Flight Question General Questions about orbiter (flight)

[Tommy]

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?

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.

In short - it's not the altitude, it the amount of velocity - and less is better for changes in direction.

Surface MFD questions
GS - Ground speed? My speed relative to sea level, right?
OS - Orbital Speed? My...????
TAS - True air speed???

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.

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 - This is short hand in the orbiter MFD as apoapsis?

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).

 

[thumper235]

I'm also a new user to Orbiter, and I'm having lots of fun. I have also realised that there is a LOT to learn. It's making my brain ache..... I've been practising with the DeltaGliderIV, watched Tex's video through a few times now and I think I'm starting to understand the principles of the physics involved. I wish I'd come across this years ago, the old brain cells aren't as good at absorbing new knowledge as they used to be!:hmm:

Hey jeffers,

When I first realized orbiter was out there I dont think my finger clicked my mouse quick enough nor did my internet connection download the core files for orbiter quick enough. It was a late night on youtube and I was watching some flight sim vids. Well, orbiter came along in the search list and the rest was history.

At first I knew very little about spaceflight. I remember thinking hey lets check some tutorials out and see what I can learn. Wow, and a few expletives came to mind. I remember thinking I would NEVER figure out ANYTHING as it looked so complicated. Well, I put the copy down. A couple weeks ago I picked it up again and this time put a little effort.

Couple newbie tips.

1) Use the forums
2) Really LOOK for an answer before asking. - People get annoyed when you ask something that is posted everywhere like a bad STD.
3) Patience!!! It will come! You will figure things out. I am guilty of coming here with //ragemode// turned on after doing something wrong over9000 times.
4) If there is anything I can help you with send me a message anytime. :thumbup:

 

[Ripley]

...3) Patience!!! It will come! You will figure things out....

Exactly!!
Patience is the keyword.
I also came to Orbiter from years of "simple" combat flight-simming (IL-2, Falcon, Jane's F-15, LongBow, etc...), and as soon as I discovered Orbiter I was blown away. I printed some tutorials and manuals and started reading, without understanding a single bit of it.

I kept reading them over and over again (together with some Orbiter practicing and much OF reading) and every single time a little bit of light was added...it's a slower process (for "normal" people at least!), that requires constance, motivation and patience.

There are also some real ninjas around here, who, after just few days since Orbiter discovery, can dock, reenter, and maybe they are practising to take a trip to Mars or beyond, but they're few.

 

[Capt_hensley]

I'm still learning after two years, still don't get most flights right. I'm finding out as an engineer, I design better than I fly. Howard Hughs, I'm not. Have fun!

 

[thumper235]

Ok an update.

I have made 3 successful re-entries. So far all of them have dropped me right at KSC to where I just had to roll around and drop my gear and land. I think with a little more practice I could do it dead-stick. My last attempt I made an error of not turning my hydraulics back on for the landing gear and did not realize it until it was too late. pulled back on the stick and mashed the throttle and just landed at runway 2. For some reason I prefer coming into KSC from the sea. For me, coming in from the water seems to make things line up a little easier.

So, 3 successful attempts and I feel as though I am qualified to handle re-entry procedures and now must move on to my moon adventure. Hopefully everything I have learned here will make this easier for me.

I was thinking about the moon and the missions in general. I have been told the landing part is not really for the novice, like myself and that I might want to consider making the landing and launch as automated as possible. In my minds eye, I thought at-least since I could launch from earth and link up with the ISS that this would be the same procedure on the moon and I would come out of the gates a little ahead of the game.

Do you all think that landing/launching from the moon is that tough?

New things I need to learn is how too...

1) proper TLI burn
2) course correction burn(s)
3) Achieve a stable lunar orbit
4) I am sure there is more I am about to figure out.

Tutorial suggestions would be MUCH MUCH MUCH appreciated. thanks guys!

 

[Tommy]

The main difficulty in landing on the Moon is knowing when to start braking so that you end up close to the base. I suggest using BurnTimeCalcMFD. It lets you select which engine (you may want to use retros so you can see ahead of you), and then you input the dV (just use your current OS plus maybe a couple hundred m/s) and it will give you a distance - begin braking that far from base.

I also suggest using a vessel that has a descent hold AP like an XR or DGIV. Otherwise trying to watch your VS and altitude (and adjust the hovers) as well as watching your speed and distance, etc gets a bit overwhelming.

