7 ms·

Why do those rockets have a fuel filter at all? I get why we have them in cars; we refuel quite often, in dirty environments and with possibly low quality fuel.

Why do those rockets have a fuel filter at all? I get why we have them in cars; we refuel quite often, in dirty environments and with possibly low quality fuel. For space rockets, it seems that it would be (comparatively) easy to ensure that the tanks are clean and the fuel is of high quality. On the other hand, if there wasn't any dirt, the filters would not have clogged, so I guess it does make sense after all.

guhidalg 3y ago
Don’t forget about all the equipment needed to fill the tanks and transport the fuel to the tanks. Probably easier to assume contamination gets introduced somewhere and just slap an in-line filter.
- foxyv 3y ago
  Imagine if the engine failed from FOD. The reaction would be "Why don't they use filters on their fuel pumps?" Whenever you make an engineering decision there are trade offs like this.
Tuna-Fish 3y ago
Most rockets don't have fuel filters, but this requires their tanks to be kept in a much more strict state of cleanliness, and any failure to do so would result in mission failures. SpaceX has long had a very different idea about FOD than most other rocket companies, famously Merlin engine qualification testing contains ingesting stainless steel nuts.
- cduzz 3y ago
  That's bonkers. What size nuts do they pass through their engine? Every engine before it goes to space has to have a nut passed through the system? Like a 12mm "go into space" grade nut or a little itty bitty nut like I might find holding down a heat sink assembly in a 1998 vintage motherboard? I'm off to the googles to find videos...
  - HPsquared 3y ago
    Probably more "qualification of the design", like those bird ingestion tests they do on airliner engines. And the test probably doesn't look too interesting, the nut might not even come out the other end.
  - toss1 3y ago
    Let us know what you find. I find it not 'bonkers' but a great idea, and hope they are qualifying with the largest nuts found upstream, in 'go to space' grades. Robustness and anti-fragility may not be so critical for an expendable vehicle that operates once for something like 12 minutes, but seems a key attribute for a refuel-able and reusable spacecraft?
    - cduzz 3y ago
      Oh I agree it's not bonkers from the perspective of "wait you want to put people on that bomb and then explode it?" You can make all sorts of rules about what's not allowed to happen, but often those things happen. I'm just ... astonished if they've got a little practice room somewhere in their factory where they give each of their new rockets a nut to process... "Okay kid here's your graduation test!". A "we need screens to cope with big things and filters to deal with small things" is probably smart for any anything that's going to be reused. A nut's probably in some ways easier to deal with than a blob of wd40 (in your lox tank).
      - infogulch 3y ago
        Rockets: "Hey what if we strap people to a giant bomb and explode it juuust slowly enough that they're launched into space alive?"
        avmich 3y ago
        Steam engines: "No problems, we're doing that already for centuries, just with less speed and mostly horizontally."
      - adastra22 3y ago
- elteto 3y ago
  I can guarantee you that “ingesting a nut” through the engine is not part of the official engine qual program. Maybe it happened once and somehow the engine survived and they were able to ascertain what had happened. But this is not a common thing. Edit: I will say that what actually is a test commonly done (also in industry in general) is doing a “roll test”. They put the stages on these massive rollers and slowly turn them… listening for any “clinks”. Pretty funny seeing it done.
  - ceejayoz 3y ago
    https://www.smithsonianmag.com/air-space-magazine/is-spacex-changing-the-rocket-equation-132285884/ https://www.smithsonianmag.com/air-space-magazine/is-spacex-... > Part of the Merlin’s qualification testing involves feeding a stainless steel nut into the fuel and oxidizer lines while the engine is running—a test that would destroy most engines but leaves the Merlin running basically unhindered. They're not running it through the engine, they're verifying that the filter is installed and functional so it doesn't get to the engine.
    - elteto 3y ago
      2012 article so it predates my time by quite a bit. Not a thing anymore though.
      - inemesitaffia 3y ago
        Things change. Maybe it's more of a type certification/ crash test thing. Test one to failure and assume it works
- justinclift 3y ago
  Wonder how many of the nuts it took to fill up that filter? ;)
- RecycledEle 3y ago
