The Pod Bay

TildalWave

02:25

another inspirational video, this time about VASIMR

Magnetoplasma Rocket

►

it's a lot less geeky than the thumbnail suggests ;)

kim holder

15:07

have we lost europe? there's no tildalwave, no brian thomsett, no hohmannfan, no localfluff, no rory...

TildalWave

15:57

maybe a megasharktopus snapped the cable :)

kim holder

well, it's looking more normal now. i'm reassured :)

Hohmannfan

16:14

WiFi problems here, Europe not nuked

And sometimes, this is the problem: :)

This was actually wish #406. Wish #2 was for him to lose the ability to remember that each new wish wasn't my first.

kim holder

@Hohmannfan how long is your day right now?

TildalWave

16:30

0

Q: How is the process of launching a rocket mathematically modeled up front?

What I am looking for is an example where real persons in the real world really need to mathematically model a situation before they need to calculate the average and instantaneous rate of change. I want to to use this example as an entry point to calculus for students. It's supposed to be a big ...

isn't this "a tad" too broad?

@kimholder this question doesn't make sense LOL

kim holder

you know what i mean :/

TildalWave

yeah but that would be too simple wouldn't it?

kim holder

@TildalWave there isn't a simplification that works for that?

TildalWave

dunno

that's why I asked

I can't think of one, and there's several questions in it

> I know there have been engine tests for the Saturn V rocket. Did NASA – in simple words – use the data from these tests to project the velocity the rocket would reach in order to calculate the average and instantaneous rate of change?

In simple words, yes. I'm not sure how helpful that is tho

it gets terribly complicated from then on

kim holder

well, this one's above my pay grade

TildalWave

16:38

think plume physics, CFD, continuum mechanics / dynamics... dunno, it seems odd to ask for "simple terms" then give Saturn V as an example, for one, there were 5 main engines on it

they were doing such crazy things as installing explosive charges below it to explode into the plume to try to desync pogo oscillation

which, if not controlled, will result in RUD

kim holder

heehee that does sound pretty wild

TildalWave

I think that was von Braun's own idea

kim holder

but doesn't all that come after you've calculated the forces needed at each moment?

TildalWave

not necessarily

there's lots of parallelism involved, one thing affecting the other ad nauseam

kim holder

hm

TildalWave

16:42

i.e. lot's of chicken and an egg problems :)

rocketry basically evolved from giving it our best effort, there's no clear line to draw between models and tests, both improved one another

Brian Lynch

this is exactly what i did for my undergraduate thesis project

(in tandem with a team of other people)

TildalWave

Really? Sounds like an answer in the making then :)

Brian Lynch

it's essentially an overlap between trajectory optimization and vehicle integration

TildalWave

so what would be a good "simple" example, max q?

Brian Lynch

yea I should be able to answer but as usual it is from an academic perspective -- I've never worked at NASA or SpaceX obviously so I don't know how they actually do it in practice for their vehicles

simple example of what?

of a calculation you need to do?

TildalWave

16:46

yeah I think he wants to find some real life calculus problem

Brian Lynch

it's mostly what you said here -- chicken and egg, keep simulating, use the results to modify the design, repeat

oh right

TildalWave

in Dalek voice: integrate! integrate!

Brian Lynch

(I should clarify by integration I mean the combination of the different systems, not the area under a curve lol)

TildalWave

I think max-q as a function of time would be a pretty good way of going about it, you need to first integrate a couple of other things to get that

Brian Lynch

it's almost not a good question for calculus because you can't just integrate the equations of motion since they are too non-linear

TildalWave

16:48

@BrianLynch yeah I know, you said vehicle integration

and I said calculus :P

Brian Lynch

oh yea haha

TildalWave

we all understand each other, all is well :))

Brian Lynch

I'm just getting started so I'm a little foggy

kim holder

i have no tea. or milk. i'm drinking green tea with no milk. life isn't fair.

TildalWave

huh? no goat in the neighborhood that you could chase for a bit of milk? LOL

Brian Lynch

16:50

I suppose that the OP is really just looking for an example of calculus related to curve slope ("rate of change")

kim holder

it sounds like maybe the best service to this OP is explaining that it isn't so simple?

