Kerbal Space Program How-to Series: 4 – A Cask of Brandy (Orbital Rescue)

(Current for KSP version 0.90)

Edit 2015-01-06: Changed the vessel design to be light enough for VAB level 1, and now uses the OKTO for SAS functionality

This chapter is written for Career mode, though it may be useful if you’ve accidentally stranded a ship in orbit in Science and Sandbox and wish to rescue the Kerbal.

In this chapter you will learn:

  • To build a rescue vessel
  • To change your orbital plane
  • To perform an orbital rendezvous

Recommended mods (makes some parts of this task a bit easier, but quite doable without):

Techs this chapter assumes you have researched: Basic Rocketry, Survivability, Science Tech, Flight Control, Advanced Flight Control.

I also recommend that you have upgraded both your Mission Control and Tracking Station to level 2.

I also recommend that you complete the in-game “Orbiting 101” tutorial from the Training menu. Practicing “EVA in Kerbin Orbit” from the Scenario menu is also helpful.

If you’re playing Career mode, you will soon encounter contracts that look like “Rescue Jedmy Kerman from Kerbin”. Accept one, and go to the Tracking Center to find him. There is a Kerbal with a spacesuit stranded somewhere in orbit, and your goal is to get him in a vessel and return him to Kerbin safely. He will always be in a low, nearly-equatorial, prograde (anti-clockwise when viewed from north) Kerbin orbit.

You will need a vessel with at least one empty seat for this. You could send two command pods up, but a Prodobodyne OKTO is much lighter than the Command Pod Mk1 (check the “mass” statistic when you hover over them in the VAB).

Place a Probodobodyne OKTO on top of a Command Pod Mk1. (The Stayputnik Mk1 is usable but not recommended as you will lack SAS capability.) Put three Z-100 Rechargeable Battery Packs around it, then place an Mk16 Parachute on top of it. The parachute shares a stage with a TR-18A Stack Decoupler under it.

Stage 1 is an FL-T400 Fuel Tank powering an LV-909 Liquid Fuel Engine. It shares a stage with a decoupler under it.

Below it are three layers of RT-10 Solid Fuel Boosters and TR-18A Stack Decouplers, staged appropriately. The top is thrust-limited to 53%, and the middle is thrust-limited to 77%.

Your craft should look like this:

 ksp-st-bernard-0-90

Or you can download the vessel file. The cost is 5,887 funds.

Save the vessel (I call it the Kerbin Orbit Rescue “St. Bernard”). Instead of launching, click new, put a pod on the pad, then “launch” that. Hit M to go to your map view.

Right-drag the view from the Tracking Center until you’re viewing downward from the North Pole, and your pod is at the 12-o’-clock position. Wait (using Time Warp if necessary) for the rescuee to reach the 1:00~1:30 position (30~45° clockwise). This ensures that after you launch, you’re slightly in front of the target. Also, take note of the target’s approximate altitude (check his apoapsis and periapsis). Once he’s in position, recover your pod.

Before you launch, save your game, and importantly, go to the VAB, switch to the Crew tab and make sure to remove any crew (looking at you Jebediah) from the command pod. (Alternatively, you can launch from the Launch Pad, which also allows you to check/edit your crew roster before launching). We need to keep that seat free for our rescuee. Then hit the launch button.

Wait a few seconds for loading, then hit T for SAS, then hit Z for full throttle, then Spacebar for launch.

Launch eastward into an equatorial orbit, just like you did in “Halfway to Anywhere”. Try to reach the same orbital altitude as the target. (If you have VOID, you may note that you can see your apoapsis height and time in your HUD, thus enabling you to launch into orbit without using the Map View.)

Locate the stranded Kerbal, click on him, and click “Set as Target”.

Now you’ll have to perform what is known as an “orbital rendezvous”. This means you will have to match orbits and positions as closely as possible to your target. Sounds difficult? Not as long as you know what to do.

Well, first, space is three-dimensional. As such, if you go to the map view and rotate around a bit, you may notice that your orbits have a small angle between them. This will not do; in space, even this slight difference could translate to dozens of kilometers; so you’d need to correct that. How?

Well, note that when you set your target, new flags appear on your orbits, “AN” and “DN”. These stand for “ascending node” and “descending node“. These are the points on your orbit that lie on the imaginary line, the “line of nodes”, where the planes of your orbits intersect. The degree measure shown when you hover over them is the angular difference between the planes. You want this to be as close to zero as possible.

To do this, you will have to perform what is known as a “plane shift”. Click on the point of your orbit with one of the nodes (pick the one that’s next in your orbit), then click Add Maneuver. Now pull or push the ksp-normal normal and ksp-antinormal antinormal axis to adjust your plane. When you get close, the AN/DN flags will tend to “run”. At this point, check them, and continue making fine adjustments to the node (you can use Precise Node, or use the scroll wheel while pointing at the axis symbol). Get this to as close to 0.0° as possible. You may sometimes get “NaN°”; that’s also acceptable.

Point your craft to the ksp-maneuver maneuver vector (if it’s not visible, there will be a blue arrow pointing to it). Then wait till you’re almost at the maneuver node. If you have the Kerbal Alarm Clock mod, click the little alarm clock button on the top right toolbar, and set a timer for it; it defaults to one minute before the node, and can be adjusted. It automatically throttles time warp when it’s about to activate. This is very helpful to avoid accidentally warping past your maneuver node.

