4 ms·

To be fair, that turbo pump and throttling control only has to work for about 5 minutes, that does make it a bit simpler by just engineering it to work for 15

by zaarn 5y ago

To be fair, that turbo pump and throttling control only has to work for about 5 minutes, that does make it a bit simpler by just engineering it to work for 15 minutes without breaking (safety margin included then).

regularfry 5y ago
Assuming single-use. Reusable rocket engines are different beasts.
- zaarn 5y ago
  Engineer it for 30 minutes and swap it regularly (SpaceX swaps turbines and pumps often enough because they do break down after only one or three launches)
NamTaf 5y ago
You're perhaps overstating how easy it is target a certain design life when constrained by other parameters. It's entirely possible that targetting '15 minutes' is actually meaningless for many of the parts because all of the other constraints are far tighter. Once you're through the stress of getting up and running, then chugging along in the steady-state is often less stressful (for something non-ablative, and of course start-stop cycles are another matter entirely). To give a far less glamourous example, I am a mech eng who works in rail. We needed some more orecars to complement an existing fleet that only had about 10 years' operational life left in them. Thus, they wanted me to design for a shorter life than the 25-30 year standard that we target, to save cost. However, trying to thin out the structure so it only had 10-15 years' of fatigue life in it meant that it fell well short of the proof load requirements needed to stop it ripping in half in a worst-case shock load. Put differently, the constraint around peak loads effectively baked about 20 years of operational life into the structure, and in turn made it difficult to save money on a shorter-life design. It would not surprise me if many of the non-ablative parts in a rocket are in fact fairly durable without the stop-start cycles. So whilst a launch may only take 5-8 minutes, I could totally believe that a 15-20 minute launch wouldn't demand heaps more from the parts. Of course, this absolutely doesn't translate to ablative parts, or items that undergo stop-start cycles. The latter of course is where the devil is for reusable equipment. Depending on the failure modes in question for turbo pumps and throttling controls, those may or may not apply.

by zaarn 5y ago

regularfry 5y ago

Assuming single-use. Reusable rocket engines are different beasts.

zaarn 5y ago
Engineer it for 30 minutes and swap it regularly (SpaceX swaps turbines and pumps often enough because they do break down after only one or three launches)

NamTaf 5y ago

You're perhaps overstating how easy it is target a certain design life when constrained by other parameters. It's entirely possible that targetting '15 minutes' is actually meaningless for many of the parts because all of the other constraints are far tighter. Once you're through the stress of getting up and running, then chugging along in the steady-state is often less stressful (for something non-ablative, and of course start-stop cycles are another matter entirely). To give a far less glamourous example, I am a mech eng who works in rail. We needed some more orecars to complement an existing fleet that only had about 10 years' operational life left in them. Thus, they wanted me to design for a shorter life than the 25-30 year standard that we target, to save cost. However, trying to thin out the structure so it only had 10-15 years' of fatigue life in it meant that it fell well short of the proof load requirements needed to stop it ripping in half in a worst-case shock load. Put differently, the constraint around peak loads effectively baked about 20 years of operational life into the structure, and in turn made it difficult to save money on a shorter-life design. It would not surprise me if many of the non-ablative parts in a rocket are in fact fairly durable without the stop-start cycles. So whilst a launch may only take 5-8 minutes, I could totally believe that a 15-20 minute launch wouldn't demand heaps more from the parts. Of course, this absolutely doesn't translate to ablative parts, or items that undergo stop-start cycles. The latter of course is where the devil is for reusable equipment. Depending on the failure modes in question for turbo pumps and throttling controls, those may or may not apply.