LambdaCactus

Something with a highly elliptical orbit like a comet would be fun.

Cut some slack, it's part of a process to eventually do the full thing... You yourself edited parts to simulate the same thing from Kerbin, so what's the difference?

Well, no. He's upfront about the parameters of the challenge so I wouldn't call it cheating: it's only restricted to the scenario of getting off of Eve. The next step in the challenge would obviously be to deliver the lander to the surface there, and the final challenge would be a full round-trip.

I'm thinking of an orbit such as when transferring from Kerbin down to Moho. When the destination at Moho is below the solar plane, what's the best way to transfer there? In a circular orbit, it's easy to think of plane changes as "twisting" around a line from the craft to the planet's center of mass, and the same happens in an elliptical orbit: If I add some "North" component to my ejection velocity at Kerbin (the apoapsis of my transfer orbit), then the plane of the transfer orbit will be inclined through the Kerbin-Sun line: the craft makes a rising arc above the solar plane as it falls towards Moho, but the periapsis will just be back on the solar plane again. Yes, I could look for a launch opportunity when Kerbin is lined up with Moho's ascending/descending nodes and do that. But let's say I launch 90° from there. In other words, the Kerbin-Moho transfer apoapsis needs to end up really far north of Kerbin, in order to get aligned with Moho's orbital plane. I'm guessing this requires more than one impulse?

What I do is plane change from a Kerbin equatorial orbit. It\'s pretty accurate and doesn\'t take much fuel. Here\'s how: 1. Get into a circular equatorial orbit 2. Focusing the map view on your craft, zoom out until both the front and back lines of the moon orbit are in your view. Line them up so you\'re looking at the moon\'s orbit 'edge-on'. You should see Kerbin in the middle, and a horizontal white line going across it for the moon\'s orbit. 3. Turn your view until the Minmus orbital plane crosses behind Kerbin (You may need to zoom out a bit more). You are now looking directly at the intersection of the two planes. 4. Check which way you need to burn: at your craft\'s next crossing of the intersection, you will either need to burn north or south to change the angle. You can orient your craft to put the navball on the horizon at 0° north or 180° south and lock it there. You can set this up ahead of time: unlike when pointing prograde in orbit, these points on the navball won\'t move, they\'ll just rotate about your ship\'s axis. 5. Zoom in until you can see the sphere of Kerbin fill the top and bottom of the screen. If you\'ve focused the map view on your craft, Kerbin will move across your view from side to side while your craft stays centered. You\'ll know you\'re at the intersection when the space on either side of Kerbin is equal. 6. When you reach the intersection, burn full power. You\'ll have to zoom out to see the Minmus orbit, and zoom back in to overlay your craft orbit. Check a few times until your orbits look like the same angle. Cut the engines. As for exactly when to start your transfer burn to Minmus, I\'m not sure. I can usually get it pretty close, but I still have to sweep around with small radial burns mid-course until I find an intercept.

Thanks for the correction, yes that\'s what I meant. I hope that part made sense anyway.

