Author Topic: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)  (Read 110122 times)

Online john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #140 on: 10/12/2017 03:17 PM »

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.
And to remain there throughout the deceleration through about 20 Mach numbers?

Maybe all that mass above the propellant tanks will do the trick and the BFS will fly like Skylon, not like HOTOL.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Online envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #141 on: 10/12/2017 03:25 PM »

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.
And to remain there throughout the deceleration through about 20 Mach numbers?

Maybe all that mass above the propellant tanks will do the trick and the BFS will fly like Skylon, not like HOTOL.

More like 30 Mach numbers, from interplanetary entry to subsonic on Earth. Will be interesting to see whether they can make it stable (or nearly so) through hypersonic, supersonic, transonic, and subsonic regimes. At least it's only hypersonic and high supersonic at Mars.

Not sure that they don't want it to fly like HOTOL; the only time lift is a significant factor is during entry and descent, where they want as much drag as possible and a very high AOA. They don't have to orient for lift during terminal landing or at all during ascent.

Offline spacenut

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #142 on: 10/12/2017 03:29 PM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Also when returning to earth through the thick atmosphere, are the wings or winglets used to keep the lower mass level with the upper mass to keep from coming in bottom first? 

I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned. 

Offline octavo

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #143 on: 10/12/2017 03:51 PM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Also when returning to earth through the thick atmosphere, are the wings or winglets used to keep the lower mass level with the upper mass to keep from coming in bottom first? 

I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned.
I think you're confusing the booster and spaceship. Only 6 engines on the the ship.

Offline Oersted

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #144 on: 10/12/2017 07:50 PM »
The BFR will be built in the Los Angeles port area.

Gwynne Shotwell Q&A. The quote below is not verbatim but from notes by Reddit-user "Sticklefront": https://www.reddit.com/r/spacex/comments/75ufq9/interesting_items_from_gwynne_shotwells_talk_at/

"Where will the BFR be built?

We're looking at building a facility by the water in LA. We thought we'd build it in our factory in Hawthorne, but we priced transport to the harbor, and it came out to $2.5m per trip. It would require taking down stoplights, and just wouldn't be worth it. So we will build a new facility by the water. We will eventually also have a number of production sites by out launch sites."

Offline lamontagne

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #145 on: 10/12/2017 09:06 PM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.  There would be no windows in the lower rooms (on the heat protected side), but this was easier to model  :-)

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

« Last Edit: 10/12/2017 09:07 PM by lamontagne »

Online envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #146 on: 10/12/2017 09:07 PM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

BFS was to be used as a an early surface hab per the 2016 presentation. That doesn't seem to have changed.

Offline lamontagne

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #147 on: 10/12/2017 09:29 PM »
40 rooms cannot evenly be distributed into 3 floors.  So do we have 3 floors with 42 spaces, two that are not rooms but some kind of common spaces?  The video shows 2 sizes of rooms, perhaps there are 4 larger rooms per floor and 10 smaller ones? And are the narrow spaces utility spaces and bathrooms?
Or did Elon just round up?
« Last Edit: 10/12/2017 09:30 PM by lamontagne »

Offline spacenut

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #148 on: 10/12/2017 09:30 PM »
Yes,  I was confusing the booster from the ship.  I'm not concerned about the booster, it will be like F9.  The ship yes will have 6 engines, 4 vacuum and 2 sea level.  Ok, that is 6 tons or so, seems like with a 50 ton down mass, the ship will be top heavy when it lands almost empty of fuel/lox.  Same with landing on the moon or Mars, with 150 tons, seems too top heavy.

Offline Lars-J

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #149 on: 10/13/2017 12:25 AM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.  There would be no windows in the lower rooms (on the heat protected side), but this was easier to model  :-)

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

Keep in mind that the Mars re-entry profile will need to be taken into account. Gravity will be shifting, from zero, to sideways (when entering the atmosphere at high angle of attack), and finally down during the landing burn and stay on Mars.

And each bed might need to double as a launch/landing couch too. So it is very likely that all couches/beds need to face in the same direction to accommodate there shifts. This is what I had in mind when trying to create some sort of interior layout for the 2016 ITS (see image)

Of course if the ship is less full and there are separate launch/entry seats, then you have much more freedom to lay things out.


Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #150 on: 10/13/2017 02:50 AM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.

That's impossible to judge unless you know how much insulation the cabin has, and how much insolation it receives (which depends on vehicle attitude).
In space the only way you lose heat is radiation. I didn't see any radiators, but they could have been hidden on the other side of the solar panels. Also, the heatshield on the bottom should be dark which will serve as a radiator. Waste heat from electronics, the life support system, chemical reactions powering human bodies, and whatever else needs energy on the ship should keep things plenty warm, they just have to keep the radiators sized and positioned correctly to balance the energy needs. If they need more heating, they just need to point the black side of the ship a bit more towards the sun.

