Tips for launching to intercept?

[goldenpsp]

So in many of my playthroughs I utilize an station orbiting Kerbin for various reasons.  One frustration I've had is that I have never been able to figure out a consistent way of timing a launch so that I get a decent intercept with the station in orbit without having to do tons of adjustments and  orbits of the planet.

Does anyone have any tips or mod suggestions to make get this more consistent?

 

[Gargamel]

1 hour ago, goldenpsp said:

mod suggestions to make get this more consistent?

Well.....   MechJeb can automate this for you.     Yes yes, MJ is cheating they say.   Who cares?   Once you've launched your 5th resupply mission to your LKO station, you really don't feel like doing it again.   With a bit of practice (not much really), you can usually get to LKO within 10km of a target from launch. 

 

But to do it manually, and I haven't done this in a long time, and only of couple times, I've waited till the target was directly overhead, and launched.  Then when I reached orbit, I measured the differences in location around the orbit.   Then I reverted, and this time waited until the target was the same distance behind the KSC as it was ahead after launch the first time.  Then you have to fly the exact same flight profile, and you should end up close enough to do a manual intercept, rather than an orbital maneuver.    Once you get good enough at flying the same profile for every launch, you can keep a little log book for the approximate angle you need to launch at to intercept for a given altitude. 

Edited October 8, 2018 by Gargamel
typos typos

[goldenpsp]

Ah I will have to look more closely at mechjeb. I already use it but it has so many features I must have missed it.  I don't consider it cheating, as you mentioned after doing tons of supply launches etc (I play usually with the various colonization mods so I spend a ton of time launching and building large ships in orbit).  I use Mechjeb to intercept while in orbit, but I didn't realize it had a feature to time the actually launch to intercept.

Otherwise the manual tips are very helpful.  Thanks.

[Gargamel]

1 hour ago, goldenpsp said:

  I use Mechjeb to intercept while in orbit, but I didn't realize it had a feature to time the actually launch to intercept.

While on the pad, select your target.   If the target is in an equitorial orbit, all is good, and select "launch to rendezvous".   It will give you some angle in the box.   Don't trust it right off the bat.  Launch your craft, and let MJ circularize.   If it's a good intercept, remember the value MJ provided.   If not, revert and try again, MJ should put the right value in the box this time.  This angle will be good for pretty much any craft, flying the identical flight profile, for that altitude.   But each time you launch, remember that angle value, you might have to fudge it slightly for different vessels.   Keep a little log book (scrap of paper works well for this) of the various altitudes and angles.   Then you don't have to do the trial and error as much. 

If it's not an equitorial orbit, Also select the "launch into plane of target" option.  Alone, this option will wait for the KSC to be right under the targets orbital plane before launching (Works wonders for Minmus missions).  Adding in the launch to rendezvous option may cause you to wait a very long time for both the orbital plane and target position to line up, if ever.  So if it's not an equitorial orbit, I usually just launch into the plane at a different altitude, and do a Hohmann intercept from there. 

Edited October 8, 2018 by Gargamel

[Kryxal]

If you miss your rendezvous, it's better to be too early than too late.  If you're going to be early, you can just raise your apoapsis and catch the station on the way back down, if it's not too far off.

[Plusck]

I've developed a habit of taking every single LKO rescue contract, and trying very hard to minimise cost and time to get them done.

The rescue orbits are probably a bit lower than where your station is, but it won't change much. If anything it's easier to correct if you wait too late to start, since you can use a lower orbit to catch up. Still, the method is:

• on the launchpad, select the station as target and right click on it so that you can see the "distance" to you clearly.
• warp ahead until the target is between 250 and 280km from KSC.
  • If your target in a higher orbit, add a bit (so for a 150km orbit, I'd add about 50km to those numbers).
  • Seen from above, the target should be a bit more than halfway across the sea separating KSC's landmass from the desert landmass.
  • If your rocket is a bit underpowered in the latter stages (so the final burn to get to orbital speed takes a long time) you should start a little earlier. If it's overpowered you can wait until the target is 220km away or so.
• go ahead and start the launch using a normal efficient gravity turn. However, your need to make the turn a touch gentler than the absolute most efficient one. What you want is some coasting time to finesse the intercept, so that you can nail it exactly with what is going to be your circularising + target orbit matching burn.
  • at the start of the launch, whenever you have enough time to switch to map view, do so and try to make sure AN/DN to target is minimal, and one of either AN or DN is near your Ap. It costs less the earlier (and slower) you do this.
• beware that at some point, the navball will probably switch to target mode. You do NOT want to be on SAS prograde follow when that happens! So, by about 30km altitude you ideally want to be able to go to stability assist. Keep burning and staging until your Ap is at the altitude of your target.
• At this stage (about 40km up) you want to go to map view and stay there if you can. If you need to stage, switch back only briefly to press the spacebar then return to map.
  • switch the NavBall to target mode;
  • ideally, your intercept chevron will show your target to be at or slightly behind you when you cross orbits. If slightly behind, raise your Ap a touch more to get the double sets of chevrons and get the second set better aligned.
  • if you're behind the target at Ap, you need to sort this quickly. Aim slightly below the horizon and burn until they close up.
• Time to "push the marble". Right-click on the target chevron to keep the intercept distance visible. In NavBall target mode, you need to push the retrograde marker over the anti-target marker: i.e. you point your rocket on the other side of the retrograde marker and burn. The marker should move away from where you're pointing. Watch as the intercept distance falls to 0.0km (or thereabouts).
  • Remember that gravity will be changing your trajectory more than the target (since you're still suborbital at this stage). Therefore the retrograde marker will not be exactly aligned vertically (radial in/out) when you're far away even if the intercept is perfect. Therefore, start by making sure it's aligned with anti-target on the normal/antinormal axis first, then watch the chevrons and keep pushing on the radial in/out axis only if the intercept distance decreases.
• And finally, the scary bit. You have MechJeb and/or KER, so you can get (and absolutely need to know) your ship's acceleration. Then check the velocity difference at the intercept chevron. Divide that velocity by your acceleration. Then divide by two. The result is the absolute minimum time to intercept that you need to burn.
  • aim squarely at the retrograde-to-target marker (which should now be right on top of the anti-target marker).
  • start burning a few seconds before that "velocity/accleration/2" time you worked out earlier.
  • keep an eye on the intercept chevron. Time to intercept should be counting down very slowly. If it starts increasing, throttle down.
  • keep "pushing the marble" if you can, to bring the intercept distance down even more.
  • if you did all that right, when you get down to 0m/s speed to target, your target should be about 100m or so away....

If you can't see your ship's acceleration, you just need to guess when to start that final burn. If time-to-intercept increases when you burn, just stop and wait for a while longer. If it keeps counting down quickly as you burn, aim away from the anti-target marker because you're too late and about to crash - hard - into your station.

Edited October 8, 2018 by Plusck

[Laie]

14 hours ago, goldenpsp said:

without having to do tons of adjustments and  orbits of the planet.

@Gargamel has covered the basics, there's nothing I can add to the technique as such. However, I wonder what you expect, and if your expectations can ever be met.

Typically, Kerbin stuff is in an equatorial orbit and you launch to an equatorial orbit, so the relative inclination is under 1° to begin with. From there, you need two maneuvers: "Hohmann transfer to target" and "match velocities with target". Doing it with fewer maneuvers calls for an ascent-to-rendezvous which, for all practical purposes, requires 100% automation.

MJ can do ascent-to-rendezvous, but usually this requires two launches: one for measurement, the next one works. Subsequent launches will only work If you re-use the LV (with same fairing size and 100% identical payload mass). Any change to the vessel will require a new test launch.

 

[Gargamel]

8 hours ago, Laie said:

ubsequent launches will only work If you re-use the LV (with same fairing size and 100% identical payload mass). Any change to the vessel will require a new test launch.

Actually, surprisingly not true.    If the launcher is completely different, not even remotely similar, then yes, you'll probably need to do a test launch.   But if the launcher could be considered a distant cousin while squinting and drunk, then there's a good chance the original data will still apply.   If your new launcher can fly the same launch profile and hit the same speed v altitude curves, then it will follow the same path into orbit.   This is a lot easier done than said, to be honest.   If you over build your ship a touch, and use the throttle limiters to limit your acceleration, then most any ship that also hits those throttle limiters will also fly the same path.    Now it won't be exact, but it will be close enough to not need an orbital maneuver to finish the docking.   

Once you get a baseline angle for launch, you can keep a note on your napkin log book on which launchers will get an offset from the base angle.  It will only be a few tenths of a degree for the most part, so even if you forget to adjust your angles from the baseline for that altitude, you'll still be close.

 

Point is, Launch angles are determined by the flight path taken, not the vessel flying it. 

Edited October 8, 2018 by Gargamel

[stelarfox]

Also it kind of depends where your station is, an you do not want to place it exactly at 70k, because any disturbance or even a dock may push it down.

