Playing career mode it was a struggle to save money to upgrade the research facility to level 3. Having an SSTO made that struggle much easier. Many of the designs people share are with R.A.P.I.E.R.s and those need a level 3 research facility. I came up with this design to solve that conundrum.

Directions to space:

go to full power and release brakes
at ~125 m/s gently rotate to 20-30 degrees.
when the apoapsis reaches ~23km (speed should be >1,000 m/s) press the space bar to turn on the rocket (whiplash will run out of air and shut off by themselves).
rotate to 45 degrees (if you want, not necessary)
throttle to zero when apoapsis reaches ~75km.
Circularize at apoapsis as usual.
You should have 200-300 m/s dv remaining.

This craft can carry the following payloads to orbit:

6 kerbals (include MK2 Crew Cabin)
2 tons of rocket fuel (include MK2 Rocket Fuel Fuselage Short)
2 tons of liquid fuel (include MK2 Liquid Fuel Fuselage Short)
1.5 tons of cargo (include MK2 Cargo Bay CRG-04, small satellites)

Replace the MK2 Crew Cabin in the design with the appropriate part to lift the appropriate payload. The ship has a reaction control system and docking port so it is capable of docking with a space station. You could remove the docking port and include the CRG-08 for a larger payload.

To land first move all fuel to the front of the plane. Perform a retrograde burn or just use the mono propellant. Nose up to ~30 degrees as you enter the atmosphere and wait until airspeed drops. The Whiplash is not very efficient so you will not travel far in the atmosphere so come down close to where you want to land.

Non-Rapier Stock SSTO