Preparation and purification of N.sub.2 O.sub.4 containing rocket propellant oxidizer

There is disclosed a process and apparatus whereby typically furnished commercially nitrogen tetroxide propellants for rocket engines may be upgraded to decrease flow decay problems and to increase the shelf life of the product; thereby providing for longer duration trouble-free rocket engine performance. The raw MON material is initially converted if necessary to the "Red" type; dissolved iron salts and other dirt particulates are removed by cooling and filtering, and thenceforth throughout the process the material is filtered at each transfer operation. Specific apparatus arrangements are provided for facilitating the process and the end product is of increased purity; whereby upon being furnished to an engine for test or vehicle launching purposes the program may proceed as scheduled and the vehicle may be maintained in space for a longer period of time.

BACKGROUND OF THE INVENTION 
It is apparent that a need exists for supply of improved grades of nitrogen 
tetroxide propellants for use as oxidizers for rocket engines during 
system design and development tests and at the launching sites of vehicles 
to be projected into space, such as satellites, space shuttles, or the 
like. Propellant supply/oxidizer flow rate decay problems such as are 
prone to interfere with design/testing programs and launch scheduling of 
the vehicle, as well as to its space flight life duration prospects have 
plagued the industry. For example, the following publications report on 
current state of the art: 
"Flow Decay in Nitrogen Tetroxide Systems", a report published at pages 
7-14 of Chemical Propulsion Information Agency Publication No. 171, July 
1968. 
"Operational Nitrogen Tetroxide Handling", a report published at pages 
41-45 of Chemical Propulsion Information Agency Publication No. 171, July 
1968. 
"Nitric Acid/Nitrogen Tetroxide Oxidizers", pages 2.3-1, 2.3-15, 3-26, 
3-27, 3-28, 3-32, 3-33, 3-40, 3-41 and 3-45 of USAF Propellant Handbooks, 
Vol. II, AFRPL-TR-76-76, February 1977. 
"Flow Decay", pages 101, 103, 104, 115, 117, 122, 124, 126, 132, 133, 135, 
136 and 139 of Final Scientific Report, June 30, 1972, AFRPL-TR-72-84. 
As a result of a professional search conducted with respect to this 
invention, Applicant is aware of the following U.S. Pat. Nos.: 2,298,255; 
2,355,817, 2,403,932; 2,759,418; 3,095,693; 3,146,139; 3,310,444; 
3,345,821; 3,383,859; 3,534,554; 3,536,543; 3,562,035; and 3,582,412. None 
of these references is considered relevant to the subject matter claimed 
herein. 
Such problems are now of major concern due to the recent emergence of 
nitrogen tetroxide propellants as major oxidizers for liquid propellant 
rocket engines of such vehicles, and occur in connection with the storing, 
transferring, testing, launching and maneuvering operations of satellites; 
MX missiles; space shuttles; and the like. This is because of the tendency 
of such oxidizers to corrode their storage/transport/transfer containers; 
as well as other steel parts of the engine oxidizer supply system. 
Commercially available nitrogen tetroxide propellants for such purposes 
are typically supplied to the test site or launch-pad-mounted vehicle, 
either into the vehicle inboard supply tanks or into close-by "ready 
storage" ground-based intermediary supply relay tanks. 
In either case, the supplied nitrogen tetroxide tends to react with the 
metal of the container and of the system components so as to cause solid 
precipitates of iron salts to form therefrom. Such solids in the supply 
stream, along with other "dirt" particulates such as are typically present 
in commercially supplied nitrogen tetroxide propellants, tend to plug the 
filters and other components of the propulsion system such as valves and 
injector orifices; thereby seriously interfering with the propellant 
transfer operations as well as the test or vehicle launching operations. 
Furthermore, and perhaps more importantly, the incidence thereof seriously 
reduces the reliable duration prospect of the rocket engine performance. 
Typical pre-storage and transfer/operational ambient temperature 
conditions contribute to these problems. 