  Those are big tanks.
  - chasd00 3y ago
    yeah and even if starship tanks are perfectly clean you would also have to assume the tanks on the trucks delivering the fuel/oxi are also perfectly clean. As well as storage tanks, plumbing, and everything else.
    - kelnos 3y ago
      Unless you filter the fuel as it's pumped into the rocket's tanks.
- codesnik 3y ago
  wow. Where I can read about those steel nuts? I wasn't able to google anything on the matter.
Ajedi32 3y ago
I was wondering about this as well. Maybe the filter isn't for dirt, but ice? (Water or solidified methane.)
dotnet00 3y ago
Also meaningful to consider that as a vehicle meant to be reusable, it's going to be pretty much impossible to keep the tanks pristine over time unless they add a cleaning process that would be expensive and time consuming to every refurbishment (which runs counter to Starship's goal of rapid reusability). Relative to all other rockets, SpaceX's are refueled pretty often. Better to just design with filters.
- skissane 3y ago
  > Better to just design with filters. I wonder how frequently the filters need to be clean/changed? After every flight? After X flight hours? And what is involved in changing them? I imagine, initially at least, they'll remove all the engines after every flight, and that will give them easy access to inspect/clean/replace the filters, when necessary. However, I expect the longer term goal is they don't want to have to remove the engines after every flight, just like how a commercial aircraft doesn't have its engines removed after every flight.
foobarian 3y ago
In my mind a rocket engine would be much more tolerant of debris since you just pipe the fuel into a giant fireball (yes I can practically see the eyes rolling, sorry! :-). For ICEs I can understand clogging tiny injectors or carbs would be a problem, do rocket engines also have injectors or other narrow parts susceptible to clogs?
- bilsbie 3y ago
  The problem is that that fuel gets routed through a thin tube around the engine bell and then goes through a turbo pump before going into the engine. Both of those would have tight tolerances.
- CarVac 3y ago
  Yes, rocket engines have many small orifices to atomize and mix the propellant. It's like a showerhead, except if the fuel side clogs, the oxidizer might eat your chamber wall.
  - pantalaimon 3y ago
    > if the fuel side clogs, the oxidizer might eat your chamber wall Ah, good old 'engine rich combustion'
    - inglor_cz 3y ago
      I still remember the exhaust plume of one of the experimental Starships turning green. "Whoa, it is eating itself!" And verily it was.
      - chasd00 3y ago
        yeah copper burns green and is commonly used in liners and other components. If it starts to burn, which it's not suppose to, you see green. You'll see a green flash when the merlin engines startup but that's because of the hypergolic fluids used to get the pumps running and ignition started, not copper burning. During Falcon night launches you can really see the green glow of the starter fluid on ignition.
s1artibartfast 3y ago
Rockets and cars have fuel filters for different reasons. You car has a fuel filter because dirt causes wear and tear on the engine over time. In a rocket, a stray particulate is a potentially explosive event. the Fuel pumps are approximately 100,000 horsepower and particles in flow can have enough kinetic energy to trigger combustion with the walls of the system the fuel is flowing through. Most metals will burn as fuel in a high oxygen environment, even at standard pressures, let alone pressures 100x higher.
deleted 3y ago
[deleted]
nordsieck 3y ago
> Why do those rockets have a fuel filter at all? 1. Launch vibration is real. There's a reason why the Merlin 1 engine has to survive a nut being fed into the fuel and oxidizer lines while running.[1] 2. My understanding with this particular issue is that SpaceX uses autogenous pressurization. It pressurizes the LOx tank with the output from the oxygen-rich preburner. Well, that output is mostly oxygen, but it contains various hydrocarbons. Which, when combined with pure oxygen can form ice and dry ice. It's generally not good to feed solid CO2 and H2O into a high performance turbopump that's designed to accept liquid oxygen. --- 1. https://www.smithsonianmag.com/air-space-magazine/is-spacex-changing-the-rocket-equation-132285884/ https://www.smithsonianmag.com/air-space-magazine/is-spacex-...
RecycledEle 3y ago
Thos
DarmokJalad1701 3y ago
> On the other hand, if there wasn't any dirt, the filters would not have clogged, so I guess it does make sense after all. Things can freeze at cryogenic temperatures. Water ice, dry ice ... or other gases solidifying. There can also be FOD knocked loose by vibrations.

