Author Topic: MCT Speculation and Discussion Thread 4  (Read 610197 times)


Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1 on: 06/14/2015 08:34 AM »
From the last thread:

So I propose that we actually refuel the MCT in LEO (something we would already routinely do) for partially propulsive Earth Entry.

Your fuel needs to get into LEO. If other MCTs on BFRs are launching fuel to LEO depots, then, pretty much by definition, those MCTs will need to be able to handle a full LEO reentry, so the TPS problem would need to be solved before you can use this to solve the program that you are trying to solve by using this. (So to speak.)

As Jim said earlier somewhere, Dragon is too small to have a rover.

Jim was wrong. Dragon's side hatch is too small for a rover like MSL, but the internal volume and modelled payload capacity to Mars is more than sufficient. Even without changing the hatch-size, you can still fit in a rover around 0.6x0.7x2m, which is large enough for the job since you will want to offload as much of the computation, power and communications to the capsule itself.

Moreso, if you want to propose using a Dragon for site-surveying for MCT landings, then even the above size limitation doesn't apply. After all, on the scale of the development costs for MCT and BFR, making a few structural modifications to a Dragon capsule is going to be a rounding off error. (Certainly less than your proposed alternative.)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2 on: 06/14/2015 03:35 PM »
From the last thread:

So I propose that we actually refuel the MCT in LEO (something we would already routinely do) for partially propulsive Earth Entry.

Your fuel needs to get into LEO. If other MCTs on BFRs are launching fuel to LEO depots, then, pretty much by definition, those MCTs will need to be able to handle a full LEO reentry, so the TPS problem would need to be solved before you can use this to solve the program that you are trying to solve by using this. (So to speak.)


Only if one tries to imagine that MCT is used as a Tanker to LEO, which is very silly and wasteful.  Tankers will be a stretched upper stage of the BFR without any cargo on top and will use its low ballistic coefficient and retro-propulsion with residual propellents and likely some parachutes to perform re-entry and landing, all while delivering far MORE propellents.

Lots of people have been pushing this idea of MCT is the ONLY thing that BFR will ever have placed on top of it and that is must do EVERYTHING we want done from LEO all the way to Mars, this is completely unrealistic and dose not save any money as the MCT would be 10x harder to design and build when it has so many requirements put on it.

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #3 on: 06/14/2015 05:38 PM »
From the last thread:

So I propose that we actually refuel the MCT in LEO (something we would already routinely do) for partially propulsive Earth Entry.

Your fuel needs to get into LEO. If other MCTs on BFRs are launching fuel to LEO depots, then, pretty much by definition, those MCTs will need to be able to handle a full LEO reentry, so the TPS problem would need to be solved before you can use this to solve the program that you are trying to solve by using this. (So to speak.)


Only if one tries to imagine that MCT is used as a Tanker to LEO, which is very silly and wasteful.  Tankers will be a stretched upper stage of the BFR without any cargo on top and will use its low ballistic coefficient and retro-propulsion with residual propellents and likely some parachutes to perform re-entry and landing, all while delivering far MORE propellents.
Why bother with the cargo?
Just leave the cargo bay empty and send up an empty MCT. Less payload to LEO = more leftover fuel in the tanks.

Lots of people have been pushing this idea of MCT is the ONLY thing that BFR will ever have placed on top of it and that is must do EVERYTHING we want done from LEO all the way to Mars, this is completely unrealistic and dose not save any money as the MCT would be 10x harder to design and build when it has so many requirements put on it.
While MCT will most likely be supplanted by a dedicated reusable tanker in the long run, in the short run its capabilities make it good enough for the task. Or a really simple, cheap, disposable tanker stage (a glorified fuel tank with a docking port and a single raptor engine).

The idea that early MCTs be used as both a tanker vehicle and an MCT stems from a very real and present fact that SpaceX does not have infinite money and thus cannot really afford development and manufacturing of multiple different reusable, earth-landable and rapidly reusable vehicle designs.

