For Questions That Don't Merit Their Own Thread

[paul_c]

I am not sure the burn rate of a SRB would vary with pressure. I could understand, maybe with temperature as the fuel heats up from conduction/radiation too, but not from pressure since solid materials don't particularly have "pressure" in the same sense that a fluid does.

Gaseous reactions' rate of reaction varies with temperature and pressure because the molecules are faster/closer so more is in the free radical state and more collisions/opportunities for molecules to interact, occur. But within a SRB, the reaction is a deflagration of solid components upon contacting the flame front.

[sevenperforce]

20 minutes ago, K^2 said:

In short, my fear is that if you try to use a terrestrial SRB on Venus it'll just blow the bleep up.

Well I don't think it would blow up because the pressure on the outside is still going to be pretty significant. The material integrity of the SRB casing is going to depend on the inside-outside pressure differential, which will probably be less severe than on Earth. Granted, temperatures of roughly 900 F aren't going to do any wonders for aluminum but it still should do okay.

20 minutes ago, K^2 said:

That said, I'm pretty sure that you can adjust the mixture and grain size for a burn rate that will work in Venusian atmosphere. The I_SP won't be great, since you wouldn't really have a nozzle, but it should still be decent. Not that you'll end up going very fast or very far, given the density of the atmosphere.

I wonder if an SRB, lowered by chute to the surface of Venus (let's ignore that Venus will melt literally every chute material) would be able to develop enough thrust to lift off.

[K^2]

20 minutes ago, sevenperforce said:

Well I don't think it would blow up because the pressure on the outside is still going to be pretty significant. The material integrity of the SRB casing is going to depend on the inside-outside pressure differential, which will probably be less severe than on Earth. Granted, temperatures of roughly 900 F aren't going to do any wonders for aluminum but it still should do okay.

Fuel is going to burn faster on Venus due to high pressure inside and exit slower than on Earth due to high pressure outside. That means the pressure differential between the inside and outside of SRB is going to be a lot higher on Venus than on Earth.

Now, I am hand-waving a lot of stuff here. Maybe higher density of combustion products will actually allow for higher rate of outflow on Venus, which would allow to maintain a reasonable pressure differential, but I'm very dubious on this. The reason I'm dubious is because runaway pressure increase is one of the modes of failure for SRBs, and if pressure increase actually increased outflow sufficiently, they'd be self-regulating under all conditions, which clearly isn't the case. So I'm still leaning towards the ka-boom. But this might require a simulation to figure out properly.

I'm also now thinking if I know anyone with pressure vessel that might let me set off a composite model rocket engine inside...

29 minutes ago, paul_c said:

I am not sure the burn rate of a SRB would vary with pressure.

Oh, it absolutely does. One of the key parameters when you are designing a solid fuel motor is the dependence of burn rate on pressure. It's often modeled as an exponent: r(P) = r₀ exp(kP). You can some times find these parameters quoted for solid motor composites.

Edited February 22, 2021 by K^2

[paul_c]

I would have thought lower flowrate (due to viscosity of fluids being greater, due to pressure inside (and out) greater) --> higher temperature for a given burn rate --> higher burn rate --> higher temperature internally --> higher pressure internally --> higher pressure differential --> (it would increase flowrate but not by much, no where near approaching flowrate of rocket in 1atm).

So, it comes down to a materials science problem - you'd need a casing which can withstand the higher (differential) pressure and temperature. 

Or alternately, you simply couldn't burn an SRB for as long, you'd need to have many stages of short-burning rockets. Because - even though I suggested some sort of equilibrium may develop - I suspect what actually happens is the temperature and internal pressure increase quite rapidly as the burn progresses, so the limitation or likelihood of uncontrolled explosion/rupture of the casing or seals is greatest towards the end of the burn. And of course, for weight/cost reasons, you'd not overengineer it, rather you'd precisely model the end temp/pressure and choose your material to withstand this (plus an engineering margin of safety).

Sure, a rocket for Venus liftoff would look and be very different to one for Earth.

[K^2]

3 hours ago, paul_c said:

So, it comes down to a materials science problem - you'd need a casing which can withstand the higher (differential) pressure and temperature. 

Well, yeah. If you can make casing strong enough, even a nuke wouldn't explode. 

But extra casing strength is extra weight, so even when building something like SRB, it usually can't take a lot more pressure than what it's designed for. There's going to be a safety margin, but if the pressure differential is several times higher, it will rupture.

There are ways to engineer around that - casing, fuel type, fuel mixture, oxidizer grain size, oxidizer to fuel ratio, bore diameter, bore shape, exhaust diameter... I'm sure you can build an SRB for Venus. I just think any that are built for operation on Earth will over-pressure and blow up.

