Why don't all SSME turbopumps gimbal with the engine?

Question

The SSME low-pressure pumps (both fuel and oxidizer) are mounted to the vehicle structure and don't gimbal. The high-pressure pumps are gimbaling with the engine, so the propellant lines between low-pressure and high-pressure pumps must be flexible. That includes not only the pump discharge ducts (2-3 MPa), but also the turbine inlet (29 MPa) on both low-pressure pumps and turbine discharge (21 MPa) on the LPFTP. This looks a like a lot of articulating joints (3 per flexible line, based on illustration below) on pressurized lines, meaning a lot of potential failure points.

Some of the joints are visible in the picture below:

What is the reason behind such engineering decision? Why are the low-pressure pumps not gimbaling with the engine? If they were mounted to the engine, all the lines downstream would be rigid. The flexibility of the propellant lines would be provided by the supply manifold upstream of the low-pressure turbopumps. This looks like a simpler solution, because of:

• Lower pressure in these ducts (0.2 - 0.7 MPa)

• Lower number of required flexible lines. I am not 100% sure by this, but it seems very likely, because there would be only 1 flexible line upstream of each low-pressure pump (2 for a single engine), instead of 3 downstream of the LPFTP + 2 after the LPOTP (that is 5 for a single engine). Multiply these 3 extra lines by the amount of articulating joints needed on each flexible line and it looks like a significant reduction of critical moving componens. There are of course other flexible lines connecting the engine with the orbiter (fuel tank pressurization, helium inlet, fuel bleed, etc.), but I think their number would be unaffected by the proposed change, so I am not counting them.

There must be some hidden reason why the SSME designers chose to fix the low-pressure turbopumps to the orbiter and forced themselves to use multiple articulating joints on high pressure lines. I would like to understand that reason.

Useful reference.

Answer 1

Start by thinking about why the low pressure pumps exist as all. Why can't the entire pumping task be done by single (multistage) pumps on the engine? Why did the engineers feel the need to have separated low pressure (LP) and high pressure (HP) pumps at all?

Because the engine gimbals, there needs to be a flexible lines to feed fuel and propellant to the engine yet still allow motion. That line also has to handle temperature (length) changes, work when iced up, have low mass, etc.

That line feeds large quantities of liquids near their boiling point to an HP pump. The pump is sucking really hard to move that mass. Because of the pressure drop of moving through that line, the end near the HP pump is below the pressure at the start of the line. If that line starts at just tank pressure, this is pretty much a recipe for cavitation(i.e. spontaneous boiling), which causes all sorts of problems up to destruction.

To avoid cavitation, the LP pumps were included. Pretty much by definition, they had to be before the long flexible line so that their output would pressurize that line; the additional pressure from the LP pump meant that that the input suction pressure of the HP pumps would never be below the vapor pressure of the fluids, so they wouldn't cavitate (i.e. spontaneously boil). So the LP pumps were mounted to the large, fixed tank piping, hence directly to the Shuttle framework.

TLDR: The LP pumps entire job was to pressurize the feed to the HP pumps. That feed starts at the bottom of the tank piping, so that's where they were put.

(Another approach to cavitation reduction is subcooling the fluids, which is interesting on its own)