Evaluating dV of a ship with changing dry mass

[Warzouz]

I'm working on a miner which bring ore back to a mothership with a refinery.

The ship is a SSTO powered by 4 LVN and 3200 units of liquid fuel. Fully fuelled it's around 40T. dV = 4000m/s. It lands on a body (all except atmo and Tylo), fill 3000 Ore and get back to the station.

dV lowers as the dead weight of the ship increase with Ore (+30T iirc). dV is nearly halved, but this depends on how much fuel I expended to land. Moho is very different from Pol

For now, I validated the concpet by trial and got this

How can I calculate the real dV of a round trip ?

DryM = dry mass of the ship

OreM = Ore mass added during trip

FuelM = Liquid Fuel mass

Engine = 4 * LVN (800)

from this formula : http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet#Basic_calculation

[cantab]

You need to split your calculation up, calculating the delta-V for separate "stages" in a loose sense. Any time your ship loses any mass other than the fuel it burns off, or gains any mass such as from mining, treat it as one stage ending and another one starting.

[Warzouz]

dV = g*ISP*ln(Mstart/Mend)

Round trip : dV1 = dv2

Going there (dV1)

MStart = DryM + FuelM

MEnd = DryM + FuelM - BurntM

Coming back (dV2)

MStart = DryM + OreM + FuelM - BurntM

MEnd = DryM + OreM

Instead of solving that, I can do it by iteration until I get dV1 = dV2 (I can even keep a safety margin for docking)

I'll try that.

[Warzouz]

OK, with my design, a 4000m/s empty miner ship can do a round trip from 1140m/s (*2 + 250m/s safety) with ore shipment = 120% of the dry mass.

So I can easily refuel :

- a Duna LO station from Ike (even starting with less fuel)

- an Eve station from Gilly if the station is on high and inclined orbit (2000km, Gilly plane)

But it's not possible to refuel a Tylo or Laythe Space Station from Pol with this design. I need a dedicated station on Pol to refuel the miner. Better, I'll use a simple tanker ship from those station. The Tylo and Laythe station won't have any miner.

Refueling from Vall could be cheaper in tranfert dV, but I'll loose on landing dV since I'll ned 2.5 landings t refill an orange tank instead of 1.2 for a Pol landing.

[Edax]

I'm working on a miner which bring ore back to a mothership with a refinery.

The ship is a SSTO powered by 4 LVN and 3200 units of liquid fuel. Fully fuelled it's around 40T. dV = 4000m/s. It lands on a body (all except atmo and Tylo), fill 3000 Ore and get back to the station.

dV lowers as the dead weight of the ship increase with Ore (+30T iirc). dV is nearly halved, but this depends on how much fuel I expended to land. Moho is very different from Pol

For now, I validated the concpet by trial and got this

http://n.lejeune.free.fr/ksp/miner.png

How can I calculate the real dV of a round trip ?

DryM = dry mass of the ship

OreM = Ore mass added during trip

FuelM = Liquid Fuel mass

Engine = 4 * LVN (800)

from this formula : http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet#Basic_calculation

In order to test the dv of my interplanetary ship that was loaded mostly with ore tanks, I tested it on the launch pad with mechjeb and launch stability enhancers. Mechjeb only measures fuel dv, so I would write down the dv amount, go back to the VAB, remove the fuel, and have the IRSU convert the ore into fuel, until the fuel tanks are full, calculate dv, and remember how empty the ore tanks are. Then replicate the ore numbers and remove the fuel in the VAB, and begin the test again until the ore tanks are empty, and I have all the dv figures, in which I add them all up to get my total dv.

Edited September 1, 2015 by Edax

[Warzouz]

@Edax : Thanks, but the point wasn't this one. I'm not converting ore during the trip. I'm just carrying it.

[Edax]

@Edax : Thanks, but the point wasn't this one. I'm not converting ore during the trip. I'm just carrying it.

Ehh, the partial point I was trying to make was that you can test delta v on the ground with full/empty ore tanks with full/empty fuel tanks with mechjeb to get the delta v values that you wanted.

[Warzouz]

True, I use this technique to evaluate my available dV if I fill every tank. The issue is the TWR though. My miner is well powered for itself. It can also push the space station, even interplanetary, but if the station is fully loaded and filled with ore, the TWR is very low (0.15 to Kerbin VAC iirc). It's a pain for traveling.

I prefer traveling with empty ore tanks and add a early interplanetary stage to push it out of Kerbin SOI quicker. It's heavier, less efficient, but easier. And my recoverable rocket SSTO is very cheap so, I don't care too much.