Why no vacuum or hydrogen based airships?

Factual Questions
181

I would like to thank you all for the effort everyone in this thread is putting into educating Scylla. I am very impressed with the range of ways that people have tried to articulate why the idea wouldn’t work. I hope it’s having an impact on its intended audience.

This part is true, but you can’t pump more air out of the jug than the jug weighs so you can’t make a buoyant jug. In fact, what people are trying to tell you is that there is no known material in the universe that will allow to create a vessel that is as light as the air it contains but strong enough not to collapse if you pump all that air out. If you try to make a negative pressure vessel that only withstands some negative internal pressure (i.e., you only partially evacuate the vessel), you can make the vessel weaker and lighter but you will also get less buoyancy because you will still have a lot of air inside. There is apparently no combination of materials and design that will allow you to make a buoyant negative pressure vessel at any level of negative pressure.

At this point, you just have a hot air balloon inside a cold air balloon. If you just eliminate the outside cold air balloon, your internal balloon doesn’t have to carry all the cold air balloon’s weight. The outside cold air balloon isn’t contributing anything to buoyancy.

Yes. And if you eliminate the jug, your hot air balloon will be able to lift more because it will not have to also lift the weight of the jug.

So now you have a heated helium dirigible with the rigid airframe made out of whatever shell material you choose. So you are proposing two elements that differentiate your machine from today’s dirigibles: (1) heated helium, and (2) rigid shell.

(1) I see no theoretical reason you couldn’t make a heated helium dirigible. I’m not sure that the engineering limitations would ever make it worthwhile to do so. According to Machine Elf’s numbers earlier, Helium has only 14.8% of the density of the air that it displaces. Heating the helium would get you some additional lift. How much extra lift you got would be a function of how much you heated it. Using real numbers from the Goodyear site (here:Current Blimps | Goodyear Blimp) and converting cubic feet into cubic meters, I know that there are 8425 cubic meters of helium in the Goodyear Lightfoot One’s envelope. All that helium at Machine Elf’s 0.178 kg per cubic meter means that the helium in the ship has approximately 1500 kg of mass. I understand from googling that hot air balloons are heated roughly 100 degrees Celsius. If we assume that you heat your balloon to 100 C (373 K), and using the gas constant for helium (2077 J/kg K) and atmospheric pressure of 101,300 kPa, we get a density of 0.1308 kg/m^3. That implies a mass of helium in your heated Goodyear Lightfoot One of about 1,101 kg. That means by heating your balloon, you could carry about 399 extra kg of payload, less whatever fuel and heating equipment you’d need to carry to maintain that temperature. I suspect in the end that you wouldn’t be able to carry much extra weight at all once you factor in the heating equipment and fuel. This is probably why there aren’t a lot of heated helium balloons.

(2) So how does your rigid shell help you? Essentially, you would have to substitute your rigid shell in place of the rigid airframe used in today’s dirigibles. For comparison, you would have to replace this lightweight Goodyear airframe (http://www.carbonfibergear.com/wp-content/uploads/2013/09/rear-frame-goodyear-zeppelin.jpg) with a shell that was superior in some way. If it were lighter, that would be great, but I doubt that you could make a shell to cover that whole airship that’s much lighter than Goodyear’s spindly structure. Maybe it would be stronger in some way that would benefit the design (like allowing a more aerodynamic shape or resisting bending better so the dirigible could turn faster or resist higher winds). I am guessing that the aeronautic engineers that designed the Goodyear blimp have a pretty good base of experience to believe that their rigid airframe would beat your eggshell design. You could try to experiment and prove them wrong. Finally, your eggshell design might help to insulate better and reduce heat loss in your heated design. Because your shell is made of imaginary material, we don’t know much about its insulating properties. For this insulating benefit to be worthwhile, it would essentially have to offset whatever weight penalty it costs with reduced heating fuel consumption. Good luck since all the heating fuel has to weigh less than 880 lbs.

No.

On the topic of aerogel negative pressure vessels:

I’d like to thank you for this post. I read through the whole thread wondering if anyone would ask or answer this question. I was guessing that the evacuated aerogel wouldn’t be strong enough to withstand the pressure. It’s kind of the corollary to the idea that there is currently no material good enough to create a pressure vessel strong enough and light enough to displace more air than it weighs. I understand that aerogels are made of materials including silica, carbon, or metals. We already know that we couldn’t build a buoyant pressure vessel out of these materials even with ideal engineering. What are the odds that aerogel would just randomly form itself into billions of little pressure vessels that could stand up to the task? Still, that it has only 2% of the strength it needs is surprising. What is the closest thing to a functional buoyant negative pressure vessel we could build? What material would it be made of and how close to buoyant would it be? A carbon-fiber sphere that’s 10 times to heavy? Is graphene aerogel as close as we can get?

