QF9E

I was too pessimistic in my previous comment: it turns out to be possible to complete the challenge without using any exploits whatsoever. No KAL hacking, no clipping, no fairing shenanigans or other dubious aero constructs. Because the craft has real drag now, I had to run the engine a couple of times to keep the orbit high enough in the atmosphere to prevent the craft from getting too low and burning up. And since the apoapsis of my orbit was substantially lower than the previous run, this run is faster as well.

Yes it is. I've since completed the challenge with a similar craft as the one in the video, with a single Vector and some fuel tanks clipped inside a fairing. I don't think it is possible without abusing fairing occlusion as the aero drag would slow the craft down too much.

It should just about be possible on Kerbin: The studio version of the song is 7:09 long (see https://www.youtube.com/watch?v=dwDns8x3Jb4). Taking x4 time acceleration into account gives an allowable mission time of 28:36. I launched my craft in the retrograde direction from the KSC, and it is comfortably past the KSC after its first orbit, at 28:42 mission time. I used KAL hacking to get infinite fuel: the craft has 2 Vector engines, one pointing forwards and one backwards, with the forward facing one at -100% thrust. It appears that you can no longer set out-of-bound values for the KAL inside the game, so I modified the craft file in a code editor, and that still works fine. The engines are inside a fairing, which is fully occluded front and back so as to give almost zero drag. I noticed that despite fairing occlusion the craft still had a bit of drag (probably because it does not have an exactly 0 degrees AoA in the lower atmosphere) so I had to fly at higher altitude than I would have liked. It might be possible to do this at a lower altitude, which will give you a lower time. The Vector thrust could also be set above 100% so as to accelerate faster at launch, that could shave off some precious seconds as well.

My Eve SSTO uses fuel cells to power the props. The fuel cells do not consume a lot of propellants. I did try solar panels but I wasn't successful with them - too much of a hassle keeping them from burning up during Eve atmospheric entry. I think a plane is better than a helicopter / drone, as the lift to drag ration of wings is way above 1. Which means that you need only a fraction of the thrust to propel a plane compared to a helicopter. Moreover, the Big-S wing and wing strake can store fuel as well, so you don't need as many fuel tanks, which also means that the mass penalty for using wings is not very high.

I've built an Eve SSTO a while ago: https://imgur.com/a/1AD3u40 Propellers are the best option for the lower atmosphere, then you will need lots of thrust to ascend to orbital altitude, and then you will need lots of dv. My craft used propellers to about 13 kilometers altitude, Vectors for the rapid boost to suborbital trajectory and NERVs to insert into Eve orbit. As to Vectors: The Mammoth is basically 4 Vectors but is 1 ton lighter than 4 Vectors so I think that is the best engine for an Eve SSTO.

I landed on Moho today without using ISRU and no gravity assists. I then returned my lander to Kerbin where it was recovered at the KSC. This mission concluded my long-running "Leave Nothing but Bootprints" campaign, in which I landed on every body in the Kerbol system without using ISRU and fully recovered all hardware used to do so. In the end I managed to land on all bodies with an SSTO, except the "big three": For Eve I used a couple of space shuttles with flyback boosters (4 stages in total): one shuttle consisted of the Eve lander, the other shuttle was a tanker to refuel the lander before landing on Eve and after its ascent from Eve. For Tylo I used a cargo SSTO to lift a lander + interplanetary stage to low Kerbin orbit (3 stages in total). The interplanetary stage then flew to low Tylo orbit with the lander, which then landed on Tylo, returned to the lander, with the entire stack returning to LKO to dock with the cargo SSTO and land. For Moho I used the same cargo SSTO to lift an ion engine powered lander to LKO (2 stages in total), which I then flew to Moho and back. The lander then flew back to LKO where it docked with the SSTO before landing the entire combination.

