New F-1B rocket engine upgrades Apollo-era design with 1.8M lbs of thrust

Dynetics and Pratt Whitney Rocketdyne rebuild the F-1 for the "Pyrios" booster.

by Lee Hutchinson - Apr 14 2013, 9:00pm EDT

• Space

http://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/

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?

Enlarge / The chart Dynetics had on hand at the gas generator test, showing major differences between the F-1 and the proposed F-1B.

Lee Hutchinson/NASA

"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."

Enlarge / A small model of the proposed F-1B design, on display at the gas generator test firing. Visible in foreground and middle are Pyrios stickers with logo. I grabbed a bunch of these.

Lee Hutchinson

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."

Enlarge / A real F-1 engine firing, from 1960. The dark jet emerging directly from the nozzle is the fuel-rich turbopump exhaust, which protects the nozzle extension from the heat of the actual rocket exhaust.

WikiMedia Commons

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.

Enlarge / Detail on the upper thrust chamber of an F-1 engine. Note tightly packed series of tubes, bound together with barrel-like hoops.

Lee Hutchinson

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.

Hydrodynamics, simplified

Another significant difference would be the incorporation of modern electronics into the engine's ignition and firing sequence. The F-1 employed an almost Rube Goldbergian system of valves and pressure checking, using what NASA engineer Nick Case referred to as "fluid mechanic logic" to get the engine firing. The flow of propellant or gas through various orifices and passageways in the engine created a cascade of conditions, tripping a valve here and closing a switch there, each building on the one before it. These reactions culminated in fuel flowing into the combustion chamber and the ignition of a hypergolic charge to actually get the fuel burning. Any missed step stops the cascade and the engine fails to ignite.
It was an ingenious system which substituted mechanical control for something that today we would do entirely in software. The F-1B would do just that—rather than relying on purely mechanical processes like a big game of Mouse Trap, the F-1B would use modern sensors and software in its start-up sequence. This will provide not just additional failsafes, but also will give the launch controllers much more visibility into exactly how the engine is operating during start-up.

Why kerosene matters

I've been saying "kerosene," but NASA doesn't tank up rockets with the same stuff you put in a portable stove. They use RP-1, a highly refined version of kerosene suitable for use in, well, rockets.
We asked R.H. Coates, lead propulsion engineer for NASA's SLS Advanced Development Office—as genuine a rocket scientist as it's possible to be!—to give us a pocket sketch of exactly why RP-1 is a good choice to use when lifting a rocket off the ground, and why at the same time it's not the best fuel to use for a rocket's entire flight.
"Refined petroleum is not the most efficient thrust-producing fuel for rockets, but what it lacks in thrust production it makes up for in density. It takes less volume of RP-1 to impart the same thrust force on a vehicle, and less volume equates to reduced stage size," he explained. "A smaller booster stage means much less aerodynamic drag as the vehicle lifts off from near sea-level and accelerates up through the more dense (thicker) part of the atmosphere near the earth. The result of a smaller booster stage is it allows a more efficient ascent through the thickest part of the atmosphere which helps improve the net mass lifted to orbit."
The fact that a rocket starts out at or near sea level and has to claw its way up out of the Earth's gravity well and push through a lot of air means that it's good to burn the dense fuel lower in the atmosphere, so you don't have to expend as much energy lifting it up higher before burning it. Engineers must make a trade-off between a dense high-thrust fuel like RP-1 and a less dense but more efficient, longer-burning fuel—something like liquid hydrogen.
"The most efficient fuel and oxidizer combination commonly used today for chemical liquid rockets is hydrogen (fuel) and oxygen (oxidizer)," continued Coates. The two elements are relatively simple and they burn easily when combined—and even better, the result of their reaction is simple water.
The measure of a rocket's fuel efficiency is called its specific impulse (abbreviated as "ISP"—or more properly I_sp), to which Doring was referring earlier. Wikipedia has an equation-heavy explanation if that's your thing, but Coates broke it down much more clearly for us: "Mass specific impulse...describes the thrust-producing efficiency of a chemical reaction and it is most easily thought of as the amount of thrust force produced by each pound (mass) of fuel and oxidizer propellant burned in a unit of time. It is kind of like a measure of miles per gallon (mpg) for rockets."

Enlarge / The gas generator firing. Visible emerging from the nozzle is the dark, fuel-rich exhaust, which takes a bit of time to completely burn. This is characteristic of gas generator exhausts.

