The results of a systematic experimental investigation on the methods of enhancing the regression rate in hydroxyl-terminated polybutadiene (HTPB) fuel used in an HTPB/gaseous oxygen hybrid motor are presented The effects of the addition of ammonium perchlorate (AP) or aluminum in the fuel, the variation of oxidizer ‐fuel ratio, and the variation of characteristic dimensions of fuel grain are presented For the extents of the parametric variations considered, while the addition of AP and/or Al and the reduction of grain port diameter enhance the regression rate, the effect due to the latter is the most signie cant one Furthermore, the regression rate increases along theaxis, and it becomes essentially constant in the port region corresponding to a fuel-rich composition The possible physical processes for all of these behaviors are discussed The experimentally obtained exponents of the variables for regression rate (oxidizer mass e ux and port diameter ) are found to be signie cantly different from those of the conventional theory The similarity between the fuel regression rate equation used in solid-fuel ramjet and that obtained in hybrid motor is discussed

Fuel Regression Rate in Hydroxyl-Terminated-Polybutadiene/ Gaseous-Oxygen Hybrid Rocket Motors

An experimental study is conducted to evaluate amine–borane materials in solid fuels that create hypergolic combustion with nitric acid. Ignition delays as short as 3.3 ms have been measured for fuels containing amine–boranes in an epoxy binder: one of the fastest ignition delays ever recorded for a hypergolic hybrid fuel. The heats of formation of the amine–boranes were measured and theoretical (ideal) performance calculated. Some amine–boranes exhibit a double peak performance allowing for a range of oxidizer/fuel combinations resulting in high performance. Amine–borane/fuel binder combinations can produce a 3.8 and 8.2% increase in Isp and ρIsp, respectively, over monomethylhydrazine and a 4–11% increase in Isp and a 17% decrease to 5% increase in ρIsp over other hypergolic hybrid rockets that use similar oxidizers. Amine–boranes also exhibit trends of being less toxic and more air stable than their base amines. In addition to the hypergolic properties, performance alone motivates the use of these mate...

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Solid amine-boranes as high performance hypergolic hybrid rocket fuels

This paper is concerned with the re-examination of the hybrid regression rate equation proposed by Marxman and co-workers and long accepted in the combustion literature. The important conclusion of the theory that the regression rate, being a weak function of the mass transfer number B, is nearly independent of the properties of the fuel and oxidiser is shown not to be borne out by the experimental studies including those of Marxman et al. Experiments specifically designed by the present authors to elucidate the dependence of the mass transfer number B are described. It is shown that the exponent on B is 0.5 instead of 0.23 from the data on rubber—oxygen+nitrogen system. A theory is proposed here to explain these results. One element of the theory is that the density variation across the boundary layer as affected by temperature and, more importantly, molecular weight has significant effect on the blocking effect (reduction in skin friction coefficient with injection). It is first shown that the theory explains the long standing results on the blocking effectwith denser gas injection. This result is then combined with the known fact that in polymer combustion environment the mean molecular weight of fueld gases near the surface is much larger than monomer molecular weight by a factor of 4–6 to obtain modified predictions on regression rate. A new regression rate law has been obtained incorporating these features and it reads as ρpr=0.056G0.8(x/μ)−0.2 (ρf/ρe)0.71 (ρw/ρe)0.14· B(1+B)−0.73+0.002ρw/ρe where ρw, ρe and ρf are the densities at the central core, wall and the flame respectively. The predicted results are shown to remove the anomalies present in the earlier prediction procedures. The new regression rate law may have far reaching consequences in hydrbrid regression rate theory as well as erosive combustion in solid propellants.

Regression rates in boundary layer combustion

A boron-based catalytic and energetic fuel additive is explored as a mechanism to enhance performance of hybrid motors using nitric-acid-based oxidizers. Ethylenediamine bisborane is highly hypergolic with nitric acid, thereby eliminating the need for a separate ignition system and providing the potential for restart. Ignition delays of 2.9±0.3 ms for ethylenediamine bisborane powder and 31.7±19.6  ms for ethylenediamine bisborane/hydroxyl-terminated polybutadiene combinations have been measured using white fuming nitric acid as the oxidizer. Theoretical specific impulse values higher than any other nitric-acid-based hypergolic hybrid combination tested are obtained; in addition, ethylenediamine bisborane is air stable and not toxic. Also presented is a method of manufacturing large quantities of high-purity ethylenediamine bisborane.

Characterization of Ethylenediamine Bisborane as a Hypergolic Hybrid Rocket Fuel Additive

Fuel regression rate enhancement studies in HTPB/GOX hybrid rocket motors

The results of a model of the combustion of monopropellants, using ammonium perchlorate as the specific example and treating the complex gas phase kinetics and diffusion by the trace diffusion approximation, are presented in this paper The species considered are HClO 4 , NH 3 , ClO 3 , NH 2 , ClOH, HNO, Cl, O, NO, N 2 O, N 2 , Cl 2 , ClO, HCl, O 2 and H 2 O, and are expected to participate in 14 reversible reactions Kinetic data for the reactions are based on the works of Ermolin 5 , Guirao, and Williams 1 and Jacobs and Pearson 18 The time-dependent governing equations are solved by the method of lines, and the results for mass burn rate, pressure index and temperature sensitivity are compared with those of experiments Some perturbational calculations show that inclusion of NO 2 and ClO 2 , in palce of atomic oxygen, leads to more realistic predictions for mass fractions and burning rate

A model of combustion monopropellants (ap) with complex gas phase kinetics

Regression ra tes of a hypergolic combination of fuel and oxidiser have been experimentally measured as a function of chamber pressure, mass flux and the percentage component of the hypergolic compound in natural rubber. The hypergolic compound used is difurfurylidene cyclohexanone (DFCH)
which is hypergolic with the oxidiser red fuming nitric acid (RFNA) with ignition dela y of 60-70 ms. The 
data of weight loss versus time is obtained for burn times varying between 5 and 20 seconds. Two methods of correlating the data using mass flux of oxidiser and the total flux of hot gases have shown that index n of
the regression law r=aGoxn or r=aGnxn-1 (x the axial distance) is about 0.5 or a little lower and not
0.8 even though the flow through the port is turbulent. It is argued that the reduction of index n is due to
heterogeneous reaction between the liquid oxidiser and the hypergolic fuel component on the surface.

H. K. Narahari

Papers

A model of combustion monopropellants (ap) with complex gas phase kinetics

Regression Rate Studies in Hypergolic System