Author Topic: MCT Speculation and Discussion Thread 4  (Read 611156 times)

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2060 on: 05/14/2016 06:38 PM »
1) Mars surface is reached by a larger version of the Dragon, with engines suitable for precision landing.

2) Mars surface is reached by a larger version of the F9R, with engines suitable for both orbital launch and precision landing.
Neither. Or rather, something in between.

You need maybe 6-7km/s to return to Earth from the surface of Mars. You can do that in a single stage with TPS, though it is hard.

Thank you!  So something in between a squat capsule and a pencil-shaped booster.

Presumably with short, stubby legs to support the weight of a fully tanked craft?

Presumably with engines angling outward (Dragon-style) to direct blast debris away from craft?

I am not sure that I understood the relevance of TPS (temperature protection system)?
Needs to be able to withstand entry to Mars, reentry back to Earth, and perhaps aerocapture. Definitely need a thermal protection system for that. But it'll have to be extremely lightweight.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2061 on: 05/14/2016 08:09 PM »
I am not sure that I understood the relevance of TPS (temperature protection system)?

Thermal protection system.  Thermal relates to heat. Though temperature and heat are related, they are not the same thing. Termerature is a measure of intensity. In comparing a match to an electric blanket, the lit match will have a higher temperature, while the electric blanket will put out more total heat, though at a lower temperature. One will keep you warm; the other will burn you, yet not keep you warm.

Offline Long EZ

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Re: MCT Speculation and Discussion Thread 4
« Reply #2062 on: 05/14/2016 09:23 PM »
Question: what is the best material to make MCT out of?

The options that I come up with are:

1) Aluminum - Lithium alloy just like Falcon.
2) Carbon Fiber composite using a honeycomb core.
3) Titanium single shell.
4) Titanium double shell with a core of some kind.

Considerations include heat tolerance, insulation needed, corrosion on Mars and Earth.
Stiffness when not pressurized. Thick core walls are much stiffer.
The material that I like is Titanium with hollow spaces to form a a 2 wall Ti sandwich with a Ti honeycomb core.
Obviously this is very difficult to make as it is hard to weld Ti alloys in air. But if that could be worked out it would seem to be the most durable.
Insulation is needed not only on Earth, but on Mars when accumulating ISRU propellants, so an inner layer of insulation is needed. For Carbon Fiber or Al-Li an outer layer of insulation would also be needed.

Offline kaoru

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Re: MCT Speculation and Discussion Thread 4
« Reply #2063 on: 05/14/2016 09:28 PM »
Cars on Earth don't have to carry their own oxygen. As RB said, you are letting your experience on Earth mislead you.
No I'm not...  I'm directly looking at the modes of the entire mission and matching it with existing technology or technology that could be applied.  Energy density per kilogram mass and the efficiency of turning that energy into useful work is the factors to determine the viability of using the solution for the mission.
The mission calls for at least two methods of power generation.  Solar will be the primary method with batteries as the primary storage to buffer normal usage.
I rule out nuclear since mixing humans with radioactive isotopes just have too many failure modes with no solution within the context of the mission. 
That leaves chemical reaction as the backup method.  This is obvious since the mission requires ISRU technology.  Since ISRU is energy storage, adding a way to make it useable in case of emergency requires very little extra mass since you already have most of it including the mass for CH4-O2/tanks.
Using batteries as a backup for X time beyond the normal usage would be more massive and you still need bigger solar panels to cover all current needs plus charging.
So in space, you have large pool of stored energy, CH4/LOX, to tap into in an emergency.  For on surface, as long as your ISRU is budgeted correctly that large pool of stored energy is available.
This is the basis of my opinion that some form of ICE or fuel cell will be included in the mission.  Since ICE is the most mature and well known technology, adds the least mass, and is flexible in application, it gets my vote.
Solar electric propulsion has vastly better "range" than any chemical rocket.
Yes, SEP has great ISP but no thrust because the amount of energy required to ionize a larger reaction mass to provide greater thrust is ridiculous.  SEP is great for low mass long duration flights measured in years.  It's not practicable for high mass "as short as possible" duration flights like a manned Mars mission.
Solar panels still work during dust storms. (Dust is actually easier for modern solar panels than water-droplet clouds on Earth.)

The power generated will drop, of course. But most of the power required on Mars will be for ISRU. You simply suspend power hungry ops during the worst of the storm. (Which, judging by MER-Opportunity, is only a few days even in a month long dust storm.)
So you lose some power...  How much power and for how long?  No one can answer this question because it's non-deterministic and we have little experience to fall back to.  Remember you have power ECLSS and ISRU because your lives depend on it.  This is why two primary methods of power generation is required.

Kaoru

Offline Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #2064 on: 05/14/2016 10:53 PM »
Question: what is the best material to make MCT out of?

The options that I come up with are:

1) Aluminum - Lithium alloy just like Falcon.
2) Carbon Fiber composite using a honeycomb core.
3) Titanium single shell.
4) Titanium double shell with a core of some kind.