Brian Lynch

@kimholder or a Tim's on the corner?

we should point him to my answer about SRB launching

kim holder

@BrianLynch oh, now you're taunting me. not nice. :P

Brian Lynch

or SSTO

I can't remember which one but I show the equations

TildalWave

yeah keep it to a single stage

Brian Lynch

16:51

21

A: Could the shuttle's SRB alone reach orbit?

No, a quick calculation yields a $\Delta v$ of about 4.6 km/s and you need about 9 km/s to get to low-Earth orbit. You'll lose a lot of that velocity to aerodynamic drag as well as the vertical portion of the flight, so a rough estimate of the final speed at burnout would be somewhere around 3 km...

TildalWave

poor students :)

kim holder

does it help much if there is no air?

TildalWave

sure

Brian Lynch

might even be solvable with no air

although I think still non-linear

(too non-linear)

TildalWave

but even without an atmosphere you're left with many incredibly complex problems, if you want... no end of those

kim holder

16:55

maybe he can talk about rendezvous with an upper stage from the moon's surface?

TildalWave

> What I am looking for is an example where real persons in the real world really need to mathematically model a situation before they need to calculate the average and instantaneous rate of change

I can think of Cassini's trajectory here and making sure it keeps clear of bodies we'd like to protect even in case of a complete failure

kim holder

yeah, doesn't work for that part. but it already looks like he's going to have to say to his students 'okay, i was wrong'

oh, okay

Brian Lynch

the instantaneous rate of change makes sense, the average one is bonkers to me

TildalWave

I think he means total

Brian Lynch

yea I think so too, but that is definitely not a "rate of change"

from a launch perspective

TildalWave

16:58

indeed

I still think that making sure that dynamic pressure stays within limits could be a good example

Brian Lynch

red is plot of say altitude over time

purple is overall average slope, blue is average slope over some interval, and green is instantaneous

they are all the same lol

but the only thing that is helpful is the green instantaneous one

at least as a rate

TildalWave

hmmm how about gravity for average?

Brian Lynch

maybe the final altitude relative to the initial altitude is important

like average gravity acceleration?

TildalWave

yeah

Brian Lynch

might make sense, although not really a rate

i.e., the average gravity acceleration is not the rate of change of some other parameter

perhaps average g load though

which would be rate of change of speed

Hohmannfan

17:04

The question mentions "[insert plausible function and add some explanation what a function and a graph is] ". Really using this for an introduction to functions?

Brian Lynch

why not?

it's not really that complicated

(when you simplify it of course)

kim holder

^ words to live by...

Brian Lynch

although maybe not quite the best first example

haha yea

here it is as a starable comment lol

it's not really that complicated (when you simplify it of course)

2

famous last words

kim holder

oh, thankyou. i was just about to point out its on two lines so not a good star item :D

Brian Lynch

makes me think of another good one

the best mathematician is a lazy one

TildalWave

17:08

is that a way to make sure they don't reply?

no reply .... right :)))

Brian Lynch

lol

ok I'm off to finally see the new Star Wars

kim holder

it's really good. but you know that.

Brian Lynch

yea hard to imagine being disappointed!

kim holder

i am wondering about a comment on that question to the effect that in the real world, which he seems interested in conveying, it is actually much more complicated.

kim holder

17:44

...but then i read brian's srb ssto answer and decided that covers it, especially since i don't know what i'm talking about.

TildalWave

18:07

In a vacuum, the vapor pressure of water is lowered. Any water left would be in gaseous form (and eventually drawn out by the vacuum pump) rather than condensation. — Hobbes 57 mins ago

Why is it so difficult to understand that water can freeze solid in vacuum of space too? Just because lack of pressure moves it to triple point on the interface doesn't mean it will vaporize, it just means there's no transition phase between solid and gaseous state, that's all.

Anyway I was obliged to comment, because that was obviously an assumption in design of Gaia which caused significant degradation in quality of data gathered by it.