SUMMARY OF THE INVENTION 
I have observed that a supply of relatively warm/hot nitrogen tetroxide 
propellant such as is typically furnished commercially to a test site or 
to a space vehicle at the launch site, precipitates solid particulates out 
of solution at progressive rates as it cools down throughout the supply 
transfer process. This problem is exaggerated whenever the supply is 
furnished from a ground-based stock after long term storage such as in 
stockpiling operations; and in any case operates to plague the engine test 
and/or vehicle launch control operations, as well as to limit the duration 
of the ensuing mission in space. This invention is concerned with the 
problem of more precisely furnishing to a propulsion system test facility 
and/or to a facility for launching a vehicle such as a satellite or the 
like into space in accordance with a prescribed test or launching program 
a more reliable propellant supply. Thus, the end-user vehicle may be 
maintained in space for a longer period of time than has heretofore been 
possible when using present day commercial grades of "Red" or "Green" 
nitrogen tetroxide (MON). The terms "Red" and "Green " MON as used herein 
refer to propellants as set forth in Military Procurement Specification 
MIL-P-26539C. 
Table I from such Mil. Spec. and which specifies chemical composition and 
physical properties is reproduced as follows: 
______________________________________ 
Limits Test 
NTO MON-1 MON-3 Para- 
Composition (Red-Brown) 
(Green) (Green) 
graphs 
______________________________________ 
Nitrogen tetroxide 
99.5 4.5.3 
assay (N.sub.2 O.sub.4)-per- 
cent by weight 
Nitric oxide (NO) 1.0 3.0 4.5.2 
content-max 
percent by weight - 
.sup.1 0.6 1.5 
min 
N.sub.2 O.sub.4 + NO - per- 
99.5 99.5 4.5.3 
cent by weight 
minimum 
Water equivalent - 
0.17 0.17 0.17 4.5.4 
percent by weight 
maximum 
Chloride content - 
0.040.sup.2 
0.040 0.040 4.5.5 
percent by weight 
maximum 
Particulate - 
10 10 10 4.5.6 
mg/liter - 
max 
______________________________________ 
.sup.1 The NO content shall be limited to that which does not change the 
specified RedBrown color of the propellant (3.4). 
.sup.2 This test need not be performed on NTO propellant if the material 
was manufactured by the ammoniaoxidation process. 
More specifically, in accordance with the present invention, the type of 
propellant being commercially furnished, whether it is "Red" or "Green", 
is first determined by appropriate analysis. If it is of the "Green" type, 
an excess of oxygen is added to it in order to convert it to the "Red" 
type for introduction into the process of the present invention. Then, in 
any case, the dissolved iron salts included in the propellant supply 
resulting from previous contact of the propellant with the steel 
components of the manufacturing, storing and/or manufacturing equipment 
are precipitated out of solution by reducing the temperature of the 
solution to within the range of +20.degree. F. to +35.degree. F. The 
referred to precipitates (as well as other "dirt" particulates such as 
typically occur in commercially furnished grades of nitrogen tetroxide 
supplies) are then filtered out so as to thereby provide to the test or 
launch site a more pure form of "Red" nitrogen tetroxide propellant. This 
improved propellant eliminates at minimum expense the currently 
frustrating problems of design modifications and of scheduling blast-off 
procedures. This product may be used "as is" for fueling engines of the 
type such as are used for example in the TITAN II missiles; or it may be 
blended to contain for example from 0.6 to 1.0 wt% nitric oxide (NO) to 
produce a purer "MON-1" type propellant; such as is used in MX type 
engines. Or, it may be blended with 1.5 to 3.0 wt% nitric oxide to produce 
a purer "MON-3" type propellant such as is used in the space shuttle 
engines, as well as in various bi-propellant satellite engines. Various 
amounts of nitric oxide may be added to the base material in similar 
manner in accordance with this invention in order to produce propellants 
such as may be required for other applications. 
Thus, the invention provides an improved base material from which may be 
blended any desired formulations of nitrogen tetroxide rocket engine 
oxidizers, which are in any case of improved purity when supplied to the 
propulsion system; thereby substantially increasing the operational 
duration functioning of such engines by reason that the propellant flow 
rate decay problems are reduced. This advance in the art is acquired 
without adding foreign matter "inhibitors" or the like, such as are 
suggested for example in U.S. Pat. Nos. 2,403,932; 3,534,554 and 
3,536,543.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In accordance with one preferred version of this invention as shown at FIG. 
