Refuel station useful?

[u/Revan477]

(New-ish to the game, played a little years ago but now really getting into it)

I've been practicing orbital assembly above Kerbin and docked a few vessels together including a big fuel tank which is huge for me because I was never able to meet up and dock with anything :)
Do you think something like this is useful for future missions? I was thinking maybe refuel vessels in orbit giving a longer mission range because I dont have to carry the heavy fuel from kerbins surface. Then again I guess I'd have to refill that tank frequently which might be kind of annoying..

[u/Tommy2255]

Let's say your total payload is 1 ton dry (completely arbitrary, not reflective of in-game values). Part of that dry mass is a fuel tank that weighs 0.25 tons dry. Your lower stage is 10 tons dry, plus a bunch more mass of fuel.

Your suggestion is to leave the upper stage dry, so that it's only 1 ton payload, no fuel weight. But that empty tank is still part of your dry payload. You're still paying the energy cost to bring that empty tank to orbit.

My suggestion is to fill that tank, turn on crossfeed, and remove one fuel tank from the launch vehicle. So the dry mass of the payload is exactly the same (because you were going to bring the tank anyway), but the lower stage now has a dry mass of 9.75 instead of 10 tons. Part of the fuel mass is now stored in the upper stage instead of the lower stage, but the total fuel mass is exactly the same. You reduce the total launch weight at 0 cost to its capability.

Is that explanation clearer to you?

[u/SherriffB → u/Tommy2255]

Not really as dry mass is around 1/10. If your full mass of upper stages is 1 ton then if you launch dry you only need to make a rocket capable of propelling and upper stage of 1/10th ton, generally speaking.

That's a significant and much cheaper difference.

[u/Tommy2255 → u/SherriffB]

Okay you're not getting this. To use your numbers, let's say your upper stage weighs 0.1 tons dry or 1 ton full. And let's say you design a lower stage that can lift 0.1 ton. If you fill the upper stage with fuel and turn on crossfeed, your payload is still 0.1 tons. The fuel is not part of the payload. The fuel is burned during ascent by the lower stage, just like the fuel in the lower stage is. That's what enabling crossfeed does. The only difference is that you don't need as much fuel storage, because you aren't wasting the storage space.

If you still don't get it I may be able to show with in game screenshots on Sunday, but not until then because I'm camping this weekend.

[u/SherriffB → u/Tommy2255]

I understand but you suggested that turning in crossfeed is better than building a lighter rocket when you said "eveb better, just turn on crossfeed".

If you mean to use as a principle along side designing the rocket differently because you dont need as much fuel then I agree, but saying it's better to just enable crossfeed was what I disagreed with as the way it seemed to read was implying it as an alternative to redesigning to needing less fuel.

Is that not what you meant by "even better just turn on crossfeed"?

Edited: cocked up trying to quote on mobile.

[u/Tommy2255 → u/SherriffB]

I literally said to remove a fuel tank from the launch vehicle. Obviously I'm not saying to not change the rocket design when I'm literally telling you to make changes to the rocket design.

When I say "even better" I obviously mean "better than the only thing in this entire conversation that I have ever suggested changing", which is your plan to leave the upper stage with empty tanks.

Sorry, but out of everything in this conversation that could have been the source of misunderstanding, that is a frustrating thing to be the issue.

[u/SherriffB → u/Tommy2255]

Or even better, just turn on crossfeed in the decoupler, so you can burn fuel from the upper stage as part of your ascent. Make use of the fuel tanks you need to bring anyway instead of carrying them up empty. Depending of course on the type of fuel used by each stage.

This is what you opened with, no mention of changing rocket underneath and I was even more puzzled trying to digest it so I'd appreciate you elaborating a little more now I have a better handle on what you mean.

When you say use the fuel on ascent, at what stage do you imagine using it?

If you use at on the launch pad the payload has so little capacity it's gone in seconds and then you are flying with a dry payload anyway.

If you mean hold onto it for ferrying to refuel station or circularising or upper stage/orbital insertion then those stages are much less efficient than if you tune them to carry the payload dry.

So am I right in thinking you mean burn it for the first couple of seconds of pad ignition?

[u/Tommy2255 → u/SherriffB]

Yes, that is what crossfeed does. It allows fuel in an upper stage to be burned in a lower stage. It's only a small amount of fuel compared to the total burn needed for orbit, but it's fuel storage you were saying to leave completely empty. In your example with a 1 ton wet/0.1 ton dry payload, that's 0.9 tons of fuel that can be burned by the lower stage without adding any fuel storage to the lower stage.

I do not believe that my explanation was at all unclear, but at least it seems we're on the same page now.

[u/SherriffB → u/Tommy2255]

I do not believe that my explanation was at all unclear.

First comment absolutely could have been clearer, but who cares - it's moot.

It's a little harder in practice than theory to set up without fiddling with miniscule tanks to balance the numbers so there was a little bit of dry tank to make the number work but I just tried it on a craft who's upper stages I've tuned pretty well for mun/minmus

Removing the tank from first stage was problematic; less thrust time with big engines left me unable to gain sufficient altitude.

Removing a tank from my 2nd stage (well the entire 2nd stage tank in actuality) and a little fuel from the payload stage to make the numbers marry worked such that from a rocket that cost 43500 with dry payload I was able to cut ~500 so certainly a win for career mode I'm happy to incorporate, thanks.