For Questions That Don't Merit Their Own Thread

[PB666]

On August 18, 2016 at 5:09 PM, wizzlebippi said:

I might be a savvy engineer, so I'll take a crack at this.  Engines work by using a heat source to cause a working fluid to expand as a means of transferring energy from fuel into mechanical energy.  The problem is most working fluids don't hold much thermal energy at useful temperatures, limiting the energy output and efficiency of the engine.  The solutions are either increasing the operating temperature or increasing the mass of the working fluid.  Increasing operating temperature usually isn't an option, at least not by enough to make a difference, due to material properties and cooling.  Increasing the size of the engine can dramatically increase energy output by increasing the volume of working fluid, but at the expense of efficiency due to difficulty heating all the working fluid.  Compressing the working fluid increases the mass being heated while keeping the volume being heated reasonable, allowing more chemical energy to be captured as heat, resulting in increased energy output and efficiency of the engine. 

Ultimately the kcal/mol of bond breaking and reformation  creates a saturation ISP, beyond which any given fuel can supercede withou the outside input of energy,msuchbas rf of laser.mThe problem is that all sources of energy have lower energy density than chemical fuels. Solar is highly weoght effocient but can on produce about 1.3 kw per kg, and agian there is the problem that efficiency of solar electric power required per N of thrust gies up linearly with ISP to 300 MW/N for a photon drive. nuclear electric power theorectically can generate huge amounts of power, but fission reactors in space are ineffeicient and require more weight in heat exchanges than solar requires panels.

SEP has the following limitation, as mass increase the two dimension profile of a three dimensional object increases as a function of f^2/3. AS a consequence as a space ship gets bigger the structure required to create a two dimension plane that can give the same amount of accelation as a smaller ship decreases. Invariably then for SEP, biggers ships will accekerate more slowly or be more inefficient. The same can be said about fission electric, it would need nore structure for heat exchangers. 

So this leaves direct nuclear propulsion as the only option more capable than chemical in terms of performance but not efficiency. And performance is an efficiency, becasue a spiralling transfer eventually cost more dV than a hohmann transfer, but the types of energy available with nuclear either fall into low performing nerva type Nuclear thermal rockets, secondary heat transfer rockets or pulse fusion rockets. Both of these have engine masses that are quite high and both need storage needy hydrogen. 

Theoretically what is needed is a rocket that converts all hydrogen and deuterium into either energy or a fusion product that is accelerated. If about 3% of the mass coverted then a stop start trip of 0.1c is plausible, though not realistic. 

 

[RocketSquid]

What's the maximum thrust power from an electrothermal engine?

[Bill Phil]

11 minutes ago, RocketSquid said:

What's the maximum thrust power from an electrothermal engine?

As much energy as you can realistically cram into it.

[RocketSquid]

56 minutes ago, Bill Phil said:

As much energy as you can realistically cram into it.

Okay, what is the practical limit given modern material science?

[DDE]

11 minutes ago, RocketSquid said:

Okay, what is the practical limit given modern material science?

None. So long as you have enough power to heat up a big enough resistor, you can keep increasing the remass flow.

[RocketSquid]

Just now, DDE said:

None. So long as you have enough power to heat up a big enough resistor, you can keep increasing the remass flow.

Wow, okay. That opens up some options.

[DDE]

Just now, RocketSquid said:

Wow, okay. That opens up some options.

Well, there will be problems transferring heat, but those aren't based around material science. It's just at some point an NTR is a better option because there's no loss of energy due to numerous conversions.

[magnemoe]

4 hours ago, DDE said:

Well, there will be problems transferring heat, but those aren't based around material science. It's just at some point an NTR is a better option because there's no loss of energy due to numerous conversions.

Max temperature who limit ISP will be limited by the material, you are limited to conductors as you need to transfer electricity. 
If you use an reactor for power you can just as well use an nerva, For solar you could use the sunlight directly, this could take you a bit higher. 

Now for fun you could use microwaves or other radiation the exhaust absorb to heat it more, however here we start getting into vasmir engines. 
If you have enough power its generally no issue getting most engines to perform serious trust. Reactors are heavy and require lots of cooling, solar panels are large and heavy and don't work well in the outer system. 
 

[TheDestroyer111]

Is the above question answered? Because it looks like it is. If it is then I have another question.

Why do propeller aircraft and many other things have power ratings instead of thrust like jets? This appears somewhat incorrect IMO, as if something is rated by its power, then the force created by it goes to infinity at zero speed. However thrust also looks as a partly incorrect way of defining an engine's output, because at infinite speed (screw relativity) a thrust-based engine (jet/rocket) would give infinite amount of kinetic energy while using the same amount of fuel as at zero speed. What is wrong here?

[Kryten]

The thrust of a prop engine will vary heavily with altitude, temperature, and propeller pitch; power-rating is a convenient constant measurement that is roughly proportional to thrust.

