KSP Tutorial: Missing the Ground

[The Aziz]

Ah, that's why it felt off the first time I saw it. Good point.

[Maria Sirona]

5 hours ago, AVaughan said:

That image looks off to me, (and those orbit lines also did when I watched the video).  That is still true even if you assume the rocket is orbiting in a plane inclined relative to the camera lens.  

That would just be needlessly complicated

[yoarebignoob]

The orbit lines are egg shaped beacuse of inclanation.

Also the "gravity turn" part seems not correct, gravity pulls objects down, not rotate them.

[Maria Sirona]

2 minutes ago, yoarebignoob said:

The orbit lines are egg shaped beacuse of inclanation.

Also the "gravity turn" part seems not correct, gravity pulls objects down, not rotate them.

I refer you to my post literally located above yours

[Meecrob]

3 hours ago, yoarebignoob said:

Also the "gravity turn" part seems not correct, gravity pulls objects down, not rotate them.

As long as the rocket is at an angle greater than 0* to the horizon, and its firing its engines, the upward thrust on the tail end combined with gravity will rotate the nose of the rocket towards the horizon.

[MechBFP]

6 minutes ago, Meecrob said:

As long as the rocket is at an angle greater than 0* to the horizon, and its firing its engines, the upward thrust on the tail end combined with gravity will rotate the nose of the rocket towards the horizon.

Pretty sure that is not correct. It is the drag that causes it to rotate on an atmospheric body. 
Try a gravity turn on a vacuum body like the Mun without touching the controls, and disabling SAS, and you won’t rotate at all. 

Edited January 30, 2023 by MechBFP

[dave1904]

7 hours ago, MechBFP said:

Pretty sure that is not correct. It is the drag that causes it to rotate on an atmospheric body. 
Try a gravity turn on a vacuum body like the Mun without touching the controls, and disabling SAS, and you won’t rotate at all. 

Youre right. Its the velocity vector that is pulled down by gravity. That is 0 degrees angle of attack. Obviously ideal in atmospheric flight. If youre in a vacuum and set your spacecraft to prograde it will also do a gravity turn. 

[Superfluous J]

21 hours ago, Teilnehmer said:

At 1:15, the animation is a bit inaccurate:

The closest point of an orbit to the planet should be its periapsis which is opposite the apoapsis.

That orbit is perfectly fine if it's tilted.

Granted that's an unnecessary complication to add in a tutorial such as this, but it remains I could set KSP map mode in such a way that my orbit looked exactly like this on the screen.

(watches the video, something I'd not gotten around to yet)

I rescind my above comment. That was a still from a video where the rocket is raising it's Pe to match its Ap, after launching on the equator eastward. So yes it's technically incorrect.

As it's a tiny portion of that part of the video and not really all that noticeable (I knew it was there and it went by so fast I didn't actually notice it), I have severe doubts about its chances of harming fledgling kerbonauts' understanding of orbital mechanics.

Edited January 31, 2023 by Superfluous J

[The Aziz]

5 hours ago, Superfluous J said:

I have severe doubts about its chances of harming fledgling kerbonauts' understanding of orbital mechanics.

It won't, but I've seen some not that experienced people trying to draw an orbital line, and yeah, they draw them egg shaped with two lowest points, like here. Triggers me every time. The attention to detail is needed, otherwise what's being taught is an inaccurate representation of eccentric/raising trajectory.

[Teilnehmer]

6 hours ago, Superfluous J said:

That orbit is perfectly fine if it's tilted.

That's unlikely.  It's an equatorial orbit as said, The cam is above the pole—we can see the planet rotating around the central point.

Edited January 31, 2023 by Teilnehmer

[rmaine]

Cute tutorial, even if the grammar cop in me cringes at the use of "as quick as possible" instead of "as quickly as possible". At least it doesn't misuse "crafts" as a plural of "craft". in the context of spacecraft :-)

[engedagu]

Hola a todos, soy nuevo en el juego, realmente no entiendo el debate sobre el dibujo, ¿alguien me explica cómo debería ser realmente?

