Help with the Rocket Equation: do I add up the Isp for multiple engines?

[Brainlord Mesomorph]

the Rockomax 55 has as Isp of 320, if I use 3 of them is my total Isp 1280? or still 320?

Edited October 27, 2014 by Brainlord Mesomorph

[EdFred]

It's still 320.

[Brainlord Mesomorph]

but the TWR has changed!

(I'm slowly getting this)

[Red Iron Crown]

If the cluster of engines is comprised of engines with the same Isp, use that Isp for the calculation.

e.g. 3xMk55 cluster has a net Isp of 320s

If the cluster of engines is comprised of engines with differing Isps, use a thrust-weighted average of the Isps to get the net Isp.

e.g. 2xMk55 + 1xLVT-30

Net Isp = (2*320*120 + 1*215*370)/(2*120 + 1*215)

= 343.6s

[TythosEternal]

For multiple engines, the combined equivalent ISP is the average Isp weighted by burn rate. However, KSP currently adjusts burn rate to maintain constant thrust, a simplification that Squad has stated they will fix in the near future. In the meantime, go with Red's advice (when is that not the answer, anyways?) and weight by thrust.

[bakanando]

This is covered in the wiki.

http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse

In your example, the thrust-to-weight ratio did change but the rate of fuel mass consumption didn't, so the Isp stays the same.

[Mars Mullo]

So do we add the thrust components together for TWR calculation?

So ISP stays the same for Delta-v claculation but for kN / m . g = TWR... do we add all the thrust values.

E.g A LV-T45 has a thrust of 250kN. say the rocket has a mass of 2 ton.

1 engine ... 250/ 2 . 9.81 = 12.7

3 engines 250 x3 / 2 . 9.81 = 38.2

Is this correct?

[ghpstage]

Your almost there, you do indeed take the combined thrust of engines active on a stage, and divide that by the weight that thrust is carrying.

Just be sure to take into account the additional mass the extra engines will add!

I think a LV-45 weighs 1.5 tons in stock, so adding an extra 2 will add 3 tons to the rockets mass!

Edited January 4, 2015 by ghpstage

[Mars Mullo]

Awesome...

I have been playing around with the TWR equation and have been using it to add sufficient SRB's to my rockets and to increase or decrease the thrust slider to achieve a ratio I want.

Reason being I notice the SRB's are more controllable in a certain ratio... which allows me to get my Scientists and engineers into orbit for their XP to increase.

And some of my rockets with one LV-T45 on a tower of 4 -5 tanks just doesn't have the oomph to get off the pad.

So dropping the thrust gives a longer burn time and a little more control. Which I need as I am also using chutes to recover funds from stages and have reached parts limits and cannot put wings or control surfaces on.

So if i desire a TWR of say 2 I can do the following:

TWR = kN / m . g

or

TWR . (m .g) = kN => 2 . (say 2 ton) . 9.81 = 39.24

then take the thrust value of the engine and divide by 100 (so for a RT-10 I get 250 / 100 = 2.5)

Then divide 39.24 by 2.5 = 15.6

so set the slider to 15.6 and then I get my TWR of 2 on take off and possibly a longer burn.

Although I have noticed in the RT-10 that the lift has a max of about 10km the sweet spot was between TWR 1.0 and 1.5 where you gained between 7km and 10km between 1m 58s and 1m 25s of burn.

Now I'm goign to go play with multiple SRB's and see what works well between having 2 - 6 SRB's

[ghpstage]

TWR = kN / m . g

or

TWR . (m .g) = kN => 2 . (say 2 ton) . 9.81 = 39.24

then take the thrust value of the engine and divide by 100 (so for a RT-10 I get 250 / 100 = 2.5)

Then divide 39.24 by 2.5 = 15.6

so set the slider to 15.6 and then I get my TWR of 2 on take off and possibly a longer burn.

You forgot to take the RT-10s mass into account again.

Bear in mind, that the SRB itself has a mass of nearly 3.8 tons, which would make up nearly 2/3rds of your craft's total. That isn't something you can just ignore if you want a meaningful calculation!

Around 15% is the thrusters self TWR=1 limit, below which the engine can't even lift itself, let alone any payload. Even just carrying a single, 0.8 tons command pod your 15.6% throttle would achieve a TWR around 0.87. While you will get a longer burn by nature of burning more slowly, you wojn't be goign far without the thrust required to lift you off the ground!

Although I have noticed in the RT-10 that the lift has a max of about 10km the sweet spot was between TWR 1.0 and 1.5 where you gained between 7km and 10km between 1m 58s and 1m 25s of burn.

I suspect the optimal TWR for the RT-10 will vary depending on the throttle % required to achieve it, but for pure RT-10 SRB rockets I have found a value of 1.5 to be extremely effective. Edited January 6, 2015 by ghpstage