Picture a normal hot air balloon, inside the balloon itself hangs a "lantern" on a fire proof rope. The latern would be a radioisotope heater unit (see link) preferably plutonium 238 due to the fact that it is easily shieldable and doesn't give off a lot of harmful ionizing radiation. (while working for NASA, I handled them myself and I haven't gained any super powers yet) each gram produces .5w of heat so about 1-2KG of fuel should provide enough energy to keep the air hot. The balloon can also be insulated to minimize heat loss. A small blow out port can control bouyancy by introducing cold air

once this balloon is in the air it will stay afloat indefinitely and wouldn't need any replenishment like hydrogen/helium weather balloons.

For bonus points thermoelectric generators could be fit onto the heater to provide power to radios or other electronic gizmos.

Perfect for running a low cost (to launch) "satellite" or spy blimp. Or simply just to run your own private google earth view.
-- metarinka, Dec 10 2010

RTG http://en.wikipedia...oelectric_generator
thermoelectric generator [metarinka, Dec 10 2010]

heater unit http://en.wikipedia...isotope_heater_unit
already used by nasa to keep space probes warm [metarinka, Dec 10 2010]

Pu-238 http://en.wikipedia.../wiki/Plutonium_238
Plutonium 238 is the preferred fuel, very safe. [metarinka, Dec 10 2010]

This can use it Ethermal_20Resting_20Place
[theircompetitor, Dec 10 2010]

Godiva device http://en.wikipedia.../wiki/Godiva_device
For the brave ... or foolhardy ... [8th of 7, Dec 11 2010]

Louis Slotin http://en.wikipedia.org/wiki/Louis_Slotin
Criticality accident [8th of 7, Dec 11 2010]

(?) http://en.wikipedia.org/wiki/Harry_K._Daghlian,_Jr. Harry K. Daghlian, Jr.
Accidental irradiation [8th of 7, Dec 11 2010]

Nuclear hot air balloon for Titan http://io9.com/5404...ear-hot+air-balloon
[ldischler, Dec 12 2010]

1kW is nowhere near enough to keep a hot air balloon afloat, but I think plutonium radiates a lot more energy than that.
-- marklar, Dec 10 2010


According to wikipedia a standard sized hot air balloon has a 2-3MW heater. 1KW would not be much lifting capacity at all.

EDIT: off by 3 decimal places.
-- DIYMatt, Dec 10 2010


1MW is 1000 x 1KW. Are we then talking about wanting 2 tons of plutonium to lift our small, lightweight balloon? Perhaps the plutonium mass should be mounted on a long pivoting telescopic stick, so its weight does not need to be carried by the balloon.
-- pocmloc, Dec 10 2010


This is clearly not feasible without a reactor.
-- marklar, Dec 10 2010


//2-3MW heater// But it's not usually on all the time, so the average power needed is less.

There are some (hideously dangerous) lightweight ways to nudge a barely subcritical lump of plutonium into non-'splodey ever-so-slightly supercriticality, and thus massively increase the power output. That way you'll only need a few kilos. Look up "criticality accidents" to see how. I hope you've got a steady hand...
-- spidermother, Dec 10 2010


In WWII, the Japanese sent fire balloons drifting across the Pacific, to fall, randomly, in North America, starting fires and theoretically striking terror into the hearts of the Americans. They did little damage. But think if they'd been able to travel farther, with a payload of toxic, radioactive plutonium.

Difficult to defend against, especially if boobytrapped with chemical explosives, to disperse the plutonium.

Edit: delete "toxic," insert "Dangerous and" before "Difficult," and delete "especially."
-- mouseposture, Dec 11 2010


//There are some (hideously dangerous) lightweight ways to nudge a barely subcritical lump of plutonium into non-'splodey ever-so-slightly supercriticality, and thus massively increase the power output//

I thought about doing this but dismissed it because it would reduce the service life of the fuel source to practically nothing (compared to its normal life). But this gave me an idea - what if the balloon had a mechanism to turn the "afterburner" on and off as needed? Balloon starts to sink, actuator jams spheres of plutonium together, balloon rises again. It conserves fuel and generates enough heat. Although anybody caught in a 200' radius would die a slow and painful death.
-- DIYMatt, Dec 11 2010


you would be surprised. If properly shielded (and being a source of mostly alpha radiation. You could practically sit on top of the fuel and not be at risk of radiation burns. I don't think there's been much luck taking fuel super critical with air cooling

Like I said when I worked with nasa we would physically (with our hands) hold RTG elements and not have to worry about radiation exposure and that shielding was not even an inch thick.

also a normal balloon is typically nylon with no insulation. Some very thin but good insulation such as mylar or aerojel could decrease your heating needs by factors of 10 as compared to normal nylon alone. And as mentioned this is a constant output instead of short bursts like a burner.