[K^2]

Indeed. With variable pitch constant speed props, there is a decent range of conditions where thrust is roughly proportional to power divided by air speed. Fixed props are a different matter, however, and there is not really a single figure that rates performance alone.

[TheDestroyer111]

Staying away from the propeller problem, what about jets? At infinite speeds, a jet engine (including rockets) gives infinite kinetic energy with the same fuel use.

[Kryten]

That's like asking why food packaging doesn't specify that the mass given is the rest mass. Nobody who works with aircraft has any reason to care about paradoxes appearing at infinite speed, simple as.

[K^2]

11 hours ago, TheDestroyer111 said:

Staying away from the propeller problem, what about jets? At infinite speeds, a jet engine (including rockets) gives infinite kinetic energy with the same fuel use.

Lets forget jets for a moment. Jets have to take in air, and that becomes less and less efficient at high speeds. Lets talk about rockets. In vacuum. Because, in vacuum, a rocket engine really does put out the same amount of thrust at any speed. And consequently, power output of a rocket standing still is zero. Power output of a moving rocket diverges to infinity as speed gets higher. It actually happens way before "infinite" speeds due to relativistic effects. Rocket engine's power output is infinite at "mere" speed of light.

But lets set aside infinities and relativism. The reason rocket has more power at higher speed is because you don't just get the chemical energy of the fuel. Fuel is also moving, and so it has kinetic energy. And because of how kinetic energy changes with speed, when you fire the engine, you actually get to use some of that kinetic energy to get more power out of the engine. Conversely, when rocket is standing still, all of the chemical energy just goes into kinetic energy of exhaust, and zero energy goes into moving the rocket. The sweet spot, where power out is exactly chemical power in, minus heat losses, is at g*I_SP/2, because KE of fuel/oxy = KE of exhaust.

So lets get back to the jet engine. Exactly the same principles apply, except, most of the exhaust mass is air sucked in by the engine. We don't have that kinetic energy to begin with. We must take incoming air, accelerate it to the engine's speed, and then we get to use part of that kinetic energy to get extra power when it is exhausted. So yeah, jet engine produces more power on output, but all of that energy first has to be put there by the same engine, which comes at the cost of drag. If instead of looking at just the net thrust of the jet engine, you take thrust minus drag produced by the engine, suddenly, you don't see power going to infinity anymore. Just the opposite, due to additional heat losses at high speeds, power of a jet engine increases for a while, peaks, and then starts going down as the speeds get higher.

More interestingly, different kinds of engines will peak at very different speeds. That's why you see high bypass turbofans on airliners, low bypass or turbojets on fighters, and ram or scram jets on hypersonic aircraft and missiles. The later don't have compressors at all, because they are trying to get the air to flow through the engine with as little resistance as possible.

[TheDestroyer111]

12 hours ago, Kryten said:

That's like asking why food packaging doesn't specify that the mass given is the rest mass. Nobody who works with aircraft has any reason to care about paradoxes appearing at infinite speed, simple as.

The paradox appears at any speed, just the infinite speed case is the only one which I can get into simple words.

@K^2 I said Screw relativity! This is a little paradox (turns out, misunderstanding) of non-relativistic physics.

BTW ramjets have compressors, just not axial/centrifugal (they are ram jets because they ram the air to compress it)

Still, thank you very much! I also have one small question. Because turbofans are essentially turboprops with ducted fans, so why not say that a turbofan has lets say 1kn thrust+1000hp power?

[K^2]

Turboprops have constant speed variable pitch props. So they maintain roughly constant power and have variable thrust. Turbofans have fixed blades in the fan/compressor, so despite many similarities they actually operate much closer to constant thrust. Everything I mentioned above about drag applies, of course, but that's usually not factored in, so a turbofan ends up a constant thrust variable power engine on paper.

A lot of it is tradition, of course. If you want to get the actual thrust/power at specific altitude and speed, you aren't going to rely on that single quoted number. You'll pull out performance charts and do some math. But as far as just giving people an idea of what the engine is capable of, turboprops are still quoted as W/Hp, like a piston prop would, and turbofans are quoted in kN/lb of thrust, like any other jet engine would.

[monophonic]

This leads me to ask a follow up: Would there be any advantage in using variable pitch fan blades in a turbofan engine compared to current engine types (fixed blade fan or variable pitch prop)? What disadvantages would there be other than the mechanical complexity? And has it been tried even just in a research setting?

[magnemoe]

5 hours ago, monophonic said:

This leads me to ask a follow up: Would there be any advantage in using variable pitch fan blades in a turbofan engine compared to current engine types (fixed blade fan or variable pitch prop)? What disadvantages would there be other than the mechanical complexity? And has it been tried even just in a research setting?

Might be some but probably not much, first it would be an complex thing as you say, main problem with turbo prop is limited speed as I understand this is because you don't want the edge of the propellers to pass the speed of sound, as the blades in an turbofan is shorter it can rotate faster. 

