Author Topic: MCT Speculation and Discussion Thread 4  (Read 610455 times)

Offline Bynaus

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Re: MCT Speculation and Discussion Thread 4
« Reply #2900 on: 09/26/2016 07:02 PM »
I think Musk's talk tomorrow will follow more or less the following logic:

0) Some remarks on AMOS-6
1) Need to make humans multiplanetary (some new twist or variant on the common theme)
2) How? Self-sufficient colony on Mars is best bet, but will not stop there. Other places just a bit harder and further.
3) How to build a colony? In steps. Argues 100 t payload per step ist best. Methalox ist best because ISRU.
4) "It all follows logically from there" -> takes us from 100 t to surface to required specs of BFR, BFS.
5) Unveils design (15 m, all that). Shows Raptor video.
6) Unveils hilariously optimistic ;) timeline to first landing:
- 2018 first BFR booster ready for initial testflights (at Spaceport America)
- 2019 first BFS ready for initial testflights.
- 2020 first orbital flight of BFR+BFS stack. First BFS-Tanker built.
- 2021 orbital refueling of BFS with tanker, lunar orbital shakedown cruise of BFS, perhaps lunar landing
- 2022 launch of first BFS (unmanned) towards Mars. First manned flight of BFS in Earth orbit, later lunar orbit (and surface?).
- 2024 launch of first BFS (manned) towards Mars.
- 2025 Landing day.
7) Makes case for public-private partnerships. "Cannot do this without NASA. But NASA might not be able to do this without us." Presents clever funding scheme which will keep them running with a small fraction of what NASA invests in SLS every year. "NASA should diversify", not only SLS but also alternatives in case rocket grounded after mishap. Multiple plans, "also Blue Origin", should keep us on track to Mars.
8 ) A bit more on the long-term plans, reiterates 500k$/pP. "Everybody who can afford a house today will be able to go." Might pool funding from wealthier passengers to enable even more people to go.

Can't wait! :D
« Last Edit: 09/26/2016 07:02 PM by Bynaus »

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2901 on: 09/26/2016 07:32 PM »
The Falcon Heavy will lift off with 27 Merlins. I don't think it is too crazy to imagine a similar number of Raptors on this baby for lift-off.

Robustness and reliability are all-important for the engines that will act as the work-horses of this space system architecture. Commonality between the first and second stage dictates a reduced maximum size of the engine.

Many smaller engines instead of five huge ones also means that SpaceX will quickly accumulate very many hours of operation. This will help improve reliability. Running the engines to "destruction" won't necessarily result in mission failure during the first years of operations, as was proven in the F9 CRS-1 launch.

A further consideration is that if they need to swap out a Raptor on Mars (bringing along a spare) the engine can't be too big.

Many factors speak in favour of using many engines, like the N-1, rather than just a few, like the Saturn V.

Offline Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #2902 on: 09/26/2016 07:41 PM »
I agree with you, but N1 isn't something that "speaks in favour" of having many smaller engines instead of few big engines, though.
« Last Edit: 09/26/2016 07:42 PM by Pipcard »

Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2903 on: 09/26/2016 08:01 PM »
I agree with you, but N1 isn't something that "speaks in favour" of having many smaller engines instead of few big engines, though.

ISTM it's worked quite well for Falcon 9, with engine-out saving CRS-1's bacon. So long as there continues to be armored engine cells to prevent fratricde, why not?
DM

Offline Mike_1179

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Re: MCT Speculation and Discussion Thread 4
« Reply #2904 on: 09/26/2016 08:03 PM »
I agree with you, but N1 isn't something that "speaks in favour" of having many smaller engines instead of few big engines, though.

The B-52 uses 8 engines (TF-33) that make 76kN each. Total thrust 608 kN. It's been flying that way since the 50's

The Boeing 787 uses 2 engines (one variant uses GEnx-1B64) which make 284 kN each. Total thrust 568 kN.

It works both ways - you define the requirements of the system (cost, performance, maintenance, reliability) and design the system from there.

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2905 on: 09/26/2016 08:18 PM »
The Raptors are designed for missions lasting several years away from maintenance services. Reliability of the system under such circumstances is a crucial requirement.

Offline dglow

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Re: MCT Speculation and Discussion Thread 4
« Reply #2906 on: 09/26/2016 08:50 PM »
I think Musk's talk tomorrow will follow more or less the following logic:

...

7) Makes case for public-private partnerships. "Cannot do this without NASA. But NASA might not be able to do this without us." Presents clever funding scheme which will keep them running with a small fraction of what NASA invests in SLS every year. "NASA should diversify", not only SLS but also alternatives in case rocket grounded after mishap. Multiple plans, "also Blue Origin", should keep us on track to Mars.

I like your thinking save for #7. Musk will, as usual, praise NASA as an excellent partner. But he will be wise to avoid any mention of SLS or NASA 'needing' SpaceX. I can think of many a Senator and Congressperson who will already feel threatened by the reveal of BFR.

Musk's best bet is to avoid either-or comparisons between The US Govt.'s Rocket and his own. Rather, emphasize that SpaceX's system will be yet another tool available in the public/private toolbox of US Space Exploration over the years to come.