182

And in case you thought that might still be significant - the Hindenbug had 200,000 cubic meters of lifting gas, resulting in up to 511,000 lb of lift. If you instead filled it with air (or air + inner helium balloon) and pumped it down by 1/400 of an atmosphere, you get about 1,300 lb of lift.

(Though for reasons mentioned above, I’m pretty sure the Hindenburg couldn’t actually withstand 1/400 atmosphere negative pressure.)

183

Tired and Cranky:

Those are good points and calculations. Thanks for taking the time to make them. One thing I believe you left out is that I need a lot less heated helium to fill to fill my envelope than unheated helium. So, whatever that difference in weight of helium is also saved. I didn’t follow your calculations enough to be able to correct for this, but I believe I did follow them enough to see that you did not take them into consideration.

My apologies if you did and I am mistaken, but if you did not and I am correct would you mind repeating your calculations taking this into account?

184

The only reason hot gas generates any lift is because it takes less gas to fill up the given volume.

185

All of the discussion in section (1) calculates exactly how much less helium you’d get to use – 399 kilograms less helium in your envelope. The reduced helium is what gives you the extra buoyancy or lifting power. This extra buoyancy would be offset by whatever heating equipment and fuel you would need to keep your helium heated to 100 degrees. If your heating equipment and fuel weigh more than 399 kg (about 880 lbs), heating the helium means your design will only carry less cargo than an unheated dirigible.

186

I don’t know how to put this more gently, but, given that there have been lots of people (often very smart) developing airships for coming up on 150 years now (including periods with all the R&D that came along with their use as important strategic military weapons) AND there haven’t been any hot air + light gas airships in that time, what does that tell you?

Your proposal doesn’t depend on any technology that wasn’t around for Count von Zeppelin, so it raises the question of is it more likely that a) you’ve come up with a simple, fairly obvious, idea, that NOBODY else in the 150 years has come up with, or b) it turns out that in the real world there’s no real advantage to this proposed set-up?

Answering that question isn’t proof it wouldn’t work, or an explanation of why, or anything, but it is maybe an indication that it makes sense to pay more attention to the arguments why it wouldn’t work, as those arguments seem to have been borne out in the real world.

187

Ok, so I get a 25 pound barn fan for circluation and place it in the interior frame of the blimp. In front of it, a high end electric space heater. I run the cable down to the gondola and plug it into their existing electrics, total weight 100 pounds or less.

Ta-da!

188

I understand. If my goal is to accept that blimps exist, and that they are well engineered this is a great way to look at it. If my goal is to understand airships than this is a fun way to learn.

I am not actually trying to build a blimp.

189

Where do you think the energy come from in the “existing electrics”?

Presumably, there’s some sort of generator, maybe one built into the engines, which means the energy comes from fuel. If you want to heat up an entire blimp envelope, you’ll need to carry more fuel and a bigger generator.

(It occurs to me that the simplest means of heating a blimp would be painting it black, with perhaps some light weight reflective covering that can be used to prevent heat absorbption when you don’t need it any more)

190

A room in your house has a lot less surface area than a blimp, and has a lot better insulation. Despite this, an electric space heater isn’t anywhere near enough to heat the room all the way up to 100 C. The temperature to which it will heat the helium in your blimp is therefore that much less yet. Plus, you still haven’t addressed the question of the energy source for the heater, beyond handwaving that it’s somewhere in the gondola, but the batteries or generator or whatever the gondola carries still adds to the weight of the vehicle.

191

As it turns out, the Rozier design for a heated lighter than air balloon is all about maintaining constant pressure so your balloon doesn’t sink at night and not about trying to get a lot more lift. The easiest way from an engineering standpoint to make a blimp that can carry more is to build a bigger blimp.

While my ideas are intriguing, many of you will be surprised to learn that they are not yet practical, and there is not likely to be a radical redesign Renaissance in the lighter than air world based on the contents of this thread.

No. No. I know that’s an unexpected development, but that’s the way it is.

I’m sorry to have let you down, and unfortunately I will not be refunding your Kickstarter pledges as the blimp research had an unexpected beer, and blackjack cost overrun.

192

Nonsense, I will simply use zero point energy, or put a small wind turbine on top of the blimp to generate electricity. There’s a lots of wind at altitude. If there’s not all I have to do is make the blimp go faster. That will spin the turbine and Voila! There’s my electricity.

193

Actually, it is quite clear that the poster in question absolutely does not comprehend the concept of what makes an object buoyant despite repeated explanations by multiple posters. This discussion has degenerated into one of those threads where the o.p. asks a seemingly reasonable question about why something isn’t done or whether it could be made workable, and then spends the rest of the thread ignoring the cogent and detailed explanations, complete with examples and calculations demonstrating why said concept doesn’t work by protesting that if he is not convinced it must not be true, essentially putting himself in the same category as creationists, Moon landing conspiranoiasts, and Holocaust deniers. It is fruitless to continue to explain to the o.p. why his concept is unworkable with actual materials, and it seems that everyone else participating in the thread already understands the basic physics of Archimedes principle.

