Author Topic: MCT Speculation and Discussion Thread 4  (Read 570914 times)

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1300 on: 01/03/2016 02:05 PM »
Getting back closer to MCT speculation.

I have been thinking about LAS function. For a long time I don't see LAS useful on MCT as a stand alone vehicle. Very little good on Mars if the landing point is not reached. Very little to no good on earth return. Redundancy and reliability will have to make it safe enough.

That leaves the question of LAS capability on earth ascend as desirable. I don't believe that carrying the LAS all the way to Mars and back to have that ability on earth ascent. Maybe a dedicated LAS capable vehicle could be designed for transfer of all 100 passengers to the waiting MCT. It could be lightweight enough that it would be possible to launch the passengers and do the final filling of the tanks for departure in one go.

Though I still believe that no dedicated LAS will be needed at all. BFR with enough thrust that it can launch even with an engine out seconds after launch. With engines that are reliable enough and equipped with sensors that shut off the engine without explosion strong enough to affect neighbouring engines.

MCT could not do LAS in the sense it speeds away from an explosion but capable of separating from BFR and doing RTLS after burning most of its fuel before landing. It would need landing gear strong enough to deal with the full weight of a launched MCT. Out of the box idea, maybe not feasible: Land MCT on a pad 10m deep in water. It destroys MCT but saves the passengers.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1301 on: 01/03/2016 08:17 PM »
(Suggest moving this entire spin-gravity subject to Non-astronauts for months in cramped environments under Advanced Concepts.)

You'd be lying flat on your back
Remember that laying flat is used as a crude proxy for weightlessness in research on Earth, precisely because it causes many of the issues experienced in space even though you are under 1g. Hence, your bike method will achieving no more than peddling on a stationary bike in weightlessness.
I like the idea of a bike track that the rider lays relatively "flat" to the outer circumference.  Quick calculations [...] for 1g

You missed my point. You don't get the benefit of 1g when you are laying down (or even sitting.)

Hence riding at 1g on an recumbent bicycle might be the equivalent of jogging at a quarter or a tenth of 1g. And probably no better than using a stationary bike.

Running has the added benefit of impact on joints and bones reducing bone-loss. (Which is why there was so much effort figuring out how to build a micro-g treadmill on ISS.)

What I like about a running track is that there's no equipment to wear out. It's very self-paced. And it's very natural.

Aside: If you insist on a bike, the track itself is unnecessary. There's this:

(the rider creates artificial gravity for the platform, which is for high-impact resistance weight training, squats, lifts, etc.)


Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1302 on: 01/03/2016 08:38 PM »
I agree that the only viable mars emergency system is to have engine-out capability ideally from the moment of lift-off.  Likewise you need lots of thrust to even think about separating from a booster even if it is not a fast separation, both these requirements would lead us to a MCT which has substantially higher thrust to weight ratio then a rocket would typically have. 

My own concept is for MCT to mass just 200 mt at launch due to being nearly empty of propellants, 4 Raptor engines give it an acceleration of 4.7 g's.   In the event of separation from the booster (which might need assistance from pressure-fed or solids to account for spin-up time of Raptor) a RTLS landing might be possible in the early phases of launch but a down range water landing is most likely.  I agree that this would likely result in a scrapping of the vehicle (just as we scrap an airplane that has had a water landing) but it will be well worth it if it saves lives.  I see the vehicle simply doing a landing burn to soft land in water (as early F9 tests did) and floating due to it's tanks, passengers will wait for rescue inside the vehicle rather then jumping into life-rafts.

On mars surface with 25 mt cargo and with full propellant tanks GLOW would be 400 mt and the vehicle would have 2.3 g's acceleration which is reduced by mars gravity to just under 2 g's upward at launch, if an engine is lost a diagonal engine must also be shut down to maintain balance and the upward acceleration is only .75 g upward at take off, still very fast by conventional launch standards and more then sufficient to reach orbit with minimal gravity losses.