1, a typical commercially supplied raw MON, such as may be in the form of 
either the so-called "Red" or "Green" type, may first be heated as is 
indicated at 10 if necessary to bring it into liquid state. For example, 
the MON may be supplied in containers under lower than 12.degree. F. 
ambient temperature conditions, thereby being in frozen solid condition. 
The liquid MON is then transferred into a reactor 12 and in event the raw 
MON material if of the "Green" type, an excess of oxygen gas is bled into 
the MON, such as from a supply tank 14 under control of valve 32. 
Accordingly, in any event the output from the vessel 12 is assured to be 
MON of the "Red" type. This is then beneficiated by cooling it down to 
within the range of +20.degree. F. to +35.degree. F. such as by passing it 
through a cooler as is shown at 16. Then it is filtered through a suitable 
device as is indicated at 18. This filter should be at least of a rating 
such as is known in the trade as "6 microns absolute"; meaning that the 
largest particle passing through the filter will be of the order of 6 
microns. If possible, a still finer filter will be employed. Thus, the 
solid precipitates which occur in the "Red" MON in the form of iron salts 
or the like resulting from previous contact with the walls of its 
containers are removed, as well as any other so-called "dirt" particles 
such as may have accumulated therein throughout its manufacture, 
transfer/storage procedures. 
A purer form of "Red" nitrogen tetroxide is thereby provided under delivery 
control by a valve 32.sub.1, from which supplies of MON-1; MON-3, etc. may 
be formulated by by-passing it under control of a valve 32.sub.2 for 
addition of appropriate amounts of NO gas under control of a valve 
32.sub.3 such as from a supply source indicated at 20 leading into a 
reactor as shown at 22 (FIG. 2). Provision is made for cooling the reactor 
prior to admission of NO gas in order to obtain an efficient reaction 
between the NO and dissolved oxygen (O.sub.2) and nitrogen dioxide 
(NO.sub.2) present in the purified "Red" MON; and a filter such as shown 
at 34 is provided downstream of the reactor to remove solid precipitates 
from the stream. Valves 32.sub.4 and 32.sub.5 are included in the delivery 
lines to cooperate with the valve 32.sub.2 to control the by-pass 
operation. Thus, the products delivered through valve 32.sub.5 are 
therefore initially substantially free of particulate inclusions such as 
would plug filters and/or feed system components in the rocket test or 
propulsion system; and therefore will exhibit longer shelf life and 
provide for longer duration trouble-free rocket engine test or engine 
performance. 
In any case, in accordance with this invention, in order to obtain 
desirably extended shelf life and trouble-free rocket engine performance, 
nitric oxide is added to the product at this stage of the process, as 
allowed by procurement specifications. However, to be assured of maximum 
shelf life and trouble-free rocket engine performance, nitric oxide would 
be added at this stage up to the maximum amount permitted by procurement 
specifications. It will be recognized that this same process may be 
employed to rectify existing stock-piled supplies of nitrogen tetroxide 
propellants before they are released for use. 
FIG. 2 also shows how the purified MON supply of the present invention may 
be furnished under control of a strategically placed conduit and valve 
system as designated generally at 31; whereby the purified propellant may 
be optionally transported either into a plant storage facility 24; a 
shipping container 26; a ready storage facility 28; or directly into the 
rocket engine test or propulsion system propellant feed tank 30 under 
control of a manifold system including valves 32.sub.6 ; 32.sub.7 ; 
32.sub.8 ; 32.sub.9 ; 32.sub.10 ; 32.sub.11 ; 32.sub.12 ; 32.sub.13 and 
32.sub.14. It is an important feature of this invention that appropriate 
filters are installed in this manifold circuitry such as are illustrated 
at 34a, 34b and 34c. Thus, it will be noted that the product is filtered 
after each cooling operation and after each transfer and storage step, as 
well as just prior to introduction into the test facility or propulsion 
system tank. FIG. 3 illustrates how the material is preferably filtered 
again at 36 just prior to its entry into the rocket engine flow control 
valve system as shown at 38 leading into the engine thrust chamber 
injector. It is to be noted that the filter 36 may preferably be of 
slightly larger pore size than the filter 18 and the 34 series of filters 
which are ground-based, and are therefore readily replaceable when 
becoming clogged. The rocket engine in toto is designated generally by the 
numeral 40. 
