Author Topic: SpaceX Starship (BFS) Engineering General Thread 2  (Read 197724 times)

Offline Norm38

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #980 on: 02/13/2019 03:57 pm »

Elon Musk ✔ @elonmusk
Starship & Super Heavy will press tanks autogenously even in version 1. Very important, as helium costs more than oxygen on Falcon, even though liquid oxygen is 2/3 vehicle mass & helium weighs basically nothing.
12:14 AM - Feb 13, 2019
https://twitter.com/elonmusk/status/1095551826668138496

There is helium rationing going on now.  My wife bought balloons for a bridal shower and was capped at 12.
I'm not surprised that helium is out from day 1.

https://www.labnews.co.uk/comment/addressing-helium-shortage-08-01-2019/

Offline CuddlyRocket

Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #981 on: 02/13/2019 06:04 pm »
On Mars: run filtered, ambient air through the same hoses that were used on Earth. It costs a bit of energy and you need a backup in case your compressor dies. But really, you need to be able to compress Mars atmosphere for fuel production anyway.

I'm assuming that by 'ambient air' you mean the Martian atmosphere ('air' is specifically the breathable mix of gases in Earth's atmosphere).

On Mars, take the 1.6% argon left over from liquefying the atmosphere for fuel production and run it through the transpiration system. That would both keep dust out and keep CO2 from freezing in the channels as you fill the tanks with LOX/LCH4. It might keep dry ice from freezing on the vehicle entirely.

The argon, and nitrogen, obtained will be valuable commodities in their own right. From the 'ISRU beyond propellants which materials in which sequence and how?' thread in the Missions to Mars (HSF) section:

Nitrogen ISRU is a by-product of atmospheric compression for the production of Methane. At the rate of 250 tonnes per ship, that's about 2.5 tonnes of methane per 100 transported colonist for Starship type vehicles. Supposing another 4 ships as cargos that's 10 tonnes of methane per traveller. The production of this methane requires 4 times as much mass of CO2 as the mass of the methane. So the CO2 production rate will be 40 tonnes of CO2 per traveller.

At 2,7% nitrogen in the atmosphere you get about 1000 kg of methane per colonist. With a density of about 1 kg/m3 for atmosphere, that's enough methane for 1300 m3 of space when you add in the oxygen. Although this seems large, a typical suburban home in the US has about 780 m3 of volume, or 1000 m3 if you include roof spaces and wall volume.  If you add in the volume required for food production, then all the nitrogen is used to fill the colony volume with [air], and it might be just a little short. ...

So nitrogen production from propellant production is just sufficient for colony development, but might be a limiting factor, or require compressing the atmosphere for nitrogen production exclusively, or using the argon that comes along as well with the compression.

There might also be some nitrogen from methane production as industrial feedstock.

Online geza

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #982 on: 02/13/2019 06:33 pm »
I get the re-entry cooling concept for the main body of Starship, what in the world are the fins and canards are going to be made out of?  They are going to be subject to some pretty rough re-entry temps.

If the fins and canards are stainless 310 and folded up (dihedral) ISTM they'd be 1) more heat resistant and 2) largely protected within the trailing methane envelope, but they may use TUFROC on hot spots. They have an SAA to work on TUFROC at Ames.

https://www.teslarati.com/spacex-new-bfr-heat-shield-advancements-nasa-assistance/

No reason they can't use same active cooling as rest of vehicle.

Are the fins still supposed to be actuated during entry? If so, then the cooling fluid must go through the actuated joints somehow. Moreover, the actuation must work, while the whole structure becomes quite hot. Does not seem to be easy. We had worried a lot about actuation even before we learned about the actively cooled hot metal thing.

Offline envy887

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #983 on: 02/13/2019 06:47 pm »
I get the re-entry cooling concept for the main body of Starship, what in the world are the fins and canards are going to be made out of?  They are going to be subject to some pretty rough re-entry temps.

If the fins and canards are stainless 310 and folded up (dihedral) ISTM they'd be 1) more heat resistant and 2) largely protected within the trailing methane envelope, but they may use TUFROC on hot spots. They have an SAA to work on TUFROC at Ames.

https://www.teslarati.com/spacex-new-bfr-heat-shield-advancements-nasa-assistance/

No reason they can't use same active cooling as rest of vehicle.

Are the fins still supposed to be actuated during entry? If so, then the cooling fluid must go through the actuated joints somehow. Moreover, the actuation must work, while the whole structure becomes quite hot. Does not seem to be easy. We had worried a lot about actuation even before we learned about the actively cooled hot metal thing.

"does not seem to be easy" applies to pretty much everything about this project...

Offline rcoppola

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #984 on: 02/13/2019 07:23 pm »
Their Titanium grid fins are the largest single piece cast & cut. The wings / legs(2) are another magnitude larger, more complex but I wonder how their titanium experience can scale to this effort?
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Offline MKremer

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #985 on: 02/13/2019 07:31 pm »
Their Titanium grid fins are the largest single piece cast & cut. The wings / legs(2) are another magnitude larger, more complex but I wonder how their titanium experience can scale to this effort?

