The crudest way of taking account of the airframe superstructure in calculating undercarriage performance is to represent it by a single concentrated mass. Crude as this method is, it is often used by aircraft firms because the only alternative known to them, apparently, is something they hesitate to face. And no wonder, because it means having to represent the wing-fuselage system by a large number of lumped masses and springs, each of which means an extra degree of freedom. This complicated dynamic system has then to be integrated with the undercarriage system, itself complicated by its non-linear characteristics.

The natural approach that suggests itself is not to consider the w/f (wing-fuselage) structure ab initio but to make use of its (usually known) normal modes and frequencies. But this is just what cannot be done by existing techniques—hence the present impasse.

The authors were requested to present short papers giving opposing views on the subject of Guided Weapon Simulators. Much of this subject is now a matter of fact and the authors found themselves in agreement on far too many points regarding the present situation to follow the requested approach, and instead have adopted the method of writing a joint paper briefly reviewing the past history of simulation and setting out the present state of the art as they see it, leaving the discussion of probable future trends to encourage controversy. It has still been difficult to be controversial, but if some of the statements made stimulate discussion then a useful purpose will have been served. It may turn out that what the authors have found difficult to produce between themselves has been created only too readily between themselves and their audience.

It is not possible to discuss simulators without bringing in the concept of modelling, indeed an apt title for this paper could have been “From Simulation to Modelling”. The model technique has been defined as “a procedure in which a representation of a system is developed in some more convenient medium and checked to show that its behaviour agrees with that of the original system for a wide variety of conditions”.

Substantial cut-backs in the national defence budget over the past five years have forced upon the Defence Ministry and missile designers alike a growing awareness of the importance of weapon costs. As a result, the ideal technical solution to a particular defence problem sometimes has to give way to the realism of what the country can afford to develop and procure—and, most important, what it could afford to sustain, if need be, in anger. A further factor has been the growing importance of exports to the country's economic position, to which arms make an important contribution. Weapon designs, therefore, have to be such that overseas customers can afford not only to purchase them but can also maintain and use them. This is particularly important in the case of developing countries.