is fed to the fuel tank to pressurise same at this point the combustion chamber pressure levels off 6 milliseconds and eventually decays 2 milliseconds as the available fuel is exhausted bearing in mind the fuel supply valve 11 is still closed upon decay of the combustion chamber pressure valve 16 closes point d and upon further decay valve 11 opens point a and the cycle is repeated each cycle resulting in a successive increase in combustion chamber pressure until the steady state condition is reached which is illustrated by the graph of fig5 a nominal fuel tank pressure of 750 p s i being assumed the points a b c and d on the graph of fig5 represent the same valve openings and closures as explained in connection with the graph of fig4 it will be seen that the cycle time is 14 milliseconds and this gives a repetition frequency of about 70 hz it will be seen that once the gas generator has been rendered operative it is self supporting in that fuel is supplied to the combustion chamber in a cyclical manner by the very operation of the combustion chamber there are no components with moving parts involved in the supply of fuel save for standard valves which are generally reliable and since only initial pressurisation of the fuel reservoir is required the means employed to this end are smaller than would otherwise be required so reducing the overall size and mass of the gas generator for a given capacity also if the initial pressurisation of the fuel reservoir involves a gas bottle the pressure therein is relatively low typically 100 1000 p s i so that no safety problems arise in as much.