Hey I've been playing KSP for quite a long time, and now for the first time I decided to do a challenge. So, meet WhiteBird:
Top recorded speed: 3301.8 m/s.
The craft is powered by two Whiplash and two Aerospike engines. After takeoff, pitch it 20 degrees and fly on Whiplashes to 20km. At that point, your speed will be around 1000m/s and whiplashes will start to choke. Turn on aerospikes and keep firing them until your apoapsis is 45km. Then shutdown the engines, point prograde to minimise drag and coast. When you are 15 seconds from apoapsis, point prograde and fire aerospikes.
At this point, watch your vertical speed. If left unchecked, it'll go from negative to positive values, blow up rapidly and put you on Kerbin escape. Make sure to lose altitude during the entire brun, if needed pitch slightly downward from prograde.
Once you're done firing and achieved your top speed, you need to aerobrake to capture around Kerbin. Do that by pointing radial in or radial out. You'll only be able to capture around Kerbin via aerobraking if you got your vertical speed management right.
After a few aerobraking paths, you'll deorbit. Glide towards the surface and land, either on land or water. At this point, to better control your pitch, you may want to set both of the two control surfaces to respond to pitch (originally one of them responds to roll only and another - to pitch).
The craft is uploaded here.
