For Questions That Don't Merit Their Own Thread

[Aerodynamic Kerbal]

Questions about Turbofans

How does bypass air produce more thrust than fast hot air?

Does that mean that the main purpose of the combusted air is to spin up the turbine at the end, so that the intake fan starts spinning?

Is the combustion in jets in general caused by the heat of the oxygen reacting with the fuel?

Do ramjets do the same heat thing?

[StrandedonEarth]

The bypass fan moves more air for more thrust. It's essentially a ducted fan powered by a combustion turbine, and is more efficient than only using the hot, fast exhaust stream for thrust (move a large amount of air at a slower speed, or use a smaller amount of hot gas moving very fast). Fast-moving gas is LOUD, so large turbofans (fan air carefully blended with the exhaust stream) are a huge help with noise abatement around airports. 

[DDE]

4 hours ago, StrandedonEarth said:

The bypass fan moves more air for more thrust. It's essentially a ducted fan powered by a combustion turbine, and is more efficient than only using the hot, fast exhaust stream for thrust (move a large amount of air at a slower speed, or use a smaller amount of hot gas moving very fast). Fast-moving gas is LOUD, so large turbofans (fan air carefully blended with the exhaust stream) are a huge help with noise abatement around airports. 

So essentially, it's a ducted counterpart of the turboprop. The propfan sits somewhere in between, mixing the rotor design elements of the two.

[magnemoe]

1 hour ago, DDE said:

So essentially, it's a ducted counterpart of the turboprop. The propfan sits somewhere in between, mixing the rotor design elements of the two.

Think you are correct, the problem with an turboprop is that the tip of your propeller blades should not go supersonic. Yes you can but not if you want an quite plane. 
But turboprop is very popular for smaller and shorter routes from short runways with smaller say 20-50 passenger planes. 
I guess that you can angle the propellers add to their performance here.
Propfans has been in the focus from time to time but are anybody using them? 

 

[DDE]

16 hours ago, magnemoe said:

Propfans has been in the focus from time to time but are anybody using them? 

The first practical propfan was built at the wrong place at the wrong time.

https://en.wikipedia.org/wiki/Progress_D-27

I also get the feeling that we're at the downturn of the airliner desogn cycle and there's just no market for a new generation of engines.

[magnemoe]

6 hours ago, DDE said:

The first practical propfan was built at the wrong place at the wrong time.

https://en.wikipedia.org/wiki/Progress_D-27

I also get the feeling that we're at the downturn of the airliner desogn cycle and there's just no market for a new generation of engines.

Looking at the specifications its faster than turboprops as up to 800 km/h, now this might not be economical velocity. 
Have an feeling its an better turboprop more than an direct competitor to the turbofan and the turboprop market it not as large an profitable. 

[K^2]

On 8/12/2023 at 1:04 PM, Aerodynamic Kerbal said:

How does bypass air produce more thrust than fast hot air?

Thrust is related to the impulse, which is proportional to velocity. Energy you get from fuel is proportional to velocity squared of the exhaust. With the same energy, if you move the air at half the speed, you can move four times as much of it. If you move four times as much air at half the speed, you get twice the thrust. So larger mass of air moving slower is more efficient that moving a small mass of air really fast.

On 8/12/2023 at 1:04 PM, Aerodynamic Kerbal said:

Does that mean that the main purpose of the combusted air is to spin up the turbine at the end, so that the intake fan starts spinning?

There's a split in both the flow and where the energy is going, which is why we have terms like high bypass turbofan and low bypass turbofan. The latter moving more air through the core and less through the bypass. But in either case, some fraction of thrust still comes from the exhaust, and in any turbine-based design, at least some of the energy goes into spinning the turbine. In a high bypass turbofan, though, yes, most of the energy is spent spinning that rotor to push more bypass air.

On 8/12/2023 at 1:04 PM, Aerodynamic Kerbal said:

Is the combustion in jets in general caused by the heat of the oxygen reacting with the fuel?

Heat is the result of the combustion. Don't confuse heat and temperature. If you put heat into the system, the system is guaranteed not to get colder, but otherwise, the relationship between heat and temperature is a complicated one.

Certain temperature is necessary for combustion to start. How high that threshold temperature is depends on what type of fuel you have, what oxidizer, and what the mixture is. Simply spraying kerosene into the air will not cause it to catch fire, so in an engine, the mixture in the combustion chamber needs to be hot enough to burn.

When fuel burns, it releases heat energy. The purpose of the engine is to convert a portion of that heat energy into mechanical energy. The rest varies wildly from one design to another. Though, some sort of a gas expansion is a factor in nearly all practical engines.

