Author Topic: MCT Speculation and Discussion Thread 4  (Read 569471 times)

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #660 on: 10/08/2015 11:53 PM »

Particularly if you are trying the use the escape vehicle's engines as landing engines for the entire MCT. The forces on the connectors, which must be instantly separable during launch-abort, would be ridiculous.


yes, a fair point about the tension forces of having the lift on top could be a problem.  It'd have to be designed for tension as well as compression.  Which may prove impractical during the actual design process.  When landing on Earth it would be relieved of all of it's cargo mass and all of it's main propellant mass, so the underslung weight would at least be at it's minimum. 

The alternative is bottom mounted landing thrusters, or figuring out a way to land on Vacuum Raptor at sea level.
« Last Edit: 10/09/2015 12:07 AM by Lobo »

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #661 on: 10/09/2015 12:21 AM »

It has to be a separate spacecraft, or it can't operate as a escape vehicle. Such a double-vehicle would become hideously complex.


Yes.  That's probably why such a cylinder-cone vehicle with a separable nose cone with a crew hab has never been thought of before.

;-)

Doesn't even need to be cylinder/cone:


Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #662 on: 10/09/2015 12:30 AM »

It has to be a separate spacecraft, or it can't operate as a escape vehicle. Such a double-vehicle would become hideously complex.


Yes.  That's probably why such a cylinder-cone vehicle with a separable nose cone with a crew hab has never been thought of before.

;-)

Doesn't even need to be cylinder/cone:



:-)

Well said.

Now obviously I'm being a little facetious with Paul.  Those vehicles don't do a side entry into an atmosphere as a stack and that will add a level of complexity.  (The the cylinder-cone geometry might be a bit easier to cover with a TPS than the Soyuz geometry.  Heh.)

But the point being, to say the the pieces being joined where they can separate in an emergency is "hideously complex" may be a tad over stated.  ;-)


« Last Edit: 10/09/2015 12:33 AM by Lobo »

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #663 on: 10/09/2015 03:08 AM »
[images of Apollo CM+SM and Dragon+trunk]

Apollo didn't and Dragon doesn't do EDL with those modules attached. The level of integration your require is an order of magnitude more complex.

It's not really two spacecraft in one, like MCT+ S2 would be two completely separate spacecraft. It's more a spacecraft, with an additional section.  Since everything below the lifeboat cannot function on it's own as it own spacecraft. So unlike your analogy, it's more like a semi tractor and trailer.

The back portion is not being "towed" like a trailer, it must be joined as a single continuous vehicle.

What I'm reacting to is that you responded to the idea of an MCT being launched to LEO unmanned, with a smaller shuttle bringing up the 100 passengers, by saying (paraphrasing) "But that means designing two spacecraft", as if you aren't required to design two (much more complex) spacecraft with your lifeboat/MCT concept. Which you continue to maintain; "it's not really two spacecraft in one".

But the point being, to say the the pieces being joined where they can separate in an emergency is "hideously complex" may be a tad over stated.

Do you not understand that a vessel that must function independently and as a deeply integrated functional part of a larger vessel (your lifeboat) is going to be much more complex than a vessel that only has to function independently (a LEO taxi)? And that a larger vessel that must be designed around a major piece that separates is going to be much more complex than a similar sized vessel that doesn't come apart?

« Last Edit: 10/09/2015 10:01 AM by Chris Bergin »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #664 on: 10/09/2015 05:09 AM »
A very rough way to estimate the cost of a vehicle development is looking at it's mass and multiplying by a factor weighted by the type of vehicle it is, for example a City buss costs less to develop them a rocket.

I think the Separate Bi-conic is attractive in this regard because rocket stages are cheaper to develop per unit of dry weight then manned capsules which are very nearly the most expensive things in aerospace development, we need only look at the relative budgets of SLS and Orion to see that Orion costs more per pound to develop.  Likewise SpaceX needed considerable NASA funding to develop Dragon and to make the 2nd version which is still short of the sophistication level likely to be employed in MCT.  We have every reason to belive the ratio in development costs in large, possibly in excess of 10:1.

I estimate a bi-conic would have a mass of around 75 mT, and the 2nd stage would be 72 mT.  I expect the Integrated version would have a mass greater then 75 but probably less the the raw additive 150 of the two separate vehicles due to some savings on redundancies, but it will probably have a development cost that is per pound equal to the smaller bi-conic I'm looking at.  This will wipe out the advantage of not developing a 2nd stage and makes the total cost greater unless the integrated vehicle has an exceedingly low mass or the cost ratio between stages/capsules is extremely low perhaps due to the difficulty of 2nd stage reuse engineering.

