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Comment by tomrod | Hacker News Reader

This is pretty cool technology. If fuel were not an issue, and no other recharges (water, food, etc.) were available, how long could a plane with passengers stay in the air?

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This is often talked about in the case of Air Force One (which is supposed to be able to keep flying in case of a nuclear war, so that the president can survive and order the military). The limiting factor is probably engine oil, e.g. this article suggests 2-3 days. https://executiveflyers.com/how-long-can-air-force-one-stay-...

The new VC-25B aren’t going to have aerial refueling capability as cost saving measure. They do have longer range.

> The new VC-25B aren’t going to have aerial refueling capability as cost saving measure.

Also: the old VC-25A did/does have it, but it was never used in ~30 years of flying.

Thanks for sharing. I never realized engine cycle times were so short on planes!

> It’s generally recommended that aircraft engine oil should be replaced every 50 hours, otherwise the oil would turn to sludge and get contaminated, which would damage the engine.

This sounds like piston engine advice. I’d be surprised if jet airliners are going in for oil changes twice a week.

Jet engines constantly pump oil under pressure between the stator and rotors to provide lubricant for the radial bearings while sealing and cleaning them. It’s a completely different kind of oil to the kind you’d find in a piston engine and operates at a much higher temperature.

Agreed but what’s the service interval on that oil? Does it get changed regularly or just topped off?

It’s a piston plane but my understanding of the DC-3 is that it runs out of fuel around the same time it runs out of oil.

Hyllion Karno is a good example for power generation unit required ultra-low maintenance.

Hybrid-electric propulsion systems can enhance flight endurance by separating propulsion and power generation tasks, individually optimizing each for specific stress levels aligned with mission requirements.

Does the marginal efficiency make up for the added weight? Aircraft engines already operate in a very narrow envelope so they can be optimized for cruise.

There was plans in the 50s/60s to have a fleet of nuclear aircraft staying airborne for 2 months. That job ended up being taken by submarines, some staying in mission for 6 months. So the logistic of keeping people and a few ICBM in a metal tube for months is pretty much sorted out. With the hypersonic craze, the money is back to fund scramjets tech too. Nuclear planes are a terrible idea, don't get me wrong, but it's not impossible to build them, it has never been. The problem is radioactive exhaust for the direct cycles ones (the easy one) or reliability for the indirect cycle (the "good" ones).

They are pretty heavy due to radiation shield material requirements.

I am pretty sure material science went a long way. No need to put a foot of lead behind the reactor. The space race provided quite a bit of funding toward lightweight protection. It was too late for those plane project as they were already ramping down in the mid 60's.

Neither that or the plane powerplant are unsolavle problem, it's more of a "why would we invest this much to solve something this silly". Apparently the russians are trying to resurrect their SLAM clone and tested one. How much of this is a realistic military project versus propaganda isn't super clear to me. It's up there along with the manned military space stations in the scale of pointless deterrent PR.

Well a passenger plane needs at most 20,000 km (20 Mm) of range, since that is the furthest distance 2 points on the surface of the Earth can be apart. Most large jet airliners cruise at around 900 km/h, which gives a maximum useful endurance of around 22 hours. There's no point having an endurance much longer than that.

AFAIK, no airliners can do that yet, though some are getting close. Some of the business jet variants can (e.g. BBJ 777-8, BBJ 777-9).

The longest duration 'scheduled' airline flights to date were the Qantas 'double sunrise' flights during WWII, which were ~32 hours long [0], though the Graf Zeppelin (airship) made passenger flights longer than this [1].

0. https://en.wikipedia.org/wiki/The_Double_Sunrise

1. https://en.wikipedia.org/wiki/LZ_127_Graf_Zeppelin

Well, the record is 64 days. They did oil changes inflight on the running engine.

https://simpleflying.com/robert-timm-john-cook-endurace-reco....

Until its next forced reboot. Rebooting a plane mid flight is suboptimal.

Until the next maintenance period? I'm probably completely wrong, but I think on some planes that might be 100's of hours before some maintenance is due.

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