This, the third part of the paper on aerospace transporters, considers their applications. It is concluded that a small nearterm vehicle would be useful for launching small satellites, for servicing satellites in orbit, for supplying space stations and as a lead-in to larger and more advanced vehicles. The latter have a potential cost per flight about 1000 times less than that of the Space Shuttle, given traffic levels sufficient to enable design and operational maturity to be achieved. Space tourism, (i.e. the making of short flights to Earth orbit by fare-paying passengers), would appear to be the first application likely to provide these traffic levels. Given high priority and an immediate start, the first space tourism flights could possibly take place this century.

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.

The method of matched asymptotic expansions is used to calculate the lift for a class of thin aerofoils with highincidence flaps or blunt trailing edges. The analysis is based on smallness of the ratio between the flap length (or trailingedge thickness) and the chord of the main foil. Results are compared with exact solutions for bent plates and for triangular and quadrilateral bodies. A procedure for extension to general trailing-edge configurations, including detached flaps, is outlined.

Turbulent incompressible heated round jets in a co-flowing stream are analysed theoretically. The normalised mean excess velocity and temperature profiles, as determined by experiments, can be shown to be well represented by appropriately defined Gaussian error-functions. The resulting solutions of the equations of motion and energy give rise to turbulent diffusivities for momentum and heat that are both functions of the stream and radial coordinates and depend parametrically on the co-flowing stream to jet velocity ratio, λ. When λ approaches zero, the eddy diffusivities become independent of the stream coordinate and a constant turbulent Prandtl number is recovered, just as previous studies have predicted. Analytical expressions are obtained for the jet growth and the centreline decay of velocity and temperature. These growth and decay laws are nonlinear functions of the stream coordinate, and show significant deviation from linearity as λ increases. Finally, it is shown that the analytical results are in fair agreement with measurements of round jets in a co-flowing stream, and correctly predict the parametric dependence of jet growth and centreline decay of velocity and temperature on λ.