I would really like to hear some exact mass number from Lobo about the whole vehicle stack, dry masses for the first stage, the integrated bi-conic dry mass (without abort systems if that's your preference) and propellant loads in each so I can plug them into the launch vehicle performance calculator I've been using http://www.silverbirdastronautics.com/LVperform.html and do an apples to apples comparison to see how gross take off weight differs.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #665 on: 10/09/2015 05:43 AM »
I think the Separate Bi-conic is attractive in this regard because rocket stages are cheaper to develop per unit of dry weight then manned capsules

Not sure I'm seeing how you use the development cost of a conventional rocket stage to compare the development cost of MCT and a manned capsule.

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #666 on: 10/09/2015 07:15 AM »
But the point being, to say the the pieces being joined where they can separate in an emergency is "hideously complex" may be a tad over stated.

Do you not understand that a vessel that must function independently and as a deeply integrated functional part of a larger vessel (your lifeboat) is going to be much more complex than a vessel that only has to function independently (a LEO taxi)? And that a larger vessel that must be designed around a major piece that separates is going to be much more complex than a similar sized vessel that doesn't come apart?

The MCT is already very complex, just think of the requirements:

1. it needs to act as its own stage (IBMCT and variants)
2. it needs to be refuelled in LEO (perhaps multiple dockings).
3. it needs to have significant loiter time in LEO
4. it needs to do the TMI
5. it needs to act as a hab, with full multiply redundant life support
6. it needs to have interplanetary comms
7. it needs a large power supply and radiators - which need to be deployed, then stowed before EDL
8. it needs to do EDL on Mars
9. it needs to land on an unprepared (or only modestly prepared) site
10. it needs a way of unloading crew and cargo - and possibly loading cargo and crew before return to Earth
11. it needs to survive on Mars for about 550 days - longer if it does not return the next synod
12. it needs to be refuelled
13. it needs to do Mars ascent and TEI - all with components that have sat at Mars for 550 days.
14. it needs to do Earth EDL (with a heat shield that has already been used for Mars EDL)
15. ideally it needs to be refurbish-able for later use.
16. it needs to do all this with a low probability of LOM and lower probability of LOC
17. the same basic design needs to be usable for cargo, crew (and as a tanker?)
18. it needs to meet cost and schedule goals during development as otherwise SpaceX won't be able to afford it.

[obviously different mission plans would lead to different requirements - these are just a sample]

These are only the top level requirements, there are many second level requirements that also have to be met (life support, fire suppression, radiation tolerance, etc.)

Each of these top level requirements add to the complexity of the MCT, so much so that the biggest argument against an IBMCT is that it has too many requirements and that they cannot all be met in one vehicle at reasonable cost and with near term technology.

My point is that MCT is already horrendously difficult to design, adding a lifeboat makes it far more difficult.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #667 on: 10/09/2015 07:48 AM »
Quote
11. it needs to survive on Mars for about 550 days - longer if it does not return the next synod

They can waive that requirement if they have to. I always anticipated that the first few MCT which have long surface times on Mars may never return. That would include the first passenger MCT that serves as habitat for at least one synod on Mars.

Those first ~3 MCT will serve as a monument for many generations of martians. :)

Later MCT will be unloaded, refuelled, checked then fly back to earth after a short stay. They will land on better prepared landing sites too.

The really hard requirements are 5) 7) 8) 9) 13) 14) But that is already plenty I agree.


Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #668 on: 10/09/2015 07:57 AM »
My point is that MCT is already horrendously difficult to design, adding a lifeboat makes it far more difficult.

And its not just a lifeboat. That would be one thing. In the system Lobo proposes, the "lifeboat" is a major section of the re-entry system for the MCT and the primary propulsion for landing.

I struggle to find an analogy that Lobo understands. It's worse than the flight-deck of the Shuttle orbiter (just that top bit) being able to jettison during launch. It's that the entire front portion of the orbiter, including heat shield, RCS, front landing gear, must be able to serve as launch abort system, and be a space-plane in its own right to do a full EDL, and still function in its original role as a significant functional part of the Orbiter.

But to then not only argue that these two spacecraft (nose-LAS and orbiter + nose-LAS) are easier to design than two equivalent separate vehicles, but actually arguing that they aren't two spacecraft, just one and a "module".

Those first ~3 MCT will serve as a monument for many generations of martians.

