|
Post by goduranus on May 5, 2017 8:10:15 GMT
Notice these two nearly identical NTRs, first delivers 8.09MN thrust using 1.59ton/second of propellant(Efficiency : 5.08MN-Second/Ton or 5.08 km/s), second one has 8.65MN of thrust using only 1.41ton/second of propellant(Efficiency : 6.13MN-Second/Ton or 6.13km/s). The delta V of an unarmored ship doesn't change when replacing the first drive with the second drive, despite the second drive being 20% more efficient This makes me think that the game only uses displayed exhaust velocity to compute DeltaV, and ignores the actual performance characteristics of the thruster itself. Btw you get more thrust/flowrate by increasing the chamber contraction ratio. Spoiler has 4k screenshots Edit: blkcandy math check.
|
|
|
Post by shiolle on May 5, 2017 9:45:24 GMT
This makes me think that the game only uses displayed exhaust velocity to compute DeltaV, and ignores the actual performance characteristics of the thruster itself. This is correct. Efficiency doesn't depend on thrust or mass flow, and neither of them is part of Tsiolkovsky rocket equation which is used to calculate delta-v. Think of it this way: two rockets with the same fuel fraction and the same exhaust velocity will have the same amount of delta-v regardless of how fast they can spend it. P.S. These two rockets can have drastically different mass too, as long as the fuel fraction and exhaust velocity are the same they will have the same delta-v (and by extension, efficiency).
|
|
|
Post by goduranus on May 5, 2017 10:00:09 GMT
This makes me think that the game only uses displayed exhaust velocity to compute DeltaV, and ignores the actual performance characteristics of the thruster itself. This is correct. Efficiency doesn't depend on thrust or mass flow, and neither of them is part of Tsiolkovsky rocket equation which is used to calculate delta-v. Think of it this way: two rockets with the same fuel fraction and the same exhaust velocity will have the same amount of delta-v regardless of how fast they can spend it. P.S. These two rockets can have drastically different mass too, as long as the fuel fraction and exhaust velocity are the same they will have the same delta-v (and by extension, efficiency). I believe the displayed exhaust velocity is the exhaust plume's thermal velocity. The rocket equation depends on the average exhaust velocity in the direction of thrust. The two are not the same. The average velocity of a wider exhaust plume will be lower. Notice how the effective velocity of the thruster in the first screenshot is 5.07km/s where as the second one is 6.31km/s, despite them both having 6.18km/s thermal velocity. But honestly 6.31km/s is probably bugged as well.
|
|
|
Post by shiolle on May 5, 2017 13:51:23 GMT
Sorry that I misread your initial argument. And after your reply I'm even more lost. Where do you get that effective velocity? I can see that the exhaust is much slower when it exits the nozzle of the second engine (mach 44.2 = 15160.6 m/s) compared to the first one (mach 52.7 = 18076.1 m/s). And thermal velocity at which point? I understand thermal velocity is proportional to the square root of the temperature? Even for pure hydrogen, thermal velocity in a single direction is lower than 6.18 km/s at reactor core temperature.
|
|
|
Post by Enderminion on May 5, 2017 14:49:36 GMT
mach numbers may be for that material, not atmosphere
|
|
|
Post by goduranus on May 5, 2017 14:59:07 GMT
Effective velocity is thrust/flow rate.
Force = Mass * Acceleration Force = Mass * dV/dt Force = Mass/dt * dV Thrust Force = Flow Rate * Propellant Velocity(in the direction of thrust)
or Propellant Velocity = Thrust/Flow Rate
With the first thruster I posted, the effective velocity must be 5.07 km/s instead of the 6.18km/s as displayed. (8.06*10^6 Newtons)/(1.59*10^3 kg/second) = 5.07*10^3 Newton-Second/kg, since a Newton is a kg*meter/Second^2.
