We usually think of a hydraulic system as fully self-contained, with a hydraulic pump, tubing, and actuators filled with a working fluid. This of course adds a lot of weight and complexity that can be undesirable in certain projects, with the Saturn V Moon rocket demonstrating a solution to this which is still being used to this day. In a blast-from-the-past, a December 1963 article originally published in Hydraulics & Pneumatics details the kerosene-based hydraulics (fueldraulics) system for the S-1C stage’s gimbal system that controlled the four outer engines.
Rather than a high-pressure, MIL-H-5606 hydraulic oil-based closed loop as in the Saturn I, this takes kerosene from the high-pressure side of the F1 rocket engine’s turbopump and uses it in a single-pass system. This cuts out a separate hydraulic pump, a hydraulic reservoir, which was mostly beneficial in terms of reducing points of failure (and leaks), ergo increasing reliability. Such was the theory at the time at least, and due to issues with RP-1 kerosene’s relatively low flash point and differences in lubricity properties, ultimately RJ-1, RP-1 and MIL-H-5606 were used during checkout leading up to the launch.
In hindsight we know that this fueldraulic system worked as intended with all Saturn V launches, and today it’s still used across a range of aircraft in mostly jet engines and actuators elsewhere of the Boeing 777 as well as the F-35. In the case of the latter it only made the news when there was an issue that grounded these jets due to badly crimped lines. Since fueldraulics tends to be lower pressure, this might be considered a benefit in such cases too, as anyone who has ever experienced a hydraulic line failure can attest to.
Featured image: Gimbal systems proposed for the F-1, oxygen-kerosene engine with a fueldraulic system. (Source: Hydraulics & Pneumatics, 1963)
Kerosene! How could they!
-t. Reader wholly ignorant about hydraulic systems
The way I see it, if you’re sat on that much hydrogen and oxygen already, you use the best solution.
If there’s a fire, a little kerosene is going to be the least of your worries.
They didn’t add any kerosene, the Saturn V was fueled by kerosene and liquid oxygen already so they just used the fuel as hydraulic fluid.
This is a great example of the K.I.S.S principle.
Sene but not forgotten?
AFAIK SpaceX rockets also use fuel for hydraulics. Simple and effective solution, no need to invent a new one. Though eventually electronic actuators may displace hydraulics in many applications.
Hydraulics have significant advantages over electronic actuators. In order to do the same work, electronic actuators need to be many times larger and made with much stronger materials than standard run of the mill motors. The amplification effect brought on by Pascal principle when using hydraulics is why a small cylinder can raise a multi-ton vehicle. Try that with electronic actuators!
+1 ….. hydraulics is used for this reason.
Roller screw actuators give them a good run for the money. Hydraulics can be still built with higher load capacities for a single unit, but that’s about it. The main problem with hydraulics is the low efficiency: typically more than half the power is just wasted on churning oil around the hoses.
For space (at least the launch phase) you have enormous amounts of power to waste and NO mass budget to waste. Different priorities than many kinds of engineering
Hydraulics aren’t as inefficent as you might think. Only the most simple and old systems “churn” oil uselessly.
Most newer systems that use proportional control valves and have a decent hydraulic power unit are smarter than that and don’t run at full bore while expecting a pressure regulator to do all the work.
Electric crew actuators and worm drives however are very inefficent. Even more so for when an actuator needs to hold a position. Any electric actuator that can be backdriven will have to be in a locked rotor condition acting like an electric heater.
If you only count the end actuator and not the pumps and pipes leading up to it. If you count the entire system, electric actuators beat hydraulics on power density, efficiency and size/weight.
Not mass when you don’t use pumps and instead have an open-cycle pressurized system that would have already been on the vehicle anyway
or if you use the fuel which is already pressurized by the fuel pump that needed anyway
You’re trading control valves, tubing and hydraulic fluid for copper, power electronics and motor drivers….also a ton of permanent magnets and silicon steel.
All of those things are both costly and usually very temperature sensitive on the electric motor end of things. They also are a lot harder to source than materials needed for fluid power systems.
A better comparison is the hydraulic cylinders + control valves versus motors + motor-controllers.
The power density you’re talking about is also usually worse because you will inevitably end up needing to add in gearboxes to be able to produce similar amounts of force….which only further reduces efficiency.
It’s okay if you just don’t like working with fluid power systems because they really do have some downsides and safety concerns that are valid. But most of the comments I’ve seen you post on this article don’t match up with my experiences working with both fluid power and electric systems and/or just straight up are factually incorrect.
Well yeah, because SpaceX learned all its tricks from NASA and JPL. All the work was largely done for them.
Like landing boosters?
Yes. DC-X and DC-XA, 1996. They did the whole vertical takeoff and soft landing under rocket power, and could be re-fueled for flight in 26 hours.
Too bad they gave up after DC-X fell over.
Don’t forget “caught the booster on pins.” DC, much as I like it, didn’t do that.
Because flying a 12 meter, 20-ton demo to 10k feet and landing it is just as hard as landing a 20-story, 200-ton skyscraper from 60km+…
Seriously, SpaceX certainly benefitted from earlier research, but saying “it’s all been done before” is extremely disingenuous, and I would consider it offensive to the engineers who just caught said 200-ton skyscraper with a pair of mechanical arms.
