Ram Accelerator Research Articles

ラム加速器の熱閉塞モードにおける不始動の形態

Failures of acceleration in thermally chocked ram accelerator are investigated using visualization by shadowgraph pictures. It is made clear in the present paper that not only the support from thermal choking pressure but also the flame holding in the boundary layer on the projectile surface has very important role for holding the combustion on the projectile afterbody. In the lower projectile velocity case, combustion behind the projectile is not so strong and falls off despite the good surface flame holding. Besides lower velocity also makes thermal choking easily at the projectile shoulder and wave unstart occurs. When the projectile velocity goes up to the 90% of CJ detonation velocity of the mixture, wave unstart occurs because combustion becomes too strong and thermal choking occurs at the projectile shoulder. If the heat release of the mixture is made lower to avoid this wave unstart, combustion region tends to falls off.

ENERGY ADDITION METHOD FOR THE ELECTROTHERMAL RAMJET

The electrothermal ramjet is a ramjet-in-tube device that has been proposed for accelerating masses to high speeds for applications such as direct space launch and hypersonic projectile studies. The electrothermal ramjet is similar to the ram accelerator, though one difference is that heat is to be added to the propellant electrically rather than via combustion. This article presents a method for adding heat to the propellant in the electrothermal ramjet configuration via magnetically balanced traveling arc discharges. Calculations indicate that current and magnetic field requirements for this method are within attainable ranges and that electrical heat addition should provide advantages over combustion heat release under certain conditions.

Strategies to protect ram accelerator projectiles from in-tube gasdynamic heating

A serious problem in advancing ram accelerator technology is the very high in-tube heat transfer rate to the projectile. Herein, we examine a number of strategies for protecting the projectile from gasdynamic heating. Radiation cooling of the projectile and flying the projectile through alternating regions of fuel-oxidizer-diluent drive gas and pure hydrogen are found to be totally unworkable. The ablative cooling technique has serious problems with a substantial retreat of the projectile surface. A transpiration cooling technique using liquid ammonia is calculated to provide adequate protection of the projectile for ram accelerator missions from 3 to 7 or 8 km/sec. Techniques for flying the projectile in pure hydrogen are also examined. One may have a vortex arrangement with a pure hydrogen core surrounded by a fuel-oxidizer-diluent mixture. The projectile may also fly in pure hydrogen while the driving energy is supplied by a deflagrating or detonating solid coating on the tube wall or by electrical energy input. The techniques for flying the projectile in pure hydrogen are judged to be extremely complex and expensive to implement. The transpiration technique appears to be the most viable way to protect projectiles flying in the 4 - 7 km/sec range.

Progress of ram acceleration with ISL's RAMAC 30

Smeets [1] published in 1988 a new concept for a ram accelerator with guiding tube rails for firing rail stabilized projectiles. This concept replaces Hertzberg et al.'s [2] fin stabilized projectiles accelerated in a cylindrical bore. The rail tube idea offers some advantages, e.g., no sabot is necessary as required for fin guided projectiles, simple projectile geometry, and possibility of varying the inner tube geometry. This principle was tested in 1993 and 1994 in rail tube version I and is now again under investigation since the beginning of 1997 in our RAMAC 30 in version II. In the rail tube concept, circular and finless projectiles are guided in a ram-tube equipped with five inner rails. At the moment we use a ram-section with a length of about 4.8 meters. A conventional powder gun serves as pre-accelerator. In the gun tube with a length of 2.8 meters, projectiles of about 150 grams are accelerated to a muzzle velocity of approximately 1800 m/s which is the initial velocity at the entrance of the ram-section. For successful operating a ram accelerator, the heat release must be limited to avoid thermal choking followed by an unstart. This choking phenomenon will be investigated in detail in this paper from the gasdynamic point of view in order to predict the right mixture for the given flow conditions around the ram projectile. Moreover, to avoid a firing failure, the material point of view must also be considered. Some recent firings have been done using aluminium, titanium and steel as test materials and its behaviour is discussed herein in detail. The first outcome is for example, for a given projectile geometry and a given gas mixture with a steel cowling no ignition occurs, whereas with aluminium or titanium as combustor surface material the ignition starts well followed by a projectile acceleration.