How did Shuttle flap joints survive repeated reentries without needing replacement?

Offline livingjw

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #986 on: 02/14/2019 12:40 am »
Their Titanium grid fins are the largest single piece cast & cut. The wings / legs(2) are another magnitude larger, more complex but I wonder how their titanium experience can scale to this effort?

How did Shuttle flap joints survive repeated reentries without needing replacement?

Titanium's max usable temperature is lower than 310S and even it will need cooling in areas.

Online john smith 19

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #987 on: 02/14/2019 06:14 am »
Their Titanium grid fins are the largest single piece cast & cut. The wings / legs(2) are another magnitude larger, more complex but I wonder how their titanium experience can scale to this effort?

How did Shuttle flap joints survive repeated reentries without needing replacement?
Shuttle control surfaces were all passive cooled. Either coated with tiles or RCC.

Key issues with control surfaces are a) Where do you put the actuator? b)How do you prevent (reduce) leakage through the joints between the upper and lower surfaces?

IIRC shuttle surfaces could move +/- 45 deg from normal. Flexure bearing systems can be built with that much rotation while being made out of a single continuous shape.
BFS. The worlds first Methane fueled FFORSC engined CFRP stainless steel structure A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structure booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline GWH

Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #988 on: 02/14/2019 06:27 am »
Are the fins still supposed to be actuated during entry? If so, then the cooling fluid must go through the actuated joints somehow. Moreover, the actuation must work, while the whole structure becomes quite hot. Does not seem to be easy. We had worried a lot about actuation even before we learned about the actively cooled hot metal thing.

If they use (vane rotary type) hydraulic actuators with liquid methane as the working fluid that cooling problem would be much easier.

Offline Hominans Kosmos

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #989 on: 02/14/2019 08:22 am »
Their Titanium grid fins are the largest single piece cast & cut. The wings / legs(2) are another magnitude larger, more complex but I wonder how their titanium experience can scale to this effort?

The aerodynamic control surfaces (brakerons/flaperons) and legs are not likely to be made of titanium.

Now that we have the Starship being built out of high temperature stainless steel alloys: what's the radiator performance potential of the single fixed landing leg / vertical fin stabilizer for heat rejection for in-space operations? Does it make the discussion of having large radiator surfaces on the back fuselage over the propellant tanks unnecessary? Higher radiating temperature times twice single leg-fin area = sufficient radiator performance?
« Last Edit: 02/14/2019 08:29 am by Hominans Kosmos »

Offline Hominans Kosmos

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #990 on: 02/14/2019 08:26 am »
Are the fins still supposed to be actuated during entry? If so, then the cooling fluid must go through the actuated joints somehow. Moreover, the actuation must work, while the whole structure becomes quite hot. Does not seem to be easy. We had worried a lot about actuation even before we learned about the actively cooled hot metal thing.

If they use (vane rotary type) hydraulic actuators with liquid methane as the working fluid that cooling problem would be much easier.
 

As I understand the past BFR/Starship control surfaces and actuators discussion, methane makes for a very poor hydraulic working fluid. Probably going to be electric, IMO.

Offline Hominans Kosmos

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #991 on: 02/14/2019 08:38 am »
I get the re-entry cooling concept for the main body of Starship, what in the world are the fins and canards are going to be made out of?  They are going to be subject to some pretty rough re-entry temps.

If the fins and canards are stainless 310 and folded up (dihedral) ISTM they'd be 1) more heat resistant and 2) largely protected within the trailing methane envelope, but they may use TUFROC on hot spots. They have an SAA to work on TUFROC at Ames.

https://www.teslarati.com/spacex-new-bfr-heat-shield-advancements-nasa-assistance/

No reason they can't use same active cooling as rest of vehicle.

Are the fins still supposed to be actuated during entry? If so, then the cooling fluid must go through the actuated joints somehow. Moreover, the actuation must work, while the whole structure becomes quite hot. Does not seem to be easy. We had worried a lot about actuation even before we learned about the actively cooled hot metal thing.
 

Seems so, yes.

<snip>

https://twitter.com/elonmusk/status/1091147092016254977
Tweet Content  Yes, ship needs pitch/roll/yaw control across wide Mach regime & angle of attack

Offline RobLynn

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #992 on: 02/14/2019 09:32 am »
As I understand the past BFR/Starship control surfaces and actuators discussion, methane makes for a very poor hydraulic working fluid. Probably going to be electric, IMO.

I think that is unlikely.  Electric is far heavier, requires large gearing ratios for reasonable power density, and is far more sensitive to temperature,  more than 150°C is generally a problem.

Hydraulics can operate to at least 300°C, and perhaps up to 400°C, and can use cryogenic methane with heat added to create high pressure gas to power it - it's nearly free power as can still use it for transpiration cooling afterwards, and eliminates heavy batteries and motors that likely need peak powers of 100's of kW.  Hydraulics can deliver tremendous peak power with relatively light weight through the use of accumulators.

