US3208891A - Monopropellant comprising an alkyl nitrate and a nitroparaffin additive - Google Patents

Description

United States Patent 3,208,891 MONOPROPELLANT COMPRISING AN ALKYL NITRATE AND A NITROPARAFFIN ADDITIVE Paul C. Eschrich, Wallingford, and Glenn R. Dixon, Hamden, Conn., assignors to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Mar. 2, 1964, Ser. No. 348,881 6 Claims. (Cl. 149-91) This application is a continuation-in-part of application Serial No. 255,694, filed Feb. 1, 1963, which was forfeited.

This invention relates to a propellant composition and more particularly to an alkyl nitrate liquid propellant for energizing devices of the explosive actuated type.

It is sometimes desirable to make a liquid propellant more sensitive to ignition in order to decrease the energy requirements to ignite it. This is especially true for the alkyl nitrate propellants in adverse environmental conditions. The addition of small amounts of a nitroparaffin such as tetranitromethane alternatively referred to as TNM or hexanitroethane alternatively referred to as HNE to alkyl nitrate propellants such as 60 mole percent ethyl nitrate-40 mole percent propyl nitrate, or N-propyl nitrate substantially reduces the energy required to ignite the propellant by adiabatic compression and by electric spark discharge.

One object of the invention is to provide a suitable alkyl nitrate mix capable of being readily ignited by direct electric spark or by adiabatic compression ignition, in contradistinction to ignition by priming and detonator explosives set off percussivelyor by an electric bridge wire.

Another object is to provide such an alkyl nitrate mix having not only the requisite sensitivity and reliability, but also the safe progressive burning character needed for discharging a shotgun, or for actuating a tool of the explosive actuated type such as a fastener driving tool.

Other objects and advantages will be apparent from this description of details of the invention in alkyl nitrate propellant modified by addition of a nitroparaflin additive to render the nitrate, which is quite resistant to ignition by electric spark or adiabatic compression, ignitable by such a spark or compression in any suitable ignition chamber, particularly when the propellant is loaded in a preferred density range with respect to the volume of the chamber.

Any nitroparaflin may be used in accordance with the present invention. Preferably, the nitro-lower-parafiins are employed, i.e. those saturated aliphatic hydrocarbons 0f the methane series C H containing less than 7 carbon atoms and optimally TNM or I-INE. The amount of nitroparaffin which may be used is at least 0.5% by weight. Preferably less than 15% by weight is employed and still more preferably a range of 1% to 10% by weight of the nitroparafiin is used.

One of the various lower alkyl nitrates contemplated in accordance with this invention, such as ethyl nitrate, isopropyl nitrate, normal propyl nitrate, amyl nitrate, hexyl nitrate, and the like, a mixture of ethyl nitrate and n-propyl nitrate in about a 60:40 mole ratio, has been found very promising for the purpose, provided there is incorporated a small but suflicient amount of tetranitromethane, a liquid nitroparaflin component providing the advantage ofimproved ignitability while improving the oxygen balance of the whole mixture because it is a compound with up to 48% available oxygen.

It is preferred in accordance with the present invention to utilize a mixture of lower alkyl nitrates. Naturally all proportions are contemplated. The preferred embodiment of the present invention as stated above is the mixed ethyl-n-propyl nitrates. The preferred ratio of ethyl to n-propyl is from 3:1 to 1:1.

Specifically, a 60:40 mixture of the mixed ethyl-npropyl nitrates having a boiling range of from about 190 F. to 231 F., and a freezing point below 148" F., at standard conditions of temperature and pressure, a closed cup flash point of about 50 F., and a density of about 1.085 grams/cc. is contemplated for modification according to this invention. A mixture containing somewhat more ethyl nitrate, e.g. as high as but preferably less than mole percent, and requiring a bit less tetranitromethane or other nitroparaflin additive, is contemplated as well as one containing somewhat less ethyl nitrate such as one having at least 50 mole percent in n-propyl nitrate and requiring somewhat more of the additive. The percent by weight figures for tetranitromethane are given herein for the 60:40 mixed ethyl propyl nitrate, hereinafter referred to as 60:40 EPN.

With the additive, the 60:40 EPN has about the same density as that noted above. This is true within the limits of mixed liquid nitrates, contemplated such as the 70:30 EPN and 50:50 EPN.

Tests carried out with a fuel comprising a single alkyl nitrate such as n-propyl nitrate or comprising a mixture of alkyl nitrates such as 60:40 ethyl-n-propyl nitrate clearly indicate that the addition of a small amount of a nitroparafiin such as TNM or HNE appreciably improves the ignition characteristics of the alkyl nitrate or mixed alkyl nitrates either by electric spark ignition or by adiabatic compression ignition. Generally, the greater the percentage by weight of nitroparafiin added to the alkyl nitrate fuel, the greater will be the effect on improved ignition.

Based on the test data obtained, it would be expected that the addition of a nitroparafiin would improve the ignition sensitivity of most lower alkyl nitrates or mixtures thereof in all proportions, and at all temperature ranges, and in all environmental conditions.

