vyznev

By the way, while experimenting with asymmetrical propellers, as inspired by @AHHans's planes above, I noticed something funny: Apparently the Breaking Ground propeller blades have their center of mass well outside the blade itself! I assume this is a kluge intended to reduce instability and/or centripetal stresses — when mounted on an appropriately sized rotor, the offset CoM ends up pretty close to the rotor axis. It's as if all the blades had built-in invisible counterweights. Conveniently, it also means that designing a well balanced asymmetrical propeller isn't nearly as hard as it might seem. I wonder if an asymmetrical helicopter could work…

So I realized that I never submitted anything for this challenge back when it was first posted. I remember trying to build an asymmetrical SSTO without using the Rapier engine, but finding it really hard to keep a stable heading while switching over to rocket propulsion in the upper atmosphere. Eventually I just gave up on that particular project and just kind of forgot about this thread. Anyway, looking at all the crazy weird-looking contraptions people have designed here, I thought about trying to subvert the challenge by designing the most normal-looking and airworthy plane possible that would still technically qualify for this challenge. Thus, let me hereby present the Hapax — a plane built out of 18 parts, each of them unique: It flies great with SAS off — in fact, it flies better with SAS off than on, as the stock SAS for some reason doesn't seem to like my use of the FAT-455 control surface as a tailplane, and tends to develop pitch oscillations. Or maybe it just doesn't like that fact that this plane has no reaction wheels at all — even the cockpit's built-in reaction wheel is disabled. With SAS off, the plane does have a slight tendency to roll spontaneously, but it's slow and easy to correct for. In any case, this is a very nice and agile plane, capable of going both fast and slow as the situation demands. Flying perfectly well under physics warp, and with enough fuel to cruise for several hours, this might just become my new favorite survey plane. Craft file: https://pastebin.com/Brg0YYy1

Given Kerbin's treacle-like water, I suspect that the best way to do this — if it's possible at all — is to have as little of the boat in the water as possible. So basically make a supersonic plane that will fly straight and almost level at 0 meters ASL, and then dip a couple of small "hydrofoils" in the water to provide a tiny bit of extra lift. And make those hydrofoils as sturdy as possible. …which I suspect may be the showstopper here. I'm not sure exactly how KSP models objects interacting with water, but I have a hunch that any part hitting water at any speed higher than its "maximum impact tolerance" is just going to shatter. And I don't think there are any stock parts with impact tolerances higher than 300 m/s.

Damn, that's a tricky one. On one hand, you clearly touched the ground before landing. On the other hand, I'm pretty sure that, according to KSP, the ramp you bounced off counts as part of the "launchpad" microbiome, so one could argue that it's within the rules. On the gripping hand, if I said yes, tricks like that would almost certainly become the new meta. So the real question is whether or not that would make the challenge more enjoyable, and also how I'd personally feel about judging the inevitable follow-on questions about whether it would be OK to just land on the ramp, or even on the ground next to it, and where exactly does the border of the launchpad area lie etc. So, alas, I think I'm going to have to be strict and make a ruling that the only permitted landing area is the flat central part of the launch pad, and that bouncing off the ground anywhere outside that area is not allowed. Sorry. But your first run is definitely valid, and puts you in the lead in any case.

Nice work! I must say I didn't expect to see SRBs used in this challenge, but in hindsight it makes sense. Those Sepratrons have some pretty crazy TWR. I guess you'll just have to try for 19 seconds, then… …ok, nevermind. (Also, I love how the two of you are so closely neck to neck despite using such radically different craft designs and flight controls.)

…and just as I'd managed to update the leaderboard again. Now I'm mildly curious about how you're controlling your engines. One set bound to throttle and the other to a custom control axis? A nice looking design in any case.

Agreed, this looks like a perfectly valid entry to me. You're now first on the leaderboard again. And while I must say that I didn't expect you all to push the time quite this far down, I was aware that the faster you went, the less you'd be actually hovering and the more it'd just feel like rocket-assisted ballistic jumping. So you could say that this was sort of expected, at least qualitatively if not quantitatively. Also, nice legs. After seeing last night's videos, I was wondering when somebody would decide that more landing legs would be better. For that matter, I wonder if there's anything to be gained by using plane landing gears instead — they have some pretty impressive impact tolerances…

Wow! I go away from the site for an hour and this happens. Anyway, you're all on the leaderboard now. Indeed. I wasn't expecting the landing to be the most difficult part of this challenge, but that's the way it looks to be going. To paraphrase Tom Petty, coming down indeed seems to be the hardest thing.

