Flight Question Better low-fuel Rendezvous technique

[Zatnikitelman]

In practicing for my upcoming OFSS Mission 7, I've dusted off the ol' shuttle and been flying mission 4 as it's reasonably similar to what I'll have to do. Unfortunately, I seem to have forgotten something about low-fuel rendezvous ops. My problem probably starts after I've caught up with the station, but am still 700km from it. This is where I usually engage Rendezvous MFD and hunt for a low-initial Dv solution. The problem here comes when it's time to brake. So far, I've not managed to intercept with a rvel less than 100m/s, taking waay too much fuel for my taste. I've searched the forums, but "Rendezvous" and other terms give me fully half the forum's posts Does anyone remember the link to a topic (or two or three...) that has better rendezvous strategies? I've tried several methods including IMFD without much success.
Thanks in advance,
Zat
P.S. Don't worry Tex, even if I had to fly the mission today, I could easily pull it off with fuel to spare, I just like having big fluffy cushions

 

[RisingFury]

To minimize your capture Delta-V, make sure your orbit is only slightly lower or higher in one extreme, and exactly the same in the other.

If your PeA matches station's PeA, make sure your ApA is like 10 km higher or lower then station's ApA. It will take a whole lot longer to sync up, but it will save you fuel. Correct towards this kind of setup once you're within a few orbits of capture. That will extend the time only by a few more orbits...

 

[Zatnikitelman]

Ok, I've spent most of today trying to improve my rendezvous method. Unfortunately, Rendezvous MFD doesn't seem to want to give me a low dv solution for both ends of the transfer so I've tried the equation on OrbiterWiki's Rendezvous topic. I built a calculator in C# for it, but I'm not sure it's giving me good data. I can get within a hundred or so km on the target's trajectory rather than just below it, and once I even managed 12km, but I still think I'm missing something.
Here's the code I'm using for the calculator:

        targetradius = (tapoapsis + tperiapsis) / 2;
        eccentricity = (1 - (2 / ((targetradius / periapsis) + 1)));

        SMA = tapoapsis/(1+eccentricity);
        temp1 = (((2*orbits)-1));
        temp2 = Math.Pow((SMA/targetradius),(3/2));
        angle = PI*(1-(temp1*temp2));

The target's data is either explicitly labeled target or has a t in front of it. I use the average of the Apoapsis and Periapsis whenever the equation wants radius because the equation assumes a circular orbit.
Now, I can do the rendezvous, but it's fairly inefficient (though I have enough fuel to complete the mission) and takes a while as I have to make constant corrections as far as 10km or so out from the target.
Thanks for any assistance,
Zat

 

[n72.75]

I typicialy follow a 30 km rule where i put my spacecraft ~30km below the orbit of the station, assuming circular orbits.

 

[agentgonzo]

Ok, I've spent most of today trying to improve my rendezvous method. Unfortunately, Rendezvous MFD doesn't seem to want to give me a low dv solution for both ends of the transfer so I've tried the equation on OrbiterWiki's Rendezvous topic.

I only use part of Rendezvous MFD's functionality. I don't know whether it will help or confuse the matter, but in case it helps, here is what I do for Rendezvous.

Get into a parking orbit that is 'below' the ISS and align planes to 0.00° RIinc. To minimise the Δv requirement, most of the alignment should be done with your launch trajectory.

Perform a burn to raise the apogee of your orbit so that it just intersects the orbit of the ISS. An easy way to do this is to open up OrbitMFD and SyncOrbitMFDs. At the start, SyncOrbitMFD will show no intersection with the ISS. As you burn prograde, watch your ApA in OrbitMFD until it is getting close to the height of the ISS, then start watching SyncOrbitMFD. As soon as SyncOrbitMFD is showing an intercept, kill your engines. Your ApA will be just intersecting the orbit of the ISS, which means that at the point of intersect, you will have almost zero vertical speed and be at the same altitude as the ISS. The aligned planes mean that you will have zero lateral relative speed to the ISS as well - very handy for Rendezvous.

Watch SyncOrbitMFD and time-accel until you are getting close to the ISS, then switch to RendezvousMFD. You'll need to understand how RendezvousMFD works. Watch RendezvousMFD (and update it if necessary) and wait until you get to the final apogee that will have you 'behind' the ISS. When you burn prograde, you will raise your perigee, thus increasing your orbital period and 'pulling back' the next apogee wrt the ISS. When the next apogee is 'behind' the ISS, stop your burn. You will have raised your perigee a bit and thus decreased the relative speed of rendezvous between you and the ISS. Repeat this last step a few times until your perigee is about 10km or so below the ISS. At one of these stages (it doesn't matter which one), burn (either retrograde or prograde, it depends on what your current orbit is like) at perigee to raise/lower your apogee so that it is at the correct altitude for the rendezvous (I normally shoot for a couple of hundred feet below the ISS). At the 'last' apogee, perform your prograde burn such that the apogee corresponds exactly with the same position (x component only, it will still be below the ISS) as the ISS. When you next get to apogee, you should pop up underneath the ISS by the target amount with very little relative speed. From here, you can perform the RPM, TORVA or any other docking mechanism you choose.

It takes a couple of run-throughs to get it so that it all makes sense, but this is my normal docking operation.

 

[Ark]

I just use Sync MFD and Docking MFD. For a low-fuel rendezvous, I'll set the intersection, align planes to 0.0 degrees, and set my PeA at 30-50 km below the target station. Then, just kick back and wait until I'm close enough to rendezvous to use attitude thrusters to tweak the PeA until DTmin is as close to zero as possible. If done right, I pass within half a km of the station, and (last time I tried it) a total orbit matching DV of 50 m/s or so. With XR and DGIVs you can afford to be a bit wasteful and rendezvous in a couple of orbits, but for Shuttle Fleet you just have to be patient.

 

[EtherDragon]

The planning of this would be a great project for IMFD, if you're inclined to use it.

1. (As Stated Before) setup a parking orbit ~30km below ISS with ECC ~0.0.
2. Use IMFD to plan a transfer. You can fidgit with the begin / encounter times and see what your Delta-V will be.
2a. You should be able to setup a perfect Hohmann Transfer Orbit this way.