The high-performance multi-stage rockets require greater accuracy in prediction and control of the starting and the tail off thrust transient of all stages for devising a smooth stage separation. The multistage rockets allow improved payload capability for vehicles with a high AV requirement such as launch vehicles or interplanetary spacecraft. Since each tank is discarded when empty, energy is not expended to accelerate the empty tanks, so a higher total AV is obtained. Note that keeping a burnt-out lower stage attached to the upper stage doesn't hurt until it's time to start the next stage. In fact, keeping spent lower stage attached until shortly before it's time to ignite the next upper stage can improve the total launch AV. Note that in the absence of required power level retro rockets, this sequence can invite possible re-contact between the upper stage and the lower spent stage if the upper stage is not accelerating as per prediction and/or the lower spent stage over performing with momentum. Evidently for a profitable mission and also for an efficient stage separation more precise predictions of the upper stage (Solid Rocket / Liquid Rocket) nozzle flow choking time and the tail off thrust transient of the lower spent stage are inevitable. In this paper parametric analytical studies have been carried out for examining the intrinsic flow properties pertaining to the nozzle flow choking time of high-performance solid and liquid rockets. Numerical simulations have been carried out for estimating the nozzle flow choking time of both rockets using a validated two-dimensional, axisymmetric, pressure based realizable k-ε turbulence model accompanied by standard wall treatment approach. We concluded that at the pre-designed conditions with identical geometry, among the conventional inlet jet properties the low turbulence intensity (0 - 10 %) fluctuations during the starting transient period of operation is more sensitive to nozzle flow choking time of both solid and liquid rockets. We also concluded that flow choking time prediction is more significant for upper stage dual-thrust solid rocket motors. This paper is a pointer towards for the optimization of stage separation sequence and coasting time of multistage rockets for multiple maneuvering lucratively without sacrificing the mission requirements.
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