Tutorial:Spaceplane to Laythe

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This tutorial will cover spaceplane design, flight, docking maneuvers, inter-planetary transfers, and aerocapture/aerobraking. We're going to take advantage of the efficiency of spaceplanes to take a kerbal to Laythe and back without discarding whole stages of fuel tanks and engines. Everything in this tutorial is re-usable or recoverable (except fuel, that is strictly one-time use only). It assumes familiarity with Orbital Terminology, use of Maneuver nodes, and Basic Maneuvers. This tutorial contains a lot of general information presented during the course of a specific mission to illustrate its use.

Take a Spaceplane to Laythe (and Back)

Specifications

Laythe is the only body other than Kerbin where jet engines will operate, so it is a natural target for spaceplane missions.

  • Length: 6-10 hours
  • Difficulty: Advanced
  • For version: 0.25, 0.90, and above (probably)

This tutorial is designed to maximize fuel efficiency both in the air (by using jet engines and wings) and in space (using a highly efficient rocket). Once we are in space this tutorial can be applied just as easily to a rocket as to a spaceplane.

Steps

Step 1 - The Space Craft

There will be two space craft used for this tutorial. A spaceplane which will land on Laythe and a cargo version which will refuel our plane and deliver a range extension fuel tank so we can make it to Laythe and back. I'm including a fully designed spaceplane for this tutorial, but feel free to use your own, or to customize the one provided.

Spaceplane

Before assembling the plane I recommend reading keptin's aircraft design forum post or Scott Manley's aerodynamics tutorial video. They explain the effects of design decisions better than I could and are excellent guides to putting together a spaceplane without focusing on a specific design. Building a spaceplane is an exercise in compromise and balance, for instance a stable plane will generally be less maneuverable. Everyone will pick a slightly different balance of aerodynamic characteristics so you'll often find different guides give you contradictory advice, which is why I prefer guides that focus on how choices affect aerodynamic characteristics over those that build a specific plane. This tutorial does include a specific plane, but it is included more as a reference and there is enough room to customize it to your needs.

This spaceplane is designed for long-range, decent flight characteristics, and fuel efficiency. With the exception of some of the struts every part of the plane is visible and I've tried to capture enough angles to make this build-able from the pictures. This plane can actually make a trip to Laythe and back without refueling under ideal circumstances (you probably can't afford to land unless you were extremely efficient). This extra fuel does mean it is a little heavy, however it can still glide to a landing fully fueled.

Action Groups
  1. Toggle TurboJets and Air Intakes
  2. Toggle LV-N Atomic Rocket Motor
Assembly
Spaceplane in SPH

Start with the RC-001S Remote Guidance Unit. This will be a manned spaceplane, but I always put a probe core on my craft so I retain control when on EVA. There is an added benefit that when flight testing your plane you don't have to risk a kerbal in an unproven spaceplane. Build out the main fuselage (Advanced Inline Stabilizer, Mk2 to 1.25m Adapter, Mk2 LF+O Fuselage, Mk2 Liquid Fuel Fuselage, Mk2 Cargo Bay CRG-04, and LV-N Atomic Rocket Motor) and forward cockpit (Mk1 Inline Cockpit, Clamp-O-Tron Shielded Docking Port). I put the monopropellant tank and RTG under the Mk1 cockpit since they represent a fair amount of mass and having them forward helps balance out the engines in the rear a little. I use a short cargo bay to house the LV-N Atomic Rocket Motor, because this compensates for the mass and length of the LV-N Atomic Rocket Motor by allowing you to mount your wings further back to keep your center of lift (CoL) behind the center of mass (CoM). This also makes it easier to layout the plane so you can avoid a tail strike. 1 LV-N Atomic Rocket Motor should be enough, but you do need to get to a very high altitude before it is sufficient on its own. Add fuel tanks on the side and the TurboJet Engines and the first four air intakes. Turn on the CoM and CoL indicators. Add in the vertical stabilizer/rudder (Small Delta Wing,Elevon 5) and the RCS thrusters. Add the wings and control surfaces and the last pair of air intakes (use a Cubic Octogonal Strut to attach them to the wings). You generally want a CoL that is slightly behind the CoM which should be fairly easy with this design, but can take some fine tuning. Don't be afraid to move the side fuel tanks to get your wings into a better position. Once you have the wings in the proper place add struts to strengthen them, particularly between wing segments that are in front of one another as they are not linked at all unless you add struts. Add struts between the main fuselage and the side fuel tanks. Now that your CoM is pretty well defined add the rear landing gear slightly behind the CoM and your front landing gear wherever you want (within reason). Connect the FL-T400 tank to the FL-T800 fuel tank with a fuel line and adding a pair of fuel lines between the FL-T800 and the Mk2 LF Fuselage so that fuel can flow in both directions. Finally add the ladder at an angle which allows it to barely touch the ground and puts you on top of the plane instead of on the side, so you fall onto the plane when you reach the top instead of on the ground.

