Nice work! Cool to see this unusual application of jet engines; I never thought the intake air storage would be significant enough to do anything. Glad to see you making more videos too!
When I first did the math I was rather surprised with how much dv you can get, max theoretical is about 2000m/s so in theory it might be possible to make a craft that reaches orbital velocity in Duna's atmosphere. And I'm glad to be back to making videos. I kinda petered off on KSP for a bit but I'm back now!
I think I remember suggesting this to you at one point, but the issue was that I was considering it for getting into orbit, and you correctly pointed out that the engines require air pressure, so they wouldn't work in space.
I guess you can still air-hog after all, it’s just a lot harder than it was. That’s really impressive. Also, KSP looks really good in that ultra ice aspect ratio.
This is brilliant. You sound so casual when you explain how this is possible. But behind the scenes, you probably had to compare all the intakes and engines to see what is the most efficient. And I bet it was pretty counterintuitive ;-)
Oh definitely! When you factor in air usage, the math more most efficient engine flips on its head, rapier is best, Goliath is worst! And I had to use the radial scoop intake, as it has the best mass ratio, 20 kg of intake to store 10kg of air (ouch).
@@lt_duckweed There's actually a term for this in propulsion called specific thrust - a higher specific thrust engine can develop a higher thrust with lower airflow. Engines designed for high performance generally have greater specific thrust
What I always wondered is why is there no oxygenless jet engine concept in the game. (While there are ridiculously overpowered electric propellers) It should be pretty efficient comparing to just a plain rocket engine. Maybe like only two times more mass intensive than an ordinary jet as you just need to carry the oxidiser and mix it into the atmosferic air.
that is just a very inefficient rocket engine then ? main difference between a jet and a rocket engine is that the rocket carries its own oxygen, while the jet has that big compressor on the front to shovel in ambient oxygen. Due to lower max pressure and heat, exhaust velocity and therefore isp will be much lower than if you just stuck the fuel into a normal rocket
@@user-si5fm8ql3c no, it is not? The difference in effective isp is that a jet exhaust has many times more mass than the fuel spent so the exhaust velocity rule doesn't apply the same way. The oxidizer portion is not that significant (it doesn't even need all 20% of oxygen to be burned)
@@AbsoluteHuman What you are really doing is just adding inert mass into the exhaust stream, if one optimizes this system further, you end up back at a good old rocket engine, but with a special nozzle. By entrenching ambient air into the exhaust stream *inside* the nozzle, then allowing it to expand, you get the same effect, but in a lighter package. Downsides to using ambient air in any way, jet with onboard oxygen or rocket, is that its not as effective at higher airspeeds, but for plane operation, this is no problem.
@@user-si5fm8ql3c so what you are saying is that a jet engine is only so much more efficient as how much dead oxidizer weight is ommited? Sounds not true at least according to the numbers KSP uses for its jets... I haven't really seen any real data but find it hard to believe. Is a jet airplane really only half that inefficient as a space rocket? What about a prop plane in that case? It still uses fuel to propel air in the opposite direction. You can measure the ISP of that too.
@@AbsoluteHuman if you look at the real isp, you get only about 50-150s for a typical jet, if air is considered in the calculations aswell, the only advantage is all that inert mass it throws out too, a heavier, but cooler, slower exhaust stream means more raw thrust, but due to having to slow the incoming air stream to subsonic speeds, they run out of power and efficiency at higher speeds, you could also just entrench air in rocket exhaust due to slots of sort in the nozzle, then allow the air to heat and expand to get more thrust that way. It really is just mainly more efficient due to not having to carry around its own oxygen
It would have been a ton easier with LOX, with intake air you pay through the nose both in mass and part count. For LOX, for every 1kg of tank mass you can carry 8kg of LFO mix. For intake air, for every 1kg of intake, you can carry 0.5kg of air. And your largest "tank" only holds 10kg of air. Nearly 20% of the takeoff mass for this craft was just the dry mass of the intakes! Thats even more than the mass of all the engines! And that only bought me about 650 m/s worth of rapier delta v once on Duna. Still, its a interesting trick, and few things are as kerbal as running your jet engines off of bottled air on another planet.
