BACKGROUND OF THE INVENTION
This invention relates generally to rocket propellants
and, more particularly, to gelled propellants. It is well known in the field of
rocket propulsion that gelled propellants offer significant advantages over solid
and liquid propellants. Solid propellants have inherently high energy but offer
no mission flexibility because once ignited they must normally be burned to completion.
Liquid propellants are less energetic than solids, as measured by specific impulse,
but offer high mission flexibility because the flow of liquid fuels can be controlled
as desired. Gels combine the advantages of solids and liquids and have additional
advantages that are well known to designers of rocket engines for use both in space
and within a planet's atmosphere.
Although the advantages of gelled propellants are widely
appreciated, gelled propellants available prior to this invention have been produced
by mixing essentially inert solids with liquids. A commonly used gellant is silicon
US Patent No. 6,165,293
entitled "Thixotropic IRFNA Gel," discloses a gelled monomethyl hydrazine
(MMH) fuel in which cellulose is the principal gallant and aluminum is added to
increase energy density, and a gelled oxidizer in which the gellant is silicon dioxide
and lithium niobate.
US Patent No. 6,063,219
entitled "Higher Density Inhibited Red Fuming Nitric Acid (IRFNA) Oxidizer
Gel," discloses an oxidizer gellant is either silicon dioxide, an unspecified metallic
oxide, or an unspecified swellable polymer. Polymers that are typically considered
as gellants for propellants are cellulose or cellulose derivatives.
Because the materials previously considered as gellants
add little or nothing to the energy content of a gelled fuel, there is still a significant
need for a gelled propellant that uses a gellant with a combustion enthalpy that
adds energy content to the propellant, as well as serving to form a gel that supports
the uniform suspension of other materials added to load the propellant with more
dense particles. The present invention satisfies this need.
SUMMARY OF THE INVENTION
The present invention resides in the use of a polymeric
gellant that satisfies the physical requirements for a gelled propellant, but also
adds energy content to the propellant. Briefly, and in general terms, the invention
may be defined as a gelled propellant, comprising a polymeric nanogellant formed
from a monomer having molecular properties that promote three-dimensional polymerization;
and a propellant to which the polymeric nanogellant is added. The resulting gelled
propellant has desirable rheological properties and the polymeric nanogellant adds
energy content to the propellant.
Monomers suitable for use in the invention may be generally
characterized by the chemical formula:
-CH3, -C2H5, or -C3H7,
2 ≤ n ≤
y ≤ 2n.
In one embodiment of the invention, the polymeric nanogellant
is added directly to the propellant as a monomer and is polymerized in situ
to form a gel. More specifically, in one disclosed example of the invention the
propellant is monomethyl hydrazine (MMH) and the monomeric form of the nanogellant
is bis-trimethoxysilylethane (BTMSE), which is mixed with the propellant and water.
The propellant catalyzes polymerization of the nanogellant, resulting in the gelled
form of the propellant. By way of example, the relative proportions of propellant,
monomeric gellant and water are approximately 94%, 5% and 1% by weight, respectively.
In another embodiment of the invention, the polymeric nanogellant
is polymerized before being added to the propellant. The polymenc nanogellant is
first polymerized in a solvent different from the propellant, then recovered from
the solvent and dried before being added to the propellant as a gellant. In this
embodiment of the invention, the monomeric form of the nanogellant may also be bis-trimethoxysilylethane
(BTMSE). The propellant may be monomethyl hydrazine (MMH), or some other liquid
fuel, such as a cryogenic liquid fuel.
In terms of a novel method, the invention may be defined
as a method for producing a gelled propellant, comprising the steps of placing a
propellant in a reaction vessel; mixing a selected monomer with the propellant in
the reaction vessel; and polymerizing the monomer in the reaction vessel, and thereby
forming a gelled propellant containing a nanogellant that provides the propellant
with desired rheological properties and adds energy content to the propellant.
More specifically, the selected monomer is characterized
by a molecular structure that promotes formation of a three-dimensional polymer.
For this embodiment of the invention, the selected monomer is soluble in the propellant
and the propellant catalyzes the polymerizing step. In a disclosed example of the
method, the selected monomer is bis-trimethoxysilylethane (BTMSE) and the propellant
is monomethyl hydrazine (MMH). More specifically, the mixing step mixes the monomer
in the amount of approximately 5% by weight of the total mixture, and further adds
water in the amount of approximately 1% by weight.