Logic tells me that with some spreadsheet math you can find the equilibrium point where so much heat input is needed to keep a certain spherical volume of air a certain temperature.

you could also use a thermometer or pressure based switch to raise or lower a nuclear poison to moderate reactivity, but I think it would be easier to have constant reactivity and just have a valve that moderates pressure.
-- metarinka, Dec 11 2010


With that power density, it can be used to trickle charge electric cars. Never aving to plug them in would be much better, especially when there is no plug...

None the less, if these cars had a large baloon on the roof, then the doom fuel could have 2 purposes. Excelent, flying cars with unlimited fuel:O)
-- saedi, Dec 11 2010


// I don't think there's been much luck taking fuel super critical with air cooling //

Define "luck" in this context.

<links>
-- 8th of 7, Dec 11 2010


...could we see some figures on energy/weight....
-- WcW, Dec 12 2010


This is already the plan for the exploration of Titan by hot air balloon. See link.
-- ldischler, Dec 12 2010


//1KW would not be much lifting capacity//

Since Titan is so cold, much less heat is needed for the same lifting capacity. The estimate is around 1% of the heat required to lift the same payload on Earth.
-- ldischler, Dec 12 2010


The ambient temperature has nothing to do with what heating capacity you will need, as you will always require a temperature X degrees above it and the cooling effects of the surrounding atmosphere will depend only on the temperature gradient to the inside.
-- marklar, Dec 12 2010


Wrong, marklar. If you are at 100K and you increase the temp to 200K, you've halved the density of the air, and you only have to deal with a 100k differential for heat loss. To achieve the same lift at 300K, you'd have to increase the temp. to 600K and deal with a 300K differential. (The temp on Titan is 94K, and on Earth, about 300K.) So 1/3 of the heat requirement and 1/3 the heat loss, which gives about 10% of the power needed on Earth. But that is still ten times higher than this 1% estimate.
-- ldischler, Dec 12 2010


Wrong, ldishler. It's the difference between inside and outside densities that determines lift, not the ratio. If you heat from 300K to 600K you achieve only 1/3 the lift, compared with heating from 100K to 200K(assuming the same pressure and composition of the atmosphere). So the benefit of a cold atmosphere is even greater than you suggest.
-- spidermother, Dec 12 2010


What I'd missed before: The density on Titan is almost ten times that on Earth, so a smaller balloon is required on Titan for the same payload and temperature ratio. The diameter would have to be 2.15 times greater on Earth, and the surface area 4.64 times greater. So the heat loss is 4.64x3=14 times greater for the earth balloon, given the same temperature ratio. But then you have to factor in the higher heat loss at a higher pressure which will reduce the Titan advantage, and the lower weight of the balloon, which will increase it.
-- ldischler, Dec 12 2010


Err... Ah yes, well done, you passed my test and didn't fall for that little joke of mine <walks away whistling>.

But seriously, the calculations are still not quite right. You can't claim density as a benefit and then not use it to also calculate the additional heat loss. If the density is x10, the Earth balloon would have only 1.4x heat loss.

Also, assuming the same atmosphere (which obviously it isn't on Titan) 100k air is more dense than 300k air, so the 100k outside the 200k balloon would definitely cool more than 300k outside a 400k balloon, but I don't know about a 600k balloon, my knowledge of fluid dynamics does not extend nearly far enough to know the formula.

edit: Haha, [ldischler] by the time I posted this, you'd edited your last line to mention it.
-- marklar, Dec 12 2010


As for reactors, the US SP-100, designed for space use, puts out 2 MW thermal and 100 kW electric.
-- ldischler, Dec 12 2010


But how much did the SP-100 weigh? Pictures suggest several tons.
-- DIYMatt, Dec 13 2010


you guys are also forgetting that gravity is much less on titan, so bouyancy has a higher effect, and it's composition isn't that of air. it's 98% nitrogen 1.4 methane and then "other" The gravity is so much less, that they say humans could fly by flapping wings attached to our arms.

If they are seriously considering it for Titan I'm sure some smarty pants have done to math that demonstrates its feasability. I think it would be possible on earth IF you use a good insulator like aerojel and a closed volume of air, you can also use cells or other internal structures to slow convective loss.
-- metarinka, Dec 13 2010



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