Post by JayUtah on Nov 22, 2005 19:15:13 GMT -4
It seems people are itching to know what that was all about. There are a few non-Apollo space threads in this section, and since this is all derived from the work on the J-2, it might fit.
As you can imagine, the flow of propellants from the STS tank into the orbiter and thence into the engines is a fluid dynamics problem of acute proportions. Flow rates and pressures are colossal. The design constraints are fairly heinous.
The fuel flow starts to become turbulent as it hits the manifold. And this is expected. There is never any such thing as the perfect manifold. In practice each engine, because of its geometric relationship to the manifold, experiences a slightly different turbulence pattern in the fuel.
As in most of these kinds of fuel systems, there are bellows joints that provide mechanical isolation from engine vibration and to allow for gimballing. A nasty side effect of the bellows joint is increased turbulence. This is common-sensical. It's a bumpy ride along a corrugated tube. And so there is a flowliner -- a sleeve on the inside of the bellows joint -- to mitigate turbulence. The flowliner is one of several internal structures in the fuel lines designed to address specific known instances of nonlaminar flow. Others include vanes that correct radial flow after a bend in the pipe; otherwise the fuel will want to swirl in the plane of the bend.
In 2002 the orbiter fleet was grounded because cracks were discovered in the flowliner. Repairs were possible, but the root cause of the crack remained unknown. This means that the flowliners have to be inspected for every flight.
Of notable concern is the proximity of the low-pressure fuel pump, which lies just downstream of the flowliners. Most propellant feed systems use staged pumps, each one adding an incremental bit of pressure to that generated by the upstream pump. This prevents individual pump elements from cavitating. The LPFP casing is only slightly wider than the fuel line itself, and is the first pump encountered by the fuel. It employs an axial impeller that looks very much like the prop on a motor boat. There are several rows of blades, each downstream blade having a larger angle of attack. The pump turns at about 35,000 rpm.
As the liquid fuel approaches this assembly, it begins to swirl in the same direction as the impeller rotation. Further -- and this is what we lately discovered -- some portion of the fuel actually travels upstream as the result of the interaction between the swirl, the blade tip, and the pump casing.
The flowliner sleeve has a series of vent slots that allow fluid to access the space between the flowliner and the bellows. This is so that the flowliner does not have to bear the feedline pressure, but also provides access for assembly and repair. The vent slots allow boroscoping to inspect the inside surfaces of the bellows, and to attach the flowliner to the inner surface of the pressure carrier. The flowliner is made of Inconel. The slots are an inch and a half or so long and a half inch or so wide, with half-circle end caps (i.e., racetrack shaped). The long axis is aligned with the fluid flow.
Normally the fluid flow would not be affected materially by these slots.
The cracks began at the slot edges and propagated circumferentially -- i.e., perpendicular to the main flow. Thus swirl was suspected. Early fluid dynamics work did not support this hypothesis because of the direction and acoustics of the suspected degree of swirl. However, with better equipment and some finer modeling (e.g., 30-50 million finite elements and time steps on the order of 0.0001 second) NASA (with our modest help, and that of SGI and Fluent) was able to determine that the confluence of the gross swirl and the small-scale backflow created an acoustic condition at the flowliner. This resulted in fluid flow that, at certain power settings, was almost perfectly circumferential for sustained periods.
This flow caused high-frequency oscillation in the long slot edges. Sustained for long enough, this caused cracks to develop in the slot edges. This represents a potential LOV (loss-of-vehicle) condition because if fracture completes, the engine will ingest the pieces of flowliner causing (at best) premature engine cutoff and (at worst) catastrophic engine failure and orbiter deflagration. A Bad Day.
Mechanical resurfacing of the flowliner slot edges seems to have little effect on the problem.
For now the likely remedy will be changing out the flowliner and gimbal bellows on each flight. Since the SSME will be used in the return to the moon, there will likely have to be a design change in order to accommodate this effect. First, the geometry of the flowliner vent slots will be changed; earlier work confirmed that acoustic loading is affected by slot geometry. Second, the LPFP will be moved farther downstream to reduce the swirl effect at the flowliner. Third, the LPFP impeller will be modified to reduce backflow and swirl -- this is a purely fluid-dynamics driven solution.