Taking off is easy - just give yourself just enough hover thrust to lift off and climb slowly (less than 5 m/s) or use the descent AP set to about 2.5 m/s positive rate. Once off the ground, use RCS to rotate to the heading, and once you are about 100m up engage the throttle (keep vessel level throughout) until your Ap is established.

As for the Moon, it depends on which tool you want to use. It can be done with the stock TransferMFD, but there are three better options - LTMFD, IMFD, and TransX.

 

[ADSWNJ]

Um, no. You have to match your velocity with the target's BEFORE you can dock - otherwise you'ld just smash into it. When you null the RVel during rendezvous, you are are matching the target's orbit - so you'd be in the 300km - 600km orbit before you actually docked.

I would start from a circular parking orbit of about 210km, and raise my Apo to match the target's Periapsis - same location, but a couple km higher. Then, at my new Apo, I would raise my Periapsis to sync at some future orbit. All adjustments will be made that way, at my Apo (be careful not to lower your PeA too much!).

As I approach the target I may use RCS translation to adjust my approach, and when I'm as close as I'm going to get I use the Docking HUD to null the relative velocity. From there it's just a matter of patiently moving in.



It's more efficient to match the target's orbit at the target's periapsis. Remember - changes in the AMOUNT of your velocity should be made when you are closer to a gravity source - changes in direction are best made farther away from the gravity source.

So summarizing - to get to 300 x 600 from a parking orbit, you get up to say 300 x 302 with your Apo aligned to Tg Peri. Then on the rv burn, nulling out the rvel brings you to close to 302 x 600, ready for docking run. The only issue I have with that is that you have potentially quite a decent rvel to null out on the RV, especially if you have a low-power engine. If you needed to break it down to keep the rvel below a specific limit, then you would presumably be back towards my approach, with the Apo's aligned.

In a more extreme example, and thinking about what you said about adjusting velocity closer to the gravity source, what would be the lowest possible fuel burn option in this following scenario:

Parking orbit: 200km x 200km.
Target: 2200km x 8200km.

Do you burn from parking to 200 x 2200, then to 2200 x 2200, then to 2200 x 8200, or you you burn from parking to 200 x 8200, then to 2200 x 8200?

 

[Tommy]

So summarizing - to get to 300 x 600 from a parking orbit, you get up to say 300 x 302 with your Apo aligned to Tg Peri. Then on the rv burn, nulling out the rvel brings you to close to 302 x 600, ready for docking run. The only issue I have with that is that you have potentially quite a decent rvel to null out on the RV, especially if you have a low-power engine. If you needed to break it down to keep the rvel below a specific limit, then you would presumably be back towards my approach, with the Apo's aligned.

You would still start by raising your Apo to match the target Periapsis. If you had low thrust, you sould then make burns at your Apo - eventually raising the periapsis until it became your Apo, and perhaps even a burn or two after that at what is now your periapsis until the RVel was manageable. However, you wouldn't do that for efficiency - only because the vessel was incapable of rendezvous with any significant RVel. Mechanical limitations cause you to take a less efficient path.

In a more extreme example, and thinking about what you said about adjusting velocity closer to the gravity source, what would be the lowest possible fuel burn option in this following scenario:

Parking orbit: 200km x 200km.
Target: 2200km x 8200km.

Do you burn from parking to 200 x 2200, then to 2200 x 2200, then to 2200 x 8200, or you you burn from parking to 200 x 8200, then to 2200 x 8200?

In a case this extreme eccentricity, other factors come into play. It's possible the most efficient method would be to raise your apo to match the target's (8200), then raise the periapsis as needed for rendezvous. This is because after you've raised your Apo, the amount of energy needed to raise the PeA is a smaller fraction of the total orbital energy. Essentially, you would be doing the most work (raising orbit to 8200) when it's most efficient - and that would also make raising the PeA less work. This is only the case in orbits with a rather high eccentricity - I'm not sure where the "cut-off" would be. Altitude is also a factor - the farther from the gravity source the less dV it takes to raise the other side of the orbit by any given amount.

The relationship between velocity and energy in an orbit can be tricky - there are many variables so there isn't a "one rule" for anything. You also need a good understanding of the concept of "work" as it applies to physics.

 

[ADSWNJ]

Nice one! I learned a lot on this thread. More things to try out as well .