The key question: does this fact about jet engines also hold for rocket engines?
If so, increasing a turbopump rocket engine power just 10% to 15% cuts engine life in half. And conversely, decreasing it by 10% to 15% doubles engine life. And if so, would this still work if we repeated the concept multiple times? If we reduced the thrust by .9^5 = .60, i.e., to 60%, which most turbopump engines can manage, then we could increase the lifetime by a factor of 2^5 = 32 times? Then a Merlin engine with a lifetime of, say, 30 reuses by running it only 60% power could have its lifetime extended to 1,000 reuses?
This would be in the range of number of reuses of the type of jet engines used on long haul flights. Rocket engines with that number of reuses probably also would allow “gas and go” operation. That is, no major refurbishment needed in between flights, as with jet engines.
Is reduced temperature the key?
In examining this question of rocket engine longevity versus jet engine longevity I once hypothesized it had to do with the high temperatures rocket engines operated at, typically ca. 3,000 °C, whereas jet engines might only operate at ca. 1,200 °C to 1,500 °C.
It might be thought it would be the high pressures of rocket engines but that can’t be the primary reason since automobile diesel engines can operate at hundreds of bars of pressure, above even that of rocket engines for many hours of service:
The pump pressures in rockets are impressive, but let's not forget that the injection system in modern diesel engines operate at 2500 bar. They are also fast enough to accomplish up to eight separate fuel injections with each cylinder cycle.
Bosch CRS3-25.
https://youtu.be/T7o2hvoJE-Q
About 31 minutes in Elon suggests the current version V1 would be capable of 40 to 50 tons to orbit. This is bad because SpaceX sold NASA on the idea the Starship HLS could serve as an Artemis lander based on 150 tons to orbit reusable and “10ish” refueling flights. If the capability is max 50 tons, then it would take “30ish” refueling flights.
If they intend to use version V2 then this is bad because it would require further qualification flights for the larger version and more importantly further qualification of the more powerful Raptor 3 engine needed.
This last is doubly bad because I’d be willing to bet dollars to donuts that they never informed NASA that the current version couldn’t do it and further development would be required for the larger version.
SpaceX needs a true Chief Engineer. Elon once said that early on when there were still doubts about its viability, they tried to recruit a Chief Engineer for SpaceX but no one good was willing to come. So Elon designated himself Chief Engineer. It is not a role Elon is well suited for. A good Chief Engineer should be scrupulously forthright. He would not refer to the little 5 or 10 second static burns SpaceX does for the SuperHeavy or Starship as "full duration".
A true Chief Engineer would be aware that "full duration" in the industry is short for "full mission duration". These static fires in the industry are conducted at the full length and the full thrust of an actual flight and are meant to give confidence to potential customers that the engines can perform as expected for the promised capabilities of the launchers.
However, SpaceX in using the term "full duration" for these little few-second burns, doesn't even tell the public, or its major customer NASA for which they have a billion-dollar contract, if these little burns are even conducted at full thrust.
This has had majorly negative consequences. The FAA had great concerns in the Raptor reliability after the first test flight. In the "corrective actions" they required of SpaceX prior to a second Starship test flight, at the top of the list was correcting the tendency of the Raptor of leaking fuel and catching on fire while in flight.
I have argued multiple, independent lines of evidence suggest SpaceX intentionally reduced the throttle of the Raptors on the booster on the second test flight, IFT-2, to improve reliability of the engines:
Running an engine at reduced throttle reduces the pressure levels within the engine, high pressure being a major cause of engine fuel leaks. The Starship upper stage though was run at near full throttle on IFT-2, perhaps because performance would be reduced too much if it also was run at reduced throttle.
The result was the booster engines worked fine, at least on ascent, while the Starship exploded on ascent on IFT-2. SpaceX has said the Starship RUD was due to an intentional LOX dump they performed to keep that flight as suborbital. However, many knowledgeable observers doubted the LOX dump alone would have caused a RUD. They argue due to the tendency of the Raptor to leak fuel, it's more likely that plus the LOX dump caused the RUD.
For the third test flight, IFT-3, after reviewing both propellant burn rates and the acceleration profile of the flight, I'm suggesting SpaceX learned their lesson from the second test flight, and this time both stages were run at reduced throttle on this flight. And this time both stages were able to complete the ascent stage of the flight successfully.
However, this does reduce the payload capability of the launcher. Elon has acknowledged this radically reduced payload capability in his recent update. But it needs to be explained by SpaceX why the payload is so greatly reduced. If it is because the Raptor needs to be run at reduced thrust in order to be reliable then that is an extremely important thing to acknowledge, and to inform NASA on it, because the thrust levels of a rocket go into assessing what its actual capabilities are.
There is another very important issue about Raptor reliability. Multiple times a Raptor has undergone a RUD doing a relight during prior testing of the Starship planned landing procedure. And on this last Superheavy/Starship test flight as well Raptors underwent a RUD during the booster landing procedure. The boostback back burn appeared to have occurred successfully. But there was venting gas after the bostback back burn suggesting there may have been a fuel leak here as well.
Note for a successful reuse of the Starship and booster, successful relights have to occur both for boostback burns and landing burns. Then in none of the prior Starship landing tests nor of the Superheavy/Starship flight tests have any flights shown successful Raptor relights without leaking fuel and catching fire, and often undergoing a RUD.
SpaceX has called one test of the Starship landing test, SN15, successful because it managed to land without exploding. But it is important to note even in that test a Raptor leaked fuel and caught fire prior to landing. It's just on that test SpaceX managed to extinguish the fire before the ship exploded:
Note that in the SpaceX plans for a reusable Starship it absolutely can not work if the Raptor can not be made to relight reliably. SpaceX in not publicly providing full mission duration, full thrust testing information on the Raptors have not shown this also for relights of the Raptor.
That is why it is so important for a launch company to publicly provide details on full mission duration, full thrust level static engine testing.
SpaceX needs a true Chief Engineer to provide such details in a forthright manner.
