Author Topic: MCT Speculation and Discussion Thread 4  (Read 503517 times)

Offline MichaelBlackbourn

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Re: MCT Speculation and Discussion Thread 4
« Reply #1200 on: 12/13/2015 05:31 PM »
. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

?? gravity is pretty much the same at 160km up as it is on the surface. Gravity losses starting from 0 or 160km up are very similar.  It's all about the orbital velocity.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1201 on: 12/13/2015 06:01 PM »
. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

?? gravity is pretty much the same at 160km up as it is on the surface. Gravity losses starting from 0 or 160km up are very similar.  It's all about the orbital velocity.

As I have already stated twice, I know that it is all about orbital velocity. I also know that gravity is almost as strong at 160km altitude. But I was not talking about gravaity, but about gravity losses. The difference is the vector at which the engines fire. However the first stage would fire vertical or almost vertical while the second stage could fire horizontal.

I will just wait for the first flights. I expect that MCT will stage at less than 3km/s, but we will see.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1202 on: 12/13/2015 06:08 PM »
The point I tried to make is that this is a lot of delta-v needed. MCT might be able to do it, if its designed to stage from LEO, but in that case it might be almost empty when it reaches orbital speed. You will then need to transfer more fuel to it so that it can start the mission to Mars.

Not necessarily. With the different trajectory it could have more fuel, the first stage less. It is a trade, and one, I openly admit, I cannot make. I am sure they will make that trade at SpaceX very thoroughly and suspect the optimum for the system with full reuse will be at less than 3km/s horizontal speed.

At the same time, the "slingshot to orbital height S1" might make re-usability easier, but you would still have to design it so that it could carry other payloads than MCT (for example, a tanker, or a hub, or mars infrastructure, or commercial payloads etc). If S1 is designed to launch more things than strictly MCT, then it would not be easy (I think) to merge both capabilities in the same structure.

Good point. However it was quite clearly said, BFR/MCT will be optimized only and really only for Mars. It also has so much capability that it can afford to operate at less than optimum in cislunar space. I am sure it will be a very inefficient system to deploy satellites to GTO without refuelling.

Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1203 on: 12/13/2015 07:18 PM »
The one design criteria most overlooked is minimum operating costs. This is probably the most significant item for the design and delta -V requirements that the BFR must meet. These operating costs are for sending an MCT to Mars meaning less tanker flights means much lower costs. An increase of the BFR's deliverable delta V by 750m/s increases the tanker prop load from 100mt to 200mt. This nearly halves the total cost for an MCT to Mars by almost 50% from 10 flights to only 5 flights. This is without any change to the design of the BFS itself. The BFS is fairly easily sized because the three design flight uses: as second stage, as EDS, and as Mars SSTO all are very close to the  same propellant loads requirements.

Online hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #1204 on: 12/13/2015 07:41 PM »
. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

?? gravity is pretty much the same at 160km up as it is on the surface. Gravity losses starting from 0 or 160km up are very similar.  It's all about the orbital velocity.

As I have already stated twice, I know that it is all about orbital velocity. I also know that gravity is almost as strong at 160km altitude. But I was not talking about gravaity, but about gravity losses. The difference is the vector at which the engines fire. However the first stage would fire vertical or almost vertical while the second stage could fire horizontal.

I will just wait for the first flights. I expect that MCT will stage at less than 3km/s, but we will see.

"less than 3km/h" is much more than almost zero horizontal velocity.

Making the first stage launch vertically and just eat the gravity and atmospheric losses would mean that
1) second stage would have to do all the work for the 7.5km/h horizontal delta-v
2) The trajectory of the rocket would be very inefficient. Doing the vertical and horizontal part of the acceleration totally separately means much more total delta-v is needed, pythagoras is our friend here. Just to get to 200km altitude 2km/s delta-v is needed for the first stage. And this is ignoring the gravity losses. With 45 degrees burn direction,  2.8km/s delta-v gives 2kms/s vertical AND 2km/s horizontal delta-v. The gravity turns real rockets are doing are even much better.