I did it! My mission parameters: [li]Stock parts[/li] [li]No calculator or other nav add-ons: totally kowboy[/li] [li]Circular(ish) orbit at Kerbin before transfer burn[/li] [li]Orbital insertion at Mun (i.e. controlled speed/low perigee landing)[/li] [li]Nice atmospheric re-entry[/li] I was just able to complete this after a few tries. It\'s actually quite thrilling to fly totally seat of the pants. I had a rough idea of the kind of numbers I would need, so I did the whole run by feel. 3 attempts total: The first attempt looked good until I skimmed into lunar terrain... that little lander engine was too little too late once I noticed my orbit insertion was going to be sub-surface. Second attempt, I had a good efficient launch with a bit of a low orbit. I was mostly circular around 75k, but then misjudged the velocity I\'d need for lunar transfer. I was aiming for 3000m/s which might be fine from 120-150k altitude, but a lot of that speed bleeds off coming up from a low orbit... I ended up in a long elliptical orbit to about 6000km. I was a bit surprised at first when I noticed I was no longer climbing, so I peeked at the map to see how far off I was (it was a failed pass anyway). Ok, maybe just get these guys home then: I made a short burn to drop my periapsis down from 75k into the atmosphere, thinking I would just come back for a landing. However, once I was in the atmosphere I could see my speed was still too high and I would skim off. At 45km my descent levelled out and my speed dropped to 2300-ish. This is too perfect! I still had plenty of fuel, and I could circularize this and make another run! I managed to get the aerobraking pass circularized around 100k, waited for the mun to come around again, and then screwed up the burn again: still too slow! I left the crew in elliptical orbit as a souvenir of a failed but really fun double-attempt. Successful mission spoilers below. Final attempt: good on launch but I was too slow to accelerate into orbit: I missed the apogee and dropped back into the atmosphere a bit. Still, I saved it and set up a slow climb up from 63km. At moonrise I was nearly level around 75km, and accelerated to 3150m/s. I reached the lunar gravity going a bit fast so I made a bit of retro-burn so I wouldn\'t just pass by. I watched my altitude drop until about 720km where it levelled out. I took a wild guess: retrograde burn, 160m/s sounds good. Honestly this was such a lucky guess that I couldn\'t believe it... I just checked the numbers in Orbit Mechanic and I was dead on! This would bring me in to a low perigee at 3 km above the surface: perfect! I still had my second stage with some fuel for slowing down, and when I burned it up, I was sitting 3km up, in level flight, going 65m/s. Couldn\'t have done it any better. Here\'s how I actually checked if that approach from 720km was sane as I was coming in: when you\'re in an elliptical orbit, the prograde/retrograde markers oscillate up and down in a sine wave: at apogee/perigee they are on the horizon, and at the ascending/descending nodes (half way between apogee/perigee at the steepest descent or climb), the markers reach the furthest away from the horizon. Let\'s say it oscillates between 40 degrees above the horizon and 40 degrees below the horizon... like a sine wave, it slows down as it reaches the highest and lowest points, and it passes through the horizon quickest. I knew I had to get from my horizon at 720km altitude to my low horizon somewhere near the surface. This means the prograde marker would dive down to some angle, pause there, and start returning to the horizon as I approach perigee. I watched that marker as it dipped. I could see it pause at -40 degrees, and I checked my altitude once I confirmed that it slowly started returning: the altitude was 350km, so I knew I was close since this is half-way between 720km and zero. This is harder to do when the approach is not so steep since the movements are smaller... If you want precision instead of seat-of-the-pants flying, use a calculator! My lander had full fuel at touchdown because of that perfect approach... Return trip: I knew Kerbin was just over the horizon from my landing spot and I was near the equator, so the direction I needed to go was straight up (perpendicular from the Mun-Kerbin line, aka the horizon). With 2 full tanks of fuel, I could just burn straight up instead of doing the more efficient method of orbiting before transferring off. I accelerated to 900m/s, which left me about 60m/s Kerbin-relative speed after leaving mun gravity (actually it was too much the other way, I was coming back retrograde). This set me up for a really steep descent, Ideally I would\'ve liked to decelerate in the high atmosphere rather than coming straight down... oh well, still landed by parachute. Fun! The map feels like cheating now.

Ah, that\'s what I was wondering... so you basically skipped trying to orbit the mun. (Not criticizing, I\'m just trying to understand the parameters of your challenge.) Well, since the mun\'s orbital speed around kerbin is about 540 m/s, you want to burn away from the mun so you basically follow its orbital trail backwards and effectively cancel out that orbital speed and fall into kerbin... (If kerbin is straight overhead in the sky at your landing spot, that means burn east.) You also need to launch with more speed so you get 500m/s at the edge of the SOI (2400km) altitude. A hofmann transfer to just pop out of the mun\'s SOI sounds like it\'s a bit too soft... I expect you\'d end up in an elliptical orbit, and like you said, burn a lot of fuel to slow down at the bottom. Sure, I would watch it! I\'m off to try this myself...

Yes, this makes sense... it can be worked out with some trial and error and approximate numbers too. 2250m/s to orbit Kerbin; 3000m/s to shoot for the moon; 600m/s to orbit the moon. (Just off the top of my head, might be off a little bit.) Mmm, yes a little bit, but not really. For orbital insertion, you will know your speed, but not your true vector. Given the same speed, it\'s either a collision into the moon, or a flyby. Small difference in angle, big difference in results. But I wouldn\'t want to hit the moon. :-( Ok, I just thought of something. If you watch the prograde marker as you reach the moon\'s gravity, it will either be near 90° straight down, or off by some degrees. (Straight down is bad.) A burn to the side should move it off to an angle, but I have no idea how much, how far out. A good approach means the marker will slowly move away from -90° as you come around, eventually crossing the horizon at perigee... (retro-burn here). So if you watch your approach rate towards the moon along with the movement of the marker, you might be able to kind of estimate which one will happen sooner: impact, or marker on the horizon. Very rough idea, but it might be doable.