Lee Hutchinson

He continued: "The specific impulse [of the liquid hydrogen and liquid oxygen-powered RS-25 Space Shuttle Main Engines] at sea-level (lift off) conditions is slightly over 365 seconds...the engine produces about 365 pounds of thrust at sea-level for each pound of hydrogen and oxygen burned together each second. The gee-whiz part is the engine burns propellant at an extremely large rate, just under 1100 pounds (over half a ton) each second, so each engine produces around 400,000 pounds thrust (force) at sea-level."
The numbers are different with RP-1. "The best demonstrated I_sp performance for hydrogen is almost 365 seconds and kerosene is 311 seconds," he went on. "So, if we were to design our two engines to the same thrust level we would see that the hydrogen fueled engine is about 17% (1.17 times) better at producing thrust per unit of mass flow into the engine. That means if both engine cycles were sized for the same thrust, the more efficient hydrogen engine would use 17% less mass in propellants to push on the vehicle with the same force."
This brings us back to the question of density versus efficiency. A 17% bump in efficiency and decrease in mass is a big deal—individual kilograms count when dealing with rockets. But that more efficient fuel takes up a lot more space, and Coats outlined that trade-off very clearly. "Liquid hydrogen has a density of about 4.3 pounds per cubic foot, or to put it differently, each gallon of liquid hydrogen only weighs a little over a half pound. A gallon of water weighs in at about 8.3 pounds. Kerosene fuel is much more dense than hydrogen at about 50 pounds per cubic foot or just over 6.7 pounds per gallon. The kerosene fuel is well over 1100% (11 times) more dense than hydrogen fuel."
So, why different fuels in different stages, with RP-1 kicking off the launch? "In the final comparison of kerosene and hydrogen fuel on a similar type engine at the same thrust," finished Coates, "although the kerosene-fueled system is less efficient on a mass basis, meaning more propellant must be loaded on the booster stage to deliver the same thrust impulse, however, the kerosene fuel is so overwhelmingly more dense that the net result is a smaller fuel tank (and boost stage) for the vehicle."
The most popular efficient fuel choice for rockets is liquid hydrogen (LH), which is a light and long-burning propellant. A LOX/LH engine (like the J-2s which powered Saturn V's upper stages) is more efficient than a LOX/RP-1 engine, meaning it burns longer and converts more of the fuel directly into propulsive energy. However, there's a cost: liquid hydrogen is less dense than RP-1. A kilogram of liquid hydrogen occupies more volume than a kilogram of RP-1. This impacts the dimensions of the rocket and the physical size of its tanks.
So, for that initial big push off the ground, a high thrust kick is needed, and that's the role of the LOX/RP F-1 engines in the Saturn V (and, similarly, the solid boosters in the Space Shuttle). The heavy fuel is burned to carry up the more efficient fuel to where it can be used most effectively. After the rocket has been hammered up through a big chunk of the atmosphere and given a good start, the high-thrust stage is dumped and the more efficient, higher ISP engines take over to pour on the majority of the orbital speed.
These lessons apply to today's launch vehicles just as they did in the past. The SLS rocket currently in the design phase will have its "core" stage built by Boeing, which intends to use the same RS-25 engines that powered the space shuttle. The choice of RS-25s is a practical one: there's a not-insignificant inventory of RS-25s in storage, and adapting them for use in SLS means a potential large cost savings. But while the RS-25s are powerful, they are high-ISP engines without the necessary thrust to get the vehicle moving, and so SLS will also use a pair of high-thrust strap-on boosters, like the Space Shuttle before it.

Piggy back testing

I also wanted to know whether or not letting Dynetics/PWR use MSFC facilities and equipment for testing represented a conflict of interest for NASA. After all, they're the customers in this competition—I was curious as to whether letting one of the competitors use NASA's gear was a normal thing.
Janet Felts, the Dynetics media relations specialist who helped organize the interview, assured me that contractors using NASA facilities for testing is a regular occurrence, regardless of the contract. "We are actually using NASA Marshall, NASA Stennis, and a small amount of NASA Langley as subcontractors. We have letters of agreement with NASA, and we are paying NASA Marshall to do our series of gas generator testing." She continued, "It's a really good example of where I believe industry and government need to head. NASA has assets that industry doesn't need to recreate...they've got state of the art testing facilities, and the ability for us to come and purchase that capability from NASA and really work as a team is a way to make all of this affordable."
Chris Kelsey, the NASA project manager overseeing Dynetics for the contract, agreed. "[Space Shuttle solid rocket booster manufacturer] ATK was also awarded an Advanced Booster research and development contract for an advanced solid booster. Certainly there are some facilities they requested to use....Any NASA or government assets that they felt would help them, it becomes part of the contract.

But will it ever fly?

And so we come back to the Advanced Booster Competition, and the F-1B. Even though solid-fuel boosters are the front-runner in the contest, Dynetics and their PWR subcontractor are hopeful that they can demonstrate an alternative with their F-1B-powered booster that doesn't just meet the terms of the competition, but also demonstrates cost savings and practicality.
It is also impossible to deny the romance of the F-1B. In spite of its underlying military and political genesis, Project Apollo is unquestionably the greatest engineering achievement in the history of humanity. For five years, between December of 1968 and December of 1972, we—not NASA, not America, but we, us, humans—left our planet behind and actually went somewhere else. In view of the overall size of the solar system it was the equivalent of a walk across the back yard, but it was an incredibly expensive trip that required billions of dollars and uncountable hours—and a not-inconsiderable number of lives. Those trips were each made on the backs of five howling, monstrous F-1 engines, and the idea of seeing them come back to life pushing a rocket even bigger than the Saturn V stirs the heart.
The question, though, is whether or not the practical side of the equation can balance the romantic. The Advanced Booster competition will run through 2015, at which point a winner will be chosen, solid or liquid. The F-1B could be the engine sending astronauts to Mars—or it could wind up as one more Wikipedia footnote.

Want to know more about the Marshall team behind the F-1's rebirth? Check out our feature, How NASA brought the monstrous F-1 'moon rocket' engine back to life. And we've also got a 40-picture photo gallery from my tour of Marshall right here.