Author Topic: MCT Speculation and Discussion Thread 4  (Read 621607 times)

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1620 on: 02/06/2016 08:04 AM »
Don't forget to save fuel for reuse.

Yes, for the tanker. The MCT going forward is something else.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1621 on: 02/06/2016 08:30 AM »

You are not alone in getting such "not that big" results.  Right now we're all over the map with possibly conflicting  years old Elon statements and crazy big BFR/MCT rumors on Reddit.  But the bottom line is that it does not even take the "mid sized" :) 15 million LB thrust vehicle to meet the LEO payload claims.

Postponing the architecture announcement for the 3rd time indicates to me that everything is in a high state of flux.  They've likely done some next level of detail of engineering analysis and arrived at some key numbers different than expected, which iterates revisions.  With Musk the decisions will not be just space cadet tech driven but will also have a strong best economic model (as he best believes it) influencing size/capability tradeoffs.

Further analysis and high levels of design flux would mean to me that the architecture is not the kind of brutally simple von Braun eat-your-heart-out kind of thinking most people are imagining.  If that was the design they could announce it now and no one would care if the rockets final size is +/-10%.  I think Musk is going to come up with something vastly more creative then that, something that is to rocketry what hyper-loop is to trains.

I've always said that I see SEP as a big part of the solution and I'd also new and radical EDL solutions, automated propellant production, containerized cargo logistics in the mix as well.  To get anywhere near an efficient low cost system requires full integration of all of these elements together into a fundamentally new arrangement.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #1622 on: 02/06/2016 12:37 PM »
BFR is much more than 100 tons to LEO. That much is clearly established.

The 100 tonnes figure was the useful payload landed on Mars IIRC. This is by a ship/lander that will refuel and depart sans 100 tonnes payload. The mass to LEO will vary considerably(though always be much larger than 100 tonnes) depending how refueling is incorporated and how you account for the spaceship itself.
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1623 on: 02/06/2016 01:32 PM »
BFR is much more than 100 tons to LEO. That much is clearly established.

The 100 tonnes figure was the useful payload landed on Mars IIRC. This is by a ship/lander that will refuel and depart sans 100 tonnes payload. The mass to LEO will vary considerably(though always be much larger than 100 tonnes) depending how refueling is incorporated and how you account for the spaceship itself.

Yes. The 100 metric tons is payload landed on Mars according to Musk.  The understandable controversy is the dry mass of the stage that makes it to LEO.  I think most estimates are too optimistic (light) for the 1st versions.
 
I see a largish 1st stage booster that goes relatively low & slow as being the easier of the engineering challenges.  The 2nd stage of this supposed TSTO has near but not equal to Earth SSTO performance and is a SSTO when departing Mars' surface.  Add in life support, re-entry speeds and mechanisms for landing on unprepared Mars sites and it's a huge architectural and developmental challenge.  I think that's the part that's in flux, not so much the 1st stage.

I'm also intrigued by the possibility of SEP (mentioned by Gwynne Shotwell) for interplanetary transit and EDL as well, but I'm skeptical because it's yet another R&D $ challenge and I sense from his philosophical comments that Musk wants to keep development architecture as simple, at least in its first decade.  Especially with Musk's avowed 2015 target, I don't see lots of sexy stuff being part of the architecture.  I hope he makes a crewed landing by 2033.
« Last Edit: 02/06/2016 01:37 PM by philw1776 »
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Offline kicaj

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Re: MCT Speculation and Discussion Thread 4
« Reply #1624 on: 02/06/2016 04:57 PM »
Hello.
I have a question, I know you had probably talk about this but I cant find it.
What kind of propulsion will have second stage?

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1625 on: 02/06/2016 05:06 PM »
Hello.
I have a question, I know you had probably talk about this but I cant find it.
What kind of propulsion will have second stage?
We think a bunch of Raptors.  The Falcon 9 uses nine Merlin 1D's in the first stage and a single slightly different Merlin 1D (Vacuum Optimized) in the second stage.

The number of Raptors in the upper stage is a bit dependent on the scale of BFR, and we're in quite some disagreement about that.  With that said, 7 is a nice number that permits a fairly large upper stage (2500 tons or so at Mars Ascent for MarsG+15%, 1000 tons or so at Earth Ascent for EarthG+15%) even with one engine-out & the opposite number throttled.  It's also a convenient packing to fit a vacuum-optimized nozzle as large as possible.

I also harbor admittedly exotic theories that some SuperDracos might come in handy as secondary propulsion.
« Last Edit: 02/06/2016 05:08 PM by Burninate »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1626 on: 02/06/2016 05:22 PM »
Technical query for my BFS models.
What's a rational target range for T/W of the 2nd stage at ignition taking into account gravity losses, etc.?
I would expect it could be less than a 1st stage's preferred T/W.
How low can it be at ignition?

