Author Topic: MCT Speculation and Discussion Thread 4  (Read 626160 times)

Offline spacenut

  • Senior Member
  • *****
  • Posts: 2458
  • East Alabama
  • Liked: 423
  • Likes Given: 241
Re: MCT Speculation and Discussion Thread 4
« Reply #2000 on: 05/11/2016 01:11 AM »
I too have asked in the past about concentric nested cylinders for metholox.  Both are about the same temperature, I think about 20 Degrees different.  Plumbing would be short and would add to vertical strength of the rocket or stage. 

Offline envy887

  • Senior Member
  • *****
  • Posts: 4831
  • Liked: 2716
  • Likes Given: 1446
Re: MCT Speculation and Discussion Thread 4
« Reply #2001 on: 05/11/2016 02:18 AM »
Strength isn't as important as strength/weight or strength/volume ratios. A short nested cylinder is basically a common bulkhead. A full length nested cylinder loses most of the volumetric advantage because the inner cylinder has a slender aspect ratio.

If you think about what a nested cylinder becomes if you take the exact same parts and stack them, it's easier to visualize: you would have one half-full large tank and one completely full smaller tank. It will be hard for this arrangement to beat two properly sized tanks.

That said, there is an advantage in that the smaller tank has very little pressure or temperature differential radially... It could even be an inflatable membrane while the outer tank is the pressure vessel. I'll do a fist pass later with real(ish) numbers to see how the strength, volume and weight optimize...
« Last Edit: 05/11/2016 02:21 AM by envy887 »

Offline matthewkantar

  • Full Member
  • ****
  • Posts: 908
  • Liked: 617
  • Likes Given: 713
Re: MCT Speculation and Discussion Thread 4
« Reply #2002 on: 05/11/2016 05:35 AM »
The volume of LOX would be much smaller than the volume of the Methane, so the inner tank would be sized so that tanks of the same length contain the proper ratio of propellants.

Matthew

Offline envy887

  • Senior Member
  • *****
  • Posts: 4831
  • Liked: 2716
  • Likes Given: 1446
Re: MCT Speculation and Discussion Thread 4
« Reply #2003 on: 05/11/2016 12:33 PM »
What you said is true for hydrolox. It is not true for methalox. It takes 3.4kg of lox to burn 1kg of liquid methane, but lox is only about 2.6 times denser. If you have equal volume tanks you will have methane left over.

If your outer cylinder diameter is 10m, the same-length inner cylinder needs to be 7.5m in diameter to hold enough lox for complete combustion.

Offline matthewkantar

  • Full Member
  • ****
  • Posts: 908
  • Liked: 617
  • Likes Given: 713
Re: MCT Speculation and Discussion Thread 4
« Reply #2004 on: 05/11/2016 01:31 PM »
What you said is true for hydrolox. It is not true for methalox. It takes 3.4kg of lox to burn 1kg of liquid methane, but lox is only about 2.6 times denser. If you have equal volume tanks you will have methane left over.

If your outer cylinder diameter is 10m, the same-length inner cylinder needs to be 7.5m in diameter to hold enough lox for complete combustion.

I may have used incorrect numbers. If I understand your numbers correctly, LOX/liquid methane tanks would have about 1.3 cubic meters of LOX per cubic meter of liquid methane?

Matthew

Offline CyclerPilot

  • Member
  • Posts: 97
  • USA
  • Liked: 23
  • Likes Given: 0
Re: MCT Speculation and Discussion Thread 4
« Reply #2005 on: 05/11/2016 02:30 PM »
For a wider BFR and BFS, I think two nested cylinders would be the best design.  Cross section would be two concentric circles.  There would be no horizontal common bulkhead.  The side walls would no longer have to support the weight of the upper tank.  The new vertical common bulkhead would actually add to compressive strength of the rocket.

Offline envy887

  • Senior Member
  • *****
  • Posts: 4831
  • Liked: 2716
  • Likes Given: 1446
Re: MCT Speculation and Discussion Thread 4
« Reply #2006 on: 05/11/2016 04:41 PM »
Matt, yes it should be about 1.3 or so. The exact volume ratio will depend on the temperature of each fuel (subcooled is denser) and the optimum engine fuel:oxidizer mass ratio which in turn depends on chamber pressure.