Just make one that is good enough and build as many as you can. An MCT without the cargo will do just fine for refuelling. Not perfect, but good enough.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #4 on: 06/14/2015 05:42 PM »
Take a BFR which is equivalent to 300 tonnes to LEO as an ELV.

As a two stage fully reusable launcher this can place about 150 tonnes of payload into LEO (+ the mass of the reusable upper stage + landing fuel).

As a two stage launcher with only the first stage reused it could place about 210 tonnes of payload into LEO + the mass of the upper stage (say about 30 tonnes).

If the MCT acts as its own upper stage, then MCT + payload can be up to 240 tonnes. This allows the MCT to have significant mass growth margin.

If the MCT were just payload to a fully reusable two stage BFR, the BFR would have to be 60% larger.

A larger BFR would cost significantly more to develop, not only is it larger and development and intrastructure cost scales more than linearly with size but a separate reusable upper stage would need to be developed. It would have the advantage of needing few tanker flights per MCT flight to Mars. A tanker based on the upper stage is probably going to be more efficient than one based on the MCT (but see mission kit discussion below). Having a MCT act as its own upper stage also has the disadvantage of putting extra design constraints on the MCT which might tip it from being difficult to design to being impossible.

If possible I think SpaceX will optimise for low development cost, even at the expense of some loss of efficiency.

One possibility for the MCT is that it is basically just an upper stage + fairing to which mission kits can be added. So there would be a tanker mission kit, a propellant mission kit and a Mars mission kit (other possible mission kits are long duration, lunar landing, science lab, etc.). The crew and cargo MCT to Mars would be identical with differences being confined to the payload.

If this is the correct way of looking at the MCT, the tanker mission kit would not be that much more inefficient than a dedicated tanker second stage, while the cost of developing a dedicated tanker would be considerably more.

Although I've used a 300 tonne ELV equivalent BFR in the example above, it looks like it could be 10% smaller and still allow adequate mass for the MCT. My guess is that BFR will be 270 tonnes to LEO ELV equivalent with a MCT which acts as its own upper stage, but there are significant factors which are unknown to anyone outside SpaceX and it is still possible that a bigger two stage reusable BFR might be used.

A more radical suggestion for the MCT is to have it having a payload of only 50 tonnes. Two of these mini-MCT would be launched they would join up in orbit, refuel and then each perform half of the TMI burn. The advantage of this arrangement is that there are always 2 independent habitats available to the crew and in an emergency all the crew could land on Mars in one of these MCT. Cargo missions would not need to join up in LEO and can do the TMI independently. This would greatly increase safety and significantly reduce development costs as the BFR and MCT would only need to be half the size. The cost would be a less efficient MCT (due to scaling and the need for a docking port) and more complex LEO operations. If SpaceX went this route than the BFR would be only about 135 tonnes to LEO ELV equivalent, barely bigger than SLS block II.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #5 on: 06/14/2015 05:58 PM »
MCT and BFR are supposed to be based on the same kind of platform. MCT will HAVE to have similar mass efficiency of an upper stage (in fact Musk has said MCT needs to be capable of Mars surface to earth in a SINGLE stage, though with far less payload), and essentially that's what it is. So whether you call the tanker a modified MCT or a stretched BFR upper stage may be a distinction without much difference.
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #6 on: 06/14/2015 06:59 PM »
Dang it, we really need one spot that has all known information about MCT that comes from SpaceX. I'm not sure if he said 80-100 or 50-100. Not that it makes an enormous difference, but it's annoying.
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Offline Llian Rhydderch

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Re: MCT Speculation and Discussion Thread 4
« Reply #7 on: 06/14/2015 08:01 PM »
Dang it, we really need one spot that has all known information about MCT that comes from SpaceX. I'm not sure if he said 80-100 or 50-100. Not that it makes an enormous difference, but it's annoying.