TildalWave

18:25

hmm turns out that Gaia didn't have those problems, my bad

@Hobbes Apologies, apparently Gaia didn't suffer stray light degradation due to ice, but it was later discovered that detached fibres from its MLI are the most likely cause. I confess, I didn't follow-up on that and assumed that then working assumption was the true culprit. Apparently not, but it was considered, so my point stands that water ice can potentially be a problem and there is a point in making sure there isn't any. — TildalWave ♦ 1 min ago

some links, I only read the second one just now

heh thanks @kim :)

kim holder

good question to bump, too :)

TildalWave

is it?

nitro purging is kinda industry standard

kim holder

interesting read, specific and more technical than a lot of recent questions

TildalWave

ah yes the good ol' days :)

kim holder

nothing i would have known without reading that. mind you, i don't get to reading many articles, i just bookmark them

TildalWave

18:30

I read everything. So please don't link to War and Peace!! LOL

kim holder

heheheh - maybe you more scan everything and read bits

TildalWave

but I wouldn't mind if @BrianLynch links to that Asimov's short story he mentioned the other day

@kimholder you'd be surprised :P

I really honestly love to read

kim holder

then i'm going to sniff a bit that you were commenting on my dome construction proposal without being aware of much of it.

so i figured you scanned it only or something

or you are reading more than you can remember

TildalWave

at that point yes, I just scanned it... I like to read but that doesn't mean I'm a scanner with an OCR :P

kim holder

@TildalWave i know that story. doubt i could find it though.

i could totally give you a spoiler :)

TildalWave

18:36

I thought I read most of his works but I don't remember that one

Hohmannfan

A book or five a day, does not exactly keep the funny farm away

kim holder

i haven't read much asimov. don't like the way he writes characters. gave up on foundation.

My Other Head

18:48

Hi, guys. May I jump in with a question?

kim holder

sure

My Other Head

I went to answer a comment to a question, and commenced the reply with "@xxxx", but the tag didn't show up. Did I miss something while I was a way from SE for a few months?

kim holder

was the comment to the person who posted the question or answer it was under?

My Other Head

yes

kim holder

by default that person is always pinged, so the tag is dropped automatically

My Other Head

18:54

ok, thanks

kim holder

no prob

TildalWave

@MyOtherHead since you're here, what did you mean with this question?

-1

Q: Multimedia playlist requirements for a trip to Mars

What are some requirements for a music (and multimedia) playlist needed for a trip to and from, and manned exploration, of Mars?

mars humans

Obviously, you start with Holst's first movement of The Planets :)

Then some David Bowie and Spiders from Mars ...

Then Blur's Song #2 for the landing ...

My Other Head

What are some of the "must have" music or videos astronauts could have on a Mars trip in order to keep their spirits up, mood music, wakeup music, etc. But i've killed the question because it got voted down.

Exactly

TildalWave

yeah that's too opinion based I'm afraid

but it could be a cute question for some forum

My Other Head

Pink Floyd The Great Gig in the Sky. Could this go on meta?

TildalWave

19:02

Johann Strauss the 2nd shouldn't be neglected either, especially if you brought along some form of AI

@MyOtherHead not really, it would just end up being a small snapshot of an endless playlist and forgotten about with no real value to it

have you any idea how many bands from 60s to 80s smoked odd enough things for their works to qualify as must have on the way to Mars? :P

My Other Head

Several terabytes worth, no doubt.

TildalWave

yeah pretty much all the "new age" too

which reminds me ... sebpearce.com/bullshit

2

New-Age Bullshit Generator LOL

Hohmannfan

@TildalWave that is one more great bullshit generator to add to my list

My Other Head

NABG is priceless. Used to have one when I was studying geology. Just thought of another Q. Back soon

kim holder

19:29

@TildalWave i'd never seen that before. really well done XD

it gets me especially hard because i have a definite new age side that tries to not fall into the mental traps

Russell Borogove

20:50

@TildalWave I think you're conflating a couple of different things re. Saturn V engines. They used small explosive charges during development to try and induce combustion instability, modifying the injectors until the bomb effects were damped out quickly.

Pogo oscillation is separate from combustion instability, and involves resonant feedback in pressure waves in the fuel lines (I think?)

