Can't sync orbits w/ ISS after aligning them

[Dojo the Mouse]

So I just accedently discovered Orbiter the other day but I'd say I'm already suffeciently addicted; I spent my one day off a week just glued to my computer. So my question is about sync orbits.

I launched in the XR2 Ravenstar without much regard for where the ISS was. Now I'm trying to get to it. I've aligned the orbits to the relative incline is around 0*. I found the ISS's altitude at my periapsis and matched it. The problem is that I can't work out a delta-v that actually makes us meet. Per my sync MFD the DTmin is 453.6 with a relative velocity of 3.077k. If I slow enough to meet it I loose orbit and I can't seem to speed up enough to catch it. I've been trying everything, any advice would help! Thanks!

 

[Goth]

Welcome to the awesome world of Orbiter, and welcome to the forum.

If I slow enough to meet it I loose orbit and I can't seem to speed up enough to catch it.

You are not supposed to catch the space station in just an orbit; it will take some orbits, 2, 3, or more. Anyway have you tried the Tutorials/DG-ISS Sync Doc scenario? Here Martin explains very well how to do what you're trying to do.

 

[Dojo the Mouse]

Yeah, so I've watched that a couple times. The thing is, he starts off WAY closer than I am (with a DTmin of only a few minutes) making is look down-right easy. I think I'm making some progress though; I can see the ISS whizzing by around me now :facepalm:

 

[Goth]

Take your time, to learn Orbiter is not instant.

The thing is, he starts off WAY closer than I am (with a DTmin of only a few minutes)

This should not be a problem, because as I said you can took some orbits before reaching the target. You can also try different encounter points using the Sync MFD.
Maybe post the scenario with your current state so we can find something specific to suggest.

 

[Wishbone]

If you have enough delta-V you can intercept and dock within one orbit regardless of the phase difference, by doing a large prograde burn to wait up there for the station to catch up with you. Otherwise, you have to do the phasing, and it takes time (not fuel). If you're using Sync MFD, fire at the opposite end from your rendezvous point to move the rendezvous point around, and _at_ the rendezvous point to reduce DTmin to 0.0.

 

[Tex]

Dojo the Mouse: Have you seen my KSC to ISS video tutorial? Here are more tutorials:

http://www.orbiter-forum.com/tutorials.php

 

[Grover]

basically, do this (easy version)

• start at a LOWER orbit altitude to the ISS (but above 200k)
• get your plane aligned with the ISS
• get the sync orbit MFD open
• burn prograde untill the highlighted line is in the orbit that you want, and DTmin is 0

explanation:
burning prograde changes your course to intercect the orbit of the ISS, you want to pass the intercection (one of them anyway) at the same time as the ISS, so you arent hundreds of KM away; burning prograde ensures that your orbits intersect, and that you dont hit the atmosphere. DTmin is the time, in seconds (or minutes, ive heard this, but th numbers add up to make it seconds) between you passing the intersect and your target passing it, so if its 0, you will pass at the same time (and you will crash if you are aligned perfectly) for the given orbit. once you have set up your sync orbit, time warp until the highlighted line is at the top (it moves up a line every time you pass the intersect) and you are close to the intersect.

then you burn RELATIVE RETROGRADE, this is not ACTUAL retrograde, but it is in fact the retrograde of your relative velocity to your target. the aim is to reduce it to 0 as you are close to the target (about 1km if you're good, about 100m if you're incredible), the way i do this is to use the IMFDs orbit program (in velocity match mode) to work out the time taken to achieve the delta-V, then beggining the burn when the time to intersect (sync orbit) is just over half of the burn time, this usually leaves me close enough that the orbital difference doesnt affect relative velocities.

from there you just move into position and dock, simple

good luck!

 

[Dojo the Mouse]

Thanks for the help everyone! After working with it for a while, I basically came up with a solution very similar to the above post. Still not perfect (my altitude tends to be off by a few km) but I'm getting there :thumbup:

 

[Grover]

if you manage to dock at all, then you're making progress. keep practising and work out your own techniques, but bear in mind that there are many ways do approach a station, you may find that your ideal one isnt the most efficient, and in fact you have to be able to do many different methods if you want an efficient approach every time.

keep going! once you can do that, try some lunar transfers, but brace yourself for a world of pain (its quite hard before you get the hang of it)

 

[Bonanza123d]

reduce it to 0 as you are close to the target (about 1km if you're good, about 100m if you're incredible)

if you are 10m

 

[Urwumpe]

If you are , you won't reduce it to 0. You dock with 15 km/s relative velocity.

 

[Wishbone]

This is called asat... does it at 0.9999c

 

[Tommy]

The main "skill" used in syncing orbits is patience! Don't be in a hurry, or you'll find yourself flying past your target at a very high speed.

The first thing is to get the planes aligned - RInc should be no more than .03, less is better.

Assuming you start in a lower orbit (which is usually the case when you launch, then rendezvous) there are a few steps to take.

1. Wait until you are near the target's Apoapsis (use OrbitMFD, and if you have "non-spherical gravity sources" enabled, this will move around some - just eyeball for middle of the area the target Apo moves around in). At this point you will burn prograde. This will raise your Apoapsis, raise it until it is slightly higher than the target's Periapsis.