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #1303 on: 01/03/2016 10:03 PM »
Not sure if this video has been posted yet..but a schematic of a 100MT lander is illustrated at 46 min mark.
Interesting vid but warning lots of physics!! :)
Cheers
Jb


The author of the thesis, Max Fagin, has been discussing it on this Reddit thread: https://www.reddit.com/r/spacex/comments/3z6tr2/thesis_defense_supersonic_retropropulsion_for/
(He says he cannot comment on his work for SpaceX though)
 

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1304 on: 01/04/2016 12:31 AM »
I agree that the only viable mars emergency system is to have engine-out capability ideally from the moment of lift-off.  Likewise you need lots of thrust to even think about separating from a booster even if it is not a fast separation, both these requirements would lead us to a MCT which has substantially higher thrust to weight ratio then a rocket would typically have. 

My own concept is for MCT to mass just 200 mt at launch due to being nearly empty of propellants, 4 Raptor engines give it an acceleration of 4.7 g's.   In the event of separation from the booster (which might need assistance from pressure-fed or solids to account for spin-up time of Raptor) a RTLS landing might be possible in the early phases of launch but a down range water landing is most likely.  I agree that this would likely result in a scrapping of the vehicle (just as we scrap an airplane that has had a water landing) but it will be well worth it if it saves lives.  I see the vehicle simply doing a landing burn to soft land in water (as early F9 tests did) and floating due to it's tanks, passengers will wait for rescue inside the vehicle rather then jumping into life-rafts.

On mars surface with 25 mt cargo and with full propellant tanks GLOW would be 400 mt and the vehicle would have 2.3 g's acceleration which is reduced by mars gravity to just under 2 g's upward at launch, if an engine is lost a diagonal engine must also be shut down to maintain balance and the upward acceleration is only .75 g upward at take off, still very fast by conventional launch standards and more then sufficient to reach orbit with minimal gravity losses.

7 engines at Mars surface just about fit into a 15m diameter.  With that many, engine failure where you (hypothetically) have to shut off the opposite thruster is at most a 28% drop, rather than a 50% drop.  I like the notion of abort to water during Earth ascent.

Launching with MCT nearly empty would be required to achieve high-G abort, but this would mean the BFR would nearly need to reach orbit.   That eliminates or makes very expensive the possibility of a 1-stage BFR.

I think we're probably going to see crew & bulk cargo launch separately from the MCT structure.  The largest mission feasible is going to be bottlenecked by the welded-together structural mass that can be sent up on a single launch vehicle.  You can only build tanks so big given a fixed structural mass, you can only build a hab so big, you can only land so much mass onto the Martian surface.

Hanging an abort requirement onto that shrinks the bottlenecking variable by an order of magnitude or so.  Putting bulk cargo (mostly food) in with the crew subtracts further.
« Last Edit: 01/04/2016 12:44 AM by Burninate »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1305 on: 01/04/2016 12:49 AM »

7 engines at Mars surface just about fit into a 15m diameter.  With that many, engine failure where you (hypothetically) have to shut off the opposite thruster is at most a 28% drop, rather than a 50% drop.  I like the notion of abort to water during Earth ascent.

Launching with MCT nearly empty would be required to achieve high-G abort, but this would mean the BFR would nearly need to reach orbit.   That eliminates or makes very expensive the possibility of a 1-stage BFR.

I think we're probably going to see crew & bulk cargo launch separately from the MCT structure.

What kind of GLOW at Mars do you see that will utilize the 5 remaining engines?  I assume the engine arrangement honey-comb with a central engine.

A 1-stage BFR is of course not what I'm proposing as it by your own logic makes abort at Earth launch impossible, not to mention it is inefficient for propellant delivery to LEO.  I expect BFR to be 2 stages with a reusable orbital 2nd stage.

