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Diaphragm opening processes with a finite time affect the generated shock waves and shock tube flows. This effect is dominant in short and low-pressure shock tubes. In this research, a high-speed visualization of the opening process of a cellophane diaphragm, which is a suitable material for operation in a low-pressure shock tube, is conducted. The self-shaping opening morphology of cellophane diaphragms is observed at the initial stage with an approximate crack propagation speed of 807 m/s, which is greater than the speed of sound of the test gas. The initial opening motion of the petals, which are formed by cracks, is successfully modeled by the rotational equation of motion. Through analysis of the generation and coalescence of compression waves, whose characteristic velocities are calculated from the measured pressure histories, the shock formation distance is reasonably estimated. From the visualized diaphragm opening processes and analysis of coalescence of compression waves, we revealed that the initial stage of the diaphragm opening process plays a dominant role in shock wave formation rather than the entire process.

In this study, an electrostatic thruster was newly developed, in which a diverging magnetic field with a cusp around a cathode was applied by using two solenoid coils. The effects of the magnetic field strength on thrust performance under a similar applied-magnetic-field configuration were investigated. Because of its light weight and lower price, argon was used as a propellant. By increasing the magnetic field strength, the thrust efficiency was improved owing to the suppression of the discharge current while an almost constant thrust was maintained. A specific impulse of 3800 s with thrust efficiency greater than 30% was obtained; the corresponding mass-averaged exhaust velocity exceeded the value by full-potential electrostatic acceleration of singly charged ions. From the thrust performance, ion beam current, and ion energy distribution function that were experimentally measured under the representative operation condition, 32% of the ion beam current and 43% of the total thrust were evaluated as the contribution of doubly charged ions.

Research interest has been growing in recent years in supersonic transport, particularly supersonic propulsion systems. A key component of a commonly studied propulsion system, ramjets, is the air intake. For supersonic propulsion systems a major factor in the overall efficiency is the intake pressure recovery. This refers to the ratio of the average total pressure after the intake to that of the freestream. One phenomenon that can have a large effect on this performance index is flow separation at the inlet. The aim of this work is to examine how pulsed laser energy deposition can be used to improve pressure recovery performance by reducing flow separation at the inlet. This research examines the effects of pulsed laser energy deposition upstream of an intake with an axisymmetric centrebody in a Mach 1.92 indraft wind tunnel. Laser frequency was varied between 1 and 60 kHz with an energy per pulse of 5.6 mJ. Schlieren photography was used to examine the fundamental fluid dynamics while total and static pressure downstream of the intake diffuser were measured to examine the resulting effect on the performance. Schlieren imaging shows that the interaction between the laser generated thermal bubble and the leading edge shock produced by the centrebody results in a significant reduction in separation along the intake cone. Analysis of the schlieren results and the pressure results in tandem illustrate that the average separation location along the length of the centrebody directly correlates to the pressure recovery observed in the intake. At the optimal laser frequency, found for this Mach number to be 10 kHz, the pressure recovery is found to increase by up to 4.7%. When the laser power added to the system is considered, this results in an overall increase in propulsive power of 2.47%. Keywords: Laser energy deposition, Supersonic, Flow dynamics, Intakes, Flow separation

The effects of hollow cathode position on thrust performances of a diverging-magnetic-field electrostatic thruster, which had a diverging magnetic field between a ring anode and a hollow cathode, were investigated. The hollow cathode was located in either a central or an external configuration. Based on the discharge current and direct thrust measurements, electrostatic and magnetic hybrid plasma acceleration was observed in the central cathode operation and even in the external cathode operation especially when magnetic field strength was less than 315 mT. However, the operating points were different: low discharge voltage×large discharge current in the central cathode operation, and high discharge voltage×small discharge current in the external cathode operation. This difference comes from electron-Hall parameter; the estimated electron-Hall parameter was about 50 in the central cathode operation, which was much lower than that of 3000 in the external cathode operation. By combining electromagnetic acceleration, the generated thrust can increase even with lower electron-Hall parameter operation. As a result, similar thrust performances were achieved in both cathode positions. Because of the hybrid plasma acceleration method, thrust density reached 50 N/m^2, which is comparable or even higher than that of the conventional electrostatic thrusters., Published Online:14 Nov 2019

