RAMAC25

Abstract

With the increasing interests in ram accelerators [5] throughout the world, the Shock Wave Research Center (SWRC), Institute of Fluid Science (IFS), Tohoku University, Sendai, Japan, decided to construct their own ram accelerator. Research results are presented in this section.

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1 Background

With the increasing interests in ram accelerators [5] throughout the world, the Shock Wave Research Center (SWRC), Institute of Fluid Science (IFS), Tohoku University, Sendai, Japan, decided to construct their own ram accelerator. With limited resources, at first the bore diameter was determined from existing unused tubes made of stainless steel. The name ‘RAMAC25’ came from its bore diameter as was done in other institutes. The first publication appeared as an institute internal report in 1996 [7]. Even before installing RAMAC25 and also thereafter, SWRC received valuable support from the group of University of Washington through researcher exchanges by Abe Hertzberg, Adam P. Bruckner, Carl Knowlen, realized by the funds from Ministry of Education, Japan, via their technical advice and collaboration projects.

Using RAMAC25, we conducted experimental examination of its propulsion performance, obtaining a muzzle speed of a projectile of 2.3 km/s and a world-highest acceleration averaged over the ram acceleration section of 4 × 10⁵ m/s² by using an ‘open-base’ projectile. Also, the building-up processes of a high-temperature slug in the transition region to the ram acceleration section were visualized.

2 Apparatus

The schematic illustration of RAMAC25 in its final configuration is shown in Fig. 1 [3, 10]. A powder gun (Fig. 2) was used as the pre-launcher to the ram acceleration section, where the projectile was accelerated to a maximum velocity of 1.3 km/s. Smokeless power (NY500, Nippon Oil and Fats Cooperation) was used as the pre-launcher propellant. In the pre-launcher, the projectile was backed by a perforated obturator, which in turn was backed by a back plate that plugged the perforation [1, 7, 8]. The obturator assists the initiation of the ram acceleration by moderately compressing and heating the mixture on the entry to the ram acceleration section. Before the ram acceleration section, the burnt gas of the smokeless powder was vented through ventilation holes fabricated along the acceleration tube of the prelauncher.

Fig. 1

25-mm-bore ram accelerator (RAMAC25)

Fig. 2

Propellant chamber

The ram acceleration section consists of a 0.6-m-long entrance tube followed by five 1.2-m-long tubes. The inner diameter of the ram acceleration tubes is 25 mm. The instrumentation units, composed of a pickup coil and a piezoelectric pressure transducer, were placed with a separation distance of 0.3 m. When the projectile passage is sensed using the pick-up coils, the projectile velocity is determined by the method of time-of-flight. There is a projectile catcher in the dump tank.

Figure 3 schematically illustrates the conventional-type projectile used in RAMAC25. It comprises two pieces, a nose and an after-body, which are hollowed and threaded together. The projectile is made of the magnesium alloy and weighs 18.5 g.

Fig. 3

Conventional-type projectile with fins

3 Visualization of Starting Processes

Since RAMAC25 was the world-smallest-circular-bore ram accelerator, the effect of finite-rate chemical reactions under a modest fill pressure could lead to difficulties in igniting the propellant mixture. This problem was solved after careful investigation of the starting processes with optical visualization of a high-temperature slug built-up by repeated shock wave reflections [2, 9]. The visualization was done using an aspherical lens (Fig. 4, [9]) through which the in-tube flow image was radially magnified with a uniform magnification. The slug was built up before a projectile entered the ram acceleration section (Fig. 5). Then, during the transition, the propellant mixture was successfully ignited. Figure 5 shows high-speed framing photography of projectile entry process observed through the aspherical lens. The images are vertically magnified by a factor of 2.06. Frame interval; 10 μs. The initial pressure of the upstream (air), 240 Pa; downstream (N₂), 0.8 MPa, single layer of 50-μm-thick Mylar diaphragm, projectile speed, 1120 m/s. A shock heated region is observed as an illuminating slug. This visualization was backed up by the pressure measurement, see Fig. 6 [9].

Fig. 4

Visualization experiment showing projectile transition using aspherical lenses. a Light path through aspherical lens, b test section

Fig. 5

Visualized shock wave refection processes during the transition with the impingement against the diaphragm

Fig. 6

In-tube pressure histories near the diaphragm. Pressure histories in projectile entry process under the same fill condition as of Fig. 5. Projectile speed of 1140 m/s. The labels ‘i’ indicate the moment in Fig. 5. x, distance from the diaphragm; labels ‘A’, ‘B’, ‘C’ correspond to the locations shown in Fig. 5

Using a holographic interferometer, the transition processes of the projectile entering the high-pressure section filled with inert gas are visualized in Figs. 7 and 8. Upon piercing of the Mylar diaphragm by the projectile conical nose, a conical shock wave is attached to the sharp nose. Then the shock wave repeats reflection between the acceleration tube and projectile walls. With this configuration, the pressure and the temperature of the gas around the conical nose are not excessively increased. Even if the gas on the right-hand side of the diaphragm was a propellant mixture, the ignition would be delayed and occur behind the projectile base. In this way, the ram acceleration can be successfully started.

