Ending mass?

[Jonda]

So I'm trying to calculate delta v, however, with this you need the starting and ending mass. I have the starting mass as 56,892 t and on Mechjeb the ending mass is 47,862 t. The problem is that I can't calculate final mass because if I drain both tanks i get around 42,000 t. How can I calculate this as the Oxidizer does not drain the same rate as the liquid fuel. Btw I am using R.A.P.I.E.R. Engines. (305 ISP).

[Kerbalstar]

Are you saying that the tanks won't fully drain? If so, do you know how empty they will be? I assume that you don't. Then I have no idea. I hope someone will, though!

[steuben]

They don't get used at the same rate. But, there is proportionality more oxidizer than liquid fuel in the tanks, equivalent to the difference in the usage rates.

And in a weird quirk of design and engineering, the sum of the usage rate of liquid fuel and oxidizer, is two mass units per unit time. 

 

[Arugela]

Unless something has changed. Calculate pure LF and ox/LF tanks separately by weight.(By this I mean weight LF+ox seperate from just LF tanks. Although either logical method works.)

Ox and LF each seperately weigh 0.005 tons per unit I think. They weight the same amount to simplify this calculation a bit.

Proportion of fuel is:

OX: 11/20=.55

LF: 9/20=.45

It is proportional just each other. Note, 9+11=20/20. 8) Unless I reversed them. it's the same as the original small tank. It has one at 9 and one at 11 units. That is the numerator for each one in proper ration. And the denominator is 20. IE their sum.

 

You can use this to figure out if you used too much LF compared to ox. Or see how much ox you can burn if you are low on LF.

Example:

If you have 55 units of ox you need= 55/.55=100*.45= 45 LF to be able to burn it all.

Mono weighs 0.004 tons per units and ore weights 0.01 tons per units. I think Ox and LF weight 0.005. It's the same number of zeros as the mono. Xeno weighs 0.0005 I think. It has one more zero than the ox/lf/mono. Ore has one less zero than lf/ox/mono.

 

Edited September 25, 2018 by Arugela

[Rocket In My Pocket]

On the off chance you are calculating DV because you think you have to, here's KER: It will calculate it for you.

Make sure to get the latest version for KSP 1.4.5. https://github.com/jrbudda/KerbalEngineer/releases

If you are calculating DV because; realism/fun/roleplay/etc then by all means carry on.

Edited September 25, 2018 by Rocket In My Pocket

[Jonda]

2 hours ago, Arugela said:

Unless something has changed. Calculate pure LF and ox/LF tanks separately by weight.(By this I mean weight LF+ox seperate from just LF tanks. Although either logical method works.)

Ox and LF each seperately weigh 0.005 tons per unit I think. They weight the same amount to simplify this calculation a bit.

Proportion of fuel is:

OX: 11/20=.55

LF: 9/20=.45

It is proportional just each other. Note, 9+11=20/20. 8) Unless I reversed them. it's the same as the original small tank. It has one at 9 and one at 11 units. That is the numerator for each one in proper ration. And the denominator is 20. IE their sum.

 

You can use this to figure out if you used too much LF compared to ox. Or see how much ox you can burn if you are low on LF.

Example:

If you have 55 units of ox you need= 55/.55=100*.45= 45 LF to be able to burn it all.

Mono weighs 0.004 tons per units and ore weights 0.01 tons per units. I think Ox and LF weight 0.005. It's the same number of zeros as the mono. Xeno weighs 0.0005 I think. It has one more zero than the ox/lf/mono. Ore has one less zero than lf/ox/mono.

 

So if i had lets say 1246 units of oxidizer t would be 1246/.55=2265.5 x .45= 1020 units of Liquid fuel?

 

19 minutes ago, Rocket In My Pocket said:

On the off chance you are calculating DV because you think you have to, here's KER: It will calculate it for you.

Make sure to get the latest version for KSP 1.4.5. https://github.com/jrbudda/KerbalEngineer/releases

If you are calculating DV because; realism/fun/roleplay/etc then by all means carry on.

Yeah I do it for fun but thank you!

[Arugela]

Yea my calculator had 1019.45454545. Not sure how the game rounds things though.

You can also do a multiplier:

Ox= 1/0.55*.45=0.81818181818181(IE 9/11); 1246*0.818181818=1019.45454545454

LF=1/0.45*.55=1.222222222222(IE 11/9); 1019.45454545454*1.222222222=1246

This is technically simpler and easier to remember if you realize the numerator values of the other formula are 9|11. (ox=smaller number first as it is the larger value; LF=bigger number first as it is the smaller number)

Although, I never remember that...

Edited September 25, 2018 by Arugela

[Snark]

Moving to Gameplay Questions.

12 hours ago, Jonda said:

How can I calculate this as the Oxidizer does not drain the same rate as the liquid fuel. Btw I am using R.A.P.I.E.R. Engines.

One unit of propellant (either liquid fuel or oxidizer) is 0.005 tons (i.e. 5 kg).  Therefore, if you just add the number of LF units to the number of oxidizer units, and multiply that by 0.005, that will tell you the total propellant mass.

Since you're using RAPIERs and are presumably switching between LF-only propulsion in air versus closed-cycle propulsion in vacuum, this means your LF and O will likely not be perfectly balanced, i.e. you'll likely run out of either LF or O first, rather than both of them running out simultaneously.  So in reality, your maximum amount of delta-V will be determined by whichever propellant is in the shortest supply.

In my own gameplay, this generally doesn't come up because I almost never use RAPIERs or fly spaceplanes.  So all my rocket engines are LFO engines, which means that I don't need to worry about LF-versus-O ratios ever, since all the fuel tanks have LF and O in precisely the correct ratio that the LFO engines use.  So the calculation is simple-- I just look at the amount of LF and divide by 90, and that's how many tons of propellant I have.  "Oh, I've got 900 units of LF plus a corresponding amount of oxidizer.  900 / 90 = 10, so I have 10 tons of propellant."  I find that the divide-by-90 rule is easy to remember and pretty quick for doing mental math when I'm calculating dV in my head.