Please, try this in KSP. You will see that how inefficient your trajectory idea is.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #1205 on: 12/13/2015 08:02 PM »
The one design criteria most overlooked is minimum operating costs. This is probably the most significant item for the design and delta -V requirements that the BFR must meet. These operating costs are for sending an MCT to Mars meaning less tanker flights means much lower costs. An increase of the BFR's deliverable delta V by 750m/s increases the tanker prop load from 100mt to 200mt. This nearly halves the total cost for an MCT to Mars by almost 50% from 10 flights to only 5 flights. This is without any change to the design of the BFS itself. The BFS is fairly easily sized because the three design flight uses: as second stage, as EDS, and as Mars SSTO all are very close to the  same propellant loads requirements.

Up til now we (or at least I have, and I think just about everyone else) have sized the BFS for the Mars mission, then sized the BFR to lift that, then sized the tanker based on what the BFR can lift.

If instead we size the BFR and tanker to be most efficient (cost wise) to refuel the FBS, it could quite well lead to a bigger BFR. The tanker would then make full use of the BFR's lift capability and the BFS only a part of it.

If this is the case I don't see any easy way of outsiders using analysis to determine the optimum BFR size, as this would depend on internal SpaceX cost data (size vs development facilities, manufacturing and operations costs).

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1206 on: 12/13/2015 08:11 PM »
. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

?? gravity is pretty much the same at 160km up as it is on the surface. Gravity losses starting from 0 or 160km up are very similar.  It's all about the orbital velocity.

Is it still inefficient if you take in to account the 1st stage needs to RTLS, and therefor needs some sort of burn to negate it's horizontal DV? In fact it needs to impart twice the horizontal DV to rtls as it used to get to separation. It's carrying less fuel so is a lot lighter however, so maybe its not that big of an ask.

As I have already stated twice, I know that it is all about orbital velocity. I also know that gravity is almost as strong at 160km altitude. But I was not talking about gravaity, but about gravity losses. The difference is the vector at which the engines fire. However the first stage would fire vertical or almost vertical while the second stage could fire horizontal.

I will just wait for the first flights. I expect that MCT will stage at less than 3km/s, but we will see.

"less than 3km/h" is much more than almost zero horizontal velocity.

Making the first stage launch vertically and just eat the gravity and atmospheric losses would mean that
1) second stage would have to do all the work for the 7.5km/h horizontal delta-v
2) The trajectory of the rocket would be very inefficient. Doing the vertical and horizontal part of the acceleration totally separately means much more total delta-v is needed, pythagoras is our friend here. Just to get to 200km altitude 2km/s delta-v is needed for the first stage. And this is ignoring the gravity losses. With 45 degrees burn direction,  2.8km/s delta-v gives 2kms/s vertical AND 2km/s horizontal delta-v. The gravity turns real rockets are doing are even much better.



Please, try this in KSP. You will see that how inefficient your trajectory idea is.

Offline joek

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Re: MCT Speculation and Discussion Thread 4
« Reply #1207 on: 12/13/2015 09:01 PM »
Making the first stage launch vertically and just eat the gravity and atmospheric losses would mean that
...

I would not be too quick to dismiss.  A TSTO "pop-up" trajectory has advantages in some situations.  There are several variations from straight up ("elevator") to simply more vertical than would otherwise be optimal from a total orbital dV perspective.

These trajectories are typically discussed in the context of RLVs (e.g., Kistler K-1 and various fly-back and boost-back designs).   Such designs may be sub-optimal from overall dV, but optimal for other reasons.  Two common attributes are: a relatively oversized booster than typical; and a second stage with greater dV capability than typical.  BFR + BFS may fit that description.

Jon Goff over at Selenian Boondocks did a nice writeup in "Orbital Access Methodologies Part III: Pop-up TSTO". A number of other papers discuss specific RLVs.
« Last Edit: 12/13/2015 10:10 PM by joek »

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1208 on: 12/13/2015 09:19 PM »
I argued one extreme, straight up. I see the 3km/s I see used frequently as the other extreme and believe the optimum will be somewhere inbetween. It may be very hard to determine an optimum. To reach the best value both stages need to share the work and that work includes RTLS of the first stage. So the overall package will be designed for an optimum including RTLS. I guess Falcon was designed with that in mind but not yet really exclusively optimised for reuse. BFR/MCT will and I am looking forward to see how SpaceX decided.