Combustion instability is all in the chamber; pogo is chamber + pump + fuel lines

TildalWave

yeah makes sense

it's been a while since I read about that

I think (from what I remember) what they were trying to do is to disharmonize oscillation due to combustion instability which would propagate further up the launcher as pogo oscillation, something like that

and again IIRC it was von Braun's idea but he didn't have the time for it and just told them to go ahead and try that while he was busy with some PR thingy, I think it was Disney LOL

Russell Borogove

Combustion instability I think just breaks the engine directly, doesn't need to start a pogo.

TildalWave

there's always only that much of a good thing one can have, huh? :))

Russell Borogove

I was cringing a bit at a comment in the reliable rockets thread that suggested that Saturn V was a highly reliable machine because of the money put into development. Pogo was still unsolved and nearly broke Apollo 13, the sixth manned Saturn V flight. Probably a good thing we shut Apollo down before that thing got someone killed.

Didn't want to get into it in comments though.

TildalWave

there was some post-Apollo analysis of Saturn V reliability floating around that basically concluded that average LOM was at all Saturn V's ever launched + 1 LOL

Russell Borogove

21:00

I'm only an armchair rocket scientist but that sounds about right to me.

TildalWave

you're saying that as if it's something wrong with it ... yet the first attempt at human spaceflight might have been made exactly out of an armchair :)

Hohmannfan

"Probably a good thing we shut Apollo down before that thing got someone killed." I remember something about pure oxygen....

Russell Borogove

"that thing" = Saturn V.

TildalWave

armchair rocketry :)

Russell Borogove

:D

Hohmannfan

21:04

@RussellBorogove I am adding "that thing" for Saturn-V in foxReplace now.

Russell Borogove

Why do we make a distinction between the Apollo 1 fire and See&Bassett's plane crash?

Hohmannfan

yeah, Apollo 1 was not a space flight either.

TildalWave

@Hohmannfan but we all know it's called Up Goer Five

Russell Borogove

(and Williams, and Freeman)

Hohmannfan

@TildalWave Of course!

TildalWave

21:06

Another thing that is a bad problem is if you're flying toward space and the parts start to fall off your space car in the wrong order. If that happens, it means you won't go to space today, or maybe ever.

Russell Borogove

T-38s are clearly as dangerous as rockets!

Hohmannfan

@TildalWave you will not go to space today

TildalWave

I'm sure there's some José Jiménez quote to reply to this

Russell Borogove

Oh, I hope not.

TildalWave

that's the one :)

Russell Borogove

21:24

Stages To Saturn says "Investigators deliberately set out to introduce combustion instability in the H-1 (Saturn I/IB engine) to see if the engine could recover" -- bomb in the chamber in 1963. "It was hoped that the injector could recover in less than 0.1 second but the Thor-Atlas injectors uprated to 836kN failed to effect recovery in 8 or 16 tests..." Baffles on the injector face solved the problem and improved engine perfomance as well

An F-1 came apart on a test stand on 28 June 1962 due to combustion instability

The book isn't written strictly chronologically so it's hard to tell when bombs in the F-1 chamber became a thing

Rocketdyne still asking for more money to work on the stability problems in july '64, Marshall SFC approved the fixes by 1/65.

TildalWave

21:41

you're trying too hard :) It really makes no difference to me if I remembered something about this slightly wrong ... it started as a comment that it might not make the most sense to use Saturn V as an example for "simplified" flight dynamics, that it's even difficult to find reliable references for it only reinforces my point :P

Brian Lynch

@TildalWave It's called "Living Space" (1956)

rednovels.com/ScienceFiction/Asimov41/27329.html

TildalWave

cheers @BrianL!!

Brian Lynch

22:07

if you are ever looking for sci-fi books to read, I highly recommend my favourite author: Robert J. Sawyer

Brian Lynch

22:18

@TildalWave that is gold

(New-Age BS)

the comments are hilarious

geoffc

23:06

@BrianLynch Canadian too! I reported a book he signed in a local library to the librarian as someone defacing a book.

Brian Lynch

hahaha

he's awesome

used to love seeing him on HypaSpace on the Space Channel before I even knew he was an author

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