Early settlers tend to be less sentimental than "future generations" would wish. So expect those MCTs to be quickly stripped for parts.
« Last Edit: 10/09/2015 08:01 AM by Paul451 »

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #669 on: 10/09/2015 08:01 AM »
Quote
11. it needs to survive on Mars for about 550 days - longer if it does not return the next synod

They can waive that requirement if they have to. I always anticipated that the first few MCT which have long surface times on Mars may never return. That would include the first passenger MCT that serves as habitat for at least one synod on Mars.

Those first ~3 MCT will serve as a monument for many generations of martians. :)

Later MCT will be unloaded, refuelled, checked then fly back to earth after a short stay. They will land on better prepared landing sites too.

The really hard requirements are 5) 7) 8) 9) 13) 14) But that is already plenty I agree.

Waive a requirement to last for 550 days on Mars, replace with requirement to last indefinitely ?!?!

And the crew needs to get back somehow - first few missions won't have colonists.


Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #670 on: 10/09/2015 09:11 AM »
Waive a requirement to last for 550 days on Mars, replace with requirement to last indefinitely ?!?!

And the crew needs to get back somehow - first few missions won't have colonists.

Waive the requirement to fly back after that time. The ISS shows that a habitat can be maintained for decades. That would be more valid for a habitat that can at some point be connected to an external ECLSS specifically designed for Mars and an expanding local power supply.

Part of the crew will go back with the passenger MCT that brings the crew that will man the station for the next synod and flies back after a few weeks. There will likely not be a life boat MCT. What would it be good for? It cannot go before the return window opens and that will be when the next MCT has arrived.

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #671 on: 10/09/2015 10:00 AM »
Waive a requirement to last for 550 days on Mars, replace with requirement to last indefinitely ?!?!

And the crew needs to get back somehow - first few missions won't have colonists.

Waive the requirement to fly back after that time. The ISS shows that a habitat can be maintained for decades. That would be more valid for a habitat that can at some point be connected to an external ECLSS specifically designed for Mars and an expanding local power supply.

Part of the crew will go back with the passenger MCT that brings the crew that will man the station for the next synod and flies back after a few weeks. There will likely not be a life boat MCT. What would it be good for? It cannot go before the return window opens and that will be when the next MCT has arrived.

So it is still a requirement to fly back one synod later. Not much of a gain there, at the cost of adding extra requirements about long duration surface stays and connection to external ECLSS.

Also I don't think short stays are possible without adding more requirements. They usually need a long duration return leg which expose the crew to higher radiation levels and adds requirements onto the MCT for long duration transits.

http://nssdc.gsfc.nasa.gov/planetary/mars/marsprof.html
Quote
The Short-Stay Mission - often referred to as an opposition-class mission, this mission profile provides Mars stay times of 30 to 90 days with a round trip total time of 400 to 650 days. This mission class requires a large amount of energy to be expended in transit, even after taking advantage of either a Venus swingby (on either the inbound or outbound leg) or a deep space propulsive maneuver in order to limit Mars and Earth entry speeds.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #672 on: 10/09/2015 10:14 AM »
So it is still a requirement to fly back one synod later. Not much of a gain there, at the cost of adding extra requirements about long duration surface stays and connection to external ECLSS.


Sounds almost like you are deliberately misunderstanding me.
« Last Edit: 10/09/2015 10:14 AM by guckyfan »

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #673 on: 10/09/2015 11:13 AM »
So it is still a requirement to fly back one synod later. Not much of a gain there, at the cost of adding extra requirements about long duration surface stays and connection to external ECLSS.


Sounds almost like you are deliberately misunderstanding me.

Let me restate it again, then.

There are two types of Mars mission classes, conjunction (long-stay) and opposition (short-stay). Opposition class missions spend over a year in space (often about 500 days on the return leg) and come closer to the Sun than the Earth (e.g. Venus swing-by). The short duration transits are conjunction class missions.

Let us assume that the initial crew come on a conjunction class mission, if they return on an opposition class mission then they will spend excessive time in space (zero-g and radiation), this adds requirements for more supplies, better ECLSS, Venus distance thermal control, etc.

Opposition class missions might be used for cargo at total duration is generally enough less than 2 years that they may be reused again at the next synod. I see no way of using Opposition class missions for crew transport without adding extra requirements to the MCT and increasing LOC probability.

So there is always a requirement for conjunction class missions for crew, which is what I meant by fly back one synod later. Any stay of the MCT (actively being used) on Mars longer than one synod adds extra requirements, as does connection to an external ECLSS.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #674 on: 10/09/2015 12:06 PM »
As far as we know all MCT, both cargo and passenger, will go on trajectories that allow them to return after a short stay on Mars and return in the same synod. At least that is what Elon Musk has set as a goal for his transport system to reuse them every synod instead of every second synod.
That means the return leg will be significantly longer than the leg earth-mars. You are introducing a new requirement for shorter return flights that would mean that manned MCT could not be reused every synod. That may or may not be the case. IMO it is just a reason to reduce the number of people who return to earth to a minimum, maybe have more water as shielding for at least a part of the crew space for the return leg. But that is problematic as the mass budget available for return is much smaller.