Investigation of obturator and ignitor effects on low velocity starting of the ram accelerator

An experimental investigation of the effects of obturator geometry, propellant chemistry, and onboard pyrotechnic ignitor systems on the starting characteristics of the ram accelerator at low launch velocity has been conducted at the University of Washington 38-mm-bore facility. The ram accelerator was successfully started at entrance velocities as low as 760 m/s using stoichiometric methane/oxygen propellants with various levels of carbon dioxide dilution and obturator configurations at a fill pressure of 2.5 MPa. Experiments using an onboard pyrotechnic ignitor demonstrated that ignition of the propellant and starting of the ram accelerator could occur without the presence of the obturator-driven normal shock.

Laser-propelled ram accelerator

The concept of laser-propelled ram accelerator (L-RAMAC)' is proposed. Theoretically it is capable of achieving a higher launch speed than that by a chemical ram accelerator because a higher specific energy can be input to the propellant gas. The laser beam is supplied through the muzzle, focused as an annulus behind the base of the projectile. The performance of L-RAMAC is analized based on generalized Rankine-Hugoniot relations, suggesting that a superorbital muzzle speed is achievable out of this device.

Ram accelerator operating characteristics at elevated fill pressures

An experimental investigation of the starting and operational characteristics of a 38-mm bore ram accelerator with propellant fill pressures greater than 5 MPa is in progress. Successful starts with 60 - 124 gm projectiles have been achieved using methane/oxygen/nitrogen propellants at fill pressures ranging from 6 to 15 MPa. At fill pressures of 8.5 MPa and above, it was found that projectiles having the nominal throat diameter (29 mm) required a minimum entrance velocity of 1250 m/s, which is about 100 m/s faster than the minimum needed to successfully start the ram accelerator at fill pressures below 7.5 MPa. Projectiles with a reduced throat diameter (25 mm) and a solid magnesium nose cone were successfully started at fill pressures up to 15 MPa with entrance velocities around 1300 m/s. The average accelerations achieved using high pressure stages were in general less than predicted for thermally choked operation by the Boltzmann equation of state.

On the possibility to modify the performance of propellant mixtures used in RAMAC by addition of metallic particles

To explain the experimental results obtained in the 30 mm Ram accelerator at ISL, showing that in the subdetonative mode, the terminal velocity of magnesium projectiles at the end of Ramac tube is higher than the CJ detonation of the propellant gaseous mixture, whereas the projectile undergoes a mass loss during the propagation, the assumption is made that metallic particles ablated from the projectile react with the gaseous species and modify the performances of the propellant mixture. The problem is examined by comparing experimental and calculation results in an analogous situation of fine aluminum particles suspended in hydrogen-air mixtures. It is demonstrated that the classical thermodynamic model is unsuitable for explaining correctly the experimental observations. Prediction of changes in performances of the propellant mixture requires that two-phase flow effects be taken into account. These changes depend on the mass concentration, size and shape of the solid particles.

Numerical study of shock induced ignition and combustion of CH4-O2-CO2 mixtures in a two dimensional ram accelerator

The numerical simulation for the starting process of the ram accelerator in Hiroshima University (HURAMAC) has being made for almost the same conditions as the experiments, where CH 4 -O 2 -CO 2 gas mixtures are used. The finite difference method is used for solving the Navier-Stokes equations including chemical reactions in the two-dimensional Cartesian coordinates. At first a test simulation is carried for the case of a cold shot with the same condition of our experiment. The structure of shock waves and the formation of expansion waves are clearly demonstrated. The simulation of a starting process of a hot shot is done for the similar conditions to one of the experiments. Numerical results show that the ignition source is formed just in front of the igniter although another high temperature region appears in the rear area of projectile. The normal shock wave is produced in front of the igniter, then it propagates forwards depending on the combustion heat release. Thermally choking mode is gradually established when the flame propagates whole cross section of the tube with the sabot ejected away from the projectile. The calculated pressure history is compared with the measured one at the middle point of the ignition tube. Very good agreement is found both in the time scale and the pressure amplitude.

Computational modeling of high pressure combustion mechanism in scram accelerator

A computational study was carried out to analyze a high-pressure combustion in scram accelerator. Fluid dynamic modeling was based on RANS equations for reactive flows, which were solved in a fully coupled manner using a fully implicit-upwind TVD scheme. For the accurate simulation of high-pressure combustion in ram accelerator, 9-species, 25-step fully detailed reaction mechanism was incorporated with the existing CFD code previously used for the ram accelerator studies. The mechanism is based on GRI-Mech. 2.11 that includes pressure-dependent reaction rate formulations indispensable for the correct prediction of induction time in high-pressure environment. A real gas equation of state was also included to account for molecular interactions and real gas effects of high-pressure gases. The present combustion modeling is compared with previous 8-step and 19-step mechanisms with ideal gas assumption. The result shows that mixture ignition characteristics are very sensitive to the combustion mechanisms, and different mechanism results in different reactive flow-field characteristics that have a significant relevance to the operation mode and the performance of scram accelerator.