Air Intakes

Air intakes are an important part of a spaceplane. There are a lot of air intake options, but for a spaceplane (as opposed to an airplane) you need to use one that has good high altitude characteristics. The Ram Air Intakes and Shock Cone Intakes both have excellent high altitude characteristics. The shock cone has slightly better performance, but they are so close they can almost be used interchangeably. The various cylindrical air intakes are not as good, but if you need a cylindrical mount and maybe a little jet fuel you might as well use one with an air intake. The radial mounted air intakes are not as effective at high altitude, but they are easy to mount. With enough of them and a proper ascent profile they can be used, but I wouldn't recommend it. 2-3 Ram Air or Shock Cone Intakes per air breathing engine is a good rule of thumb (two radial intakes roughly equals one of these intakes). Adding more intakes than this adds weight with relatively little increase in operating envelope.

Fuel Flow

A headache with a lot of spaceplane designs is that your aerodynamic stability changes as you burn fuel. With a rocket you generally only need to worry about how your thrust vector interacts with your center of mass and unless you have an asymmetric design you probably didn't even know you needed to worry about it. Even though the CoM will move as fuel burns the thrust vector is generally on the same line it is moving in, so it will still point through it. With a spaceplane you have to worry about gravity interacts with two force vectors (thrust and lift), which are generally at right angles to one another. Like rockets your thrust vector will naturally point towards the center of mass in most designs intuitively. The center of lift however tends to be at a right angle to the geometric plane or line the CoM moves in which means that you basically have a lever with a fulcrum that is moving all the time. If it moves too far or in the wrong direction your spaceplane will become unstable. You can empty tanks to simulate a flight to see how the CoM will move or use a mod like RCS Build Aid which adds a DCoM (Dry Center of Mass marker). Keep in mind how the CoM will change as fuel flows, not just full and empty. If the CoM starts and ends at the same location, but moves 1 meter in between you're probably going to be unstable at some point in the flight. The front to rear fuel flow that KSP uses tends to result in CoM that moves backwards and once it moves behind the CoL your plane wants to pitch up which increases the angle of attack, which increases lift, pushing the nose further up, often resulting in a stall or loss of air intake and asymmetric flameout. This spaceplane design uses fuel lines to change the flow so that it goes outside-in which means the CoM moves less and because the outer fuel tanks are at the rear the CoM tends to move forwards. It would be better if it didn't move, but this is one of the design concessions I made along the way. You shouldn't need to rebalance your fuel with this design except when you are refueling. I highly recommend designing your fuel flow so that it is naturally stable since there are a lot of things you need to keep track of when getting a spaceplane to orbit and trying to transfer fuel manually during that is not easy.

