For Questions That Don't Merit Their Own Thread

[K^2]

2 hours ago, Jacke said:

or sometimes one of the previous characters reversed (Math's use of backward's E meaning "Exists", upside-down A meaning "For every")  or on its side (8 on its side meaning the Infinity of Integers or Aleph₀).

Yeah, back in the day, the limitation was, "They have this in the type-set," so turning a character 180° was always an option. 90° was only available for some characters. These days, of course, it's all about what you can represent in LaTeX, and that opens up a lot more possibilities.  is a perfectly valid way to represent a quantum state that wouldn't have been easy to print a few decades ago. (Irony is that I had to insert it as an image after entering it in the editor, because this board doesn't allow arbitrary emoji, which is what they had to do with non-standard formulae back in the printing days as well. XD)

[Guest]

I hope the previous question was answered. Anyone know where this footage came from? I found it in this video today:

 

[cubinator]

15 hours ago, The Doodling Astronaut said:

I hope the previous question was answered. Anyone know where this footage came from? I found it in this video today:

I don't know about the video, but the "SPEEN" comes from Vinesauce...

[Guest]

1 hour ago, cubinator said:

I don't know about the video, but the "SPEEN" comes from Vinesauce...

haha yeah, but hopefully somone knows the footage

[kerbiloid]

Logo of Associated Press.

[DDE]

1 hour ago, The Doodling Astronaut said:

haha yeah, but hopefully somone knows the footage

Uhm

 

[Guest]

ah so this one, thanks

[YNM]

On 3/9/2021 at 4:13 AM, K^2 said:

this board doesn't allow arbitrary emoji

You can probably do kaomoji instead ?

cat ➞ =^._.^=

cat_alive ➞ (=^･ω･^=)

cat_dead ➞ (=✖ᆽ✖=)

[munlander1]

If something is a good oxidizer, excluding costs, reactivity with everything besides a fuel, safety concerns including but not limited to spills, explosions, and exhaust products, chemical stability, and quantities available, would it make a good rocket fuel?

[kerbiloid]

If it has great atomic mass, ot can be ineffective.

It can produce compounds extinguishing fire or breaking the structure with shockwaves or overheat it or something else.

It can have low ISP or produce low thrust, so be partially inappropriate depending on altitude (sea level / vacuum).

[DDE]

42 minutes ago, munlander1 said:

If something is a good oxidizer, excluding costs, reactivity with everything besides a fuel, safety concerns including but not limited to spills, explosions, and exhaust products, chemical stability, and quantities available, would it make a good rocket fuel?

Well, one of the most potent oxidizers ever known is ClF5, which was explored as a rocket propellant.

[K^2]

47 minutes ago, munlander1 said:

If something is a good oxidizer, excluding costs, reactivity with everything besides a fuel, safety concerns including but not limited to spills, explosions, and exhaust products, chemical stability, and quantities available, would it make a good rocket fuel?

What you care about is exhaust velocity, which comes down to 1) Amount of energy released, 2) Mass distribution of the products, and 3) Mechanical degrees of freedom in products.

Being a strong oxidizer doesn't guarantee any of these things. While the reaction is likely to be very exothermal, it doesn't mean that a more reactive oxidizer gives you more energy. And if it gives you more energy but at a cost of heavier compounds in the exhaust, or if a lot of that energy goes into vibrational degrees of freedoms of complex molecules, then you're not getting more thrust out of it.

There are other considerations too, but you might have ways to work around them with engine design. If you don't get high kinetic energy of combustion products, then you aren't going to fix it. And because of that, you can't just look at oxidizer in isolation. Same oxidizer will produce very different amounts of energy and complexity of products with different fuels. So you have to consider oxidizer/fuel as a package when discussing whether it has a potential to make good rocket propellant.

[JoeSchmuckatelli]

58 minutes ago, K^2 said:

What you care about is exhaust velocity, which comes down to 1) Amount of energy released, 2) Mass distribution of the products, and 3) Mechanical degrees of freedom in products.

Being a strong oxidizer doesn't guarantee any of these things. While the reaction is likely to be very exothermal, it doesn't mean that a more reactive oxidizer gives you more energy. And if it gives you more energy but at a cost of heavier compounds in the exhaust, or if a lot of that energy goes into vibrational degrees of freedoms of complex molecules, then you're not getting more thrust out of it.

There are other considerations too, but you might have ways to work around them with engine design. If you don't get high kinetic energy of combustion products, then you aren't going to fix it. And because of that, you can't just look at oxidizer in isolation. Same oxidizer will produce very different amounts of energy and complexity of products with different fuels. So you have to consider oxidizer/fuel as a package when discussing whether it has a potential to make good rocket propellant.

So - how far below C-4 and DetCord do we have to be?  Are the fuels currently in use the best possible, or are there other chemical combinations that are really, really close... but we're waiting on materials science to get to a point where they can handle one or more components?

