Rocket propellant research had its heyday in the mid-20th century, when the space race and the Cold War meant chemists had plenty of money and long leashes. Only a few of their most interesting ideas ended up in working rockets, but they charted new areas of chemical space, some of which, like boron chemistry, have proved useful in other fields. Geopolitical shifts, along with a growing emphasis on health, safety, and the environment, put a damper on propellant chemistry in the last decades of the 1900s. But the need for high-performance propellants hasnt gone away, and neither has chemists interest in pushing the envelope. In this episode of Stereo Chemistry, we hear from chemists who lived through the heady days of the 50s and 60s and the ones carrying rocket chemistrys torch today.

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The following is the script for the podcast. We have edited the interviews within for length and clarity.

Kerri Jansen: Back in 2012, astronaut Chris Hadfield was getting ready to blast off to the International Space Station. Before he did, though, he got on Reddit to host an Ask Me Anything, where users of the social news site could ask Chris all of their most burning questions. And during the Q&A, Chris described what its like to blast off.

Launch is immensely powerful, he said, and you can truly feel yourself in the centre of it, like riding an enormous wave, or being pushed and lifted by a huge hand, or shaken in the jaws of a gigantic dog. . . . The weight of over 4 Gs for many minutes is oppressive . . . until suddenly, after 9 minutes, the engine[s] shut off and you are instantly weightless. Magic.

Today on Stereo Chemistry well be talking about that magic. Or the chemistry behind the magic, I should say. Specifically, the chemistry of rocket fuel. And Ive got the perfect copilot here to propel our journey. Hi, Sam.

Sam Lemonick: Hey, Kerri.

Kerri: So, Sam, the idea for this episode came from you. What got you interested in rocket chemistry?

Sam: Well, a lot of the space stories I write rely on rockets. Rovers wouldnt be roving on Mars, telescopes like Hubble wouldnt be exploring the universe, if rockets hadnt put them there. So what Id like to say is that I developed a deep respect for these workhorses and the unsung chemistry that makes them work. But the truth is I read a really smart and fun book about rocket chemistry, and I wanted to dig in and learn more.

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Kerri: And what is this book you speak of?

Sam: Its called Ignition!, with an exclamation point. It was first published in the early 70s and written by rocket chemist John Clark, who was also a sci-fi author who palled around with people like Isaac Asimov.

Clarks accounts of rocket research are completely captivating. The book was out of print for a while, although there were excerpts circulating on the internet, which is where I first found it. Anyway, its back now and if you want to learn more about the heyday of American propellant chemistry than we could include here, you should definitely go check it out.

Clark died in 1988, so unfortunately I couldnt talk with him, but I did the next best thingI talked to some of the other rocket chemists who were there at the beginning. Well, the beginning of modern rocket science.

Kerri: Oh, cool. So how do you define modern rocket science, and when did it start?

Sam: Modern rocketry is basically what let humans escape Earths gravity for the first time, and it started in the late 1800s.

Kerri: Okay, but unless you have a Ouija board youre not telling me about, Im guessing the scientists you talked to would have been active a bit later than that. So when did they come in? And big picture, what did they tell you?

Sam: Yeah, youre right. The most seasoned people I talked to did their work in the 1950s and 60s. That was a really wild, unique period when almost unlimited funding for rocket chemistry was available. Unfettered by budgets and, in some cases, by what you might think of as common sense, rocket scientists during that era pursued some truly wild chemistry looking for better propellants. What was fascinating to me is that even though the scientists during that time made a lot of really important discoveries, very few of those molecules ended up being used in working rockets. Instead, those important discoveries have transformed multiple fields of chemistry.

Kerri: But were still gonna talk about the rockets, though, right?

Sam: Well definitely talk about rocket chemistry. Unfortunately, there arent that many scientists still living from the early part of that heavily funded era. But the ones still with us have some amazing stories. As you might expect, there were accidents. And they tested some fascinating substances. But what I also learned is that rocket chemistry isnt a done deal. It seems like a second stage of rocket research is now taking place, with scientists in the US, China, and elsewhere pushing into new areas of chemical space.

Kerri: Before we get into all of that, though, lets start with the basics: What exactly is rocket fuel, and how does it work?

