled in the tanks, which also means that the boiloff from the tanks is much less due to lower heat loss.

This is more important when you have H2 stages, since H2 is so cold that you'll have significant boiloff otherwise which you now avoid by the constant tiny thrust available.  For methane and O2, it's less clear, since with good sunshading/orientation you can maintain low enough temperatures to avoid most/all of the fuel boil off problem of long missions.

The really huge benefits of ACES?IVF are on several fold: 1) much better mass fractions, as you save a lot of weight over conventional stages, 2) flexibility without reconfiguration of the stage, since so long as you have fuel available, you can power your stage, for days, weeks, or months; with existing upper stages, if you can do the mission at all, you may need different amounts of helium/tanks, etc, making a lot of non-leo missions one-off problems, and 3) you can get a lot of power when you need it.

So I'm a real fan of the idea, and find the slowness of ULA to get it into flight frustrating.  I had the chance to have dinner once at a meeting with the IVF inventor, Frank Zegler and hear a talk from him; IVF is a true step forward, and innovation of first order.

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1878 on: 04/03/2016 11:19 AM »
JG has it mostly right about IVF.

IVF seems to have gone through several revisions over the years as ULA have refined the concept. Key parts of it are.

Thrusters drive by the boiloff can be run in a pulsed mode to settle the propellants into 2 big single balls. This cuts boiloff by 1/2. Longer missions were always expected to have more layers of multilayer insulation that current ones. I think the current level is 5 but ULA have suggested somewhere between 19 and 34 IIRC.

The biggest discovery (when ULA analyzed all their peripheral systems for a stage) was the GHe tanks to do tank pressurization were the biggest item, yet with cryogenic propellants all you need to pressurize the tanks is something to heat them. In a normal ICE heat is a waste products you send to the radiator. In IVF it is a critical resource to the whole mission.

BTW IVF does have a battery sized mostly for (internal combustion) engine startup.

I think it's a tricky modelling task to trade IVF alone against IVF + solar for an uncrewed stage.

However once you have crew on board your power needs rise substantially and some kind of PV system seems pretty much essential unless you have a convenient sized nuclear reactor available. Very nice to have. Very doubtful you'll get it. :(

Moving to more use of momentum control devices for attitude control seems a pretty good idea but the preferred device is a Control Moment Gyroscope, not a Momentum Wheel. They run at constant (high) speed on 2 axis bearings. Such units were used on Skylab and do run on the ISS.

Replacing hypergols with O2/CH4 is an excellent idea to create a single system that can be reloaded through ISRU. Actually I think you can do hypergols from the Martian atmosphere but the very low level of N2 present would make it a very slow process.

The issue with O2/CH4 is ignitions. Mission planners like their RCS to be reliable and O2/CH4 probably complicates each thruster with spark plugs and support electronics unless you have a catalyst that can operate on the fluids as they come out of the tanks, or maybe a resonance tube ignitor, which needs enough tank pressure on at least one of the propellants to get supersonic flow in a nozzle.   
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Offline the_other_Doug

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Re: MCT Speculation and Discussion Thread 4
« Reply #1879 on: 04/03/2016 02:08 PM »
In terms of RCS fuel (where you really like the hypergolic, no-igniter-required aspect of current fuels), and in terms of ISRU production, what about hydrogen peroxide?  It has been successfully used as a monopropellant in RCS systems, notable for Mercury spacecraft but in many others, as well.

You do need a catalyst for peroxide monopropellant systems, which I imagine would reduce their service lives, but you don't need to find scarce nitrogen to make it with ISRU techniques.  And it is easier to maintain for long timeframes since it is not cryogenic and doesn't have the boiloff issue you have with LOX.
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