In accordance with a second embodiment of the method, the
invention comprises the steps of placing a selected monomer in a reaction vessel
with a selected solvent; polymerizing the selected monomer in the reaction vessel,
to produce a nanogellant polymer in solution with the selected solvent; recovering
the nanogellant polymer from the solvent by a process that utilizes solvent processing
or drying methods that effectively reduce or eliminate liquid surface tension during
solvent removal and recovery of dry nanogellant materials. These methods include:
use of surfactants, freeze drying or solvent sublimation, and super-critical or
near critical point fluid processing; and dispersing the recovered nanogellant polymer
in a selected propellant to form a gelled propellant. In this embodiment of the
method, the selected monomer may be bis-trimethoxysilylethane (BTMSE) and the propellant
may be monomethyl hydrazine (MMH). However, the propellant may also be selected
from other liquids, such as other forms of hydrazine and such as cryogenic liquid
fuels, including liquid propane or liquid ethane.
It will be appreciated from this brief summary that the
invention provides a significant advance in the field of propellants for rocket
engines and the like. In particular, the invention provides a technique for gelling
liquid propellant fuels and oxidizers using a gellant of nanometer proportions,
which provides desirable rheological and other physical properties not obtainable
using conventional gallants. Other aspects and advantages of the invention will
become apparent from the following more detailed description, taken in conjunction
with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is graph showing the rheological properties of monomethyl
hydrazine (MMH) propellant that has been gelled in accordance with the invention,
using bis-trimethoxysilylethane (BTMSE) as the gellant.
FIG. 2 is diagram showing the chemical structure of bis-trimethoxysilylethane
(BTMSE) before and after polymerization.
FIG. 3 is a temperature-pressure phase diagram showing
recovery of nanoparticulate gellant by solvent sublimation / freeze dry and critical
fluid processing techniques.
FIG. 4 is a pair of graphs showing the rheological properties
of cryogenic fuels, namely liquid propane and liquid ethane, which have been gelled
in accordance with the invention using bis-trimethoxysilylethane (BTMSE) as a gellant.
DETAILED DESCRIPTION OF THE INVENTION
As shown in the drawings by way of illustration, the present
invention is concerned with gelled rocket fuels. In the past, gellants for rocket
propellants have been formed by mixing practically inert solid particles in suspension
with a liquid propellant. Although these mixtures or suspensions have provided the
desired physical properties of a gelled propellant, they have not added energy content
to the fuel, which therefore does not perform as efficiently as it might.
In accordance with the present invention, a gelled propellant
is produced using a polymeric gellant that also adds energy content to the fuel.
There are two basic embodiments of the invention. In one embodiment, the gellant
is formed by a process of polymerization that takes place in the liquid fuel itself.
This is referred as the in-situ method. In the other embodiment, the polymeric
gellant is formed separately and later added to the fuel. This is referred to as
the ex-situ method.
In the in-situ method, the polymer gellant material
is produced directly in the liquid propellant by carrying out a polymerization reaction
involving a selected monomer species dissolved in the liquid rocket propellant.
The liquid propellant is thereby converted to a semi-solid or gel, which consists
of a meso-porous nano-fibril structure with entrapped liquid. The gel exhibits a
low yield stress compared with regular solids but it is sufficient for stably dispersing
micrometer-scale and larger-scale metal and other energetic solids in the liquid
propellant. The monomer is converted to the nanofibril structure during the course
of the polymerization reaction and typically no further processing is required,
except perhaps for the addition and mixing of other energetic materials. With the
in-situ approach, the liquid rocket propellant may be a monopropellant such as hydrazine,
either mono-, di-, tri-,or tetra-methyl hydrazine, or one of the fuel-oxidizer components
for a bipropellant system, such as the fuels designated RJ-4, RJ-5, RJ-7, JP-4,
JP-5, JP-9, JP-10. These are fuels specified by the US military for various ram-jet
(RJ) and other jet engine powered missiles.
An efficacious gelling agent for mono-methyl hydrazine
(MMH) is the in-situ polymer derived from the hydrolysis and condensation
polymerization reaction of bis-trimethoxysilylethane (BTMSE) as shown in the following
+ 3nH2O → Polymer (Si2C2H4O3)n
The in-situ grown polymer is able to produce a clear
MMH gel at polymer concentrations as low as 5% by weight, and unlike silica gellants
has a combustion enthalpy that adds energy content to the propellant. The polymerization
reaction is carried out at ambient temperatures and requires a low moisture content
for initiation and completion. Typical rheological properties for the MMH gel are
presented in FIG. 1.
