ISP at sealevel from bodies other than kerbin

[PrvDancer85]

Hi,

I searched for a sheet or something that tell u the ISP and thrust of engines for planets with atmosphere that are Not kerbin, but i didnt found One.

Does anyone One if something like this do exist?

I'm on Console, so sadly i cant use mods

 

Edited February 15, 2018 by PrvDancer85

[Spricigo]

 

 

 

[PrvDancer85]

19 minutes ago, Spricigo said:

 

 

 

Thanks, but i have forgotten to say that i'm a consoleguy. 

[Spricigo]

5 minutes ago, PrvDancer85 said:

Thanks, but i have forgotten to say that i'm a console guy. 

 .. seems that your options are:

• go to the places to get the number 
• calculate from the data on configs. (which you can find in the wiki)

While no one can offer better help, some general guidelines:

Duna atmosphere is thin and don't affect much ISP. Vaccum engines works resonable well even at "sea" level.

Eve atmosphere grabs you and want to crush you, only a handful of engines (Mammoth, vector, Dart, TwinBoar) have a passable performance.

Laythe, in comparison with kerbin, is a bit thinner at Sea level, and more similar at higher altitudes.

[Streetwind]

The scaling is linearly proportional, I believe - do correct me if I'm wrong.

Let's say you have an imaginary engine with 250s at kerbin sea level and 300s in a vacuum. Kerbin sea level has 1.0 standard atmospheres pressure. Laythe sea level has 0.8 standard atmospheres pressure. Therefore, sea level Isp of this engine on Laythe would be 260s. You calculate it by taking the difference between Kerbin sea level and vacuum Isps, and multiplying that with Laythe's relative pressure. (300 - 250) * 0.8 = 40. This is the loss compared to vacuum pressure, so 300 - 40 = 260.

This method works for all worlds with an atmosphere less dense than Kerbin. It doesn't work for Eve, because engines scale differently at pressures above 1.0 standard atmospheres than they do below it.

[Starman4308]

14 minutes ago, Streetwind said:

The scaling is linearly proportional, I believe - do correct me if I'm wrong.

There's an atmosphereCurve entry in the part .cfg file. IIRC, they're used to define a cubic Hermite spline.

Unfortunately I don't know of anything in the stock VAB/SPH GUI that provides arbitrary-pressure specific impulse measurements, and it's not like you can just open your favorite text editor and have a looksee at the game files on console.

[Streetwind]

16 minutes ago, Starman4308 said:

There's an atmosphereCurve entry in the part .cfg file. IIRC, they're used to define a cubic Hermite spline.

Yeah, I realize this, which is why I said it's linearly proportional. There are just two float curve keys defining 1.0 standard atmospheres and zero pressure. So the path between them is linear.

Of course, atmosphereCurve doesn't have tangents specified, and there is usually a third key above 1.0, so the curve may or may not be entirely straight in the vicinity of the 1.0 key. But it's probably a negligible error, especially at lower pressures. Probably.  

Edited February 15, 2018 by Streetwind

[Zhetaan]

@PrvDancer85:

Sea level I_sp depends on two things:  the engine's configuration file and the atmospheric pressure.

The sea level pressure for each body is:

Eve:  5.0 atm
Kerbin:  1.0 atm (naturally)
Duna:  .0666667 atm
Laythe:  .6 atm (its gravity is .8 g; apologies, @Streetwind)
Jool:  15.0 atm (not that you would want to go to 'sea' level, and no rocket will have usable thrust that low, but the value exists)

I picked a selection of popular first-stage engines (and the Poodle for comparison) and pulled the atmospherecurve values from their configuration files (on the wiki, which may not be completely up-to-date, but it's close enough for now).

Mammoth:
0:  315
1:  295
12:  .001

Twin Boar:
0:  300
1:  280
9:  .001

Mainsail:
0:  310
1:  285
9:  .001

Dart:
0:  340 -50 -73.71224
1:  290 -21.23404 -21.23404
5:  230 -10.54119 -10.54119
10:  170 -13.59091 -13.59091
20:  .001

Swivel:
0:  320
1:  270
6:  .001

Reliant:
0:  300
1:  280
7:  .001

Poodle (for comparison):
0:  350
1:  90
3:  .001

With these values, you can either use a Unity floatcurve editor to get exact intermediate values, or you can take a few guesses and interpolate for rough intermediate values.  The first two values for most of these engines give an exact I_sp for vacuum and Kerbin sea level; the third is the effective cutoff pressure in atmospheres.  The Dart has a more complex curve because it has to maintain near-constant I_sp over a variety of pressures, that's mathematically tricky, so it has extra numbers.

