Author Topic: MCT Speculation and Discussion Thread 4  (Read 621772 times)

Offline Lampyridae

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Re: MCT Speculation and Discussion Thread 4
« Reply #440 on: 08/21/2015 11:08 AM »
Potential interpretations:
A)
the consumable budget is going to be considerably closer to refined food powder & oils than ISS' partially-dehydrated whole food panty...

Is the food really that bad on the ISS? Are they that desperate with the cargo ship losses?

j/k, couldn't resist.

But in all seriousness, remember that the ships will not fly on their own but launched as a rag-tag fugitive fleet. There is no need for there to be any food at all on the passenger MCTs. One logistics MCT would provide enough chow for the entire 1000 person fleet. The passengers could also fly in cold sleep, which I see as a viable option as far as colonisation goes.

@Burninate

I vote for D) None of the above. 100 is the number of people going to Mars in colonization mode. It is not anticipated and planned for that so many people will ever go back to earth. That number may be closer to 10 max. That could be provided for with 25t return mass. Provided the ECLSS for 100 people does not have too much weight by itself which could reduce the max number of people going back further or part of the ECLSS would need to be removed and go back on empty cargo MCT to maximize passenger capacity.

If the ECLSS on passenger MCT was made up of, say, 5 identical modules then after arrival on Mars, 3 modules could be removed for use in ground habitats leaving the other 2 for the return trip.  Saves weight and also rotates new equipment on every subsequent flight.  Similarly, gas and water storage tanks.

The 100 passenger phase is likely to be deep into the 21st century, 2070 or thereabouts. Rocketry has seen very little radical development since the 60s so we could be seeing the MCT still flying, but there could also be a whole host of unforeseen events. Bigelow building a cycler to serve the Earth-Mars routes.
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #441 on: 08/21/2015 12:05 PM »
If the ECLSS on passenger MCT was made up of, say, 5 identical modules then after arrival on Mars, 3 modules could be removed for use in ground habitats leaving the other 2 for the return trip.  Saves weight and also rotates new equipment on every subsequent flight.  Similarly, gas and water storage tanks.

I agree that everything really useful on Mars would be removed. Quite possibly storage tanks and bunks and dividers for the passengers may be useful. The ECLSS units not so much. ECLSS on Mars would be based on plants. Greenhouses that produce food enough to eat will also produce enough oxygen. Technical recycling like on spacecraft is not needed. So even if some of them need to be removed to maximise passenger capacity they would go back on cargo MCT for reuse unless single components like pipes, valves or fans are valuable on Mars.

It also depends on how much the weight of this equipment is. If the 25t return mass allow return of everything to earth and 10 people and supplies they may remove very little, only items of really high value on Mars.

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #442 on: 08/21/2015 12:29 PM »
If the ECLSS on passenger MCT was made up of, say, 5 identical modules then after arrival on Mars, 3 modules could be removed for use in ground habitats leaving the other 2 for the return trip.  Saves weight and also rotates new equipment on every subsequent flight.  Similarly, gas and water storage tanks.

I agree that everything really useful on Mars would be removed. Quite possibly storage tanks and bunks and dividers for the passengers may be useful. The ECLSS units not so much. ECLSS on Mars would be based on plants. Greenhouses that produce food enough to eat will also produce enough oxygen. Technical recycling like on spacecraft is not needed. So even if some of them need to be removed to maximise passenger capacity they would go back on cargo MCT for reuse unless single components like pipes, valves or fans are valuable on Mars.

It also depends on how much the weight of this equipment is. If the 25t return mass allow return of everything to earth and 10 people and supplies they may remove very little, only items of really high value on Mars.

Primary O2 source would likely be plants, but you must have a secondary system in case you have a plant disease issue.

Everything on an MCT will be high value to Mars, at least for some decades!

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #443 on: 08/21/2015 02:12 PM »
Secondary source would be the fuel ISRU. Plenty of oxygen and nitrogen produced there. Some CO2 scrubbing would be needed.

Some ECLSS units may be needed at outlying mining or research stations without their own plant LSS.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #444 on: 08/21/2015 04:02 PM »
The 100 passenger phase is likely to be deep into the 21st century, 2070 or thereabouts. Rocketry has seen very little radical development since the 60s so we could be seeing the MCT still flying, but there could also be a whole host of unforeseen events. Bigelow building a cycler to serve the Earth-Mars routes.