Offline cscott

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Re: MCT Speculation and Discussion Thread 4
« Reply #1209 on: 12/14/2015 12:23 AM »
Huh, that's interesting. I should have read TFA.  But I think there is a little ambiguity: the writer appears to have prompted the answer, with the obscenity context, and they end up discussing the BFG from Doom, where the F definitely stood for "frakking".  We've heard SpaceX mince the middle F into Falcon, which is actually quite a nice fit when you consider it.  I'm certainly not convinced that SpaceX will use an obscenity when they finally introduce the BFR.  I suspect they will continue to (at least occasionally) use Falcon in public, if they give any expansion for BFR at all.

(Now that I think of it, if might have been Gwynne or Hans who used "Falcon" in the middle.  So maybe this is another of the nomenclature variances within SpaceX, like Dragon V2/2/Crew Dragon etc.)

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1210 on: 12/14/2015 02:03 AM »
The one design criteria most overlooked is minimum operating costs. This is probably the most significant item for the design and delta -V requirements that the BFR must meet. These operating costs are for sending an MCT to Mars meaning less tanker flights means much lower costs. An increase of the BFR's deliverable delta V by 750m/s increases the tanker prop load from 100mt to 200mt. This nearly halves the total cost for an MCT to Mars by almost 50% from 10 flights to only 5 flights. This is without any change to the design of the BFS itself. The BFS is fairly easily sized because the three design flight uses: as second stage, as EDS, and as Mars SSTO all are very close to the  same propellant loads requirements.

Could not agree more.
I was in a high-tech industry engineering products where low production cost was paramount.  We did the same cost driver analysis on every single aspect that Musk does.
I see the 1st stage as going low & slow under 3 Km/sec while the delta V is in the MCT stage two which makes it utilitarian for all Mars purposes.  Large propellant tanks make 'mods" for a tanker minimal. 
I also think those speculating that the BFR might be larger than minimal models may be onto something as they reduce # of flights to refuel the MCT in LEO or wherever.  It's a complex system analysis whether to make a minimum parts cost re-useable BFR vs making one a bit larger (a bit more expensive per unit) that reduces # of flights to fuel up an MCT in orbit for a Mars journey.
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Online GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #1211 on: 12/14/2015 04:25 AM »
I'm certainly not convinced that SpaceX will use an obscenity when they finally introduce the BFR.  I suspect they will continue to (at least occasionally) use Falcon in public, if they give any expansion for BFR at all.
I don't think it'll ever be an official name, but given Musk's personality I'd say it was definitely that at first. There'll be a rename, unless "Big Falcon" sticks.

Offline LastStarFighter

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Re: MCT Speculation and Discussion Thread 4
« Reply #1212 on: 12/14/2015 06:15 AM »
Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ

I beleive you are misinterpreting him simplifying things for the magazine readers. He seems to just be saying there will be a independant spacecraft and rocket. Just like there is a Dragon spacecraft and Falcon "booster" rocket. Thats just my take on it though. I could be wrong and perhaps they have found a reliable supplier of unobtainium and dilithium crystals from which to build this single stage booster rocket out of.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1213 on: 12/14/2015 09:37 AM »
I beleive you are misinterpreting him simplifying things for the magazine readers. He seems to just be saying there will be a independant spacecraft and rocket. Just like there is a Dragon spacecraft and Falcon "booster" rocket. Thats just my take on it though. I could be wrong and perhaps they have found a reliable supplier of unobtainium and dilithium crystals from which to build this single stage booster rocket out of.

I am at a loss here. I cannot understand where the idea of a second stage comes from.

The abilities of a second stage are a subset of what MCT needs to do going from LEO to Mars surface and, after refuelling, back to earth. So why duplicate that existing capability with a separate second stage?

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #1214 on: 12/14/2015 01:57 PM »
. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

?? gravity is pretty much the same at 160km up as it is on the surface. Gravity losses starting from 0 or 160km up are very similar.  It's all about the orbital velocity.

As I have already stated twice, I know that it is all about orbital velocity. I also know that gravity is almost as strong at 160km altitude. But I was not talking about gravaity, but about gravity losses. The difference is the vector at which the engines fire. However the first stage would fire vertical or almost vertical while the second stage could fire horizontal.

I will just wait for the first flights. I expect that MCT will stage at less than 3km/s, but we will see.

"less than 3km/h" is much more than almost zero horizontal velocity.