guhidalg 3y ago

Don’t forget about all the equipment needed to fill the tanks and transport the fuel to the tanks. Probably easier to assume contamination gets introduced somewhere and just slap an in-line filter.

foxyv 3y ago
Imagine if the engine failed from FOD. The reaction would be "Why don't they use filters on their fuel pumps?" Whenever you make an engineering decision there are trade offs like this.

Tuna-Fish 3y ago

Most rockets don't have fuel filters, but this requires their tanks to be kept in a much more strict state of cleanliness, and any failure to do so would result in mission failures. SpaceX has long had a very different idea about FOD than most other rocket companies, famously Merlin engine qualification testing contains ingesting stainless steel nuts.

cduzz 3y ago
That's bonkers. What size nuts do they pass through their engine? Every engine before it goes to space has to have a nut passed through the system? Like a 12mm "go into space" grade nut or a little itty bitty nut like I might find holding down a heat sink assembly in a 1998 vintage motherboard? I'm off to the googles to find videos...
- HPsquared 3y ago
  Probably more "qualification of the design", like those bird ingestion tests they do on airliner engines. And the test probably doesn't look too interesting, the nut might not even come out the other end.
- toss1 3y ago
  Let us know what you find. I find it not 'bonkers' but a great idea, and hope they are qualifying with the largest nuts found upstream, in 'go to space' grades. Robustness and anti-fragility may not be so critical for an expendable vehicle that operates once for something like 12 minutes, but seems a key attribute for a refuel-able and reusable spacecraft?
  - cduzz 3y ago
    Oh I agree it's not bonkers from the perspective of "wait you want to put people on that bomb and then explode it?" You can make all sorts of rules about what's not allowed to happen, but often those things happen. I'm just ... astonished if they've got a little practice room somewhere in their factory where they give each of their new rockets a nut to process... "Okay kid here's your graduation test!". A "we need screens to cope with big things and filters to deal with small things" is probably smart for any anything that's going to be reused. A nut's probably in some ways easier to deal with than a blob of wd40 (in your lox tank).
    - infogulch 3y ago
      Rockets: "Hey what if we strap people to a giant bomb and explode it juuust slowly enough that they're launched into space alive?"
      - avmich 3y ago
        Steam engines: "No problems, we're doing that already for centuries, just with less speed and mostly horizontally."
    - adastra22 3y ago
elteto 3y ago
I can guarantee you that “ingesting a nut” through the engine is not part of the official engine qual program. Maybe it happened once and somehow the engine survived and they were able to ascertain what had happened. But this is not a common thing. Edit: I will say that what actually is a test commonly done (also in industry in general) is doing a “roll test”. They put the stages on these massive rollers and slowly turn them… listening for any “clinks”. Pretty funny seeing it done.
- ceejayoz 3y ago
  https://www.smithsonianmag.com/air-space-magazine/is-spacex-changing-the-rocket-equation-132285884/ https://www.smithsonianmag.com/air-space-magazine/is-spacex-... > Part of the Merlin’s qualification testing involves feeding a stainless steel nut into the fuel and oxidizer lines while the engine is running—a test that would destroy most engines but leaves the Merlin running basically unhindered. They're not running it through the engine, they're verifying that the filter is installed and functional so it doesn't get to the engine.
  - elteto 3y ago
    2012 article so it predates my time by quite a bit. Not a thing anymore though.
    - inemesitaffia 3y ago
      Things change. Maybe it's more of a type certification/ crash test thing. Test one to failure and assume it works
justinclift 3y ago
Wonder how many of the nuts it took to fill up that filter? ;)
RecycledEle 3y ago
Those are big tanks.
- chasd00 3y ago
  yeah and even if starship tanks are perfectly clean you would also have to assume the tanks on the trucks delivering the fuel/oxi are also perfectly clean. As well as storage tanks, plumbing, and everything else.
  - kelnos 3y ago
    Unless you filter the fuel as it's pumped into the rocket's tanks.
codesnik 3y ago
wow. Where I can read about those steel nuts? I wasn't able to google anything on the matter.

