Question on Kerbal Engineer Redux data

[hhatch]

Attached are 2 photos of a rocket I built in VAB and on the launch pad.

 

I'm having some trouble understanding some of the numbers. Why does stage 5 and 6 have so much delta V when all they have are decouplers. The total delta v shown on the launch pad is different than in the VAB. Is the delta V shown in the VAB not for the vessel ? If not what is it ?

 

Also, I've been looking for a site that explains what all the numbers mean but I have not been able to find one. If someone knows of one I would appreciate a link.

[GuyWithGlasses]

Hello!

 

In the VAB, you have configurated KER with non-atmospheric data, and on the KSC, KER shows engines with atmospheric data (cause youre in the atmosphere).

The first issue I think is could be cause you've enabled crossfeed between stages (in the decoupler menú).

 

Regards!

Edited December 11, 2017 by GuyWithGlasses

[James Kerman]

1 hour ago, hhatch said:

Why does stage 5 and 6 have so much delta V when all they have are decouplers

The figure on the left of the slash under delta v is stage dv, right of the slash is total rocket dv.  Isp means the efficiency of your engine when using fuel and is measured in seconds.  Isp is different for most engines, some are optimized for atmospheric conditions and some are better in a vacuum - you can find out the difference between engines in by right clicking them in the VAB or space plane hangar menu's (atm=atmosphere/vac=vacuum).  For a rocket like yours I would use a Terrier as the last stage engine as it has a higher Isp (vac=345s) than the reliant you have there now (vac=310s) and it's also less than half the weight.  These two factors mean you can go further with an equal amount of fuel.

I can see by the KER data that you likely have a fuel problem but there is no need to use hoses as you can assign priority to whichever fuel tanks you want to drain first by using advanced tweekables.  Press escape, go to settings, scroll down until you see the gameplay heading and click on advanced tweekables.  Now you can right click on any fuel tank and assign it a number (highest number drains first or two tanks with the same number will drain together).

The only other thing I worry about is the TWR (thrust to weight ratio).  It must be greater than 1 if you are launching vertically and I have read that 1.3 to 1.4 is optimal however I've found 1.6 to 1.8 to be better for the rockets I build.  Now that you are running KER you can fine tune everything and find your own optimum.

Edited December 11, 2017 by James Kerman
Grammar

[Plusck]

KER gives the information per stage based on what you have after staging that stage and until you need to stage the next stage.

So your stage 7, which starts you off, only lasts until you get rid of the first set of boosters. It therefore won't last all that long and won't give a large dv change.

By far the greatest dv change will come once you get rid of all your boosters and have the core stage with full tanks. That happens when you stage number 4, so therefore that's where all the dv is.

 

KER currently cannot suss out how you plan to time your staging if there is no absolute end to a given stage. If you enable crossfeed, then even if the boosters' tanks are empty, the engines will still have fuel available to them via crossfeed. Therefore KER fails to recognise that you intend to drop the booster (by watching the fuel tanks and dropping the stage when empty), so it vastly understates your dv.
The same thing often happens if you have drop-tanks.

The only workaround, I think, is to make the build with fuel lines and fuel crossfeed off, until the numbers are where you want them. You then just have to accept that in-flight numbers won't be right, remove the fuel lines and enable crossfeed for the finished ship.

Likewise with drop-tanks: to get the numbers, add fuel lines and stick a tiny engine on that stage to get an approximate number for dv.

 

As for all the numbers, they should be fairly self-explanatory. "Torque" in the VAB info is vital if you have assymetric builds and need to know whether your thrust is offset. "Burn" is vital when putting fuel tanks on SRBs: you need to set the SRB thrust limiting so that the SRB stage lasts exactly as long as fuel in the tanks will, and thr "burn" time lets you know about that.
For the other numbers, you'll have to ask specifically which ones you want to know about

[hhatch]

3 hours ago, James Kerman said:

The figure on the left of the slash under delta v is stage dv, right of the slash is total rocket dv.  Isp means the efficiency of your engine when using fuel and is measured in seconds.  Isp is different for most engines, some are optimized for atmospheric conditions and some are better in a vacuum - you can find out the difference between engines in by right clicking them in the VAB or space plane hangar menu's (atm=atmosphere/vac=vacuum).  For a rocket like yours I would use a Terrier as the last stage engine as it has a higher Isp (vac=345s) than the reliant you have there now (vac=310s) and it's also less than half the weight.  These two factors mean you can go further with an equal amount of fuel.

I can see by the KER data that you likely have a fuel problem but there is no need to use hoses as you can assign priority to whichever fuel tanks you want to drain first by using advanced tweekables.  Press escape, go to settings, scroll down until you see the gameplay heading and click on advanced tweekables.  Now you can right click on any fuel tank and assign it a number (highest number drains first or two tanks with the same number will drain together).

The only other thing I worry about is the TWR (thrust to weight ratio).  It must be greater than 1 if you are launching vertically and I have read that 1.3 to 1.4 is optimal however I've found 1.6 to 1.8 to be better for the rockets I build.  Now that you are running KER you can fine tune everything and find your own optimum.

I enabled advanced tweakables and If I change a setting for 1 or more tanks all tanks empty at the same time. The six tanks I have were loaded in symmetry. My original setup was asparagus staging which did work except for the funny numbers I got in KE. I wanted to dump dead weight as soon as possible. I can't seem to do it with the advanced tweakables. Also what are the autostrut and rigid attachment options  ?

[James Kerman]

To asparagus you need to add your boosters 2x2x2 instead of 6 at once (I think that's what you've done).  The first tanks you want to drain and dump, make them 3.  The next 2 and the last 1.  You should see them draining in that order, highest to lowest.

Autostrut is an automatic strut from the part you select it on to the heaviest part of your spacecraft - good for wobbly ships. 

Rigid attachment makes a joint between 2 parts inflexible, but is more prone to breaking.

P.S.
The reliant engine has no gimbal (the ability to move around a bit and assist your rocket's controllability) so I would recommend the swivel instead, even though it has a little less thrust. 

Edited December 11, 2017 by James Kerman
Added p.s.