If it is info based on some sort of "reliable source" (news article, speech captured on video and available on the web, etc.), then wikis seem much better than discussion board threads for compiling the info. 

Wikipedia has such an article:  Mars Colonial Transporter.  (also, a section of that article has it's own link for the BFR:  MCT launch vehicle .

If people don't want to actually add text and a source to the article in that encyclopedia, then just summarize the factoid and add the source in raw URL on the Talk page associated with the MCT:  the  Mars Colonial Transporter (Talk page)
Re arguments from authority on NSF:  "no one is exempt from error, and errors of authority are usually the worst kind.  Taking your word for things without question is no different than a bracket design not being tested because the designer was an old hand."
"You would actually save yourself time and effort if you were to use evidence and logic to make your points instead of wrapping yourself in the royal mantle of authority.  The approach only works on sheep, not inquisitive, intelligent people."

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #8 on: 06/14/2015 08:36 PM »
Dang it, we really need one spot that has all known information about MCT that comes from SpaceX. I'm not sure if he said 80-100 or 50-100. Not that it makes an enormous difference, but it's annoying.

My recollection is 80-100 people but my memory is not a very safe source.

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #9 on: 06/15/2015 01:26 AM »
A second attempt at a Mars Colonial Transporter.  Not a first generation ship, but perhaps second or third generation of 10m core rockets.  Youtube playlist should show 18 little videos covering the whole trip.  A few very speculative items have crept in, for fun.
For some unknown reason, you need to restart the playlist after the first video. Sorry.

https://www.youtube.com/watch?v=s0zokCWXYE4?list=PLBU9UJfqaRooKnHY8QtQ399qqRwBqU6W3

Michel Lamontagne
« Last Edit: 06/17/2015 04:11 AM by lamontagne »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #10 on: 06/15/2015 05:15 AM »
MCT and BFR are supposed to be based on the same kind of platform. MCT will HAVE to have similar mass efficiency of an upper stage (in fact Musk has said MCT needs to be capable of Mars surface to earth in a SINGLE stage, though with far less payload), and essentially that's what it is. So whether you call the tanker a modified MCT or a stretched BFR upper stage may be a distinction without much difference.

Show me the quote for this, because I've never heard any such thing.  Rather I think direct single stage Earth return is rather a possible (and the most aggressive possible) interpretation of some of Musks statements but it is far from set in stone.

And even if Elon had said this was his goal we should have SERIOUS doubts if such a goal would survive contact with real engineering as the vehicle capable of doing all that would put a single stage to Earth orbit vehicle to shame.  You can hand wave away the incredible difficulty and mass costs of EDL on Mars and Earth and the costs of keeping a vehicle alive during interplanetary transit.

All this talk about avoiding costs by not developing a 2nd stage are silly, SpaceX MUST have a use for the BFR other then launching for Mars related travel.  The rocket would be completely useless for any other purpose if it's payloads were volumetricly constrained by needing to be inside a MCT cargo-hold which is likely no more then 500 m^3,  SLS should have a payload fairing in excess of 2000 m^3.

Offline sublimemarsupial

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Re: MCT Speculation and Discussion Thread 4
« Reply #11 on: 06/15/2015 05:47 AM »
MCT and BFR are supposed to be based on the same kind of platform. MCT will HAVE to have similar mass efficiency of an upper stage (in fact Musk has said MCT needs to be capable of Mars surface to earth in a SINGLE stage, though with far less payload), and essentially that's what it is. So whether you call the tanker a modified MCT or a stretched BFR upper stage may be a distinction without much difference.

Show me the quote for this, because I've never heard any such thing.  Rather I think direct single stage Earth return is rather a possible (and the most aggressive possible) interpretation of some of Musks statements but it is far from set in stone.

