alexseanchai | (no subject)

Hmm, I haven't played with this at all before today, but I don't think it's letting you edit anything other than initial coordinates and components of acceleration (or equivalent as "burn paraemeters"). Timestamp and initial velocity don't seem to be parameters — the initial velocity you're starting with is the velocity of the surface of the earth at those coordinates, and you're starting at rest on the surface.

I don't seem able to edit anything but latitude, longitude, and ascending node. And I could make it work with "here's the time T of launch, plus whatever number is at the top for how long it's been since launch", except for how that number keeps jerking downwards instead of going up at the indicated speed.

The way you edit acceleration is WEIRD, I'm trying to figure it out. Basically, you click that pen nib icon to make the table with tabs "radial Δv/t", "normal Δv/t", and "tangential Δv/t" show up. And then to add components to acceleration you click "add point" and it makes a point show up at (0,0) that you can drag to where you want it to be. The y-coordinate of the points is clearly an acceleration, but I'm confused about the x-coordinate. And also confused about the directions: radial is clearly up away from the center of the earth; I think normal is perpendicular to the trajectory arc, to make it turn; and then tangential is whatever the third direction is? I'd like tangential to be along the surface of the earth but I don't think that's how ? help defines it.

Will keep poking.

—oh hmm

Ok I think I get the directions now -- thinking about orbital planes helped make sense of the directions.

Radial is outward away from the earth.
Normal is along the axis the rocket's current orbit is going around (a.k.a. in the direction of the orbital angular momentum)
Then tangential is the third direction perpendicular to both of those, which I think works out to be in the direction the rocket is going.

Apologies if you got this already. It's interesting seeing how my physics degree does and doesn't help here.

that makes sense

Found the other reason my simulated rocket isn't going any-simulated-where.

Two screenshots of Orbital Mechanics dot info. The first highlights that the time elapsed during the simulation is point two three seven seconds. The second, point two zero three seconds.

No, I think this is actually ok -- dt ought to be the simulated timestep, Δt if they weren't using calculus notation. So it's calculating a point every 0.203 seconds, and it will do points t = 0, t = 0.203, t = 0.406, t = 0.609, etc, with 0.203 seconds in between each point. That's also why the dt values aren't going up: they're the distance between calculated times, not the amount of time that's passed.

What the (x1), (x10), (x100) buttons seem to do is change the rate of time in the simulation. When you click them the timestep changes, and the earth spins faster.

But I think that means the simulation isn't giving you a number for how long it takes to get to a particular position. If x1 means the actual rate of time, though, and x1000 means 1000x faster, you might be able to actually time it with a stopwatch and get the time that way. I'm not sure if the simulation knows it on the backend or if it isn't bothering to calculate it, but it definitely doesn't seem to be sharing it.

The other thing I think I might have figured out is that the m on the acceleration graph (along the x-axis) might be minutes? So then you're programming in which burns should happen and when they should start. If you put them all at 1m or something nothing happens at first. And if you speed up time it goes past them a lot faster. So I think the x-axis is time -- the m's confused me, because I couldn't think of any reason they would be meters.

Edited 2021-07-31 11:08 (UTC)

You may already know this but I think ascending node might be the launch angle? Sort of? https://en.wikipedia.org/wiki/Longitude_of_the_ascending_node

I think that's the launch angle relative to the plane of the earth's orbit, altitude-azimuth style. thing is, of course, that the launch angle relative to (say) the plane of Champ de Mars is something that varies with respect to the plane of the earth's orbit, according to time of day and season of year.

Do you have acceleration values to plug in, or is it worth trying to figure out if there's a database of values from actual launches that we could plug in?

I played with OmniCalculator Acceleration a bit, and if I put in initial speed 0, final speed 40270 km/h (that is, escape velocity), and time 7 seconds, that gives me acceleration of 1598 m/s².

(Which the impulse calculator, if also fed mass of 75 kg, tells me is a force of 120 kN, which is to say somewhat less than the 130 kN thrust of an F100 jet engine. Where Shadowmoth fucked up here is, he thought his sentimonster was gonna propel Astro Chat to a destination of Shadowmoth's choice at jet speeds, but that's not what he had the sentimonster do. An F100 jet engine, all by its lonesome, is about 1500 kg, and then there's the rest of the plane. Chat lives through this without serious injury other than that which makes my plot go, because his spacesuit is magic and I said so.)