Methane pneumatics might be another option to save weight and eliminate hydraulic fluid (hard to replace on Mars), and could go to far higher temperatures - though compressibility is a problem for control it could perhaps be combined with a locking carbon carbon brake system.

Consider that Falcon 9 uses hydraulics for gimballing and grid fins - they could have used electric.  Modern aircraft are still mostly all hydraulic too with power by wire only making some inroads.
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline Hominans Kosmos

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #993 on: 02/14/2019 10:36 am »
As I understand it liquid methane bulk modulus is comparable to that of methanol, which leaves it thee to four times as squishy as water and hydraulic-oil respectively. Does using inefficient hydraulic fluid not increase the mass budget of the hydraulic system? 

Additionally, as I understand it, normal hydraulics don't circulate fluid in a fashion that would be required to mitigate methane boil-off, the working fluid can never turn to gas in the system. This is why low vapor pressure fluids are used for hydraulics, is it not? Something like a radically novel hydraulic fluid circulation system would be needed, or more likely: submerging all the actuators inside a cryogenic methane bath. This does not bode well for the mass budget of hydraulics. 

Pneumatics, it is suggested, is not a good tool for actuating stabilizers. Maybe there's a way around that.

Electric actuators do not have a problem of overheating when you can probably quite easily cool the machinery with, let's say cold gaseous methane? Something like that. Controllability is key.

What's traditional is irrelevant. Consider: there are no oil refineries on Mars to make RP1 or hydraulic oil with. 

Consider that Falcon 9 uses hydraulics for gimballing and grid fins - they could have used electric.  Modern aircraft are still mostly all hydraulic too with power by wire only making some inroads.
« Last Edit: 02/14/2019 10:37 am by Hominans Kosmos »

Offline Eer

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #994 on: 02/14/2019 12:40 pm »
Talk about alternative fluids makes me wonder if SpaceX won’t go whole hog steam punk and use water steam to drive actuators. Easy enough to replace on Mars. Considerable experience working with it. Capable of generating quite a bit of pressure for use on demand and heat doesn’t sound like it will be a problem. Just speculation.

Offline Hominans Kosmos

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #995 on: 02/14/2019 12:53 pm »
Could be water-based hydraulics. Electrical heating as anti-freeze? Easier to mitigate than having cryogenic squishy ready to turn to gas methane hydraulics?

How to prevent ice-damage during periods between use, when the vehicle will go into cold-soak? Bleed lines to space while heater is on? Re-fill with water while heater is on before you need to use actuators again?
« Last Edit: 02/14/2019 12:57 pm by Hominans Kosmos »

Offline DusanC

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #996 on: 02/14/2019 02:11 pm »
IMHO hydraylic pump will be driven by gasseous CH4 pneumatic motor from the autogenous pressurisation pipeline. That hydraulic pump will drive aerospace grade "" plain" hydraulic system with synthetic oil and hydraulic acumulator as seen in that Raptor render from IAC2016.

Offline livingjw

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #997 on: 02/14/2019 02:28 pm »
IMHO hydraulic pump will be driven by gasseous CH4 pneumatic motor from the autogenous pressurisation pipeline. That hydraulic pump will drive aerospace grade "plain" hydraulic system with synthetic oil and hydraulic accumulator as seen in that Raptor render from IAC2016.

- Autogenous pressurant is not available unless engines are firing. Engines are not firing during reentry when actuators are needed.

- Will probably use RCS high pressure tanks to run a small turboshaft burner. They probably will match this to an electric generator, and use electro-hydraulics for the actuators.

- I agree with: ....That hydraulic pump will drive aerospace grade .... hydraulic system....

Offline DusanC

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #998 on: 02/14/2019 02:45 pm »
IMHO hydraulic pump will be driven by gasseous CH4 pneumatic motor from the autogenous pressurisation pipeline. That hydraulic pump will drive aerospace grade "plain" hydraulic system with synthetic oil and hydraulic accumulator as seen in that Raptor render from IAC2016.

- Autogenous pressurant is not available unless engines are firing. Engines are not firing during reentry when actuators are needed.

- Will probably use RCS high pressure tanks to run a small turboshaft burner. They probably will match this to an electric generator, and use electro-hydraulics for the actuators.

- I agree with: ....That hydraulic pump will drive aerospace grade .... hydraulic system....
$DEITY knows why I was thinking about Raptor hydraulics while writing a post about flaps/gridfin hydraulics.


P. S. Why use burner-turbine-generator-e.motor setup when you can just use pneumatic motor driven by RCS gas?
« Last Edit: 02/14/2019 02:49 pm by DusanC »

Offline RoboGoofers

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #999 on: 02/14/2019 02:58 pm »
Another example of 'nothing is easy with Starship'. The hydraulic system is going to spend a lot of time at very low temps and vacuum/near vacuum, and then spikes of very high temps. Here's a fluid that might work but it's rather pricey (minimal google search for "cryogenic hydraulic fluid")
Are there any examples of hydraulic systems used on deep space hardware?

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