Sam: Okay, so Chinese inventors made the first rockets in the 13th century, powered by gunpowder. But remember when I said modern rocket chemistry started in the late 1800s? That was thanks to Russian scientist Konstantin Tsiolkovsky. In 1896 he published a paper titled Exploration of Cosmic Space by Means of Reaction Devices, meaning chemical rockets. In it, Tsiolkovsky showed mathematically that gunpowder doesnt have enough energy to put a rocket into space. He proposed instead reacting liquid oxygen and liquid hydrogen as a propellant.

Kerri: Oxygen and hydrogenwhy those two?

Sam: Well, any propellant is going to need two basic components: a fuel and an oxidizer. In Tsiolkovskys proposal, the hydrogen is the fuel and the oxygen is the oxidizer. They react by combusting. Now, you can think of a really basic rocket as a chamber that controls the geometry of the reaction. As the reactants combust, the rocket shoots hot gas, the reaction products, out in one direction. That produces a force that pushes the rocket in the opposite direction. Thats Newtons third law for you physics nerds.

Now, back to Tsiolkovsky. He came up with an equation that can tell you if your rocket will make it to space. To be fair, other scientists also independently derived the same formula to describe propulsion, but scientists call it the Tsiolkovsky rocket equation because he was the one thinking explicitly about rockets going into orbit and beyond. And what that equation tells you is that, if you want to escape gravity, you want a chemical reaction that runs hot and generates low-weight products. High temperature means reactions that release a lot of energy. Combustion ticks that box.

Kerri: Okay, I see. And the product of the hydrogen and oxygen combustion reaction is water, which is a small molecule, low-weight. But why is it important to have low-molecular-weight products? Wouldnt more massive molecules push the rocket harder?

Sam: Actually, no. The way several rocket scientists explained it to me is that lighter, smaller products means the exhaust can be denser. And that means more force.

Kerri: Got it. So, that was more than 100 years ago. What are we using now?

Sam: So remember how I told you that a lot of the propellants that chemists tested during the 50s and 60s didnt make it into rockets? Well, this past August, when the US Air Force launched a GPS satellite into orbit, they used a rocket powered by, you guessed it, liquid hydrogen and liquid oxygen.

Kerri: Okay, so a century ago a scientist proposed using hydrogen and oxygen to propel rockets into space. And were still using those propellants? Thats our shortest episode ever.

Sam: Dont worry, theres still a lot of the story left to tell. First of all, not all rockets today run on those propellants. Those scientists
in the 50s and 60s did actually change rocket chemistry. To understand how rocket chemistry ended up where it started, we need to understand the things those chemists did, and what happened after.

I want to start with Fred Hawthorne. He might be the living person who best represents the arc of 20th century rocket chemistry. Hawthorne is the winner of a National Medal of Science and an inorganic chemistry expert. Hes 91, and some people call him Mr. Boron, which youll understand soon. He was a rocket chemist at the company Rohm and Haas when they were leaders in the propellant world. Later, he was a chemistry professor. But before all that, he was a kid with a chemistry set.

Fred Hawthorne: When I was about 12 years old, I got a chemistry set. A Gilbert chemistry set. And it fascinated me.

Sam: You can probably picture a Gilbert chemistry set. They came with test tubes and vials of all kinds of different chemicals. The sort of thing that could never be sold to kids today.

Fred Hawthorne: And I spent all my free time learning chemistry. I was just very drawn to it.

Kerri: Hang on. Let me do some quick math here. If hes 91 now, that means that when he was 12 it was like, what, 1940? So this is right around the beginning of World War II.

Sam: Yeah, and the timing is important. Two years before Fred was born, American physicist Robert H. Goddard launched the worlds first liquid rocket using liquid oxygen as an oxidizer and gasoline as a fuel. That set off a flurry of rocket research, in the US, Europe, and Russia. Fast forward a few years and the German Werhner von Braun started his rise to prominence and infamy in rocket science. He was a member of the Nazi party. Thousands of von Brauns V-2 rockets killed civilians in Allied European cities. Thousands more died in the concentration camps that built the rockets.

At the end of the war, von Braun surrendered to the Americans, who were keen to use his expertise in their own rocket program. In 1950, von Braun moved to the Armys Redstone Arsenal in Huntsville, Alabama, to lead the countrys rocket program.