You want to do roughly half your burn before the node, and half after, to be as accurate as possible. You can use the “Estimated Burn Time” calculation the game gives you, however it is often inaccurate, and may display “N/A”. You can either press Shift to run your engine at minimum throttle for a few seconds to get an adjustment (then Ctrl to throttle back down to zero), or if you have VOID, its burn time estimate would be much more accurate.

Ensure you’re still aligned to the ksp-maneuver maneuver vector, wait for half your projected burn time before your node (or if you have VOID, you can simply wait for the “half done at node” counter to tick down to zero), then press Z for full throttle. Watch the green bar to the right of your navball deplete as you complete your maneuver, and keep your craft aligned to the maneuver. As soon you’re almost out, or when your maneuver symbol starts to “run”, press X to kill your engines. Then rotate to the maneuver vector, and use Shift and Ctrl to carefully finish your maneuver. You want to be as precise as possible, and get the counter down to 0.1 or 0.0 m/s. Once you’re done, click the checkmark to delete your node.

OK, now you’re in similar orbits, but he’s still some ways behind you. What should you do now?

If you said “slow down”… that’s exactly the wrong answer. You’d get closer at first, but would soon fall victim to the “orbital rendezvous paradox”. You see, slowing down would put you in a lower orbit, which would decrease your orbital period. You would then end up orbiting faster, and speeding away from the target! This is of course ignoring the fact that since you’re in a low orbit, slowing down will likely just deorbit you. Oops.

You do want to be careful though; just pointing to prograde and speeding up would make it difficult to plan your encounter precisely. Instead, make a maneuver node somewhere in front of you (you do want to give yourself a few minutes to make the required adjustments). Pull the ksp-prograde prograde axis some ways until you see the arrows indicating your closest projected encounter approach one another. Soon they’ll point at almost the same spot. Hover over it to see your projected distance.

Now you want to fine tune your maneuver to get your encounter as close as possible. Use either Precise Node or your scroll wheel on both the ksp-prograde prograde/ ksp-retrograde retrograde axis, as well as the ksp-radialradial/ ksp-antiradialantiradial axis, to get as close an encounter as possible. (I do very much recommend using Precise Node, as changes are easier to undo with it, it allows you to edit your node from off-screen, and the scroll wheel is prone to “slipping off” the axis symbol and changing your camera zoom instead.) For best results, alternate between adjusting one axis, then the other, then back to the first one, and so on.

(More experienced players can combine the plane-shift with this adjustment; however, while adjusting these axes it may be necessary to readjust the ksp-normalnormal/ ksp-antinormalantinormal axis to keep the planes aligned)

You want to get your encounter under 1km. If you’re having trouble getting your encounter closer, you can always perform your burn first, then create another node roughly halfway to your intercept point. Rinse and repeat till you’re happy with it. Remember to delete your nodes after you’ve performed them. And remember that you want to be as precise as possible when burning these.

Once you have a good intercept, switch your navball to “Target” mode. Then wait until you’re almost at your intercept point (Kerbal Alarm Clock can set a timer for that too). Press M to exit map view, and locate your target – there should be a small reticle somewhere, and if you hover over it you can see your distance.

Since you set your navball to “Target” mode, your displayed velocity is now relative to your target. You want to zero this out when you’re close to the target. Point your ship to the ksp-retrograde retrograde symbol. Wait until your target is almost at the projected distance, then press Z to fire. As usual, throttle down when you’re almost there. You don’t have to be too precise with this though; leaving 0.1-0.2 m/s is fine here.

You’re now in almost the same orbit as the target. You can switch to the Kerbal as long as you’re less than 2.5 km away, but it is much easier and faster if you bring the ship closer first. Point your ship ksp-target-prograde toward your target (the opposite symbol points ksp-target-retrograde.svg away from the target) . Give yourself a small impulse; not too much, or you’d waste fuel and may miss your target, or worse, not miss him! About 10 m/s is plenty. Turn about to the ksp-retrograde retrograde symbol again, and slow down when you get close. Zero it out when you’re less than 50 meters away.

Then point your ship to the zenith (white dot in the center of the blue half). This is because EVA’d kerbals have a preferred “up” direction, for some strange reason, and have no “roll” controls; aligning your ship to the zenith would make it easier to board.

You can now switch to the Kerbal using the [ or ] keys. Collect an EVA report; if you’re lucky, it’d be considered a new biome, netting you a few more science points. Then press R to activate his EVA pack, and L to activate his headlights. Press V to change your camera mode; change this to Chase mode. Go to the orbital map, and set the rescue vessel as the target. Slowly maneuver your Kerbal toward the pod. Press F when the prompt appears to grab the ladders, then press F again to board.

Hooray! Nothing left to do but quicksave and deorbit. You should know what to do by now. Hopefully you land in a new biome, and thus can do some crew reports and EVAs; once in the air (it’s safest to EVA just after your parachute is fully deployed), and once on the ground (don’t forget to collect a surface sample). Then recover your vessel to complete your contract!

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