Making the first stage launch vertically and just eat the gravity and atmospheric losses would mean that
1) second stage would have to do all the work for the 7.5km/h horizontal delta-v
2) The trajectory of the rocket would be very inefficient. Doing the vertical and horizontal part of the acceleration totally separately means much more total delta-v is needed, pythagoras is our friend here. Just to get to 200km altitude 2km/s delta-v is needed for the first stage. And this is ignoring the gravity losses. With 45 degrees burn direction,  2.8km/s delta-v gives 2kms/s vertical AND 2km/s horizontal delta-v. The gravity turns real rockets are doing are even much better.



Please, try this in KSP. You will see that how inefficient your trajectory idea is.
1) Given that the dV needed to pull a SSTE from Mars surface is about 8 km/s, I'd say you're set in the dV department.

2) This is less about having an efficient trajectory than it is simplifying first stage reuse. This way, you go directly up and directly down, with very little correction needed, as opposed to boosting back from a gravity turn.



A brief simulation in KSP confirms that a high delta V upper stage is required for this purpose (which then requires refueling to get anywhere but orbits and halfways and whatnot), but this method does in fact simplify the construction of the first stage (basically you don't need to obsess so much about the mass budget and you can fix most first stage problems by just throwing additional mass at them) and eases first stage recovery, at the expense of making the upper stage more mass sensitive.

Basically since the BFS seems to have to be a Not-Quite-SSTO by design (SSTE from Mars surface is a requirement), a pop-up TSTO makes a lot of sense, especially given how much it simplifies first stage recovery.

Offline Dante80

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Re: MCT Speculation and Discussion Thread 4
« Reply #1215 on: 12/14/2015 03:25 PM »
Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ

I beleive you are misinterpreting him simplifying things for the magazine readers. He seems to just be saying there will be a independant spacecraft and rocket. Just like there is a Dragon spacecraft and Falcon "booster" rocket. Thats just my take on it though. I could be wrong and perhaps they have found a reliable supplier of unobtainium and dilithium crystals from which to build this single stage booster rocket out of.

I'm willing to guess that SX will not make a S2 and put the spaceship on top of it. It will actually use the spaceship to finish the orbit, and then maybe re-fuel it there to get to mars (or an even higher staging point).

Remember, the spaceship we are talking about should be capable of some serious dv if it can reach mars from LEO/HEO and land (or launch from mars and return). You are not going to need another stage on the rocket to get it to LEO, unless in-orbit refueling is out of the question. A re-usable S1 is enough, dilithium crystals would be overkill at this point.. ;)   

Moreover, a second stage that reaches orbit is either very expensive (expendable), or very expensive and complex (re-usable). If BFR stays as an exclusive MCT booster (as has been hinted by SpaceX in the past), then its more safe to assume that there won't be a second stage.

In any case, we will have to see how this unfolds. We may get some information by Elon on the architecture by Q1 2016.
« Last Edit: 12/14/2015 03:29 PM by Dante80 »

Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1216 on: 12/14/2015 05:00 PM »
The one design criteria most overlooked is minimum operating costs. This is probably the most significant item for the design and delta -V requirements that the BFR must meet. These operating costs are for sending an MCT to Mars meaning less tanker flights means much lower costs. An increase of the BFR's deliverable delta V by 750m/s increases the tanker prop load from 100mt to 200mt. This nearly halves the total cost for an MCT to Mars by almost 50% from 10 flights to only 5 flights. This is without any change to the design of the BFS itself. The BFS is fairly easily sized because the three design flight uses: as second stage, as EDS, and as Mars SSTO all are very close to the  same propellant loads requirements.

Could not agree more.
I was in a high-tech industry engineering products where low production cost was paramount.  We did the same cost driver analysis on every single aspect that Musk does.
I see the 1st stage as going low & slow under 3 Km/sec while the delta V is in the MCT stage two which makes it utilitarian for all Mars purposes.  Large propellant tanks make 'mods" for a tanker minimal. 
I also think those speculating that the BFR might be larger than minimal models may be onto something as they reduce # of flights to refuel the MCT in LEO or wherever.  It's a complex system analysis whether to make a minimum parts cost re-useable BFR vs making one a bit larger (a bit more expensive per unit) that reduces # of flights to fuel up an MCT in orbit for a Mars journey.
The range of LEO capability given by SpaceX way back for the MCT was 180-250mt. As we have discussed tremendous about what the BFS dry weight is possible 80mt is sort of a average or consensus value + 100mt of payload, making the 180mt to LEO the absolute minimum that the system must meet. But what if the performance was closer to the other end 250mt. That means that about 70mt of extra propellant is delivered per flight.