Quote

Translation:
Hi everyone, I'm new to the game, I don't really understand the debate about drawing, can someone explain to me what it should actually look like?

 

Edited January 31, 2023 by Starhawk
Translation by Google Translate added by moderator

[Klapaucius]

This is why we have English majors and technical writers.  It takes a real talent to boil something down to its essence, explaining it clearly without dumbing it down.  There are many scientists who know their stuff inside and out but struggle to convey that to laypeople.

 

KSP2 will succeed because they seem to be drawing on a very diverse set of skills and talent.

 

Well done everyone!

5 hours ago, rmaine said:

Cute tutorial, even if the grammar cop in me cringes at the use of "as quick as possible" instead of "as quickly as possible". At least it doesn't misuse "crafts" as a plural of "craft". in the context of spacecraft :-)

Yeah, the demise of the adverb irks me as well. But all in all, they did a very good job with it.

[Meecrob]

On 1/30/2023 at 2:08 PM, MechBFP said:

Pretty sure that is not correct. It is the drag that causes it to rotate on an atmospheric body. 
Try a gravity turn on a vacuum body like the Mun without touching the controls, and disabling SAS, and you won’t rotate at all. 

"A gravity turn or zero-lift turn is a maneuver used in launching a spacecraft into, or descending from, an orbit around a celestial body such as a planet or a moon. It is a trajectory optimization that uses gravity to steer the vehicle onto its desired trajectory."

From: https://en.wikipedia.org/wiki/Gravity_turn

No atmosphere required...hence why the gravity turn is usually done when above the majority of the atmosphere.

Just think about it like a free body diagram; gravity pulls the rocket towards the "ground" through the CoM. With the rocket's engine engaged, we have another force applied to the vehicle. Divide this force into horizontal and vertical components. Now we have one downward force acting through the CoM and one upward force acting through the engine nozzle. This is an imbalance and the rocket will rotate nose down.

 

Edited February 1, 2023 by Meecrob

[rmaine]

40 minutes ago, Meecrob said:

"A gravity turn or zero-lift turn is a maneuver used in launching a spacecraft into, or descending from, an orbit around a celestial body such as a planet or a moon. It is a trajectory optimization that uses gravity to steer the vehicle onto its desired trajectory."

From: https://en.wikipedia.org/wiki/Gravity_turn

No atmosphere required...hence why the gravity turn is usually done when above the majority of the atmosphere.

Just think about it like a free body diagram; gravity pulls the rocket towards the "ground" through the CoM. With the rocket's engine engaged, we have another force applied to the vehicle. Divide this force into horizontal and vertical components. Now we have one downward force acting through the CoM and one upward force acting through the engine nozzle. This is an imbalance and the rocket will rotate nose down.

 

Sorry, but just no! Actual NASA engineer (well, retired) speaking here. True that air isn't required for a gravity turn. But *SOMETHING* has to create some torque or the vehicle won't rotate. Air is a convenient way to do it automatically, assuming the vehicle is aerodynamically stable. (It's not drag as one post above describes it, but that's a separate quibble). If not in an atmosphere, the rotation would be either from engine gimballing or RCS jets (or for KSP, gyros, though that's not going to cut the mustard for most real rockets). And most emphatically no, the engine nozzle does not magically produce an upward force. It produces a force along the vehicle centerline (or however else the engine is aligned, but that's most commonly the centerline). Engine gimballing can change that, but that requires active control and almost certainly would not be just an upward force; pretty hard to get into an orbit if the force from the engine was somehow always upward. Oh, there are some relatively minor aerodynamic effects making the thrust not quite along the centerline when the vehicle has a non-zero angle of attack (or sideslip), but those can be neglected for current purposes.

