Delta-V of my orbiting stage has change when I come back

[K_mu]

Hello everyone,

I have been playing KSP for a few weeks now and today I finally reach Duna. The trip is going pretty well and I still have 1700 m/s of delta V left for the return as my ship is orbiting at 61km from Duna's surface. I then detach my lander, proceed to land it on the planet, make some science, and then go back for an orbital encounter with the transfer stage. But when the encounter is done, the delta V displayed by my vessel is 700 m/s instead of the previous 1700 ! I'm pretty confuse because I see no reason why my ship could be heavier, especially after burning almost all the fuel of my lander.

Is there any explanation to it ? Any way to avoid it ?

Sorry if this question have already been asked, I've looked in the forum for a few key words and was unable to find anything related.

Thanks for taking the time to read this, and have a nice day :)

[Geonovast]

Welcome to the forum, @K_mu!

Without pictures, it's going to be hard to tell, but from your description I have two guesses that may both be true:

1. The initial reading of 1700 m/s may have been taking into account the fuel in the lander.

2. The lander's engine is still activated and presumably pointing the opposite direction as the orbiter's engine, so they would be pushing against each other.

 

Screenshots would help a lot if you can get them.

 

[paul_c]

41 minutes ago, K_mu said:

But when the encounter is done

Encounter or docking?

[K_mu]

Hello Geonovast,

 

Thanks you very much for you (very quick !) answer ! I think the first point is the one, It makes sense...

Is there anyway to avoid the delta V "displayer" adding both values from the lander and the transfer stage ?

@ paul_c : I meant docking, you're right

Here are a few screenshots. First picture is before the separation, second is after the separation, third is before docking and the last one is the result of the docking, with 800 m/s of delta V.

https://imgur.com/DOIpYVd

 

 

 

 

[paul_c]

One way might be to "disable crossfeed" on the docking port(s), that way the delta V should be calculated only from the fuel it can reach via direct connections/its own stages etc. Docking ports are set to enable crossfeed by default, and stage separators are disabled by default.

Sometimes KSP won't calculate the deltaV of a craft with extra fuel/engines/whatever connected by docking ports, because it doesn't take into account that you might undock something to reduce weight and improve performance.

[Fraktal]

I don't know about the stock dV display, but the KER one doesn't combine dV values if you disable crossfeed on either docking port.

Damn ninja kraken...

Edited February 13, 2021 by Fraktal

[king of nowhere]

in general, the more complex your craft and your mission becomes, the less reliable is the game's calculation. nowadays i mostly run very complex stuff, and i always make my calculations by hand.

[K_mu]

Disabling crossfeed does work, thank you very much all . This will allow me to be more precise in my rocket design ! Fortunately I found a launch window allowing me to bring Jebediah back with those 700 m/s ^^

18 minutes ago, king of nowhere said:

i always make my calculations by hand.

My rockets are still pretty basics but I will have a look at the maths, out of curiosity.

Again thank you everyone for responding so quick !

[king of nowhere]

25 minutes ago, K_mu said:

My rockets are still pretty basics but I will have a look at the maths, out of curiosity.

 

to calculate the deltaV available there is a simple equation:

deltaV=Isp*g*ln(m₀/m_f)

where Isp is the specific impulse of the engines, g is the gravity of earth (yes, always of earth; it's because of the weird way Isp is defined), m₀ is the current mass of the rocket, and m_f is the empty mass. g can be easily approximated to 10

So, when I am in map view and not focusing view on any specific target, i can see the mass of my ship. By looking at my resources, I can also look at how many units of fuel and oxidizer i have, and each one of those is 5 kg. So, here is my really complex ship. the game has calculated 112 m/s in some weird way. it doesn't even have stages on its own, it's just that it has other ships docked, and those sometimes have stages.

anyway, i take the final mass, 1732.7 tons. then i sum up my oxidizer and liquid fuel and multiply by 5 kg, i get 543.5 tons of fuel. so i subtract the fuel to the current mass; the ship will be 1189.2 tons when dry. so I can divide the initial mass with the empty mass: 1732.7/1189.2=1,457. Now I make the natural logarithm of this number, I get 0.376. I know my ship is mounting 20 wolfhounds, which have Isp 380. So I multiply the logarithm I just got by 380*10.

0,376*380*10=1430 m/s. More info on wikipedia

As for TWR, I know each wolfhound is 375 kN. And 10 kN are basically equivalent to 1 ton, i.e. they will get TWR 1 on a 1 ton mass. with 20 wolfhounds I have 375*20=7500 kN = 750 tons. I just divide this by my mass, 750/1733=0.43.

But then, TWR 1 (in kerbin gravity, as calculated with this method) means 10 m/s of acceleration. So my ship will accelerate 4.3 m/s². It means that for my 360 m/s manuever, it will need 360/4.3=84 seconds. So I know to start the engines some 40 seconds before the node.

 

now, this calculation has limits. it works for a single ship, it gets more complicated otherwise. say you go to duna, then drop a lander, recover it, and return. you may want to consider the lander fuel as dead weight in your calculation. assume your lander will use 5 tons fo fuel, you will remove 5 tons from your fuel while making the calculations. this will give a wrong result: when you are returning, that fuel will be gone, your ship will be lighter, you will have more deltaV. but it is a decent approximation, and it will underestimate your deltaV, which is always good for an approximation. you can use that value when designing your duna craft.

but say that you want to go to jool, then you drop two shuttles to orbit every moon, and those shuttles will have smaller shuttles to land, and then they will rejoin your mothership, where one of them will stay, and the other, after binding two more other docked vessels, will get more fuel and go to Eeloo, while your mothership will go back to duna. what is your deltaV for your mothership? will that be enough to get to jool and back? the answer to that question is "screw that, i'll just eyeball it".  and afterwards, you'll find yourself 300 m/s short, spending hours trying to find some gravity assist to make it back...

[Spricigo]

7 hours ago, K_mu said:

 

Here are a few screenshots. First picture

Striking resemblance with Gossiper makes me instantly like your lander.

Anyways, it may be quite laborious to get a reliable deltaV amount for a rocket like your. Not only you need to consider that the fuel in the lander is not available for the previous stage, you also need to take in account the reduced weight when the lander rejoins the mothership and that any fuel left in the lander will now be available if needed. Is not really difficult, just there is several points where a mistake can happen.

7 hours ago, K_mu said:

Fortunately I found a launch window allowing me to bring Jebediah back

That is nice.

Had any trouble with capturing/reentry? The craft seems to not be optimized for taking the heat from a interplanetary approach, on the other hand, coming  from Duna may not get it to the point where it become an issue.

[K_mu]

On 2/13/2021 at 9:23 PM, king of nowhere said:

to calculate the deltaV available there is a simple equation

Thanks, I'll try to plan my next trip using this and your example.

On 2/14/2021 at 4:08 AM, Spricigo said:

Had any trouble with capturing/reentry?

Not that I can remember. The Mk1 Lander Can was equiped with four Mk12 Radial parachutes, one  MK16 parachute and had its own heat shield, the reentry worked just like the one I had done from the mun/minmus so far.

Once again tkanks, seems to be a great community around here :)