My impression was, this design exercise was for a vehicle & mission architecture which would work without thus-far-imaginary technologies like induced torpor, would be capable of equipping for 100-person missions and most of Musk's other requirements, and would have first mission in the 2030's and first launch in the 2020's.
« Last Edit: 08/21/2015 04:06 PM by Burninate »

Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #445 on: 08/21/2015 04:21 PM »
The 100 passenger phase is likely to be deep into the 21st century, 2070 or thereabouts. Rocketry has seen very little radical development since the 60s so we could be seeing the MCT still flying, but there could also be a whole host of unforeseen events. Bigelow building a cycler to serve the Earth-Mars routes.

My impression was, this design exercise was for a vehicle & mission architecture which would work without thus-far-imaginary technologies like induced torpor, would be capable of equipping for 100-person missions and most of Musk's other requirements, and would have first mission in the 2030's and first launch in the 2020's.

Agreed, but that doesn't mean SpaceX can pull it off from an engineering or cost point of view. We might see a smaller first generation MCT to get things rolling and a later second generation MCT that can handle 100-person missions.

Until SpaceX releases a PowerPoint showing what they really have in mind, we don't have much to go on.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #446 on: 08/21/2015 07:04 PM »
What if, some 100t cargo deliveries to Mars were made with a one way EDL system but launched from LEO with a reusable booster that could put 50 modules on a course to Mars each Synod?

So I have been assuming that a first generation Cargo MCT would be something like:

Dry weight 50t
Payload 100t
ISP 360
ΔV  6000m/s
Propellant 670t

And I presumed that at first several are sent that won't ever be returning (though their raptors and other systems could be spares to service future vehicles and their tanks the first local propellant stores).  However in this assumption I also assumed that we would have passenger carrying MCTs that had a significant enough different design that many systems and even possibly vehicle shape might be different.

Along with these I presumed an alternate BFR 2nd stage that was a reusable tanker, also presumed a LEO depot probably built from tankers, or even possibly a BFR first stage launched with no payload as an SSTO.

But lets imagine if we go this route:

A tanker, cylindrical with an aerodynamic fairing.

An MCT, the first version 10 - 20 passenger model that also carries significant amount of cargo.

And rather than sending cargo MCTs, imagine a cargo "capsule" that weighs in at 25t dry and includes the TPS just capable of the one way over Mars escape velocity entry and landing and has 100t of cargo. The total weight of this unit is 200t with 75t of propellant for 1300m/s ΔV (ISP 280). It is launched with full cargo and no fuel as the payload of a regular tanker craft. A stripped down tanker is used on orbit (2 engines instead of 4 or 5, no TPS) it is partially fueled (about 650t out of 800) and the 75t of propellant is loaded on the cargo pod. It boosts to TMI (4300m/s roughly) lets the cargo pod go, immediately boosts most of the velocity off backwards and does a little maneuver at apogee around 12,000km or so for a near rendezvous perigee and then rendezvous at perigee. As long as the fuel was ready at the depot and there were cargo pods to go it could do 50 launches per synod easily.

So I figure the pod would have some super draco style landing engines though fueled by Methalox, however it would save significant structural and equipment mass being designed for the one way trip and not be carrying raptors, as much reduced TPS, and even less robust landing gear. So I envisage 25t dry weight, 100t cargo and 75t propellant. The vehicle that launches it is based on a tanker and can be loaded with the appropriate amount of propellant for the mission requirement (based on when in the launch window the individual pod is going).  In the long run MCTs will bring most of the people and freight but in that initial build up period for the first few synods this might be an effective supplement.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #447 on: 08/22/2015 03:43 AM »
I don't see how any large lander can avoid having large amounts of retro-propulsion, a few Draco's might be nice for toughing down on at a thrust level just equal to gravity, but they can't exert enough force during the short time before impacting the surface.  Raptor rockets are definitively going to be necessary for something of that tonnage.

People keep throwing out this idea of some kind of vehicle which is designed to be expendable maximum efficiency cargo haulers, that's not going to happen.  If their are vehicles that don't come back to Earth it will be because they are demonstration vehicles (probably sub-scale) for gathering data and testing concepts.  Falcon-1 equivalents designed to be pathfinders rather then haulers, they will be overbuilt as an expendable and will try to incorporate and test return capabilities as soon as possible and any cargo they bring will be part of testing and perfecting return rather then trying to deliver any supplies for people.