Ram accelerator operations at acceleration level up to 6 x 104 g

Experiments were conducted using a 25-mm-bore ram accelerator. By varying the projectile mass and the mixture fill pressure, the projectile acceleration level was varied by a factor of three. The acceleration performance is evaluated based on a ballistic efficiency. Over a velocity range from 80% to 110% of Chapman-Jouguet detonation velocity, the ballistic efficiency was kept almost constant, and did not depend on the acceleration level.

An expansion-Ludwieg tube for accelerating combustible gas mixtures to superdetonative speeds

Stoichiometric methane/air and ethylene/air gas mixtures had been successfully accelerated to superdetonative speeds using the classical expansion tube with buffer tube developed at ISL. Hydrogen/air gas mixtures could not be accelerated by this principle. For this purpose a special set-up consisting of an expansion tube using an additional Ludwieg tube as hydrogen reservoir was developed and tested at the ISL Shock Tube Laboratory. With this set-up hydrogen/air gas mixtures can be successfully accelerated to a flow having a superdetonative speed with static pressures up to 215 kPa at 305 K. The experiments in the expansion tube are carried out for basic research purposes on ram-accelerator-related combustion phenomena and also for supporting and optimising the operation of the 30 mm and 90 mm ram accelerator facilities at ISL. We shall discuss problems encountered while using the expansion tube and some preliminary experiments performed with this new facility to prove the homogeneity of the flow.

Determination of the choke pressure of a ram accelerator projectile in subdetonative regime

Operation of the thermally choked propulsive mode provides a high level of acceleration at velocities below the Chapman-Jouguet detonation speed. A one-dimensional modeling of this process has been elaborated which takes into account the real gas effects and yields results that are in good agreement with experimental data at this velocity and acceleration. It also provides an estimate for the pressure level at the thermal choking point that is assumed to occur in the region where the pressure rapidly drops and the oscillatory character of the signal abruptly smoothes out. A fairly good agreement between experimental data and those derived from the one-dimensional modeling based on the ideal gas equation of state was observed. The effects of increasing the initial pressure, however, suggest that a real gas equation of state should be used. The present paper is aimed at providing experimental data on the thermal choking pressure. These values are compared to calculations based on real gas equations of state, i.e. Boltzmann and Percus-Yevick. The thermal choke point pressure in many high-pressure experiments is much lower than the calculated values which brings into question the validity of the criteria for identifying the choke point and/or the accuracy of the pressure measurements under these conditions.

Numerical modeling and simulation of RAMAC 30 experiment carried out at the French-German research institute of Saint-Louis

A ram accelerator experiment which was done in ISL's RAMAC 30, was numerically simulated in this study. The case of shot 225 was considered and the initial projectile launching speed was 1.8 km/s which corresponded to the superdetonative speed of the stoichiometric H 2 /O 2 mixture diluted with 5CO 2 . CFD code had some enhanced features suitable for the high-pressure gaseous combustion. In this study, it was found that neither shock heating nor viscous heating was sufficient to ignite the mixture at the low speed of 1.8 km/s, as was found in the experiments using a steel-covered projectile. However, we could succeed in igniting the mixtures by imposing an amount of additional heat to the combustor section and simulate the positive acceleration of the shot 225 with an aluminum-covered projectile in reasonable agreement. Some additional studies were preformed concerning the location of the forced ignition and the conical angle of the projectile. The results gave some insights into the combustion mechanism and the condition of positive acceleration in a scram accelerator.

