The Explosive History of Cryogenic Propellants
Like the beasts of the Wild West, the taming of a new technique can be a strenuous task. You have to respect it or you might get burnt, or worse. In the 1950’s the United States was actively in the business of taming new propellants for its fledgling rocket industry. The work began after World War II with the seizure of German V-2 missiles and was fully running even before the Sputnik launch of October 1957.
Following the trail blazing success of its genocidal Nazi adversaries, the United States originally planned to fuel its ballistic missiles with the same ethyl alcohol that fueled the German V-2. This same spirit is still available and awaiting your abuse at the local liquor store, but that is not important. The United States succeeded with the 1958 release of the Redstone ballistic missile that could hurl a three-ton thermonuclear warhead two hundred miles.
Nonetheless ethyl alcohol is not without its problems. Servicemen have been known to abuse it and the performance is pathetic. Engineers tried substituting it with methanol and isopropanol alcohol and couldn’t solve either problem.
However the 1950s witnessed the inception of more than just missiles. Both the military and commercial manufactures were actively launching jet aircraft, and they all needed vast amounts of fuel. At the time J-4 jet fuel was available everywhere, and jet engines can burn just about anything. This isn’t the reality with rocket engines. The engines are much more complex and burn really hot. J-4 is known to polymerize in the hot cooling channels of engines, causing self-destruction. However they really needed the performance of a hydrocarbon to get a Thor missile warhead to its destination nearly 2,000 miles away. The solution should have been obvious, formulate a special blend for missiles that will be fueled once at most. It took the Department of Defense until 1957 to write the specifications for RP-1. This fuel is still used today to power missiles, and you can always tell by the prominent bright flame of the booster.
Yet rockets burn more than just kerosene. Engineers including Von Braun have been working with liquid oxygen since 1936, but oxygen is a beast in itself. The engines burn very hot and they can be tricky to get started. The original V-2, Thor, and Jupiter used a pyrotechnic ignition, but it was never reliable. Engineers later developed the hypergolic start using a sealed glass capsule of triethyl boron and triethyl aluminum. The ampoule is placed in the fuel line of the missile and ruptured by the pressure surge at ignition. The trick worked so well that it became the standard for the Atlas missile and all liquid oxygen and kerosene engines.
Some might say that you can’t go wrong mixing liquid oxygen with liquefied hydrogen. The performance is extraordinary, leading engineers to use this mix on the Space Shuttle and the modern Delta 4 rocket. It blend always burns clear, with the exception of the RS-68 engine on the Delta rocket. This is on account of the engines simplicity. Regenerative cooling is expensive. Instead the RS-68 engine nozzle is cooled by its own slow ablative vaporization in the heat. It turns out that engineers have been aware of hydrogen performance since World War II. Nonetheless no one knew how to manage it. Hydrogen is a very low-density fuel, and this equates to the need for massive missile tankage. Then there is the formidable problem of its low boiling point. Hydrogen is super-cryogenic with a boiling point 21K lower than oxygen, requiring elaborate insulation schemes and it gets better. Quantum Mechanics will also get in the way. Hydrogen fuel H2 is in reality two protons orbiting one another. Quantum Mechanics demands that each atom can orbit in parallel to one another or opposite to one another. You could say that it is academic until you have to store it. The real kicker is its ability to transition from the opposing state to the parallel state, and then release 337 calories of heat for each mole. Since you only need 219 calories to vaporize one mole of hydrogen, that is plentiful energy to evaporate a liquefied mass even in a sealed thermos. P.L. Barrick solved the problem with a hydrated ferric oxide at the University of Colorado. The catalyst accelerates the transaction, and since it introduction several other catalysts have become available. The J-2 engine that powers the Centaur upper stage is the longest running and most successful hydrogen-oxygen rocket. The propellants are easily ignited by electrical ignition of the gaseous components in a pilot chamber above the main combustion chamber.
Not all the stories of the early space race are glamorous. Much of the research went nowhere. Many of the chemists involved could show early on that many propellants are to be too toxic, overly corrosive, or explosive. Liquid fluorine is doubtless the only exception. In Ignition, John Clark says that research was active by 1947. The Jet Propulsion Laboratory was a frontrunner in the study, and that is impressive because liquid fluorine is very tricky to manipulate. It is hypergolic (self-igniting) with just about every material. Despite this, Bill Doyle of North American successfully fired engines as early as 1947. Still the work went nowhere until Scott Kilner of The Office of Naval Research proved that liquid fluorine is of a higher density than initially believed. It is in fact 1.54 instead of the falsely believed 1.108. The result of this discovery was that engineers could utilize additional energy in their performance calculations. This work encouraged engineers to experiment burning fluorine with ammonia and hydrazine, and the result was fantastic. The only problem with this arrangement is contamination. The slightest grease or dirt in the plumbing would ignite on contact, and with it the entire engine. John said that the only solution to that is a pair of good running shoes. Notwithstanding these problems, fluorine was successfully fired with Hydrazine. Bell even built the Chariot for the third stage of the Titan III missile. The stage burned a mixture of monomethyl hydrazine, water, and hydrazine with fluorine. General Electric also got in on the game with the X-430 fluorine hydrogen motor. Neither of these stages was ever flown for reasons that should be obvious, the mixture was too hypergolic. Then someone at Rocketdyne had a bright idea, dilute the fluorine in liquid oxygen. The two oxidizers will fully dissolve into each other and have boiling points only a few degrees apart. The Flox mixture was thirty percent fluorine. It was even compatible with the hardware of the Atlas missile, and boosted its performance by five percent. Yet it too was never used on an operational missile.
Despite these achievements, fluorine will probably always be punk rock. With its exhaust rich in hydrogen fluoride it is too offensive for people in the audience and the launch pad. Just wait until a rocket crashes! Fluorine is also profoundly more expensive than liquid oxygen. If it is ever used in a booster, expect it in an upper stage.
In conclusion, the taming of the chemicals used in modern rocketry was an epic feat. Kerosene was much more energetic than alcohol, but its propensities to polymerize stalled success for years until new formulas were created. The relatively common propellant liquid oxygen was also challenging, because it was tricky to burn until the adoption of hypergolic ignition. Liquefied hydrogen was only domesticated with the mastery of quantum mechanical chemistry. Fluorine is too hypergolic and corrosive to likely ever be used in a rocket stage.
