Comments on: Todays Tasks http://jpaerospace.com/blog/?p=429 Our Journey to Space Sun, 11 Dec 2022 19:44:38 +0000 hourly 1 http://wordpress.org/?v=3.0.1 By: Jeffrey Bridge http://jpaerospace.com/blog/?p=429#comment-23581 Jeffrey Bridge Tue, 10 Feb 2009 23:28:25 +0000 http://jpaerospace.com/blog/2009/01/24/todays-tasks/#comment-23581 A hot air balloon gets lift because there is a lesser density of air on the inside than the outside. The heating is what keeps the air inside less dense, and forces excess air to exit from inside the envelope. Because the bottom is open, the pressure is exactly the same on the inside as the outside. If you tried to close it off to keep sea level pressure inside the envelope, you'd have a few problems: 1. You can no longer just send the propane flame straight up into the envelope, as you would quickly increase the pressure (and mass!) inside the envelope until it burst. Some sort of (less efficient) heat exchanger would be required. 2. As you began going up towards 13 miles or whatever, the outside air pressure and density both drop dramatically. There is half as much pressure by about 18 Kft, a quarter by ~34 Kft, and about 1% by the time you reach 100 Kft. The only way for the hot air balloon to retain lift, regardless of the pressure inside, is for the mass of air inside to be less than if you filled it with the surrounding air. A quick calculation... assumptions: 1. air is an ideal gas 2. outside air density is 1% of sea level 3. pressure inside is 1atm In order for the inside density to equal the outside density (at which point there is zero lift; it needs to be less), the temperature would need to be about 30,000 Kelvin, or five times hotter than the surface of the sun, at which point the amount of lift being generated would be the least of your concerns. A hot air balloon gets lift because there is a lesser density of air on the inside than the outside. The heating is what keeps the air inside less dense, and forces excess air to exit from inside the envelope. Because the bottom is open, the pressure is exactly the same on the inside as the outside. If you tried to close it off to keep sea level pressure inside the envelope, you’d have a few problems:

1. You can no longer just send the propane flame straight up into the envelope, as you would quickly increase the pressure (and mass!) inside the envelope until it burst. Some sort of (less efficient) heat exchanger would be required.

2. As you began going up towards 13 miles or whatever, the outside air pressure and density both drop dramatically. There is half as much pressure by about 18 Kft, a quarter by ~34 Kft, and about 1% by the time you reach 100 Kft. The only way for the hot air balloon to retain lift, regardless of the pressure inside, is for the mass of air inside to be less than if you filled it with the surrounding air. A quick calculation…

assumptions:
1. air is an ideal gas
2. outside air density is 1% of sea level
3. pressure inside is 1atm

In order for the inside density to equal the outside density (at which point there is zero lift; it needs to be less), the temperature would need to be about 30,000 Kelvin, or five times hotter than the surface of the sun, at which point the amount of lift being generated would be the least of your concerns.

]]> By: BalloonWhisperer http://jpaerospace.com/blog/?p=429#comment-21846 BalloonWhisperer Fri, 30 Jan 2009 22:57:02 +0000 http://jpaerospace.com/blog/2009/01/24/todays-tasks/#comment-21846 Yep, that would work. Well, there might be a problem with holding on to the string while the rocket is exhausting burning gas, but you could build a capsule that would withstand it. Of course, as with most balloons that are exhausting gases, your capsule might be whipped around this way and that, or it may act as a stabilizing tail much as a kite tail does. And, while the adventurers would welcome a ride on a burning balloon, many of us old folk would prefer to watch them from the comfort of our spacious (pun intended) flying wing as it ascends under controlled power on its albeit longer flight to orbit. And, I suspect your head for physics is not much different from mine. I have rested on many such thoughts of new-fangled space travel, and never been found wrong in practice because no one has ever disproved an idea that hasn't been tried (except to those who are easily disapproving). Keep the ideas coming. Eventually we'll find that simple combination of on/off the shelf techno that will achieve our goal of CATS (cheap access to space). Yep, that would work. Well, there might be a problem with holding on to the string while the rocket is exhausting burning gas, but you could build a capsule that would withstand it. Of course, as with most balloons that are exhausting gases, your capsule might be whipped around this way and that, or it may act as a stabilizing tail much as a kite tail does. And, while the adventurers would welcome a ride on a burning balloon, many of us old folk would prefer to watch them from the comfort of our spacious (pun intended) flying wing as it ascends under controlled power on its albeit longer flight to orbit. And, I suspect your head for physics is not much different from mine. I have rested on many such thoughts of new-fangled space travel, and never been found wrong in practice because no one has ever disproved an idea that hasn’t been tried (except to those who are easily disapproving). Keep the ideas coming. Eventually we’ll find that simple combination of on/off the shelf techno that will achieve our goal of CATS (cheap access to space).

