Theoretical Specific Impulse Research Articles

Evaluation of Ethanol-Blended Hydrogen Peroxide Monopropellant on a 10 N Class Thruster

The evaluation of an ethanol blended hydrogen peroxide monopropellant thruster was carried out. The specific impulse of hydrogen peroxide was increased by blending it with ethanol. Ethanol was selected because it presented no significant problem in terms of storability. An oxidizer-to-fuel ratio of 50 was selected considering the thermal characteristics of the material used and the higher specific impulse that was obtained in comparison to the theoretical specific impulse of hydrogen peroxide. For the thruster material, 316L stainless steel was chosen based on compatibility test results and other considerations. Barium hexaaluminate was used as a catalyst support because the chamber temperature increased owing to the combustion of ethanol. The surface area of barium hexaaluminate was approximately six times higher than that of alumina after heating at 1200°C. Platinum and manganese oxide with lead oxide catalysts were evaluated on a 10 N class thruster and the catalyst capacity, specific impulse, and efficiency characteristic velocity were almost similar. However, the fluctuation of the chamber pressure increased in the case of manganese oxide with lead oxide catalyst. The decomposition performance of both catalysts showed no remarkable difference, but the platinum catalyst was more reliable.

Numerical study on operating characteristics of a magnesium-based fuel ramjet

A two-dimensional axis symmetrical model for multi-phase turbulent combustion simulation has been developed in a magnesium-based fuel ramjet, the stochastic model is applied to track the motion and transportation of discrete phase in the whole flow field in Lagrangian frame. Firstly, the flow field parameters distribution including temperature, velocity, Mg particle burning rate, main components are obtained to describe the basic operating characteristics, and the thermoacoustic oscillation during the combustion process is predicted due to the observed vortex. Then combined with the developed thermodynamic calculation program, the influencing rules of water/fuel ratio and magnesium-based fuel composition ratio on ramjet operating characteristics are analyzed. The results obtained show that the distribution of two water/fuel ratios exerts a direct impact on the temperature fluctuations in combustor, further influences the combustion stability, as for theoretical specific impulse, a maximal value is observed along with the total water/fuel ratio, while the monotonous trends for thermal efficiency and propulsive efficiency are achieved respectively. Moreover, the magnesium-based fuel composition ratio is a dominant factor to influence the operating characteristics, and it is proved that the high-proportion Mg in fuel is beneficial to specific impulse augment through validation of both simulation results and thermodynamic calculation results.

Thermodynamic analysis of solid-fuel mixtures glycidyl azide polymer (GAP)/RDX for miniengines of microelectromechanical systems

On the basis of thermodynamic calculations we show that solid-fuel mixtures glycidyl azide polymer/RDX are promising for use in miniengines made on the basis of technologies of microelectromechanical systems of semiconductor microelectronics. It has been shown that small (up to 20 mass percent) additives of RDX to the glycidyl azide polymer markedly increase the values of the theoretical specific impulse and the thermal efficiency of the engine and decrease the quantity of undesirable solid carbon formed in combustion products of the mixed fuel. In so doing, these mixtures provide fairly low combustion temperatures not exceeding the thermostability limit of crystal silicon from which the miniengine case is made. The physicochemical factors influencing the value of the theoretical specific impulse of the mixed solid-fuel charge have been elucidated, and methods for its maximization have been proposed.

Molecular Dynamics Study of RDX/AMMO Propellant

Molecular dynamics simulations have been performed to investigate well-known energetic material cyclotrimethylene trinitramine (RDX) crystal, 3-azidomethyl-3-methyloxetane (AMMO) and RDX/AMMO propellant. The results show that the binding energies on different crystalline surface of RDX changes in the order of (010) > (100) > (001). The interactions between RDX and AMMO have been analyzed by means of pair correlation functions. The mechanical properties of RDX/AMMO propellant, i.e. elastic coefficients, modulus, Cauchy pressure, and Poisson's ratio, etc., have been obtained. It is found that mechanical properties are effectively improved by adding some amounts of AMMO polymers, and the overall effect of AMMO on three crystalline surfaces of RDX changes in the order of (100) > (010) > (001). The energetic properties of RDX/AMMO propellant have also been calculated and the results show that compared with the pure RDX crystal, the standard theoretical specific impulse of RDX/AMMO propellant decrease, but they are still superior to those of double base propellant.