If bulk cargo is launched separately from the MCT vehicle then that mandates a 2 stage BFR as something other then the MCT vehicle needs to push the cargo to orbit.  This would be a very attractive architecture as it would cut the size of the BFR nearly in half.  I've thought it likely that this is going to be sufficiently difficult that SpaceX will not try it initially, but will rather defer it to later and use it to just ramp up the rate of delivery once it is mastered.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1306 on: 01/04/2016 06:04 PM »


My own concept is for MCT to mass just 200 mt at launch due to being nearly empty of propellants, 4 Raptor engines give it an acceleration of 4.7 g's.   In the event of separation from the booster (which might need assistance from pressure-fed or solids to account for spin-up time of Raptor) a RTLS landing might be possible in the early phases of launch but a down range water landing is most likely.  I agree that this would likely result in a scrapping of the vehicle (just as we scrap an airplane that has had a water landing) but it will be well worth it if it saves lives.  I see the vehicle simply doing a landing burn to soft land in water (as early F9 tests did) and floating due to it's tanks, passengers will wait for rescue inside the vehicle rather then jumping into life-rafts.

On mars surface with 25 mt cargo and with full propellant tanks GLOW would be 400 mt and the vehicle would have 2.3 g's acceleration which is reduced by mars gravity to just under 2 g's upward at launch, if an engine is lost a diagonal engine must also be shut down to maintain balance and the upward acceleration is only .75 g upward at take off, still very fast by conventional launch standards and more then sufficient to reach orbit with minimal gravity losses.

I'm interested in just what the numbers are for the lower stage or stages that put the nearly empty of propellant 200mT MCT into LEO.
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1307 on: 01/04/2016 11:05 PM »
It's 200 mt total, 75 for the vehicle, 100 for cargo and 25 propellant which would be just for final orbital insertion and maneuvering or for the landing burn into water in case of an abort.

1st Stage boosters is comparable to what you have discussed, 54,900 kN thrust, 180 tons dry, 3150 mt Propellant.  2nd Stage 16,100 kN, 72 mt dry, 1200 mt propellant.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1308 on: 01/05/2016 05:43 PM »
So the MCT is the 3rd stage or am I mis-understanding?
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1309 on: 01/05/2016 05:59 PM »
The BFS (fka MCT) is the 2nd stage according to recent comments by Musk. I don't know what it is in other peoples' fantasy architectures.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1310 on: 01/05/2016 07:46 PM »
So the MCT is the 3rd stage or am I mis-understanding?

Basically yes though the first 2 stages are a full orbital launch system capable of putting any payload into LEO.  Despite other folks desire to interpret Musk statements as unconditional support for their positions it is nothing of the sort, Musk simply said the mars spacecraft needs a booster on Earth, he did not specify the stages in said booster. 

The impracticality of a 1 stage booster that is dependent on being paired with a fully interplanetary spacecraft of gigantic size to do any launch at all is so high that the idea should be discounted until explicitly confirmed by Musk, and even then I would be highly doubtful such an architecture would successfully get out of development.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1311 on: 01/05/2016 09:36 PM »
So the MCT is the 3rd stage or am I mis-understanding?

Basically yes though the first 2 stages are a full orbital launch system capable of putting any payload into LEO.  Despite other folks desire to interpret Musk statements as unconditional support for their positions it is nothing of the sort, Musk simply said the mars spacecraft needs a booster on Earth, he did not specify the stages in said booster. 

The impracticality of a 1 stage booster that is dependent on being paired with a fully interplanetary spacecraft of gigantic size to do any launch at all is so high that the idea should be discounted until explicitly confirmed by Musk, and even then I would be highly doubtful such an architecture would successfully get out of development.

I think you are making a straw man there. There could be one booster paired with different reusable upper stages. An general purpose one for most LEO/GTO payloads (developed first), and a more specialized Mars variant. That's how I see it.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1312 on: 01/05/2016 10:31 PM »
So the MCT is the 3rd stage or am I mis-understanding?

Basically yes though the first 2 stages are a full orbital launch system capable of putting any payload into LEO.  Despite other folks desire to interpret Musk statements as unconditional support for their positions it is nothing of the sort, Musk simply said the mars spacecraft needs a booster on Earth, he did not specify the stages in said booster. 

The impracticality of a 1 stage booster that is dependent on being paired with a fully interplanetary spacecraft of gigantic size to do any launch at all is so high that the idea should be discounted until explicitly confirmed by Musk, and even then I would be highly doubtful such an architecture would successfully get out of development.

