totm nov 2023 SpaceX Discussion Thread

[sevenperforce]

Random aside: does anyone have any idea how the proposed BFS is supposed to do the flip from prograde to retrograde for the landing burn?

I've tried modeling it, and...yuck.

[mikegarrison]

4 hours ago, tater said:

Sending 7 crew to station would apparently double the science done, as 5 out of 6 crew days is spend on maintaining ISS, and other non-science activity.

That assumes that crew availability is the limiting factor for science productivity. I'm not convinced that the real limiting factor isn't just the lack of any actual need for scientists to be working in space....

[HebaruSan]

20 minutes ago, sevenperforce said:

Random aside: does anyone have any idea how the proposed BFS is supposed to do the flip from prograde to retrograde for the landing burn?

I've tried modeling it, and...yuck.

Last time I saw an animation of it, it looked like they were using engine gimbal.

[sh1pman]

23 minutes ago, mikegarrison said:

That assumes that crew availability is the limiting factor for science productivity. I'm not convinced that the real limiting factor isn't just the lack of any actual need for scientists to be working in space....

It is. Astronauts and cosmonauts have to do a lot of busy work just maintaining ISS systems running and performing various equipment checks. Have to work on weekends even. Having more people will either ease the individual workload or massively increase the science productivity. 

[sevenperforce]

34 minutes ago, HebaruSan said:

Last time I saw an animation of it, it looked like they were using engine gimbal.

Looking at this, it doesn't seem so:

Looks like prograde/radial-in for initial entry, with a slow yaw to normal during ascent, then around to high AoA prograde during approach, followed by a pitchback to retrograde before main engines are ignited for the landing burn.

Can't for the life of me figure out how it is supposed to work. The yaw maneuver looks like a controlled tailspin. It looks like they expect to do some kind of gliding pitch-up-to-stall to bleed off velocity, then drop through the stall (somehow), then pitch back. But with the heavy crew module up front and those winglets at the back, that thing should be a lawn dart.

Maybe this is just the Mars entry version, where aerodynamic forces on approach are far lower, and they are doing something completely different at Earth EDL?

Edited January 17, 2018 by sevenperforce

[mikegarrison]

13 minutes ago, sh1pman said:

It is. Astronauts and cosmonauts have to do a lot of busy work just maintaining ISS systems running and performing various equipment checks. Have to work on weekends even. Having more people will either ease the individual workload or massively increase the science productivity. 

Or ... having more people will increase the strain on the systems and cause there to be more work needed to keep them running.

[sh1pman]

3 minutes ago, mikegarrison said:

Or ... having more people will increase the strain on the systems and cause there to be more work needed to keep them running.

Not true. Note how there are now 2 cosmonauts instead of 3 on ISS. Sergey Ryazansky said in one of his videos from ISS that he has to work considerably more because of the crew reduction.

Also, this.

[mikegarrison]

12 minutes ago, sh1pman said:

Not true. Note how there are now 2 cosmonauts instead of 3 on ISS. Sergey Ryazansky said in one of his videos from ISS that he has to work considerably more because of the crew reduction.

Also, this.

I don't think I want to follow you down this rabbit hole.

My actual point (which was rather snarky, I admit) is that having more crew will only allow them to do more science if there is in fact any need to do more science. I'm not terribly convinced there is any *need* to do any science at all. As far as I can tell, almost all ISS science is circular in nature -- it's primarily about the effects on materials and people of being on a space station.

[tater]

While I agree the science work is marginal, the simple math is that 5/6 of every working day is “not science,” and I have read NASA people saying they’d get more of the real (science) work done with even a single extra crew member, literally doubling the possible work they could do. 

ISS science is mostly human factors stuff, so it’s not hard to imagine them able to do more.

[HebaruSan]

1 hour ago, sevenperforce said:

Looking at this, it doesn't seem so:

Fair enough, and the other references I've been able to find are not particularly enlightening. "Earth to Earth" conspicuously skips this step entirely; the ship is prograde in orbit one frame and descending retrograde the next:

Spoiler

The earlier ITS video shows the engines firing at just about the end of Mars atmospheric entry, but it goes on to land almost immediately after that, so we can probably take that as not a complete physics simulation:

Spoiler

If they build on established tech, then the obvious choice is to do what the F9 booster does, flip around with RCS puffs. But you'd obviously have to do that before atmospheric entry.