Starting Envelope of the Subdetonative Ram Accelerator

An experimental investigation has been undertaken to explore the effects of propellant composition, entrance Mach number, and projectile throat area variation on the subdetonative ram accelerator starting process. Upper and lower limits on these parameters provide bounding conditions under which a supersonic projectile can successfully initiate and stabilize the combustion-supported shock system on the projectile necessary for acceleration. Relatively energetic propellant combinedwith high entrance Mach number are observed to push the shock system ahead of the projectile, whereas low energy release together with low entrance Mach number are conducive to the shock system falling behind. Decreasing entrance Mach number also causes the shock system to propagate ahead of the projectile under some circumstances, suggesting that fundamentally different start failure mechanisms are responsible. Increasing the  ow area at the throat lowers the Mach number for choking the diffuser, but causes difŽ culty in containing the combustion-supported shock system unless the Mach number is increased or the propellant energy release is decreased. Trends observed in this investigation and prior research are used to develop a generalized starting envelope, illustrating where the success and failure boundaries lie in relation to one another on the propellant energy release vs Mach number plane.

Temperature solution of one-dimensional modeling of rapid transient heat transfer around a cylindrical body

The aim of the present paper is to provide a numerical solution of the heat transfer equation into and along the wall of a cylindrical profile in a very high-speed flow. A series of diagrams are derived. They allow to calculate the time-dependent temperature at any point on the surface as well as inside the body. The profile, which is initially at a uniform temperature, is suddenly submitted to a convective and/or radiative heat flux around its surface. These diagrams provide useful data on extremely rapid heat transfer phenomena, i.e., at very low Fourier (Fo) numbers. They can be used regardless the dimensions, thermophysical characteristics of the body, and thermal characteristics of the wall, i.e., at any Biot (Bi) number. They provide a reliable shortcut for the understanding of the thermal behavior of the projectile abrasion in a ram accelerator operation. The analytical aspect of the problem is detailed in the case of a long, i.e., one dimensional, cylindrical geometry, but the diagrams can be used for 2D transient phenomena, in relation with the solution of the infinite plate, which has been developed earlier for the same range of Fourier ans Biot numbers. The general equation that is presented in the paper is quite complex to solve for some Fo-Bi combinaisons. Therefore, a numerical procedure was developed in order to account fore some important divergence problems that may occur during the calculation in these specific cases. The accuracy of the present diagrams has been strongly improved in comparison with that of the same type of plots, which are available in the literature and turn out to yield a poor accuracy for some specific Fo-Bi combinaisons.

Review of Propulsion Applications of Detonation Waves

Applications of detonations to propulsion are reviewed. First, the advantages of the detonation cycle over the constant pressure combustion cycle, typical of conventional propulsion engines, are discussed. Then the early studies of standing normal detonations, intermittent (or pulsed) detonations, rotating detonations, and oblique shock-induced detonations are reviewed. This is followed by a brief discussion of detonation thrusters, lasersupported detonations and oblique detonation wave engines. Finally, a more detailed review of research during the past decade on ram accelerators and pulsed detonation engines is presented. The impact of the early work on these recent developments and some of the outstanding issues are also discussed.

Recent advances in laser-based diagnostics for gaseous flows

Laser-based diagnostic techniques offer unique capabilities for experimentation on gaseous flows. In this paper, we overview recent progress of two concepts: spectrally resolved absorption and planar laser-induced fluorescence (PLIF) imaging. The absorption measurements utilize tunable diode lasers (TDLs) as light sources. Recent TDL applications include a wavelength-multiplexed system for rapid temperature sensing for use in combustion control, and absorption probes for time-resolved measurements of temperature, velocity and species concentrations in a hypersonic shock tunnel. Recent PLIF work includes applications to supersonic, exothermic flowfields relevant to ram accelerators, and development of a method for imaging temperature in air flows using acetone seeding.

00/01534 Real gas effects in ram accelerator propellant mixtures: theoretical concepts and applied thermochemical codes

00/01534 Real gas effects in ram accelerator propellant mixtures: theoretical concepts and applied thermochemical codes

The applications of unsteady, multi-dimensional studies of detonation waves to ram accelerators

Since much of the early work on the concepts on which ram accelerators are based dates back to the 1960s, although many of these are still being actively pursued, it is difficult to formulate a completely logical approach. This situation is compounded by the use of presently unacceptable treatments of unidimensional detonations in the early work and unfortunately extended to some of the more modern treatments. My approach has been to start by dealing with the early work and recent work impinging upon it, then to re-emphasise recent work on detonations, particularly that dealing with the influence of changes in confinement on quenching and re-initiation of detonations. However, some knowledge of this is inferred in suggestions made in Part 2 for possible improvements in the techniques. Latter sections cover the development of the ram accelerator, the use of various types of projectiles, developments in experimental techniques and finally on areas in space flight where the results from ram accelerators might be utilised.