Author Topic: SpaceX: Mars Colonial Transporter "MCT" -- Speculation (not Raptor)  (Read 676060 times)

Offline go4mars

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Vehicles going out will be carrying habitats, consumables, surface equipment.  Vehicles coming back will be carry people, and fewer of them then the ones going out.
The assumption, IMO, is only that you're returning the vehicles to reduce cost.
It would be pretty easy to find heavy things to ship back to Earth if there was capacity.  Rock core weighs a lot for example. 

What's coming back is just a minimally fueled tank and the engines.  Maybe not even all of the tanks.  (Tanks are also useful on Mars)
Though tanks are useful on Mars, I find it hard to believe that the added complexity of a divisible "spaceship" would be worth it.  I'm thinking of the quote where Elon mentions something like, "and if you think
a 650klb raptor engine is cool, wait until you see the spaceship it's attached too."  Or some such. 

The only thing I'd change on your picture is that likely the central CCB first stage will not be recoverable, at least for the first iteration of the F9HR, due to the crossfeeding and higher speed that it will attain.
It's possible that the entire middle core is part of the "spaceship".  The big tanks and engines go all the way to Mars surface and back rather than having a separate upper stage.  That would certainly increase the utility of a "mars hopper" for collecting ice, exploring, moving drilling rigs, base elements, and other things around the surface, etc...

Then again, the middle core could also be designed to orbit the earth once, low, re-enter and land back at the pad while the "spaceship" above goes 4 mars.
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Online meekGee

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Vehicles going out will be carrying habitats, consumables, surface equipment.  Vehicles coming back will be carry people, and fewer of them then the ones going out.
The assumption, IMO, is only that you're returning the vehicles to reduce cost.

It would be pretty easy to find heavy things to ship back to Earth if there was capacity.  Rock core weighs a lot for example. 

So....  to even things out, you're going to add dead weight ?!
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Offline Lobo

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The MCT diagram: http://twitpic.com/d1rwr6.

The second stage appears to do TMI, Mars landing, Mars ascent and TEI. That's some potent second stage!

It could be a modified Mars Direct plan.  But instead of a separate Hab/Lander and ERV (Earth Return Vehicle), there is just one common vehicle. 
One is sent uncrewed first to land and refuel itself, as Zubrin proposed with the ERV.  (Booster and core of LV land back at launch site and are reused).
Then another is sent with the crew (Booster and core of LV are reused) to lander near the first one, and it will refuel itself while the crew is on the surface.  Then there is redundancy for the crew if there's a problem with either vehicle.  The crew leaves in one and the next crew lands nearby for the next mission.  They could stagger the landings as Zubrin proposed to slowly expand the exploration range, but a crew landing will always be close enough to the previous lander to get there by rover. 

I imagine there will be some expendable items however.  I doubt the entire ERV will do EDL at EArth and then be reused.  Maybe it will have a DRagon capsule on it that could be though.  I would imagine the MCT upper stage would be expended, although I suppose theoretically if it was part of the lander, it could be filled, and it's Raptor engines be designed with enough thrust to lift itself back off the surface of Mars and do TEI?  Not sure how you'd get that stage through Mars EDL though. And it would need enough residual fuel after doing TMI to propulsively land the stack on Mars after Entry and Descent.  I see a lot of technical hurles there.

And I'm not sure how Red Dragon would factor into this. 


Offline go4mars

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So....  to even things out, you're going to add dead weight ?!
I'm a geologist, so maybe it's the bias of having looked at thousands of tonnes of rock core during my career.  I don't consider it dead weight, and neither would any planetary scientist.  It isn't to ensure that equal mass must depart from either end.  "if there is capacity"     With the lower gravity and drag losses, less people and consumables, there might be even more capacity for "dead weight" on return journeys than outgoing from Earth.  The architecture could even be something novel like stacked spaceships on Mars that act as boosters or stages for eachother when launching back to Earth.  I don't know.  Do you?   
« Last Edit: 07/16/2013 10:15 pm by go4mars »
Elasmotherium; hurlyburly Doggerlandic Jentilak steeds insouciantly gallop in viridescent taiga, eluding deluginal Burckle's abyssal excavation.

Online meekGee

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  I don't know.  Do you?   

Nobody knows, this is conjecture, but - I was pointing out that you'd like to optimize the system for more upmass then downmass, since this is the nature of building a colony.

Seems to me that running a less optimized system, and carrying back rocks (hundreds or thousands of tons, mind you) seems, how shall I say, not well aligned with the main design goals of a colony.

When building a colony, I'd think that anything that makes it to Mars and can be used there, should stay there.  If there was a use for the spaceships on Mars, I'd say keep them too, but since there isn't, the next best option is to re-use them.  But if the trip up requires 2-3x as much propellant as the trip back, you might consider having two sets of tanks, and leaving one on Mars for use as (for example) ISRU tanks - otherwise you'll just need to haul separate tanks up with you instead.

