Author Topic: MCT Speculation and Discussion Thread 4  (Read 625568 times)

Online Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #2740 on: 09/09/2016 04:06 AM »
[Keep in mind, I am still both excited and skeptical at the same time about MCT. But I am talking about theoretical plans here.]

I know that Musk wants to do a Mars round trip within a Earth/Mars synodic period (26 months or 780 days) for easy reuse. This would necessitate a short-stay, high delta-v opposition class travel profile, as opposed to a long-stay conjunction class profile.

But for the first mission, the MCT is supposed to be inhabited for the entire journey (presumably, with pre-landed cargo MCTs for in-situ propellant production), unlike the fully operational phase in which MCT is primarily used for one-way colonization (it seems to be designed that way).

An opposition-class first mission would require astronauts to stay in microgravity (if artificial gravity is not involved) and completely exposed to cosmic radiation (unlike the Martian surface which at least offers some protection) for the return trip (which would last about a year, unlike the 100-day trip to Mars). So might the first mission be conjunction-class?
« Last Edit: 09/09/2016 04:09 AM by Pipcard »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2741 on: 09/10/2016 11:42 PM »
My predictions are still going against the grain, I think we will see mostly BFR details with only modest information on the actual mars transit and landing and what ever details are revealed of that phase of the mission will be subject to change as SpaceX actually gets landing experience on Mars, it's too soon to completely commit to the mode or design of the mars landing vehicles at this time.

BFR will be narrower then the 15m diameter many have speculated, I think closer to 12m diameter as the thrust density of the Raptor engine will be high and a narrower vehicle makes for a lighter more efficient thrust structure to transmit the thrust of many engines to the tank side walls and makes for lower air resistance on assent as well.  The first stage will be recovered right from the first orbital flight and will be well tested on it's own in sub-orbital flights with a mass-simulating 2nd stage used for testing.

A recoverable second stage with vacuum raptor engines, probably 6 so it has engine out capability, will be used along with a conventional payload fairing to launch large payloads of high volume and make the BFR a direct competitor to SLS.  This 2nd stage will also act as the LEO propellant tanker with just the attachment of a minimal nosecone.  Recovery of the 2nd stage still be slightly experimental at the time the first orbital launches and the fist few launches will be refine the recovery process for the 2nd stage and several may be lost in the process.  The 2nd stage will orbit the Earth one or more times before recovery and might not land at the launch site.

Overall vehicle height is be over 200 ft at the top of the 2nd stage, with payload fairing around 300 ft.  Any mars bound spacecraft will be introduced several years after BFR itself is flying and will not fly the super-direct flight plan of departing from LEO directly to mars surface and then from Mars surface directly back to Earth surface, this requires too much delta-V and propellant production on Mars, rather their will be a refueling step somewhere between likely in a high Earth orbit and/or high Mars orbit.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2742 on: 09/11/2016 03:18 PM »
My predictions are still going against the grain, I think we will see mostly BFR details with only modest information on the actual mars transit and landing and what ever details are revealed of that phase of the mission will be subject to change as SpaceX actually gets landing experience on Mars, it's too soon to completely commit to the mode or design of the mars landing vehicles at this time.

Agree completely.  MCT/BFS will be more conceptual and under development than big 1st stage.

BFR will be narrower then the 15m diameter many have speculated, I think closer to 12m diameter as the thrust density of the Raptor engine will be high and a narrower vehicle makes for a lighter more efficient thrust structure to transmit the thrust of many engines to the tank side walls and makes for lower air resistance on assent as well.  The first stage will be recovered right from the first orbital flight and will be well tested on it's own in sub-orbital flights with a mass-simulating 2nd stage used for testing.

Believe that # engines and room for architectural growth makes > 12m more likely.  I say closer to 15m than 12m.
Do the tooling expense once and use it "forever".

Stage one will be recovered from the get go and may actually fly with less than the standard # of engines for 1st test flights.