On 8/12/2023 at 1:04 PM, Aerodynamic Kerbal said:

Do ramjets do the same heat thing?

Yes. The only difference is that in a ram jet, the supersonic shock performs the same function as a compressor in a turbojet engine, so you end up with no moving parts.

[magnemoe]

10 hours ago, K^2 said:

Thrust is related to the impulse, which is proportional to velocity. Energy you get from fuel is proportional to velocity squared of the exhaust. With the same energy, if you move the air at half the speed, you can move four times as much of it. If you move four times as much air at half the speed, you get twice the thrust. So larger mass of air moving slower is more efficient that moving a small mass of air really fast.

This, and this is true for rockets to, an hydrolox engines has lower trust than an RP1 engine of similar size because hydrogen is so light, but this makes it go fast so you get impressive ISP, nuclear thermal engines takes this to the extreme as they only heat hydrogen so isp might be 3 time higher, now unlike in KSP trust might not be low as its an nuclear reactor powering this but still not an heavy lift engine.  
If you need high trust for an short time like use rockets to get an plane up to takeoff speed you want an solid rocket with pretty heavy propellant for more trust. 
Now you can not go supersonic with high bypass ratio engines as then the turbine blades has to be supersonic. 

[K^2]

5 hours ago, magnemoe said:

This, and this is true for rockets to, an hydrolox engines has lower trust than an RP1 engine of similar size because hydrogen is so light, but this makes it go fast so you get impressive ISP, nuclear thermal engines takes this to the extreme as they only heat hydrogen so isp might be 3 time higher, now unlike in KSP trust might not be low as its an nuclear reactor powering this but still not an heavy lift engine.

This gets into the weeds of what it is that you are optimizing for. While we stuck to the atmospheric jet engines, we were purely in a world were the propellant is plentiful and energy (from fuel) is limited. Which is the only world in which pushing more mass to save energy really makes sense. Because more mass is always free.

Clear on the other side of this spectrum are the NTRs, which might as well be infinite energy for flight planning purposes, and you are limited by the propellant. Here, you don't want to be energy-efficient. You want to be propellant-efficient, and so yeeting the propellant as hard as you can makes a better sense. It's extremely energy inefficient to use Hydrogen compared to another propellant, sure, but why do you care? You have a nuclear reactor. What matters is that you need less of hydrogen by mass to get the same delta-V than anything else. So you go with that.

The chemical rockets are in the midway. The propellant-to-energy is pre-determined for you, so all you really can do is convert as much of that energy into kinetic energy of propellant as you can. Any attempt to dilute the mixture either way ends up reducing efficiency. There is no choice of exhaust velocities here. You take what you get from your fuel.

 

One final special case is the energy-bound case where you bring propellant with you, but it's not your energy source. This is similar to the NTR situation, except we can't count energy as unlimited. It only really makes sense if your energy source is ultra-dense, so probably nuclear, but your I_SP and target delta-V are so high, that you are going to burn through your reactor fuel. This is not remotely realistic for an NTR, but some sort of a really beefy plasma propulsion engine running for many years, perahps? And what's interesting here is that there is an optimal I_SP to aim for depending on your target delta-V to optimize the energy use. To be precise, exhaust velocity of approximately 0.63 * delta-V is the target. It results in m_f/m_i of about 4.9.

The only way I can see this become relevant in the near future is if we figure out mini-fusion of some sort and the fusion fuel is very expensive, like He3, or something. Which might be the case with the Helion reactors. In which case, we might have nuclear electric rockets with the right sort of a target range...

[SunlitZelkova]

Does innovation in science and engineering work better when development is divided up, or when everyone works together on one thing?

I’ve been thinking about this not only for the purposes of writing stories where humanity unites prior to the Space Age, but also in relation to the string of failed lunar landers we’ve seen in recent years.

I.e. Would we see a lower number of failures if we combined every space program into one location and entity, or does having multiple agencies doing the same thing, but not cooperating, increase innovation?

I wonder if the international fusion project gives an indication of what a united, international space program would be like.

[kerbiloid]

4 hours ago, SunlitZelkova said:

I.e. Would we see a lower number of failures if we combined every space program into one location and entity, or does having multiple agencies doing the same thing, but not cooperating, increase innovation?

1. Space programs of different countries are based on their local abilities and constraints, and thus differ very much.
Unlikely there is something common in the US and SU space programs, except biology and aims.

2. One boss nerd isn't what the other nerds need to breathe. It never ends well.
On the other hand, many nerd teams devalue each of them.

4 hours ago, SunlitZelkova said:

I wonder if the international fusion project gives an indication of what a united, international space program would be like.

