Author Topic: MCT Speculation and Discussion Thread 4  (Read 621559 times)

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1380 on: 01/21/2016 07:33 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.

You're absolutely right that business reasons make 3x10m a much, much easier sell than 15m.  But they have stated in the past three years that they changed their plans from the former to the latter (or at least, to "one big core").  My inferrence is either that they ran into trouble with mounting point load paths, or crossfeed was very important for their plans and that was dropped (unlikely given deep multiengine throttling & reliable restart requirements), or that they just needed the bigger fairing for the reusable Mars lander & MAV.
« Last Edit: 01/21/2016 09:20 PM by Burninate »

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #1381 on: 01/21/2016 08:50 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.

One of the key principles of MCT design is that by designing it explicitly for Mars (not for anything else), it enables an optimisation that will make a big difference in costs.  If you're always launching for MCT, the costs of getting 3 cores ready for launch, retrieving and refurbing 3 cores is much higher than using a single more powerful core.  The only reason for doing it is to enable smaller launches, many of which the FH should be able to cover anyway.

(Before the standard replies that BFR will have to launch other payloads for whatever reasons - Yes, I suspect it will. The principle above was not to only USE the BFR for Mars, it was to DESIGN the BFR based on Mars requirements not other payloads.)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1382 on: 01/21/2016 10:32 PM »
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)

Lars you seem to have mentally dropped half of the proposal in your responses so that you can rail against throttling, I said gimbaled engines on the outer perimeter of the vehicle would be used and a vehicle so equipped would only need to vary thrust if these gimbaled engines are insufficient, just as ANY rocket with multiple engines can and would vary thrust beyond what it's gimbaling provides.

Your N-1 comparison is completely disingenuous by your own admission, the failure were not the result of insufficient control authority provided by fixed engines, the Russians knew this and continued to use fixed engines for decades because they are mechanically simpler.  And when engines do gimbal they generally do so on only one axis outward to provide maximum leverage again simplifying all the mechanics.  All engines being provided with 2 axis gimbaling is just an American excess not worth the cost and complexity much like Hydrogen fuel.

A 30 engine booster will have around a dozen engines on the out perimeter that can all gimbal on one axis.  Only the center engine would need 2 axis gimbaling for landing.  That leaves some ~18 engines that can be fixed and packed closer together for maximum thrust density.  This is very similar to the configuration the Soyuz uses 4 fixed nozzles and 4 small single axis vernier nozzles, and as we all know the Soyuz is the most successful Russian vehicle and one SpaceX would be wise to copy.

Online KelvinZero

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Re: MCT Speculation and Discussion Thread 4
« Reply #1383 on: 01/21/2016 11:57 PM »
Lars you seem to have mentally dropped half of the proposal in your responses so that you can rail against throttling, I said gimbaled engines on the outer perimeter of the vehicle would be used and a vehicle so equipped would only need to vary thrust if these gimbaled engines are insufficient, just as ANY rocket with multiple engines can and would vary thrust beyond what it's gimbaling provides.
I liked the idea of gimbaling but I have wondered about the effect of variations in thrust between different engines. They are still actually at full thrust so if variation is 1%, then if engines are gimballed fully outwards you still have sideways 'noise' in your signal equivalent to 1% full thrust, whereas a smaller downwards pointing engine would equivalently only have variation of 1% of its current thrust, and this variation almost entirely straight down in one dimension instead of sideways.

Also I imagine that balancing the thrust in the event of an engine out would be extremely difficult. Things could go sideways very rapidly ;)
« Last Edit: 01/22/2016 12:02 AM by KelvinZero »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1384 on: 01/22/2016 12:01 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.

"Work" would be subjective. ejecting and landing on mars with no supplies would leave you just as dead.
After a couple synods of crewed missions (assuming the number of BFSes each synod grows), there will be a sizable presence on Mars, and it would soon be cheap to have a "launch on need" BFS available (that would be used for a cargo flight back if not used for rescue during that synod, so doesn't use any resources other than space near the pad).

And an early abort would leave you close to base, within range of a rover. There's also the ability for the survivors to walk over and hook up into a group survival rebreather that would extend survivability on the surface for days (not fun, of course, but better than dead), allowing time for a rescue. Or for early flights with few passengers, even a small inflatable hab/"lifeboat" could be available.

I'm just spit-balling here. My main point is that abort modes for SpaceX's architecture are nothing like NASA's would be (where you wouldn't be able to afford the extra weight of an abort lifeboat, etc, and where there's no way you would have a launch-on-need ascent vehicle available).
« Last Edit: 01/22/2016 12:02 AM by Robotbeat »
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Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1385 on: 01/22/2016 12:12 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)

Lars you seem to have mentally dropped half of the proposal in your responses so that you can rail against throttling, I said gimbaled engines on the outer perimeter of the vehicle would be used and a vehicle so equipped would only need to vary thrust if these gimbaled engines are insufficient, just as ANY rocket with multiple engines can and would vary thrust beyond what it's gimbaling provides.