This is awesome. I can guess at how most of it can be done, but how did you accomplish orbital insertion? (Or did you skip it?) What I mean is, how can you tell if you\'re coming in for a low perigee or an impact, and how do you know which way to correct for that?

I\'ve done this. The trick is to build more horizontal speed early, then reduce thrust as you approach your orbital altitude. What I do: [list type=decimal] [li]Slowly start pitch-over at 10-15km, depending on the ship\'s speed and stability.[/li] [li]Maintain a slow pitch-over, your thrust vector ('nose') should follow your prograde marker as it falls over toward the horizon. You\'ll want to time it so you\'re pretty much horizontal around 50-60km. You can aim higher, it depends how high you want to establish your orbit. Just remember you will keep burning and climb up to it by building horizontal velocity.[/li] [li]When approaching horizontal attitude, reduce thrust so you\'re just pushing the Apoapsis ahead of you, perhaps gaining on it slowly, but not pushing it any further away.[/li] [li]Keep adjusting your attitude to point at the horizon. Keep low thrust. As long as you keep apoapsis slightly ahead of you, you will be climbing. If you have a longer way to climb, you can push apoapsis further ahead: just don\'t let it get too far away from you, or you\'ll get too elliptical, and that means cuttiing the engines for a while.[/li] [li]The trick to finish it off is to reduce thrust so you approach and hit apoapsis at your target altitude with zero thrust. Since you\'ve been burning horizontally the whole time and slowly gaining orbital speed, it should be pretty much circular when you get there. Check your periapsis and add a little burn if necessary.[/li]

As long as you\'re just dropping SRB\'s for takeoff assistance then yes.

That was a beautiful video, the music was a nice touch. What does the 21.8° refer to?

I assume 'get into orbit' means to establish the orbital trajectory, not complete the whole orbit: I got Apogee 108k, perigee 72k, then immediately deorbited. Total time to splashdown: 12:16 (game clock).

Either that or the silvery 'water' is very dense!

It can be hard to hit the shutoff at just the right time when the m/s numbers are flying by. If you burn 100% throttle and try to cut it using the X key at the right moment, it\'s hard to hit the right number, and then it immediately starts bleeding off. If you slow your burn as you approach the target speed, or make small adjustments after the burn, it seems like you\'re 'chasing' a target number, or trying to maintain a speed that should have already bled off, in effect ending up faster than you should be. Maybe I\'m overthinking it. It\'s just that after the theoretical 'instantaneous impulse', the speed naturally starts to drop as it gets exchanged for altitude. I haven\'t yet measured how much speed might be lost in the first 90 seconds of a steep transfer orbit, but if that\'s how long my ship takes to accelerate to the transfer orbit and I\'m trying to match a calculated speed I should\'ve theoretically had 90 seconds ago, then shouldn\'t my target speed be slower by that much? 90 seconds is 5% of the low orbit period; it\'s 18-ish degrees. I guess I\'m wondering what does that margin play out to? Is the difference just the extra distance travelled in my low orbit, or is the error magnified at the far end at all? I meant a FRT approach to the moon, where you arrive in front of the moon, then insert into orbit coming around the far side (retrograde). The question was about the differences in the return transfer from different orbit directions. What I meant was if you do your return transfer burn from the far side, the moon\'s gravity curves the path inward (towards home). If you\'re orbiting the other way (prograde) and your return transfer burn happens on the moon\'s near side, then the moon\'s gravity curves the path outward (away from home). In practice, the paths off the Mun are pretty straight once you get hyperbolic speed and the game is quite forgiving, so it\'s inconsequential here, really. But would transferring off a more massive outer planet make this significant? I\'m just curious how the calculations work, and what they might/might not account for. I never studied the maths, but lately I\'ve been picking it up a bit of recreational learning so it\'s fun to have toys like this to see it in action!

Thanks, that helps! I understood that the equations rely on an instantaneous impulse, so that\'s why I was wondering whether the couple minutes to accelerate needed any tuning. For a moderately massive ship, it takes about 90 seconds to get up to speed. A larger ship or a ship with low thrust could take more than a few minutes. I guess I was wondering how you know when you\'ve hit the target speed. Close enough; as quick as possible seems to be the answer: I shouldn\'t try and chase the last 50 m/s as I watch the speed bleed away when I cut the engines. What I was asking was whether this works for either direction orbiting the moon. A free-return trajectory with lunar orbit insertion will put you in retrograde orbit, so you have to burn on the far side to transfer back. A prograde lunar orbit would have you burn on the near side. Since transfer path gets curved in opposite directions by the moon\'s gravity in each of these situations, how much does it matter?