This study experimentally investigated the effects of repetitive laser energy deposition using a supersonic intake model with a central conical compression surface and a 19×19 mm^2 cross-sectional duct in a Mach 1.92 indraft wind tunnel: especially in the subcritical and buzz modes. A single-pulse energy deposition was observed to suppress the flow separation at the compression surface by “sweeping” a shock wave system with the thermal bubble generated by the energy deposition. The duration of the sweeping effect was approximately 160 μs in the subcritical mode and 100–420 μs in the buzz mode. Furthermore, repetitive deposition of laser pulse energies was observed to moderate instabilities in both modes, and it increased the pressure recovery by as much as 8%; also, the occurrence of buzz was delayed, thereby widening the stable, subcritical regime. In both modes, there was a threshold value for the laser pulse repetition frequency fL, thd, which corresponded to the duration of the sweeping effect (e.g., fthd=6 kHz in subcritical mode). Below this frequency, the increase in the pressure was proportional to the repetition frequency; whereas above fthd, the effect per single pulse was reduced., Published Online:26 Oct 2019

Gas and liquid are termed fluid because they can flow with changes to their shape. Most flows occurring in our daily life are not significantly influenced by compressibility. However, in high-speed flows and/or when velocity or pressure rapidly varies, compressibility is significant. The compression and expansion of a fluid, which correspond to variations of its density, lead to the propagation of pressure waves. In compressible fluid dynamics, we study the relationship between pressure-wave propagation and its impact on flow properties. In this chapter, we will learn the basic concepts of pressure-wave propagation through several examples.

An experimental study was conducted to examine the effects of repetitive energy deposition on the supersonic flow characteristics over a duct system with a central conical compression surface. Boun...

Published online28 October 2018

We evaluate the energy conversion efficiency of an electrical exploding foil accelerator that accelerates a thin dielectric foil (called the flyer) to more than 1 km/s, which is propelled by electrically exploded bridge material. The effective flyer mass ejected from the accelerator is estimated by impulse measurements obtained using a gravity pendulum as well as by time-resolving flyer velocity measurements obtained using a photonic Doppler velocimetry system. For two different bridge sizes (0.2 and 0.4 mm), the flyer velocity and impulse increase with the input energy at the bridge section. The maximum flyer velocity and impulse, that is, 4.0 km/s and 67 µN s, respectively, are obtained by supplying 0.33 J of input energy. Upon increasing the input energy, the effective flyer mass also increases and exceeds the designed-bridge mass for both bridge sizes. The energy conversion efficiency from input electrical energy to flyer kinetic energy is calculated based on the effective flyer mass, velocity, and input energy. Both bridge sizes show comparable efficiencies: 27% and 30% for 0.2 and 0.4 mm bridges, respectively. The efficiency increases with increasing specific input energy at least up to 15 MJ/kg for the 0.4 mm bridge, whereas the efficiency of the 0.2 mm bridge above 30 MJ/kg decreases. This is owing to the excessively high input energy density in the 0.2 mm bridge, which causes the effective flyer mass to increase by including surrounding materials. These results indicate that the specific input energy should be optimized for obtaining maximum efficiency.

The effects of a radio-frequency (RF) power on the ion generation and electrostatic acceleration in a helicon electrostatic thruster were investigated with a constant discharge voltage of 300 V using argon as the working gas at a flow rate either of 0.5 Aeq (Ampere equivalent) or 1.0 Aeq. A RF power that was even smaller than a direct-current (DC) discharge power enhanced the ionization of the working gas, thereby both the ion beam current and energy were increased. However, an excessively high RF power input resulted in their saturation, leading to an unfavorable increase in an ionization cost with doubly charged ion production being accompanied. From the tradeoff between the ion production by the RF power and the electrostatic acceleration made by the direct current discharge power, the thrust efficiency has a maximum value at an optimal RF to DC discharge power ratio of 0.6 – 1.0.

