The channels bringing in the oxidiser are expanded within the drive block via an opening not screened from the fuel over part or all their length so as to give a number of small independent combustion chambers (2) within the block which are injected with oxidiser from above. The combustion gases from these chambers intersect with those from the main combustion chamber at an angle to ensure optimum mixing of the two flows until they leave the main chamber and thus enlarge the fuel surface sustaining the combustion process. The oxidiser is fed in lengthways from the top of the block via a distributor (5). It is introduced axially and thus centrally (6) and so distributed between individual downward fall-tubes (7) guided in longitudinal block grooves.