A recoverable second stage with vacuum raptor engines, probably 6 so it has engine out capability, will be used along with a conventional payload fairing to launch large payloads of high volume and make the BFR a direct competitor to SLS.  This 2nd stage will also act as the LEO propellant tanker with just the attachment of a minimal nosecone.  Recovery of the 2nd stage still be slightly experimental at the time the first orbital launches and the fist few launches will be refine the recovery process for the 2nd stage and several may be lost in the process.  The 2nd stage will orbit the Earth one or more times before recovery and might not land at the launch site.

Good case for 6th engine for engine out capability.  Mostly needed on launch to LEO.

If not landing @ launch site which is certainly possible, it will have to be someplace where water transport can move this expensive 12m+ asset back to the launch site.  Launch site will be the preferred landing site.

Overall vehicle height is be over 200 ft at the top of the 2nd stage, with payload fairing around 300 ft.  Any mars bound spacecraft will be introduced several years after BFR itself is flying and will not fly the super-direct flight plan of departing from LEO directly to mars surface and then from Mars surface directly back to Earth surface, this requires too much delta-V and propellant production on Mars, rather their will be a refueling step somewhere between likely in a high Earth orbit and/or high Mars orbit.

Reasonable height.  Still under 100m.

Agree couple years of BFR flights before 2nd stage ready for Mars transit.

Although the big risk/problem is making sufficient qty of propellant on Mars, sending "tanker" flights to Mars orbit is very resource consuming (read expensive) and leaves you with empty tanker(s) stuck in Mars orbit.  Sure they could be landed IF they had heat shielding and some propellant left, but then they too need surface and orbital refueling.  I think tankers would not have propellant expensive wasted mass heat shielding, so landing these near empty tankers on Mars is a moot point.

Glad to read good specific predictions before the IAC talk.
Anyone else have predictions/updates?
« Last Edit: 09/11/2016 03:27 PM by philw1776 »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2743 on: 09/11/2016 10:03 PM »
Thx 4 reply philw1776 I have a few counter and clarifying points.


Planning for future upgrades to BFR is interesting, but I'm doubtful that a vehicles diameter is made larger then necessary to allow for the insertion of extra engines at a later date.  First the extra engines and thrust would require a complete redesign of the plumbing structures and thrust structures which is a significant part of the first stages total design.

Second the traditional means to add more engines is with boosters and I could see the addition of say 6 Falcon boosters much like thouse from FH each running 4 Raptors each, for a total of 24 additional engines as a more straightforward way to make a super powerful version of BFR.  I expect the falcon family to eventually be converted to Raptors, plus a small landing engine, for it's own reasons so this booster will be available to BFR as the path of least resistance.

For 2nd stage landing, the launch site is certainly the preferred location but because the 2nd stage can not possibly boost back to the launch site and must instead do at least one Earth orbit the ground track may be several hundred miles away from the launch site.  So the vehicle either needs to have a lot of cross-range when doing a once around entry like the shuttle, or it needs to remain in orbit until ground track intersects the launch site again, that requires a longer on orbit life-span.  Which will be simpler I'm not sure and as I expect the 2nd stage to be getting developed while in service some transition may occur during development.  If the landing is not at the launch site then your right it had better be coastal so the stage can be put on a barge for transport back to the launch site.

Lastly I believe it may be possible to send tankers from Earth to a high mars orbit, have them offload propellants and still return to Earth orbit either via chemical propulsion or SEP so all vehicles are recovered.  This would allow the Mars landing and relaunch vehicles to be much lighter at liftoff from Mars, using something like 1/3rd as much propellant, then refill in orbit and make a fast return to Earth. 

The exact details of how and where refueling occurs aren't clear yet but what I'm sure of is that the direct return approach is not viable, the fixation many have with it is analogous to the fixation on direct earth return in the early days of Apollo, it's attractive for it's simplicity but it a bridge too far in a lot of engineering areas.