This discussion is just a counter-argument to the idea that you should "equate the dVs" and therefore build a symmetrical back-and-forth system, which simply doesn't add up.
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Offline mlindner

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Folks:

Yep, that was my tweet. I have posted some of my ideas here but didn't make much of an impact so I decided to go to the source. I hope this helps you further along down the path. :)


http://forum.nasaspaceflight.com/index.php?topic=30103.msg1074626#msg1074626

I give tinker credit for a good hyperloop idea. "Pretty much" for MCT from Elon means almost nothing. There's no credit for tinker to take other than getting a response from Elon. The drawing is generic as hell and pretty much covers hundreds of possible designs. Doesn't really help us here at all.

Grats tinker though on at least getting a response.
LEO is the ocean, not an island (let alone a continent). We create cruise liners to ride the oceans, not artificial islands in the middle of them. We need a physical place, which has physical resources, to make our future out there.

Offline mlindner

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Folks:

If you do a little research, you'll discover I published the MCT flowchart before Elon dropped that clue on Saturday at Teslive. :)

I have decades of research experience. Sometimes lives depended on me getting it right the first time with insufficient data. Considering the minimal consequences on me getting things wrong here, I can safely say I'm doing this for fun. I do take all your comments and criticisms seriously though! :)

tinker

Just don't take that attitude with people on this forum. There are lots of people here who have worked on things that lives depended on. (Disclaimer: I haven't yet.)
LEO is the ocean, not an island (let alone a continent). We create cruise liners to ride the oceans, not artificial islands in the middle of them. We need a physical place, which has physical resources, to make our future out there.

Offline Patchouli

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I would assume the long-term $500,000 ticket goal would leverage a cycler-type infrastructure where the transit ship is roomy, permanent, and doesn't do any major trajectory changes.  You'd be crazy not to, since that's essentially a space station which we're (at least fundamentally) able build and maintain now.  Prior to that you'll probably have a smaller in-space transit vehicle that loiters at Mars.  I don't know which the term "MCT" refers to. 



To offer either a ticket for 500K or transport hundreds of settlers a year a fleet of Mars cycler craft probably would be a necessity.

The MCT shown in the diagram at best would be able to land ten colonists at once and that likely would be really pushing things.

Though that vehicle may be just the cargo carrier.

If you're going to build a colony you'll probably want to land several hundred tons of supplies before the first colonists arrive.

Hab sections,a power plant,clothing,rovers,and enough consumables such as food to last several years.
« Last Edit: 07/17/2013 10:53 pm by Patchouli »

Offline Joel

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Tinker suggests (if I'm able to decypher his ramblings) that the MCT is sized as a SSTO Mars-Earth return vehicle. That sounds like a reasonable starting point to me.

Back-of-the-envelope calculation: if you assume 7000 m/s delta-v from Mars' surface and 380 s Isp, you get a propellant mass fraction of ~0.85. Assume a T/W of 1.5 at take-off, 2.5 MN thrust for a single Raptor and 3.7 m/s2 gravity on Mars. That gives you a gross take-off weight of 450 (metric) tons, out of which 380 tons is propellant.

EDIT: Adjusted up delta-v, but might still be too low.
« Last Edit: 07/18/2013 08:56 am by Joel »

Offline MikeAtkinson

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Tinker suggests (if I'm able to decypher his ramblings) that the MCT is sized as a SSTO Mars-Earth return vehicle. That sounds like a reasonable starting point to me.

Back-of-the-envelope calculation: if you assume 7000 m/s delta-v from Mars' surface and 380 s Isp, you get a propellant mass fraction of ~0.85. Assume a T/W of 1.5 at take-off, 2.5 MN thrust for a single Raptor and 3.7 m/s2 gravity on Mars. That gives you a gross take-off weight of 450 (metric) tons, out of which 380 tons is propellant.

EDIT: Adjusted up delta-v, but might still be too low.

Working with those numbers for the Earth-Mars direction.

Assume 6000 m/s delta-v and 380 tonnes propellant. Propellant mass fraction ~0.80, gives IMLEO is 475 tonnes.

If we guess that the stage + ship has a dry mass of 50 tonnes, playload Earth-Mars is 45 tonnes and Mars-Earth is 20 tonnes.

Launcher needed is 95 tonnes to LEO for the stage + ship + payload. It then takes 5 launches (4 for fuel) per mission.

Offline Joel

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1.5 T/W on Mars corresponds to 0.57 T/W on Earth. According to
http://www.spacelaunchreport.com/falcon9v1-1.html, current F9v1.1 upper stage can be estimated to have a T/W of 45/(78.1+16) = 0.48. Lower than that for FH.