50 years of "next 10 years"?

Edited August 22, 2023 by kerbiloid

[FleshJeb]

4 hours ago, SunlitZelkova said:

Does innovation in science and engineering work better when development is divided up, or when everyone works together on one thing?

I’ve been thinking about this not only for the purposes of writing stories where humanity unites prior to the Space Age, but also in relation to the string of failed lunar landers we’ve seen in recent years.

I.e. Would we see a lower number of failures if we combined every space program into one location and entity, or does having multiple agencies doing the same thing, but not cooperating, increase innovation?

I wonder if the international fusion project gives an indication of what a united, international space program would be like.

This is a fascinating question.

I'm just going to brain dump here. No energy for structure, apologies:

Pooled budgets, pooled knowledge. Systems/Standards interoperability. To a certain extent we're already sharing a lot of knowledge and standards with "competing" organizations. We're also already benefiting sociologically and politically from collaborating with other nations.

I think you get into the "design-by-committee" problem when organizations get too large.

The group that provides the largest percentage of the budget has a bigger vote. How do priorities get set in monolithic organizations?

What missions get prioritized, what instrument packages go on those missions?

I think there's a bigger benefit in having different sets of priorities and instrument designs. Uniformity means that maybe every planetary mission is carrying the same design of spectrograph for efficiency's sake. That instrument may have unknown flaws, or less-appropriate specifications than a custom-built one.

Even within existing large programs, they're already subdivided. JPL is a subset of NASA, Soviet Design Bureaus had a lot of independence. I'd suggest that one of the drivers for this is preventing the stagnation of uniformity.

Does a worldwide space program end up with the equivalent of the UN Security Council, where SOME participants get to veto everyone else? Is that veto power a pro or a con? (I'm inclined to think the latter.)

I think it would be a good idea to look at the structure and history of NATO to see what the kind of organization you're proposing would look like.

Brain dying, thanks for the question. REALLY thought-provoking.

[TheSaint]

6 hours ago, SunlitZelkova said:

Does innovation in science and engineering work better when development is divided up, or when everyone works together on one thing?

Or will it turn into a complete disaster?

Yes.

Too many variables. It all depends on the humans involved. Sometimes they will work well together, sometimes they will do nothing but play politics all day and accomplish nothing. You don't need to try to meld international space programs to see this. All you have to do is try to get two churches together for a potluck. Ask me how I know.

[magnemoe]

11 hours ago, SunlitZelkova said:

Does innovation in science and engineering work better when development is divided up, or when everyone works together on one thing?

I’ve been thinking about this not only for the purposes of writing stories where humanity unites prior to the Space Age, but also in relation to the string of failed lunar landers we’ve seen in recent years.

I.e. Would we see a lower number of failures if we combined every space program into one location and entity, or does having multiple agencies doing the same thing, but not cooperating, increase innovation?

I wonder if the international fusion project gives an indication of what a united, international space program would be like.

If everything works well I say combined works better.  But division makes mistakes much easier to recover from, if all goes in the same direction momentum will move it along, also competition helps keep the pressure up. 

[Terwin]

Competition gave us Apollo, cooperation gave us the ISS.

I expect that 'fear of falling behind ' is a much more effective motivatior than thirst for discovery, at least when it comes to national budgets.

Edited August 23, 2023 by Terwin
Autoincorrect

[magnemoe]

2 hours ago, Terwin said:

Competition gave us Apollo, cooperation gave us the ISS.

I expect that 'fear of falling behind ' is a much more effective motivatior than thirst for discovery, at least when it comes to national budgets.

Also completion gave us falcon 9 with reusable first stages and moving the goalposts forward to second stage reuse.
So completion probably works better for things with multiple solutions, cooperation works better if its so expensive you can only afford one. 
And  two smaller space stations had not worked better I say. 

[JoeSchmuckatelli]

I think, honestly, that the best comes after the government pushed breakthrough gets normalized and for profit competition takes over. 

[SunlitZelkova]

So I'm curious about the plausibility in real life of part of a novel called Radio Free Albemuth.

In the book, an alien satellite has been orbiting Earth for thousands of years, being responsible for various religious phenomena in the past. It isn't discovered until the 1970s, and then the Soviet government, with approval from their American puppet president, proceed to blow it up using an interceptor satellite.

Would it be possible for an object to remain in Earth orbit for that long- effectively being a second satellite of Earth prior to real artificial satellites- without being detected during the early days of the Space Race?

The satellite is past GEO (it took "two or three days" for the interceptor satellite to reach it) and it's characteristics are otherwise unclear. Would it be possible to build it utilizing some material making it less visible to telescopes and radar?