These are all cubic Hermite splines, as has been said (@Starman4308), but I'm not at the right computer for that.  For values between 0 and 1, the curve is usually linear enough that a linear fit will work, but for values greater than 1 (Eve), the curve is no longer linear enough, so I used a quadratic fit instead.  However, that is not necessarily the best fit, especially for values close to the cut-off pressure (Eve's surface is usually in the suspect zone).  The Poodle has the most extreme drop in I_sp and no clamping on the nodes, so its curve is, in a word, crazy:  I tried a logarithmic fit for it as well.  Though it gave a better curve than the quadratic, the intermediate values were different enough that I chose to use the average value and indicated the approximation with the tilde (~).  It should also be noted that the Dart uses a clamped curve, which is a lot more difficult to draw on my one-lung work computer, so you should probably consider both the Duna and Laythe values to be suspect; unlike other engines, however, the Dart has a defined value for Eve sea level, so that value, at least, is exact.

Note further that the Poodle drops out at three atmospheres; it has no significant thrust on Eve.

Note further than that that Duna's atmosphere is almost negligible; vacuum I_sp can be approximated reasonably as sea-level I_sp for any first-stage lifter.  Second-stage and dedicated vacuum engines probably have more significant differences.

All stock engines have negligible thrust at 15 atm; Jool take-offs are just as impossible as Jool landings.

I'll see about doing some more work on this at home later today.

Engine	Vacuum	Duna	Laythe	Kerbin	Eve
Mammoth	315	313.7	303.1	295	203.6
Twin Boar	300	298.8	288.4	280	166.7
Mainsail	310	308.4	295.3	285	161.4
Dart	340	335-336	307-308	290	230
Swivel	320	316.7	290.1	270	56.7
Reliant	300	298.9	288.9	280	123.8
Poodle	350	328.2	176.8	90	<<.001

EDIT:  I tried this with a spline interpolator and was able to refine the values somewhat.  Everything for which a quadratic spline was sufficient changed at the hundredths or less; the values given in the table stand.  The Poodle was very close at Duna but I revised by about ten for Laythe.  The interpolator I used didn't allow me to use tangents, so I need to wait until later to better refine the values for the Dart.

Edited February 15, 2018 by Zhetaan

[PrvDancer85]

@Zhetaan yeah thats the kind of stuff i was looking for. Thanks for the effort, if u Really want to make a complete sheet would u be that nice and link me?

Also to the others @Streetwind @Starman4308 @Spricigo thanks for helping

Edited February 15, 2018 by PrvDancer85

[Streetwind]

51 minutes ago, Zhetaan said:

Laythe:  .6 atm (its gravity is .8 g)

Derp!  Shows how often I go to Jool...

Great analysis, by the way!

[Zhetaan]

@Streetwind, @PrvDancer85, @Starman4308, @Spricigo:

Here's the update.  I kept getting weird numbers when I tried to fit the curve.  My initial guess values tended to be off a bit--which I expected from trying to use a quadratic fit on a cubic spline--but when I tried to fit a cubic, I started to see values that didn't make any sense, especially for Eve.  Either KSP or Unity is doing something odd with the spline, and since I don't need to bother figuring out the equation when I want obtainable data points, I decided to get empirical data instead.  That means that I strapped every engine in the game onto a sandbox monstrosity, turned on all the cheats, and went to every planet with an atmosphere.  Duna has no sea-level terrain; I believe the lowest elevation is about 170 metres but I don't know exactly where it is, so I decided to use a spot at about 270 metres.  Cutoff pressure is taken from the configuration files.