It's a little bit hidden, but basically click your username at the top right corner of the page, then select "Account Settings" from the menu, and then "Signature" of the left side of the settings page. (If you're on mobile and don't see your username on the top right, click the three-line "hamburger icon" instead, then "Account", then "Account Settings", then "Signature" on the new page just as usual.) Or just follow this link: https://forum.kerbalspaceprogram.com/index.php?/settings/signature/

That's pretty clever! I've added you to the top of the leaderboard. Congratulations!

No need for a pilot, and yes, SAS is allowed. (FWIW, my own attempt was uncrewed and used SAS.)

That's 100 meters plus the altitude shown before launch (which will be the altitude of the launch pad, plus the height of your command pod / chair / probe core above the launch pad). That is to say, the bottom of your rocket (which will be sitting on the launch pad before you launch) should not rise more than 100 meters above its original altitude, since the top of the VAB helipad is about 100 meters higher than the center of the launch pad. But since we can't easily measure the altitude of the bottom of your rocket (unless you place your core part there, which could be kind of inconvenient), we'll just assume that your rocket stays more or less vertical and measure from whatever KSP treats as its reference point, which I believe will normally be whatever part you're controlling it from. (I actually went and measured it by having Jeb climb up to the helipad, and the actual altitude difference between the launchpad and the VAB helipad seems to be about 102 meters. So you can have an extra 2 meters of margin if you absolutely need it. But 100 meters is a nice round number, so I'll keep it as the official limit.) Also, I went and made some badges! (ping @sturmhauke) Feel free to pick your preferred size, or just download this 1000 by 1000 px image and rescale it. (Note that the caption below the sign is invisible on imgur, since it's black on black. View on a light background for best results.)

Awesome, congrats! That was way faster than I was expecting. Looks like the race is indeed on. Nice use of the Vernor engines for steering. It looked like you were cutting it pretty close to the altitude limit there at 1:10 to 1:20, but I checked the numbers and it looks like you're OK. So onto the leaderboard you go. Sorry, forgot to mention those. Thanks for the reminder! But no, parachutes are not allowed. I guess they would be primarily useful for landing, but that's part of the challenge.

As per the challenge submission guidelines, here's my first attempt at completing this challenge myself. Spoiler: I didn't make it back, but I did manage to get about 3/4 of the way around before hitting the mission control building and crashing. In any case, despite the failure, I believe this attempt should be sufficient to demonstrate that, with careful piloting, the challenge should in fact be possible to complete:

This is a spinoff from @Petabyte's Longest Hover challenge. Your task, should you choose to accept it, is simple but probably not easy: Launch a rocket from the launchpad. It must have no jet engines, parachutes or aero surfaces (wings, fins, elevons, propeller blades, etc.) of any kind. Hover around the VAB while keeping the bottom of your rocket below the roof level. Hitting the ground or the roof of any building is of course not allowed either. Land back on the launch pad. As this is a race, entries will be ranked by time from launch to landing, but the real challenge is just making it around the VAB and back to the launchpad at all. Anyone who completes the challenge above will earn a place on the leaderboard, a cool badge and eternal fame and glory.* (* Amount and duration of fame and glory may vary.) Rules and clarifications: All entries should be built with stock parts. DLCs are allowed. (I may add a separate category for modded entries if there's demand for it.) As this is a piloting challenge, using any kind of autopilot-like mods (including Mechjeb autopilot or kOS scripts) is forbidden. Use of stock SAS is allowed. (Again, if people want to submit entries using kOS etc., I'll add a separate category for those.) Purely cosmetic or informational mods are OK, as is anything else that doesn't significantly affect physics or piloting. I'll even allow FAR, since it shouldn't make much difference for this challenge. If in doubt, please note in your entry what mods you're using. Also, any physics cheats like reducing gravity are obviously also forbidden. So are ladder drives and other glitch exploits. The challenge is to complete the course while hovering on rocket engines. Jet engines and any aero surfaces (wings, fins, elevons, propellers, etc.) are forbidden, as are parachutes. (Yes, that includes using the EVA parachutes on Kerbals.) All kinds of rocket engines are allowed, including RCS thrusters and SRBs. If it would work in a vacuum, it's OK. Both crewed and uncrewed craft are allowed. Staging is allowed (but shouldn't be required, if you're fast enough). However, crashing is not. Your craft should be able to take off again after landing, at least if refueled. Also, the VAB and the launchpad have to be still intact at the end of your flight. As the challenge is to fly around the VAB and not over it, the bottom of your rocket should never rise above the level of the helipad on the roof of the VAB, which is 108 meters above ground level. In particular, as long as your indicated altitude above sea level never exceeds your launch altitude + 100 m, you should be good. Also, as this is a flight challenge, your rocket should not touch the ground (or the roof of any buildings) except while on the central flat area of the launchpad. Bumping into walls is OK, I guess, but e.g. bouncing off the launchpad ramp is not. To show that you've completed the challenge, post either a video of your flight or screenshots showing (at least) your craft 1) on the launchpad before launch, 2) hovering on the west side of the VAB, and 3) landed back on the launchpad. If you don't have video, also post a screenshot of the F3 popup at the end of the flight showing the highest altitude reached (which should be at most 100 meters above your indicated altitude at launch). There's probably something else that I've forgotten. If you have any questions or notice any omissions, please let me know. This is my first challenge here, so it's probably not 100% perfect. Thanks! Categories: As of April 10, 2020, this challenge now has two leaderboard categories: a general category, with the rules as specified above, and a new Ultra Low TWR category! To qualify for the Ultra Low TWR category, in addition to following the rules for the general category, your craft's TWR (thrust-to-weight ratio) must not exceed 1.1 at any time during your flight. The easiest way to show your TWR during flight is to click the delta-v indicator button below the staging menu on the left side of the screen, which will expand the staging menu to show (among other things) the TWR of your current stage. You are also strongly recommended to record a video of your flight with the TWR display visible, in order of verify that your TWR stays under the limit at all times. (Alas, KSP does not show your peak TWR in the F3 popup window.) Leaderboards: General: @jshu — 0:16 with a fully automated craft using Breaking Ground robotic parts and a KAL controller! (link) @ralanboyle — 0:18 with four Cub engines and a Spark and extra landing legs (link; earlier runs: 0:49, 0:25, 0:20) @jimmymcgoochie — 0:19 using two upward and two horizontal Twitch engines (link; earlier runs: 1:05, 0:57, 0:22) @Petabyte — 0:36 with four Twitch engines (link) @sollarflare — 2:21 with a whole bunch of Twitch engines, almost qualifies for Ultra Low TWR (link) @DRAG0Nmon — 2:22 with four Twitch engines, piloted by Jebcal Kerman (link) @sturmhauke — 2:36 with an aerospike and Vernors for lateral control, piloted by Jeb in a command chair (link) … Ultra Low TWR (NEW!): @ralanboyle — 1:14 with one Spark, piloted by Herdock Kerman (link) @vyznev — 1:27 with a Spark and four Spiders, piloted by Val (link) … Honorary mentions: @Klapaucius with "Sophocles", an SRB-only entry! While the 2 minute flight did not quite meet the letter of all the rules, the sheer kerbal awesomeness of this crazy flame-spewing contraption definitely deserves an honorary mention.

