Delta-V problems

[DunaManiac]

Help! I can't calculate Delta-V correctly. I've tried everything! It never works. I can't calculate the ISP of two NERVA rockets. Please help! 

 

Okay, So I'll break it down for you.  So I did this equation 800*(17.5/10.5)*9.81. I got 5667, but the actual rocket had only 1700 m/s when I flew it. Also, I did 800*(17.5/10.5) and got 567. I didn't know how so I winged it. And I failed. So please help. 

[HebaruSan]

You're missing a natural logarithm ("ln").

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

https://en.wikipedia.org/wiki/Natural_logarithm

[OhioBob]

If you are using a Nerv, are you using tanks that have both fuel and oxidizer?  If so, then that's also part of your problem.  A Nerv uses liquid fuel only, so oxidizer is just dead weight. 

The ISP of two Nerv is the same as one.  The only time you have to worry about figuring out combined ISP is when you have multiple engines with different ISP.

 

Edited July 29, 2017 by OhioBob

[OhioBob]

41 minutes ago, DunaManiac said:

So I did this equation 800*(17.5/10.5)*9.81. I got 5667 ... Also, I did 800*(17.5/10.5) and got 567. 

There's something more wrong with your math than just leaving out the natural logarithm.  Neither one of those equations equal the answer you got.  If you have a starting mass of 17.5 t, an ending mass of 10.5 t, and an Isp of 800 s, then the delta-v is,

dV = 800*9.80665*LN(17.5/10.5) = 4008 m/s

If you are really getting only 1700 m/s, then there is something else wrong.  That's why I asked if you might be including oxidizer in the calculation.

[DunaManiac]

NO! I had three NERVAS on it.

[OhioBob]

22 minutes ago, DunaManiac said:

NO! I had three NERVAS on it.

No to what?  My question about oxidizer?

The number of Nervs (I assume you are using the stock engine LV-N "Nerv", not NERVA) doesn't make any difference.  ISP is not additive.

Also, please explain where the 1700 m/s number came from.  That number doesn't mean much to us it we don't know from where it was derived.

It would also be helpful if you could post an image of vessel.  A lot of problems can be diagnosed from a simple picture.
 

Edited July 29, 2017 by OhioBob

[Spricigo]

 

By any chance are you using Nerv in the atmosphere? They are absolute garbage in the atmosphere (i_sp , 1atm 185)

Another possibility is that your maneuvers (e.g long maneuvers in a curved trajectory) are so inefficient that much of the theorical deltaV is lost. (

Edited July 29, 2017 by Spricigo

[DunaManiac]

OH! I did have Oxidizer.  And it was a Interplanetary transfer stage, so yeah.

Edited July 30, 2017 by DunaManiac

[Spaceman17]

just make an sls block 2 cargo

[OhioBob]

34 minutes ago, DunaManiac said:

OH! I did have Oxidizer. 

OK, that's not good when using Nerv engines.  The Nerv will use up the liquid fuel, and when that's gone, it will stop running.  The oxidizer is nothing but a bunch of useless ballast that destroys your performance.  You should redesign using liquid fuel tanks (the ones that don't have any oxidizer).  Of course you still need oxidizer for any stage using a chemical rocket engine.  Also, if you're using fuel cells you will still need to carry a small amount of oxidizer for those.