Molecular Dynamics Simulation of Binding Energies, Mechanical Properties and Energetic Performance of the RDX/BAMO Propellant

Molecular dynamics (MD) simulations were performed to investigate the well-known energetic material cyclotrimethylene trinitramine (RDX) crystal, 3,3'-bis-azidomethyl-oxetane (BAMO) and the RDX/ BAMO propellant. The results show that the binding energies of RDX with BAMO on different crystalline surfaces change as follows: (010)>(100)>(001). The interactions between RDX and BAMO were analyzed by pair correlation functions g(r). The mechanical properties of the RDX/BAMO propellant, such as the elastic coefficients, modulus, Cauchy pressure, and Poissonʹs ratio, were obtained. We find that the mechanical properties are effectively improved by adding some BAMO polymer and the overall effect of BAMO on the three crystalline surfaces of RDX changes as follows: (100)>(001)>(010). The energetic performance of the RDX/BAMO propellant was also calculated and the results show that compared with the pure RDX crystal, the standard theoretical specific impulse (Isp) of the RDX/BAMO propellant decreases but it is still superior to that of the double base propellant.

Molecular dynamics study of the structures and properties of RDX/GAP propellant

Molecular dynamics simulations have been performed to investigate well-known energetic material cyclotrimethylene trinitramine (RDX) crystal, glycidyl azide polymer (GAP) and RDX/GAP propellant. The results show that the binding energies on different crystalline surface of RDX change in the order of (010)>(100)>(001). The interactions between RDX and GAP have been analyzed by means of pair correlation functions. The mechanical properties of RDX/GAP propellant, i.e., elastic coefficients, modulus, Cauchy pressure, and Poisson's ratio, etc., have been obtained. It is found that mechanical properties are effectively improved by adding some amounts of GAP polymer, and the overall effect of GAP on three crystalline surfaces of RDX changes in the order of (100)>(010)>(001). The energetic properties of RDX/GAP propellant have also been calculated and the results show that compared with the pure RDX crystal, the standard theoretical specific impulse (Isp) of RDX/GAP propellant decrease, but they are still superior to those of double base propellant.

Efficient solid rocket propulsion for access to space

Space launch activity is expected to grow in the next few years in order to follow the current trend of space exploitation for business purpose. Granting high specific thrust and volumetric specific impulse, and counting on decades of intense development, solid rocket propulsion is a good candidate for commercial access to space, even with common propellant formulations. Yet, some drawbacks such as low theoretical specific impulse, losses as well as safety issues, suggest more efficient propulsion systems, digging into the enhancement of consolidated techniques. Focusing the attention on delivered specific impulse, a consistent fraction of losses can be ascribed to the multiphase medium inside the nozzle which, in turn, is related to agglomeration; a reduction of agglomerate size is likely. The present paper proposes a model based on heterogeneity characterization capable of describing the agglomeration trend for a standard aluminized solid propellant formulation. Material microstructure is characterized through the use of two statistical descriptors (pair correlation function and near-contact particles) looking at the mean metal pocket size inside the bulk. Given the real formulation and density of a propellant, a packing code generates the material representative which is then statistically analyzed. Agglomerate predictions are successfully contrasted to experimental data at 5 bar for four different formulations.

Novel High Nitrogen Propellant Use in Solid Fuel Micropropulsion

The use of two novel materials, 3,6-bis(1H-1,2,3,4-tetrazol-5-ylamino)-s-tetrazine (BTATz) and mixed N-oxides of 3,3'-azo-bis(6-amino-1,2,4,5-tetrazine) (DAATO3.5, where the 3.5 indicates the average oxide content), as solid fuels in micropropulsion systems is investigated. These materials were selected due to their ease of ignition, relatively good safety characteristics, long-term storage capability, low combustion temperature, noncorrosive combustion products, and reasonable projected material cost. Material safety data including impact, friction, differential scanning calorimetry, and electrostatic sensitivity are reported and compared to the previously implemented micropropulsion fuels, lead styphnate, glycidyl azide polymer (GAP) and 3,6-diamino-1,2,4,5-tetrazine-1,4-dioxide. Burn rate data are also reported for both materials. Whereas BTATz and DAATO3.5 have very high burn rates, DAATO3.5 is believed to have the highest burn rate of any known stable organic solid. Additionally, DAATO3.5 has a very desirable pressure exponent of 0.28. Test stand results using a quartz microthruster are reported for these materials. Estimated chamber pressures reached 18 atm with peak thrust levels around 0.1 N. Measured specific impulses of approximately 14% of the theoretical specific impulse (218 s for BTATz and 228 s for DAATO3.5) were obtained. This level of inefficiency is believed to be acceptable for micropropulsion systems and is comparable to other, independent work.