You see this in uses of the two types, turboprop is most used on short routes often with short runways and smaller planes, lots of this in Norway, on the longer trips you use turboprop you spend more time at optimal speed and attitude, running the engine sub-optimal during takeoff don't matter so much as its an faction of the trip, on an 30 minutes flight its the other way around. Or an 2 hour trip there the plane land 4 times
The speed difference 600 against 800 is not an huge issue unless you fly intercontinental. 

[p1t1o]

14 hours ago, monophonic said:

This leads me to ask a follow up: Would there be any advantage in using variable pitch fan blades in a turbofan engine compared to current engine types (fixed blade fan or variable pitch prop)? What disadvantages would there be other than the mechanical complexity? And has it been tried even just in a research setting?

There are indeed examples of this: https://en.wikipedia.org/wiki/Variable_pitch_fan

One example I saw stated that it increased fuel efficiency by up to 14%.

[ChrisSpace]

On what day of the year (on average) would the Northern Hemisphere's food stockpiles be at their lowest amount? How much would this change during a nuclear winter?

[p1t1o]

1 hour ago, ChrisSpace said:

On what day of the year (on average) would the Northern Hemisphere's food stockpiles be at their lowest amount? How much would this change during a nuclear winter?

The nuclear winter hypothesis has waxed and waned in its credibility over the years, and effects on agriculture can be hard to predict, but this passage from the wiki about a 2014 study is quite recent and apt:

 

In 2014, Michael J. Mills (at the US National Center for Atmospheric Research, NCAR), Owen B. Toon (of the original TTAPS team), Julia Lee-Taylor, and Alan Robock published "Multi-decadal global cooling and unprecedented ozone loss following a regional nuclear conflict" in the journal Earth's Future.^[130] The authors used computational models developed by NCAR to simulate the climatic effects of a regional nuclear war in which 100 "small" (15 kt) weapons are detonated over cities. They concluded, in part, that

global ozone losses of 20-50% over populated areas, levels unprecedented in human history, would accompany the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30-80% over Mid-Latitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10-40 days per year for 5 years. Surface temperatures would be reduced for more than 25 years, due to thermal inertia and albedo effects in the ocean and expanded sea ice. The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine.

[ChrisSpace]

20 hours ago, p1t1o said:

The nuclear winter hypothesis has waxed and waned in its credibility over the years, and effects on agriculture can be hard to predict, but this passage from the wiki about a 2014 study is quite recent and apt:

 

In 2014, Michael J. Mills (at the US National Center for Atmospheric Research, NCAR), Owen B. Toon (of the original TTAPS team), Julia Lee-Taylor, and Alan Robock published "Multi-decadal global cooling and unprecedented ozone loss following a regional nuclear conflict" in the journal Earth's Future.^[130] The authors used computational models developed by NCAR to simulate the climatic effects of a regional nuclear war in which 100 "small" (15 kt) weapons are detonated over cities. They concluded, in part, that

global ozone losses of 20-50% over populated areas, levels unprecedented in human history, would accompany the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30-80% over Mid-Latitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10-40 days per year for 5 years. Surface temperatures would be reduced for more than 25 years, due to thermal inertia and albedo effects in the ocean and expanded sea ice. The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine.

Congrats, you have answered literally everything else I was going to ask except my original question.

[p1t1o]

2 hours ago, ChrisSpace said:

Congrats, you have answered literally everything else I was going to ask except my original question.

You're welcome?

Its probably the best answer you are likely to get, its a recent study and its not like nuclear winters have a huge research topic behind them.

Not all nuclear winters are the same, how many weapons? What yield? Globally dispersed or more localised? Are they detonating in urban areas or over more remote targets? Which atmospheric model are you using to predict airflow? Which conflagration model are you using to predict smoke/soot particle size and distribution? Which weather model are you using to predict effects on agriculture?

And FYI, if you ever do find a model that can accurately give you an answer to your question given certain atmospheric properties, you will probably be a billionaire.

Its a hard question, its why you are asking it here.

 

Edited September 13, 2016 by p1t1o

[RainDreamer]

What star system that we have discovered has the most planets ?

[Spaceception]

On 9/14/2016 at 9:26 AM, RainDreamer said:

What star system that we have discovered has the most planets ?

Most planets overall: http://www.openexoplanetcatalogue.com/planet/HD 10180 b/ 9 planets.

Largest Kepler system: http://www.openexoplanetcatalogue.com/planet/Kepler-90 h/ 7 planets.

Closest confirmed Multiplanetary system: https://en.wikipedia.org/wiki/Wolf_1061 3 planets

Closest unconfirmed Multiplanetary system: https://en.wikipedia.org/wiki/Epsilon_Eridani#Planetary_system 2 planets

Although, if Proxima has more than 1 planet, than that would obviously be the closest Multiplanet system.

Edited @RainDreamer

Edited September 15, 2016 by Spaceception