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S. Department of Defense, including the Nike,Honest John, Sparrow, and Terrier missiles.7 Muchless seems to be known about these developments.Other Developments and Key Questions• ln a separate line of development, a group ofresearchers at the Guggenheim AeronauticalLaboratory at Caltech (which formed the nucleus ofwhat became JPL in 1944) was working on jetassisted takeoff units.

In June of 1942, a chemistnamed John Parsons had the idea of combiningasphalt (as a binder and fuel) with potassiumperchlorate (as an oxidizer) to make the firstcastable composite solid propellant.8Many more discoveries were behind these large solidrocket developments than just these propellantcontributions. Integral to the stories of the propellantsused on large rockets and missiles, smaller tacticalmissiles, and a host of smaller rockets for a variety ofrockets and spacecraft were the various binders, fuels,and oxidizers that went into the propellants. Forexample, the motors for the Polaris A1 missile designed• JPL engineer Charles Bartley improved on Parsons’basic discovery by replacing the asphalt with aThiokol polysulfide polymer, LP-2.

CombiningLP-2 with an internal-burning, star-shaped grainand case-bonding the combination in a thin case led†Telephonicinterview of Col. Edward N. Hall (United States AirForce, Retired) by J. D. Hunley, Nov. 13, 1998.‡Hall, Edward N., “USAF Engineer in Wonderland Including theMissile Down the Rabbit Hole,” undated typescript provided by Hall.3American Institute of Aeronautics and Astronautics(Photo courtesy of NASA.)Figure 1.

A Polaris A2X test missile on the launch pad at the Atlantic Missile Test Range, Cape Canaveral, Florida.The A2X is the prototype for the 1500-nmi Polaris A2 missile that became operational in 1962.4American Institute of Aeronautics and Astronautics(Photo courtesy of NASA.)Figure 2.A Titan 3E launch vehicle with a Centaur upper stage. This vehicle featured the five-segment solid rocketmotors shown firing in this photograph together with three liquid-propellant stages, including the Centaur.

Thisconfiguration launched the Viking, Helios, and Voyager spacecraft from 1974 to 1977.oxidizer in the late 1940’s and used it in the early 1950’son the RV-A-10 missile, an important precursor of thePolaris missile.¶ A significant part of the history ofrocketry involves the full details of how ammoniumperchlorate successfully came to be used and how thevarious ingredients in the Polaris motors came to becombined in the proportions that ultimately wereemployed.by Aerojet featured a cast, case-bonded polyetherpolyester-polyurethane composition with 15 percentaluminum and ammonium perchlorate.

Karl Klager atAerojet has been credited with being largely responsiblefor developing both the grain and the propellant forthese motors, but the story of their development isevidently quite complex. Klager received the U. S. NavyDistinguished Public Services Award in 1958 for hiswork on the Polaris missile, but the development ofsome of the propellant ingredients predates when Klagerjoined Aerojet in 1950.For subsequent missiles and rockets, similar questionsexist about the development of the propellant grains.How, for instance, did polybutadiene–acrylic acid(PBAA); acrylic acid, acrylonitrile, and butadieneterpolymer (PBAN); carboxyl-terminated polybutadieneIn 1948, Aerojet had replaced potassium perchloratewith ammonium perchlorate in certain aeroplexpropellants to reduce smoke and increase specificimpulse. Problems existed with combustion instabilityand the aeroplex binder was not case-bondable, leadingAerojet to convert to a polyurethane binder in 1953 and1954.

But the process had started.§ JPL and Thiokolwere also working with ammonium perchlorate as an§Klager, Karl and Albert O. Dekker, “Early Solid CompositeRockets,” unpublished paper, Oct. 1972.¶Wiggins, Joseph W., “The Earliest Large Solid Rocket Motor—The Hermes,” presented at the AIAA 9th Annual Meeting andTechnical Display, Washington, D.C., Jan. 8–10, 1973.5American Institute of Aeronautics and Astronautics(NASA photo 81-HC-829.)Figure 3. Space Shuttle Columbia climbs skyward on its second mission into space from NASA’s Kennedy SpaceCenter, Florida, on November 12, 1981.

This photograph shows one of the two giant solid rocket boosters firing toprovide the lion’s share of the orbiter’s lift for the first 24 nmi of ascent into space.6American Institute of Aeronautics and Astronautics(CTPB); and hydroxyl-terminated polybutadiene(HTPB) come to be developed as binders? How didtetramethylene tetranitramine (HMX) come to replace atleast some of the ammonium perchlorate in some highenergy propellants? The technical literature seems toleave these kinds of questions unanswered.

Moreresearch is needed.Karl Klager, who is credited with the development ofHTPB, was asked how he came to develop this lowcost, low-viscosity propellant that has become anindustry standard. He said only that he starteddevelopment in 1961 but waited until 1969 to proposethe propellant to NASA for the Astrobee D andAstrobee F sounding rockets on which it flewsuccessfully. Perhaps, however, Klager’s responseregarding how he came to discover unsymmetricaldimethylhydrazine (UDMH) (which is a liquidpropellant used on the Bomarc missile, Titan 2 missile,Titan 3 and Titan 4 rockets, and other missiles androckets) applies equally to HTPB.

