Author Topic: MCT Speculation Thread  (Read 602732 times)

Offline QuantumG

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MCT Speculation Thread
« on: 12/13/2013 02:41 AM »
Some new information available:



Quote from: Elon Musk
Mars is, if you have a low energy trajectory, like a minimum energy trajectory is about 6 months. I think that can be compressed down to about 3 months, and it gets exponentially harder as you go lower than that - 3 to 4. It's important to actually be at that level because then you can send your spaceship to Mars and then bring it back on the same orbital synchronization. Earth and Mars synch up every two years and then they're only kinda in synch for about 6 months. Then, ya know, they're really too far apart. So you've got to be able to go there and back in one go. That's important for making the cost of traveling to Mars an affordable amount.

http://shitelonsays.com/transcript/raw-science-elon-musk-on-mars-2013-12-09

.. and if I transcribe any more today I'm going to have to hit many hands with a hammer to make them feel better.

So, MCT is supposed to go to Mars and back in under 6 months. W.T.F.
« Last Edit: 12/13/2013 11:19 AM by Chris Bergin »
I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Online meekGee

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Re: MCT Speculation Thread
« Reply #1 on: 12/13/2013 02:47 AM »
Hohmann transfer orbits minimize the required delta-V for a one-way trip.

Musk is saying - suppose you want to go out and return ASAP.  Optimize that.

If you use Hohmann transfer going one way, you're in a bad orbital position for the return trip.

So you (presumably) end up having to do two "equally inefficient" trips.

Interesting.



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Offline QuantumG

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Re: MCT Speculation Thread
« Reply #2 on: 12/13/2013 02:58 AM »
Yeah, what's the delta-v from LEO?

Table 4.2 in The Case For Mars indicates that 7.93 km/s will get you to Mars in 130 days (over 4 months), and describes the aeroentry as "impossible".

So, 8 km/s from LEO with propulsive entry? That sounds, ummm, ambitious.

I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

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Re: MCT Speculation Thread
« Reply #3 on: 12/13/2013 03:16 AM »
Indeed.   But as he said - the more you push it, the harder it gets, and very quickly.

The 3 months is not set in stone.

The interesting point is the posing of the optimization metric:  The sum of the dV of the two trajectories

What I don't understand is why it is so important to get your spacecraft immediately, vs. getting it on the next cycle.     In the latter case you still get your spacecraft back, but only later, so you need maybe 2x as many spacecraft.   Sounds equivalent to taking a 50% payload penalty, for example.
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Offline QuantumG

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Re: MCT Speculation Thread
« Reply #4 on: 12/13/2013 03:19 AM »
What I don't understand is why it is so important to get your spacecraft immediately, vs. getting it on the next cycle.     In the latter case you still get your spacecraft back, but only later, so you need maybe 2x as many spacecraft.   Sounds equivalent to taking a 50% payload penalty, for example.

I don't get that either. Especially when you consider the massive delta-v penalty of trying to get there and back in a single window.

This Elon Musk doesn't sound like the "show a little leg" Elon Musk of a year ago. I guess he still doesn't have any more idea what MCT is going to be than we do  ;D
I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

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Re: MCT Speculation Thread
« Reply #5 on: 12/13/2013 03:25 AM »
:)  I dunno.

He obviously spent time on it.

My question is:  "Why does he want the spacecraft back on the same cycle".

Sunk capital cost is still missing from your balance sheet.  He wants to get as much throughput per spacecraft.

If the choice is "50% payload penalty" or "2x cycle time", then the tie breaker is "shorter time in transit".

(And, we don't know if it's really 50% and really 2x.)

My guess is that he's been hacking at this for how many years now?  He probably got further along than we did in the last 20 minutes.
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Offline starsilk

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Re: MCT Speculation Thread
« Reply #6 on: 12/13/2013 03:28 AM »
What I don't understand is why it is so important to get your spacecraft immediately, vs. getting it on the next cycle.     In the latter case you still get your spacecraft back, but only later, so you need maybe 2x as many spacecraft.   Sounds equivalent to taking a 50% payload penalty, for example.

I don't get that either. Especially when you consider the massive delta-v penalty of trying to get there and back in a single window.

I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

faster means less exposure to radiation, and less zero-G bone loss/muscle loss etc. remember these are not intended to be high-function astronauts (at least: not when it comes to colony time). they're not going to deal with long space travel as well as hand-picked astros.

there are going to be people going to Mars AND returning - not necessarily the same people in the same window, but you do need the fast transit in both directions.