Flight Worthiness Test

You'll want to test out the plane on Kerbin, because the last thing you want to do is send a spaceplane to Laythe only to discover that it can't land or takeoff on a less than ideal runway or is unstable in flight. My worthiness test is pretty simple:

  1. Empty the pilot seat. We don't want to kill a kerbal for a test flight.
  2. Takeoff from the runway. You should be able to pull up once you reach 85 m/s or greater and get into the air.
    • The plane must be able to takeoff before the runway ends. If it doesn't the landing gear is probably not optimally placed.
    • If you strike your tail you either need to move the landing gear back or pull up more gradually.
  3. Turn around and head towards the open field on the opposite side of the KSC from the runway.
    • You should be able to turn at a reasonable rate without stalling. If not check your wing placement.
    • Check for wing deflection when you turn. The main wing should behave as one unit. If not you probably need more struts.
  4. Land on the open field.
    • You should be able to land without striking the tail.
    • You might want to land with the engines on to maintain low vertical speed (less than 10 m/s).
    • Watch as you land and make sure nothing flexes too much.
  5. Point your plane towards an unobstructed, flat section and takeoff again.
  6. If you are new to piloting planes in KSP repeat the takeoff and landing steps until you are comfortable with the process.
  7. Land on the rougher fields behind the KSC.
  8. Take your plane into orbit.
    • As you climb watch to see if the plane tends to pull to either side. This usually indicates your wings are flexing and you probably need to add strusts somewhere. Some drifting is normal, but SAS should be able to keep you pretty much straight.
  9. Re-enter and land.
  10. If you can get through all of these steps it is probably a worthy spaceplane and your pilot skills are up to the task.

Cargo Conversion

Cargo conversion in orbit with cargo bay open

Basically this is the same plane with the forward LF+O tank replaced with a Mk2 Cargo Bay CRG-08 and the rear LF tank replaced with a Mk2 Cargo Bay CRG-04 segment and a Mk2 Liquid Fuel Fuselage Short tank. I would literally take the other plane and convert it to create the cargo plane. This should be an easy conversion if your side tanks ended up connected to the Mk2 Cargo Bay CFG-04 used for the LV-N Atomic Rocket Motor because it means you can detach the bulk of the rear section without having to re-tune everything once your replace the middle parts. You will need to re-add the forward wings, but that should be straightforward. The cargo bay contains our fuel extension tank for this mission. The node attachment mode (activated when you press the modifier key) is very important when building inside a cargo bay, particularly with the Clamp-O-Tron Docking Port which has a tendency to surface attach. Make sure to strut the cargo in so that it doesn't flop around in flight.

Remove the oxidizer from the two FL-T800 tanks, because we won't be needing that much oxidizer. I didn't replace them with jet fuel tanks just because I do use this cargo plane for missions where more oxidizer is needed.

Step 2 - Kerbin Orbit

General Notes

Getting a space plane into orbit is more complicated than a traditional rocket and you will find a large variance in recommendations between tutorials. Every plane is going to be a little different, so don't expect to be able to use the same ascent profile for every plane. They will be similar, but every plane has different weight, air intake count, lift rating, etc. and these are all factors that will change when you need to perform maneuvers during the ascent. Below I describe the ascent profile I use for this plane. No matter what plane you use, to get into orbit using as little fuel as possible you want to maximize the Δv you get from your air breathing engines before they cut out (where this happens varies from plane to plane and pilot to pilot). You must carefully balance your vertical and horizontal velocity especially as the air thins. You want to climb out of the thicker air to avoid drag, but you also need to keep a high airspeed so that you can force enough air through your intakes to keep the engines fed, and of course getting into orbit means you want a lot of horizontal velocity while you can use your efficient air breathing engines. This means that how you pilot a plane can dramatically influence how high you can go using jet power and how efficiently you get there. If you climb too fast you won't have enough airspeed and you will lose thrust due to intake starvation before you can build up your airspeed to offset it. If you climb too slowly you'll burn a lot of fuel heating up the atmosphere. As a rule of thumb, you shouldn't go fast enough to see re-entry or Mach effects below 20 km and they should never be very pronounced if you want to minimize fuel use.

