Serious Scientific Answers to Absurd Hypothetical questions

[sevenperforce]

3 hours ago, ARS said:

So here's my question:

There are 2 hypersonic fighter jet, they have the same max speed, performance, etc. But they have different propulsion method

Craft 1 use hypersonic scramjet engine attached on rear section that provide thrust that push the aircraft forward

Craft 2 use an advanced phlebotinum engine powered by handwavium (basically fictional engine), that propel the craft without actually provide thrust, but accelerate all parts of the craft at the same time forward

Both craft travel at the same speed.

Craft 1 moves by being pushed by the engines like normal jet configuration

Craft 2 moves by accelerating all of it's parts at the second time forward

My question is, does craft 2 has better performance in terms of withstanding the air resistance? Since in craft 1, the whole airframe is being pushed, but in craft 2, the whole airframe parts is moving at once

The scramjet engine must take in atmospheric air as an oxidizer and burn it with a fuel. This results in significant drag. In fact, the net thrust of a scramjet (or any airbreathing engine) is the difference between the product of exhaust mass flow and exhaust velocity and the product of intake mass flow and forward velocity, in keeping with the following equation:

F_{thrust_net} = MF_exhaust * v_exhaust - MF_intake * v_vehicle

Looks complex, but it's intuitive if you think about it. What's providing forward impulse? Well, it's the momentum of whatever you're pushing out of the back end of the engine. But what are you pushing out of the back end of the engine? Fuel, yes, but also all the air you pulled into your intake to mix and burn with your fuel. And that air entered your engine at the forward airspeed of your vehicle, so you have to subtract the momentum it already had in order to find out what your actual effective net thrust is.

All that to say -- a scramjet, or any other airbreather, has to deal not only with drag on the vehicle, but also with intake drag. If the handwavium engine has the same total thrust as the scramjet, then the handwavium aircraft will have far superior drag performance because you don't have to deal with an intake. If the handwavium engine has the same net thrust as your scramjet, the scramjet will have lower total drag because net thrust factors in intake drag losses, and the intake of a scramjet is typically a large component of your vehicle body.

Another thing to deal with is mass. The scramjet will lose mass as it flies while the handwavium engine will not. However, the scramjet will also lose thrust as it accelerates, due to that v_vehicle value increasing, so that needs to be taken into account as well.

But none of these differences have anything to do with the mechanical transference of force/acceleration. One can presume the scramjet's airframe is rigid enough that force on the engine mount is transferred to the rest of the craft as well as if each component were being accelerated together.

[DAL59]

How strong of an capacitor could you make that weighs less than 100 pounds?  

[Green Baron]

Me ? If i'd coil one up it'll probably go kerbal on first sight of a free electron :-)

Old style communication via smoke signs ...

 

Depends on the technology and what you mean with "strong".  A quick search reveals numbers for the energy and power density.

[YNM]

7 hours ago, DAL59 said:

How strong of an capacitor could you make that weighs less than 100 pounds?  

I guess this would comes down to dielectric coefficient and density of the isolator.

 

Conveniently, for "current" tech, these are allegedly the range power supplies can provide.

[wumpus]

14 hours ago, YNM said:

I guess this would comes down to dielectric coefficient and density of the isolator.

Expect that ceramic capacitors beat electrolytics in W/kg and lose badly in J/kg (they should be even closer to "ideal" capacitors with less equivilent series resistance).  In practice this is meaningless (matching the equations makes them great for filters and similar where you don't want resistance at all, but you want more capacitance and can easily eat the electrolytic's resistance).

I've never heard of Li-ion capacitors.  I want a hyrbrid car with them (plus LiFePO4 reserve), and I want it now!  It would even let a Leaf-style battery system have Tesla-level pep (assuming you could afford the motors.  But I suspect that the battery system is the the hard part right now).  The wiki for Li-ion capacitors is pretty weak, and if those are the real applications (because of other issues with the things) then it is dead (many of those would be better suited for even lead-acid batteries).

[YNM]

On 06/02/2018 at 8:45 AM, wumpus said:

Expect that ceramic capacitors beat electrolytics in W/kg and lose badly in J/kg (they should be even closer to "ideal" capacitors with less equivilent series resistance).  In practice this is meaningless...

Yeah. Capacitance are measured in farads, which is sort of equivalent to the amount of electrical charge (in coulombs) - for the slight caveat that they do vary with the voltage. Discharging a capacitor is a jolting process where all you know you'll just get ridiculous current in a super short time.

Translating them into joules is easy, but getting the watts can be hard.

Given that the charge goes with voltage (the capacitance being fixed here) the limit is in breakdown voltage as well. I found that ceramic capacitors are used mainly in RF devices, if such is true then they don't have a high breakdown voltage, which means a low electric charge, despite the fast response.

[magnemoe]

7 minutes ago, YNM said:

Yeah. Capacitance are measured in farads, which is sort of equivalent to the amount of electrical charge (in coulombs) - for the slight caveat that they do vary with the voltage. Discharging a capacitor is a jolting process where all you know you'll just get ridiculous current in a super short time.