@@lt_duckweed I'm using this to run jets at Tekto (that Outer Planets Mod moon with thick atmosphere and ethane lakes), using a simple "drain valves take intake air as they did in 1.9.0" mod to not run out of air (still need enough intakes to feed the jets, tho) and justify it with fluff text that those are jet engines taking ethane vapor from the atmosphere and burning it with stored oxigen - right the other way around than normal. Just because of how Kerbal it is doing that..
would be pretty neat to stage the intakes off, that'd help a bit. The only other oxygenless atmospheres in the stock system (eve and jool) seem like poor targets due to how expensive it is to leave them, but it'd be great to see this used in Outer Planets Tekto. I just love how instead of bringing oxidizer, you just brought oxygen X3
How much "air time" could you get with the stored intake air powering the jets on Duna? I counted only 90 seconds, which seems quite unfortunately short :/ Also, which intake did you use to store the air? I image some have better mass/air fractions than others
I used the XM-G50 Radial Air Intake, it has the best ratio (which is still quite bad, 20kg and can hold 10kg of air). If you got rid of all the other stuff like the fuel to get to and from Duna, and the Nerv, and used a lower thrust to weight ratio so you could really load it down with intakes I imagine you could get, idk, 5 min or so?
@@lt_duckweed Cool stuff. I did my own tests and saw the jets worked on Eve; have you tried them there? They pump out full power to ~18K on Eve. I haven't done the math but I really wonder if Eve jets could be viable. They are certainly a lot more powerful and fun than propellers.
Well yes but the amount of airflow real life jet engines need is enormous...so in theory it could work but it would never be viable to carry that amount of air with you. Imagine storing the oxidizer for a rocket engine as a gas instead of a liquid and adding 78 % nitrogen on top for no good reason...you're not going to go far ^^
@@2nd2lastdodo I see no reason you cannot store liquid O2 as you would in a rocket, then have some sort of pipe and heating element or other way to convert it into a pressurized gas and feed it into the combustion chamber(s). Unless the jet somehow relies on all of that extra nitrogen?
@@danburrykerman6826 good point...I'm not an engineer but I guess it would work. But it leaves another question: do you stick to the principle of a jet engine where you have to burn just a little fuel to spin the turbine because you use the air as reaction mass? That would mean you'd have to dump a massive amount of your O2 unburnt out the back if you use it out of tanks instead of the ambient air. If on the other hand you decide to burn it all and use the resulting gasses as reaction mass, you just end up with a rocket engine and have to carry the corresponding amount of fuel. I guess you probably could make it fly either way, I see no reason why you couldn't stick wings to a rocket and go horizontally in an atmosphere but it would be terribly unefficient by jetplane standarts. Rockets usually reach a burn time of 2 - 3 minutes...assuming your thrust to weight ratio can stay below one because your wings provide lift and you are not going straight up you can probably stretch that, maybe even double or triple it but you won't come anywhere close to the hours of burntime a conventional jet engine has
Would it theoretically be possible for a jet engine to generate its own intake air using that KAL-1000 glitch where engines make their own fuel with negative throttle?
Nice work! Cool to see this unusual application of jet engines; I never thought the intake air storage would be significant enough to do anything.
Glad to see you making more videos too!
When I first did the math I was rather surprised with how much dv you can get, max theoretical is about 2000m/s so in theory it might be possible to make a craft that reaches orbital velocity in Duna's atmosphere.
And I'm glad to be back to making videos. I kinda petered off on KSP for a bit but I'm back now!
I think I remember suggesting this to you at one point, but the issue was that I was considering it for getting into orbit, and you correctly pointed out that the engines require air pressure, so they wouldn't work in space.
I guess you can still air-hog after all, it’s just a lot harder than it was. That’s really impressive. Also, KSP looks really good in that ultra ice aspect ratio.