Ajedi32 3y ago

I was wondering about this as well. Maybe the filter isn't for dirt, but ice? (Water or solidified methane.)

dotnet00 3y ago

Also meaningful to consider that as a vehicle meant to be reusable, it's going to be pretty much impossible to keep the tanks pristine over time unless they add a cleaning process that would be expensive and time consuming to every refurbishment (which runs counter to Starship's goal of rapid reusability). Relative to all other rockets, SpaceX's are refueled pretty often. Better to just design with filters.

skissane 3y ago
> Better to just design with filters. I wonder how frequently the filters need to be clean/changed? After every flight? After X flight hours? And what is involved in changing them? I imagine, initially at least, they'll remove all the engines after every flight, and that will give them easy access to inspect/clean/replace the filters, when necessary. However, I expect the longer term goal is they don't want to have to remove the engines after every flight, just like how a commercial aircraft doesn't have its engines removed after every flight.

foobarian 3y ago

In my mind a rocket engine would be much more tolerant of debris since you just pipe the fuel into a giant fireball (yes I can practically see the eyes rolling, sorry! :-). For ICEs I can understand clogging tiny injectors or carbs would be a problem, do rocket engines also have injectors or other narrow parts susceptible to clogs?

bilsbie 3y ago
The problem is that that fuel gets routed through a thin tube around the engine bell and then goes through a turbo pump before going into the engine. Both of those would have tight tolerances.
CarVac 3y ago
Yes, rocket engines have many small orifices to atomize and mix the propellant. It's like a showerhead, except if the fuel side clogs, the oxidizer might eat your chamber wall.
- pantalaimon 3y ago
  > if the fuel side clogs, the oxidizer might eat your chamber wall Ah, good old 'engine rich combustion'
  - inglor_cz 3y ago
    I still remember the exhaust plume of one of the experimental Starships turning green. "Whoa, it is eating itself!" And verily it was.
    - chasd00 3y ago
      yeah copper burns green and is commonly used in liners and other components. If it starts to burn, which it's not suppose to, you see green. You'll see a green flash when the merlin engines startup but that's because of the hypergolic fluids used to get the pumps running and ignition started, not copper burning. During Falcon night launches you can really see the green glow of the starter fluid on ignition.

s1artibartfast 3y ago

Rockets and cars have fuel filters for different reasons. You car has a fuel filter because dirt causes wear and tear on the engine over time. In a rocket, a stray particulate is a potentially explosive event. the Fuel pumps are approximately 100,000 horsepower and particles in flow can have enough kinetic energy to trigger combustion with the walls of the system the fuel is flowing through. Most metals will burn as fuel in a high oxygen environment, even at standard pressures, let alone pressures 100x higher.

deleted 3y ago

[deleted]

nordsieck 3y ago

> Why do those rockets have a fuel filter at all? 1. Launch vibration is real. There's a reason why the Merlin 1 engine has to survive a nut being fed into the fuel and oxidizer lines while running.[1] 2. My understanding with this particular issue is that SpaceX uses autogenous pressurization. It pressurizes the LOx tank with the output from the oxygen-rich preburner. Well, that output is mostly oxygen, but it contains various hydrocarbons. Which, when combined with pure oxygen can form ice and dry ice. It's generally not good to feed solid CO2 and H2O into a high performance turbopump that's designed to accept liquid oxygen. --- 1. https://www.smithsonianmag.com/air-space-magazine/is-spacex-changing-the-rocket-equation-132285884/ https://www.smithsonianmag.com/air-space-magazine/is-spacex-...

RecycledEle 3y ago

Thos

DarmokJalad1701 3y ago

> On the other hand, if there wasn't any dirt, the filters would not have clogged, so I guess it does make sense after all. Things can freeze at cryogenic temperatures. Water ice, dry ice ... or other gases solidifying. There can also be FOD knocked loose by vibrations.