Author Topic: MCT Speculation and Discussion Thread 4  (Read 621862 times)

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1360 on: 01/16/2016 04:10 AM »
If you were to have an abort system on MCT, it'd have to work for Mars ascent (as well as Earth), and probably even terminal landing as well. No, this is not impossible. Hard, but not impossible.

Or just not have an abort system.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1361 on: 01/16/2016 06:54 AM »
On the issue of an abort capsule I agree with guckyfan and Robotbeat, it is impractical and offers very little use at Mars while on Earth a whole vehicle abort is more practical.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1362 on: 01/16/2016 09:43 AM »
If you were to have an abort system on MCT, it'd have to work for Mars ascent (as well as Earth), and probably even terminal landing as well. No, this is not impossible. Hard, but not impossible.

Or just not have an abort system.

Possible, no doubt. But with a massive payload penalty. Worst case payload back to earth would approach zero, meaning no return for humans.

I believe it will be engine out capability, high reliability and very, very low risk of explosive failure.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1363 on: 01/16/2016 01:16 PM »
On the issue of an abort capsule I agree with guckyfan and Robotbeat, it is impractical and offers very little use at Mars while on Earth a whole vehicle abort is more practical.
That's not what I think. I think you can do abort on Earth AND Mars. And "whole vehicle" abort is likely not appropriate.

But you don't HAVE to have an abort capability.
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1364 on: 01/20/2016 01:21 PM »
Shotwell mentioned about BFR a few months ago at the South Summit 2015 (Oct 7-9), in Madrid, " [Falcon Heavy] This one is about 4M pounds of thrust, and the mock... the vehicle that takes us to Mars will be three or four times that size"

https://www.youtube.com/watch?v=omBF1P2VhRI?t=10m46s

(original video, mostly Spanish-language conference proceeding, but Shotwell's voice still appears beneath a title graphic for the first ten minutes, though not her face.  The video I linked above seems to have been created a while after this one was promoted, and does a proper cut to her presentation alone)

I also vaguely remember her mentioning offhand that they were developing a 180-210t to LEO superheavy launcher.  I've been trying to find the interview, but can't turn anything up.

Above Quote from the Update thread

3 to 4 times FH thrust fits well with the 12.7-14.7 million pounds thrust BFR models I've built that conform to prior SX statements. 

It contradicts the ludicrous 15m wide 300+m tall !!! behemoths posted on Reddit which would require over 400 Raptors to lift off if the posters had simply calculated the propellant volume and subsequent mass.
« Last Edit: 01/20/2016 01:35 PM by Chris Bergin »
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1365 on: 01/20/2016 02:54 PM »
Shotwell mentioned about BFR a few months ago at the South Summit 2015 (Oct 7-9), in Madrid, " [Falcon Heavy] This one is about 4M pounds of thrust, and the mock... the vehicle that takes us to Mars will be three or four times that size"

https://www.youtube.com/watch?v=omBF1P2VhRI?t=10m46s

(original video, mostly Spanish-language conference proceeding, but Shotwell's voice still appears beneath a title graphic for the first ten minutes, though not her face.  The video I linked above seems to have been created a while after this one was promoted, and does a proper cut to her presentation alone)

I also vaguely remember her mentioning offhand that they were developing a 180-210t to LEO superheavy launcher.  I've been trying to find the interview, but can't turn anything up.

Above Quote from the Update thread

3 to 4 times FH thrust fits well with the 12.7-14.7 million pounds thrust BFR models I've built that conform to prior SX statements. 

It contradicts the ludicrous 15m wide 300+m tall !!! behemoths posted on Reddit which would require over 400 Raptors to lift off if the posters had simply calculated the propellant volume and subsequent mass.
15m wide is pretty reasonable, but the main utility of that is to prevent it from being anywhere near 300m tall.  I don't expect it to be anywhere near the fineness ratio of F9.  A cylindrical rocket doesn't have infinite room to scale up;  The base grows as n^2 while mass grows as n^3, and eventually the base's space to fit rocket motors limits further growth.  Additionally, we have raised issues with eg the launch facilities here.