Given it is an alien satellite, I suppose there technically could be some undiscovered physical process and materials being used to shield it from Terran eyes.

[magnemoe]

16 minutes ago, SunlitZelkova said:

So I'm curious about the plausibility in real life of part of a novel called Radio Free Albemuth.

In the book, an alien satellite has been orbiting Earth for thousands of years, being responsible for various religious phenomena in the past. It isn't discovered until the 1970s, and then the Soviet government, with approval from their American puppet president, proceed to blow it up using an interceptor satellite.

Would it be possible for an object to remain in Earth orbit for that long- effectively being a second satellite of Earth prior to real artificial satellites- without being detected during the early days of the Space Race?

The satellite is past GEO (it took "two or three days" for the interceptor satellite to reach it) and it's characteristics are otherwise unclear. Would it be possible to build it utilizing some material making it less visible to telescopes and radar?

Given it is an alien satellite, I suppose there technically could be some undiscovered physical process and materials being used to shield it from Terran eyes.

As i understand satellites out at GEO will stay in orbit for thousands of years. And if out say 60.000 km and an inclined orbit it would be hard to spot. Far more so if it tried to be stealthy as in non reflecting materials towards earth. 
Now blowing it up would be stupid, you want to loot all the alien technology. in the 70's you could do an manned mission to it. 

[kerbiloid]

Read the nearest GSO satellite passive emission signature, emulate it, and destroy it.

They will think, it's a dead sat.

Upd.
Won't work now. They read this forum.

Edited September 6, 2023 by kerbiloid

[magnemoe]

13 hours ago, kerbiloid said:

Read the nearest GSO satellite passive emission signature, emulate it, and destroy it.

They will think, it's a dead sat.

Upd.
Won't work now. They read this forum.

Downside is that some might point an telescope at the satellite, GEO satellites was pretty rare in the 70's, some astronomer might point an telescope at it. Also you now only see one part of earth. 
Now being responsible for various religious phenomena is weird, could be done with some sort of reentery probe with holograms but here I say magic.  
Still trying to board it to loot the alien technology would be very tempting. 

[kerbiloid]

2 hours ago, magnemoe said:

Also you now only see one part of earth. 

The former real sat is blown up and looks like debris, the Dr. Evil craft doesn't need to be exactly GSO, it drifts on a tilted elliptic orbit and passively emits like the sat body would.

[sevenperforce]

11 hours ago, magnemoe said:

Downside is that some might point an telescope at the satellite, GEO satellites was pretty rare in the 70's, some astronomer might point an telescope at it. Also you now only see one part of earth. 

If it's farther than GEO then it will still be able to see all of Earth over the course of its orbit.

Yes, some amateur astronomer might point a scope at it, but it would be difficult to pick up again if it was in an orbit like that. Also limited reflectivity would make it virtually invisible. 

11 hours ago, magnemoe said:

Now being responsible for various religious phenomena is weird, could be done with some sort of reentery probe with holograms but here I say magic.  
Still trying to board it to loot the alien technology would be very tempting. 

I don't see how it could be responsible for anything on Earth but then again I haven't read the book.

[SunlitZelkova]

13 hours ago, sevenperforce said:

I don't see how it could be responsible for anything on Earth but then again I haven't read the book.

At the time the author wrote it, he was still trying to explain his religious experiences of 1974 with pseudoscientific theories like thought transference.

I don’t recommend it unless you are a big fan of the author (Philip K. Dick). He actually canned this project, it wasn’t published in his lifetime and was only a first draft.

Note that in the novel he did end up publishing after reworking Radio Free Albemuth, called VALIS, he decided to make the proponents of the “satellite theory” insane, while leaving it an open question as to whether anything he experienced was real.

In contrast, Radio Free Albemuth establishes an “end all be all” set of facts and reality.

[darthgently]

On 9/7/2023 at 10:36 PM, SunlitZelkova said:

At the time the author wrote it, he was still trying to explain his religious experiences of 1974 with pseudoscientific theories like thought transference.

I don’t recommend it unless you are a big fan of the author (Philip K. Dick). He actually canned this project, it wasn’t published in his lifetime and was only a first draft.

Note that in the novel he did end up publishing after reworking Radio Free Albemuth, called VALIS, he decided to make the proponents of the “satellite theory” insane, while leaving it an open question as to whether anything he experienced was real.

In contrast, Radio Free Albemuth establishes an “end all be all” set of facts and reality.

I got drawn into the entire VALIS  related series a few years back. If you find reality bending PKD based movies like Blade Runner and Total Recall  intriguing you might get drawn into VALIS.  PKD on steroids