Offline Chris Bergin

Re: MCT Speculation and Discussion Thread 4
« Reply #675 on: 10/09/2015 12:29 PM »
I think that's a good point about the difference between NASA Mars and SpaceX Mars. NASA Mars is very much focused on rotation. SpaceX Mars is to create a colony.

I doubt they would be short of customers who would be willing to spend some cash (probably affordable to a lot of people via selling their home) and up sticks and become a resident of Mars.....and not return (the element of increasing the population, as opposed to several years stays and coming back).

Offline BroncoBill

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Re: MCT Speculation and Discussion Thread 4
« Reply #676 on: 10/09/2015 12:38 PM »
With you having been given advanced sight of the SpaceX MCT architecture, every word you post on this "speculation" thread from now onwards will be scrutinised with a fine tooth coombe until the grand reveal ! not that I'm suggesting in anyway that you should refrain from posting on this thread until the reveal, PLEASE keep on posting  :)

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #677 on: 10/09/2015 01:03 PM »
With you having been given advanced sight of the SpaceX MCT architecture, every word you post on this "speculation" thread from now onwards will be scrutinised with a fine tooth coombe until the grand reveal ! not that I'm suggesting in anyway that you should refrain from posting on this thread until the reveal, PLEASE keep on posting  :)

Did CB say anything about MCT? Not sure his tweet did....

I repeat.

"DEATH STAR"

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #678 on: 10/09/2015 01:18 PM »
As far as we know all MCT, both cargo and passenger, will go on trajectories that allow them to return after a short stay on Mars and return in the same synod. At least that is what Elon Musk has set as a goal for his transport system to reuse them every synod instead of every second synod.
That means the return leg will be significantly longer than the leg earth-mars. You are introducing a new requirement for shorter return flights that would mean that manned MCT could not be reused every synod. That may or may not be the case. IMO it is just a reason to reduce the number of people who return to earth to a minimum, maybe have more water as shielding for at least a part of the crew space for the return leg. But that is problematic as the mass budget available for return is much smaller.

A short return trip adds few extra requirements onto the MCT as there already are requirements for a short trip to Mars. However a long return trip which goes into the orbit of Venus adds requirements that are not needed on other phases of the mission. For cargo it adds a few extra requirements, but for crew it adds far more, especially as the MCT will be limited to 25% payload on the return trip.

It is helpful to make a distinction between mission requirements and MCT requirements. Using a short duration return is a mission requirement, as is returning N crew at a time. Handling the heat at the distance of Venus from the Sun and keeping a crew (of size N) healthy for a 500 days in space are MCT requirements.

I think Elon will be satisfied if the 90% cargo missions can be reflown in 1 synod while the 10% crew missions are reflown in 2 synods. Crew MCT probably need far more refurbishment than cargo MCT so it would be a push to get them reflown the next synod anyway.

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #679 on: 10/09/2015 01:37 PM »
I think that's a good point about the difference between NASA Mars and SpaceX Mars. NASA Mars is very much focused on rotation. SpaceX Mars is to create a colony.

I doubt they would be short of customers who would be willing to spend some cash (probably affordable to a lot of people via selling their home) and up sticks and become a resident of Mars.....and not return (the element of increasing the population, as opposed to several years stays and coming back).

I think most people commenting on this thread realise that.

We see a distinction between early MCT missions which will likely have small crew sizes of professional astronauts, having an emphasis on exploration, basic engineering research on how to use ISRU especially for fuel and life support, and setting up an initial base. ISS style economy - crew do not pay for the resources they use. Most of these astronauts are likely to return home after one or more synods on Mars.

Later synods where the emphasis in on building the base(s) and modest scale mining, where the cost per person is still high. These people are likely to be sponsored (i.e. hired on Earth to do specific jobs), when their contracts are over they may return, and I think many will (think mines in the outback),  with a few very wealthy individuals. Company town economy. MCT and BFR are likely to have been updated.

Then finally 20+ years after the first landing mass colonisation at $500k per trip. Mixed economy. Economic migrants, going to make a better life for themselves (in terms of challenge and contributing to a frontier). Large colonial fleets.

Recent discussion on this thread has been on the first stage.

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