[K^2]

27 minutes ago, JoeSchmuckatelli said:

Are the fuels currently in use the best possible, or are there other chemical combinations that are really, really close... but we're waiting on materials science to get to a point where they can handle one or more components?

Per weight, LH2/LOX is pretty close to as good as it gets. Both F2 and ClF3 are better as oxidizers, but so much nope for being just a little better than oxygen here that I don't think anyone's even considering building an engine with these. I don't think there is any way to beat HF as your exhaust if we disregard engine materials, pumps, tanks, storage, etc. So that's probably as good as it gets.

If it's important to you that your fuels are liquid at room temperature or you want higher density for smaller tanks, that's where the field opens up a bit, but we are already talking about combinations that are a lot worse than the above right away for these use cases.

[ARS]

If a craft is flying very fast in atmosphere, it experiences atmospheric drag and excessive heating, in addition to causing a sonic boom effect. Is there any equivalent for it in underwater scenario? I know that there's a supercavitation effect that allows torpedoes to travel at highspeed underwater, but does the effect is the same like in the air, such as the underwater equivalent of sonic boom? Does being underwater mitigates the heating caused by drag or it does nothing?

[K^2]

@ARS Yeah, you can have a supersonic shock in water. The speed of sound is a lot higher, however, so it happens at much, much higher speed.

[DDE]

4 hours ago, ARS said:

I know that there's a supercavitation effect that allows torpedoes to travel at highspeed underwater

I'm not sure about the US-made series of bullrts, but the effect on the Shkval is sometimes said to be artificially enhanced by injecting some of the rocket exhaust through the forward nozzle.

[magnemoe]

3 hours ago, DDE said:

I'm not sure about the US-made series of bullrts, but the effect on the Shkval is sometimes said to be artificially enhanced by injecting some of the rocket exhaust through the forward nozzle.

Know the US made some bullets with an cavity in the tip a bit like an hollow point bullet but this would obviously need to be steel.  As I understand purpose here was to shoot up mines from an helicopter. 
Injecting gas at the nose sounds smart. 

It has been some talk of creating plasma and use as an buffer during reentry, also using electricity to control plasma to brake at high attitudes so you move slower then the atmosphere start heating you up a lot.

[kerbiloid]

Usually they inject gas behind the projectile, to fill the vacuum bubble behind the bottom and prevent being sucked back.
https://en.wikipedia.org/wiki/Rocket-assisted_projectile

Anti-helicopter mines launch unguided explosively formed penetrators (i.e. a supersonic jet of melted metal)

Edited March 11, 2021 by kerbiloid

[DDE]

2 hours ago, magnemoe said:

It has been some talk of creating plasma and use as an buffer during reentry, also using electricity to control plasma to brake at high attitudes so you move slower then the atmosphere start heating you up a lot.

https://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-i-introduction/

58 minutes ago, kerbiloid said:

Anti-helicopter mines

"Equipment" 

[kerbiloid]

34 minutes ago, DDE said:

1 hour ago, kerbiloid said:

Anti-helicopter mines

"Equipment" 

Remote Tactile Contact Initiation Device,

Upd.
Chopper Slapper,

Edited March 11, 2021 by kerbiloid

[DDE]

1 hour ago, kerbiloid said:

Upd.
Chopper Slapper,

[Ziggy Kerman]

On 3/9/2021 at 8:01 PM, The Doodling Astronaut said:

I hope the previous question was answered. Anyone know where this footage came from? I found it in this video today:

 

Most of my payloads

[Shpaget]

On 3/8/2021 at 8:59 PM, JoeSchmuckatelli said:

 

On the page was a reference to 'Jerk' 

Is this legit?  https://en.m.wikipedia.org/wiki/Jerk_(physics)

Note: "Snap, crackle and pop" 

It certainly is legit, and I'd wager you're experiencing jerk mitigation practices in your everyday life.

For example, when a train approaches a curve, the track does not instantly transition from straight to whatever the radius is required to make the turn. It gradually changes and tightens the turn to ease into the final radius.  If it wasn't like that, passangers would experience sudden lateral acceleration which would be uncomfortable (and damaging  to equipment).

More directly observable example of the same thing is while driving a car. If you suddenly turn the steering wheel it's quite unpleasant, even though the steering wheel is not turned all that much. Slowly turning the wheel is reducing the jerk.

[JoeSchmuckatelli]

NewQ: after seeing the clip of a satellite being spun at deployment, I began to wonder why.  I presumed most satellites would want internal gyros to keep the outside body + science, comms & solar, etc. stable... So what kind of satellite would be spin stabilized? 

 

Edit: @kerbiloid - if you say 'planets' so help me... 

Edited March 14, 2021 by JoeSchmuckatelli