US3698191A

US3698191A - Nonsustaining hybrid propellant grain

Info

Publication number: US3698191A
Application number: US660869A
Authority: US
Inventors: Robert W Ebeling Jr
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1967-08-01
Filing date: 1967-08-01
Publication date: 1972-10-17
Anticipated expiration: 1989-10-17

Abstract

A nonsustaining hybrid propellant grain is provided for use in hybrid rocket systems employing halogen and interhalogen oxidizers. The propellant grain comprises an organic binder and a nitrate oxidizing salt in amounts substantially stoichiometric with the carbon content of the grain. The preferred nitrate is ammonium nitrate.

Description

United States Patent 1 1 3,698,191 Ebeling, Jr. 1451 Oct. 17, 1972 [54] NONSUSTAINING HYBRID 3,068,641 12/1962 Fox ..60/220 PROPELLANT GRAIN 3,083,526 2/1963 Hudson ..60/220 3,147,161 9/1964 Abele etal ..149/19 [721 g Kutzmwm 3,234,729 2/1966 Altman et a1 ..60/220 3,257,802 6/1966 Kaufman ..60/220 [73] Assignee: United Aircraft Corporation, Ea t 3,305,523 2/1967 Burnside ..149/19X Hartford, Conn. l Filed: g 1967 Primary Exammer-Ben am1n R. Padgett Appl. No.: 660,869

us. 131 ..60/220, 60/219 Int. c1. ..C06d 5/10 Field 61 Search ..60/218, 219; 149/2, 19

Attorney-Steven F. Stone [57] ABSTRACT A nonsustaining hybrid propellant grain is provided for use in hybrid rocket systems employing halogen and interhalogen oxidizers. The propellant grain comprises an organic binder and a nitrate oxidizing salt in amounts substantially stoichiometric with the carbon content of the grain. The preferred nitrate is ammonium nitrate.

6 Claims, N0 Drawings NONSUSTAINING HYBRID PROPELLANT GRAIN BACKGROUND OF THE INVENTION Hybrid propellant systems generally consist of a combustion chamber containing a solid fuel grain and an injector assembly for injecting a fluid oxidizer into the combustion chamber to react with the fuel grain and produce combustion gases which are then exhausted through a nozzle to produce thrust. Hybrid propellant systems are simpler than liquid bipropellant systems in that tankage and flow control systems are required for only one propellant and have a decided advantage over solid propellant systems in that throttling and on-off capability can readily be obtained by control of the flow of the fluidoxidizer. In order to increasethe performance of such systems, the prior art has progressed from the relatively low energy oxygen type oxidizers such as nitric acid and oxygen to high energy halogen type oxidizers such as fluorine and the interhalogen type compounds of which ClF BrF and BrF are exemplary.

With respect to the fuel component, the simplest fuel system consists of a cured polymeric grain made from a suitable polymeric system such as polyethylene, polypropylene, polybutadiene, polyurethane, polyester, and polyisobutylene, for example. In some cases, an amount of an inorganic oxidizing agent is added to increase the regression rate of the grain. It is known, however, that higher performance can be obtained if, in addition to the polymeric binder, various oxidizable materials such as metals; e.g., aluminum and magnesium; metal hydrides; boron; carborane compounds; and high energy organic compounds such as organic substituted hydrazine and boron hydride compounds, and various heterocyclic nitrogen containing compounds such as, for example, tetraformaltrisazine (TFTA), triaminoguanidineazide, triaminoguanidine, and the double salt of triaminoguanidineazide and hydrazine azide are loaded into the binder. Burning rate modifiers can also be incorporated in the binder.

When fluorine and interhalogen type oxidizers are employed, it was found that, with the exception of carbon, all of the components of the grain are either oxidized or liberated in gaseous form. The carbon, however, is liberated in the form of finely divided elemental carbon (i.e., soot) which not only produces an objectionable black smoky flame, but, also, reduces the performance from that which would have been obtained if the carbon were oxidized.

Since oxygen is needed to oxidize the carbon, an approach to the problem has been to modify the fluid oxidizer by including sufficient oxygen to react with the carbon. Such fluid oxidizer systems include OF F LOX, and various interhalogen-oxygen compounds such as, for example, CIO F. These systems, however, have not proven satisfactory, particularly where storability of the system is needed.

Another approach to the problem is to include an oxygen-containing compound in the fuel grain. However, I have found that when the oxygen-containing compound is selected from, for example, the high performance perchlorate type oxidizers now generally used in solid and hybrid propellants, such as ammonium perchlorate, it is not possible to incorporate sufficient oxidizer to react with all the carbon without causing the fuel grain to sustain combustion after oxidizer flow has been stopped. If a grain sustains, it is not suitable for use in an on-off system.

SUMMARY OF THE INVENTION According to this invention, it has been found that a hybrid propellant grain for use with halogen oxidizers can be loaded with an amount of oxygen-containing oxidizer stoichiometric with the carbon content of the grain, without producing a sustaining grain if the oxidizer is selected from inorganic nitrates such as the alkali and alkaline earth nitrates and ammonium nitrate.

It is, accordingly, an object of this invention to provide a hybrid propulsion system employing a halogen or interhalogen oxidizer that produces a carbon-free exhaust.

It is another object of this invention to provide a hybrid propulsion system employing a halogen or interhalogen oxidizer that is capable of stop-start operation.