Normal LFO engines:

Engine	Vacuum	Duna	Laythe	Kerbin	Eve	Cutoff Pressure (atm)
Ant	315.0	299.4	160.0	80.0	0	3.0
Spider	290.0	288.1	272.5	260.0	114.1	8.0
Twitch	290.0	287.4	266.1	250.0	83.8	7.0
Spark	320.0	316.4	289.5	270.0	88.9	7.0
Reliant	310.0	307.1	278.9	265.0	88.2	7.0
Swivel	320.0	315.7	268.9	250.0	48.4	6.0
Terrier	345.0	327.7	173.4	85.0	0	3.0
Vector	315.0	313.7	303.5	295.0	193.3	12.0
Dart	340.0	335.3	303.6	290.0	230.0	20.0
Thud	305.0	303.1	286.6	275.0	139.7	9.0
Mainsail	310.0	308.4	295.8	285.0	147.8	9.0
Skipper	320.0	317.4	297.2	280.0	57.4	3.0
Poodle	350.0	332.7	160.2	90.0	0	6.0
Twin-Boar	300.0	298.7	289.1	288.0	147.6	9.0
Rhino	340.0	331.1	252.9	205.0	0	5.0
Mammoth	315.0	313.7	303.3	295.0	193.3	12.0

 

Solid Rocket Boosters:

Engine	Vacuum	Duna	Laythe	Kerbin	Eve	Cutoff Pressure (atm)
Flea	165.0	163.4	150.2	140.0	28.3	6.0
Hammer	195.0	193.4	180.2	170.0	57.4	7.0
Thumper	210.0	207.7	188.9	175.0	35.0	6.0
Kickback	220.0	218.4	205.4	195.0	66.7	7.0

 

Jet Engines:

Jets operate differently from other engines.  Their I_sp is always the Kerbin sea level value; the thrust varies with the velocity of the engine and a curve called atmCurve which is distinct from atmosphereCurve.  Of course, there needs to be enough oxygen entering the engine to keep it from starving for the lack, but technically, the I_sp on Eve or in space is the same as on Kerbin.

 

Utility, Odd, and Other Engines:

Engine	Vacuum	Duna	Laythe	Kerbin	Eve	Cutoff Pressure (atm)
Nerv	800.0	759.3	375.7	185.0	0	2.0
Rapier	305.0	303.1	283.3	275.0	139.7	9.0
Puff	250.0	241.4	165.5	120.0	0	4.0
Dawn	4200.0	3927.2	1480.9	100.0	0	1.2
Sepratron	154.0	151.6	131.5	118.0	22.6	6.0
Launch Escape System	180.0	178.7	168.2	160.0	69.7	8.0

 

There were a few interesting discoveries here.  One thing to note is that, in terms of performance, the Vector really is one engine from a Mammoth cluster; everything is exactly the same.  Another is that the Spider is much, much more capable than the Ant in terms of atmospheric operation.  This by no means is an assertion that the Spider is a valuable launching engine, but it does mean that you could probably make some fun glider drones with it, and these things would work on Eve, too--the Spider can operate at some truly high pressures.  The Hammer cuts off at a higher pressure than the Thumper; if you are silly enough to take SRBs to Eve, then take Hammers over Thumpers because Thumpers at Eve sea level are operating too close to their limit, and the I_sp values reflect this.  The Dart is the only engine that operates at 15 atm; if you're even crazier than the one who took SRBs to Eve, then the Dart is the only thing that works at Jool's datum (zero altitude point).  I don't know whether any rocket can climb out of that far down Jool's gravity well (pressure may be a problem before you get that low anyway), but the Dart is the only thing that can try.

Edited February 16, 2018 by Zhetaan

[PrvDancer85]

2 hours ago, Zhetaan said:

   

Engine	Vacuum	Duna	Laythe	Kerbin	Eve	Cutoff Pressure (atm)
Ant	315.0	299.4	160.0	80.0	0	3.0
Spider	290.0	288.1	272.5	260.0	114.1	8.0
Twitch	290.0	287.4	266.1	250.0	83.8	7.0
Spark	320.0	316.4	289.5	270.0	88.9	7.0
Reliant	310.0	307.1	278.9	265.0	88.2	7.0
Swivel	320.0	315.7	268.9	250.0	48.4	6.0
Terrier	345.0	327.7	173.4	85.0	0	3.0
Vector	315.0	313.7	303.5	295.0	193.3	12.0
Dart	340.0	335.3	303.6	290.0	230.0	20.0
Thud	305.0	303.1	286.6	275.0	139.7	9.0
Mainsail	310.0	308.4	295.8	284.6	147.8	9.0
Skipper	320.0	317.4	297.2	280.0	57.4	3.0
Poodle	350.0	332.7	160.2	90.0	0	6.0
Twin-Boar	300.0	298.7	289.1	288.0	147.6	9.0
Rhino	340.0	331.1	252.9	205.0	0	5.0
Mammoth	315.0	313.7	303.3	295.0	193.3	12.0

