Flight Question What is the Normal?

[Hurricane]

Hello everyone. I have seen that in Lunar Transfers you use the Normal(+ or -) more than once, and I know when to use which. But my question is, what is the Normal and how can I visualize it so I better understand it?

~Thanks in advance, Oz. :tiphat:

 

[Spacethingy]

Hello, "Normal" means 90 degrees out (perpendicular to...).

In Orbiter that means 90 degrees to your prograde (sort of forward) direction.

Check out the Orbiter manual, which is in "your_orbiter_install/doc/Orbiter.pdf". Page 57 has some good diagrams. Also, "Go Play in Space" has some good advice, it's a kind-of walkthrough guide.

 

[Grover]

yes, normal means perpendicular to your orbit. so if you imagine prograde as forward, and retrograde as backwards, normal + is upwards (towards the north pole of earth in normal orbits) and normal _ is downwards (towards the south pole in normal orbits) but going into a retrograde orbit (where your equitorial inclination is higher than 90 or lower than -90, normal + points to the south pole and vice versa.

the main purpose of normal orientations is to align your orbital plane with your target, to simplify your orbital math to 2 dimensions, rather than dealing with 3

 

[Urwumpe]

better said: The reference of normal is the orbit plane. If you point at the normal, you point parallel to the "rotation axis" of your orbit. Anti-normal points in the opposite direction, but is still perpendicular to your orbit plane.

 

[Hurricane]

So if I'm on Prograde, Normal + would be pulling up by 90 degrees?

 

[Urwumpe]

So if I'm on Prograde, Normal + would be pulling up by 90 degrees?

No, first of all, up and down are pretty useless qualities in orbit. Ignore them if you can.

Next, you should remember the important directions in orbit: You have your radius vector (r) and its direction vector equivalent (r-bar), which gives you the direction to the center of the planet you are orbiting. The next important direction is your velocity vector (v) and its direction (v-bar). The plane that is spanned by radius and velocity vector is your orbit plane - all your orbital motion happens inside this flat plane, if you have no other forces than (spherical) gravity.

So, radius is the closest you will get to up and down. up means increasing radius, down means decreasing it. But since the orbit plane normal is 90° to the plane, it also means that it is both 90° away from your radius vector and your velocity vector at the same time: It is 90° to the left side (right hand rule applies, point thumb up, pointer finger forward, then when you have your middle finger perpendicular to both previous fingers, it shows you where the normal is).

 

[Jarvitä]

Or, to put it formally: Orbit-normal unit vector is the cross product of the velocity unit vector and radius unit vector. Orbit-antinormal unit vector is the cross product of radius unit vector and velocity unit vector:

[math]\hat{v}\times\hat{r}=\widehat{N+}[/math]
[math]\hat{r}\times\hat{v}=\widehat{N-}[/math]

 

[Hurricane]

No, first of all, up and down are pretty useless qualities in orbit. Ignore them if you can.

Next, you should remember the important directions in orbit: You have your radius vector (r) and its direction vector equivalent (r-bar), which gives you the direction to the center of the planet you are orbiting. The next important direction is your velocity vector (v) and its direction (v-bar). The plane that is spanned by radius and velocity vector is your orbit plane - all your orbital motion happens inside this flat plane, if you have no other forces than (spherical) gravity.

So, radius is the closest you will get to up and down. up means increasing radius, down means decreasing it. But since the orbit plane normal is 90° to the plane, it also means that it is both 90° away from your radius vector and your velocity vector at the same time: It is 90° to the left side (right hand rule applies, point thumb up, pointer finger forward, then when you have your middle finger perpendicular to both previous fingers, it shows you where the normal is).

So how do I visualize Normal + in relation to Orbiter's Prograde direction?

 

[Urwumpe]

So how do I visualize Normal + in relation to Orbiter's Prograde direction?

prograde = in direction of the velocity vector.

 

[TSPenguin]

IMO nothing beats the excellent Orbital Operations tutorial with regards to making these kind of things understandable.

 

[jedidia]

Why not just tell the poor man that angles are measured counter-clockwise? :lol:

Normal + is 90 degrees counter-clockwise to your prograde direction, and also perpendicular to your orbital plane. Normal - is 90 degrees in the other direction, or 270 degrees counter-clockwise. But yes, the tutorial that TSPenguin posted is probably the most helpfull...

 

[Jarvitä]

Why not just tell the poor man that angles are measured counter-clockwise? :lol:

Because learning the mathematics behind it is much more engaging and satisfactory than being given an un-intuitive solution without even understanding why that's the solution.

 

[Ripley]

1 image is worth more than 1.000 words...

From [ame="http://www.orbithangar.com/searchid.php?ID=3074"]Tutorial: DG to ISS[/ame]

 

[Hurricane]

prograde = in direction of the velocity vector.

That, I know.

IMO nothing beats the excellent Orbital Operations tutorial with regards to making these kind of things understandable.

Thanks!

Why not just tell the poor man that angles are measured counter-clockwise?

Normal + is 90 degrees counter-clockwise to your prograde direction, and also perpendicular to your orbital plane. Normal - is 90 degrees in the other direction, or 270 degrees counter-clockwise. But yes, the tutorial that TSPenguin posted is probably the most helpfull...

Haha so true!

1 image is more worth than 1.000 words...

So true! And by the way, that's JUST what I asked. If I were to go on prograde and pull up 90 degrees (relative to the SHIP'S CURRENT direction) that would be Normal+!

 

[n0mad23]

lol - but if you turned your craft "upside down" what then? Urwumpe's advice,

up and down are pretty useless qualities in orbit. Ignore them if you can

is actually quite important, as "up" and "down" are totally arbitrary when you're in any kind of orbit. In other words, it's all about your orientation. Trying to hold onto this kind of paradigm will only make thinking in orbital mechanics terms more difficult.

 

[Jarvitä]

So true! And by the way, that's JUST what I asked. If I were to go on prograde and pull up 90 degrees (relative to the SHIP'S CURRENT direction) that would be Normal+!

Not true. "Prograde" can describe any number of orientations (for example, in the picture above, the deltaglider's "bank" angle is independant in the prograde direction), therefore, "up from prograde" isn't even logically defined.

 

[Hurricane]

Not true. "Prograde" can describe any number of orientations (for example, in the picture above, the deltaglider's "bank" angle is independant in the prograde direction), therefore, "up from prograde" isn't even logically defined.

I said, Orbiter's Prograde. That is, facing your vector and with the left wing towards the earth.

 

[Moach]

actually, it's easier if you can visualize your orbit path as a "ring" around the earth (or whateverit is you're orbiting)

then, try to visualize a direction in which your orbit is "spinning" around.... as if it were an axis that at any point of your orbit, would be found perpendicular to your "prograde" direction


THAT - is your normal axis, or normal vector as it's most often called

so yeah, it's basically "up" - but only if you assume the ring described by your orbit your orbit as a reference to define what would be "flat"

look:

that image makes it a bit more clear, i hope...

 

[Hurricane]

actually, it's easier if you can visualize your orbit path as a "ring" around the earth (or whateverit is you're orbiting)

then, try to visualize a direction in which your orbit is "spinning" around.... as if it were an axis that at any point of your orbit, would be found perpendicular to your "prograde" direction


THAT - is your normal axis, or normal vector as it's most often called

so yeah, it's basically "up" - but only if you assume the ring described by your orbit your orbit as a reference to define what would be "flat"

look:

that image makes it a bit more clear, i hope...

Thank you very much! So JUST to see if I get it right: If I were aligned horizontally with my orbital plane, with the nose facing into the tangent vector, and I pulled up, that would be the normal?

 

[jedidia]

So true! And by the way, that's JUST what I asked. If I were to go on prograde and pull up 90 degrees (relative to the SHIP'S CURRENT direction) that would be Normal+!

No. You have to pull counter-clockwise relative to your prograde direction. "Up" might be anywhere.