the rear of the container 13 to pressurize the particulate material towards the nozzle conveniently the container 13 and propulsion nozzle 12 are a unitary member but not necessarily so the container can have a light insulating wall 23 in fig2 the two pumps 17 and 19 of fig1 are replaced by a single high pressure pump items common with fig1 are given the same reference numerals in fig2 the fluid is fed from the tank 15 via pipe 30 to a high pressure pump 31 from the pump 31 fluid is fed via pipe 32 to a position adjacent to the throat 22 of the nozzle and also to the rear of the container 13 by pipe 33 a servo valve 34 can be provided in the pipe 32 to control the flow in the example the container or vessel 13 is a high pressure construction and may have an insulating wall 23 the particulate material 14 is given a heat content this can be obtained in various ways it can be heated in a storage system for example by solar heating a heat source can be provided in the vehicle or heat such as solar heat and can be applied to the material as it is used other ways of providing heat can be provided it is also possible to use material that has a heat content at its source using its actual temperature a martian moon for example fig3 and.