Online KelvinZero

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Re: MCT Speculation and Discussion Thread 4
« Reply #2744 on: 09/12/2016 03:57 AM »
Agree completely.  MCT/BFS will be more conceptual and under development than big 1st stage.
...
Anyone else have predictions/updates?
My totally uninformed and unsubstantiated pet hope is a sudden plan for a falcon heavy replacement:
How short could you plausibly make the full-diameter BFR? Im just thinking a lot shorter and symmetrically remove some engines.
If it is only needs 12 meter diameter instead of 15 that is great. However dumb my idea was at 15m, it is probably a bit less dumb at 12m :)

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2745 on: 09/12/2016 06:46 AM »
Agree completely.  MCT/BFS will be more conceptual and under development than big 1st stage.
...
Anyone else have predictions/updates?
My totally uninformed and unsubstantiated pet hope is a sudden plan for a falcon heavy replacement:
How short could you plausibly make the full-diameter BFR? Im just thinking a lot shorter and symmetrically remove some engines.
If it is only needs 12 meter diameter instead of 15 that is great. However dumb my idea was at 15m, it is probably a bit less dumb at 12m :)
I'm not sure how valid this is, but the tanks have to be some shape.  And that shape will probably have hemispherical endcaps for mass efficiency.   You can't make a tank with hemispherical endcaps smaller than a sphere, and it is far easier to build a cylinder than other shapes.  If this is a crippling constraint, it seems like you probably have to use multiple smaller subscale tanks.

For monolithic common-wall tankage, though, a synergy shows up at approximately:
~4000 ton S1 mass
3.8kg LOX to 1kg LNG
7.5m radius spherical LNG tank, sharing a wall with a
7.5m radius, 15m height cylindrical-with-hemispherical-endcap LOX tank (volume calcs are cylindrical)

~2000 ton S2 mass
3.8kg LOX to 1kg LNG
6m radius spherical LNG tank, sharing a wall with a
6m radius, 12m height cylindrical-with-hemispherical-endcap LOX tank (volume calcs are cylindrical)
51.2% the volume of S1 based on simple geometry

Problems unresolved:
* In a common thermal environment, a LOX tank will be higher pressure than an LNG tank.  Will this work with the common wall under compressive stress, or is the common wall on the wrong way around?
* Are multiple tanks (probably 2.5m radius tanks) going to be easier than a monolithic tank?
* Does the thin-wall pressure vessel approximation (Ideal pressure vessel mass scales directly with mass contained at the same pressure) hold at either scale, or are the walls instead dictated by a minimum gauge or some other constraint?  Does it hold at one third scale (2.5m radius)?  If it did we would be dealing with seven long cylinders of the same mass as a monolithic tank.
* Will the slosh on a 7.5m radius tank, especially a 7.5m radius spherical tank, be manageable through a fairly high-rotation 'swan dive' maneuver?
« Last Edit: 09/12/2016 07:01 AM by Burninate »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2746 on: 09/12/2016 08:51 AM »
I think it is likely that the first stage tank bottom will be flattened or perhaps be a Diffuser head https://en.wikipedia.org/wiki/Head_(vessel), aka concave like the bottom of a soda can.

This would provide room for plumbing between the thrust structure and tank as well as allowing the thrust structure to be very close to the tanks bottom rim again minimizing it's size and mass.

The second stage would likely just have the normal tank shape because it's smaller engine count would mean the engines are basically all along the periphery and the thrust structure can just be a ring.

 In any case perfect hemispheres aren't the only option for tank bottoms or tops, modified ellipsoids are common.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2747 on: 09/12/2016 05:20 PM »
Thx 4 reply philw1776 I have a few counter and clarifying points.

Lastly I believe it may be possible to send tankers from Earth to a high mars orbit, have them offload propellants and still return to Earth orbit either via chemical propulsion or SEP so all vehicles are recovered.  This would allow the Mars landing and relaunch vehicles to be much lighter at liftoff from Mars, using something like 1/3rd as much propellant, then refill in orbit and make a fast return to Earth. 

The exact details of how and where refueling occurs aren't clear yet but what I'm sure of is that the direct return approach is not viable, the fixation many have with it is analogous to the fixation on direct earth return in the early days of Apollo, it's attractive for it's simplicity but it a bridge too far in a lot of engineering areas.