Stranger

194

True. You can’t educate someone who declares he’s going to ignore naysayers.

195

Not conversant with thermodynamics and conservation of energy, either, eh?

Stranger

196

The energy to run that electric heater doesn’t come from nowhere. It comes from the fuel that you carry on the blimp.

Hot air balloons don’t carry electric generators and then use electric space heaters to heat the balloon because that’s less efficient and would add weight. Hot air balloons burn lightweight fuel to heat their gas. You should probably do the same.

I’m not an engineer so I’m just doing a first order approximation of how much fuel you might need. I’ll let the real engineers here correct my numbers if they are interested.

This site says that a typical hot air balloon has about 90,000 cubic feet of air, or about 2548 cubic meters (http://socalballoons.org/ballooning/faq/) and it needs 30-40 gallons of propane, or roughly 126 to 168 lbs of fuel for a flight. So from what I can tell, this is about the amount of fuel necessary to keep 2548 cubic meters of air at about 100 Celsius. At 100 Celsius, the mass of the air in the hot air balloon is about 2411 kg by my estimate (using the air gas constant from this site Gases - Specific Heats and Individual Gas Constants and a density of air of 1.225 kg/m^3).

By comparison, the envelope in the Goodyear Lightfoot One contains about 3.3 times more volume of gas. It is also full of helium which has less mass than a comparable volume of air but which has a higher specific heat – that is, it takes more energy to raise the temperature of a kilogram of helium by one degree than it takes to raise a kilogram of air by the same amount. As I noted before, the mass of your hot helium is 1,101 kg. This means mass of the helium in the dirigible is only about 45.7% of the mass of the hot air in the balloon (1,101 kg of helium versus 2411 kg for hot air). But, it takes about 19% more energy to raise the temperature of a kg of helium than it does to raise the temperature of a kilogram of air. In the end, that means you need only about 54% of the energy (fuel) to heat the helium in the dirigible than you need to heat the volume of air in the hot air balloon. So, you need roughly 68 to 91 lbs of fuel for heating the helium (or about 16 to 22 gallons of propane). You also need the burner and the tanks. A 20-gallon titanium tank apparently weighs about 40 lbs empty. Wikipedia says a burner weighs about 50 pounds.

So, with minimum fuel and equipment, you’ve already used up roughly 180 lbs of the theoretical 880 extra pounds that you get to carry. But you’ve created another problem. Hot air balloons can burn their fuel right in the envelope. You can’t because your envelope is filled with helium that won’t burn. So you need to find a way to move the heat from 7 million BTU burner into your helium-filled envelope. That’s more weight, and if the heat transfer process is less than perfectly efficient, more fuel that you would need, adding yet more weight. I don’t have a good idea how you would accomplish the heat transfer. All of the helium heating adds complexity with seemingly modest theoretical benefit. The hot helium idea seems worse and worse the more I look at it and I thought it was terrible to begin with.

197

Engineering went almost nowhere from the era of the Egyptian pyramids to the mid 1700s. It was the advent of serious math and math education that elevated man’s ability to build stuff from rote hand-me-down empiricism to deliberate design.

Until you can write and explain the equation for it, you don’t understand it. Certainly not well enough to build it. You can have some vague qualitative notions about how the shin bone connects to the arm bone, but that’s about it.

For truly primitive stuff like building a garden shed empiricism is plenty good enough. If after 2 years the roof sags, add some more 2x4s. Flying machines are much harder because gravity is relentless.

198

This is true. However you did need what you call empiricism to steer the number crunching, otherwise you never doing any new or exciting. You just make more efficient blenders.

199

That’s not true at all, and if anything the opposite is true. Elon Musk is famous for promoting “first principles” type thinking. Instead of saying “electric cars suck because electric cars have always sucked”, he went and actually did the math on how many batteries you can actually fit in a car, how much range you would get out of it, and so on–and concluded, correctly, that you can build a great electric car. Everybody else was operating on intuition and analogy, but it took (really a tiny amount of) math to prove that it’s indeed possible.

200

Ok. That one is trounced. The version they used in the Breitling orbiter essentially had a hot air balloon surrounding a helium balloon. Don’t think that would work in a blimp.
How about this. According to fueleconomy.gov. 58-62% of an automobile engines efficiency is lost as waste heat. I could not find figures for blimp engines, but assume that there is some correlation.

How about instead of putting the engines outside the gondola, we put them on top of the gondola inside the pressure envelope. We run the air intakes back outside. Next we run the exhaust through a heat exchanger before venting it outside. We run the propellers out on drive lines.

That should add very little to no weight.

Using Kyles converter (found on web) that converts hp to btu I find that 400 hp equal over 1,000,000 btus/hr. If the twin 200 hp engines on the GY blimp are equivalent in their thermal efficiency that means they throw off 1,200,000 btus of waste heat per hour.

Let me try to figure out what that does for us.