[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 (R
b = 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.
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.