]]> By: Peter Merel http://jpaerospace.com/blog/?p=429#comment-21718 Peter Merel Fri, 30 Jan 2009 07:05:30 +0000 http://jpaerospace.com/blog/2009/01/24/todays-tasks/#comment-21718 I have a kind of stupid idea. At least I expect you to shoot it down pretty quick. But just asking ... What if instead of helium you just used a hot air balloon. A big one with a lot of tensile strength. Enough to hold, say, 14 PSI. Don't ask me what that should be made of because I don't know. So you heat this thing until it can float to say 13 miles, which I read is the height record for a 44 ton hot air balloon launched in India in 2005. Now assuming that you still have 14 PSI inside the balloon (no doubt a bad assumption) you just open up some sensibly placed nozzles and use the entire contents of the balloon as reaction mass to get the thing the rest of the way to orbit. And beyond - it seems like the surface to volume ratio is going to favor very large balloons so it's pretty easy to get a lot more pressurized air involved for the price of just a little more high-tensile-strength fabric. If for some reason there's not enough power to weight in this you could of course fill an appropriate fraction of the balloon with hydrogen and use a chemical rocket to accelerate the escaping gas. No idea if that's necessary ... Anyway I'm probably daft and there's probably some very good reason you can't use a blimp as a rocket. But since I have no head for physics I'd be obliged if you'd let me know what that reason might be. I have a kind of stupid idea. At least I expect you to shoot it down pretty quick. But just asking …

What if instead of helium you just used a hot air balloon. A big one with a lot of tensile strength. Enough to hold, say, 14 PSI. Don’t ask me what that should be made of because I don’t know.

So you heat this thing until it can float to say 13 miles, which I read is the height record for a 44 ton hot air balloon launched in India in 2005.

Now assuming that you still have 14 PSI inside the balloon (no doubt a bad assumption) you just open up some sensibly placed nozzles and use the entire contents of the balloon as reaction mass to get the thing the rest of the way to orbit.

And beyond – it seems like the surface to volume ratio is going to favor very large balloons so it’s pretty easy to get a lot more pressurized air involved for the price of just a little more high-tensile-strength fabric.

If for some reason there’s not enough power to weight in this you could of course fill an appropriate fraction of the balloon with hydrogen and use a chemical rocket to accelerate the escaping gas. No idea if that’s necessary …

Anyway I’m probably daft and there’s probably some very good reason you can’t use a blimp as a rocket. But since I have no head for physics I’d be obliged if you’d let me know what that reason might be.

]]> By: Robert A Vollrath http://jpaerospace.com/blog/?p=429#comment-21713 Robert A Vollrath Fri, 30 Jan 2009 06:28:34 +0000 http://jpaerospace.com/blog/2009/01/24/todays-tasks/#comment-21713 Great blog. Just finished reading Floating to Space for the second time. I think the JP designs from the book would make great toys or even a story vehicle for a graphic novel. Is anyone doing this? Great blog. Just finished reading Floating to Space for the second time.

I think the JP designs from the book would make great toys or even a story vehicle for a graphic novel. Is anyone doing this?

]]>