Author Topic: MCT Speculation and Discussion Thread 4  (Read 621553 times)

Offline RotoSequence

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Re: MCT Speculation and Discussion Thread 4
« Reply #1940 on: 04/20/2016 11:13 AM »

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Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1942 on: 04/20/2016 06:42 PM »
BFS?

MCT = BFR + BFS.

People previously assumed just the Mars stage was the "MCT", but apparently Musk uses that name for the whole system. IMO, it's less confusing for everyone to adopt Musk's nomenclature, since he's not likely to adopt ours; unfortunately most posters persist in using "MCT" for just the BFS.

Offline RanulfC

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Re: MCT Speculation and Discussion Thread 4
« Reply #1943 on: 04/20/2016 08:41 PM »
"Best Friend Spaceship?" No, I suspect down that path we'd end up somewhere dark and dangerous... Spaceship Friendship is Magic and all that... :::Shudder::: ;)

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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1944 on: 04/21/2016 12:28 AM »
It may not be part of the existing design plan, but why couldn't MCT have some temporarily expandable section, a la Bigelow, which could be temporarily expanded during transit to Mars in order to provide necessary interior space during the months-long journey, and which could then be un-expanded or jettisoned before Mars arrival?
Because 1) un-expanding is problematic. Needs to be carefully refolded. 2) jettisoning it makes it expensive.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1945 on: 04/21/2016 01:17 AM »
BFS?

MCT = BFR + BFS.

People previously assumed just the Mars stage was the "MCT", but apparently Musk uses that name for the whole system. IMO, it's less confusing for everyone to adopt Musk's nomenclature, since he's not likely to adopt ours; unfortunately most posters persist in using "MCT" for just the BFS.

I remember back a year or so ago when people argued that Elon's MCT nomenclature implied a single monolithic vehicle which was clearly absurd.  Unfortunately most speculation today is hardly much improved with a compulsive desire to make simplistic and monstrously large 'direct' approaches.

Offline Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #1946 on: 04/21/2016 02:14 AM »
BFS?

MCT = BFR + BFS.

People previously assumed just the Mars stage was the "MCT", but apparently Musk uses that name for the whole system. IMO, it's less confusing for everyone to adopt Musk's nomenclature, since he's not likely to adopt ours; unfortunately most posters persist in using "MCT" for just the BFS.

I remember back a year or so ago when people argued that Elon's MCT nomenclature implied a single monolithic vehicle which was clearly absurd.  Unfortunately most speculation today is hardly much improved with a compulsive desire to make simplistic and monstrously large 'direct' approaches.
They still do, you know? They think the development will cost less because it is only one vehicle design.
« Last Edit: 04/21/2016 02:16 AM by Pipcard »

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #1947 on: 04/21/2016 08:22 AM »
And yet Musk himself has said that the concept is a bit off the wall (cannot remember exact phrase). Just building a big rocket is not 'off the wall'.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1948 on: 04/21/2016 05:38 PM »
Landing the same spacecraft on Mars that was launched from Earth and returning it to Earth or LEO would be "off the wall".  Even with LEO refueling before Mars transit.
I think there will be additional "enhancements" in the plan.
Elon says 2025, but in Elon time that means mid 2030s, even his 2:1 schedule slip would be fantastic.
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Offline sanman

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Re: MCT Speculation and Discussion Thread 4
« Reply #1949 on: 04/21/2016 10:38 PM »
Landing the same spacecraft on Mars that was launched from Earth and returning it to Earth or LEO would be "off the wall".  Even with LEO refueling before Mars transit.
I think there will be additional "enhancements" in the plan.
Elon says 2025, but in Elon time that means mid 2030s, even his 2:1 schedule slip would be fantastic.

Meh, a guy like him won't be satisfied with waiting until the 2030s - and SpaceX will be continually ramping its talent and expertise over the next 10 years - so how will SpaceX maintain the buzz if they don't keep making steady progress toward Mars? MCT and Mars colonization are the brass ring or the carrot that Musk keeps holding out in front of everyone, so he can't afford to lose cred on that.

In the meantime, I'd like to ask if MCT would make use of Propellant Densification, or would that be too difficult to manage for methalox, given that it also has to be refueled and launched from the Mars environment?
Would it be possible to do Propellant Densification just for the Earth-Mars leg, and go with regular undensified propellant for the return journey from Mars to Earth? (Then later on, as Mars-side infrastructure builds up, you could upgrade to Propellant Densification for Mars liftoff as well.)





Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1950 on: 04/21/2016 10:57 PM »
I like the MCT with the 8 engines around.  I still think that sending 10-20 people with cargo would be the norm for several years in order to build the colony.  Lots of equipment, habitats, greenhouse units, roving vehicles, earth moving vehicles, and lots of solar panels to power everything, etc, to unload.  I also think they will not send 100 people and 100 tons of cargo to Mars on one spacecraft, but far more cargo than people initially.  Like Musk said, 10 cargo flights for every 100 people.  So, why not put 10 people on each cargo flight?  Then some type of LES capsule could be at the top, and if something happened on the way, only 10 people would be in danger instead of 100. 

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1951 on: 04/22/2016 02:07 AM »
BFS?

MCT = BFR + BFS.

People previously assumed just the Mars stage was the "MCT", but apparently Musk uses that name for the whole system. IMO, it's less confusing for everyone to adopt Musk's nomenclature, since he's not likely to adopt ours; unfortunately most posters persist in using "MCT" for just the BFS.

I remember back a year or so ago when people argued that Elon's MCT nomenclature implied a single monolithic vehicle which was clearly absurd.  Unfortunately most speculation today is hardly much improved with a compulsive desire to make simplistic and monstrously large 'direct' approaches.
They still do, you know? They think the development will cost less because it is only one vehicle design.
It probably would. If you're going to do refueling, and you're going to be doing ISRU, and you're going to be building a high-performance TSTO fully reusable HLV, then you basically can skip much the rest. No need for a Battlestar Galactica style architecture with 10 different elements to satisfy the 10 different NASA centers, incorporating everyone's pet technology, etc.

Refueling makes it possible to reuse the same hardware for multiple purposes without ballooning the costs. Several years ago, I realized (with another member... mmeijeri?) that a reusable 5km/s lander/ascender (I think we picked a lander with balloon tanks to improve mass fraction) can do a whole bunch of mission tasks and simplify the overall architecture dramatically, including serving as its own upper stage, etc. Something like this is the logical conclusion of refueling and reuse.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1952 on: 04/22/2016 03:44 AM »
RB:  You completely inverted the meaning of 'Battlestar Galactica'.  That was Zubrin's pejorative term for a single huge vehicle that carries all propellant from Earth which was the NASA plan in the 90 day report.

Zubrin's argument was two fold, use insitu propellant on mars to reduce outbound propellant needs (which we all agree Musk is doing) AND using a number of smaller modular elements that serve specific roles and provide redundancy.  The Battlestart Galactica term was specifically about not making a single all inclusive budget busting vehicle designed to perform multiple tasks.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1953 on: 04/22/2016 04:22 AM »
Another thing that virtually no one here talks about is MARS ORBITAL RENDEZVOUS.

If the BFS can be refueled in LEO then it can likewise be refueled in LMO if propellant can be brought their.

This would break the DeltaV budget for Earth return into two legs, assent to LMO and then TEI resulting in a vastly smaller vehicle AND a faster Earth return then would be possible with a single direct launch of even a huge vehicle.

To make the vehicle capably of departing for Mars it must start at EML-1 fully fueled which again will allow for a faster transit then a large vehicle starting in LEO.

To get the propellants to LMO and the vehicle to EML-1 you use the same solution a SEP tug, fist it moves the BFS to EML-1, picks up and drops off fuel between LEO and EML-1, then makes a fuel run all the way out to mars to rendezvous with the BFS a second time and finally returns to Earth to repeat the cycle.

The BFS would only need around 4-5 km/s DeltaV capability in this scenario which makes it hugely smaller and simpler, to launch to LEO the BFR is a 2 stage rocket like F-9 but with reusable 2nd stage which can carry payloads other then the BFS on top.

Offline jsgirald

Re: MCT Speculation and Discussion Thread 4
« Reply #1954 on: 04/22/2016 12:58 PM »
The Battlestar Galactica thing made me think and put some numbers, just plug data in the rocket equation to see what happens. I'm assuming the 'land the whole thing' architecture because, in order to transport a lot of cargo and people to Mars, you need a big and heavy lander anyway, so it does make sense to use the same ship to make the interplanetary part as well. I'm also assuming a pure chemical rocket propulsion system, although G Shotwell mentioned other possibilities being studied.

As a reference I used Saturn V (I'm taking Apollo XVII numbers), obviously the only comparable succesfull system so far, yes the Nova designs where more like it, but they remained paper rockets, no ppt in those happy days.

Plugging the Saturn IC data in the rocket equation shows that deltaV was just over 3.3 Km/s, it also staged at a pretty low altitude (good for the RTLS thing). Let's assume that the MCT booster does likewise, that leaves about 7 Km/s deltaV to the second stage (aka BFS) which is ok if you plan to refuel and do a TMI burn.

