Helium balloons and 1 tonne.

You're so funny. Liquid helium is heavier than air. So you will not have any buoyant lifting capability at all with liquid helium.

I take it, that if you have a "bottle" of liquid helium and opened the valve, the liquid would convert to its gaseous state, which in turn would inflate the balloon.

So, how much liquid would be required to give sufficient gas in order to solve the problem?

Just taking a quick stab here:

The density difference between air and helium gas is roughly 1 Kg per cubic meter (closer to 1.05 Kg per cubic meter) ; so to displace a weight (mass) of 1 tonne (1000 Kg) you'd need roughly 1 tonne (1000 Kg) of helium gas.

In liquid form helium has a density of 145 grams per liter, so to get 1 tonne of liquid helium you'd need about 7000 liters.

_{/If I did my math right.}

Ummm - well your looking to displace a 'ton of air' (at STP ~ 1230g/m³ ) with the volume of helium @ 178.6g/m³ (at STP)

Even-so there is a fair bit of heat to find, to get the liquid to gas at STP, before impact.

^{Might be a better idea to invest in a parachute as thermal grenades tend to ruin balloons}

The heat of vaporization is very low. Also, helium has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. I'd have to drag out my old Physics books to calculate the amount of energy needed, but I'm thinking it wouldn't be an outrageous amount.

The latent heat of vaporization for helium is very low but don't forget that this is just the energy required to go from liquid to gas at the same temperature. In this scenario one must get the helium gas warmed to ambient temperature or it will not displace enough of a volume of air to create the 1 tonne of bouyancy. So you must also include the specific heat required to get helium to warm the approximately 300° C temperature increase. Yes, the atypical negative Joule-Thomson coefficient will induce some self heating as the helium expands but this will still require some considerable amount of heat to get this balloon inflated so that the balloon and payload can be pushed out of a cargo door of an airplane.

Speaking of the airplane, I suspect that not even the Airbus Beluga will be large enough to carry an inflated balloon like this that can carry a ton aloft.

Yeah - OP would want one a lot shorter than that

True, and seeing it's around 178 kg per 1000 m³ - plus a bit more to carry the balloon, harness, etc - ostensibly, not much to heat. However, it's being chucked out at 450 feet AGL, so there's not much time to get all the He in the envelope up to ambient.

I imagine if it wasn't quite quick, the payload descent velocity would - if not tear the balloon apart - tend to compress the He.

Hence the thermal grenade comment.

And I think you'd need a radiant source, there being no time for conduction or convection - (though I haven't checked the conductivity, or even time to impact)

Another thing is, I think heat - or over-heating - would be a way of achieving greater volume to curb the descent, but avoid it 'not landing' and then disappearing to the stratosphere.

I suppose you could do a 'drag-out deployment (like with chutes). I imagine the drag of the deflated balloon, would have the payload out the back, well before meaningful inflation took place.

And obviously the balloon is going to be quite long for the volume required, so that in turn 'shortens' the distance to impact - so time to inflate.

So I hope the above explains, more or less, my thought train, behind original comment.

How much have you figured out already?

Without this balloon, how much time elapses until the one tonne impacts the ground?

Without this balloon, what is the velocity of the one tonne at impact?

And, why would you take it the He changes phase? We do not know the temperature; is it part of the homework question you have not shared with us?

"based on various locations around the world with an average height of 50 metres above sea level;"

I don't know how much He you'll need, but let's just hope none of those "average" locations around the world is over 501 meters above sea level.

Once you get your required information here in CR4 you will ask the next question.

"How large will the balloon have to be and who builds these?"

I once saw what it took to lift one person. I need not formulas I just close my eyes and imagine 100 people being lifted, just lifted! It would be bloody huge!

http://blogs.chicagotribune.com/news_columnists_ezorn/2009/06/the-physics-of-up-how-many-balloons-would-it-really-take-to-lift-a-house.html

Hope all goes well, Ky.

Ahhhh nothing like watching a LAPES extraction outta the rear of a C-130 Herky Bird! But with Helium-filled balloon? Nadda!

As 34point suggested, pull the pins on those grenades, jump out and kiss your arse bye bye! LOL

[LAPES = Low Altitude Parachute Extraction System, for those not in the know....]. I'm sure there may be a U-Tube video showing how a palletized M-2 Bradley IFV is dispatched from the Herky! Just don't stand in the way of that puppy!

http://www.youtube.com/watch?v=3jYqSUl0Vn0 - Way too cool - just watch - Loupy

Well actually this is LAPES

I think you want more altitude as you can see in this one there is 'inflation' time involved.

Thank you, for all your advice.

My nephew saw a program on TV, where someone drove a car out the rear of a plane, a balloon inflated, and he continued when he landed.

I said it was Hollywood, he said it wasn't, I could not explain why (not that clever).

So once again, thank you all.

Ian.

That depends upon how fast the <...aircraft...> is going, which has been withheld, as has whether it is in a climb, level flight, or a dive.

It also depends upon the size and elasticity of the <...balloon...> into which the <...liquid helium...> were jettisoned and the speed at which the <...balloon...> were filled, which have been withheld.

It also depends upon the ambient temperature and the thermal conductivity and thickness of the material forming the <...balloon...>, which has also been withheld.

It depends upon the cloud cover and the local rain or snow precipitation, which have also been withheld.

What also needs to be withheld is any invitation to other CR4 readers try it out to obtain the time that it takes by experiment.