The preparation of gelled monomethyl hydrazine (MMH) using
1,2 Bis(trimethoxysilyl)ethane is based on the equation above, which has been rewritten
in different form as follows:
+ 3H2O → 6CH3OH + Si2C2H4O3
The three-dimensional polymerization of this reaction is depicted in FIG. 2. It
should be noted that the high effective pH of MMH catalyzes the reaction. In addition,
the polymerization requires water and produces methanol (CH3OH). It will
be observed from the diagrammatic representation of the polymer structure that each
of the silicon atoms in the structure provides tetrafunctional branch points from
which four linear chains emanate. One of the four possible chains provides a link
to a C2H4 group and the other three provide links to oxygen
atoms. The tetrafunctional property of the structure allows it to grow efficiently
in three dimensions and provide the desired mechanical gelling properties.Prior
to gelling the MMH, the initial concentration of water in the MMH is determined,
preferably using a gas chromatographic method. An amount of water to add to the
MMH is then calculated to make the final mix 1.0% percent by weight water. The gelling
of the MMH is then carried out in a manner to exclude exposure to atmospheric moisture,
carbon dioxide and oxygen. These gases can be absorbed by the MMH and detract from
its value as an eventual fuel.
Working in an inert atmosphere (void of moisture, carbon
dioxide and oxygen) the weight percentages of the ingredients listed in Table 1
(below) are combined. The mixture is stirred and allowed to react for 24 hours at
ambient temperature. The mixture must be stored in a container that is compatible
with the ingredients and prevents exposure to atmospheric moisture, carbon dioxide
Table 1: Ingredients of Gelled MMW
In the ex-situ method, the polymerization reaction
is carried out in a different solvent from the liquid propellant itself and the
nanometer particulate reaction product is subsequently recovered from this reaction
medium or some exchange solvent in a manner that preserves its high specific surface
area and morphological structure. The recovered dried nanoparticulate material may
then be re-dispersed in the desired rocket propellant to produce a gelled propellant.
Nanoparticulate recovery in the ex-situ method uses either a drying process or a
solvent elimination process in which the liquid surface tension forces are minimized
or near completely eliminated. This is accomplished by taking advantage of the solvent's
phase diagram and operating in a cyclic manner around either the reaction medium's
or exchange solvent's triple or critical points. In FIG. 3, the paths ABCD and AB'C'
are possible temperature-pressure cycles around a triple point (TP) and a critical
point (CP), respectively, in the solvent's phase diagram. These product recovery
methods are commonly referred to as freeze drying and critical point drying. Alternatively,
surfactants or solvents with inherently very low surface tensions under ordinary
ambient conditions may be used, provided these materials are selected to be effective
and compatible with the monomer and polymer products.
In the gellant synthesis procedure, a dilute solution of
BTMSE in ethanol is prepared, a small quantity of water is added and the reaction
mixture warmed to approximately 60° C and allowed to react at this temperature
for approximately 12 hours. A soft alcohol gel is formed. To recover the nanogellant
via freeze drying, the alcogel is exchanged with water and then frozen and sublimated
at -10° C. Recovery of nanogellant via critical fluid processing requires exchanging
alcohol with liquid carbon dioxide (critical temperature 33° C) or some other
fluid with a conveniently low critical temperature in a pressure vessel. Upon exchange
of alcohol, the temperature of the CO2 -exchanged gel is raised to 40°
C and the elevated pressure allowed to vent slowly. Other exchange fluids with conveniently
low critical temperatures may also be used in place of CO2.
FIG. 4 shows the rheological properties of ethane and propane
fuels after gelling with BTMSE gellant in accordance with the ex-situ method
It will be understood that the foregoing examples of the
invention have focused for purposes of illustration on the use of BTMSE as a suitable
monomer. Many other monomers also fall within the scope of the invention, including,
for example, bistrimethoxysilylmethane, bis-trimethoxysilylpropane, and so forth,
and also including various amine compounds (derivatives of ammonia in which one
or more hydrogen atoms are replaced by alkyl groups). Monomers suitable for use
in the invention may be generally characterized by the chemical formula:
-CH3, -C2H5, or -C3H7,
2 ≤ n ≤
y ≤ 2n.
Similarly, although the invention is described by way of example as gelling monomethyl
hydrazine (MMH) propellant, it will be understood that other propellants, such as
hydrazine, di-methyl hydrazine, tri-methyl hydrazine and tetra-methyl hydrazine,
are also candidates for gelling in accordance with the invention. As also mentioned
above, cryogenic propellants such as liquid methane, liquid ethane, liquid butane
and liquid hydrogen may also be used.
It will be appreciated from the foregoing that the present
invention represents a significant advance in the field of gelled rocket propellants.
In particular, the invention provides a gellant that can be produced either
in situ to form a gel in certain categories of fuels, such as monomethyl
hydrazine, or the gellant can be produced ex situ and added to various fuels.
Regardless of which technique is used to produce the gelled fuel, it has desirable
rheological properties that render it more useful than liquid fuels. It will also
be appreciated that, although specific embodiments of the invention have been described
in detail for purposes of illustration, various modifications may be made without
departing from the spirit and scope of the invention. Accordingly, the invention
should not be limited except as by the appended claims.