/sci/ - Science & Math » Thread #13026037

14KiB, 468x512, unnamed (1).jpg

View Same Google iqdb SauceNAO

Anonymous Sat 24 Apr 20:02:48 2021 No.13026037 View Reply Original Report

Quoted By: >>13026045 >>13026075 >>13026098 >>13027271 >>13031644 >>13032035

Hi /sci/, I've been trying to figure something out with physics and I just don't get it.
I'm in university and my task is to design a theoretical helium aerostat (something that looks like pic related) that can reach the 30km altitude with an 8kg payload. No matter how I try to use the Buoyancy formula, I can't reach this altitude, because at some point, the air density becomes so low that it's lower than helium, making helium not be able to rise anymore. I've been trying to look things up, but no matter how I try, I get around 26km.
Am I missing something simple? I'm admiteddly pretty bad at physics, as my course is on programming.

Anonymous

Anonymous Sat 24 Apr 2021 20:04:52 No.13026045 Report

Quoted By:

>>13026037
slap rockets on it.

Anonymous

Anonymous Sat 24 Apr 2021 20:15:19 No.13026075 Report

Quoted By: >>13026089

>>13026037
The density of helium is not constant with height. Most helium balloons are not filled to the maximum volume at the surface. This then allows the helium to expand at higher elevations, reducing the density of the helium.

Anonymous

Anonymous Sat 24 Apr 2021 20:21:15 No.13026089 Report

Quoted By: >>13026136 >>13026705 >>13031997 >>13033297

>>13026075
I forgot to mention, but my exercise also mentions that the skin of the balloon is made out of a non-stretchable material, which I believe makes this approach impossible.
However, this is weird, because in the example they give us, the balloon did work according to what you described - the weather balloon method. I want to keep searching for a way to do this without a stretchable skin, but I guess I can fall back on this if it's impossible.

Elon Musk Fanboy

Elon Musk Fanboy Sat 24 Apr 2021 20:23:43 No.13026098 Report

Quoted By:

>>13026037
Cheat, attach a prop to it

Anonymous

Anonymous Sat 24 Apr 2021 20:31:36 No.13026136 Report

Quoted By: >>13026191

>>13026089
Non-stretchable does not imply non-compressible i.e. it has a maximum size but it can be folded and not be filled up completely, right?

Anonymous

Anonymous Sat 24 Apr 2021 20:45:28 No.13026191 Report

Quoted By: >>13026465

>>13026136
Hmm, you are correct I believe. But how would such an aircraft work? You'd have to have pretty strange shape that would somehow expand naturally.

Anonymous

Anonymous Sat 24 Apr 2021 21:38:48 No.13026465 Report

Quoted By: >>13026509

>>13026191
Bud, it's literally just a partially inflated balloon. You can have a bunch of loose material, that won't cause any issues, such that the maximum size is larger than the size whilst at the surface.

Anonymous

Anonymous Sat 24 Apr 2021 21:47:31 No.13026509 Report

Quoted By:

>>13026465
Look man, I'm a bit of a dumbass, so imagining a slightly deflated balloon is difficult. Still, I think I'll go with that strategy. I'll see how much I need to reduce the density and figure out how much gas there needs to be at the start. The whole project isn't too serious anyway.
Thanks for the help man, I don't know how long threads stay up on /sci/, but I'm going to go to sleep.

Anonymous

Anonymous Sat 24 Apr 2021 22:28:14 No.13026705 Report

Quoted By:

>>13026089
Many balloons are partially filled so that they will expand to fill the envelope at altitude. Look up the Echo satellites, essentially huge helium balloons in space.

Anonymous

View Same Google iqdb SauceNAO 1577462d7ad07e410b146c25ad9c8449.jpg, 129KiB, 850x1202

Anonymous Sat 24 Apr 2021 23:26:39 No.13026911 Report

Quoted By: >>13028193 >>13028627

At 30 km, https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html says air has a density of $\rho=0.01841\textrm{ m}^3$ . Using the buoyancy principle, we can get a force balance of the balloon at altitude: $<br />m_{He}+m=\rho V$ . Where V is the volume of the balloon (assumed to be constant since it cannot stretch), m is mass of payload, and m_He is the total helium used. Because this last mass is greater than zero, we can say $\rho V-m=m_{He}>0,\ V>m/\rho=8/0.01841=434.5\textrm{ m}^3$ . Or equivalently, the radius of your (unrealistic) balloon MUST be greater than 4.7 meters.