 

But in any case, you aren't going up from Venus on an SRB. Aerodynamic losses for conventional ascent on Venus are on the order of 20km/s. Given that you are unlikely to scrape up even 200s from SRBs in that environment, it's just not going to happen.

[paul_c]

Pragmatically, you could do it with something other than a rocket, eg a paddle wheel or propeller. At 95 bar, the viscosity of Venus' atmosphere it going to be like a thick mud, so blasting a rocket to high speed seems terribly inefficient.

[K^2]

Honestly, a weather balloon is probably your best bet, but a turboprop might work pretty well for the first stage too. Either way, by the time you're ready to light the boosters, you're in much, much thinner atmosphere. In that case, the only concern is having these survive the heat without damage, which is still a challenge, certainly, but a far more manageable one. And with this, you can do all the testing on Earth. You can put the boosters into a heated pressure chamber with the right gas composition to "weather" them and then see if they still fire under normal atmospheric conditions, which is about where you'd be switching to rockets.

[JoeSchmuckatelli]

6 hours ago, K^2 said:

Well, yeah. If you can make casing strong enough, even a nuke wouldn't explode

What would it do? 

[paul_c]

Produce (very polluted) thrust from the nozzle?

[K^2]

3 minutes ago, JoeSchmuckatelli said:

What would it do? 

It's still going to turn into a miniature sun inside the shell, but having nowhere to expand, would linger much longer as hot plasma. Depending on how much radiation and heat escapes, it might still evaporate significant chunk of surroundings, but if that impossible material is also a good radiation blocker and poor heat conductor, which it might have to be, come to think of it, then it's just going to be a very hot sphere that will take a very long time to cool down.

[wumpus]

5 hours ago, K^2 said:

Honestly, a weather balloon is probably your best bet, but a turboprop might work pretty well for the first stage too. Either way, by the time you're ready to light the boosters, you're in much, much thinner atmosphere. In that case, the only concern is having these survive the heat without damage, which is still a challenge, certainly, but a far more manageable one. And with this, you can do all the testing on Earth. You can put the boosters into a heated pressure chamber with the right gas composition to "weather" them and then see if they still fire under normal atmospheric conditions, which is about where you'd be switching to rockets.

But liquid rockets will have just as much back pressure at the nozzle, so you might want the efficiency of solid rockets producing >>1g thrust until you got too close to supersonic (is there nothing that more thrust won't cure?).  That isn't much delta-v, but it should help you take you out of the worst of the atmosphere that the first stage can't escape.  Those little boosters that ring some rockets might only provide thrust for ~1 minute, but they provide a significant change in the amount of mass lifted to orbit (presumably by assisting the rocket during the low acceleration phase).

I can't be sure if you'd want Venus SRBs to assist the main rocket or simply be the second stage.  Hauling low-Isp rockets to Venus (and lowering them to the surface) is expensive, but you'd also have to design your nozzle for a much thicker atmosphere (for a stage likely dealing with mostly vacuum).

Finally, SRB Isp isn't that bad, that's mostly a KSPism.  It isn't hyrdalox or metholox, but I doubt you are getting off the ground with hydrolox (getting off Earth with just hydrolox is a challenge).  You'd probably use a methalox engine combined with your SRBs.  Dealing with cryofuels (or oxidizers) on Venus is an exercise left to the reader.  For additional fun, consider the additional challenges of an air-augmented rocket (as far as I know, the air isn't used as an oxidizer) with Venusian atmosphere (it will help your Isp, but bring the Hell of Venus's atmosphere inside the rocket.  You want the Isp, don't you?

[sevenperforce]

47 minutes ago, wumpus said:

You want the Isp, don't you?

The answer is always yes.

[magnemoe]

4 hours ago, K^2 said:

It's still going to turn into a miniature sun inside the shell, but having nowhere to expand, would linger much longer as hot plasma. Depending on how much radiation and heat escapes, it might still evaporate significant chunk of surroundings, but if that impossible material is also a good radiation blocker and poor heat conductor, which it might have to be, come to think of it, then it's just going to be a very hot sphere that will take a very long time to cool down.

Yes, now this is quite possible to do, underground test of nuclear weapons worked like this but they was hundreds of meter underground. 
They left an cavity.  I guess lots of the vaporized rock got pushed into cracks in the surrounding rock kind of like fracking on steroid.  