Gwynne has said, "We're looking at SEP" so this is a possibility.
On the downside if it's SEP, it is yet another vehicle to be developed and tested.

I haven't looked deeply at delta Vs, masses and run the rocket equation to comment cogently on the feasibility of SEP, etc. tankers to HMO & back.  I'm certain that SX has done so in their architectural bakeoffs.
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Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2748 on: 09/12/2016 06:32 PM »
As the Big Reveal is nearly upon us (hopefully!), I think that someone (not me) should generate an official Mars MCT Architecture Prediction contest.  This would be composed of a list of questions with multiple choice answers, with a fixed value for each question.  Prizes TBD.

One problem is that L2 members may have 'insider information' on this topic.  So I guess they will have to be just guided by their own conscience as to their level of participation.

Anyway, I'll kick it off with a few suggested questions (but will be happy for someone else to officially take this over and come up with the canonical list, as well as being judge, jury, etc.)

1) Overall Launch Architecture
     a)  MCT is composed simply of a BFR 1st stage and BFS 2nd stage/spacecraft
     b)  Boost phase consists of 2 stages, which put the BFS into orbit
     c)  Other: 3rd stage, 'half' stages, drop tanks, etc.

2) Number of Raptor Engines on BFR (1st stage)

3) Diameter of BFR (1st stage)

4) Total Raptor 1st stage thrust (sl)

5) LAS Architecture
     a) No LAS - BFS is the escape mechanism
     b) Traditional LAS - above BFS and is nominally jettisoned during launch phase
     c) BFS contains smaller 'ejection pod' where humans reside during launch
     d) Other, non-traditional LAS design

6) Shape and Landing Mode of BFS
     a) Capsule (perhaps elongated), w/ TPS on base
     b) Cylindrical or biconic - horizontal landing
     c) Cylindrical or biconic - vertical landing
     d) Other

7) Mars and Earth return
     a) BFS does direct entry into Mars and Earth atmosphere
     b) BFS does orbital capture before performing entry burn and landing
     c) Same as b, but upon Earth return, stays in orbit for next synod

8)  Use of non-chemical thrust
     a) Not part of the plan
     b) Will use SEP for some/all of the big transits
     c) All chemical for now, but plans to incorporate SEP down the road


Can anyone think of more/better questions?

Offline Bob Shaw

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Re: MCT Speculation and Discussion Thread 4
« Reply #2749 on: 09/12/2016 06:40 PM »
Thx 4 reply philw1776 I have a few counter and clarifying points.


Planning for future upgrades to BFR is interesting, but I'm doubtful that a vehicles diameter is made larger then necessary to allow for the insertion of extra engines at a later date.  First the extra engines and thrust would require a complete redesign of the plumbing structures and thrust structures which is a significant part of the first stages total design.

Actually, that is *exactly* the approach taken with the S-1C in the days of Saturn development!

Offline DJPledger

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Re: MCT Speculation and Discussion Thread 4
« Reply #2750 on: 09/12/2016 07:06 PM »
As the Big Reveal is nearly upon us (hopefully!), I think that someone (not me) should generate an official Mars MCT Architecture Prediction contest.  This would be composed of a list of questions with multiple choice answers, with a fixed value for each question.  Prizes TBD.

One problem is that L2 members may have 'insider information' on this topic.  So I guess they will have to be just guided by their own conscience as to their level of participation.

Anyway, I'll kick it off with a few suggested questions (but will be happy for someone else to officially take this over and come up with the canonical list, as well as being judge, jury, etc.)

1) Overall Launch Architecture
     a)  MCT is composed simply of a BFR 1st stage and BFS 2nd stage/spacecraft
     b)  Boost phase consists of 2 stages, which put the BFS into orbit
     c)  Other: 3rd stage, 'half' stages, drop tanks, etc.