The ablation spot area and impulse characteristics of various polymers were experimentally investigated against burst irradiation of Nd: YLF laser pulses with a pulse repetition frequency of 1 kHz, wavelength of 1047 nm, temporal pulse width of 10 ns, and single-pulse fluence of 6.1 J/cm 2 to 17.1 J/cm 2 . The dependences of ablation area on the pulse energy from 0.72 to 7.48 mJ and the number of pulses from 10 pulses to 1000 pulses were investigated. In order to characterize their impulse performance as a function of fluence, which should not depend on ablation material, an effective ablation spot area was defined as that obtained against aluminum, 1050 A, as the reference material. An impulse that resulted from a single burst of 200 pulses was measured with a torsion-type impulse stand. Various impulse dependences on the fluence, which were not readily predicted from the optical properties of the material without ablation, were obtained. By fitting the experimentally measured impulse performance to Phipps and Sinko's model in the vapor regime, the effective absorption coefficient with laser ablation was evaluated, thereby resulting in three to six orders of magnitude larger than that without ablation. Among the polymers examined using polytetrafluoroethylene (PTFE) as the best volume absorbers, the highest momentum coupling coefficient of 66 μNs/J was obtained with an effective absorption coefficient more than six times smaller than that of the other polymers.

The shock wave generation in a closed tube using the high-speed jet injection was experimentally investigated. The unsteady behavior of the gas compression by the jet injection was visualized by the schlieren method, and the temporal history of the overpressure was measured by the pressure transducers which flush mounted in a tube. The injected jet can compress the gas in the tube, and the shock wave generation was observed by schlieren visualization and the pressure measurement. The characteristics of the jet and the relation between the cross-sectional area of the jet and the tube had an important role for the shock wave generation.

Impacts of energy deposition approach on performance of supersonic flow over a duct system were experimentally examined. Without using energy deposition, in the case of duct’s normal operation, supersonic flow was found being separated and weakly fluctuating at the duct’s compression surface. Additionally, in the case of unstable operation, supersonic flow was found strongly oscillating at both around the duct entrance and inside the duct chamber. To control these unfavorable influences to duct performance, energy deposition approach with repetition frequency up to 60 kHz was applied. Single energy pulse was confirmed to be able to suppressed flow separation. On the other hand, repetitive energy deposition not only could moderate flow separation but also increase total pressure of the internal flow, though the effectiveness highly depends on applied deposition frequency. Moreover, by applying repetitive energy pulses at 60 kHz, flow oscillation in the case of the duct’s unstable operation could be suppressed, and flow was stabilized.

The ion beam energy performance in a helicon electrostatic thruster with various anode geometries was investigated. For discharge voltages from 100 to 300 V, the ion beam energy increased almost linearly with the discharge voltage. The rate of increase strongly depended on the anode inner diameter (ID). However, the ion beam energy was fit to a single linear function of the total input power irrespective of the anode ID. The effect of an insulation cover set on the side wall of the anode depended on the anode ID; while with the anode ID being equaled to that of the helicon source tube, i.e., 27 mm, the ion beam energy was decreased by about 30%, with the anode ID of 40 mm the electrostatic acceleration became negligible. Under typical operation conditions, the ion beam energy had a flat-hat profile with a diverging angle of 47°; the specific impulse, thrust efficiency, and propellant utilization efficiency evaluated from probe survey measurements were 2000 s, 9.7%, and 98%, respectively.

Shock wave–boundary layer interaction (SWBLI) causes serious problems against realizing supersonic transport, such as flow unsteadiness which leads to degradation of engine performance, wing lift capacity and control surfaces effectiveness, and heat transfer and pressure loads which reduce the endurance of aircraft structures.

