Problem Is this a bug? Apoapsis and periapsis changing on their own

[DrEStrangelove]

I watched quite a few YT videos before trying Orbiter, most with all perturbations turned on, and I was successful in getting to ISS before. The main problem is that MFDs can be extremely unreliable.

I can get to about 0.3 inclination difference by the time I get to orbit, and I get it to 0.00 as soon as I can. I then circularise my orbit and get my periapsis to where I want to intercept the ISS. The problem is that, unlike in YT videos I’ve seen, even if I have 18 orbits shown, it will always decide that the first one on the list is the best one. This doesn’t change with time and new orbits - the first one is always the yellow one. It seems very unreliable - others seem to make adjustments to get time difference to 0 or as close as possible, and it doesn’t seem to be working for me - adjustments needed are so extreme that in order to meet them, I need to change my apoapsis by 80-100k. I assume that it’s not a bug, but why is it so unreliable for me?

 

[Urwumpe]

Just to set the record straight, there is no exhaust at all for XR APUs (i.e., there is no "APU thruster" or anything like that). Running the APU does reduce your total mass (very) slightly as it runs and burns the fuel, but that's all. :tiphat:

I wasn't sure, I assumed there is conservation of mass and was not sure if it was vented non-propulsive (by having two opposing vents).



The Shuttle APUs are for example propulsive, though very weakly....

 

[asbjos]

The problem is that, unlike in YT videos I’ve seen, even if I have 18 orbits shown, it will always decide that the first one on the list is the best one. This doesn’t change with time and new orbits - the first one is always the yellow one. It seems very unreliable - others seem to make adjustments to get time difference to 0 or as close as possible, and it doesn’t seem to be working for me - adjustments needed are so extreme that in order to meet them, I need to change my apoapsis by 80-100k. I assume that it’s not a bug, but why is it so unreliable for me?

This could happen if you were e.g. in front of ISS, and below it, thus moving faster than ISS. Then, each orbit, you will travel further and further away from ISS, until you are a half revolution in front of it, and start to get closer again.

In this case, the SyncOrbit MFD will show you that the first orbit is the "best" in the way that it will be when you are the closest to ISS.

If you are in front of the ISS, you need to be higher up (longer orbital period) to get closer, and vice versa.

Also, remember that the Space Shuttle (and Soyuz, previously) mostly spent up to two days of orbital flight before docking with ISS. 18 orbits is just a bit more than one day. So no good interception available is no big deal, if you just take the time to wait.

The difference in orbital period between a 360 km orbit (typical ISS height in Orbiter) and 200 km orbit is only 3 minutes. So for the 90 minute orbit of ISS, you can be up to 30 orbits away from overtaking ISS if you are just in front of it.

 

[Buck Rogers]

The Shuttle APUs are for example propulsive, though very weakly....

They had to recalculate the trajectory on the Apollo 13 because they had neglected to take into account the propulsive effect of their urine jettisons!

I would recommend using InterplanetryMFD and RendevousMFD for orbital interceptions.

 

[DaveS]

I wasn't sure, I assumed there is conservation of mass and was not sure if it was vented non-propulsive (by having two opposing vents).



The Shuttle APUs are for example propulsive, though very weakly....

Yes, 23 lbf(102.12 N) per APU. And the exhausts of the APUs are visible at night. They're yellow pulsing lights you can see by the vertical stabilizer. If you're wondering what all the noise is coming from, it's the ground purge and coolant vehicles.

 

[DrEStrangelove]

Thank you all, you're awesome!

I've just completed my first full ISS mission in XR2, carrying 11t of LOX and some fuel to the base and returning safely to Cape Canaveral. My only question is, is there a way to transfer fuel/LOX from XR2 to ISS, not the other way around? For immersion purposes I simply dumped some fuel and LOX while docked but perhaps there is a way to do it properly.

 

[asbjos]

Check out ReFuel MFD!

 

[DrEStrangelove]

Check out ReFuel MFD!

It’s great, thanks!