Kerri: So this is what kicked off that unique period of rocket research in the US, when rocket scientists were just rolling in money. Where does Fred Hawthorne come in?

Sam: Right. Redstone is also where Fred ended up in 1954 after getting his PhD. He was working as a research chemist for the Rohm and Haas chemical company, which had its rocket labs on the Army base. The US military funded Rohm and Haass rocket research, and it was going all-out in pursuit of higher performing rockets, because the US didnt want to get beaten by Russia into space and in the nuclear missile race.

Fred Hawthorne: Money was not a big problem. Time was a problem, because we were competing with the Russians. So things were very crude and a little bit sloppy at first.

Sam: Redstone was a little frustrating for Fred as a scientist. He says there wasnt time or interest in understanding exactly what made a good propellant. People werent really interested in the fundamental chemistry.

Fred Hawthorne: They simply threw a lot of things together, had a lot of troubles and very few real successes.

Kerri: But you said at the start that Fred eventually did do some fundamental research that would change chemistry, even if it didnt necessarily change rocket science.

Sam: True. Fred would eventually work with compounds called carboranes, which are caged molecules made of carbon, boron, and hydrogen. But at first he was working with a propellant called petrin acrylate. And if youre listening to this episode to hear about rockets blowing up, youll want to hear Freds petrin acrylate story.

Credit: CEN

Petrin acrylate is a polymer with a hydrocarbon backbone and side chains sprouting from it that are made from esters of PETN, which is one of the molecules used in plastic explosives. And petrin acrylate is a solid propellant, so its not in a tank like liquid hydrogen or kerosene would be; its poured into the rocket and then hardens into a rubber. Fred describes petrin acrylate as a little twitchy. And the Army wanted a lot of this twitchy propellant for a test rocket. Six thousand pounds to be exact.

Fred Hawthorne: Thats about three tons of stuff. Its a hell of a lot of explosive material.

Sam: Fred and other Rohm and Haas chemists and engineers managed to build the rocket, and they set it up on the test range. Because they didnt want it to actually launch, Fred says they buried it in dirt, concrete, and anything else they could put on top of it. They also wired it up with instruments to learn more about how this new propellant performed, which made the test rocket a very valuable piece of equipment.

On the day of the launch, Fred and a couple dozen other people gathered on a grandstand about 300 meters from the rocket, excited to watch the test fire. The engineer who filled the rocket with propellant was sitting in front of Fred, and Fred asked how the propellant looked.

Fred Hawthorne: And he said, Well, its got a crack in it, but we filled the crack with epoxy, and that should be okay.

Sam: Fred means there was a crack in the surface of that rubber column of propellant. And as you might guess, it was not okay.

Fred Hawthorne: We started counting down. Five, four, three, two. And when we got down to zero, we were too far away to hear anything yet. And then we saw a shock wave coming through the grass and then that came through and hit us. So we got a pretty good ride out of that.

Sam: Despite the shock wave and supersonic rocks whizzing over the crowd, Fred says nobody was hurt. An office building about 500 feet away was destroyed, but it had been evacuated before the test. And about 50 cars in a nearby parking lot were crushed by falling concrete.

Even closer to the rocket was a trailer full of equipment collecting data from the instrumented rocket. Fred says it was shot through with holes, but somehow the two technicians inside were unhurt, and they managed to collect the data as well.

It turned out that instead of burning from the bottom up, the petrin acrylate had started burning up the surfaces of that crack in the propellant. The rocket wasnt designed to handle pressures of hot gases there, thus the explosion.

This wasnt the only petrin acrylate mishap at Redstone, and shortly after, Rohm and Haas decided to abandon the molecule, which was apparently just too twitchy to pursue further.

Kerri: I mean, if my research project exploded and threw a bunch of rocks and concrete at me, Id be inclined to abandon it, too. So this is when Fred switched to carboranes?

Sam: Yeah. So Rohm and Haas brought in a new director of chemistry research at Redstone, Warren Niederhauser. He set his chemists on two new lines of research. One targeted inorganic compounds, specifically boron. This was the group Fred was in charge of.

Fred Hawthorne: You see, boron is next to carbon in the periodic table. It ought to behave very much like carbon. There ought to be a corresponding chemistry there that is waiting to be developed. That was my thinking. And sure enough, it worked.