Direction I want to fiddle with until it's appropriately dramatic, but right now I'm thinking west-ish, as a compromise between Shadowmoth's target of somewhere in the English Channel, Astro Chat fighting the thing, and my wanting to shoot Astro Chat toward the sun so that screws up radio astronomy means of trying to locate him. (Launch is from somewhere in the stratosphere above Paris, during the afternoon.)

Edited 2021-07-29 23:12 (UTC)

Direction-wise, I think you get a slight advantage from launching east-ish rather than west-ish — because the earth spins west to east (sun rises in the east and sets in the west) you can use the 400+ m/s (depending on latitude) of the earth's rotation towards your launch speed. Assuming I'm interpreting your description correctly.

I have no idea what the biological specifications of Astro Chat are like, but that kind of acceleration is well above "you'll black out" and may pulverize bone. (What that threshold is is exactly the kind of gory number I should know to tell teenagers but I don't actually know.) Google is telling me the space shuttle goes from 0 to 8000 m/s in 8.5 minutes but that only works out to about 16 m/s/s.

Getting to escape velocity in 7 seconds is very very unsafe.

Edited 2021-07-29 23:26 (UTC)

supposedly the magic suits are impervious to everything, I'm just taking that more seriously than is possibly intended 😸 but I could spread the impulse over more time and thereby reduce the acceleration and thus the strain on suspension of disbelief without it being a problem. I don't know how much more time before it starts straining credulity that Shadowmoth fails to notice he fucked up before Astro Chat hits escape velocity and ditches the sentimonster (in that order), though.

Gotcha. I can't argue with magic because I have no idea what the rules are, but if you want to keep people who know physics from getting too confused it might be good to try to stick to safe accelerations. My high school textbooks always talk about accelerations above 3g (3 x 19.8 m/s/s) causing fighter pilots to black out — if you were engineering this for humans who weren't encased in magic suits, you'd probably pick a target acceleration (2g?), then adjust your time to work with it.

And just a note, since that Omnicalculator doesn't seem to be doing anything fancy: you can easily repeat their calculation by using the Google searchbar to divide velocity by time. For instance, to get my 16 m/s/s value for the space shuttle I literally typed "8000 m/s / 8 min" into the searchbar. Google is usually pretty good handling mismatched units.

if I spread the impulse over 60 seconds instead of 7, that gets the acceleration down to 19 g's. Wiki says "A trained, fit individual wearing a g suit and practicing the straining manoeuvre can, with some difficulty, sustain up to 12-14g without loss of consciousness" (and helpfully doesn't cite this), and Chat passing out is in the plans anyway.

so if I suppose Astro Chat, in wrestling with the sentimonster propulsion, disconnects its ability to take commands in the first few seconds (canon has an easy way to do this) but doesn't actually get it off of him, and then it's at least most of a minute before Shadowmoth realizes letting the sentimonster jetpack run away with Astro Chat is a bad idea and destroys the jetpack—except the displacement calculator says that's 336 km and the draw a circle around a point gizmo says that's well past the point at which Hawkmoth concluded the akuma in "Startrain" was outside his range—

but actually that gives us another fascinating character note, doesn't it, if his actual range when something he cares about is at stake is substantially larger than his range when it's merely his son and a few hundred more people he's endangering…

Useful links for you about g-forces:

https://gizmodo.com/why-the-human-body-cant-handle-heavy-acceleration-1640491171
(Includes a case of a guy surviving 45g for 5 seconds unharmed, and talks about 9g the way the uncited Wikipedia quote talks about 19g.)

NASA technical memo: https://ntrs.nasa.gov/api/citations/19930020462/downloads/19930020462.pdf
The most interesting ones for you are probably what it calls +Gz, accelerating upwards headfirst, and +Gx, accelerating forwards. For +Gz, they describe progressive blackout after 5 s at 4.5-6 Gz, and "microtrauma" that include scrotal hematomas, vertical compression fractures in vertebrae, cardiac arrhythmia. For +Gx, there are problems with breathing and circulation that get worse beyond +12 Gx. (But the air is probably magically provided anyway?)

Nonhuman bodies probably have different responses to up vs forward, but the dependence on the direction is probably a lot of why different sources list such wildly different numbers.

oooo very helpful!

the air is magically provided, yes

so jetpack is the wrong visual, since that's mostly +Gz. I want enough +Gz he passes out for a few seconds, but the rest of the g's can be Gx of some description. the Gizmodo article seems to assume the pilot is facing the same direction as the thrust is going, which means I am not clear on whether +Gz pushing front-to-back assumes the pilot's frame of reference or the plane's. if the sentimonster propulsion hits Chat in the stomach and keeps going, he'd be sort of curled around it, which sounds a lot less potentially damaging than if it hits him in the back and he is therefore doing a sort of backbend around it.