I suspect you can't build it much smaller than 12m and still have realistic height.

If you try to build it larger than a number somewhere in the 15m-18m range, then you run into issues where bridges need to be rebuilt to get the parts in place, and propellant tanks lose their cylindrical character.
« Last Edit: 01/20/2016 03:01 PM by Burninate »

Offline gadgetmind

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Re: MCT Speculation and Discussion Thread 4
« Reply #1366 on: 01/20/2016 02:56 PM »
Why not make it into a flattened disk, and get it to spin while we're at it!

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1367 on: 01/20/2016 04:24 PM »

Above Quote from the Update thread

3 to 4 times FH thrust fits well with the 12.7-14.7 million pounds thrust BFR models I've built that conform to prior SX statements. 

It contradicts the ludicrous 15m wide 300+m tall !!! behemoths posted on Reddit which would require over 400 Raptors to lift off if the posters had simply calculated the propellant volume and subsequent mass.
15m wide is pretty reasonable, but the main utility of that is to prevent it from being anywhere near 300m tall.  I don't expect it to be anywhere near the fineness ratio of F9.  A cylindrical rocket doesn't have infinite room to scale up;  The base grows as n^2 while mass grows as n^3, and eventually the base's space to fit rocket motors limits further growth.  Additionally, we have raised issues with eg the launch facilities here.

I suspect you can't build it much smaller than 12m and still have realistic height.

If you try to build it larger than a number somewhere in the 15m-18m range, then you run into issues where bridges need to be rebuilt to get the parts in place, and propellant tanks lose their cylindrical character.

Yes.  I start my models at 12.5m minimum (fit the engines) but believe they'll go for 15m but no larger, for reasons like future growth in # of engines.  I believe that the 15m "airframe" will be fabricated on-site as 15 is too large for ground transport on public roads and will also have problems on waterways with bridges.  The BFR will blow out steam singing, "I'm a little teapot, short and stout!"  Well under 100m tall with the MCT.
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1368 on: 01/21/2016 12:57 AM »
~30 engines (+/-50%) sounds about right to me. Around the same as Falcon Heavy, but in a single core (which makes engine-out management a little simpler).

15m, too.


...by the way, we can know for a fact that if it's only 15m in diameter, it is going to be limited to around, say, 50 engines of about 500klbf each if they are squished together as close as they can go and have an exit pressure of around near sea level and a nice high chamber pressure of about 2000-3000psi. You cannot physically fit in more engines. For a lift-off T/W of 1.1 or more, that puts a hard upper bound of around 10,000 tons. So if 15m is the diameter (it probably is), then the lift-off mass cannot be more than 10,000 tons. And that leaves ZERO room for gimballing.

A more sane packing arrangement would leave them at around 6000 tons or less.
« Last Edit: 01/21/2016 01:05 AM by Robotbeat »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1369 on: 01/21/2016 02:35 AM »
With ~30 engines you probably don't need gimbaling, or at least not on most of the engines.  Some variable thrust and a few engines along the edge that gimbal on just one axis should be sufficient and is the Russian way of doing it.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1370 on: 01/21/2016 04:08 AM »
With ~30 engines you probably don't need gimbaling, or at least not on most of the engines.  Some variable thrust and a few engines along the edge that gimbal on just one axis should be sufficient and is the Russian way of doing it.

That worked sooo great for the N1. :) I know that was failure of testing, but using variable thrust for control is a truly *terrible* idea. That's when you need more power, not less of it.

A few central engines may be fixed, but most should fully gimbal for general control authority and engine out capability. Having all the engines be identical (including their gimbal setups) also simplifies mass production and testing procedures, as SpaceX illustrate whenever their F9 rocket launches.

(it seems to be a VERY common misconception that some F9 engines are fixed, they are NOT)

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1371 on: 01/21/2016 04:34 AM »
With ~30 engines you probably don't need gimbaling, or at least not on most of the engines.  Some variable thrust and a few engines along the edge that gimbal on just one axis should be sufficient and is the Russian way of doing it.