Just a few questions about using this for the Hohmann transfers: Do you need to factor in any time for your ship to accelerate? At the end of the burn when the engines are cut, the ship immediately starts losing speed since it\'s in a steeply climbing orbit. How do you know you\'re at the right speed if the speed is being drained away during the whole burn? Is there a way to fine-tune it? What about the angle? If the ship is gyroscopically pointed at the horizon (aligned with the flat orbit) at the start of the burn at the intercept time, does it matter how long the acceleration goes in that constant direction, or should the ship be continuously adjusted to the horizon during the whole burn? Do the high-to-low transfers work for prograde only? Which side of the orbit do you calculate from?

Ok, here\'s my run: Level flight: Engine cutoff in the mountains: Safe! Final landing spot: So I can just barely make it to the snowy mountains with those 3 tanks and 1 engine (with JATO for safety, although it takes off fine without it too), depending how well I prepare my approach... The distance runs seem to work better with an initial slow climb and then cruise on a long descent path with reduced throttle. As for agility, I think I\'ll need a joystick to make it any fun... it\'s too jumpy otherwise. I\'ve attached the craft file so you\'re welcome to try it.

Well, there\'s some room for interpretation of what makes an airplane... in the game physics, 'does it glide like a brick' is good enough for me. It\'s certainly possible though. nextgenerationliberty\'s plane counts! Again, agility is relative in this game... the idea is that you can even turn well enough to point at different targets reliably... I\'m open to change the parameters if you think it\'s too easy.

Seriously? 1 fuel tank wouldn\'t get you out of the atmosphere, let alone to the moon and back.

This game started with a stock lander design, using fins for legs. As I was test-piloting it around the KSC, landing on the tower, the VAB, the fuel drum, I found I could almost keep the thing flying when it pitched over... so I evolved my design to make it flyable, and started fooling around in the mountains west of KSC. I even took a few bombing runs, playing the mountain climbing challenge, but I don\'t have the range for any snowy peaks higher than 300m: I\'ve been able to just barely fly to one of the closest white peaks on the edge of the icefield to the west. Still working on refining the piloting, I\'ll post my own entry later. Can you make something vaguely resembling an airplane from junkyard parts, and make it fly? Challenge Description: Build and fly an airplane using only stock parts. The goal is to launch, climb, turn, achieve level and controllable flight as long as your fuel lasts, then pull the eject handle and parachute safely. Rules: No ballistic flight except the initial launch before you gain aerodynamic control and pitch over. Flight ceiling of 5000m. SRBs for JATO will be allowed, but otherwise single stage only. You fly what you launch with. RCS may earn you funny looks since this is supposed to be an airplane, but let\'s allow it for assistance as long as it\'s not your primary source of lift. The Kerbals\' pod must parachute safely at the end of the journey, so you might want a decoupler (not required if your design uses crumple zones: the criteria here is survival of the crew). Players may challenge one of two categories: [list type=decimal] [li]Agility: how many lakes can you fly over in the mountains to the west or north of KSC? To count a lake, altitude must be under 1200m.[/li] [li]Endurance: how far can you fly in a more-or-less straight line to the west of KSC?[/li] Suggested bonus agility challenge: fly a figure-8 between the VAB and the tower. (I have yet to try this, but I believe my plane can do it.) Proof of completion: Screenshot of your plane in flight, showing level flight and angle of attack < 45°. Agility scoring: shot of each mountain lake you fly over. Endurance scoring: Screenshot of final parachuted destination on the map. Allowed parts: Stock parts only. Have fun!

Nice trick! I didn\'t realize about the zooming, I\'ll have to figure out how it works on my Mac trackpad. I read somewhere here that the way the parts objects are modelled, the engine requests units of fuel from the part above them in the stack, and this request repeats to the next part above until the upper object says 'false: I have no fuel'. This allows the upper fuel tanks to return their fuel down the stack first. The fuel lines essentially work like jumpers so when the engine asks the ASAS, 'gimme fuel?', it responds with 'yes, I got some from above', thanks to its fuel line.

Good job! Looks like you still had enough fuel there to circularize your orbit if you waited until reaching Apo before dumping the lifter stage. Also, you should be able to lighten your orbiter by just using RCS to deorbit instead of wasting mass on a whole engine and a nearly-full tank.

Can you clarify how you do that? So does the fuel line attach on the bottom of the fuel tank? But then what does it attach to? I wouldn\'t mind trying it, how tight is the tolerance on hitting the flight plan just right?