Maybe they haven't solved it yet, maybe the CoM is a lot further back than we think, maybe they'll do aerocapture and descent separately with flip in vacuum, or maybe it's a trade secret.

[ment18]

2 hours ago, sevenperforce said:

Looking at this, it doesn't seem so:

Looks like prograde/radial-in for initial entry, with a slow yaw to normal during ascent, then around to high AoA prograde during approach, followed by a pitchback to retrograde before main engines are ignited for the landing burn.

Can't for the life of me figure out how it is supposed to work. The yaw maneuver looks like a controlled tailspin. It looks like they expect to do some kind of gliding pitch-up-to-stall to bleed off velocity, then drop through the stall (somehow), then pitch back. But with the heavy crew module up front and those winglets at the back, that thing should be a lawn dart.

Maybe this is just the Mars entry version, where aerodynamic forces on approach are far lower, and they are doing something completely different at Earth EDL?

I think they want to use the manuevering thrusters, since the methane thrusters will be pretty powerful, but I haven't done the math so that could be unreasonable.  I think the BFR has serious lift in the front, Elon said during the presentation that the wings were required to keep the vehicle stable during reentry, which suggests that they only pull the center of lift to approximately the center of mass of the vehicle.

[CatastrophicFailure]

Don’t ask me where, but I’m positive I recall someone from SpaceX saying they’re fairly certain they can do the flip with aerodynamic surfaces alone. Big speed brake on the back, maybe? Deployable canards, akin to the slats they keep wayward backwards stock cars from going airborne?

In more ways than one, these videos aren’t showing us the complete picture. I’m sure the internal ones SpaceX has are much better.  like BulgariaSat *cough cough*

[PB666]

2 hours ago, tater said:

While I agree the science work is marginal, the simple math is that 5/6 of every working day is “not science,” and I have read NASA people saying they’d get more of the real (science) work done with even a single extra crew member, literally doubling the possible work they could do. 

ISS science is mostly human factors stuff, so it’s not hard to imagine them able to do more.

I can just see this some space admin hanging around JSC in the middle of the night watching the cleaning people do shop

' hey friend, would you like to go into space' , a kind of cold hard stare comes back, then a little fear ' . . . . no . . . . no , I don't think so. definitely not'.
It reminds me of Garrett Morris and the 3-mile island skit on Saturday Night Live.

They really do need young people with low blood pressure and alot of energy to do then. Just convert all the spacemeals to starbucks coffee.

 

[tater]

As long as it's decaf (otherwise their BP goes up).

[PB666]

1 hour ago, tater said:

As long as it's decaf (otherwise their BP goes up).

They are young so . . . . .but you in the ISS when they some one is bouncing off the walls, they literally would be bouncing off the walls.

[Nefrums]

8 hours ago, sevenperforce said:

Random aside: does anyone have any idea how the proposed BFS is supposed to do the flip from prograde to retrograde for the landing burn?

I've tried modeling it, and...yuck.

It has to have CoM an CoL close, and have a powerful rcs system.

The landing burn propellant tanks is far back, inside one of the main tanks.

They probably have to light the engines halfway through the flip to gain more control through gimbal.

 

[mikegarrison]

39 minutes ago, Nefrums said:

It has to have CoM an CoL close, and have a powerful rcs system.

The landing burn propellant tanks is far back, inside one of the main tanks.

They probably have to light the engines halfway through the flip to gain more control through gimbal.

 

That's way too complicated. I say they send Jeb out on a ladder....

[kerbiloid]

11 hours ago, sevenperforce said:

Looking at this

In this animation: are those fins  on the top or on the bottom side of the ship?

I mean: are they aerobraking or landing with heads down + rotating around the longitudinal axis?

Will their brains stay inside or are considered expendable?