Actually, I had an idea for a spinoff challenge. I won't spoil it quite yet, but it feels interesting (and way more challenging that just hovering in place). I'll just need to record a video to show that it's doable… Edit: Here we go…

BTW, I worked out the actual theoretical upper limit on the hover time. It turns out that I got very close to it. For those interested, let me briefly explain the math: So it turns out that the theoretical maximum time a rocket with a single Mammoth engine (which should be the optimal choice among the stock engines) can hover at Kerbin's sea level is 567.2 seconds, or 9 minutes 27.2 seconds. In principle, we could do slightly better by sticking together multiple Mammoth engines and fuel tanks with just one probe core. But all this achieves is spreading the tiny 0.04 t mass of the OKTO2 probe core over multiple engines. In the limit, this would be equivalent to just removing the mass of the probe core from the equation, which turns out to increase the hover time by less than 0.2 seconds. But wait, how was my rocket still hovering at 9:28 seconds into the flight, then? Well, the reason is because I wasn't hovering at sea level. Even a couple hundred meters of extra altitude is enough to slightly decrease the air pressure and increase the thrust that the engine produces for a given fuel consumption rate. For example, a quick check using the delta-v menu in the VAB shows that at 500 m above sea level the maximum thrust of the Mammoth engine increases from 3446.03 kN to 3766.9 kN. Plugging that improved thrust into the formulas above gives an Isp of 2909.07 m/s and a maximum hover time of 570.9 seconds, or 9 minutes 30.9 seconds. Of course, I clearly didn't quite manage to reach that theoretical optimum. Besides imperfect flying, one reason is that my rocket was slightly underweight at only 375.04 t of wet mass at launch, almost 7 tonnes short of the optimal sea-level m(0) = 381.858 t, which is enough to drop my rocket's theoretical maximum hover time (at 500 m ASL) down to just 569.06 seconds. Adding a few more fuel tanks (say, an FL-T800 and an FL-T400, for a total launch mass of 381.79 t) could've maybe allowed me to squeeze out that extra second or two of hover time. OK, so can this magic limit of nine and a half minutes be beaten? Well, maybe, sort of: We could try using DLC parts. The Mastodon engine from Making History has a better sea-level TWR than the Mammoth, although its Isp is worse. Alas, a quick calculation suggests that, even clustered, it can only hover for 559.65 seconds at sea level, which is slightly worse than the Mammoth. Obviously, if the altitude limits are to be measured from the ground, then we could haul the rocket to a mountain top and hover there. This would not only increase the rocket's thrust, but also (very slightly) reduce the force of gravity. Off the top of my head, I'm not sure what the Mammoth engine's thrust would be 500 m above Kerbin's highest mountain top, and I've spent way too much time researching this already to bother trying to find out, but just plugging in the engine's vacuum thrust of 4000 kN gives a theoretical hover time of 610.66 seconds. It might be possible to break the 10 minute limit this way. We could go full Kerbal and try using SRBs instead! Alas, it would seem that, despite their impressive sea-level TWR, they all lose to LF engines both in Isp and in their dry/wet mass ratio (which is fixed, since all the fuel comes with the engine and you can't add more). FWIW, the Clydesdale would probably be the best of the lot, with a theoretical hover time of 404.17 seconds at sea level. In any case, since you can't throttle SRBs (and since stock KSP won't even let you adjust the thrust curve), the only way to hover with them would be to cluster multiple engines with different burn rates and fire them in a carefully planned sequence. (I'll leave adjusting the figures above for the new 1 km height limit as an exercise for now. You can probably squeeze out a couple more seconds of hover time from that.)

After seeing your entry, I realized that my plane had way more wing area than it really needed. So I cut it down. I also added one more pair of engines after realizing I had over 100 units of EC left after my first test flight. At this point, I think the limiting factor is actually my own piloting skill. I could probably save well over a minute just by landing faster. Anyway, here's my new run. Final time 09:15. This time I even recorded a video! Craft file: https://pastebin.com/BWU3NzvH