The impulse characteristics of an aluminum ablator against repetitive Nd:YLF laser pulses with a 1047 nm wavelength and 5 to 15 ns pulsewidth were investigated. The laser was operated in burst mode varying the pulse repetition frequency, single-pulse energy, and number of pulses in a burst. The burst was repeated until the impulse reached saturation. A torsion pendulum was used to measure the total impulse generated in a single burst with an accuracy higher than 2%, even with a 1 s burst period. The impulse increased, and then it gradually reached saturation as the total laser energy increased. In the operation range examined, the saturation value of the impulse increased with increasing single-pulse fluence.

An impulse generated by laser ablation bas been implicitly assumed to be directed in the normal direction to the irradiation surface. In this study, however, the impulse induced by an Nd:YAG laser pulse with a wavelength of 1.064 μm irradiated on to a macroscopically flat aluminium plate ablator was enhanced in the direction of the laser beam incidence due to microscopic surface undulation, the characteristic height and pitch of which are tens of the beam wavelengths. The inner product of the incident laser beam vector and the normal vector to the surface is locally enhanced on the 'sunny' side. In the vapor regime in which the impulse is an increasing function of the fluence, the effective fluence and the impulse component in the incident direction are enhanced. With repetitively irradiating laser pulses on to the same spot with a fluence of 4.5 J cm^−2 nominal to a macroscopically flat surface, the impulse gradually increased due to this 'sunflower' effect, then became saturated. The momentum coupling coefficient in the saturation stage became an increasing function of the angle of incidence. This impulse performance will become considerably important in the application of laser ablation to the remote motion control or even deorbiting of space debris., ファイル公開：2020-08-01

We investigate the drag characteristics of truncated cones in Mach 1.94 flow with repetitive laser pulse energy depositions with a frequency of up to 80 kHz. The drag decrement is almost in proportion to the laser pulse repetition frequency, and scales with a greater-than-square power of the truncation diameter. The performance of the latter is associated with the effective area of pressure modulation and the effective residence time of vortices which are baroclinically generated after the interaction between laser-heated gas bubbles and the bow shock wave. With employing a concave head, the drag decrement is enhanced. With increasing the truncation diameter, the efficiency of energy deposition becomes higher; yet, within the operation range of this study the drag coefficient still remains high.

Repetitive pulse laser energy deposition can modulate the shock wave oscillation frequency, which caused by shock wave and boundary layer interactions. Without energy deposition, the shock oscillation frequency is lower than 10 kHz. This frequency can be modulated to the repetition frequency of energy deposition. However, the oscillation frequency does not correspond to the energy deposition frequency when the frequency of the repetitive energy deposition become high enough to interact successive vortex rings. At 40 kHz energy deposition, the shock oscillation frequency is lower than 10 kHz, and the strong peak is observed at 1 kHz.

Ceramics will be used for power generating systems in the next generation. When they are used in this system, damage due to foreign object is inevitable. However, few systematic and comprehensive investigations have been reported on this subject. Various ceramics including fiber-reinforced mortars were investigated to understand their behavior when impacted by a spherical projectile. The volume of the cone cracks was large in ceramics which underwent transgranular fracture, while it was small in which underwent an intergranular one. Even though the energy consuming ability by the formation of surfaces was low up to 3.5% of the kinetic energy of a projectile, this ability increased with the ratio of the intergranular fracture to the transgranular one. Boron carbide showed a lower pressure as compared to the other ceramics. Fiber reinforcing increased the ballistic limits, but no clear advantage was suggested when absorbing the kinetic energy of a projectile far over its limits.