It is contemplated that the monopropellant mixture of this invention is to be used in an ignition system in the presence of a gaseous phase, i.e. at loading densities less than 1.085 grams/co, and that the system may in one embodiment be in the form of a blank cartridge, shot shell cartridge, or the like, and in another embodiment the monopropellant may be put into a chamber for ignition independently of any cartridge.

Other additives compatible with the fuel mix such as nitroglycerin and ethylene oxide are also contemplated which may be used as well as the nitroparafiin.

This invention will be more clearly understood in connection with the following examples which are given as illustrations of the invention and not as limitations.

EXAMPLE I Advantageous ignition characteristics were found when various amounts of tetranitromethane; herein after referred to as TNM, were added to 6:40 EPN at various loading densities in a closed spark ignition chamber utilizing a spark gap of about 0.095 of an inch, a typical dimension in the range of about 0.1 of an inch understood to be variable downwardly to a slight extent to further reduce the voltage and energy requirements to a limited extent for safety and ease of ignition. Data obtained under the conditions described above is shown in Table I below.

Table I Mixture Wt. Percent Loading Energy wt. percent (30/40 EPN desnity (joules) TN M (g./cc.)

The data obtained from Table I can be summarized as follows:

At loading densities in the range from about 0.3 gram/ cc. to about 0.4 gram/cc., mixtures having a TNM content ranging from about 2 per cent to about 6 per cent by weight in 60:40 EPN have distinct advantage. For example, by using a mixture of 6 percent TNM and 94 percent of the 60:40 EPN, with the mixture at around a 0.4 gram/ cc. loading density in the chamber, there was experienced the greatest reduction in voltage required and energy required.

From about 4 to about 6 percent, TNM is remarkably effective as an ignition aid in 60:40 mixtures loaded to densities over a broader range of from about 0.3 or a bit less to about 0.5 gram or a bit more per cubic centimeter of chamber. Desirable characteristics are also obtained in this range from 0.3 gram/cc. to 0.5 gram/cc. loading density, with the 60:40 EPN containing from 3 to 4 percent TNM. Optimum performance occurred at a loading density of 0.40 gram/ cc. with 60:40 EPN having 4 percent TNM with respect to economical use of TNM, and ease of ignition without too much sensitivity for safe handling.

At a loading density of about 0.3 gram per cubic centimeter, a 4 percent addition was found to reduce the energy required to less than 5 joules; at the same density a 6 percent addition lowered the energy required to about 3 joules. At a loading density of about 0.5 gram/cc. the energy required was a little over 4 joules and a little less than 3 joules with additions of 4 percent and 6 percent of TNM, respectively. With additions of from about 3 percent to about 4 percent TNM, the electrical energy required was between 3 and 4 joules at a monopropellant loading density of about 0.4 gram per cubic centimeter.

Additions of a few percent TNM such as about 2 percent TNM in the monopropellant, when loaded to a density of from about 0.25 to about 0.4 permitted spark ignition at an energy represented by less than -12 joules; with a 60:40 mix having 2 percent additive, ignition was achieved at an energy as low as about 5 joules. In any event, with monopropellant having from about 4 percent to about 6 percent TNM and loaded to suitable densities, ignition is readily achieved electrically at energy levels as low as between 2 and 4 joules.

Additions of from about 1 percent to about 8 percent TNM is Plated, For a 50:50 EPN system as high as 8 percent is contemplated. Less TNM is required for a 70:30 EPN system, for comparable results. Additions of from about 3 percent to about 5 percent are preferred in the above-noted operative ranges of ignition chamber loading density. Addition of too much tetranitromethane, produces a detonation rather than progressive burning upon ignition.

Addition of a nitroparaffin such as TNM, in the disclosed limits, makes possible a reduction of electrical energy required by a factor of at least ten; it provides the basis for direct ignition in a simple and economic way, free from undesired side effects of intervening secondary explosives of percussion and thermal sensitive primers.

EXAMPLE II Experiments were performed with a standard compression ignition test apparatus to show the improved sensitivity of alkyl nitrate propellants with nitroparaffin additives to compression ignition.

The apparatus consist of a piston, closed chamber, O-ring, steel diaphragm and falling weight. The O-ring was placed in the closed chamber with the test sample mixture, and the steel diaphragm was placed on the O- ring. On top of this assembly the piston was placed, being held at a fixed distance, about 0.050 inch, above the fuel mixture.

The falling weight impacted the piston, driving it down into the chamber against the diaphragm and against the O-ring; thereby, effecting adibiatic compression of the air above the fuel mixture which ignited the fuel. Upon ignition, the combustion pressure of the fuel burst the diaphragm, and the gases were vented through a small hole in the piston.

In Table II, sensitivity E (kg-cm.) indicates a 50% probability of ignition of the mixture occurring when a given weight is dropped a given distance. The lower the E (kg-cm.) number the more sensitive to ignition is the mixture. The data shown in this table is grouped in three sections for the purpose of comparison.

The data obtained in these experiments are summarized in Table II below.

Table II COMPRESSION IGNITION TEST DATA Mixture Tomp., Drop Sensitivity F. \vt., kg. E50 (kg.cn1.)