2) Number of Raptor Engines on BFR (1st stage)

3) Diameter of BFR (1st stage)

4) Total Raptor 1st stage thrust (sl)

5) LAS Architecture
     a) No LAS - BFS is the escape mechanism
     b) Traditional LAS - above BFS and is nominally jettisoned during launch phase
     c) BFS contains smaller 'ejection pod' where humans reside during launch
     d) Other, non-traditional LAS design

6) Shape and Landing Mode of BFS
     a) Capsule (perhaps elongated), w/ TPS on base
     b) Cylindrical or biconic - horizontal landing
     c) Cylindrical or biconic - vertical landing
     d) Other

7) Mars and Earth return
     a) BFS does direct entry into Mars and Earth atmosphere
     b) BFS does orbital capture before performing entry burn and landing
     c) Same as b, but upon Earth return, stays in orbit for next synod

8)  Use of non-chemical thrust
     a) Not part of the plan
     b) Will use SEP for some/all of the big transits
     c) All chemical for now, but plans to incorporate SEP down the road


Can anyone think of more/better questions?
SL thrust of each Raptor on BFR.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2751 on: 09/12/2016 09:01 PM »
Thx 4 reply philw1776 I have a few counter and clarifying points.

Lastly I believe it may be possible to send tankers from Earth to a high mars orbit, have them offload propellants and still return to Earth orbit either via chemical propulsion or SEP so all vehicles are recovered.  This would allow the Mars landing and relaunch vehicles to be much lighter at liftoff from Mars, using something like 1/3rd as much propellant, then refill in orbit and make a fast return to Earth. 

The exact details of how and where refueling occurs aren't clear yet but what I'm sure of is that the direct return approach is not viable, the fixation many have with it is analogous to the fixation on direct earth return in the early days of Apollo, it's attractive for it's simplicity but it a bridge too far in a lot of engineering areas.

Gwynne has said, "We're looking at SEP" so this is a possibility.
On the downside if it's SEP, it is yet another vehicle to be developed and tested.

I haven't looked deeply at delta Vs, masses and run the rocket equation to comment cogently on the feasibility of SEP, etc. tankers to HMO & back.  I'm certain that SX has done so in their architectural bakeoffs.

First I find focusing on vehicle count as a measure of difficulty is a poor metric which I think has led many people astray, when the mission requires a high performance in a number of conflicting areas trying to achieve all these performance values out of one vehicle is usually far more difficult then breaking up the problem using specialized vehicle. 

That has been the lesson of all space travel, it's why rockets have stages, it's why the LEM rather then the command module landed on the moon and it's why I consistently foresee more complex mission modalities then the majority.

Second It's more likely that for retrieving tankers we would be looking at a SEP engine section carried on top of the 2nd stage like a payload and which remains attached to it for the duration of the mission.  Effectively creating a bi-modal-propulsion tanker.  The SEP propellants can used to return the tanker to LEO after it's offloaded all of the chemical propellants in the vicinity of Mars, the low dry mass at that point and the fact their is no propellant to chill means all power can be directed to the engines so the SEP unit dose not need to be very large at all. 

Alternatively if your willing to just throw chemical propellant at the problem, two tankers full in LEO should be able to go to high mars orbit and offload propellant to a manned return and then themselves return, though they would face high entry velocities.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2752 on: 09/13/2016 10:24 AM »
As the Big Reveal is nearly upon us (hopefully!), I think that someone (not me) should generate an official Mars MCT Architecture Prediction contest.  This would be composed of a list of questions with multiple choice answers, with a fixed value for each question.  Prizes TBD.

One problem is that L2 members may have 'insider information' on this topic.  So I guess they will have to be just guided by their own conscience as to their level of participation.

Anyway, I'll kick it off with a few suggested questions (but will be happy for someone else to officially take this over and come up with the canonical list, as well as being judge, jury, etc.)

1) Overall Launch Architecture
     a)  MCT is composed simply of a BFR 1st stage and BFS 2nd stage/spacecraft
     b)  Boost phase consists of 2 stages, which put the BFS into orbit
     c)  Other: 3rd stage, 'half' stages, drop tanks, etc.