I am not aware of ANY rocket steering with differential throttling, other than N-1. It just isn't done, because most engines don't have responsive enough throttling, nor accurate enough throttling.

Your N-1 comparison is completely disingenuous by your own admission, the failure were not the result of insufficient control authority provided by fixed engines, the Russians knew this and continued to use fixed engines for decades because they are mechanically simpler. And when engines do gimbal they generally do so on only one axis outward to provide maximum leverage again simplifying all the mechanics.  All engines being provided with 2 axis gimbaling is just an American excess not worth the cost and complexity much like Hydrogen fuel.

You are only looking at older designs from the early space age, where gimbaling was not developed for larger engines. But Energia and Zenit used/uses two axis gimbal, Angara uses two-axis gimbal, as will all future Russian LVs.

It is not an "American excess", it is common sense, especially if you want to have engine out capability.

A 30 engine booster will have around a dozen engines on the out perimeter that can all gimbal on one axis.  Only the center engine would need 2 axis gimbaling for landing.  That leaves some ~18 engines that can be fixed and packed closer together for maximum thrust density.  This is very similar to the configuration the Soyuz uses 4 fixed nozzles and 4 small single axis vernier nozzles, and as we all know the Soyuz is the most successful Russian vehicle and one SpaceX would be wise to copy.

Fixed engines with vernier nozzles was a limitation of early rocketry, not a desired outcome in itself, nor something that increased reliability. American LV's moved away from them vernier engines quickly, Russian LV's did as well too - it is just taking longer for Soyuz to be phased out due to other reasons. Falcon 9 is a clear demonstration that your line of thinking is opposite of SpaceX - and the rest of the industry.

Will all engines gimbal in a BFR? Maybe not - but those that do will use two-axis gimbal to allow the vehicle to survive engines out situations.

Edit: Added some images... First the RD-170 from Energia strap-ons (all 4 nozzles of the same engine had independent 2-axis gimbal), and Second - a picture from the SpaceX M1D production line, showing how the standard gimbal actuators look when installed.
« Last Edit: 01/22/2016 03:24 PM by Lars-J »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1386 on: 01/22/2016 12:28 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.

The only reason I mentioned "no gimballing" is just to say what the physical limits of 15m diameter rocket would be for lift-off weight. I definitely don't think SpaceX is going to pack the engines that close, and I don't think they'll rely on differential throttling at all. They'll gimbal the engines, because that's way more sensible and efficient.

I kind of wish I didn't bring that up. Just trying to cover the bases.
« Last Edit: 01/22/2016 12:29 AM by Robotbeat »
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Online Chris Bergin

Re: MCT Speculation and Discussion Thread 4
« Reply #1387 on: 01/22/2016 02:07 PM »
Let's all use our inside voices. We're all rocket friends here.

(Really don't want to trim out long posts because one line was "OMG, OMG, O.M.G!" but we need to enforce civility here otherwise it'll quickly turn into the SDC days....and for you new people, that wasn't fun on that no-longer-around site!)

Online launchwatcher

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Re: MCT Speculation and Discussion Thread 4
« Reply #1388 on: 01/22/2016 04:03 PM »
"Work" would be subjective. ejecting and landing on mars with no supplies would leave you just as dead.
After a couple synods of crewed missions (assuming the number of BFSes each synod grows), there will be a sizable presence on Mars, and it would soon be cheap to have a "launch on need" BFS available (that would be used for a cargo flight back if not used for rescue during that synod, so doesn't use any resources other than space near the pad).
Another possibility would be an unmanned land-on-need BFS (or other vehicle) kept in Mars orbit with reserve supplies; after an abort you could drop it next to the survivors to give them additional supplies, shelter, and/or surface mobility.  It could also bail out the main settlement if something bad happened to their storage on the surface.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1389 on: 01/22/2016 05:54 PM »
It's been suggested before that a small number of the very first BFS's (formerly MCT) landers on Mars will not return to Earth. It shouldn't be hard to modify one or two into a point-to-point planetary hopper. It's a useful tool to have, and serves as a rescue vehicle.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1390 on: 01/22/2016 09:07 PM »
It's been suggested before that a small number of the very first BFS's (formerly MCT) landers on Mars will not return to Earth. It shouldn't be hard to modify one or two into a point-to-point planetary hopper. It's a useful tool to have, and serves as a rescue vehicle.

If they can do such hops, they can probably return to earth as well.