The point is, you couldn't run 20 hair dryers on 35 KW. Shrinking the design, power, fuel and everything else but not shrinking the passenger count tends to lead to less technical credibility. As far as tilting the ship, I've never seen any indication that the solar fins pivot. They are always rendered in the same position. So, tilting the ship just exacerbates the power situation. For comparison, the Shuttle supported a crew of 8 and the 3 fuel cells could produce 21 KWs of continuous power.
I feel to see why everyone would need to be running a hairdryer full blast. That's more energy than even a closed loop ECLSS.

Hair dryers use a LOT of power. Equivalent to like 200 bright LED light bulbs.
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Offline livingjw

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #151 on: 10/13/2017 05:19 AM »

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.
And to remain there throughout the deceleration through about 20 Mach numbers?

Maybe all that mass above the propellant tanks will do the trick and the BFS will fly like Skylon, not like HOTOL.

- The CP needs to be near the CG to be controllable, so they added the wing to drag the CP towards the tail. The wing also gives them some more volume in the back for subsystems. Design is all about lining up you CG and CP and providing adequate control authority.

- The CP will be nearer the bottom (heatshield side) than the CG, hence BFS will not be stable, nor should it be. It has to fly AoAs from -10 to 190 degrees.

- BFS will be actively controlled using the flaps for roll and pitch and RCS thrusters for yaw.

- But it does need adequate aerodynamic control authority in pith and roll. Flap pitch deflections must be able to move the CP from in front of the CG to behind it as commanded by the autopilot.

- Mars entry, down to about 600 m/s, will be flown somewhere between 45 and 65 degrees AoA, maybe even a little higher. The vehicle maintaining high AoA to maximize drag. AoA can be set within these limits to extend or shorten downrange landing position.

-  Roll angle is the principle control. Lift is controlled by rolling alternately left and right. Roll angles vary as much as 360 degrees to control g's, dynamic pressure, altitude and position (similar to the shuttle). The easiest way to think about it is to picture and arrow sticking out of the vehicle in the direction of flight. It will be sticking through the CG and out of the forward bottom of the BFS at an angle of about negative 45-65 degrees and yaw = 0. You will always be flying in the direction of this arrow. If you want to get to higher dynamic pressure, you kill off lift by rolling alternately left and right. If you need down force, you roll nose down and alternate left and right. You can see the BFS nose down early in its Mars entry, keeping it from skipping out of the atmosphere.

- One nice thing about hypersonics is that much above Mach 4 flow becomes Newtonian and aerodynamics don't change much, they just get hotter. Stability derivatives tend to not change much.

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #152 on: 10/13/2017 05:36 AM »
The BFR will be built in the Los Angeles port area.

Gwynne Shotwell Q&A. The quote below is not verbatim but from notes by Reddit-user "Sticklefront": https://www.reddit.com/r/spacex/comments/75ufq9/interesting_items_from_gwynne_shotwells_talk_at/

"Where will the BFR be built?

We're looking at building a facility by the water in LA. We thought we'd build it in our factory in Hawthorne, but we priced transport to the harbor, and it came out to $2.5m per trip. It would require taking down stoplights, and just wouldn't be worth it. So we will build a new facility by the water. We will eventually also have a number of production sites by out launch sites."

This does slightly mess up the notions that
a) 9m BFR was chosen to fit existing factory
b) F9 production will have to end to allow BFR to be built

I don't know how flexible the factory floor is. Can they use the existing facility to produce the first test articles and prototypes for BFR, or would that require removing all the F9 equipment?
Waiting for joy and raptor

Offline Patchouli

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #153 on: 10/13/2017 06:09 AM »

TBF the Dragon reentry is "fast and furious" due to it being quite dense.

The BFS and the other variants have large surface areas and (in principal) much lower ballistic coefficients (or "wing loadings"  if they were aircraft) so in principle should have lower wear.

I think SX are aiming at a part of the size/TPS solution space that has not really been looked at Historically "reusable" has actually meant, winged/longish entry/heat re-radiated. But what if you built a vehicle with the "fluffiness" of a winged, internally tanked vehicle (Like a Shuttle but with the ET internal) but brought it down with a profile more like a capsule? Coming down fast(ish) sacrifices cross range (but how much of that do you really need?) but reduced heat soak to your TPS (and reduced ablation to negligible levels?)

This maybe another area where the lore of the subject (driven by assumptions made by engineers in a tearing hurry to make something work?) is about to be challenged.

I think BFS can partly use propulsive capture into a low energy orbit on Earth return as it would leave Mars with full propellant load and much less cargo.
If Spacex can meet the mass fractions claimed it probably can almost get into LEO by propulsion alone.
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.


This does slightly mess up the notions that
a) 9m BFR was chosen to fit existing factory
b) F9 production will have to end to allow BFR to be built

I don't know how flexible the factory floor is. Can they use the existing facility to produce the first test articles and prototypes for BFR, or would that require removing all the F9 equipment?

They'll have to keep F9 upper stage production going as this is not reusable.
This also opens up an option for an interim vehicle if there are delays with BFR/BFS in that it might be possible to fly a F9 upper stage on a half length BFR stage.
« Last Edit: 10/13/2017 06:18 AM by Patchouli »

Offline Lars-J

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #154 on: 10/13/2017 06:19 AM »
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.