1. (a) 60/40 EPN 70 l 2. 5 ([7) 60/40 EPN, 96%; TNM, 4% 70 l 2.1 ((1)60/40 EPN, 94%; TNM, 6%- 70 1 1.7 2. (a) n Propyl nitrate V 1 15.9

(b) n-Propyl nitrate, 95%; TNM,

5% 70 V 6. 7 (c) n'Propyl nitrate, 90%; TNM,

10 o 70 2 4. 4 3. (a) 60/40 EPN 0 5. 2 (1)) 60/40 EPN, 90%; TNM, l0% 0 3. 8 (0) 60/40 EPN, 95%: hexanitroethane, 5% 0 4. 7

1 (1 kg. 11.5). 2 (1 kg. 3.2).

b, and c, it is shown that the addition of small amounts of tetranitromethane (TNM) to a mixture of 60/40 EPN lowers the E sensitivity point. In 2a, b, and c, it is shown that the addition of TNM to a single alkyl nitrate, n-propyl nitrate, reduces its E sensitivity number substantially. In 311, b, c, it is shown that at a sub-normal temperature (0 F.) the E sensitivity of 60/40 EPN is greater than at 70 F. and this E sensitivity is decreased by the addition of the nitroparaffins TNM and HNE.

In 1a,

EXAMPLE III Further experiments were carried out to show the improved sensitivity of alkyl nitrates with nitroparaffin additives in a new compression ignition apparatus designed to test ignition sensitivity of fuel mixtures in a combustion gas atmosphere. The sensitivity to ignition is indicated as percent ignition failures at different levels of energy produced by a falling weight.

The apparatus consist essentially of a closed chamber, a moving piston which always remains in the chamber, and a falling weight. The fuel mixture was injected in the chamber and the piston was set at a fixed distance above the fuel (.5 inch). The falling weight drove the piston downward, compressing the atmosphere above the liquid and, thus, igniting the fuel. Upon ignition, the piston was driven upward exposing exhaust ports leading to closed chamber, thereby, trapping the exhaust gases. The next test was then conducted in an atmosphere of the combustion gases.

Table III shows the data obtained from this test in evaluating the performance of a TNM 60/40 EPN fuel mixture, a HNE 60/40 EPN fuel mixture and a 60/40 EPN fuel mixture without an additive. The test data is recorded as percent of ignition failures which occurred when a given weight was dropped from a given height to ignite the propellant mixture.

Table III COMPRESSION IGNITION TEST APPARATUS NO. 2

90% 60/40 EPN; Percent ignition Inch- TNM, failures: 90% 60/40 EPN, pounds percent 60/40 EPN; 10% percent hexanitroethane The difficulty of maintaining exactly the same atmosphere from test to test may account for the unexpected variation in the experiments using HNE where a small increase in percent failures was actually recorded when going from 8 to 9 inch pounds. However, maintaining similar procedures in each test, yielded data which clearly shows that less energy is required to ignite fuel mixtures efiiciently with mixtures containing the nitroparaffin additives of TNM and HNE.

What is claimed is:

1. A monopropellant liquid mixture consisting essentially of a lower alkyl nitrate and a nitroparafiin additive containing 1 to 7 carbon atoms in an amount of 0.5% to 15% by weight, said liquid being readily ignitable by electric spark or by adiabatic compression.

2. A mixture according to claim 1 in which said additive is tetranitromethane or hexanitroethane.

3. A monopropellant liquid mixture consisting essentially of ethyl nitrate and normal propyl nitrate to which is added a nitroparaflin containing 1 to 7 carbon atoms in an amount of 0.5% to 15% by weight, sufficient to render said mixture more sensitive to ignition by electric spark or adiabatic compression.

4. The mixture of claim 3 containing to 10% by weight of tetranitromethane or hexanitroethane.

5. The mixture of claim 3 containing more than mole percent but less than 70 mole percent ethyl nitrate and containing less than 50 mole percent but more than 30 mole percent normal propyl nitrate.

6. The mixture of claim 3 containing not less than 1% but not more than 10% tetranitromethane in a :40 mixture of ethyl nitrate and normal propyl nitrate.

References Cited by the Examiner UNITED STATES PATENTS 2,338,120 1/44 Lawrence 14990 X 2,551,554 5/51 Wood 14990 2,993,768 7/61 Holzl 149-88 3,116,187 12/63 Scanlon et al. 144-90 X CARL D. QUARFORTH, Primary Examiner. BENJAMIN R. PADGETT, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noe 3,208,891 September 28, 1965 Paul c. Eschrich et al It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 18, after "containing" insert M 1% Signed and sealed this 5th day of July 1966c (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims (1)

1. A MONOPROPELLANT LIQUID MIXTURE CONSISTING ESSENTIALLY OF A LOWER ALKYL NITRATE AND A NITROPARAFFIN ADDITIVE CONTAINING 1 TO 7 CARBON ATOMS IN AN AMOUNT OF 0.5% TO 15% BY WEIGHT, SAID LIQUID BEING READILY IGNITABLE BY ELECTRIC SPARK OR BY ADIABATIC COMPRESSION.