Author Topic: MCT Speculation and Discussion Thread 4  (Read 621155 times)

Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #180 on: 06/20/2015 08:38 PM »
The reason the MCT dry weight is so important is that everything else is dependent on that. The 1st stage must be able when combined with the MCT spacecraft be able to put the MCT into orbit with 100mt of payload. Without the dry weight of the MCT you cannot specify how much propellant the MCT must hold (this is an iterative process with dry weight) and thus how large and how many engines are necessary for the 1st stage BFR. Everything returns back to the dry weight. The Raptors have less unknowns than the MCT dry weight. For the most part 5 Raptors on the MCT itself should be able to support quite a range of MCT weight unless it gets to heavy then the number would probably increase to 7 or 9 if things are really bad. Once you get to 9 that means that 15m for the diameter of the 1st stage is not going to be large enough you will need significantly more than 30 engines.

When I broke out everything except the heat shield as separate estimates the estimate did not change very much it reduced to 97mt from the 99mt earlier value. Unless there is some magic weight savings from somewhere I think the dry weight is going to be close to 100mt. It has been estimated way back that the reason SpaceX has hinted at the BFR being able to orbit 200+mt is that the MCT will weigh 100mt and carry a 100mt payload. I am beginning to believe that may be the right value to use since our estimates are narrowing in on the same value of ~100mt for the MCT dry weight.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #181 on: 06/20/2015 08:45 PM »
Why would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?

I would guess more like 30-35 tons.
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Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #182 on: 06/20/2015 08:49 PM »


Perhaps a mass estimate based on adding up the parts of the MCT would be more reasonable.

You'll need,

# of Raptors and mass of Raptor (lets assume 100:1 thrust:weight which would make them 2.3 mT each.
Thrust structure mass, probably proportional to thrust, few good examples to base a comparison on
Tank volume and tankage fraction, F9 tanks are reasonable basis for comparison
Surface Area and mass per unit area of Thermal protection
Structural mass, probably proportional to internal volume and peak g-forces.
Landing legs, I've read that these are generally 10% of touch down mass.
Auxiliary systems, solar panels, radiators, batteries, avionics etc etc, again hard to estimate.
I like this method for estimation better than just scaling up Dragon (or just random guessing ;) .)

Was that 10% of touch down mass or touchdown weight?  On Mars, it will have 100 tons of cargo but at a fraction of the gravity.  Earth landing should be less 0-20 tons of cargo.

The structural mass depends a lot on design details.  For example if the crew/cargo section is below the propellant tanks, the walls have to be beefed up to support the full propellant load (~1000 tons) through max Q.  Also, having the TPS somewhere other than the bottom (top or side), requires it to be beefed up further to handle load in multiple directions.

I think that there will only be a pressurized version (as opposed to an un-pressurised cargo version) for reasons of the cargo/crew pressure vessel doing double duty as the support structure, just like the propellant tanks.

Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #183 on: 06/20/2015 09:07 PM »
Why would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?

I would guess more like 30-35 tons.

Just the propellant tank portion, 5 Raptor engines and possibly landing legs too would weight ~40mt. Now add the reentry shield and the cargo bay structure. Of course the cargo variant will not have as high a dry weight as the crew variant but where is the tradeoff in crew payload size and crew vehicle dry weight increase. If you could get the cargo variant to have a dry weight as low as 60mt then reduce the payload size of the crew variant (crew + supplies) to only 60mt on a crew variant that dry weight 100mt things will work out better in that the overall system becomes smaller. You shrink the size and maybe some savings on the propellant tank dry weight due to smaller tanks.

My only problem with the estimates is that the more detail we go the heavier the MCT gets.

Offline Oceanbluesky

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Re: MCT Speculation and Discussion Thread 4
« Reply #184 on: 06/20/2015 09:30 PM »
Sorry this is a basic but what are "rendings"? Illustrated separations of rocket stages and their components (or should the links at the top of these threads read "renderings")??

Quote
L2 MCT Rending Effort (ongoing, large collection):
http://forum.nasaspaceflight.com/index.php?topic=35307.0

Thanks for the clarification

Offline Oceanbluesky

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Re: MCT Speculation and Discussion Thread 4
« Reply #185 on: 06/20/2015 09:45 PM »
Musk has said 80,000 people per year (and ten times as many cargo shipments), which is 1000 Passenger MCTs at once, plus 10,000 cargo MCTs (or actually, there ways around this, but it remains to be seen if they're worth it). So yeah, at any one time, there would need to be thousands of MCTs.

No, he has suggested a mature colony developed over decades would reach a population of 80k, not that eighty thousand settlers would be sent every year....

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #186 on: 06/20/2015 09:54 PM »
It shouldn't have worse dry mass per wet ton than a Falcon 9 first stage plus the fairing (not counting the heatshield).
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Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #187 on: 06/20/2015 09:56 PM »
No, he has suggested a mature colony developed over decades would reach a population of 80k, not that eighty thousand settlers would be sent every year....

