For Questions That Don't Merit Their Own Thread

[kerbiloid]

It's a sword-shaped shield.

[KSK]

On 6/12/2022 at 6:04 PM, JoeSchmuckatelli said:

Hey - Physics People! 

Torus-hab on a spacecraft question:  is there any difference in the ability of a craft to maneuver that depends upon the orientation of the 'wheel'? 

Most of the time when artists depict a craft with an attendant torus habitat, the axis of rotation is in line with the engines and thrust.  But what would happen if the torus was in line with the direction of travel (axis of rotation perpendicular to thrust) ?  The engines are placed either side of the axis or the ship is built like a bicycle fork (placing the engines centerline but outside of the torus)?

Given approximately the same mass, does orientation of the wheel affect maneuvering in spaaaaaaaace? 

Yes. The hab wheel will behave like a gyroscope and precess. In other words, applying a torque about one axis will cause a rotation about a different axis. A fly-by-wire system should be able to compensate though - if the pilot commands a pitch manoeuvre, for example, the system might respond by firing the yaw thrusters instead.

A bicycle fork configuration could be made to work but it’s harder to design compared to having the hab rotation axis along the axis of thrust.

Hab spins one way, so the rest of the ship spins the other way. In a bicycle fork configuration, the ship is now tumbling end over end, which isn’t great for executing manoeuvres. You would need to cancel out the rotation using a flywheel (which goes where?) or RCS. 

[JoeSchmuckatelli]

2 hours ago, KSK said:

wheel will behave like a gyroscope and precess.

 

2 hours ago, KSK said:

bicycle fork configuration, the ship is now tumbling end over end, which isn’t great for executing manoeuvres. You would need to cancel out the rotation using a flywheel (which goes where

These are great descriptions, thanks! 

I never realized how much handwavium was used to allow ships like this in SciFi.  Just operating the rotating section during the coast phase with zero maneuvering while rotating makes absolute sense (now that I think about it with the right frame of mind!). Between you and kerbiloid I now 'get it' and feel kinda silly for even asking the question. 

Funny how you don't know what you don't know! 

 

[KSK]

5 hours ago, JoeSchmuckatelli said:

These are great descriptions, thanks! 

I never realized how much handwavium was used to allow ships like this in SciFi.  Just operating the rotating section during the coast phase with zero maneuvering while rotating makes absolute sense (now that I think about it with the right frame of mind!). Between you and kerbiloid I now 'get it' and feel kinda silly for even asking the question. 

Funny how you don't know what you don't know! 

You're welcome - glad they helped!

And it's a cliche but hey - there are no such things as silly questions.   Heck, I work in IP - much of my job involves asking daft laddie questions of people who are a whole lot smarter than me.

[TheSaint]

On 6/20/2022 at 7:34 AM, JoeSchmuckatelli said:

These are great descriptions, thanks! 

I never realized how much handwavium was used to allow ships like this in SciFi.  Just operating the rotating section during the coast phase with zero maneuvering while rotating makes absolute sense (now that I think about it with the right frame of mind!). Between you and kerbiloid I now 'get it' and feel kinda silly for even asking the question. 

Funny how you don't know what you don't know!

Just for reference, there are at least a couple of ways around this:

1. Your spin habitat can incorporate a flywheel, a circular counterweight that spins in the opposite direction of the spin hab in its hub. It doesn't have to have the same mass as the spin hab, it can actually have a much lower mass, it will just spin at a much faster rate to counteract the momentum of the spin hab. This is actually not a bad idea anyway, because if your spin hab spins up against the flywheel instead of the rest of the ship then it imparts its angular momentum on the flywheel instead of the body of the ship, thus at least partially preventing a spin-up of the hab from causing the ship to start rotating. A major downside of having a rapidly spinning flywheel in your spacecraft is that it holds a very large amount of kinetic energy. So if something goes wrong with it Very Bad Things will happen.

2. If your ship is large enough to justify the mass expense, you can incorporate two spin habs, each rotating in opposite directions. That way their gyroscopic forces will cancel each other out. All sorts of options there as your ships get larger, as long as you mount them in counter-rotating pairs. 

[kerbiloid]

While it would neutralize the total momentum, though probably would still be a problem to maneuver while the twin hab is counter-rotating, as they are both gyroscopes.

(So, their axis tilting would make a stress in the bearings.)

Edited June 25, 2022 by kerbiloid

[cubinator]

Are gas particle collisions really elastic?

[kerbiloid]

Nothing is, as the energy is partially absorbed and/or dissipated.

(I.e. entropy takes its tax.)

[Jacke]

5 hours ago, cubinator said:

Are gas particle collisions really elastic?

Only under Ideal Gas Law.  Even Classic Physics and Chemistry have an adjustment to be more realistic if more complex in calculation.

https://en.wikipedia.org/wiki/Van_der_Waals_equation

[Nuke]

On 6/24/2022 at 9:17 PM, kerbiloid said:

While it would neutralize the total momentum, though probably would still be a problem to maneuver while the twin hab is counter-rotating, as they are both gyroscopes.

