#### Ark

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- Thread starter Ark
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[ame="http://www.orbithangar.com/searchid.php?ID=1407"]BurnTimeCalcMFD (BTC) 1.5[/ame]

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BurnTimeCalc is a great MFD. I recommend it highly. A must have in my book.

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It lets you enter all the relevant parameters, as well as calculate more than just delta-v (so long as you have the right numbers available)

How do I calculate D-V for a thruster group where thrusters have different ISP? Will the result be correct if I just take average ISP or will that be way of?

Just for clarification, I need to put this into code, so a tool won't help me. If I can just take average ISP, I'll have no trouble at all, otherwise thigs might get a bit complicated...

Simple answer: Don't burn all the thrusters at the same time.

For the sake of keeping it simple, let's say that we have a ship with two thrusters, one with an x amount of ISP and the other with x/2.

I think that it's more complicated than just taking the average ISP.

There are some distinctions that have to be made, for example are the thrusters burning from the same fuel tank or does each thruster have it's own tank(s)/propelant?

If it's the second case then the answer is simple, you burn the thruster with the higher initial mass/final mass ratio first and then the other one, this way you get the maximum dV from the available fuel. (When you burn the second thruster, you'll have less mass to "move").

Now arises the question, what if they have the same initial mass/final mass ratio ? In that case, it doesn't really matter which one you burn first. The total dV you'll get will be the sum of the dV each thruster can produce.

In the case that they burn from the same tanks, for the same amount of fuelmass:

A thruster with x amount of ISP will give you a y amount of dV.

A thruster with x/2 amount of ISP will give you y/2 amount of dV.

So, it would be better to burn the thruster with the higher ISP longer, than burning both of them at the same time.

Now, suppose that you did burn both engines at the same time, what would be the total dV you'd get?

Well... things get complicated at this point, since we have to calculate how much fuel the lower ISP thruster would burn -during the burntime of the higher ISP thruster- and then subtract the fuelmass (of that burntime) from the higher ISP one; and keep repeating the process untill the values converge. Newton's method

Things would be simpler if you had thrusters with different thrusts but the same ISP.

I hope this post helped.

I hope this post helped.

Well, it helped in so far as it confirmed my suspicions that the thing is a bit more complicated.

Trouble is, the code is for IMS, and people will assign thrusters to groups by themselves. Those thrusters might or might not have the same ISP, they might or might not share a fuel source, and I wanted to provide a display that shows the total DV tey can burn with any thruster group... I guess I'll just check the thrusters and make the DV-status N/A if there are mixed thrusters in the group.

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Dgatsoulis pointed me to this thread, so I gave the issue with a group of thrusters a bit of thought.

If I'm correct, when thrusters with different ISP don't share the fuel source, the total DV should be the sum of DVs for each of them. Things get a bit problematic when they share a fuel source, because then DV you get depends on how you use them. If you burn all the fuel using a thruster with higher ISP, you will get more DV than if you burned all of it using a thruster with lower ISP. You can still calculate maximum DV and minimum DV ,though (assuming the highest and the lowest ISP, respectively).

EDIT: On the second thought, it's not that simple even if they don't share the fuel source. It starts to depend on the way you use them, too. It looks like you'll get more DV by burning the engines with lower ISP first. So again, you can only calculate maximum and minimum DV.

EDIT2: If we burn them at the same time, additional questions arise - will they burn all their fuel in the same time? Or will one of them stop working first? Anyway, the answer can be found by integrating the acceleration, but the exact form of the equations and the result will depend on the assumptions.

If I'm correct, when thrusters with different ISP don't share the fuel source, the total DV should be the sum of DVs for each of them. Things get a bit problematic when they share a fuel source, because then DV you get depends on how you use them. If you burn all the fuel using a thruster with higher ISP, you will get more DV than if you burned all of it using a thruster with lower ISP. You can still calculate maximum DV and minimum DV ,though (assuming the highest and the lowest ISP, respectively).

EDIT: On the second thought, it's not that simple even if they don't share the fuel source. It starts to depend on the way you use them, too. It looks like you'll get more DV by burning the engines with lower ISP first. So again, you can only calculate maximum and minimum DV.

EDIT2: If we burn them at the same time, additional questions arise - will they burn all their fuel in the same time? Or will one of them stop working first? Anyway, the answer can be found by integrating the acceleration, but the exact form of the equations and the result will depend on the assumptions.

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