Offline matthewkantar

  • Full Member
  • ****
  • Posts: 908
  • Liked: 617
  • Likes Given: 713
Re: MCT Speculation and Discussion Thread 4
« Reply #2007 on: 05/11/2016 05:24 PM »
I guess the fuel has to go in the inside tank. If the inner tank is 10 meters in diameter, then the outside tank would be 15.1 meters or so. Conventional stacking architecture, for the same tankage, would of course be the same height as the nested design, so aside from getting rid of the feed tube from the top tank to the engine cluster, the nesting design would have a lot more surface area of aluminum. The nesting design could use thinner elements all around though and and have strengths not present in the stacked design.

The BFR is likely in my opinion, to have a center engine plus two concentric rings of engines. The inner tank can be the load path for the inner engines, saving structure that would carry the load out to the outer tank walls in a conventional design. That structure is potentially quite heavy. I don't have the chops to spec the whole design out, but I think there is something here.

What I would really like to know is if this has been explored before by professional rocket designers. On the one hand this seems likely, but on the other, how many steely eyed rocket designers have set out to design a first stage with dozens of engines? A few for sure, (N-1) but not many.

Matthew

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Re: MCT Speculation and Discussion Thread 4
« Reply #2008 on: 05/11/2016 08:39 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.
Your logic is correct, but your statement isn't correct.
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 TomH

  • Senior Member
  • *****
  • Posts: 2230
  • Vancouver, WA
  • Liked: 930
  • Likes Given: 336
Re: MCT Speculation and Discussion Thread 4
« Reply #2009 on: 05/11/2016 10:19 PM »
The BFR is likely in my opinion, to have a center engine plus two concentric rings of engines

My guess is that they will consider the ideal T/W for landing and base the configuration decision on that. If one engine is ideal, there will be one center engine. If three engines burning at landing is ideal, I would not be surprised to see an equilateral triangular cluster in the center with that surrounded by concentric rings. The engines fired for boost back and/or entry may not even be the same engines used for landing.

Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 363
  • Likes Given: 0
Re: MCT Speculation and Discussion Thread 4
« Reply #2010 on: 05/11/2016 11:10 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.
Your logic is correct, but your statement isn't correct.

I can assure that is my real opinion and I am not secretly expecting a different size, so this would be considered a correct statement of my opinion.  Now if you a different opinion then by all means present it.

Also with regard to this idea of nested propellant tanks, my understanding of pressure vessels is that the outer tank would need to have the full mass and strength of a single tank with the combined volume of them both because the mass in the inner tank still contributes to the forces trying to burst the outer tank.

With regard to engine arrangement the central engine with 2 (or more) concentric rings of engines around it makes for the most flexible arrangement so long as the rings are hexagonal (which is most efficient anyway).  You can fire one central engine, 2 opposite flanking engines, 3 engines in a triangle, 4 engines etc etc.  In fact you should be able to fire a symmetrical group of engines in every single increment from one up to the total engine count so I see this as the optimal arrangement unless it presents some difficulty in propellant piping.

Offline philw1776

  • Full Member
  • ****
  • Posts: 1111
  • Seacoast NH
  • Liked: 762
  • Likes Given: 325
Re: MCT Speculation and Discussion Thread 4
« Reply #2011 on: 05/12/2016 03:38 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

IF the Rvac engine approaches the 5m width some have computed, 10m will be too small.
I figure the BFR/BFS as just under 100m tall but maybe 15m wide, certainly at least 12.5m.  If narrower, then taller.
In any case, it will look very different from the pencil necked geek Falcon9.
« Last Edit: 05/12/2016 03:40 PM by philw1776 »
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Oli

  • Senior Member
  • *****
  • Posts: 2245
  • Liked: 426
  • Likes Given: 57
Re: MCT Speculation and Discussion Thread 4
« Reply #2012 on: 05/12/2016 03:47 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

I doubt the tank is going to dominate the structural mass of the vehicle.