Oh wait, had that been done before, too?
They demonstrated all the moves and showed it was possible, and how to do it. The problem is the same. Someone had to write down the math and figure out the parameters to apply it to the small scale model, which SpaceX then got and scaled it up.
Or do you think NASA just made a rocket and flew it by the seat of their pants?
That’s a genuine first. However, it’s not such a big leap from landing on legs to landing on supports. In fact it’s a bit of a backwards step. The main problem is the pinpoint landing and guidance back to the landing site, which had been done before. If you can land on a moving barge on the sea, you can land into a stationary tower.
The landing legs are a technological challenge and a weak point in the system that risk failures and toppling over; grabbing the rocket from up high is a more stable position and makes the landing easier, and you don’t have to include the landing legs because the grabbers are in the tower.
So, they kinda just made things easier for themselves, but without the landing legs you can no longer land on a barge down the range, so you can’t lift heavier cargo or reach higher orbits because you always need enough fuel left over to get back to base.
It landed on legs first and then fell over after multiple flights. SpaceX did it the other way around when they started.
The reason they gave up was because NASA leadership was jealous that their expensive pet space plane project wasn’t going anywhere while the DC-X project started leap-frogging ahead with a tiny budget, so they cut the budget and refused to repair it. In a sense they weren’t wrong, since a VTOL rocket only solves one problem – returning the rocket – while lacking in other properties such as having a smaller payload fraction and not being able to return anything from space, and NASA wanted a new shuttle instead.
grabbing the rocket from high up also means it is mostly in tension where it stronger so you can make it lighter
Isn’t that how NASA is supposed to function?
Yes and no…
In the past, NASA paid private companies to do the research and development for new technology. This time around NASA just handed them the technology and said “make more of this, but bigger”.
Yes they did not have to reinvent spaceflight, kind of how NASA and JPL learned from Ze Germans. Still the incremental improvements they made were better than NASA has made in half a century
What good would any of us be if we didn’t learn from the work AND mistakes of others???
Pretty sure the Falcon 9 uses hydraulics (an early crash was caused by running out of fluid while using an open, no return system). The first Starship booster had hydraulic controls but they later switched to electro-mechanical control for gimbaling the engines. Since both the booster and orbiter run on liquid methane and oxygen fueldraulics wasn’t an option.
Full flight simulators have also been switching to electronic actuators versus hydraulics.
Yeah pretty sure it’s the usual (for launch vehicles) open-cycle system of incompressible fuel pumped through the system by pressurized gas used to full up the remainder of the fuel tank. Iirc that one Falcon crash was because of the pressurant gas running out prematurely.
Tsiolkovsky sez: Every gram counts!
There exists a YouTube video of the guy who designed the SR71 engines talking about the hydraulic computer and the benefits such as surviving and thriving in super high temperature environments and being purely mechanical with check valves and stuff. Amazing.
Taking apart an old cooked automatic transmission can be a great learning opportunity and inspiration. The hydraulic “computer” section with its labyrinth of oil passages, solenoids, cylinders, clutches et cetera is a delight to reverse-engineer
It might be overly picky to ask if Apollo 1 counts as “a launch”.
The Apollo 1 capsule and it’s tragic fire has nothing to do with the ten successful Saturn 1 launches.
European cars through the 80s used their Gasoline in a hydraulic metering circuit. It balanced a vane in the intake to measure the air entering the engine. It ran at 75PSI and the electric Bosch fuel pump was capable of 500hp or more on an engine using more common 35-40PSI electronic fuel injectors.
These were the last good German cars!
Despite having to bleed them if you ran out of gas.
In retrospect it makes a lot of sense If you already have a high volume, high pressure pump and lots of liquid to also reuse that for some hydraulic actuators.
An additional factor is that longevity / endurance is not a big issue. “Earth bound” hydraulic systems (bucket excavators, bench press, etc) need to work reliable over many years and hundreds of thousands of cycles, while when used in rockets, it’s a limited amount of movements in a few minutes. This greatly reduces the demands on the lubricity of the fluid.
I worked on the modern day example of this system. It does save mass and reduce part count which is great in a rocket, but it has some interesting disadvantages. The main fuel tank of a rocket is not as easy to keep clean as a small closed hydraulic system. Hundreds of people walk in that fuel tank during fabrication and ground ops. All sorts of debris is created during the build process that is ideally cleaned out, but perfection is difficult to achieve. The fluid cleanliness can be a really big deal to the precision servo valves that control the steering actuators, and can be responsible for all sorts of valve issues. The other issue with Kerosene or RP-1 as a hydraulic fluid is that it lacks the lubricity that standard oils have. Parts wear out faster than they do with hydraulic fluid and can get jammed more easily. Poor lubrication coupled with potential dirt in the fluid can have huge consequences for hydraulic components.
My dad machined parts that went on the v 5 rocket motors, for all units. They only made so many and he machined every one required. Until they closed the order out. That man taught me my mechanic skills. Also, I had a customer that was a rocket scientist and his guru in college was Niel Armstrong. All of his rocketry problem solving skills were instructed by The Man that walked on the moon. And his rolls Royce was display of that working knowledge,lol
From my skills to my income the world of space travel has continued to enrich my life