JOHN F. GALL Research & Development Department, Pennsalt Chemicals Corp., Philadelphia, Pa.
Fluorine-Derived Chemicals as Liquid Propellants Fluorine resources available to the missile industry are adequate for any test program contemplated for the near future
T H E TEST requirements for a new chemical to be considered for missile use start from a few pounds for property testing and miscellaneous preliminary phase studies, then move to a few dozen pounds or, if the expense is not too great in the large laboratory phase, to a few hundrzd pounds. Here the chemical is tested in small engines of a few hundred to a few thousand pounds of thrust. Assuming acceptance thus far, engines now have to be designed around the novel and presumably beneficial character of the new compound, and tested on a scale approximating that of the intended tasks which the new engines are to perform. At this point, the demand for the chemical can become very large. Consider a hypothetical but typical example. Suppose that the engine designer is interested in a thrust of 100,000 pounds, and that the specific impulse (defined as thrust per weight flow of propellant combination) is 250 seconds. This engine, the simple calculation shows, will consume 400 pounds per second of propellant combination, which might be divided, for example, into 250 pounds per second of oxidizer and 150 pounds per second of fuel. I n the early testing program of a n important new propellant chemical, running times of 10 to 20 seconds-i.e., consumptions of 2 to 4 tons of propellant combination-may be typical. However, in later stages, where life tests or acceptance programs are involved, full-duration runs are made and these may last 1 or 2 minutes. I n a test of 100-second duration, for example, 40 tons of propellant chemical are burned up; and part of this, perhaps the oxidizer, is the hard-won new chemical being discussed, Some of the government sponsors of the missile development program expect each test stand for large engines to carry on about 20 firings per month. As a testing program may employ a half dozen test stands in different locations, the de-
mand for our propellant combination may soar to hundreds of tons per month. Within a small number of years, assuming favorable results throughout, the testing program is completed and the propellant chemical becomes operational. At this time, during peaceful conditions, and of course, prior to frequent space travel schedules, the demand for our propellant chemical will sag substantially, and taper off to a low figure for replacement, training, and minor development programs. Although this is not a good mar et curve for a new chemical, it gives t e product a good opportunity to become familiar both to manufacturers and to people who have to handle the materials. The chemical, as indicated by the theme of this conference, thus becomes available for commercial use. I t is a respon-
1
Properties of Oxidizers Melting Point, Substance Fluorine Nitrogen trifluoride Oxygen difluoride
c.
-220
Boiling Density Point, a t B.P., ’ C. G./Cc. -188 1.51
-207
-129
1.54
-224
-145
1.50
sibility of the company which manufactures the missile chemical for rocket engine development to seek commercial opportunities during the years of the missile market. From its p6sition in the periodic classification of elements, it is clear that fluorine and its derivatives should be among the most powerful oxidizers for use with the appropriate fuels to propel rockets. I n addition to fluorine itself, such compounds as the halogen fluorides, nitrogen trifluoride, and oxygen difluoride should be considered. These are all compounds of fluorine with the lighter elements and possess relatively low heats of formation. The calculated performance of an oxidizer-fuel combination is measured by specific impulse, and by the product of this with the mean propellant density. Selecting fuels which are of special value for the indicated oxidizer, we find the comparative performances shown in the box. Fluorine, with a n appropriate fuellike hydrazine or ammonia, is very substantially superior in performance to currently used oxygen and nitric acid oxidizers, and would be equaled only by some fuel combinations with ozone. Fluorine is not an inexpensive oxidizer, and it seems wrong to use a premium oxi-
Performance of Selected Rocket Propellant Combinations”*b Oxidizer Fluorine Oxygen difluoride Nitrogen trifluoride 50:50 Chlorine trifluoride Bromine pentafluoride Oxygen RFNA (22% NOn)
+
Fuel Hydrazine n-Octane
Specific Impulse, Seconds 315 300
Specific Impulse X Density 410 365
Ammonia Hydrazine Ammonia n-Octane JP-4
295 255 240 265 240
340 365 420 255 305
fluorine,
Expansion 500 to 14.7 lb./sq. inch gage frozen equilibrium. “Rocket Engine Propellants,” Rocketdyne, a Division of North American Aviation, Inc.
VOL. 49, NO. 9
SEPTEMBER 1957
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dizer to burn a low-cost fuel such as hydrocarbon mixtures. However, in some circumstances-for example, where fluorine is to be used interchangeably with some other oxidizer-this may be desirable. The heat of formation of the carbon-fluorine bond, however, is not high and the major energy release from combustion of fluorine and hydrocarbon lies in the formation of the hydrogenfluorine bond. A superior performance can be obtained if enough oxygen is mixed with fluorine to consume carbon, so that the full benefit of the hydrogen-fluorine reaction can be realized. In a typical hydrocarbon, about two atoms of fuel should be present per atom of oxygen, and this suggests the use of fluorine-oxygen mixtures (which can be prepared readily, and are stable); or the readymade combination of oxygen difluoride can provide a similar energy resource, a constant and correct oxygen-fluorine ratio, and some improvement in physical properties. I t is unfortunate that at present no good way to make oxygen fluoride economically is a t hand. The goal would be oxygen fluoride a t a cost equal to or less than that of the oxygenfluorine mixture. The halogen fluorides are hindered by the high atomic weight of the ligand halogen and, although this relation is best in chlorine trifluoride, this oxidizer is only intermediate in performance between nitric acid and oxygen. The big benefit of chlorine trifluoride is the convenient 12' C.) vapor pressure (boiling point and the good density, combined with permanent storability in standard materials of construction. In bromine pentafluoride, the drawback of high atomic weight is greater, but here we have an oxidizer which, in combination with a selected fuel, can give a propellant combination of notably high density which needs to be considered seriously whenever over-all performance includes the total missile dimensions as significant factors. Finally, in this survey of fluorine-derived chemicals, we reach nitrogen trifluoride, which has good handling characteristics but for which, like oxygen
+
Melting Point,
Substance Fluorine Chlorine trifluoride Chlorine monofluoride Bromine trifluoride Bromine pentafluoride Iodine pentafluoride Iodine heptafluoride
c.
