Lighter-Than-Air Accelerator from Stacked Balloons
To reach Earth's orbit cheaply, we may try to stack torus-like lighter-than-air balloons filled with propellant, and trigger their ignition in rapid succession like a chain of dominoes. The chain could be triggered by the approaching payload. The balloons could contain explosive gas (e.g., O2 + H2). The payload may have to be a cone-shaped to maximize thrust. The overall principle is similar to Project Daedalus, but on small scale.
The tunnel of these lighter-than-air balloons would be tapering off at higher altitudes, to create the trajectory for accessing orbit.
Alternatively, the lighter-than-air balloons could be used to construct an accelerator track, where instead of explosive, the electric field is used to accelerate smaller payloads.