Polyoxymethylene (POM) has been widely studied as a promising laser propulsion propellant when paired to CO2 laser radiation. POM is a good test case for studying ablation properties of polymer materials, and within limits, for study of general trends in laser ablation-induced impulse. Mechanisms such as vaporization, combustion, and plasma are evaluated and a description is made of the link between the fluence of the beam and the resulting temperature of the target. For characterization of propulsion parameters, almost all previous studies of POM considered limited ranges of ambient pressure and incident fluence. As a result, despite many studies, there is no general understanding of POM ablation that takes into account pressure, spot area, fluence, and effects from confinement and combustion. This paper reviews and synthesizes CO2 laser ablation propulsion work using POM targets in order to make preliminary steps to address this deficiency. Previously published data is compared in terms of ablated mass ...

In ‘in-tube propulsion,’ that is the combination of conventional rocket propulsion and ballistic range operation, propulsion performance can be enhanced with increases in thrust due to spatial confinement and in payload capability because propellant can be placed off board a projectile. This paper describes proofed concepts of in-tube propulsion by using repetitive laser pulses.

A novel preheating technique to enhance laser-ablative impulse is investigated. In this technique, target material is first heated to an appropriate temperature by irradiating with a continuous laser. Impulse is generated by irradiating an intense laser pulse on the preheated material. In the experiment, polymer materials are preheated simply using an electric heater to investigate the effect of the material temperature on the laser ablative impulse. A pulsed TEA CO2 pulse laser is used to generate impulse. The results demonstrate that the present technique is useful for Poly-vinyl Chloride (PVC), Acrylic resin (PMMA), and Carbon Fiber Reinforced Plastic (CFRP). The momentum coupling coefficient of PMMA has a peak at the temperature close to the glass transition temperature.

A drag over a flat-nosed cylinder with repetitive laser pulse irradiations ahead of it were experimentally measured in a Mach-1.92, in-draft wind tunnel. Laser pulses were focused using a plane-convex lens fabricated on the nose of the cylinder at a repetition frequency of up to 10 kHz and power of 70 W at a maximum. The drag was measured using a low-friction piston which was backed by a load cell in a cavity at a controlled pressure. Under the experimentally available operation conditions, an up-to-3 % drag reduction and an efficiency of energy deposition of about 10 were obtained. Mechanisms of the drag reduction are analyzed based on experimental flow visualization and numerical simulation.

Impulse characteristics for a polyacetal target repetitively irradiated with pulses from a transversely excited atmospheric CO 2 laser were experimentally investigated. About 10-J laser pulses were repetitively irradiated up to 110 times onto a 6.6- or 8.6-mm-diam spot on the target, which was mounted on a torsion-type impulse balance. In the first several laser pulses, the impulse and ablation rate were strongly influenced by the initial conditions of the target surface. After ten cleaning pulses, 100 pulses were irradiated in various burst modes. Successive laser pulses in a burst were irradiated at a repetition frequency of 50 Hz. The time interval between successive bursts was greater than 3 min. The momentum coupling coefficient C m was almost independent of the burst mode. With a fluence of 18.8 J/cm 2 , C m gradually increased with an increasing total number ofpulses, reaching 220 μN. s/J at an ambient pressure of 10 -2 Pa and 145 μN - ·s/J in the atmosphere. When the fluence was 31.8 J/cm 2 , C m began to decrease after about 50 pulses. C m was smaller for a smaller spot diameter. Those impulse characteristics were closely associated with target surface morphology and fluid dynamics of the ablation plume and the ambient air.

Impulse generation mechanisms in pulsed laser ablation were experimentally studied using the velocity interferometer system for any reflector and framing Schlieren visualization. The impulse was estimated from the rear surface velocity at the center of the laser irradiated spot. The fluence was from 13 to 24 J/cm 2 . In most cases, the propulsive force generated even after the primary laser power peak significantly contributed to the total impulse. With the combination of transversely excited atmospheric CO 2 laser and aluminum target, only air breakdown was induced on the target surface without ablation and the impulse level was low. With decreasing ambient pressure P 0 , the impulse also decreased, and eventually vanished. With the combination of Nd:YAG laser and aluminum target, the ablation jet contributed to impulse generation and the impulse did not vanish even at vanishing P 0 . When a transversely excited atmospheric CO 2 laser pulse was directed onto the polyacetal target, the impulse increased by a factor of 10 in comparison with the aluminum target, yielding a momentum coupling coefficient exceeding 400 μN ·s/J. When P 0 was at atmospheric pressure, the laser plasma shielded the target surface against the proceeding laser power transmission and the impulse saturated at a lower value than at P 0 = 10 -2 Pa.

Interactions between plasma and shock waves, both of which are produced by the gas-breakdown induced by a CO2 laser pulse, are observed via framing Schlieren visualization. The deformation of the plasma bubble is characterized using the ratio of the laser pulse energy to the ambient pressure, and the laser beam diameter. The energy conversion efficiency from the laser pulse to the blast wave is dependent to the ambient pressure while it is insensitive to the other parameters. The size and the deformation speed increased with the characteristic shock radius.

Aeroballistic range is a great facility for measuring a near field pressure around a supersonic free flight model. Commonly-used wind tunnel experiments enable to measure the pressure data under well controlled attitude conditions. However the obtained data are influenced by a sting and boundary layer.

Shock Wave Boundary Layer Interactions (SWBLIs) can induce separation which causes loss of a control surface effectiveness, drop of an air intake efficiency and it may be the origin of large scale fluctuations such as air-intake buzz, buffeting or fluctuating side loads in separated propulsive nozzles

L ASER ablation can be usefully employed to generate a propulsive impulse on an object not only in the atmosphere but also in vacuum [1–3]. Larson et al. [4] proposed a launch vehicle propelled by repetitively pulsed laser ablation. Several authors [5–9] show that favorable propulsion performance in the atmosphere can be achieved with a polymer material, polyacetal, which is commercially named “Delrin” or is abbreviated as “POM.” The ablated gas from this material does not contain much air pollutant. Targeting space applications, the laser-ablative-propulsion performance of metals and polymers at low ambient pressures has been intensively investigated [10–17]. For many kinds of metals and polymers, Cm ranges from 10 to 100 N-s=J, and depends strongly on the characteristics of the laser pulse. According to measurements byGregg andThomas [10], metallic materials have an optimum laser intensity that maximizes Cm. Phipps et al. [12] formulated experimental Cm characteristics of aluminum alloys and several polymers in terms of the intensity, width, andwavelength of the laser pulse.Cm can be further increased by utilizing the so-called “volume absorber” [13] or layered target [14]. Recent measurements by D’Souza and Ketsdever [17] of Cm for polyacetal at low ambient pressure using a frequency-doubled Nd:YAG pulsed laser showed peak of 110 N-s=J at a laser intensity of the order of 10 W=cm. Several authors investigated the influence of ambient pressure on Cm [18,19]. Pakhomov et al. [18]measured the impulse generated on an aluminum surface irradiated by a CO2 laser pulse whose pulse width was 200 ns; Cm decreased monotonically with decreasing ambient pressure. In Dufresne et al.’s surface-pressure measurement [19], the ambient-pressure dependence of Cm for aluminum was quite sensitive to the laser pulse width. For polymer materials, Beverly and Walters [20] measured the CO2-laser-induced shock pressure in cellulose acetate and polymethylmethacrylate. The peak shock pressures exhibited the complex dependence on the ambient pressure. The ablative impulse dependence of materials other than aluminum warrants further investigations. To quantitatively evaluate the feasibility of laser-ablativepropulsion systems, the propulsive performance of polyacetal over a wide range of ambient pressure needs to be known. In the present paper, the influence of the ambient pressure on the impulse characteristics of polyacetal is investigated experimentally using a transversely excited atmospheric (TEA) CO2 pulse laser.