Huh? It will be far cheaper to transport ANY commodity from from Earth to the Moon directly, rather than from Mars.

Offline Patchouli

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #155 on: 10/13/2017 06:56 AM »
Depends on how things work out but energy wise it's a lot easier to take something from Mars to the Moon than from Earth.

Online hkultala

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #156 on: 10/13/2017 07:11 AM »

TBF the Dragon reentry is "fast and furious" due to it being quite dense.

The BFS and the other variants have large surface areas and (in principal) much lower ballistic coefficients (or "wing loadings"  if they were aircraft) so in principle should have lower wear.

I think SX are aiming at a part of the size/TPS solution space that has not really been looked at Historically "reusable" has actually meant, winged/longish entry/heat re-radiated. But what if you built a vehicle with the "fluffiness" of a winged, internally tanked vehicle (Like a Shuttle but with the ET internal) but brought it down with a profile more like a capsule? Coming down fast(ish) sacrifices cross range (but how much of that do you really need?) but reduced heat soak to your TPS (and reduced ablation to negligible levels?)

This maybe another area where the lore of the subject (driven by assumptions made by engineers in a tearing hurry to make something work?) is about to be challenged.

I think BFS can partly use propulsive capture into a low energy orbit on Earth return as it would leave Mars with full propellant load and much less cargo.

Please, use numbers and calculations when you think. It might give you better thoughts.

Delta-V from LEO to mars tranfer orbit is only 3.8 km/s. And braking / landing to surface from the transfer orbit to mars is 99% aerobraking.
Delta-V from Mars surface to earth transfer orbit is 6.4 km/s.

This is the reason the return payload is much smaller (50 tonnes) that outgoing payload

Quote

If Spacex can meet the mass fractions claimed it probably can almost get into LEO by propulsion alone.

Delta-V from Mars surface to LEO is 10.2 km/s.

So no, far from that. Cannot do it. And try to do it with any cargo.

Quote
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.

this "some" would probably be a negative amount.
« Last Edit: 10/13/2017 07:13 AM by hkultala »

Online john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #157 on: 10/13/2017 08:41 AM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Good question. In theory as long as all the mass stays inside the base legs things should be OK, but then
there are external wind forces. [EDIT. Oops. The big joker here is how flat the landing area is (and how flat will it be after the Raptors have hosed it for a bit). That will be a recurring issue, so landing the first vehicle light doesn't help much. A permanent landing area needs to be checked for soft spots (partly landing on a sink hole would be a Very Bad Day) and flattened for future landings probably just after the crews have "dug in," which I assume is what TBC is for) ]

On Earth the stage should lock into its landing cradle (now I think of it very EE Smith, but without an inertialess drive ) on Mars wind forces (even at high speeds) should be much lower due to the pressure being 1/160 that of Earth SL (  "The Martian" is quite realistic in many ways, but not this. OTOH visibility and dust damage is likely to be a real problem. an Earth hurricane can hit an area for a few hours before moving on or dissipating. A Mars sandstorm can last for months. )
Quote from: spacenut
I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned.
Doesn't 31 engines only apply to the booster? Otherwise 1 tonne implies no better than a T/W of 174:1, although the joker is the complexity of the feed piping.
« Last Edit: 10/13/2017 09:01 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Online john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #158 on: 10/13/2017 09:01 AM »
More like 30 Mach numbers, from interplanetary entry to subsonic on Earth. Will be interesting to see whether they can make it stable (or nearly so) through hypersonic, supersonic, transonic, and subsonic regimes. At least it's only hypersonic and high supersonic at Mars.
Noted. I was still thinking in terms of Earth re-entry.  30 Machs
Quote from: envy887
Not sure that they don't want it to fly like HOTOL; the only time lift is a significant factor is during entry and descent, where they want as much drag as possible and a very high AOA. They don't have to orient for lift during terminal landing or at all during ascent.
The point about HOTOL and Skylon was the HOTOl design didn't really work precisely because of the huge shift in CoG and CoM due to the massive shift in mass properties (through propellant burn off) and lifting forces, needing an enormous set of servos on the nose canards to keep it horizontal.

BFS should be easier to manage on Mars as fully loaded I doubt it will be bottom heavy the way the BFR is but Earth re-entry may be more doubtful, assuming it's left all it's payload on Mars.
[EDIT. It depends on how much all that nose section masses. If it's too light those tail flap actuators are going to be fighting a tendency to flip all the way down. Too heavy and the won't be able to go up far enough to keep it nose front. I can't remember if those wings are expected to have trailing edge control surfaces. If they aren't, I'm betting by the time this flies they will.  ]
« Last Edit: 10/13/2017 09:10 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Online hkultala

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #159 on: 10/13/2017 12:15 PM »

This does slightly mess up the notions that
a) 9m BFR was chosen to fit existing factory

EM never talked about existing FACTORY. He talked about exsiting FACILITIES.

Launch pads, testing pads, integration facilities etc are also facilities.

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