That worked sooo great for the N1. :) I know that was failure of testing, but using variable thrust for control is a truly *terrible* idea. That's when you need more power, not less of it.

A few central engines may be fixed, but most should fully gimbal for general control authority and engine out capability. Having all the engines be identical (including their gimbal setups) also simplifies mass production and testing procedures, as SpaceX illustrate whenever their F9 rocket launches.

(it seems to be a VERY common misconception that some F9 engines are fixed, they are NOT)

Variable thrust on a big fat rocket like this is going to have fairly high control authority (unlike the high-fineness-ratio F9), so something like 'Right side -2% thrust" is enough.  Also, outward or inward-canted steering motors could be used for even greater effect, or the entire main engine bank could be inward-canted by a few degrees.

I don't think it's something to rule out either on first principles, or on the back of the N1 experience, a rushed-to-launchpad program using primitive computerization that decided to work out its kinks in live tests, but was deprived of funding and leadership before finishing its work.

With that said, I don't have any good reason to think non-gimballing engines are going to be a thing on BFR.  With MCT, on the other hand, there may be value in fixing the combustion chambers as well as the massive engine bells rigidly to the spacecraft;  Those gimbals are a source of potential failure with a few years of vacuum between uses, and whether the odds of that are greater than the odds of some other steering system failing, is a bit more of an open question than with BFR.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1372 on: 01/21/2016 05:09 AM »
With that said, I don't have any good reason to think non-gimballing engines are going to be a thing on BFR.  With MCT, on the other hand, there may be value in fixing the combustion chambers as well as the massive engine bells rigidly to the spacecraft;  Those gimbals are a source of potential failure with a few years of vacuum between uses, and whether the odds of that are greater than the odds of some other steering system failing, is a bit more of an open question than with BFR.

I strongly disagree with that. I consider fixed engines more likely on a BFR booster than a MCT. First, the fewer engines you have, the more critical it is that they all gimbal. Otherwise you really have no engine out capability at all, a *long* way from home. Yes, your gimbal actuators need to be VERY reliable, but I think they need to be there to provider multiple levels of redundancy.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1373 on: 01/21/2016 03:14 PM »
~30 engines (+/-50%) sounds about right to me. Around the same as Falcon Heavy, but in a single core (which makes engine-out management a little simpler).

15m, too.


...by the way, we can know for a fact that if it's only 15m in diameter, it is going to be limited to around, say, 50 engines of about 500klbf each if they are squished together as close as they can go and have an exit pressure of around near sea level and a nice high chamber pressure of about 2000-3000psi. You cannot physically fit in more engines. For a lift-off T/W of 1.1 or more, that puts a hard upper bound of around 10,000 tons. So if 15m is the diameter (it probably is), then the lift-off mass cannot be more than 10,000 tons. And that leaves ZERO room for gimballing.

A more sane packing arrangement would leave them at around 6000 tons or less.

I believe that they're aiming for a decent T/W, certainly far higher than 1.1.  The old slow liftoff F9 had 1.19 I believe with the current F9 Full Thrust model considerably higher.  The issue is minimizing gravity losses.  I'm estimating they're going for 1.25 or better with the BFR.

One nice thing about 25-30 something engines at decent T/W is that one millisecond after Launch OK Release is that an engine can fail, well at least not catastrophically, and the mission continue.  I believe even the F9 with it's new higher T/W can lose an engine right away and complete most missions.

15 meter diameter initial design point allows for architectural growth with easy propellant tank elongation and future room to add engines beyond an initial 25-30.  Launch support towers don't need to be skyscrapers keeping launch operations simpler and more cost effective.

I'd estimate around 5,000 metric tons GLOW.
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Offline Arb

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Re: MCT Speculation and Discussion Thread 4
« Reply #1374 on: 01/21/2016 04:01 PM »
Re the gimbal / variable-thrust steering discussion for the BFR (booster, not MCT). Given ~30 Raptors what would be the approximate weight saving of variable-thrust?

