h a l f b a k e r yYeah, I wish it made more sense too.
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I 'calculated' that for a Boeing 777 you'd save a few hundred kgs allowing greater range, increased capacity or fuel savings. The resulting squeaky voices might help calm nervous flyers especially in an emergency. Disclaimer: I made numerous assumptions and guesses in my calculation.
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Helium is too expensive. Use hydrogen. But how are you going to deal with CO2 build-up and bring in fresh oxygen without flushing your wonderfully combustible hydrogen or your very expensive helium? |
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Hmm... cryogenic filtering? CO2 conveniently turns solid at a reasonably-achievable temperature. Remove, add O2 to the He, return to cabin. |
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Running the numbers. A 777 is approx 1000m3 internal volume (excluding seats, people, cargo, packets of peanuts etc.). The O2 has to stay, so 80% of the air replaced with Helium would give just under 7kg mass of Helium. The equivalent standard air (no water vapor for the moment) would be 488kg. That's on the ground though. At cruise, the pressure altitude is set to around 10,000ft, which is 2/3rd sea level pressure. So the difference is now ~320kg. Let's assume the people/cargo/peanuts are 1/3rd of the volume. Your saving is now a couple of hundred kilos, or 2 passengers, and you have to construct a CO2 scrubber and oxygen concentrator/injector system into that system, tricky to pull off with a 200kg weight budget. |
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You can get a better weight saving by not painting the aircraft. Personally, I'd remove the life jackets. I'm not an expert, but I'm fairly well read on aviation safety and I'm unaware of an example of passenger life jackets being useful. Plenty of examples where they were detrimental to safety and the weight saving would be considerable, along with the saving of time for the little demo the cabin crew do. |
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How about filling the aeroplane with vacuum? I understand vacuum is lighter than helium. |
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//How about filling the aeroplane with vacuum?// It is. Tricky to do on the ground/at low altitude, but progressively easy as you climb. Concorde practically flew in a vacuum, ~5% normal atmospheric pressure, so you have very little pressure differential to deal with. The problem however, is that everyone always dies in the first five minutes. This is somewhat to do with a lack of pressure and a lot to do with a lack of pO2. You can compensate for this up to about 20% normal atmospheric pressure (~15,000ft) by just making the air ALL O2. You have 1/5th the pressure but 5x the concentration, so it works out, bar a few issues with CO2, humidity etc. |
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The next problem you create, is that a 100% O2 atmosphere and fires really make a synergy of destruction. See apollo 1. |
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//cabin pressure of 3.7 PSI and a lower risk of fire.// |
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On the ground? That's tricky to work with, the capusle is going to spend all of it's mission at positive pressure to the outside (because there isn't any pressure outside!). The obvious way to do it is to keep the capsule at whatever positive pressure you've chosen (3.7 PSI seems reasonable) vs the outside. So sitting on the launch pad you'd expect it to be above ambient (16.7 PSI again, makes sense) by about the same delta P you anticipate for the mission, that way any leaks become obvious, and the structure is under relatively constant stress. |
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The fire made it obvious that high pressure O2 and anything flammable were a bad idea, but I imagine the way to go is to make the air inside normal % O2 and the same 16.7 PSI, and bleed off pressure as the capsule rises, blending in a higher percentage of O2 as you go. Or did they go with a constant internal pressure switching from -ve to +ve during ascent? Seems risky from a structural/sealing point of view if you ask me, but then I'm no Apollo expert. |
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It's interesting that fires don't go out any easier at low pressures. |
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Airliners are very leaky, vents and pumps ensure continuous ventilation. Perhaps requiring passengers to wear those masks at all times would work and your mix of choice could be delivered directly to them. |
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