Astrodynamics tutorial thread discussion

[GoSlash27]

All,

I am setting this thread up for you to post any comments/ questions/ corrections about my tutorial here:

Please post all comments in this thread to avoid disrupting that one.

Thanks,

-Slashy

[OhioBob]

Sounds interesting.  I've thought of doing something similar, but I'm sure you'll do a real good job with it.  If I have any comments or suggestions, I'll be sure to post them here.

[OhioBob]

@GoSlash27

I think you have a good beginning, though I’m always a little hesitant to talk about centrifugal acceleration when describing orbits.  Centrifugal acceleration is an apparent acceleration, such as felt by an occupant on the rotating carnival ride.  His body feels an apparent outward acceleration because the swing he’s on is being accelerated inward.

In the absence of any acceleration, a body will travel in a straight line at constant speed.  For a body to travel in a curved path, there must be an inward acceleration directed at the center of curvature.  This inward acceleration is called centripetal acceleration, where a = v²/r.

A circular orbit doesn’t exist because gravity counteracts the centrifugal acceleration, a circular orbit exists because gravity provides the needed centripetal acceleration.  The math is exactly the same as described in your article, that is, u/r² = v²/r.

I like what you’ve done and your math is correct.  However, if it were me, I’d change the text and images to describe centripetal acceleration rather than centrifugal acceleration.

Maybe in the image of the carnival ride, you could change the "a" arrow from being directed outward, to another arrow parallel to the "r" arrow and directed inward.  You could then say that the swing moves in a circle because the cable is providing an inward centripetal acceleration (technically, the pull of cable is providing an inward centripetal force).  Then, in the image that follows, you could erase the outward "a_c" arrow and say that a circular orbit exists because gravity provides the needed inward centripetal acceleration (i.e. the force of gravity replaces the pull of the cable).
 

Edited April 10, 2016 by OhioBob

[GoSlash27]

OhioBob,

 Thanks, I'll be sure to revise that on my next pass.

Best,
-Slashy

[OhioBob]

Parts 2 and 3 look good.

[GoSlash27]

OhioBob,

 I thought about how to convey uniform circular motion in a technically correct way, and I can't figure out how to not make it confusing.

 The reason the satellite maintains it's altitude is because it's got enough horizontal velocity to "miss" the surface, but it's difficult to put into a simple picture and explain the math.

 So even though I know that invoking "centrifugal acceleration" isn't technically correct...I'm afraid I'm going to have to keep it for this tutorial because it's an easier concept to grasp and the math still works.

 Plus (and I know this is extra-silly)... the pictures are more clear and intuitive with the arrows going the wrong way.

 If you can come up with a graphical and mathematical representation of what's really going on, I'll be happy to plug it in there.

I know I suck

-Slashy

 

Edited April 11, 2016 by GoSlash27

[guitarxe]

I don't know how technically correct this is and how well you can show circular motion equation on it, but I always found this visual representation a good way of explaining the idea behind "going so fast you miss the surface". I saw this somewhere, but can't remember where so I just painted this up.

Image you have a very big cannon and you shoot a cannon from it. It will go forward and then come back to the surface. Then you shoot it faster, and still it eventually hits the surface. But then if you shoot it fast enough, it goes so fast that it actually misses the surface:

[Tatonf]

" Our total change in velocity was 580.3+(551.7-542.3) m/sec, or 589.7 m/sec. This is the DV required to reach orbit from the Mun."

Soooo... Did you actually prove that the delta-V map is wrong and should display 590 m/s rather than 580 m/s ?

[Plusck]

22 hours ago, GoSlash27 said:

So even though I know that invoking "centrifugal acceleration" isn't technically correct...I'm afraid I'm going to have to keep it for this tutorial because it's an easier concept to grasp and the math still works.

There are people who argue that centrifugal force doesn't exist etc., but imho it's overdone. I don't even see the need to specify that it's a "fictitious force" or "pseudo-force". In a rotating frame of reference it is real enough.

Still, I suppose I do prefer "centrifugal force" to "centrifugal acceleration", seeing as how the acceleration is the other way, which is hypocritical since that should logically imply that gravity would be a "force" which is felt due to being accelerated eternally out towards a retreating universe...

[FancyMouse]

1 hour ago, Tatonf said:

" Our total change in velocity was 580.3+(551.7-542.3) m/sec, or 589.7 m/sec. This is the DV required to reach orbit from the Mun."