2. Set SyncMFD to use "Intersect 1" or "Intersect 2", and ensure you have an intersect (it will tell you if you don't). Also, you can use the Len button to have SyncMFD display more orbits, IIRC up to 8 orbits can be displayed.

3. If you are a long ways from the target, it can take several orbits, even a dozen or more, before a good close intersect comes up. Keep orbiting until the highlighted orbit starts moving up the list.

4. Tuning the rendezvous. ALL burns to lower the DTmin will be made at your Apoapsis, which is the rendezvous point. Two orbits before the rendezvous use a prograde burn to lower the DTmin. This will often change which orbit you rendezvous at, moving it to a later orbit. That's fine, and will result in a lower relative velocity when you do reach your target. Use translational RCS to "fine tune" this to very close to zero DTmin. Again, Non-Spherical Gravity will make the DTmin change a bit, but this rarely poses a real problem.

5. Last Orbit. When you are one full orbit from the rendezvous, fine tune the DTmin one last time - and don't try to re-tune it later. DTmin may increase as you go around, but will drop back down as you get close to rendezvous, so trying to "fix" it a half orbit from rendezvous will usually just make things worse!

6. Rendezvous. About 1/4 orbit before rendezvous, target the vessel's main transponder (not an individual docking port, IDS has a much shorter range) in DockMFD, and then hit the <HUD> button in DockMFD. Point your vessel at the negative RVel indicator (the plus sign in the circle) and keep an eye on the RVel and Distance. When the Distance gets low (less than 10k) OR the distance starts to increase, burn toward the negative RVel marker until RVel is zero (less than 1m/s is fine). Then point your vessel toward the target, pick a docking port (and set DockMFD to it - then hit the <HUD> button so the Docking Approach Lane is shown). Point the vessel about halfway out the lane and give a tiny nudge of forward thrust. Keep the RVel at 10 m/s or less, and use bursts of 10x time accel to speed things up - don't use thrust or the higher RVel will cause you problems. There should be a "plus sign" visible on the hud (the Relative Velocity indicator). Use linear translation RCS to keep this centered on the approach lane. Once you get near the lane, eliminate the RVel, point toward the dock, and move in.

 

[Ripley]

...Also, you can use the Len button to have SyncMFD display more orbits, IIRC up to 8 orbits can be displayed...

Currently, up to 19.

For the rest, excellent little docking tutorial - I must try it!

While we're at it, I was thinking the other day some very basic things like....

Let'assume that:
- Our orbit is a sort of "hinged" ring;
- The "hinges" are the Apogee and the Perigee;
- So, the axis around wich this ring can pivot is the diameter "passing through" Apo and Peri;
- The points where we apply energy to more efficiently rotate the ring are at 90° to these "hinges", so they are the AN & DN nodes.

Now, according to some "space flight principles" I've read in a TransX doc (can't recall its number), wouldn't it be easier and more fuel efficient to Align Planes when our orbit is "bigger", therefore using a "longer lever"??

So the final question is: isn't it better to align orbital plane at a later point, rather than being "the first thing to do"?

Sorry if my terms are not exactly "spacetech" but it's late in the night...

 

[Wishbone]

For large orbital plane corrections, assuming there's no aerobraking option, it may be beneficial to a) burn prograde in periapsis and raise ApA very very high, b) use burns in the node which is closer to apoapsis, c) do retrograde burn to get to your target orbit again. The logic is simple: you want to turn a velocity vector, and the best way to turn it is when it's shorter.

And, there are several methods of skinning a rabbit - sorry, doing a rendezvous. With longer phasing times one may even exploit the non-sphericity to wait till RInc goes to the minimum, although you may be out of oxygen long before that.

 

[Tommy]

- Our orbit is a sort of "hinged" ring;
- The "hinges" are the Apogee and the Perigee;
- So, the axis around wich this ring can pivot is the diameter "passing through" Apo and Peri;
- The points where we apply energy to more efficiently rotate the ring are at 90° to these "hinges", so they are the AN & DN nodes.

The nodes can be anywhere in relation to the Apo and Peri, they do NOT need to be at 90 degrees - and these nodes would be the "hinge" (the orbital plane rotates around the burnpoint).

So the final question is: isn't it better to align orbital plane at a later point, rather than being "the first thing to do"?

The main reasons I advocated the plane change first are:

1. SyncMFD doesn't work well if there is a significant RInc
2. It allows you to separate the PlC and Sync into individual steps - making it a bit easier.

If your RInc is over half a degree, it's often more efficient to make the transfer burn when you are at the node nearest the target's Apoapsis, then make the plane change when you reach your Apoapsis (which will be the rendezvous point) - assuming your target's orbit isn't too eccentric.

As Wishbone stated, for a larger plane change it can be more efficient to raise your Apo quite high (much higher than the target), make the plane change at your Apo, then lower your Apo down to the target's altitude. This will only be more efficient if the RInc is high, I'd say at least 5 degrees, if not more.