As per word of Musk, the ship will be capable of putting 100 t of useful mass on Mars, so let's make it another 100 t for structure (engines, TPS, legs, you name it). So that leaves a 200 t dry mass vehicle, capable of 7 km/s deltaV, with engines giving a Isp of 380 s you need a whopping 1150 t of propellant and a total BFS mass at lift off of about 1350 t. This also pushes the BFR mass to 3200 t, more than a Saturn V GLOW.

Indeed, it's not a small ship, so the term Battlestar does apply here.
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1955 on: 04/22/2016 02:05 PM »
Another thing that virtually no one here talks about is MARS ORBITAL RENDEZVOUS.

If the BFS can be refueled in LEO then it can likewise be refueled in LMO if propellant can be brought their.

This would break the DeltaV budget for Earth return into two legs, assent to LMO and then TEI resulting in a vastly smaller vehicle AND a faster Earth return then would be possible with a single direct launch of even a huge vehicle.

To make the vehicle capably of departing for Mars it must start at EML-1 fully fueled which again will allow for a faster transit then a large vehicle starting in LEO.

To get the propellants to LMO and the vehicle to EML-1 you use the same solution a SEP tug, fist it moves the BFS to EML-1, picks up and drops off fuel between LEO and EML-1, then makes a fuel run all the way out to mars to rendezvous with the BFS a second time and finally returns to Earth to repeat the cycle.

The BFS would only need around 4-5 km/s DeltaV capability in this scenario which makes it hugely smaller and simpler, to launch to LEO the BFR is a 2 stage rocket like F-9 but with reusable 2nd stage which can carry payloads other then the BFS on top.

Questions arise...
How long does the SEP tug take to bring the BFS and/or the propellant to EML-1 and return to LEO? 
How many SEP trips to EML-1 per single BFS launch to Mars and return?
How efficient are SEP solar panels after numerous long trips thru the Van Allen belts? 
How many BFTanker flights to re-fuel the SEP tug itself for its various transits to support one Mars round trip mission?  Including bringing up the BFS propellant that the SEP carries all the way to Mars orbit.
Besides the added expense, resources and complexity of developing the SEP tug.
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Re: MCT Speculation and Discussion Thread 4
« Reply #1956 on: 04/22/2016 02:18 PM »
Another thing that virtually no one here talks about is MARS ORBITAL RENDEZVOUS.

If the BFS can be refueled in LEO then it can likewise be refueled in LMO if propellant can be brought their.

This would break the DeltaV budget for Earth return into two legs, assent to LMO and then TEI resulting in a vastly smaller vehicle AND a faster Earth return then would be possible with a single direct launch of even a huge vehicle.

To make the vehicle capably of departing for Mars it must start at EML-1 fully fueled which again will allow for a faster transit then a large vehicle starting in LEO.

To get the propellants to LMO and the vehicle to EML-1 you use the same solution a SEP tug, fist it moves the BFS to EML-1, picks up and drops off fuel between LEO and EML-1, then makes a fuel run all the way out to mars to rendezvous with the BFS a second time and finally returns to Earth to repeat the cycle.

The BFS would only need around 4-5 km/s DeltaV capability in this scenario which makes it hugely smaller and simpler, to launch to LEO the BFR is a 2 stage rocket like F-9 but with reusable 2nd stage which can carry payloads other then the BFS on top.
Orbital rendezvous and refueling does make things more mass efficient, but I think ferrying fuel from earth may not be the right approach. Instead consider a tanker variant of the BFS which is sent to Mars and uses local ISRU to loft fuel to LMO for refueling the returning BFS.
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Re: MCT Speculation and Discussion Thread 4
« Reply #1957 on: 04/22/2016 02:39 PM »
Another thing that virtually no one here talks about is MARS ORBITAL RENDEZVOUS.

If the BFS can be refueled in LEO then it can likewise be refueled in LMO if propellant can be brought their.

This would break the DeltaV budget for Earth return into two legs, assent to LMO and then TEI resulting in a vastly smaller vehicle AND a faster Earth return then would be possible with a single direct launch of even a huge vehicle.

To make the vehicle capably of departing for Mars it must start at EML-1 fully fueled which again will allow for a faster transit then a large vehicle starting in LEO.

To get the propellants to LMO and the vehicle to EML-1 you use the same solution a SEP tug, fist it moves the BFS to EML-1, picks up and drops off fuel between LEO and EML-1, then makes a fuel run all the way out to mars to rendezvous with the BFS a second time and finally returns to Earth to repeat the cycle.