Fig. 7

Reconstructed interferogram of projectile entry process by piercing Mylar diaphragm, fill pressures, 0.1 MPa (left) and 1.0 MPa (right) of the Mylar diaphragm, single layer diaphragm (thickness, 100 μm), projectile speed, 1051 m/s

Fig. 8

Reconstructed interferogram of projectile entry process by piercing Mylar diaphragm, fill pressures, 0.1 MPa (left) and 1.0 MPa (right) of the Mylar diaphragm, two-layer diaphragm (thickness, 188 μm), projectile speed, 1105 m/s

4 High Acceleration Operation Using Open-Base Projectile

The small facility had the advantage to obtain a high acceleration owing to the square-cubic law; in principle the mass of a projectile scales with the cube of its dimension whereas the acceleration with an area that scales with its square. With this background, the high acceleration operation, which may not be relevant for tactical purposes, was investigated in RAMAC25.

Most projectiles used in ram accelerators have a centerbody, which is supported either by fins or rails. For the projectile to keep its integrity during the prelaunch with a high-acceleration level of the order of 10⁴ g (gravitational acceleration) or higher, it was manufactured as a single piece. Moreover, for thermally-choked operation [1, 5, 6], the required entrance velocity is about 1.2 km/s, and the projectile experiences an acceleration level higher than 10⁴ g also in the prelaunch processes. In order to obtain this high acceleration level, the mass of the projectile needs to be as small as possible. In order to reduce mass, the center-body of a projectile is often hollowed. Usually the hollow is machined by dividing a projectile into two pieces, a nose and an after-body. After machining a hollow in each piece, the two pieces are threaded together. In this way, the mass of the projectile is decreased. However, while the pressure outside of the center-body becomes 20 times as high as the fill pressure or even higher, the inside pressure remains unchanged. The projectile experiences a large compressive load from the outside. Table 1, Figs. 9 and 10 show the experimental conditions, the acceleration tube wall pressure histories and the projectile velocity profiles obtained with the conventional type projectile. In first and second stage, the propellant mixtures are 2.8CH₄ + 2O₂ + 5.7N₂ and 4.6CH₄ + 2O₂ + 2He, respectively. The initial fill pressure is 3.5 MPa all through. In the third stage, except for the case of M _CJ = 3.4, where M _CJ denotes the Chapman-Jouguet detonation speed, unstart occur right after the transition and the projectile is decelerated right away.

Table 1 Experimental conditions of the third stage in three-stage operation with conventional projectile; fill pressure, 3.5 MPa; propellant mixture, XCH₄ + 2O₂ + YHe

Fig. 9

Inner wall pressure histories during the passage of the conventional type projectile, the time, t, corresponds to the moment of the throat

Fig. 10

Velocity profiles of three-stage operation using the conventional projectile with fins, fill pressure; 3.5 MPa

Utilizing RAMAC 25, another type, that is an open-base projectile (Fig. 11, [4]), was also used to increase the acceleration with a limited operation pressure. They are made as a single piece, of aluminum alloy, A7075-T6. Since the base of the projectile center-body is hollow, pressure inside and outside the projectile is almost balanced. Its wall thickness could be significantly reduced. A thread to connect two center-body pieces is not necessary. The minimum body thickness is 1 mm. Four fins support the center-body. The mass is about 12 g. Table 2 contains the experimental conditions. The propellant mixture of the first and second stages are the same as those of the Table 1.

Fig. 11

Open-base projectile

Table 2 Experimental conditions of the third stage in three-stage operation with the open-base projectile; fill pressure: 3.5 MPa

Figure 12 shows inner wall pressure histories measured during the passage of a projectile. A high pressure was maintained around the projectile after-body, corresponding to thermally choked operation.

Fig. 12

Inner wall pressure histories during the passage of the open-base projectile

Figure 13 shows projectile velocity histories measured with the open-base projectile. The operation has three stages. Various fuel mixture ratios are investigated in the third stage (4.8 m in length). Hereafter the ratio of a heat release from the combustion to the initial static enthalpy will be designated by Q. The entrance Mach number in the third stage, M ₃, was varied from 3.1 to 3.5. For Q = 3.4, wave fall-off occurs right after entering the third stage. With Q = 4.2, ram acceleration lasted for the longest distance. However, the average acceleration was lower than that with Q = 4.6. With Q larger than this value, wave un-start occurs before reaching the achievable velocity. However, when M ₃ was decreased from 3.5 to 3.2, the effective acceleration length increased, and a higher velocity was achieved. With this decreased value of M ₃, not only the maximum velocity but also the maximum Mach number became higher. Although the lower M ₃ operation was superior to that in the earlier mentioned acceleration performance, usually starting reliability was improved with increasing the entrance Mach number. The value of 3.2 for the entrance Mach number was close to this critical condition.

Fig. 13

Velocity profiles of three-stage operation using the open-base projectile, fill pressure; 3.5 MPa

Operation of the system at M ₃ = 3.2 yielded the highest performance: Through the 6.6-m-long, three-stage-ram-acceleration section, a velocity increment from 1.3 to 2.3 km/s and an average acceleration of 4 × 10⁴ g was achieved.

5 Summary

The investigations of RAMAC25 were conducted with a global collaboration, exchange of personal, know-how, data and information. Using this world-smallest-circular-bore ram accelerator, understanding the initiation processes of ram acceleration was improved by the visualization of starting processes. Also, the high ram acceleration with a modest fill pressure was demonstrated using the open-base projectile.