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #448 on: 08/23/2015 01:50 PM »
A) Are there going to be a propellant depots, or are MCT's going to be refueled by a succession of tanker rendezvous?
A) Elon Musk mentioned depots. They will be needed when many flights go every launch window. However I believe that they won't need them early on. Two or three MCT, one of them passenger, can easily be fuelled directly.
That's consistent with the Wait-but-Why article, which says that Elon described the early process as sending up the MCT, returning the BFR and sending up tankers to refuel. With more MCTs it'll make sense to make a depot so that this can allow the MCT to be sent up last, getting a refuel quickly before departing. But at the beginning using small reusable tankers makes more sense.

Early missions:
a) BFR launches the MCT then BFR returns
b) BFR launches tanker then returns
c) Tanker refuels the MCT then returns for another run
Repeat b-c
d) BFR launches another MCT.

1 BFR, 1 Tanker, 2 or 3 early version MCTs.
I know others frequently say reusability is key, and it was clear to me (I thought), but the effect on the early planning only really just dawned on me.

Q: "How do you launch your fleet"
A: "We have a rocket, a big one"
Q: "Just one rocket?"
A: "Yes. And one tanker".
Q: "So you're sending just one spacecraft"
A: "No, we're sending 5".

Of course the reality is that redundancies are needed (though in today's world if one crashes it will hold up the sister rockets enough to miss the Mars window anyway).

Really once SpaceX builds a single BFR and tanker they can launch and relaunch, and relaunch. That first one will be expensive. When they have prototype MCTs they can launch them and refuel them. This could be a fun time for Elon :) And except for manned MCTs, other MCTs can stay in orbit a year waiting for the Mars window, to depart.

Getting a brilliant BFR and tanker early is more important than the MCT... with the exception that the BFR needs to know the MCT mass etc. That's why the Falcon reusability results are critical (while Dragon v2 can be a few years behind). And perhaps the early BFR and tanker can help with a Dragon mission to Mars.

(edit: sorry if that's really obvious!)
« Last Edit: 08/23/2015 01:55 PM by GregA »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #449 on: 08/23/2015 08:51 PM »
I’ve been running spreadsheets & rocket equation calcs to speculate about the MCT. I’ve taken these Musk’s statements as a given.
100mT land on Mars cargo
Land the whole thing and re-use it; i.e. return it to Earth
Raptor thrust over 230mT; use a lot of them.
363 seconds ISP vacuum

10 meter stage 1 diameter won’t get it done; we need a bigger boat.
BFR is 12.5m diameter, making a relatively squat vehicle just over 50m tall, no worries about pad towers for 100m high rockets.  BFR is shorter than F9 but 10 times more massive.  BFR’s 12.5m diameter is a nice size to fit living quarters and various colonial heavy equipment.

Don’t need 15 million LBS thrust, but with mass ratios 4.25% range need just over 11 million LBS (50 million newtons) thrust, 21engines.  CORRECTION:  Typo should be 22 engines

Given the huge delta V requirements for both Mars departure from refueling at LEO AND later functioning as a SSTO taking off from Mars’ surface and return to Earth, I put the Km/sec budget into the 2nd stage.  Stage one goes Low & Slow, just 3.4 Km/sec boosting the heavy 2nd stage before return to launch site, RTLS.

Total BFR mass 4100mT or 9 million LBS.  LEO mass fraction 4.25%.

Stage One:
12.5m diameter with 21m length propellant tanks
2950mT  6.5 million LBS  1st stage fueled mass
230mT thrust engines  506K LBS
21 engines 50 million Newtons  11.1 million LBS Thrust; T/W 1.23
Rings of 14 engines, 7 engines, plus a center engine
Avg ISP from sea level to vacuum 325
Only 3.4 Km/sec Delta V via Rocket equation
0.5 Km/sec additional Delta V for RTLS

Stage Two The MCT:
Dry Mass 175mT; 100mT is cargo
12.5m diameter with 7.5 m length propellant tanks. 
Cargo/passengers 12m; 1470 m3 volume
1150mT fueled mass (2.5 million LBS)
363 seconds ISP vac; Rvac engines 10% higher thrust as with F9
6 Rvac engines   3.3 million LBS Thrust
6.7Km/sec Delta V capability, via Rocket eq.