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1391 on: 01/22/2016 11:29 PM »
It's been suggested before that a small number of the very first BFS's (formerly MCT) landers on Mars will not return to Earth. It shouldn't be hard to modify one or two into a point-to-point planetary hopper. It's a useful tool to have, and serves as a rescue vehicle.

If they can do such hops, they can probably return to earth as well.

Depends on the size of the hops Paul was going for, and... mathematically that doesn't seem logical. What's your reasoning, Guckyfan? Whilst I agree with you all BFSs will RTE, the margins to hop and the margins for Earth return are considerably far apart from each other.
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1392 on: 01/23/2016 08:47 AM »
Depends on the size of the hops Paul was going for, and... mathematically that doesn't seem logical. What's your reasoning, Guckyfan? Whilst I agree with you all BFSs will RTE, the margins to hop and the margins for Earth return are considerably far apart from each other.

My reasoning. BFS are capable of return to earth when functioning. They need to be functioning to make the hops.

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1393 on: 01/23/2016 09:09 AM »
It's been suggested before that a small number of the very first BFS's (formerly MCT) landers on Mars will not return to Earth. It shouldn't be hard to modify one or two into a point-to-point planetary hopper. It's a useful tool to have, and serves as a rescue vehicle.

If they can do such hops, they can probably return to earth as well.

Airplanes can 'hop' between continents. They cannot get in to orbit. One does not imply the other

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1394 on: 01/23/2016 10:58 AM »

Depends on the size of the hops Paul was going for, and... mathematically that doesn't seem logical. What's your reasoning, Guckyfan? Whilst I agree with you all BFSs will RTE, the margins to hop and the margins for Earth return are considerably far apart from each other.

My reasoning. BFS are capable of return to earth when functioning. They need to be functioning to make the hops.

Being functioning and being supplied by an active ISRU facility is a wholly separate thing. Without propellant it doesn't matter.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1395 on: 01/23/2016 12:47 PM »

Depends on the size of the hops Paul was going for, and... mathematically that doesn't seem logical. What's your reasoning, Guckyfan? Whilst I agree with you all BFSs will RTE, the margins to hop and the margins for Earth return are considerably far apart from each other.

My reasoning. BFS are capable of return to earth when functioning. They need to be functioning to make the hops.

Being functioning and being supplied by an active ISRU facility is a wholly separate thing. Without propellant it doesn't matter.

Both return to earth and hops assume fuel ISRU in place. A safe assumption as this is the first thing to be installed. Operational according to Elon Musk before humans arrive.

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #1396 on: 01/23/2016 01:43 PM »
Or ISRU methane production is built into the early BFS's, or maybe all of them for redundancy. How large would the device  be?
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1397 on: 01/23/2016 01:57 PM »
Or ISRU methane production is built into the early BFS's, or maybe all of them for redundancy. How large would the device  be?

This is one of the things we don't have a good credible estimate for. If you're harvesting water from the atmosphere, the device and its power needs get extremely large (eventually impacting mission design decisions), but if you're mining water... I'm not sure a mining layout has been defined, and what few people I have seen assuming numbers have been shots in the dark.

From a business reusability perspective, and from a safety perspective, the conservative option is for every single MCT to carry enough ISRU equipment that it can return to Earth, or at least return to LMO, after landing.  If you refrain from that you either need to hang a multiplier onto your MCT construction rate (eg send two for every one you get back, eliminating most of the point of reusability), or reuse existing landed ISRU gear to an extreme degree... with unknown wear, spare & repair characteristics.  On Earth, mining equipment starts out heavy as hell & large parts of it get beat up and replaced at a predictable rate.

An alternate safety precaution would be to land an MCT with a separating capsule tip of <2m3/person, not enough volume to live in, but enough to get to an LMO escape vehicle, that's landed with enough propellant that you could return without the habitat/lander.  This would resemble an Orion in accomodation level, and rendezvous with a hab & prop tank in orbit that returns to Earth.  It would be a reserve capacity, not usually used... although in some contingencies (like an atmospheric water-harvesting ISRU possibility that turns out to have low performance), it might be employed in a split mission capacity with the lander returning the following synod.
« Last Edit: 01/23/2016 02:07 PM by Burninate »

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #1398 on: 01/23/2016 01:59 PM »
All the more reason for FH precursor missions. Wonder if he has Tesla building the drive systems for the necessary 'bots.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1399 on: 01/23/2016 03:10 PM »
About a Megawatt of power is needed to produce enough propellant for a MCT in a year. 40 tons unless you hook up to base-side infrastructure. If you have nuclear, you need huge radiators (or ground infrastructure) and shielding of some kind. Solar of 1MW requires even larger deployment on the ground. Electrolysis and Sabatier are a little smaller but still substantial.

It simply isn't a good idea to keep it on the MCTs.
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