Author Topic: SpaceX FX/FXX/BFR Speculation Thread.  (Read 376161 times)

Offline Mongo62

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #660 on: 05/09/2014 03:19 am »
For example, a Falcon 9 v1.0 could get 10 tonnes to LEO.
If it had been re-usable - your choice would have been either
a) only get 7 tonnes to LEO (expensive in terms of payload), or
b) build a bigger Falcon 9 v1.1 that can get 10 tonnes to LEO (expensive in terms of bigger rocket & more fuel)

I believe that the listed F9 v1.1 payload of 13,150 kg (14.5 tons) to LEO already has first-stage RTLS built into the number.
« Last Edit: 05/09/2014 03:24 am by Mongo62 »

Offline Vultur

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #661 on: 05/09/2014 03:41 am »
I know it's guesswork, but if the comment that "the whole rocket could land on mars" is right, then there is no centre stage to RTLS, right?

I think the whole MCT lands, not the whole rocket -- it's not single-stage-to-Mars.

Offline GregA

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #662 on: 05/09/2014 10:29 am »
I believe that the listed F9 v1.1 payload of 13,150 kg (14.5 tons) to LEO already has first-stage RTLS built into the number.
That's right. Knew I should have used mythical rockets to illustrate my point.

edit:
A slightly bigger reusable version can be cheaper and with greater payload than original.
« Last Edit: 05/09/2014 10:41 am by GregA »

Offline GregA

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #663 on: 05/09/2014 10:42 am »
I think the whole MCT lands, not the whole rocket -- it's not single-stage-to-Mars.
Fair point. I certainly don't think all the boosters go to Mars.

Offline bilbo

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #664 on: 05/11/2014 01:26 am »
I think the whole MCT lands, not the whole rocket -- it's not single-stage-to-Mars.
Fair point. I certainly don't think all the boosters go to Mars.
I think this would be likely, I think MCT is probably just the payload, the boosters themselves probably just reenter earths atmosphere and land somewhere

Offline SteveKelsey

Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #665 on: 05/11/2014 08:55 am »
Something is bugging me and I don't have the information to resolve the question. Assuming 15 metres dia and a two stage architecture this has an interesting implication for costs. If the booster can be reused say 10 times, a big if I agree, one booster can launch 10 MCT's before replacement/ refurb. This further reduces the assets needed to achieve colonisation and therefore cost. Adjust ratio of booster build to MCT build according to taste.
Given this saving on booster build would the 15 metre diameter architecture be cheaper in use than the three core through simpler processing ? Although the components are bigger and more expensive, does the simplicity of two stages deliver benefits over the three core approach or does the infrastructure costs of the 15 metre diameter architecture outweigh the benefit of simplicity?
If it does offer a cost saving, although audacious, it might be the optimal solution. SpaceX embraces audacious if it delivers cost benefit.
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Offline Lars_J

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #666 on: 05/11/2014 11:15 pm »
Why 15m? The bigger you go, the higher your ground handling cost and difficulties are. 10m seems big enough.

Keep in mind that an oversized vehicle doesn't help if your launch rate isn't gift enough.

Offline GalacticIntruder

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #667 on: 05/12/2014 01:59 am »
Kind of chicken or egg. We don't know what the MCT total mass and size is, so we don't know what capability might be needed to launch it.  I would guess squatty is better, since I posit that MCT has to be ginormous, like A380 size!

I guess around 150mT reusable single core, but I think that is too small for their ambitions, unless they have a Heavy version after that. I favor single core over tri core. 

It would be safe to assume that it has to be more capable than SLS Block(x) and any Chinese or Russia paper rocket.
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Offline Burninate

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #668 on: 05/12/2014 07:14 am »
Kind of chicken or egg. We don't know what the MCT total mass and size is, so we don't know what capability might be needed to launch it.  I would guess squatty is better, since I posit that MCT has to be ginormous, like A380 size!

I guess around 150mT reusable single core, but I think that is too small for their ambitions, unless they have a Heavy version after that. I favor single core over tri core. 

It would be safe to assume that it has to be more capable than SLS Block(x) and any Chinese or Russia paper rocket.
Raptor will be used to launch "100 tons to Mars" according to SpaceX.  150mT to LEO is numerically insufficient for that.  ~450mT to LEO is in the right neighborhood though.  So a Heavy is just about right.

Offline SteveKelsey

Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #669 on: 05/12/2014 08:16 am »
original post was

"Thank you Lars_J. For colonisation isn't a high flight rate a given. Musk will want to carry as many fee paying colonists as possible with the assets in use to make it viable."

...but this is an extension to my post because in the original question I did not unpack my thinking sufficiently, apologies for that!

SpaceX favors cost efficiency so it seems likely that the BFR/MCT will be designed to achieve the ultimate objective of Mars colonization and used ‘sub-optimally’ for the initial phase. This is likely to be more efficient than developing an mixed architecture as only one booster and one MCT has to be designed and developed and manufacturing costs for the fleet would be spread over a greater number. This raises the question of how the ultimate objective affects the BFR/MCT design.

