Every turn, you lose half your velocity, so if you start at velocity 2, at the start of next turn your velocity will be 1.
To be a bit more clear, I'd phrase that as, "...at the
EDIT start EDIT of the next your velocity MIGHT fall to 1 UNLESS you spend 1 thrust point to offset aerodynamic drag." Aircraft don't actually go through a see-sawing velocity.
The reason for this is air movement in BT is modeled on a modified spacecraft movement system. In BT, when a spaceship in open space expends 1 thrust point in 1 turn, and then expends no more thrust, the ship will continue to drift along at 1 hex per turn forever. If it spend 3 thrust points in a later turn, it will then start moving 4 hexes per turn, and continue drifting at 4 hexes per turn forever.
BT air movement is the same except for the addition of aerodynamic drag (which removes half your velocity the following turn, round down, unless you fight it with more thrust). A fighter that expends 4 thrust points on Turn 1 will move 4 hexes. On turn 2, if its engines are idled, it will slow down from drag so that it only covers 2 more hexes (for a total of 6). On turn 3, if the engines are still at idle, it will slow down to 1 hex per turn (half its previous velocity of 2), covering a 7th and final hex. On turn 4, with a velocity of 0, it will stall and plummet.
Or that fighter could then sustain its velocity. On turn 1, it spent 4 thrust points and accelerated to 4 hexes per turn, covering 4 hexes. On turn 2, it can sustain that cruising speed by expending half its velocity in thrust: 2 points will keep it moving at 4 hexes per turn.
This system means a fighter can reach a velocity equal to twice its thrust points. A 'Mechbuster, limited to 6 thrust per turn, could spend 6 thrust on turn 1 to reach a velocity of 6 and cross 6 hexes. On turn 2, it could expend 6 more thrust. 3 thrust points would be spent offsetting drag, so it would reach a velocity of 9 (6 from turn 1, -3 for drag, plus 6 thrust = 9), and would have crossed a total 15 hexes by the end of turn 2 (6 on turn 1, 9 on turn 2). On turn 3, it could expend another 6 thrust points to reach velocity 11 (9 velocity from turn 2, -4 for drag, plus 6 equals velocity 11). On turn 4, it could expend another 6 thrust points to reach velocity 12 (11 velocity from turn, -5 for drag, plus 6 equals velocity 12). The 'Mechbuster couldn't accelerate any further because the drag at velocity 12 is 6 points, which is all its engines could give.
That's quite different than ground movement, where momentum is not simulated.