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Re: MCT Speculation and Discussion Thread 4
« Reply #2013 on: 05/12/2016 03:49 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

I doubt the tank is going to dominate the structural mass of the vehicle.
Efficient rocket stages are pretty much just giant tanks. Think again.
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 envy887

  • Senior Member
  • *****
  • Posts: 4831
  • Liked: 2716
  • Likes Given: 1446
Re: MCT Speculation and Discussion Thread 4
« Reply #2014 on: 05/12/2016 04:12 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

I doubt the tank is going to dominate the structural mass of the vehicle.
Efficient rocket stages are pretty much just giant tanks. Think again.

The Shuttle orbiter structure massed twice as much as it's external tank. That may not reflect MCT's design, but there aren't a lot of other baselines for very large reusable upper stages.

The MCT booster, however, will certainly be a giant tank with lots of engines and little else.

Offline Oli

  • Senior Member
  • *****
  • Posts: 2245
  • Liked: 426
  • Likes Given: 57
Re: MCT Speculation and Discussion Thread 4
« Reply #2015 on: 05/12/2016 04:15 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

I doubt the tank is going to dominate the structural mass of the vehicle.
Efficient rocket stages are pretty much just giant tanks. Think again.

I thought he meant BFS, my fault.

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5602
Re: MCT Speculation and Discussion Thread 4
« Reply #2016 on: 05/12/2016 04:22 PM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

I doubt the tank is going to dominate the structural mass of the vehicle.
Efficient rocket stages are pretty much just giant tanks. Think again.

I thought he meant BFS, my fault.
BFS requires rocket-stage-like performance and so will have to be built like a rocket stage. Tank mass is very important.
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 aameise9

  • Member
  • Posts: 55
  • Potsdam, Germany
    • Cognitive Biology
  • Liked: 14
  • Likes Given: 67
Re: MCT Speculation and Discussion Thread 4
« Reply #2017 on: 05/13/2016 12:33 AM »
May I ask if anyone has a conception how MCT is supposed to RETURN from Mars?

It was mentioned a few times that the return leg, surface to LMO and LMO to TEI, constitutes the most challenging aspect ('long pole') for the entire architecture.  Yet I have read almost no discussion of how this could be achieved.  Basically, there seem to be two extreme possibilities, plus a range of intermediates.

1) Mars surface is reached by a larger version of the Dragon, with engines suitable for precision landing.  But can the same engines take the craft back to orbit?

2) Mars surface is reached by a larger version of the F9R, with engines suitable for both orbital launch and precision landing.  But can such a craft launch without a launch complex and its infrastructure?  And can such a craft return to earth without replenishing propellants from a tanker in LMO?

Apologies if I have overlooked any information pertaining to these points!






Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 363
  • Likes Given: 0
Re: MCT Speculation and Discussion Thread 4
« Reply #2018 on: 05/13/2016 01:01 AM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

IF the Rvac engine approaches the 5m width some have computed, 10m will be too small.
I figure the BFR/BFS as just under 100m tall but maybe 15m wide, certainly at least 12.5m.  If narrower, then taller.
In any case, it will look very different from the pencil necked geek Falcon9.

2nd stage engine bells might indeed be an issue, but we need to know how many engines the 2nd stage would have.  If it's 4 engines then the 12.5m should work, if it's 5-7 then 15m would indeed be necessary.

Offline envy887

  • Senior Member
  • *****
  • Posts: 4831
  • Liked: 2716
  • Likes Given: 1446
Re: MCT Speculation and Discussion Thread 4
« Reply #2019 on: 05/13/2016 01:50 AM »
A lot of hints point to BFR as a F9 scaled up 10x. 15m lbf thrust vs. 1500; 100t to LEO with RTLS vs. 10t plus a margin for reuse. That would make BFR a 12.6m diameter at the same height if methalox is 20% less dense.
But Raptor is supposed to be scaled up about 3x, so for the same staging points the US needs 3 Raptors to match the Mvac. Three 5m bells should fit nicely on a 12.6 m stage.

Tags: