Tommy
Well-known member
Changing the plane should be done when the vessel is traveling at it's slowest. This usually means a higher altitude. So, if I had a 180k by 325k orbit, I would change the plane at the node closest to the Apoapsis where my speed is lower. If this is during an ascent, then it may be cheaper at 180k on the way up - since I'll be adding velocity when I circularize at 325k.Inclination - Suppose I have to change my inclination by 2 degrees. Whether I do the burn at 180km or 325km does it make a difference in the fuel needed to complete it?
In short - it's not the altitude, it the amount of velocity - and less is better for changes in direction.
GS - Ground Speed. The horizontal component of my velocity, relative to the ground. Assuming you were maintaining a constant actual velocity, as you gain altitude ground speed will reduce to zero, then increase again. While it is still a positive number, the direction will have changed. For instance, the Moon actually orbits "west to east", but it appears to travel east to west because the Earth is rotating faster than the moon orbits. This mode is relative to the Earth's surface, so it is subject to the Earth's rotation.Surface MFD questions
GS - Ground speed? My speed relative to sea level, right?
OS - Orbital Speed? My...????
TAS - True air speed???
OS - Orbital Speed. Your velocity relative to the non-rotating center of planetary mass. It is NOT subject to the earth's rotation - so a vessel sitting on the pad at KSC will have over 400 m/s velocity when using OS - because the vessel is affected by the Earth's rotation but the reference isn't.
TAS - True AirSpeed. In Orbiter, this is your true speed relative to a rotating planetary mass. It is independant of any atmosphere, so it is accuate even on the Moon, etc. This is unlike IAS (Indicated AirSpeed, which simulates the old "pitot tube" system) is dependant on density and pressure so it shows lower than true speeds at higher altitudes, won't work in a vacuum, etc.
I pretty much just use TAS, maybe switch to OS late in an ascent.
Equatorial plane - I wiki's this and for some reason I can wrap my head around it. They even showed a picture. Can anyone break it down just a little bit for me?
I don't think it's the Equatorial Plane that's confusing you. It's just a plane that passes through the equator, and it's perpendicular to the Earth's axis of rotation. It's the plane that allows for a fully geo-stationary orbit.
I suspect it's the Equatorial Frame that may be confusing you. One of the hardest things about spaceflight is learning how the different frames of reference work together. Eventually, after you've read enough tutorials, and practiced enough, you'll get a "Eureka" moment and it will make sense.
Here on Earth, we navigate using one frame of reference. The ground beneath our feet. Except for long flights, we don't even need to acknowledge that the world isn't flat! In spaceflight, we need other frames of reference. The two most common are Ecliptic and Equatorial. Both define a specific plane as the reference. Great confusion results from mixing them up.
The Sun rotates, so it has an axis and an equator. The Ecliptic plane is the Sun's equatorial plane. This provides a single reference frame that can be used anywhere in the solar system - and there is only one Ecliptic plane in the solar system. It's best used for interplanetary flight.
Every planet and moon in the solar system also have their own equatorial plane. The Equatorial Frame of reference uses the local equatorial plane as it's reference. It isn't very useful beyond the bodies Sphere of Influence, but it is well suited to "local" flights like LEO or the Moon. Remember - this is a "local frame" and will be different from other the Ecliptic and from other Equatorials as well.
ApR on Orbit MFD is Apoapsis Radius - the distance from the center of the planet. On the right, there's a button marked DIST, hit that and altitudes will be displayed instead of radius. ApR will change to ApA, which means Apoapsis Altitude (distance above the ground).APR - This is short hand in the orbiter MFD as apoapsis?