Let’s say you are at a children’s party. They run out of helium for the balloons! OH NO! Uncle Joe has an idea: He calls his buddy from the Hydrogen Plant and gets a tank. Now everyone has ultra-floaty balloons… until the moment that a 737 is flying over the party at the exact same time Little Timmy loses grip of his balloon. The balloon is flying high, right in front of the plane’s engines! OH NOOOOOOOOO! What would happen if the plane ingested a balloon with flammable gas? What about… 100 balloons?
Well, to start, the jet’s engines are always ingesting a flammable gas called “oxygen”, if they didn’t they’d stop running.
Second, even hypothesizing a child’s balloon filled with some other-than-oxygen flammable gas, the volume contained in the balloon would probably be insignificant compared to the total volume of air rushing into the engine and it probably wouldn’t even be noticed. Any effect would be minor at most.
Of course, if a real jet pilot or engineer chimes in with a correction I am willing to revise my stance.
Assuming a gallon of Hydrogen per balloon, you will need 357 balloons to have equivalent energy to a gallon of gasoline.
An 8 hour flight uses around 300,000 lbs of Jet fuel, so roughly 10 lbs of fuel per second or 12 gallons of fuel per second.
So you see it wont be a big deal to have such an small amount of energy added even if you could have 357 balloons go through the engines in 1 second.
There are gas turbines (and aero derivatives - which are airplane engines modified for power production or use as a drive) that run on pure hydrogen (in refineries) or high hydrogen content fuel - it is no big deal.
The number you need to watch out for fuels in gas turbines (air plane engines) is the Wobbe Number. As long as the fuel meets the Wobbe number, the turbine has no problems.
Oxygen is not ‘flammable’ in the typical sense; it does not act as a fuel in the combustion reaction. It supports flame (as the quintessential oxidant,) but oxidants are not themselves flammable, as I understand it.
However yes, the hydrogen in the balloon probably isn’t much compared to the jet fuel that’s already being burned in the engine, and I doubt that the balloon will be shredded, releasing the hydrogen fuel, significantly earlier in the process that it could damage a part of the engine that’s not supposed to be exposed to flame. That’s a WAG though.
(This was in reply to Broomstick)
I’d think the concern would be getting plastic gunk inside the engines (which would apply regardless of hydrogen, helium, or hot air). Even there, I am most definitely not an aeronautical engineer but I wouldn’t think there would be enough gunk to matter.
Wouldn’t the more likely problem be the lack of Oxygen to burn the fuel, if you somehow had a whole bunch of balloons lined up in front of one of the engines.
Me, I would worry more about what the rubber might do in the engine than about the hydrogen. I suspect 357 balloons worth of rubber might bind or gum something up.
Maybe I calculated wrong. According to my google, hydrogen has 319 BTU per cubic foot or about 38 BTU per gallon at normal air pressure. That would take about 3500 gallons of hydrogen gas to equal one energy gallon of Jet fuel.
Ok, so… forget the hydrogen shenanigan. Would a flock of balloons do anything to a plane?
Ahem…
Is the kid who releases the balloons running on a conveyor belt?! 
Thank you, enjoy the veal…
Jet fuel density is around 6.7 pounds per gallon, so 10 lbs per second is actually about 1.5 gals / sec.
Most of the balloons are going to be knocked away from the plane before they could have a chance of getting into the engine. The very few that get in would have less impact than a duck, as they aren’t heavier than said fowl.
ducks in an engine would be a fowl fouling.
They might freak the pilots out for a second, on a “Hey, what’s that goat doing up in a cloud” level, which could be problematic if they were on final approach or takeoff on manual control.
You are right - I calculated it incorrectly.
Xema - you are right too - I did the calc in a hurry and made lot of mistakes. Maybe I’ll get some time tomorrow to rerun the calcs.
The plane as a whole? Not much. Each hydrogen-filled balloon is lighter than the parcel of air it displaces, so the notion of “impact” upon the airframe is pretty much meaningless.
As far as hydrogen balloons going into an engine, not sure. Hydrogen has an upper explosion limit of 75%. That is, you could have a blend that is 75% hydrogen and 25% air, and it would support a flame front. And spheres have a maximum packing factor of about 74%. So even if you had balloons spaced together as tightly as possible (without squeezing them), you’d be introducing a flammable mixture into the engine upstream of where it’s normally found.
Hydrogen has a considerably higher flame speed than most hydrocarbons, so I’d guess once the balloons got shredded by the compressor fan and made it back to the burners and got lit, you might flame out the engine. In the worst case, maybe the brief overpressure would split the combustion cans; I’m guessing it’s more likely you’d just end up with a brief flow reversal, something akin to a compressor stall, followed by resumption of normal engine operation.
Modern civilian jet engines have high bypass ratios, meaning that most of the air that enters the nacelle never goes through the core. It is accelerated aft by the fan (which is what you see when you look into a jet engine) and bypasses the rest of the engine completely. The core has a much smaller diameter than the fan, and anything solid that hits the fan is flung outward toward the nacelle wall and away from the core. My not-completely-WAG: the balloons themselves would be shredded by the fan blades and pass harmlessly through the bypass duct. The majority of the hydrogen would do the same. The small amount of hydrogen that makes it into the engine core wouldn’t do much more than cause a very slight increase in turbine temperature for a fraction of a second. It probably wouldn’t even be noticeable to the pilots.
I’m thinking along the same lines. The plastic could meld onto a rotor blade, inducing a vibration into the rotation. That vibration would likely be picked up by sensors, reported in the cockpit and worst case scenario would probably only be the need to shutdown the engine and make a precautionary landing.
Now, I suspect a single balloon isn’t enough to even cause a detectable vibration, and it’s likely to be spread out and not a “point mass” on a single rotor blade (which I think would be worst case scenario), so the risk would still be pretty small. I have no idea how much mass added to a rotor (or removed, if it broke) would cause a detectable vibration,and I’m sure it depends on which rotor/stage, the engine type and size, etc.
This is just a WAG though. I don’t do much work with engines.
What if there were 99 of them - and they were red?
Hitting balloons (at least in reasonable numbers) with a jet is a nonevent. As explained above, almost all will deflect around the aircraft.
Somebody who tried to do some fuel consumption calcs upthread messed up a a bit. A reasonable fuel burn for a cruising airliner is on the order of 100 lbs of fuel per minute. A bigger jet will burn more, as will an older less efficient one. Newer smaller RJ-type jets might be half that. 100lbs/minute is very very roughly 2 lbs per second. Or very very roughly 1 quart of kerosene per second.
The fuel energy in a kid’s balloon-worth of gaseous hydrogen is a tiny fraction of the fuel energy in a quart of kerosene. Even eating a balloon right down the engine throat where all the hydrogen went into the combustor would be a non-event.
One of the design challenges with building hydrogen powered airliners is that even if the hydrogen is cooled to a liquid or a slush, it still doesn’t have the energy density (calories/unit volume) of kerosene. So although the stuff is light, we’d need many more cubic feet of fuel tankage versus the same craft powered with conventional fuel. And we’d have to deal with all the other fun hassles of cryogenics.