Incorrect.

https://twitter.com/elonmusk/status/273483420468932608
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Offline Oceanbluesky

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Re: MCT Speculation and Discussion Thread 4
« Reply #188 on: 06/20/2015 11:05 PM »
Incorrect.
https://twitter.com/elonmusk/status/273483420468932608

Wow!! Thanks so much for taking your time to link to his tweet. (If it were not his account I wouldn't believe he'd said that - especially after reading the article he links to in various venues several times...it seems as if the author of that article is unclear as to the timeframe of 80k and Musk is tweeting a correction. Amazing! Sorry to have wasted your time, much appreciated. 

This is a phenomenal figure. Doable, but, ten times the passenger capacity of Disney's entire cruise fleet. Their largest ship carries about four thousand passengers in luxury for a couple of weeks, with quite a bit of superfluous amenities...eventually humanity will create interplanetary cruise lines but that is still a staggering figure. Glad someone is thinking boldly.

Did this figure initially take you aback?? Were you surprised by it?? (Jeez I must be thinking small in so much of my life...that still shocks me! Anyways thank you for your time!)
« Last Edit: 06/20/2015 11:11 PM by Oceanbluesky »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #189 on: 06/21/2015 02:24 AM »
Did this figure initially take you aback??

Honestly it still does.

But see it that way: combine 80000 people a year, 10 cargo flights for each colonist flight and 50 Million $ per flight the total cost of 440 Billion $ would still be below the present US military budget so certainly doable if there is a sufficiently strong motivation, like impending destruction of the earth. In that case this number could even be exceeded. But short of that?

I believe however that colonization and forming of a truly self sustaining civilization can be achieved with much more modest numbers IF the effort is sustained for a sufficiently long period. Like the present NASA budget for 100 years should be enough.
« Last Edit: 06/21/2015 10:08 AM by guckyfan »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #190 on: 06/21/2015 04:20 AM »


I like this method for estimation better than just scaling up Dragon (or just random guessing ;) .)

Was that 10% of touch down mass or touchdown weight?  On Mars, it will have 100 tons of cargo but at a fraction of the gravity.  Earth landing should be less 0-20 tons of cargo.

The structural mass depends a lot on design details.  For example if the crew/cargo section is below the propellant tanks, the walls have to be beefed up to support the full propellant load (~1000 tons) through max Q.  Also, having the TPS somewhere other than the bottom (top or side), requires it to be beefed up further to handle load in multiple directions.

I think that there will only be a pressurized version (as opposed to an un-pressurised cargo version) for reasons of the cargo/crew pressure vessel doing double duty as the support structure, just like the propellant tanks.

The landing gear mass is almost certainly driven by the force of impact with the surface NOT the static weight of the vehicle, in other words objects still have inertia irregardless of gravity.  And even if static weight weight were the concern you would need to size the legs based on the gross take off weight which we all agree will be greater then landing weight.

F9 first stage has 8% of dry mass in the leg system, and this is designed for flat artificial surfaces and is not carry precious human cargo.  The LEM had around 3% of touch down mass in legs, but that was a soft-touchdown with a deeply throttling engine, not the SpaceX 'hover-slam'.

I think Gross take off Weight will be ~450 mT total, not these monstrous 1000 ton figures.  And their would not be any kind of integral habitat in a 'crew' version.  Their will just be a single version with an unpressurized cargo bay into which a habitat module would be placed.

Max Q is aerodynamic pressure peak, in the Martian atmosphere it is an almost irrelevant force compared to the force experienced during launch from Earth, it is not the same as max g-forces which is what would be relevant for not crushing the vehicle.
« Last Edit: 06/21/2015 04:55 AM by Impaler »

Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #191 on: 06/21/2015 09:09 AM »
Did this figure initially take you aback??

Honestly it still does.

But see it that way: combine 80000 people a year, 10 cargo flights for each colonist flight and 50 Million $ per flight the total cost of 440 Billion $

So .. uh .. the cargo is free?


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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #192 on: 06/21/2015 09:20 AM »
So .. uh .. the cargo is free?

You are right, I miscalculated by one order of magnitude, sorry. Yes it is much higher than the US defense budget. Corrected my post.
« Last Edit: 06/21/2015 10:10 AM by guckyfan »

Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #193 on: 06/21/2015 09:47 AM »
So .. uh .. the cargo is free?

You are right, I miscalculated by one order of magnitude, sorry. Yes it is much higher than the US defense budget. Corrected my post.

Your original flight cost was calculated correctly (800 passenger flights, 8000 cargo flight, 8800 total at $50M a pop yields the $440B.

The cost of the cargo is presently unknown. My gut feeling is that the answer to question "what do you have to pack  in order to live on Mars" is quite lengthy, complex and thus expensive.
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #194 on: 06/21/2015 10:12 AM »

Your original flight cost was calculated correctly (800 passenger flights, 8000 cargo flight, 8800 total at $50M a pop yields the $440B.

The cost of the cargo is presently unknown. My gut feeling is that the answer to question "what do you have to pack  in order to live on Mars" is quite lengthy, complex and thus expensive.

True, I did not include the material value of the cargo.

Edit: Seems I am seriously slow this morning. Of course R7 was referring to the value of the cargo. I included only the cost of the flights.

