NASA has spent a lot of time and money resurrecting the F-1 rocket engine that powered the Saturn V back in the 1960s and 1970s, and Ars recently spent a week at the Marshall Space Flight Center in Huntsville, Alabama, to get the inside scoop on how the effort came to be. But there's a very practical reason why NASA is putting old rocket parts up on a test stand and firing them off: its latest launch vehicle might be powered by engines that look, sound, and work a whole lot like the legendary F-1.
This new launch vehicle, known as the Space Launch System, or SLS, is currently taking shape on NASA drawing boards. However, as is its mandate, NASA won't be building the rocket itself—it will allow private industry to bid for the rights to build various components. One potential design wrinkle in SLS is that instead of using Space Shuttle-style solid rocket boosters, SLS could instead use liquid-fueled rocket motors, which would make it the United States' first human-rated rocket in more than 30 years not to use solid-fuel boosters.
The contest to suss this out is the Advanced Booster Competition, and one of the companies that has been down-selected as a final competitor is Huntsville-based Dynetics. Dynetics has partnered with Pratt Whitney Rocketdyne (designers of the Saturn V's F-1 engine, among others) to propose a liquid-fueled booster featuring an engine based heavily on the design of the famous F-1. The booster is tentatively named Pyrios, after one of the fiery horses that pulled the god Apollo's chariot; the engine is being called the F-1B.
The F-1B and how it differs
Ars was on-hand to observe one of the fiery F-1 gas generator tests in Huntsville, and after the test I was able to speak at length with the Dynetics/PWR folks about the engine. Dynetics had set up a display next to the test viewing area featuring a small model of the proposed F-1B rocket engine, along with a chart highlighting the differences between the F-1B and the F-1 and a small model of an SLS rocket with two Pyrios boosters hanging from its sides.
Available to answer my questions were Kim Doring and Andy Crocker, the program manager and assistant program manager for Dynetics' space launch systems group. What would the F-1B look like, I asked them?
"The first thing you'd notice is that it's large. It's just going to be a very, very large piece of machinery," explained Doring. "In the F-1, they needed every bit of performance they could get, and so they took the exhaust from the turbine and dumped it into the nozzle and got a little extra performance out of that. That made the engine a bit bigger...but when you look at the intricate way they had to build that, it was really, really difficult, and very expensive."
No more exhaust recycling
"One major difference that most people would notice right away is that...we've decided to do away with that turbine exhaust that feeds into the nozzle, and that part of the nozzle that comes after where the turbine exhaust manifold would dump in," Doring continued. The gas generator's rocket exhaust, which I'd just watched, was used to drive the fuel pump turbine, but then had to be directed somewhere; the exhaust manifold took those gasses and coated the inside of the thrust chamber with them. This turbine exhaust was still fuel-rich and so didn't burn as quickly as the more balanced fuel/oxidizer mixture being sprayed into the F-1's thrust chamber. The slower-burning turbine exhaust rolled down the inside of the nozzle, protecting it from the much hotter thrust reaction and keeping it cool. This dense, slower-burning exhaust is easily visible in the F-1's thrust pattern—it is the darker-colored plume exiting the nozzle for a short distance before the much brighter primary exhaust.
The turbine exhaust manifold is one of the F-1's most distinctive features—it branches off of the side of the nozzle and then wraps around the nozzle at approximately its visual midpoint. Doing away with it would change the look of the engine significantly. "So the chamber nozzle would be smaller—would look smaller even to the common person, even though it's still huge," he continued. "That specifically will save a lot of money and complexity in the way we're deciding to build the engine to address NASA's specific goals of affordability and performance."
"This will be somewhat different," finished Doring. "You'll see the hot exhaust coming out of a tube right next to the nozzle, and then you'll have the big nozzle plume coming out of the main nozzle."
Fortunately, the removal of the turbopump exhaust manifold and its complex series of ducts and baffles and tubes doesn't particularly compromise the engine's performance. Doring is quick to point out that even without ducting in the turbopump exhaust, the F-1B is being designed to have as much thrust as the uprated F-1A concept from the 1960s: about 1.8M lbs of thrust, with the goal of being able to loft 150MT of cargo into low Earth orbit with four engines on two boosters (coupled with the other RS-25 and J-2X engines in the SLS stack). There's also enough head-room in the overall booster design to add another 20MT of total lift capacity without requiring significant engineering changes, to meet other SLS design goals a bit down the road.
Dynetics and PWR are trying to hew as closely as possible to the operating characteristics of the old engine's uprated F-1A variant, which was extensively tested in the 1960s but never actually flown. The original hardware worked very well, and changes are only being made where it's necessary to cut costs. "The flow paths will be the same," as the F-1A, Doring elaborated when I asked for details. "The chamber pressure will be about the same, and the thrust will be about the same. It's about a 1.8 million pound thrust engine, and if you look at the F-1A specs, it's going to be about the same."
"This is even after ditching the recycling of the gas generator exhaust?" I asked.
"You lose very little thrust," confirmed Doring. "You lose a little bit of specific impulse, but you lose very little thrust. The booster flies for just a couple of minutes and drops off and then the vehicle flies on, so specific impulse matters very little."
No longer a series of tubes
Another clear difference is the construction of the exhaust nozzle itself. The F-1's nozzle was made up of two parts: the first portion was actually an extremely complex series of tubes brazed together and bound by hoops, like staves in a barrel. Kerosene fuel was circulated through the tubes to absorb heat and cool the exhaust. The tubes stretched down to the distinctive turbopump exhaust manifold, and then looped back up. Below the manifold, which wrapped around the engine like a pair of fingers, was a removable nozzle extension that focused the engine's combustion and helped the engine deliver additional thrust.
Advances in manufacturing techniques will allow the F-1B to dispense with the complicated upper nozzle tubing; as it's currently envisioned, the new rocket will feature a much simpler thrust chamber and nozzle made of steel—according to Andy Crocker of Dynetics, the nozzle will consist of an inner liner and outer jacket, brazed together, with cooling provided by fuel flowing through simple slots in the inner liner. This is far easier and less expensive to build than the labor intensive "barrel hoop" tube wall design of the original F-1.
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