In Silico Studies on Truncated Aerospike Nozzle with Optimum Cowl Length for Single Stage to Orbit Vehicles

Abstract

The aerospike nozzle belongs to the class of altitude compensating nozzles, which offers various advantageous over traditional convergent-divergent (CD) nozzles and/or bell nozzles. In light of the recent theoretical discovery of external flow choking due to the streamlines compression (V.R.S.Kumar et al., Physics of Fluids, 33(3), 2021), the diagnostic investigation of fluid flow characteristics of aerospike nozzles received considerable attention in the aerospace industries for the lucrative design optimization of truncated aerospike nozzles at various nozzle pressure ratios (NPRs). The decisive demonstration of the roots and aftermaths of the boundary layer persuaded flow choking (Sanal flow choking) and/or Streamtube flow choking in reacting/non-reacting flows and humans’ circulatory system (V.R.S.Kumar et al., Physics of Fluids, 34(4&10), 2022) sheds lights on the diagnostic investigation of Streamtube flow choking in the free external flow domain of the truncated aerospike nozzles with an optimum cowl length. Such studies help for the worthwhile design optimization of the mission specific altitude compensation spike nozzles with a most favorable truncation of its spike to increase the payload capabilities of single-stage-to-orbit (SSTO) vehicles. Altitude adaptability plays a significant role in designing SSTO vehicles. Herein, comprehensive in silico studies have been carried out to examine the causes and effects of Streamtube flow choking in the external flow domain of an aerospike nozzle with a cowl and with different percentage of truncation having a nozzle pressure ratio (NPR) of 2.57. We have observed that Streamtube flow choking is more significant in aerospike nozzles without truncation. Therefore, achieving the optimum truncation is a design objective to negate the undesirable Streamtube flow choking. Streamtube flow choking due to streamline compression leads to normal shock wave generation causing an enhancement of entropy. Streamlines compression occurs because of spike and/or cowl geometry. An increase in entropy increases the gas temperature and viscosity. An increase in viscosity increases the flow stickiness causing the streamlines compression. It leads to subsequent Streamtube flow choking. The cascade effect of Streamtube flow choking due to sonic fluid throat effect can be negated through spike truncation. We have observed that the sonic fluid throat effect occurs at a critical total-to-static pressure ratio. It creates high entropy and turbulence level due to undesirable shock wave generation due to the CD nozzle shaped Streamtube. We have comprehended that the Streamtube flow choking due to sonic fluid throat effect and the undesirable shock wave would possibly lead to system failure due to high entropy wave towards the spike surface. We have conjectured that the sonic fluid throat effect can be negated by injecting suitable fluid to the Streamtube flow choking region with high heat capacity ratio (HCR) than the operating gas because the flow choking pressure ratio is uniquely controlled by HCR. Additionally, the Streamtube flow choking effect can be negated by increasing the static pressure through the contour optimization of aerospike nozzles. This study is a pointer towards for the design optimization of truncated aerospike nozzles with an optimum cowl length for achieving the altitude compensation lucratively for the single-stage-to-orbit vehicles.