Sam: The US military had actually been investigating boron compounds as potential jet fuels because they burn about 50% hotter than hydrocarbons. But burning boron compounds also damages jet engines and produces toxic boron oxides. So Freds group got involved in carboranes, which were discovered by another group of rocket chemists. Remember, these are caged molecules made of carbon, boron, and hydrogen. These were more stable than the original boron compounds. Freds group figured out how to make acrylate esters of carboranes, among other compounds. And it was all slow-going at first. They were testing everything in small batches and they made their starting material, decaborane, from scratch. Decaborane has, you guessed it, 10 borons atoms in its caged structure.

Credit: CEN

Just to illustrate once again how this period in rocket science was fueled by extreme amounts of research funding and a
desire to compete with the Russians, lets go back to Fred. He says one day, he got invited to give a talk to a group of Air Force scientists studying solid rockets.

Fred Hawthorne: I talked about 20 minutes, and the guy said, Hold it, Ill be back. And he left the room. And he came back about 15 minutes later and said, Ive just given orders for you to receive a long tonthats 2,200 poundsof high-grade decaborane to be delivered to Rohm and Haas.

Sam: Before that, Fred says his team was spending about $10 a gram on decaborane. A long ton translates to almost 100,000 g, or $10 million worth of the stuff.

Kerri: I see what you mean. Money was really flying around back then. So did their investment in Fred pay off?

Sam: Well, in some ways, yeah. One of the carborane compounds he made burned 10 times as fast as petrin acrylate, the culprit in that spectacular test failure. And it was easier to handle, too. But it didnt end up delivering any more energy than petrin acrylate in their experiments.

But, after Fred left Rohm and Haas in 1962 and went into academia, he took boron chemistry to new heights. Fred figured out how to make metallic compounds with carborane ligands, complexes that have proven useful as chiral catalysts and radioactive markers for medical imaging. Hes also explored carborane derivatives that could be used to selectively target tumors with radiation therapy. Today, hes an emeritus professor at the University of Missouri.

I asked him if its fair to say that propellant chemistry is one of the reasons boron chemistry developed the way it did.

Fred Hawthorne: Yeah. Oh yeah.

Kerri: Hence the nickname Mr. Boron.

Sam: Right. Its a similar story for other rocket chemists pushing the envelope at that time. Emil Lawton was a contemporary of Freds. Emil worked on fluorine chemistry at Rocketdyne, a rocket engine company in Southern California. He made a whole bunch of fluorine compounds, including wild molecules like chlorine pentafluoride and oxychlorine trifluoride.

Kerri: Whats so wild about those?

Sam: These interhalogen compounds are incredibly strong oxidizers. Theyre known for combusting with basically anything they touch. Chlorine trifluoride, a tetrahedral molecule made of a chlorine atom and three fluorines, is maybe the most reactive of the bunch. Its hypergolicmeaning it ignites on contactwith wood, cloth, and most metals, but also with sand, asbestos, and even water.

But, like Fred, Emil told us these fluorine compounds were dead ends, at least for rocket chemistry. I asked him if any of his molecules ever made it into rockets.

Emil Lawton: Surprisingly, none of my molecules did.

Kerri: Did Emil say why not?

Sam: Emils group never scaled up their fluorine reactions because the compounds were too reactive to be practical. He says people were initially interested in these molecules because they had high performance for their weight, which meant rockets that could carry more payload for their size. But engineers found ways to miniaturize electronics and guidance components, and those weight savings made it so that rockets didnt need the dangerous fluorine propellants. Emil was done with halogens, but he did stay in rocket chemistry, and later in his career would help the military investigate rocket accidents.

Kerri: Okay, so the chemistry Fred and Emil worked on didnt end up in todays rockets. But . . . something did. So what did end up taking off?

Sam: Well, to answer that, I have to tell you about what happened later on, after this period of lavish spending we just talked about. Rocket chemistry entered a sort of dark ages. The sense of urgency was gone, and so was the funding support.

Kerri: Wow. So what happened to bring about the dark ages of rocket chemistry?

Sam: Ill tell you. But after the break. Well hear about that plateau, and where rocket chemistry went next. Stay tuned.

Giuliana Viglione: Hi there. This is Giuliana Viglione, C&ENs editorial fellow. I hope youre enjoying this explosive episode as much as I am. We at C&EN work hard to bring you the very best stories on Stereo Chemistry. And we wanted to take this opportunity to ask for your feedback. What do you like? What can we do better?

