After making a successful round trip to the Mun, Jebediah decided he would spend some time figuring out how to get to that moon and back without having to space walk and burn his suits thrusters while pushing against the pod nine times inorder to adjust the trajectory of his pod enough to return to Kerbin because the damn vessel Mun mk 6 ran out of gas early. NASA requires constant progress so Jeb tagged Bob Kerbin "Your it!" and ran off.
Now, Bob has been spooked about going to other orbital bodies since he and a friend got stuck on Munis three times and Mun once when their damn Rover flipped every time they got one. Bob headed over to the aircraft hangar.
Simple cockpit, two horizontal wings and six continous burn rockets. Good for about a 5 second engine burn and then the 'aircraft' goes in to glide mode and managed to remain in the air for about 5 minutes before landing on the steel girder attached to the belly of the cockpit as makeshift landing gear. Hey, NASA was nervous about Bob after the multiple Rover flip incident across two moons and limited his creative budget significantly.
The budget increase not being as much as he felt he deserved, Bob instead decided to simply improve how the mk1 performed. Thus the vertial stabilizer as well as actual stabilizers on the horizontal wings. He used this model to buz the headquarters building four times and refused to come down until they gave him a respectable budget increase.
And to his suprise, they gave him a budget equivalent to a probe launch on munis last month. Happily Bob started from scratch, completely ditching rocket engine technology. Three liquid jet fuel tanks, ram air scoop on top, basic jet engine on the back side. Main wings carried over from previous models as well as the vertical stabilizer but this model due to its weight increase received adjustable winglets on the front and solid winglets on the rear. The rear ones simply add more stability, the fronts how ever work in conjunction with the main horizontals forcing the jet to gain altitude. Solid design, capable of insane flight lengths, max height just over 10km before air runs out.
NASA, now aware of Bob's skills with atmospheric aircraft sent him several understudies and working together they came up with the mk4. Single fuel tank, ditched the rear horizontal sabilizers, doubled up on the vertical stabs. Proudly, this bird flies faster than the mk3 though seems to encounter control issues at top speed and during excessive acceleration. None the less, safest model to date, landed her on a small abandoned landing strip on an island just off NASA's shoreline.
The mk5 marks NASA's successful convincing of Bob that his next step is orbital. The mk5 runs with the mk4's atmospheric engine design but adds two traditional rocket engines to the mix as well as three circular air inlets. The jet engine carries the bird up to around 11km at which the rocket engines kick in. Unfortunately NASA hasn't acquired the best of the best of hardware, most is still in testing so the rocket engines, jet engine and air inlets aren't at their top models just yet. This bird got to about 45km before she ran out of rocket fuel.
NASA considers this a resounding success with one side note. Bob... well there was an incident during re-entry. Everything went fine, reentered, jet engine re-engaged... but sadly when they had launched this mission they didn't take in to account the time of day. Poor BoB flew in total darkness for about forty minutes before realizing the engines would not carry him in to morning and day light. Dropping down to less than 100 feet from the surface he flew until water gave way to land. Landing gear out, landing gear lights on he killed the engines and glided to a touch down. Everything went fine until he enthusiastically put on full brakes.
Want to know what happens when you full break on a jet moving at 200m/s across the surface?
Fortunately Bob was successfully retrieved and NASA still considers the jet a success... but won't let Bob fly at night ever again.