About 8 fuel cargo MCTs needed to refuel the MCT for LEO Mars departure

S1 avg ISP    325   
S2 vac ISP   363   
1st Stage T/W   1.23   
BFR DIA   12.5   m
MCT Mass   175   mT
1st Stage Tank Length   21.0   m
S1 Propellant Volume    2576   m3
1st Stg Airframe Weight   190   mT
Propellant Weight   2731   mT
S1 Engines Weight   33   mT
S1 Total Weight mT   2954   mT
S1 Total Weight LBS   6.5   Million LBS
S1 empty + S2   1373   mT
DRY Weight   223   mT
%  DRY WEIGHT     7.5%    %
RTLS Propellant   40   mT
RTLS Delta V   0.53   Km/sec
Stage One Km/sec   3.40   Km/sec Rocket Equation
2nd Stage Tank Length   7.5   m
Propellant Volume   920   m3
Propellant Mass   975   mT
S2/S1 mass   0.39   
S2 Mass w/MCT   1150   mT
S2 Mass w/MCT   2.5   Million LBS
Calc # Vac Raptors   6.00   1.32
Stage 2 Thrust   3.3   Million LBS
Stage 2 Km/sec   6.70   Km/sec Rocket Equation
S2 Mars 25mT Cargo    8.7   Km/sec Rocket Equation
TOTAL WT mT   4104   mT
TOTAL WT LBS    9.0   Million LBS
THRUST Needed    11.1   Million LBS
THRUST Needed    49.5   Million Newtons
1st STAGE # ENG    21.9    
Eng 14+7+1 = 22
LEO Mass Fract   4.26%   
« Last Edit: 08/24/2015 12:26 PM by philw1776 »
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #450 on: 08/23/2015 09:13 PM »
I am quite sure I recall ISP of the Raptor vac engine as 380 which would make it somewhat less heavy. Can someone confirm or correct me?

Edit: here the reference, planned ISP is 380

http://forum.nasaspaceflight.com/index.php?topic=37839.msg1390942#msg1390942

It is interesting to hear that 15 million pound thrust are on the high side. 11 and a smaller number of engines are certanly preferable.
« Last Edit: 08/23/2015 09:17 PM by guckyfan »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #451 on: 08/24/2015 12:13 AM »
380 vacuum ISP will definitely reduce both stage's mass.  Rocket equation.  i'll update.
Any other errata?
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #452 on: 08/24/2015 03:23 AM »
I'm doubtful this 2nd stage can be built, the vehicle mass is a mere 6.5% of the liftoff mass, while a F9 first stage with legs is 6% without anywhere near the entry velocity demands, and SpaceX has a lot of trouble getting that first stage to not "explode on impact with the atmosphere" at a mere 1.3 km/s and that's with retro-propulsion.  This vehicle would need to survive re-entry at Earth of at least 11 km/s possibly more.

If the vehicle is cylindrical and comes into the atmosphere like a bullet with either the top or bottom facing forward it could conceivably get away with having TPS on only that side and it would be compressed along the naturally strong long axis.  But it would probably not decelerate fast enough and either impact the martian surface or 'impact' the lower atmosphere on Earth and be crushed by dynamic pressure.  On the other hand if the vehicle comes in on it's side it needs TPS over a much larger area and is being compressed on a much weaker axis again causing it to buckle.

Offline Owlon

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Re: MCT Speculation and Discussion Thread 4
« Reply #453 on: 08/24/2015 04:11 AM »
380 vacuum ISP will definitely reduce both stage's mass.  Rocket equation.  i'll update.
Any other errata?

380 vacuum ISP for the vacuum Raptor, 363 vacuum ISP for the sea level Raptor. I'm pretty sure the 363 just came from forum estimates (albeit probably from actual propulsion engineers).

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #454 on: 08/24/2015 01:07 PM »
Revised the #s based on comments.  Changed ISPs for S1 average, sea level to vac & S2 ISP vac.
S2 added 5mT for TPS to the MCT and reduced the mass of S2 so that the ratio was more realistic. Side benefit was eliminating one Rvac engine so nearly another couple mT added for S2 TPS as well.  So S2 now has "only" 5 Rvac engines.  The 150mT MCTs cited here that carried 100mT cargo seem infeasible.

