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Question about Fuel usage and staging


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I am using the dV map to build ships to travel to Dune and Eve.  My current build can get to an 80km Kerbal orbit on the first stage and has about 80 m/s dV left over after reaching orbit.  

I then try to set a maneuver node to get an encounter with my target but it inevitably takes much more dV than I have remaining in that first stage.  

Would I be better off to just separate my lift stage and waste that fuel so I can try to make my transfer in one burn,  or should I use that little bit of fuel to get it started and then plot a second burn after the tanks are emptied, or should I rework my lift stage to not have as much fuel and drop it before reaching LKO?

Not sure what the best choice is.  

Tell me your opinions and techniques.  

 

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In general, I would suggest keeping the fuel, doing your burn until it runs out, then staging off and continuing the burn with the next engine.  I'm unclear whether you're considering this one burn or two, but there's really not much disadvantage of doing so (the time it takes to stage and light the next engine should be trivial).  You should not need to do a separate maneuver node; you can just keep using the original.  The one tricky thing is it can be tough to know how long the next part of the burn will take, since your new stage will likely have different TWR.  You should be able to get a feel for this over time, but you can always also save the game, stage, check out the burn time gauge, and then reload.  

All that said, 80m/s is not a lot, so it's not a big deal if you just ditch it.  But with, say, 400 m/s left, it's probably worth dealing with a multi-engine burn.

Sometimes you want to split long burns anyway ("periapsis kicking"), so if you have about half the delta-v in one stage and half in the other, it can make sense to burn out one stage, and then start the next stage on the next orbit.  But again, with only 80m/s, it's probably not worth worrying about that.  

Final note: some folks like to organize their stages so as to minimize debris remaining in orbit.  Those people would probably try to set up the staging so one stage runs out just before you hit LKO, and will reenter and burn up.  (Like with the Space Shuttle external fuel tank).  But there's no essential gameplay reason for doing so.  If you don't care about debris, best to go with whatever design provides the best mix of delta-v, cost, thrust, etc.

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I like to gather and attach my spent boosters into a giant fuel depot in orbit. To do this, it helps if they have a little fuel left in them, so they can maneuver a bit on their own.

Also, the interplanetary ship probably already has enough fuel. But the only reason why a ship would go to my fuel depot is if it's desperately short on fuel. So fuel in the depot is actually sometimes more important than a little extra kick for an interplanetary ship.

 

Edited by bewing
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1 hour ago, BentRim said:

Would I be better off to just separate my lift stage and waste that fuel so I can try to make my transfer in one burn,  or should I use that little bit of fuel to get it started and then plot a second burn after the tanks are emptied, or should I rework my lift stage to not have as much fuel and drop it before reaching LKO?


Myself, I'd consider staging when your peri is still in atmosphere, and finish circularizing on the interplanetary injection stage.   Either way, with so little d/v remaining, I'd just get of the rid of it just to make things easier.

Edited by DerekL1963
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Thanks for the replys.  I was just trying to get a feel for the best way to handle the situation and I guess there really isn't a "best" way.  

@bewing Could you elaborate a little more on how you gather the boosters and get the fuel out of them?  I have a station around Kerbin with some large tanks that could use refilling and if I could do that by gathering debris it would be helpful.  

Always learning something new.  

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Well, there are several ways.

When I launch my station into LKO, it does not have any tanks. I expect to get the tanks by attaching spent booster stages. I also use klaws, instead of docking ports -- but you can use docking ports to do the same thing.
So the hub of my station tends to look something like this:

deorbiter.png

This kind of station has an additional use -- each little klaw is capable of deorbiting and landing pods, engines or other small parts for contracts.

 

Instead of using a decoupler on the top of your booster, you can put a docking port there instead. But the basic point is that if you can maneuver a spent booster stage over to this station and dock with it, then you can suck all the remaining fuel out with a manual transfer -- or put fuel into it for storage. I use small nuclear spacetugs with klaws on their noses for pushing things around in LKO also.

 

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23 hours ago, bewing said:

Well, there are several ways.

When I launch my station into LKO, it does not have any tanks. I expect to get the tanks by attaching spent booster stages. I also use klaws, instead of docking ports -- but you can use docking ports to do the same thing.

 

Instead of using a decoupler on the top of your booster, you can put a docking port there instead. But the basic point is that if you can maneuver a spent booster stage over to this station and dock with it, then you can suck all the remaining fuel out with a manual transfer -- or put fuel into it for storage. I use small nuclear spacetugs with klaws on their noses for pushing things around in LKO also.

 

 

Ah,  I see now...

 

I have only played KSP in career and don't have everything available yet so my knowledge and thought process is based on only the technology I have used so far in my personal career.

 

I guess I really need to go into sandbox for a bit and see what all the other parts do that I haven't played with yet.  

 

I appreciate your patience with all my novice questions...

 

 

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8 minutes ago, BentRim said:

I guess I really need to go into sandbox for a bit and see what all the other parts do that I haven't played with yet.

