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Kerbal Challenge: Landing the MkX Space Plane

February 13th, 2014 · 4 Comments · Gaming

Welcome to the walk through guide for my Kerbal Challenge that shows you how to land the MkX Reusable Orbiter that I designed in the Kerbal Space Program. This is a very text-heavy and barebones checklist-style I made on very little sleep with some assumed knowledge of Kerbal game and orbital mechanics, but it is all based on the threepeat test flights I did recently to ensure this craft is capable of performing everything it says it can do. I say it can do. Whatever. Any questions? I will be happy to answer them on the thread or respond to you directly if I feel it will give people who don’t want to be told what to do extra information.

I would suggest reading everything before giving it a go, as some items happen in quick succession.

To Space and Back Again

Let’s begin by loading the craft onto the launch pad.

SAS Activated. To assist with takeoff control, I highly recommend the use of the SAS. While the flight to orbit can be flown under complete manual control, it does require some fancy flying and I wouldn’t do it my first time up – unless you want to see what happens.

Throttle Up. As hilarious as it is, your poor kerb won’t appreciate getting blowed up because the heavy lift engines failed to ignite upon launch causing the craft to shake apart on the pad.

Stage 5 Activated. This will ignite all booster engines and release the support couplings to allow the craft to begin its ascent.

Begin Roll. Immediately after clearing the support towers, start a nice easy roll towards a heading of 090 (East). This will make it easy to get back to the KSC later on.

Stage 4 Activated. This is the solid-booster separation stage. All remaining engines will continue their burn

Gravity Turn. Allow Kerbin to assist us into orbit – immediately after jettisoning the solid booster rockets, start to angle your ascent 45º while maintaining your heading of 090.

RCS Activated. At this point you’re going to want the extra kick of the RCS to help you control your ascent into thinner and thinner air. Of course you could have activated it earlier with the SAS instead of remembering to do it in the middle of a burn with a stage release fast approaching and trying to maintain your heading/inclination but where’s the fun in that?

Stage 3 Activated. The center booster rockets will flame out next and can be immediately jettisoned via this stage. By this time you should be at your 45º inclination.

Stage 2 Activated. The four liquid booster engines will be the last to go and this stage will jettison them. Besides being to either side of the MkX’s wings, these 4 boosters are joined with structural supports that can further damage the MkX if care is not taken at this stage. You do not need to immediately activate this stage after engine flame out – take a few seconds to ensure your craft is stable and won’t spin into the boosters as they fall away. To help escape the confines of the boosters the MkX’s orbital engine is tied into this stage and will begin to burn immediately upon booster separation.

Cut Throttle. Remain at full throttle for Stage 2 activation and immediately cut throttle afterwards – the MkX will quickly fly free of the boosters. Confirm you are completely clear of the boosters (especially on the night side) before attempting any further orbital maneuvers.

RCS Deactivated. This is a personal choice of mine. There’s really no need for it now, but it’s up to you – plenty of spare propellant onboard. I leave SAS on but again, use at your discretion.

Establish 70km Circular Orbit. Once clear of the boosters you will be on a trajectory high above Kerbin that will result in you crashing right back down onto the planet if action is not taken. Use basic orbital trajectory adjustment by performing prograde/retrograde burns at both Apsis and Periapsis positions along your orbit to turn the highly-eccentric one you begin with into a nice nearly-circular (+/-500m) one just over 70km (+1km) above the planet’s surface at the very outer edge of the atmosphere. You have just under 5 minutes worth of fuel to use to accomplish this task.  You shouldn’t need more than 1/2. You can use the orbital burn tool to plan out your adjustments, but I found it just as easy to turn the right way at the marked locations for Ap and Pe on the map and burn until I saw the results I wanted.

Goof Off. You’re in orbit! Since you’re just falling forever at the moment, might as well spend some time playing around with the MkX’s various doodads and thingamabobs assigned to the special action keys 1-6 (#4 is supposed to extend the solar panels even though these panels can’t extend even though the actions menu in the VAB lets you add an extension action). Go ahead and let your kerb stretch his legs with an EVA – don’t forget to extend the ladders so he can have an easier time getting back aboard. Switch on the floodlight to make it easier to see the ship on the night side of Kerbin.

Calculate Re-entry Burn. Okay back to work, and this is the real meat of the mission. You need a reliable tool to calculate when you should enter Kerbin’s atmosphere to land at the KSC. The purpose of attaining a circular orbit was so that we could perform this burn at any point along our trajectory, so that’s one problem out of the way. The other problem is figuring out how to make the orbital burn tool indicate to us when and where we need to burn to drop out of orbit within gliding range of the KSC. I won’t go into detail on how I figured this out but I will show you how to do it and admit that it is highly tailored to this specific spacecraft – you’ll have to do your own trial and error if you want to do something similar. But the technique at least is the same.