So you could technical do it, you might need an steel sphere 50 meter in diameter however and don't think that will contain an megaton blast. Yes it will vaporise  a lot of the material near the bomb and you will get an idiotic high pressure. 
Now for more crazy stuff to do with nukes, think orion pulse nuclear but as an IC engine. 
Some here talked about sapping power of an orion engine pusher plate stroke, it would make sense for an warship as in combat it might be under trust to evade and using power hungry lasers and railguns. 
The IC engine would get way more power out of each stroke  

2 hours ago, wumpus said:

But liquid rockets will have just as much back pressure at the nozzle, so you might want the efficiency of solid rockets producing >>1g thrust until you got too close to supersonic (is there nothing that more thrust won't cure?).  That isn't much delta-v, but it should help you take you out of the worst of the atmosphere that the first stage can't escape.  Those little boosters that ring some rockets might only provide thrust for ~1 minute, but they provide a significant change in the amount of mass lifted to orbit (presumably by assisting the rocket during the low acceleration phase).

I can't be sure if you'd want Venus SRBs to assist the main rocket or simply be the second stage.  Hauling low-Isp rockets to Venus (and lowering them to the surface) is expensive, but you'd also have to design your nozzle for a much thicker atmosphere (for a stage likely dealing with mostly vacuum).

Finally, SRB Isp isn't that bad, that's mostly a KSPism.  It isn't hyrdalox or metholox, but I doubt you are getting off the ground with hydrolox (getting off Earth with just hydrolox is a challenge).  You'd probably use a methalox engine combined with your SRBs.  Dealing with cryofuels (or oxidizers) on Venus is an exercise left to the reader.  For additional fun, consider the additional challenges of an air-augmented rocket (as far as I know, the air isn't used as an oxidizer) with Venusian atmosphere (it will help your Isp, but bring the Hell of Venus's atmosphere inside the rocket.  You want the Isp, don't you?

However an balloon and the compressed gas is not that heavy.  Other benefits, the expanding gas will cool the lander, The thing going up is just the sample return container, the balloon and tracking and control. 
Might even use the expanding gas to power an rover, you have an airship or plane with an rocket to pick it up and return it to orbit. 

[JoeSchmuckatelli]

How do lights reflect off of 'liquid' textures (or other 'reflective' textures) in games that do not have Ray Tracing?

[Terwin]

22 minutes ago, JoeSchmuckatelli said:

How do lights reflect off of 'liquid' textures (or other 'reflective' textures) in games that do not have Ray Tracing?

In graphics class they spoke about how the first 'mirrored' enemy in a computer game worked: they just grabbed pixels from a different portion of the screen as 'good enough' because the processor was not powerful enough to do the ray-tracing real-time.  It was lauded as a clever fix that was just as effective for the intended purpose as ray-tracing would have been.

I do not see any reason for them to have bothered to change the approach for later games.

[JoeSchmuckatelli]

1 minute ago, Terwin said:

In graphics class they spoke about how the first 'mirrored' enemy in a computer game worked: they just grabbed pixels from a different portion of the screen as 'good enough' because the processor was not powerful enough to do the ray-tracing real-time.  It was lauded as a clever fix that was just as effective for the intended purpose as ray-tracing would have been.

I do not see any reason for them to have bothered to change the approach for later games.

So - I'm playing WOT, which actually has some pretty impressive lighting effects.  Shadows look cool; they move if you knock something down, so I don't think they're baked in.  The dappling of light is really well done... but what caught my attention today was seeing 'Sunlight' reflect off of a pool of 'oil' as I moved the camera around.  It had a very specific relation to the sun in the skybox.

 

Pretty nice.

[ARS]

Is there any reason why firearms magazine has particular shape? (I'm talking specifically about the regular 30-rounds magazines, not drums) There are magazines that's curved forward (M4 or AK), straight (FAL or G3) and swept backward (Vector). Is there any reason for the shape or it's just for aesthetic purpose? Considering all of them do the same thing: Push the bullets inside upwards

[RyanRising]

24 minutes ago, ARS said:

Is there any reason why firearms magazine has particular shape? (I'm talking specifically about the regular 30-rounds magazines, not drums) There are magazines that's curved forward (M4 or AK), straight (FAL or G3) and swept backward (Vector). Is there any reason for the shape or it's just for aesthetic purpose? Considering all of them do the same thing: Push the bullets inside upwards

I think it depends on the casing - if your round is tapered and you try to line them up wall-to-wall, you'll get a curve. If your magazine is long enough and the round is tapered enough, matching the magazine to the curve the rounds naturally make will improve feeding. For an extreme example of this, see the Chauchat magazine.

Straight magazines, on the other hand, are easier to pack, so if the magazine is short enough relative to the taper of your cartridge that you can get away without curving it it's usually easier to handle. Additionally, some cartridges aren't tapered at all, so a curve won't help those.