The BFS would only need around 4-5 km/s DeltaV capability in this scenario which makes it hugely smaller and simpler, to launch to LEO the BFR is a 2 stage rocket like F-9 but with reusable 2nd stage which can carry payloads other then the BFS on top.
Orbital rendezvous and refueling does make things more mass efficient, but I think ferrying fuel from earth may not be the right approach. Instead consider a tanker variant of the BFS which is sent to Mars and uses local ISRU to loft fuel to LMO for refueling the returning BFS.

Is LMO refueling likely to be necessary? Musk has stated that the Mars return payload will be on the order of 25t. Staging velocity to LEO and LEO to TMI with 100t are both considerable more constraining than Mars surface direct return of 25t. Any ship that can sent 100t through TMI (about 3.6 kms dV) or from BFR staging to LEO (at least 4.5 kms dV) will be able to easily send 25t from Mars surface to TEI (about 5.3 kms dV but 1/4 the payload).

The most constraining leg seems to be getting from BFR staging velocity to LEO with 100t of payload.

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Re: MCT Speculation and Discussion Thread 4
« Reply #1958 on: 04/22/2016 03:30 PM »
The Battlestar Galactica thing made me think and put some numbers, just plug data in the rocket equation to see what happens. I'm assuming the 'land the whole thing' architecture because, in order to transport a lot of cargo and people to Mars, you need a big and heavy lander anyway, so it does make sense to use the same ship to make the interplanetary part as well. I'm also assuming a pure chemical rocket propulsion system, although G Shotwell mentioned other possibilities being studied.

As a reference I used Saturn V (I'm taking Apollo XVII numbers), obviously the only comparable succesfull system so far, yes the Nova designs where more like it, but they remained paper rockets, no ppt in those happy days.

Plugging the Saturn IC data in the rocket equation shows that deltaV was just over 3.3 Km/s, it also staged at a pretty low altitude (good for the RTLS thing). Let's assume that the MCT booster does likewise, that leaves about 7 Km/s deltaV to the second stage (aka BFS) which is ok if you plan to refuel and do a TMI burn.

As per word of Musk, the ship will be capable of putting 100 t of useful mass on Mars, so let's make it another 100 t for structure (engines, TPS, legs, you name it). So that leaves a 200 t dry mass vehicle, capable of 7 km/s deltaV, with engines giving a Isp of 380 s you need a whopping 1150 t of propellant and a total BFS mass at lift off of about 1350 t. This also pushes the BFR mass to 3200 t, more than a Saturn V GLOW.

Indeed, it's not a small ship, so the term Battlestar does apply here.

SpaceX pushes mass fractions to the limit so a 100t empty mass is probably conservative, but lets go with that. However, Saturn V was a 3 stage to orbit system, so the comparisons to a TSTO aren't really accurate.

BFR will likely have mass fractions much closer to Falcon 9 than Saturn V, but with a better ISP than either. Falcon 9 stages at around 2.2 to 2.5 kms, and the upper stage adds about 5.3 kms to get it to LEO. With Raptor's higher ISP, it only takes about 625t of prop to get a 200t ship+payload into LEO.

If they can stage at 3.5 kms (with enough prop in the BFR to boostback to reentry at a survivable Mach 6 or 2 kms), then the upper stage/ship only has to add about 4.5 kms to LEO, which happens to be about the same performance it needs to sent 100t through TMI and do a 1 kms (ish) EDL burn at Mars. That only takes about 500t of prop.

The other question is how they will want to size the Raptor. At the currently stated 2300kN a single Raptor would be rather underpowered for throwing 100t into LEO, although probably not worse than the Centaur US. A single 2300 kN engine would be ideal for TMI and Mars return though, and dragging extra engines to Mars and back isn't ideal.

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Re: MCT Speculation and Discussion Thread 4
« Reply #1959 on: 04/22/2016 04:36 PM »

The other question is how they will want to size the Raptor. At the currently stated 2300kN a single Raptor would be rather underpowered for throwing 100t into LEO, although probably not worse than the Centaur US. A single 2300 kN engine would be ideal for TMI and Mars return though, and dragging extra engines to Mars and back isn't ideal.

You're going to need multiple engines to propulsively land BFS back at KSC, Brownsville or wherever, which is absolutely critical for the economics of the architecture. I'm assuming extendable/discardable nozzle extensions on the BFS raptor, which may be a distinctive variant from the initial upper stage raptor vac.

Besides, if I was the commander of a 100 individual interplanetary spaceship, I'd either want engines with a ludicrous reliability or a few with a wide gimbal so I have engine out redundancy. Nobody wants to be marooned.

They are going to be carting a few unnecessary engines with them, I bet the most edible of my cowskin hats on it.
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