totm nov 2023 SpaceX Discussion Thread

[Xd the great]

Ahh thanks.

7 minutes ago, Nibb31 said:

Now, vertically launching with a spaceplane on top (X-20 DynaSoar, DreamChaser, or Hermes) is much more problematic, because those vehicles are designed to generate lift. You are going to need strong control authority to keep it flying straight.

That's why the Space Shuttle flew to orbit "upside down". It could use some of the lift generated by the wings to rotate the flight curve rather than fight the lift with the engine gimballing.

Thats why all my space shuttles crash in ksp. Never knew that. 

[wumpus]

3 hours ago, Nibb31 said:

You just have to keep in mind that although it looks like it has wings, the BFS is not a spaceplane.

Is it supposed to come in "backwards", or "forwards" and eventually flip for a powered landing?  Doing tricks like that in KSP, I've always come in backwards (largely because engines have OP heatshields).

[Spaceception]

18 minutes ago, wumpus said:

Is it supposed to come in "backwards", or "forwards" and eventually flip for a powered landing?  Doing tricks like that in KSP, I've always come in backwards (largely because engines have OP heatshields).

From the video in the presentation (41:17 onward roughly), it has a high angle of attack, and comes in right side up, heatshield forward. Once it's lower in the atmosphere, it looks like it levels off on its belly (roughly) to bleed off more velocity, rights itself again, and uses the engines to land.

[tater]

Yeah, shuttle-like in many respects, then it falls belly down, and given the canard, it can then fly the nose to a high AoA again, making the not-really-a-flip easier.

[magnemoe]

11 hours ago, Xd the great said:

I doubt it. Assuming the bfr uses a launch profile similar to a falcon 9, stage seperation at 6000km/h, about 6000m/s delta v will be needed. Which is about 2/3 delta V of a full bfs. 

So, at most the bfs will be half empty. I did not do the math, but rely on kerbal ecperience for this.

As I understand BFS can abort, that is probably one of the reasons for 7 non vacuum engines to. 
TWR would not be very impressive however but once you are away from first stage you should be able to RTLS as you have more dV than first stage who took you up as that you could bleed out lots of speed with aerodynamic. 

[tater]

7 minutes ago, magnemoe said:

As I understand BFS can abort, that is probably one of the reasons for 7 non vacuum engines to. 
TWR would not be very impressive however but once you are away from first stage you should be able to RTLS as you have more dV than first stage who took you up as that you could bleed out lots of speed with aerodynamic. 

It cannot abort.

TWR is no where near enough to escape a conflagration, nor is engine chill-down and spool up fast enough.

In addition, if it did manage to pull away, then what? Could it land with full tanks? Where would it land?

[NSEP]

10 minutes ago, tater said:

In addition, if it did manage to pull away, then what? Could it land with full tanks? Where would it land?

It would do cool flips and tricks untill its light enough.

sarcasm

[RCgothic]

BFR is very much larger than anything else that's ever flown. Perhaps that affords more time for an abort. Cut main engine, deploy grid fins, drag separation sufficient?

Doesn't help on the ground, I suppose.

[Mad Rocket Scientist]

Great video simulating the lunar flyby trajectory:

Wildly underrated channel too.

[magnemoe]

3 hours ago, tater said:

It cannot abort.

TWR is no where near enough to escape a conflagration, nor is engine chill-down and spool up fast enough.

In addition, if it did manage to pull away, then what? Could it land with full tanks? Where would it land?

No it does not have escape system performance so if first stage blew up violently they would be gone, not sure if they have enough TWR to even seperate at max Q but assume the first stage would fall behind faster. An splashdown would also not work, you are also toast if you get issues with the BFS outside of engine out. I assume you could land on two engine if you loose the core one. 

For fuel at worst case you hover and dump, just hovering would also work. But yes the two falcon 9 explosion would kill the crew, if nothing as it was upper stage but even if lower stage blew up like that I think they would be dead. 

[sh1pman]

 

[Spaceception]

Now, I'm preeeeety sure this article has already been shared https://arstechnica.com/science/2018/09/inside-the-eight-desperate-weeks-that-saved-spacex-from-ruin/, but I really like this quote "Outside of the USSR, from 1957 through 1967, and NASA from 1961 to 1971, it is difficult to find a country or company that has had a more dynamic decade in space than this." And this is why I think we'll see the BFR launch. Pretty much all of the comments were interesting to read as well (Surprisingly), and despite what SpaceX has ahead of itself, I'm excited for the day when we cheer on the first BFR test launch.

Now I just want more updates of the BFR progression. How many times have the Raptors been tested, and for how long in total? How hard was it to build the first carbon fiber cylinder? Do they see building the first fuel tank as faster and/or easier since they already built a larger one a couple years ago? What's the progress on the refueling procedures? Ugh... I wish there were more people like Tim in the audience asking questions. Or even publically submitted ones entered by SpaceX employees or something.

Edited September 23, 2018 by Spaceception

[CatastrophicFailure]

6 hours ago, magnemoe said:

But yes the two falcon 9 explosion would kill the crew, if nothing as it was upper stage but even if lower stage blew up like that I think they would be dead

It is perhaps worth noting that neither of the two Falcon 9 failures could occur on the BFR, there’s no helium system, it will use the fuel and oxidizer themselves to pressurize the tanks, so no worries about burst helium bottles. 

There could, of course, still be other failure modes, but the technical stuff we are seeing demonstrates more and more that SpaceX has done and continues to do their homework, and then there’s all the stuff going on behind the scenes that we don’t see. I think it’s the integrity of the tanks themselves that’s the biggest risk of a RUD right now. 

[Xd the great]

26 minutes ago, CatastrophicFailure said:

I think it’s the integrity of the tanks themselves that’s the biggest risk of a RUD right now. 

