When doing a docking mission, do your time your launch to achieve a rendezvous, or edit your velocity while already in orbit to achieve it?

[coyotesfrontier]

Title, I usually use the latter method, as while it's unrealistic and wasteful of delta-v, it's much easier to do then timing your launch.

Edited September 2, 2019 by coyotesfrontier

[IncongruousGoat]

9 minutes ago, coyotesfrontier said:

...as while it's unrealistic and wasteful of delta-v...

Not true at all. For any rendezvous that's ever been conducted in the history of human spaceflight, but let's take a flight to the ISS from Cape Canaveral as an example. Now, the Cape is at 28 degrees north, but the orbit of the ISS is inclined 51.5 degrees relative to the equator to allow launches from Baikonur to rendezvous with it. This means that the optimal time, from a delta-V perspective, to launch to the ISS from the Cape is when the Cape is passing under the ISS's orbit, in order to avoid needing an extremely expensive plane-change burn. This in turn means that you can't time your launch around where the ISS is in its orbit, which means that you've got to slowly match its orbit over the course of several hours (or over a day in the case of Progress/Soyuz).

EDIT: But, to answer your question, I usually take the slow approach, because odds are I'm launching to something inclined where I can't launch directly to rendezvous.

Edited September 2, 2019 by IncongruousGoat

[vossiewulf]

I time my launch so in maybe half an orbit I'll be in the correct phasing position for a Hohmann transfer. So a bit ahead of the correct phasing point if your transfer will be to a lower orbit, and slightly behind the correct phasing point if you're transferring to a higher orbit.

If nothing else you need half an orbit or so to match planes. If you're launching manually you will always need at least a small plane-match burn.

[Superfluous J]

I just launch. Once you're in space (in aligned orbits) the rest is laughably cheap.

[KerikBalm]

When you say "edit", do you just mean "change" with rocket thrust? or do you go into your save file and edit the numbers there?

I normally try to time my launches to get in the ballpark, just so that things go faster than warping through multiple orbits.

However, in most cases the other way is not that expensive as long as no plane change is involved.

[Curveball Anders]

I almost always use a standard set of lifters that put my payload into 75km orbit.

Regardless of where it's supposed to go after that.

 

[adsii1970]

Just as @5thHorseman and @Curveball Anders have said, I just launch.

I see every mission which requires docking as a three-stage mission:

1) Achieve orbit and similar to Curveball Anders, every launch goes to a 78Km orbit.

2) Rendezvous with the targeted vessel. I consider this complete when I'm about 250m to 100m from my target vessel.

3) Actual docking. I have a mod installed that gives me a nice metallic thud and the sound of mag locks sealing. Nothing beats hearing that sound after a successful capture.

 

[Guest]

I launch when the target is almost directly overhead so that I will always be trailing it, and park in a lower orbit to catch up for a transfer. This is reasonably expedient without being number-crunchy.

My lifters are rated for 125km and 250km orbits for timewarp purposes, and upper stages generally don't go higher than this. Spacecraft detach and continue upward once they get a transfer. All space below the rather high 300km standard station orbit is kesslerised for fun; ascending through a thick ring of debris with Distant Object Enhancement is quite a sight and safer than it looks, since paint chips don't get everywhere in KSP.

Edited September 2, 2019 by Guest

[magnemoe]

Try to time it, but always launch a bit early so I can put Ap a bit higher on circulation to get an intercept after one orbit. 
On bodies with no atmosphere I launch a bit late and then plot an intercept during circulation or later. 
 

[moogoob]

Launch to a temporary orbit, then a simple hohmann transfer, fine tune and then a velocity match at closest approach, courtesy of Mechjeb's maneuver planner. Sometimes I put too much delta-v in my orbital maneuvering stage, any excess helps top up fuel depots and whatnot. :p

I'm finally getting back into a career save, for the first time in several updates. Just finished setting up my mobile Minmus orbiter station, which is a dry run for a later manned Duna craft. Each station/spacecraft/mothership/thing is four parts so I've been doing nothing but rendezvouses recently.

Now I just have to somehow get another 2500 kilos of liquid fuel there to finally claim that "build a station around Minmus" objective. :p

[natsirt721]

For equatorial launches, timing isn't important, except for reducing travel times. Phasing is pretty cheap if you're willing to wait for it, so I usually try to get close but err on the side of behind. For LKO intercepts like saving kerbals in <80km orbits, you probably have to go high to phase anyway, so timing is less important.

For inclined intercepts, timing is incredibly important. Plane changes are hands down the most expensive maneuver, so if you can inject close to (preferably directly into) the target's orbital plane, you can save a ton of propellant.

