Why would you invest in a spaceplane stock that has no actual spaceplanes to fly?Space roller-coaster stock Virgin Galactic (SPCE 3.50%) took investors on a wild ride this week, soaring 10% Monday on news of progress building its Delta-class spaceplane, then tumbling 7% early Wednesday after reporting how much money its current Unity-class spaceplane is losing -- only to land right near where it began the week, around $1 a share.But I think investors are overestimating Virgin Galactic's chances of success. ...