The whole reason for the larger BFR and less flights would be the same regardless of whether the BFR is reusable or expendable. If the minimum design BFR takes 10 flights to accomplish sending a single BFS to Mars but a bigger BFR with the exact same design BFS that takes only 5 flights although the BFR costs 50% more per flight still gives a reduction to per mission of 75% of the 10 flights configuration. There are other advantages to requiring half the flights and that is pad availability. In order to support sending 4 BFS's to Mars the 10 flights minimal BFR configuration would require 40 launches in the 780 day period (a launch every 19.5 days).  In order to support sending 4 BFS's to Mars the 5 flights large BFR configuration would require 20 launches in the 780 day period (a launch every 39 days). For the 10 cargo to 1 crew ratio of missions making the possibility of 20 cargo and 2 crew missions in a synod (780 day period) that would require
a) 10 flight config-> a launch every 3.5 days
b) 5 flight config-> a launch every 7 days

As mission counts increase the number of launches becomes a greater cost factor than any other consideration.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1217 on: 12/14/2015 07:29 PM »
Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ

I beleive you are misinterpreting him simplifying things for the magazine readers. He seems to just be saying there will be a independant spacecraft and rocket. Just like there is a Dragon spacecraft and Falcon "booster" rocket. Thats just my take on it though. I could be wrong and perhaps they have found a reliable supplier of unobtainium and dilithium crystals from which to build this single stage booster rocket out of.

Their is a large group of people who are simply dead-set on this 'super-direct' architecture and always have been and will interpret everything as confirmation of that architecture, no amount of technical arguments on my part about it's in-feasibility, enormous cost and risk have dissuaded them.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #1218 on: 12/14/2015 08:38 PM »
Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ

I beleive you are misinterpreting him simplifying things for the magazine readers. He seems to just be saying there will be a independant spacecraft and rocket. Just like there is a Dragon spacecraft and Falcon "booster" rocket. Thats just my take on it though. I could be wrong and perhaps they have found a reliable supplier of unobtainium and dilithium crystals from which to build this single stage booster rocket out of.

Their is a large group of people who are simply dead-set on this 'super-direct' architecture and always have been and will interpret everything as confirmation of that architecture, no amount of technical arguments on my part about it's in-feasibility, enormous cost and risk have dissuaded them.

I think it's more of a case that the configuration of a single stage BFR booster with BFS on top, then BFS does the 'super-direct' stuff as you call it, is just the simplest configuration that happens to also be consistent with everything that SpaceX and Elon have stated.  Especially this last statement from Elon. 

I agree with you that eventually they will employ some type of cycling SEP craft for the 'long trip across the  pond'(and they will probably say that this was always planned for the MCT system), but that doesn't seem to be in the cards right now.  And it seems clear at this point that there will be no 2nd stage between the BFR booster and the BFS.

It will be interesting to see how your technical objections are met when Elon eventually unveils the system design.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1219 on: 12/15/2015 02:01 AM »
From another thread, here is a recent quote by Musk in an interview:

Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ:

"Well, there's two parts of it—there's a booster rocket and there's a spaceship. So the booster rocket's just to get it out of Earth's gravity because Earth has quite a deep gravity well and thick atmosphere, but the spaceship can go from Mars to Earth without any booster, because Mars's gravity is weaker and the atmosphere's thinner, so it's got enough capability to get all the way back here by itself. It needs a helping hand out of Earth's gravity well. So, technically, it would be the BFR and the BFS." As in "Big frakking Spaceship."

http://www.gq.com/story/elon-musk-mars-spacex-tesla-interview?utm_source=10370

So... I guess we are back to basics, what many of us have arguing, it seems? This thread has been devalued (and made less interesting) in the last few months by people wanting to discuss their own architectures (yes, you know who you are), so hopefully this can narrow down the discussion again.
Let me just say, in a totally mature manner (regarding BFS--what we used to call MCT--being the integrated second stage):

Neener.

:)
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