I'd also assumed RTLS in my past calculations but have come to believe that the 1st stage will instead land on a floating platform, as less propellant is needed with no boost-back.
« Last Edit: 02/06/2016 05:23 PM by philw1776 »
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Offline dror

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Re: MCT Speculation and Discussion Thread 4
« Reply #1627 on: 02/06/2016 05:34 PM »

I'd also assumed RTLS in my past calculations but have come to believe that the 1st stage will instead land on a floating platform, as less propellant is needed with no boost-back.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1628 on: 02/06/2016 06:30 PM »
Technical query for my BFS models.
What's a rational target range for T/W of the 2nd stage at ignition taking into account gravity losses, etc.?
I would expect it could be less than a 1st stage's preferred T/W.
How low can it be at ignition?


That really depends on the amount of energy imparted by the first stage, and to a lesser extent the angle: if the first stage leaves off at 3km/s at a 45° angle, then the vertical component is 2.1km/s which on its own would allow the craft to coast to an altitude 210 km higher over a period of 210 seconds (rounded and ignoring the already reduced gravity loss from it trajectory around a sphere). If we had a 2nd stage with an initial T/W ratio of .75 and a final one of 3 then after 210 seconds it would roughly have added 3km/s, be travelling at a velocity that already cuts further gravity losses by more than 50% as it goes around the curved surface of the Earth and have a T/W ratio of more than 1.5 and need to simply maintain altitude so at the start of this period it is moving about 5.1km/s down range, needs to add another 3km/s as its T/W goes from 1.5 to 3 so initially the angle it needs at 5.1km/s headed down range with a 1.5 T/W ratio is 23° to stay at the same altitude which reduces to 0° a mere 2 minutes and 20 seconds later. The gravity loss would be something like 120m/s.

If instead the separation occured at 5km/s at a 20° angle then the vertical component is still over 1.5km/s but your downrange component is around 4.5km/s so you only need another 3.5km/s while you have more than 5 minutes coasting upward on the current trajectory a T/W ratio of less than .25 going to 1 would give you close to the same gravity loss as the previous case.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1629 on: 02/08/2016 05:55 AM »
The 3 km/s separation seems to be the more likely scenario and the T/W ratio sounds reasonable at 0.75 so we can extrapolate different masses for the 2nd stage at separation based on an engine count and the target Raptor thrust.

7 Raptors:  16100 kN thrust,  2200 mt mass at separation.
6 Raptors:  13800 kN thrust,  1880 mt mass at separation.
5 Raptors:  11500 kN thrust, 1570 mt mass at separation.
4 Raptors:  9200 kN thrust, 1260 mt mass at separation.

I believe the 5 engine configuration is getting on the small end, my bet would be to use a hexagonal 6 engine arrangement which would provide a space for a smaller central landing engine (I dub this mini engine 'Robin').

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1630 on: 02/08/2016 06:02 AM »
The 2nd stage of this supposed TSTO has near but not equal to Earth SSTO performance and is a SSTO when departing Mars' surface.

Can you clarify if you actually mean SSTLMO (to Low Mars Orbit) or SSTTEI (to trans-Earth-Injection).  The latter is what everyone claiming integrated 2nd stage seems to be aiming for and it tends to yield a mars take off mass >1000 mt.  The former is my position and involves SEP transit vehicles or refueling in mars orbit with SEP delivered propellants and keeps mars take-off mass to ~400 mt.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1631 on: 02/08/2016 02:04 PM »
The 2nd stage of this supposed TSTO has near but not equal to Earth SSTO performance and is a SSTO when departing Mars' surface.

Can you clarify if you actually mean SSTLMO (to Low Mars Orbit) or SSTTEI (to trans-Earth-Injection).  The latter is what everyone claiming integrated 2nd stage seems to be aiming for and it tends to yield a mars take off mass >1000 mt.  The former is my position and involves SEP transit vehicles or refueling in mars orbit with SEP delivered propellants and keeps mars take-off mass to ~400 mt.

Sorry about my mis-statement.  I meant SSTEI, the latter with the larger mass at take off.
I'm simply trying to model vehicles based on as I best understand what Musk has said.
However, I believe that the September 2016 mission architecture reveal, raptor thrust, whatever will have changed in the years since the early statements.
It's possible that SEP which SX says they're "looking at" is part of the architecture but given the incredibly early 2025 target date I doubt that additional R&D expense for an interplanetary SEP vehicle will be included.
I continue to believe that available cash flow will be a serious limit to any Mars architecture taking hardware form.
« Last Edit: 02/08/2016 02:05 PM by philw1776 »
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Offline Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #1632 on: 02/09/2016 03:29 PM »
My part in the architecture speculation is the LEO propellant depot. Propellant transfers can be accomplished with little or no depot as long as there are only a few missions during each synod. But when will the volume of traffic to Mars require a larger depot?

