(19)
(11) EP 0 520 104 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication:
30.12.1992 Bulletin 1992/53

(21) Application number: 91305823.6

(22) Date of filing: 27.06.1991
(51) International Patent Classification (IPC)5C06B 45/10
(84) Designated Contracting States:
DE FR GB IT SE

(71) Applicant: THIOKOL CORPORATION
Ogden, Utah 84401-2398 (US)

(72) Inventors:
  • Lund, Gary K.
    Utah 84401 (US)
  • Cragun, Richard B.
    Pleasant View, Utah 84414 (US)
  • Doll, Daniel W.
    North Ogden, Utah 84414 (US)

(74) Representative: Bankes, Stephen Charles Digby et al
BARON & WARREN 18 South End Kensington
London W8 5BU
London W8 5BU (GB)


(56) References cited: : 
   
       


    (54) Non-self-deflagrating fuel compositions for high regression rate hybrid rocket motor application


    (57) Hybrid rocket motor fuel grain compositions with high regression rates (0.05 to 0.15 ips) are obtained in which conventional oxidizers are used at relatively low oxidizer mass fluxes. The compositions include an energetic polymer, preferably a polymer of glycidyl azide in a matrix of inert polymer such as polybutadiene. A preferred composition comprises glycidyl azide polymer (24%), hydroxy terminated polybutadiene (56%) and metallic Al (20%). The composition is cured with an isocyanate.


    Description


    [0001] This invention relates to non-self-deflagrating fuel compositions for high regression rate hybrid rocket motor application. More particularly, it relates to the use of energetic self-deflagrating polymers blended with non-energetic,self-deflagrating polymeric materials to produce hybrid rocket motor solid fuel grains with enhanced regression rates and other improved properties which are non-self-deflagrating.

    [0002] The invention allows high regression rates to be achieved in a solid fuel hybrid rocket motor operated with an injected auxiliary oxidizer. This results in mass flows high enough to make a large hybrid rocket motor feasible for booster and large launch vehicle applications. The use of non-self-deflagrating fuels results in retention of throttleability and greatly reduces safety risks in operation and handling.

    [0003] Hybrid rocket motor development has been evolving for a number of years, primarily with applications targeted at small tactical motor devices. One of the most difficult technologies encountered during development of hybrid rocket motors has been the achievement of sufficiently high solid fuel regression rates during motor operation to allow simple grain geometries and high fuel mass fractions to be employed in motor design without rendering the fuel self-deflagrating. In achieving this end, a multitude of fuel additives and formulations have been investigated in hybrid motor development programs using liquid or gaseous oxidizer injection. These efforts and the results generated are summarized in the open literature with the most thorough discussions being: (1) United Technology Center, "Investigation of Fundamental Phenomena in Hybrid Combustion" Final Technical Report UTC 2097-FR, UTC, Sunnyvale, Ca, 1965: (2) Lockheed Propulsion Company, "Low Hazard Hybrid Fuel Development Program" Final Report NWC TP 6617, Naval Weapons Center, China Lake, Ca., 1974; and (3) U.S. Army Rocket and Guided Missile Agency, "Feasibility of Hybrid Propulsion Systems", ARGMATR ZE3R, U.S. Army Ordinance Missile Command, Redstone Arsenal, Alabama, 1961.

    [0004] Based on these reports and the literature in general, increased regression rates may generally be achieved by: (1) Including a solid oxidizer (e.g. ammonium perchlorate or ammonium nitrate) in the fuel formulation along with various metals (Al, Zr, ZrH₂, etc.), catalysts (ferrocene, catocene, etc.); and exothermic, low decomposition temperature additives such as dicyandiamide, tetraformyl trisazine, etc.; or (2) Including reactive metals (Li, Mg), or oxidizers such as FLOX (O₂/F₂) which greatly increase the combustion temperatures and reactivity of the oxidizer.

    [0005] The above propellant combinations are capable of producing regression rates of 0.1 to 0.2 inches per second under motor operating conditions of 200 to 1000 psi with total oxidizer mass flux levels of 0.1 to 0.6 lb per second per square inch. These regression properties are approximately 10 times greater than obtained in the absence of the additives and represent ballistic properties adequate for practical motor design and application.

    [0006] Unfortunately, the above approaches suffer from deficiencies in that by resorting to inclusion of either very reactive fluorinated oxidizers and fuels such as Li or LiH, or the use of 20+ percent solid oxidizer in the fuel grain, a number of safety and handling considerations are compromised. In general, the use of solid oxidizer at levels sufficient to achieve the desired regression rate enhancements in the fuel grains results in compositions capable of sustaining low level combustion in the absence of supplemental oxidizer, making these behave as conventional solid propellants (i.e. self-deflagrating). Use of lithium metal and hydride leads to difficulties in fuel grain processing and storage since these materials are reactive with moisture and air. The use of fluorine or perchlorates in the propellant system leads to acidic and toxic hydrogen halides in the exhaust, which can result in environmental damage, particularly with large booster motor applications, and they are toxic themselves. Thus, the most promising methods of improving hybrid motor ballistic properties available in the literature suffer from undesirable side effects, such as component toxicity and hazards, and environmental effects from exhaust products.