It is another object of this invention to provide a fuelrich grain for a hybrid propulsion system that is nonsustaining.

It is another object of this invention to provide a nonsustaining hybrid fuel grain containing amounts of oxidizer stoichiometric with the carbon content of the grain.

These and other objects of this invention will be readily apparent from the following description of the invention.

DESCRIPTION OF THE INVENTION This invention can be employed with any polymeric binder system such as those set out above, however, for ease of understanding, one binder system will be referred to throughout. The binder employed in the following examples consists of hydroxy-terminated polybutadiene cured with toluene diisocyanate which produces a tough, rubbery matrix incorporating the various additives.

In the combustion process, the carbon present is oxidized to CO and the following equivalent weights are of use in determining the amounts of the various materials needed to achieve stoichiometry.

25.0g binder 1.85 equivalents of C 144.0g TFTA 4.00 equivalents of C 80.5g NH NO 3.00 equivalents of O l 17.0g NI-I CIO 4.00 equivalents of 0 Thus, in the simplest fuel system in which merely the fuel and oxidizer are present, it is seen that for 25g of binder, approximately 50g of NH NO are required for stoichiometric oxidation of the carbon present, whereas approximately 54g of NH CIO would be required. On a percentage basis, this yields grains comprising, respectively, about 33 percent binder and 67 percent NI-I NO and 32 percent binder and 68 percent NI-I CIO It is important to note that in no instance was it found possible to produce a nonsustaining grain using NI-I CIO in amounts greater than 40 percent by weight and, in most cases, sustaining was noted in the range between 22 percent to 30 percent NH,CIO

As was noted above, it is generally desirable to incorporate various high energy additives into the grain to enhance the performance of the system. The amount of these additives is generally determined by the solids loading capability of the binder used, which, with techniques presently available, is in the range of 75 percent to 90 percent by weight depending on the binder and the castability modifiers used. Thus, it is readily apparent that the performance of the basic system of this invention can be enhanced materially since the solids loading capability of the binder is not realized with the 33 percent binder 67 percent oxidizer grain. In this connection, it is also apparent that an amount of oxidizer substantially greater than that required to be stoichiometric with the carbon content of the grain is undesirable. The excess oxidizer takes the place of fuel material which could otherwise be utilized and decreases the performance that could otherwise be obtained. The following examples are illustrative of various high performance systems according to this invention. Grains having the formulations of Table l were fired with ClF and were found to be nonsustaining upon shut-off of oxidizer flow. The percentages are by weight. The Nl-l NO used was Monsanto Dense E-2 fertilizer grade, average practice size about 2000p.

TABLE I No. Binder Nl-LNO; TFTA Boron Al Additional experiments with grains containing boron or boron compounds have indicated that heated boron appears to undergo an exothermic surface reaction with Nl-l NO which causes the grain to sustain if it is fired for a sufficient period of time to heat the boron to the temperature required to initiate the reaction. Accordingly, it is preferable to limit the total boron content of the grain to a maximum of 5 percent in those cases in which prolonged firing periods are contemplated.

This invention has been described with respect to several embodiments thereof. These embodiments are illustrative and not limiting of this invention. Various modifications will be apparent to workers skilled and can be made without departing from the scope of this invention which is limited only by the following claims wherein:

I claim:

1. In a method of hybrid propulsion wherein a fuelrich grain is burned with a fluid oxidizer selected from the group consisting of fluorine and interhalogen compounds to produce hot combustion products which are exhausted through a thrust producing nozzle and the flow rate of the fluid oxidizer is terminated at least once prior to the total consumption of said fuel-rich grain whereby intermittent propulsion may be obtained; the improvement wherein said fuel-rich grain consists of a cured carbonaceous polymeric binder having solid additives dispersed therethrough, said solid additives comprising a material selected from the group consisting of ammonium nitrate, alkali metal nitrates, and alkaline earth metal nitrates, said material being present in an amount such that the oxygen content of the material is substantially stoichiometric with the carbon cont nt of the r in whereb combustion roducts substantially free o? elemental carbon are btalned and said grain will extinguish upon termination of said fluid oxidizer flow.

2. The method of claim 1 wherein said material is ammonium nitrate.

3. The method of claim 1 wherein said additives further comprise solid high energy fuel materials selected from the group consisting of metals, metal hydrides, boron, boranes, organic borane derivatives, organic hydrazine derivatives and heterocyclic nitrogen compounds.

4. The method of claim 3 wherein the boron content of said grain is no greater than 5 percent by weight.

5. The method of claim 1 wherein said grain comprises at least about 10 percent binder.

6. The method of claim 1 wherein said grain comprises from 20 33 percent by weight of hydroxy-terminated polybutadiene cured with toluene diisocyanate.

Claims

2. The method of claim 1 wherein said material is ammonium nitrate.
3. The method of claim 1 wherein said additives further comprise solid high energy fuel materials selected from the group consisting of metals, metal hydrides, boron, boranes, organic borane derivatives, organic hydrazine derivatives and heterocyclic nitrogen compounds.
4. The method of claim 3 wherein the boron content of said grain is no greater than 5 percent by weight.
5. The method of claim 1 wherein said grain comprises at least about 10 percent binder.
6. The method of claim 1 wherein said grain comprises from 20 -33 percent by weight of hydroxy-terminated polybutadiene cured with toluene diisocyanate.