Considerations include heat tolerance, insulation needed, corrosion on Mars and Earth.
Stiffness when not pressurized. Thick core walls are much stiffer.
The material that I like is Titanium with hollow spaces to form a a 2 wall Ti sandwich with a Ti honeycomb core.
Obviously this is very difficult to make as it is hard to weld Ti alloys in air. But if that could be worked out it would seem to be the most durable.
Insulation is needed not only on Earth, but on Mars when accumulating ISRU propellants, so an inner layer of insulation is needed. For Carbon Fiber or Al-Li an outer layer of insulation would also be needed.
The simplicity of your initial question is daunting. For me, it would have to be broken down into a long list, just for the major components, because each one will require drastically different engineering considerations. (Then I would duck and defer to a materials specialist.)
 
Mars Pioneers will require our continued support.

Offline Long EZ

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Re: MCT Speculation and Discussion Thread 4
« Reply #2065 on: 05/15/2016 08:38 PM »
I had in mind the structure of the propellant tanks and possibly the crew area. If the vehicle does atmospheric entry nose first, then it either needs a heat shield or be a material that can take the heat. Also of concern is micrometeorite  protection. There is a region just short of Mars that has a high concentration of them. A double wall structure is preferred for that. A double wall also has better stiffness for the weight.

Offline jjwaDal

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Re: MCT Speculation and Discussion Thread 4
« Reply #2066 on: 05/15/2016 08:44 PM »
Basically for the MCT to be real SpaceX has  to answer one question : « How do you make a HLV (possibly larger than the « Saturn-5 » rocket) both cheaper and reliable ?

There's an easy answer which is that to absorb the cost of development and the overall costs to keep  the whole system going you need to fly often. Mandatory but not sufficient.
Reusability is also an important point and they are working on that.
But strategy is paramount.

Looking at the numbers given for Falcon Heavy is telling…
If anyone had proposed at ESA or ULA or Roskosmos 7 years ago to build a rocket capable to lift 54 mT to LEO or 22 mT to GTO he would have been told there is no market for those capacities and the cost of development for a few launches (possibly to deep space or the army) doesn't justify that.
Well SpaceX is telling us FH can do that and will probably never do that.
But… Can lift 20 to 30 mT to LEO or 8 mT to GTO for 90 million $ and there's a market for that.
Because it is based on a rocket often flown, because it will have commercial customers and fly often too, and because first stages will be recovered and refly, Shotwell told us they were thinking of flying rocket in the long run for less than 10 million $…

Let's consider they can refly their first stages on a regular basis.

The next step should be the recovery of the second stage and RTLS for all first stages, and FH can't do that without losing too much lift capacity and too many customers.
But Raptor will come along and we can think of a single stick able to essentially return all stages to launch site and make more than the max capacities of FH in full expendable mode.
Full reusability and a very large fairing (let's say 10m +) should give it a commercial use given its price (including price/pound).
But such a rocket would enable travel to Mars in a Mars Direct style 10 years from now. Anyhow Elon is not going to send to Mars 100 people at a time  without lots of stuff on the ground and selected first tiny crews to tend the « shop »…
Bind three of these boosters and you would have the BIG ONE, cheap and reliable before sending anybody to Mars.

I think it would be suicide to build directly the BFR.  :o

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2067 on: 05/21/2016 10:00 PM »
Trip Harris is celebrating 9 with SpaceX today, and is currently is Manager of Falcon landings. The YouTube clip he chose to link to his celebratory tweet is....interesting.

https://twitter.com/SpaceXTrip/status/733869950067036160

Quote
@SpaceXTrip
Today I celebrate nine years @SpaceX! It's amazing how much it's changed, and I am excited for what the future holds https://t.co/oNLgPQM9my

Quote
Picard:"Somehow I doubt this will be the last ship to carry the name Enterprise."

DM

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #2068 on: 05/22/2016 01:06 PM »
Trip Harris is celebrating 9 with SpaceX today, and is currently is Manager of Falcon landings. The YouTube clip he chose to link to his celebratory tweet is....interesting.

https://twitter.com/SpaceXTrip/status/733869950067036160

Quote
@SpaceXTrip
Today I celebrate nine years @SpaceX! It's amazing how much it's changed, and I am excited for what the future holds https://t.co/oNLgPQM9my

Quote
Picard:"Somehow I doubt this will be the last ship to carry the name Enterprise."

There were a lot of other good quotes in this strip. Doesn't mean its this one. And maybe he just liked the scene a lot without deeper meaning. I wouldn't read too much into it.

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #2069 on: 05/22/2016 01:30 PM »
Solar panels for MCT could be made to flip upside down at night to keep potential dust off, then flip back during the day.  They are going to be packed for flight, so a few adjustments on the design of them can flip the panels.  I also think a mixture of materials for MCT.  Seems like titanium around the bottom sensitive areas would be better than aluminum, based on what we have seen with Falcon 9.  At one time on the proposed X-33 or Oriental Clipper a titanium wire fabric was to be used on the skin of this spacecraft. 

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2070 on: 05/22/2016 05:27 PM »
Such frequent cleaning is not needed and dust is likely to have enough electrostatic cling that it will not fall off simply by flipping something upside down (try this with a dusty object on Earth if your not convinced).

