USS Benfold engineering casualty

[thump]

USS Benfold (DDG 65) is a Flight I. The Flight IIA (hangar) ships start with DDG 79. But also, I'm not sure how you move a helo out to the flight deck without power for RAST. I only did a short stint on a DDG, but I don't think running the APU in the hangar is a thing either.

Manual straightening is an emergency / operational necessity item in the 80T-122, but with MCO all things are possible. You can prybar the RSD and push the aircraft around.

One also wonders if the external power cable could be run backwards, such that current left a spinning aircraft and powered the ship. I don’t know if this is possible. Honestly sounds like something Kirk would ask Scotty.

 

[hscs]

One also wonders if the external power cable could be run backwards, such that current left a spinning aircraft and powered the ship. I don’t know if this is possible. Honestly sounds like something Kirk would ask Scotty.

Likely not - circuits would not support the flow as well as 110 output vs ship 440 VAC, 400hz output, and power excess of the aircraft.

 

[Gatordev]

Not a helo expert, but I imagine it's like a huffer start in reverse. I can easily see how there would be enough bleed air from the helo to start a GTG, or at least an aux generator that lets you charge the main air accumulator. It's pretty smart if that's how it works.

I agree the concept makes sense, but practically speaking, I'm still struggling to understand where the bleed air is coming from. And as the discussion continued, even more questions have come to light.

Those air hook ups also look HUGE. It's been a minute since I've done any fluid dynamics equations, but the bleed air lines coming off the APU are pretty small (about 1.5-2"), plus running some sort of line from the helo to the valve...I just wonder if you'd even have enough ass behind the air flow. I guess maybe if you can build up pressure over time.

Still...it's an interesting that someone actually thought of the idea, even if they didn't think through the execution.

 

[PhrogPhlyer]

Not much banter on lets try this out and see if we can do it.

Might take a little effort, so... unless there's going to be some extra money in trying?
At least that's the attitude I saw in Portsmith.

 

[sevenhelmet]

I agree the concept makes sense, but practically speaking, I'm still struggling to understand where the bleed air is coming from. And as the discussion continued, even more questions have come to light.

Those air hook ups also look HUGE. It's been a minute since I've done any fluid dynamics equations, but the bleed air lines coming off the APU are pretty small (about 1.5-2"), plus running some sort of line from the helo to the valve...I just wonder if you'd even have enough ass behind the air flow. I guess maybe if you can build up pressure over time.

Still...it's an interesting that someone actually thought of the idea, even if they didn't think through the execution.

That's a fair point. I'd think the helo has enough bleed air to directly start one of the GTGs, but probably not sufficient flow & pressure to charge up the massive air accumulator the ship uses.

Good system design suggests electrical power would be more valuable than the bleed air- you'd think there's some APU type thing on the ship that allows them to bootstrap themselves up from basically a battery starter somewhere or backup diesel power. OTOH, the fact that they have FOUR primary gas turbines plus backup diesels suggests this kind of power failure SHOULD be extremely rare...

I sailed once on a diesel-electric ship (Oiler USNS Yukon). The power system used two medium speed diesels, two dual-purpose generator/motors, and two high speed diesel generator engines for electrics and propulsion. The props could be direct driven or electrically driven by any one of the four diesels (albeit at reduced speed if you had less than two), full speed at any combination of two engines, and the ship also had two emergency diesel generators that could cover all electrical loads, minus the shafts. It was a pretty ingenious system, and a complete power loss was pretty unlikely (bad fuel would certainly be a concern).

 

[Gatordev]

I'd think the helo has enough bleed air to directly start one of the GTGs,

I'm continuing the questions for my own education, not because I'm arguing your point... In the grand scheme of things (think 10K' foot view), how much bleed air is actually needed to start a turbine? The H-60 can start both of its engines simultaneously (at lower altitudes) with the APU, so that's certainly a notable amount of pressure. But any idea on how that compares to, say, a F-18? Or since we're talking about a ship GTG, an C-130 engine?

I understand a F-18 doesn't have a free turbine (or at least I didn't think it did), but generally what I'm asking is do most engines tend to need roughly the same amount of air? I understand it's not apples to apples, but it's interesting that most engines tend to start at around the same Ng (~12-20%), but obviously some engines are working harder than others.

I'm mostly thinking out loud at this point, but I know you and IKE can out-pocket-protect most of us here on the engineering stuff.

 

[sevenhelmet]

I'm continuing the questions for my own education, not because I'm arguing your point... In the grand scheme of things (think 10K' foot view), how much bleed air is actually needed to start a turbine? The H-60 can start both of its engines simultaneously (at lower altitudes) with the APU, so that's certainly a notable amount of pressure. But any idea on how that compares to, say, a F-18? Or since we're talking about a ship GTG, an C-130 engine?

