Junnai Zhai
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Research Subjects
Junnai Zhai has a total of 10 co-authors. Here are the top co-authors by number of shared publications:
- Ali Gülhan
Supersonic and Hypersonic Technologies Department, Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR E.V.), Cologne, Germany
3 shared publications - Ansgar Marwege
Supersonic and Hypersonic Technologies Department, Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR E.V.), Cologne, Germany
3 shared publications G. Guidotti
1 shared publicationH. Gaible
1 shared publication- J.-L. Vérant1 shared publication
- Josef Klevanski
Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR), Köln, Germany
1 shared publication
Latest Publications
- Open Access
- 8 November 2026
- Published by Springer Nature in CEAS Space Journal
- p. 1-44
Cold gas wind tunnel experiments of a descending launcher first stage and generalization of the engine scaling parameter
In the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage with three Prometheus engines was investigated, the so-called T3. It shall be recovered with a downrange landing approach. In this paper, the results of cold gas experiments are presented where the engine plume was simulated with pressurized air. For that purpose, the engine scaling parameter, which is used to scale the engine parameters and thrust to the experimental conditions, was generalized. It is shown that the matching reduces to a single term, $${\gamma}_{\text{e}}{M}_{\text{e}}^{2}$$ . With the parameter in this simplified form, it can be ensured that the relevant similarity parameters for retro-propulsion such as thrust coefficient, momentum flux ratio, and ambient pressure ratio can be matched at the same time. The test campaign showed that switches between the short and the long penetration mode occur for T3, since the re-entry burn is performed at comparatively low thrust coefficients. They cause significant changes of the pressure coefficients on the base area, the short penetration mode giving the lower values, and the lower pressure fluctuations. The transition was quantified from the bow shock stand-off distance extracted from high-speed Schlieren recordings. It exhibits a hysteresis in the thrust coefficient, at Mach 2.0 between C T = 1.896 and 2.905, so that the flow state is not determined by the instantaneous flight condition alone. The upper limit of the long penetration mode decreases monotonically with increasing Mach number, and the mode is confined to a narrow band of angles of attack around zero incidence. Consequences for the design of the flight trajectory are discussed.- 8 November 2026
- Published by Springer Nature in CEAS Space Journal
- p. 1-44
Open AccessCold gas wind tunnel experiments of a descending launcher first stage and generalization of the engine scaling parameter
In the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage with three Prometheus engines was investigated, the so-called T3. It shall be recovered with a downrange landing approach. In this paper, the results of cold gas experiments are presented where the engine plume was simulated with pressurized air. For that purpose, the engine scaling parameter, which is used to scale the engine parameters and thrust to the experimental conditions, was generalized. It is shown that the matching reduces to a single term, $${\gamma}_{\text{e}}{M}_{\text{e}}^{2}$$ . With the parameter in this simplified form, it can be ensured that the relevant similarity parameters for retro-propulsion such as thrust coefficient, momentum flux ratio, and ambient pressure ratio can be matched at the same time. The test campaign showed that switches between the short and the long penetration mode occur for T3, since the re-entry burn is performed at comparatively low thrust coefficients. They cause significant changes of the pressure coefficients on the base area, the short penetration mode giving the lower values, and the lower pressure fluctuations. The transition was quantified from the bow shock stand-off distance extracted from high-speed Schlieren recordings. It exhibits a hysteresis in the thrust coefficient, at Mach 2.0 between C T = 1.896 and 2.905, so that the flow state is not determined by the instantaneous flight condition alone. The upper limit of the long penetration mode decreases monotonically with increasing Mach number, and the mode is confined to a narrow band of angles of attack around zero incidence. Consequences for the design of the flight trajectory are discussed. - Open Access
- 30 September 2026
- Published by Springer Nature in CEAS Space Journal
- p. 1-47
Aerodynamic Behaviour of a Descending Launcher First Stage Demonstrator with Grid Fins in the Aerodynamic Phase
In the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage with three Prometheus engines was investigated, the so-called T3. It shall be recovered with a downrange landing approach. In this paper the aerodynamic phase of the descent trajectory was investigated using wind tunnel experiments of the full launcher configuration in the Trisonic Wind Tunnel Cologne (TMK). The grid fins proved to be efficient to trim and control the vehicle. Increasing the size of the grid fins by 10% does not directly transfer into a larger trimmable angle of attack range. Modelling the engine nozzles as closed surfaces instead of open cavities introduces a systematic offset in the axial force coefficient which vanishes at zero incidence and grows to approximately 0.05 at the extremes of the tested angle of attack range, and it affects the roll moment of the undeflected configuration at the supersonic Mach numbers. This simplification is therefore acceptable for low-fidelity databases, but the nozzles should be modelled as open cavities for high-fidelity databases and for the validation of CFD computations. A Reynolds number variation showed a small but systematic effect on the axial force coefficient and on the roll moment, while no discernible effect was found in the coefficients governing the trim and the static stability. A superposition approach defined in earlier work was validated with the wind tunnel data. During ascent, the increase in length of the configuration influences the pitch moment coefficient considerably more than the deployment of the grid fins, while the deployed fins increase the axial force coefficient by approximately 17% at Mach 3.0.
