3 ms·

VMAX adds 2% to the max RPM of the engines, giving a bit of extra performance for when it is really needed. In the process it increases wear and tear on the eng

VMAX adds 2% to the max RPM of the engines, giving a bit of extra performance for when it is really needed. In the process it increases wear and tear on the engine, necessitating an expensive very thorough inspection and possibly repair of the engines. If your life depends on it, by all means use it. In all other circumstances, save the maintenance crew the extra work and the taxpayer the extra costs.

tgsovlerkhgsel 6y ago
I'm surprised they bothered providing that option for such a small benefit. Was there some specific enemy fighter model that this allowed them to outfly?
- GuB-42 6y ago
  I think it is cheap to add and it may safe your life so why not? It is just a matter of raising a limit. On the mechanical side of things each part has a certain lifetime, you can increase it by reducing strain and vice versa. So they probably already know how much the engines can be pushed if you only need a total lifetime of 1h instead of hundreds or hours. They most likely extrapolated that 2% from this.
- sturmeh 6y ago
  I assume that's why they restrict its use now, because the overhead of repair/stress isn't worth the 2% thrust. During WW2 there's a good chance they had no idea what they'd be up against whilst they were developing this jet, so they built in a theoretical 2% improvement switch with a large caveat incase the 2% improvement ever gave them the edge. I think the use of the VMAX switch is only warranted if the ship would otherwise be destroyed or lost if it were not used, which seems like a very unlikely scenario.
  - vlasev 6y ago
    Reminds me of how the Space Shuttle Main Engines gained improvements over time so you could then set them to 104.5% thrust. Higher modes were later available. From Wikipedia on RS-25 engines: - Block IIA (RS-25C): First flown on STS-89, the Block IIA engine was an interim model used whilst certain components of the Block II engine completed development. Changes included a new large throat main combustion chamber (which had originally been recommended by Rocketdyne in 1980), improved low pressure turbopumps and certification for 104.5% RPL to compensate for a 2 seconds (0.020 km/s) reduction in specific impulse (original plans called for the engine to be certified to 106% for heavy International Space Station payloads, but this was not required and would have reduced engine service life). A slightly modified version first flew on STS-96. - Block II (RS-25D): First flown on STS-104, the Block II upgrade included all of the Block IIA improvements plus a new high pressure fuel turbopump. This model was ground-tested to 111% FPL in the event of a contingency abort, and certified for 109% FPL for use during an intact abort.
yborg 6y ago
There's no possible situation I can think of except for a record speed attempt for using this switch, it's baffling that it exists. Even if it increased engine thrust by 10% somehow, compared to the 80% boost from reheat it's a trivial addition. And if there were any serious combat scenarios in which a 2% RPM boost at the turbine for a few minutes would mean the difference between victory and defeat I would think they would have redesigned the engine to make that available ALL the time. I'm very surprised it wasn't just removed from the aircraft.

tgsovlerkhgsel 6y ago

I'm surprised they bothered providing that option for such a small benefit. Was there some specific enemy fighter model that this allowed them to outfly?

GuB-42 6y ago
I think it is cheap to add and it may safe your life so why not? It is just a matter of raising a limit. On the mechanical side of things each part has a certain lifetime, you can increase it by reducing strain and vice versa. So they probably already know how much the engines can be pushed if you only need a total lifetime of 1h instead of hundreds or hours. They most likely extrapolated that 2% from this.
sturmeh 6y ago
I assume that's why they restrict its use now, because the overhead of repair/stress isn't worth the 2% thrust. During WW2 there's a good chance they had no idea what they'd be up against whilst they were developing this jet, so they built in a theoretical 2% improvement switch with a large caveat incase the 2% improvement ever gave them the edge. I think the use of the VMAX switch is only warranted if the ship would otherwise be destroyed or lost if it were not used, which seems like a very unlikely scenario.
- vlasev 6y ago
  Reminds me of how the Space Shuttle Main Engines gained improvements over time so you could then set them to 104.5% thrust. Higher modes were later available. From Wikipedia on RS-25 engines: - Block IIA (RS-25C): First flown on STS-89, the Block IIA engine was an interim model used whilst certain components of the Block II engine completed development. Changes included a new large throat main combustion chamber (which had originally been recommended by Rocketdyne in 1980), improved low pressure turbopumps and certification for 104.5% RPL to compensate for a 2 seconds (0.020 km/s) reduction in specific impulse (original plans called for the engine to be certified to 106% for heavy International Space Station payloads, but this was not required and would have reduced engine service life). A slightly modified version first flew on STS-96. - Block II (RS-25D): First flown on STS-104, the Block II upgrade included all of the Block IIA improvements plus a new high pressure fuel turbopump. This model was ground-tested to 111% FPL in the event of a contingency abort, and certified for 109% FPL for use during an intact abort.

yborg 6y ago

There's no possible situation I can think of except for a record speed attempt for using this switch, it's baffling that it exists. Even if it increased engine thrust by 10% somehow, compared to the 80% boost from reheat it's a trivial addition. And if there were any serious combat scenarios in which a 2% RPM boost at the turbine for a few minutes would mean the difference between victory and defeat I would think they would have redesigned the engine to make that available ALL the time. I'm very surprised it wasn't just removed from the aircraft.