(So, their axis tilting would make a stress in the bearings.)

i think my solution is to dispense with the rest of the ship, put everything in the torus and either get rid of the hub or put your lander there endurance style. keeping things like engines and fuel tanks in spin gravity helps solve ullage issues. you might need to beef up the structural components though. but it seems like it would be a bit better than carrying bearings, motors and crew access tunnels (and all the airlocks and hatches involved). you can also put a big aeroshell on one side and use it for aerocapture maneuvers. 

[ARS]

Is there a reason why muzzle brake constructions are wildly different? On one hand, we have the muzzle brake that vents the gas sideways, on the other hand, we have star-shaped muzzle brake that vents the gas circularly on 5 directions, and on the other hand we have the one with multiple smaller holes around the tip of the muzzle like a mic. Does "it works as long as it has holes on the muzzle" (so for example you can use assault rifle muzzlebrake on sniper rifles) or it's specifically tailored with set parameter for specific gun (meaning the one for assault rifle is inefective or useless when put on sniper rifles)?

Also, does putting muzzle brakes beneficial if you're gonna fire that gun in space?

Edited June 27, 2022 by ARS

[kerbiloid]

42 minutes ago, ARS said:

Also, does putting muzzle brakes beneficial if you're gonna fire that gun in space?

Not necessary.

Spoiler

***

Depends on the recoil force, on the recoil-powered automatics, and other reasons.

[ARS]

13 minutes ago, kerbiloid said:

Not necessary.

  Hide contents

***

Depends on the recoil force, on the recoil-powered automatics, and other reasons.

Isn't that Almaz gun?

[kerbiloid]

13 minutes ago, ARS said:

Isn't that Almaz gun?

Yes.

https://topwar-ru.translate.goog/163699-zagadki-kosmicheskoj-pushki-artillerijskaja-ustanovka-schit-1.html?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=ru

[TheSaint]

3 hours ago, ARS said:

Is there a reason why muzzle brake constructions are wildly different? On one hand, we have the muzzle brake that vents the gas sideways, on the other hand, we have star-shaped muzzle brake that vents the gas circularly on 5 directions, and on the other hand we have the one with multiple smaller holes around the tip of the muzzle like a mic. Does "it works as long as it has holes on the muzzle" (so for example you can use assault rifle muzzlebrake on sniper rifles) or it's specifically tailored with set parameter for specific gun (meaning the one for assault rifle is inefective or useless when put on sniper rifles)?

So, a muzzle device serves one or more of three different purposes:

• It can serve to obscure the muzzle flash of the firearm. This is your classic "birdcage" or three-prong flash hiders that you see on a lot of assault rifles. It hides the actual muzzle of the rifle from direct view to make the flash of the rifle firing less visible, especially at night.
• It can help to reduce muzzle climb. When a rifle is fired fully automatic, the muzzle will tend to pull upwards with each successive shot due to the rifle's recoil. Many muzzle devices are engineered to vent the propellant gasses in an upward direction to help counteract this. A good example is the simple slant cut muzzle device on classic AKs.
• It can also help reduce felt recoil. This is what is properly called a "muzzle brake". These muzzle devices direct the propellant gasses backwards, pulling the rifle forwards when the bullet leaves the barrel and helping the reduce the impact of recoil on the shooter.

You will also see muzzle devices that accomplish two or more of the above tasks. For example, if you look at the classic birdcage flash hider on the M16/M4, you will notice that it omits several ports on the bottom side of the flash hider. This directs more of the gasses upwards and helps reduce muzzle climb in automatic fire.

Generally, the two things that size a muzzle device are bullet diameter and barrel threading. Barrel threads go without saying; if it won't thread on your barrel then it won't work, full stop. Bullet diameter/cartridge fitting is a bit more complicated. Using a larger muzzle device with a smaller bullet is going to let propellant gasses vent around the bullet and at least partially defeat the purpose of the muzzle device. Using a muzzle device that is designed for a bullet that is smaller than the one your firearm is chambered for will lead to Very Bad Things, at a minimum destroying the muzzle device. Using, say, a .22 Long Rifle muzzle device with .223 Remington (which have the same bullet diameter, but wildly different cartridge sizes and propellant loadings) is going to give you...interesting results, to say the least. In general, muzzle devices are engineered at least for a specific cartridge, if not for a specific firearm, and it's best to stick with the manufacturer's recommendations.

3 hours ago, ARS said:

Also, does putting muzzle brakes beneficial if you're gonna fire that gun in space?

It would seem to me that the principles would be the same, it would simply be a matter of engineering it to handle zero back pressure instead of 1 ATM back pressure. Along with all of the other issues of engineering a gun to operate in space, which have been covered in other posts.

Edited June 27, 2022 by TheSaint
Probably should answer the actual question.