So a MCT designed to be able to be used as a SSTO Mars-Earth return vehicle, should also have enough thrust to be a (reusable) upper stage on Earth.

Offline MikeAtkinson

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So a MCT designed to be able to be used as a SSTO Mars-Earth return vehicle, should also have enough thrust to be a (reusable) upper stage on Earth.

Yes, although I think that other consideration will mean that the launcher upper stage and the SSTO Mars-Earth stage (I'll call it the MCT stage here) will not be identical.

The three major ones are:
1. the MCT will be more efficient if the stage and ship (cargo carrying) parts are integrated into one vehicle.
2. the MCT stage will need low boil-off and refueling technology, which is not needed for a second stage.
3. there needs to be a tanker to refuel the MCT stage, the MCT launcher can possibly do that most efficiently with a stretched second stage holding the extra propellant.

Offline R7

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current F9v1.1 upper stage can be estimated to have a T/W of 45/(78.1+16) = 0.48. Lower than that for FH.

Ed has the tweet pic of 1Dvac but appears to have forgotten to update Elon's thrust comment (80 tons) to the table.
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Offline Joel

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Well, my guess is that SpaceX wants to have the development of their Mars architecture payed for and reach maturity by launching satellites to LEO/GTO/GEO. And that it will stage earlier than todays USs, hence requiring more thrust.

Of course, there might be different variants for LEO and Mars as well as for crewed/non-crewed. But they could probably all use the same engine (the Raptor), tanks (possibly of different lengths), avionics, reentry architecture etc.

Offline Joel

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current F9v1.1 upper stage can be estimated to have a T/W of 45/(78.1+16) = 0.48. Lower than that for FH.

Ed has the tweet pic of 1Dvac but appears to have forgotten to update Elon's thrust comment (80 tons) to the table.

OK. That gives significantly higher T/W. So, a MCT used as US would probably need shorter tanks than a MCT going to Mars.

Offline QuantumG

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What delta-v are you using for direct return from Mars?
Human spaceflight is basically just LARPing now.

Offline Jim

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1.5 T/W on Mars corresponds to 0.57 T/W on Earth. According to
http://www.spacelaunchreport.com/falcon9v1-1.html, current F9v1.1 upper stage can be estimated to have a T/W of 45/(78.1+16) = 0.48. Lower than that for FH.

So a MCT designed to be able to be used as a SSTO Mars-Earth return vehicle, should also have enough thrust to be a (reusable) upper stage on Earth.

from another thread

The issue is that it is not intuitive, even though the delta V is the same going either direction. The allocation of delta V between aerobraking and propulsive landing on Mars is not the same as Earth plus throwing in Mars ascent and final LEO injection delta V's.  Not to mention aerodynamic and structural  differences for the aerobraking at the two planets.  There is no optimization. The vehicle is going to have to be designed to handle the most stressing environment or propulsive requirement at either planet with the inherent mass.  If the aero thermo heat loads are worse for earth than Mars, then the TPS will be designed for that.  The propellant load will have to be sized for either LEO injection, TMI and Mars landing or Mars ascent, TEI, and earth landing.  The optimal aero design for entry and launch is not conducive for mounting and deploying solar arrays and radiators for cruise or landed operation.  Nuclear power negates the need for solar arrays but not radiators.

Offline Joel

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What delta-v are you using for direct return from Mars?

7 km/s, it that too low? Jim's diagram suggests 4.1+0.9+0.3+0.2+0.9 = 6.4 km/s.

Online meekGee

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Joel, I'd modify the logic.

Assume for a second that the MCT is coming back mostly empty.  The reason it's coming back is just re-use.  It might carry some science samples, but that's negligible.

So allocate all the "payload" mass to the empty structure.

Now on the Earth-Mars trip, you'd want to throw a fully-loaded MCT, which requires a much larger impulse.  How much payload per empty structure?  Not sure.  A lot of the payload (e.g. solar panels) barely requires any structure at all, might even be able to travel un-pressurized, so will be limited by what you can hide behind the heat shield on Mars.

Also, as Jim points out - the dV for Mars-to-Earth should be lower since you get much more aerobraking, since a) larger gravity well, and b) more atmosphere.
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Offline Joel

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Assume for a second that the MCT is coming back mostly empty.  The reason it's coming back is just re-use.  It might carry some science samples, but that's negligible.

Let's be more realistic. First of all, there won't be many trips going to Mars in the foreseeable future, if any. And if any spacecraft will return, it will do so so to carry people back. Bringing back spacecrafts just for reuse is way further into the future.

Also, as Jim points out - the dV for Mars-to-Earth should be lower since you get much more aerobraking, since a) larger gravity well, and b) more atmosphere.

The numbers I quoted are for Mars-to-Earth, taking aerobraking and different gravity wells into account. Do you have a better delta-v estimate?

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