Why were the SSMEs at risk of damage if run dry?

[photogineer]

So, I was reading up on the RS-25 engines that flew on the shuttle and are slated for SLS and I read in multiple places that they were deliberately shutdown before fuel exhaustion because if they were left to run dry they would be destroyed. I am curious why this is though? In my reading I've come up with two theories:

1) The sudden loss of fluid pressure inside one or both of the turbopumps would cause cavitation that could damage or destroy the pumps

or

2) Since the cryogenic fuels were circulated through multiple heat exchangers to heat the fuel and cool the engine, before reaching combustion, that a sudden loss of fuel would result in abrupt loss of cooling while the combustion was still occurring, thus damaging or destroying the uncooled components.

Are either of these even close to being true? Is it another reason? Enlighten me please

Matt

[SargeRho]

I'm guessing that the turbopumps would shredd themselves if they don't have the resistance caused by fuel being pumped by them.

[Red Iron Crown]

The first reason sounds more likely to me than the second. Sure, no more fuel means no more coolant, but it also means no more burning and generating heat. It probably wouldn't heat up significantly more than when it was shut off with fuel remaining.

[softweir]

iirc, the SSMEs' turbo-pumps used the fuel or oxygen as lubricant, and if they ran dry they would probably seize up - which at those revs would be severely destructive!

[sgt_flyer]

the risk of cavitation in turbopumps is dreaded by rocket scientists - that's one of the most destructive things you could put on a turbopump. For non reusable engines, that's not a big deal, but SSME's were meant to last for several flights.

Basically, cavitation is the formation of a vapour bubble behind the pump's blade (because the pressure is too low) When the bubble grows too large, it collapses, and the liquid hits the back of the blade with a lot of force - enough to damage the lightweight structure & material used in the making of the turbopump.

Here's what cavitation can do to a beer bottle.

Here's more infos on the process and a damaged turbopump blade image. (And that's an industrial grade blade - heavy & solid )

http://www.processindustryforum.com/article/cavitation-overcoming-cavitation-damage-control-valves-pipelines

Edited July 24, 2014 by sgt_flyer

[rdfox]

Another issue is that, in the event of an oxygen-rich shutdown situation (i.e., the fuel supply runs out first), the cold LH2 fuel being pumped through the cooling passages in the engine nozzles will run out before the combustion ends, resulting in severe burning of the engine nozzles themselves due to lack of cooling. (This was actually specifically called out in the Rogers Commission report on the Challenger disaster as further evidence that the SSMEs were not the point of failure, as all three showed the sort of nozzle burns that were to be expected from oxygen-rich shutdowns. That came due to the LOX tank in the external tank being the upper tank, with the longer line supplying it to the engines, so when the ET disintegrated, the LOX supply in the plumbing lasted longer than the LH2 supply.)

If you ablate the nozzles enough, you'll get a burn-through structural failure, which is a Bad Thing (it can result in Rapid Unplanned Disassembly) even in an expendable engine. This is why just about all rocket designs have included low-level sensors in both the fuel and oxidizer tanks which would, when a certain number of them simultaneously indicated a low-level condition, initiate a controlled shutdown of the engines; it meant that shutdown would not come due to starvation of one propellant or the other and result in a loss of regenerative cooling of the nozzle and potential burn-through. With the SSMEs designed to fly 20-30 times, this clearly becomes even more important than it is in a simple pressure-fed expendable engine, as damage suffered in one flight may result in failure on a later flight; between that and the various turbopump issues (cavitation when pressure gets too low, loss of lubrication from loss of the pumped fluid, and overspeeding due to loss of resistance of the pumped fluid), a controlled shutdown with significant residual propellant remaining was the safest option.

(Interesting note: it's not just the turbopumps on rockets that use the fluid that they pump to restrict their speed and provide lubrication. The in-tank electric fuel pumps on modern cars are designed to use the gasoline they pump into the fuel lines to both keep the pump from overheating, and to lubricate it so that the bearings don't fail. Thus, it's really a bad idea to let your car run out of gas, because it may do some damage to the fuel pump... and refilling the fuel lines when you try restarting it after getting fuel in the tank almost certainly will. It won't *break* it, not the first time, but it can make it weaker, and more likely to eventually fail.)

[Westi29]

Also, the SSME's have a really interesting design in that their inner cones have hundreds of micro tubes carrying the fuel into the center, where it is sprayed out. Basically, the fuel travels along tiny pipes on the inside edge of the rocket nozzle and the heat of the rocket exhaust combusts the fuel. So if the rockets were to run dry, they would have destroyed the cones as the liquid fuel kep the engines intact in the first place.

Imagine if the combustion in your car was caused by lining the hottest parts of the engine with fuel lines to cool it, and if you ran it dry the fuel lines would cease to cool the engine and everybody has a bad day.

"RS-25 schematic.

A functional diagram showing the flow of propellant through an RS-25 engine. Very high flow rates are needed in the main combustion chamber, so the energy to achieve this is generated in upstream combustion "pre-burner" chambers that generate a partial burn to drive the high pressure turbopumps on both the fuel and oxidizer sides. Pressure feeding the preburners is maintained by the low-pressure turbopumps. Routing fuel through the nozzle serves to both cool the nozzle as well as pre-heat the fuel."