Engine	Vacuum	Duna	Laythe	Kerbin	Eve	Cutoff Pressure (atm)
Flea	165.0	163.4	150.2	140.0	28.3	6.0
Hammer	195.0	193.4	180.2	170.0	57.4	7.0
Thumper	210.0	207.7	188.9	175.0	35.0	6.0
Kickback	220.0	218.4	205.4	195.0	66.7	7.0

Engine	Vacuum	Duna	Laythe	Kerbin	Eve	Cutoff Pressure (atm)
Nerv	800.0	759.3	375.7	185.0	0	2.0
Rapier	305.0	303.1	283.3	275.0	139.7	9.0
Puff	250.0	241.4	165.5	104.1	0	4.0
Dawn	4200.0	3927.2	1480.9	100.0	0	1.2
Sepratron	154.0	151.6	131.5	118.0	22.6	6.0
Launch Escape System	180.0	178.7	168.2	160.0	69.7	8.0

Awesome, you are best

[OhioBob]

On ‎2‎/‎15‎/‎2018 at 9:36 AM, Zhetaan said:

Engine	Vacuum	Duna	Laythe	Kerbin	Eve
Mammoth	315	313.7	303.1	295	203.6
Twin Boar	300	298.8	288.4	280	166.7
Mainsail	310	308.4	295.3	285	161.4
Dart	340	335-336	307-308	290	230
Swivel	320	316.7	290.1	270	56.7
Reliant	300	298.9	288.9	280	123.8
Poodle	350	328.2	176.8	90	<<.001

If you evaluate it exactly using a cubic Hermite spline, the values are:

Engine	Vacuum	Duna	Laythe	Kerbin	Eve
Mammoth	315.0	313.7	303.9	295.0	188.6
Twin Boar	300.0	298.7	289.9	280.0	141.7
Mainsail	310.0	308.4	296.4	285.0	143.7
Dart	340.0	335.2	303.6	290.0	230.0
Swivel	320.0	316.7	290.5	270.0	54.1
Reliant	300.0	298.8	291.3	280.0	95.0
Poodle	350.0	332.1	173.4	90.0	0.001

[OhioBob]

On ‎2‎/‎15‎/‎2018 at 8:52 AM, Streetwind said:

The scaling is linearly proportional, I believe - do correct me if I'm wrong.

Let's say you have an imaginary engine with 250s at kerbin sea level and 300s in a vacuum. Kerbin sea level has 1.0 standard atmospheres pressure. Laythe sea level has 0.8 standard atmospheres pressure. Therefore, sea level Isp of this engine on Laythe would be 260s. You calculate it by taking the difference between Kerbin sea level and vacuum Isps, and multiplying that with Laythe's relative pressure. (300 - 250) * 0.8 = 40. This is the loss compared to vacuum pressure, so 300 - 40 = 260.

This method works for all worlds with an atmosphere less dense than Kerbin. It doesn't work for Eve, because engines scale differently at pressures above 1.0 standard atmospheres than they do below it.

What you describe is the way it works in real life, but in KSP specific impulse defined by a curve in the engine config file.  This is a case were Squad made things more complicated than the simple real life reality.

The equation for thrust is,

F = q * V_e + (P_e - P_a) * A_e

where q is the propellant mass flow rate, V_e is the exhaust gas velocity, P_e is the exhaust gas pressure at the nozzle exit, P_a is the ambient air pressure, and A_e is the cross-sectional area of the nozzle exit.  For a given engine running at a constant throttle setting, q, V_e, P_e, and A_e are all constants (assuming a bell nozzle; not true for an aerospike) .  The only thing that changes is P_a.  Therefore thrust varies by an amount proportional to P_a.  We have,

F = F_vacuum - P_a * A_e
 

Edited February 17, 2018 by OhioBob

[OhioBob]

On ‎2‎/‎15‎/‎2018 at 9:25 AM, Streetwind said:

Of course, atmosphereCurve doesn't have tangents specified...

It's my understanding that when no slopes are specified, the slopes used are computed as follows:  (1) The out slope of the first point in the slope between the first and second points, (2) the in slope of the last point is the slope between the second-to-last and last points, and (3) the in/out slopes for all points in between is the slope between the two points straddling it.