 

3 hours ago, Klapaucius said:

There are many scientists who know their stuff inside and out but struggle to convey that to laypeople.

There are also scientists (and engineers) who know their stuff, but struggle to convey it to other scientists and engineers. Back before I retired, I often made the point that it didn't matter what great technical insights you had, they weren't worth much if you couldn't express them well enough for other people to understand. This was a lesson I didn't learn myself until well after I left school. My eventual appreciation for that is probably a significant contributing factor to why I ended up as editor of an international programming language standard.

Edited February 1, 2023 by rmaine
typos

[MechBFP]

45 minutes ago, Meecrob said:

"A gravity turn or zero-lift turn is a maneuver used in launching a spacecraft into, or descending from, an orbit around a celestial body such as a planet or a moon. It is a trajectory optimization that uses gravity to steer the vehicle onto its desired trajectory."

From: https://en.wikipedia.org/wiki/Gravity_turn

No atmosphere required...hence why the gravity turn is usually done when above the majority of the atmosphere.

Just think about it like a free body diagram; gravity pulls the rocket towards the "ground" through the CoM. With the rocket's engine engaged, we have another force applied to the vehicle. Divide this force into horizontal and vertical components. Now we have one downward force acting through the CoM and one upward force acting through the engine nozzle. This is an imbalance and the rocket will rotate nose down.

 

EDIT: I get the difference but I am trouble finding the words to explain the difference between that and what I was referring to originally. 

Edited February 1, 2023 by MechBFP

[Kerbart]

11 minutes ago, rmaine said:

... But *SOMETHING* has to create some torque or the vehicle won't rotate. Air is a convenient way to do it automatically, assuming the vehicle is aerodynamically stable. (It's not drag as one post above describes it, but that's a separate quibble). ...

Of course I called BS, drew a diagram... and you're right. Keyword here is torque. There's no doubt that, once ascending at an angle, gravity creates a sideward force—absolutely. That force is there. But that force will go straight through the center of gravity. There's no fulcrum to create torque.

Now if only there was a way to simulate this.

[MechBFP]

Just now, Kerbart said:

Of course I called BS, drew a diagram... and you're right. Keyword here is torque. There's no doubt that, once ascending at an angle, gravity creates a sideward force—absolutely. That force is there. But that force will go straight through the center of gravity. There's no fulcrum to create torque.

Now if only there was a way to simulate this.

Yes thank you. That is what I was attempting to explain in my edit but failing. 
 Now to be completely accurate while gravity doesn’t rotate the ship itself it does rotate the ships vector (I think, correct me if I am wrong here.) which is perhaps what was originally intended but misinterpreted or misunderstood. 

[rmaine]

Just now, MechBFP said:

Yes thank you. That is what I was attempting to explain in my edit but failing. 
 Now to be completely accurate while gravity doesn’t rotate the ship itself it does rotate the ships vector (I think, correct me if I am wrong here.) which is perhaps what was originally intended but misinterpreted or misunderstood. 

Yes.

[The Aziz]

Man, if any newbies would come across this thread, they'd be scared off by you guys. Simple effective tutorial and here we are with torque, vectors, AoA...

Actually, one new player already arrived and doesn't understand what you're talking about. Good job.

Edited February 1, 2023 by The Aziz

[Superfluous J]

8 hours ago, engedagu said:

Hola a todos, soy nuevo en el juego, realmente no entiendo el debate sobre el dibujo, ¿alguien me explica cómo debería ser realmente?

Hi everyone, I'm new to the game, I don't really understand the debate about drawing, can someone explain to me what it should actually look like?

orbits are an ellipse with the thing being orbited at one of the ... uh... vertices (?) of the ellipse, NOT the center or anywhere else. In the video, for about 10 frames, this is not the case. It's not an ellipse and the planet is not correctly offset from the center.