And even if Elon had said this was his goal we should have SERIOUS doubts if such a goal would survive contact with real engineering as the vehicle capable of doing all that would put a single stage to Earth orbit vehicle

Do you anything more than handwaving to support this? Because the people on this site that have tried to put numbers to this have shown that such a stage has a LESS demanding dry mass fraction than an SSTEO vehicle.
« Last Edit: 06/15/2015 05:48 AM by sublimemarsupial »

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #12 on: 06/15/2015 07:05 AM »
MCT and BFR are supposed to be based on the same kind of platform. MCT will HAVE to have similar mass efficiency of an upper stage (in fact Musk has said MCT needs to be capable of Mars surface to earth in a SINGLE stage, though with far less payload), and essentially that's what it is. So whether you call the tanker a modified MCT or a stretched BFR upper stage may be a distinction without much difference.

Show me the quote for this, because I've never heard any such thing.  Rather I think direct single stage Earth return is rather a possible (and the most aggressive possible) interpretation of some of Musks statements but it is far from set in stone.
It will have to be a Mars SSTO in any case, which means 4.5 km/s of delta V minimum. That's very much in 2nd stage territory.

All this talk about avoiding costs by not developing a 2nd stage are silly, SpaceX MUST have a use for the BFR other then launching for Mars related travel.  The rocket would be completely useless for any other purpose if it's payloads were volumetricly constrained by needing to be inside a MCT cargo-hold which is likely no more then 500 m^3,  SLS should have a payload fairing in excess of 2000 m^3.
But what will use that excessive volumetric capability? The BA2100? Who would use a BA2100 and why? Would you ever need to launch multiple BA2100s? If so, for what reason? If you need something big put in space, wouldn't you rather design it according to the volumetric constraints of the vehicle you'll be using instead of the other way around?

In any case, the bread and butter of most commercial launch service companies is and has always been communication satellites.

A cargo bay which can hold 100 tonnes of cargo for mars can most definitely hold a comms satellite, and the excessive delta V that a MCT is required to pull (even/especially if it doesn't act as its own 2nd stage or do a one burn from Mars to Earth since it still needs at least 4.5 km of delta V to rendezvous with a transfer tug in LMO) make it more than capable of acting as a GTO delivery vehicle.

The volume constraint argument is a red herring. If something big enough to fill the volume constraints of a SLS fairing comes along and requires a launch it might as well get a stage specifically designed for it or even the SLS, if that ever goes into commercial launches. You design things based on the constraints you are given, not the other way around.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #13 on: 06/15/2015 07:40 AM »
A second attempt at a Mars Colonial Transporter.  Not a first generation ship, but perhaps second or third generation of 10m core rockets.  Youtube playlist should show 18 little videos covering the whole trip.  A few very speculative items have crept in, for fun.
For some unknown reason, you need to restart the playlist after the first video. Sorry.

Michel Lamontagne

A lot of thought and work has gone into this. The engineering of an MCT like this would be formidably difficult, but it addresses concerns about zero gravity and abort that other conceptual designs do not.

One improvement might be to have the capsule part of the MCT nominally land attached to the cargo/transit part. Then it can perform an abort during landing.

Do you have mass estimates to go with the animations?

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #14 on: 06/15/2015 07:59 AM »
All this talk about avoiding costs by not developing a 2nd stage are silly, SpaceX MUST have a use for the BFR other then launching for Mars related travel.  The rocket would be completely useless for any other purpose if it's payloads were volumetricly constrained by needing to be inside a MCT cargo-hold which is likely no more then 500 m^3,  SLS should have a payload fairing in excess of 2000 m^3.

I disagree. Estimates in other threads have about 22 m^3 per person of pressurised volume, so for 100 passengers that is 2200 m^3. If the crew accommodations are payload to MCT, this means that the payload volume would need to be 2500 m^3 or above.

With suitable mission kits MCT could perform the following missions:

- Tanker flights to LEO
- Propellant depot
- Satellite and space station (up to BA 2100 size at least) delivery
- Tourist launch to LEO (~300 passengers)
- Cargo/crew delivery to space stations anywhere in cis-lunar space
- Moon landings
- NEO visits.