Ascent Profile

Climb to 18 km as quickly as you can. Usually I pitch up 45 degrees, which is aggressive, but this plane does have the TWR to support it on air breathing engines. At 18 km start leveling out and at 20 km you want a vertical speed between 0 and 10 m/s. The air is starting to thin, but we still have plenty of air intake, so this is a good point to build up the airspeed we need to keep going higher. You should be able to get up to 950 m/s easily. As you fly your pitch and vertical speed will naturally increase, stop counteracting this and slowly climb up to 25 km and level out again. Pick up horizontal speed again (your total surface velocity should get up to around 1450 m/s). Pitch up sharply to about 45 degrees and build up vertical speed (over 100 m/s) and keep this up until you reach 30-31km and pitch down so that you are pointing prograde or a little above it. You should keep enough vertical momentum to continue climbing while building up horizontal velocity with your air intakes pointed directly into the airflow. If you get this right you should be able to accelerate to 2000 m/s on air breathing engines alone and if not your LV-N Atomic Rocket Motor has a thinner atmosphere to push you through. Generally you don't need to use this trick and it can be a little hard to master, but it does allow this spaceplane to go much higher on TurboJets than it otherwise could given its TWR and air intake requirements at this point in the flight. Once your turbojets are no longer sufficient activate the LV-N Atomic Rocket Motor, but leave the TurboJets on. Once the Turbojets are no longer useful shut them off. Circularize into a Kerbin orbit at 100km. This is a good orbit for a rendezvous. Generally the plane can reach a 100km orbit with over 3/4ths of its pre-flight fuel, but this may take some practice and anything over 1/2 should be good enough for this tutorial.

Notes on Maintaining Air Intake

As you ascend you will lose air intake. Initially you need to maintain intake by lowering your pitch which will increase your airspeed and put your air intake directly into the air flow which will improve their ability to take in air. Eventually this will be insufficient and you will need to reduce your throttle to decrease your air intake requirement. Due to the fact that we have an even number of TurboJets we must decrease the throttle very deliberately to avoid asymmetric thrust. You can right click on the engines quickly to see if they are getting out sync before it turns into a spin. The hardest part is getting from 25km to 35km because you need a high speed to maintain the air intake required by the engines at full throttle. Above 35km you don't need much thrust to keep climbing and you can usually recover from a spin. At some point the turbojets will be inadequate and you'll need to activate your rockets. This will increase your airspeed and force more air into the intakes which will allow your TurboJets to continue working for a little longer. You still need to decrease throttle as you continue to lose air intake. Keep an eye on your apoapsis and make sure it doesn't significantly exceed your desired orbit. You're generally still low enough in the atmosphere that you will lose airspeed and your apoapsis will decrease without the engines, so don't shut them off, just decrease thrust whenever your apoapsis starts to go too high. When you can no longer run your air breathing engines use group 1 to shut them off and decrease the air intake drag.

Step 3 - Cargo Plane Rendezvous

There are a number of tutorials on rendezvous. This is a technique I like, but feel free to substitute your own or use any of the other docking tutorials out there for this step.

Intercept

Load the cargo plane. For this launch go into the crew tab and select none. We don't need this to be manned flight and we might not have enough fuel to safely return. Put the cargo conversion on the runway and time warp until the space plane is overhead. Launch at this point and enter a circular 80 km orbit (same basic ascent profile, but the cargo conversion doesn't have quite as wide a margin for pilot error, so be careful). Set the spaceplane as a target and match orbital inclination. When you set the spaceplane as a target you should see "AN" and "DN" tabs on your orbit. These indicate optimal points for matching inclination and if you hover over them you will see how many degrees off your orbits are. Create a maneuver node on the nearest AN or DN and add normal or anti-normal velocity until your inclination is off by 0 degrees (or better yet NaN) and perform the maneuver. Create a maneuver node and add enough prograde velocity on the node to barely have two intercepts. Increase your orbit count on the node until your intercept is as close as possible. If your launch was in the right position it shouldn't be more than an orbit or two. Drag the node around until the first intercept is less than or equal to 0.5 km. Unfortunately there is a bug that often results in the orbit count changing when you drag a maneuver node, which is frustrating when using this technique, but I still prefer it because you generally only need one burn to get an intercept and it uses the minimum amount of fuel (you can mitigate it by timewarping so the maneuver doesn't require increasing the orbit count). Execute the burn and if you over or under burn use RCS to correct. I usually keep using RCS as long as my intercept distance continues to decrease. You should now be headed for a close encounter with your target.

Docking

Spaceplane and Cargo plane about to dock.