I think you are making a straw man there. There could be one booster paired with different reusable upper stages. An general purpose one for most LEO/GTO payloads (developed first), and a more specialized Mars variant. That's how I see it.

No, many people have explicitly said that no separate 2nd stage will ever exist and all launches will be using MCT as the one and only 2nd stage.
 
If a reusable upper stage is produced first (which I expect) then it make no sense not to utilize it when launching the MCT vehicle regardless of how much deltaV the MCT itself may be capable of, you would simply design a larger MCT, or a smaller BFR if this was your plan and you would always use the 2nd stage.
« Last Edit: 01/05/2016 10:32 PM by Impaler »

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1313 on: 01/05/2016 10:49 PM »
Then you are deliberately misreading people and interjecting your own ideas about how it must be done.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1314 on: 01/06/2016 12:11 AM »
Then you are deliberately misreading people and interjecting your own ideas about how it must be done.

Misreading?  I'll provide quotes if you like, but it is indisputable that many people have adamantly rejected the idea of a 2 stage BFR and insist on an exclusive MCT as the only 2nd stage. 

MCT may be very stripped down when in cargo mode, essentially a reusable second stage with a long-loiter package and refueling capability. Don't know why you'd insist on yet another stage.

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

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

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

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

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

In any case, we will have to see how this unfolds. We may get some information by Elon on the architecture by Q1 2016.

If you think I am misreading Musk statements I would argue that the other side has done the same and with far less justification on technical merits.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1315 on: 01/06/2016 03:23 AM »
Geez, not this again. Read what I wrote closer. (Hint: I did not state that you misread *all*) Some have advocated that, but I certainly have not.

I have always advocated a BFR upper stage with a cargo bay, so good luck putting an MCT in or on top of that. An MCT would be an evolved version of such a stage. There is no reason to launch a Mars capable vehicle for most BFR missions, which will be LEO, GTO, and propellant deliveries.

But why do I bother. Will it be Impaler vs the world for another few weeks here? If so, let me know when the dust has settled.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1316 on: 01/06/2016 03:50 AM »
Misreading?  I'll provide quotes if you like, but it is indisputable that many people have adamantly rejected the idea of a 2 stage BFR and insist on an exclusive MCT as the only 2nd stage. 

You're going to have to switch to the new nomenclature or things are going to get very confusing.

Musk's last statement makes it clear that what we've been calling "the MCT", Musk is obviously calling BFS. Hence, "MCT" is the whole system, BFR plus BFS plus whatever the system needs (fuel depots, etc.)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1317 on: 01/06/2016 04:46 AM »
Geez, not this again. Read what I wrote closer. (Hint: I did not state that you misread *all*) Some have advocated that, but I certainly have not.

I have always advocated a BFR upper stage with a cargo bay, so good luck putting an MCT in or on top of that. An MCT would be an evolved version of such a stage. There is no reason to launch a Mars capable vehicle for most BFR missions, which will be LEO, GTO, and propellant deliveries.

But why do I bother. Will it be Impaler vs the world for another few weeks here? If so, let me know when the dust has settled.

I think you should likewise re-read, I said 'many people' had said their would be no separate 2nd stage, you are basically saying that is correct.  But never said it was your position.  It seems you are staking out some kind of middle ground in which a less then mars capable vehicle is the normal 2nd stage for the many LEO/GTO launches (which in principle I agree with but most have rejected), but the MCT launch doesn't make use of this 2nd stage when it launches (which still seems illogical to me).

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1318 on: 01/06/2016 04:47 AM »
Impaler: There's literally no difference in my mind between a reusable second stage and a sufficiently stripped-down MCT (and I expect the first version they fly will be stripped down). That's what I meant, and I don't know what else you could take from what I said.
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Offline dror

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Re: MCT Speculation and Discussion Thread 4
« Reply #1319 on: 01/06/2016 05:40 AM »
A human rated MCT needs a launch abort system.
The LAS will probably eject the crew compartment only, not the big engines and fuel tanks.
So
A mct\bfs will be built of two seperate parts - the propultion module and the utility module.
The propultion module is practically the BFR's 2nd stage and the utility module will be mission specific.
That's how I see it. Does it makes sence?
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