This fourth and final part of the paper on aerospace transporters attempts to explain why, if aerospace transporters are as technically and commercially viable as has been suggested in the previous parts of the paper, they were not developed several years ago.

A brief historical survey of space transportation reveals two unique features compared with other forms of transportation. First, technically straightforward methods of greatly reducing cost have persistently failed to be adopted. The feasibility of winged flyback boosters was demonstrated as long ago as 1945 when the Germans flew two experimental V-2s fitted with wings to increase range. Winged flyback boosters have never been built. Second, all significant development funding has come from government agencies - there has been very little private investment in launch vehicle development. It is suggested that the link between these two features is that space policy was initially dominated by considerations of defence and national prestige, and that the commercial uses of space have not become significant enough to give priority to cost reduction.

In this situation, it is proposed that developing a small aerospace transporter would assist the permanent manned space stations now being planned by government agencies, by providing safe and low-cost transport for their crews; and would accelerate the commercial exploitation of space by greatly reducing launch costs and thereby permitting the start of a space tourism industry. However, at present there are no firm plans for aerospace transporters, and passenger transportation is not being considered by government agencies.

The main conclusion of the paper is that feasibility studies of aerospace transporters should be undertaken soon to include particular emphasis on the prospects for commercial space passenger transportation.