For Questions That Don't Merit Their Own Thread

[USB4]

I wanted to make an engine that would operate in atmospheres without oxygen (like eve) for long periods of time in a realistic (non cheaty) fashion. Ive looked at electric motors, but given how out of scale ksps electrical system is im unsure any motor can be really balanced (no mass to electricity, weird scaling across parts). Next I thought about systems that brought their own oxidizers, however those seem to all be heavy or use up their fuel far too quickly. 

 

I managed to stumble upon this however, which was my inspiration for modeling this monopropellant based 2 cylinder engine.

 

Spoiler

What Im asking here, is if anyone here would know how to get a somewhat accurate config for this in terms of ISP and power for its size in kerbal terms.

 

Currently with a 0.625m part, I have it with a max of 15kn at around 0.4 mach at sea level, and isp, Im unsure of.

[munlander1]

Saw this on @Matt Lowne's discord, but I couldn't find the article after a quick google search. Can anyone verify this?

[James Kerman]

1 hour ago, munlander1 said:

Can anyone verify this?

I can find no confirmation of planet 9 however the post may relate to a new paper on planet 9 detection based on time resolved coadds covering 3 quarters of the sky and incorporating 4 years out of seven of WISE Observations.

https://arxiv.org/pdf/1712.04950.pdf
Published 2 days before the above post.

[Aghanim]

Question for anyone who frequently visits Atomic Rockets website:

! WARNING STAR WARS THE LAST JEDI SPOILER !

Spoiler

In The Last Jedi, the battle cruiser bridge was shot by 2 Imperial fighters, blowing it up and killing everyone inside, including Admiral Ackbar And then I remembered that somewhere in Atomic Rockets website he specifically warns us to not put the bridge in front of the ship, where it is extremely vulnerable to this kind of thing, but I forgot where it is, so can someone point me to there?

 

Edited December 18, 2017 by Aghanim

[K^2]

On 12/17/2017 at 6:15 PM, USB4 said:

What Im asking here, is if anyone here would know how to get a somewhat accurate config for this in terms of ISP and power for its size in kerbal terms.

Currently with a 0.625m part, I have it with a max of 15kn at around 0.4 mach at sea level, and isp, Im unsure of.

While sitting on the ground, a Cessna 150/152 will have about 15kN at effective I_SP of about 20,000s. This will vary a bit with the engine, but it's in the ballpark. However, it's not going to be able to get to anything close to mach 0.4. Even a variable pitch prop will lose thrust and I_SP rapidly as it picks up the speed. I would expect about half the thrust and I_SP by the time a variable pitch prop gets up to these speeds, assuming the whole thing is designed to operate like that in the first place. A fixed pitch prop will be just chopping air by about mach 0.2.

Turboprops are designed to operate into transonic regions, but even their I_SP will drop to something in the 5,000-6,000 range by that point. So if you want to go for something almost ridiculously Kerbal, I'd go with 20,000s at mach 0, 10,000s at mach 0.5, 5,000s at mach 1, and 0s at mach 1.5. Thrust should scale the same, keeping fuel consumption constant. You can add second scaling on altitude, which will not impact the I_SP, but will scale thrust proportionally to air density.

[USB4]

11 minutes ago, K^2 said:

While sitting on the ground, a Cessna 150/152 will have about 15kN at effective I_SP of about 20,000s. This will vary a bit with the engine, but it's in the ballpark. However, it's not going to be able to get to anything close to mach 0.4. Even a variable pitch prop will lose thrust and I_SP rapidly as it picks up the speed. I would expect about half the thrust and I_SP by the time a variable pitch prop gets up to these speeds, assuming the whole thing is designed to operate like that in the first place. A fixed pitch prop will be just chopping air by about mach 0.2.

Luckily for me the model Ive already designed has variable pitch  (though having them at the right angle for any given speed would Im assuming need firespitter or something like it)

Quote

Turboprops are designed to operate into transonic regions, but even their I_SP will drop to something in the 5,000-6,000 range by that point. So if you want to go for something almost ridiculously Kerbal, I'd go with 20,000s at mach 0, 10,000s at mach 0.5, 5,000s at mach 1, and 0s at mach 1.5. Thrust should scale the same, keeping fuel consumption constant. You can add second scaling on altitude, which will not impact the I_SP, but will scale thrust proportionally to air density.

I had my config with it going to mach 8. Guess Il change that. Though correct me if Im wrong, but dont most ksp stock engines unrealistically high top speeds? The big question though, is how close to aircraft fuel is monopropellant efficiency wise. I dont know what the values here convert to. I suppose Il try (likely incorrectly) to figure it out myself from the numbers there, but I was hoping someone who knows the know would be able to far more easily than I could.