Click to embiggen the image above, which shows a re-entry burn plot for the KSC. There are three important things you need to take note of. One is the Δv of 500m/s – this is a constant burn rate used every time. The next is the target (not current) Pe altitude, a variable in this equation, not a constant. The third are the labels assigned to the various grab handles in the burn calculator, which I will explain shortly. First though you need to perform a few steps:

Plot Maneuver at Pe. Open the burn calculator at the current Pe of your orbit, making sure that this point is well ahead of you in your current orbit.

Degrade Orbit. Grab the handle that controls the degradation of the orbit and pull it out until the dotted orbit intersects Kerbin about halfway to the plot node. Note that in doing so the Pe mark will swing around to the opposite side of the orbit (inside the planet) and be replaced momentarily by the Ap mark until the orbit expands enough.

Ellipsate Orbit. It’s a word, ok? Grab the blue handle with the lines inside the circle and pull until you start to see the dotted line rising up from the planet.

Prograde Orbit. Now grab the other green handle and pull – shortly after you should see the Pe marker pop back out of the planet. Commence gleeful cackle.

Set Re-entry Altitude. Right so here is where the red labels come into play, as they define how you can massage the orbital line to produce the results you want. Those results are a Pe altitude of 23km (+/-1oom – maybe more but I was always within 50m) with a Δv of 500m/s (my own margin of error was +/-0.5m/s). You can indeed be extremely precise with this if you’re anal enough like I am. Manipulate the orbit as shown to get the 23km of Pe with a Δv of 500m/s (NOTE: this image is of my second orbital re-entry test altitude of 25km, before I found the 23km Sweet Spot). Yes, it’s a bit tricky at first but soon becomes intuitive. For example if the current altitude is only 22,767m while the Δv is 500m/s then you can use the (+m/s +alt) handle to delicately add a few tenths of a second to the Δv and then use the (-m/s +alt) handle to take those tenths back and return the Δv to 500m/s while gaining the altitude you need to make the Pe within 50m of 23km.

Find the KSC. This is actually pretty easy if you don’t know where to look or can’t see because the night side has no lights. Mouse over the top center of the orbital tracking screen to bring down the menu from which you can select to show Orbital Debris. Some of your lift stages will have crashed back down to the KSC and thus will show you exactly where it is on the globe. Don’t forget you need to switch views back to see the position of your orbiter.

Check Orbital Declination. Grab the entire plot (the white circle around your orbit) and drag it around until the dotted path just before the Pe marker appears to be over the KSC. Now move the camera so the dotted orbit plot becomes a single line on your screen and you are on the same plane as it. Check to make sure this line properly crosses over the KSC. It can probably be off quite a bit since you’ll have time in your glide to correct but personally I prefer the straight-in approach. If you do too, then use the purple grab handles of the orbital plot to fix the declination of your orbit so that it passes over the KSC again. Note that if you performed properly in your burns and liftoff your orbital path should be right over the KSC. Note also that if you modify the inclination you will need to slightly re-adjust the Pe and Δv.

Plot Re-entry Burn. This entire time spent fiddling around with the burn tool you should have been dragging it around your orbit to keep it ahead of the MkX so it doesn’t become invalid and you lose all your work (extremely frustrating, let me tell you). Another potential issue is if you drag it too far off the orbit line, the white circle will turn red. If this happens do not release the mouse as it will cancel the plot. Just stay calm and work the plot around until it finds the line again and goes back to white. If the KSC is on the night-side and you want to make a day landing, you’ll just have to compress time and keep dragging the plot ahead of the spacecraft until the planet turns around to bring light back to the KSC. When you are finally ready to go, drag the plot ahead so the Pe marker covers the tiny piece of flat plain (or the icon of spaceship debris if you’re using that) that denotes the KSC’s location. Now compress time again until your craft starts to catch up. Notice the rotation of Kerbin has moved the KSC ahead of your plot, so drag and readjust. Eventually your craft will catch up to the plot while it is covering the KSC and that is your re-entry insertion point. Make sure as you adjust the plot you are ready and aimed to initiate burn.

Perform Re-entry Burn. It’s go time. This burn of Δv 500m/s should take the MkX around 1:30 to complete, although this may vary slightly depending on how much fuel you used (and thus mass you’ve lost) to get to this point. Orbital tests have shown that with full fuel the MkX will burn a Δv of 100m/s in around 20 seconds. So use that information as you will if you want to take a stab at calculating your own burn time estimates. The ones put up on the HUD don’t always turn out to be true for me. Anyways, aim to start your burn at T-45s to the plot node. Stick to the orbital map to watch your trajectory and confirm it is shaping up as you expect – it will elongate a bit at first before the Pe labels come to intersect so don’t panic that you’re on a burn to overshoot. Ideally your trajectory should be at or very near your target altitude of 23km by the time the KSC location rotates fully ahead of the Pe label. Having a throttle axis for small/slow burn adjustments is very handy.