Translating them into joules is easy, but getting the watts can be hard.

Given that the charge goes with voltage (the capacitance being fixed here) the limit is in breakdown voltage as well. I found that ceramic capacitors are used mainly in RF devices, if such is true then they don't have a high breakdown voltage, which means a low electric charge, despite the fast response.

How many watt you get out depend on how parallell you system is, you can double the watt strength using two capacitors. 
Same is true for all systems with limited output as in moar boosters

[p1t1o]

Lets say you put a slab of metal in space, floating a long way from anything.

Lets add electrons to it to give it a charge.

Lets keep adding them until they start spraying off into space and let it settle to a constant charge.

Lets call it one electrode of a capacitor.

Lets have a similar slab from which we have extracted some maximum amount of electrons, giving it a positive charge.

We can discharge the capacitor by bringing the two slabs quickly into the vicinity of each other, at some distance the charge will arc across space.

Is this a hypothetically maximum-performance capacitor?

 

[Spaceception]

Alrighty, so let's say there's an intelligent alien race that develops on a world with a dim sun, and hazy atmosphere, making visible light vision crap; and let's say they use radar, or sonar, or something similar to communicate with each other - could that evolve, or be technologically developed to be similar to telepathy? Just the communication part, not the whole 'mind reading' or 'psychic' parts. Or would it be just outside the realm of possibility, unlikely, but not completely impossible? Or just straight up impossible?

[DAL59]

2 hours ago, Spaceception said:

Alrighty, so let's say there's an intelligent alien race that develops on a world with a dim sun, and hazy atmosphere, making visible light vision crap; and let's say they use radar, or sonar, or something similar to communicate with each other - could that evolve, or be technologically developed to be similar to telepathy? Just the communication part, not the whole 'mind reading' or 'psychic' parts. Or would it be just outside the realm of possibility, unlikely, but not completely impossible? Or just straight up impossible?

Since you can get dental implants that make you hear radio waves, there is no reason why not.  

[magnemoe]

4 hours ago, Spaceception said:

Alrighty, so let's say there's an intelligent alien race that develops on a world with a dim sun, and hazy atmosphere, making visible light vision crap; and let's say they use radar, or sonar, or something similar to communicate with each other - could that evolve, or be technologically developed to be similar to telepathy? Just the communication part, not the whole 'mind reading' or 'psychic' parts. Or would it be just outside the realm of possibility, unlikely, but not completely impossible? Or just straight up impossible?

Radar would be very hard, sonar is plausible as bats and dolphins uses it.
However max bandwidth is not an limiting factor for human communication, language is limited to far below 10 byte / second, the fastest modems using phone lines was 28K bit or 3.5KB/s. 

The first real revolution for humans was the ability to store knowledge as in writing, second was to mass produce this, last was to make everything available for everyone as in internet. 

Social insects have very low communication bandwidth, they are also stupid, food this direction works. well enough. The queen has no real authority 

On the other hand you have humans in action
https://www.youtube.com/watch?v=TDgVm_Ffe1s
No this is not done often as the workers mostly stepped on each other, you prefer to do it over time until you connect. However in settings like saving trapped people after an earthquake its pretty common as wasted man-hours is irrelevant to saving people.
 

[kerbiloid]

6 hours ago, magnemoe said:

On the other hand you have humans in action
https://www.youtube.com/watch?v=TDgVm_Ffe1s

Would be nice to watch those "9 hours" from begin to end (with time compression of course), rather than several shots of working people and a slogan.
Second question: how long will the 9-hour station stay intact before it requires a complete remastering.

Edited February 9, 2018 by kerbiloid

[DAL59]

What is you made a BFR heavy?  

[kerbiloid]

18 minutes ago, DAL59 said:

What is you made a BFR heavy?

In won't lift.

Or you mean make it triple?

[DAL59]

1 minute ago, kerbiloid said:

Or you mean make it triple?

yes

[kerbiloid]

It will be the first rocket with 93 engines to run at once and probably they will face problems with joints.

[Spaceception]

20 hours ago, magnemoe said:

Radar would be very hard, sonar is plausible as bats and dolphins uses it.
However max bandwidth is not an limiting factor for human communication, language is limited to far below 10 byte / second, the fastest modems using phone lines was 28K bit or 3.5KB/s. 

The first real revolution for humans was the ability to store knowledge as in writing, second was to mass produce this, last was to make everything available for everyone as in internet. 

Social insects have very low communication bandwidth, they are also stupid, food this direction works. well enough. The queen has no real authority 

On the other hand you have humans in action
https://www.youtube.com/watch?v=TDgVm_Ffe1s
No this is not done often as the workers mostly stepped on each other, you prefer to do it over time until you connect. However in settings like saving trapped people after an earthquake its pretty common as wasted man-hours is irrelevant to saving people.
 

Well, I was thinking of something that could be long-distance (Many kilometers at least), because the aliens have very rare conflicts due to being so interconnected, and due to their accelerated communication skills, they advanced 6x faster than humanity time-wise. I was wondering if that would be realistic, or if I would have to stray more into the fiction side of things.

