SSTO spaceplanes in upscaled systems?

[Norcalplanner]

I did some SSTO spaceplane stuff in the past, and did OK - not great, but OK. My current career is 3x scale with a 1.2x atmosphere and stock aero and heating, and I've been unsuccessful in making one for a larger Kerbin.  Is it possible to do an SSTO at 3x scale? If so, are there any particular tips? Orbital velocity is about 4 km/s and a typical rocket needs 5.4 km/s of delta v to make it to LKO. Any help would be appreciated.

[Streetwind]

This is actually not unexpected behavior. You see, the only reason why spaceplanes are even a thing in KSP in the first place is the fact that Squad has given us jet engines operating at better-than-RL speeds, for use on a planet where orbital velocity is less than one third of RL speed.

Jet engine top speed on Kerbin: roughly Mach 5
Kerbin orbital velocity: roughly Mach 8

Jet engine top speed on Earth: roughly Mach 3.5
Earth orbital velocity: roughly Mach 25

^--- That's why we don't have spaceplanes yet IRL. While technically possible, the atmospheric engines make so little difference that you might just as well build a full-on rocket. Only better jet engines will potentially fix that (see SABRE).

And this is also the reason you're failing to build a classical SSTO spaceplane in an upsized solar system. Your jet engines continue to have the same performance, but orbital velocity increases; therefore, the advantage that using jet engines over a pure rocket stage brings with it shrinks rapidly. In stock KSP, a spaceplane is basically "a jet aircraft with a little bit of onboard rocket assistance". In an upscaled system, a spaceplane will trend more and more towards being "a rocket that airlaunches itself". Trying the former approach instead will not yield results, because you won't be able to reach orbital velocity if you rely on jets as much as stock spaceplanes do.

My recommendation is therefore simple: stop trying to build an aircraft that goes to space. Start building a capable rocket instead, and then figure out if you can improve it by attaching wings and jet engines. You'll want to budget at least 2500 m/s worth of acceleration on rocket engines alone. Try and build a rocket stage out of spaceplane LFO fuselages that has about 3.5k dV. Then, see if you can get that thing transsonic under jet power, without the vessel's total rocket dV falling below 2.5k due to the added weight. I'll wager a guess and say that breaking the sound barrier with such a craft will be the biggest challenge; due to the way ramjets work, once you're past that, you're probably good to go.

 

Edited March 6, 2016 by Streetwind

[Norcalplanner]

2 hours ago, Streetwind said:

My recommendation is therefore simple: stop trying to build an aircraft that goes to space. Start building a capable rocket instead, and then figure out if you can improve it by attaching wings and jet engines. You'll want to budget at least 2500 m/s worth of acceleration on rocket engines alone. Try and build a rocket stage out of spaceplane LFO fuselages that has about 3.5k dV. Then, see if you can get that thing transsonic under jet power, without the vessel's total rocket dV falling below 2.5k due to the added weight. I'll wager a guess and say that breaking the sound barrier with such a craft will be the biggest challenge; due to the way ramjets work, once you're past that, you're probably good to go.

 

Streetwind,

Thanks for the great explanation.  I'll give that a whirl. 

I suspect I'm going to need to upscale a bit.  It might also have to turn into a semi-staged vehicle, with a pair or two of SRBs hung like missiles under the wings (thinking about the 0.625m ones from SpaceY to minimize drag) or doing some sort of Shuttle / Buran type setup with a cheap disposable lifter that can be recovered with Stage Recovery.