Edit: Atmosphere? What?
|
|
|
Post by shiolle on May 5, 2017 16:40:26 GMT
Thank you for the explanation. I can reproduce these results. Two additional comments: 1. Thrust power is calculated from displayed exhaust velocity (as F*Ve/2) 2. If we calculate flow rate from displayed exhaust velocity and thrust (as mDot = F/Ve) we get 1.309 t/s for the first rocket and 1.4 t/s for the second.
Edit: I opened one of the standard NTRs, "6.18 km/s Methane Gimballed Nuclear Thermal Rocket" and it has the same problem. Listed propellant flow 805 kg/s while calculations predict 663.4 kg/s.
|
|
|
Post by ash19256 on May 6, 2017 1:07:09 GMT
So are engines using more propellant than they should be or something? Can someone simplify what has been figured out for someone who isn't all that good with math?
|
|
blkcandy
Junior Member
Burn complete. Crawling back to bed.
Posts: 78
|
Post by blkcandy on May 6, 2017 3:06:17 GMT
Something like that.
The thrust impulse per ton propellant (specific impulse) of engine 2 is higher than engine 1, yet he got the same dV. goduranus calculated the specific impulse of both engines by using thrust(MN) divided by mass flow rate (Ton/seconds).
Now this impulse is pretty much the 'ability to change momentum'. 1Ns impulse gives 1Ns momentum or gives 1m/s dV for a mass of 1kg. Higher impulse = more dV.
In godurnus case: for every ton of propellant, engine 1 can creates an impulse of 5.07MN-second. Engine 2 can creates 6.31.
So, engine 2 should gives more dV, but it doesn't.
|
|
blkcandy
Junior Member
Burn complete. Crawling back to bed.
Posts: 78
|
Post by blkcandy on May 6, 2017 4:06:50 GMT
So let me try this:
Engine 1: In-game exhaust velocity: 6.18km/s (This should mean 6.18MNs impulse per ton propellant if this is the effective exhaust velocity.) Thrust: 8.09MN Mass flow rate: 1.59t/s Impulse per propellant (Specific impulse) calculation: If thrust = specific impulse (in ms) * mass flow rate specific impulse = thrust/mass flow rate = (8.09MN)/(1.59t/s) = 5.0881MNs/t = 5.0881km/s
Engine 2: In-game exhaust velocity: 6.18km/s Thrust: 8.65MN Mass flow rate: 1.41t/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (8.65MN)/(1.41t/s) = 6.1348MNs/t = 6.1348km/s
I honestly don't know why it doesn't match, but this could be the problem. Either we got this wrong, or the game got the thrust wrong, or it got the exhaust velocity wrong, or the exhaust velocity wasn't an effective exhaust velocity yet still mistakenly used in calculation.
|
|
|
Post by goduranus on May 6, 2017 4:55:24 GMT
Doh, I made a typo with the second one, but yeah they still mismatch.
|
|
blkcandy
Junior Member
Burn complete. Crawling back to bed.
Posts: 78
|
Post by blkcandy on May 6, 2017 5:10:59 GMT
So I checked my engines:
Engline 1 (Decane NTR): In-game exhaust velocity: 6.76km/s Thrust: 20.2kN Mass flow rate: 2.99kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (20.2kN)/(2.99kg/s) = 6.7559kNs/kg = 6.7559km/s (0.06% error)
Engine 2 (Decane Resistojet): In-game exhaust velocity: 6.76km/s Thrust: 248kN Mass flow rate: 36.8kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (248kN)/(36.8kg/s) = 6.7391kNs/kg = 6.7391km/s (0.31%) Tested on 1kt unarmored tanker drone: Engine 1 dV: 67.2 km/s Engine 2 dV: 66.9 km/s
All errors looks very minor for me.
