Castor 4B: The Engine for Our Ride to Space
As we near the launch date, I wanted to write a bit about the rocket, specifically, the rocket motor, that is going to lift our experiment to space. I have not been able to get in close proximity with the rocket yet, but on the flight over to Sweden, I re-read the technical papers on its history and engineering development.
The motor used on MAXUS 9 is the Castor 4B, produced by Orbital ATK (previously, Thiokol) in the US. The story of its development is going to take some space, but crosses paths with some thought-provoking history and intriguing personalities.
In 1926, Joseph C. Patrick, a Kansas City, Missouri, doctor, was finding he enjoyed odd jobs involving chemistry more than he did practicing medicine. While trying to find ways to turn ethylene, a byproduct of cracking petroleum, into ethyl glycol (antifreeze), Patrick discovered a reaction between sodium polysulfide and ethylene dichloride that resulted in a rubbery solid that had excellent chemical resistance, making him the first person in the US to develop a synthetic rubber. By 1928, he had started a company and, with some knowledge of Greek, decided to call it “Theion Kolla” (“sulfur glue”), to become “Thiocol” and eventually “Thiokol.” His company limped along through the Depression years, moving from Kansas to New Jersey, finding niche applications of his rubber in gaskets, coatings, and sealants. Continuously trying to improve his product, in 1942 Patrick and his employees developed a way to make synthetic rubber that did not involve a volatile solvent (his original product did, and as a result smelled terrible), which opened up enormous applications for the rubber. The timing coincided with the massive World War II mobilization, and Patrick found his synthetic rubber products increasingly in demand for sealing fuselages, fuel tanks, gun turrets, and so on.
The first application of Thiokol’s products to rocket propulsion came from the storied GALCIT lab at CalTech under the direction of Theodore von Kármán. Some of von Kármán’s graduate students were, following Robert Goddard, the first people attempting to develop liquid-fueled rockets in the US, and after some accidents in the lab, were banished to an empty plateau in the hills behind Caltech to become the Jet Propulsion Laboratory (better known today as NASA JPL, which alas no longer does propulsion research but operates most of NASA’s robotic spacecraft exploring the solar system).
While mostly working on liquid propellant rockets, one of von Kármán’s “suicide squad”, John Whiteside “Jack” Parsons, got the idea for a new kind of solid propellant, reportedly while watching a roof being tarred. Rather than a solid propellant made by pressing powders together, which is how the Chinese and everyone since had been doing it for centuries, Parsons’ concept was a propellant that could be comprised of a tar or rubber-like fuel with particles of oxidizer trapped inside. Parsons himself is an utterly mesmerizing character who is the subject of biographies in his own right, but the story here will remain on the rubber that the JPL team hit upon to realize Parsons’ idea: Thiokol’s polysulfide-based synthetic rubber. During the War, JPL developed the earliest versions of JATO units (Jet-Assisted Take Off, with “jet” in those days meaning “rocket”) that would be strapped onto aircraft and fired on take-off to deliver a powerful kick, allowing planes to use shorter runways. Appreciating this could become a huge product during the post-War era when jet aircraft, such as the B-47 bomber, required fifty disposable JATO “bottles” for each takeoff, Thiokol began their own rocket propellant development work, initially in Elkton, New Jersey, in 1948, but a year later transferred their rocket propellant operations to Redstone Arsenal in Huntsville, Alabama, at the insistence of the U.S. Army.
JPL with US military collaboration continued to work on successively bigger rockets under the naming scheme “Private,” “Corporal,” “Sergeant,” that was an inside joke of von Kármán’s, but ran into trouble with the large solid-fueled Sargent, and in 1954 Thiokol stepped in to help with the propellant. The Sergeant became the first solid-fueled, short-ranged surface-to-surface missile to be armed with an atomic bomb warhead at the end of the 1950s. The next ten years saw incredible growth of Thiokol, from a company with $1 million in sales in 1948 to nearly $100 million in sales in 1958, almost entirely fueled by Cold War tensions. It becomes nearly impossible to separate the development of nuclear-tipped Intercontinental Ballistic Missiles (ICBMs) from the development of rockets for space launch during this period, their history being just too entwined.
The biggest project Thiokol undertook during this period was the development of the Minuteman, an ICBM that — due to its use of stable, storable solid propellants — could be readied to fire from a silo in 60 seconds, and hence the name. Solid-fueled ICBMs had this advantage over liquid-fueled ICBMs (such as the Atlas and Titan), which required considerably longer to be readied for launch, and this advantage was largely enabled by Thiokol’s expertise in propellant formulation and casting, a technology the rival Soviet Union would only master later. The Minuteman, after several generations now called the Minuteman III, remains the only land-based ICBM in the US today. To realize the massive Minuteman production line, around 1960 Thiokol set up a new operation in Utah, the state that the company is most associated with today.
The Castor was developed at this same time, deriving from the Sergeant, as the second stage of Scout, a four stage, entirely solid-fueled rocket commissioned by the brand-new NASA in 1959 for launching small satellites into orbit. The first stage was called Algol, and the third and fourth stages Antares and Altair, all names of stars. Scout was a reliable little launch vehicle, retired in the mid-1990s.