There are a bunch of ways you can let us know. If youre listening to this episode on Apple Podcasts, you can leave a review or a rating without even leaving the app. That helps other chemistry enthusiasts find this podcast and it will help us make this show better for you and all our future listeners. And you can always email cen_multimedia@acs.org with your feedback. Have an idea for a chemistry story youd like to hear? Let us know!

Thanks to everyone who has rated Stereo Chemistry already. Your support means a lot to us, and were excited to bring you even more captivating stories from the world of chemistry in the coming months.

And now, back to the show.

Kerri: Im on the edge of my seat here, Sam. What happened to put the brakes on rocket chemistry?

Sam: The scientists I talked to had a lot of ideas about why rocket chemistry research lost some speed. Money and politics definitely played a role. Rocket scientists in the 50s and 60s were awash in government cash and racing with the Soviets to build rockets that could reach the moon or deliver nuclear warheads across the globe. After the moon was in reach, and the Vietnam War sapped Americas interest in military adventurism, and the Cold War was growing stale, the political will and financial support for exotic rocket chemistry research started to dry up.

Kerri: So the funds are gone, political support has tanked. How did rocket research continue? I mean, it didnt stop completely, right?

Sam: It didnt stop, but it was definitely slower going than the 50s and 60s had been. One thing researchers had to do was get creative with their projects. They worked on the same propellant molecules but showed that those molecules had other uses as well. Karl Christies research program went this direction.

Hes now a professor at the University of Southern California. Emil hired Karl at Rocketdyne in 1967, and Karl spent almost three decades there working with halogen compounds. Later, at the Air Force Rocket Propulsion Lab, he made polynitrogen molecules. He did a lot of really wild chemistry, probably as much as Fred or Emil. Early in his career it was halogen compounds, like chlorofluoro compounds under Emil. Later, he was the first to make stable polynitrogen compounds, including pentazenium, a five-nitrogen cation.

Kerri: That sounds like a lot of nitrogens.

Sam: Yeah, which makes it super energetic. Nitrogen-containing compounds are popular propellants and explosives, because the conversion of nitrogen-nitrogen single bonds in those molecules to nitrogen-nitrogen triple bonds in molecular nitrogen gas is incredibly exothermic. In pentazenium, resonance structures make the molecule more stable than it might seem at first glance, which makes it a useful propellant. Still, even though he was able to work his way toward such an interesting new molecule, Karl is very aware that the conditions of propellant chemistry had changed. He was at Rocketdyne when winter arrived for rocket chemistry.

Karl Christe: I mean you could not get any support; after the Apollo program there was zero money for new rocket propellants.

Sam: To give you a sense of how sad this period must have been for chemists, Karl rattled off a list of exotic chemicals that were unceremoniously destroyed at the Air Forces rocket propulsion lab, where he worked for a decade after Rocketdyne, because no one was going to use them: Ten thousand pounds of pentaborane set on fire with bullets fired into the tanks. They destroyed all their chlorine trifluoride, too, and apparently the worlds supply of oxygen difluoride as well.

Karl Christe: So it gives you a good idea, you know, that people are not going to use it very much anymore.

Sam: Cost was a factor here, according to Karl. The Apollo program proved you could get to the moon o
n a combination of hydrogen, oxygen, and jet fuel, all of which are cheaper than those exotic chemicals. And actually, hydrogen is pretty expensive, too, so these days rockets like the Russian Soyuz and SpaceXs Falcon 9 just use oxygen and jet fuel.

Kerri: And so you mentioned Karl had to get creative to keep working on propellants? How did he do that?

Sam: Karls nitrogen and fluorine research program at Rocketdyne survived because his team transitioned to chemical lasers. These basically convert the chemical energy of a propellant into laser light. The lasers were meant to fly on huge jets or ride on trucks and shoot down incoming missiles. Other rocket chemists of Karls generation had similar stories.

Kerri: Okay, so rocket propellant research continued, although slowly. So what are we using today, besides the liquid hydrogen and oxygen that launched that satellite you mentioned earlier.

Originally posted here:
Podcast: When rocket chemistry blasted off and came back to Earth - Chemical & Engineering News