And for my larger engines which I am to lazy to slap on my tanker:
Engine 3 (Fluorine Hydrogen): In-game exhaust velocity: 5.15km/s Thrust: 1.78kN Mass flow rate: .345kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (1.78kN)/(.345kg/s) = 5.1594km/s (0.18%)
Engine 4 (Decane Resistojet): In-game exhaust velocity: 6.76km/s Thrust: 745kN Mass flow rate: 110kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (745kN)/(110kg/s) = 6.7727km/s (0.19%) Engine 5 (Decane MPD): In-game exhaust velocity: 210km/s Thrust: 391kN Mass flow rate: 1.86kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (391kN)/(1.86kg/s) = 210.2151km/s (0.10%) And stock engines: Engine 6 (Fluorine Hydrogen): In-game exhaust velocity: 4.92km/s Thrust: 887kN Mass flow rate: 206kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (887kN)/(206kg/s) = 4.3058km/s (12.484%)
Engine 7 (Hydrogen Deuteride NTR) In-game exhaust velocity: 9.10km/s Thrust: 1.09MN Mass flow rate: 133kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (1.09MN)/(133kg/s) = 8.1955km/s (9.9396%) Engine 8(Ammonia Resistojet) In-game exhaust velocity: 4.05km/s Thrust: 2.46kN Mass flow rate: .600kg/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (2.46kN)/(0.600kg/s) = 4.1km/s (1.23%) Engine 9(Helium MPD) In-game exhaust velocity: 24.3km/s Thrust: 172N Mass flow rate: 7.07g/s Impulse per propellant (Specific impulse) calculation: Thrust/mass = (172N)/(7.07g/s) = 24.3281km/s (0.46%)
The error of the engines I check are minor except for the 2 stock engines. I still cannot figured out why.
|
|
|
Post by goduranus on May 6, 2017 5:18:51 GMT
A low chamber contraction ratio seems to cause a large error.
|
|
blkcandy
Junior Member
Burn complete. Crawling back to bed.
Posts: 78
|
Post by blkcandy on May 6, 2017 6:42:16 GMT
To verify that:
Engine 1(6.73km/s 30.0MW Decane Resistojet) In-game exhaust velocity: 6.73km/s Thrust: 597kN Mass flow rate: 110kg/s Chamber Contraction Ratio: 1 Specific Impulse = (597kN)/(110kg/s) = 5.4727km/s (19.36% Error)
Engine 2(6.76km/s 30.0MW Decane Resistojet) In-game exhaust velocity: 6.76km/s (100.45% of Engine 1) Thrust: 745kN Mass flow rate: 110kg/s Chamber Contraction Ratio: 1000 Specific Impulse = (745kN)/(110kg/s) = 6.7727km/s (0.19% Error) (123.75% of engine 1)
Both engines are based on the same model. Only chamber contraction ratio, throat radius, and chamber thickness are adjusted.
Try those engines on 1kt tanker drone: Engine 1 dV = 55.1 km/s Engine 2 dV = 55.4 km/s (100.54% of Engine 1)
I think this should confirm your suspicion. There is something wrong with how the game calculate the effect of the chamber contraction ratio. And the game determine dV based on exhaust velocity regardless of the thrust per propellant specific impulse.
|
|
|
Post by ash19256 on May 6, 2017 15:43:18 GMT
To verify that: Engine 1(6.73km/s 30.0MW Decane Resistojet) In-game exhaust velocity: 6.73km/s Thrust: 597kN Mass flow rate: 110kg/s Chamber Contraction Ratio: 1 Specific Impulse = (597kN)/(110kg/s) = 5.4727km/s (19.36% Error) Engine 2(6.76km/s 30.0MW Decane Resistojet) In-game exhaust velocity: 6.76km/s (100.45% of Engine 1) Thrust: 745kN Mass flow rate: 110kg/s Chamber Contraction Ratio: 1000 Specific Impulse = (745kN)/(110kg/s) = 6.7727km/s (0.19% Error) (123.75% of engine 1) Both engines are based on the same model. Only chamber contraction ratio, throat radius, and chamber thickness are adjusted. Try those engines on 1kt tanker drone: Engine 1 dV = 55.1 km/s Engine 2 dV = 55.4 km/s (100.54% of Engine 1) I think this should confirm your suspicion. There is something wrong with how the game calculate the effect of the chamber contraction ratio. And the game determine dV based on exhaust velocity regardless of the thrust per propellant specific impulse. Well, that bites something fierce.
|
|