Kerbal Space Program How-to Series: 5 – Around the Mun

(Current for KSP version 0.25)

In this chapter, you will learn:

  • To build a vessel that is capable of orbiting the Mun, doing some science experiments there, and returning.
  • Some principles for medium-to-heavy-weight rocket design
  • How to use struts to stabilize your vessel
  • Plan a trajectory across spheres of influence

Recommended mods:

First, if you’re in Career mode, go to Mission Control and accept the “Explore the Mun” mission. If you have it, also accept the “Transmit or recover science data from space around the Mun” mission.

It’s time to spend some of your hard-earned science points. First research Stability. This should complete column 3 of the tech tree. In column 4, research Advanced Rocketry. If you have enough points, also research General Construction.

For this goal, you will need a rocket that’s bigger and better than anything before. It’s going to be a little more complicated too; if you continue stacking solid fuel boosters, you’ll soon find that the craft would become too heavy to even get off the ground. The problem is that you’re not producing enough thrust to counteract Kerbin’s gravity; if you have VOID, you may see a statistic called “T/W Ratio” or “T:W”. This stands for thrust-to-weight ratio, which is basically how much thrust your rocket will produce versus Kerbin’s gravity. If it’s close to 1, your rocket will visibly struggle to get off the ground. If it’s at or below 1, your rocket won’t move at all, or only after it has burned and wasted some of its fuel. Once you’re in space, T:W becomes less important (because you are no longer directly fighting gravity), except as an estimate as to how quickly you can perform your maneuvers.

What you’d need to do at that point is to build outward, so that more than one engine can fire at once. However, you’ll soon run into more problems; another downside of solid fuel is that it is heavy. This makes keeping the craft steady and making the gravity turn more difficult, and exponentially increases the amount of engines and fuel required toward the bottom. This isn’t a problem for the lighter craft previously featured, but here, you would be better served by building a liquid fuel core and engine to lift you to orbital height. Solid fuel boosters are still dirt cheap though, and are thus best used for the first lift stages (to get you to ~10,000m).

So brace yourself; here comes the design specs:

ksp-munar-orbiter ksp-munar-orbiter-b

The top consists of a Command Pod Mk1 with a Mk16 Parachute on its top and a TR-18A Stack Decoupler on its bottom, as usual. The parachute and decoupler should share stage 0. Below the decoupler is an FL-T400 Fuel Tank and an LV-909 Liquid Fuel Engine. Around the fuel tank, turn on Angle Snap (the button is near the bottom left corner, next to the symmetry button), and place three SC-9001 Science Jr.s symmetrically, and then three Mystery Goo Containment Units on their top. Another stack decoupler goes below the LV-909, and these should share stage 1.

Now below it, stack two FL-T800 Fuel Tanks, then an LV-T30 Liquid Fuel Engine. It shares stage 2 with two TT-38K Radial Decouplers, placed symmetrically around the top fuel tank.

On the radial decouplers, attach two RT-10 Solid Fuel Boosters, and thrust limit them to about 75%. Now adjust the position of both the decouplers and the boosters, so that one of the boosters sits straight below one of the Science Jr.s. This is where Angle Snap comes in handy. This is important to maintain bilateral (mirrored) symmetry of the craft as viewed from the top; without it, asymmetric drag will cause the craft to roll uncontrollably like a screw when launched. It is not a fatal error, but it would make aiming your gravity turn difficult.

Pair the RT-10s with two TR-18A Stack Decouplers below them, as stage 3. Then stick two Rockomax BACC Solid Fuel Boosters below the decouplers. These should be stage 4.

This introduces a small problem. Note that the boosters are quite long, and only attached near the top. If you launch the rocket as-is, the boosters will start swinging like pendulums… that weigh 11 tons and will likely ram right into your liquid fuel core!

Thus, you need some form of connection near the bottom, to hold them steady. In the Structural tab, find the EAS-4 Strut Connector. Ensure that symmetry is set to 2x, then click once on the BACC, and once on the bottom FL-T800, to connect them with a strut. Do the same for the other one.

Rotate your view to the other side; check to ensure that there are two more placed automatically (by the symmetry option) on the other side. You are now done! Check your staging once more, and name the craft and save it. I call it the “Munar Orbiter“. You can download the vessel file here.

(If you couldn’t research General Construction, you won’t have the struts; instead, replace the paired stack decouplers with two radial decouplers on the bottom FL-T800, and attach the BACCs to those instead. It would be slightly more expensive, but it will work.)

Go back to the Space Center and save the game. Hit the launch button. You know the drill, wait a few seconds, then hit T for SAS, then Z for full throttle, then Spacebar to fire. Turn toward the east at 10,000m, and launch into an 80,000m equatorial orbit.

Now you want to plan a transfer orbit to the Mun. To do this, first locate the Mun in the map view, right click it, and Set as Target. Secondly, click at the point of your orbit on the opposite side of the Mun, and drag the ksp-prograde prograde axis until your projected apoapsis is slightly beyond the Mun’s orbital height.

