This discrepancy formed perhaps the hardest technical challenge of the entire enterprise. In considering the range of possible modes of transportation, there are two very clear local optima among all the possibilities that it seemed extremely desirable to match. First is the family car, particularly in terms of its payload capacity and the convenience of operation it provides. Second is the flying regime of commercial jets. This is perhaps less well known.
Airliners fly just above the tropopause and just below the speed of sound.
Designing such a machine illuminates many of the considerations that will in all likelihood be typical of many early products of molecular manufacturing. First, the interface of the nano-mechanism to the macroscopic world is problematical. Surface films of moisture affect exposed nanomechanisms like pouring molasses over conventional machines. For comparison, a typical automobile engine might produce 100 kW power. Drexler motors producing the same power would occupy a volume of a tenth of a cubic millimeter, about the same as a single hair from one of my eyebrows.