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1375 on: 01/21/2016 04:07 PM »
I still think the BFR/MCT diameter will not exceed 10m.  There are many reasons for this.  I also think this version would be more economical in the long run.  A 3 core heavy for MCT launch.  Say 5.5m-8m for a single core for launching deep space probes, filling a fuel depot and other cash making activities.  The single core would be able to launch 80-100 tons.  A 3 core heavy version 250 tons.  A fully reusable 80 ton launcher might eliminate Falcon Heavy.  Say it had 9 engines at 550k lbs thrust or slightly more for a 5 million lb thrust reusable rocket.  This rocket can put up a lot of stuff to make money from the Air Force, NASA or others to help pay for MCT.  A single core could also be adapted to have two Falcon 9 cores attached for boosting slightly over 100 tons to LEO and could still launch from the cape.   

This size also would greatly expand locations for manufacturing and shipping via cheap barge and launch locations.  Otherwise if too wide it would have to be built at a shipyard and transported via ocean to various launch locations.  River and intercoastal barge widths in America are 36', or 10m maybe 11m.

This may make MCT cylindrical which might mean horizontal landing.  A try at using the vacuum Raptor in a reusable upper stage would be a good test for MCT cylinder design. 
« Last Edit: 01/21/2016 04:09 PM by spacenut »

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1376 on: 01/21/2016 04:18 PM »
Re the gimbal / variable-thrust steering discussion for the BFR (booster, not MCT). Given ~30 Raptors what would be the approximate weight saving of variable-thrust?

Not much, really. And the loss of performance would be severe. You are launching something massing on the order of thousands of tonnes, and the only way you can turn is to throttle down a lot of your engines!?!  :D That won't fly.

Steering by differential thrust is a *terrible* idea. It has always been, it will always be.
« Last Edit: 01/21/2016 04:19 PM by Lars-J »

Offline RoboGoofers

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Re: MCT Speculation and Discussion Thread 4
« Reply #1377 on: 01/21/2016 05:20 PM »
If you were to have an abort system on MCT, it'd have to work for Mars ascent (as well as Earth), and probably even terminal landing as well. No, this is not impossible. Hard, but not impossible.

Or just not have an abort system.

"Work" would be subjective. ejecting and landing on mars with no supplies would leave you just as dead.

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1378 on: 01/21/2016 05:35 PM »
I still think the BFR/MCT diameter will not exceed 10m.  There are many reasons for this.  I also think this version would be more economical in the long run.  A 3 core heavy for MCT launch.  Say 5.5m-8m for a single core for launching deep space probes, filling a fuel depot and other cash making activities.  The single core would be able to launch 80-100 tons.  A 3 core heavy version 250 tons.  A fully reusable 80 ton launcher might eliminate Falcon Heavy.  Say it had 9 engines at 550k lbs thrust or slightly more for a 5 million lb thrust reusable rocket.  This rocket can put up a lot of stuff to make money from the Air Force, NASA or others to help pay for MCT.  A single core could also be adapted to have two Falcon 9 cores attached for boosting slightly over 100 tons to LEO and could still launch from the cape.   


I buy these reasons, but a 3 core isn't what they're going for, and they've explicitly mentioned the core size .

We should work around what they're actually doing, or we have a 100% likelihood of being wrong.
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #1379 on: 01/21/2016 07:26 PM »
Attached are my latest BFR spreadsheet models. 

I have 2 core sizes, 12.5m and 15m.  I believe 15m will be the design center for SX because it allows growth.  All are TSTO with the MCT as stage 2. I have modeled various MCTs; all carry 100mT of cargo.  The different user selectable MCT masses simply mean different dry weight assumptions for the MCT.  From 25-30 Raptor engines.  You can vary most parameters including core size to design your own.

Most #s are citations from SX with others extrapolations based on F9 parameters.  Sources are below the main spreadsheet.

The BFR/MCT will likely be a stout vehicle under 100m height.
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