Time warp until you are within 4km of your target. Start a burn to match velocity. As a rule of thumb I burn until my target relative velocity starts with the same digit as my range to the target (i.e. 3.9 km -> 30 m/s, 2.9 -> 20 m/s, 1.9 -> 10 m/s). Position your burn so that your retrograde marker moves toward the target retrograde marker. When burning retrograde the marker will move away from your level indicator. In other words if you want to move the retrograde marker down orient your ship so that your level indicator is above the retrograde marker. If your retrograde marker and target marker drift too far apart go ahead and burn a little to move them closer. You want to maintain some target relative velocity to close range with the target while slowly decreasing it to avoid overshooting. Once you are within 1km decrease to 5 m/s. At 300 m burn and decrease to 1 m/s. Turn your ship around so that the docking port faces the spaceplane, open the docking port, and switch to the spaceplane, open the protected docking port and face it towards the cargo plane. Switch back to the cargo plane. Use RCS to line up your approach and when you are within 50 m use RCS to slow to <0.5 m/s and dock.

Transferring Fuel/Cargo

Transfer the remaining fuel in the cargo plane to the spaceplane. If your launches were fairly efficient you should have enough to completely refill the spaceplane, however we don't want to do that. This spaceplane doesn't have a very high TWR when using the LV-N Atomic Rocket Motor, so we only want enough fuel to get where we want with some margin for error. We only want about 400 units of LF in the LF only tank (saves a literal ton of dead weight since we won't need it on Laythe), you could get by with even less, but this gives us fuel we can use to fly around if we don't de-orbit where we wanted to. Completely refill all fuel tanks that take liquid fuel and oxidizer. Leave only half the fuel in the range extender (equivalent to the FL-T400). Decouple the range extender from the cargo plane. Decouple the cargo plane from the spaceplane and use RCS to move it out of the way (ideally while leaving the range extender basically where it is). Switch to the range extender and use RCS to dock with the spaceplane. You should have enough fuel in the cargo plane to de-orbit it. Land it on Kerbin either now or whenever you get around to it. Put as much fuel forwards as you can while keeping the side tanks balanced. This will compensate for the shift in center of mass now that we've dropped our cargo.

I'm sure someone will run the numbers and figure out that we don't necessarily need the range extender. If we just refuel the spaceplane it should be able to handle this journey without the added range provided by the fuel tank. However, having the range extender decreases the Δv we need on Laythe by allowing us to leave some fuel in orbit and we can add more fuel to make this tutorial safer (we pay for it with longer burns). Feel free to do this tutorial without the range extender if you want.

Step 4 - Jool Transfer

Escape

Use Alex Moon's Launch Window Planner to determine the next launch window for a Kerbin to Jool transfer. We'll perform an aerocapture when we arrive, so make sure to check that box. I usually select the mid-course plane change. There should be a transfer requiring less than 2000 m/s, if not click on advanced settings and increase the departure time a couple of years. Once we have our launch window calculated, time warp to a couple of days before the launch window. Zoom out and set Jool as your target. Create a maneuver node at roughly the Ejection angle and set your node for a prograde burn of the ejection velocity. Drag you maneuver node to find the point where you are at your closest approach. Our TWR is not high enough to make this in one burn, so we'll split the transfer burn into two phases. The first will be a burn to raise our apoapsis to 9 Mm. This will be an eccentric orbit that retains the 100 km periapsis we currently have. Start with the maneuver for a Jool intercept and then reduce the Δv so that your apoapsis is 9 Mm (approximately 820 m/s). Perform the first burn splitting the burn time so that half is before the node and half is after the node. Once that is complete, setup a new one at the same spot (your new periapsis should be in roughly the same location as your old node) set to the remainder of the ejection velocity (ejection Δv - 820 m/s) and adjust it until you get an encounter with Jool and the lowest periaspsis you can achieve without altering your Δv. Take the burn time for the new maneuver and divide by two, this is when you should start the second burn in the transfer. At the appropriate time start the second burn. After the second burn you should only need minor corrections to achieve the approach you had planned with your maneuver.