See this for more, or just look at your non-circular orbits in KSP map view.

https://www.everythingrf.com/community/what-is-an-elliptical-orbit

 

[Meecrob]

2 hours ago, rmaine said:

Sorry, but just no! Actual NASA engineer (well, retired) speaking here. True that air isn't required for a gravity turn. But *SOMETHING* has to create some torque or the vehicle won't rotate. Air is a convenient way to do it automatically, assuming the vehicle is aerodynamically stable. (It's not drag as one post above describes it, but that's a separate quibble). If not in an atmosphere, the rotation would be either from engine gimballing or RCS jets (or for KSP, gyros, though that's not going to cut the mustard for most real rockets). And most emphatically no, the engine nozzle does not magically produce an upward force. It produces a force along the vehicle centerline (or however else the engine is aligned, but that's most commonly the centerline). Engine gimballing can change that, but that requires active control and almost certainly would not be just an upward force; pretty hard to get into an orbit if the force from the engine was somehow always upward. Oh, there are some relatively minor aerodynamic effects making the thrust not quite along the centerline when the vehicle has a non-zero angle of attack (or sideslip), but those can be neglected for current purposes.

 

There are also scientists (and engineers) who know their stuff, but struggle to convey it to other scientists and engineers. Back before I retired, I often made the point that it didn't matter what great technical insights you had, they weren't worth much if you couldn't express them well enough for other people to understand. This was a lesson I didn't learn myself until well after I left school. My eventual appreciation for that is probably a significant contributing factor to why I ended up as editor of an international programming language standard.

Looks like I'm pretty crap at explaining things I don't mean gimballing or anything of the sort. Basically I'm saying something is pushing the aft end up (the engine) while there is no balancing force on the nose once away from vertical orientation. The engine acts through the longitudinal axis while gravity pulls straight down from the CoM.

Wiki doesn't say much, but there is a free body diagram there. It looks like I explained it backwards.

https://en.wikipedia.org/wiki/Gravity_turn#Downrange_acceleration

 

 

[Ashandalar]

1 minute ago, Meecrob said:

Looks like I'm pretty crap at explaining things I don't mean gimballing or anything of the sort. Basically I'm saying something is pushing the aft end up (the engine) while there is no balancing force on the nose once away from vertical orientation. The engine acts through the longitudinal axis while gravity pulls straight down from the CoM.

Wiki doesn't say much, but there is a free body diagram there. It looks like I explained it backwards.

https://en.wikipedia.org/wiki/Gravity_turn#Downrange_acceleration

 

 

That diagram is explaining that the direction of the velocity vector (prograde) rotates during a gravity turn, not that the craft itself rotates.

[rmaine]

Just now, Meecrob said:

Looks like I'm pretty crap at explaining things I don't mean gimballing or anything of the sort. Basically I'm saying something is pushing the aft end up (the engine) while there is no balancing force on the nose once away from vertical orientation. The engine acts through the longitudinal axis while gravity pulls straight down from the CoM.

Wiki doesn't say much, but there is a free body diagram there. It looks like I explained it backwards.

https://en.wikipedia.org/wiki/Gravity_turn#Downrange_acceleration

 

 

Again, no. The engine is *NOT* pushing the aft end up. Nothing is pushing the aft end up. As you say, the engine is pushing along the longitudinal axis. That axis goes right through the center of mass. This does *NOT* cause any rotation; it just doesn't.

[Meecrob]

Ok let me try a different way. Say we have a pen stood on end vertically. It is stable, all forces are balanced. If I tip it slightly from the vertical axis, it will fall over because the forces are now unbalanced. Now imagine I do the same experiment, only the surface I am conducting this experiment on can accelerate upwards and also rotate (pitch) so that it is always acting through the longitudinal axis of the pen as it tips. The pen still tips as in the first experiment. The only difference as far as the pen is concerned between the first and second experiments is that in the second, the pen is now moving upwards and/or downrange...there is no force to prevent the pen from tipping. There is no force to restore the pen to a stable condition.