 

I always play career, but I am constantly switching back to sandbox mode for a launch or two or five to try new things out. Testing different parts, crazy rocket designs, intercept techniques .... If I have no idea whether something will work the way I want or not, a quick trip to the sandbox helps a lot. :)

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For 80 m/s I would not use it on the ejection burn.  Without careful fiddling your burn time is going to be wrong and thus your ejection angle will be wrong.  You could easily waste more fuel on the correction than you saved from using the booster.

However, I wouldn't jettison, either.  Instead I would raise my apoapsis however much I could with 40m/s of fuel, then circularize again.  I would jettison the virtually empty booster before setting up my ejection burn.  It's slightly less efficient but it's a lot less hassle.

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I don't build many rockets ,  however two things to consider

  • by 15km, an upper stage engine like the Terrier (Sea level ISP 85, Vacuum ISP 345) is giving better ISP than a first stage engine like the Reliant (265 Sea Level ISP, 310 Vacuum ISP)
  • by the time you're over 1600m/s and 35km, drag is negligible, orbital freefall is already counteracting most of the gravity, and your craft is much lighter than on launch due to fuel burnoff. 

What this means is i try to design my lower stage to burn out before reaching full orbit.  Not only is this good housekeeping (avoids littering LKO with spent boosters),  it should be more efficient, because a) the upper stage engine gives better ISP than the booster one  b)  avoids accelerating the heavy, powerful lower stage engines and their empty tanks the rest of the way to orbital velocity

So, ideally the OP's dilemma never arises.    The upper stage has the fuel for completing orbit insertion and the interplanetary mission.

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Ideally, yes. But it tends to be a lot cheaper to use SRBs for your lower stages. And they burn for a particular time with a particular thrust -- and by the time they burn out, you're halfway to orbit.

And the big LF engine on the booster does such a nice quick job of circularizing you after that. :D 

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23 hours ago, bewing said:

Ideally, yes. But it tends to be a lot cheaper to use SRBs for your lower stages. And they burn for a particular time with a particular thrust -- and by the time they burn out, you're halfway to orbit.

And the big LF engine on the booster does such a nice quick job of circularizing you after that. :D 

Well, you can tweak the fuel amount and thrust limit. (usefull if you standardize Launch Vehicles and Payloads. Otherwise will require a different setting for each launch, so probably more trouble than its worth). But agreed that often SRBs gives more Bang for Bucks (specialy because it tends to do the same job in less time, a more important currency than funds)

Another point is that usually is easier to deal with the problem of a overpowered booster(debris in orbit) than the problem of a underpowered booster(spaceship not in space). Also a overengineered first stage don’t cause the domino effect of a overengineered upper stage. So if you will have "too much rocket" somewhere, better to be in the first stage.

 

 

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On 05/03/2017 at 9:03 PM, bewing said:

Ideally, yes. But it tends to be a lot cheaper to use SRBs for your lower stages. And they burn for a particular time with a particular thrust -- and by the time they burn out, you're halfway to orbit.

And the big LF engine on the booster does such a nice quick job of circularizing you after that. :D 

I tend to overbuild the upper stage then.  I suspect if you're getting to orbit on the lower stage you overbuilt the lower stage and could have done with a little less fuel on the lower and a little more on the upper.    But I take the point that a lot of folks just like to have a general purpose lifter for missions and the payload mass varies.   Not enough lower stage risks not getting to space, so people just overbuild it and litter transmunar space with all sorts of boosters :-)

However, these days the fuel flow rules reduce the dilemma somewhat.

To take a simple example, I'll have a terrier powered upper stage with far more fuel than needed,  a decoupler , with crossfeed set to "enable", then some lower stage fuel tanks that are only enough to get us to 15km or so,  then a reliant.

The lower stage tanks empty first then the Reliant pulls fuel from the upper.   As soon as i'm confident of getting to orbit,  I fire the decoupler and go the rest of the way on the lighter. more efficient Terrier.   I put aero surfaces on both stages so i can re-enter with the complete upper stage rather than having to discard everything that is not the capsule, and maintain aerodynamic control.

Liquid fuel / oxidizer tanks don't weigh much when empty but are worth over half their full value,  the oxidizer tanks must be lined with platinum or something.    So if i can bring a few back, why not :-)

 

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6 hours ago, AeroGav said:

I tend to overbuild the upper stage then.  I suspect if you're getting to orbit on the lower stage you overbuilt the lower stage and could have done with a little less fuel on the lower and a little more on the upper.    But I take the point that a lot of folks just like to have a general purpose lifter for missions and the payload mass varies.   Not enough lower stage risks not getting to space, so people just overbuild it and litter transmunar space with all sorts of boosters :-)

In both cases the rocket is overbuilt in the sense that a smaller more balanced, light and cheap  craft would do the job. However KSP is forgiving enough that both can be made efficient(ish).

Also there is the matter of which parameter we choose to determine efficiency, different people will make different compromises.

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