sevenperforce

My PSA about Percy:

ROCKET INTENDED FOR LAUNCHING ASTRONAUTS EXPLODES IN OCEAN

That sounds like a fun story but I cannot find tail nor feather of it anywhere -- tell me more!

RIP B1059. That was definitely the largest amount of plasma I've ever seen coming off the engine cluster post-entry-burn. It's possible that an early shutdown might have screwed with the trajectory, leaving the booster too far off-course to maneuver to the droneship. Lucky birbs.

That is freaking amazing. In other news, a PSA about why we have to wear masks even after getting the vaccine:

We’ve... ...got... ...a... SUPERHEAVY GRIDDDDD FINNNNNN!!!!!!!!

The volume of the collective Schwarzschild radius scales with the cube of the total mass. If the cross-section only scales with the square of the total mass, then the collective gravity well is going to grow faster than the collision cross-section. It’s almost like a mean free path analysis, isn’t it? You have a bunch of “particles” that basically only interact via elastic collisions, with a VERY small inelastic collision cross-section. Near-misses alter the paths, but since the motion is chaotic they don’t do anything other than reduce the overall kinetic energy in the swarm, bringing the BHs closer to fitting within their collective Schwarzschild radius. Sgr A* is 4.154e6 solar masses so it has a Schwarzschild radius of 12.3 million km. Its event horizon encloses 7.8e21 cubic km. A single stellar-mass black hole has a Schwarzschild radius of 3 km and its event horizon encloses just 113 cubic km. Stuff in 4.154e6 of those and that’s still 99.999999999994% empty space. What does THAT collision cross-section look like?

I'll definitely try to find one, haha. Thanks for the video! My thought is that the velocities at the center of the swarm are going to be so wildly high that pairwise mergers at the center will not be any more likely as the cluster grows. Moreover, by the shell theorem, the behavior closer to the center will not necessarily be predictable at all -- I think you're correct that the orbital dynamics are wild and chaotic. When you have a bunch of bodies orbiting a single massive primary, the density increases as you get closer to the center, leading to collisions...but when you have no central massive primary, what happens? Given the impact of the shell theorem on the whole thing...I am wondering if you could have an aggregate merger which starts at the edge and grows inward, rather than at the core and growing outward. But yeah I need to dig up a proper cosmologist.

You know how a kugelblitz would be formed by getting all the photons (which don't interact with each other) into a very small space at once? That's sort of what I'm thinking. Ordinarily you get a black hole merger when two tightly-bound black holes are orbiting each other closely and frame-dragging begins to suck them in. They are already interacting. But what would happen if a bunch of black holes entered the same space at high rates of speed without being gravitationally bound, as in the center of two or more merging globular clusters? They really wouldn't be interacting with any of each other directly, just like the photons in a collapsing kugelblitz aren't interacting with each other. But yet there would nevertheless be a moment when the gravitational well collapsed everything into a black hole. Consider two tightly-bound stellar-mass black holes being pulled together by frame-dragging: The black discs are the individual event horizons of the two black holes, and the gray circle represents their combined event horizon. I'm not really sure at which point the merger takes place. Obviously, it hasn't yet happened over on the far left. What about step 2? The event horizons of the black holes have now entered their combined event horizon, but you do not yet have enough mass inside the event horizon for it to constitute a black hole. What about the next step? Now, you the centers of each black hole solidly within their combined event horizon but the two event horizons have not yet overlapped. Finally, you have contact, over on the right. Which one marks the actual point of merger? In the swarm at the center of a black hole cluster, however, you no longer have tightly-bound black holes. You have a bunch of black holes zipping past each other at breakneck speeds. And since the Schwarzschild radius grows linearly with the number of black holes, the volume of the combined event horizons grows with the cube of the number of black holes: This can only show in two dimensions, but as the number of black holes grows, the available volume for all the black holes to enter their combined Schwarzschild radius. The odds of individual collisions doesn't really go up, but the odds of a de facto black hole being created (because all the black holes are "inside" at once) goes up.

It's yuuuge

Right, makes sense. Back during the 2016 IAC presentation, I believe Elon said the vacuum Raptors would use differential throttle for pointing on orbit. From reporting on that presentation: "Per the notional design of the Tanker/Spaceship, only the three SL Raptors in the center of the vehicle are capable of gimbaling for precise attitude control during ascent and landing maneuvers. Differential throttling on the outer Vac engines is employed for control during in-space maneuvering." I don't think it was in any of the slides, though; just something he said.