[Piper]

The vast majority of my rendezvous and dockings occur with stations/spacecraft in polar orbit.  In which case I wait for my launch site to line up with the orbiting spacecraft, and then launch either north or south (depending on which side of the orbit I launch on), and try and get as close to the inclination of my target as possible.  In most cases, I launch to an altitude below that of my target, so I can play "catch-up" with it.  Once in orbit, I adjust my alignment with the target and get to 0 relative inclination.  After that I do one of two things.  If the position of the rendezvous in orbit is important (ie, I want to make sure it happens on the day side of the planet) I perform a burn so that my orbit intersects the target's orbit where I want it to.  After a few orbits of catching up, I then perform another burn at that intersection, raising my periapsis, so that on my next time around I arrive at that point at the same time my target does.  If it doesn't matter where in the orbit I meet my target, then I simply time my apoapsis raising so that I meet my target at that first intersection.

[Incarnation of Chaos]

I don't think iv'e ever actually conducted docking mission where i launched directly into the encounter; i always go into a parking orbit and adjust from there.

[DStaal]

19 hours ago, Incarnation of Chaos said:

I don't think iv'e ever actually conducted docking mission where i launched directly into the encounter; i always go into a parking orbit and adjust from there.

I’ve never done one intentionally.  I've had a few cases where it was 'oh, the transfer launch window is *now*' at my circilization burn.

[papuchalk]

It is not important to launch at certain time.

More important is to follow the same orbital inclination as target. (and not only for docking, but also when flying to a planet, especially Minmus has quite a remarkably different inclination than eqatorial)

 

So i follow only orbital inclinations

Edited September 11, 2019 by papuchalk

[XLjedi]

Yeah, I pretty much eyeball it like @Lu K.

...and don't often do a very good job.  So I voted "Time Launch" but it's very crude at best.

Edited September 11, 2019 by XLjedi

[Snark]

4 hours ago, Lu K. said:

for those LKO rescue missions i usually just wait until the target is in a specific spot above the desert then launch and try to get a fairly close intercept before the circularisation burn. i'm impatient and prefer to eyeball things than do maneuver nodes.

This is pretty much me. LKO rescues are always at approximately the same height, give or take a kilometer or two, which means it's pretty easy to eyeball:

1. I launch my rescue ship to the pad, shift to map view, rotate the camera so that it's looking straight down at the south pole (so that orbits go from left to right) and zoom in a bit.
2. When the target gets at the "right spot" (about five degrees or so above KSC's western horizon), I launch and do my usual gravity turn on ascent.
3. As soon as my Ap gets high enough for the "target closest approach" markers to appear on the map, that tells me how well I estimated my launch timing.  Did I hit it spot on, or am I slightly leading or trailing the target?
4. If I'm leading the target, pitch slightly above as I continue to burn.  If I'm trailing the target, pitch slightly below and continue to burn.  (Yes, this means a bit of cosine loss, but it really is only slightly off prograde, and it's a fairly minor amount of dV, so this is fine.)
5. Wait for the navball to auto-switch to target-relative mode (i.e. when the target gets within 60 km or so of me), and then just complete the rest of the rendezvous in the usual way by watching the relative positions of the and markers and burning appropriately.

Works like a charm.  The reason it's easy is that I always launch when the target is in the same spot over the western horizon, which makes it easy to eyeball.  The reason why it's always the same spot is that, 1. the rescuees are always in pretty much the same orbit, and 2. I always design all of my rockets to have exactly the same launchpad TWR, which means my ascent profiles are pretty consistent.

This lets me retrieve kerbals without even completing an orbit.  I launch, directly rendezvous, EVA-transfer the kerbal, retro-burn, and re-enter.  End up landing without even getting halfway around Kerbin from KSC (unless I deliberately delay my reentry burn so as to try to land the kerbal right next to KSC, as a pointless-but-fun bit of target practice, which I sometimes do.)

[Dan Kerman]

I time the launch to be close...  lower altitude and behind.. get into a stable orbit, adjust inclination if needed, then burn to rendezvous.

[RizzoTheRat]

I'll time my launch to hit the same plane as the target (except when launching from the mun to a station on an inclined plane as that would take ages), but usually launch to a lower orbit and play catch up.

[Espatie]

I time my launches. For my usual LKO rescuer rocket, I launch at the point when the target craft switches to comms direct to KSC (the green line) - which is about halfway across the ocean west of KAfrica.

 

The closest one I've even done I never even went orbital. Just launch, intercept of 300m, bail the rescuee out, dive across the intervening space, get into the capsule just as the altimeter ticks to 69,999. Landed in the ocean just west of KIndia.

 

The real trick is knowing how to time and angle your KSC launch to get the right inclination of orbit for a Mun rescue.