This is brilliant. You sound so casual when you explain how this is possible. But behind the scenes, you probably had to compare all the intakes and engines to see what is the most efficient. And I bet it was pretty counterintuitive ;-)
Oh definitely! When you factor in air usage, the math more most efficient engine flips on its head, rapier is best, Goliath is worst! And I had to use the radial scoop intake, as it has the best mass ratio, 20 kg of intake to store 10kg of air (ouch).
@@lt_duckweed Does closing intakes actually reduce their drag or does it only store air?
@@Boomchacle it has no effect on drag
@@lt_duckweed There's actually a term for this in propulsion called specific thrust - a higher specific thrust engine can develop a higher thrust with lower airflow. Engines designed for high performance generally have greater specific thrust
I saw the title and clicked on this out of curiosity. Needless to say, not disappointed. This was quite very unique, excellent work!
always a treat when you upload
OMFG I NEVER ACTUALLY THOUGHT ABOUT THIS
What I always wondered is why is there no oxygenless jet engine concept in the game. (While there are ridiculously overpowered electric propellers) It should be pretty efficient comparing to just a plain rocket engine. Maybe like only two times more mass intensive than an ordinary jet as you just need to carry the oxidiser and mix it into the atmosferic air.
that is just a very inefficient rocket engine then ?
main difference between a jet and a rocket engine is that the rocket carries its own oxygen, while the jet has that big compressor on the front to shovel in ambient oxygen.
Due to lower max pressure and heat, exhaust velocity and therefore isp will be much lower than if you just stuck the fuel into a normal rocket
@@user-si5fm8ql3c no, it is not? The difference in effective isp is that a jet exhaust has many times more mass than the fuel spent so the exhaust velocity rule doesn't apply the same way. The oxidizer portion is not that significant (it doesn't even need all 20% of oxygen to be burned)
@@AbsoluteHuman What you are really doing is just adding inert mass into the exhaust stream, if one optimizes this system further, you end up back at a good old rocket engine, but with a special nozzle.
By entrenching ambient air into the exhaust stream *inside* the nozzle, then allowing it to expand, you get the same effect, but in a lighter package.
Downsides to using ambient air in any way, jet with onboard oxygen or rocket, is that its not as effective at higher airspeeds, but for plane operation, this is no problem.
@@user-si5fm8ql3c so what you are saying is that a jet engine is only so much more efficient as how much dead oxidizer weight is ommited? Sounds not true at least according to the numbers KSP uses for its jets... I haven't really seen any real data but find it hard to believe. Is a jet airplane really only half that inefficient as a space rocket? What about a prop plane in that case? It still uses fuel to propel air in the opposite direction. You can measure the ISP of that too.
@@AbsoluteHuman if you look at the real isp, you get only about 50-150s for a typical jet, if air is considered in the calculations aswell, the only advantage is all that inert mass it throws out too, a heavier, but cooler, slower exhaust stream means more raw thrust, but due to having to slow the incoming air stream to subsonic speeds, they run out of power and efficiency at higher speeds, you could also just entrench air in rocket exhaust due to slots of sort in the nozzle, then allow the air to heat and expand to get more thrust that way.
It really is just mainly more efficient due to not having to carry around its own oxygen
Man I've been wondering if that could work ever since 0.90. Glad to see someone else got curious.
Underrated channel!
Really cool idea! Is this actually practical beyond a proof of concept or would this mission have been easier with LOX instead of intake air?
It would have been a ton easier with LOX, with intake air you pay through the nose both in mass and part count. For LOX, for every 1kg of tank mass you can carry 8kg of LFO mix. For intake air, for every 1kg of intake, you can carry 0.5kg of air. And your largest "tank" only holds 10kg of air.
Nearly 20% of the takeoff mass for this craft was just the dry mass of the intakes! Thats even more than the mass of all the engines! And that only bought me about 650 m/s worth of rapier delta v once on Duna.
Still, its a interesting trick, and few things are as kerbal as running your jet engines off of bottled air on another planet.