Lastly, the reason the Vector has a magazine that's swept backwards is because it uses Glock mags. Glock magazines are made to fit in the grip of a handgun, and we've generally found that giving those an angle greater than 90 degrees to the barrel is more comfortable to hold. That limitation doesn't apply to the Vector and other guns using pistol magazines outside of the grip, but they keep the sweep to maintain compatibility.

Edited February 23, 2021 by RyanRising

[magnemoe]

1 minute ago, RyanRising said:

I think it depends on the casing - if your round is tapered and you try to line them up wall-to-wall, you'll get a curve. If your magazine is long enough and the round is tapered enough, matching the magazine to the curve the rounds naturally make will improve feeding. For an extreme example of this, see the Chauchat magazine.

Straight magazines, on the other hand, are easier to pack, so if the magazine is short enough relative to the taper of your cartridge that you can get away without curving it it's usually easier to handle.

Lastly, the reason the Vector has a magazine that's swept backwards is because it uses Glock mags. Glock magazines are made to fit in the grip of a handgun, and we've generally found that giving those an angle greater than 90 degrees to the barrel is more comfortable to hold. That limitation doesn't apply to the Vector and other guns using pistol magazines outside of the grip, but they keep the sweep to maintain compatibility.

Good point however all rifle rounds I have seen are straight walled up to the point they tapper in for the bullet. Now the curved magazine will extend less downwards who is important as you often lie down shooting. 
Think this is the reason, its also the reason many WW2 machine pistols has the magazine sideways and some machine guns had an top mounted magazine.

Drum magazines tend to be unreliable in the field because more friction in them 

[RyanRising]

2 minutes ago, magnemoe said:

Good point however all rifle rounds I have seen are straight walled up to the point they tapper in for the bullet. Now the curved magazine will extend less downwards who is important as you often lie down shooting. 
Think this is the reason, its also the reason many WW2 machine pistols has the magazine sideways and some machine guns had an top mounted magazine.

Drum magazines tend to be unreliable in the field because more friction in them 

Looks to me like they are tapered, though very slightly, on the case walls.  This is less pronounced in 5.56x45 than, say, 7.62x39, but I think it's still present. That being said, I'm only reading the documentation here, not actually poring over handful of ammo. I understand, though, that you'd want some amount of taper to help with extraction? If they were perfectly straight-walled, seems to me the cases would really like to stick in the chamber no matter how hard you pull on the end.

[kerbiloid]

Wider aft - curvier mag.

[magnemoe]

5 hours ago, RyanRising said:

Looks to me like they are tapered, though very slightly, on the case walls.  This is less pronounced in 5.56x45 than, say, 7.62x39, but I think it's still present. That being said, I'm only reading the documentation here, not actually poring over handful of ammo. I understand, though, that you'd want some amount of taper to help with extraction? If they were perfectly straight-walled, seems to me the cases would really like to stick in the chamber no matter how hard you pull on the end.

You are correct I looked it up  5.56x45 tapers 0.6 mm, this is an 6.6% tapper.  https://military.wikia.org/wiki/5.56×45mm_NATO, the AK-47 ammo might be more tapered. 

 

[JoeSchmuckatelli]

CMB question:  From what I understand, we have looked in every direction and detected the Cosmic Microwave Background everywhere.  Are the interpretations of this that by measuring the CMB that we are seeing 'the end' of the universe, or is it more like the CMB is a fog, and that's merely how far we can see through it (i.e the distance at which it becomes opaque from our point of view)?

 

I know there is more to the world than this... but this is as far as I can see.

(Is this 'trees in fog' a good analogy to understand the CMB - or is the interpretation that it's a skybox beyond which lies nothing)?

[magnemoe]

45 minutes ago, JoeSchmuckatelli said:

CMB question:  From what I understand, we have looked in every direction and detected the Cosmic Microwave Background everywhere.  Are the interpretations of this that by measuring the CMB that we are seeing 'the end' of the universe, or is it more like the CMB is a fog, and that's merely how far we can see through it (i.e the distance at which it becomes opaque from our point of view)?

 

I know there is more to the world than this... but this is as far as I can see.

(Is this 'trees in fog' a good analogy to understand the CMB - or is the interpretation that it's a skybox beyond which lies nothing)?

No, the end of the observable universe is there galaxies moves at light speed because the expansion of the universe. 
The CMB is the leftover heat of the big bang and is pretty uniformly distributed. 

[DDE]

On 2/21/2021 at 10:22 PM, magnemoe said:

Is it other materials who float in their own liquid? 

"Bismuth, gallium and certain alloys"

https://mipt.ru/dbmp/upload/5df/OXF_4-arphlf42saz.pdf

Wiki also mentions germanium, silicon and antimony.