...and I assume staging is also not allowed? Anyway, with those restrictions, I figured the optimal craft ought to be fairly simple: take the engine with the highest sea-level specific impulse, which turns out to be the Mammoth.* Stack enough fuel on it to put the sea-level thrust-to-weight ratio just over 1.0 and add the lightest probe core available. Then just turn on SAS, launch at full throttle and slowly throttle down as the fuel is depleted and the TWR goes up. *) The Vector engine has the same sea-level Isp as the Mammoth, but the Mammoth produces 4 times as much thrust as the Vector while weighing slightly less than 4 times as much, so using the Mammoth saves a bit of dead weight. And it worked. This was really supposed to be just a test run, so I didn't bother recording video, and I did go briefly above 500 m at the beginning because I didn't throttle down soon enough. But this thing hovers beautifully. So, yeah, pretty nice flight, if I may say so myself. Nine and a half minutes is way longer than I expected. In fact, I was quite surprised that my hover time was longer than the Mammoth engine's Isp of 295 seconds (i.e. just under 5 minutes), since I originally assumed that this should be the theoretical upper limit. However, I realized my mistake afterwards: the Isp gives the maximum time that the engine could theoretically support its initial fuel load while hovering in normal Earth (or Kerbin) gravity, assuming that the weight of the engine itself and any fuel tanks etc. was negligible. But since the craft gets lighter as the fuel is depleted, and that allows the engine to be throttled down, the actual maximum possible hover time is longer.

By "spacecraft", I assume you mean a rocket-powered one? Because I'm pretty sure I could build a solar and/or RTG powered helicopter than can hover forever. Possibly even without using DLC parts, although it'd certainly be much easier with the Breaking Ground motors. Also, I assume this should be done on Kerbin, and that the altitude is to be measured from the surface and not from sea level. (If you wanted, you could specify an airless celestial body like the Mun or Tylo instead to definitely rule out any non-rocket solutions. But you do need to require a specific body, or everyone will choose Gilly.)

Do I have to be on the ground while turning around? Because on my best run so far, I kinda wasn't. (Came down a bit too fast, did a touch-and-go bouncing off the island runway, turned around in the air and managed to land on the second attempt. I did come to a stop before taking off again, but forgot to turn around a second time.) Anyway, nice challenge. I was playing around with Breaking Ground electric props anyway (for an Eve ascent vehicle), so this was a perfect fit. I'm still trying to figure out exactly how to make the best use of all these DLC parts, so this plane probably won't be breaking any speed records. But at least I can set a baseline for others to beat. This thing is basically a small ore tank with wings and a tail. 300 units of ore, 300 units of electric charge. It flies just like any normal plane, and is stable enough that SAS is strictly optional. (I flew this mission without it, if only to conserve power.) Stage to start the engines, throttle controls torque. The main quirks are that there's no movable rudder, so steering off the ground is by roll and pitch only (which is how I tend to usually fly anyway), and that the H/N keys ("translate forward/back") adjust propeller pitch. For optimal thrust, you'll typically want to try to keep the prop AOA between 5° and 10°. Final elapsed time: 10:06. I could've almost certainly got it under 10 minutes if I hadn't messed up the first landing. More pictures: https://imgur.com/a/iY8LgRq Craft file: https://pastebin.com/3erfS5GS

Hmm. Have you considered a two-stage recoverable design? Like flying the craft to LKO in one piece, then decoupling and having the second stage fly off to the Mun while the first stage re-enters and lands back on Kerbin. It's a bit tricky to design both stages so that they can safely land separately after decoupling, but it's doable. It's also not strictly necessary for the first stage to actually reach orbit — if you're a few hundred m/s short of orbital velocity with an apoapsis around 80–100 km or so, you can decouple the second stage and have it boost itself to orbit before the first stage re-enters. You can't really land back at KSC that way, though. (Here's an example of the concept, although that one uses higher-tech parts and the second stage isn't designed to land since it's an interplanetary probe.) It's even possible to fly a Falcon 9 style mission without reloading in KSP by launching the first stage into a sufficiently high suborbital trajectory that the second stage can circularize before the first stage re-enters. With a steep enough ascent trajectory, even return-to-launch-site is possible. (Ps. Your screenshot doesn't show up for me unless I first visit the link directly. It seems your image host doesn't allow hotlinking. Try e.g. imgur instead.)

My experience with the run I posted above (and the various more or less successful practice landings before it) is actually that I probably started decelerating too soon, and would've saved some time by waiting until I was almost above the KSC before making a pretty steep descent. And I still cleared the mountains with plenty of room to spare. Of course, a perfect 90° turn just above the KSC isn't optimal either (even if your craft could do it), but you really do want to minimize the time spent flying (relatively) slowly through the lower atmosphere. Anyway, I'll probably try a new flight in the retrograde direction, and with a steeper descent, at some point. My gut feeling is that I might be able to get below 30 minutes that way, but that'll require optimizing the ascent too.