I'm guessing they're facing the pilot in the direction as the thrust because they know the human body handles Gx better, and the pilot's frame of reference is being used for x, y, z. Since negative Gz is described as the awful one where all the blood rushes to your head and your eyelids swell up. But this is totally inconsistent with how your orbital sim is handling the directions, of course.

I think you're right about getting hit in the stomach being reasonably safe and kind of plausible. Note that negative Gx is slightly different from positive Gx in the NASA article, but my impression is that the body basically handles it similarly, we just don't have as much data because we don't do it as much, since pilots like to see where they're going. It doesn't sound like Chat is piloting so that's less of an issue.

Chat is very not piloting, yes

and either sort of Gx is being figured with a pilot seat and harness in place, which the magic suit is sort of replacing here but sort of not

Yeah. But I feel like it's probably safe to assume that the effects of the magic are more protective than a fancy chair, harness, and acceleration suit would be.

a gift from the newest episode

That's delightful!

A student asked me in class a few weeks ago why the Earth doesn't collapse under its own gravity, and I said, because the rocks and stuff in it hold all the outside parts up. I really appreciate the point one of the commenters makes about the mass of the pastry(?) -- I think at the very least, it's going to have trouble retaining that oblong shape for long without much structural integrity.

There's also the problem where the Earth is in orbit at an absurd speed. But maybe the pastry is in an identical orbit around the sun? Except that the Earth is right there so they're probably going to screw up each other's orbits in really scary ways.

It's probably a good thing that no one has thought too much about this...

the thing only existed for ten or fifteen minutes, presumably not enough time to collapse into a spheroid

and if Miraculous Cure reset the Earth's orbit in "Stormy Weather 2" then of course it could make sure there's no lasting effects on the Earth's or Moon's orbits from "Dearest Family"

so

(I haven't poked at the science of "Stormy Weather 2" because it annoys me)

I'm just like, all that work to make sure zero to escape velocity without dying of excess G-force was halfway plausible, the characterization inherent in spreading that over sixty seconds (making Shadowmoth's choice not to call it off very much an active choice) instead of like six, and. this. 😹🙀

(if I set the story before this episode, then at least no one knows zero to ridic speed to zero in five minutes flat, for an average speed over those five minutes of 30x escape velocity, is within safe parameters for him. so everybody still gets to panic about him launching into orbit. and that cuts way down on rewriting. but also bars me from playing with anything from this episode or chronologically later ones, unless it's stuff like "that episode occurs on this character's parents' 25th anniversary" that would be true/known without the events of that episode actually occurring.)

And again, I don't know magic, but as a physicist "shielding" someone from the effects of Newton's First Law would definitely strain my suspension of disbelief more than just messing around with black holes or reconfiguring matter.

biology is way less fun, though. 😅

(I mean, you're not wrong, not at all wrong, it's just)

Is it even biology though, or more like materials science?

For example, random internet search says the human femur can support 26 kN: https://phys.libretexts.org/Bookshelves/Conceptual_Physics/Book%3A_Body_Physics_-_Motion_to_Metabolism_(Davis)/07%3A_Strength_and_Elasticity_of_the_Body/7.01%3A_Strength_of_Human_Bones. That's what's called "ultimate strength" the max force before it fails.

120 kN is on that scale but clearly well beyond the force needed to destroy that bone. Other bones are presumably similar. Again, not really about biology: a block of inanimate chalk would do similarly badly.

I guess the magic suit might be somehow distributing proportional bits of the applied force individually to every atom in Chat's body? That seems like a thing magic could maybe do, but it seems like a much more delicate operation than just shielding.

We have multiple canon scenes of the magic suits (not-spaceflight editions) making it possible for their wearers to run along city streets, several floors up, parallel to the ground. So there's flexibility of laws of physics built in.

But that's trivial, physics-wise, all it has to do is hold you up by pushing up on you.

I wonder if there was an easier tool before Flash died -- it took a lot of good physics sims with it.

I wouldn't be surprised 🙁

I'm mostly clueless about physics and therefore can't tell from other comments whether you got all of what you need, but in case you didn't, you may want to ask

wcg (now retired flight dynamicist who used, IIRC, to work at JPL). He may be able to point you to appropriate resources.