MCT may not be the most efficient system to perform such missions, but it is capable enough. Capability will win out over efficiency in my opinion because dedicated efficient systems of the size of MCT will cost a lot to develop. The one exception to this might be a dedicated tanker because of the large number of tanker flights required for anything beyond LEO.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #15 on: 06/15/2015 11:34 AM »
MCT and BFR are supposed to be based on the same kind of platform. MCT will HAVE to have similar mass efficiency of an upper stage (in fact Musk has said MCT needs to be capable of Mars surface to earth in a SINGLE stage, though with far less payload), and essentially that's what it is. So whether you call the tanker a modified MCT or a stretched BFR upper stage may be a distinction without much difference.

Show me the quote for this, because I've never heard any such thing.  Rather I think direct single stage Earth return is rather a possible (and the most aggressive possible) interpretation of some of Musks statements but it is far from set in stone.

And even if Elon had said this was his goal we should have SERIOUS doubts if such a goal would survive contact with real engineering as the vehicle capable of doing all that would put a single stage to Earth orbit vehicle to shame.  You can hand wave away the incredible difficulty and mass costs of EDL on Mars and Earth and the costs of keeping a vehicle alive during interplanetary transit.

All this talk about avoiding costs by not developing a 2nd stage are silly, SpaceX MUST have a use for the BFR other then launching for Mars related travel.  The rocket would be completely useless for any other purpose if it's payloads were volumetricly constrained by needing to be inside a MCT cargo-hold which is likely no more then 500 m^3,  SLS should have a payload fairing in excess of 2000 m^3.
The quote is in the thread the moderators deleted for some reason.
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Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #16 on: 06/15/2015 11:42 AM »
With suitable mission kits MCT could perform the following missions:
- Tanker flights to LEO
- Propellant depot
- Satellite and space station (up to BA 2100 size at least) delivery
- Tourist launch to LEO (~300 passengers)
- Cargo/crew delivery to space stations anywhere in cis-lunar space
- Moon landings
- NEO visits.
MCT may not be the most efficient system to perform such missions, but it is capable enough.

Hmmm, it looks like many people (myself included) are reading into MCT that it will be what the Space Shuttle was supposed to be. A low-cost, general-purpose, reusable space truck.

Others, like Impaler, are (perhaps more realistically) assuming it will be a specialised single-purpose vehicle, barely capable of what is being asked of it.

Not specifying which group you fall into is bound to result in pointless arguing past each other.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #17 on: 06/15/2015 11:58 AM »
What difference is there between MCT and a reusable upper stage? Just the habitable portion on top. They will need similar performance (~6.5-7km/s). Both need reentry and landing capability (legs, etc).

The problem with Shuttle is there were only a few of them made, no custom ones. With MCT, thousands will be made, so no problem making some that lack the habitable portion or that act as tankers or that are only used for cargo. The requirements for these things are similar but the MCTs can be modified to fit the purpose instead of having one vehicle type do everything at once.
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #18 on: 06/15/2015 12:11 PM »
Imagine a reusable F9 upper stage and Dragon melded together into a more structurally efficient whole and using methane instead of kerosene (slightly stretched to compensate for the lower bulk density of methane/oxygen). That would have roughly 7km/s of performance, which is basically what you need for Mars surface to Earth. Make that bigger, and you have a rough sketch of MCT.

(And MCT will need it's crew quarters to be far lower density than Dragon... Dragon is 500-1000kg per m^3 of pressurized volume. MCT will need to be more like 120-250kg per m^3, comparable to a passenger jet.)
« Last Edit: 06/15/2015 12:34 PM by Robotbeat »
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #19 on: 06/15/2015 12:14 PM »
I do expect the tanker to be different. No payload or crew quarters. Just stretched main tanks. That's a lot more mass efficient. But early on for the first few missions or in a test phase they may use MCT for that purpose too.

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