[Delay]

What if you made a continuous ring of connected rockets all the way around Earth and ignite all engines at the same time?

My guess is that they would immediately break apart because the circumference to cover has grown while the size/number of rockets has not.

[Steel]

15 minutes ago, Delay said:

What if you made a continuous ring of connected rockets all the way around Earth and ignite all engines at the same time?

My guess is that they would immediately break apart because the circumference to cover has grown while the size/number of rockets has not.

Which way are they all pointed? If they all point along the tangent of the ring the ring would spin (no net thrust, only a net rotation) and the ring would then break apart from the strain, unless it is a magical unbreakable ring

[peadar1987]

On 08/02/2018 at 6:41 PM, Spaceception said:

Alrighty, so let's say there's an intelligent alien race that develops on a world with a dim sun, and hazy atmosphere, making visible light vision crap; and let's say they use radar, or sonar, or something similar to communicate with each other - could that evolve, or be technologically developed to be similar to telepathy? Just the communication part, not the whole 'mind reading' or 'psychic' parts. Or would it be just outside the realm of possibility, unlikely, but not completely impossible? Or just straight up impossible?

To a deaf alien race our ability to hear would be considered "telepathy". Basically what you're asking for is the aliens to communicate using means not obvious to humans. They could use infrasound, magnetic fields, subtle visual cues, UV light...

[magnemoe]

2 hours ago, Spaceception said:

Well, I was thinking of something that could be long-distance (Many kilometers at least), because the aliens have very rare conflicts due to being so interconnected, and due to their accelerated communication skills, they advanced 6x faster than humanity time-wise. I was wondering if that would be realistic, or if I would have to stray more into the fiction side of things.

Yes, reach is an issue with voice, we used lot of visual stuff earlier like smoke signals, problem is even lower bandwidth. 
You could get longer reach underwater.

now for faster development I would rather invent the printing press way earlier, no issue making one in the ancient Egypt, ink, copper melting into forms and papyrus. 
Not sure how much impact it would have back then, literacy rates was very low.
However during roman times an printing press would have very high impact.

[DAL59]

How big would the moon look if it orbited just above the roche limit?

[wumpus]

53 minutes ago, DAL59 said:

How big would the moon look if it orbited just above the roche limit?

"Vulcan doesn't have a moon, it has a nightmare" [some Star Trek book I read in the 1980s.  Almost certainly "Spock's World", because I didn't read many (and will admit to even less)].

Now for some googling... stoops to wiki [hey, if I'm going to quote licensed books, I might as well site wiki].  Wiki claims the Moon is 21 times the Roche limit.

Big.  Really big.  I'm guessing the naive guess of 400 (square of 20) should be pretty close, but can't be bothered to check the Moon's angular size and work out if the angles are two big for simple approximation.

[Zeiss Ikon]

On 2/8/2018 at 8:31 AM, p1t1o said:

Lets say you put a slab of metal in space, floating a long way from anything.

Lets add electrons to it to give it a charge.

Lets keep adding them until they start spraying off into space and let it settle to a constant charge.

Lets call it one electrode of a capacitor.

Lets have a similar slab from which we have extracted some maximum amount of electrons, giving it a positive charge.

We can discharge the capacitor by bringing the two slabs quickly into the vicinity of each other, at some distance the charge will arc across space.

Is this a hypothetically maximum-performance capacitor?

 

This is far from a maximum performance capacitor.  Capacitance increases as the plates get closer together, providing your dielectric doesn't break down and permit an arc.  Vacuum (at least the imperfect kind you find close to planets) is, relatively speaking, a terrible dielectric (ultra-pure water is better, just as an example); you need far too much distance between plates to prevent arcing in order to get maximum electric field density (which is where the energy is stored in a capacitor).

if you're doing something that needs maximum capacitance, you'll usually select/fabricate your capacitor to have the biggest plate area possible, the least separation between plates you can manage (i.e. thinnest dielectric) and still isolate your working voltage, and in some cases, the lowest self-inductance possible (don't roll the thing up, let it lay flat instead).  Leyden jars can run to extremely high voltages (tens or hundreds of thousands of volts) because glass has a very high breakdown voltage -- but the glass in common jars is too thick, limiting the charge they can store.  If you're building a TEA laser, you'll usually fabricate your capacitor(s) from foil plates lying flat with food wrap or projection transparency sheets, and live with a limiting voltage between 10k and 25k (both of those dielectrics will break down after a time even at 10kV, and they'll break down quickly at 30 kV).  The dielectric choice here is a compromise; thinness improves capacitance and reduces self-induction, and the thing is easy enough to dismantle and rebuild that a short working life is an acceptable tradeoff.

This kind of capacitor doesn't store huge amounts of energy, however; it's optimized for voltage and charge/discharge rates (the foil/acetate type used for TEA lasers can charge and discharge between ~0V and 10kV 120 times per second if the power supply is up to it).

[kerbiloid]

And if we take two non-charged metal slabs and put them enough close to each other, we'll get Casimir Effect.