Added S1 propellant mass & adjusted dry mass to near 7%.  Added 5mT to RTLS propellant since I am clueless as to how much Delta V is needed.

S1 avg ISP SL to MECO   330   
S2 vac ISP    380   
1st Stage T/W   1.25   
BFR DIA   12.5   m
MCT Mass   180   mT
1st Stage Tank Length   21.5   m
S1 Propellant Volume   2637   m3
1st Stg Airframe Weight   175   mT
Propellant Weight   2796   mT
S1 Engines Weight   33   mT
S1 Total Weight mT   3004   mT
S1 Total Weight LBS   6.6   Million LBS
S1 empty + S2   1233   mT
DRY Weight   208   mT
%  DRY WEIGHT     6.9%   
RTLS Propellant     45   mT
RTLS Delta V   0.63   Km/sec
Stage One Km/sec   3.71   Km/sec Rocket Equation
2nd Stage Tank Length   6.5   m
Propellant Volume   797   m3
Propellant Mass   845   mT
S2/S1 mass   0.34   
S2 Mass w/MCT   1025   mT
S2 Mass w/MCT   2.3   Million LBS
Calc # Vac Raptors   4.98   1.23 T/W
Stage 2 Thrust   2.8   Million LBS
Stage 2 Km/sec   6.48   Km/sec Rocket Equation
S2 Mars 25mT Cargo    8.5   Km/sec Rocket Equation
TOTAL WT mT   4029   mT
TOTAL WT LBS   8.9   Million LBS
THRUST Needed    11.1   Million LBS
THRUST Needed    49.4   Million Newtons
1st STAGE # ENG    21.9     Eng 14+7+1 = 22
LEO Mass Fract   4.47%   
MCT Payload length   12   m
MCT Payload Vol m3   1472   m3
« Last Edit: 08/24/2015 01:12 PM by philw1776 »
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Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #455 on: 08/24/2015 02:15 PM »
MCT Mass   180   mT

S2 Mass w/MCT   1025   mT

S2 Mars 25mT Cargo    8.5   Km/sec Rocket Equation

I don't think those three go together.

A stage with 180mt dry mass, 8.5km/s delta v and 25mt payload has a wet mass of 1980mt.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #456 on: 08/24/2015 02:48 PM »
MCT Mass   180   mT

S2 Mass w/MCT   1025   mT

S2 Mars 25mT Cargo    8.5   Km/sec Rocket Equation

I don't think those three go together.

A stage with 180mt dry mass, 8.5km/s delta v and 25mt payload has a wet mass of 1980mt.

In the context of what he wrote it was 80t dry mass and 100t cargo outbound and 25t cargo inbound
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #457 on: 08/24/2015 02:54 PM »
MCT Mass   180   mT

S2 Mass w/MCT   1025   mT

S2 Mars 25mT Cargo    8.5   Km/sec Rocket Equation

I don't think those three go together.

A stage with 180mt dry mass, 8.5km/s delta v and 25mt payload has a wet mass of 1980mt.

In the context of what he wrote it was 80t dry mass and 100t cargo outbound and 25t cargo inbound

80t dry mass for landing 100t on the surface? That's impossible.

Not with a blunt body, certainly not with a slender body.

In fact you get only close to that with HIAD.

The problem is, going with retropropulsion instead of more aerobraking (e.g. with HIAD) does not reduce your dry mass, to the contrary, because you're coming in heavier and consequently your structural mass increases.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #458 on: 08/24/2015 04:12 PM »
So we're saying that the oft stated 200mT MCT landing on Mars is impossible?
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #459 on: 08/24/2015 04:18 PM »
MCT Mass   180   mT

S2 Mass w/MCT   1025   mT

S2 Mars 25mT Cargo    8.5   Km/sec Rocket Equation

I don't think those three go together.

A stage with 180mt dry mass, 8.5km/s delta v and 25mt payload has a wet mass of 1980mt.

In the context of what he wrote it was 80t dry mass and 100t cargo outbound and 25t cargo inbound

That was what I was attempting to say. 

I am correcting a spreadsheet formula error (rocket equation) for Mars takeoff.  EDIT formula was OK.
« Last Edit: 08/24/2015 05:02 PM by philw1776 »
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