This really is Apollo-era technology. All the components I mentioned were in the original J-2 propellant feed systems for the S-II and the S-IVB.
As you can imagine, the flow of propellants from the STS tank into the orbiter and thence into the engines is a fluid dynamics problem of acute proportions. Flow rates and pressures are colossal. The design constraints are fairly heinous.
The fuel flow starts to become turbulent as it hits the manifold. And this is expected. There is never any such thing as the perfect manifold. In practice each engine, because of its geometric relationship to the manifold, experiences a slightly different turbulence pattern in the fuel.
As in most of these kinds of fuel systems, there are bellows joints that provide mechanical isolation from engine vibration and to allow for gimballing. A nasty side effect of the bellows joint is increased turbulence. This is common-sensical. It's a bumpy ride along a corrugated tube. And so there is a flowliner -- a sleeve on the inside of the bellows joint -- to mitigate turbulence. The flowliner is one of several internal structures in the fuel lines designed to address specific known instances of nonlaminar flow. Others include vanes that correct radial flow after a bend in the pipe; otherwise the fuel will want to swirl in the plane of the bend.
In 2002 the orbiter fleet was grounded because cracks were discovered in the flowliner. Repairs were possible, but the root cause of the crack remained unknown. This means that the flowliners have to be inspected for every flight.
Of notable concern is the proximity of the low-pressure fuel pump, which lies just downstream of the flowliners. Most propellant feed systems use staged pumps, each one adding an incremental bit of pressure to that generated by the upstream pump. This prevents individual pump elements from cavitating. The LPFP casing is only slightly wider than the fuel line itself, and is the first pump encountered by the fuel. It employs an axial impeller that looks very much like the prop on a motor boat. There are several rows of blades, each downstream blade having a larger angle of attack. The pump turns at about 35,000 rpm.
As the liquid fuel approaches this assembly, it begins to swirl in the same direction as the impeller rotation. Further -- and this is what we lately discovered -- some portion of the fuel actually travels upstream as the result of the interaction between the swirl, the blade tip, and the pump casing.
The flowliner sleeve has a series of vent slots that allow fluid to access the space between the flowliner and the bellows. This is so that the flowliner does not have to bear the feedline pressure, but also provides access for assembly and repair. The vent slots allow boroscoping to inspect the inside surfaces of the bellows, and to attach the flowliner to the inner surface of the pressure carrier. The flowliner is made of Inconel. The slots are an inch and a half or so long and a half inch or so wide, with half-circle end caps (i.e., racetrack shaped). The long axis is aligned with the fluid flow.
Normally the fluid flow would not be affected materially by these slots.
The cracks began at the slot edges and propagated circumferentially -- i.e., perpendicular to the main flow. Thus swirl was suspected. Early fluid dynamics work did not support this hypothesis because of the direction and acoustics of the suspected degree of swirl. However, with better equipment and some finer modeling (e.g., 30-50 million finite elements and time steps on the order of 0.0001 second) NASA (with our modest help, and that of SGI and Fluent) was able to determine that the confluence of the gross swirl and the small-scale backflow created an acoustic condition at the flowliner. This resulted in fluid flow that, at certain power settings, was almost perfectly circumferential for sustained periods.
This flow caused high-frequency oscillation in the long slot edges. Sustained for long enough, this caused cracks to develop in the slot edges. This represents a potential LOV (loss-of-vehicle) condition because if fracture completes, the engine will ingest the pieces of flowliner causing (at best) premature engine cutoff and (at worst) catastrophic engine failure and orbiter deflagration. A Bad Day.
Mechanical resurfacing of the flowliner slot edges seems to have little effect on the problem.
For now the likely remedy will be changing out the flowliner and gimbal bellows on each flight. Since the SSME will be used in the return to the moon, there will likely have to be a design change in order to accommodate this effect. First, the geometry of the flowliner vent slots will be changed; earlier work confirmed that acoustic loading is affected by slot geometry. Second, the LPFP will be moved farther downstream to reduce the swirl effect at the flowliner. Third, the LPFP impeller will be modified to reduce backflow and swirl -- this is a purely fluid-dynamics driven solution.
This really is Apollo-era technology. All the components I mentioned were in the original J-2 propellant feed systems for the S-II and the S-IVB.