    [0007] It is desirable to formulate hybrid rocket motor fuel grain compositions capable of providing high (0.05 to 0.15 ips) regression rates with conventional injectable oxidizers such as oxygen, which do not result in self-sustaining combustion or undue handling and environmental hazards.

    [0008] The use of GAP/HTPB blends, as described below, allows these goals to be achieved by providing regression rates of 0.1 ips to be achieved at relatively low oxidizer mass fluxes (0.3 to 0.4 lb/sec/in.) with oxygen. The exhaust products do not contain any obviously toxic products in large amounts, being typical of conventional liquid propellant systems.

    [0009] One of the more promising energetic polymers for application to rocket motors or propellants is a polymer of glycidyl azide (GAP), the use of which in compositions for extinguishing fires is described in United States Patent 4,601,344. In that patent the energetic azide polymer is utilised in compositions containing a high nitrogen content solid additive for the purpose of generating large amounts of nitrogen gas.

    [0010] In the present invention one or more energetic azide polymers such as polymeric glycidyl azide (GAP) or polymers of other azide compounds is homogeneously blended with and retained by an inert (non-self deflagrating) polymer matrix based on a suitable polymer such as polyethylene, polyacrylics, polytetrahydrofuran or polybutadienes such as hydroxy terminated or carboxy terminated polybutadiene (HTPB or CTPB).

    [0011] Hydroxy terminated polybutadiene (HTPB) based binders are preferred in the hybrid rocket motor fuel compositions of this invention. One such suitable binder material is the liquid resin R45M supplied by Arco Chemical Company. Other binder materials which are suitable include carboxy or epoxy terminated polybutadienes, copolymers such as polybutadiene/ acrylic acid, or polybutadiene/acrylic acid/acrylonitrile, or other liquid polymers such as polybutene, polyisobutylene, liquid polysulfide polymers, polyethylene, rubbers both natural and synthetic, such as butylrubber, ethylacrylate/methylvinylpyridine copolymers, and polyvinyl resins.

    [0012] Where required, conventional curing agents are selected and employed to effect cure of the binder. For example, polyisocyanates are employed to cure hydroxy or epoxy terminated resins, and diaziridines, triaziridines, diepoxides, triepoxides and combinations thereof readily effect cures of carboxy terminated resins. Normally an amount of curing agent up to about 5% by weight of all the combined propellant ingredients is sufficient for curing. The selection of the exact amount of curing agent for a particular propellant combination will be within the skill of one experienced in the art and will depend, of course, upon the particular resin, the curing time, the curing temperature, and the final physical properties desired for the propellant.

    [0013] The finished binder may include various compounding ingredients. Thus it will be understood herein and in the claims that unless otherwise specified, or required by the general context, that the term "binder" is employed generically and encompasses binders containing various compounding ingredients. Among the ingredients which may be added, for example, is a plasticizer such as dioctyl adipate, so as to improve the castability of the uncured propellant and its rheological properties after cure. The binder content of the fuel grain composition will usually range from about 8 1/2 to 99% by weight.

    [0014] The energetic polymer of glycidyl azide (GAP) has been found to be self-deflagrating under pressure (Rb = 0.765 ips at 1000 psi chamber pressure). If it is blended with from 30 to 99.99% by weight of HTPB, a homogeneous castable fuel mixture is produced which may be cured (gelled ) by reaction with a multifunctional isocyanate such as Desmodur N-100.

    [0015] To produce a composition suitable for rocket motor applications, additional ingredients and fillers such as free metallic aluminium, zinc, magnesium, etc, and nitrogen containing compounds, (tetrazoles, triazoles, nitriles, etc.) and the like may be included. One such composition comprised a mixture consisting of:
       24% GAP
       56% HTPB (HT) and,
       20% Aluminum Powder, cured with an isocyanate.

    [0016] The invention will be more fully understood from the examples which follow and from the accompanying drawings, in which:

    Figure 1 depicts firing data for a GAP/HTPB blend fuel with gaseous oxygen;

    Figure 2 shows similar data for a GAP only fuel grain test; and

    Figure 3 is a graph of hybrid combustor fuel regression rates as a function of oxygen mass flux, showing the effect of GAP on fuel regression rates.



    [0017] Hybrid fuel formulations were evaluated with gaseous oxygen in a small combustor. Fuel grain cartridges (1.5 in. diameter by 2.5 in. long with a 0.85 in. central bore) were fabricated and the combustor was charged with from one to five grains at a time. Gaseous oxygen was injected into the bore of the combustor with the oxygen mass flow controlled by means of a calibrated sonic orifice. The fuel/oxygen mixture was ignited by means of a small pyrotechnic ignitor and the combustor operated for from one to ten seconds. Combustor operation was terminated by stopping oxygen flow, immediately followed by purging with nitrogen to sweep residual oxygen from the fuel bore. Fuel regression rate was calculated by weight loss of the fuel grain(s) during combustor operation.