Finally, I would add one to your list:

2.5) Red Dragon in ~2018; emphasize SX-NASA cooperation, demonstration of ISRU on-planet.

Offline DJPledger

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Re: MCT Speculation and Discussion Thread 4
« Reply #2907 on: 09/26/2016 09:03 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.

We'll know in just over a day (I hope).
You can have 27 engines with a single center engine. The configuration for BFR would be a single center engine surrounded by rings of 8 and 18 engines.

Offline Bynaus

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Re: MCT Speculation and Discussion Thread 4
« Reply #2908 on: 09/26/2016 09:14 PM »
I think Musk's talk tomorrow will follow more or less the following logic:

...

7) Makes case for public-private partnerships. "Cannot do this without NASA. But NASA might not be able to do this without us." Presents clever funding scheme which will keep them running with a small fraction of what NASA invests in SLS every year. "NASA should diversify", not only SLS but also alternatives in case rocket grounded after mishap. Multiple plans, "also Blue Origin", should keep us on track to Mars.

I like your thinking save for #7. Musk will, as usual, praise NASA as an excellent partner. But he will be wise to avoid any mention of SLS or NASA 'needing' SpaceX. I can think of many a Senator and Congressperson who will already feel threatened by the reveal of BFR.

Musk's best bet is to avoid either-or comparisons between The US Govt.'s Rocket and his own. Rather, emphasize that SpaceX's system will be yet another tool available in the public/private toolbox of US Space Exploration over the years to come.

Finally, I would add one to your list:

2.5) Red Dragon in ~2018; emphasize SX-NASA cooperation, demonstration of ISRU on-planet.

You are certainly right that he will praise NASA, and you are probably right that its perhaps be better if he would not mention NASA needing SpaceX - but then, if they cannot do it on their own, he has to make the case why it would be a good thing for NASA to invest some money in the SpaceX plan: and that is difficult to do if SpaceX don't have anything to offer that NASA wants/needs. Although, thinking of it, redundancy might be that thing...

I didn't want to imply he would do an "either-or" comparison with SLS, more like you said, another tool in the box (hence the "NASA should diversify").

There is one more thing I forgot to list, somewhere around 5) : badass video of the BFR/BFS-architecture. :D

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2909 on: 09/26/2016 09:24 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.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2910 on: 09/26/2016 09:28 PM »
I think Musk's talk tomorrow will follow more or less the following logic:

...

7) Makes case for public-private partnerships. "Cannot do this without NASA. But NASA might not be able to do this without us." Presents clever funding scheme which will keep them running with a small fraction of what NASA invests in SLS every year. "NASA should diversify", not only SLS but also alternatives in case rocket grounded after mishap. Multiple plans, "also Blue Origin", should keep us on track to Mars.


I like your thinking save for #7. Musk will, as usual, praise NASA as an excellent partner. But he will be wise to avoid any mention of SLS or NASA 'needing' SpaceX. I can think of many a Senator and Congressperson who will already feel threatened by the reveal of BFR.

Musk's best bet is to avoid either-or comparisons between The US Govt.'s Rocket and his own. Rather, emphasize that SpaceX's system will be yet another tool available in the public/private toolbox of US Space Exploration over the years to come.

Finally, I would add one to your list:

2.5) Red Dragon in ~2018; emphasize SX-NASA cooperation, demonstration of ISRU on-planet.

Mostly agree with 1-6, but 7 doesn't sound like Elon.

I think he will talk about Mars ground systems (habs, pressurized and unpressurized rovers, power, ISRU other than for fuel, research, mining, etc. ) and how there is a need for partners to implement them. I think the fact that Elon is presenting at IAC means that he hopes some of those partners will be international.


Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2911 on: 09/26/2016 09:43 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.

The 15 m diameter speculation had little to do with individual engine thrust.

1. Various attempts have been made to optimise the BFR mass, these give least tank mass for a 13-15 m stage.

2. The BFS has been said by Musk to be very large, a scaled Dragon capsule design might have to be over 23 m, which would fit much more easily on a 15 m stage. Biconic or semi-lifting body BFS designs would seem to need 12-15 m diameter to enclose the inidcated volume.

3. L2 info and various leaks on reddit

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #2912 on: 09/26/2016 09:46 PM »

...



SpaceX has what NASA needs. SX are being paid 1.6B for it. And then there's CRS-2. That's quite some money invested by NASA into the SpaceX plan
« Last Edit: 09/26/2016 09:49 PM by mfck »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2913 on: 09/26/2016 10:22 PM »
There are a few things that have stopped me from publishing my full plan other than poor time management.

Not wanting to assume an unprecedented chamber pressure was one of them, despite hints that FFSC was aimed at that.  I couldn't get RPa Lite to spit out the right numbers at ~20MPa;  While 360-370s is easy at lower pressures with realistic combustion inefficiencies, 380s is not.  I guess that's out of the way.  I'll see how much I can write tonight, before it is completely technically obsolete.
« Last Edit: 09/26/2016 10:34 PM by Burninate »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2914 on: 09/26/2016 10:33 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.
Raptor could be tiny and still obtain 3x as much thrust if you're willing to tolerate low Isp.  Nozzles get exponentially larger as you raise Isp closer to its maximum possible figure;  That max figure itself scales with chamber pressure (though not only chamber pressure, and not linearly).  The base area of F9 is 3.66m, so a 15m BFR would be a factor 16.8x as large.

