Converting an SSTO to TSTO

[Jestersage]

After realizing how much the ITS resembles a KSP spaceplane, I realized a lowpart count Stock ITS can be made by converting some existing SSTO. However, I am wondering: assuming it was tested to be able to reach a 100x100 LKO, do mounting it on a rocket increase its DeltaV and allow it to travel to Beyond Duna?

[Streetwind]

Depends on what kind of SSTO it is.

If it is a spaceplane that relies on airbreathing engines to attain the majority of orbital velocity, mounting it on a rocket won't do you any good, because all that that rocket is doing is turning the whole airbreathing equipment into useless dry mass.

If it's a rocket SSTO, then absolutely yes - all you just did was add another rocket stage, turning a one-stage rocket into a two-stage rocket. The dV provided by that new lower stage is dV not consumed by the spacecraft during ascent.

[Foxster]

Something like this can be fun...

 

[AeroGav]

Spaceplane on rocket designs are hard to do though.    It is probably easier to take a successful SSTO with nuke engines and good LF capacity, and find a way to mount its jet engines and intakes on decouplers.  Easiest would be to just hang them under the wings over the CG, so there is no handling change when they drop off, like this design (somewhat outdated, need to get rid of those draggy fuel ducts for 1.2.1)

https://kerbalx.com/AeroGav/Astrojet-Citation-X

As you can see , the underwing Panthers and the Rapier are gone by the time we reach orbit.  The fuel tanks in the engine nacelles are drained first by the duct systems.  Not having the weight of jet engines to carry around, and having nearly full internal fuel upon reaching orbit, brings the design up from 2200 to 3700 dV.  Not bad for something so heavily laden with extras.

 

Alternatively if you decouple the engine off the back, you need to put a winglet on the nacelle to not have it's weight affect the handling when present.   Like this one - 

This can fly to the surface of Duna and back.  Video here is rather long but shows the engine decoupling right at the beginning.  Not using SAS or anything,  I'm flying with keyboard control using pitch trim.  Doesn't even flinch when the decoupler fires

Finally,   you could go full air launch to orbit and have your spaceplane ride up there on the back of a carrier jet.