It is really hard to make an educated guess. But it would be nowhere near the cost of present NASA payloads. Much would be COTS equipment, maybe slightly modified. Also what is dedicated Mars would not be single production items but produced in quantities for so many people.

Sometimes I use a first approximation and say the cargo is as much as transport cost. That would average 500000 $ for one t. Averaged between simple tools and Intel CPUs.
« Last Edit: 06/21/2015 10:26 AM by guckyfan »

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #195 on: 06/21/2015 04:19 PM »
I would think once the flight rate of 80, 000 colonists per year is reached, the cargo requirements per colonist fight will be much less than 10:1.

By that time, the mars industrial base should be able to produce anything they need short of integrated circuits.

Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #196 on: 06/21/2015 04:30 PM »
I would think once the flight rate of 80, 000 colonists per year is reached, the cargo requirements per colonist fight will be much less than 10:1.

By that time, the mars industrial base should be able to produce anything they need short of integrated circuits.
See this earlier post
 http://forum.nasaspaceflight.com/index.php?topic=37808.msg1390322#msg1390322


Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #197 on: 06/21/2015 04:35 PM »
I would think once the flight rate of 80, 000 colonists per year is reached, the cargo requirements per colonist fight will be much less than 10:1.

By that time, the mars industrial base should be able to produce anything they need short of integrated circuits.

You may well be right. A colony needs to be well advanced to be able to absorb that many people per year. I really don't see that mass exodus happen, ever. It is just that it is not completely impossible with available ressources.

Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #198 on: 06/21/2015 05:14 PM »
I would think once the flight rate of 80, 000 colonists per year is reached, the cargo requirements per colonist fight will be much less than 10:1.

By that time, the mars industrial base should be able to produce anything they need short of integrated circuits.

You may well be right. A colony needs to be well advanced to be able to absorb that many people per year. I really don't see that mass exodus happen, ever. It is just that it is not completely impossible with available ressources.

As I outlined earlier it is possible with using just the BFR/MCT as the workhorse for getting to and from he Earth and Mars gravity wells in the 100mt increments. Fo getting to amd from Earth and Mars a bigger/safer vehicle with better mass/people or mass/cargo ratios would bring down costs tremendously and since the MCT's would be used almost exclusively as a quick up/down vehicle with high reuse the whole endever remains fairly cheap.

A follow on to the MCT weight design issue I developed this chart to get a handle on the design space relative to propellant and dry weights relationships for the various profiles that the MCT would have to be capable of. The highest delta v of 9km/s is a L2 return from Mars and the 7.5km/s is a direct fiery reentry return from Mars. I also added the estimated tank prop amount based on a prop tank weighing 25% of the total dry weight of the MCT. This seems to be good guideline in the MCT design. Some iteration to optimize from that point would get an accurate weight model.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #199 on: 06/21/2015 05:32 PM »
Why would MCT weigh that much dry, particularly in a cargo config (because you mentioned 100mt payload, and Musk keeps talking about cargo flights as separate from passenger flights) and without yet counting the heat shield?

I would guess more like 30-35 tons.

Just the propellant tank portion, 5 Raptor engines and possibly landing legs too would weight ~40mt. Now add the reentry shield and the cargo bay structure. Of course the cargo variant will not have as high a dry weight as the crew variant but where is the tradeoff in crew payload size and crew vehicle dry weight increase. If you could get the cargo variant to have a dry weight as low as 60mt then reduce the payload size of the crew variant (crew + supplies) to only 60mt on a crew variant that dry weight 100mt things will work out better in that the overall system becomes smaller. You shrink the size and maybe some savings on the propellant tank dry weight due to smaller tanks.

My only problem with the estimates is that the more detail we go the heavier the MCT gets.

My visualization for the MCT version of the BFR upper stage is 4 raptors, but the hardware to cant them for Mars landing/take off.   I think 60t works for the dry weight of a cargo only version, and I am not committed one way or the other yet as to whether the passenger ECLSS and quarters are just cargo 'modules' that fit on an otherwise standard MCT or a seperately designed and built MCT.  What I do expect is that a passenger MCT is less loaded with payload than the cargo only one so that it has more ΔV partly for slighlty shorter transit time, partly for more safety margin.

My visualization for a reusable Earth orbit tanker Upper Stage for the BFR is a slightly smaller volume all fuel vehicle that adds little to the launch weight of the BFR and has about 10% lower dry weight than the MCT cargo. Running some numbers this morning I am only seeing 120mt of propellant left over (after margins) to transfer to a depot.  Note the idea is that there doesn't need to be active cooling on the tanker since it goes immediately to its offload rendezvous.  I want this vehicle to be specially designed because operationally it flies the most. With 4.1 flights per MCT going to Mars, plus whatever flights get made to satisfy other BLEO business the depot, MCTs and other LEO and Earth based Mars infrastructure gets used for.  So a small fleet of these makes sense. Cargo MCTs may well work for just launching cargo bound anywhere in LEO or beyond and I suspect that there will  be far less volume of this than keeping the depot topped up so no need to specifically develop a LEO cargo MCT.
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