The drivers for move of the rockets are designed in form that or the other version of the Laval nozzle.The reactive force of the traction which is created by such driver can essentially be increased if in the outer section of nozzle is placed an active shock wave of stretching (depression): then the reactive force of the traction becomes
R=ζRL , ζ=Uoff/US ,
where US is the gas velocity on the front side of outer section of nozzle (e. g. in the gas flow running against on it) and Uoff is gas velocity on the back side of it.
The strict detailed description of this phenomenon had provided in the (renovated) author's monograph "Сильные ударные волны в сплошных телах" where it is shown that theoretically ζ≤1+2/(γ-1) , γ is the adiabat exponent of the gas. In English the information about it author, as usually, had sent on his site http://www.leonid-philippenko.narod.ru/index.html , but the hosting administration decline to place it there by blockading the site.