Course Corrections

You are now on course to an encounter with Jool, but you will need to correct your inclination and periapsis. The launch window planner recommends a point at which to correct the inclination, but it assumes and equatorial orbit which may or may not be true. Still it is a good first approximation. Put a maneuver node where it recommends and experiment to find the maneuver that matches inclination with Jool with the least fuel. Try moving your node around to find the fuel optimal point. Time warp and carry out the maneuver. Now decrease your Jool periapsis to 120 km +/- 50 km (you're probably too far out to be more accurate than that). Again experiment until you find the fuel optimal maneuver. Make sure to try both prograde/retrograde and radial in/out changes as one will usually have a similar effect with significantly less fuel.

Step 5 - Jool Aerocapture

Time warp until you are about to enter Jool's sphere of influence. As a rule I always stop time warp before an encounter because warping through the encounter tends to result in appreciable changes in your periapsis. Once you are within the Jool's SOI enter your orbital details into KSP Aerobraking Calculator. You want an apoapsis is low enough to avoid Vall's SOI ( < 34.6 Mm), but high enough that you will end up with a Laythe encounter ( > 20 Mm). The tool ignores lift surfaces and for this spaceplane the amount of lift we have is enough that at least for capture you need to put in a lower target orbit than you really want. I usually use 21 Mm since you will end up with a Laythe encounter even if you don't experience the lift you are expecting and if you do it is below Vall's SOI. Adjust your periapsis based on the tool. I usually time warp to within 10 days, make a correction, warp within 5 days make a minor correction and then warp until I'm about to hit the atmosphere. Generally I use RCS in precision mode due to the very small Δv required to make the changes and high precision we need for an aerocapture. Once you are at your new apoapsis burn to raise your periapsis out of the atmosphere. I suggest a generous increase (up to 200-400 km) since it will give us room to adjust our approach to Laythe.

Step 6 - Laythe Aerocapture

Approach

Set Laythe as your target and match inclination. Set a maneuver node and advance the orbit until you have a Laythe encounter. Adjust your node to make your Laythe periapsis 22 km +/- 4km in a prograde direction (counter-clockwise). Again try multiple different combinations and positions. You may also want to reduce your orbit count to make an earlier adjustment (you can use a second maneuver node to see the effect on the latter orbit). If you are burning prior to your Jool periapsis make sure that the maneuver doesn't decrease your periapsis below 138 km or you'll have an unintended aerobraking that will throw off your approach (among other things). Make your burn, time warp up to the encounter, and cross into Laythe's SOI.

Aerocapture

Go back to the KSP Aerobraking Calculator and plug in your orbital details. For this one we want an apoapsis of 100 km. Due to lift effects we won't get our desired orbit on the first pass. With an aerocapture we generally have enough velocity to burn off that we need to go deep into the atmosphere which means the difference between our desired orbit and a crash is relatively small, so don't try to compensate by going lower. Perform the aerocapture. Just after you exit the atmosphere go back into the aerobraking calculator, plug in your new orbital details, and set an apoapsis of 100 km again.

Aerobrake

Now that we've reduced our velocity with the first aerobrake maneuver the second one should be at a much higher altitude where it is much safer to aerobrake. At your apoapsis adjust your periapsis to the value output by the tool (the tool will tell you delta-v for an immediate maneuver, but doing it at apoapsis requires less and doesn't significantly change the outcome). Perform the second aerobrake maneuver. Generally a third aerobrake is required before we are close enough to our desired orbit. Once we are close enough circularize at a 100 km orbit.

Step 7 - Undocking

Now that we are at our destination we want to separate what we are taking down to the surface from what we're leaving in orbit. First you want to move fuel around. You don't want the plane full of rocket fuel when you land. We need most of the remaining rocket fuel in the spaceplane for our return trip and it is just a waste of fuel to take it down into a gravity well and bring it back up. It is also easier to fly with less weight. Completely fill the range extender with fuel from the rear tanks. Undock the fuel extension tank from the spaceplane. Make sure your CoM as determined by your fuel keeps you aerodynamically stable. You need the side tanks equally full and you want the majority of your fuel in the central forward tanks.