The other early spaceflight application of the Castor was in Little Joe, a booster quickly thrown together to test reentry of the Project Mercury capsule prior to the first flights of the Mercury astronauts.
The main use of later generations of the Castor family, however, have been as “strap-on” boosters that provide the initial thrust to lift other (usually, liquid-fueled) rockets. Use of Castor strap-ons allowed the Thor medium range ballistic missile to be upgraded to the Delta launch vehicle, successive generations of which went from using two Castors to three, then six, and eventually as many as nine solid rockets attached around a liquid-fueled core, increasing the total payload mass the Delta could loft into orbit and making it a workhorse launch vehicle that still delivers satellites to orbit today. More than two thousand Castors have flown in space-launch-related missions, with a nearly flawless record of reliability.
The Castor 4B is the specific model that will launch MAXUS 9. It is a 30-ft-long (10-m-long) steel cylinder that is 40 inches in diameter (about a meter) with a tenth of an inch wall thickness. Inside is 22,000 pounds of propellant. The propellant is a hydroxyl-terminated polybutadiene (HTPB) binder, an elastomer that reflects Thiokol’s continued advances in synthetics during the 1960s, that has trapped inside it particles of ammonium perchlorate (AP, NH4ClO4) and 20% aluminum powder. The AP, in the form of white, powdery crystals, is a nasty, strong oxidizer that is also a touch toxic. It provides the oxygen to burn the aluminum powder and the HTPB binder (itself, not a bad fuel). The use of aluminum as fuel is particularly fitting for our McGill team, since, as I will discuss in the coming days, the study of aluminum as a fuel is what started our research group down the path that led us to the MAXUS 9 experiment. Aluminum, when burned with an oxidizer, releases more heat on a volumetric basis than any hydrocarbon fuel. The real advantage here is its density: It releases about twice as much energy as an equal volume of hydrocarbon fuel when burned with oxidizer. While not as impressive on an energy-per-mass basis, if you are interested in the amount of energy you can store in a given volume, aluminum is your fuel of choice. Volumetric energy density is usually what matters for propellant applications in missiles (that need to fit under the wing of a jet fighter, for example) and strap-on boosters.
Aluminum burns hot, at more than 3000 C/5500 F, at which it is blindingly bright white. The burning aluminum in the two Solid Rocket Boosters (SRBs) of the Space Shuttle, again courtesy of Thiokol, is what made Shuttle launches so visually impressive, and something we won’t see again for the foreseeable future, as most big boosters under development now (for example, Falcon Heavy) favor liquid hydrocarbon fuels that burn with a nearly invisible flame. This aspect of aluminum fuel was well illustrated in a memo we received from the project manager of the sounding rocket division of the Swedish Space Corporation that operates Esrange, shortly before we came here:
Dear all — one of the actions from the ESB concerns eye safety during the Maxus launch. As you are aware the Castor series motors have 20% Aluminium as part of their fuel. This means it burns very bright and anyone watching the flight should take precautions. It is recommended that anyone attending the flight have good quality sunglasses to watch the launch. Please can you distribute to your teams.
Our McGill team was tempted to all go and buy $400 pairs of Ray-Bans and charge them to the grant supporting the project, but then thought better of it. After all, we do this in the lab from time to time, and we’re used to having a purple blotch temporarily imprinted on our retinas (like you get from a photographic flash) from watching aluminum burn.
Inside the Castor 4B is a cylindrical hole running up the middle of the propellant, and near the exhaust end approaching the nozzle, slots that expand out from the hole. This geometry is used so that, as the propellant burns over its exposed surface and is consumed, the burning area remains more-or-less constant, providing constant thrust. Exactly how to realize this is part of the dark artistry that is solid propellant design, reflecting Thiokol’s’ decades of expertise forged out of the Cold War and Space Race. Most Castors that are used as strap-ons have canted nozzles, so the thrust is vectored both up and onto the core rocket, but our Castor on MAXUS 9 will be a straight nozzle that can be vectored to steer the flight of the rocket. It takes just over a minute to burn the 10 tons of propellant inside, during which time the thrust generated is almost five times the weight of the rocket motor. While carrying our MAXUS 9 payload, the acceleration of the entire vehicle will be a maximum of 15 gees (that is, 15 times Earth’s gravity). Designing the experimental apparatus to survive this load was one of the major challenges our colleagues at Airbus faced in building the PERWAVES hardware.
If you are interested in buying a Castor, you can download the catalogue from Orbital ATK here (when teaching rocket propulsion, I always enjoy giving an assignment wherein I send students online to spec out various rocket engine options, just like buying a car). In the 1980s, Thiokol was acquired by the Morton Salt company, and at the end of the Cold War went through a bewildering series of name changes due to corporate acquisitions and reshufflings, to become Orbital ATK Inc. The “ATK” is for Alliant Techsystems, not Thiokol, so the “sulfur glue” name might vanish for good. Just before posting this, I bumped into the team from Orbital ATK in the lobby of our dormitory here at the Esrange Space Center. The Orbital ATK team is here to oversee the preparation of the Castor 4B for today’s launch. “You guys from Thiokol?” I asked. They smiled warmly and didn’t correct me, so maybe the name will not be lost to history after all.