Colonist numbers and the optimum Mars launch window will drive the BFR/MCT design. Musk has stated a target colony size of 80,000. Actually he has given higher figures but lets take the more ‘conservative’ number as the target.

The MCT target capacity is 100 colonists, so to get our 80,000 we need 800 launches. 

Lets take a large timeframe for the colonization period, 20 years sounds like a reasonable number for now. The optimal Martian launch window happens every 26 months so lets extend the timeframe a little to accommodate ten launch windows. The length of the launch window is dictated by the BFR/MCT’s capability. Lets say a launch window of two months can be built into the BFR/MCT performance.
Unless I have made some outrageously bad assumptions that suggests 40 launches a month, which usefully redefines ‘busy’. You can extend the launch window by increasing the performance of the BFR/MCT to ease the launch rate but at some point the cost of the extra performance reduces cost efficiency. I am assuming it is not cost efficient to design a system that can launch all year round.  In this admittedly invented context it suggests that the fewer components in the fleet, the easier the launch rate is to accomplish. Achieving efficient throughput outweighs infrastructure cost and architecture cost where volume is high. This argues for a two stage design rather than a Heavy, despite the  additional  cost of the two stage design.

One more question. Is the cost of a 15 meter core launch pad more or less expensive than a 3 x 10 meters core design? I would place a bet that they are evens but then I am a really bad Poker player.
« Last Edit: 05/12/2014 12:30 pm by SteveKelsey »
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Offline Llian Rhydderch

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #670 on: 05/12/2014 01:27 pm »

...  it suggests that the fewer components in the fleet, the easier the launch rate is to accomplish. Achieving efficient throughput outweighs infrastructure cost and architecture cost where volume is high. This argues for a two stage design rather than a Heavy, despite the  additional  cost of the two stage design.


Interesting observation, Steve.  I'm a bit agnostic on the stages/single-core-vs-tri-core question, but your broader point about cost-efficient throughput trumping infrastructure capital cost and development cost when flight volume is high is a good one for us to keep in mind.

Welcome to the forum.
« Last Edit: 05/12/2014 01:28 pm by Llian Rhydderch »
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Offline 216pi

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #671 on: 05/12/2014 01:58 pm »
original post was

...

Colonist numbers and the optimum Mars launch window will drive the BFR/MCT design. Musk has stated a target colony size of 80,000. Actually he has given higher figures but lets take the more ‘conservative’ number as the target.

The MCT target capacity is 100 colonists, so to get our 80,000 we need 800 launches. 

...

According to his tweet Elon targets 80000 colonists per year : https://twitter.com/elonmusk/status/273483420468932608

Offline SteveKelsey

Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #672 on: 05/12/2014 03:48 pm »
Thank Llian, been lurking longtime : )

Hi 216pi

He did make that claim which I ignored as it seemed extreme. I don't know how his claim squares with the 26 month optimal launch window for Mars. A year round launch approach would make such a large volume 'manageable'  but how would this square with a reasonable design performance for the BFR/MCT?
If using the optimal launch window that implies a crazy launch rate and a substantially larger fleet to meet demand, which loads costs. Neither scenario seems optimal from a system designed to meet the costs which are needed to make colonisation possible. Some new thinking is required which admittedly SpaceX seem to be quite good at.
« Last Edit: 05/12/2014 03:50 pm by SteveKelsey »
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Offline ImUtrecht

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #673 on: 05/12/2014 08:27 pm »

...  it suggests that the fewer components in the fleet, the easier the launch rate is to accomplish. Achieving efficient throughput outweighs infrastructure cost and architecture cost where volume is high. This argues for a two stage design rather than a Heavy, despite the  additional  cost of the two stage design.


Interesting observation, Steve.  I'm a bit agnostic on the stages/single-core-vs-tri-core question, but your broader point about cost-efficient throughput trumping infrastructure capital cost and development cost when flight volume is high is a good one for us to keep in mind.

EM said that the BFR will be based on the Facon Heavy design.

Welcome to the forum.

Offline GregA

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #674 on: 05/12/2014 11:29 pm »
I feel confident in saying 80,000 a year will take a while :) That comment from EM, and GS comment that in 12 years they'd be looking for many launch sites to move lots of people, does show it's a grand vision. Though it won't start at 80,000 a year.

Given the bigger vision, rather than looking at the MCT is the main goal and theorising a "simpler" pre-colonisation vehicle to prepare the way (as some have done), perhaps the MCT is the simpler vehicle. Something that can be accomplished before the big things like in-orbit construction, and transfers to other ships etc.

On another note:
In this admittedly invented context it suggests that the fewer components in the fleet, the easier the launch rate is to accomplish. Achieving efficient throughput outweighs infrastructure cost and architecture cost where volume is high. This argues for a two stage design rather than a Heavy, despite the  additional  cost of the two stage design.