Edited January 18, 2018 by kerbiloid

[sh1pman]

8 minutes ago, kerbiloid said:

In this animation: are those fins from on the top or on the bottom side of the ship?

I mean: are they aerobraking or landing with heads down + rotating around the longitudinal axis?

Will their brains stay inside or are considered expendable?

"Astronaut brains will be expendable at first, but we are expecting to achieve partial reusablity with BFS block 3"

[PB666]

2 hours ago, kerbiloid said:

In this animation: are those fins  on the top or on the bottom side of the ship?

I mean: are they aerobraking or landing with heads down + rotating around the longitudinal axis?

Will their brains stay inside or are considered expendable?

I actually don't know how two winglets pointing in the direction of oncoming airflow won't twist around.

[sevenperforce]

12 minutes ago, PB666 said:

I actually don't know how two winglets pointing in the direction of oncoming airflow won't twist around.

Yeah, that's my experience in testing. There's no roll authority at all; those winglets are bound to go shuttlecock.

11 hours ago, ment18 said:

I think they want to use the manuevering thrusters, since the methane thrusters will be pretty powerful, but I haven't done the math so that could be unreasonable.  I think the BFR has serious lift in the front, Elon said during the presentation that the wings were required to keep the vehicle stable during reentry, which suggests that they only pull the center of lift to approximately the center of mass of the vehicle.

Thrusters are good for initiating pitch, roll, and yaw movements, but I don't think they can be relied upon to continuously fire to oppose aerodynamic forces.

I tried modeling a BFR with more lift in the front, by adding some wing strakes up near the nose, and it gave me a pretty balanced entry and descent but its stall speed was so high that it went into a stall and tailspin at the very beginning of the landing approach.

[PB666]

I can only imagine the look on this guys chief extraterrestrial engineer when he tells him "I want a city of mars by 2028"

The word ambitious is a gross understatement of his plan . . . . but anyway let him try, I can't see the other vaporwares going anywhere. 

Edited January 18, 2018 by PB666

[Nibb31]

This is just another of the problems that are being handwaved away, just like ISRU, ECLSS, colonization, etc... The normal way of doing this would be to

• Come up with a plan on paper
• Test in simulation or lab
• Develop a subscale prototype
• Test in real conditions
• Develop BFS

Of course, all that has a certain cost and means that there is no way you are landing 100 tons of payload on Mars by 2028. Instead, Musk's approach is:

• Develop BFS.
• Realize that is much harder than we thought.
• Delay or backscale the original plan.

BFS is too ambitious on too many levels with too many new technologies, techniques, and procedures. If one of these technologies fails to deliver or underperforms, the whole plan goes to waste.

Edited January 18, 2018 by Nibb31

[kerbiloid]

26 minutes ago, PB666 said:

I actually don't know how two winglets pointing in the direction of oncoming airflow won't twist around.

Me too
 

Spoiler

 

[PB666]

12 minutes ago, Nibb31 said:

This is just another of the problems that are being handwaved away, just like ISRU, ECLSS, colonization, etc... The normal way of doing this would be to

• Come up with a plan on paper
• Test in simulation or lab
• Develop a subscale prototype
• Test in real conditions
• Develop BFS

Of course, all that has a certain cost and means that there is no way you are landing 100 tons of payload on Mars by 2028. Instead, Musk's approach is:

• Develop BFS.
• Realize that is much harder than we thought.
• Delay or backscale the original plan.

BFS is too ambitious on too many levels with too many new technologies, techniques, and procedures. If one of these technologies fails to deliver or underperforms, the whole plan goes to waste.

Maybe, i'm not sure about this, but maybe they think that if the BFR cannot achieve its 2022/24 goals that it will still be profitable for missions within Earth's extended SOI. So there could be a point if someone says 'we can't do this' then they are doing something else, like providing an assist for asteroid capture missions or refueling someone's gateway.

BTW, for the 2022 mission to be useful, all they need to do is get a probe into Mars orbit with mars sample on it, so it could be joined to NASA's mission, in which case you could get someone to pay for it.

Edited January 18, 2018 by PB666