Edited December 19, 2017 by USB4

[K^2]

Monoprop is definitely worse. But why are you using that, and not fuel/ox mix?

[USB4]

48 minutes ago, K^2 said:

Monoprop is definitely worse. But why are you using that, and not fuel/ox mix?

I noticed that apart from electric engines there arent really any other ways to fly on planets with atmospheres without oxygen. While electric engines are fine, due to the weirdness of electricity in ksp, I dont think they work quite right or ever can be really properly balanced energy density/use wise. That, and they also cant register for fuel remaining or run time. 

 

As for why monopropellant over lx/ox, I havent found an engine capable of using both in real life with long endurance more typical of turbojets or piston engines, but I did find a nasa project from the 70s which used mono propellant to power a little one cylinder engine, so I used that roughly for inspiration and a guide for power/efficiency.

[sevenperforce]

17 hours ago, Aghanim said:

Question for anyone who frequently visits Atomic Rockets website:

! WARNING STAR WARS THE LAST JEDI SPOILER !

  Hide contents

In The Last Jedi, the battle cruiser bridge was shot by 2 Imperial fighters, blowing it up and killing everyone inside, including Admiral Ackbar And then I remembered that somewhere in Atomic Rockets website he specifically warns us to not put the bridge in front of the ship, where it is extremely vulnerable to this kind of thing, but I forgot where it is, so can someone point me to there?

 

http://www.projectrho.com/public_html/rocket/spacewartactic.php

 

[USB4]

13 hours ago, K^2 said:

Monoprop is definitely worse. But why are you using that, and not fuel/ox mix?

So For anyone else reading, is my math wrong or is does kerosene really have a Specific energy (MJ/kg) of more than 20 x  than that of hydrazine?! Surely thats wrong or Im misinterpreting something right? 

 

 

[K^2]

1 hour ago, USB4 said:

So For anyone else reading, is my math wrong or is does kerosene really have a Specific energy (MJ/kg) of more than 20 x  than that of hydrazine?! Surely thats wrong or Im misinterpreting something right?

Enthalpy of Hydrazine is 50.63kJ/mol at molar mass of 32g/mol. So I'm getting ~1.5MJ/kg. And yeah, kerosene is about 46MJ/kg, so it's actually close to 30x difference. However, a lot of it is due to oxidizer coming from air. The stoichiometric ratio of kerosene to pure oxygen is about 1:3.276. So the net specific energy of kerlox is only about 11mJ/kg, and that's "only" 6-7 times better than Hydrazine.

The reason for the difference is because you're using Hydrazine as monoprop. So all you get out of it is enthalpy of the fuel, with products being N₂ and H₂ gasses, which have enthalpy of zero by definition. On the other hand, were you to burn the hydrogen, the enthalpy of water vapor is -241.8kJ/kg, and you'd get two moles of that for every mole of hdyrazine. This is why anything with hydrogen in it makes for such a fantastic bipropellant fuel. If you were to burn hydrazine with oxygen, the net specific energy is going to be 8.3MJ/kg, and that's only 20% less than kerlox.

But yeah, monoprops are pretty pitiful in their performance compared to bipropellant fuels.

[USB4]

7 minutes ago, K^2 said:

"only" 6-7 times better than Hydrazine.

Thats still a whole lot better than 30x at 30 x you might as well just use a rocket.

 

The thing is though, isnt it still about 30x then given it doesnt have to carry oxidizer? If I am getting that right, then Im totally confused by how the test plane got to 20000ft in 34 mins of flight with such bad efficiency. Im confused arent I.

[Rickert-A33]

how to make a nice and warm egg that doesnt need to be baked.

[Green Baron]

57 minutes ago, Rickert-A33 said:

how to make a nice and warm egg that doesnt need to be baked.

Question or recipe ?

If q then any reptile/bird could answer this ...

[K^2]

5 hours ago, USB4 said:

The thing is though, isnt it still about 30x then given it doesnt have to carry oxidizer? If I am getting that right, then Im totally confused by how the test plane got to 20000ft in 34 mins of flight with such bad efficiency. Im confused arent I.

Do you have a link with some additional information, perhaps? Because without details I'm just speculating. That said, the aforementioned C152 takes about 5 gallons of 100LL for an hour of flight. A little over 6 if you are climbing, and it will climb to 20k feet in 30 minutes. That is not a plane designed for efficiency. It's mostly designed for ease of operation and fun of flying. If you build something with larger wings out of better materials, you'll have no trouble cutting it to 4 gallons per hour without sacrificing much in terms of performance. So we're looking at 2 gallons or ~6kg of kerosene for that climb. At 30x flying on Hydrazine, that's 180kg of fuel. That's a bunch, but you can get quite a bit more into a two-seater if you really need to. And a good chunk of that will be compensated by a lighter engine, I bet.

