ISP of engines and atmosphere height efficiency question.

[Moonfrog]

On career mode I've finally got around to building something with the Skipper engine. But I don't really want to use it because I can get the job done better with 3 of the LV 215 thrust engines. I have the structural 3 engine attachment thing.

The LV engines have better isp at 320 -370 compared to the skipper engine of 300-350. Also 3 times 215 is 645, only 4 less than the skipper engine at 650. the engines also weigh less combined than the skipper.

On the description it says the skipper excels as a mid section booster. Got me thinking about the ISP. Heres what i'm interesting in knowing.

At what point during take off in the atmosphere does the skipper become 320 ISP engine and maxes out at 350? Since it seems the skipper engine if used during that period would be just as efficient as the 215 LV t engines. Only when LV -T's go above 350 that it really makes a difference. I imagine u could ignore the first 300-320 bit since its probally shortest part of the journey, or u could use boosters to get it up to that point.

Interesting in knowing when ISP changes at different altitudes so I can build the rocket to match.

Also would it be worth it? if I did use a skipper engine then swapped it out at 350 isp height for more efficient engine would it be worth it? Since this would seem to be a lot of trouble to go through, be a shame if it was for nought.

Edited November 13, 2013 by Moonfrog

[blizzy78]

At what point during take off in the atmosphere does the skipper become 320 ISP engine and maxes out at 350?

You can right-click an engine while it's running (ie. during ascent) and look up its current Isp.

[Nao]

Skipper achieves 320 ISP quite early in launch, but that is easy to test by yourself (I don't have KSP open right now) 350 is for vacuum although above 10km it is already pretty close to it ~345+

It also has 2 advantages over 3 LVT30's , less parts and gimbaling which can help a lot in controling the rocket.

While ISP is important you shouldn't build rockets that engage more efficient engines as you reach higher altitudes to gain performance, as it will actually cost you some. The more engines fairing at launch the better, because any engine that waits for its turn is a dead weight that costs fuel to get to the point of ignition.

In the end even LV-N engine with its isp at sea level of ?225 in many cases is better off burning at launch even thou it uses fuel less efficiently because it adds thrust. The mass cost of fuel burned inefficiently will still be smaller than additional mass (and fuel cost for that mass) of more engines required to reach same thrust to weigh ratio (TWR) without the LV-N burning.

edit: @ blizzy78, uhh didn't think of that lol, sometimes veteran players think of stuff as granted >_< good hint.

[Johnno]

It also has 2 advantages over 3 LVT30's , less parts and gimbaling which can help a lot in controling the rocket.

A third advantage is stackability. You're going to end up with an overly complex craft if you want to stack tricoupled engines. If you however forego stacking alltogether and use a core launcher with asparagus staged (or onion, or any other veggie type) boosters you might as well drop the tricoupler entirely and go for 6 or even 8 LV-T30 surrounding a central LV-T45 for its gimbaling goodness. Like this:

Higher thrust and better ISP than a mainsail, but more mass and parts.

[Moonfrog]

So weight is more important than ISP?

Also when right clicking on engine I found.

Fuel Flow with lots of decimal points.

Specific Impulse with very large numbers.

thrust.

I don't know how any of that translates into ISP. hmm after looking at it again, is Specific impulse meant ot be ISP? if so why is it called ISP and not SIP?

Edited November 13, 2013 by Moonfrog

[blizzy78]

Because it's not "Isp", but "I" with a subscript "sp":

Edited November 13, 2013 by blizzy78

[Godot]

So weight is more important than ISP?

...

Depends on the relation to the total rocket weight ...

often, especially with small landers, replacing an engine with a (relatively) heavy weight but good Isp with one that has a lower Isp, but also a lower weight,

may result in an increase in dV

[kahlzun]

In the end even LV-N engine with its isp at sea level of ?225 in many cases is better off burning at launch even thou it uses fuel less efficiently because it adds thrus

The LV-N gains Isp really fast due to the massive change from 220 to 800 as it leaves the atmosphere. After you pass about 1700m altitude, the air pressure has dropped enough that its Isp exceeds 390, the 3rd best Isp available after the Ion and LV-N.

Behold:

To answer the OP's question:

All engines (except jet engines) have Isp that diminishes linearly with air pressure. For example, the skipper starts with 300 at 1-or-more atmospheres (sea level) and goes up to 350 (vacuum).

If you've climbed to an altitude where the pressure is 0.5 atm, the isp will be 325.

Air pressure drops very fast, put a barometer sensor on a BACC SRB and see for yourself.