Think about the problem of sealevel overexpansion, which bounds the upper end of the nozzle size range for a first stage engine;  The overexpansion threshold will occur at the same nozzle thrust density, so you have strong specific impulse incentive to make nozzles bigger if you had a hypothetical "Merlin-sized but 3x as thrust-dense".
« Last Edit: 09/26/2016 10:37 PM by Burninate »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2915 on: 09/27/2016 12:15 AM »
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.

The 15 m diameter speculation had little to do with individual engine thrust.

1. Various attempts have been made to optimise the BFR mass, these give least tank mass for a 13-15 m stage.

2. The BFS has been said by Musk to be very large, a scaled Dragon capsule design might have to be over 23 m, which would fit much more easily on a 15 m stage. Biconic or semi-lifting body BFS designs would seem to need 12-15 m diameter to enclose the inidcated volume.

3. L2 info and various leaks on reddit

Thrust density not individual engine thrust is the problem, people have been assuming Merlin thrust density which was never going to be the case.  Tank mass is not sole consideration in deciding a rockets width, minimizing thrust structure mass and air resistance makes the thinnest possible rocket the optimum shape.

Large dose not mean wide, the vehicle is quoted at twice the Saturn V thrust, that fulfills any 'large' descriptor independent of diameter.  Their is no need for the booster to match the width of it's payload either, all rockets routinely fly with payload fairings wider then the rockets core.  Estimates of huge BFS diameters sound like attempts to retroactively justify the 15 m diameter booster and are not credible in my opinion.

I'm saying these reddit and L2 'leaks' were nothing more then poor speculations and rumors based on naive extrapolations of F9 thrust density.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2916 on: 09/27/2016 08:10 AM »
but then, if they cannot do it on their own, he has to make the case why it would be a good thing for NASA to invest some money in the SpaceX plan

There's no need for him to publicly try to "sell" MCT and his Mars architecture to NASA at the first presentation. There's already plenty of interest and there's years of development ahead, with plenty of opportunity to sell NASA on the idea of investing in some aspect of the capacity. (Similar to getting USAF interested in a Raptor-based upper-stage for FH. Gives USAF redundancy for heavy-lift/high-orbit, gives SpaceX extra funding for Raptor development and access to more USAF payloads.)

But using this presentation to, in essence, publicly mock NASA's flagship program merely creates unnecessary enemies and hence unnecessary resistance to later cooperation.

{shrug} 10 hours.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2917 on: 09/27/2016 08:18 AM »
SpaceX has what NASA needs. SX are being paid 1.6B for it. And then there's CRS-2. That's quite some money invested by NASA into the SpaceX plan

That's NASA purchasing services at a very good price.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2918 on: 09/27/2016 02:57 PM »
SpaceX has what NASA needs. SX are being paid 1.6B for it. And then there's CRS-2. That's quite some money invested by NASA into the SpaceX plan

That's NASA purchasing services at a very good price.
Heck, even if it was just NASA giving SpaceX money, NASA will be paid back in spades just Red Dragon succeeds.
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Offline Snake

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Re: MCT Speculation and Discussion Thread 4
« Reply #2919 on: 09/27/2016 03:39 PM »
My final design. All numbers are metric.

                             TOTAL      BFR      BFS   LAS

Total Dry Mass (t)   408+100   266   142+100   21.5
Total Wet Mass (t)   5,522   3,722   1,800   29.8
Height (m)                56.6   23.0   33.6   10.2
Raptors                           36   30   6   16 (SuperDracos)
Raptor Thrust (t)      ---       252 (SL)   310 (Vac)   7.3 (SuperDraco)
Thrust (t)                 ---    7,560   1,860   117
TWR                           ---    1.37   1.03   3.9

                                                                   RTLS
Burn Time (m:s)      2:05+17   5:29   24.6   17
DeltaV (m/s)         11,000   3,500   7,500      0
DeltaV with RTLS   10,250   2,750   7,500      750
---------------------------------------------------------------------------------------------
            
Observed Velocity at MECO (m/s)   1,385
Observed Height at MECO (km)      42
Time to MECO (sec)                  2:05

BFS can make it to orbit with 2 opposing engines out with extra gravity losses of ~400 m/s.

LAS Capsule is twice the size (8 times the volume) of Dragon 2.
So can seat at least 8 x 7 = 56 crew.

SuperDracos *could* be used for final descent (200 m) to Mars Surface.

BFR looks too short. Probably because I haven't adjusted fuel density to be accurate.

Images of similar ship reaching orbit of a 6.4x Kerbin. http://imgur.com/a/hKOpG

Benjamin Franklin Class Ship
« Last Edit: 09/27/2016 03:55 PM by Snake »

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