So again, pretty terrible, but not so much as to be entirely impractical. Although, I bet the conclusion was, "Well, this works, but where are we going to fly it?" Anywhere in Sol system you could, you definitely want a longer range. Moreover, in the real world, modern batteries can actually do considerably better than this, which is why I don't see us coming back to the concept.

[peadar1987]

3 hours ago, Green Baron said:

Question or recipe ?

If q then any reptile/bird could answer this ...

Pretty sure most reptiles/birds would tell you that all of their eggs really don't need to be baked!

[Green Baron]

Exactly ! :-)

They know how to make a nice and warm egg that does not need to baked

Edited December 20, 2017 by Green Baron

[USB4]

1 hour ago, K^2 said:

Do you have a link with some additional information, perhaps? Because without details I'm just speculating. That said, the aforementioned C152 takes about 5 gallons of 100LL for an hour of flight. A little over 6 if you are climbing, and it will climb to 20k feet in 30 minutes. That is not a plane designed for efficiency. It's mostly designed for ease of operation and fun of flying. If you build something with larger wings out of better materials, you'll have no trouble cutting it to 4 gallons per hour without sacrificing much in terms of performance. So we're looking at 2 gallons or ~6kg of kerosene for that climb.

This nasa pdf is pretty much the most information I can find on it. Thats not to say its isnt detailed enough at least for this purpose Im guessing.

 

1 hour ago, K^2 said:

At 30x flying on Hydrazine, that's 180kg of fuel. That's a bunch, but you can get quite a bit more into a two-seater if you really need to. And a good chunk of that will be compensated by a lighter engine, I bet.

They did it at under 90 kg total.

[KG3]

On ‎12‎/‎17‎/‎2017 at 9:15 PM, USB4 said:

I wanted to make an engine that would operate in atmospheres without oxygen (like eve) for long periods of time in a realistic (non cheaty) fashion. Ive looked at electric motors, but given how out of scale ksps electrical system is im unsure any motor can be really balanced (no mass to electricity, weird scaling across parts). Next I thought about systems that brought their own oxidizers, however those seem to all be heavy or use up their fuel far too quickly. 

 

I've been wondering if you could run a jet engine in an atmosphere that had fuel in it with a vehicle had the oxidizer stored in tanks.  Maybe if you were on Saturn's moon Titan and flying through a cloud of Methane or something.  

[Grand Ship Builder]

12 hours ago, Rickert-A33 said:

how to make a nice and warm egg that doesnt need to be baked.

eat loads of kentucky fried chicken

[Green Baron]

Winter solstice ("Sun stands still") today (Dec 21.). Sun at its lowest point for everybody above the tropic of cancer. Or, in general terms; the daylight times are getting longer again for the vast majority of us. Except for those head first, who are in the opposite situation :-)

There seems to be a slight difference between southern and northern hemispheres in solstice times. Knows somebody how this comes ? Only because of our calendar or is there an astronomical reason ?

Edited December 21, 2017 by Green Baron

[K^2]

On 12/20/2017 at 4:33 AM, USB4 said:

They did it at under 90 kg total.

They must have squeezed more efficiency out of plane, engine, prop, or some combination. If you design plane for slower flight, it can climb just as quickly at lower fuel consumption. I'll try to find some more specific info. The numbers you're giving would imply 3x energy efficiency of a Cessna 152, which isn't all that crazy.

18 hours ago, Green Baron said:

There seems to be a slight difference between southern and northern hemispheres in solstice times. Knows somebody how this comes ? Only because of our calendar or is there an astronomical reason ?

The exact moment of solstice is the same around the world. It's the moment when Sun and Earth's axis share a plane. It's winter solstice here and summer solstice in the southern hemisphere, or vice versa, but it's the same exact moment. All of the differences can only come from offset of local time from universal time.

[shynung]

Is it possible to make a megaton-class nuclear bomb that doesn't leave radioactive fallout? Say, for mining purposes.

[Green Baron]

Yeah, with a megaton of TNT.

You want to mine with a bomb ? That does not sound like a good idea to separate the useful stuff from the waste ;-)

[shynung]

AFAIK, in mining, explosives are used to break a chunk of ground into small-enough pieces that the diggers can scoop up into the trucks.

So, use case for a nuke mining bomb would be to blow open the side of a mountain, to access the ore within.