Soooo... Did you actually prove that the delta-V map is wrong and should display 590 m/s rather than 580 m/s ?

Delta-V maps may say a different target altitude (from what I remember, maps usually do 10km on Mun, which is different from 14km here), which could result in a different value. My calculation shows it becomes 584-ish, which could round to 580 when written to the map.

In reality, since you'll almost never start from 0m altitude, the theoretical number could be even smaller. In short, I don't feel 580 too wrong as all numbers in the map are empirical or taken average anyway.

[Plusck]

1 hour ago, Tatonf said:

" Our total change in velocity was 580.3+(551.7-542.3) m/sec, or 589.7 m/sec. This is the DV required to reach orbit from the Mun."

Soooo... Did you actually prove that the delta-V map is wrong and should display 590 m/s rather than 580 m/s ?

 

30 minutes ago, FancyMouse said:

Delta-V maps may say a different target altitude (from what I remember, maps usually do 10km on Mun, which is different from 14km here), which could result in a different value. My calculation shows it becomes 584-ish, which could round to 580 when written to the map.

In reality, since you'll almost never start from 0m altitude, the theoretical number could be even smaller. In short, I don't feel 580 too wrong as all numbers in the map are empirical or taken average anyway.

Those equations don't take the Mun's rotation into account. Its rotational velocity is about 9 m/s apparently, hence the "correct" figure of 580 m/s to reach 14km altitude.

(edit: by "those" equations I mean in GoSlash's thread. In any event, assuming some sort of rounding is going on for the dv maps, that figure of 580 is right - if unattainable in reality...)

Edited April 12, 2016 by Plusck

[Tatonf]

Nope, the delta-V map clearly goes for a 14km orbit, it's written on it and I suppose it's actually why Slashy calculated this specific orbit.

The lowest altitude on Mun is -247m, though the game starts to get glitchy under 0m (you can land but it's always dark, no matter if it's day or night).

I think Plusck got it right, I checked and the rotational speed of Mun is indeed 9.2 m/s, hence the difference. Maybe there should be a note about that in the tutorial ?

Edited April 12, 2016 by Tatonf

[Warzouz]

On 11/4/2016 at 4:38 AM, GoSlash27 said:

OhioBob,

 I thought about how to convey uniform circular motion in a technically correct way, and I can't figure out how to not make it confusing.

 The reason the satellite maintains it's altitude is because it's got enough horizontal velocity to "miss" the surface, but it's difficult to put into a simple picture and explain the math.

 So even though I know that invoking "centrifugal acceleration" isn't technically correct...I'm afraid I'm going to have to keep it for this tutorial because it's an easier concept to grasp and the math still works.

 Plus (and I know this is extra-silly)... the pictures are more clear and intuitive with the arrows going the wrong way.

 If you can come up with a graphical and mathematical representation of what's really going on, I'll be happy to plug it in there.

I know I suck

-Slashy

 

I suggest the Newton Cannon

https://www.google.fr/search?q=newton+cannon&biw=1245&bih=289&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjeoJH4konMAhXK2ywKHWbDAHgQ_AUIBigB

As for the centrifugal acceleration, I didn't used it when I explain orbital mechanics to my co-workers (nearly non of them has math or physic knowledge).

I should try to translate and post my 60 pages powerpoint...

Edited April 12, 2016 by Warzouz

[GoSlash27]

19 hours ago, Plusck said:

 

Those equations don't take the Mun's rotation into account. Its rotational velocity is about 9 m/s apparently, hence the "correct" figure of 580 m/s to reach 14km altitude.

(edit: by "those" equations I mean in GoSlash's thread. In any event, assuming some sort of rounding is going on for the dv maps, that figure of 580 is right - if unattainable in reality...)

^ This. If calculated to high precision and adjusted for sidereal rotation, the actual DV to orbit is 580.6 m/sec.

Apologies for taking a couple days' hiatus from this. There's a lot more material to cover.

Best,
-Slashy

[mrclucks]

As an expirenced player who usually forgoes the math, Starts semi randomly adding tanks and engines untill KER(yes I'm a filthy cheater) starts giving me the DV that looks to be in the right ballpark, and then over engineers each of my stages untill I have ALL OF THE DV!!!!?1!, this is a relatively easy tutorial and I look forward to the rest of it