Offline QuantumG

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Re: MCT Speculation Thread
« Reply #7 on: 12/13/2013 03:32 AM »
I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

Nope, he specifically says the fast transit is to get the spacecraft back in the same window to reduce the cost. Don't tell me you didn't even read the transcript I painstakingly typed out for you.
I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Offline QuantumG

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Re: MCT Speculation Thread
« Reply #8 on: 12/13/2013 03:33 AM »
My guess is that he's been hacking at this for how many years now?  He probably got further along than we did in the last 20 minutes.

No doubt, but in this interview he sounds like he's just thinking out loud, and probably is.
I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Online Robotbeat

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Re: MCT Speculation Thread
« Reply #9 on: 12/13/2013 03:34 AM »
You also might be able to put the MCT (or whatever it is) to work doing cislunar work when you aren't near Mars. (And by that I primarily mean shuttling stuff to Earth orbit.) That way, not only do you get to use the rocket more often for Mars, but you can also use it many more times in general. If you can only use the MCT for Mars, it's going to be a lot more expensive.
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Re: MCT Speculation Thread
« Reply #10 on: 12/13/2013 03:35 AM »
I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

faster means less exposure to radiation, and less zero-G bone loss/muscle loss etc. remember these are not intended to be high-function astronauts (at least: not when it comes to colony time). they're not going to deal with long space travel as well as hand-picked astros.

there are going to be people going to Mars AND returning - not necessarily the same people in the same window, but you do need the fast transit in both directions.

That's true, but if that was the primary consideration, you could optimize for an even faster transit, if you're willing to let the spacecraft wait at Mars for another cycle.

There's more to this story...
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Offline starsilk

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Re: MCT Speculation Thread
« Reply #11 on: 12/13/2013 03:40 AM »
I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

Nope, he specifically says the fast transit is to get the spacecraft back in the same window to reduce the cost. Don't tell me you didn't even read the transcript I painstakingly typed out for you.

yes, I read it. I feel your pain.

BUT there's more to the 'cost' of the spaceship than getting it there and back. there's the cost of how big it has to be to avoid people killing one another on a 6 month voyage, the cost of consumables for that length of flight, cost of radiation shielding, cost of dealing with zero-g incapacitated astronauts when they arrive at Mars etc etc.

if it was just about getting the spaceship back it's got to be cheaper just to build two.

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Re: MCT Speculation Thread
« Reply #12 on: 12/13/2013 03:40 AM »
I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

faster means less exposure to radiation, and less zero-G bone loss/muscle loss etc. remember these are not intended to be high-function astronauts (at least: not when it comes to colony time). they're not going to deal with long space travel as well as hand-picked astros.

there are going to be people going to Mars AND returning - not necessarily the same people in the same window, but you do need the fast transit in both directions.

That's true, but if that was the primary consideration, you could optimize for an even faster transit, if you're willing to let the spacecraft wait at Mars for another cycle.

There's more to this story...
I think the "more" is that the MCT can be used on the Earth-side for more regular launch operations.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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Re: MCT Speculation Thread
« Reply #13 on: 12/13/2013 03:50 AM »
I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

faster means less exposure to radiation, and less zero-G bone loss/muscle loss etc. remember these are not intended to be high-function astronauts (at least: not when it comes to colony time). they're not going to deal with long space travel as well as hand-picked astros.

there are going to be people going to Mars AND returning - not necessarily the same people in the same window, but you do need the fast transit in both directions.

That's true, but if that was the primary consideration, you could optimize for an even faster transit, if you're willing to let the spacecraft wait at Mars for another cycle.

There's more to this story...
I think the "more" is that the MCT can be used on the Earth-side for more regular launch operations.

Maybe.

Also, in my mind, I always see a prolonged "equipment only" campaign before there's a people campaign.

However, Elon may be thinking that each MCT is 95% equipment and 5% people, and they build the infrastructure as they get there (well, each group build the infrastructure hauled over by the previous MCT) - and so the "tie breaker" is significant.

QG - can you copy over more data on dV requirements for different transit times?  Also - isn't there another transfer orbit (not as low-dV as Hohmann) that is open in the "other" years?
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Offline guckyfan

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Re: MCT Speculation Thread
« Reply #14 on: 12/13/2013 04:02 AM »
This is great stuff. Thanks QuantumG for the work you put in.

But I don't think this is about MCT, it is more of a future plan. He is not getting to 500.000 Dollar per ticket with MCT. If they ever get there it will need a few more development cycles.


Offline QuantumG

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Re: MCT Speculation Thread
« Reply #15 on: 12/13/2013 04:06 AM »
QG - can you copy over more data on dV requirements for different transit times? 

You just want my fingers to fall off.