[TheSaint]

10 hours ago, Nuke said:

i think my solution is to dispense with the rest of the ship, put everything in the torus and either get rid of the hub or put your lander there endurance style. keeping things like engines and fuel tanks in spin gravity helps solve ullage issues. you might need to beef up the structural components though. but it seems like it would be a bit better than carrying bearings, motors and crew access tunnels (and all the airlocks and hatches involved). you can also put a big aeroshell on one side and use it for aerocapture maneuvers. 

This works, for certain applications. Your structure does get a lot heavier, since everything (including all of your heaviest components, like engines and fuel) need to be braced against spin gravity. I don't know if there is enough trade-off there to make it worth it. You might be better off with a "pencil through a ring" configuration. You put the living quarters in the ring, everything else in the pencil, then spin the whole thing. The ring is at gravity, but the stuff in the pencil is seeing a lot less force, simply because the diameter of the pencil is so much less.

Personally, I usually imagine that spin habs are a lot smaller than most sci-fi imagine them to be. I don't see the entire living quarters being in the spin hab. We already know that humans can live for quite some time, days if not weeks, in zero-G with little or no ill effects. So I see most of the living quarters being in zero-G, with a smaller spin hab for recreation/exercise. So you eat and sleep in the zero-G portion of the living quarters, then you spend a mandatory two or three hours a day in the spin hab: exercising, relaxing, etc. It can be a lot smaller because it doesn't contain the kitchen, dining room, sleeping quarters, etc. It's basically just a big rec room with a running track going through it. This has the added benefit of allowing the spin hab to be redundant. If something does go wrong with it you can just lock the door and you still have your normal living quarters.

Edited June 27, 2022 by TheSaint

[Gargamel]

On 6/27/2022 at 2:18 AM, Nuke said:

i think my solution is to dispense with the rest of the ship, put everything in the torus and either get rid of the hub or put your lander there endurance style. keeping things like engines and fuel tanks in spin gravity helps solve ullage issues. you might need to beef up the structural components though. but it seems like it would be a bit better than carrying bearings, motors and crew access tunnels (and all the airlocks and hatches involved). you can also put a big aeroshell on one side and use it for aerocapture maneuvers. 

I’m seeing the SpongeBob Patrick meme here.   “We solve all our problems over here, by creating a whole new set of issues over there.”      Doesn’t mean it’s not a bad idea though.  
 

Although, I don’t ever recall seeing a frisbee flying face first.  

[Nuke]

pretty sure you can do something with the aerodynamics to keep that thing flying straight. also its spun up so that should help spin stabilize it. 

[DDE]

Why don't stage-and-a-half rockets use aerospikes? Thinking specifically about the Space Shuttle.

[ARS]

5 hours ago, DDE said:

Why don't stage-and-a-half rockets use aerospikes? Thinking specifically about the Space Shuttle.

The most critical problem is cooling system. By default, SSME uses fuel flow through the main fuel valve into regenerative cooling systems for the nozzle. Considering the heat the engine needs to withstand (aside from the hot gas from the engine itself) from launch into reentry, it's A LOT.  The pressure (and temperature) of the gas remains very high all along the spike surface in order to maintain the consistent engine performance across all altitudes, and the sharp tip of the engine design itself leaves very little room for cooling systems, unlike the bell-shaped nozzle of SSME. Some other problems includes the fact that SSMEs are not exactly fully reusable, since despite being advertised as reusable, the engine needs to be swapped for each new launches, and considering the cooling problems above, it would considerably jack up the maintenance cost

Edited July 1, 2022 by ARS

[mikegarrison]

[StrandedonEarth]

@Gargamel: Since you're a machinist, ever made one of these?

short version:   https://i.imgur.com/dD7nLcv.mp4

Long vvveeerrrsssiiiooonnn:::: 

 

[JoeSchmuckatelli]

Question for the pedantics: when describing the amount of starlight received at any given point on an exoplanet, will we need a different word than insolation? 

 

(Pluto) 

Edited July 5, 2022 by JoeSchmuckatelli

[ARS]

40 minutes ago, JoeSchmuckatelli said:

Question for the pedantics: when describing the amount of starlight received at any given point on an exoplanet, will we need a different word than insolation? 

Yes, although it's very insignificant. Depending on Pluto's orbit, the planet's position is between 30 and 50 times more distant than the Earth's from the Sun, and since brightness decreases with the square of the distance, sunlight on Pluto is between 900 - 2500 times dimmer than sunlight on Earth (again, depending on Pluto's location on it's orbit), though with that amount of brightness, it's still quite a bit brighter than full moonlight on Earth - but definitely kinda dim. You’d be able to see fairly well around you (after all the real photo of Pluto is still quite well-lit), but the Sun itself won't be any brighter than other stars around it and for things like plant growth or generating electricity from solar power - forget it, it’s nowhere near enough

[Gargamel]

12 hours ago, ARS said:

though with that amount of brightness, it's still quite a bit brighter than full moonlight on Earth - but definitely kinda dim.

That didn’t sit right with me, but I think the answer is actually “sort of”.    As you said it varies, but there is an approximate analog here On earth: 

https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/plutotime/

 

This tells you the next time your location under a clear sky will look like Pluto at noon.