I test-flew an old SSTO that I made for a low part count challenge back in August. The challenge asks for a minimalist plane to fly from KSC to the Island Runway and back, and it turns out that - unknown to me until today - my SSTO can do so while reaching Low Kerbin Orbit on both legs of the journey. Here's my completely accidental SSTO2: Craft file here: https://kerbalx.com/QF9E/Low-part-count-SSTO

It turns out my little 9 part spaceplane is quite a bit more capable than I originally thought: it is able to reach orbit twice without refueling. So here's a flight from KSC via Low Kerbin Orbit to the Island Runway, then back to the KSC via a second LKO.

Let's do this! No SAS, no reaction wheels, no GUI and I use the pointy-nosed Mk1 cockpit, which has much reduced visibility compared to the Mk1 inline.

I've gone to Duna and back without leaving the (stock) cockpit, which means you cannot use maneuver nodes. It is also possible to go to Duna without doing any steering, see https://imgur.com/a/Y9QpQsg I did use a maneuver node there, but I think it should be possible to do without.

I'm using version 1.12.5.3190 which as far as I know is the current version. I do run a couple of mods, but nothing that should change parts. But to be on the safe side I just tried without mods: my craft still takes off like it should. The impact tolerance for the Girder XL should be 80 m/s, as shown here: https://wiki.kerbalspaceprogram.com/wiki/Modular_Girder_Segment_XL. My SPH shows the same value: In light of all this I don't think it is something on my side. Maybe the girders have been inadvertently changed by some mod or other in your install? Or perhaps there's an issue with your craft. What is the takeoff speed of your craft with normal landing gear? Mine takes off at about 60 m/s, and the girder does explode as expected at 80 m/s if I don't take off in time. And how did you place the girder? It should be placed such that the back end of the girder is only a little behind the center of mass of your craft. Too far back and it will be impossible to pull up the nose during the takeoff run. Not saying you do this, but I see many videos of KSP players who place the main landing gear of their SSTOs (or, in this case, the back end of the girder) so far back that it is impossible to pull up the nose during a take-off run. Many players just run their SSTOs straight over the end of the runway, only pulling up once over the edge. This technique will not work when you're using girders for landing gear as the speed will be too high by then. Thanks for the heads-up. I renamed the craft in KerbalX and I did not realize that this changes the link to the craft. The correct link is https://kerbalx.com/QF9E/Low-part-count-SSTO

9 part SSTO: Craft file: https://kerbalx.com/QF9E/Low-part-count-SSTO I used a girder as a skid rather than a conventional landing gear, which saved me 2 parts compared to @OJT's submission. For the wing I used a single Big-S wing which doubles as a fuel tank, which saves me another 2 parts. And I flew the entire mission on the capsule battery, which saved a further part. To save on electricity I switched off the reaction wheel during atmospheric flight - I didn't need it as the control surfaces offer ample control by themselves. And I carefully balanced my craft so that on re-entry only a small amount of electricity was needed for the reaction wheels. I used the Precise Position mod to position my wing so that its center of mass / lift is eactly on the centerline of the craft. The lack of landing gear means you don't have much control over the craft on the ground, which necessitated a careful approach and landing on the Island Runway, in order not to run out of runway for the subsequent takeoff. And I almost overshot the KSC runway as my speed was a bit high - I knew from testing that the landing skid tends to explode when touching down at over 80 m/s.

You can also replace the 3-part landing gear with a single modular girder segment XL. Or any other part that has a high impact tolerance, forms a relatively stable platform and has limited drag. To prevent excess drag, you could use a Mk2 or Mk3 cargo bay as impromptu landing gear - you can land a craft on their open cargo bay doors. Although the Mk2 and Mk3 form factors may not be the best choice for low part craft.

Three part submission: I mainly used the parachute to turn the craft retrograde so that it can use its rocket engines for a landing burn. Note that I ran out of fuel a second or so before touching down on the KSC runway. If that is sufficient grounds for disqualifying me under rule 1: "running out of fuel", so be it. There's a couple of times in the flight where the flight was less efficient than it could have been, so it should be possible to perform the flight and not run out of fuel. Therefore I hope you will accept my submission as-is.