Author Topic: MCT Speculation and Discussion Thread 4  (Read 569095 times)

Offline hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #2920 on: 09/27/2016 03:46 PM »
O.K. I want to change my guess for number of engines from 31 to 27.

And this isn't just because Musk informed us that the thrust is going to be higher than previously hinted at. ;)

It's also because I realized that, while 31 gives a very nice, minimum diameter packing pattern with a single center engine (like the F9), having a single center (landing) engine is probably not appropriate or desired here.  It's unlikely that the BFR could land on a single engine - just too heavy.  3 centralized engines would probably be appropriate.  And the nice symetrical, minimum pattern for 3 center engines comes out to be 27.

3 center engines is much worse than 1.

If any of those 3 center engines fail, then it's crash and burn.

But with many more engines, the landing burn can be done with center engine + ANY two engines that are on oppisite sides of the craft.




Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2921 on: 09/27/2016 04:26 PM »
O.K. I want to change my guess for number of engines from 31 to 27.

And this isn't just because Musk informed us that the thrust is going to be higher than previously hinted at. ;)

It's also because I realized that, while 31 gives a very nice, minimum diameter packing pattern with a single center engine (like the F9), having a single center (landing) engine is probably not appropriate or desired here.  It's unlikely that the BFR could land on a single engine - just too heavy.  3 centralized engines would probably be appropriate.  And the nice symetrical, minimum pattern for 3 center engines comes out to be 27.

3 center engines is much worse than 1.

If any of those 3 center engines fail, then it's crash and burn.

But with many more engines, the landing burn can be done with center engine + ANY two engines that are on oppisite sides of the craft.





I think the nearly empty BFR (with some landing propellent) would be too massive for a single Raptor.  And I think if any of the 3 central engines fail, there would be a backup engine in line with the failing engine and the center of the vehicle which could be called into service.  Throttling and gimballing employed to null out any resultant moments.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2922 on: 09/27/2016 05:17 PM »
One engine should be enough. The empty first stage does not weigh that much. I hope for a configuration with 3 central engines. The 3 fire but if one fails two are enough for braking and stability by gimballing. For the final approach any one of the three engines will do. Engines 2m from center with a 12-15 m stage should be able to land.

Offline BSenna

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Re: MCT Speculation and Discussion Thread 4
« Reply #2923 on: 09/27/2016 05:18 PM »
Last try:
« Last Edit: 09/27/2016 05:18 PM by BSenna »

Offline Hauerg

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Re: MCT Speculation and Discussion Thread 4
« Reply #2924 on: 09/27/2016 05:31 PM »
I'm revising BFR diameter down to a range of 10 - 12 m based on Musk comment on raptor being comparable to Merlin size but carrying 3x thrust.

That completely rules out 15 m diameters, a vehicle that size is approximately 9x the base area of F9, with triple thrust density of Raptor would lead to 27x the thrust of F9 or something close to 45 million pounds of thrust, 3x times what Musk has aimed for.

Fundamentally the 15 m speculation was nothing more then a crude attempt to multiply the thrust of F9 by a factor of 9 while completely ignoring the thrust density improvements that come from the full-flow staged combustion cycle.

We wil know within 90 minutes. But iirc there was indeed a quote from elon, or second hand reports of it, regarding 15m tank tooling or something like that.

Offline malu5531

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Re: MCT Speculation and Discussion Thread 4
« Reply #2925 on: 09/27/2016 05:43 PM »
Target BFR thrust stated by EM and Raptor announced thrust seems to indicate 25-27 Raptors which is still too many for my liking.

Looks like my guess was closer.  :D
« Last Edit: 09/27/2016 05:44 PM by malu5531 »

Offline dasmoth

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Re: MCT Speculation and Discussion Thread 4
« Reply #2926 on: 09/27/2016 05:45 PM »
Those who didn't want it to be like the N1 have got their wish.

I make that 42 engines!

Offline vandersons

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Re: MCT Speculation and Discussion Thread 4
« Reply #2927 on: 09/27/2016 05:54 PM »
So, if I got that right the MCT is a VTOL lifting body spacecraft. Very impressive!

Online ZachF

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Re: MCT Speculation and Discussion Thread 4
« Reply #2928 on: 09/27/2016 05:57 PM »
42 engines!!!! good god!