When a pulsed laser beam is focused onto a metal plate in water, a high-pressure pulse can be generated. This paper investigates the formation and behavior of bubbles that appear after the high-pressure-pulse generation. A 200-mJ, second-harmonic Nd:YAG laser pulse (duration of 7 ns) is incident on a stainless steel (SUS 304) plate to form a 1.1-mm-diameter spot. A bubble is formed and expands over the spot, which generates a hemispherical blast wave around the spot. The bubble grows in a period of the order of 400 μs after the laser pulse to reach a radius of about 5 mm. The behavior of the bubble is strongly affected by the thickness of the plate. When the plate thickness is 10 mm, the bubble keeps its hemispherical shape during the period of the first bounce. However, with a thin plate, for example of 0.1-mm thickness, the bubble is pinched and its head separates; moreover, another bubble is generated on the other side of the plate. Since these bubbles have different bounce motions, the acoustic field between 400 and 900 μs significantly differs depending on the thickness of the plate.

The effectiveness of repetitive laser pulses energy deposition on the control the interaction between a shock wave and a boundary layer over a hemisphere-cylinder-flare model using is investigated. With symmetrical energy deposition, the flow separation was suppressed with the energy deposition, and decreasing the pressure loss over the flare. The impact of asymmetrical energy deposition was evaluated with force measurement and optical flow visualization.

Experimental study is conducted in a 25-mm-bore ram accelerator at mixture Ž ll pressures of 2.5 and 3.5MPa. The obturator motion is analyticallymodeled to understand the effects of perforation ratio and entrance velocity on the drag coefŽ cient and the mixture ignition delay time. The ram acceleration is achieved at both pressures. At 2.5 MPa the starting and operation characteristics are marginal, whereas increasing the pressure to 3.5 MPa results in better starting reliability and ram acceleration characteristics.

A new type of propellant, a mixture of ammonium nitrate and alcohol, was examined in ballistic experiments. Withprilledammoniumnitrateandethanol,increasedballisticperformancewasobtainedforthemaximumenergy release conditions. From the measured chamber pressures and measured in-tube velocity proe le, the location of energy release and pressure wave motions are documented. This propellant is characterized by low-product molecular mass, high-specie c energy, and high e exibility in controlling its performance both by changing the type of alcohol and by varying the reactants’ molar ratio.

Flow characterization experiments of stagnation enthalpies up to 37 MJ/kg are carried out in the recently commissioned expansion tube facility. The features and the effective duration time of the test flowfield are determined not only from static and pitot pressures but also by observing the temporal variation of radiating emission from a shock layer around a spherical forebody. The thickness of the radiating layer matches well with the numerical results. The arrival time of the contact surface between the acceleration and test gases is better identified both as a decrease in the radiating layer thickness and from the emission spectroscopy. The useful test time is determined by integrating the mentioned measurements

An experimental study was conducted to develop a system for superorbital high-enthalpy pow simulation with gas injection. The expansion tube X-2 was used as the base facility. The synchronization of the gas injection to the expansion tube operation and the effect of the gas injection on the high-enthalpy flow generation performance are carefully discussed. When hydrogen was injected, the performance of the acceleration tube operation was not degraded, whereas with nitrogen injection the shock speed was significantly decreased. Hypersonic shock layers with gas injection were visualized by the schlieren method. This system is expected to be used as a tool to simulate a hypersonic shock layer with ablation for a reentry capsule.

Theoretical and experimental studies are conducted on the quantitative evaluation of a friction force at the interface between a projectile and the launch tube wall in a ballistic range. From a mechanical balance relation applied to the projectile, the friction force is expressed theoretically as a function of the net acceleration of the projectile. With this formulation, experimentally measured in-bore projectile velocity proe les obtained using a velocity interferometer system for any ree ector are well interpreted. If the initial projectile motion is signie cantly suppressed by the friction force while the burning rate of the propellant remains signie cant, the base pressure becomes much higher than that with a negligibly small friction force, thereby enhancing the net acceleration. However, if the projectile-tube e t is excessively high, the main function of the friction force is to decrease the muzzle velocity.