SpaceX has tested and burst their giant balls with liquid oxygen. 

On a serious note, cryogenics and composites dont mix well, but i doubt the fuel needed to be stored cryogenically during most interplanetary flight, since they burned most of it already.

Edited September 23, 2018 by Xd the great

[DAL59]

On 9/20/2018 at 1:55 PM, Ultimate Steve said:

The one with the theorized giant slab of rock that could fall and cause a giant tsunami that could wipe out the East coast of the US?

Quote

Lateral collapse events at stratovolcanoes, similar to the current threat posed by the western flank of Cumbre Vieja, could increase due to the physical effects of global warming on the Earth from increases in deviatoric stress from post-glacial rebound, while the size and frequency of eruptions are also likely to increase.

Oh no

[Ultimate Steve]

48 minutes ago, DAL59 said:

Oh no

Not to be off topic any more (last post I promise!) but the first story I tried to write that was both decent and long was about the La Palma Megatsunami. I never finished it, I made some bad plot choices (okay, the main characters are wandering through a lifeless wasteland, now what?) but for a long time that was the best thing I'd written!

Back to SpaceX, the next launch is on October 6, so there's quite some time to twiddle our thumbs.

[cubinator]

1 hour ago, Ultimate Steve said:

Back to SpaceX, the next launch is on October 6, so there's quite some time to twiddle our thumbs.

Uuuuugggghhhh 

I'll be fiddling with rocket parts.

[CatastrophicFailure]

1 hour ago, cubinator said:

Uuuuugggghhhh 

I'll be fiddling with rocket parts.

But ya can’t ask for better timing for once.  Reasonable hours on a Saturday AND the first west-coast RTLS! 

3 hours ago, Xd the great said:

SpaceX has tested and burst their giant balls with liquid oxygen. 

On a serious note, cryogenics and composites dont mix well, but i doubt the fuel needed to be stored cryogenically during most interplanetary flight, since they burned most of it already.

They’ve made a very impressive test piece, but it remains the big unknown as far as mechanical issues. Falcon 9 has already demonstrated they can build a rocket that can survive an engine exploding, a failing Raptor likely wouldn’t cause a RUD, there are no helium tanks to break loose or burst outright, but how is that huge composite structure going to handle repeated hot/cold cycles, flight pressures, etc?

They’ll still need to bring some cryogenic methane/oxygen all the way to Mars for landing/maneuvering. 

[Xd the great]

7 minutes ago, CatastrophicFailure said:

But ya can’t ask for better timing for once.  Reasonable hours on a Saturday AND the first west-coast RTLS! 

They’ve made a very impressive test piece, but it remains the big unknown as far as mechanical issues. Falcon 9 has already demonstrated they can build a rocket that can survive an engine exploding, a failing Raptor likely wouldn’t cause a RUD, there are no helium tanks to break loose or burst outright, but how is that huge composite structure going to handle repeated hot/cold cycles, flight pressures, etc?

They’ll still need to bring some cryogenic methane/oxygen all the way to Mars for landing/maneuvering. 

Yes, the fuel tanks is a big problem, they are working on it.

If they have such huge fuel tanks, and it will be almost empty when they arrive on Mars, do they really need to keep it cryogenic? Or is it done to increase engine efficiency?

[CatastrophicFailure]

4 minutes ago, Xd the great said:

Yes, the fuel tanks is a big problem, they are working on it.

If they have such huge fuel tanks, and it will be almost empty when they arrive on Mars, do they really need to keep it cryogenic? Or is it done to increase engine efficiency?

AFAIK the engines need to run on liquid. If you tried to run them on gas, well, you’d find out exactly how a BFS tolerates exploding engines. 

[Xd the great]

34 minutes ago, CatastrophicFailure said:

AFAIK the engines need to run on liquid. If you tried to run them on gas, well, you’d find out exactly how a BFS tolerates exploding engines. 

So cryogenic, but not super-cooled 

[magnemoe]

3 hours ago, Xd the great said:

So cryogenic, but not super-cooled 

Super-cooled is just an tricks to get 10% more fuel in, its not needed afterward. 
They has two smaller tanks inside the fuel tank, this is to hold fuel and oxygen for the return and landing, benefit in that the fuel tank will insulate the header tanks well. 

[RedKraken]

Aside from the sub-cooling density bonus...

" it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant." - elon

https://space.stackexchange.com/questions/18486/how-actually-do-sub-cooled-propellants-reduce-cavitation-within-turbo-pumps-an

Elon mentioned it a couple of years back (IAC2016?)

I wonder what the penalties are for trying the run the raptors on non-subcooled but still liquid propellant?

Less mass flow and thrust for sure.....decreased life expectancy of pump parts? chugging - > exploding engine?

https://en.wikipedia.org/wiki/Combustion_instability

Chapter 10 of Sutton's Rocket Propulsion Elements :

"It is important for pumps not cavitate during operation because cavitation produced bubbles reduce the steady nominal propellant mass flow and cause combustion instabilities"

Edited September 23, 2018 by RedKraken
sutton

[DAL59]

2 hours ago, RedKraken said:

It is important for pumps not cavitate during operation because cavitation produced bubbles reduce the steady nominal propellant mass flow and cause combustion instabilities

Is this the same thing as Vapor Lock caused by lack of ullage?  

Edited September 23, 2018 by DAL59

[RedKraken]

Not sure Dal...sounds like the symptoms may be similar....unwanted gas in the liquid feed pipes ..... but very different magnitude.

But i don't think cavitation bubbles would be big enough to block the mass flow.

The engine would just run rough until something important breaks.

 

Edited September 24, 2018 by RedKraken