Klager said that hesimply brought his knowledge of the science ofchemistry to bear on the need for a propellant. He hadearned a Ph.D. in chemistry from the University ofVienna in 1934 and had worked for several chemicalfirms in Europe from 1931 to 1948 before moving to theUnited States and starting work for Aerojet in, , ††1950.# **Ernie Sutton’s privately printed and very usefulhistory of Thiokol# provides partial answers to some ofthese questions.

According to Sutton, Thiokol begansearching in 1952 for propellants that would raiseperformance while lowering temperatures of the burninggrain. Funded by the U. S. Army, Thiokol chemistssought to reduce or eliminate the sulfur content of itspolysulfides by preparing liquid hydrocarbon polymers.The chemists tried several polymers and combinationsof copolymers but found adding functional groups thatwould cure readily was problematic. Eventually, in1954, the chemists in Huntsville, Alabama, discovered acopolymer of butadiene and acrylic acid—PBAA.

Using32-oz soda bottles for mixing, the chemists succeeded ingetting a liquid epoxide resin to react with the carboxylgroups yielded from the acrylic acid and thus produceda cured polymer binder. Sutton does not reveal,however, who the chemists were or exactly how theyarrived at their discovery, although his narrativesuggests that they did so by trial and error with likelypolymers.Klager’s explanation is similar to the answer given byCharles Henderson about the discovery that largeamounts of aluminum—on the order of approximately16–20 percent—substantially added to the specificimpulse of solid propellants. Henderson and KeithRumbel, both chemical engineers trained at theMassachusetts Institute of Technology (Cambridge,Massachusetts), were aware that other researchers,including some at Aerojet, had calculated that thespecific impulse of composite solid propellants could beraised by including aluminum in the ingredients. Theseother researchers’ calculations, following contemporarytheory, had showed that the aluminum would increasethe specific impulse only within a narrow range.

Whenthe amount of aluminum in the propellant exceeded thelevel of 5 percent of the total content, the calculationsindicated that the specific impulse again declined.Sutton notes that PBAA was a definite improvement,but that it did not possess good tear strength.# Hence,Thiokol introduced PBAN, which offered betterphysical properties, in 1954. According to Sutton,although Thiokol originally developed PBAN, theAmerican Synthetic Rubber Corporation in Louisville,Kentucky (later known as Kentucky Synthetic RubberCorporation), somehow—he doesn’t say how—beganproducing the binder in the late 1950’s.

Used inMinuteman missiles, Space Shuttle solid rocketboosters, and Poseidon missiles, PBAN has accumulatedthe largest production tonnages in the industry.#Rumbel and Henderson apparently had betterinformation about the actual nature of the combustionprocess and went well beyond the 5-percent level,finding that the specific impulse of the Atlantic Researchpolyvinyl chloride propellant climbed significantly asthat level was exceeded. In addition to a lack of falseassumptions about the then–still largely unexplorednature of the combustion process, what permittedRumbel and Henderson to make their discovery wasLater in the 1950’s, Thiokol developed CTPB,although Sutton does not say how.

He notes CTPB hasbetter mechanical properties than PBAN but never fullysupplanted the latter, partly because of its higher costand partly because of the emergence of a better polymer,known as HTPB.#**Shortbiography of Dr. Karl Klager, provided by him toJ. D. Hunley, Mar. 1997.††Telephonic interview of Dr. Karl Klager by J. D. Hunley,Mar. 15, 1997.#Sutton,E. S., “How a Tiny Laboratory in Kansas City Grew into aGiant Corporation: A History of Thiokol and Rockets, 1926–1996,”privately printed, Jan. 1997.7American Institute of Aeronautics and Astronauticsgood empirical technique. They and their colleagues atAtlantic Research apparently had the proper mix ofskills and knowledge to employ the correct proceduresfor testing the effects of a 21-percent concentration ofaluminum combined with 59 percent ammoniumperchlorate and a binder of 20 percent plasticizedpolyvinyl chloride in test stands at Atlantic Research.Aerojet later verified their findings in an actual 100-lbrocket in early 1956.mixed with other propellant ingredients and cast ineither cartridge-loaded grains or case-bonded rocketcasings because, when subjected to moderate heat, the##nitrocellulose converted into a solid.Henderson did not furnish a date for thisdevelopment, but he said that in 1955 he and Rumbelbegan scale-up work and propellant development basedon Sloan and Mann’s discovery.

Atlantic Researchconstructed a pilot plant and began development of twoCMDB formulations in 1956. Table 2 shows whatcomprised the two formulations.Rumbel and Henderson had found that severalconditions were necessary for good combustion ofaluminum: small particles (fortunately available fromboth the Aluminum Company of America, Alcoa,Tennessee; and Reynolds Metals Company, Richmond,Virginia), proper chamber pressure (approximately725–889 psi with measured specific impulse at thosetwo pressures being 230 and 247 lbf-sec/lbm,respectively) and oxidation ratio, and sufficient energycontent in the propellant mixture to ignite thealuminum. They used test motors 5 in.