The easiest and simplest SSTO (in sandbox, anyway)

[thereaverofdarkness]

Just start with a large probe core, put two orange tanks below it, and stick a mainsail engine on the bottom. Fins/reaction wheel/battery pack optional. It makes it to orbit pretty easily if you fly straight up until 10km, turn 45º east, then burn until your apoapsis is just above the atmosphere.

#2.) It might be possible to make a SSTO rocket with just the MK-1B command casule, LV-T30 engine, and fuel. I know my very first rocket in career mode was a thin margin from reaching orbit when I launched her. I used 7 of the starter fuel tanks.

I'm not sure why it's easier to make a rocket-only SSTO than a jet+rocket SSTO. Seems to defy logic. I discovered these rocket SSTOs accidentally just recently, and it makes all the hours I have spent on jet SSTOs seem kind of wasted--though my latest and greatest SSTO jet IS prettier than a rocket.

[Levelord]

Small probe core, half 1m fuel tank, 2 rockomax mini engines.

[UmbralRaptor]

Command part (probe core, Mk1 pod, doesn't matter), rockomaxx x16 tank (the medium one), LV-T30. If you want something bigger, use an orange tank and a skipper.

Getting the most payload out of a single engine SSTO means fueling it to rather poor (1.3ish) TWR, though.

[numerobis]

The payload fraction of a rocket SSTO is abysmal compared to a jet one. But they are much simpler.

[thereaverofdarkness]

It doesn't make sense to me that the rockets would be easier. I burn half of the fuel required to get to orbit by the time I reach upper flight level. But a plane can get up to 26km and have only burned 5% of its fuel. Why, then, is it so hard for it to get into orbit from there when rockets can do it so easily from the ground?

[Hunne]

I have yet to come up with a neat design that uses jet engines and e.g. the aero spike...I too get me contraptions most of the time up to 25km or beyond but getting to orbit seems impossible...well, career mode...but i´m almost done with the techtree and I really don´t need bigger plane parts^^

[UmbralRaptor]

The payload fraction of a rocket SSTO is abysmal compared to a jet one. But they are much simpler.

Well, yes. Even with the 48-7S, you're unlikely to break 11% with a rocket. Meanwhile, something like KwirkyJ's HELPr series can hit 70%...

It doesn't make sense to me that the rockets would be easier. I burn half of the fuel required to get to orbit by the time I reach upper flight level. But a plane can get up to 26km and have only burned 5% of its fuel. Why, then, is it so hard for it to get into orbit from there when rockets can do it so easily from the ground?

Isp != simplicity

In KSP rockets can (excluding payload) have as few as 2 parts, show consistent thrust in all environments, and are relatively forgiving in terms of control and ascent path. Those 2 part designs are even practical lifters! (eg: LV-T30 + x200-16, Skipper + orange tank) When adding a rocket to a rocket, you can also easily synergize (eg: asparagus staging), getting enhanced performance. Even with bad staging, current parts mean that you're more likely to see minor performance improvements rather than losses.

Jets are none of these. At a minimum, you need 3 parts. Practically, you'll be looking at over a dozen. Lift and drag must be dealt with in greater detail to produce a stable and controllable craft. Thrust is affected directly by speed, and indirectly by environment (altitude, speed, angle of attack) and craft design (intake count, directions, and types). Worse-still, synergizing is hard and mostly impractical. If you're using jets, it's well worthwhile to push their performance to the point of giving you a 30 x 75 km orbit. Switching to rockets at 25 km and 1000 m/s (mostly horizontal) leaves you in a rather high drag situation, with a velocity vector poorly suited to establishing orbit. Fairly often, craft show up that would see greater performance by switching to fewer/smaller engines or less fuel(!)

[thereaverofdarkness]

actually, I figured out why rockets do it easier. It's because when the rocket is down to 25% of the launching mass, it has 15% fuel--the plane at 25% remaining mass is empty. Considering the weight of the dry rocket to the remaining fuel, tha bit of fuel can actually grant almost as much delta-v as the plane gets from start to finish under its rockets.

That's why.