Edited November 13, 2013 by kahlzun

[capi3101]

Kerbin's atmosphere has a scale height of 5,000 meters, with an atmospheric pressure of 1 atmosphere at the surface. I_sp in the game is setup so that its minimum level is reached at an atmospheric pressure of 1 atmosphere (so for places like Jool and Eve, which have substantially higher surface atmospheric pressures, an engine will still have the same I_sp at the surface as it does on the surface of Kerbin.

No, that's not how it would work on RL, but let's move on anyway.

The formula for Kerbin's atmospheric pressure with height looks like this, according to the wiki:

P_k = 1atm * e^(-a/5000), where P_k is the atmospheric pressure, atm means "atmospheres" (=1013 millibars or 101.3 kiloPascals), a is altitude and e is the inverse natural logarithm.

So atmospheric pressures on Kerbin look like this (rounded to five digits):

1,000 m: 0.81873

2,000 m: 0.67032

3,000 m: 0.54881

4,000 m: 0.44933

5,000 m: 0.36788

6,000 m: 0.30119

7,000 m: 0.2466

8,000 m: 0.2019

9,000 m: 0.1653

10,000 m: 0.13534

15,000 m: 0.04979

20,000 m: 0.01832

25,000 m: 0.00674

30,000 m: 0.00248

35,000 m: 0.00091

40,000 m: 0.00034

45,000 m: 0.00012

50,000 m: 0.00005

55,000 m: 0.00002

60,000 m: 0.00001

65,000 m: 0.00000

70,000 m: (in vacuum)

Since I_sp scales linearly with height, what you can do to guesstimate it at a particular altitude is take the difference between the two given I_sp ratings, multiply it by one minus the atmospheric pressure at the altitude, and add that amount to the atmospheric I_sp rating. As a formula, it looks like this:

I_sp = I_sp(atm) + ((I_sp(vac) - I_sp(atm)) * (P_k(sfc) - P_k(current))

Let's take the Skipper for example: I_sp is 300 at the surface and 350 in space. 350-300 = 50. At 3,000 meters, the atmospheric pressure is .54881 atmospheres (as per above), so we have the following:

I_{sp(3000 m)} = 300 + ((350-300) * (1-.54881) = 300 + (50*0.45119) = 300 + 22.5595 = 322.5595

If you want to know the specific altitude at which the I_sp is exactly a specific amount, you have to work both the atmospheric scale formula and the formula above backwards. Like so for the Skipper:

I_sp = I_sp(atm) + ((I_sp(vac) - I_sp(atm)) * (P_k(sfc) - P_k(current))

320 = 300 + ((350-300) * (1 - P_k)

320 - 300 = 50 * (1 - P_k)

1 - P_k = 0.4

-P_k = -0.6, therefore P_k = 0.6

P_k = e^(-a/5000)

ln(P_k)= -a/5000

a = -5000 * ln(P_k)

a = -5000 * ln(0.6) = -5000 * -.51083 = 2,554.128 meters (the answer to your original question, if I'm not mistaken)

From those atmospheric scales above, you can see that by about 25,000 meters any engine is just about at its vacuum Isp. This is, incidentally, completely consistent with the structure of Earth's atmosphere; standard surface pressure is 1013 mB, at 10 kilometers - the tropopause region - it's around 10 mB.

Edited November 13, 2013 by capi3101

[Nao]

So weight is more important than ISP?

Also when right clicking on engine I found.

Fuel Flow with lots of decimal points.

Specific Impulse with very large numbers.

thrust.

Fuel flow, is shown in total fuel+oxidizer (or in case of jet engines, fuel+air) in tons, so for the kahlzun's image its 0,47kg/s. And since Thrust = Isp * g0 * fuel flow, (where g0 is a constant of 9,81 m/s^2) we have relation between the numbers shown there 0,00047 *9,81 * 391 = 1802N or 1,8kN

In general, for ascents and landings on heavy bodies, weight is more important as engines take big part of total craft mass, but for orbital operations, transfers, insertion burns, corrections etc. high isp is more Important as you can get away with small number of engines making fuel the biggest part of ship's mass.

In the end it all comes down to burn time of the particular engine (how much energy or dV it produces) if it's working for a long time, giving large amount's of dV it may be better to pick heavier more efficient engine. (rough estimate of "long time": 3-4 minutes)

Hmm that said this have given me idea about a chart, maybe ill edit one in later.

[Tank Buddy]

I was using LV-909s for s SSTO and they has an Isp of 390 (vaccum Isp) at around ~20 Km, so just keep that in mind.