The only reliable way to remove dust is brush or blow it off, as the solar panels need to have a retraction system simply put at air-knife along the hull so that the panel is blown clean when retracted.  Periodically retract and redeploy the panel as needed and you avoid any need to turn or rotate the panel either under gravity or in space.

Offline sanman

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Re: MCT Speculation and Discussion Thread 4
« Reply #2071 on: 05/25/2016 06:52 AM »
I don't know if it's been talked about yet - but what about MCT/BFS and the hoverslam? As F9R is showing, hoverslam can be pretty damn rough/tough on the rocket. So what would MCT/BFS face during descent on Earth or Mars, and would hoverslam be feasible? Or are you just going to need much more deeply throttleable engines?


Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2072 on: 05/26/2016 12:18 AM »
I don't know if it's been talked about yet - but what about MCT/BFS and the hoverslam? As F9R is showing, hoverslam can be pretty damn rough/tough on the rocket. So what would MCT/BFS face during descent on Earth or Mars, and would hoverslam be feasible? Or are you just going to need much more deeply throttleable engines?
It's not necessarily the hoverslam that is rough on the rocket, it's the reentry. Shield from reentry heating, and hoverslam is simply a more efficient way to land (though maybe a bit sketchy with crew).
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

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #2073 on: 05/26/2016 02:26 AM »
I don't know if it's been talked about yet - but what about MCT/BFS and the hoverslam? As F9R is showing, hoverslam can be pretty damn rough/tough on the rocket. So what would MCT/BFS face during descent on Earth or Mars, and would hoverslam be feasible? Or are you just going to need much more deeply throttleable engines?
It's not necessarily the hoverslam that is rough on the rocket, it's the reentry. Shield from reentry heating, and hoverslam is simply a more efficient way to land (though maybe a bit sketchy with crew).

Brace everyone properly in comfy chairs (with lots of straps), ensure you have maximal confidence, have engines that throttle deeper than merlins, and try not to think about the three figure ton bullet you're sitting in hurtling towards the concrete. Hopefully (going out on a limb to say certainly) BFS will have a lower thrust-to-weight ratio compared with a F9 stage one.

It's -just- a D2 (with a foot about a 13th of the area of a Manhattan city block and about as heavily populated) with less margins and no chutes - for that reason, they better be damn sure everything works according to plan.

That's partly why we have D2 and F9r, of course. I for one look foreward to watching gigantic martian apartments dropping out of the sky. It'd probably be the most watchable component of the whole program.
« Last Edit: 05/26/2016 02:37 AM by The Amazing Catstronaut »
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2074 on: 05/27/2016 02:59 PM »
...
Hopefully (going out on a limb to say certainly) BFS will have a lower thrust-to-weight ratio compared with a F9 stage one.

We can put some rough bounds on that. Musk said the Raptor is targeting about 230t of thrust at SL, which should give about 280t thrust in vacuum. To deliver 100t to Mars surface will take about 100t of dry mass (more or less). The same dry mass is supposed to return 25t to Earth. BFS is supposed to have "multiple" engines, which based on possible core diameters and vac bell sizes probably means 3 engines.

Put all those together (total thrust / total mass - local gravity) and the net acceleration at the Mars surface is 3.8g, and the net acceleration at the Earth's surface is 4.5g.


Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2075 on: 05/28/2016 02:08 AM »
What do you mean 3.8g and 4.5g, thouse are incredible acceleration rate and likely beyond what crew could tolerate, did you mean m/s of net upward acceleration at launch, aka how fast you will move away from the ground?  Even then they are incredible and imply crushing burnout acceleration rates, please show your formula for deriving these values.

When looking at vehicle acceleration rates the acceleration at burn out is very important too as it determines a lot of the strength requirements of the vehicle and it's cargo.  Throttling down or shutting down engines can keep the acceleration reasonable a triangular 3 engine configuration is poor as it permits no shutdown.
« Last Edit: 05/28/2016 03:15 AM by Impaler »

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #2076 on: 05/28/2016 07:02 AM »
What do you mean 3.8g and 4.5g, thouse are incredible acceleration rate and likely beyond what crew could tolerate,

I don't think so... 4 g isn't that big of a deal with correct positioning. The Shuttle was limited to 3, but Apollo astronauts took way more g's than that.

Offline Elmar Moelzer

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Re: MCT Speculation and Discussion Thread 4
« Reply #2077 on: 05/28/2016 07:52 AM »
Some rollercoasters pull higher g loads (I heard of up to 7 though only for a short time).

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2078 on: 05/28/2016 01:00 PM »
Humans can handle 8-9 gees just fine if oriented the correct way.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Hyperion5

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Re: MCT Speculation and Discussion Thread 4
« Reply #2079 on: 05/28/2016 02:04 PM »
Humans can handle 8-9 gees just fine if oriented the correct way.

Not for very long, though.  NASA limited sustained gees on Apollo flights to 4.5 g and Shuttle flights to 3 g for very good physiological reasons.  Spacex should be aiming for a similar sustained g load for manned launches and reentries for the same reasons.

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