2) Number of Raptor Engines on BFR (1st stage)

3) Diameter of BFR (1st stage)

4) Total Raptor 1st stage thrust (sl)

5) LAS Architecture
     a) No LAS - BFS is the escape mechanism
     b) Traditional LAS - above BFS and is nominally jettisoned during launch phase
     c) BFS contains smaller 'ejection pod' where humans reside during launch
     d) Other, non-traditional LAS design

6) Shape and Landing Mode of BFS
     a) Capsule (perhaps elongated), w/ TPS on base
     b) Cylindrical or biconic - horizontal landing
     c) Cylindrical or biconic - vertical landing
     d) Other

7) Mars and Earth return
     a) BFS does direct entry into Mars and Earth atmosphere
     b) BFS does orbital capture before performing entry burn and landing
     c) Same as b, but upon Earth return, stays in orbit for next synod

8)  Use of non-chemical thrust
     a) Not part of the plan
     b) Will use SEP for some/all of the big transits
     c) All chemical for now, but plans to incorporate SEP down the road


Can anyone think of more/better questions?
SL thrust of each Raptor on BFR.

Implied by the combination of questions 2 and 4.  But maybe this would be a better question than #4.

Online Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #2753 on: 09/13/2016 06:24 PM »
First I find focusing on vehicle count as a measure of difficulty is a poor metric which I think has led many people astray, when the mission requires a high performance in a number of conflicting areas trying to achieve all these performance values out of one vehicle is usually far more difficult then breaking up the problem using specialized vehicle. 

That has been the lesson of all space travel, it's why rockets have stages, it's why the LEM rather then the command module landed on the moon and it's why I consistently foresee more complex mission modalities then the majority.
I thought this too, and you still have a point about conflicting performance requirements, but the reason why Apollo was broken up into separate spacecraft instead of a direct ascent architecture was because expensive expendable rockets forced constraints on mass, so there were compromises.

The difference is that MCT speculators assume that a fully reusable launcher would reduce costs such that
IMLEO is no longer the primary constraint

The idea (with an all-chemical MCT) is that only a single monolithic vehicle needs to be developed, refueled by tankers (of mostly the same design) in Earth orbit before going to Mars (where it gets refueled by pre-placed ISRU before going back to Earth).
« Last Edit: 09/13/2016 11:14 PM by Pipcard »

Online KelvinZero

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Re: MCT Speculation and Discussion Thread 4
« Reply #2754 on: 09/15/2016 09:23 AM »
Gwynne has said, "We're looking at SEP" so this is a possibility.
On the downside if it's SEP, it is yet another vehicle to be developed and tested.
Random thought: What about a vehicle that can do missions with small crews without SEP, but you have a plan to add SEP later (as a stage, cycler or L2 propellant tug, whatever) when you have multiple flights to spread the costs over and want closer to that 500k/passenger goal, and also to achieve that fast return to earth for immediate reuse?

« Last Edit: 09/15/2016 09:25 AM by KelvinZero »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2755 on: 09/15/2016 01:57 PM »
Gwynne has said, "We're looking at SEP" so this is a possibility.
On the downside if it's SEP, it is yet another vehicle to be developed and tested.
Random thought: What about a vehicle that can do missions with small crews without SEP, but you have a plan to add SEP later (as a stage, cycler or L2 propellant tug, whatever) when you have multiple flights to spread the costs over and want closer to that 500k/passenger goal, and also to achieve that fast return to earth for immediate reuse?
Sure, just change how many tankers are used to refuel (& where) the BFS in orbit. SEP can be to push the fuel to a higher energy orbit before refueling.
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2756 on: 09/15/2016 08:36 PM »
Gwynne has said, "We're looking at SEP" so this is a possibility.
On the downside if it's SEP, it is yet another vehicle to be developed and tested.
Random thought: What about a vehicle that can do missions with small crews without SEP, but you have a plan to add SEP later (as a stage, cycler or L2 propellant tug, whatever) when you have multiple flights to spread the costs over and want closer to that 500k/passenger goal, and also to achieve that fast return to earth for immediate reuse?
Sure, just change how many tankers are used to refuel (& where) the BFS in orbit. SEP can be to push the fuel to a higher energy orbit before refueling.