Step 8 - Laythe Landing

Laythe is a great place to fly a spaceplane, but a challenging place to land one. There is very little land and most of it is covered with mountains. Fortunately there are a few decent landing spots on the equator, but there is really only one I would recommend. It is the largest equatorial island and as you orbit it is the last island before a large expanse of ocean. If you look at the topographical map on the Laythe page there is a peninsula at approximately 165°W and 2°N that is probably the best landing spot in my opinion. Time warp until this island is on the light side. Create a maneuver node 180 degrees from your landing site for a retrograde burn resulting in a periapsis of 28,800 m directly above the island. I discovered this through trial and error, but as is often the case someone on the forums has calculated this and created charts. Once the atmosphere slows you down this should roughly match where you land. Time warp until your about ready for your maneuver. Remember that the planet is rotating, so you will probably need to move your node before you burn. Make your burn. Use RCS to correct for any under/over-burning. If you everything goes right you should enter the atmosphere, slow down, and decelerate just about at our landing site. Before you decelerate switch the turbojets on and the LV-N Atomic Rocket Motor off. You might over or undershoot the landing site depending on a number of factors. If you undershoot use the Turbojets to fly to the site. If you overshoot you can probably perform a gliding U-turn or spiral depending on how far you overshoot. If you overshoot by more than what you can glide, the middle of the island is also a good secondary landing site. This plane is capable of landing in a glide, but I recommend landing at 1/3rd thrust at least until you are familiar with the terrain. While this is one of the better landing sites it is not without its hazards and you may need to pull up to avoid a sand dune or sloping terrain which is much easier when the engines are running. Once you touch down do whatever exploration you want and get back into the plane.

Step 9 - Laythe Takeoff

Turn your plane towards 90 degrees. Find a relatively flat area long enough for a takeoff and hit full throttle. Getting into Laythe orbit is much easier than Kerbin orbit because of the lower gravity and because we've burned a lot of our fuel or left it in orbit. Start with a steep climb. Keep pitching down to maintain a vertical velocity below 100 m/s and level out at 20 km then accelerate until you reach your target apoapsis (I recommend 80 km). Be careful because it is very easy to overshoot if you aren't watching closely.

Step 10 - Redocking

We need some of the 4 metric tons of fuel we left in the range extender, so redock with it. At this point you have a choice to make. You can transfer some the fuel into the spaceplane and leave the range extender in Laythe orbit for later missions, transfer all the fuel and deorbit it, or you can take it back with you. Abandoning the range extender will decrease your dry weight and give you several hundred m/s of Δv (if you have needed to do a lot of maneuvers you may not have a choice). If you take the range extender back to Kerbin you have the option to refuel it and re-use everything for the next mission. If you leave it in Laythe orbit you can probably leave enough fuel that you don't need to over provision a future mission (you can also temporarily store fuel for the return trip like we did here). I generally take it back with me, but the choice is yours.

Step 11 - Escaping Laythe

This isn't necessarily the most efficient way to do this, but it is easy to do without precise timing. Setup a maneuver node with just enough Δv to escape Laythe. Move the node around until you maximize your Jool apoapsis. This should be around 75 Mm. You might need to add a little bit of Δv to achieve this, but with the proper ejection angle you should be close. This will put you out past the other moons of Jool where you can avoid accidental slingshot maneuvers or accidental abrupt lithobraking as we time warp later on. Once you have set up a good maneuver time warp as needed and burn to escape Laythe. I don't usually split this burn because it is usually short and Laythe moves fast enough that in the next orbit the ejection angle will already be sub-optimal if we try to split it. Once you have escaped Laythe's SOI time warp to your Jool apoapsis and raise your periapsis to the same altitude.

Step 12 - Return To Kerbin

Basically this is the reverse of your trip to Laythe. Use the launch window calculator to find out when, how much Δv, and at what ejection angle. Time warp to within a few days and set up your maneuver node. Kerbin has a much smaller SOI, so it is a little harder to find an intercept. You'll probably need to include an inclination change to get one at all. Once you have a good maneuver planned execute the maneuver at the proper time. As you return to Kerbin correct your inclination and lower your periapsis to about 30 km +/- 10 km. Once you are within Kerbin's SOI use the aerobraking calculator to figure out your aerocapture altitude. Use the same repeated aerocapture/aerobraking steps we used on Laythe to get into your desired orbit (or keep braking until you land).

