Delta v calculating

[Spaceman83]

Okey i have 3 solid fuel boosters and 2 liquid engines so how i get Isp count them for delta v do i just add them all and  use that number to count for delta v calculation like 100+100+200+200+200=800 and then delta v like Δv = ln ( Mstart / Mdry ) * 800 * g

[LN400]

Here are the equtions you want:

http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

Look under Math Examples. Basically, you need to calculate effective Isp (as shown on that page) and use that figure in your calculations. 

[ElWanderer]

If you have engines with different Isp values firing at the same time, you need to calculate a combined, thrust-weighted Isp, as per LN400's link.

However, you mention having SRBs. Unless they burn for roughly the same length of time as the liquid engines, then you have effectively staged (and chances are you will actually have staged by dropping the boosters). For each stage, you need to do a separate calculation. This is more complicated if engines from more than one stage are firing at the same time. You need to calculate how long the SRBs will be firing and from that calculate how much fuel mass the liquid engines will use. Then you can calculate the delta-v of the first stage with the SRBs attached, and the delta-v of the remaining stage after they have burned out / been jettisoned.

This can be worked out by hand, but it's much easier with a tool of some kind. I wrote myself a spreadsheet to do the calculations but I also use KER now.

[Snark]

Do you run your SRBs and your liquid-fuel engines at the same time as each other?  Or do you run the SRBs until they burn out, and then use the liquid-fuel?

If you're running them at the same time, you need to do some math as the previous posters have indicated.

However... it's worth noting that SRBs have a much lower Isp than most liquid-fueled engines.  In terms of maximizing your dV, it's generally best to burn your lowest-Isp fuel first:  i.e. if possible, take off on SRBs alone, and kick in the liquid engines only after the SRBs are gone.

[LN400]

18 minutes ago, Snark said:

Do you run your SRBs and your liquid-fuel engines at the same time as each other?  Or do you run the SRBs until they burn out, and then use the liquid-fuel?

If you're running them at the same time, you need to do some math as the previous posters have indicated.

However... it's worth noting that SRBs have a much lower Isp than most liquid-fueled engines.  In terms of maximizing your dV, it's generally best to burn your lowest-Isp fuel first:  i.e. if possible, take off on SRBs alone, and kick in the liquid engines only after the SRBs are gone.

I can not argue directly against that, as it is sound. However, personally I found the best use for stock SRB to add them when the start TWR of the mains were just not enough for a good acceleration, or below 1. Fire the SRB with the main, with the main at reduced power perhaps so that by the time the SRBs go out, the TWR for the mains have increased sufficiently. This allows for steering (I'm just too much of a cheapscate for the controllable fins, they are horribly expensive) while enjoying the raw power of the SRB. With mod SRBs things are different, with them having vector steering. Stock, I prefer to fire them all.

[Snark]

10 hours ago, LN400 said:

I can not argue directly against that, as it is sound. However, personally I found the best use for stock SRB to add them when the start TWR of the mains were just not enough for a good acceleration, or below 1. Fire the SRB with the main, with the main at reduced power perhaps so that by the time the SRBs go out, the TWR for the mains have increased sufficiently. This allows for steering (I'm just too much of a cheapscate for the controllable fins, they are horribly expensive) while enjoying the raw power of the SRB. With mod SRBs things are different, with them having vector steering. Stock, I prefer to fire them all.

I prefer to take off on SRBs only, with simple fixed fins for aero stability, and let the gravity turn do all the steering. 

Other than the tiny eastward nudge right off the pad, rockets shouldn't need any actual steering until they're above atmosphere.

[foamyesque]

I usually prefer to do co-firing and carry extra fuel in my core stage; it gets you good TWR off the pad and excellent control authority at the same time. Particularly on larger rockets, or ones susceptible to wobbly-rocket problems (radial attachments, probes in fairings, long stacks) some steering is needed.

[LN400]

8 hours ago, Snark said:

I prefer to take off on SRBs only, with simple fixed fins for aero stability, and let the gravity turn do all the steering. 

Other than the tiny eastward nudge right off the pad, rockets shouldn't need any actual steering until they're above atmosphere.

If only I could. Independent on design (assuming symmetric mass distribution) the rocket WILL invariably lean westwards right after liftoff with north/south thrown in more randomly. Without gimbals or controllable fins, the tiny reaction wheel in the pod can not save the rocket from going the wrong way. Under no circumstances have I been able to send a symmetric rocket on a due east ascent unless I have gimbals or fins too make the early corrections. Once the eastwards course is established gravity is enough but I still have the issue with the rockets random falling over either north or south of due east.

Edited August 5, 2016 by LN400
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