totm nov 2023 SpaceX Discussion Thread

[Nibb31]

1 hour ago, tater said:

 

 

I think they only need extend enough to cover small terrain changes. Think shock absorbers, not long, spindly legs.

It's not the length that bothers me, it's the surface area in contact with the ground. 100 tons per leg, on a surface of a few square centimeters, is bound to sink into the ground, even on a prepared surface. It needs feet or pads with a wider surface area.

[Jaff]

Those petals could be movable heatshield segments. Ie they fold out and then come together in the middle to cover the engines. 

 

The mechanism for that that wouldn’t be too complicated 

[sh1pman]

Or they could be spikes that pierce the ground. How's that for stability.

Edited September 14, 2018 by sh1pman

[magnemoe]

9 minutes ago, Nibb31 said:

It's not the length that bothers me, it's the surface area in contact with the ground. 100 tons per leg, on a surface of a few square centimeters, is bound to sink into the ground, even on a prepared surface. It needs feet or pads with a wider surface area.

Yes, length is nice, the pad is at least 70 cm might be as much as an meter, it will land pretty dry outside of moon, you could make some sort of foot for it. if you wanted, this would be most relevant for moon landings and it could be droppable after moon takeoff. 

Make me wonder about the landing profile for this, wings straight for reentry. then Y for landing? will it launch with it in Y or V? 

16 minutes ago, Jaff said:

Those petals could be movable heatshield segments. Ie they fold out and then come together in the middle to cover the engines. 

 

The mechanism for that that wouldn’t be too complicated 

Might be as this will come down sideway you don't need an complete cover , might also be for aerodynamic stability, and for blast protection if landing on moon or mars. 

[Xd the great]

10 minutes ago, magnemoe said:

Make me wonder about the landing profile for this, wings straight for reentry. then Y for landing? will it launch with it in Y or V? 

Probably Y shaped, like most missiles with canards. Or whatever they are called.

[Mitchz95]

13 hours ago, IncongruousGoat said:

I'm of the opinion that Elon is just messing with us at this point. That render doesn't make much sense as a new version of BFR for a number of reasons, not least of which that it lacks any sea-level-optimized nozzles, which makes it incapable of landing on Earth. This is Elon Musk's Twitter account, people. It's not exactly a reputable news source. Hopefully this will all be cleared up in the stream on Monday.

It's on SpaceX's official account, not Elon's.

[tater]

It lacks vacuum nozzles, not sea level ones.

[Xd the great]

Just now, Mitchz95 said:

It's on SpaceX's official account, not Elon's.

Yes, but does it make a difference?

SpaceX makes so many modifications to their rockets that the final product is barely recognizable as the preliminary idea.

Just now, tater said:

It lacks vacuum nozzles, not sea level ones.

Maybe the petals work as one, along as acting like a heat shield, landing pads, and whatever else.

[tater]

36 minutes ago, Nibb31 said:

It's not the length that bothers me, it's the surface area in contact with the ground. 100 tons per leg, on a surface of a few square centimeters, is bound to sink into the ground, even on a prepared surface. It needs feet or pads with a wider surface area.

I'm confident they've considered this. Whatever Kremlinology we do until we have more details, I would say that all changes have been made for real engineering reasons.

Part of the underside of the fins could fold down as the legs deploy diagonally, for example (making the leg the width of the pad at the end, but several meters long.

(note that it could fold down the bit without deploying diagonally at all)

Infinitely better drawing here, same idea:

https://forum.nasaspaceflight.com/index.php?topic=46371.msg1856002#msg1856002

 

[magnemoe]

1 hour ago, Xd the great said:

Probably Y shaped, like most missiles with canards. Or whatever they are called.

Yes,  the V profile it might be to minimize lift so they can go from sideways to engines down, fold up wing and use the canards to generate maximum lift and drag on nose might even use engine gimbal and offset to help. 
Then you are vertical you extend fins to Y and lower legs. 

Benefit of this design over the old is far more aerodynamic control, however you got an rocket with huge fins like an 1950 car, Elon did not want that
Even if its make landing more stable up into crumble zones. 
KSP style, we are happy to announce our first Mun landing, and also our first Mun base, the landing legs, engine and tanks worked better than expected as crumble zones. 

[RedKraken]

https://www.reddit.com/r/SpaceXLounge/comments/9fqrrk/wildass_speculation_thread_30_1_what_the_hell_is/

 

[tater]

I can't see it as a deployable nozzle, makes little sense.

If they were gonna do something deployable, why not just make a center plug and not worry about it? (go completely Phil Bono!)

[sevenperforce]

12 minutes ago, tater said:

I can't see it as a deployable nozzle, makes little sense.

If they were gonna do something deployable, why not just make a center plug and not worry about it? (go completely Phil Bono!)

They don't want a complete engine redesign.

[tater]

25 minutes ago, sevenperforce said:

They don't want a complete engine redesign.

Would it need an engine redesign at all? Seems like it becomes a layout issue.

 

 

(Firefly Space Systems plug nozzle design, above)

 

Alternately, they could have a few of the Raptors have extensible nozzles. (say 3 of the surrounding 6).

[sevenperforce]

13 minutes ago, tater said:

Would it need an engine redesign at all? Seems like it becomes a layout issue.