I am rather good in doing recovery missions (specially near kerbin), and found out a way to do it more or less good. Even so you need a few things.

1) Be able to do a lunch to orbit without with the same rocket doing exactly the same path:

in other words if you want to be able to time it, you need to actually be abel to do it in repeated ways and match it. if not you cannot time it.

2) as somoene say it before, its better to be early than late (why, because if you are earlier is easier to wait), 

3) also depends on how much time you want to wait for the randevous, waiting 4 hours i manage to do it with very small dv's from the original ship (and it saves a lot that way).

4) you can do it even mathematically once you are in orbit, if you want an explanation on how to do that please pm me.

although if you are looking for refueling you want to do the 1 and not the point 3. (and perhaps the type 4 to use as less fuel as possible).

something i made a while ago for refueling purposes, was a terrible small ship with almost nothing but 2 hooks, really small fuel and the best engine to use at least fuel as possible. and of course unmanned.

with that i sent such ship to take the fuel from any ship in orbit (even partially depletted debris with fuel left), then, docked and moved it to the space station (instead moving all the space station), after draining the fuel depot made a sling shot (rotating the station really fast and opening the hook to gain speed) and of course later remove the empty fuel depot from debris. (you can easily gain 2 to 20 depending on your station). is not much but a bigger orbit means less fuel to do maneuvers.

but point one is the most important of everything. and a good ship to make it happen too.

[phantom000]

I have a similar problem with my new asteroid. Its in a stable orbit but I can't seem to reach it because i just don't have enough Delta-V. I tried adjusting my orbit during take off but I can't get it right. Once i had the AN/DN down to about 25 but then it shot up to over a hundred.

[Gargamel]

2 hours ago, phantom000 said:

Its in a stable orbit but I can't seem to reach it because i just don't have enough Delta-V.

Well, there's a couple solutions to this:

1)  Build a bigger ship with more dV. 

2) Launch to a lower orbit, and Hohmann transfer up to the asteroid.    (You know why I prefer asteroids to comets? Cause they're a little meteor!).

What altitude is your asteroid at.   There is a point, and a complete guess tells me it's somewhere around 250km-300km altitude, where a direct launch to intercept is less efficient than a LKO launch and Hohmann transfer.    So if your asteroid is in a higher orbit, you probably want to launch to LKO.  At worst, a one way trip should be less than 4500 dV all said, and that's if the asteroid is somewhere around minmus. 

[lunardog15]

gosh darn it

that pun was bad

[Tyko]

On 10/7/2018 at 4:15 PM, goldenpsp said:

So in many of my playthroughs I utilize an station orbiting Kerbin for various reasons.  One frustration I've had is that I have never been able to figure out a consistent way of timing a launch so that I get a decent intercept with the station in orbit without having to do tons of adjustments and  orbits of the planet.

Does anyone have any tips or mod suggestions to make get this more consistent?

 

It's pretty easy once you get the hang of it and doesn't require any autopilot. The only mod I'd suggest would be one of the maneuver node mods that give you better control - Precise Maneuver or Precise Node.

Here's how I do it:

1. Make sure your station is well above minimum orbital altitude. For this example let's use 175km as a good altitude for stations.
2. from Map mode time warp until your station a bit past the KSC. You want to launch so that you end up about 1/8 orbit behind the station when you circularize. You may have to try a few times to get the timing right, but once you've figured it out you can do it over and over.
   1. If you come up behind the station you might have to wait an orbit or two.
   2. If you come up AHEAD of the station that's when you might have to wait days.
3. Launch into a lower orbit. For our example, launch into a 125km orbit. 
4. Circularize at 125km
5. Set the station as Target
6. Drop a manuever node in front of your ship.
7. Raise the AP to the altitude of your station
8. Drag the maneuver node forward around your orbit until you get an intercept. 
   1. Depending on how far behind the station you may have to drag it around a full orbit or more. That's okay, the game knows you're an orbit ahead even if you drag past your ship's current position
9. Fine tune your maneuver to get as close as possible.
10. Execute the burn when you hit your maneuver node.

Hope this helps. I do it all the time. If you have any questions let me know.

EDIT: Keep in mind that NASA/Roscosmos with all their computers and flight engineers don't attempt to launch to rendezvous. Even the fastest flight plan involves the craft orbiting the earth several times while lining up a rendezvous.