Departure Velocity |  Orbit Period | Time to Earth Return | Transit to Mars | Mars Aeroentry
A 3.34 km/s | 1.5 years | 3 years | 250 days | Easy
B 5.08 km/s | 2.0 years | 2 years | 180 days | Acceptable
C 6.93 km/s | 3.0 years | 3 years | 140 days | Dangerous
D 7.93 km/s | 4.0 years | 4 years | 130 days | Impossible
I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Online meekGee

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Re: MCT Speculation Thread
« Reply #16 on: 12/13/2013 04:07 AM »
I love you man.
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Re: MCT Speculation Thread
« Reply #17 on: 12/13/2013 04:12 AM »
I'd say it's not about the spacecraft, but the people inside it. 6 months is a long, long time in tight confines, and the radiation and zero-g are going to make a mess of people by the time they get to Mars.

faster means less exposure to radiation, and less zero-G bone loss/muscle loss etc. remember these are not intended to be high-function astronauts (at least: not when it comes to colony time). they're not going to deal with long space travel as well as hand-picked astros.

there are going to be people going to Mars AND returning - not necessarily the same people in the same window, but you do need the fast transit in both directions.

That's true, but if that was the primary consideration, you could optimize for an even faster transit, if you're willing to let the spacecraft wait at Mars for another cycle.

There's more to this story...
I think the "more" is that the MCT can be used on the Earth-side for more regular launch operations.

Maybe.

Also, in my mind, I always see a prolonged "equipment only" campaign before there's a people campaign.

However, Elon may be thinking that each MCT is 95% equipment and 5% people, and they build the infrastructure as they get there (well, each group build the infrastructure hauled over by the previous MCT) - and so the "tie breaker" is significant.

QG - can you copy over more data on dV requirements for different transit times?  Also - isn't there another transfer orbit (not as low-dV as Hohmann) that is open in the "other" years?
I really wonder if the payloads actually need to travel along with the MCT. Equipment and consumables aren't necessarily in a huge hurry, so you could send them on long, lofted trajectories (perhaps ones that took a few years), and after the initial burn to send them on their way, the MCT would boost back to Earth (or Earth orbit) to be refueled and used again. This way, the MCTs could be used all the time, not just once every couple years. Sure, not all the trips would be ideal from either a payload or transit perspective, but from a capital utilization perspective this allows you to get an order of magnitude improvement in utilization.

(You could imagine MCTs on the Mars side that "catch" the payloads and deliver them to the Martian surface, then are refueled and go catch some more payloads.)

Anyway, that's my pet idea of how to get a lot more use out of a reusable Mars rocket stage than just one trip every couple years (which is just a little better than expendable).
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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Re: MCT Speculation Thread
« Reply #18 on: 12/13/2013 04:23 AM »
So...

The faster you leave, the longer the orbital period

Earth return time is not monotonic with it - I assume this is about whether you can return after a partial or whole orbit.  It's irrelevant anyway - since we're stopping at Mars.

Transit to Mars monotonically decreasing as it should, and regrettably the last column is qualitative instead of listing the actual relative velocity....

k, got it.

Elon says "three-to-four", since Earth and Mars are in position for "about 6 months".  But going to Mars in 130 days in the most efficient manner is different than going to Mars before the optimum time, and returning after the optimum time.  So even more dV.

Each doubling of the required dV is a 2.7x on the wet/dry ratio of the rocket.   If we're talking chemical propulsion, once you get to 8-9 km/sec, you're doing another earth launch.

Definitely does not add up.

And let's assume Elon did his numbers at a better level than we did, so what can make sense?

Electrical propulsion?   I'd be shocked (!)

Here's the thing though.  I always discounted electrical propulsion (This is just musings at this point - I am not ready to convert yet) since the dV to Mars is not great enough to justify it.  You pay a lot for dry mass, and if the mission dV is not sufficient, you actually end up worse off than with a lightweight lower-ISP chemical rocket.

But if you change the problem to one in which you want to execute a dV that's Jovian scale, then maybe it starts making sense.

So I'm willing to listen again if anyone heard loud buzzing or fizzing sounds while driving near Hawthorne late at night.
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Offline go4mars

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Re: MCT Speculation Thread
« Reply #19 on: 12/13/2013 04:28 AM »
I really wonder if the payloads actually need to travel along with the MCT.
I'll echo that thought.  It's in line with earlier suggestions of launching a bunch of stuff to near Earth via BFR during the ~2 years between windows, then roughly on the day of maximum mass transfer, they all shove off together for Mars from where they were parked.  Some of that "stuff" could potentially be fuel loads.  Rapidly cheaply reusable rockets...

Perhaps the "there and back in 6 months" bits are ships specifically for only transiting diapers, doritos, and doers. 

« Last Edit: 12/13/2013 04:33 AM by go4mars »
Elasmotherium; hurlyburly Doggerlandic Jentilak steeds insouciantly gallop in viridescent taiga, eluding deluginal Burckle's abyssal excavation.

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