- 30 September 2026
- Published by Springer Nature in CEAS Space Journal
- p. 1-47
Open AccessAerodynamic Behaviour of a Descending Launcher First Stage Demonstrator with Grid Fins in the Aerodynamic Phase
In the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage with three Prometheus engines was investigated, the so-called T3. It shall be recovered with a downrange landing approach. In this paper the aerodynamic phase of the descent trajectory was investigated using wind tunnel experiments of the full launcher configuration in the Trisonic Wind Tunnel Cologne (TMK). The grid fins proved to be efficient to trim and control the vehicle. Increasing the size of the grid fins by 10% does not directly transfer into a larger trimmable angle of attack range. Modelling the engine nozzles as closed surfaces instead of open cavities introduces a systematic offset in the axial force coefficient which vanishes at zero incidence and grows to approximately 0.05 at the extremes of the tested angle of attack range, and it affects the roll moment of the undeflected configuration at the supersonic Mach numbers. This simplification is therefore acceptable for low-fidelity databases, but the nozzles should be modelled as open cavities for high-fidelity databases and for the validation of CFD computations. A Reynolds number variation showed a small but systematic effect on the axial force coefficient and on the roll moment, while no discernible effect was found in the coefficients governing the trim and the static stability. A superposition approach defined in earlier work was validated with the wind tunnel data. During ascent, the increase in length of the configuration influences the pitch moment coefficient considerably more than the deployment of the grid fins, while the deployed fins increase the axial force coefficient by approximately 17% at Mach 3.0.
- Open Access
- 29 July 2026
- Published by Elsevier in Acta Astronautica
- Vol. 249, 882-909
Aerodynamic numerical reconstruction of experimental wind tunnel tests of an optimized inflatable heat shield in undeformed and deformed configurations for upper-stage recovery within the EFESTO-2 project
- 29 July 2026
- Published by Elsevier in Acta Astronautica
- Vol. 249, 882-909
Open AccessAerodynamic numerical reconstruction of experimental wind tunnel tests of an optimized inflatable heat shield in undeformed and deformed configurations for upper-stage recovery within the EFESTO-2 project
- Open Access
- 10 July 2026
- Published by Springer Nature in CEAS Space Journal
- p. 1-50
Unsteady loads on a descending launcher first stage demonstrator in the aerodynamic phase
Within the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage equipped with three Prometheus engines (T3) was investigated. The vehicle is designed for recovery using a downrange landing approach. This paper analyzes the aerodynamic phase of the descent trajectory based on the wind tunnel experiments conducted with the full launcher configuration in the Trisonic Wind Tunnel Cologne (TMK). Unsteady pressure fluctuations were measured using high-frequency pressure sensors. Significant fluctuations were observed, particularly in the base region of the vehicle. In this region, amplitudes reached approximately 30% of the dynamic pressure and occasionally increased to 40%. In the vicinity of the grid fins, the amplitudes were lower but remained on the order of 10%. The measured pressure signals did not exhibit distinct dominant frequencies and were predominantly broadband in nature.
- 10 July 2026
- Published by Springer Nature in CEAS Space Journal
- p. 1-50
Open AccessUnsteady loads on a descending launcher first stage demonstrator in the aerodynamic phase
Within the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage equipped with three Prometheus engines (T3) was investigated. The vehicle is designed for recovery using a downrange landing approach. This paper analyzes the aerodynamic phase of the descent trajectory based on the wind tunnel experiments conducted with the full launcher configuration in the Trisonic Wind Tunnel Cologne (TMK). Unsteady pressure fluctuations were measured using high-frequency pressure sensors. Significant fluctuations were observed, particularly in the base region of the vehicle. In this region, amplitudes reached approximately 30% of the dynamic pressure and occasionally increased to 40%. In the vicinity of the grid fins, the amplitudes were lower but remained on the order of 10%. The measured pressure signals did not exhibit distinct dominant frequencies and were predominantly broadband in nature.