FIGS. 4 and 5 illustrate graphically how changes in solubility of corrosion 
products in nitrogen tetroxide propellants occur as functions of NO 
content and temperature. For example, FIG. 4 graphs typical solubility 
changes in relation to nitric oxide content, whereas FIG. 5 illustrates 
solubility changes in relation to temperature changes. Incidentally, just 
prior to initial loading an/or reloading of the tank and plumbing 
components of the propellant handling system, all parts thereof are 
preferably cleansed of any metal corrosion/oxidation by-products such as 
may exist on their propellant contacting surfaces. This may be done by 
flushing them with an appropriate cleaning solution. For example, 
components made of stainless steel may be "pickled" with an aqueous 
solution of nitric and hydrofluoric acids. Then, the cleansed surfaces are 
passivated such as with an aqueous nitric acid solution. 
EXAMPLE #1 
Approximately 200 grams of typically commercially furnished "Green" MON 
(mixed oxides of nitrogen containing an equilibrium amount of nitrogen 
dioxide and nitric oxide) were cooled down to a temperature of 
approximately 32.degree. F. for beneficiation. A small quantity 
(approximately 50 grams) of the chilled material was then analyzed in 
order to determine its nitric oxide content. The remainder of the material 
was then weighed into a tared glass pressure vessel. The vessel and 
contents were then cooled to approximately 32.degree. F. and contents 
bleached by bubbling an excess of commercially pure oxygen gas into the 
liquid, thus converting the nitric oxide present in the liquid to nitrogen 
tetroxide, according to the equation: 
EQU 2NO+O.sub.2 .fwdarw.N.sub.2 O.sub.4 
Addition of more than the stoichiometric amount of oxygen was then 
confirmed by analysis. The cooled product (impure "Red" MON) was then 
transferred by nitrogen gas pressure through a chilled plastic filter 
assembly rated at 6 microns absolute, into a clean, chilled glass 
receiver. This product was pure "Red" MON, which by analysis substantially 
exceeded the requirements for "Red-Brown" propellant as set forth in 
Military Procurement Specification, MIL-P-26539C. 
EXAMPLE #2 
100 grams of "Red" MON (mixed oxides of nitrogen) as received from a plant 
which commercially produces a mixture of nitrogen tetroxide containing an 
equilibrium amount of nitrogen dioxide and dissolved oxygen, was cooled 
down to 32.degree. F. and then passed by nitrogen gas pressure through a 
chilled plastic filter assembly rated at 6 microns absolute into a clean, 
chilled glass receiver. The product was pure "Red" MON, which by analysis 
exceeded the requirements for "Red-Brown" nitrogen tetroxide type 
propellant as set forth in Military Procurement Specification 
MIL-P-26539C. 
EXAMPLE #3 
A quantity of pure "Red" MON, as produced in Example 1 or Example 2, was 
cooled down to 32.degree. F. and then transferred into a glass pressure 
vessel. The vessel was weighed before and after charging. The vessel and 
contents were then again cooled down to 32.degree. F. Commercially pure 
nitric oxide was then bubbled into the liquid to convert any excess oxygen 
into nitrogen tetroxide, and to convert some of the nitrogen dioxide 
present to dinitrogen trioxide according to the equation: 
EQU NO+NO.sub.2 .fwdarw.N.sub.2 O.sub.3 
Addition of the nitric oxide was monitored by weight change and terminated 
when analysis confirmed the desired degree of dinitrogen trioxide 
formation. The method of analysis employed is provided in Military 
Procurement Specification MIL-P-26539C. The product was pure "Green" MON 
containing the equivalent of 0.6-1.0 wt% nitric oxide, therefore exceeding 
the purity of the MON-1 required in the aforementioned Military 
Procurement Specification. 
EXAMPLE #4 
Approximately 100 grams of the product obtained in Example 3 was furnished 
additional nitric oxide as in Example 3, so that sufficient nitric oxide 
was added so as to produce a final value equivalent to 1.5-3.0 wt% nitric 
oxide. The final product was a pure grade of MON-3 propellant exceeding 
the requirements of Military Procurement Specification MIL-P-26539C.