Finishing word

If all went according to plan you have now taken a spaceplane from Kerbin to Laythe and back again.

Additional Ideas

  • Refuel in orbit and go again. To make it more interesting send ahead a rover and/or a fuel tanker.
  • Leave the range extender in Laythe orbit and build it out in subsequent missions into a re-fueling station.
  • Take the cargo conversion above and replace the fuel extension tank with a minimal lander. I've used this for a KSC to Mun and back mission. Remember to add that oxidizer back into the FL-T800 fuel tanks.

Math (Optional)

  • LF == Liquid Fuel (fuel units, in other words what you see in the resource dialog)
  • O == Oxidizer (fuel units)
  • Mass is in metric tons (1000 kg)

Fuel Mix

One downside of using jet engines is that your liquid fuel and oxidizer are no longer guaranteed to be in a ratio that rocket engines can burn. You can end up with either excess liquid fuel (a.k.a. jet fuel) or excess oxidizer (a.k.a. dead weight). This makes calculating your Δv at any given point a little more difficult, but using some simple equations you can make it easier.

If LF > O / 11 * 9 you have excess liquid fuel.

Jet Fuel Units (excess liquid fuel) =

Jet Fuel Mass (excess liquid fuel) =

Rocket Fuel Mass (excess liquid fuel) =

Rocket Fuel Mass (excess oxidizer) =

Note: 200 fuel units per t = 1000 kg per t / 5 kg per fuel unit

Delta-V Budget

For a mission of this complexity it is a good idea to create a Δv budget to figure out whether you can get where you want to and how much margin for error you have. Since we are refueling in orbit I'm ignoring the Δv needed to get into Kerbin orbit. We're also saving a lot of Δv by using aerocapture/aerobrake maneuvers where we can. Most of the values are approximate since they can vary based on launch window, inclination, eccentricity, and lot of other factors (basically I rounded up to the nearest 100 m/s). The Δv maps on the Cheat sheet page is helpful in building the budget, though they are just a first step. This budget assumes that you are trying to perform fuel optimal maneuvers where practical (with some extra margin since this is a tutorial).

  • Low Kerbin Orbit To Jool - 2000 m/s
  • Pre-SOI Orbit Corrections (Jool) - 100 m/s
  • Orbital Insertion (Jool) - Aerocapture/Aerobrake + 200 m/s
  • Pre-SOI Orbit Corrections (Laythe) - 300 m/s (Can vary wildly depending on where you intercept Laythe)
  • Orbital Insertion (Laythe) - Aerocapture/Aerobrake + 200 m/s
  • De-Orbit (Laythe) - 100 m/s
  • One-way Total = 2900 m/s
  • Re-Orbit (Laythe) - 3100 m/s (LF) + 200 m/s
  • Escape (Laythe) - 600 m/s
  • Circularize (Jool) - 500 m/s
  • Escape (Jool) - 1200 m/s
  • Pre-SOI Orbit Corrections (Kerbin) - 100 m/s
  • Orbital Insertion (Kerbin) - Aerocapture/Aerobrake + 200 m/s
  • De-Orbit (Kerbin) - 100 m/s
  • Two-way Total = 5800 m/s (LF+O) + 3100 m/s (LF)
  • Rocket Δv = 6645 m/s
  • Over provision >= 845 m/s

I usually just guess on the jet fuel (LF) portion because jets are so efficient that I don't need to be too accurate and their ISP varies quite a bit in flight. This is a rough estimate which we're going to add a safety margin to, so we don't need to be very precise or accurate.

Once you have a total Δv you can use the rocket equation re-written to solve for the fuel mass to avoid over provisioning too much. Sending too much fuel means you need to burn longer than you otherwise need to and if you are landing it means that you need more upward force (parachutes, wings, or descent engines) to land safely.

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