That will be like 25 million pounds of thrust at liftoff!!!!  :o 8)  ;D
« Last Edit: 09/27/2016 05:57 PM by ZachF »

Offline inventodoc

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Re: MCT Speculation and Discussion Thread 4
« Reply #2929 on: 09/27/2016 06:00 PM »
Yup 42.  Going full N1 on this thing.

Online ZachF

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Re: MCT Speculation and Discussion Thread 4
« Reply #2930 on: 09/27/2016 06:01 PM »
Just noticed the video says 28,730,000 pounds of thrust (127,800kN)

That rocket will be putting out more power than entire large nations when lit off!

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2931 on: 09/27/2016 06:08 PM »
Showing my cards before they're technically obsolete:
https://docs.google.com/spreadsheets/d/1CGSeJCfWXDO9wWFW6DmuluSWdhvvkiES6XkJiJHyxaI/edit?usp=sharing
https://docs.google.com/document/d/1SJyYrgMfc1baz28EXOUPBfdBGKvXaWwLyv4stwxTZ_M/edit?usp=sharing

Illustrations were a rush-job, but they work as a diagram, and my figures are vetted against pressure, temperature, tank-volume, propellant-mass, thrust-threshold, Isp, delta-V, and some components of dry mass.

The capsule tip didn't get much elaboration because I didn't have time to finish the math, but it's important to the architecture and I'm convinced you could fit it into 150mT dry mass provided enough carbon fiber.  The legs also got left out of the document (six of them for redundancy, rather spidery and shock-absorbing).

This thing fits a token amount of reliable hypergols for each trip to maneuver.
« Last Edit: 09/27/2016 06:16 PM by Burninate »

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2932 on: 09/27/2016 06:44 PM »
42 engines!!!! good god!

That will be like 25 million pounds of thrust at liftoff!!!!  :o 8)  ;D

Yup.  Glad I revised my engine count downwards just in time!  :o

Offline matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2933 on: 09/27/2016 07:22 PM »
1000 ships!

Offline BSenna

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Re: MCT Speculation and Discussion Thread 4
« Reply #2934 on: 09/27/2016 07:46 PM »
12 meter diameter. This one was considering 15m.

Offline hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #2935 on: 09/27/2016 08:58 PM »
So, 3 SL engines and  6 fixed-mount vacuum engines.

So the 3 SL engines handle landing back to earth. And also for liftoff from mars and after staging when ging from earth the extra thrust of these is needed.

Interplanetary transport burns can be done with just the fixed vacuum engines, if differential thrusting and manouvering thrusters give enough control authority.

Offline matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2936 on: 09/27/2016 09:05 PM »
3 center engines is much worse than 1.

If any of those 3 center engines fail, then it's crash and burn.

But with many more engines, the landing burn can be done with center engine + ANY two engines that are on oppisite sides of the craft.

You need to tell SpaceX right away.

Matthew

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #2937 on: 09/27/2016 11:07 PM »
Musk just finished a press conference, and was asked about abort capabilities:

https://twitter.com/jeff_foust/status/780896313676148737
Quote
Musk: spaceship can serve as own abort system from booster, but on Mars, either you’re taking off or you’re not. #IAC2016

https://twitter.com/NASAWatch/status/780896600294064128
Quote
Asked about abort modes for launcher @elonmusk said "make it very reliable ... you do not have parachutes for commercial airliners" #IAC2016

I have argued exactly this on these MCT threads many times, I'm glad he shares the same point of view. At this scale such systems are impractical.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2938 on: 09/27/2016 11:09 PM »
Looks like a got closest on the booster,

12 m diameter, check
tall and skinny, not short and wide, contrary to consensus
carbon fiber, check but obvious
some engines fixed rather then gimbaling, only person to predict that
possible manufacturing a Mchoud, check
landing directly on launch pad with 2nd stage put on by crane, speculated on that about a month ago
engine count undershot by 11 and thrust by about half, but did anyone predict this much thrust

Almost everything above the booster I got wrong, and I'm very skeptical of that part of the vehicle.

Offline matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2939 on: 09/27/2016 11:31 PM »
I guessed 36 engines, did not see anybody guessing more, so if we use The Price Is Right rules....

Matthew

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