@@lt_duckweed I'm using this to run jets at Tekto (that Outer Planets Mod moon with thick atmosphere and ethane lakes), using a simple "drain valves take intake air as they did in 1.9.0" mod to not run out of air (still need enough intakes to feed the jets, tho) and justify it with fluff text that those are jet engines taking ethane vapor from the atmosphere and burning it with stored oxigen - right the other way around than normal. Just because of how Kerbal it is doing that..
would be pretty neat to stage the intakes off, that'd help a bit. The only other oxygenless atmospheres in the stock system (eve and jool) seem like poor targets due to how expensive it is to leave them, but it'd be great to see this used in Outer Planets Tekto. I just love how instead of bringing oxidizer, you just brought oxygen X3
Could you share the craft file - if only to save us the tedium to clip 400 intakes manually to replicate your stunt?
Sure, I'll try to remember to upload it after work.
How much "air time" could you get with the stored intake air powering the jets on Duna? I counted only 90 seconds, which seems quite unfortunately short :/
Also, which intake did you use to store the air? I image some have better mass/air fractions than others
I used the XM-G50 Radial Air Intake, it has the best ratio (which is still quite bad, 20kg and can hold 10kg of air).
If you got rid of all the other stuff like the fuel to get to and from Duna, and the Nerv, and used a lower thrust to weight ratio so you could really load it down with intakes I imagine you could get, idk, 5 min or so?
@@lt_duckweed Cool stuff. I did my own tests and saw the jets worked on Eve; have you tried them there?
They pump out full power to ~18K on Eve. I haven't done the math but I really wonder if Eve jets could be viable. They are certainly a lot more powerful and fun than propellers.
Woah, that’s wacky, very cool
if engines work better in denser atmosphere, does that mean that they work really well on eve or jool?
Unfortunately, the thrust curves for jet engines end at kerbin sea level. So the thicker atmospheres of Eve and Jool don't actually help any :(
amazing design, do they push you in a vacuum?
Unfortunately no, jet engines have 0 thrust in a vacuum, even if you use this trick to bring air with you.
how do you get waterfall configs for jet engines
What if you run air from compressed air tanks through a real life jet engine? Will it work?
No idea tbh, but seems to me like it wouldn't end well lol.
@@lt_duckweed Didn't they do something like that though to test some of the first ramjets?
Well yes but the amount of airflow real life jet engines need is enormous...so in theory it could work but it would never be viable to carry that amount of air with you. Imagine storing the oxidizer for a rocket engine as a gas instead of a liquid and adding 78 % nitrogen on top for no good reason...you're not going to go far ^^
@@2nd2lastdodo I see no reason you cannot store liquid O2 as you would in a rocket, then have some sort of pipe and heating element or other way to convert it into a pressurized gas and feed it into the combustion chamber(s). Unless the jet somehow relies on all of that extra nitrogen?
@@danburrykerman6826 good point...I'm not an engineer but I guess it would work. But it leaves another question: do you stick to the principle of a jet engine where you have to burn just a little fuel to spin the turbine because you use the air as reaction mass? That would mean you'd have to dump a massive amount of your O2 unburnt out the back if you use it out of tanks instead of the ambient air.
If on the other hand you decide to burn it all and use the resulting gasses as reaction mass, you just end up with a rocket engine and have to carry the corresponding amount of fuel. I guess you probably could make it fly either way, I see no reason why you couldn't stick wings to a rocket and go horizontally in an atmosphere but it would be terribly unefficient by jetplane standarts. Rockets usually reach a burn time of 2 - 3 minutes...assuming your thrust to weight ratio can stay below one because your wings provide lift and you are not going straight up you can probably stretch that, maybe even double or triple it but you won't come anywhere close to the hours of burntime a conventional jet engine has
Would it theoretically be possible for a jet engine to generate its own intake air using that KAL-1000 glitch where engines make their own fuel with negative throttle?
I've never tried it, but it seems like it should work
Ok
Me reading the title: "Wait. That's illegal"