Effect of aluminium in propellant composition on acoustic emission parameters (Short Communication)

The study reports the variation in acoustic emission signals acquired during combustion of typical propellants with varying aluminium and ammonium perchlorate content. It was observed that when propellant strands having the same composition undergo combustion under similar conditions, they produce consistent acoustic emission signals. To study the effect of variation of aluminium content in the propellant composition on the acoustic emission produced during combustion, the aluminium content was varied from 6 per cent to 18 per cent in a HTPB-based composite propellant with 86 per cent solid loading. Experiments were carried out with propellant strands under the same conditions for a comparative study. Acoustic emission parameters, such as peak amplitude, ring-down counts, average frequency, hits and energy were studied as functions of time. Among these parameters, only energy ring-down counts and frequency varied significantly with aluminium content. The effect of cumulative values of energy, frequency and ring-down counts, the effect of burn rate and theoretical specific impulse against the aluminium percentage variation, and the variation of specific impulse against acoustic energy can all be correlated. The clear trend is indicative of possible prediction of propellant performance parameter like specific impulse from acoustic emission parameters.

Burn rate measurements of HMX, TATB, DHT, DAAF, and BTATz

Abstract : In this paper we present new burn rate results for several energetic materials. The burn rates of octahydro- 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), HMX and estane-based binder (PBX 9501), 1,3,5-triamino- 2,4,6-trinitrobenzene (TATB), and TATB and KelF binder (PBX 9502), are reported and compared with existing data. Burn rate data of these common explosives complement and extend existing data sets. Burn rate data of three novel high-nitrogen materials are presented in this work. Specifically, the high-nitrogen monopropellants considered are 3,6-dihydrazino-s-tetrazine (DHT), 4,4'-diamino-3,3' -azoxyfurazan (DAAF), and 3,6-bis(1H-1,2,3,4-tetrazol-5-amino)-s-tetrazine (BTATz). High-nitrogen compounds may be key to meeting the advanced performance objectives of next-generation solid propellants. High-nitrogen solids offer the possibility of high performance, reduced emissions, and lower plume signature (low temperature and no HCl) than current propellant systems. The theoretical specific impulse is comparable to HMX. In contrast to HMX, however, high-nitrogen materials tend to be insensitive to impact. Because high-nitrogen energetic materials have intrinsically large positive heats of formation and produce low-molecular-weight reaction products, they may be suitable for consideration in high-performance propellant applications. BTATz appears particularly interesting because of its rapid burn rate, relatively low-pressure exponent, and high heat of formation. The effect of a small amount of binder is investigated for all but one of these materials.

Reactive impulse from the explosion of a gas mixture in a semiinfinite space

The reactive impulse from the explosion of an acetylene-oxygen mixture in a cylindrical chamber is calculated and measured. The gas mixture was charged into the closed end of the chamber and filled all or part of the length of the chamber. In the latter case, air filled the remaining part of the chamber volume. With a variation in the ratio of chamber and charge lengths from 1 to 7–10, the specific impulse from detonation of the mixture increases from 160 to 500–540 sec. In the case of fuel-air mixtures, the theoretical specific impulse is several times lower per unit mass of the mixture and several times greater per unit mass of the fuel.

A systematic procedure for estimating the standard heats of formation in the condensed state of non aromatic polynitro-compounds

Abstract An additive method for predicting the standards heats of formation at 298 K of aliphatic and alicyclic polynitro-compounds is described. It consists in three steps (i) the evaluation of the enthalpy of formation in the gaseous state (ii) the estimation of the enthalpy of vaporization for a liquid or enthalpy of sublimation for a solid and (iii) the calculation of interaction terms, mainly between NO2 groups. The uncertainty on predicted values are generally within a few kcal/mol. As an example of application, it is shown that the theoretical specific impulse of high-energy solid propellants can be calculated with a relative uncertainty less than 1%.

On underwater detonations, 1. A new method for predicting the CJ detonation pressure of explosives

Abstract The experimental detonation pressure of explosives has been found to correlate reasonably well with the theoretical specific impulse and density of these compositions. Two data sets were subjected to linear regression analysis, giving a number of possible correlations; however, the equation containing Isp × Density2, a Kamlet-Jacobs like term, was found to show the best relationship for predicting, today's plastic bonded explosives. The complete equation is:

Free radical propulsion concept

ANEW propulsion concept which may have particular usefulness for transfer of loads from low Earth orbit (LEO) to geosynchronou s Earth orbit (GEO) is presented. Electrical energy in the form of microwaves is used to heat a propellant gas, thus providing thrust. An outline of the concept and experimental results in support of it are presented. Contents A variety of possible future space missions involve the transfer of substantial masses from LEO to GEO. 1 A crucial factor in propulsion systems designed to meet this need is the ratio of total mass required in LEO to payload mass in GEO, due to the fact that Earth-to-orbit cost is a major fraction of total cost. Systems presently considered for such missions include chemical systems and electrostatic ion propulsion systems. Figure 1 presents payload mass ratio as a function of specific impulse for these systems. Chemical systems provide short trip times at the expense of mass ratios of 3 or greater. Electrostatic ion propulsion systems can achieve mass ratios of 1.25-2.5, but require trip times of 50 days or longer. There is an intermediate range of specific impulse not accessible to present systems which would allow moderate trip times while maintaining mass ratios similar to those of electrostatic systems. The free radical propulsion concept presented here may prove capable of operating in that regime. The concept is illustrated schematically in Fig. 2. Electric power from solar panels is used to operate a microwave generator, and microwave energy is coupled to a propellant gas by means of a resonant cavity. The gas is ionized partially by the microwave fields, and inelastic collisions of gas molecules with the resultant electrons serve to dissociate the gas. The gas is allowed to recombine downstream, with the recombination energy heating the gas. The hot gas is expanded through a nozzle to produce thrust. Molecular hydrogen, with a theoretical specific impulse of 2100 s (100% conversion), is the most attractive propellant. Sample calculations on a hydrogen thruster for a thrust of 1 N and a specific impulse of 1500 s yield the following results: mass flow rate, 6.8xlO~ 5 kg/s; minimum input power, 7.3 kW; minimum flow area (nozzle throat), 4x 10 ~2 to 4x 10 ~4 m2 for pressures of 13.3 to 1330 N/m2, respectively; and thrust densities of the order of 100 N/m2. Attractive features of the free radical propulsion concept in comparison to electrostatic ion propulsion or similar systems are increased thrust, thrust density, and thrust-to-power ratio.

Performance Optimization of Metallized Solid Propellants

AbstractThe object of this paper is to present a method for determining, for a typical metallized solid propellant system, the optimum combination of the reactants so as to arrive at a composition which can yield the maximum specific impulse. A direct approach of this kind for optimization of propellants does not appear to have been attempted so far. The optimization is sought to be carried out by expressing the theoretical specific impulse of the propellant under frozen flow conditions as a function of the different variables involved and maximizing the same, subject to the usual linear and non‐linear constraints of mass balance, mass action, pressure balance, energy conservation and entropy conservation. The gradient projection technique due to Rosen has been adopted for carrying out this optimization. The method was programmed on the IBM 360/Model 44 Computer in FORTRAN IV language and the results of the computations for a typical metallized solid propellant system are presented in the paper. The investigations reveal that the gradient projection technique can be a useful computational device for determining the optimum composition and the corresponding performance parameters of propellant systems containing a large number of reactants and products with or without condensed phases.

Requisite Temperatures for the Stabilization of Atomic H in Solid H2

If an atomic hydrogen/molecular hydrogen propellant containing at least 15% free H atoms by weight may be used, values for the theoretical specific impulse near and above 750 s may be predicted. The tritium-impregnation concept has been applied to manufacturing such an H/H2 propellant, and a phenomenological rate process theory has been derived for the matrix-isolation storage and equilibrium stability of atomic H produced at such ultralow temperatures in tritium-impregnated H2. It is suggested that an energy storage efficiency greater than 0.30 may be obtained at temperatures below 100 mK. So that the storage of atomic H is stable with respect to arbitrary small perturbations, the surface temperature must be less than a critical value dependent on sample volume, tritium weight fraction, and energy storage efficiency. A derivation of the formula for this critical surface temperature is presented, noting that the energy storage efficiency is to be fixed by experiment.

A Solid-Liquid Rocket Propellant System

S years ago, an investigation of a hybrid (solidliquid propellant) rocket propellant s}^stem was conducted by the General Electric Company as part of an Army Ordnance Contract. The system studied involves the use of 90 per cent hydrogen peroxide, the oxidizer, in combination with a solid fuel, generally polyethylene. The purpose of this report is to review the reasoning which led initially to the interest in such a system and to the choice of the particular materials used; in addition, some of the results will be briefly discussed. The advantages to be gained from systems of this kind are not immediately obvious; in fact, there are probably not many solid-liquid propellant combinations which do offer any advantages without introducing other difficult problems. The following summary of the desirable characteristics of the system described here will show why it appeared to be interesting after some rocket engine tests: 1 The theoretical specific impulse is good, with a maximum value of 228 lb-sec/lb at 300 psi and oxidizer/fuel weight ratio of about 6. 2 The average density is high; at the ratio corresponding to the maximum specific impulse, the average density is 1.34 g/cc. 3 By decomposing all of the 90 per cent H202 catalytically, spontaneous ignition of the fuel is obtained with a delay of usually less than 0.5 sec. In this way, the combustion of the fuel may be considered as augmenting the performance of hydrogen peroxide as a monopropellant. Ignition is generally reliable and combustion is smooth over a wide range of oxidizer/fuel ratios. 4 With proper design of the engine and fuel charge, hard or explosive starts should never occur; the peroxide decom-