I'm not sure if this is a different perspective - but I'm trying to imagine the MCT launching from Mars in one configuration, and from Earth in a different configuration. It would seem boosters from Earth is the easiest way to allow for the increased requirements from earth. (But maybe the first stage could look different on each planet? Or one less stage on Mars?)
« Last Edit: 05/12/2014 11:30 pm by GregA »

Online guckyfan

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #675 on: 05/13/2014 06:39 am »
I'm not sure if this is a different perspective - but I'm trying to imagine the MCT launching from Mars in one configuration, and from Earth in a different configuration. It would seem boosters from Earth is the easiest way to allow for the increased requirements from earth. (But maybe the first stage could look different on each planet? Or one less stage on Mars?)

They don't need a launch stage on Mars. The gravity and low atmosphere will allow MCT to be the launch vehicle as well as the Mars departure stage going back to earth. So it is just one vehicle, the MCT that leaves LEO, lands on Mars and then after refuelling goes back to the launch site on earth for servicing and relaunch. That's IMO but to me nothing else fits what Elon Musk has said.


Offline SteveKelsey

Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #676 on: 05/13/2014 12:19 pm »
I agree with that, which still leaves the question of the most cost effective booster system to launch the MCT.
I am still not convinced by the 'Heavy' model.
Moving 80,000 people to Mars, per annum, is an industrial process and subject to the same principles. Cost per seat is driven by fixed costs ( assets and infrastructure costs) and marginal costs ( processing, refurb, logistics etc). A triple core architecture increases both, especially when the further restrictions of peak demand being driven by a limited launch window is considered.
it depends of your perspective. If you are making rockets to turn a profit the modular principle works well, it is sweating your factory assets the right way hence the heavy makes sense. But if your objective is to ship a mass population to Mars it looks different and I would expect the two stage to win out all other things being equal.
« Last Edit: 05/13/2014 12:20 pm by SteveKelsey »
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Online guckyfan

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #677 on: 05/13/2014 12:38 pm »
......... and I would expect the two stage to win out all other things being equal.

You mean two stage to earth orbit with MCT as a payload? I think that wins out only if the launch vehicle is capable of lifting the fully fuelled MCT in one launch. And I doubt that they will have a fully reusable launch vehicle capable of ~500t to LEO. That would require a capability of 800t to 1000t expendable, dwarfing Sea Dragon.

Offline Nilof

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #678 on: 05/13/2014 01:57 pm »
I think it's fairly clear that the MCT will be using depots. They significantly decrease the size of the rocket needed, and they would allow SpaceX to actually benefit from rapid reuse for mars missions. My conservative guess would be something on the order of:

MCT: carries 100 tons payload, has a 75 tons dry mass, and contains 775 tons of propellant when fully loaded. Has an Isp of 340 and a Delta-V of ~5.6 km/s with the full payload, 6.5 km/s with a 50 ton payload, 7.2 km/s with a 25 ton payload, and 8 km/s with no payload.

Launches as the upper stage of a HLV. Depending on payload, is capable of building and refueling the fuel depots it uses between launch windows.

For Mars missions, gets refueled at LEO for:
a) minimum energy transfer: gets fully refueled at the LEO depot, performs a ~4 km/s burn to mars, areocaptures at Mars, and then lands using up to ~1.6 km/s of Delta-v. Comes back with up to 25 tons of payload, by lifting off(4 km/s), performing a burn for Earth (2.5 km/s), and areocapturing, areobraking significantly, and landing at Earth(up to 700 m/s).

b) Mars toss transfer missions: gets fully refueled at LEO, places a 100 ton payload in a Mars transfer orbit with a 4 km/s burn, separates from the payload with ~200 tons of propellant(so ~4km/s delta-v) left. Slows down and lands at the launch site or docks with the depot within a day or two for a second mission.

c) Fast Mars transfer. Essentially the same as the first part of a), but uses an L2 depot and an Earth swingby maneuver to get an extra ~3 km/s of delta-v allowing for a faster trajectory to Mars. Can launch later than a) or b), and thus combines very well with b). May use a third depot in Mars orbit refueled with Martian fuel to get back to Earth faster as well.

The nature of the lower boost stage is something that could vary. My guess would be that in regular operation it would operate as a two stage to orbit with a payload in the 50-60 ton range. For a Mars mission with a larger payload requirements it may use a heavy configuration with side boosters, or simply launch as a TSTO twice splitting the payload up if possible.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline GregA

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Re: SpaceX FX/FXX/BFR Speculation Thread.
« Reply #679 on: 05/13/2014 02:10 pm »
I think it's fairly clear that the MCT will be using depots. They significantly decrease the size of the rocket needed, and they would allow SpaceX to actually benefit from rapid reuse for mars missions.
I do think the MCT needs a full tank of fuel in LEO. That either means refueling in LEO, or not using any fuel in the MCT to get there. Could 4 boosters (that do a full re-entry and landing) leave a full MCT in LEO?

The next question then is just how much fuel the MCT can hold. A full central core could get up quite some speed to Mars, and stop, and land… but my thinking there is stuck (wrongly) on the current "picture" of the BFR core.

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