Prepare for Re-entry. Enable SAS/RCS at your discretion. You really don’t need either. Switch the camera to Free View mode so you can get a tail perspective of the craft and maneuver it upright and facing prograde, level with the horizon. Right-click the orbital engine and shut it down. Ensure that your throttle is fully closed and activate Stage 2, which will enable the four retro-thrusters radially-mounted on your rear fuselage near the tail fin.

Ride Out Re-entry. This isn’t as bad as it sounds, since the current stock version of KSP doesn’t really model re-entry, it just shows it visually. Still, it can cause your craft to enter some gnarly spins if you are not upright and level with the horizon when you hit around 30km. If you are, the craft will bob up and down gently but otherwise remain stable and docile through the entire re-entry burn. Stay level with the horizon.

Establish Glide. Once you’re about 20km up you’ll start to get a bit of wind rushing past you and notice some small aerodynamic effects starting to take control of the orbiter. Again, simply keep the nose level until you are 10km up and into thicker atmosphere before beginning to gradually pull all the way back on the stick and waiting patiently for the nose to begin to rise as the atmosphere continues to thicken. Eventually you will notice the vertical speed indicator start to show a decrease in your rate of fall, and soon you will be in a glide with the stick fully back and dropping at around -10m/s. Maintain this position as you continue to bleed off speed and altitude, looking ahead to spot the runway as it comes into view. As you drop lower your speed loss will begin to decrease as will your vertical speed. Remember though that you need this speed to maintain a good endurance glide so don’t let the vertical speed needle rise above -5m/s (the -10m/s mark is good) or your ground speed drop lower than around 60m/s. No attempts at flying should be made, especially if you chose not to re-align your orbit if needed – you’ll need all the height you can get to maneuver in line with the runway.

Prepare for Landing. Make sure you are lined up with the runway once it comes into view. If you took the time to line up your orbit and launched eastward as directed then you should be able to simply fly straight in. Do not aim for the beginning of the runway! This will cause you to glide at an angle that probably will end up landing short. Instead, aim for the far end of the runway to help ensure that you come in high with room to spare. Once you are positive you can make the runway, drop your landing gear and toggle your lights if it’s dark out. Enter a dive that aims you at a point about 50 feet in front of the runway. As you reach this point you will want to gradually level out so that by the time you cross the runway threshold the speed of your dive has you flying (that is, your vertical speed is nil) straight down the runway. Maintain your attitude as level to the horizon as possible – do not flare for touchdown. Instead gradually engage the retro-thrusters to slow yourself down and allow the craft to settle onto the runway on all wheels. Flaring on touchdown could potentially damage or destroy your orbital engine. Increase thrust and engage wheel brakes to slow to a stop. Get out and kiss the sweet ground. Oh and take a picture.

In The Event of Emergency. The MkX is equipped with the potential to save the life of your kerb should you wish to spare him death and destruction if a catastrophic failure occurs at some point in the mission. Toggling Stage 1 will decouple the command module from the rest of the craft and deploy the two parachutes attached to it. Depending on the situation, this may or may not work. But at least you can say you tried. If you still have some stages to cycle through, a similar and more immediate effect can be had by toggling the Abort action.

Additional Altitudes

The first altitude I tried was 30km, which was way too high. I then dropped in at 20km and that turned out to be too low. Next I tried 25km, which was still too high. Naturally, 22.5km was my next target, and that brought my craft low between the mountains – very cool but I did not think I could make the runway and ended up managing to glide in after several long minutes. 23.5km will give you plenty of cushion in your glide – I had to engage my retro-thrusters several km up to avoid an overshoot but was able to slow down enough to land without needing to circle around to the opposite runway end. The Goldilocks number is 23km.

On the 500m/s Δv

This was partially a nice round number, partially a guesstimate and partially a calculation based on the rotation of Kerbin at the KSC (174.6m/s) and the MkX’s orbital speed at 70km (2295m/s). Ok that last one was a total lie. But the numbers are true. Oh it was also a reasonable late-mission burn time for the amount of fuel that might be remaining. As a constant, this allowed the altitude adjustments to have some meaning in where I ended up after re-entry. Being the one and only case in which I used this technique of applying the Pe marker to plot a re-entry burn I can’t really say how easy it will be to apply to other craft. But you are certainly welcome to give it a go now that you (hopefully) understand what to do. Cause I don’t.

Safe flight!

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