The next requirement is that your balloon must be inflated on the ground. This means the absolute pressure of the helium within the balloon on the ground MUST be at least 1 atmosphere, or 101 kPa. Inflating to atmosphere is obviously the easier design. So, considering He as an ideal gas at standard temperature (288 K) this means $\rho_{He}RT/M_{He}=P_0=101\textrm{ kPa}$ where R and M_He are the universal gas constant and molar mass of He, respectively. This means $m_{He}=\rho_{He}V=\rho V-m=\frac{P_0M_{He}}{RT},\ \ V=\frac{P_0M_{He}}{RT\rho}+\frac{m}{\rho}=\frac{(101\cdot10^3)(4.003\cdot10^{-3})}{(8.314)(288)(0.01841)}+\frac{8}{0.01841}=443.7\textrm{ m}^3$ The radius of this balloon is closer to 4.8 meters.

In other words, a design with a 4.8 meter radius, non-stretchable balloon pressurized with 1 atmosphere of standard temperature helium on the ground is sufficient for your task.

Anonymous

Anonymous Sun 25 Apr 2021 01:25:00 No.13027271 Report

Quoted By: >>13028193

>>13026037
Vacuum airship. Or does it HAVE to us Helium?

Anonymous

Anonymous Sun 25 Apr 2021 06:00:42 No.13028193 Report

Quoted By: >>13028197

>>13027271
Specified in the task that it has to use helium
>>13026911
I see now, I didn't realize that m{He} could be a changeable value, I just calculated it based on the volume of the balloon
Though the whole calculation looks good, I have one question - here $m_{He}+m=\rho V$ you don't use g like this $m_{He}+m=\rho V g$ and I'm not sure why. I do believe your formula, since I saw other places use something similar to the buoyancy formula but without the g, I just don't know why
Either way, thanks a lot, this will really help. I doubt I would have figured this out on my own.

Anonymous

Anonymous Sun 25 Apr 2021 06:02:51 No.13028197 Report

Quoted By: >>13028239

>>13028193
g cancels out

Anonymous

Anonymous Sun 25 Apr 2021 06:19:11 No.13028239 Report

Quoted By:

>>13028197
Aaah, now I get it, I just remembered that weight is on the left side not mass

Anonymous

Anonymous Sun 25 Apr 2021 09:11:59 No.13028627 Report

Quoted By: >>13030078

>>13026911
Do you know of any methods I could use to calculate how long it would take for the balloon to reach 30km?

Anonymous

Anonymous Sun 25 Apr 2021 18:30:44 No.13030078 Report

Quoted By: >>13032036

>>13028627
Go here: http://hyperphysics.phy-astr.gsu.edu/hbase/airfri2.html#c5
Enter the values and you get about 5m/s.terminal velocity, and I guess you reach that quickly. Then just 30000 m / 5m/s = 6000 s = 100 minutes.

Anonymous

Anonymous Mon 26 Apr 2021 01:10:50 No.13031644 Report

Quoted By:

>>13026037
idfk i googled the maximum height a helium balloon can reach and it said 9km

Anonymous

Anonymous Mon 26 Apr 2021 02:40:50 No.13031997 Report

Quoted By:

>>13026089
underinflate the balloon baka!

Anonymous

Anonymous Mon 26 Apr 2021 02:53:33 No.13032035 Report

Quoted By:

>>13026037
Are you allowed to heat the helium up?

Anonymous

View Same Google iqdb SauceNAO 35f75eaefd56590d6c65f9225058af14.jpg, 142KiB, 850x666

Anonymous Mon 26 Apr 2021 02:53:36 No.13032036 Report

Quoted By:

>>13030078
Just keep in mind that quadratic drag is a function of air density, and so terminal velocity changes as it rises.

Anonymous

Anonymous Mon 26 Apr 2021 11:05:51 No.13033297 Report

Quoted By:

>>13026089
Make internal strechable chambers
The inner balloon will compress the air within the gap, effectively giving you an air-rocket if you make some wholes at the bottom of the ballon.

You could also have your inner ballon hyper compressed by using successive chambers, but that is way more time consuming

Capcode	All Only User Posts Only Moderator Posts Only Admin Posts Only Developer Posts
Show Posts	All Only With Images Only Without Images
Deleted Posts	All Only Deleted Posts Only Non-Deleted Posts
Ghost Posts	All Only Ghost Posts Only Non-Ghost Posts
Post Type	All Only Sticky Threads Only Opening Posts Only Reply Posts
Results	All Grouped By Threads
Order	Latest Posts First Oldest Posts First

Your latest searches