    [0018] Figures 1 and 2 are graphs showing combustor pressures vs time for GAP/HTPB hybrid fuel compositions at medium pressure, high O₂ flux (Fig. 1) and GAP alone at low pressure, low O₂ flux (Fig. 2).

    [0019] As shown in Figure 1, the GAP/HTPB blends do not self-deflagrate and motor operation ceases upon termination of oxidizer flow. Use of neat GAP as the fuel grain leads to uncontrollable deflagration with no response to oxidizer flow. As shown by the data in the tables which follow, dramatic increases in motor regression rate (R) are obtained by inclusion of GAP in the fuel formulation as compared to HTPB or other inert materials alone. These increases are much greater than obtained by simple metallization or through use of solid additives alone.

    [0020] Both HTPB and Poly THF respond to metallization with A1 as shown by the results in tables I and II. Similar results are obtained with Mg. Other metals which may be used are Zn and W.
    Table I
    Effect of A1 on inert Binder Regression Rate
    % Metal HTPB Binder POLY THF Binder
    Al R(IPS) Pc(PSIA) R(IPS) Pc(PSIA)
    0 0.035 375 0.023 320
    7 0.041 400 - -
    10 - - 0.026 335
    20 0.040 375 0.033 370
    30 0.043 400 0.042 410
    40 0.045 425 0.048 425
    50 - - 0.048 435
    Footnote to Table I
    R = Regression Rate
    Pc = Chamber Pressure


    [0021] Metallization of HTPB, poly THF, and GAP/HTPB with A1 coupled with aft chamber mixing as provided by the 5 grain body configuration, increases regression rates by more than 50% over those observed without GAP in the formulation as shown by the data in Table II.

    [0022] A comparison of the baseline, HTPB(HT), and metallized HTPB/GAP binder (Al or Zn) regression rate behavior-vs-oxygen mass flux is graphically illustrated in Figure 3. As can be seen, similar oxidizer dependencies are observed for all fuels with a direct dependance on GAP concentration being evident.
    Table II
    Effect of GAP and Al on HTPB Binder Fuel Regression Rates
    FUEL R(IPS) Pc(PSIA) COMMENTS
    HTPB 0.035 375 3 Grain/3 Grain Body
    30/70 GAP/HTPB 0.056 410 3 Grain/3 Grain Body
    50/50 GAP/HTPB 0.083 475 3 Grain/3 Grain Body
    70/30 GAP/HTPB 0.200 410 1 Grain+2HDPE/3 Grain Body
    30/70 GAP/HTPB+10% Al 0.059 445 3 Grain/3 Grain Body
    30/70 GAP HTPB+40% Al 0.058 470 3 Grain/3 Grain Body
    30/70 GAP/HTPB+40% Al 0.070 500 3 Grain/5 Grain Body
    Footnote to Table II:
    R = Regression Rate
    Pc = Chamber Pressure


    [0023] Having now described preferred embodiments of the invention it is not intended that it be limited except as may be required by the appended claims.


    Claims

    1. A hybrid rocket motor fuel composition comprising a liquid azide polymer blended with or co-cured with an inert polymeric binder for the same, the proportions of azide polymer in the composition being between 1% and 70% by weight and the proportions of inert polymeric binder in the composition being between 8 1/2 and 99% by weight, the relative proportions being such that the composition is non- self-deflagrating.
     
    2. The composition claimed in claim 1, wherein the azide polymer is a polymer of glycidyl azide.
     
    3. The composition claimed in claim 1 or 2, wherein the binder is a polymer or copolymer selected from the group consisting of polybutadiene, substituted polybutadienes, polybutadiene copolymers, polybutene, polyisobutylene, polysulfide polymers, polyethylene, natural and synthetic rubbers, and polytetrahydrofuran.
     
    4. The composition claimed in claim 3, wherein the binder is a substituted polybutadiene.
     
    5. The composition claimed in claim 3, wherein the binder is a hydroxy terminated or a carboxy terminated polybutadiene.
     
    6. The composition claimed in claim 3, wherein the binder is polytetrahydrofuran.
     
    7. The composition claimed in any preceding claim, including, in addition, a free metal.
     
    8. The composition claimed in claim 7, wherein the metal is selected from the group consisting of Al, Mg, Zn and W.
     
    9. The composition claimed in any preceding claim, including, in addition, at least one nitrogen compound selected from the group consisting of tetrazoles, triazoles, aliphatic nitriles, nitrocellulose, ammonium nitrate and mixtures thereof.
     
    10. The composition claimed in claim 9, in which the compound is 5-aminotetrazole.
     
    11. The composition claimed in claim 9, in which the compound is 3-amino-1,2,4-triazole.
     
    12. The composition claimed in any preceding claim, including, in addition, carbon black.
     
    13. The composition claimed in any preceding claim, wherein the binder comprises between 8 1/12 and 95% by weight of the composition.
     
    14. The composition claimed in any preceding claim, including, in addition, a curing agent for said polymeric binder.
     
    15. A hybrid rocket motor fuel composition comprising by weight:
       24% glycidyl azide polymer
       56% hydroxy terminated polybutadiene and
       20% aluminum powder.
     




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