This paper gives a broad and simplified theoretical treatment of the effects on performance of a scramjet engine due to adding a precoolant to the air flow somewhere in the initial compression process. The study suggests that the largest increase in thrust can be derived from injection as far upstream as practical. In particular, it can reduce the lowest Mach number at which a scramjet can usefully operate, although at some cost in propellant (i.e. fuel plus coolant) consumption. Liquid ammonia seems a particularly suitable airflow coolant, not least in that it can also release heat in downstream combustion.