So, little-known fact: "aerospike" actually refers to a "plug" nozzle, the point being that you do not need a full-length spike because gases entrained at the base of the plug act as a stagnation zone which generate a spike effect. The spike is made of "air" (gases), thus aero-spike.

If the outer engines are all gimbaled inward, then perhaps the same effect is possible, with the exhaust plume of the core engine acting as the virtual aerospike.

13 minutes ago, tater said:

Alternately, they could have a few of the Raptors have extensible nozzles. (say 3 of the surrounding 6).

Extensible nozzles a la RL-10 save length but not diameter. The BFS is diameter-limited.

[tater]

4 minutes ago, sevenperforce said:

So, little-known fact: "aerospike" actually refers to a "plug" nozzle, the point being that you do not need a full-length spike because gases entrained at the base of the plug act as a stagnation zone which generate a spike effect. The spike is made of "air" (gases), thus aero-spike.

If the outer engines are all gimbaled inward, then perhaps the same effect is possible, with the exhaust plume of the core engine acting as the virtual aerospike.

This is why I said plug, and not aerospike.

 

4 minutes ago, sevenperforce said:

Extensible nozzles a la RL-10 save length but not diameter. The BFS is diameter-limited.

If it's 3 of the outside engines it might be possible (though the layout actually shown clearly limits this as you say).

Gotta wait until Monday, I guess...

 

[NSEP]

https://arstechnica.com/science/2018/09/spacex-says-its-bfr-will-fly-someone-around-the-moon-we-have-questions/

[Mad Rocket Scientist]

Maybe raptor has been downscaled again, and those are vacuum nozzles. Or maybe there are two sizes of raptor, and there is still a mix of SL and vac engines, they just have the same nozzle size?

I imagine the folding lower fins are so it can reenter stably like the shuttle, then fold up the fins and flip backwards to land.

[sevenperforce]

8 minutes ago, Mad Rocket Scientist said:

Maybe raptor has been downscaled again, and those are vacuum nozzles. Or maybe there are two sizes of raptor, and there is still a mix of SL and vac engines, they just have the same nozzle size?

I imagine the folding lower fins are so it can reenter stably like the shuttle, then fold up the fins and flip backwards to land.

The dev scale Raptor is about the size of a Merlin, albeit heavier with much higher thrust. An MVac is roughly the size of a SL full-size Raptor.

I wonder if they decided to mix 3 SL full-size raptors with 4 dev scale Vacuum Raptors and use the same engine bells for everything.

[Mad Rocket Scientist]

1 minute ago, sevenperforce said:

The dev scale Raptor is about the size of a Merlin, albeit heavier with much higher thrust. An MVac is roughly the size of a SL full-size Raptor.

I wonder if they decided to mix 3 SL full-size raptors with 4 dev scale Vacuum Raptors and use the same engine bells for everything.

They'd have to convert Mvac to methane though, and it still wouldn't be staged combustion.

[RedKraken]

No solar panels on 2018 BFS? ....methane/lox generators for power?

[tater]

2 minutes ago, RedKraken said:

No solar panels on 2018 BFS? ....methane/lox generators for power?

Given that this is related to a near(ish) term Apollo 8 sort of mission, with possibly just one crew member, I doubt solar is needed.

[sevenperforce]

15 minutes ago, Mad Rocket Scientist said:

They'd have to convert Mvac to methane though, and it still wouldn't be staged combustion.

Nah, you misunderstand. The Raptor they've shown tests for was a scaled-down Raptor:

Quote

By August 2016, the first integrated Raptor rocket engine, manufactured at the SpaceX Hawthorne facility in California, shipped to the McGregor rocket engine test facility in Texas for development testing.^[12]The engine had 1 MN (220,000 lb_f) thrust, which makes it approximately one-third the size of the full-scale Raptor engine planned for flight tests in 2019/2020 timeframe. It is the first full-flow staged-combustion methalox engine ever to reach a test stand.

So the development scale Raptor is a third the thrust of the full-size Raptor.

Presumably, then, you could connect a dev scale Raptor to a full-size Raptor's engine bell assembly and you'd end up with a vacuum-optimized dev scale Raptor.

[Mad Rocket Scientist]

9 minutes ago, sevenperforce said:

Nah, you misunderstand. The Raptor they've shown tests for was a scaled-down Raptor:

So the development scale Raptor is a third the thrust of the full-size Raptor.

Presumably, then, you could connect a dev scale Raptor to a full-size Raptor's engine bell assembly and you'd end up with a vacuum-optimized dev scale Raptor.

Sorry, I didn't read your post carefully enough.

[sevenperforce]

19 minutes ago, sevenperforce said:

The development scale Raptor is a third the thrust of the full-size Raptor.

Presumably, then, you could connect a dev scale Raptor to a full-size Raptor's engine bell assembly and you'd end up with a vacuum-optimized dev scale Raptor.

Such a configuration would really decrease the maximum thrust of the stage.

• Dev Raptor, SL nozzle, SL: 567 kN
• Dev Raptor, SL nozzle, space: 612 kN
• Dev Raptor, Vac nozzle, space: 633 kN
• Raptor, SL nozzle, SL: 1,700 kN
• Raptor, SL nozzle, space: 1,834 kN
• Raptor, Vac nozzle, space: 1,900 kN

You know, looking at this, I don't think they need vacuum nozzles at all.

The Raptor has such a ridiculously high chamber pressure that its SL pressure losses are really small, and it's only about 5% underexpanded in vacuum.