The Basics

[Blazer Ayanami]

What if I have the glitch where the closest approach line keeps spasmijg out around the orbit of an object you want to dock to?

I have that glitch too. All the time. But its not important you can dock perfectly without it. That indicator is useful just If you wanna hit a target, or docking on your first orbit

 

[Realistic Mods]

ISP
Specific Impulse

Sticking with the car theme, ISP is simply how fuel efficient an engine is. In car terms, that’s distance/fuel (miles per gallon for example). In space, that doesn't work because theoretically once you're in deep space with no forces acting on you, then you will continue in motion forever at a constant speed covering all the distance in the universe for free. (I know technically that’s the same for a car but you have drivetrain resistance, rolling resistance, drag etc. all acting on the car, trying to slow it, thus you need to keep applying fuel to maintain a speed).
ISP is essentially a measure of how much of fuel quantity it takes to produce an amount of thrust. The less fuel an engine burns to achieve a specific impulse, the more efficient it is.
Now, 2 things about ISP

1. It's measured in seconds
2. It does take gravity into account.

Why seconds, and not lbs, kg, N or whatever? Because NASA, when they were working all this stuff out, was a mix of 'borrowed' German scientists and Americans. One side (Zee Germans) wanted to use Metric. The Americans wanted to use Imperial. After what I'd like to assume was a good natured fight involving a lot of spanners and slide rules, they decided on a unit of measurement everyone knew, but wasn’t in either system. Seconds.
And with gravity, you use Earths gravity 9.8 regardless of where you are. Why? Because its used as a constant, a benchmark for comparison. It allows you to compare an ISP of say a Rocketdyne F1 Engine with a titan without worrying if someone has used...Mars...as a constant for the titan.
How to work out ISP then. Easy, the information pops up when you select an engine will tell you its ISP. And as long as you only use that engine, regardless of how many, the rockets ISP will be that number.
Things get complicated when you're using engine combinations, or boosters with different engines. Even though each engine is using is own ISP, when it comes to working out the Delta V (coming soon...) of that rocket, you need a combined ISP of the engines that are being used at the time.
There are 2 ways of doing this.
Hard Way:

1. you need the exhaust velocity of an engine. To work this out, you multiply thrust by fuel consumption.
2. Then, you multiply that, by g (9.8) to get an individual engine ISP
3. Do that for all engines
4. Now, for each individual engine, divide its force by its own ISP. Do this for all of the engines and add the totals together. Then take the combined total of all the thrust your engines create and divide that by the all the totals you've just worked out.

Or

Easy Way.

1. Add up the combined thrust of your engines
2. Add up the combined fuel consumption of your engines.
3. Take your gravitational constant (9.8) and multiply it by your total consumption.
4. Finally, take your combined thrust and divide it by the answer you just got.
5. BOOM. ISP Baby.

I have a doubt,
According to sfs, Ion engines have more Isp than Valiants. But how is that so? If we keep an valiant engine at low throttle, it will consume less fuel, just like the ion engine? Even if we keep at low throttle, it should consume the same amount of fuel to do a manuever, but this time at a slower rate (Is that so?) Please answer

 

[Marmilo]

I have a doubt,
According to sfs, Ion engines have more Isp than Valiants. But how is that so? If we keep an valiant engine at low throttle, it will consume less fuel, just like the ion engine? Even if we keep at low throttle, it should consume the same amount of fuel to do a manuever, but this time at a slower rate (Is that so?) Please answer

Well, no. If you reduce throttle, you will reduce consumption, but also thrust. So, if you set your throttle to 50%, you will be using half the fuel, but also having half the thrust, and thus half the acceleration. So, to achieve the same velocity change, you will need to burn for twice as long. Burning with half the consumption for double the time is the same as burning with normal fuel consumption for normal time

 

[Altaïr]

I have a doubt,
According to sfs, Ion engines have more Isp than Valiants. But how is that so? If we keep an valiant engine at low throttle, it will consume less fuel, just like the ion engine? Even if we keep at low throttle, it should consume the same amount of fuel to do a manuever, but this time at a slower rate (Is that so?) Please answer

I'll answer for Horus because it's been long that he is inactive, but I'm sure he would tell you the same.

Firstly, yes, when you lower the throttle the fuel consumption is decreased in proportion. But that's not what makes an engine efficient, it's not because your thrust is low that your engine is efficient. This is not related. For example the Frontier engine has more thrust than the Valiant while being also more efficient.

ISP tells you more than that. To take your example of a Valiant vs ion engine, it means that even if you lower the Valiant thrust to equalize it with an ion engine, the Valiant will still consume more fuel.

There's a simple way to illustrate it. A Valiant has as much thrust as 27 ion engines. So let's see what happens if we make them work against eachother:

As no side is more powerful than the other one, this thing shouldn't move right? This is the result:

Indeed, the "chariot" didn't move. But if you look at the fuel, you'll notice that the Valiant side consumed much more fuel despite providing the same thrust. This is what efficiency means. An engine with a higher ISP will consume less fuel at equal thrust.