I just wanted to leave this here. More detailed flight report coming later. Maybe. Sleep first. Statistics summary: Launch mass: 8.485 tonnes (+ 1 kerbal in command seat, optional) Part count: 16 Cost: 23,880 Passengers: 1 Flight duration: 35 minutes 38 seconds Highest altitude reached: 34.66 km ASL Fuel left in tank: 0.61 units Basically, I thought my old 5 part LF only SSTO might be a good starting point for this challenge, so I tweaked it until I got something that worked and met the challenge requirements. The idea is to minimize drag and heat issues by having just a single stack of 1.25 m parts. The craft climbs up to just under 35 km using the whiplash jet engine, then turns around and uses the nuke to build up and sustain orbital velocity at that altitude. The initial version had no aero surfaces at all, but I ended up including two advanced canards for more flight control and to stabilize the craft when flying backwards using the nuke (which this craft does for most of the flight; only the initial climb up to 30 km and the final glide back to the runway are done in the "forward" orientation). Takeoff and landing are both vertical (using parachutes for the latter) because, while this thing actually glides quite well and is surprisingly agile in the air, it has no landing gears. Craft file here. Flying it is quite tricky, not least due to flip-over maneuver needed to switch from jet to nuke propulsion. Said maneuver is really only practical above 30 km, and of course the challenge forbids going above 35 km. Also, the whiplash jet starts losing thrust above 15–20 km and flames out around 25 km, and you really want to have both the correct apoapsis altitude and a decent fraction of sideways orbital velocity built up by then. Oh, and while turning around, don't forget to reverse the control authority slider on the canards or the thing will be quite unstable indeed. And remember to do it again when turning back for the final glide. During the cruise portion, you only need about 1.5% thrust on the nuke to counteract drag. I recommend dragging the thrust limiter on the nuke way down for finer control. You'll basically want to set SAS to retrograde hold (since you're flying backwards, of course) and control your orbit using the throttle. Only use the W/S keys if your climb rate is way off and needs to be corrected ASAP. A useful rule of thumb is to try to keep your apoapsis just under 35 km and about 1 minute ahead of your current position. Although once your orbit gets circular enough, the time to apoapsis kind of stops being meaningful. Hotkey 1 toggles the jet engines and 2 toggles the nuke. Hotkey 3 opens and closes the cargo bay; it makes a decent airbrake when open, and you also must open it before deploying the chutes. Just don't try to slow down even faster by turning sideways at high speed with the cargo bay open, or this can happen: This particular flight was hardly optimal. In fact, it was originally meant to be just a practice flight, but it turns out I got a pretty good climb and ended up in a nice cruise orbit. Still, I could probably shave at least a minute off each end. The real challenge, I feel, is getting below 30 minutes. The orbital period around Kerbin at 34.5 km is about 28 minutes, so it's technically possible even without any downward thrust tricks. But there's not a lot of margin left for takeoff and landing.

Actually, this should be (theoretically) doable on Gilly. According to the wiki, a synchronous orbit around Gilly needs a semi-major axis (= average of periapsis and apoapsis, measured from the center of the body) of 55.14 km. That's less than half of Gilly's SOI radius of 126.12 km, so a synchronous orbit of any eccentricity (that doesn't intersect the surface) should be possible around Gilly. Also, a semi-synchronous orbit (i.e. one with a period 1/n times the main body's rotation period, for some integer n > 1) could also work, and would make this possible on most (probably all) moons in KSP. The periapsis of such an orbit would cycle between n different points on the surface, but as long as you made sure to build the "hoop" at the highest one of those points, it should be fine. Also, probably the easiest way to do this would just be to build the hoop at the highest point on the moon's equator and put the satellite in a perfectly circular equatorial orbit. That should work on any moon, and would have the added advantage of not having to worry about getting the phase of the orbit and the argument of periapsis exactly right. Just zero out the inclination, drop the periapsis to the desired altitude and then do a burn at periapsis to circularize. Fine tune with tiny radial burns if needed.