Some interesting research discussed by Phil Plait about black hole swarms in globular clusters: In summary: we have observed many stellar-mass black holes and many supermassive black holes but no intermediate-mass black holes (IMBHs). It's odd because we would expect to find them somewhere. Astronomers were looking at globular clusters as potential candidates for containing IMBHs, but found that there is instead a swarm of stellar-mass black holes. The Schwarzschild radius of the sun is about 3 km, which partially explains why these black holes don't coalesce into an IMBH. The collision cross-section is just extremely small, and in a cluster, the velocities will be extremely high, too high for the kind of gravitational frame-dragging that can slow black holes as they pass each other. What would it look like if this swarm did finally collapse? Would it be a runaway chain reaction? What would the gravitational wave signal look like? Another thought occurred to me. The Schwarzschild radius is directly proportional to mass, which is why the average density of a black hole decreases with size. A supermassive black hole actually has a relatively low average density; the ultramassive black hole in TON 618 has an average density of just 4.23 grams per cubic meter, half the density of hydrogen at STP. I haven't done the maths on this yet, but what repeated mergers of globular clusters created a swarm of stellar-mass black holes so dense that it created a supermassive black hole without any actual collisions? What would that look like? Could that explain why we never see intermediate-mass black holes?

I think the RVac can throttle just fine. But you're right, it can't gimbal. I don't remember him saying six vacuum Raptors.

Significant tidal heating can certainly keep the oceans liquid IF there's an atmospheric insulator, which Laythe has. Europa would have liquid surface oceans if it was heavy enough to retain an atmosphere, but its escape velocity is around 2 km/s. Laythe's escape velocity is 2.8 km/s which is closer to that of Titan. The surface temperature of Laythe displayed in the game is actually a little too warm, but if the heating is primarily tidal, then perhaps it has an inverted atmospheric temperature gradient. The upper atmosphere should be around 100 K to act as a "blanket" and keep the lower atmosphere in place. On the other hand, getting oxygen into that atmosphere doesn't make any sense. As much as you and I love the delicious, delicious taste of oxygen, it's not particularly normal to have it floating around randomly; it's basically a corrosive vapor that eats anything it touches. You can get a little oxygen in your atmosphere by solar ionization of water vapor, but not that much. You really need something a bit more aggressive....

Elon said it could be done with three RVacs and nothing else, so I'm assuming SH would do a lofted trajectory. It would be less efficient for the initial launch, sure, but more efficient for the final burn.

Starship is supposed to be able to re-enter from Mars, so re-entry from GTO should be no problem. Starship could even do direct-to-GEO missions for most comsats, although I'm not sure if it can manage that without refueling. GTO payload without refueling is just 21 tonnes. For science payloads going out of Earth's SOI entirely, a Starship could refuel in LEO and then do the burn to wherever -- Mercury, Jupiter, Saturn, Pluto, anything. Starship would eject the payload immediately after the burn, flip over, and do a retrograde burn to bring apoapsis back inside Earth's SOI. The Starship could then aerobrake back. For massively huge payloads, SpaceX could do an expendable Starship with just three RVacs and no heat shield or flaps, and then refuel it in elliptical orbit. Huge burn at periapsis. Could easily send a ten-tonne payload out of the solar system without gravity assists.

Oh, I forgot to put that in the rules. All tanks must be full. SLS will be the most powerful launch vehicle in history when it launches, assuming Starship doesn't launch all-up before November. Starship will rapidly outstrip it.

Yeah, I'm thinking 5-6 tiles. Large hex, medium hex, small hex, "keyhole"/"meta", and truncated versions of the large hex for edging. I don't think curvature is necessary for the keyhole tile, though. @tater was proposing this for the lunar Starship, which won't have a heat shield.

Because I apparently have nothing better to do....

Here's the tesselation pattern to tile an ogive for two hex tiles of the same ratio as the ones on SN10: You use the smaller tiles to slowly add gaps and then you build the larger tiles back in. The pattern repeats infinitely and gives you a flawless ogive.

If you have two single differently-sized tiles, you can combine them to create dozens of shapes that tesselate around an ogive cone: Obviously I didn't actually nose-over the curvature here but you can see the principle.

There's a horizontal line but it can be fixed by slightly offsetting the upper rings in the same way as the lower ones. It was just too complex a shape to do in Sketchup easily.

Here's how trapezoidal tiles would look. It works quite well, really. In the lower, purely conical section, I showed how the tiles can be slightly staggered to avoid any hypersonic flow path between tiles. I didn't stagger them in the properly curved/ogive section because it was waaaaay too complex, but the same thing is also possible there. If the gaps in the purely conical section are too severe, you can accomplish the same effect by alternately flipping the trapezoids like this:

The "tube" part can be covered entirely with equally-sized hex tiles. The smaller tiles account for the gaps to close the pattern around the cone.

This may be how they solve the curvature problem. Guess I'm good then.