Hazards The handling of fluorine and its derivatives is admittedly difficult. Fluorine and halogen fluorides are violent incendiaries and they possess toxicity at least comparable with that of other halogens. I t is fortunate, however, that most of the useful structural metals form a ready and self-healing film of solid reaction product which prevents further attack up to temperatures well beyond those to be concerned with in practical use. Compressed fluorine, for example, and liquid chlorine trifluoride have now been transferred, stored, and shipped in steel containers, and liquid fluorine can be similarly handled in vessels of nickel alloys designed to maintain loiv temperature strength (fluorine boils in the liquid air range, where common steels are severely weakened by low temperatures). When liquid fluorine is to be explored in rocket engine development, we must Contemplate a large volume of liquid in storage at the test stand, very large liquid flows, and the possibility of abrupt pressure changes. These factors introduce severe local hazards from catastrophic leaks or sudden bursting of the liquid fluorine lines. However, the current practice in rocket engine testing involves the use of massive fireproof test stands and adequate personnel protection by means of heavy-walled and separately ventilated control buildings. A severe leak of liquid fluorine in one of these areas would be a spectacular demonstration, but, apart from local destruction of metal parts and combustion damage to wiring,
Boiling Point,
c.
- 188
- 220
11.8
-76.3
- 100.8
- 154
125.8 40.5
8.8 -61.3 9.6 5
fluoride, effective manufacturing procedures are not yet available. The oxidizer derived from fluorine will probably find special niches in the missile propellant catalog. The need for
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fluorine itself seems evident. This is the oxidizer of the greatest possible performance, and it is easy to recognize that rocket engineers will be meeting tasks for which the highest ratio of thrust to weight will be needed. I t is inevitable that fluorine w-ill become an important rocket chemical, and thereremainsmerely the task of learning to produce this element cheaply, to handle and ship it safely, and to exploit it effectively in reliable engines.
98 Sublimes 4.5
Density, G./Cc. 1.51 at B.P. 1.7 at 25' C.
...
2.8 at 25' C. 2.46at25' C. 3.19 at 25' C.
...
insulation, and other flammables nearby, no widespread damage should be expected. The rocket engineer, in protecting himself from the engine and from the hazard of fuel fires and explosions, has
INDUSTRIAL AND ENGINEERING CHEMISTRY
already taken steps to safeguard himself and his expensive control equipment from the hazards of liquid fluorine, A problem of a different character arises in considering the nature of exhaust gases issuing from the rocket engine. A ton of fluorine, for example, will produce about a ton of hydrogen fluoride from a hydrogen-containing fuel and this is a corrosive and toxic product. Here, again, however, the necessary location of a rocket test installation in unpopulated regions gives the protection that is needed. The exhausts from fluorine engines are very hot and composed of low molecular weight species. They thus form a nonpersistent gas which can be expected to move rapidly upward and to disperse quickly in prevailing winds. With some attention to location and weather conditions, therefore, the rocket engineer should be able to carry on his testing program without hazard to personnel or equipment. Two special comments are to be made. The local condensate on the test stand may develop a substantial concentration of hydrogen fluoride in water solution; this is a significant hazard because of the insidious corrosive action of strong hydrofluoric acid applied to the skin. There should be a recommended standard practice of flushing the test stand with water following a fluorine engine run. The cascade systems which are standard equipment on rocket test stands will perform this function. The second point is the effect of fluorine and fluorides on vegetation. The problems here have been quantitatively assayed by the fluorine chemicals industry, and the possible effect on valuable plant life in the area of the test stand can be evaluated. Fluorine Requirements
The resources of fluorine availablr 10 the missile industry are adequate for an)contemplated rest program, requirements for which can now be predicted. Xf a fluorine demand, for example, of 150 tons per month is considered, this should be compared with the total U. S. production of acid grade fluorspar which i s about 20,000 tons per month, equivalent to about 10,000 tons per month of elemental fluorine. The missile program \uould thus require only a small percentage of the currently used fluorine. T h e fluorine chemist recognizes also the number of alternative sources of fluorine, including very large amounts available uith difficulty from phosphate rock and a substantial resource from by-product topaz. These are resources of the United States. In addition, if necessary, large amounts of fluorspar might be drawn from Mexico and Canada. Commercial Chemical Development Association, French Lick, Ind., May 14,1957.