Actually in this case the experience is not perfect, because the ion engines pump out the fuel from both tanks, but you can recreate that experiment by comparing a Hawk vs 3 Valiants, or a Titan vs 4 Frontiers. Those 2 combinations are balanced, you will notice the same phenomenon.

That's why a maneuver performed with a more efficient engine will consume less fuel, and as you said the thrust has nothing to do with this. The thrust only dictates the rate at which the maneuver is executed.

 

[Realistic Mods]

I'll answer for Horus because it's been long that he is inactive, but I'm sure he would tell you the same.

Firstly, yes, when you lower the throttle the fuel consumption is decreased in proportion. But that's not what makes an engine efficient, it's not because your thrust is low that your engine is efficient. This is not related. For example the Frontier engine has more thrust than the Valiant while being also more efficient.

ISP tells you more than that. To take your example of a Valiant vs ion engine, it means that even if you lower the Valiant thrust to equalize it with an ion engine, the Valiant will still consume more fuel.

There's a simple way to illustrate it. A Valiant has as much thrust as 27 ion engines. So let's see what happens if we make them work against eachother:
View attachment 78100
As no side is more powerful than the other one, this thing shouldn't move right? This is the result:
View attachment 78101
Indeed, the "chariot" didn't move. But if you look at the fuel, you'll notice that the Valiant side consumed much more fuel despite providing the same thrust. This is what efficiency means. An engine with a higher ISP will consume less fuel at equal thrust.

Actually in this case the experience is not perfect, because the ion engines pump out the fuel from both tanks, but you can recreate that experiment by comparing a Hawk vs 3 Valiants, or a Titan vs 4 Frontiers. Those 2 combinations are balanced, you will notice the same phenomenon.

That's why a maneuver performed with a more efficient engine will consume less fuel, and as you said the thrust has nothing to do with this. The thrust only dictates the rate at which the maneuver is executed.

Thank you very much. You spent such a time just typing to answer my question!
Thanks

 

[Altaïr]

Thank you very much. You spent such a time just typing to answer my question!
Thanks

You're welcome

 

[Realistic Mods]

Altaïr
is fuel consumption that ISP? like 240s?

 

[Altaïr]

Altaïr
is fuel consumption that ISP? like 240s?

No but they are related, via the following relation:
Thrust = FC × g0 × Isp

FC is fuel consumption, g0 = 9.8 m/s² is the gravitational constant of Earth, and Thrust is... thrust obviously, but expressed in kilonewtons.

With the thrust expressed in tons, the relation reduces to:
Thrust(tons) = FC × Isp

So to get the fuel consumption, you just have to divide the thrust as displayed by the game by Isp.

For example, a Hawk has 120 tons of thrust, and 240 seconds of specific impulse, so its fuel consumption is 120/240 = 0.5 tons/second.

 

[Realistic Mods]

No but they are related, via the following relation:
Thrust = FC × g0 × Isp

FC is fuel consumption, g0 = 9.8 m/s² is the gravitational constant of Earth, and Thrust is... thrust obviously, but expressed in kilonewtons.

With the thrust expressed in tons, the relation reduces to:
Thrust(tons) = FC × Isp

So to get the fuel consumption, you just have to divide the thrust as displayed by the game by Isp.

For example, a Hawk has 120 tons of thrust, and 240 seconds of specific impulse, so its fuel consumption is 120/240 = 0.5 tons/second.

Thanks!

 

[Realistic Mods]

Your Dv is 9.8 x 1020 x LN(35 [overall mass at the start] /20 [mass at the end of the burn]) = 5596.157879.

I am getting about 47,599.425 by calculating by these values

(just to verify cause I am making a calculator)

 

[Marmilo]

g = 9.81
ISP = 1020
Wet mass/ dry mass = 35/20 = 1.75
Ln(1.75)=0.5596157879
∆V= ISP * g * ln(wet mass/dry mass) = 1020 * 9.81 * 0.5596157879 = 5599.6274972

How on earth did you get 47000?

Here's a working calculator so you can compare results: https://www.google.com/url?sa=t&sou...YQFnoECAQQAQ&usg=AOvVaw2shXTK508QyN9LvyyyS1LV

 

[Realistic Mods]

g = 9.81
ISP = 1020
Wet mass/ dry mass = 35/20 = 1.75
Ln(1.75)=0.5596157879
∆V= ISP * g * ln(wet mass/dry mass) = 1020 * 9.81 * 0.5596157879 = 5599.6274972

How on earth did you get 47000?

Here's a working calculator so you can compare results: https://www.google.com/url?sa=t&sou...YQFnoECAQQAQ&usg=AOvVaw2shXTK508QyN9LvyyyS1LV

Thanks!, I mistook Ln for e ie. 2.718 approx, highly changed the answer

 

[Marmilo]

Yes, I thought it was something with ln, but I didn't know what exactly got messed up. Let's hope your calculator works now!

 

[Realistic Mods]

Yes, I thought it was something with ln, but I didn't know what exactly got messed up. Let's hope your calculator works now!

It does! Thank you!

 

[Marmilo]

It does! Thank you!

No problem