You might see intercept markers, but they would be pretty far apart. That’s because the Mun is still orbiting Kerbin; by the time you get there, it would have moved from its current position. As such, drag the center of the maneuver node to move it along your orbit. Drag it counter-clockwise until your orbit breaks, and you see lines indicating your projected path into the Mun’s sphere of influence (SOI). This is where the Mun’s gravity dominates over Kerbin’s gravity, and is where it is possible to orbit the Mun instead of Kerbin. (Though the sphere of influence has a real world counterpart, KSP’s orbital model is somewhat simplified, in that it only considers gravity from one object at a time, using what is known as the “patched conic approximation”.)

Carefully position the maneuver node so that you will enter the Mun from the right (counterclockwise) side, with a periapsis between 12,000m and 55,000m. It can be helpful to rightclick on the Mun and click “Focus View”, to better view your eventual path around it. (If you have Precise Node, you can adjust the position of your node by adjusting the “UT” [Universal Time] value.)

Perform the maneuver. As you get close, hit X to kill your engines. Then, instead of viewing the maneuver node marker, focus view on the Mun instead, and burn ksp-prograde prograde slowly (with Shift/Ctrl) to get your periapsis to the desired height. If it’s too low, turn around and burn ksp-retrograde retrograde a little. Then delete the maneuver node.

If you exit map view now, you’d notice that you are quite visibly moving from Kerbin toward the Mun. Don’t forget that you’re here for science; in fact, an opportunity is coming up shortly. Once you’re above 250,000m, you will be considered “in space high above Kerbin”, and should thus open one of the Science Jrs and the Mystery Goo Container on top of it. Don’t forget to do a Crew Report, and an EVA Report. Also remember to Take Data from your pod, so that you can make more Crew Reports later.

It will be several hours before you enter the Mun’s SOI, so you’d probably want to use time warp. However, be careful; time-warping past SOI changes can result in inaccuracies, potentially giving you a rather different orbital path than is projected. Kerbal Alarm Clock has an option to set a timer to just before the SOI change; for best results, set a timer for 10 minutes before, then set one for 1 minute before, then wait for the SOI change either at normal speed or at 5x (two arrow) speed.

When you enter the Mun’s SOI, your navball will suddenly change, as it switches reference frames from Kerbin’s to the Mun’s. Your orbital path might move slightly, so check the periapsis again. if it’s too low, make a small ksp-radialradial burn to raise it, or if it is too high, make a small ksp-antiradialantiradial burn to lower it.

You will always enter Mun’s SOI on an escape trajectory. To put yourself in a stable orbit, create a maneuver node at the periapsis. Then drag the ksp-retrograde retrograde axis to shrink your orbit. As usual, aim for a circular orbit.

Don’t time warp to it just yet! There is still more science to do. As long as you’re above 60,000m, you are considered “in space high above the Mun”, and can thus open the second Science Jr and Mystery Goo Container, and perform another Crew Report and EVA Report.

Then perform the circularization maneuver. You are now “in space near the Mun”, thus you can now use the third and final set of science equipment, and yet another crew report and EVA report. During your EVA, collect all the science from all the equipment and store it back in the pod. You should now have 12 stored experiments in the pod.

After a couple of victory laps around the Mun if you so wish (you may be able to wring out some more science points by EVA reporting above different biomes), it is time to return home. You could just exit the Mun’s SOI, then try to lower the periapsis to return, but it is more efficient to plan a direct return path. To do this, locate the Mun’s orbit. The Mun orbits Kerbin counterclockwise (all planets and moons in KSP orbit counterclockwise). Orient your view so that the direction in which the Mun is going (the Mun’s “prograde”) is in the 12-o’clock position. Then create a node slightly in front of it, at the 11-o’-clock position. Pull the ksp-prograde prograde axis until you get an escape trajectory. Continue pulling until your Kerbin periapsis falls to about 30,000m. If you do this right, you should have a Kerbin apoapsis slightly under the Mun’s orbit, and your escape path should be roughly parallel to the Mun’s orbit, but going backward. Essentially, you are speeding up relative to the Mun, and slowing down relative to Kerbin, with the same burn, thus saving fuel and time. The effect is that the Mun will “leave you behind” in its orbit, letting Kerbin’s gravity pull you back toward it.

ksp-mun-return
Return trajectory from the Mun (click to zoom)

Quicksave the game with F5, then perform the maneuver. Again, avoid warping too quickly past the SOI change. (The map view may sometimes glitch out near the SOI change; just wait it out, it will soon switch normally) When warping in Kerbin’s SOI, pay attention to the altimeter, and be careful not to warp too fast when you’re due to reenter Kerbin’s atmosphere (~70,000m), or you might just zoom right past it.

As usual, open parachutes at 10,000m; if you are in a new biome, you can do a crew report and EVA report once after the parachutes fully deploy, and once on the ground. Don’t forget to take a surface sample too. Then recover the vessel. Note that the further you get from Kerbin, the more science you receive! This will give us quite a bit of freedom for future rocket designs.

Tip: turning off SAS after you partially deploy your parachutes will let your pod naturally hang off it, saving electricity. If the ground has a significant slope, turn it on again near the ground and adjust your heading just before touching down.

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