To address this question I have been developing a worksheet - see the file attachment below.

The number of launches to LEO and the number of Mars trips will depend on the volume of MCTs of the various types that are produced, the current inventory of each type on Earth, and the rate of return of vehicles from Mars. In table 1 below is a projection over 4 synods, beginning with the first introduction of a prototype during “Synod 0.” In this somewhat conservative projection I assumed a constant production rate of 2 MCTs per year or 4 per 26-month synod with no increase in the rate. I also do not account for any contribution from preliminary Dragon missions that could set up small ISRU propellant facilities on Mars. How realistic is this?
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1633 on: 02/09/2016 04:19 PM »
... How realistic is this?

I don't expect an MCT to return until there are people there, various bits of logic for that, but one is inspecting engines, and another is that in the timeline the way I see it that by the time one fully fueled MCT is ready to go, the people are arriving and that means that fuel is reserve for returning crew in a catastrophic situation.

I don't see them sending just one passenger MCT when they first send people. Having an MCT that can support say 24 people to Mars loiter and return to Earth should then suggest sending 24 people in two such MCTs, or if you sent 3 that could hold 12 each then send 8 per MCT for a total of 24. The logistics of returning people then would allow for return flights of manned MCT's part way through the 2nd synod of manned occupation while still maintaining a 100% evacuation fleet on Mars through the next 5 synods, while still returning a large portion of the passenger MCTs (though I see sending them back at conjunctions not doing the rapid return thing).
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1634 on: 02/09/2016 04:28 PM »
I think the first MCT will be cargo only.  Solar panels for power and ISRU equipment.  The second one may have a small crew 6-12 people to stay for about 18 months to two years.  They would make sure all equipment works.  MCT for themselves is refueled for return, and also the second one would carry additional equipment and habitat.  This will probably be true for the first few arriving.  Then as equipment for ISRU is operational for fuel, oxygen, water, and modules for living and possibly growing algae with some food, people will start arriving with additional MCT's with even more equipment. 

Maybe even 9-10 MCT's landing robotically with all equipment food, etc, necessary for 100 people to arrive.  Depends on how many MCT's can be built before the first synod, to how many can be build between each synod.  Some people might want to return home, some will stay, some will rotate.  A lot of unknowns. 

Offline CraigLieb

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Re: MCT Speculation and Discussion Thread 4
« Reply #1635 on: 02/09/2016 06:17 PM »
...
I read that novel too.  But...  Colonists are people. It will only take one going koo-koo to jeopardize everyone and the ship.   

And if you catch the kook in time, you run a quick experiment to determine how long someone lasts outside the airlock.
Colonize Mars!

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1636 on: 02/09/2016 07:40 PM »
...
I read that novel too.  But...  Colonists are people. It will only take one going koo-koo to jeopardize everyone and the ship.   

And if you catch the kook in time, you run a quick experiment to determine how long someone lasts outside the airlock.

HAL9000 did just this in '2001 A Space Odyssey'.  His altruistic research motives were mis-understood by the audience.
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Offline QuantumG

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Re: MCT Speculation and Discussion Thread 4
« Reply #1637 on: 02/10/2016 11:22 PM »
I like the way he listed space tourism as something *other* companies are doing.. and he specifically mentioned orbital space tourism in that list. I'd love to know when SpaceX decided they were too good for this market.
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Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1638 on: 02/10/2016 11:32 PM »
I like the way he listed space tourism as something *other* companies are doing.. and he specifically mentioned orbital space tourism in that list. I'd love to know when SpaceX decided they were too good for this market.

Probably because space tourism doesn't help them with their long term projects. They may have deduced that the cost for SpaceX to get a working tourism architecture going wouldn't equate to a big enough end profit to be worth it in the end. Besides, they have enough projects in the pipeline as-is, including trying to get their LVs off the ground (which has been a little shaky this year so far).

It makes sense for Blue and for VG because they can repurpose architectural elements to help them learn enough to get to orbital launch. SpaceX already knows this. Why would they go back? It doesn't help them to get to BFR.


And yes, Steve counts as a source since he's a major SpaceX investor along with a close business friend of Elon. He watches the space sector closely and has at least a moderate knowledge of how the field works from a technical level and a dominant one from a financial level.  It's safe to hazard that he has access to information that we do not, since he has access to Elon and SpaceX generally that we do not.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1639 on: 02/11/2016 12:14 AM »
It counts as a source, but this page isn't for discussion.
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