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What does she mean about the VMAX? They're "technically forbidden" to use it, but it's understood that in a combat situation they may well need it to escape an aggressor?

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This extra boost of power is also known as "War Emergency Power", and goes back to World War II planes. In the P51 Mustang, WEP increased the horsepower by 61%. But after using WEP, the engines needed to be inspected before flying again, so it was only used when necessary.

https://en.wikipedia.org/wiki/War_emergency_power

Related is the "Battle short" switch on Navy computers, which shorts the fuses with copper bars and disables over-temperature faults, so you can keep the computer running during a battle even if things are going bad.

I have heard that the air force bought backup generators to power field installations for ground controllers, etc. These backup generators had a maximum continuous operation time, after which they would shut themselves off. The air force required an override button, so that they could be run longer, even if it meant the generator would destroy itself. If it's powering ground control, and it's the critical moment of the battle, and you can get ten more minutes power but then the generator breaks, you take the ten minutes and let the generator die. And you have to have the ability to do that if you need to.

> In the P51 Mustang, WEP increased the horsepower by 61%.

In his youth, my late dad flew P51 Mustangs in the Korean War. He told the story of how he and another guy were flying a photo reconnaissance mission (I think it was) when the other guy was shot down by ground fire, crashing not far in front of Chinese lines. My dad then shot it out with the Chinese ground troops, hoping to keep them away from his downed colleague until help arrived. Back at the base, my dad's best friend mistakenly heard that it was my dad who'd been shot down; as my dad described it, his friend climbed into another P51 and flew all the way across Korea at "full military power" (the term he used) to get in the fight.

My dad's shot-down colleague was killed in the crash, it turned out. I vividly remember my dad telling the story late in his life and getting pretty emotional: He was certain that, in trying to protect his colleague, he had killed scores of young men who had little choice about being there, who probably were just farm boys like him and had parents and maybe wives and children at home. I don't think he ever got over that.

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.

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?

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.

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.

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.

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.

I'd guess only situation where you'd use it is when you'd lose the aircraft otherwise. In normal conditions escaping an aggressor would not be such situation.

Think it as redline in a car; take away the limit and the engine can rev higher and possibly get few hp more, but the engine won't be in any good shape after that.

A lot of fighters have "emergency power" controls that will push the engine past its normally rated power band for a short time at the cost of drastically shortening the life of the engine. I would guess that's what this is and why pilots are normally forbidden from using it.

It's very interesting that engineers can compute this threshold to a 2% error.

"Any fool can build a bridge that won’t fail, but it takes an engineer to build a bridge that just barely won’t fail"

Bridge building video games suggest that in fact fools mostly build bridges that fail spectacularly and are astonished that things that can't work don't work.

As a more historical example of this, a lot of WWII fighters had a max power setting on the throttle that had a wire strung across it, but the pilot could break past the wire to go to 'War Emergency Power' and gain anywhere from 15-50%+ more horsepower from the engine. The use had to be logged and after a few hours of this the engine would need to get a complete tear-down and rebuild.

It is basically a 'you are trading away this engine to save your life' setting, but sometimes that was a necessary thing to do.

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