I understand a F-18 doesn't have a free turbine (or at least I didn't think it did), but generally what I'm asking is do most engines tend to need roughly the same amount of air? I understand it's not apples to apples, but it's interesting that most engines tend to start at around the same Ng (~12-20%), but obviously some engines are working harder than others.

I'm mostly thinking out loud at this point, but I know you and IKE can out-pocket-protect most of us here on the engineering stuff.

Regarding the F-18 engines, what do you mean, "free turbine"? That normally describes a gas generator that is separated from the mechanical loads it supports (e.g. PT-6). Both major variants of the F-18 use dual-spool turbofans (F-404 and 414 respectively), so the term as I understand it doesn't really apply to those engines.

As far as "how much" starting air is required, that's a great point- bigger turbofans take more volume of air (vs. pressure) to start, so larger APUs or huffers are required. I once saw a ground huffer nearly expire trying to external air-start a Global 7500 during "minimum spec" GSE testing. The engine started, but the huffer was very unhappy about it. I think they even changed their AFM limits because of it.

Air volume delivered might be the limiting factor for starting a DDG's prime mover, since it's similar to a large airliner turbofan (DC-10 IIRC). Exactly how much air that needs vs. what a helo APU can provide is in the weeds enough that I'd have to phone a friend for more specifics.

Paging skipper @IKE ...

 

[IKE]

Paging skipper @IKE ...

As much as I'd love to do the math on this, and call sign notwithstanding, I don't know.

So...I did what every self-respecting nerd does and asked AI.

Gemini reports the GE LM2500 shipboard gas turbine requires 2,000-2,500 CFM to start, while the H-60 APU produces 650-900 CFM. Assuming similar pressure (35-45 psig)

 

[sevenhelmet]

As much as I'd love to do the math on this, and call sign notwithstanding, I don't know.

So...I did what every self-respecting nerd does and asked AI.

Gemini reports the GE LM2500 shipboard gas turbine requires 2,000-2,500 CFM to start, while the H-60 APU produces 650-900 CFM. Assuming similar pressure (35-45 psig)

Right to the point, as always. Thanks for taking the time to respond to that, IKE.

...even if it involved use of AI...

 

[haubby]

As much as I'd love to do the math on this, and call sign notwithstanding, I don't know.

So...I did what every self-respecting nerd does and asked AI.

Gemini reports the GE LM2500 shipboard gas turbine requires 2,000-2,500 CFM to start, while the H-60 APU produces 650-900 CFM. Assuming similar pressure (35-45 psig)

LM2500's are the GTM's. The GTG's are Allison 501-K34's. GTM's are started by bleed air from the GTG's. We really need two GTG's running in parallel to get the adequate bleed air required. One GTG will do it but you will have more slow or aborted starts and the cooling fans take alot of amperage to start for the GTM's.

 

[IKE]

LM2500's are the GTM's. The GTG's are Allison 501-K34's. GTM's are started by bleed air from the GTG's. We really need two GTG's running in parallel to get the adequate bleed air required. One GTG will do it but you will have more slow or aborted starts and the cooling fans take alot of amperage to start for the GTM's.

Thanks for the correction and to hell with AI. But also, Gemini says the 501-K34 require even more power at 3,000-3,500 CFM, so 5ish H60 APUs...or 6-7 kickass leaf blowers?

 

[haubby]

Thanks for the correction and to hell with AI. But also, Gemini says the 501-K34 require even more power at 3,000-3,500 CFM, so 5ish H60 APUs...or 6-7 kickass leaf blowers?

Helm Yeah!

 

[Gatordev]

Both major variants of the F-18 use dual-spool turbofans (F-404 and 414 respectively), so the term as I understand it doesn't really apply to those engines.

That's all I was getting at...different design.

Gemini reports the GE LM2500 shipboard gas turbine requires 2,000-2,500 CFM to start, while the H-60 APU produces 650-900 CFM. Assuming similar pressure (35-45 psig)

Gracias. But as haubby mentioned, it should be the 501.

Thanks for the correction and to hell with AI. But also, Gemini says the 501-K34 require even more power at 3,000-3,500 CFM

...which seems strange. Presumably the smaller engine that has the more limited start capabilities would be less, not more. But who am I to argue with AI?

 

[sevenhelmet]

But who am I to argue with AI?

A human being.

I will die on this hill: human judgment and experience cannot be replaced by large language models or anything else the tech bros are slinging bags of money to bring about. If they spent 10% of what they've put into AI on worker education and welfare, their employees would be ecstatic.

To hell with AI and social media (and yeah, I use both. They have us addicted unfortunately...)

 

[Gatordev]

A human being.

I will die on this hill: human judgment and experience cannot be replaced by large language models or anything else the tech bros are slinging bags of money to bring about.

False. If a picture shows Margot Robbie in a thong and she has 11 fingers and three elbows, then clearly Margo Robbie has 11 fingers and three elbows. There's no arguing it.