Edited October 30, 2018 by Tyko

[Kryxal]

33 minutes ago, Tyko said:

EDIT: Keep in mind that NASA/Roscosmos with all their computers and flight engineers don't attempt to launch to rendezvous. Even the fastest flight plan involves the craft orbiting the earth several times while lining up a rendezvous.

Then again, NASA tends to be launching into an inclined orbit, don't they?  If you're doing that, lining up the orbital plane as best you can matters more than a direct intercept...

[Dafni]

12 minutes ago, Kryxal said:

Then again, NASA tends to be launching into an inclined orbit, don't they?  If you're doing that, lining up the orbital plane as best you can matters more than a direct intercept...

That is true too. Hence the short windows to launch to ISS for example, recurring 24hrs later if they miss one as far as I understand.

[Tyko]

1 hour ago, Kryxal said:

2 hours ago, Tyko said:

EDIT: Keep in mind that NASA/Roscosmos with all their computers and flight engineers don't attempt to launch to rendezvous. Even the fastest flight plan involves the craft orbiting the earth several times while lining up a rendezvous.

Then again, NASA tends to be launching into an inclined orbit, don't they?  If you're doing that, lining up the orbital plane as best you can matters more than a direct intercept...

Inclination changes cost a lot of DeltaV. The shuttle could only perform a few degrees of inclination change with all of its on-board fuel. Essentially they had to get the inclination right at launch. All of the orbits were to give them time to precisely rendezvous. The shuttle entered a number of lower phasing orbits, each one getting it closer to the station's orbit. 

Gemini attempted direct rendezvous a number of times. Gemini 11 actually pulled it off.

Here's some great info from this Rendezvous and Docking guide

Going through a phase

When you launch, the odds are slim that your target vehicle, such as the ISS, will be in exactly the correct spot in its orbit for you to go right up and dock with it. Most likely, it will be too far ahead or behind. Thus, you could launch into the correct orbital plane and directly to the correct altitude, and discover your target is on the other side of the earth.

Mission planners could postpone the mission until a day when the target is in the perfect spot at the exact launch time, but a much better solution is to allow the chaser spacecraft to phase, or catch up to, the target spacecraft after launch.

A basic tenent of orbital mechanics dictates that the higher the orbit, the slower the orbital speed, whether it’s a satellite around the earth or a planet around the sun. It’s why a martian year is longer than an earth year. Thus, the target vehicle will be moving slower than a spacecraft at a lower altitude. The greater the difference in altitude, the greater the difference in speed. The lower, faster spacecraft thus catches up to the higher, slower one at a rate that depends on the heights, or energy, of their respective orbits.

It’s often not practical to raise the target vehicle to a higher altitude, nor can a chaser spacecraft safely orbit lower than about 100 nautical miles (185 km) altitude because of our atmosphere. Therefore, the chaser altitude range of 100 nm up to the target’s altitude defines the range of “catch-up rates” for chasing and rendezvousing with the target.

(Note: You could launch the chaser into a higher, slower orbit and let the target vehicle catch up to it and then rendezvous from above, but this is a waste of energy and therefore, of propellant, payload, and money.)

So when you get into orbit, the target will be some fraction of an orbit ahead of you, possibly close to 360°. This distance is called the phase angle. The lower the initial, or phasing, orbit, the faster the catch-up rate and the less time it takes to complete the phasing.

Now we add in the human element.

In the “good old days” of Gemini, the spacecraft was so simple that the crew could transition from launch to rendezvous almost immediately upon reaching orbit, and the rendezvous could be completed very quickly. (Gemini 11 achieved rendezvous on the first orbit!)

 

Edited October 30, 2018 by Tyko

[Gargamel]

7 hours ago, Kryxal said:

Then again, NASA tends to be launching into an inclined orbit, don't they?  If you're doing that, lining up the orbital plane as best you can matters more than a direct intercept...

Yeah, I setup a station on a 35' inclination this play through, just for fun I thought.   After the third construction mission was launched, I just left the tug attached and deorbited the whole dang thing.  Too much a PITA. 

[MaverickSawyer]

A little late to the show, but hey.

I almost always run my stations in slightly higher orbits than my typical insertion orbit (125km for the station, 100km for insertion.). I wait until the target is 45-60* above the west horizon, then launch. Get to the parking orbit, wait a few minutes until the proper time for the Hohmann transfer, then burn to intercept. Correct for any inclination errors and drift during the first burn about 6-8 minutes later, then match velocities at closest approach. Usually I only need ~60 m/s to pull this all off, and I'm happily docked to the station in ~30 minutes or so. I have actually pulled as short as 20ish minutes, but that was rare.