Staging fuel at EML1/2 or similar HEO with SEP would cut refueling launches from Earth by about 50% for crewed flights.

SEP tugs could also put cargo landers directly into TMI from LEO, and then quickly return to LEO. That would cut refueling Earth launches for cargo flights by 65% to 80%. And there will be a lot more cargo flights than crew flights.

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Re: MCT Speculation and Discussion Thread 4
« Reply #2757 on: 09/15/2016 10:00 PM »
Gwynne has said, "We're looking at SEP" so this is a possibility.
On the downside if it's SEP, it is yet another vehicle to be developed and tested.
Random thought: What about a vehicle that can do missions with small crews without SEP, but you have a plan to add SEP later (as a stage, cycler or L2 propellant tug, whatever) when you have multiple flights to spread the costs over and want closer to that 500k/passenger goal, and also to achieve that fast return to earth for immediate reuse?
Sure, just change how many tankers are used to refuel (& where) the BFS in orbit. SEP can be to push the fuel to a higher energy orbit before refueling.

Staging fuel at EML1/2 or similar HEO with SEP would cut refueling launches from Earth by about 50% for crewed flights.

SEP tugs could also put cargo landers directly into TMI from LEO, and then quickly return to LEO. That would cut refueling Earth launches for cargo flights by 65% to 80%. And there will be a lot more cargo flights than crew flights.

I think that the Mars transport system will evolve considerably over time by the addition of new vehicles and operations that augment and multiply the effectiveness of the initial vehicle.

The BFR as the primary LEO launch vehicle and the vehicle that lands on mars will likely remain as they are in form and function as the constraints imposed during launch and landing force these vehicles to be very dedicated to these roles. 

But the entire middle of the transit will be subject to upgrading by adding one or more vehicles dedicated to that role as well as more complex rendezvouses, propellant transfers and cargo-handling operations.  So while initial missions are likely to see the mars lander just depart from LEO using large amounts of propellant the ultimate goal would be to keep the lander at Mars permanently and use an in-space only transfer vehicle. 

This is another reason that I reject the direct return modality, in creating a vehicle system designed for direct flight and completely independent operation it makes upgrading nearly impossible.  It is better to have the initial role out of the system be more flexible even if it makes for a more complex mission modality and higher IMLEO because it provides a smooth transition into expanded capacity and lower eventual prices.


Lastly SEP can not perform a TMI and then rapidly return to Earth.  If a SEP tug took a payload to the very edge of Earths gravity well and then accelerated to just above escape velocity and released the cargo the heliocentric orbit would still be very Earthlike and would not intersect Mars, the SEP tug has to push the payload a good deal beyond Earth escape to actually give a Mars intersecting trajectory to the payload, and this would take time, a few weeks at least even using a very high acceleration, low ISP SEP system, by that time the tug would be well away from the Earth with considerable velocity and return would take quite a long time via retro propulsion, it might be better to just let the tug fly on to mars and try to slingshot around mars back towards Earth as this would require virtually no propellant until your back in the Earths gravity well when you can try to decelerate and capture into an elliptical orbit.

Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2758 on: 09/17/2016 12:42 AM »
Elon Musk ✔ @elonmusk
Turns out MCT can go well beyond Mars, so will need a new name…
8:22 PM - 16 Sep 2016

Offline Chris Bergin

Re: MCT Speculation and Discussion Thread 4
« Reply #2759 on: 09/17/2016 12:55 AM »
Elon Musk ✔ @elonmusk
Turns out MCT can go well beyond Mars, so will need a new name…
8:22 PM - 16 Sep 2016

*Sirens* Elon has begun talking about MCT *Sirens* ;D

I just hope there aren't any stupid name suggestion tweets in the replies.......oh ;)

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